The effect of post-weld heat treatment on the mechanical properties of 2024-T4 friction stir-welded joints

Materials and Design 31 (2010) 2568–2577

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The effect of post-weld heat treatment on the mechanical properties of 2024-T4 friction stir-welded joints _ Hakan Aydın a,*, Ali Bayram a, Ismail Durgun b a b

Uludag˘ University, Faculty of Engineering and Architecture, Department of Mechanical Engineering, 16059 Görükle-Bursa, Turkey Tofasß-Fiat, Türk Otomobil Fabrikası A.Sß., 16369 Bursa, Turkey

a r t i c l e

i n f o

Article history: Received 12 August 2009 Accepted 13 November 2009 Available online 18 November 2009 Keywords: C. Heat treatments D. Welding E. Mechanical

a b s t r a c t In this study, the effect of post-weld heat treatment (PWHT) on the mechanical properties of friction stirwelded 2024 aluminum alloys in the T4 temper state was investigated. Solution heat treatment and various ageing treatments were given to the welded joints. The PWHT procedures caused abnormal coarsening of the grains in the weld zone, which resulted in a drop in micro-hardness at the weld zone compared to the base material of the joints. T6 (190 °C – 10 h) ageing treatment after welding was found to be more beneficial than the other heat treatments in enhancing the mechanical properties of the 2024T4 joints. However, the T6 (190 °C – 10 h) heat treatment led to significant ductility deterioration in the joint. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Aluminum alloys are increasingly used in many important manufacturing areas, such as the automobile industry, aeronautic and military, because of their low-density and good mechanical properties [1]. However, the welding of aluminum and its alloys has always represented a great challenge for designers and technologists. Many difficulties are associated with this kind of joint process [2]. It is obvious that serious problems, such as tenacious oxide layer cavities, hot cracking sensitivity, and porosity, may occur when fusion welding is applied to aluminum and its alloys. Moreover, the conventional techniques, such as fusion welding, often lead to significant strength deterioration in the joint because of a dentritic structure formed in the fusion zone [3]. Friction stir welding (FSW) is a solid state metal joining technique that was developed and patented by The Weld Institute of Cambridge, UK, in 1991 [4], and took place during the solid state phase; the above-mentioned problems were not observed [3,5–8]. FSW is well suited for joining aluminum alloys, especially those that are typically considered to be un-weldable, such as 2XXX and 7XXX series aluminum alloys [3]. 2024 Aluminum alloy is an age-hardenable alloy that possesses enhanced strength because of the precipitation of the Al2CuMg phase upon solutionising and artificial ageing. Precipitate strengthened alloys show a worsening of mechanical properties in the weld

* Corresponding author. Address: Uludag˘ Üniversitesi, Mühendislik Mimarlık Fakültesi, Makine Mühendislig˘i Bölümü, 16059, Görükle-Bursa, Türkiye. Tel.: +90 224 294 19 88; fax: +90 224 294 19 03. E-mail address: [email protected] (H. Aydın). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.11.030

zone because of the dissolution and growth of strengthening precipitates during the welding thermal cycle [5,9–12]. Though FSW joints yield better joint efficiency compared to fusion welding processes, the gap between the strength values of the base metal and the weld metal is large [13]. FSW gives rise to softening of the joints and results in the degradation of mechanical properties [9,14,15]. To recover the loss of mechanical properties in the weld zone, one option is to fully post-weld, reheat, and treat welded components [16]. Few studies have been performed to determine the effect of PWHT on the mechanical properties of friction stir-welded aluminum alloys [8,13,16,17]. Chen et al. [8] observed that the tensile strength of the friction stir-welded joints of 2219-O aluminum alloy could be significantly improved by the PWHT process. Elangovan and Balaubramanian [13] also examined the influence of PWHT on the tensile properties of friction stir-welded 6061 aluminum alloys. A 40–45% decrease in strength for the friction stir-welded joints compared to that of the base material (BM) was observed in this study. This strength loss of the friction stir-welded joints in this study has been significantly improved by the artificial ageing treatments after welding. Krishnan [17] investigated the effect of PWHT on the properties of friction stir-welded 6061 alloys. He observed that the weld (stir) region exhibited very coarse grains after the PWHT, and the samples failed after PWHT during the root bend test. Sullivan and Robson [16] also applied a PWHT to friction stir-welded 7449 aluminum alloys. They found that this joint has a dramatic effect on the nugget zone (NZ) and thermo-mechanically affected zone (TMAZ) microstructures. The hardness in the NZ and TMAZ also decreased.

