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Laser Welding of Low Weldability Materials
Autogenous pulsed laser welding of aluminium alloy AA6082-T651
Rui Ravail Luz Rodrigues
Thesis to obtain the Master of Science Degree in
Materials Engineering
Supervisor: Prof. Maria Luísa Coutinho Gomes de Almeida
Co-supervisor: Prof. Maria de Fátima Reis Vaz
Examination Committee
Chairperson: Prof. Maria Amélia Martins de Almeida
Co-supervisor: Prof. Maria de Fátima Reis Vaz
Members of the Committee: Prof. Eurico Gonçalves Assunção
Prof. Inês da Fonseca Pestana Ascenso Pires
December 2015
I
Acknowledgements
It would not have been possible to realize this master thesis without the involvement of Prof.
Dr. Maria Luísa Coutinho Gomes de Almeida hence I must express my deepest gratitude for her
dedication and guidance throughout the semester. Her patience and encouragements kept me
focused and helped me finish on time; I could not have imagined having a better advisor. I would also
like to thank Prof. Dr Maria de Fátima Reis Vaz for her contribution and support during all my
experimental work at Instituto Superior Técnico.
Then, I would like to thank Dr. Phill Carr for his hospitality and Eng. João Miguel Martins Silva
for his unwavering support. Both were essential for the realization of this project. This was truly an
edifying experience and I was really fortunate to have had this opportunity.
A final word to my family and close friends who were always present whenever needed.
II
III
Abstract
Autogenous laser welding of the AA6082-T651 aluminium alloy was investigated with 3 lasers,
namely a pulsed laser of 300W, a disk laser of 4kW and a laser marker of 70W. First, the hot cracking
susceptibility was studied with two conventional laser welding equipment, without using filler material
or heat treatment. Additionally, an attempt was made to weld with a laser marking equipment. Most of
the welds made were laser seam welds but, some continuous and laser spot welds were also tested.
As each laser was operated with different parameters, more than 400 welds were obtained with a wide
range of parameters. Selected welds were studied with visual inspection, dye penetrant inspection
(DPI), optical microscopy and scanning electron microscopy (SEM) with energy dispersive X-ray
spectroscopy (EDS). Results indicated that welds with pulse laser beam tend to develop hot cracking,
due to the segregation of silicon-rich low melting point eutectics to the grain boundaries and the
development of contraction stresses during solidification. On the other hand, positive results were
found with continuous welding since this process results in much longer solidification times and lower
stresses. Finally, promising results were obtained with the laser marking machine, which produced
high aspect ratio laser seam welds without hot cracking and a penetration of 1 mm using 99.9% of
overlap factor. These welds were crack free due to their small weld pool, avoiding segregation effects,
and to the heat build-up of successive spots, what had similar effect to solidification in continuous
welding.
Keywords:
Aluminium alloy AA6082-T651
Autogenous laser welding
Hot cracking
Nanosecond pulse welding
Pulsed laser welding
IV
Resumo
O objetivo deste trabalho foi estudar a viabilidade da soldadura laser sem material de adição
ou tratamentos térmicos da liga de alumínio AA6082-T651, atendendo a sua suscetibilidade à
fissuração a quente. A pesquisa foi efetuada com 3 lasers diferentes: um laser pulsado de 300W, um
laser de disco de 4000W e um laser de marcação de 70W. Foram efetuadas maioritariamente
soldaduras pulsadas em costura e adicionalmente soldaduras contínuas e por pontos. Mais de 400
soldaduras foram produzidas e testaram uma larga variedade de parâmetros de funcionamento. As
soldaduras selecionadas foram analisadas por inspeção visual, DPI, microscopia ótica e SEM com
EDS. Os resultados indicaram que a soldaduras pulsada tendem a desenvolver fissuração a quente
devido à segregação de compostos eutécticos de baixo ponto de fusão, ricos em silício, para as
zonas de limite de grão e ao desenvolvimento de tensões de contração durante a solidificação. Foram
obtidos resultados positivos com soldadura contínua, visto que este processo resulta em maiores
tempos de solidificação e menores tensões. Finalmente, foram obtidos resultados promissores com o
equipamento de marcação laser que neste caso produziu soldaduras em costura, com elevado rácio
de penetração/largura, sem fissuração a quente e com penetração de 1 mm usando 99.9% de fator
de sobreposição. Estas soldaduras não evidenciaram fissuração, devido ao reduzido tamanho do
banho de soldadura, que evitou os efeitos da segregação e à acumulação de calor dos sucessivos
pulsos, produzindo efeito semelhante ao que ocorre na solidificação da soldadura continua.
Palavras-chaves:
Liga de alumínio AA6082-T651
Soldadura laser autogénea
Fissuração a quente
Soldadura com pulsos de nano-segundos
Soldadura com laser pulsado
V
Table of Contents
Acknowledgements .................................................................................................................................. I
Abstract................................................................................................................................................... III
Resumo .................................................................................................................................................. IV
Table of Contents .................................................................................................................................... V
List of Figures ....................................................................................................................................... VIII
List of Tables ........................................................................................................................................... X
Abbreviations .......................................................................................................................................... XI
1. Introduction ...................................................................................................................................... 1
1.1 Objectives and motivation ....................................................................................................... 1
1.2 Document structure ................................................................................................................. 1
2. Literature Review ............................................................................................................................. 3
2.1 Aluminium ................................................................................................................................ 3
2.1.1 Commercial alloys ............................................................................................................... 3
2.1.2 Common welding processes ............................................................................................... 4
2.2 Laser welding........................................................................................................................... 5
2.2.1 Laser physics ....................................................................................................................... 5
2.2.2 Advantages of laser welding ................................................................................................ 6
2.2.3 Laser types and parameters ................................................................................................ 7
2.2.4 Welding modes .................................................................................................................... 7
2.2.4.1 Conduction mode welding ........................................................................................... 8
2.2.4.2 Keyhole or deep penetration welding mode ................................................................ 8
2.3 Weldability of aluminium alloys................................................................................................ 8
2.3.1 Typical welding problems .................................................................................................... 8
2.3.2 Hot cracking ......................................................................................................................... 9
2.3.2.1 Metallurgical factors ................................................................................................... 10
2.3.2.2 Mechanical factors ..................................................................................................... 12
2.3.2.3 Reducing hot cracking ............................................................................................... 13
2.3.3 Hot cracking in laser welding ............................................................................................. 14
2.4 Summary ............................................................................................................................... 16
3. Experimental Procedure ................................................................................................................ 18
3.1 Base material characteristics ................................................................................................. 18
3.1.1 Composition ....................................................................................................................... 18
3.1.1.1 Alloying elements: silicon and magnesium ................................................................ 18
3.1.1.2 Alloying element: manganese ................................................................................... 20
3.1.2 Temper .............................................................................................................................. 20
3.1.2.1 Temper steps ............................................................................................................. 20
3.1.2.2 Precipitation hardening .............................................................................................. 21
3.1.2.3 Ageing process .......................................................................................................... 22
3.1.3 Properties and applications ............................................................................................... 23
3.2 Laser equipment and trials .................................................................................................... 23
3.2.1 Technical data – AL 300 laser ........................................................................................... 23
VI
3.2.2 1st to 3
rd trials – AL 300 laser ............................................................................................. 24
3.2.2.1 1st trial ........................................................................................................................ 25
3.2.2.2 2nd
trial ....................................................................................................................... 25
3.2.2.3 3rd
trial ........................................................................................................................ 26
3.2.3 Technical data – TruDisk 4002 laser ................................................................................. 26
3.2.4 4th to 6th trials – TruDisk 4002 laser ................................................................................. 27
3.2.4.1 4th trial ........................................................................................................................ 28
3.2.4.2 5th trial ........................................................................................................................ 28
3.2.4.3 6th trial ........................................................................................................................ 29
3.2.5 Technical data – G4 Series Z Type laser .......................................................................... 29
3.2.6 7th trial – G4 Series Z Type laser ....................................................................................... 30
4. Results and Analysis ..................................................................................................................... 31
4.1 Penetration analysis .............................................................................................................. 31
4.1.1 Spot size of 0.2 mm ........................................................................................................... 31
4.1.2 Spot size of 0.3 mm ........................................................................................................... 33
4.1.3 Spot size of 0.4 mm ........................................................................................................... 34
4.1.4 Spot size of 0.5 mm ........................................................................................................... 36
4.1.5 Overall penetration results ................................................................................................. 37
4.2 Hot cracking analysis ............................................................................................................. 38
4.2.1 Visual inspection 2nd
trial ................................................................................................... 39
4.2.1.1 Spot size 0.2 mm ....................................................................................................... 39
4.2.1.2 Spot size 0.3 mm ....................................................................................................... 40
4.2.1.3 Spot size 0.4 mm ....................................................................................................... 41
4.2.1.4 Spot size 0.5 mm ....................................................................................................... 42
4.2.2 Dye penetrant inspection 2nd
trial ...................................................................................... 43
4.2.2.1 Results of DPI ............................................................................................................ 43
4.2.2.2 Observations .............................................................................................................. 44
4.2.3 Visual inspection 3rd
trial .................................................................................................... 45
4.2.3.1 Spot size 0.2 mm ....................................................................................................... 45
4.2.3.2 Spot size 0.5 mm ....................................................................................................... 46
4.2.4 SEM examination............................................................................................................... 48
4.2.4.1 Welding speed 1 mm/s .............................................................................................. 48
4.2.4.2 Welding speed 2.5 mm/s ........................................................................................... 49
4.2.4.3 Welding speed 4 mm/s .............................................................................................. 50
4.2.5 Overall results .................................................................................................................... 50
4.3 Chemical analyses ................................................................................................................ 51
4.3.1 Chemical compositions of the face of the welds ............................................................... 51
4.3.1.1 Welding speed of 1 mm/s .......................................................................................... 52
4.3.1.2 Welding speed of 2.5 mm/s ....................................................................................... 53
4.3.1.3 Welding speed of 4 mm/s .......................................................................................... 54
4.3.1.4 Prepared sample surface .......................................................................................... 55
4.3.1.5 Face of the welds vs. prepared sample surface ........................................................ 56
VII
4.3.2 Chemical compositions of the fusion zone of the welds .................................................... 56
4.3.2.1 Penetration ................................................................................................................ 57
4.3.2.2 Welding speed of 1 mm/s – AL 300 laser .................................................................. 58
4.3.2.3 Welding speed of 2.5 mm/s – AL 300 laser ............................................................... 59
4.3.2.4 Welding speed of 4 mm/s – AL 300 laser .................................................................. 60
4.3.2.5 Welding speed of 4.5 mm/s – TruDisk 4002 laser ..................................................... 61
4.3.2.6 Material with cracks vs. material without cracks ........................................................ 62
4.3.2.7 Final interpretation of the fusion zone ....................................................................... 64
4.3.3 Continuous and spot welds ............................................................................................... 64
4.3.3.1 Continuous welds ...................................................................................................... 65
4.3.3.2 Spot welds ................................................................................................................. 66
4.4 Optical microscopy analysis .................................................................................................. 68
4.4.1 Characterization of the welds ............................................................................................ 68
4.4.2 Penetration results ............................................................................................................. 69
4.4.3 Research interpretation ..................................................................................................... 70
5. Conclusions ................................................................................................................................... 72
References ............................................................................................................................................ 75
Annexe A – Bloc sample cutting and mounting .................................................................................. A - 1
Annexe B – Sample grinding, polishing and etching .......................................................................... B - 1
Annexe C – SEM with EDS equipment ............................................................................................. C - 1
Annexe D – Pulsed shapes ............................................................................................................... D - 1
Annexe E – Dye penetrant inspection of spot size 0.3 and 0.4 mm .................................................. E - 1
VIII
List of Figures
Figure 1 Worldwide evolution of recycled and primary aluminium [2] ..................................................... 3 Figure 2 Schematic of (a) absorption, (b) spontaneous emission, and (c) stimulated emission [10]...... 6 Figure 3 Difference between the output power of CW and PW [18] ....................................................... 7 Figure 4 Effect of chemical composition of weld metal on relative crack susceptibility in various aluminium alloys [25], [36] ..................................................................................................................... 11 Figure 5 Effect of composition on crack susceptibility [25] .................................................................... 11 Figure 6 Guide to choose the filler metals for minimizing hot cracking in welds of high-strength aluminium alloys [25] ............................................................................................................................. 13 Figure 7 Effects of alloying elements on cracking sensitivities [56] ...................................................... 16 Figure 8 Al-Si-Mg alloys: (left) Al corner of ternary phase diagram; (right) Al-Mg2Si pseudo-binary section [60] ............................................................................................................................................ 19 Figure 9 Variation of main alloying elements in different Al-Mg-Si alloys [61] ...................................... 19 Figure 10 Quasi-binary section through the aluminium-rich corner of the ternary Al-Mg-Si phase diagram [61] ........................................................................................................................................... 21 Figure 11 The relationship between phases observed and ageing condition [63] ................................ 22 Figure 12 Strength evolution during artificial (and natural) ageing [61] ................................................ 22 Figure 13 AL 300 laser with the AL-T 500 ............................................................................................. 24 Figure 14 TruDisk 4002 laser with the KR16......................................................................................... 27 Figure 15 G4 Series Z Type laser ......................................................................................................... 29 Figure 16 Penetration vs. peak power for spot size of 0.2 mm ............................................................. 31 Figure 17 Refining of penetration vs. peak power for spot size of 0.2 mm ........................................... 32 Figure 18 Weld profiles for spot size of 0.2 mm .................................................................................... 32 Figure 19 Penetration vs. peak power for spot size of 0.3 mm ............................................................. 33 Figure 20 Refining of Penetration vs. peak power for spot size of 0.3 mm ........................................... 33 Figure 21 Weld profiles for spot size of 0.3 mm .................................................................................... 34 Figure 22 Penetration vs. peak power for spot size of 0.4 mm ............................................................. 34 Figure 23 Refining of penetration vs. peak power for spot size of 0.4 mm ........................................... 35 Figure 24 Weld profiles for spot size of 0.4 mm .................................................................................... 35 Figure 25 Penetration vs. peak power for spot size of 0.5 mm ............................................................. 36 Figure 26 Refining of penetration vs. peak power for spot size of 0.5 mm ........................................... 36 Figure 27 Weld profiles for spot size of 0.5 mm .................................................................................... 37 Figure 28 – 2
nd trial with spot size of 0.2 mm ........................................................................................ 39
Figure 29 – 2nd
trial with spot size of 0.3 mm ........................................................................................ 40 Figure 30 – 2
nd trial with spot size of 0.4 mm ........................................................................................ 41
Figure 31 – 2nd
trial with spot size of 0.5 mm ........................................................................................ 42 Figure 32 – 2
nd trial DPI results with spot size of 0.2 mm ..................................................................... 43
Figure 33 – 2nd
trial DPI results with spot size of 0.5 mm ..................................................................... 44 Figure 34 – 3
nd trial welds with spot size of 0.2 mm .............................................................................. 46
Figure 35 – 3rd
trial welds with spot size of 0.5 mm .............................................................................. 47 Figure 36 SEM images of 2
nd trial with welding speed of 1 mm/s ......................................................... 48
Figure 37 SEM images of 2nd
trial with welding speed of 2.5 mm/s ...................................................... 49 Figure 38 SEM images of 2
nd trial weld with welding speed of 4 mm/s ................................................ 50
Figure 39 Example of EDS spectrum obtained for the chemical analysis ............................................ 51 Figure 40 EDS locations of the face of the weld for 1 mm/s of welding speed ..................................... 52 Figure 41 EDS chemical compositions of the face of the weld for 1 mm/s of welding speed ............... 52 Figure 42 EDS locations of the face of the weld for 2.5 mm/s of welding speed .................................. 53 Figure 43 EDS chemical compositions of the face of the weld for 2.5 mm/s of welding speed ............ 53 Figure 44 EDS locations of the face of the weld for 4 mm/s of welding speed ..................................... 54 Figure 45 EDS chemical compositions of the face of the weld for 4 mm/s of welding speed ............... 54 Figure 46 EDS locations of the prepared sample surface ..................................................................... 55 Figure 47 EDS chemical compositions of the prepared sample surface .............................................. 55 Figure 48 Mean chemical composition of the face of the welds vs. prepared sample surface ............. 56 Figure 49 SEM images of the fusion zone of the welds ........................................................................ 57 Figure 50 EDS locations of the fusion zone of the weld for 1 mm/s of welding speed ......................... 58 Figure 51 EDS chemical compositions of the fusion zone of the weld for 1 mm/s of welding speed ... 58 Figure 52 EDS locations of the fusion zone of the weld for 2.5 mm/s of welding speed ...................... 59 Figure 53 EDS chemical compositions of the fusion zone of the weld for 2.5 mm/s of welding speed 59 Figure 54 EDS locations of the fusion zone of the weld for 4 mm/s of welding speed ......................... 60
IX
Figure 55 EDS chemical compositions of the fusion zone of the weld for 4 mm/s of welding speed ... 60 Figure 56 EDS locations of the fusion zone of the weld for 4.5 mm/s of welding speed ...................... 61 Figure 57 EDS chemical compositions of the fusion zone of the weld for 4.5 mm/s of welding speed 61 Figure 58 Mean chemical compositions of the locations with vs. without cracks.................................. 62 Figure 59 Mean chemical compositions of locations with cracks for each welding speed ................... 62 Figure 60 SEM images of the continuous weld with 2.8 kW of power .................................................. 65 Figure 61 SEM images of the continuous with 3.4 kW of power ........................................................... 66 Figure 62 SEM images of spot welds with 0.38 mm of spot size and 1 ms of pulse duration without cracks at the face .................................................................................................................................. 67 Figure 63 SEM images of spot welds with 0.38 mm of spot size and 1 ms of pulse duration with visible cracks at the face .................................................................................................................................. 68 Figure 64 – 7
th trial with spot size of 0.051 mm ..................................................................................... 69
Figure 65 – 7th trial with pulse duration of 350 ns and 3.6 mm/s of welding speed .............................. 70
Figure 66 Classical beam matter interaction [75] .................................................................................. 71
X
List of Tables
Table 1 Composition series of wrought and cast aluminium alloys [1] ................................................... 4 Table 2 Typical welding problems in aluminium alloys [25] .................................................................... 9 Table 3 Chemical composition of aluminium alloy 6082 [1], [57] .......................................................... 18 Table 4 Physical properties for aluminium alloy 6082 [57] .................................................................... 23 Table 5 Mechanical properties for aluminium alloy 6082 [57] ............................................................... 23 Table 6 Summary of laser equipment and trials .................................................................................... 23 Table 7 Technical data of the AL 300 laser with the AL-T 500 [64], [65] .............................................. 24 Table 8 Technical data of the TruDisk 4002 laser with the KR16 [66]–[68] .......................................... 27 Table 9 Technical data of the G4 Series Z Type laser [69], [70] ........................................................... 29 Table 10 Penetration results of initial welds plots ................................................................................. 37 Table 11 Penetration results of refined plots ......................................................................................... 38 Table 12 Penetration, heat input and power density of the 4 welds...................................................... 58 Table 13 Results of the visual inspection of the spot welds .................................................................. 67 Table 14 Penetration results of the single pass welds .......................................................................... 70
XI
Abbreviations
AC Alternating Current
BTR Brittle Temperature Range
CW Continuous Wave
DPI Dye Penetrant Inspection
EBW Electron Beam Welding
EDS Energy Dispersive X-ray Spectroscopy
FZ Fusion Zone
GB Grain Boundary
GMAW Gas Metal Arc Welding
GP-zone Guinier-Preston zone
GTAW Gas Tungsten Arc Welding
HAZ Heat Affected Zone
MIG Metal Inert Gas
Nd:YAG Neodymium Yttrium Aluminium Garnet
PAW Plasma Arc Welding
PMZ Partially Melted Zone
PW Pulsed Wave
RSW Resistance Spot Welding
SEM Scanning Electron Microscope
SSSS Supersaturated Solid Solution
TIG Tungsten Inert Gas
XII
1
1. Introduction
1.1 Objectives and motivation
Laser welding is becoming the main welding choice for many industries. Mass production and
cutting edge industries like automotive and airspace evidence this trend. This technology offers
important mechanical and economical advantages. However considering aluminium alloys, process
parameters to assure adequate laser welding results have yet large room for improvement unlike in
more conventional welding processes. Hence, one of the objectives of this thesis is to complement the
existing knowledge about laser welding of low weldability aluminium alloys, precisely the AA6082-
T651 alloy.
