Effect of welding parameters on microstructure and mechanical properties of aluminum ...nopr.niscair.res.in/bitstream/123456789/42931/1/IJEMS 24... · 2017-10-13 · Indian Journal

Indian Journal of Engineering & Materials Sciences Vol. 24, June 2017, pp. 215-227

Effect of welding parameters on microstructure and mechanical properties of aluminum alloy AA6082-T6 friction stir spot welds

H Aydin*, O Tuncel, Y Umur, M Tutar & A Bayram

Mechanical Engineering Department, Engineering Faculty, Uludag University, 16059, Gorukle-Bursa, Turkey

Received 2 March 2016; accepted 27 January 2017

Friction stir spot welding is an alternative joining technique for lightweight materials such as aluminum alloys. In this study, the influence of welding parameters, namely rotational speed, plunge depth, dwell time and travel speed, on weld microstructure and weld strength of friction stir spot welded AA6082-T6 aluminum alloy have been investigated. The joined samples were investigated by the methods of microstructural observations, microhardness tests, tensile shear tests and fractography. The hardness values of weld zone decreased significantly after FSSW process. Among the weld zones, HAZ showed the lowest hardness values. Higher rotational speed and lower travel speed led to the lower hardness values in the weld zone. The plunge depth and dwell time did not significantly affect the hardness values in the weld zone, except the SZ near TMAZ. Larger plunge depth and higher dwell time promoted higher hardness values in the SZ near TMAZ. The tensile test results showed that tensile shear load increased with increasing plunge depth and dwell time and with decreasing tool rotational speed and travel speed. The most effective welding parameter was found as dwell time. Higher rotational speed, higher plunge depth, higher dwell time and lower travel speed resulted in relatively ductile fracture.

Keywords: AA6082 Aluminum alloy, Friction stir spot welding, Microhardness, Microstructure,Tensile shear load,

Welding parameters Recently, due to the shortages of global resources and environment problems, lightweight materials have been used extensively in automotive industry1-3. Aluminum alloys find a wide variety of applications in mechanical constructions and automotive industry because of its remarkable combination of characteristics, such as specific strength, high corrosion resistance, good formability, high recycle and high electrical and heat conductivity4-7. The use of aluminum alloy sheets in the automotive industry inevitably involves welding. Resistance spot welding (RSW) is the traditional joining technique for automobile body parts made of metal sheets. Unfortunately, RSW of aluminum alloy sheets have many disadvantages because of physical and metallurgical properties of aluminum alloys such as high conductivity, low strength at elevated temperature and particularly surface oxide film (Al2O3), which melt at extremely high temperatures. Furthermore, alternative spot joining processes like self-piercing rivets have high consumable costs8-10.

Friction stir spot welding (FSSW) was developed as an alternative joining technology to replace RSW and riveting for fabrication of light-weight metals with high thermal conductivity such as aluminum alloy sheets.

FSSW is a solid state welding method developed for use in automotive and aerospace industries11. FSSW is a derivative process of friction stir welding (FSW), which was developed by Mazda Motor Corporation and Kawasaki Heavy Industries in 2003 based on FSW to lap join aluminum sheets12,13. Compared with RSW that is commonly used welding method in the automotive industry, FSSW offers 90% energy saving and 40% equipment saving due to its minimal equipment requirement14-16. A schematic illustration of the FSSW process is shown in Fig. 117. The process respectively consists of three distinct phases: (a) plunging, (b) stirring and (c) retracting. In FSSW, the joint is created by plunging a rotating tool into a work-piece until the welding tool’s shoulder reaches a desired penetration depth. It remains at this plunge depth for a specified length of time (dwell time), at which point the tool is retracted18,19. The tool rotational speed, plunge depth, dwell time and travel speed are the most important process parameters that affect the heat generation, joint formation and mechanical properties of FSSWed joints. Moreover, the welding tool geometry plays a crucial role in material flow and mixing. Researches show that welding with concave tool shoulder produce higher static strength compared the tools with convex or flat tool shoulders20.

