The optimisation of process parameters for friction stir spot-welded AA3003-H12 aluminium alloy using a Taguchi orthogonal array

The aim of the present work is to optimise the welding parameters for friction stir spot welded non-heat-treatable AA3003-H12 aluminium alloy sheets using a Taguchi orthogonal array. The welding parameters, such as the tool rotational speed, tool plunge depth and dwell time, were determined according to the Taguchi orthogonal table L9 using a randomised approach. The optimum welding parameters for the peak tensile shear load of the joints were predicted, and the individual importance of each parameter on the tensile shear load of the friction stir spot weld was evaluated by examining the signal-to-noise ratio and analysis of variance (ANOVA) results. The optimum levels of the plunge depth, dwell time and tool rotational speed were found to be 4.8 mm, 2 s and 1500 rpm, respectively. The ANOVA results indicated that the tool plunge depth has the higher statistical effect with 69.26% on the tensile shear load, followed by the dwell time and rotational speed. The tensile shear load of the friction stir spot welding (FSSW) joints increased with increasing plunge depth. Additionally, examination of the weld cross-sections, microhardness tests and fracture characterisation of the selected friction spot welded joints were conducted to understand the better performance of the joints. All the fractures of the joints during tensile testing occurred at stir zone (SZ), where the bonded section was minimum. The tensile shear load and tensile deformation of the FSSW joints increased linearly with increasing the bonded size. The finer grain size in the SZ led to the higher hardness, which resulted in higher fracture strength. When the tensile shear load of the joints increased approximately 3-fold, the failure energy absorption of the joints increased approximately 15-fold.     

Defect features and mechanical properties of friction stir lap welded dissimilar AA2024–AA7075 aluminum alloy sheets

5 mm-Thick dissimilar AA2024-T3 and AA7075-T6 aluminum alloy sheets were friction stir lap welded in two joint combinations, i.e., (top) 2024/7075 (bottom) and 7075/2024. The influences of process conditions (welding speed and joint combination) on defects (hook and voids) features and mechanical properties of joints were investigated in detail. It was found that the hook deflects largely upwards into the stir zone (SZ) at lower welding speeds (50, 150 mm/min) in both combinations. The process conditions significantly affect the hook geometry which in return affects the lap shear strength. In all 2024/7075 joints, voids appear and the joints fracture from the tip of hook on AS along the SZ/TMAZ (thermomechanically affected zone) interface in lap shear test (tensile fracture mode). In 7075/2024 joints, the hook on RS horizontally extends a large distance into the bottom stir zone at higher welding speeds (225, 300 mm/min). The joints fracture in three modes: shear fracture along the lap interfaces, tensile fracture and the mix fracture of both. In both joint combinations, the lap shear strength generally increases with the increase of welding speed. 7075/2024 Joints show higher failure load than 2024/7075 joints at lower welding speeds while the opposite result appears at higher welding speeds.     

Effect of tool rotational speed and pin profile on microstructure and tensile strength of dissimilar friction stir welded AA5083-H111 and AA6351-T6 aluminum alloys

The relatively new welding process friction stir welding (FSW) was applied in this research work to join 6 mm thick dissimilar aluminum alloys AA5083-H111 and AA6351-T6. The effect of tool rotational speed and pin profile on the microstructure and tensile strength of the joints were studied. Dissimilar joints were made using three different tool rotational speeds of 600 rpm, 950 rpm and 1300 rpm and five different tool pin profiles of straight square (SS), straight hexagon (SH), straight octagon (SO), tapered square (TS), and tapered octagon (TO). Three different regions namely unmixed region, mechanically mixed region and mixed flow region were observed in the weld zone. The tool rotational speed and pin profile considerably influenced the microstructure and tensile strength of the joints. The joint which was fabricated using tool rotational speed of 950 rpm and straight square pin profile yielded highest tensile strength of 273 MPa. The two process parameters affected the joint strength due to variations in material flow behavior, loss of cold work in the HAZ of AA5083 side, dissolution and over aging of precipitates of AA6351 side and formation of macroscopic defects in the weld zone.     