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H. Aydın et al. / Materials and Design 31 (2010) 2568–2577 Table 1 Chemical composition of 2024 aluminum alloy used in this study (wt.%). Cu

Mg

Mn

Zn

Fe

Si

Ti

Cr

Al

4.50

1.50

0.50

0.20

0.40

0.41

0.12

0.07

Balance

360

Advancing Side Retreating Side Welding Tool

Weld and Rolling direction

Rotating Direction 3

100 100

Fig. 1. Schematic illustration of FSW process used in this study.

2.9

20

The aim of this study is to improve the deteriorated mechanical properties in the FSW zone with the PWHT procedures, which are W (8 months), T6 (100 °C – 10 h), and T6 (190 °C – 10 h) ageing treatments and O-temper.

2. Experimental procedures 2.1. Material and friction stir welding conditions

Ø4 Left screw 0.8 mm pitch

The experiments were conducted on 2024 aluminum alloy, whose composition is given in Table 1. The friction stir-welded joints were fabricated from nominally 3-mm thick plates. The welding samples were longitudinally butt-welded parallel to the rolling direction using a manual vertical milling machine. The welding procedure can be seen schematically in Fig. 1. The plates were clamped to a steel backing plate. The FSW process was performed with a tool with a truncated cone pin (Fig. 2). The tool was made of 1.2367 (X38CrMoV5-3) hardened steel. The welding tool rotated clockwise, and it was tilted 2–3° from the direction normal to the plate. The tool rotated at 2140 rpm, and the tool advance rate was 40 mm/s.

Ø5 Ø17 Ø20

Fig. 2. Geometry of the welding stir tool.

20

12.5

R20

50 65 200

Fig. 3. Tension test specimen geometry used to measure the transverse strength of the welds.

3

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a

b

d

c

Retreating Side

Advancing Side 2 mm

Fig. 4. The macrostructural classification of the different regions in the FSW zone of the as-welded joints. (a) NZ, (b) TMAZ, (c) HAZ, and (d) BM.

2.2. Post-weld heat treatments The original metal used in this study is 2024 aluminum alloy in the T4 temper state, which results from a water quench at approximately 0 °C after a solution treatment of the alloy at 510 °C for 2.5 h, followed by several years of natural ageing. After FSW, solution heat treatment and various ageing treatments were performed on the welded joints. The PWHT procedures include solutionising at 510 °C for 2.5 h, quenching in water at approximately 0 °C followed by various ageing treatments at room temperature for 8 months (W joint), at 100 °C and 190 °C for 10 h (T6 joints). The O-temper joint was also obtained by cooling in static air after solutionising the as-welded joint.

a

2.3. Metallography Following FSW and heat treatments, transverse cross sections were observed optically and with scanning electron microscope (SEM). The specimens were cut perpendicular to the welding direction using an electrical discharge machine and were then etched in a solution with 5 ml nitric acid, 2 ml hydrofluoric acid, 3 ml hydrochloric acid, and 190 ml distilled water.

0.5 mm

b

2.4. Mechanical tests The Vickers micro-hardness tests were carried out along the centerlines of the cross-sections perpendicular to the welding direction with a distance between neighbouring measured points of 1 mm in the weld zone under a load of 50 g for 10 s. Tensile tests were carried out perpendicular to the welding direction to determine the tensile properties of the welded joints. The specimens for the tensile tests were prepared according to the TS 138 EN 10002-1 standard [18] for sheet material (Fig. 3). The tensile properties of each joint were evaluated using four tensile specimens cut from the same joint. The room-temperature tensile tests were carried out at a crosshead speed of 10 mm/min using a Zwick Z-050 testing machine.