___________ Corresponding author (E-mail: [email protected])

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A variety of studies have focused on the effect of process parameters on the microstructure and mechanical properties of FSSWed aluminum alloy joints. Buffa et al.21 presented the experimental results of FSSWed 1.5 mm thick AA6082 aluminum alloy sheets and reported that the decrease in the failure load observed with increasing tool rotation speed due to the enlargement of the area characterized by low microhardness at the boundary between HAZ and TMAZ. Yoon et al.22 investigated the effect of the tool plunge speed and the tool plunge depth on the surface appearance, macrostructure and mechanical properties of the FSSWed AA5454-O plates. They stated that the increase of the tool plunge depth resulted in the increase of the tensile shear load. However, the change of the tool plunge speed did not lead to the remarkable variation in the tensile shear load of the FSSWed plates. Babu et al.23 showed that the mechanical performance of a FSSW is mainly governed by its geometrical features (hook height and bond width). Maximize bond width and minimize hook height is a good basis for process optimization and mechanical properties of friction stir spot welds are not very sensitive to the base material temper condition. Badarinarayan et al.24 reported that the tool geometry played a major role in formation of hook. They found that FSSW made with concave tool shoulder yielded higher static strength than flat shoulder convex at similar plunge depth.

Recently, AA6082 aluminum alloy sheets are gaining popularity in automotive body structure applications. Therefore, performing FSSW of AA6082 alloy may be a demand in the modern automotive industry. The application of this alloy requires a more complete understanding of the issues associated with FSSW of AA6082 aluminum alloy sheets. It is thus important to study the welding behavior of FSSWed joints of AA6082 alloy sheets. Although there are several studies on the microstructure and mechanical properties of FSSWed various aluminum alloys in the

literature2,5,8,11,15,17,19,21-24, only a few limited studies focused on the properties of the FSSWed AA6082 aluminum alloy sheets21,25. Therefore, in the present study, the authors focused on the microstructure and mechanical properties of FSSWed AA6082-T6 aluminum alloy, and so the relationships between the mechanical properties and main process parameters (rotational speed, plunge depth, dwell time and travel speed) were investigated.

Experimental Procedure In this study, FSSW was performed on 3 mm thick

AA6082-T6 aluminum alloy sheets. The chemical composition and mechanical properties of AA6082-T6 aluminum alloy sheet used in this study are listed in Tables 1 and 2, respectively. Samples were cut into pieces in dimensions of 100 mm × 40 mm to prepare the shear tensile test samples according to ANSI/AWS/SAE/D8.9-97 standard26 (see Fig. 2).

Fig. 1 A schematic illustration of FSSW process (a) plunging, (b) stirring (dwell) and (c) retracting17

Fig. 2 Welding sample installed as lapping joints for the shear-tensile test Table 1 Chemical composition of AA6082-T6 aluminum alloy

used in this study (wt%) Si Fe Cu Mn Mg Cr Zn Ti Al 0.9 0.44 0.08 0.54 0.7 0.03 0.04 0.04 Balance

Table 2 Mechanical properties of AA6082-T6 aluminum alloys used in this study

Ultimate tensile strength (MPa)

0.2% Proof strength (MPa)

Elongation (%)

342 298 16

AYDIN et al.: FRICTION STIR SPOT WELDING

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These pieces were friction stir spot welded in the overlapping configuration using a vertical CNC milling machine. A non-consumable tool made of H13 hot work tool steel, having a 10 concave shoulder of 15 mm in diameter was used for all welds. A threaded cylindrical pin used for the welds has a length 3.5 mm along with a right-hand screw of 0.8 mm pitch. Two curvilinear grooves, opposite to each other, were also milled on the pin by using a milling tool with a diameter of 2 mm (see Fig. 3). The welding tool was rotated in a clock-wise direction during the welding process. Additionally, a specific clamping fixture for the work-pieces was fabricated to ensure correct axial positions and obtain precise plunge depth values (see Fig. 4). An experimental design was created to determine the effects of the following welding parameters; rotational speed, plunge depth, dwell time and travel speed. The welding parameters used in this study are given in Table 3.

