cmt thesis

A Feasibility Study of “Cold Metal Transfer” – Gas Metal Arc Welding

(CMT-GMAW) Nickel Base Superalloy Inconel 718™

by

Timothy Patrick Hasselberg

A Thesis Submitted to the Graduate

Faculty of Rensselaer Polytechnic Institute

in Partial Fulfillment of the

Requirements for the degree of

MASTER OF SCIENCE

Major Subject: MECHANICAL ENGINEERING

Approved:

_________________________________________ Ernesto Gutierrez-Miravete, RPI Thesis Adviser

_________________________________________ Samuel Christy, Pratt and Whitney Thesis Adviser

Rensselaer Polytechnic Institute Hartford, Connecticut

April 2009

ii

© Copyright 2009

by

Timothy Patrick Hasselberg

All Rights Reserved

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CONTENTS “Cold Metal Transfer” – Gas Metal Arc Welding: CMT-GMAW Material Joining Characterization of Nickel Base Superalloy Inconel 718™ …...………...i LIST OF TABLES ……………………………………………………………………......v

LIST OF FIGURES …………………...…………………………………………………vi

LIST OF EQUATIONS..…………………………………………………………………xii

NOMENCLATURE..……………………………………………………………………xiii

ACKNOWLEDGMENT ..……………………………………………………………….xv

ABSTRACT.....………………………………………………………………………….xvi

1. Introduction ……………………………………………………………………………1

1.1 Fusion Welding Overview……………………………………………… …….3

1.2 Arc Welding Precipitation-Hardenable Nickel-Base Superalloys… ………….5

2. The Cold Metal Transfer-Gas Metal Arc Welding Process ……………………………7

2.1 Arc Length ….………………………………………………………………..11

2.2 Wire Feed Speed ……………………………………………………………..12

2.3 Traverse Speed ………...……………………………………………………..13

2.4 Electrode Orientation ………...………………………………………………13

3. Method of Approach …………………………………………………………………..15

3.1 Pre- and Post-Weld Thermal Treatments …………………………………….17

3.2 Weldment Specimen Fabrication …………………………………………….18

3.3 Post-Weld Non-Destructive Evaluation ……………………………………...21

3.4 Material Property Characterization ………………………………………….24

3.4.1 Micro-Analysis ……………………..……………………………...24

3.4.2 Specimen Mount Preparation ………………………………………26

3.4.3 Mechanical (Hardness) Analysis …………………………………..27

3.4.4 Mechanical (Tensile) Analysis …………………………………….28

4. Results and Discussion ………………………………………………………………..31

4.1 Non-Destructive Analysis of CMT-GMAW Weldments………………… …31

4.2 Destructive Analysis …………………………………………………………34

4.2.1 Representative Weldment Microstructure …………………………34

4.2.2 Macrostructural Comparison to GTAW …………………………...38

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4.2.3 Microstructural Comparison to GTAW ……………………………39

4.3 Hardness Properties ………………………………………………………….42

4.4 Tensile Properties ……………………………………………………………43

4.4.1 Room Temperature Properties ……………………………………..45

4.4.2 Elevated Temperature Properties ..…………………………………47

4.4.3 General Comments and Property Comparison .……………………50

5. Conclusions ……………………………………………………………………………52

6. References ……………………………………………………………………………..54

Appendix A …………………………………………………………………………….A-1

Appendix B. ……………………………………………………………………………B-1

Appendix C.……………………………………………………………………………. C-1

Appendix D ...…………………………………………………………..………………D-1

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LIST OF TABLES

Table 2-1: Effect of changes in process variables on weld attributes [7]..………………10

Table 3-1: Material specification for PMET 818 (AMS 5832)...………………………..15

Table 3.2-1: Various parameters examined for CMT-GMAW representative geometry

weldments …..……………………………………………………………………………19

Table 3.2-2: Parameters used for tensile weldments ...…………………………………..21

Table 3.4.2-1: Grinding and polishing procedure used for micro-sections ……………..27

Table 3.4.4-1: Machining procedure for tensile specimen extraction ...…………………29

Table 4.1-1: Visual observations made during processing of representative geometry

weldments ………………………………………………………………………………..31

Table 4.3-1: Material hardness results for GTAW and CMT-GMAW weldments ……..43

Table 4.4.1-1: Room temperature (65°F) tensile properties …………………………….45

Table 4.4.2-1: Elevated temperature (1100°F) tensile properties ...…………………….48

Table A-1: Parameters used for weld trials………………………………………….....A-1

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LIST OF FIGURES

Figure 1-1: High speed photography image of a CMT cycle with real time stamps [1] ...2

Figure 1.1-1: Fusion welding processes tree diagram [2] ..………………………………3

Figure 1.1-2: Arc welding processes tree diagram [2]………………...………………….4

Figure 1.1-3: Basic arc welding circuit diagram [2] .……………………………………..5

Figure 1.2-1: Diagram showing the effect of aluminum and titanium hardener content on

the tendency to PWHT cracking [3]……………………………………………………….6

Figure 2-1: Fronius CMT-GMAW process control diagram [1] .………………………...7

Figure 2-2: Schematic representation of short circuiting metal transfer [8] ..…………….8

Figure 2-3: CMT-GMAW wire retraction process [1]......………………………………..9

Figure 2-4: Comparative thermal inputs for various metal transfer processes [1]………10

Figure 2.1-1: Constant-potential power source illustration [8] .…………………………12

Figure 2.4-1: Effect of electrode position and welding technique [8] .………………….14

Figure 3-1: Illustration of welding equipment used for robotic trials [4] ………………16

Figure 3.2-1: Illustration of straight single-line beads, straight-line-weave pattern beads,

circular weld build-ups for CMT-GMAW representative geometry weld trials ..……….20

Figure 3.2-2: Illustration of edge weld build-ups for CMT-GMAW representative

geometry weld trials ..……………………………………………………….……………20

Figure 3.2-3: Stock geometry used for tensile weldment fabrication ...…………………21

Figure 3.2-4: Illustration of tensile weldment post-welding ...………………………….21

Figure 3.3-1: Types of gas porosity commonly found in weldments [9] ..……………...22

Figure 3.3-2: Lack of fusion in various weld joints [9]...……………………………….22

Figure 3.3-3: Identification of cracks according to location in weld and base metal [9]..22

Figure 3.4.1-1: Illustration of CMT-GMAW representative geometry weldments with

indicated cut planes ...…………………………………………………………………….25

Figure 3.4.1-2: Illustration of micro section plane definitions [10]……………………..25

Figure 3.4.1-3: Illustration of tensile weldments with indicated tensile extraction and cut

planes ….…………………………………………………………………………………26

Figure 3.4.3-1: Illustration depicting the various locations for hardness testing ………..28

Figure 3.4.4-1: Illustration of tensile specimen used for testing………………………...29

Figure 3.4.4-2: Illustration of tensile specimen extraction ……………………………...29

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Figure 3.4.4-3: Illustration of tensile specimen loading scheme ...……………………...30

Figure 4.1-1: White light macro image (post thermal treatment) of representative CMT-

GMAW weldment specimen #3 ..………………………………………………………...32

Figure 4.1-2: White light macro image (post thermal treatment) of representative CMT-

GMAW weldment specimen #5 .………………………………………………………...32

Figure 4.1-3: X-ray image of representative geometry CMT-GMAW weldment specimen

#3……………………………………………………………………………….………...33

Figure 4.1-4: X-ray image of representative geometry CMT-GMAW weldment specimen

#5……………………………………………………………………………….………...33

Figure 4.2.1-1: Transverse micro-section location 3 from the circular weld build-up,

specimen #3 (post thermal treatment), etchant: electrolytic 10% oxalic acid…………....34

Figure 4.2.1-2: Planar micro-section location 8 from the straight-line-weave pattern

bead, specimen #3 (post thermal treatment), etchant: electrolytic 10% oxalic acid ……..35

Figure 4.2.1-3: Longitudinal micro-section location 12 from single straight-line pattern,

specimen #3 (post thermal treatment), etchant: electrolytic 10% oxalic acid …………...35

Figure 4.2.1-4: Transverse micro-section location 4 from edge weld build-up, specimen

#5 (post thermal treatment), etchant: electrolytic 10% oxalic acid………………………36

Figure 4.2.1-5: Planar micro-section (post thermal treatment) showing primary dendrite

growth, etchant: electrolytic 10% oxalic acid ……………………………………………37

Figure 4.2.1-6: Longitudinal micro-section (post thermal treatment) demonstrating the

randomness of dendrite growth, etchant: electrolytic 10% oxalic acid ………………….38

Figure 4.2.2-1: Macro image analysis of tensile weldments (post thermal treatment)

showing weld profile and distortion of manual GTAW and automated CMT-GMAW …39

Figure 4.2.3-1: Longitudinal micro-sections of both CMT-GMAW and GTAW tensile

weldments (post thermal treatment), etchant: electrolytic 10% oxalic acid ……………..40

Figure 4.2.3-2: Transverse micro-sections of both CMT-GMAW and GTAW tensile

weldments (post thermal treatment) exhibiting deposit height, etchant: electrolytic 10%

oxalic acid ………………………………………………………………………………..41


weldments (post thermal treatment) exhibiting deposit depth, etchant: electrolytic 10%

oxalic acid………………………………………………………………………………...42

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Figure 4.4-1: Macro image of flat specimen used for testing, specimen CMT-1 ……….44

Figure 4.4.1-1: Representative macro photographs, room temperature specimens GTAW-

1 and CMT-2 …………………………………………………………………………….46

Figure 4.4.1-2: SEM photographs of fractured surfaces, room temperature specimens

GTAW-1 and CMT-2 ……………………………………………………………………47

Figure 4.4.2-1: Representative macro photographs, elevated temperature specimens

GTAW-6 and CMT-4…………………………………………………………………….49

Figure 4.4.2-2: SEM photographs of fractured surfaces, elevated temperature specimens

GTAW-6 and CMT-4 ……………………………………………………………………49

Figure A-1: Specimen 25-1 (20°, Pull Angle). ………………………………………...A-2

Figure A-2: Specimen 25-3 (0°, Neutral Angle) ……………………………………...A-2

Figure A-3: Specimen 25-6 (20°, Push Angle)..……………………………………….A-2

Figure B-1: White light macro image (post thermal treatment) of representative

weldment specimen #1 (wire brushed). ………………………………………………...B-2


weldment specimen #2 (wire brushed)....……………………………………………... .B-2


weldment specimen #3 (as welded). ……………………………………………………B-3


weldment specimen #4 (wire brushed). ………………………………………………...B-3


weldment specimen #5 (as welded). ……………………………………………………B-4

Figure C-1: Planar micro-section location 1 from the circular weld build-up, specimen