c

3. Results and discussion 3.1. Microstructures Friction stir-welded 2020-T4 joints consist of several zones involving different microstructures: (a) NZ, (b) TMAZ, (c) HAZ, and (d) BM (Fig. 4). Each zone experiences a variety of thermal cycles (and deformation in the NZ and TMAZ), which results in a complex mixture of microstructural processes [16]. The FSW process transforms the slightly elongated and larger grains in the BM to fine dynamically re-crystallised and equiaxed grains in the NZ (Figs. 5 and 6a, b, and c). The TMAZ lies outside the NZ and is distinguishable from the NZ (Fig. 6c). The grains in the TMAZ are significantly rotated and elongated. The plastic deformation in the HAZ is absent or is unable to modify the initial grain structure (Fig. 6e). This zone has only been subjected to thermal alterations. The precipitate state and grain size in the BM were not affected by the welding (Figs. 5 and 6). Age hardenable 2024 aluminum alloys

Fig. 5. Optical (a) and SEM (b and c) micrographs of the un-welded parent metal.

in the T4 temper state contain fine hardening precipitates (Fig. 5b and c). The fine hardening precipitates become coarse in the weld zone after the FSW process (Fig. 6a, b, and d). This, in turn, leads to a vast difference in mechanical properties across the weld, and such variation is detrimental to the joint performance. The cross-sectional macrographs and micrographs of the joints subjected to various heat treatments after welding can be seen in

H. Aydın et al. / Materials and Design 31 (2010) 2568–2577

a

b

c

d

2571

0.5 mm

e

0.5 mm

Fig. 6. Optical (c and e) and SEM (a, b, and d) micrographs of FSW zone of the as-welded joint. (a and b) NZ; (c) the transition zone from NZ to TMAZ; (d) TMAZ; (e) HAZ.

a b

Retreating Side

Advancing Side 4 mm

Fig. 7. Cross-sectional macrographs of the FSW zones of the W (8 months) joint (a) and T6 (100 °C – 10 h) joint.

Figs. 7 and 8. Following the solution heat treatment, a significant change took place in the friction stir-welded zone of the joints. Large grains that approach several hundred microns to few millimetres were observed in the FSW zone of the joints. This kind of microstructural instability has been identified as abnormal grain

growth. The vast majority of the grain boundaries in friction stir-welded microstructures have a high angle in nature (>15°); their microstructure must be regarded as re-crystallised. Grain growth appears to be a natural consequence of heat treatment [19]. The primary factors leading to abnormal grain growth in the friction stir-welded zone are associated with the inhomogeneous deformation pattern during the friction stir process. The following factors contribute to the onset of abnormal grain growth: (a) anisotropy in grain boundary energy and mobility, (b) reduction of pinning forces due to dissolution of particles, and (c) thermodynamic driving forces emanating from the grain size distribution. A critical aspect to consider is the balance between thermodynamic driving forces for abnormal grain growth and the pinning forces that impede grain boundary migration [20]. The solution temperature has a significant effect on the stability of the grains of the friction stir-welded joints. The extent of grain

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a

b

0.5 mm

0.5 mm

c

d

0.5 mm

0.5 mm

e

0.5 mm

g

0.5 mm

f

0.5 mm

h

0.5 mm

Fig. 8. Optical micrographs of the FSW zones of the PWHT joints. (a) The FSW zones of T6 (190 °C – 10 h) joint and (b) T6 (100 °C – 10 h) joint; the transition zone from FSW zone to BM of (c) T6 (190 °C – 10 h) joint, (d and e) T6 (100 °C – 10 h) joint, (f) W (8 months) joint; (g and h) O-temper joint.

H. Aydın et al. / Materials and Design 31 (2010) 2568–2577

a

2573

ture of the joints in O-temper state has very coarse hardening precipitates (Figs. 9c and 10g). Therefore, the lowest micro-hardness and strength values were obtained in these joints (Figs. 11 and 12). 3.2. Micro-hardness

b

c

Fig. 9. SEM micrographs of the FSW zones of the PWHT joints. (a) T6 (100 °C – 10 h) joint, (b) T6 (190 °C – 10 h) joint, (c) O-temper joint.

growth increases as the solution temperature increases [21]. The solution heat treatment in this study for all PWHT joints is conducted in the same conditions: 510 °C for 2.5 h. Therefore, there is no appreciable difference in the grain sizes in the friction stir-welded zones of PWHT joints. However, there are significant differences in the size and distribution of the hardening precipitates in the friction stir-welded zone and BM of the joints depending on the PWHT conditions (Figs. 9 and 10). The micro-hardness values and tensile properties of the PWHT joints also prove these differences (Figs. 11 and 12). The static properties of the friction stir-welded joints of the precipitation-hardening alloys depend on the distribution of the hardening precipitates rather than the grain size [6]. The most effective precipitation for hardening occurred in the T6 (190 °C – 10 h) ageing treatment. This leads to considerable hardening and strengthening compared to the other PWHT joints (Fig. 11). On the other hand, the microstruc-