The welded samples were carefully cross-sectioned through the weld center using a low speed precision

cutting machine for microstructural examination and microhardness measurements. Then, the cross-sectioned samples were mounted in bakalite, and then ground manually using SiC emery papers. Subsequently, the samples were mirror polished successively through 1 µm and 0.3 µm diamond pastes on a polishing machine, and then washed and blow dried. Mirror-polished samples were characterized to obtain their cross-sectional morphologies using a Nikon DIC microscope with a Clemex image analysis system after etching in a reagent with the following composition for 7 min: 5 mL hydrofluoric acid and 95 mL distilled water. The mechanical properties of the welds were evaluated through microhardness measurements and tensile shear tests. Vickers microhardness measurements were performed on the metallographic samples using a DUROLINE-M microhardness tester with a 300 g load for 20 s of dwell time. The hardness values were measured spacing of 0.5 mm along the 1 mm above the central-line. In order to evaluate the mechanical strength of the welds, the lap shear tensile tests were carried out on a UTEST-7014 tensile testing machine, at room temperature and with a crosshead displacement speed of 5 mm/min. Two additional sheet pieces having a thickness equal to 3 mm were also fixed on the clamping sides of the tensile shear test samples in order to avoid the possible parasitic effects of the bending moment (see Fig. 5). The tensile lap-shear load for each weld was obtained by averaging four test results. Following the tensile shear tests, the fracture surfaces of the joints were examined using Zeiss EVO 40 XVP scanning electron microscope (SEM).

Fig. 3 The welding tool used in this study

Fig. 4 Details of clamping fixture used in this study

Fig. 5 The tensile shear test samples with additional sheetpieces

Table 3 Welding parameters used in this study

Sample Rotational

speed (rpm)

Plunge depth (mm)

Dwell time (s)

Travel speed (mm/min)

S1 1000 5 7 50 S2 1500 5 7 50 S3 2000 5 7 50 S4 2500 5 7 50 S5 1500 4 7 50 S6 1500 4.5 7 50 S7 1500 5.5 7 50 S8 1500 5 2 50 S9 1500 5 4 50 S10 1500 5 9 50 S11 1500 5 0 50 S12 1500 5 0 43 S13 1500 5 0 38 S14 1500 5 0 32


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Results and Discussion

Microstructure The weld cross-sections of the FSSWed AA6082-

T6 aluminum joints exhibited four main distinct microstructural zones, including the dynamically recrystallized stir zone (SZ), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and base metal (BM), as shown in Fig. 6. Additionally, a typical “keyhole” in the center of the spot welded joint and the upward curved “Hook” formed by the upward bending of the interface of the sheets due to the penetration of the tool into the lower sheet can be

clearly seen in Fig. 6. The BM microstructure revealed elongated and larger grain structure belonging to the rolling operations, including relatively coarse (Fe,Mn)3SiAl12 particles and very fine dispersed Mg2Si precipitates (see Fig. 7a)27. There is no evidence of BM microstructure in the SZ (see Fig. 7b). The larger and elongated grain structure in BM was dynamically recrystallized in the SZ, which had experienced high temperatures and extensive plastic deformation, including relatively coarse strengthening Mg2Si precipitates. This precipitate coarsening in the SZ may be attributed to the dissolution (solutionizing) and growth of strengthening precipitates during the welding thermal cycle, followed by cooling in air. Meanwhile, the grains in the TMAZ were significantly rotated and deformed along the flow of the materials during stirring process (see Fig. 7c). On the other hand, the grains in the HAZ, which is affected by the heat but not by deformation, were slightly overgrown as a result of the exposure to the welding heat (see Fig. 7d). The thermal exposure in the TMAZ and HAZ during the welding process resulted in coarsening of the strengthening precipitates (overaging).