Laser welding equipment is split in pulsed and continuous lasers and these types of
equipment serve different purposes. Continuous lasers are widely used in the industry due to their
very high productivity and the quality and strength of the welds it achieves. Initial investment cost is
usually higher than for pulsed lasers but they require less maintenance and need fewer spare parts
replacements which can balance its life cycle cost. Pulsed lasers, on the other hand, cannot compete
with continuous lasers in terms of productivity but are particularly appropriate for micro-welding
(precision welding) and other special applications including repairs of expensive mechanical parts like
injection moulds or also for small series manufacturing. These two last examples are the core
business of Carr’s Welding Technologies Ltd. which is a laser job shop located in Kettering (England)
where most of the welding for this thesis was performed. This company owns both continuous and
pulsed lasers and its request for this study was to investigate the possibility of laser welding AA6082-
T651 aluminium alloy, using one of their pulsed lasers, specifically the AL 300 of Alpha Laser to obtain
quality welds with 1 mm of penetration and more than 60% overlap factor.
The use of filler material, dedicated welding mounts or special heat treatments significantly
increase the difficulty and cost of a welding procedure. Therefore, from a cost reduction perspective
this research centred its tests on attempting to accomplish pulsed laser welding without heat treatment
or filler material.
To resume, this thesis is focused on researching the possibility of pulsed laser welding the
AA6082-T651 aluminium alloys without filler material, referred to as pulsed autogenous laser welding.
It is known that the viability of this objective is conditioned by the high probability of occurrence of hot
cracking. This has led to a complementary research using scanning electron microscope (SEM) with
energy dispersive X-ray spectroscopy (EDS) detector and other inspection methods to study the
metallurgical causes of hot cracking when welding this aluminium alloy.
1.2 Document structure
This document is organized in 5 chapters and several subsections. The 1st chapter,
Introduction, presents the motivation behind this work and what outcomes were expected followed by
a brief description of the chosen document structure of this study. The 2nd
chapter, Literature Review,
goes through the related work to outline the essential and current knowledge about aluminium, laser
2
welding and hot cracking. The information regarding aluminium and laser welding is introductory
whereas emphasizes was given on the hot cracking explanation since it holds particular importance
for this thesis analysis. The 3rd
chapter, Experimental Procedure, begins with the specifications and
characteristics of the AA6082-T651 aluminium alloy, highlighting the importance of composition and
temper on this material. Subsequently the lasers and trials description starts by presenting and
characterising each of the 3 laser equipment used, followed by the sequence of the respective trials.
With the 3 distinct lasers used, 7 trials were made that summed a total of 412 welds. The 4th chapter,
Results and Analysis, include results of the visual inspection, dye penetrant inspection (DPI), SEM
images, EDS analysis and optical microscope images. In this chapter, these results were interpreted
with references that support the analysis performed. Finally, the 5th chapter, Conclusions, resumes the
central ideas that were stated and discussed in this thesis and suggests possible future work
regarding autogenous pulsed laser welding of aluminium alloys.
3
2. Literature Review
2.1 Aluminium
Since the end of the 19th century aluminium became an interesting option for many
engineering applications. As it is the second most plentiful metallic element on earth, its extraction and
production grew worldwide over the past 60 years (Figure 1). Nowadays, the aluminium industry is the
largest non-ferrous metal industry in the world economy [1].
Figure 1 Worldwide evolution of recycled and primary aluminium [2]
Aluminium is a metal that offers a high level of versatility, with more than 300 alloy
compositions developed. These alloys offer a wide range of physical and mechanical properties
including low density, high specific strength, good corrosion resistance, good workability, high thermal
and electrical conductivity, attractive appearance, and intrinsic recyclability [3]. Aluminium alloys can
be found in many applications, namely in the markets of transportation (planes, trains, automobiles,
bicycles), buildings and construction (doors, windows, frames, siding), packaging (cans, packaging
foil), engineering applications and cables (heat sinks, electrical transmission lines, aluminium
conductor steel-reinforced cables) [2], [4], [5].
2.1.1 Commercial alloys
As mentioned, there are many aluminium alloys commercially available. As the major
producing countries established their own classification, each alloy found itself to have multiple
references. However, these classifications share the same structure. Consequently the explanation of
a single one, namely the broadly recognized “Aluminium Association” system, reviews all others.
The Aluminium Association system divides aluminium alloys between wrought and cast alloy
compositions. These different terminologies have been established to differentiate the respective
alloys according to their composition. Subsequently, cast and wrought alloys are divided in 9
composition series (Table 1), each with a different selection of alloying elements. Finally, in terms of
heat treatment a single terminology is applicable for wrought and cast alloys alike [1].
4
Table 1 Composition series of wrought and cast aluminium alloys [1]
Wrought composition series Cast composition series
1xxx Controlled unalloyed (pure) compositions
2xxx Alloys in which copper is the principal alloying element, though other elements, notably magnesium, may be specified
3xxx Alloys in which manganese is the principal alloying element
4xxx Alloys in which silicon is the principal alloying element
5xxx Alloys in which magnesium is the principal alloying element
6xxx Alloys in which magnesium and silicon are principal alloying elements
7xxx Alloys in which zinc is the principal alloying element, but other elements such as copper, magnesium, chromium, and zirconium may be specified
8xxx Alloys including tin and some lithium compositions characterizing miscellaneous compositions
9xxx Reserved for future use
1xx.x Controlled unalloyed (pure) compositions
2xx.x Alloys in which copper is the principal alloying element, but other alloying elements may be specified
3xx.x Alloys in which silicon is the principal alloying element, but other alloying elements such as copper and magnesium are specified
4xx.x Alloys in which silicon is the principal alloying element
5xx.x Alloys in which magnesium is the principal alloying element
6xx.x Unused
7xx.x Alloys in which zinc is the principal alloying element, but other alloying elements such as copper and magnesium may be specified
8xx.x Alloys in which tin is the principal alloying element
9xx.x Unused
The two major categories of aluminium alloys, wrought and cast, are fundamentally different in
terms of mechanical properties and so generally they cannot be used for the same purposes.
Considering the applications of aluminium in the industry, cast alloys are less used than wrought
alloys. Furthermore, the casting process has already been thoroughly investigated. As a result, more
studies have been recently found on wrought aluminium alloys than on cast aluminium alloys and
particular attention has been given to relatively new welding processes such as friction stir welding
and laser welding.
2.1.2 Common welding processes
While many welding processes have been industrially employed to weld both cast and
wrought aluminium alloys such as arc, resistance, friction, electron beam, and laser welding [3], the
most common welding processes for aluminium welding are [6]–[8]:
a) Gas-Tungsten Arc Welding (GTAW) or Tungsten Inert Gas (TIG)
This process creates an electric arc between a tungsten electrode and the workpiece to reach
the necessary melting temperatures for welding. Inert gas protection is necessary and usually argon is
used. Helium and mixtures of helium with argon can also be used [7].
b) Plasma Arc Welding (PAW)
This is an advanced version of the gas-tungsten arc welding process. It uses a specific torch
with a constricted nozzle and 2 gas flows to form a concentrated plasma arc which delivers a more
concentrated welding heat source than the gas-tungsten arc welding [6].
5
c) Gad Metal Arc Welding (GMAW) or Metal-Inert Gas (MIG)
Once more, this is an arc welding process. The main difference is that this process uses
consumable metal electrodes (wire) with either inert or active gas protection to create an electric arc
for welding [7].
d) Electron Beam Welding (EBW)
In this process welding is achieved by bombarding the workpiece with an intense beam of very
highly accelerated electrons (0.3 to 0.7 of the speed of light). The impact of these electrons against
the workpiece creates the necessary heat for welding. Usually vacuum is used but it is not an
essential requirement [7].
e) Resistance Spot Welding (RSW)
This is a process where 2 (or more) metal sheets are welded together at particular spots with
heat generated by electrical resistance. This process uses a pair (or more) of electrodes which deliver
the necessary current to weld each spot and impose a clamping force on the sheets, to maintain a
tight seal [7].
Aluminium can be welded with any other welding process, given that the necessary
preparations are made and the adequate parameters are used. Nevertheless, due to its inherent
characteristics when compared to all other processes, laser welding stands out as one of the most
promising welding methods for aluminium alloys [3].
2.2 Laser welding
"LASER" is an acronym for "Light Amplification by Stimulated Emission of Radiation" and a
laser is a device which generates or amplifies light. The light of these devices, known as laser beam,
has some unique properties specifically [9]:
Monochromatic: has a single wavelength
Directional: shows low divergence
Intense: has a high density of photons
Coherent: has the same phase relationship
2.2.1 Laser physics
The theoretical understanding of the process of generation or amplification of light was
developed by Plank and Einstein in the beginning of the 20th century and is known as the quantum
theory of light. This theory has multiple statements regarding the nature of light, one of which allowed
the development of the laser devices. This concept relates to the generation of photons triggered by
the transition of an atom or a molecule from an excited, more energetic, state (E2), to a lower and less
energetic state (E1). To generate a sustainable laser beam emission, 3 different processes must
happen simultaneously: stimulation absorption, spontaneous emission and stimulated emission
(Figure 2) [9]–[13].
6
Figure 2 Schematic of (a) absorption, (b) spontaneous emission, and (c) stimulated emission [10]
The photons emitted by stimulated emission have the same energy, wavelength, phase and
direction of the incident stimulating photons. These collective photons create a light emission called
laser beam which has the properties mentioned above [9]–[13].
2.2.2 Advantages of laser welding
Currently, lasers are used extensively in the processing industry, notably to join materials,
which is one of the earliest recorded applications. Laser joining involves the use of a high power laser
beam as heat source to fuse or join two solids. Joining can be made through different techniques
which include welding, brazing, soldering and micro welding [14]. The use of a laser for welding offers
certain advantages over the more conventional arc welding processes such as [7], [15]:
High processing speeds with instant start and stop
High energy density output
Join difficult-to-weld materials (ex.: titanium or quartz)
Creates lower distortion thus the workpiece requires no fixation
No electrodes or filler materials are required but filler material is optional
Narrow and very precise welds can be made
Welds with little or no contamination can be produced
The heat-affected zone adjacent to the weld is very narrow
However, this welding method also has some unfavourable aspects such as [7]:
Part fit-up and alignment are critical
Investment cost for laser welding is expensive
These disadvantages may have been a problem earlier, when this new technology was more
expensive and less reliable. But today’s popularity of lasers, reflected by their growing use, indicates
that this technology is becoming increasingly more profitable. Some industrial segments have already
implemented this option, namely the automotive and, in particular, the aerospace industry [15]–[17]
due to their necessity to join dissimilar materials with different section thickness, compositions,
physical or chemical properties [14], [15].
7
2.2.3 Laser types and parameters
There are two main types of lasers, pulsed wave (PW) lasers and continuous wave (CW)
lasers. Both are used to weld aluminium alloys and the key difference between them is the power
output. For the same the average power, pulsed laser are capable of delivering a high output power
within a short time through what is called a pulse. Comparatively continuous laser is capable to
maintain a continuous laser beam emission indefinitely but with a lower output (Figure 3) [18].
Generally CW lasers are used for high speed welding whereas PW lasers are used for precision
welding [3]. Furthermore, the welds obtained, with one type of equipment or the other, are very
different. Welding aluminium alloys with CW lasers produces welds free of porosity, cavity, or hot
cracking and with good bead appearance, while the welds obtained with PW lasers may have worse
bead appearances and can also have underfill, porosity, cavities, and cracks [19], [20]. Having a
different laser output influences the welds obtained but also the process parameters that are used to
define each type of laser. The parameters normally used in CW laser welding are welding speed,
power and spot size (or beam diameter) while the parameters normally used in PW laser welding are
pulse duration, pulse frequency, pulse shape, peak power spot size and welding speed [3], [18], [21],
[22]. These differences of process parameters can be problematic when attempting to compare or
replicate the results of a CW laser with a PW laser and vice versa [18].
Figure 3 Difference between the output power of CW and PW [18]
Besides these differences between types of lasers there are other process parameters which
affect the quality, size and properties of the welds. Other important parameters include the welding
speed for both continuous and seam welds and the welding time for seam welds. Additionally some
parameters are related to the gas used to shield the welding process, namely the type of gas, the flow
rate of gas, the angle of gas flow to the workpiece and the nozzle design [21].
2.2.4 Welding modes
Laser welding has 2 principal modes (or regimes) which are conduction and keyhole (or deep
penetration) mode but for some studies an additional mode can be defined between those two
namely, transition mode [23]. For conduction and keyhole modes, it is generally accepted that below a
8
certain power density, around 106 W/cm
2 for aluminium, the welding process is within conduction
mode while above this value it is within keyhole mode. However the transition between modes is a
complex subject which does not depends solely of the power density of the welding process.
Therefore it is preferable to describe these modes with their respective characteristics [18].
2.2.4.1 Conduction mode welding
Conduction mode is stable process with no vaporization which results in accurate control of
the heat input [18]. In conduction mode welding, the surface of the material is heated above its melting
point but below its vaporization temperature. In a similar way to conventional fusion welding
processes, the fusion with laser occurs only by heat conduction and a semicircular weld bead with an
aspect ratio of 1.2 or less is formed [3]. Typically this mode uses larger beams that have a good gap
bridging ability. Consequently, there is no need for laser systems with high quality beams.
Furthermore, the welds made in conduction, generally do not present porosity, cracks, undercuts and
spatter. However, this process is slow and has a lower coupling efficiency which results in a lower
productivity. Moreover it has a higher heat input which results in higher distortion [18]. Lastly, this
welding mode is limited to materials with relatively thin thickness [3].
2.2.4.2 Keyhole or deep penetration welding mode
Keyhole mode uses a higher power density to obtain partial vaporization of the material in
order to reach much higher penetrations [3]. This welding mode is typically unstable and tends to
produce welds with high level of porosity, high amount of spatter and loss of alloying elements.
Additionally, the degradation of mechanical properties of the base material is possible. Unlike
conduction, this mode has a low gap bridging capability and requires a laser system with a relatively
good beam quality. On the other hand, this welding mode can make deep penetration, high aspect
ratios welds using low heat input and low distortion. Finally, keyhole welding mode has a much higher
productivity than conduction welding mode [18].
2.3 Weldability of aluminium alloys
For conventional welding methods, the main factors that affect the welding of aluminium
include parent metal, consumables, design, welding procedures, welding equipment and joint
preparation. Since conventional welding processes like MIG, TIG or PAW are used regularly and have
been studied extensively over the years, all the necessary information to make good quality welds is
already detailed in different standards [24]. However, this is not yet the case of laser welding as
information regarding difficult welding situations is still quite limited. Thus, much on-going research is
focused on studying and improving the weldability of aluminium alloys using laser welding equipment.