The micro-hardness profiles of the joints can be seen in Fig. 11. The hardness in the precipitation-hardening aluminum alloys greatly depends on the size and distribution of the hardening precipitates. The micro-hardness profile across the section of the weld zone of as-welded joint shows a general softening in the TMAZ and HAZ in contrast to the BM and NZ. The transition zone between the TMAZ and HAZ of the as-welded joint has a hardness value of 90 HV, while the hardness value in the BM is about 114 HV. The reason for this softening in the TMAZ and HAZ is the coarsening and overaging of hardening precipitates due to the weld thermal cycle during the FSW process. On the other hand, a very fine grain structure allows a partial recovery in the NZ, where the hardness value is 108 HV. The hardness values across the weld zone of PWHT joints vary depending on the PWHT procedures (Fig. 11). The hardness values in the friction stir-welded zone for all PWHT joints are lower than those in the BM of the joints. This low hardness in the friction stirwelded zones is associated with the very coarse grain structure (Figs. 7 and 8). The hardness values in the TMAZ, HAZ, and BM of the as-welded joint were significantly improved by the T6 PWHT (190 °C – 10 h) ageing treatment. In the four-PWHT condition, the joint in the T6 (190 °C – 10 h) ageing treatment exhibited the highest hardness values of about 108 HV in the NZ and 125 HV in the BM. The hardness values in the TMAZ and HAZ of the joint in T6 (190 °C – 10 h) ageing condition were increased by an average of 18 HV compared to those of the as-welded joint. The microstructure of the joint in the T6 (190 °C – 10 h) temper state contains fine and uniformly distributed hardening precipitates. This is the main reason for the enhanced hardness and improved tensile properties of the joint in T6 (190 °C – 10 h) temper state. However, the hardness values of the as-welded joint in the NZ were not significantly improved by T6 (190 °C – 10 h) ageing treatment due to abnormal grain coarsening in the NZ. It can be seen in Fig. 11 that the other PWHT procedures, which are W (8 months) and T6 (100 °C – 10 h) ageing treatments, are insufficient to harden the as-welded joints. As expected, the minimum hardness values were obtained in the PWHT O-temper joint with very coarse hardening precipitates (Figs. 10g and 11). 3.3. Tensile properties and fracture surfaces The tensile properties of the joints are given in Table 2 and Fig. 12. The un-welded original metal showed a yield strength and tensile strength of 351 MPa and 492 MPa, respectively. However, the yield strength and tensile strength of the as-welded joint are 279 MPa and 389 MPa, respectively. This indicates a 20–21% reduction in strength for the 2024-T4 friction stir-welded joints compared with that of the un-welded parent metal. The tensile tests revealed the significant improvements in the tensile properties for the PWHT joints. The strength of as-welded joint was significantly increased by the PWHT T6 (190 °C – 10 h) ageing treatment. In the four-PWHT procedure, the T6 (190 °C – 10 h) ageing treatment offered the greatest improvement in the strength values. However, the T6 (100 °C – 10 h) ageing treatment, and especially the O-temper treatment, further reduced the strength values of the as-welded joint. On the other hand, the strength values of the joint in the W (8 months) temper state are slightly higher than those of the as-welded joint. The yield and tensile strength of the joint in the T6 (190 °C – 10 h) temper state are

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a

b

0.5 mm

0.5 mm

c

d

0.5 mm

e

f

g

Fig. 10. Optical (a–c) and SEM (d–g) micrographs of the BMs of the PWHT joints. (a and d) W (8 months) joint; (b and e) T6 (100 °C – 10 h) joint; (c and f) T6 (190 °C – 10 h) joint; (g) O-temper joint.