The effect of rotational speed on macrostructure of a friction stir spot-welded AA6082-T6 joint can be seen in Fig. 8. Higher rotational speed resulted in

Fig. 6 A typical macrostructure of a friction stir spot-welded AA6082-T6 joint (S1 sample)

Fig. 7 The typical microstructures of a friction stir spot-welded AA6082-T6 joint (S1 sample) (a) BM, (b) SZ(c) TMAZ and (d) HAZ


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coarser grain structure in the SZ, especially near TMAZ, and lower hook height due to the higher heat input. In addition, the hook curvature was fairly broken down with increasing rotation speed. Any significant difference in bonded section size was not observed with the increase of rotational speed. SZ microstructure with the higher rotational speed exhibited relatively larger grain structure and coarser strengthening precipitates, especially in the grain boundaries owing to the higher heat input (see Fig. 9).

The effect of plunge depth on macrostructure of a FSSWed AA6082-T6 joint is given in Fig. 10. Larger

plunge depth resulted in larger bonded section size, lower hook height and wider SZ due to the increased material mixing during the FSSW process. Similar to the effect of the rotational speed, larger plunge depth has led to coarse SZ microstructure, i.e., larger grains and coarser strengthening precipitates especially in the grain boundaries, due to the higher heat input (see Fig.11).

The effect of dwell time on macrostructure of a friction stir spot-welded AA6082-T6 joint is shown in Fig. 12. Higher dwell time resulted in larger bonded section size and wider SZ due to the higher heat input. However, any significant difference in hook height

Fig. 8 The effect of rotation speed on macrostructure of a friction stir spot-welded AA6082-T6 joint (a) S1 Sample (1000 rpm), (b) S4 Sample (2500 rpm). (HH: hook height)

Fig. 9 Optical micrographs of the SZ of the spot welds (a) S1 sample (1000 rpm) and (b) S4 sample (2500 rpm)

Fig. 10 The effect of plunge depth on macrostructure of a friction stir spot-welded AA6082-T6 joint (a) S5 sample (4 mm) and (b) S7 sample (5.5 mm)


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was not observed with the increase of dwell time. In addition, it should be expected that higher dwell time has led to coarser SZ microstructure, i.e., larger grains and coarser strengthening precipitates, due to the increased heat input.

The effect of travel speed on macrostructure of a friction stir spot-welded AA6082-T6 joint can also be seen in Fig. 13. Any significant difference in macrostructure of the joints was not stood up with the increase of travel speed. However, it should be noted

that lower travel speed may result in relatively larger grain size and coarser strengthening precipitates in SZ microstructure due to the relatively higher heat input. Microhardness

The microhardness profile of the FSSWed joints is a direct indicator of microstructural evolution during FSSW process. The transverse hardness profiles on the cross-sections of FSSW joints in precipitate-hardened alloys are mainly governed by thermal

Fig. 11 Optical micrographs of the SZ of the spot welds (a) S5 sample (4 mm) and (b) S7 sample (5.5 mm)

Fig. 12 The effect of dwell time on macrostructure of a friction stir spot-welded AA6082-T6 joint (a) S8 sample (2 s) and (b) S2 sample (7 s)

Fig. 13 The effect of travel speed on macrostructure of a friction stir spot-welded AA6082-T6 joint (a) S14 sample (32 mm/min) and (b) S11 sample (50 mm/min)


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exposure during FSSW28,29. The hardness profile evolution across the weld, covering SZ, TMAZ, HAZ and BM, is shown in Fig. 14. The hardness of the BM is around 105 HV0.3 for the T6 temper condition. It can be clearly seen that the hardness values in the weld zone, including SZ, TMAZ and HAZ, fell dramatically. The lowest hardness values (around 65 HV0.3) for the weld zone were observed in HAZ owing to the significantly over aging due to frictional heat during welding process. A hardness drop in HAZ is roughly 38% when compared with BM hardness. The hardness values in SZ of the spot weld were higher than that in the TMAZ and HAZ owing to the finer grain structure, comparatively finer precipitates and some degree of work hardening (intensive stirring) during FSSW process (see Figs 7 and 14)30,31. Some of the precipitates in the SZ may have also taken into solution during process, then reprecipitated as finer during cooling, it might follow the increase in hardness in SZ29,32. On the other hand, the hardness of TMAZ was higher than that of HAZ due to the high

density of dislocations induced by the plastic deformation during FSSW process31,33,34.