2.3.1 Typical welding problems
Aluminium alloys are used for numerous applications and are considered to have a good
weldability. However, some alloys, under certain circumstances are vulnerable to the appearance of a
number of defects. Typical problems of aluminium alloys include most notably porosity inside the weld,
hot cracking at the fusion zone (FZ) or at the partially melted zone (PMZ), loss of ductility in the PMZ
9
and softening in the heat affected zone (HAZ) (Table 2). These problems can be more or less severe
(or even eliminated) depending on the factors previously listed. Even so, as a general rule, some
series of wrought aluminium alloys are particularly susceptible to some weld defects. Higher strength
aluminium alloys like the 2xxx, 6xxx and 7xxx are examples of series which, mainly because of their
composition, are particularly susceptible to have hot cracking [25]. Given the importance of hot
cracking with these alloys the subsequent heading is entirely dedicated to its detailed explanation.
Table 2 Typical welding problems in aluminium alloys [25]
Typical problems Alloy type Solutions
Porosity
Al-Li alloys (severe) Surface scraping or milling
Thermovacuum treatment
Variable-polarity keyhole PAW
Powder-metallurgy alloys (severe)
Thermovacuum treatment
Minimize powder oxidation and hydration
during atomization and consolidation
Other types (less severe) Clean workpiece and wire surface
Variable-polarity keyhole PAW
Hot cracking in FZ Higher-strength alloys
(2xxx, 6xxx, 7xxx)
Use proper filler wires and dilution
In autogenous GTAW, use arc oscillation
or less susceptible alloys
Hot cracking and low ductility in PMZ
Higher-strength alloys Use low heat input
Use proper filler wires
Low-frequency arc oscillation
Softening in HAZ
Work-hardened materials Use low-heat input
Heat-treatable alloys Use low-heat input
Postweld heat treating
2.3.2 Hot cracking
This section focuses on explaining the hot cracking of aluminium alloys while referring some
studies that show evidence of the different causes of this problem. The studies made on laser welding
are excluded since they are reviewed in the next section.
Definition
Hot cracking or solidification cracking is a weld-cracking failure mechanism. It usually occurs
in the weld metal at elevated temperatures during cooling. Hot cracking also can be found in the HAZ,
where it is known as liquation cracking [26].
Hot cracking occurs predominantly at the weld centreline or between columnar grains for the
reason that the fracture path of a hot crack is intergranular [26]. There are several theories of
solidification cracking which include the “strain theory”, the “brittleness temperature range theory”,
Borland’s “generalized theory” and the “critical speed theory” [27]. All these theories accept that hot
cracking is caused by the formation of a coherent interlocking solid network that is separated by
almost continuous thin liquid films. This solid network ruptures because of the tensile stresses inherent
to the solidification of the metal thus forming deep centreline cracks characteristic of this welding
defect.
10
To understand the causes of hot cracking in aluminium alloys the simplest approach is to
separate between the metallurgical factors and the mechanical factors [25], [28].
2.3.2.1 Metallurgical factors
The metallurgical factors that have been known to affect the solidification cracking
susceptibility of aluminium welds include [25], [28]:
a) Solidification temperature range
The solidification temperature range, also referred to as freezing temperature range, defines
the temperature interval of a phase diagram where both liquid and solid coexist. It delimits the
temperature interval during which the weld metal solidifies. A longer temperature range is more
harmful to the solidification of the weld because it creates a larger region where solid and liquid
coexist and allows more time for the liquid to spread and to form a detrimental thin liquid film that
drastically weakens the resistance of the material to accommodate the stresses of contraction.
Therefore, the hot cracking sensitivity of an alloy increases with increasing solidification temperature
range. In aluminium alloys, eutectic reactions can occur during the terminal stage of solidification and
extend the solidification temperature range [25]. Such undesirable aspect was observed multiple times
with GTAW welding, spot GTAW and GMAW of AA6061, AA6082, AA6261 and AA6351 aluminium
alloys using different filler metals. Multiple welds of these studies revealed that the base metal solidus
temperature was below the weld metal solidus temperature. As a result the welds developed cracks in
the HAZ [29]–[32].
b) Amount and distribution of liquid at the terminal stage of solidification
The crack sensitivity of aluminium and aluminium alloys is generally determined by the
composition and so, the selection of the alloying elements and their respective percentages have a
critical influence (Figure 4). As shown, with either pure aluminium or highly alloyed aluminium (i.e.
more than 6 wt. %) the crack sensitivity is very low. This is because with pure aluminium there are no
low melting-point eutectics that form at the grain boundaries whereas with highly alloyed aluminium
there is abundant eutectic liquid between the grains that is able “heal” occasional cracks that appear.
The main problem arises between those two compositions because the volume of liquid between
grains is sufficient to create a thin, continuous grain boundary film that is insufficient for healing the
cracks, making those alloys susceptible to solidification cracking. The formation of such thin films was
confirmed with alternating current (AC) TIG welding of multiple alloys, including the ZL101, AA5083
and AA6082 [33]. Additionally, the mentioned lack of enough liquid was studied in castings of
AlSi7MgCu-alloys [34] and again in castings of commercial alloys like the AA1050, AA3104, AA5182
and AA6111 and in Al-Si binary alloys [35].
11
Figure 4 Effect of chemical composition of weld metal on relative crack susceptibility in various aluminium alloys [25], [36]
Additionally, welds with fine equiaxed dendritic structure and with abundant liquid between
grains deform more easily under stresses than welds with coarse columnar dendritic structure
consequently they also have lower susceptibility to cracking (Figure 5) [25]. Hence, this is another
example which shows that the liquid distribution and amount plays an essentially role in the cracking
susceptibility of the welds.
(a) weld
(b) crack susceptibility curve
(c) pure metal
(d) low solute
(e) more solute
(f) much more solute
Figure 5 Effect of composition on crack susceptibility [25]
c) Ductility of solidifying weld metal
During solidification the ductility of the weld metal is a concern. This is because in a
determined temperature range, the solidifying weld metal has a much lower ductility than the weld pool
and the completely solidified weld metal. This temperature range is called brittle temperature range
(BTR) [25] and has been studied with 16 different commercial alloys welded with GTAW. In this study
the solidification crack susceptibility was ranked according to the measure of this BTR [37]:
( ) * ( )
( ) ( )+
12
Complementary, a study on GTAW of AA2024 aluminium sheets showed that solidification
cracking was avoided by directing liquid nitrogen behind the weld pool increasing the cooling rate and
therefore reducing the influence of the ductility factor [38].
d) Surface tension of grain boundary liquid
Surface tension essentially determines how the liquid wets the solid grains during the
solidification. If the surface tension between the solid grains and the grain boundary liquid is very low,
a liquid film will form between the grains causing a reduction of the strength of the solid network which
increases the hot cracking susceptibility. On the other hand, if the surface tension is high, the liquid
phase will be globular and will not wet the grain boundaries and as a result the cracking susceptibility
will be lower [25]. Complementary, a study of 16 different commercial alloys welded with GTAW
confirmed the importance of the surface tension on the crack susceptibility by observations of the
dihedral angle of eutectic products in the grain boundary [37].
e) Grain structure of weld metal
As previously mentioned, fine equiaxed grains are less susceptible to solidification cracking
than coarse columnar grains. This occurs because, unlike columnar grains, fine equiaxed grains can
freely deform to accommodate contractions. Furthermore, during solidification of fine-grained materials
the liquid at the grain boundary can more easily feed the incipient cracks which can effectively heal the
welds. Finally, fine-grained materials offer more grain boundary area for the low melting-point
segregates to be dispersed consequently they are less concentrated which is less detrimental to the
cohesive strength of the solid network during solidification [25]. With the study of the 16 different
commercial alloys welded with GTAW the mean grain size showed a weak correlation with the
cracking susceptibility but, nonetheless, the crack susceptibility decreased with decreasing grain size
[37]. Another study made about GTAW of 1050A-H14, AA6082-T6 and AA5083-H111 aluminium
alloys used specific filler materials to reveal that the refinement of the microstructure prevented the
formation of centreline solidification cracks. These studies corroborate that a smaller grain structure is
beneficial to the weld metal [39].
2.3.2.2 Mechanical factors
The more relevant mechanical factors to be taken into consideration in hot cracking are [25]:
a) Contraction stresses
The presence of stresses acting on adjacent grains during solidification is essential to the
formation of cracks. Therefore, materials with high thermal contraction and high solidification
shrinkage will be propitious to hot cracking. As aluminium alloys have high thermal expansion
coefficients and high solidification shrinkage they tend to develop high levels of stress which explains
their high cracking susceptibility, especially in alloys with wide solidification temperature ranges [25].
13
b) Degree of restraint
The degree of restraint of a workpiece is another mechanical factor that can cause hot
cracking. For a specific joint design and material, the imposition of greater restraints will increase the
hot cracking susceptibility [25] since it will difficult the accommodation of deformation which could
otherwise decrease stresses in the workpiece.
2.3.2.3 Reducing hot cracking
To reduce hot cracking in the aluminium alloys several aspects can be influenced, namely:
a) Control of weld metal composition
When welding aluminium alloys, it is desirable to have a weld metal composition that is far
from the peak of the crack sensitivity curve (Figure 4) and to reach this desired weld metal
composition, a filler metal of a proper composition (Figure 6) must be used and specific welding
parameters must be selected in order to achieve the desired dilution ratio [28].
Figure 6 Guide to choose the filler metals for minimizing hot cracking in welds of high-strength aluminium alloys [25]
b) Control of solidification structure
As already discussed, hot cracking can be avoided by grain refining. This can be achieved
through the use of small amounts of refining agents such as titanium and zirconium in the filler metal
[25] or with the control of some aspect of the welding process such as, for example using magnetic arc
oscillation in GTAW of AA6061 commercial aluminium sheets [40].
c) Use of favourable welding conditions
Favourable welding conditions can be obtained by either reducing strains or improving weld
geometry. Reducing thermally induced strains can be achieved simply by using high-intensity heat
sources, like electron or laser beams, which significantly reduces the distortion of the workpiece.
Additionally, less joint restraint and proper preheating of the workpiece can help reduce strains
therefore avoiding the hot cracking [25].
14
2.3.3 Hot cracking in laser welding
To review the knowledge concerning the hot cracking of aluminium alloys with laser welding, a
chronological approach was taken. Starting in 1988, Cieslak et al. investigated the autogenous laser
welding of crack sensitive aluminium alloys like the AA6061-T6, AA5456-H116 and AA5086-H32 with
different types of neodymium yttrium aluminium garnet (Nd:YAG) lasers and found that cracking was
clearly process dependant (continuous vs. pulsed). With this research it became clear that, from a hot
cracking perspective, pulsed welding is more detrimental than continuous welding. Furthermore, it was
determined that for laser welding of Al-Mg-Si alloys, the data accumulated about hot cracking, when
arc welding or in castings, were not adequate [41]. Hence, to fill this lack of reliable data concerning
laser welding of aluminium alloys many recent studies were undertaken. These studies are
subsequently referred, to list some alloys which are susceptible to hot crack and detail the parameters
that were used to solve this problem.
In 2004, Hector et al. characterized the texture of continuous autogenous welds with Nd:YAG
laser of AA5182-O and AA6111-T4. A close examination of the welds showed that grain boundary
liquation occurred in the AA6111-T4 welds and caused the alloy to develop fine hot cracks [42]. In
2005, Cicalӑ et al. studied the influence of operating parameters in continuous welds (autogenous
welds and welds with filler material) made with Nd:YAG lasers of AA6056-T4 aluminium alloy. This
work found that the most influential factors in avoiding hot cracking were the welding speed, the
fastening system and the wire parameters. In this work the best results were given by low welding
speeds and a uniform compression fastening system. Furthermore, optimum values for wire feed rate
and position were also determined [43].
Four years later, in 2008, Zhang et al. examined the effects of pulse shaping on the
solidification susceptibility of pulsed autogenous laser welding made with a Nd:YAG laser in AA6061-
T6 aluminium alloy. It was found that pulse shaping using a ramp-down gradient could eliminate the
solidification cracking in this alloy [44]. Also in 2008, Chen et al. compared single and dual beam
autogenous laser welds made with Nd:YAG and diode lasers of AA5052-H19 aluminium alloy. The
results showed that unlike single-beam welds, which displayed some defects (namely hot cracking,
voids and spatter) the dual-beam welds exhibited smooth surfaces with no evidence of the commonly
observed aluminium weld defects such as hot cracking, porosity, spatter, and depletion of magnesium
[45].
In 2009, Malek Ghaini et al. worked on pulsed autogenous laser welding with a Nd:YAG laser
of AA2024-O aluminium alloy. This work investigated whether solidification cracks and liquation cracks
acted independently or were related with each other in terms of initiation and propagation. It was found
that two types of partially melted zones and fusion lines can be identified in the weld metal and these
react differently depending on the energy of the pulses. It was also made clear that, in this case, the
location of the cracks resulted of the opposing effects of crack healing through backfilling and crack
propagation due to the mechanical stresses [46]. Also in 2009, Sánchez-Amaya et al. made
continuous welds with a diode laser in AA5083-T0 and AA6082-T6 aluminium alloys under conduction
15
regime, in an attempt to increase the maximum penetration reached while maintaining good quality. It
was found that cracks were due to the tensions formed during the solidification. Additionally, it was
observed that the extension of the cracks diminished as the laser power increased and as the welding
speed decreased [47].
A year later, in 2010, Chang et al. compared pulsed and continuous autogenous welds made
with a Nd:YAG laser in dissimilar joints of AA6061-T651 with A3003-O. Similarly to the work of Cieslak
et al. the results showed that the presence of defects was worse in the pulsed welds than in the
continuous welds. Furthermore, adequate parameters of shielding gas and flow rate, focusing position
and structure design were determined [48]. In 2010 as well, Katayama et al. studied the laser welding
phenomena and the factors affecting weld penetration and welding defects in AA5083 aluminium
alloys using a continuous CO2 laser, continuous YAG disk lasers and fibre lasers. It was found that
cracks were easily formed at higher welding speeds in thicker plates [49].
In 2011, Pakdil et al. studied the microstructural and mechanical properties of continuous
welds of AA6056-T6 aluminium alloy using a CO2 laser and an AlSi12 wire filler material. In this study,
grain boundary liquation was detected in the FZ and in the HAZ although, there was no liquation
cracking [50]. In 2011, similarly to the work of Cicalӑ et al. Silva located a processing window for
continuous autogenous welding of AA6082-T651 aluminium alloy using a YAG disk laser. This
investigation confirmed the importance of the welding speed and of the solidification period to avoid
gas entrapment and hot cracking [6].
In 2013, Somonov et al. studied induction heating with the intent to prevent hot cracking during
laser welding. Although no welds were actually made, computer modulation showed that prevention or
reduction of the formation of hot cracks was theoretically possible by induction heating, thanks to the
thermally induced compressive stress created in the weld area [51]. In 2014, Zhao et al. made pulsed
autogenous welds of Al−Mn−Mg alloys with varying silicon contents. This study found that no cracking
existed in the weld pool when silicon content was below 0.34 wt. % however, when the silicon content
increased to 0.47 wt. % cracking happened in the weld pool, due to the distribution of liquid eutectic
phases in the grain boundaries [52].
Finally, in the year 2015, Sheikhi et al. studied pulsed welding of AA2024 with a Nd:YAG laser
and complemented the previous work of Malek Ghaini et al. by developing a prediction model. This
model demonstrated that it is possible to avoid solidification cracking with proper control of the pulse
ramp down shape [53]. Also in 2015, Witzendorff et al. studied the hot crack formation in pulsed laser
welding of AA6082-T6 with a Nd:YAG laser. In this study, the hot cracking was evaluated using high-
speed cameras that captured visible and infrared radiation assessing the strain rate, strain, and
metallurgical outcome. It was found that different pulse shapes, within the conduction welding regime,
reduced hot cracking. Alternatively, within the keyhole welding regime there was a high susceptibility
to hot cracking regardless of parameters tested. Finally, it was found that extensive hydrogen diffusion
at the solid–liquid interface promoted the crack initiation [54].
16
Considering the above mentioned studies, some essential notions can be made regarding
aluminium laser welding. Although autogenous welding is a better choice for industrial applications
[55], some aluminium alloys do not make good quality welds this way, which is the case of hot crack
susceptible alloys. Such alloys include the AA2024 of the 2xxx series, the AA5456, AA5086, AA5182,
AA5052 and AA5083 of the 5xxx series and the AA6061, AA6111, AA6056, AA6082 of the 6xxx
series. With these alloys it might be necessary the use a filler material with proper dilution ratio to
avoid the crack sensitive compositions (Figure 4). That is to say, for each of these series there is a
minimum content of alloying elements to have good quality welds (Figure 7).
Figure 7 Effects of alloying elements on cracking sensitivities [56]
Concerning autogenous welds, the most influential choice to avoid hot cracks is the laser type.
Continuous lasers are capable of providing longer solidification times to avoid hot cracks and are
therefore a better choice. Pulsed lasers, on the other hand, typically make pulses with a few
milliseconds and as a result the solidification times are smaller and the welds are more susceptible to
hot cracking [41]. Furthermore, to avoid hot cracks with pulsed lasers conduction welding mode is
preferred and in some cases pulse shaping is also recommended. Thus, if more penetration is
required and the welding mode enters within the keyhole regime, a solution is yet to be found to avoid
hot cracks [54]. Finally, for any welds a good joint design is an important consideration to reduce
unnecessary stresses [55].