#3 (post thermal treatment), etchant: electrolytic 10% oxalic acid. ……………………C-2

Figure C-2: Longitudinal micro-section location 2 from the circular weld build-up,

specimen #3 (post thermal treatment), etchant: electrolytic 10% oxalic acid. …………C-2

Figure C-3: Transverse micro-section location 3 from the circular weld build-up,




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Figure C-7: Longitudinal micro-section location 7 from the straight-line-weave pattern

bead, specimen #3 (post thermal treatment), etchant: electrolytic 10% oxalic acid. …...C-5

Figure C-8: Planar micro-section location 8 from the straight-line-weave pattern bead,


Figure C-9: Transverse micro-section location 9 from the straight-line-weave pattern

bead, specimen #3 (post thermal treatment), etchant: electrolytic 10% oxalic acid …....C-6


bead, specimen #3 (post thermal treatment), etchant: electrolytic 10% oxalic acid …....C-6


bead, specimen #3 (post thermal treatment), etchant: electrolytic 10% oxalic acid …...C-7

Figure C-12: Longitudinal micro-section location 12 from single straight-line pattern


Figure C-13: Planar micro-section location 13 from single straight-line pattern bead,

specimen #3 (post thermal treatment), etchant: electrolytic 10% oxalic acid …………C-8

Figure C-14: Transverse micro-section location 14 from single straight-line pattern

bead, specimen #3 (post thermal treatment), etchant: electrolytic 10% oxalic acid ……C-8





Figure C-17: Longitudinal micro-section location 1 from edge weld build-up, specimen

#5 (post thermal treatment), etchant: electrolytic 10% oxalic acid …………………...C-10

Figure C-18: Transverse micro-section location 2 from edge weld build-up, specimen




x


#5 (post thermal treatment), etchant: electrolytic 10% oxalic acid……………………C-11


#5 (post thermal treatment), etchant: electrolytic 10% oxalic acid……………………C-12

Figure D-1: Post-test macro image of room temperature specimen CMT-1 ………….D-2

Figure D-2: Macro image of fractured surface, specimen CMT-1 …………………....D-2


Figure D-4: Macro image of fractured surface, specimen CMT-2 ……………………D-3



Figure D-7: Post-test macro image of elevated temperature specimen CMT-4 ………D-5


Figure D-9: Post-test macro image of elevated temperature specimen CMT-5 ………D-6

Figure D-10: Macro image of fractured surface, specimen CMT-5 …………………..D-6

Figure D-11: Post-test macro image of elevated temperature specimen CMT-6 ……..D-7

Figure D-12: Macro image of fractured surface, specimen CMT-6 …………………..D-7

Figure D-13: Post-test macro image of room temperature specimen GTAW-1 ……....D-8

Figure D-14: Macro image of fractured surface, specimen GTAW-1 ………………...D-8

Figure D-15: Post-test macro image of room temperature specimen GTAW-2 ……...D-9

Figure D-16: Macro image of fractured surface, specimen GTAW-2.……………….. D-9

Figure D-17: Post-test macro image of room temperature specimen GTAW-3 ……..D-10

Figure D-18: Macro image of fractured surface, specimen GTAW-3 ………………D-10

Figure D-19: Post-test macro image of elevated temperature specimen GTAW-4 ….D-11

Figure D-20: Macro image of fractured surface, specimen GTAW-4 ……………….D-11





Figure E-1: Stress-Strain curve of room temperature CMT specimens ……………….E-2

Figure E-2: Stress-Strain curve of elevated temperature CMT specimens ……….…...E-3

Figure E-3: Stress-Strain curve of room temperature GTAW specimens ……………..E-4

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Figure E-4: Stress-Strain curve of elevated temperature GTAW specimens ………….E-5

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LIST OF EQUATIONS

Equation 4.2.1-1 ………………………………………………………………………….38

Equation 4.4-1 ……………………………………………………………………………44

Equation 4.4-2 ……………………………………………………………………………44

Equation 4.4-3 ……………………………………………………………………………42

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NOMENCLATURE

A Arc Current, amps

Af Final Area

Ao Original Area

AC Alternating Current

AMS Aerospace Material Specifications

CG Columnar Grain

CMT Cold Metal Transfer

CMT-GMAW Cold Metal Transfer-Gas Metal Arc Welding

DC Direct Current

E Elastic Modulus

EGW/ESW Electrogas Welding/Electroslag Welding

e Elongation

FCAW Flux Core Arc Welding

FPI Florescent Penetrant Inspection

G Temperature Gradient (°C/cm)

gf gram-force

GMAW Gas Metal Arc Welding

GTAW Gas Tungsten Arc Welding

HAZ Heat Affected Zone

HRC Rockwell C Hardness

HV Vickers Hardness

HT Heat Treatment

ipm Inches Per Minute

L Arc Length, inches

Lf Final Length

Lo Original Length

LOF Lack of Fusion

LOP Lack of Penetration

LLC Limited Liability Company

LST Local Solidification Time

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N Newton

NDE Non-Destructive Evaluation

ODS Oxide Dispersion Strengthening

Pmax Maximum Force

PAW Plamsa Arc Welding

PWHT Post-weld Heat Treatment

R Solidification Rate (cm/min)

RA Reduction in Area (%)

RPM Revolutions Per Minute

Su Ultimate Strength (Tensile)

SAW Submerged Arc Welding

SEM Scanning Electron Microscope

SMAW Shielded Metal Arc Welding

TL Liquidus Temperature (°C)

Ts Solidus Temperature (°C)

V Arc Voltage, volts

γ’ Gamma Prime Precipitate

γ” Gamma Double Prime Precipitate

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ACKNOWLEDGMENT

I would like to express my heartfelt appreciation to my Pratt and Whitney advisor,

Dr. Samuel Christy, for providing continuous technical guidance and support throughout

this research thesis. I would like to extend my appreciation to my Rensselaer Polytechnic

Institute advisor, Dr. Ernesto Gutierrez-Miravete, for his additional support and

acknowledgement of this research.

I would also like to thank the following people who provided support and

encouragement throughout this thesis, all of whom are my colleagues at Pratt and

Whitney: Mr. Bruce Saxton, Mr. Russell Melnick and Mr. William Rose for providing

the ideas and funding for this research; Mr. David Rutz for providing endless amounts of

technical guidance; technician, Mrs. Daria Palladino, for providing the various weldment

mounts; Mr. David Gaudreau for providing micro-imaging analysis, and finally Mr. John

Finn for providing his GTAW artisanship.

I would also like to extend my appreciation to Mr. Shaun Relyea of Fronius USA

LLC for providing guidance on weld parameter development and optimization.

Finally, a special thanks to my friends and family for without their support and

direction, I would not have been able to complete this research endeavor. Lastly, I would

like to extend a special thanks to Erin Altman for editing this thesis.

xvi

ABSTRACT

The subject of this research is the metallurgical and structural characterization of

welding wrought nickel base superalloy Inconel 718™ (AMS 5596) using a new Gas

Metal Arc Welding (GMAW) process dubbed “Cold Metal Transfer” (CMT). It was

hypothesized that CMT-GMAW would provide an equivalent, if not increased, weld joint

yielding improved metallurgical and structural capabilities when compared to the Gas

Tungsten Arc welding (GTAW). The CMT-GMAW weldment in question was placed

though a series of tests, varying weld parameters with the objective of obtaining a

standardization of the process’s critical inputs, Non-Destructive Evaluation (X-ray and

Florescent Penetrant Inspection [FPI]); micro hardness; metallographic review

(macroscopic and microscopic, including scanning electron microscopy); tensile (room

and elevated temperature). Prior to welding, the wrought AMS 5596 substrate material

was solutioned. After welding, the material was placed through a conventional post weld

heat treatment cycle including a solution and precipitation heat treatment. Under the

various test conditions previous mentioned, the CMT-GMAW process was directly

compared to the current industry standards for GTAW.

According to the parameter optimization study it was noted that the key attribute

of the CMT-GMAW process is its electronically controlled short circuit droplet

detachment method, which is dictated by the weld synergic line. The synergic line is a

linear mathematical relationship, proprietary to Fronius International LLC, which

incorporates voltage and amperage process controls into the wire feed speed.

All non-destructive and destructive analyses of the CMT-GMAW weldments showed

little evidence of the porosity that is commonly inherent when using a conventional

GMAW process. Macro and micro-analysis of the CMT-GMAW weldments exhibited a

columnar grain microstructure similar to those obtained with conventional GTAW with

the exception of a reduced substrate consumption, Heat Affected Zone (HAZ), and less

distortion weld.

Mechanical hardness properties of the CMT-GMAW were evaluated at various

metallographic locations, including the substrate, HAZ, and weld filler locations. All

measurements confirmed equivalency to those obtained from GTAW and were consistent

with the current AMS standards.

xvii

Tensile properties of the CMT-GMAW weldments were at both room temperature

(65°F) and a serviceable elevated temperature of 1100°F. The results obtained indicated

equivalency to GTAW, with the exception that the CMT-GMAW specimens exhibited

approximately 2x increase in the percentage of Reduction in Area (RA) at both room and

elevated temperatures.

On the basis of overall evaluation, it was concluded that CMT-GMAW provides the

following benefits: excellent weld quality on wrought Inconel 718™; comparable

metallographic structure to those commonly seen in fusion welded deposits; increased

deposition rates when compared to GTAW; a reduction in overall thermal input by

achieving almost current-free metal transfer; virtually spatter-free metal transfer by

controlling the short circuiting; comparable material hardness to GTAW weldments; and

analogous tensile strength with increased RA when compared to GTAW.

1. Introduction The purpose this research is to metallurgically and structurally characterize the

welding of nickel base superalloy Inconel 718™ using a Cold Metal Transfer-Gas Metal

Arc Welding (CMT-GMAW). During the research there were several milestones or

objectives that were sought to be met including: welding parameter optimization; solution

and precipitation heat treatment; Non-Destructive Evaluation (X-ray and Florescent

Penetrant Inspection [FPI]); micro hardness; metallographic review (macroscopic and

microscopic); and tensile (room and elevated temperature).