The effect of welding parameters, i.e., rotational speed, plunge depth, dwell time and travel speed, on microhardness profile of a FSSWed AA6082-T6 joint can be seen in Figs 15-18. The microhardness values in the weld zone (SZ, TMAZ and HAZ) was reduced with increasing rotational speed due to the increased thermal softening (grain coarsening, dissolution and over aging) (see Fig. 15). From the microhardness values in Fig. 15, it can be said that SZ size increases with the increase of rotational speed owing to higher input. The plunge depth did not show any significant effect on the hardness of the weld zone, except the SZ near TMAZ (see Fig. 16). The hardness in the SZ near TMAZ at larger plunge depth was fairly higher. This may be attributed to the age-hardening effect and effective material mixing (plastic deformation) at the larger plunge depth during the FSSW process. The HAZ hardness at larger plunge depth was also slightly lower. Additionally, in Fig. 16, it can be seen that SZ size at larger plunge depth was noticeably larger

Fig. 14 A typical microhardness profile of a friction stir spot-welded AA6082-T6 joint. (S1 sample)

Fig. 15 The effect of rotation speed on microhardness profileof a FSSWed friction stir spot-welded AA6082-T6 joint. (S1 sample: 1000 rpm; S4 Sample: 2500 rpm)

Fig. 16 The effect of plunge depth on microhardness profile of a friction stir spot-welded AA6082-T6 joint. (S5 sample:4 mm; S7 sample: 5.5 mm)

Fig. 17 The effect of dwell time on microhardness profile of a friction stir spot-welded AA6082-T6 joint (S8 sample:2 s; S2 sample:7 s)


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(see Fig. 16). Similar to the effect of plunge depth, the hardness in the SZ near TMAZ at higher dwell time was higher due to relatively effective age-hardening effect during the FSSW process (see Fig. 17). In addition, it can be seen that higher dwell time resulted in wider SZ due to the higher heat input. At lower travel speed, the hardness values in weld zone (SZ, TMAZ and HAZ) were relatively lower and SZ size was wider owing to relatively higher input (see Fig. 18).

Tensile shear performance To determine the quality of the friction stir spot

welds, the tensile shear loads of the joints were experimentally determined using tensile tests in our previous study35,36. The tensile shear performance of spot-welded joints is a significant factor in vehicle design37,38. The tensile test results obtained from our previous study exhibited that the welding parameters significantly affected the tensile shear performance of the FSSWed AA6082-T6 joints35,36 (see Table 4 and Fig. 19). The tensile shear load of the FSSWed heat treatable AA6082-T6 aluminum alloys decreased almost

linearly with increasing rotational speed (see Fig. 19a). The tensile shear load of the spot-welded joints decreased roughly 19% when the rotational speed was increased from 1000 rpm to 2500 rpm. This decrease in tensile shear load of the spot welds is primarily

Fig. 19 — The effect of welding parameters on tensile shear load of a friction stir spot-welded AA6082-T6 joint35,36 (a) rotational speed,(b) plunge depth, (c) dwell time and (d) travel speed

Table 4 Tensile shear load values of the friction stir spotwelded joints35,36 (average values)

Sample

Tensile shear load (kN)

Standard deviation

S1 7.89 0.35 S2 7.01 0.60 S3 6.92 0.69 S4 6.39 0.73 S5 6.46 0.33 S6 7.08 0.13 S7 7.27 0.48 S8 4.87 0.05 S9 5.00 0.28 S10 6.75 0.27 S11 3.21 0.12 S12 3.37 0.30 S13 3.76 0.14 S14 4.55 0.08

Fig. 18 The effect of travel speed on microhardness profile of a friction stir spot-welded AA6082-T6 joint (S14 sample: 32 mm/min; S11 sample: 50 mm/min)