2.4 Summary
This review has tried to clarify some important notions:
Aluminium and aluminium alloys can be found in a wide variety of applications and can be
welded with many different techniques, one of which is laser welding.
Although laser welding offers many potential advantages, as it is a relatively recent welding
process, the existing data is still very limited.
Most aluminium alloys are easy to weld however, some of the 2xxx, 5xxx and 6xxx series
alloys, can be susceptible to hot cracking, in this case they are consider low weldability alloys.
For the alloys susceptible to hot cracking, the simplest solution recommended when laser
welding (or arc welding) is to use a filler material.
17
Pulsed laser welding is more susceptible to cause hot cracking than continuous laser welding.
Even so, controlling pulse durations and pulse shapes can be enough to solve this problem.
Pulsed laser welding in keyhole mode is more susceptible to cause hot cracking than in
conduction mode, hence no solution has been found to reach higher penetrations.
The last point of this enumeration is the central objective of this thesis. In other words, this
investigation was focused on doing pulsed laser welding outside the conduction regime in order to
produce welds with 1 mm of weld penetration without hot cracks, which was yet to be reported.
18
3. Experimental Procedure
Objective
This work attempted to pulsed laser weld the aluminium alloy AA6082-T651 in an industrial
environment. The main difficulty of this work revolved around the hot cracking susceptibility of this
alloy with the increased difficulty of using pulsed lasers instead of continuous lasers. Additionally, the
complementary objective was to obtain a laser seam weld with a minimum of 1 mm of penetration and
more than 60% of overlap and, if possible, to establish a processing interval that would show welds
with minimal or no hot cracking.
In this chapter, first of all the aluminium alloy AA6082-T651 is presented. Then, since 3
different lasers were employed, the description of the different tests was divided according to each
laser system and their respective trials.
3.1 Base material characteristics
When studying the physical metallurgy of aluminium alloys it is worth to assess the effects of
the composition, the mechanical working, and the heat treatment on the mechanical and physical
properties of the alloy. Consequently the subsequent evaluation of the aluminium alloy AA6082-T651
starts with the description of its composition, follows with the description of its temper and finally
indicates some of its general properties.
3.1.1 Composition
The aluminium alloy AA6082 belongs to the 6xxx series of the wrought aluminium alloys and
can be found commercially under multiple standard designations and specifications notably: AA6082,
HE30, HP30, HS30, DIN 3.2315, EN AW-6082, ISO: AlSi1MgMn and A96082 [57]. For all these
designations the recognized chemical composition is the identical [1], [57]:
Table 3 Chemical composition of aluminium alloy 6082 [1], [57]
Composition, wt. %
Si Fe Cu Mn Mg Cr Zn Ti Unspecified other elements
Al min. Each Total
0.7-1.3 0-0.5 0-0.1 0.4-1 0.6-1.2 0-0.25 0-0.2 0-0.1 0.05 0.15 Balance
3.1.1.1 Alloying elements: silicon and magnesium
The alloys of the 6xxx series contain primarily magnesium and silicon (up to 2 wt. % of each)
[58] in the proportions required for the formation of magnesium silicide: Mg2Si (atomic ratio of 2:1 and
wt. % ratio of 1.73:1). The Mg2Si particles are formed as a result of a ternary peritectic reaction but
instead of using a ternary system to study the average composition of these alloys, it is simpler to use
the pseudo-binary Al-Mg2Si system (Figure 8). In view of this pseudo-binary system the main concept
to understand is that the alloys of the 6xxx series precipitate Mg2Si in a given temperature range.
Furthermore these precipitates tend to form particles that confer an increase in strength and so these
alloys are heat treatable [1], [59].
19
Figure 8 Al-Si-Mg alloys: (left) Al corner of ternary phase diagram; (right) Al-Mg2Si pseudo-binary section
[60]
The maximum solubility of Mg2Si is 1.85 wt. %. Moreover wrought aluminium alloys of the
6xxx series usually have compositions with approximately 1.2 wt. % Mg2Si [60] and can be divided
between compositions in the following way. In the 1st group the sum of magnesium plus silicon is
below 1.5 wt. %. In this group there are the 6060 and 6063 alloys. In the 2nd
group the sum of
magnesium plus silicon is equal or more than 1.5 wt. % and other additions such as 0.3 wt. % of Cu
are possible. In this group there is the 6061 alloy. In the 3rd
group the content of magnesium and
silicon can vary and there is a substantial excess and silicon. In this group there are the alloys 6082
and 6005 (Figure 9) [1].
Figure 9 Variation of main alloying elements in different Al-Mg-Si alloys [61]
The alloys of the 3rd
group, due to their excess of silicon, have typically a higher strength than
other 6xxx alloys. For example, an alloy with 0.8 wt. % of Mg2Si, which has an excess of 0.2 wt. % of
silicon has an increase in strength of about 70 MPa. However, this excess of silicon can bring
20
disadvantages, as in these alloys the silicon tends to segregate to the grain boundaries causing
cracking. To counteract this problem, elements such as magnesium, chromium, or zirconium are
generally added to the composition in order to prevent the recrystallization and control the grain
structure during the heat treatment [1].
3.1.1.2 Alloying element: manganese
In the aluminium alloy 6082 the manganese has two effects. It increases the strength of the
alloy by being in solid solution or as a finely precipitated intermetallic phase or it prevents
recrystallization and grain growth by precipitating in coarse dispersoids which increase the quench
sensitivity of the alloy [1], [61].
3.1.2 Temper
The aluminium alloy 6082 is commercially found with different tempers which the most
commons are [57]:
O – Annealed wrought alloy
T4 – Solution heat treated and naturally aged
T6 – Solution heat treated and artificially aged
T651 – Solution heat treated, stress relieved by stretching and then artificially aged
3.1.2.1 Temper steps
The T651 temper that was made in this 6082 allows to obtain better mechanical properties by
precipitation hardening. The T651 temper involves the subsequent steps (Figure 10) [60], [61]:
Solution heat treatment consists of heating to an elevated temperature (T1≈530°C) and
maintaining that temperature for a prescribed period of time (around 1 hour).
Quenching the alloy (in water) is the process of cooling fast enough to retain the same
microstructure as during the solution heat treatment.
Stress relieving involves relieving the tensions created during quenching with stretching,
compressing or a combination of both.
Artificial aging entails maintaining the alloy at a lower temperature (T2≈175°C) for a prescribed
period of time (for T6 it is usually about 24h).
21
Figure 10 Quasi-binary section through the aluminium-rich corner of the ternary Al-Mg-Si phase diagram
[61]
The combination of solution heat treatment, quenching and aging (natural aging: T4 or
artificially aging: T6) is called precipitation hardening. Each step has a different purpose:
The solution heat treatment allows the alloying elements to form a solid solution with
aluminium.
When the temperature is quickly reduced by quenching, a supersaturated solid solution is
created.
Artificial aging is then used to precipitate (in a controlled way) the phases which cause
strengthening of the alloy.
3.1.2.2 Precipitation hardening
The generally accepted precipitation sequence which occurs during the precipitation
hardening of the supersaturated Al-Mg-Si alloy is the following [62]:
( )
There are many precipitates that can form. The most effective hardening precipitates are the
coherent needle like shape precipitates. Additionally the coarse rod-shaped precipitates can
also make an important contribution to the increase in strength of the precipitation hardening. Finally,
for a set composition, the precipitation sequence depends on the temperature and time used in the
aging process (Figure 11) [61].
22
Figure 11 The relationship between phases observed and ageing condition [63]
3.1.2.3 Ageing process
As it was above mentioned some precipitates contribute more than others for the final
hardening effect. Therefore, depending on the density and size distribution of the hardening
precipitates, the alloy has a peak value of strength and hardness that can be reached which is called
the peak-aged condition. This condition is attained with the T6 heat treatment and contains a balance
of the two metastable and precipitates (Figure 12). Resuming, the aging process causes
precipitation within the grains which improves the mechanical properties of the alloy at expense of
ductility [61].
Figure 12 Strength evolution during artificial (and natural) ageing [61]
23
3.1.3 Properties and applications
The AA6082 is a relatively new alloy whose higher strength has replaced the AA6061 in many
applications. This alloy has the following generic physical properties:
Table 4 Physical properties for aluminium alloy 6082 [57]
Property Value
Density 2700 g/cm3
Melting Point 555°C
Modulus of Elasticity 70 GPa
Electrical Resistivity 0.038x10-6
Ω.m
Thermal Conductivity 180 W/m.K
Thermal Expansion 24x10-6
K-1
The mechanical properties of AA6082 plates with a T651 temper and with a thickness
between 6 to 12.5 mm are:
Table 5 Mechanical properties for aluminium alloy 6082 [57]
Property Value
Proof Stress 255 Min MPa
Tensile Strength 300 Min MPa
Elongation A50 mm 9 Min %
Hardness Brinell 91 HB
As a final note the aluminium alloy AA6082 is typically used in transport application and other
high stress applications like, for instance trusses, bridges, cranes, ore skips, beer barrels and milk
churn [57].
3.2 Laser equipment and trials
The following section details the 3 laser equipment used and their respective trials. As
mentioned before, the 412 welds made are divided in 7 trials which are presented in a chronological
order (Table 6).
Table 6 Summary of laser equipment and trials
Lasers Ownership Trials Nº of welds
AL 300 Carrs Welding Technologies Ltd 1st to 3
rd trial 372
TruDisk 4002 Carrs Welding Technologies Ltd 4th
to 6th
trial 16
G4 Series Z Type Welding Engineering and Laser Processing Centre 7th
trial 24
3.2.1 Technical data – AL 300 laser
The 1st laser employed for the tests the AL 300 of ALPHA LASER GmbH installed at Carrs
Welding Technologies Ltd (located in Kettering, England). This 1st equipment was a typical Nd:YAG
flash lamp pumped solid state laser with an emission wavelength of 1064 nm limited to pulsed laser
welding. This laser was set up on top of a motorized workbench, the AL-T 500 from the same
manufacturer (Figure 13).
24
Figure 13 AL 300 laser with the AL-T 500
The complete laser system featured the following technical data:
Table 7 Technical data of the AL 300 laser with the AL-T 500 [64], [65]
Technical data of AL 300
Average power 300 W
Peak power 9 kW
Pulse energy 90 J
Pulse duration 0.5 – 20 ms
Pulse frequency Max. 100 Hz
Welding spot diameter 0.2 – 2.0 mm
Focusing optics 150 mm
Pulse shape Adjustable power-shaping of a pulse
Technical data of AL-T 500
Workpiece motion Motorized
Welding speed Max. 25 mm/s
3.2.2 1st to 3rd trials – AL 300 laser
All the tests made with this laser were conducted at Carrs Welding Technologies Ltd in an
industrial environment. Therefore, due to the limited equipment and resources available at the
company, these tests had to be conducted following the trial and error method. After each trial, the
results were interpreted to select the most promising parameters for the next trial. Following this
procedure a sequence of 3 trials were made with this 1st welding equipment.
The most critical problem to weld this alloy is its strong hot cracking susceptibility. This is
typically solved either by pre and post heating, using adequate filler material or changing the welding
parameters in order to control the heat input. While the effects of pre and post heating could not be
tested, because there was no oven at the company and it was not possible to use another laser beam
to pre or post heat by performing passes, the other two choices were still available [6]. The use of filler
25
material had already been verified and was common practice at these installations therefore this
research focused on testing different welding parameters to attempt to obtain positive results.
Consequently, the 3 trials made were focused on changing the welding parameters in an attempt to
locate a viable processing window where hot cracking would not occur or be minimized.
Before starting making trials with different parameters it was necessary to establish the initials
ones for this AL 300 laser. A common way of picking those parameters is choosing similar ones
referred literature that showed positive results. For this laser, the subsequent values of pulse duration
and shape, shielding gas and flow rate, laser inclination, pulse frequency and welding speed of the 1st
trial were selected according to the parameters of similar studies [44], [54] and taking into account the
company’s parameters in previous tests.
3.2.2.1 1st
trial
Experimental parameters:
Pulse duration and shape: 4 ms with rectangular pulse shape
Gas and flow rate: Argon with 15 L/min and Heliweld21 with 25 L/min
Laser inclination: 15°
Pulse frequency: 10 Hz
Welding speed: 2.5 mm/s
Spot sizes: 0.2, 0.3, 0.4 and 0.5 mm
Peak powers 1st step: 1.67, 2.15, 2.70, 3.28, 3.90, 4.60, 5.32 and 6.06 kW
Peak powers 2nd
step: 3.28, 3.40, 3.53, 3.65, 3.78, 3.90, 4.04, 4.18, 4.32, 4.46 and 4.60 kW
This trial was made in 2 steps. In the 1st step, the peak powers tested were from 1.67 to 6.06
kW for all spot sizes with both shielding gases. Then, the peak powers closest to 1 mm penetration
were refined only with argon. In the 2nd
step, the peak powers tested were from 3.90 to 4.60 kW for
spot size 0.2 and 0.5 mm and from 3.28 to 3.90 kW for spot size 0.3 and 0.4 mm. Testing spot sizes in
conjunction with peak powers for both gases determined the best parameters to reach about 1 mm of
penetration while assessing the superficial quality of the welds. Weld penetrations were measured and
the quality of the welds was assessed visually. The 1st trial identified the following peak powers for the
subsequent trial: 3.78, 3.90, 4.04, 4.18 and 4.32 kW.
3.2.2.2 2nd
trial
Experimental parameters:
Pulse duration and shape: 4 ms with rectangular pulse shape
Gas and flow rate: Argon with 15 L/min
Laser inclination: 15°
Pulse frequency: 10 Hz
Welding speed: 1, 1.5, 2, 2.5, 3, 3.5 and 4 mm/s
Spot sizes: 0.2, 0.3, 0.4 and 0.5 mm 1 This is a shielding gas mixture of 75% helium and 25% argon.
26
Peak powers: 3.78, 3.90, 4.04, 4.18 and 4.32 kW
The 2nd
trial tested welding speeds from 1 to 4 mm/s, spot sizes of 0.2 to 0.5 mm and peak
powers of 3.78 to 4.32 kW with argon in order to identify the influence of heat input and pulse overlap
on the hot cracking susceptibility of the welds. For such evaluation all welds were photographed
before and after being inspected with a liquid dye penetrant, which is capable of detecting small
cracks on the welds. In this trial, higher welding speeds improved the weld quality thus 3, 3.5 and 4
mm/s were chosen for the next trial.
3.2.2.3 3rd
trial
Experimental parameters:
Pulse duration and shape: 4 ms with rectangular pulse shape and 3 tailored pulse shapes
Gas and flow rate: Argon with 15 L/min
Laser inclination: 15°
Pulse frequency: 8, 10, 12, 14, 16 and 18 Hz
Welding speed: 3, 3.5 and 4 mm/s
Spot sizes: 0.2 and 0.5 mm
Peak power: 4.04 kW
The 3rd
trial using argon gas and tested frequencies from 8 to 18 Hz, spot sizes of 0.2 and 0.5
mm, welding speeds of 3, 3.5 and 4 mm/s and 4 pulse shapes: 1st, 2
nd, 3
rd pulse
shapes and
rectangular pulse shape (Annexe D). These welding speeds with different frequencies and pulse
shapes assessed, once more, the influence of heat input and pulse overlap on the hot cracking
susceptibility of the welds. The welds were photographed, and then examined with a microscope to
check for hot cracks. In this trial, it seemed that higher pulse frequencies produced better welds and
so, pulse frequency around 16 Hz appeared adequate for the following trial with the 2nd
laser.
3.2.3 Technical data – TruDisk 4002 laser
The 2nd
laser used was the TruDisk 4002 of TRUMPF GmbH + Co. KG also found at the same
company (Carrs Welding Technologies Ltd). This 2nd
equipment was a solid state laser which instead
featured diode pumped Nd:YAG disks with an emission wavelength of 1030 nm capable of both
continuous and pulsed laser welding. The focusing system of this laser was mounted on a robotic arm,
the KR16 from Kuka that controls all movements, specifically the welding speed of the trials (Figure
14).
27
Figure 14 TruDisk 4002 laser with the KR16
The complete laser system featured the following technical data:
Table 8 Technical data of the TruDisk 4002 laser with the KR16 [66]–[68]
Technical data of TruDisk 4002
Emission wavelength 1030 nm
Laser Power 4 kW
Beam Parameter Product (BPP) 8 mm x mrad
Minimum diameter laser light cable 200 µm
Technical data of KR16
Rated payload 16 Kg
Horizontal distance (Lz) 150 mm
Vertical distance (Lxy) 120 mm
Interference radius 1611 mm
Speed of motion of axis 156 °/s
3.2.4 4th to 6th trials – TruDisk 4002 laser
For a second time, all tests made with this laser were conducted at Carrs Welding
Technologies Ltd in an industrial environment with limited equipment and resources available and
following the trial and error method.
As the TruDisk 4002 is designed to be used as a continuous laser it has completely different
capabilities than the previous AL 300 laser. Namely, this disk laser is capable to generate very long
(virtually unlimited) pulse durations and the software allows a much greater control of pulse shaping.
Also, this laser presented more accurate focal positioning which allowed for precise spot sizes to be
obtained. With these advantages in consideration 3 intrinsically different trials were made.
The AL 300 laser only tested seam welds to solve the hot cracking problem but due to the
equipment malfunction it was necessary to change of laser equipment. Hence, the 4th trial (TruDisk
4002 laser) was focused on testing longer pulse durations with a pulse shape. Secondly, the 5th trial
28
confirmed that continuous welding with adequate welding parameters would not have hot cracking.