Influence of a non-rotating shoulder on heat generation,microstructure and mechanical properties of dissimilar AA2024/AA7050 FSW joints

Friction stir welding (FSW) and stationary shoulder friction stir welding (SSFSW) were carried out for the butt joining of dissimilar AA2024-T3 and AA7050-T7651 aluminium alloys with thicknesses of 2 mm. A comparison between the two processes was performed by varying the welding speed while keeping the rotational speed constant. Through the analysis of the force and torque produced during welding and a simple analytical model, it was possible to show that in SSFSW there is more effective coupling with the tool and the heat produced is more efficiently distributed. This process decreases both the welding area and the diffusion at the interface of the two alloys compared with FSW. The minimum microhardness occurred at the advancing side (AS) at the interface between the thermo-mechanically affected zone (TMAZ) and the stir zone (SZ) in both processes, although the decrease was more gradual in SSFSW. This interface is also where all specimens failed for both welding technologies. An increase in tensile strength was measured in SSFSW compared with standard FSW. Furthermore, it was possible to establish the mechanical performance of the material in the fracture zone using digital image correlation.     

Effect of welding parameters on microstructure and mechanical properties of AA6061-T6 butt welded joints by stationary shoulder friction stir welding

Stationary shoulder friction stir welding (SSFSW) butt welded joints were fabricated successfully for AA6061-T6 sheets with 5.0 mm thickness. The welding experiments were performed using 750–1500 rpm tool rotation speeds and 100–300 mm/min welding speeds. The effects of welding parameters on microstructure and mechanical properties for the obtained welds were discussed and analyzed in detail. It is verified that the defect-free SSFSW welds with fine and smooth surface were obtained for all the selected welding parameters, and the weld transverse sections are obviously different from that of conventional FSW joint. The SSFSW nugget zone (NZ) has “bowl-like” shapes with fairly narrow thermal mechanically affected zone (TMAZ) and heat affected zone (HAZ) and the microstructures of weld region are rather symmetrical and homogeneous. The 750–1500 rpm rotation speeds apparently increase the widths of NZ, TMAZ and HAZ, while the influences of 100–300 mm/min welding speeds on their widths are weak. The softening regions with the average hardness equivalent 60% of the base metal are produced on both advancing side and retreating side. The tensile properties of AA6061-T6 SSFSW joints are almost unaffected by the 750–1500 rpm rotation speeds for given 100 mm/min, while the changing of welding speed from 100–300 mm/min for given 1500 rpm obviously increased the tensile strength of the joint and the maximum value for welding parameter 1500 rpm and 300 mm/min reached 77.3% of the base metal strength. The tensile fracture sites always locate in HAZ either on the advancing side or retreating side of the joints.     

Microstructure and mechanical optimization of probeless friction stir spot welded joint of an Al-Li alloy

In this work, a third generation Al-Li alloy has been successfully spot welded with probeless friction stir spot welding (P-FSSW), which is a variant of conventional friction stir welding. The Box-Behnken experimental design in response surface methodology (RSM) was applied to optimize the P-FSSW parameters to attain maximum tensile/shear strength of the spot joints. Results show that an optimal failure load of 7.83 kN was obtained under a dwell time of 7.2 s, rotation speed of 950 rpm and plunge rate of 30 mm/min. Sufficient dwell time is essential for heat conduction, material flow and expansion of the stir zone to form a sound joint. Two fracture modes were observed, which were significantly affected by hook defect. In addition to mechanical testing, electron backscattering diffraction (EBSD) and differential scanning calorimetry (DSC) were used for microstructure evolution and property analysis. The precipitation of GP zone and Al₃Li as well as the ultrafine grains were responsible for the high microhardness in the stir zone.     

Formation and mechanical properties of stationary shoulder friction stir welded 6005A-T6 aluminum alloy

6005A-T6 aluminum alloy is welded by stationary shoulder friction stir welding (SSFSW). At a constant rotational velocity of 2000 rpm, the effect of welding speed on mechanical properties of SSFSW joint are investigated in detail. Defect-free joint with gloss surface and small flash is attained and no cracks appear at the bending angle of 180°. Compared with traditional friction stir welding (FSW), width of rotational shoulder affected zone is relatively small because of the smaller diameter of rotational shoulder. Increasing welding speed is benefit for reducing the width of softening region and the softening degree. The fracture position of welding joint locates in thermo-mechanically affected zone and the fracture surface morphology presents the typical ductile fracture. The maximum tensile strength of joint at the welding speed of 400 mm/min reaches 82% of base metal (BM).     