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attributed to the increased thermal softening in SZ (grain and precipitate coarsening) and lower thickness of the upper sheet underneath the shoulder owing to the higher input although higher rotational speed resulted in lower hook height. Ohashi et al.39 and Xie et al.40 stated that the mechanical properties of the FSSW joints are directly related to both the bonded section size and the thickness of the upper sheet underneath the shoulder. It should also be noted that precipitate-free zones (PFZ) around the grain boundaries arising from precipitate coarsening might have reduced the strength of SZ. The relationship between tensile shear load and the plunge depth of FSSW joints can be seen in Fig. 19b. The tensile shear loads of the joints increased almost linearly with increasing plunge depth. The tensile shear load of the spot welded joints increased approximately 13% when plunge depth was increased from 4 mm to 5.5 mm. This increase can be attributed to larger fully bonded section size and lower hook height. However, it should also be noted that the maximum plunge depth is constrained by the thickness of the sheets17. The effect of dwell time on the tensile shear load of FSSW joints is shown in Fig. 19c. The tensile shear loads of the joints increased almost linearly with increasing dwell time. The tensile shear load of the spot welded joints increased roughly 44% when dwell time was increased from 2 s to 7 s. This increase can be associated especially with larger fully bonded section size. It should also be noted that the tensile shear load of the spot welds decreased above dwell time of 7 s. In addition, it can be said that dwell time is the most effective parameter on strength of the FSSW AA6082-T6 aluminum alloy joints in this research. On the other hand, when the dwell time was zero, the tensile shear loads of the spot welded joints were fairly lower values (see Fig. 19d). The effect of travel speed on the tensile shear load of FSSW joints can also be seen in Fig. 19d. Although the lower travel speed resulted in lower microhardness values in weld zone, the tensile shear loads of joints decreased almost linearly with increasing travel speed. The tensile shear load of the spot welded joints decreased approximately 30% when the travel speed was increased from 32 mm/min to 50 mm/min. This may be attributed to improper stirring action around the tool pin due to insufficient plasticization of the base metals owing to the lower heat input at higher travel speed. Fracture surfaces

Under the tensile loading conditions of the FSSW joints, the crack initiates in the partially bonded region and propagates in the bonded section, so the

final fractural location of the FSSW joints may occur 2 possible crack extending paths in the SZ (Fig. 20).

The macro SEM images for the fracture surfaces of the top view of the lower sheet of the joints are given in Fig. 21. The bonded sections, which correspond to the SZs, can be clearly seen in Fig. 21. There was not any significant difference in fully bonded section size with the increase of rotational speed (see Fig. 21 (a) and (b)). Larger plunge depth, higher dwell time and lower travel speed resulted in relatively larger fully bonded section size due to the higher heat input. The fracture surface of the joint with the highest travel speed exhibited partially bonded section with the propagated radial cracks owing to improper flow and insufficient plasticization of the base metals (see Fig. 21h). Therefore, this spot welded joint had the lowest tensile shear load with 3.21 kN.

The micro SEM images of the fracture surfaces of the bonded section of the FSSW joints are given in Figs 22-25. The fracture surfaces of the bonded sections of the joints with relatively lower heat input, which correspond to SZs that consist of typical very fine grains, exhibited intergranular fracture, and so, reflected the SZ grain structure (see Figs 22a, 23a, 24a and 25b). The fracture surfaces of these joints were relatively brittle with cleavage fracture. In addition, brittle partially bonded section with the propagated radial cracks can be seen in the fracture surface of the joint with the highest travel speed, indicating the low quality metallurgical bonding in this zone (see Fig. 25c). On the other hand, a higher volume fraction of small voids and elongated dimples of various sizes in the loading direction were observed in the fracture surfaces of the bonded

Fig. 20 Possible fractural locations of a FSSW joint (S1 sample)


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sections of the joints with relatively higher heat input, which indicates that the failure is the result of a ductile fracture (see Figs 22b, 23b, 24b and 25a). Higher rotational speed, higher plunge depth, higher

dwell time and lower travel speed resulted in relatively ductile fracture and led to the coarse grain structure in the fracture surface of the SZ due to the higher heat input. Thus, it may be concluded that

Fig. 21 Macroscopic SEM images for the fracture surfaces of the joints (a) S1, (b) S4, (c) S5, (d) S7, (e) S8, (f) S2, (g) S14 and (h) S11


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these joints failed with higher deformation and behaved in a more ductile manner.