Until now, aircraft engine manufacturers have faced a choice between GTAW or

GMAW as a low cost option for arc welding gas turbine components during

manufacturing and repair. GTAW has successfully captured much of the market, as the

limiting factors of traditional GMAW have always been the joining of unique material

types (including non-ferrous materials) and consistent quality (i.e. the weld spatter

inherent in traditional GMAW). Fronius International LLC, a European market leader in

arc-welding technology, introduced the concept of CMT-GMAW welding to the public in

2005. By incorporating the traditional concepts of the GMAW method – that is, applying

a wire consumable – Fronius was able to produce a process of arc welding with high

levels of accuracy on various materials which were typically reserved for GTAW

welding. The CMT-GMAW is a fully digital, micro-processor-controlled inverter

welding process that results in the introduction of a reduced amount of residual heat to

the workpiece and produces a virtually spatter free weld. The improved weld quality is

obtained via a digital process-control that detects a short circuit, and then retracts the wire

being feed so as to help detach and deposit a single molten droplet at a time, as shown in

figure 1-1. Figure 1-1 shows one cycle of the CMT-GMAW wire retraction process

using high-speed photography. It is important to note the short duration of arcing period,

approximately 1/3 of the total cycle time (14.31ms). [1]

1

Figure 1-1: High-speed photography image a CMT cycle with real time stamps [1]

While it is well known that GTAW is the industry standard, it also requires a

highly skilled operator when performed manually. The process typically necessitates an

operator to use both hands to execute the weld: one for holding the torch and the other for

feeding in filler to ensure maximum precision. GTAW also uses a tungsten electrode

from which an arc is generated to provide the heat for the addition of the filler material,

creating a large temperature differential between the filler and the workpiece.

Additionally, there is a potential for transfer of molten tungsten from the electrode to the

weld causing contamination, resulting in a hard and brittle inclusion. However, the CMT-

GMAW uses a consumable electrode that acts not only as the filler, but also the heat

conductive electrode. This coupled to the computer controlled wire retraction mechanism

results in a process that greatly reduces the amount of heat applied to the workpiece

during joining. Additionally, the thermal input levels of CMT-GMAW are much lower

and more controllable than conventional GMAW or GTAW welding, since each of these

processes cannot go below a certain heat level to create an arc.

Speed and spatter control are the two key benefits of Fronius’ CMT-GMAW.

CMT-GMAW welding offers a speed that is four to five times faster than conventional

GTAW, with levels of consistent quality matching those of robotic automation. [1]

2

1.1 Fusion Welding Overview Fusion welding is any process that uses thermal energy to produce melting of the

metals to be joined to create a weld pool that ultimately creates a solid joint when

solidified. The melting process allows the materials to flow and mix together to form a

sound metallurgical bond upon solidification. Typically, a filler material, generally of

suitable material composition, is added in a molten form to enhance the strength of the

bond. Most fusion welding processes generally require a heat source of a sufficient

temperature and intensity to produce a localized melting of both the substrate material

and the subsequent filler material. Fusion welding of materials can be accomplished by

both chemical and electrical means, figure 1.1-1. Figure 1.1-1 is a process tree diagram

that represents the various fusion welding processes, including the break down into sub

categorical processes (e.g. flame, arc, radiation, etc.). However, the specific focus of this

research is based on arc welding processes. [2]

Figure 1.1-1: Fusion welding processes tree diagram [2]

A formal definition of arc welding is a group of joining processes that produces a

coalescence of workpieces by heating them with an electrical arc. Figure 1.1-2 is a

process tree diagram that represents the various arc welding processes, including the

break down into sub categorical processes. However, the specific focus of this research is

based on Gas Metal Arc Welding (GMAW) and Gas Tungsten Arc Welding (GTAW). A

3

fusion arc welding process basically consists of a power source; workpiece lead,

electrode lead, and a workpiece (see figure 1.1-3). The power source provides the

welding voltage and current. Welding current may be either AC (Alternating Current) or

DC (Direct Current). DC is by far the most common power source; although, many

power sources contain circuitry for converting AC electricity to DC. In order for an arc to

take place, there must be some means for attaching the welding electrode lead to the

power source. This may be initiated through fixed connection or sliding contact,

depending on the specific arc process. The workpiece lead must be connected to the

workpiece, which is usually accomplished by a clamp. Welding electrodes are either

consumable (GMAW) or non-consumable (GTAW). Consumable electrodes melt and

transfer to the weld pool as the source of filler material as well as the electrode. In

contrast, non-consumable electrodes when used properly do not melt and transfer to the

substrate material during the arcing process. Tungsten is generally used for this purpose

since it has a high melting point (6147°F). [2]

Figure 1.1-2: Arc welding processes tree diagram [2]

4

Figure 1.1-3: Basic arc welding circuit diagram [2]

1.2 Arc Welding Precipitation-Hardenable Nickel-Base Superalloys The precipitation-hardened alloys distinguish themselves by exhibiting superior

mechanical properties after undergoing precipitation treatment (aged). These materials

are characterized by their distinctively high strength at room temperature to

approximately 1300°F. Fusion often leads to dissolution of the hardening phases and their

reprecipitation in a less desirable physical form in the matrix. The objective of employing

joining processes on precipitation-hardened superalloys, particularly nickel-base

superalloys, is to find a way to keep the high strength associated with the hardening

phases, or the long-term strength associated with oxide dispersion strengthening, or ODS

from being lost due to the welding process. Depending on the actual composition, these

alloys may have good weldability, and most are formed, machined, and welded in the

solutioned condition. The processed materials are then re-solutioned after welding and

aged to obtain the desired properties. The post-weld processing of these alloys after

fusion welding may be considerably difficult, caused by a tendency to cracking. The

susceptibility to hot cracking is directly related to the aluminum and titanium contents.

Figure 1.2-1 shows a plot of weldability as a function of (Ti + Al) content, a number that

will reflect the expected level of precipitates. Here it can be seen that with increasing Ti

and Al content one can expect the weldability of the material to decrease. [3]

5

Figure 1.2-1: Diagram showing the effect of aluminum and titanium hardener content

on the tendency to PWHT cracking [3]

It is well known that some precipitation-hardened nickel-base and iron-nickel-

base superalloys are considerably less weldable than cobalt-base superalloys. Because of

the presence of the strengthening phases when fusion welded, nickel-base alloys tend to

be susceptible to hot cracking (weld cracking) and Post-Weld Heat Treatment (PWHT)

cracking, sometimes referred to as strain-age cracking. Fortunately, alloys such as

Inconel 718™ and Inconel 706™ are much less susceptible to PWHT cracking since the

age hardening develops via the Ni, Nb, γ”, precipitate, but the γ” precipitate is formed at a

much slower rate then those γ’-hardened superalloys. This allows superalloys such as

Inconel 718™ to be heated into the solution temperature range without suffering

significant aging and the resultant PWHT cracking. [3]

6

2. The Cold Metal Transfer-Gas Metal Arc Welding Process In most aerospace applications, it is highly desirable that the fusion arc welding

process has low thermal input. CMT stands for Cold Metal Transfer. Of course, the term

“cold” has to be understood in terms of a welding process: when set against conventional

GMAW, CMT-GMAW is indeed a cold process with its characteristic feature of

alternating thermal arc pool, i.e. hot when an arc is initiated and cold when the arc is

extinguished and the wire is retracted. This alternating hot and cold treatment has been

made possible by a new technological development from Fronius International LLC that

incorporates the wire motions into the process control via a computer monitoring system

(see figure 2-1). Some of the other features that make this unit unique when compared to

conventional GMAW units include: two separate wire-drives (front and rear) that are

separated by the wire buffer. The front drive, located in the torch, (see figure 2-1A),

moves the wire back and forth in a dabbing motion at a rate of up to 90 times per second.

Simultaneously, the rear drive pushes the wire directly from the filler spool located in the

wire drive. It is important to note that both drives are digitally controlled by the process

control. To ensure a constant wire feed, a wire buffer (see figure 2-1B) is interposed

between the two drives to decouple them from one another. [1, 4, 5 and 6]

Figure 2-1: Fronius CMT-GMAW process control diagram [1]

In the world of GMAW processes, there are essentially three types of metal

transfer mechanisms that can be used: (1) short circuiting transfer, (2) globular transfer,

7

and (3) spray transfer. The type of transfer is determined by a number of factors, the most

influential of which include (1) magnitude and type of welding current, (2) electrode

diameter, (3) electrode composition, (4) electrode extension, and (5) shielding gas. CMT-

GMAW in its most elementary form can be considered a short circuit GMAW process.

Short circuit GMAW consists of the lowest range of welding currents and electrode

diameters associated with GMAW. This type of transfer produces a small, fast-freezing

weld pool. During the arcing process, metal is transferred from the electrode to the

workpiece only during a short period when the electrode is in contact with the weld pool,

hence the term “short circuiting transfer.” It is important to note that no metal is

transferred across the arc gap. [7 and 8]

The sequence of events in the transfer of metal and the corresponding current and

voltage for a typical short circuit transfer GMAW process can be seen in figure 2-2. As

the wire touches the weld metal, the current increases [(A), (B), (C), (D), in figure 2-2].

The molten metal at the wire tip pinches off at D and E, initiating an arc as shown in (E)

and (F). It is here that the rate of current increase must be high enough to heat the

electrode and promote metal transfer, yet low enough to minimize spatter caused by

violent separation of the drop of metal (one of the major disadvantages of conventional

short circuit GMAW). Finally, when the arc is established, the wire melts at the tip as the

wire is fed forward towards the next short circuit at (G). Overall, the open circuit voltage

of the power source must be low enough that the drop of molten metal at the wire tip

cannot transfer until it touches the base metal. [7 and 8]

Figure 2-2: Schematic representation of short circuiting metal transfer [8]

8

Unlike its short circuit GMAW counterpart, CMT-GMAW incorporates a digital

process-control that detects the short circuit at the workpiece, and then mechanically

retracts the wire to help detach the molten droplet. The wire retraction greatly reduces the

spatter that is typically associated with conventional short circuit GMAW. Reduced

spatter is the first essential difference from conventional short circuit GMAW processes;

the second most notable difference is a reduction in thermal input since there is virtually

current-free droplet detachment, hence the term “Cold Metal Transfer.” Lastly, unlike

conventional short circuiting GMAW -where there is a constant push motor driven

system, the CMT-GMAW uses a two motor drive system that pushes the wire forward,

and as soon as the short circuit occurs, it pulls it back (see figure 2-3). [1]

Figure 2-3: CMT-GMAW wire retraction process [1]

The rearward movement of the wire assists droplet detachment during the short

circuit. In this way, the arc itself only inputs heat very briefly during the arcing period.