Microstructure and tensile strength of friction stir welded joints between interstitial free steel and commercially pure aluminium

In the present study, the joining of interstitial free steel and commercial pure aluminium was carried out by friction stir welding (FSW) technique using tool rotational speeds of 600, 900, 1200 rpm and traverse speed of 100 mm/min. The microstructure and micro-hardness of the weld interface have been investigated. Optical microscopy was used to characterize the microstructures of different regions of friction stir welding joints. The scanning electron microscopy-back scattered electron (SEM-BSE) images show the existence of the different reaction layers in the welded zone. The Al₃Fe intermetallic compound has been observed in the weld interface and their thickness increase with the increase in tool rotational speed. Tensile strength was also evaluated and maximum tensile strength of ∼123.2 MPa along with ∼4.5% elongation at fracture of the joint have been obtained when processed at 600 rpm tool rotational speed.     

Refill friction stir spot welding of 5083-O aluminum alloy

In this work,refill friction stir spot welding(RFSSW) was used to weld 2 mm-thick 5083-O alloy.The Box–Behnken experimental design was used to investigate the effect of welding parameters on the joint lap shear property.Results showed that a surface indentation of 0.3 mm effectively eliminated the welding defects.Microhardness of the stir zone(SZ) was higher than that of the base material(BM) and the hardness decreased with increasing the heat input during welding.The optimum failure load of 7.72 k N was obtained when using rotating speed of 2300 rpm,plunge depth of 2.4 mm and refilling time of 3.5 s.Three fracture modes were obtained during the lap shear test and all were affected by the hook defect.     

Microstructure and mechanical properties as a function of rotation speed in underwater friction stir welded aluminum alloy joints

A 2219-T6 aluminum alloy was underwater friction stir welded at a fixed welding speed and various rotation speeds in order to illuminate the influence of rotation speed on the performance of underwater joints. With increasing rotation speed, the hardness of the stir zone (SZ) gradually increases due to the increase in dislocation density. The tensile strength first increases from 600 to 800 rpm and then reaches a plateau in a wide rotation speed range. After that a remarkable decrease in tensile strength occurs owing to the formation of void defect. The joint welded at lower rotation speed tends to be fractured in the SZ. At higher rotation speeds, the hardness increase in the SZ makes the fracture locations of defect-free joints move to the thermal-mechanically affected zone (TMAZ) or heat affected zone (HAZ).     

Effects of tool rotation speed on microstructures and mechanical properties of AA2219-T6 welded by the external non-rotational shoulder assisted friction stir welding

The external non-rotational shoulder assisted friction stir welding (NRSA-FSW) was applied to weld high strength aluminum alloy 2219-T6 successfully, and effects of the tool rotation speed on microstructures and mechanical properties were investigated in detail. Defect-free joints were obtained in a wide range of tool rotation speeds from 600 rpm to 900 rpm, but cavity defects appeared on the advancing side when the tool rotation speed increased to 1000 rpm. The microstructural deformation and heat generation were dominated by the rotating tool pin and sub-size concave shoulder, while the non-rotational shoulder helped to improve the weld formation. Microstructures and Vickers hardness distributions showed that the NRSA-FSW is beneficial to improving the asymmetry and inhomogeneity, especially in the weld nugget zone (WNZ). At the tool rotation speed of 800 rpm, both the tensile strength and the elongation reached the maximum, and the maximum tensile strength was up to 69.0% of the base material. All defect-free joints were fractured at the weakest region with minimum Vickers hardness in the WNZ, while for the joint with cavity defects the fracture occurred at the defect location.     