Thirdly, the 6th trial tested single spot welds. To sum up, the 4
th trial was similar to the trials of the
previous laser but the following two tested different welding situations, specifically continuous and
single spot welds.
3.2.4.1 4th
trial
Experimental parameters:
Pulse duration and shape: 20 ms with the 4th tailored pulse shape (Annexe D)
Gas and flow rate: Argon with 15 L/min
Laser inclination: 15°
Pulse frequency: 16.4 and 17 Hz
Welding speed: 4.5 mm/s
Spot size: 0.67 mm (-4 collimation)
Peak power: 3.1, 3.2, 3.3 and 3.4 kW
This 4th trial tested frequencies of 16.4 and 17 Hz with peak powers between 3.1 to 3.4 kW
and revealed that the previous results using the AL 300 laser had inadequate pulse overlap.
Additionally, the initial examination of the welds suggested that longer pulses with adequate pulse
shapes reduced the hot cracking of the welds (but a later SEM examination refuted this observation).
All welds were visually inspected and examined with a microscope. As this trial only tested 8 different
combinations of parameters no improvements could be observed between the different welds and so,
no parameters were selected.
3.2.4.2 5th
trial
Experimental parameters:
Gas and flow rate: Argon with 12 L/min
Laser inclination: 10°
Welding speed: 25 mm/s
Spot size: 0.38 mm (-2 collimation)
Laser power: 2.8 and 3.4 kW
In the 5th trial, 2 continuous welds were made with a welding speed of 25 mm/s, spot size of
0.38 mm (-2 collimation) and laser powers of 2.8 and 3.4 kW. This trial confirmed that continuous
welds can be made without occurrence of hot cracking using different laser powers and that, regarding
hot cracking, welding speed is more influential than the size of the weld pools (which is essentially
determined by the laser power). As this trial was not pulsed welding no parameters were selected.
29
3.2.4.3 6th
trial
Experimental parameters:
Pulse duration and shape: 1 and 2 ms with rectangular pulse shape
Gas and flow rate: Argon with 12 L/min
Laser inclination: 10°
Spot sizes: 0.2, 0.38 and 0.67 mm (0, -2 and -4 collimation)
Peak power: 4 kW
In the 6th trial, single spot welds were made using pulse durations of 1 and 2 ms and spot
sizes of 0.2, 0.38 and 0.68 mm. This trial tested if small pulse duration single spot welds with small
fusion zones were able to accommodate the high shrinking tensions of a fast solidification. In this trial,
it should be taken in consideration that since the surface preparation method was manual grinding the
results lacked consistency.
3.2.5 Technical data – G4 Series Z Type laser
The 3rd
laser operated was the G4 Series Z Type of SPI Lasers UK Ltd made available by the
Welding Engineering and Laser Processing Centre (located in Cranfield, England). This last
equipment is a solid state diode pumped fibre laser with an emission wavelength between 1059 to
1065 nm capable of both continuous and pulsed laser welding (Figure 15).
Figure 15 G4 Series Z Type laser
This laser featured the following technical data:
Table 9 Technical data of the G4 Series Z Type laser [69], [70]
Technical data of G4 Series Z Type
Average power 70 W
Maximum pulse energy 1 mJ
Maximum peak power 13 kW
Pulse duration 10 – 500 ns
Emission wavelength 1059 – 1065 nm
Beam quality factor (M2) 1.6
Full angle divergence 80 – 120 mrad
Pulse frequency Max. 1 MHz
30
3.2.6 7th trial – G4 Series Z Type laser
Experimental parameters:
Pulse duration and shape: 240, 350, 520 ns with 3 pre-programed pulse shapes
Gas and flow rate: no shielding gas
Laser inclination: no fixed inclination
Pulse frequency: 70 kHz
Welding speed: 3.6, 10.5, 35, 35.7, 70, 105 mm/s
Spot size: 0.051, 0.1 and 0.15 mm
Peak power: 5, 7, 13 kW
Number of passes (or cycles): 1, 10 and 20
This 7th trial tested spot sizes of 0.051, 0.1, and 0.15 mm with welding speeds of 3.6 and 35.7,
35 and 70, 10.5 and 105 mm/s respectively and with 3 specific set of parameters of pulse duration,
pulse shape and peak power. Each spot size was tested with 240 ns of pulse duration, pulse shape
nº0 and 13 kW of peak power, then with 350 ns of pulse duration, pulse shape nº 32 and 7 kW of peak
power and finally with 520 ns of pulse duration, pulse shape nº 36 and 5 kW of peak power (pulse
shapes in Annexe D).
Furthermore, welds with multiple passes (or cycles) were also tested. The parameters used for
the welds with 10 passes were:
Pulse duration of 240 ns, pulse shape nº0, peak power of 13 kW with both welding speeds of
70 mm/s and 105 mm/s for spot size of 0.1 mm and 0.15 mm, respectively.
Welding speed of 35.7 mm/s with spot size of 0.051 mm and 2 different set of parameters,
namely pulse duration of 240 ns, pulse shape nº0 and peak power of 13 kW and also pulse
duration of 520 ns, pulse shape nº36 and peak power of 5 kW.
The parameters used for the welds with 20 passes were:
Pulse duration of 240 ns, pulse shape nº0, peak power of 13 kW with both welding speeds of
70 mm/s and 105 mm/s for spot size of 0.1 mm and 0.15 mm, respectively.
The results of this equipment were unexpected. With a visual inspection it appeared that the
laser marking equipment simply etched the surface of the samples. However, when assessing the
penetration of the welds with spot size of 0.051 mm, it showed that positive weld profiles were
obtained.
31
4. Results and Analysis
4.1 Penetration analysis
This research began with the evaluation of the penetration results of the AL 300 laser. All the
welds made during the 1st trial (AL 300 laser) were measured and registered, and then the resulting
values of penetration vs. peak power were plotted. From these plots and for each spot size, the peak
powers closest to the 1 mm penetration objective were pointed and selected. Those plots allowed as
well, evidencing the transition from conduction to keyhole welding regimes. This transition is typically
characterized by noticeable increase of penetration with peak power and modifications of the weld
profile from a rounded to a pear shape.
The subsequent sections for each spot size respected the following order of presentation. For
each spot size, the penetration measurements made with both argon and Heliweld2 gas were
described. Then, the penetration measurements of the argon gas welds were measured in trimmed
intervals to validate the previous results. Finally, relevant weld profiles were presented to illustrate the
evolution of the weld profiles according to the welding regime.
4.1.1 Spot size of 0.2 mm
The penetration achieved with spot size of 0.2 mm was slightly higher with argon than with
Heliweld2. Also, with both gases a noticeable increase of the penetration was observed at 3.28 kW
indicating that the transition of conduction to keyhole happened at this peak power. Secondly, the
selected peak power to reach about 1 mm of penetration was initially 4.6 kW (Figure 16). However,
refining the penetration measurements with argon between 3.9 and 4.6 kW showed that 4.32 kW was
sufficient to reach the established objective of 1 mm penetration (Figure 17).
Figure 16 Penetration vs. peak power for spot size of 0.2 mm
y = 0,0843x2 - 0,2148x + 0,2884 R² = 0,9862
y = 0,046x1,9218 R² = 0,9441
0
0,5
1
1,5
2
2,5
1,5 2,5 3,5 4,5 5,5 6,5
Pen
etr
ati
on
(m
m)
Peak power (kW)
Argon Heliweld2 Polinomial (Argon) Potencial (Heliweld2)
32
Figure 17 Refining of penetration vs. peak power for spot size of 0.2 mm
On the other hand, with the observation of the weld profiles a distinctive alteration was
identified. The weld changed from a rounded shape with low penetration at 2.7 kW to a pear shape
with higher penetration at 3.28 kW which confirmed the transition of welding regime mentioned before
(Figure 18). Identificar (transição entre regime identifier conduction e keyhole)
Conduction Keyhole
2.7 kW with Argon
3.28 kW with Argon
2.7 kW with Heliweld2
3.28 kW with Heliweld2
Figure 18 Weld profiles for spot size of 0.2 mm
0,76 0,89
0,81
1,05 1,1
1,95
0,4
0,6
0,8
1
1,2
1,4
1,6
1,8
2
3,8 4 4,2 4,4 4,6 4,8
Pen
etr
ati
on
(m
m)
Peak power (kW) Argon
33
4.1.2 Spot size of 0.3 mm
The penetration achieved with a spot size of 0.3 mm with argon gas was very slightly higher
than with Heliweld2 gas. Once again, with both gases a noticeable increase of the penetration started
at 3.28 kW. It is noticeable that this spot size with 6.06 kW of peak power and argon gas reached the
maximum penetration depth of all the measurements at 2.46 mm of penetration. Finally, the peak
power closest to the 1 mm of penetration was about 3.9 kW for both gases (Figure 19). This was then
approved by the refined penetration measurements with argon between 3.28 and 3.9 kW (Figure 20).
Figure 19 Penetration vs. peak power for spot size of 0.3 mm
Figure 20 Refining of Penetration vs. peak power for spot size of 0.3 mm
Like previously, the weld profiles changed from a rounded profile to a pear profile when
increasing the peak power from 2.7 to 3.28 kW. This modification confirmed that a transition of welding
regimes occurred between these welds (Figure 21).
y = 0,0522x2 + 0,1719x - 0,3898 R² = 0,9633
y = 0,0219x2 + 0,3137x - 0,5678 R² = 0,967
0
0,5
1
1,5
2
2,5
3
1,5 2,5 3,5 4,5 5,5 6,5
Pen
etr
ati
on
(m
m)
Peak power (kW)
Argon Heliweld2 Polinomial (Argon) Polinomial (Heliweld2)
0,59 0,59 0,73
0,87 0,78
1,09
0,4
0,6
0,8
1
1,2
1,4
1,6
1,8
2
3,2 3,4 3,6 3,8 4
Pen
etr
ati
on
(m
m)
Peak power (kW) Argon
34
Conduction Keyhole
2.7 kW with Argon
3.28 kW with Argon
2.7 kW with Heliweld2
3.28 kW with Heliweld2
Figure 21 Weld profiles for spot size of 0.3 mm
4.1.3 Spot size of 0.4 mm
The penetration achieved with a spot size of 0.4 mm with both gases was almost identical.
Once more, the increase of penetration depth with both gases started at 3.28 kW. An almost ideal 1
mm penetration was reached with a peak power of 3.9 kW (Figure 22), and such penetration was
confirmed by the refined penetration measurements with argon between 3.28 and 3.9 kW.
Nevertheless, a weld made with argon and 3.78 kW reached 1.08 mm of penetration, hence this lower
peak power was considered appropriate to accomplish the 1 mm objective (Figure 23).
Figure 22 Penetration vs. peak power for spot size of 0.4 mm
y = 0,0353x2,3625 R² = 0,9584
y = 0,0506x2 + 0,1354x - 0,3441 R² = 0,9594
0
0,5
1
1,5
2
2,5
3
1,5 2,5 3,5 4,5 5,5 6,5
Pen
etr
ati
on
(m
m)
Peak power (kW)
Argon Heliweld2 Potencial (Argon) Polinomial (Heliweld2)
35
Figure 23 Refining of penetration vs. peak power for spot size of 0.4 mm
Yet again, the change of the weld profiles between 2.7 and 3.28 kW confirmed the presence of
a transition of welding regimes between these welds (Figure 24).
Conduction Keyhole
2.7 kW with Argon
3.28 kW with Argon
2.7 kW with Heliweld2
3.28 kW with Heliweld2
Figure 24 Weld profiles for spot size of 0.4 mm
0,44 0,51 0,59 0,63
1,08 1,01
0,4
0,6
0,8
1
1,2
1,4
1,6
1,8
2
3,2 3,4 3,6 3,8 4
Pen
etr
ati
on
(m
m)
Peak power (kW) Argon
36
4.1.4 Spot size of 0.5 mm
Finally, the penetration achieved with both gases for a spot size of 0.5 mm was almost
identical. With this spot size, the increase of penetration started at 3.9 kW with both gases, indicating
that the transition of welding regime occurred at this higher peak power. The peak power necessary to
reach the 1 mm penetration objective was recognized between 3.9 and 4.6 kW (Figure 25). The
refined penetration measurements with argon between 3.9 and 4.6 kW determined that 4.04 kW was
sufficient to attain the 1 mm penetration (Figure 26).
Figure 25 Penetration vs. peak power for spot size of 0.5 mm
Figure 26 Refining of penetration vs. peak power for spot size of 0.5 mm
In this spot size, the weld profiles changed between 3.28 and 3.9 kW confirming that, this time,
the transition of welding regimes required a higher peak power (Figure 27).
y = 0,0403e0,7027x R² = 0,9322
y = 0,0455e0,6745x R² = 0,9378
0
0,5
1
1,5
2
2,5
3
1,5 2,5 3,5 4,5 5,5 6,5
Pen
etr
ati
on
(m
m)
Peak power (kW)
Argon Heliweld2
Exponencial (Argon) Exponencial (Heliweld2)
0,86
1,05
1,38 1,24
1,14
1,38
0,4
0,6
0,8
1
1,2
1,4
1,6
1,8
2
3,8 4 4,2 4,4 4,6 4,8
Pen
etr
ati
on
(m
m)
Peak power (kW) Argon
37
Conduction Keyhole
3.28 kW with Argon
3.9 kW with Argon
3.28 kW with Heliweld2
3.9 kW with Heliweld2
Figure 27 Weld profiles for spot size of 0.5 mm
4.1.5 Overall penetration results
To offer an overall perspective of the weld penetrations achieved in this trial, all the
measurements were compiled in 2 separate tables (Table 10 and Table 11).
Table 10 Penetration results of initial welds plots
Argon Heliweld2 (75% He+25% Ar)
Pulse energy Peak power Spot size (mm) Spot size (mm)
0.2 0.3 0.4 0.5 0.2 0.3 0.4 0.5
6.67 J 1.67 kW 0.17 0.18 0.15 0.13 0.14 0.15 0.15 0.16
8.59 J 2.15 kW 0.22 0.21 0.19 0.19 0.19 0.19 0.20 0.20
10.80 J 2.70 kW 0.21 0.27 0.26 0.24 0.23 0.24 0.24 0.22
13.10 J 3.28 kW 0.66 0.50 0.53 0.27 0.42 0.65 0.43 0.30
15.60 J 3.90 kW 0.72 1.36 1.09 0.82 0.81 0.94 0.95 0.73
18.38 J 4.60 kW 1.04 1.52 1.67 1.63 1.18 1.57 1.63 1.65
21.26 J 5.32 kW 1.49 2.10 1.91 1.84 1.00 1.78 1.91 1.69
24.24 J 6.06 kW 2.12 2.46 2.10 2.00 1.29 2.02 2.18 2.12
38
Table 11 Penetration results of refined plots
Argon
Pulse energy Peak power Spot size (mm)
0.2 0.3 0.4 0.5
13.10 J 3.28 kW - 0.59 0.44 -
13.60 J 3.40 kW - 0.59 0.51 -
14.10 J 3.53 kW - 0.73 0.59 -
14.60 J 3.65 kW - 0.87 0.63 -
15.10 J 3.78 kW - 0.78 1.08 -
15.60 J 3.90 kW 0.76 1.09 1.01 0.86
16.16 J 4.04 kW 0.89 - - 1.05
16.71 J 4.18 kW 0.81 - - 1.38
17.27 J 4.32 kW 1.05 - - 1.24
17.83 J 4.46 kW 1.10 - - 1.14
18.38 J 4.60 kW 1.95 - - 1.38
The refining penetration measurements were made solely with argon due to three simple
reasons: the choice of shielding gas did not have any significant influence on the penetration results;
the superficial quality of the welds was slightly better with argon gas, notably in terms of roughness,
where it provided welds with smother surfaces; finally, argon gas was the cheapest option.
With all the previously results two conclusions were made regarding this analysis. The
shielding gas did not influence relevantly the penetration depth achieved, so all subsequent trials used
solely argon gas. To assure the 1 mm penetration objective, four different peak powers, one for each
spot size, were selected. The interval of peak powers tested in the 2nd
trial was from 3.78 to 4.32 kW.
4.2 Hot cracking analysis
The 2nd
and 3rd
trial (AL 300 laser) attempted to solve the hot cracking problem using different
welding speeds, pulse frequencies and pulse shapes while maintaining pulse duration of 4 ms. Since
these trials were made in industrial environment providing limited characterisation equipment
(sandpapers, chemical etchants and a microscope) the approach to follow could not rely on accurate
characterisation of each weld. Therefore, numerous welds (i.e. 284 welds) were made to determine
the best parameters that would minimize the hot cracking problem. As well, the limited time available
did not permit all welds to be equally studied. Even though all welds were visually inspected and
photographed but only a part of the results were subjected to complementary inspections, specifically
the welds of the 2nd
trial were inspected with dye penetrant and the welds of the 3rd
trial were
examined with the company’s optical microscope.
To sum up, this section studied the overall quality of the welds made in the 2nd
and 3rd
trial.
First the results of the 2nd
trial, which include visual and dye penetrant inspection (DPI), were
reviewed. Then, the visual inspection of the welds of the 3rd
trial was revised. Furthermore, 3 welds of
the 2nd
trial showing positive characteristics were examined with a scanning electron microscope
(SEM), from a certified laboratory. The images obtained with this more powerful equipment offered
unprecedented visual details of the welds to assess weld quality and crack location.