The thermal input is immediately reduced after arc is extinguished, creating an oscillating

hot/cold weld pool. During the CMT-GMAW process, the average current is kept very

small by controlling the short circuit, resulting in virtually spatter free metal transfer.

Figure 2-4 illustrates the reduced thermal input required for metal transfer as compared to

other conventional metal transfer processes. Also worth mentioning, precision droplet-

detachment of the CMT-GMAW ensures that after every short circuit, a near-identical

quantity of filler metal is melted off. [1, 4, 5 and 6]

9

Figure 2-4: Comparative thermal inputs for various metal transfer processes [1]

The most important variable of any GMAW process, including CMT-GMAW,

which affects the weld penetration, bead geometry, and overall weld quality are: (1)

welding current (wire feed speed), (2) polarity, (3) arc voltage (arc length), (4) travel

(traverse) speed, (5) electrode extension, (6) torch angle, and (7) electrode diameter.

Knowledge and control of these variables are essential in order to consistently produce

welds of acceptable quality. However, changing one variable generally requires altering

additional parameters to retain an acceptable quality weld because the variables are not

completely independent of each other. The effects of these variables on deposit attributes

are shown in table 2-1. [7]

Table 2-1: Effect of changes in process variables on weld attributes [7]

10

2.1 Arc Length Arc voltage and arc length are related terms that are often used interchangeably,

however they are quite different in practice. Arc voltage is the electrical potential

between the electrode and the workpiece. Arc voltage is generally lower than the voltage

measured directly at the power source because the voltage drops at the connections and

along the length of the welding cable. Consequently, an increase in arc voltage will result

in a longer arc length. It is also important to note that excessively high arc voltage can

cause weld porosity, spatter, and undercut; therefore, arc length is a variable of interest

and should be controlled as it can have a profound impact on the overall weld quality. [7

and 8]

In conventional GMAW, the surface of the workpiece (i.e. jagged or flat) and the

welding speed can both have a marked effect on the stability of the arc. The arc length is

acquired and adjusted mechanically with the CMT-GMAW. This means that the arc

remains stable, regardless of the surface condition of one’s workpiece. In addition to a

mechanical response, the CMT-GMAW utilizes a self-correction mechanism via a

constant-potential power source as illustrated in figure 2.1-1. Where L is the arc length,

the length between the melting electrode tip and the base metal. As the contact-to-work

distance increases, the arc voltage and arc length would tend to increase with a

conventional GMAW power source. However, with the CMT-GMAW power source, the

welding current decreases with a slight increase in voltage, while the mechanical drives

in the torch adjust appropriately to maintain a consistent arc length. Conversely, if the

distance is shortened, the lower voltage would be accompanied by an increase in current

and a mechanical adjustment in wire feed speed to compensate for the shorter wire

stickout. [7 and 8]

11

Figure 2.1-1: Constant-potential power source illustration [8]

2.2 Wire Feed Speed In conventional GMAW welding, as the electrode feed speed (wire feed speed) is

varied, the welding current varies in a similar manner with the arc length. This occurs

because the current output of the power source fluctuates dramatically with the slight

changes in the arc voltage when alterations are made to the wire feed speed. If all other

variables were held constant, an increase in welding current would result in the

following: (1) an increase in the depth and width of the penetration, (2) an increase in the

deposition rate, and (3) an increase in the size of the weld bead. [7 and 8]

Unlike conventional GMAW units where current and voltage can be changed

independently, these two key parameters are linked together in the CMT-GMAW via a

digital process control, or an arc synergic line. An arc synergic line is a linear

mathematical relationship (proprietary to Fronius International LLC) which incorporates

the voltage and amperage process controls into the wire feed speed, and is dependent on

the thermal and electrical resistivity properties of the material substrate/filler used.

Therefore, each synergic line is uniquely different: that is, every synergic line consists of

several points on a voltage current diagram (U-I diagram), which are formed from the

connection of a series of certain current and pertinent voltage levels for any given wire

composition and gas. Each point on the synergic line is recorded with the same arc

12

length, despite different performance, through the whole power range. An arc synergic

line is experimentally determined via high-speed video techniques. [6, 7 and 8]

2.3 Traverse Speed As with any GMAW process, including CMT-GMAW, the traverse or travel

speed has a profound impact on the weld quality. Travel speed is the linear rate at which

the arc is moved along the surface of the workpiece. The filler metal deposition rate per

unit length increases when the travel speed is decreased. The welding arc impinges on

the molten weld pool rather than the base metal at very slow speeds, thereby reducing the

effective penetration and resulting in a wider bead. As travel speed is increased, the

thermal energy per unit length of weld transmitted to the base metal from the arc is at

first increased because the arc acts more directly on the base material. However, further

increases in travel speed impart less thermal energy to the base metal. Melting of the base

metal therefore first rises and then decreases with increasing travel speed. As the travel

speed increases, there is a tendency for undercutting along the edges of the weld bead

because of insufficient deposition of filler metal to fill the path melted by the arc. [7 and

8]

2.4 Electrode Orientation Electrode orientation affects bead shape and penetration to a greater extent than

arc voltage and travel speed. The electrode orientation is described in two ways: (1) by

the relationship of the electrode axis with respect to the direction of travel and (2) the

angle between the electrode axis and the adjacent workpiece surface. When the electrode

points in a direction opposite to the travel direction, it results in a trail angle, which is

known as the backhand method. Similarly, when the electrode points in the direction of

travel, it results in a lead angle and is called the forehand method. The electrode

orientation and its effect on the width and penetration of the weld bead are illustrated in

figure 2.4-1. [7 and 8]

13

Figure 2.4-1: Effect of electrode position and welding technique [8]

When the electrode is changed from perpendicular to the lead angle technique

with all other conditions unchanged, the penetration decreases and the weld bead

exhibited is wider and flatter. Maximum penetration is obtained in the flat position with

the drag technique, at a drag angle of about 25 degrees from perpendicular. The drag

technique also produces a more convex, narrower bead, an increasingly stable arc, and

less spatter on the workpiece. [7 and 8]

14

3. Method of Approach The weld filler chosen for the weld trials performed in this research was 0.035-

inch diameter PMET 818 (AMS 5832) weld wire provided by Polymet Corporation (see

table 3-1 for filler material specifications). The arc synergic line used during all trials was

ER NiCrMo-3, and is based on 0.045-inch diameter weld filler AMS 5582, Inconel

718™. The subsequent wire feed and traverse speeds were altered according to weld

geometry and base material thickness, and will be discussed in further detail in the

subsequent chapters.

Table 3-1: Material specification for PMET 818 (AMS 5832)

From previous weld experience using 0.025-inch thick wrought Inconel 718™

plate, it was determined that the ideal torch angle for these trials would be between 5-10

degrees (from perpendicular) and using a backhand (pull) technique (see appendix A for

trial details). A backhand technique is used to produce a weld with maximum penetration

and a narrow, convex surface configuration. This angle also provides maximum shielding

of the molten weld pool. The shielding gas for all weldments was 100% pure argon, with

a flowrate of 30-35 CFH. A Plexiglas gas chamber was also used, as applicable, to ensure

an inert atmosphere during welding. An illustration of the robotic weld set-up and listing

of the equipment used during all automated CMT-GMAW weld trials is shown in figure

3-1 below.

15

Figure 3-1: Illustration of welding equipment used for robotic trials [4]

Several weldments with varying parameters were prepared in order to meet the

parameter optimization, heat treatment, metallographic review and NDE (X-ray and FPI),

objectives of this research. Each weldment represents the various weld profiles and

geometries. These geometries include straight single-line beads, straight-line-weave

pattern beads, circular weld build-ups, and material edge build-ups which are

representative of the most common weld styles in aerospace applications. All specimens

were heat treated accordingly, with a pre- and post-weld solution heat treatment and

finally an age treatment. All weldments were then macro-evaluated using common NDE

techniques - X-ray and FPI - to ensure weld quality (e.g. porosity, lack of fusion, and

cracking).

16

Once NDE was complete, all specimens were micro-sectioned with a minimum of

21 cuts were made to examine the weld joint in the axial, transverse, and longitudinal

planes of view. All weld microstructure and subsequent Heat Affected Zone (HAZ) were

evaluated at up to 200x magnification and were compared to conventional GTAW. The

micro-sections were evaluated for agreement of weld quality and microstructural

equivalency, including phases, grain structure, grain size, and solidification patterns.

Several micro hardness readings were taken in varying locations starting from the

fusion zone and emanating toward the HAZ and finally into the parent material and weld

material. These measurements were taken throughout each weldment and were compared

to the AMS standards (Aerospace Material Specifications) to determine the mechanical

properties and meet the objectives of the research.

Additional weld plates were fabricated so as to obtain the tensile properties of the

CMT-GMAW weldments. Tensile specimens were extracted through the welded region

of the several weld plates. These tensile specimens were pulled perpendicular to the

direction of the weld travel and tested at both room temperature (65°F) and at the

elevated engine service temperature (1100°F). These results were compared to those

obtained using the same testing scheme for the GTAW weldments.

3.1 Pre- and Post-Weld Thermal Treatments Thermal (heat) treatment is any application of a temperature, for any amount of

time, sufficiently high enough as to accomplish one of the following: (1) reduce stresses,

(2) allow atom movements to redistribute existing alloy elements, (3) promote grain

growth, (4) promote new recrystallization, (5) dissolve phases, (6) produce new phases

owing to precipitation from solid-solutioning, or (7) support alloy surface chemistry. [3]

Prior to welding, the following thermal treatment was performed in order to place

the base material in a fully solutioned (weldable) condition and to obtain complete

recrystallization and maximum softness. The pre-weld solution was performed in an

vacuum atmosphere where (1) the base material specimens were heated to a temperature

of 1750°F ± 25°F and held for a period of 60 minutes, then (2) cool to a temperature of

1100°F at a rate of 35°F/minute proceeded by (3) continued cooling to a temperature of

1000°F at a rate of 15°F /minute; all remaining subsequent cooling rates were

17

50°F/minute. All cooling procedures were produced via back-filling with 100% argon

gas.