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As-welded Joint

W (8 months) Joint

T6 (100 ºC-10h) Joint

O-Temper Joint

T6 (190 ºC-10h) Joint

100

130

80

Efficiency [%]

120

Microhardness [HV 0,05]

110 100 90

60 40 20

80

YSE [%]

70

As-welded Joint

60 50

TSE [%]

EE [%]

0

Advancing Side

NZ

Retreating Side

W (8 months) Joint

T6 (100 ºC-10h) Joint

T6 (190 ºC-10h) Joint

O-Temper Joint

Fig. 13. Efficiencies of the joints.

25 22 20 18 16 14 12 10 9 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 9 10 12 14 16 18 20 22 25

Distance from the weld zone center [mm] Fig. 11. Micro-hardness distributions in the joints.

55

550

Strength [MPa]

500

Rm

A [%]

50

450

45

400

40

350

35

300

30

250

25

200

20

150

15

100

10

50

5

Elongation [%]

Rp0,2

0

0 2024-T4 Parent Metal

As-welded Joint

W (8 months) Joint

T6 (100 ºC-10h) Joint

T6 (190 ºC-10h) Joint

O-Temper Joint

Fig. 12. Tensile properties of the tested samples.

345 MPa and 430 MPa respectively, but the elongation value is only 6%. The yield strength of the T6 (190 °C – 10 h) joint is almost equivalent to that of the un-welded original metal, but the tensile strength and elongation show 12.6% and 72.6% reductions, respectively, compared with the un-welded original metal. However, the yield and tensile strength of the T6 (190 °C – 10 h) joint are about 24% and 11% greater, respectively, than those of the as-welded joint. The low elongation in this joint is attributed to low fracture toughness, equiaxed grains and the presence of precipitate free zones [17]. Many studies have been carried out to explain the presence of a precipitate free zone with brittleness [22–25].

The elongation value of the un-welded parent metal was 21.9%. However, the as-welded joint exhibited an elongation value of 9%. This was an approximately 60% reduction in the elongation of the as-welded joint compared with the un-welded original metal. The elongation values of the W (8 months), T6 (100 °C – 10 h), T6 (190 °C – 10 h), and O-temper joints were 12.1%, 11.3%, 6%, and 5.2%, respectively. The W (8 months) and T6 (100 °C – 10 h) ageing treatments after welding lead to significant improvement in the elongation compared with the as-welded joint: 34.4% and 25.6% increases, respectively. However, the ductilities of the O-temper and T6 (190 °C – 10 h) joints significantly deteriorated compared with the as-welded joint: 42.2% and 33.3% reductions, respectively. The joint efficiency can be defined as the ratio of a tensile property of a welded joint to that of the un-welded original metal. The yield strength efficiency (YSE), the tensile strength efficiency (TSE) and the elongation efficiency (EE) of the joints can be seen in Fig. 13. The strength efficiencies of the as-welded joint were about 79%, while the EE of this joint was 41%. The highest YSE and TSE, 98% and 87.4%, respectively, were obtained in the T6 (190 °C – 10 h) joint, but the EE of this joint was only 27.4%. The highest EE, 55.3%, was obtained in the W (8 months) joint, while the Otemper joint had the lowest EE: 23.7%. The strength loss of the as-welded joint occurred in the FSW zone. The as-welded joint was fractured near or at the interface between the NZ and the TMAZ on the advancing side [9]. The interface between the NZ and the TMAZ is clearly visible in Fig. 6c, and this causes a weak region at the joint; thus, the joint is fractured at this interface during tensile testing. The fracture location of the PWHT joints, except for the O-temper joint, also occurred in the FSW zone on the advancing side and corresponds to the lowest value of micro-hardness (Fig. 11). On the other hand, the fracture in the O-temper joint occurred in the friction stir-welded zone on the retreating side. In the PWHT joints, the FSW zone is a weak part of the joints because of the serious grain coarsening; thus, the fractures occurred in the FSW zone. This implies that the PWHT procedures examined in this investigation have no significant effect on the fracture locations of the joints.

Table 2 Tensile properties of the un-welded parent metal and the as-welded and PWHT joints (average values). Sample

0.2% Proof strength, Rp0.2 (MPa)

Ultimate tensile strength, Rm (MPa)

Elongation, A (%)

Fracture locations of the joints

2024-T4 Un-welded parent metal As-welded joint

351

492

21.9

–

279

389

9

W (8 months) joint T6 (100 °C – 10 h) joint T6 (190 °C – 10 h) joint O-Temper joint

289 248 345 105

402 379 430 227

12.1 11.3 6 5.2

The interface between the NZ and the TMAZ on the advancing side The FSW zone on the advancing side The FSW zone on the advancing side The FSW zone on the advancing side The FSW zone on the retreating side

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a

b

c

d

Fig. 14. Fractographs of the joints: (a) W (8 months) joint, (b) T6 (100 °C – 10 h) joint, (c) T6 (190 °C – 10 h) joint, and (d) O-temper joint.