39
4.2.1 Visual inspection 2nd trial
The core parameter tested in the 2nd
trial was welding speed. First and foremost, the welds of
the 2nd
trial had weld defects notably undercuts, burned edges and blowholes. These welding defects
appeared somewhat irregularly across the welds therefore, for each spot size, we identified the welds
with the best quality.
The results are presented in figures with an identical configuration. Each figure includes all
welds made with a determined spot size. Also, each figure has 5 images, viz. one image for each peak
power (from 3.78 to 4.32 kW), and every image has 7 welds, viz. one weld for each welding speed (4,
3.5, 3, 2.5, 2, 1.5, and 1 mm/s).
4.2.1.1 Spot size 0.2 mm
Using spot size of 0.2 mm, it was noted that with welding speed of 1 mm/s and peak power of
4.32 kW there were problems of burned edges coupled with undercuts. All lower welding speeds and
peak powers revealed positive results with just occasionally some blowholes. Due to its good weld
quality, the spot size of 0.2 mm was selected for the 3rd
trial (Figure 28).
3.78 kW
3.90 kW
4.04 kW
4.18 kW
4.32 kW
left to right: welding speeds of 4, 3.5, 3, 2.5, 2, 1.5 and 1 mm/s
welds start at the top
Figure 28 – 2nd
trial with spot size of 0.2 mm
40
4.2.1.2 Spot size 0.3 mm
Using spot size of 0.3 mm it was noted that with welding speeds of 1 and 1.5 mm/s in all peak
powers, burned edges and undercuts persistently appeared across the welds. Also, blowholes
randomly appearing in the welds and created small localized burned undercut sections, making this
spot size an inadequate choice for the 3rd
trial (Figure 29).
3.78 kW
3.90 kW
4.04 kW
4.18 kW
4.32 kW
left to right: welding speeds of 4, 3.5, 3, 2.5, 2, 1.5 and 1 mm/s
welds start at the top
Figure 29 – 2nd
trial with spot size of 0.3 mm
41
4.2.1.3 Spot size 0.4 mm
The welds with spot size of 0.4 mm also presented a bad quality as those with spot size of 0.3
mm. Welding speeds of 1 and 1.5 mm/s at 4.04, 4.18 and 4.32 kW peak powers showed serious
burned edges and undercuts. Apart from welding speeds of 3.5 and 4 mm/s at 3.78 kW of peak power,
severe blowholes randomly occur throughout the other welds, which forced the exclusion of this spot
size as well for the 3rd
trial (Figure 30).
3.78 kW
3.90 kW
4.04 kW
4.18 kW
4.32 kW
left to right: welding speeds of 4, 3.5, 3, 2.5, 2, 1.5 and 1 mm/s
welds start at the top
Figure 30 – 2nd
trial with spot size of 0.4 mm
42
4.2.1.4 Spot size 0.5 mm
For this last spot size of 0.5 mm, the welds with 3.78 kW of peak power displayed a silver glow
suggesting that the laser beam barely penetrated the aluminium oxide layer. These welds brought
suspicion on the integrity of the laser components resulting in the inspection of the laser pumping
cavity, which confirmed that the equipment was not working properly as the pumping flash lamp was
partially damaged. At this point it is important to mention that certainly a significant part of the tests
made so far were influenced by the use of a flash lamp that was not in good condition. Even so, with
welding speeds of 1 and 1.5 mm/s and for all peak powers, except the 3.78 kW, the welds exhibited
undercuts and burned edges. Also, with peak powers of 4.18 and 4.32 kW the welds had more
blowholes. As the overall quality of the welds was slightly better with spot size of 0.5 mm than with 0.3
and 0.4 mm and the doubts about the influence of the condition of the flash lamp, this spot size was
selected for the 3rd
trial (Figure 31).
3.78 kW
3.90 kW
4.04 kW
4.18 kW
4.32 kW
left to right: welding speeds of 4, 3.5, 3, 2.5, 2, 1.5 and 1 mm/s
welds start at the top
Figure 31 – 2nd
trial with spot size of 0.5 mm
43
4.2.2 Dye penetrant inspection 2nd trial
The dye penetrant inspection (DPI) had two purposes which complemented the previous
observations. This inspection assessed the gravity of the hot cracking problem of this alloy.
Additionally it helped to select the peak powers, spot sizes and welding speeds for the 3rd
trial. DPI is
a procedure with low reproducibility that offers a qualitative analysis of the hot cracking problem. This
inspection begins by presenting all the image results and then performs the analysis.
Although the DPI was performed for the 4 spot sizes (0.2, 0.3, 0.4, and 0.5 mm), a preliminary
observation has determined that only the results of the spot sizes 0.2 and 0.5 mm are presented and
analysed, due the manifest bad quality observed on the welds of the 0.3 and 0.4 mm spot size
(Annexe E). Consequently these spot sizes were not selected for the 3rd
trial.
4.2.2.1 Results of DPI
The dye penetrant results are presented in 2 figures with the same configuration as in the
visual inspection. Each of the following figures includes the welds for a single spot size. Also, each
figure has 5 images, viz. one image for each peak power (from 3.78 to 4.32 kW), and every image has
7 welds, viz. one weld for each welding speed (4, 3.5, 3, 2.5, 2, 1.5, and 1 mm/s).
3.78 kW
3.90 kW
4.04 kW
4.18 kW
4.32 kW
left to right: welding speeds of 4, 3.5, 3, 2.5, 2, 1.5 and 1 mm/s
welds start at the top
Figure 32 – 2nd
trial DPI results with spot size of 0.2 mm
44
3.78 kW
3.90 kW
4.04 kW
4.18 kW
4.32 kW
left to right: welding speeds of 4, 3.5, 3, 2.5, 2, 1.5 and 1 mm/s
welds start at the top
Figure 33 – 2nd
trial DPI results with spot size of 0.5 mm
4.2.2.2 Observations
All the welds inspected with the dye penetrant revealed the presence of cracks which
confirmed the high hot cracking susceptibility of this AA6082-T651 alloy (Figure 32 and Figure 33).
This inspection also proved that these variations of parameters were insufficient to solve the hot
cracking problem. However, some welds showed better results than others thus some progress was
made.
Comparing the dye penetrant results of spot sizes 0.2 and 0.5 mm we noted that the influence
of the peak power in the hot cracking problem was unclear (Figure 32 and Figure 33) and for that
reason the intermediate value, 4.04 kW was chosen for the 3rd
trial.
Finally, at all peak powers in both spot sizes, it appeared that less hot cracking occurred with
welding speed of 3, 3.5 and 4 mm/s therefore these welding speeds were selected for the 3rd
trial
(Figure 32 and Figure 33).
45
4.2.3 Visual inspection 3rd trial
In this 3rd
trial new samples were tested using the parameters selected by the inspections of
the 1st and 2
nd trial. It should be noted that a new pumping flash lamp was used consequently any
comparison with previous trials is not reliable. The core parameters tested in the 3rd
trial were
frequencies and pulse shapes. Again, the welds of the 3rd
trial occasionally displayed weld defects and
so this inspection started by identifying the welds with the best quality for each spot size.
The results were once more grouped by spot sizes (i.e. 0.2 and 0.5 mm), thus just two figures
include all welds made in this trial. Each figure has 12 images. There are 4 different pulse shapes (i.e.
no shape + 3 pulse shapes), each pulse shape has 3 images, viz. one for each welding speed (3, 3.5
and 4 mm/s), and every image has 7 welds, viz. one weld for each frequency (18, 16, 14, 12, 10 and 8
Hz).
4.2.3.1 Spot size 0.2 mm
The evaluation for this spot size is presented grouped by frequencies (Figure 34).
At frequencies of 8 and 10 Hz: the welds made with all welding speeds and all pulse
shapes presented undercuts and the surfaces were unexpectedly rougher than in the
previous trials.
At frequency of 12 Hz: the welds made with welding speeds of 3 and 3.5 mm/s and
rectangular pulse shape, as well as with welding speeds of 4 mm/s and the 3rd
pulse
shape presented similar defects characteristics at the beginning of the welds (from the top
to almost the middle) but then the process stabilized and these defects presented a
positive improvement.
At frequencies of 14, 16 and 18 Hz: the welds made with all welding speeds and all pulse
shapes presented positive results.
46
No shape
3 mm/s
3.5 mm/s
4 mm/s
1st shape
3 mm/s
3.5 mm/s
4 mm/s
2nd
shape
3 mm/s
3.5 mm/s
4 mm/s
3rd
shape
3 mm/s
3.5 mm/s
4 mm/s
left to right: frequencies of 18, 16, 14, 12, 10 and 8 Hz
welds start at the top
Figure 34 – 3nd
trial welds with spot size of 0.2 mm
Globally no improvement of the quality of the welds could be noticed with the use of a
particular pulse shape. Finally, all these welds were observed on the company’s (Carrs Welding
Technologies Ltd) optical microscope. This examination confirmed that all welds presented hot cracks.
4.2.3.2 Spot size 0.5 mm
The evaluation for this spot size presented worse results than the previous one (Figure 34). All
the welds made with rectangular pulse shape presented more defects, particularly burned edges. At
frequencies 14, 16 and 18 Hz the welds of the 3rd
pulse had also serious burned edges. Comparatively
47
the best results were obtained with the 1st and 2
nd pulse shapes, though these welds revealed weld
defects randomly in all frequencies and welding speeds. Therefore, no pattern could be clearly
identified except that, unlike the previous trial, the frequencies of 14, 16 and 18 Hz did not present a
better weld quality (Figure 35).
No shape
3 mm/s
3.5 mm/s2
4 mm/s
1st shape
3 mm/s
3.5 mm/s
4 mm/s
2nd
shape
3 mm/s
3.5 mm/s
4 mm/s
3rd
shape
3 mm/s
3.5 mm/s
4 mm/s
left to right: frequencies of 18, 16, 14, 12, 10 and 8 Hz
welds start at the top
Figure 35 – 3rd
trial welds with spot size of 0.5 mm
2 Exclude the weld with a cross mark at the top.
48
Lastly, these welds were also examined on the company’s optical microscope and confirmed
that all welds had hot cracks. To resume, this trial showed that neither pulse shape nor frequency
could solve or improve the hot cracking susceptibility of the alloy.
4.2.4 SEM examination
The welds of the 3rd
trial were observed with the company’s (Carrs Welding Technologies Ltd)
optical microscope. With this procedure some differences were noted between the cracks, specifically
some cracks spread at the centred of the weld, some started at the centre but deviated to the sides
and others appeared at the edges of the welds. Consequently, to illustrate these different cracking
behaviours detailed images were taken with a SEM equipment of the MicroLab (at Instituto Superior
Técnico of Lisbon, Portugal).
The welds of the 2nd
trial with 0.2 mm of spot size, 4.32 kW of peak power and welding speeds
of 1, 2.5 and 4 mm/s were chosen because they had fewer defects than the others and revealed
promising results with the DPI. The SEM used was a JEOL JSM 7001F (Annexe C).
For each weld, 2 images with different amplifications of the same location were presented.
From the 50x magnification we perceived the general crack location and from the 200x magnification
we assessed the severity of the crack (depth and width).
4.2.4.1 Welding speed 1 mm/s
For a welding speed of 1 mm/s the images revealed lots of cracks on the weld and showed
they appeared not only in the centre but also closer to the edges (Figure 36).
It is known that the cooling stresses concentrate at the centre of the welds consequently
cracks tend to be located at the centre. However, low welding speed increased the heat input and the
solidification time, which allows more time for low strength precipitates and impurities to segregate to
different locations, probably explaining the spread like distribution of cracks across the welds.
200x
Figure 36 SEM images of 2nd
trial with welding speed of 1 mm/s
49
4.2.4.2 Welding speed 2.5 mm/s
For a welding speed of 2.5 mm/s fewer cracks were present but the weld displayed a
continuous and significant centreline crack.
This aspect suggests that the problem with this welding speed is related to the stress
accumulation at the middle of the weld during solidification of the melt pool and cooling of the material
(Figure 37).
200x
Figure 37 SEM images of 2nd
trial with welding speed of 2.5 mm/s
50
4.2.4.3 Welding speed 4 mm/s
For a welding speed of 4 mm/s the weld had fewer cracks than for a welding speed of 1 mm/s
but with a similar pattern as they appeared also in the centreline and on the sides of the weld,
although these cracks were narrower.
Higher welding speed lowered the heat input and reduced the appearance of cracks in the
material. Still, this was insufficient to avoid hot cracking at locations favouring the segregation of lower
strength precipitates and impurities. At these locations the increase of stress combined with the
precipitation of lower strength material caused cracks to appear (Figure 38).
200x
Figure 38 SEM images of 2nd
trial weld with welding speed of 4 mm/s
4.2.5 Overall results
Many different parameters were tested during the 3 trials of this AL 300 laser, including spot
sizes, peak powers, welding speeds, frequencies, pulse shapes and shielding gases. The purpose of
this approach, which had already been successful with another aluminium alloy 3 , was to select
adequate parameters which would produce welds with no hot cracking and overall good quality.
Unfortunately, using the AL 300 laser to weld AA6082-T651 aluminium alloy without hot cracking was
unsuccessful but some parameters provided decent weld quality.
On a different note, the burned pumping flash lamp compromised the accuracy of the peak
powers mentioned in the 1st and 2
nd trial. Nevertheless, the hot cracking problem could still be studied
since the cracks of welds allowed different interpretations of cracking propagation. Besides, the results
were inconsistent and consequently the laser was subjected to further maintenance which identified
that one of the mirrors of the optical system was misaligned. Because of this, the spot sizes mentioned
were also incorrect and so, the rest of the trials were made with 2 other lasers (TruDisk 4002 and G4
Series Z Type).
3 This procedure was previously carried out with success by Carrs Welding Technologies Ltd with an AA3003 alloy.
51
4.3 Chemical analyses
This section presents the results of the analyses performed on the trials of the AL 300 laser
and on the trials of the TruDisk 4002 laser.
With the AL 300 laser the pulse duration was kept constant at 4 ms and a large number of
parameters were tested (Figure 39). A significant number of samples of those tests were analysed
using SEM with EDS. Since all tests at this pulse duration revealed hot cracks, the most logical
change in other to solve the hot cracking problem in subsequent trials, with the TruDisk 4002 laser,
was to test different pulse durations. As a result, both longer and smaller pulse durations, namely 1
ms, 2 ms and 20 ms were tested. Due to the limited availability of this 2nd
laser (TruDisk 4002) and of
material samples, it was not possible to test as many different parameters as it was done with the AL
300 laser. Therefore, the approach for the analysis of the trials of 2nd
laser (TruDisk 4002) was
focused in search of any positive results, using a SEM with EDS on the welds available. To conclude,
some of the welds of the 1st laser (AL 300) and some of the 2
nd laser (TruDisk 4002) were examined
with a SEM with EDS in order to identify possible causes of the hot cracking.
Figure 39 Example of EDS spectrum obtained for the chemical analysis
To resume, the SEM images and the EDS chemical compositions at multiple locations in the
welds are presented in this section:
First, we determined the chemical composition of the face of the welds of the 2nd
trial (AL
300 laser) and we compare it with a prepared sample surface.
Secondly, we studied the images and chemical compositions of the fusion zone of the 2nd
trial welds (AL 300 laser) and of the 4th trial welds (TruDisk 4002 laser).
Finally, we reviewed the SEM images of the continuous welds and the spot welds made in
the 5th and 6
th trial (TruDisk 4002 laser), respectively.
4.3.1 Chemical compositions of the face of the welds
The face of the welds of the 2nd
trial (AL 300 laser) with spot size of 0.2 mm, peak power of
4.32 kW and 1, 2.5 and 4 mm/s of welding speed were chemically analysed at 6 different locations.
52
Specifically, 3 locations with cracks and 3 locations without cracks were examined to identify
differences of chemical composition. And so, the exact locations analysed on each figure are marked
by pink rectangles followed by their respective chemical compositions.
4.3.1.1 Welding speed of 1 mm/s
Location 1, 2 and 3 had no cracks while locations 4, 5 and 6 had cracks (Figure 40).
Figure 40 EDS locations of the face of the weld for 1 mm/s of welding speed
The analysis discovered mostly aluminium with small, varied amounts of oxygen, magnesium,
and silicon. As it can be observed, no clear pattern was found between the chemical compositions
with and without cracks (Figure 41).
Figure 41 EDS chemical compositions of the face of the weld for 1 mm/s of welding speed
3,8
5
4,5
7
4,0
9
4,2
10
,37
6,3
4
1,1
3
0,8
2
0,9
7
0,7
6
1,3
7
0,8
7
94,34 93,46 93,86 93,72
85,95 91,84
0,6
8
1,1
6
1,0
9
1,3
2
2,3
1
0,9
6
0
10
20
30
40
50
60
70
80
90
100
Spectrum 1 Spectrum 2 Spectrum 3 Spectrum 4 Spectrum 5 Spectrum 6
Wt.
%
Locations
O
Mg
Al
Si
53
4.3.1.2 Welding speed of 2.5 mm/s
Location 1, 2 and 3 had no cracks while locations 4, 5 and 6 had cracks (Figure 42).
Figure 42 EDS locations of the face of the weld for 2.5 mm/s of welding speed
This second analysis revealed the same elements with similar amounts. Correspondingly, no
clear pattern could be identified between the chemical compositions with and without cracks (Figure
43).