After welding, the following solution thermal treatment was performed to resolve

any residual stresses and dissolve any secondary phases and prepare the alloy for

subsequent aging. The aging is performed to obtain maximum ductility, this process also

homogenizes the microstructure prior to aging. Like the initial solutioning treatment, the

weldment specimens were once again (1) heated to a temperature of 1750°F ± 25°F and

held for a period of 60 minutes, then (2) cooled to a temperature of 1100°F at a rate of

35°F/minute and proceeded by (3) continued cooling to a temperature of 1000°F at a rate

of 15°F /minute; all remaining subsequent cooling rates were 50°F/minute and cooling

procedures were produced via back-filling with 100% argon gas.

Immediately following the solution treatment, the weldment specimens were

placed through the following precipitation (aging) thermal treatment to bring out the

desirable strengthening precipitates and control other secondary phases, including

carbides and detrimental topographically close packed phases. Some of the

topographically close pack phases include σ, μ, and Laves which are variably detrimental

when more than trace amounts are present. The weldment specimens were (1) heated to a

temperature of 1325°F ±25°F and held for a period of eight hours, then (2) cooled at a

rate of 100°F ±25°F/hour to a temperature of 1150°F ±25°F and again held for a period

of 18 hours; all remaining subsequent cooling rates were 50°F/minute and all cooling

procedures were produced via back-filling with 100% argon gas.

3.2 Weldment Specimen Fabrication Nickel alloys are commonly susceptible to embrittlement by lead, sulfur,

phosphorus, and other low-melting-point elements. These materials have been known to

exist in grease, oil, paint, marking crayons, or ink and from lubricants, cutting fluids,

shop dirt, and processing chemicals. Another important variable is the possibility of

surface oxides which may have formed from previous processing, e.g. thermal

treatments. Therefore, it is good practice to ensure that all specimens are completely free

of foreign materials before they are welded. [7]

18

To ensure quality weldments, comprehensive cleaning procedures were

performed prior to welding. The localized areas to be welded were wire brushed using

handheld pneumatic stainless steel bristle wheels to remove any possible oxides and then

followed by wiping the area with a clean acetone swab.

Five weldments with varying wirefeed parameters (see table 3.2-1), were made

utilizing standard 4-inch x 4-inch x 1/8-inch thick AMS 5596 shear cut sheet stock. Each

parameter was chosen from previous trials on similar geometries. Each weldment

exhibited several different weld profiles/geometries, including straight single-line beads,

straight-line-weave pattern beads, circular weld build-ups (figure 3.2-1), and edge build-

ups (figure 3.2-2). These diverse geometries are representative of the most common weld

styles in aerospace applications. A clear Plexiglas chamber was used during processing

to note “real-time” observations for parameter optimization of the weld deposition during

processing. The visual observations made during processing were in regards to overall

arc stability, weld wetting, and solidification.

Table 3.2-1: Various parameters examined for representative CMT-GMAW

geometry weldments

19

Figure 3.2-1: Illustration of straight single-line beads, straight-line-weave pattern

beads, circular weld build-ups for CMT-GMAW representative geometry weld trials

Figure 3.2-2: Illustration of edge weld build-ups for CMT-GMAW representative

geometry weld trials

Several manual GTAW (using a Lincoln Electric Square Wave TIG -355 welder)

and CMT-GMAW weldments, with constant parameters (see table 3.2-2), were made

utilizing shear cut AMS 5596 sheet stock with two 3/8 flat tip end mill machine trenches

spanning the entire length of the specimen (see figure 3.2-3). After machining the base

stock, a weld was placed along the trench length of the specimen, as shown in figure 3.2-

4.

20

Table 3.2-2: Parameters used for tensile weldments

Figure 3.2-3: Stock geometry used for tensile weldment fabrication

Figure 3.2-4: Illustration of tensile weldment post-welding

3.3 Post-Weld Non-Destructive Evaluation There are essentially four categories of flaws which can occur in GMAW and

GTAW weldments: (1) gas porosity, figure 3.3-1; (2) Lack of Fusion (LOF); (3) Lack of

Penetration (LOP), figure 3.3-2; and (4) cracks, figure 3.3-3. The form, location, and

21

orientation of each type are governed by several factors, including joint configuration,

welding process, base metal, filler metal, and other variables associated with the joint. [9]

Figure 3.3-1: Types of gas porosity commonly found in weldments [9]

Figure 3.3-2: Lack of fusion in various weld joints [9]

Figure 3.3-3: Identification of cracks according to location in weld and base metal

[9]

22

Although visual (white light) inspection is an invaluable and easy-to-perform

assessment method, it should be mentioned that is also unreliable for detecting subsurface

flaws and finite surface flaws. Consequently, it should not be the only inspection method

performed when ensuring weld integrity. [9]

All visual inspection were performed prior to Florescent Penetrant Inspection

(FPI) and X-ray using a standard 14X eye loop. The primary purpose of this examination

step was to verify the following: (1) conformity of welds with regards to surface

roughness, weld spatter, and cleanliness; (2) presence of obvious surface flaws such as

lack of fusion, undercuts, overlaps, and cracks.

Florescent Penetrant Inspection is a non-destructive method of revealing

discontinuities that are open to the surfaces of solid and essentially nonporous materials.

Indications of a wide spectrum of flaw sizes can be found regardless of the configuration

of the workpiece and flaw orientations. The florescent liquid penetrates deep into various

types of minute surface openings by capillary action. [9]

The florescent penetrant used for this research was ultra-high-sensitivity; simply

meaning, it can penetrate into the smallest of indications. The primary purpose of this

inspection step was to check for the presence of non-obvious surface flaws such as lack

of fusion, undercuts, overlaps, and cracks which may not have been visible using white

light inspection.

The major limitation of visual inspection and FPI is that both can detect only

imperfections that are open to the surface. As a result, an X-ray inspection was used for

detecting subsurface flaws. X-rays can detect flaws that are completely internal and

located well below the surface of the part. Although X-ray will reveal the interior of

opaque objects, it cannot detect all types of discontinuities, such as small defects in thick

objects and very fine cracks. Weldments were also metallographically sectioned and

examined to ensure a complete analysis. [9]

The primary purpose of this inspection step was to check for the presence of sub-

surface flaws such as (1) gas porosity; (2) Lack of Fusion (LOF); (3) Lack of Penetration

(LOP); and (4) cracks that could not be detected via the previous inspection techniques.

Secondly, any areas with indications would be marked as a location for micro sectioning.

23

3.4 Material Property Characterization During this research it was imperative to classify the material properties of the

weldments since welds (essentially a cast structure) generally behave differently from

wrought structures (the base material). Wrought structures are generally more

homogeneous and finer-grain than cast structures. The greater homogeneity enables more

of the hardener elements to be taken into solution and to thus effectively converted to γ’

and γ” phases. Moreover, the distribution of the γ’ and/or γ” in wrought alloys is

generally more uniform and finer in size. Additionally, wrought structures tend to have

finer grain sizes than cast structures, and subsequently have enhanced tensile strengths. It

is important to understand the metallurgy behind the solidified weld structure so as to

provide the associated strength debit involved when placing a welded (cast structure) on a

wrought base material. [3] Microstructural phase morphology can vary widely (e.g. script versus blocky

carbides, cuboidal versus spheroidal γ’, cellular versus uniform precipitation, and discrete

γ’ versus γ’ envelopes). [3]

The solidification patterns of welded Inconel 718™ is governed by the laws of the

phase diagram (as is the solidification of all metals). The kinetics of the solidification

process determines the microstructure that is actually formed. However, what makes the

solidification of superalloys like Inconel 718™ exceptionally different from less

sophisticated alloys is that the solute content of these alloys is very high. [3]

3.4.1 Micro-Analysis

The purpose of the micro-analysis metallurgical review of this research was to

classify the general microstructure of the solidified weld pool formed during the CMT-

GMAW process. This analysis was accomplished by examining the weld geometry in

various planes of view (i.e. longitudinal, planar, and transverse) at magnification levels

up to 200x using standard microscopy techniques. Additional micro-analysis was

performed using electron microscopy to analyze the fractography of tensile specimens.

The microstructure was compared to conventional GTAW for weld quality and

microstructural equivalency, including phases, grain structure, solidification patterns, and

24

process induced defects including, porosity, Lack of Fusion (LOF), and Lack of

Penetration (LOP) and cracks.

After welding, all of the representative geometry weldments were sectioned

according to figure 3.4.1-1 using handheld pneumatic silicon carbide cut off wheels. A

total of 21 micro-sections were made, allowing for such visual orientations as,

longitudinal, planar and transverse planes. Refer to figure 3.4.1-2 for view plane

orientation definitions.

Figure 3.4.1-1: Illustration of CMT-GMAW representative geometry weldments with

indicated cut planes

Figure 3.4.1-2: Illustration of micro section plane definitions [10]

After welding, one representative tensile weldment from each weld process

(yielding two in total,) - manual GTAW and automated CMT-GMAW - was sectioned

according to figure 3.4.1-3 using handheld pneumatic silicon-carbide cut-off wheels.

25

These sections were taken to confirm hardness and microstructural equivalence of the

CMT-GMAW with GTAW prior to tensile testing.

Figure 3.4.1-3: Illustration of tensile weldments with indicated tensile extraction and

cut planes

3.4.2 Specimen Mount Preparation

Specimen preparation is crucial for the overall microscopic analysis of a

material’s microstructure. In order to be able to adequately analyze a material, the micro-

specimen’s surface must be properly prepared to maintain good edge retention and be

free of scratches, smears, and contaminants. Therefore only one skilled in the field should

prepare micro-sections.

All cut sections were mounted in a two-piece castable clear epoxy resin. Castable

epoxies consist of two or more liquid resins that are mixed in certain proportions. In a

two-piece epoxy, one liquid is the resin and the other liquid is the hardener (also called

activator or catalyst). After curing, the following procedures (table 3.4.2-1) were used to

prepare the cut surface for metallographic examination. Each mount was ultrasonically

cleaned prior to each subsequent step to ensure no grit cross-contamination. After all

polishing procedures were performed; the micro-section mounts were submersed in a

10% oxalic acid and water bath and electrolytically etched using a 12v power source.

26

Table 3.4.2-1: Grinding and polishing procedure used for micro-sections

3.4.3 Mechanical (Hardness) Analysis

The term hardness as commonly used, may be defined as the ability of a material

to resist permanent indentation or deformation when in contact with an indenter under

load. Hardness testing is perhaps the simplest and the least expensive method of

mechanically characterizing a material since it does not require an elaborate specimen

preparation, involves rather inexpensive testing equipment, and is relatively quick. The

theoretical and empirical investigations have resulted in fairly accurate quantitative

relationships between hardness and other mechanical properties of materials, such as

ultimate tensile strength, yield strength, and strain hardening coefficient and fatigue

strength and creep. [11]

All micro-hardness readings were taken via the Vickers micro hardness method.