SZ hardness has taken as basis for relations between the results of hardness, tensile shear performance and fracture behavior, since all the fractures of the joints during tensile testing occurred at the SZ, where the bonded section was minimum. In general, lower hardness in SZ resulted in lower tensile performance. But, as mentioned earlier, higher travel

speed resulting in higher hardness in SZ and lower tensile performance gave rise to improper stirring action around the tool pin due to insufficient plasticization. However, the main effective factor on the tensile performance of FSSW joints is primarily the bonded section size. If the bonded section size of a FSSW joint is the same, then other factors come into play in, such as thickness of the upper sheet underneath the shoulder, thermal softening in SZ,

Fig. 22 – Microscopic SEM images for the fracture surfaces of the bonded section of the FSSW joints (a) S1 sample (1000 rpm) and(b) S4 sample (2500 rpm)

Fig. 23—Microscopic SEM images for the fracture surfaces of the bonded section of the FSSW joints (a) S5 sample (4 mm) and(b) S7 sample (5.5 mm)

Fig. 24 – Microscopic SEM images for the fracture surfaces of the bonded section of the FSSW joints (a) S8 Sample (2 s and,(b) S2 sample (7 s)


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hook height and improper stirring action. So, as a result of the obtained results in this study, although there is no clear relation between hardness and fracture behavior or tensile shear performance and fracture behavior, a good relationship between heat input and fracture behavior could be established. As known, heat input during FSSW can be identified to be the best parameter index to correlate with post-weld properties. Higher heat input resulted in relatively ductile fracture behavior.

Conclusions The microstructure and mechanical properties of the

friction stir welded joints of the heat-treatable AA6082-T6 aluminum alloy in different welding conditions are investigated. The following conclusions are drawn:

(i) Higher rotational speed resulted in wider SZ, coarser microstructure in the SZ, lower hook height and broken hook curvature, but did not noticeably affect the bonded section size. Larger plunge depth led to larger bonded section size, lower hook height, wider SZ and coarse SZ microstructure. Higher dwell time resulted in larger bonded section size, wider SZ and coarse SZ microstructure, but did not affect the hook height. Travel speed did not noticeably affect the macrostructure of the joints.

(ii) FSSW process of the heat treatable AA6082-T6 aluminum alloy significantly decreased the hardness in the weld zone, including SZ, TMAZ and HAZ. HAZ had the lowest hardness. The hardness in SZ was higher than that in the TMAZ and HAZ.

(iii) Higher rotational speed and lower travel speed resulted in the lower hardness in the weld zone. Except the SZ near TMAZ, the plunge depth and dwell time did not significantly affect the hardness in the weld zone. Larger plunge depth and higher dwell time led to higher hardness in the SZ near TMAZ.

(iv) The tensile shear loads of the FSSWed heat treatable AA6082-T6 aluminum alloys decreased almost linearly with increasing rotational speed or travel speed. On the other hand, the tensile shear loads of the joints increased almost linearly with increasing plunge depth or dwell time. In addition, dwell time was the most effective parameter on strength of the FSSW AA6082-T6 aluminum alloy joints.

(v) Higher rotational speed, higher plunge depth, higher dwell time and lower travel speed resulted in relatively ductile fracture due to the higher heat input. On the other hand, lower rotational speed, lower plunge depth, lower dwell time and higher travel speed resulted in relatively brittle with intergranular cleavage fracture due to the lower heat input.

Acknowledgement The authors are deeply grateful to the Uludag

University Scientific Research Fund (BAP) for its financial support to this research (Project Contract No. OUAP MH 2014/24).

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Effect of welding parameters on microstructure and mechanical properties of aluminum ...nopr.niscair.res.in/bitstream/123456789/42931/1/IJEMS 24... · 2017-10-13 · Indian Journal

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