Figure 43 EDS chemical compositions of the face of the weld for 2.5 mm/s of welding speed
0 2,7
3
10
,33
6,8
7 1
9,2
9
6,7
4
1,3
3
0,9
4
1,0
7
0,9
8
1,1
1,3
3
97,85 95,7
87,58 91,28
78,4
90,58
0,8
2
0,6
2
1,0
1
0,8
6
1,2
1
1,3
4
0
10
20
30
40
50
60
70
80
90
100
Spectrum 1 Spectrum 2 Spectrum 3 Spectrum 4 Spectrum 5 Spectrum 6
Wt.
%
Locations
O
Mg
Al
Si
54
4.3.1.3 Welding speed of 4 mm/s
Likewise, location 1, 2 and 3 had no cracks and locations 5, 6 and 7 had cracks (Figure 44).
Figure 44 EDS locations of the face of the weld for 4 mm/s of welding speed
Once more, the chemical composition revealed the same elements with similar amounts and
no distinguishable pattern (Figure 45).
Figure 45 EDS chemical compositions of the face of the weld for 4 mm/s of welding speed
0
0 4
,74
3,2
9
4,3
1
3,0
9
0,8
9
1,4
4
1,0
3
0,9
1
0,9
6
1,0
8
98,46 97,32 93,34 94,74 93,92 95,11
0,6
5
1,2
4
0,8
9
1,0
6
0,8
1
0,7
3
0
10
20
30
40
50
60
70
80
90
100
Spectrum 1 Spectrum 2 Spectrum 3 Spectrum 5 Spectrum 6 Spectrum 7
Wt.
%
Locations
O
Mg
Al
Si
55
4.3.1.4 Prepared sample surface
Following the identical procedure the prepared sample surface was also chemically
characterized. This time, 2 locations were selected (Figure 46).
Figure 46 EDS locations of the prepared sample surface
The chemical composition of both locations were closely matched indicating that the surface,
after preparation was homogeneous (Figure 47).
Figure 47 EDS chemical compositions of the prepared sample surface
6,4
5
5,5
4
0,9
9
1,2
90,89 91,42
1,6
7
1,8
4
0
10
20
30
40
50
60
70
80
90
100
Spectrum 1 Spectrum 2
Wt.
%
Locations
O
Mg
Al
Si
56
4.3.1.5 Face of the welds vs. prepared sample surface
With these EDS chemical compositions the following comparison was made between the face
of the welds, regardless of the location (that is with or without cracks) and the prepared sample
surfaces. This comparison was based on the mean wt. % of oxygen, magnesium and silicon (Figure
48).
Figure 48 Mean chemical composition of the face of the welds vs. prepared sample surface
The comparison indicated that almost no vaporization took place since the amount of
magnesium was nearly the same with an average of 1.05 wt. % on the face of the weld and 1.10 wt. %
on the prepared sample surface which is a decrease of about 4.5%. The amount of oxygen was also
similar with a decrease close to 12.2% which attested that the gas protection was adequate. Lastly,
the silicon content variation was more noticeable because it decreased significantly, about 40.9%.
Since silicon was not lost by evaporation it is believed that the silicon segregated to the fusion zone of
the welds.
4.3.2 Chemical compositions of the fusion zone of the welds
The fusion zone of the welds of the 2nd
trial (AL 300 laser) with 1, 2.5 and 4 mm/s of welding
speed, and of the 4th trial (TruDisk 4002 laser) with 3.4 kW, 16.4 Hz and 4.5 mm/s were inspected with
the SEM and chemically analysed with EDS. Those welds were cut, mounted, contrasted (Annexe A
and B) and examined to study their chemical composition, more precisely the surfaces of the cracks
and the surrounding material without cracks. For each weld 2 different cracks in the fusion zone were
selected and for each crack 2 locations were selected. Specifically, it was analysed 1 location with a
crack and 1 location without any crack. It was also assessed the penetration of these welds through
the images taken with the SEM. So, first these images and respective penetrations are presented, and
then the EDS locations and respective chemical compositions are detailed.
5,27 6,00
1,05 1,10 1,04 1,76
0,001,002,003,004,005,006,007,00
Face of thewelds
Prepared samplesurface
Weig
ht%
(%
)
Locations
O
Mg
Si
57
4.3.2.1 Penetration
The following SEM images were taken with a low magnification of 100x to allow the full
penetration to be measured (Figure 49).
1 mm/s (AL 300 laser)
2.5 mm/s (AL 300 laser)
4 mm/s (AL 300 laser)
4.5 mm/s (TruDisk 4002 laser)
Figure 49 SEM images of the fusion zone of the welds
The penetrations were all similar although different heat inputs and different power densities
were used. Heat input and power density was calculated with the following equations:
[71]
[17]
First, the use of different heat inputs for the welds of the AL 300 laser did not influence the
penetration at all, which indicates that penetration is not a function of heat input [17]. Secondly, it is
known that the power density of these welds was lower than the estimated values because the laser
was defective and compromised both peak powers and spot sizes. As a result, the TruDisk 4002 laser,
which was in proper condition, reached an equivalent penetration with a power density much lower
(Table 12).
58
Table 12 Penetration, heat input and power density of the 4 welds
Laser Spot size Welding speed Penetration
(mm) Heat input
(J/mm) Power density
(MW/mm2)
AL 300 0.2 mm
1 mm/s 0.72 172.8
13.75 2.5 mm/s 0.75 69.1
4 mm/s 0.77 43.2
TruDisk 4002 0.67 mm 4.5 mm/s 0.74 128.2 0.96
4.3.2.2 Welding speed of 1 mm/s – AL 300 laser
There are 2 images of cracks: in the 1st crack, locations 1 and 2 are respectively with and
without crack; in the 2nd
crack, locations 4 and 5 are respectively with and without crack (Figure 50).
1
st crack at 3500x
2
nd crack at 2500x
Figure 50 EDS locations of the fusion zone of the weld for 1 mm/s of welding speed
As expected, the EDS revealed that the fusion zone was mainly aluminium and small amounts
of oxygen, magnesium and silicon. Interestingly, there was a clear difference between the chemical
compositions in the locations with and without cracks, specifically without cracks the material was 100
wt. % of aluminium, while with cracks the other elements mentioned were also present (Figure 51).
Figure 51 EDS chemical compositions of the fusion zone of the weld for 1 mm/s of welding speed
2,75 0 0 0 1,02 0 1,45 0
93,89 100 97,07 100
2,34 0 1,48 0 0
10
20
30
40
50
60
70
80
90
100
Spectrum 1 Spectrum 2 Spectrum 4 Spectrum 5
Wt.
%
Locations
O
Mg
Al
Si
59
4.3.2.3 Welding speed of 2.5 mm/s – AL 300 laser
There are 2 images of cracks: in the 1st crack, locations 3 and 5 are respectively with and
without crack; in the 2nd
crack, locations 1 and 3 are respectively with and without crack (Figure 52).
1
st crack at 2500x
2
nd crack at 2500x
Figure 52 EDS locations of the fusion zone of the weld for 2.5 mm/s of welding speed
Like previously, this EDS analysis found the same elements and the same pattern between
locations with and without cracks. Also, it was noted that the amounts of oxygen and silicon present at
the locations with cracks increased when compared to the welding speed of 1 mm/s (Figure 53).
Figure 53 EDS chemical compositions of the fusion zone of the weld for 2.5 mm/s of welding speed
4,68 0
7,45
0 1,1 0 1,1 0
92,08 100
87,11
100
2,15 0 4,34
0 0
10
20
30
40
50
60
70
80
90
100
Spectrum 3 Spectrum 5 Spectrum 1 Spectrum 3
Wt.
%
Locations
O
Mg
Al
Si
60
4.3.2.4 Welding speed of 4 mm/s – AL 300 laser
There are 2 images of cracks: in the 1st crack, locations 3 and 1 are respectively with and
without crack; in the 2nd
crack, locations 2 and 3 are respectively with and without crack (Figure 54).
1
st crack at 2500x
2
nd crack at 2500x
Figure 54 EDS locations of the fusion zone of the weld for 4 mm/s of welding speed
This EDS indicated similar results to the previous welding speeds of 1 and 2.5 mm/s. Namely,
the amounts of oxygen, magnesium and silicon increased in comparison to both 1 and 2.5 mm/s of
welding speed and, except the presence of a small amount of magnesium on the location 3 of the 2nd
crack, the pattern for this welding speed was identical (Figure 55).
Figure 55 EDS chemical compositions of the fusion zone of the weld for 4 mm/s of welding speed
11,3
0
8
0 2,27 0 1,25 1,17
76,35
100
87,31
98,83
10,07
0 3,44
0 0
10
20
30
40
50
60
70
80
90
100
Spectrum 3 Spectrum 1 Spectrum 2 Spectrum 3
Wt.
%
Locations
O
Mg
Al
Si
61
4.3.2.5 Welding speed of 4.5 mm/s – TruDisk 4002 laser
There are 2 images of cracks: in the 1st crack, locations 1 and 2 are respectively with and
without crack; in the 2nd
crack, locations 1 and 2 are respectively with and without crack (Figure 56).
1
st crack at 3500x
2
nd crack at 3500x
Figure 56 EDS locations of the fusion zone of the weld for 4.5 mm/s of welding speed
Finally, the EDS chemical analysis of the TruDisk 4002 laser weld was consistent with the
pattern formerly observed. To be specific, this EDS showed, once more differences of chemical
composition between location with and without cracks. Oxygen and silicon are present at the location
with cracks, but unlike the pattern for welding speeds of 1 and 2.5 mm/s it was noticed that there is
magnesium in the locations without cracks (Figure 57).
Figure 57 EDS chemical compositions of the fusion zone of the weld for 4.5 mm/s of welding speed
5,87 0
10,49
0 1,34 1,5 1,49 1,25
89,12 98,5
83,9
98,75
3,67 0
4,13 0
0
10
20
30
40
50
60
70
80
90
100
Spectrum 1 Spectrum 2 Spectrum 1 Spectrum 2
Wt.
%
Locations
O
Mg
Al
Si
62
4.3.2.6 Material with cracks vs. material without cracks
The welds of the 2nd
trial (AL 300 laser) and of the 4th trial (TruDisk 4002 laser) were different
in almost every parameter, including frequencies, peak powers, pulse duration, pulse shape, spot
sizes and welding speeds. Due to the malfunction of the 1st laser, both reference peak powers and
spot sizes were compromised and as a result, any analysis using these metrics is not reliable.
Therefore, this research could not study the influence of heat input and power density on composition
which is usually the standard. So it was necessary to focus on comparing chemical compositions
between the locations with and without cracks or between different welding speeds. To sum up, with
all the EDS results of the fusion zone the mean chemical composition in wt. % was calculated for
locations with and without cracks (Figure 58). Additionally, the evolution of mean wt. % of oxygen,
magnesium and silicon in the locations with cracks for each welding speed was also determined
(Figure 59).
Figure 58 Mean chemical compositions of the locations with vs. without cracks
With the mean chemical composition of both locations with and without cracks it is clear that,
in average, oxygen, magnesium and silicon are present in substantial amounts in the cracks with an
average of 6.32 wt. % of oxygen, 1.38 wt. % of magnesium and 3.95 wt. % of silicon. On the other
hand, the locations without cracks are free of those elements, except of magnesium but in smaller
amounts with only 0.49 wt. %. These results suggest that predominantly oxygen and silicon are
responsible for, or in correlation to the hot cracking of the welds.
AL 300 laser
TruDisk 4002 laser
Figure 59 Mean chemical compositions of locations with cracks for each welding speed
6,32 0 1,38 0,49
88,35 99,51
3,95 0 0,00
20,00
40,00
60,00
80,00
100,00
With crack Without crack
Mean
wt.
%
Locations
O
Mg
Al
Si
1,38
6,07
9,65
1,24 1,10 1,76 1,91
3,25
6,76
0,00
2,00
4,00
6,00
8,00
10,00
1 mm/s 2,5 mm/s 4 mm/s
Mean
wt
%
Welding speed
8,18
1,42
3,90
0,00
2,00
4,00
6,00
8,00
10,00
4,5 mm/s
O
Mg
Si
63
First, it was noted that increasing the welding speed also increased the amounts of oxygen
and silicon at the locations with cracks. It is known that unlike magnesium and silicon, which can only
come from the weld itself, oxygen may come from the various surface oxides, as for instance Al2O3,
MgO, Mg(OH)2 [72], [73], or directly from the atmosphere when gas protection is ineffective. On this
point, the results of the characterization of the face of the welds pointed towards oxygen coming from
the surface oxides. On the other hand, faster welding speeds lead to lower heat inputs and faster
solidification times. Additionally, with faster welding speeds, elements that are present at the surface
and that can vaporize, like oxygen, are more susceptible to be trapped inside the fusion zone. Hence,
the presence of oxygen in the welds can be related to the solidification times in the following manner:
a lower welding speed increases the solidification time which allows more oxygen to escape from the
metal weld pool and less oxygen to be left inside the FZ and segregate to the locations causing
cracks. This explains why higher concentrations of oxygen were found with faster welding speeds.
The silicon and magnesium come from the weld itself, mainly from the inorganic compound
Mg2Si, which precipitates during the precipitation hardening treatment and contributes to the
mechanical properties of the alloy [63]. The chemical composition of the locations without cracks
indicated 99.51 wt. % of aluminium. This suggests that the silicon and some of the magnesium
detected at the locations with cracks came from the surrounding weld metal causing the surrounding
material to be almost depleted of any other element besides aluminium. Interestingly, the amount of
silicon increased significantly with increasing welding speeds but the amounts of magnesium did not
follow the same pattern. This increase of silicon content with welding speed can be related to the
solidification time. Lower welding speeds lead to higher heat inputs and longer solidification times.
These longer solidification times allow more of the silicon to precipitate into the bulk of the material.
Hence, forming these precipitates decreased the amount of silicon found in solid solution at the
surface of the cracks with these analyses.
Overall, the results of the chemical compositions of the fusion zone suggest that the hot
cracking problem arises from the depletion of hardening elements from the neighbouring metal to the
grain boundary location through segregation during solidification. The amount of silicon found at the
crack locations was irregular, with results going from 1.45 wt. % to 10.07 wt. % and was overall high
with an average of 3.95 wt. %. In contrast, at the location without cracks there was no silicon detected.
Thus complete depletion of silicon around the cracks weakened the strength of the material in these
locations while other locations, probably the grain boundaries, accumulated high amounts of silicon,
became brittle and cracked with the solidification stresses. On the other hand, the magnesium at the
locations with cracks was roughly the same for all welding speeds with amounts going from 1.02 wt. %
to 2.27 wt. % and an average of 1.38 wt. %. Additionally magnesium was also found at the location
without cracks with amounts up to 1.5 wt. %. Thus, unlike with silicon, it is difficult to relate hot
cracking with the amount of magnesium and further analysis is necessary to clear this possibility.
In conclusion, this analysis shows that the hot cracking is partially caused by the presence and
accumulation of oxygen and silicon at specific locations which are likely the grain boundaries. It was
also shown that oxygen must come from surface of the material whereas silicon is one of the main
64
alloying elements of this aluminium alloy. Consequently, while oxygen can be reduced or even
completely eliminated with proper surface preparation the same cannot be done with silicon.
4.3.2.7 Final interpretation of the fusion zone
This chemical analysis showed that there is a difference in the compositions of the locations
with cracks and the locations without cracks, namely the location with cracks have higher contents of
silicon and oxygen. It is known that there are many factors affecting the hot cracking susceptibility of
an alloy and that one of these factors is the possibility to form low melting point eutectics during
welding and solidification. Considering the possible causes of hot cracking in literature for the
aluminium alloys of the 6xxx series, it is believed that this chemical analysis found evidence that the
hot cracking of this alloy can be explained by the formation of silicon rich low melting point eutectics
during welding and solidification. These low melting point eutectics belong to the following Al-Mg-Si
alloy systems [44]:
Al-Mg2Si;
Al-Si;
Al-Mg2Si-Fe-Mg3Si6Al8-Si
Al-Mg2Si(CrFe)4Si4Al13-Si;
Al-CuAl2-Mg2Si,
AlCu2Mg8Si6Al5-CuAl2-Si
These different systems have low melting point eutectics with melting temperatures ranging
from 514 to 595°C. In comparison, the melting point of the aluminium alloy AA6082 is about 660°C
[44]. Therefore, with this difference of temperatures it may have caused a liquid film to be formed
resulting in the hot cracking of the welds.
4.3.3 Continuous and spot welds
Lastly, on one hand, we confirmed that using continuous welding (5th trial) with adequate
welding speeds was able to solve the hot cracking in the welds [6]. On the other hand we tested if
single spot welds (6th trial) with shorter pulse durations could also solve hot cracking due to their much
smaller weld pools and faster solidification times which, in theory, reduces the possibility to
accumulate of oxygen and silicon in stress sensitive locations. These 5th and 6
th trials (TruDisk 4002
laser) were studied with the SEM without EDS because there were no cracks to analyse (5th trial) and
it was not feasible on the available time schedule (6th trial).
65
4.3.3.1 Continuous welds
In this 5th trial, it was confirmed that continuous welds with adequate parameters had no hot
cracking. The penetration of the welds was about 2.30 mm with 2.8 kW of power (Figure 60) and 2.73
mm with 3.4 kW of power (Figure 61).
Fusion zone (30x)
Face of the weld (30x)
Intersection of the fusion zone with the heat affected zone (400x)
Figure 60 SEM images of the continuous weld with 2.8 kW of power
66
Fusion zone (30x)
Face of the weld (30x)
Intersection of the fusion zone with the heat affected zone (400x)
Figure 61 SEM images of the continuous with 3.4 kW of power
These 2 welds had different powers and the same welding speeds. This shows the importance
of adequate cooling rate for the welds to accommodate the stresses of the solidification. Unfortunately,
it was not possible to recreate similar conditions with pulse welding, even with pulses durations of 20
ms, because each spot tends to solidify during off times.