The Vickers test method uses a diamond shaped indenter pressed into a material using a

force of 500 gf. The resulting indentation diagonals are measured, and the hardness

number is calculated, by dividing the force by the surface area of the indentation. [11]

In order to fully characterize the weld processes, hardness readings were taken

through three unique areas of interest including, the base material (this was used as a

base-line), the HAZ (which is defined as the area between the fusion line and the

unaffected parent material), and the weld material (see figure 3.4.3-1).

27

Figure 3.4.3-1: Illustration depicting the various locations for hardness testing

3.4.4 Mechanical (Tensile) Analysis

Short-time properties are always under consideration for property-microstructure

relationships. Short-time properties include tensile (or compressive) strengths, (yield and

ultimate) produced by continuous fairly rapid application of a steady load to reach plastic

deformation (proportional limit, 0.02% yield, 0.2% yield) or fracture. Elongations during

and at the conclusion of a test are also measured, reductions in area at the conclusion of a

test are also recorded, and finally elastic modulus is generally measured as well. It

should be noted that tensile properties are usually at issue up to the region of about 1400

°F. [3]

Once welding was complete, standard (flat) tensile specimens (figure 3.4.4-1)

were extracted from the weldments according to figure 3.4.4-2. All specimens were

machined according to the procedure set-forth in table 3.4.4-1. It should be noted on

figure 3.4.4-2 that the entire gage width and length is made of weld deposit. This was

done in an effort to ensure fracture would be induced solely along the welded material.

Extracting specimens in this manner also allotted a direct strength comparison of the

GTAW deposit with CMT-GMAW.

28

Table 3.4.4-1: Machining procedure for tensile specimen extraction

Figure 3.4.4-1: Illustration of tensile specimen used for testing

Figure 3.4.4-2: Illustration of tensile specimen extraction

29

Tensile properties of the CMT-GMAW weldments were obtained using flat

tensile specimens that were tested at both room temperature (65°F) and at Inconel

718™’s serviceable elevated temperature (1100°F). The tensile specimens were pulled

perpendicular to the plane of the weld solidification pattern (figure 3.4.4-3). The

properties that were obtained via tensile testing include tensile strength, yield strength,

elongation, reduction in area, strain and break, and modulus. After testing was complete,

the fractured surfaces of both the GTAW and CMT-GMAW specimens were evaluated

using Scanning Electron Microscopy (SEM) to obtain a general topographic image

analysis of the fracture surfaces.

Figure 3.4.4-3: Illustration of tensile specimen loading scheme

30

4. Results and Discussion 4.1 Non-Destructive Analysis of CMT-GMAW Weldments

White light visual and subsequent ultraviolet light high-sensitivity Florescent

Penetrant Inspection of all representative geometry weldments and tensile weldments

using a standard 14X eye loop yielded no evidence of weld-induced surface defects. Of

the seven representative geometry weldments evaluated, specimen numbers three (figure

4.1-1) and five (figure 4.1-2) exhibited the most consistent results in to overall weldment

appearance. This was similar to the observations made during the welding process, table

4.1-1. The overall visual appearance of weldment with regard to surface roughness and

bead shape appeared to be very uniform. This observation is consistent with automated

weld processes. See appendix B for macro images of all representative geometry

weldment specimens.

Table 4.1-1: Visual observations made during processing of representative geometry

CMT-GMAW weldments

31

Figure 4.1-1: White light macro image (post thermal treatment) of representative

CMT-GMAW weldment specimen #3

Figure 4.1-2: White light macro image (post thermal treatment) of representative

CMT-GMAW weldment specimen #5

Upon further evaluation of all weldments, it was noted that there was no evidence

of process-induced weld spatter. This confirms the ability of the CMT-GMAW droplet

32

detachment method to produce wide bead weldments with virtually no weld spatter.

Lastly, visual observation and FPI confirmed no presence of obvious surface flaws such

as lack of fusion, undercuts, overlaps, and cracks.

X-ray inspection of the representative geometry weldment specimens and tensile

weldments using million-volt x-ray produced no evidence of subsurface weld-induced

defects such as such as porosity, Lack of Fusion (LOF), Lack of Penetration (LOP), and

cracks. This confirms the ability of CMT-GMAW process to produce a variety of

weldment geometries with no weld-induced porosity. Figures 4.1-3 and 4.1-4 are images

of the x-ray films taken of representative weldment specimens three and five verifying no

subsurface indications.

Figure 4.1-3: X-ray image of representative geometry CMT-GMAW weldment

specimen #3

Figure 4.1-4: X-ray image of representative geometry CMT-GMAW weldment

specimen #5

33

4.2 Destructive Analysis

4.2.1 Representative Weldment Microstructure

Destructive analysis is performed to view a material at a microscopic level.

Destructive analysis is often only employed for critical tasks, as it is impractical to

destructively analyze all components; hence the use of the previous mentioned NDE

methods. To fully characterize the microstructure obtained using the CMT-GMAW weld

process, of 21 micro-sections were taken from the representative geometry of specimens

three and five. Sections were extracted through the following unique areas of interest, arc

start (up slope), arc stop (down slope), and multi-pass build-up interfaces in three

different orientations (longitudinal, planar, and transverse), reference figure 3.4.1-2.

Upon initial analysis, it was found that the microstructure of each weldment was uniform

within each group (weldment geometry) and cut orientation. Therefore, for the purposes

of this discussion, only one representative micro-section from each orientation

(independent of bead type) is shown in the following images below, figure 4.2.1-

(circular build-up), figure 4.2.1-2 (straight-line-weave pattern bead), figure 4.2.1-3

(single straight-line pattern), and figure 4.2.1-4 (edge weld build-up). Note: additional

substantiating microphotographs can be found in appendix C.

Figure 4.2.1-1: Transverse micro-section location 3 from the circular weld build-up,

specimen #3 (post thermal treatment), etchant: electrolytic 10% oxalic acid

34

Figure 4.2.1-2: Planar micro-section location 8 from the straight-line-weave pattern

bead, specimen #3 (post thermal treatment), etchant: electrolytic 10% oxalic acid

Figure 4.2.1-3: Longitudinal micro-section location 12 from single straight-line

pattern, specimen #3 (post thermal treatment), etchant: electrolytic 10% oxalic acid

35

Figure 4.2.1-4: Transverse micro-section location 4 from edge weld build-up,


In common with other weld processes, the major intermetallic phase found inside

the CMT-GMAW weld and at the boundaries in the HAZ is the hexagonal Laves phase.

Laves morphology in HAZ ranges from isolated particles to continuous sheets and

massive dendritic walls as shown in the previous high magnification images above.

Inside the weld liquidation zone, the recrystallized, dendritic network within weld grains

is formed from discontinuous, cross-linked sheets of lamellar MC carbides, mixed with

Laves precipitates. The precipitates in the weld or along the HAZ boundaries are Nb-

rich. Clusters of small carbides generally contain levels of Ti, Cr, and Ni (but not Fe)

enhanced by a few percent, possibly reflecting local Nb depletion. Apart from occasional

grown-in dislocations, the carbide crystals are generally well ordered. [12]

In general, the structure of the CMT-GMAW weldments is parallel with the

equiax casting. The fundamentals of the weld solidification are the same as those of a

casting, but with different boundary conditions. As the weld filler solidifies, the primary

dendrites reject solute into the interdendritic liquid. The low niobium primary dendrites

grow into the solidifying metal in a direction perpendicular to the solidification front.

This is best illustrated in figure 4.2.1-5 where the niobium dendrites are growing out of

the page, which is perpendicular to the solidification front. [3]

36

Figure 4.2.1-5: Planar micro-section (post thermal treatment) showing primary

dendrite growth, etchant: electrolytic 10% oxalic acid

Weldment heat extraction is not uniform, unlike conventional casting practices

where the heat extraction is generally uniform and precisely controlled to enhance

properties. Since the heat is extracted from the welded component in a uncontrolled

manner, the subsequent primary dendrite growth is random and columnar in nature, as

shown in figure 4.2.1-6 Although random in temperament, the dendrite growth is still

perpendicular to the direction of heat extraction. The dendrites form a tightly packed

columnar-grain (CG) structure.

37

Figure 4.2.1-6: Longitudinal micro-section (post thermal treatment) demonstrating the

randomness of dendrite growth, etchant: electrolytic 10% oxalic acid

During solidification, solid dendrites begin to grow in what is referred to as a

“mushy zone”. This region of solid dendrites and liquid interdendritic regions has a

temperature gradient defined by the width of the mushy zone and the liquidus and solidus

temperatures. The size of the dendrites in the mushy zone is directly related to the local

solidification time (LST). LST is defined by equation 4.2.1-1, where TL is the liquidus

temperature (°F), Ts is the solidus temperature (°F), G is the temperature gradient (°F/in),

and R is the solidification rate (in/min). Thus, with increasing heat extraction or

decreasing heat input, (rate of molten metal introduction) G and R are increased. This

intern decreases the LST, and, as is intuitively logical, increasing heat extraction or

decreasing heat input decreases the dendrite size as will be seen in section 4.2.3. [3]

(Equation 4.2.1-1)

4.2.2 Macrostructural Comparison to GTAW

At a macroscopic level, one can clearly observe the differences between the

CMT-GMAW and GTAW processes. This can be seen in the bead shape and

distribution. The GTAW process produces a bead shape typical of a multi-pass manual

38

operation and appears to be less uniform than the single-pass automated CMT-GMAW

(figure 4.2.2-1). From this figure one can also observe the massive amount of distortion

in GTAW weldment base metal (see figure 4.2.2-1). Distortion is caused by residual

(thermal) stresses induced during solidification and is generally prominent in a high heat

input arc process. This cross-section clearly indicates that the GTAW process is much

more heat invasive than the CMT-GMAW process.

