Microstructure and Mechanical Properties of Friction Stir Welds in Aluminium Alloys 2024-T3, 5083-O and 6063-T6

R. M. Leal1,a and A. Loureiro2,b

1 ESAD, R. Isidoro I. A. de Carvalho, 2500-917 Caldas da Rainha, Portugal 2 DEM – FCTUC, Pinhal de Marrocos, 3030 Coimbra, Portugal

aruileal@esad.ipleiria.pt, baltino.loureiro@dem.uc.pt

Keywords: Friction stir welding, aluminium alloys, microstructure, mechanical properties.

Abstract. The aim of this research is to study the effect of the welding process on the

microstructure and mechanical properties of friction stir welded joints in aluminium alloys 2024-

T3, 5083-O and 6063-T6. A small loss of hardness and strength was obtained in welds in alloys

2024-T3 and 5083-O as opposed to welds in alloy 6063-T6, where a substantial softening and a

drop of strength were observed. In alloy 6063-T6 a strength efficiency of only 45 to 47% was

obtained.

Introduction

Fusion welding of aluminium alloys has several difficulties which reduce productivity and quality

of the welded joints. These problems are the formation of porosity, usual in all aluminium alloys,

the hot cracking tendency, typical of some alloys of the series 2000, 6000 and 7000, an important

loss of strength in the weld metal and thermal affected zone and a significant distortion, which

increase with increasing heat-input. Friction stir welding (FSW) is a solid state joining process

developed recently which allows minimizing the problems mentioned above. Though the process

presents large potential of application in several industries, changes in microstructure and

mechanical properties of the welds have been reported in the last years [1, 2]. Some authors account

the improvement of mechanical properties in the weld region [3]. This improvement is attributed to

the dispersion of particles and to the formation of a homogeneous and refined microstructure in the

weld. Others refer a softening in the weld caused by dissolution and growth of strengthening

precipitates during the weld thermal cycle [4].

The aim of this research is to study the effect of the welding process on the microstructure and

mechanical properties of friction stir welded joints in aluminium alloys 2024-T3, 5083-O and 6063-

T6.

Experimental procedure

Friction stir welds were produced in plates of aluminium alloys 2024-T3, 5083-O and 6063-T6 with

3 mm thick. The Cchemical composition of the plates is indicated in Table 1.

Table 1 – Chemical composition of aluminium alloys (wt%)

Alloy Al Cr Cu Fe Mg Mn Si Ti Zn

2024-T3 Bal. 0.05 4.3 0.25 1.5 0.6 0.2 0.1 0.1

5083-O Bal. 0.15 0.05 0.2 4.5 0.7 0.25 0.1 0.15

6063-T6 Bal. 0.05 0.05 0.2 0.6 0.05 0.3 0.06 0.05

Welds were produced with an ESAB machine LEGIO FSW 3 UT, equipped with a screw threaded

pin of 6 mm in diameter and 2.8 mm in length. The welding parameters applied in the tests are

Materials Science Forum Vols. 514-516 (2006) pp 697-701Online available since 2006/May/15 at www.scientific.net© (2006) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.514-516.697

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indicated in Table 2. The choice of the welding parameters was based on the appearance of the

welds without surface defects.

Metallographic analysis was done in an optical microscope ZEISS HD 100. Several etchants were

used to reveal the grain size structure and the precipitates existing in the matrix of aluminium.

Kroll’s reagents was used in 2024-T3 and a modified Poulton’s reagent in 5083-O and 6063-T6

alloys to reveal the grain structure. A mixture of methanol (25ml), hydrochloric acid (25ml), nitric

acid (25ml) and a drop of hydrofluoric acid were used to reveal precipitation. Hardness profiles

HV1 were obtained transversely to the weld direction. Mechanical efficiency of the welds was

determined by tensile testing.

Table 2 – Welding parameters used in the tests.