4.3.3.2 Spot welds
In this 6th trial, spot welds were produced with 1 and 2 ms of pulse duration and 3 spot sizes.
All 6 parameters were repeated 10 times each, making a total of 60 spot welds. Unfortunately all the
parameters tested revealed spot welds both with and without cracks consequently this trial was
inconclusive (Table 13).
67
Table 13 Results of the visual inspection of the spot welds
Pulse duration Spot size Nº. of spot welds with cracks
1 ms
0.2 mm 7
0.38 mm 3
0.67 mm 10
2 ms
0.2 mm 5
0.38 mm 6
0.67 mm 9
It seems probable that some spots did not have cracks because the reduced pulse duration
limited the size of the fusion zone therefore avoiding the typical stress and segregation issues. It also
seems likely that very little penetration was achieved however this was not verified as the
heterogeneity of these results did not justify further investigation. Such heterogeneous results can be
caused by exterior factors like the superficial defects of the material or an uneven sample preparation.
It is worth to note that due to the discrete nature of spot welds, these factors greatly influenced the
quality of each spot weld.
The best parameters, that is to say the spot weld parameters with fewer cracks, used 0.38 mm
of spot size and 1 ms pulse duration and revealed 3 out of 10 spots with cracks (Figure 62 and Figure
63). This result seemed promising as each spot weld was a bit different however no clear pattern was
identified to distinguish those with or without cracks.
Magnification 40x
Figure 62 SEM images of spot welds with 0.38 mm of spot size and 1 ms of pulse duration without cracks at the face
68
Magnification 40x
Figure 63 SEM images of spot welds with 0.38 mm of spot size and 1 ms of pulse duration with visible cracks at the face
4.4 Optical microscopy analysis
The spot welds with short pulse durations of 1 and 2 ms made in 6th trial (TruDisk 4002 laser)
revealed that some spots had no visible hot cracking. This directed this study to further reducing the
pulse duration to 240, 350 and 520 ns in the 7th trial with the G4 series Z Type laser. Considering this
laser is designed for laser marking and micro-machining applications, no comparable research and no
reference values were found on laser welding aluminium alloys with such unconventional equipment.
Furthermore, having in mind that this last trial is somewhat innovative, the choice of parameters was
intended to test the full processing interval of the laser. So, this 7th trial tested wide-ranging
parameters and made a brief overview to support future investigation.
The initial visual inspection of the samples was misleading since these tests resembled simple
laser markings and for that reason, at a first approach, penetrations were evaluated with an optical
microscope. This analysis with the optical microscope revealed that proper welds with very high
aspect ratios were made. Hence, in this section first the images taken with the optical microscope are
presented, then the results of the weld penetrations are assessed and finally it is proposed an
explanation for the causes of this unexpected but favourable situation.
4.4.1 Characterization of the welds
In the 7th trial only the tests made with spot size of 0.051 mm were selected. For each spot
size, tests with single and multiple passes were made. It is worth to note that tests made with multiple
passes had a black oxidized protruded surface which is uncharacteristic of the welding process. These
tests revealed that, regarding laser welding of aluminium alloys, the use of multiple passes was not a
good option for two reasons: the first reason was that the material evaporated and the laser left a deep
hole instead of a weld and the second reason was that multiple passes did not increase penetration.
On the other hand, the tests of spot size 0.051 mm, with single passes showed that pulsed
laser welding was successfully achieved independently of the others parameters tested and revealed
no cracks. Additionally, it was noted that these welds had a high aspect ratio and that some porosity
and holes occasionally appeared though their general appearance suggested that good quality welds
69
were made when compared to previous pulsed welds of the AL 300 and the TruDisk 4002 lasers
(Figure 64).
Single pass
240 ns with 35.7 mm/s
240 ns with 3.6 mm/s
350 ns with 35.7 mm/s
350 ns with 3.6 mm/s
520 ns with 35.7 mm/s
520 ns with 3.6 mm/s
Multiple passes (10 cycles)
240 ns with 35.7 mm/s
520 ns with 35.7 mm/s
Figure 64 – 7th
trial with spot size of 0.051 mm
4.4.2 Penetration results
As stated above, the penetration results revealed welds with high aspect ratios. For instance,
the weld with pulse duration of 350 ns and 3.6 mm/s of welding speed had an aspect ratio of 17.3:1
70
(Figure 65) which suggests that the welding process was within the keyhole regime. However with
pulse durations between 240 and 520 ns the resulting keyhole is naturally instable which explains the
occasional presence of porosity and holes in those welds. The penetration results also indicated that
lower welding speeds achieved much higher penetration. Although the overlap factor is known to
influence the penetration of aluminium welds, an increase of 74.3% of weld penetration by increasing
the overlap factor of 1% (from 99% to 99.9%) was unexpected (Table 14).
Figure 65 – 7th
trial with pulse duration of 350 ns and 3.6 mm/s of welding speed
Every test has multiple weld lines (4 lines drawing a rectangle) so 2 penetration
measurements were made for the single pass welds and are detailed as follows (Table 14).
Table 14 Penetration results of the single pass welds
Tests Penetration with overlap factor 99%
Tests Penetration with overlap
factor 0.999%
Penetration increase from 99 to 99.9%
240 ns with 35.7 mm/s
0.38 mm 240 ns with 3.6 mm/s
0.84 mm 119.9%
0.38 mm 0.85 mm 127.0%
350 ns with 35.7 mm/s
0.61 mm 350 ns with 3.6 mm/s
1.00 mm 64.5%
0.55 mm 0.98 mm 75.9%
520 ns with 35.7 mm/s
0.63 mm 520 ns with 3.6 mm/s
0.96 mm 51.4%
0.45 mm 0.61 mm 35.3%
Average 0.50 mm Average 0.87 mm 74.3%
4.4.3 Research interpretation
First, it is considered the fact that proper welding was achieved. It is known that this type of
equipment is usually intended to perform laser ablation which is the removal of material from a
substrate by direct absorption of laser energy [74]. In ablation with pulsed laser radiation, depending
71
on the pulse duration, different beam-matter interaction mechanisms become dominant. With
microsecond or nanosecond laser pulses the ablation process is dominated by heat conduction,
melting, evaporation and plasma formation (Figure 66). In the ablation processes involving
nanosecond lasers, the absorbed laser energy first heats the target surface to the melting point, and
then to the vaporization temperature [75]. Moreover, metals require much more energy to vaporize
than to melt [76], therefore it was reasonable to assume that adequate control of the laser parameters
could allow to make welds with melting of the surface and minimal vaporization. In other words,
choosing parameters that would increase the surface temperature while keep it close to the
vaporization temperature would perform laser welding very close to the keyhole regime.
With the spot size of 0.051 mm, the use of a high power density of 49 J/cm2 (far above the
typical ablation threshold of metals that is from 1 to 10 J/cm2) combined with long pulse durations of
240 to 520 ns and pulse frequency of 70 kHz caused the ablation process to be inefficient for metal
surface vaporization but quite efficient in accumulating heat and in melting the metal through joule
heating [77]. Hence, these conditions resulted in welds in the keyhole regime featuring high aspect
ratios.
Figure 66 Classical beam matter interaction [75]
Remarkably, no hot cracks could be spotted in any of the welds. Considering that even at the
kilohertz repetition rate, the coupled laser energy is not dissipated until the next laser pulse arrives
[77] the accumulation effect at 70 kHz heated every spot weld multiple times, creating a linearly
temperature decrease that reduced the solidification tensions. Additionally these high aspect ratio
welds had very small melt pools consequently the segregation of critical elements such as oxygen or
silicon was much reduced, compared to normal conduction and keyhole laser welds. It is this
combination of soft temperature decrease and small high aspect ratios welds that made possible to
form autogenous pulsed seam welds of AA6082-T651 without hot cracks.
Finally, it was noticed that the weld penetration increased significantly with the overlap factor
which allowed the welds to reach the objective of 1 mm at 99.9% of overlap. The influence of
overlapping pulsed laser processing on the melting ratio has already been investigated [77]–[79] thus
these result confirmed previous studies. Finally, it must be noted that the achievement of the main
objective of this investigation was successful, specifically the autogenous laser spot welding of
AA6082-T651 featuring 1 mm of weld penetration, an overlap factor over 60% of and no hot cracks.
72
5. Conclusions
This work investigated autogenous laser welding of AA6082-T651 aluminium alloy. For such
purpose, 3 completely different lasers were used, namely a conventional pulse laser welding
equipment of 300W, a high power continuous laser welding equipment of 4000W and a laser marking
equipment of 70W. In order to widen the field of investigation of this research every laser tried a
different range of parameters and so the results were analysed independently.
AL 300 laser
This equipment tested different pulse shapes, gas compositions and flow rates, pulse
frequencies, welding speeds, spot sizes and peak power while keeping the pulse duration constant at
4 ms. The 372 welds produced had hot cracks and the equipment experienced technical problems
which made any tests with longer pulse durations impractical. From the 3 trials of this laser the main
conclusion is that typical pulsed laser welding equipment operates with particularly unfavourable pulse
durations for autogenous welding of crack sensitive alloys, such as the AA6082-T651 aluminium alloy
of this work. The problem of the pulse duration was confirmed with multiple chemical analyses which
showed that at the millisecond pulse range, the metal weld pools created are large enough to allow
segregation of lower melting point eutectics at the grain boundaries. This segregation coupled with the
high solidification stresses, typical of the pulsed laser welding process, resulted invariably in hot
cracking of the welds.
TruDisk 4002 laser
This equipment did continuous welding, pulsed seam welding and spot welding. A total of 16
welds tested different pulse durations and shapes, pulse frequencies, welding speeds, spot sizes and
peak powers. From the 3 trials of this laser different conclusions were found regarding the weldability
of the AA6082-T651 aluminium alloy. The welds made using long pulses of 20 ms presented
superficial improvements with some partially healed locations. However, SEM images and EDS
chemical analysis revealed that the welds had hot cracks due to similar reasons to the AL 300 welds.
The continuous welds made using a welding speed of 25 mm/s showed no signs of hot cracking which
confirmed that continuous lasers were a viable option for autogenous laser welding. The spot welds
results were unclear as, for every set of parameters tested, part of the spots had hot cracks.
Therefore, with both the TruDisk 4002 and the AL 300 the joint conclusion is that autogenous laser
welding using pulses with durations of milliseconds is inadequate for welding the AA6082-T651
aluminium alloy. This difference of results was due to the control of the solidification time which is
achieved differently with each type of equipment. Pulse lasers can use longer pulse durations and
pulse shapes to create a smoother solidification profile of each spot which diminishes the solidification
stresses resulting in less hot cracking. However, pulse durations of 20 ms or shorter do not decrease
sufficiently the stresses of the solidification after each pulse. Continuous lasers on the other hand, use
welding speed to control the solidification time thus achieving much slower solidification which ensures
the development of less stresses, resulting in crack free welds.
73
Experimental parameters without cracks:
Single pass
Continuous welding
Gas and flow rate: Argon with 12 L/min
Laser inclination: 10°
Welding speed: 25 mm/s
Spot size: 0.38 mm (-2 collimation)
Laser power: 2.8 kW and 3.4 kW
G4 Series Z Type laser
This laser marker made 24 different welds and tried different pulse durations and shapes,
welding speeds, spot sizes, peak powers and number of passes. The results of this laser were
unexpected and very clear. Using multiple passes caused the laser to remove material through
ablation, which is the intended purpose of this laser marking machine. However, using a single pass
produced high aspect ratio welds. Moreover, these welds revealed no hot cracks and one of the welds
achieved the 1 mm of penetration objective initially stated. This result has two explanations. First, the
parameters used were inadequate to achieve ablation therefore the irradiated material heated and
melted, resulting in welds instead of ablation marks. Secondly, the laser marker operated with a
frequency of 70 kHz consequently, each successive pulse was fired before achieving the complete
cooling of the previous spot. This caused heat to accumulate creating a weld with a similar thermal
profile to continuous welding. Furthermore, it is remarkable that no hot cracks were found. Such result
was probably due to the small metal weld pool of these high aspect ratio welds, which limited the
segregation of low melting point eutectics, in addition to longer cooling times and lower stresses
provided by the continuous heat build-up.
Experimental parameters without cracks and 1 mm of weld penetration:
Single pass
Pulsed welding
Pulse duration and shape: 350 ns with the pre-programed pulse shapes nº32 (Annexe D)
Pulse frequency: 70 kHz
Welding speed: 3.6 mm/s
Spot size: 0.051 mm
Peak power: 7 kW
Suggestion for future work
As a final point, very positive results were achieved with the laser marking equipment thus it
would be interesting for future work, to make further research with this type of equipment. Accordingly,
frequencies in the kilohertz range with different welding parameters should be studied to determine
weld penetration, aspect ratio and overall weld quality. Additionally, applying laser markers to practical
(industrial) issues would probably open a new field of possibilities for welding low weldability alloys,
including the AA6082-T651 alloy of this thesis.
74
75
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A - 1
Annexe A – Bloc sample cutting and mounting
First, the selected weld had to be cut from their respective bloc samples. Once these welds
were separated they were cut transversely to the welding direction with a certain angle, using a liquid
cooled circular saw:
Welds placed with angle to be cut by the liquid cooled circular saw
The purpose of this angle was to make the crack surfaces more accessible to the EDS signal.
The pieces obtained had about 20° of inclination:
2
nd trial
spot size of 0.2 mm
peak power of 4.32 kW
welding speed of 1, 2.5 and 4 mm/s
4
th trial
peak power of 3.4 kW
frequency of 16.4 Hz
welding speed of 4.5 mm/s
Welds after cutting process to make the angled surface
These pieces were then placed in moulds where resin was poured:
2
nd trial
3
rd trial
Mounted samples inside their moulds during the 12h curing period of the resin
Mounting the pieces in resin provides easier and homogenous grinding and polishing of the
samples, especially in the later stages of the polishing process when cloth and colloidal solutions are
used to do the fine polishing.
B - 1
Annexe B – Sample grinding, polishing and etching
It is important to prepare the sample surfaces before applying the etching agent otherwise it
will not react appropriately. For this work, the grinding steps chosen were found in the characterization
guide of the Laboratório de Caracterização de Materiais (at Instituto Superior Técnico of Lisbon,
Portugal). The following grades of sandpapers were used:
230, 320, 600, 800, 1000 and 2400-grit, for 1 to 2 min each
4000-grit, for 5 to 10 min to remove all the scratches of the 2400-grit
Then, the samples were immersed in ethanol and placed in an ultrasonic cleaner for 10 min.
Afterwards they were polished with the following particle sizes and respective cloths:
Cloth ADR II using diamond particles of 3 µm
Cloth HSB using diamond particles of 1 µm (this step is optional)
Cloth SUPRA using SPM (OPS) suspension and applying distilled water for lubrication
Before etching, the samples were, once more cleaned of any residue with the same process of
immersion in ethanol and ultrasonic cleaning for 10 min.
Finally, the polished sample surfaces were chemical etched with a Keller’s reagent which was
prepared with the following list of components:
Keller's reagent
Component Quantity
Distilled water H2O
190 mL
Nitric acid HNO3
5 mL
Hydrochloric acid HCL
3 mL
Hydrofluoric acid HF
2 mL
Multiple etching times were tested: 20, 30, 35 and 40 s. The etching time selected was 30 s
because longer etching times revealed some darken regions which are evidence of over-etching. The
etching was halted by rinsing the samples with distilled water and drying them using ethanol and a hair
drier.
As final note, the last polishing cloth and etching steps were made 20 min prior to the SEM
analysis to decrease the possibility of atmospheric or handling contamination of the samples.
C - 1
Annexe C – SEM with EDS equipment
As mentioned, a number of welds were analysed with a scanning electron microscope (SEM)
specifically the JEOL JSM 7001F. This equipment took detailed images and made multiple chemical
analyses of different locations in the samples using an acceleration voltage of 15 kV. Additionally, the
different compositions of the welds were determined by energy dispersive X-ray spectroscopy (EDS)
using backscattered secondary electrons.
SEM with EDS device JEOL JSM 7001F
D - 1
Annexe D – Pulsed shapes
rectangular pulse shape (rectangular form):
1st pulse shape, decreasing power after 80% of the pulse duration:
2nd
pulse shape, decreasing power after 60% of the pulse duration:
3rd
pulse shape, decreasing power after 40% of the pulse duration:
D - 2
4th pulse shape, with the pulse divided in welding section (4 ms) and cooling section (16 ms);
the cooling section starts at 65% of output power and decreases linearly until reaching a
minimum output power of 40 W:
pre-programed pulse shape nº 0 (WF0):
pre-programed pulse shapes nº 32 and nº 36 (WF32; WF36):
E - 1
Annexe E – Dye penetrant inspection of spot size 0.3 and 0.4 mm
3.78 kW
3.90 kW
4.04 kW
4.18 kW
4.32 kW
left to right: welding speeds of 4, 3.5, 3, 2.5, 2, 1.5 and 1 mm/s
welds start at the top
2nd
trial DPI results with spot size of 0.3 mm
3.78 kW
3.90 kW
4.04kW
4.18 kW
4.32 kW
left to right: welding speeds of 4, 3.5, 3, 2.5, 2, 1.5 and 1 mm/s
welds start at the top
2nd
trial DPI results with spot size of 0.4 mm