The tensile properties of PWHT joints are far lower than those of the un-welded base materials in the same temper states. The yield strength, tensile strength and elongation of the un-welded 2024 aluminum alloy in the T6 (190 °C – 10 h) temper state are 403 MPa, 496 MPa, and 8.6%, respectively. These values for the un-welded 2024 aluminum alloy in the T6 (100 °C – 10 h) temper state are 293 MPa, 451 MPa, and 25.5%, respectively, while the those for the un-welded 2024 aluminum alloy in the O-temper state are 126 MPa, 289 MPa, and 17.6%, respectively [9]. These values can be compared with the values in Table 2 and Fig. 12. The reason for such a decrease in the tensile properties of the PWHT joints could be explained by the abnormal grain growth in the friction stir-welded zone (Figs. 7 and 8). The fracture surfaces of the tensile tested specimens were characterised using SEM. SEM photographs were taken from the centre regions of the tested specimens, and the pictures are presented in Fig. 14. It is clear that the fractures are predominantly inter-granular. Aydın et al. [9] reported that the size of the dimples in the fracture surface section of the as-welded joint was smaller than that in the fracture surface section of the un-welded parent metal, which suggested that a smaller stretch zone was present at the tip of the crack. This led to a small plastic zone ahead of the crack and rather low ductility. The fracture surfaces of the PWHT joints are also covered with finer dimples, as shown in Fig. 14. The size of the dimples in the fracture surfaces might be indicative of the ductility of the PWHT joints when the elongation of the PWHT joints in Table 2 was not considered. As the dimples increase in size (Fig. 14), the elongation of the PWHT joints also increases (Table 2). The O-temper and T6 (190 °C – 10 h) joints exhibited the lowest ductility, while the W (8 months) and T6 (100 °C – 10 h) joints had the highest ductility. The size of the dimples in the fracture surface section of the W (8 months) and T6 (100 °C – 10 h) joints is larger than that of the other joints, so the ductility of these joints are higher. No remarkable difference in the fractographs of the W

(8 months) and T6 (100 °C – 10 h) joints was observed; thus, their ductilities are similar.

4. Concluding remarks In this study, the effect of post-weld heat treatment on the mechanical properties of friction stir-welded 2024 aluminum alloys in T4 temper state was investigated in detail. The main results are as follows. Once the FSW joints are subjected to solution heat treatment at high temperature, the welds experience abnormal grain growth within the FSW zone. Moreover, the hardness in the FSW zone of the PWHT joints is lower than that in the BM of the joints. This could be explained by the abnormal grain growth in the FSW zone. Further, the hardness values in the TMAZ, HAZ, and BM for the as-welded joint can be significantly increased by a PWHT T6 (190 °C – 10 h) ageing treatment procedure. In addition, the strength of the friction stir-welded 2024 aluminum alloys in the T4 temper state can be significantly improved with a PWHT T6 (190 °C – 10 h) ageing treatment procedure. However, this heat treatment led to significant ductility deterioration in the joint. The TSE and YSE values of the as-welded joint were increased up to 87.4% and 98%, respectively, by the T6 (190 °C – 10 h) ageing treatment. However, the EE of the T6 (190 °C – 10 h) joint was only 27.4%. The W (8 months) and T6 (100 °C – 10 h) ageing treatments offered a significant improvement in the elongation compared with the as-welded joint: 34.4% and 25.6% increases, respectively. However, the O-temper and T6 (190 °C – 10 h) heat treatments led to a significant ductility deterioration: 42.2% and 33.3% decreases, respectively, compared with the as-welded joint. The PWHT procedures examined in this investigation do not influence the fracture locations of the joints; all joints were fractured in the friction stirwelded zone. This result can be explained by the micro-hardness profiles and the inner structure of the joints. Finally, the tensile

H. Aydın et al. / Materials and Design 31 (2010) 2568–2577

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