Alloy Plunge force

[kN]

Rotation speed

[rpm]

Welding speed

[mm/s]

Indentation

time [s]

2024-T3 10 - 12 800 3.3 – 6.6 15

5083-O 7 - 10 800 - 1500 2.5 - 10 6

6063-T6 4.5 1000 9.2 6

Results and discussion

Microstructure. It is evident in all welds the formation of a thermal and mechanically affected

zone (TMAZ), originated by the large plastic deformation of the material which moves with the

rotating pin. Material is plasticized by the heat generated in the process mainly by the friction

mechanism. A region called nugget, characterized by an equiaxed and well refined microstructure,

can be observed in the centre of this zone. Fig. 1a) and 1b) illustrate respectively the

microstructures of the parent material and of the nugget of a weld carried out on aluminium alloy

2024-T3. The refinement of the microstructure is generally attributed to dynamic recover or

dynamic recrystallization [5].

The remaining of the thermal and mechanically affected zone suffers a large plastic deformation,

with large rotation of the grains, as is shown in the right side of Fig. 1 c) of the same weld. In the

Fig. 1 – Microstructures observed in a

weld done on a 2024-T3 aluminium

alloy; a) parent material (Poulton

reagent); b) nugget (Kroll reagent); c)

transition between nugget and the rest of

the thermal and mechanically affected

zone (Kroll reagent)

a) b)

c)

698 Advanced Materials Forum III

left side of Fig. 1 c) is shown the nugget in lower magnification than in Fig. 1 b). These changes of

the microstructure are accompanied by alterations of the size and density of precipitates as well the

rearrangement of remaining dislocations in all welds. Fig. 2 shows TEM micrographs of the

different regions of a weld produced on 6063-T6 alloy. A significant reduction of precipitates and

of the dislocation density, in addition to the reduction of the grain size, is obtained in the nugget, as

illustrated in Fig. 2 d).

0,5 µµµµm 1 µµµµm

2 µµµµm 5 µµµµm

Fig. 2 – TEM micrographs of several regions of a weld on 6063-T6 alloy; a) parent material; b)

thermal affected zone; c) thermal and mechanically affected zone; d) nugget.

In the thermal and mechanically affected zone, outside of the nugget, a large dislocation density is

observed showing some interaction among dislocations as depicted in Fig. 2 c). Some coarsening of

the precipitates can be observed in the thermal affected zone, see Fig. 2 b), while parent material

presents small precipitates and high dislocation density, as shown in Fig. 2 a).

Hardness. Fig. 3 illustrates the hardness evolution through FS welds in the aluminium alloys under

analysis. Closed symbols concern to welded joints and open symbols refer to the parent materials. A

small decrease in hardness was obtained in welds in alloy 2024-T3 in the TMAZ and in the HAZ.

This is because precipitates dissolved during welding partially reprecipitate during cooling which

associated to the natural ageing, give some hardness recovery. Welds on 5083-O alloy show only a

small increase of the hardness on the retreating side because it is a non heat treatable alloy and the

hardening occasioned by plastic deformation is in part removed by recover and recrystallization

phenomena promoted by the heat generated in the process. In opposition welds on 6063-T6 alloy

show a significant loss in hardness in TMAZ and HAZ. This is because no significant

reprecipitation occurs in TMAZ even after natural ageing and some coarsening of precipitates

occurs in HAZ, as illustrated in Fig. 2, becoming incoherent with the matrix. The chemical

composition and the state of treatment of aluminium alloys are determinant factors of the

degradation of mechanical properties that occurs during welding operations.

Materials Science Forum Vols. 514-516 699

0

20

40

60

80

100

120

140

160

180

-30 -20 -10 0 10 20 30

Distance [mm]

Hardness HV1

2024FSW

5083PM

5083FSW

6063PM

6063FSW

2024PM

Fig. 3 – Hardness evolution in friction stir welds (FSW) on aluminium alloys 2024-T3, 5083-O and

6063-T6; PM – parent material; TMAZ – thermal and mechanically affected zone; AS – advancing

side; RS – retreating side.