Figure 4.2.2-1: Macro image analysis of tensile weldments (post thermal treatment)

showing weld profile and distortion of manual GTAW and automated CMT-GMAW

4.2.3 Microstructural Comparison to GTAW

At the microscopic level, the microstructure of the GTAW and CMT-GMAW

appear to be equivalent in appearance. Both the GTAW and CMT-GMAW weld

microstructures have a column grain dendrite structure that emanates in a direction

perpendicular to the heat extraction, as previously described in section 4.2.1, figure 4.2.3-

1. It can also be noted from this figure that the CMT-GMAW weld microstructure

appears to be much more uniform with respect to randomness of the dendrite formation

than the GTAW microstructural. This can be attributed to the automated processing and

one pass as opposed to multiple passes, such as those performed with the manual GTAW

(3 passes). A multi-pass weld has the tendency to create several locations of merging

39

dendrites and erratic growth patterns in which new grains nucleate and grow at the

expense of original grains (the initial pass).

Figure 4.2.3-1: Longitudinal micro-sections of both CMT-GMAW and GTAW

tensile weldments (post thermal treatment), etchant: electrolytic 10% oxalic acid

When an evaluation was performed on the transverse micro-sections, one can

clearly distinguish the substantial amount of weld deposit delivered by 1-pass of the

CMT-GMAW when compared to 3-passes of GTAW. In fact, with 1-pass of the CMT-

GMAW, there appears to be almost 4x as much weld deposit when compared to the 3-

passes of the GTAW (figure 4.2.3-2). This is enevitably the case because, as previous

mentioned, the electrode is the filler in CMT-GMAW. Therefore, every time an arc is

struck the filler is melted creating a larger amount of weld deposit. This is quite

extraordinary given that the heat input transmitted from the CMT-GMAW process to the

substrate appears to be substantially less than the GTAW process (see section 4.2.2).

40


weldments (post thermal treatment) exhibiting deposit height, etchant: electrolytic

10% oxalic acid

When further evaluating the transverse micro-sections, it was also determined that

the CMT-GMAW process consumes much less of the base material than the GTAW weld

deposit. This is illustrated in figure 4.2.3-3, which is a transverse image of the weld joint

taken at the base of the tensile weldment trench (refer to figures 3.2.2-1 and 3.2.2-2 for

image orientation). The measurements shown were taken from the base of the stock

material (which was used as the base line) and followed to the bottom of the lowest point

of the HAZ from each weldment. From this image, it is clear that the GTAW weldment

consumed much more of the base material during processing (approximately two times).

This can be attributed to the fundamentals of GTAW processing which requires the

heating of the substrate via the tungsten electrode prior to the addition of the weld filler,

whereas the CMT-GMAW uses a consumable electrode. The consumable electrode in

theory requires less heat input to produce the weld. This finding additionally confirms the

observations made in section 4.2.2.

41


weldments (post thermal treatment) exhibiting deposit depth, etchant: electrolytic

10% oxalic acid

4.3 Hardness Properties To further characterize the CMT-GMAW weld process, 81 hardness readings

were taken through three unique areas of interest. The areas included the base material

(this was used as a base-line), the HAZ (which is defined as the area above and below the

line of fusion), and the weld material. This analysis was performed in three different

orientations (longitudinal, planar, and transverse) to confirm consistency and

correspondence to the GTAW process. Overall, the results indicated that the CMT-

GMAW weldments material hardness was well within the current AMS specification and

was also equivalent to the hardness obtained with the GTAW process as well (table 4.3-

1). Refer to figures 3.4.1-2 and 3.4.3-1 for view plane orientation and weld location

definitions.

42

Table 4.3-1: Material hardness results for GTAW and CMT-GMAW weldments

4.4 Tensile Properties The mechanical properties of materials are usually temperature dependent and are

subject to change with temperature. As a result, it is imperative to consider materials’

properties at different temperatures. Some of the typical material property concerns are:

ultimate strength, yield strength, modulus, and ductility. The most common technique in

characterizing the temperature dependence of mechanical properties is to conduct tensile

tests at varying temperatures. As previously discussed in section 3.4.4, tensile properties

of the CMT-GMAW and GTAW weldments were obtained using a total 12 flat tensile

specimens (figure 4.4-1) and were tested at both room temperature (65°F) and at elevated

temperature (1100°F). [11]

43

Figure 4.4-1: Macro image of flat specimen used for testing, specimen CMT-1

The tensile strength or ultimate tensile strength (su) is the maximum load divided

by the original cross-sectional area of the specimen (see equation 4.4-1). The tensile

strength is the value most frequently cited in the results of a tension test. The tensile

strength, however, is a value of little fundamental significance with regard to the strength

of a metal. For ductile metals, the tensile strength should be considered as a measure of

the maximum load that a metal can withstand under the very restrictive conditions of

uniaxial loading. This value bears little relation to the useful strength of the metal under

the more complex conditions of stress that are usually encountered during service. [11]

Equation 4.4-1

Yield strength is the stress at which plastic deformation or yielding is first observed.

With most materials, there is a gradual transition from elastic to plastic behavior and

identifying the point at which plastic deformation begins is difficult to define with

precision. [11]

Ductility specifically refers to a material's ability to deform under tensile stress and is

generally broken into two major categories: reduction in area and elongation, where

Reduction in Area (RA) is defined by equation 4.4-2 and elongation (e) is defined by

equation 4.4-3. [11]

Equation 4.4-2 Equation 4.4-3

44

The slope of the initial linear portion of the stress-strain curve is the elastic

modulus (E), or Young's modulus. Elastic modulus is a measure of the stiffness of the

material; the greater the modulus, the smaller the elastic strain resulting from the

application of a given stress. The binding forces between atoms determine the elastic

modulus. Because these forces cannot be changed without altering the basic nature of the

material, the elastic modulus is one of the most structure-insensitive of the mechanical

properties. [11]

4.4.1 Room Temperature Properties

At room temperature, it was established that the overall tensile strength of the

CMT-GMAW was very comparable to its GTAW counterpart (see table 4.4.1-1). The

average ultimate tensile strength of the CMT-GMAW was found to be within 8 ksi of the

GTAW, whereas the yield strength was within 13 ksi.

One statistic which can be clearly observed from the data is that the CMT-

GMAW exhibited an average of 2x increase in elongation %, Reduction in Area % (RA),

and strain at break when compared to the GTAW. The specifics of this phenomenon will

be later discussed in sections 4.4.3 and 4.4.4. Finally, it is also worth noting that the

modulus of both CMT-GMAW and GTAW appear to be equivalent as well.

Table 4.4.1-1: Room temperature (65°F) tensile properties

After testing was complete, the fractured surfaces of both processes were

macroscopically examined and further microscopically examined using electron

microscopy (see figures 4.4.1-1 and 4.4.1-2). At a macroscopic level, as expected with

45

room temperature fracture, both the GTAW and CMT-GMAW specimens appeared to

have fractured in a less ductile manner than what would be expected at elevated

temperatures which is evident through the rough appearance of the fractured surface.

Other substantiating macroscopic images can be seen in appendix D.

In figure 4.4.1-2, it is important to point out that the surface features of both the

GTAW and CMT-GMAW concur with those findings at a macroscopic level. At lower

magnification, it can be noted that surface of the CMT-GMAW appears to show more

cleavage than that of the GTAW specimen as evidenced by the smoother surface of the

CMT-GMAW fracture. At higher magnification of the GTAW specimen, one can

observe the striation lines which intrinsically form as the specimen begins to stretch and

deform under continual tension.

Figure 4.4.1-1: Representative macro photographs, room temperature specimens

GTAW-1 and CMT-2

46

Figure 4.4.1-2: SEM photographs of fractured surfaces, room temperature specimens

GTAW-1 and CMT-2

4.4.2 Elevated Temperature Properties

At elevated temperatures, (1100°F) it was similarly established that the overall

tensile strength of the CMT-GMAW was very comparable to GTAW (see table 4.4.2-1).

The average ultimate tensile strength and yield strength of the CMT-GMAW was found

to be within 6 ksi of the GTAW processed specimens. Additionally, it was also observed

that the CMT-GMAW once again exhibited an average increase in elongation %,

Reduction in Area % (RA), and strain at break when compared to the GTAW, though the

increase was not as extreme as those exhibited at room temperature. This will again be

later discussed in detail in sections 4.4.3 and 4.4.4. The modulus of both CMT-GMAW

and GTAW appear to be equivalent as well.

47

Table 4.4.2-1: Elevated temperature (1100°F) tensile properties

Similarly to the room temperature analysis, after testing the fractured surfaces of

the elevated temperature, specimens were macroscopically and microscopically examined

using electron microscopy (see figures 4.4.2-1 and 4.4.2-2). At a macroscopic level, both

the GTAW and CMT-GMAW specimens appeared to have fractured in a ductile manner

as seen from the extensive cleaving (sliding) appearance of the fractured surface. This

surface appearance is commonly observed at elevated temperatures since the grain

boundaries and successive dislocations tend to slip with continued tension. Other

substantiating macroscopic images can be seen in appendix D.

In figure 4.4.2-2, the surface features of both the GTAW and CMT-GMAW at a

microscopic level correspond with those findings at a macroscopic level, though the

CMT-GMAW appears to have more cleaving which also coincides with the equivalent

ductility values obtained. This is particularly evident in the high magnification image of

the CMT-GMAW sample, since the surface features appear to be smooth and continuous.

The higher magnification image of the GTAW sample focuses on the rapid tensile

overload (or final fracture) region of the fracture surface given that the surface features

appear to be much coarser.

48

Figure 4.4.2-1: Representative macro photographs, elevated temperature specimens

GTAW-6 and CMT-4

Figure 4.4.2-2: SEM photographs of fractured surfaces, elevated temperature

specimens GTAW-6 and CMT-4

49

4.4.3 General Comments and Property Comparison

In general, strength is reduced at high temperatures and materials become softer

and more ductile as temperature increases. However, the rate and direction of property

changes can vary widely. For example, the yield strength and elongation of various alloys

are functions of the temperature. The majority of these changes are due to various

metallurgical factors. Other features that often cannot be easily predicted, can also affect

mechanical behavior at high temperatures. For example, re-solutioning of phases,

precipitation, and aging (diffusion-controlled particle growth) can occur, both during

heating prior to testing and during the testing itself. These processes have also been

known to produce a wide variety of responses in mechanical behavior depending on the

material, as previously discussed in section 4.2.

As temperature increases, the strength of a material usually decreases and the

ductility increases. The general reduction in strength and increase in ductility of metals at

high temperatures is related to the effect of temperature on deformation of the material.