Tensile strength. Tensile behaviour of the friction stir welded joints produced in the three

aluminium alloys is illustrated in Fig. 4. A characteristic conventional stress-strain curve is depicted

for each parent material as well as for a welded joint in this material. Welds produced in 2024-T3

alloy have a yield stress similar to that of the parent material. The weld efficiency, defined as the

rate between the yield stress of the welded joint and the yield stress of the parent material, is of

approximately 100%. The premature fracture of welded specimens occurred in the

thermomechanicaly affected zone because of small defects (voids) formed during welding

operation. In fact this material is sensitive to defect formation due to the welding parameters (weld

travel speed and tool shoulder force), as reported previously [6]. This result is compatible with the

small degradation of hardness observed in the welds and is in agreement with the results of

Kristensen et al [7] for a welding speed of 400 mm/min.

For the alloy 5083-O a small decrease of the weld efficiency (around 13%) was obtained. The

fracture of welded specimens occurred in the TMAZ, suggesting this is the weakest region. This

result appears contradictory with the hardness measurements because hardness in the retreating side

of the weld is superior to the hardness of the parent material however in the advancing side

hardness is similar to the parent plate. This fact associated to the natural surface roughness of the

weld can justify the placement of the fracture. The decrease in elongation of the welded specimens

is due to the localization of the plastic deformation in the weakest zone of the weld.

Welds in 6063-T6 alloy have a weld efficiency of approximately 45-47%, which is in agreement

with the hardness results that show a large softening in the weld. Fracture in these specimens

occurred in the thermal affected zone, precisely the region showing the lowest hardness. These

results are in opposition to those of Luan et al [8] which refer a weld efficiency of 80-90% for this

material, though they assume a marked decrease in hardness in the weld region. The difference in

tensile properties may remain in the fact our welds were neither polished nor heat treated. However

the reduction of tensile strength is not proportional to the reduction of hardness, therefore hardness

measurements should not be used to estimate tensile strength.

TMAZ AS RS

700 Advanced Materials Forum III

0

50

100

150

200

250

300

350

400

450

0 0.05 0.1 0.15 0.2

Strain [mm/mm]

Stress [MPa]

2024-PM

2024-FSW

5083-PM

5083-FSW

6063-PM

6063-FSW

Fig. 4 – Stress-strain curves of parent plates and welded joints; PM – parent material; FSW friction

stir welded joint.

Conclusions

Friction stir welding process produces significant alterations of the microstructure in the thermal

and mechanically affected zone as well in the thermal affected zone of the welds in the aluminium

alloys tested. A small loss of hardness and of strength was obtained in welds in alloys 2024-T3 and

5083-O as opposed to welds on alloy 6063-T6, where a substantial softening and a drop of strength

were observed. In alloy 6063-T6 a strength efficiency of only 45 to 47% was obtained. Therefore

the change in mechanical properties of the welded joint is function not only of the welding

technology but depends also of the chemical composition and of the thermal and mechanical

treatments of the aluminium alloys.

Acknowledgements

The European Community, the Portuguese Government and FEDER are acknowledged for the

financial support of this research through the Programme POCTI.

References

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Metz, France.

[8] G. Luan, S. Lin, P. Chai, H. Li, The 5th Int. F.S.W. Symposium, Sep. (2004), Metz, France.

Materials Science Forum Vols. 514-516 701

Advanced Materials Forum III 10.4028/www.scientific.net/MSF.514-516 Microstructure and Mechanical Properties of Friction Stir Welds in Aluminium Alloys 2024-T3, 5083-

O and 6063-T6 10.4028/www.scientific.net/MSF.514-516.697

DOI References

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doi:10.1016/S0254-0584(02)00604-1 [6] R. Leal, A. Loureiro, Materials Science Fórum Vols. 455-456 (2004) 299-302

doi:10.4028/www.scientific.net/MSF.455-456.299 [4] Y. S. Sato, H. Kokawa, Metallurgical and Materials Transactions A, Vol. 32A, Dec. 2001, 3023-3031.

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