At room temperature, plastic deformation occurs when there are dislocations in the

material slip. The dislocations also intersect and build up in the material as they slip. This

build-up of dislocations restricts the slip, and, thus, increases the forces necessary for

continued deformation. This practice of continued deformation is known as strain

hardening or work hardening. [11]

At elevated temperatures, dislocation climb comes into play as another

deformation mechanism. The build-up of strain energy from strain hardening can be

relieved at high temperatures when crystal imperfections are rearranged or eliminated

into new configurations, also know as recovery. A much more rapid restoration process is

recrystallization in which new, dislocation-free crystals nucleate and grow at the expense

of original grains. The restoration processes can be greatly enhanced by the increase in

the thermal activity and mobility of atoms at higher temperatures. As a result, lower

stress is required for deformation, as shown in the stress-strain diagrams in appendix E.

[11]

The crucial difference between the properties of the GTAW and CMT-GMAW

weldments is the ductility. In general, it has been shown that with larger grain size, there

is a corresponding decrease in ductility. Therefore, as previously mentioned in section

50

4.2.1, the temperature exhibited during the solidification process governs grain size and

the subsequent dendrites that form (the size of the dendrites is defined by equation 4.2.1-

1).

From what has been observed through visual observations, the GTAW process

inherently induces a greater amount of heat input during welding; therefore, upon

solidification, the solidification rate (R) will be small since it will take longer for the filler

to solidify. With a smaller R value, the local solidification temperature will be larger

(equation 4.2.1-1). When the solidification conditions become sufficiently slow, the

dendrites and the separation between them become large, in addition to the grain size.

This will decrease the ductility of the weldment (as with the GTAW specimens), whereas

the CMT-GMAW has been shown to have a lower thermal input. The lower thermal

input will create a smaller local solidification temperature and will result in smaller, more

finely, precipitated grains that will increase the ductility. The above is represented in the

tensile data obtained during this research.

51

5. Conclusions

During parameter optimization, it was observed that the key attribute of the CMT-

GMAW process is its electronically controlled short circuit droplet detachment method.

This precision process control is optimized by the push-pull servomotor located within

the torch head and is controlled by the synergic line. The synergic line is a linear

mathematical relationship that incorporates the voltage and amperage process controls

into the wire feed speed. The synergic line helps with the detachment of the molten

droplet during the short-circuiting, drastically reduces weld spatter, and provides a low

thermal gradient during arcing.

During parameter optimization, it was also found that the traverse and wire feed

speeds had a profound impact on the weld quality and overall bead profile shape. As with

any arcing welding process, when all other controls were held constant, varying the

traverse speed could change the bead shape and wire feed speed. In contrast, the other

process controls parameters had little affect on the overall weld quality, since the

synergic line dictates the majority of the process controls which simplified the overall

weld process and parameter optimization. However, it was noted that fine-tuning

parameters could greatly affect the initial wetting of the filler material.

After the material was subjected to welding, it was placed through conventional

post weld heat treatment cycles. The heat treatments included a solution heat treatment at

1865°F for one hour, and a precipitation treatment at 1400°F for 16 hours.

All non-destructive analyses of the CMT-GMAW weldments showed little evidence

of spatter, porosity, surface and sub-surface cracking, Lack of Fusion, and Lack of

Penetration. The absences of the above can be attributed to the precision droplet

detachment method of the CMT-GMAW process. The CMT-GMAW process reduced

porosity and spatter because it creates less turbulence within the weld pool, as the wire

feed is not a constant forward moving motion. Surface and sub-surface cracking was

reduced by the lower thermal input properties of the CMT-GMAW process. Visual white

macro analysis of the CMT-GMAW weldments exhibited a reduced amount of substrate

distortion when compared its GTAW counterpart.

At the microscopic level, the microstructure of the GTAW and CMT-GMAW

appears to be equivalent. Both the GTAW and CMT-GMAW weld microstructures have

52

a column grain dendrite structure that emanates in a direction perpendicular to the heat

extraction. The microstructure of the CMT-GMAW was equivalent to that of GTAW

with the exception of a reduced substrate consumption level and subsequent Heat

Affected Zone (HAZ) with an increased amount weld filler deposition. With the reduced

thermal input of the CMT-GMAW, it was also noted that the consequent grain size was

reduced, and the dendrites were finer than those seen in the GTAW weldments.

Mechanical hardness properties of the CMT-GMAW were evaluated in 81 different

locations including the substrate, HAZ, and weld filler locations; all of which confirmed

equivalency to the current AMS standards and those obtained from the GTAW

weldments.

Tensile properties of the CMT-GMAW weldments obtained were tested at both room

temperature (65°F) and at Inconel 718™ serviceable elevated temperature (1100°F). The

results obtained at room and elevated temperature indicated equivalency between the

CMT-GMAW and GTAW with the exception that the CMT-GMAW specimens exhibited

approximately 2x increase in the percentage of Reduction in Area (RA) at both room and

elevated temperatures. This increase can be attributed to the finer grain and dendrite

sizes obtained when using the CMT-GMAW. When evaluating the fractured surface, it

was noted that the CMT-GMAW exhibit more cleaving during fracture, which, of course,

is a byproduct of greater ductility.

On the basis of overall evaluation, it was concluded that CMT-GMAW provides the

following benefits: excellent weld quality on wrought Inconel 718™; comparable

metallographic structure; increased deposition rates when compared to GTAW; a

reduction in overall thermal input by achieving almost current-free metal transfer;

virtually spatter-free metal transfer (as compared to conventional GMAW) by controlling

the short circuiting; comparable material hardness; comparable tensile strength with

increased RA when compared to GTAW.

53

6. References

[1] The CMT Process –A Revolution in Materials-Joining Technology. Brighton: Fronius

USA LLC, 2004.

[2] Prof. Charles Albright. "Arc Welding Processes: Short Course, Pratt and Whitney"

Edison Joining Technology Center, Ohio State University. 23 Jan. 2001.

[3] American Society for Metals. Superalloys: A Technical Guide, Second Edition,

Matthew J. Donachie and Stephen J. Donachie. Ohio: ASM International, 2003

[4] CMT: Cold Metal Transfer, MIG/MAG dip-transfer arc process. Brighton: Fronius

USA LLC, 2007.

[5] CMT: Cold Metal Transfer,MIG/MAG dip-transfer process for automated

applications. Brighton: Fronius USA LLC, 2004.

[6] Bruckner, Jergen. "Cold Metal Transfer Has a Future Joining Steel to Aluminum." American Welding Society. 2005

<http://www.aws.org />.

[7] American Society for Metals. Metals Handbook: Welding, Brazing, and Soldering,

10th Edition, Volume 6. Ed. David L. Olson. Ohio: ASM International, 2007.

[8] American Welding Society. Welding Handbook, 8th Edition, Volume 2. Ed. R. L. O’

Brien. Miami: U.S.A American Welding Society, 1991.

[9] American Society for Metals. Metals Handbook: Non-Destructive Inspection and

Quality Control, 9th Edition, Volume 11. Ed. Howard E. Boyer. Ohio: ASM

International, 1992.

[10] American Society for Metals. Metals Handbook: Metallography and

Microstructures, Volume 9. Ed. George F. Vander Voort. Ohio: ASM

International, 2004.

[11] American Society for Metals. Metals Handbook: Mechanical Testing and

Evaluation, Volume 8. Howard Kuhn and Dana Medlin. Ohio: ASM International,

2000.

54

55

[12] Thompson, R.G. "Microfisuring of Alloy 718 in the Weld Heat-Affected Zone."

Journal of Metals. July 1988: 44-48.

Appendix A

All weld trials were performed on the edge of 0.025-inch thick AMS 5596 base

material using the ER NiCrMo-3 arc synergic line. The wire feed speed, current, and

voltage held constant while varying the torch angle and technique. The shielding gas for

all weldments was 100% pure argon, with a flowrate of 30-35 CFH; a gas chamber was

also used to ensure an inert atmosphere during welding. All appropriate heat treatments

were performed pre- and post-weld.

Table A-1: Parameters used for weld trials

A-1

Figure A-1: Specimen 25-1 (20°, Pull Angle)

Figure A-2: Specimen 25-3 (0°, Neutral Angle)

Figure A-3: Specimen 25-6 (20°, Push Angle)

A-2

Appendix B

The following appendix pertains to the visual inspection of all representative

geometry weldment specimens. Reference table 4.1-1 for parameters used and

observations made during weld trials.

B-1


weldment specimen #1 (as welded)



B-2





B-3



B-4

Appendix C

The following appendix pertains to the micro-section analysis of representative

geometry weldment specimens 3 and 5. The following photographs are of the additional

substantiating micro-sections. Reference table 4.1-1 for parameters used and observations

made during weld trials.

C-1

Figure C-1: Planar micro-section location 1 from the circular weld build-up, specimen

#3 (post thermal treatment), etchant: electrolytic 10% oxalic acid



C-2





C-3





C-4

Figure C-7: Longitudinal micro-section location 7 from the straight-line-weave pattern


Figure C-8: Planar micro-section location 8 from the straight-line-weave pattern bead,


C-5





C-6



Figure C-12: Longitudinal micro-section location 12 from single straight-line pattern


C-7

Figure C-13: Planar micro-section location 13 from single straight-line pattern bead,




C-8





C-9





C-10





C-11



C-12

Appendix D

The following appendix pertains to the macro analysis of the fractured surfaces of

all tensile specimens evaluated. The following photographs are of the additional

substantiating macro-sections. Reference tables 4.4-1 and 4.4-2 for individual tensile

specimen testing results.

D-1

Figure D-1: Post-test macro image of room temperature specimen CMT-1

Figure D-2: Macro image of fractured surface, specimen CMT-1

D-2



D-3



D-4

Figure D-7: Post-test macro image of elevated temperature specimen CMT-4


D-5



D-6



D-7

Figure D-13: Post-test macro image of room temperature specimen GTAW-1

Figure D-14: Macro image of fractured surface, specimen GTAW-1

D-8



D-9



D-10

Figure D-19: Post-test macro image of elevated temperature specimen GTAW-4


D-11



D-12



D-13

Appendix E

The following appendix pertains to stress-strain curves generated during tensile

testing. Reference tables 4.4.1-1 and 4.4.2-1 for individual tensile specimen testing

results.

E-1

Figure E-1: Stress-Strain curve of room temperature CMT specimens

E-2

Figure E-2: Stress-Strain curve of elevated temperature CMT specimens

E-3

Figure E-3: Stress-Strain curve of room temperature GTAW specimens

E-4

Figure E-4: Stress-Strain curve of elevated temperature GTAW specimens

E-5

cmt thesis

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