Fatigue behavior of friction stir weld joints of Al–Zn–Mg alloy AA7039 developed using base metal in different temper condition

This paper presents the influence of base metal temper conditions (O, W and T6) on fatigue behavior of friction stir weld joints of Al–Zn–Mg alloy AA7039. Fatigue tests were performed at stress ratio of 0.1 and tensile to tensile stress in the range of 85–215 MPa to develop S–N curves. Fractured surfaces were investigated by SEM to determine the mode of fracture. On the basis of this study, friction stir welding of Al–Zn–Mg alloy AA7039 is recommended to be performed in W temper condition because of better tensile properties and superior fatigue strength.     

EBSD study on microstructure and texture of friction stir welded AA5052-O and AA6061-T6 dissimilar joint

Friction stir welded AA5052-O and AA6061-T6 dissimilar joint has a more obvious impact on microstructure and texture evolution compared to single material welding due to differences in physical and chemical parameters between two aluminum alloys. Microstructure, texture evolution and grain structure of AA5052-O and AA6061-T6 dissimilar joint were investigated by means of OM,EBSD and TEM measurements. Experimental results showed that FS weld was generalized in four regions–nugget zone (NZ),thermomechanically affected zone (TMAZ),heat affected zone (HAZ) and base metals (BM), using standard nomenclatures. NZ exhibited the complex structure of the two materials with flowing shape and mainly composed of the advancing side material Subgrain boundaries in weld nugget zone gradually transformed into high angle grain boundaries by absorbing dislocation and accumulating misorientations. Grain refinement of weld nugget zone was achieved by dynamic recrystallization. In the friction stir welding process, the presence of the shear deformation in weld made {001} < 100 > C cube texture, {123} < 634 > S texture in BM gradually transformed into {111} < 1(−)12(−) > A1¹ shear texture. HABs distribution were most significant in nugget followed by RS and then by AS. In TMAZ and NZ, numerous precipitates and lots of dislocations were observed.     

Effects of sleeve plunge depth on microstructures and mechanical properties of friction spot welded alclad 7B04-T74 aluminum alloy

Friction spot welding (FSpW) was applied to join the 7B04-T74 aluminum alloy successfully, and effects of sleeve plunge depth on weld appearance, microstructures and mechanical properties were investigated in detail. When the sleeve plunge depth was larger than 2 mm, a surface indentation with a depth of 0.2 mm should be applied in order to eliminate the defect of annular groove. The tensile shear properties of the joints were dependent on hook geometry, location of alclad layer, and hardness of stir zone (SZ). With increasing the sleeve plunge depth from 2 to 3.5 mm, the hook height increased, the alclad layer downward migrated further and the hardness of SZ decreased. The optimized FSpW joint was obtained when the sleeve plunge depth was 3 mm, and the corresponding tensile shear failure load was 11921 N. Two different failure modes, i.e. shear fracture mode and tensile-shear mixed fracture mode, were observed in the tensile shear tests.     

Investigations on the effects of friction stir welding parameters on intermetallic and defect formation in joining aluminum alloy to mild steel

Joints of Al 5186 to mild steel were performed by using friction stir welding (FSW) technique. The effects of various FSW parameters such as tool traverse speed, plunge depth, tilt angle and tool pin geometry on the formation of intermetallic compounds (IMCs), tunnel formation and tensile strength of joints were investigated. At low welding speeds due to the formation of thick IMCs (which was characterized as Al₆Fe and Al₅Fe₂) in the weld zone the tensile strength of joints was very poor. Even at low welding speeds the tunnel defect was formed. As the welding speed increased, the IMCs decreased and the joint exhibited higher tensile strength. The tunnel defect could not be avoided by using cylindrical 4 mm and 3 mm pin diameter. By using a standard threaded M3 tool pin the tunnel was avoided and a bell shape nugget formed. Therefore tensile strength of the joint increased to 90% of aluminum base alloy strength. At higher welding speed and lower tool plunge depth, the joint strength decreased due to lack of bonding between aluminum and steel. Based on the findings, a FSW window has been developed and presented.     

Evaluation of microstructures and mechanical properties of friction stir welded lap joints of Inconel 600/SS 400

The microstructures and mechanical properties of friction stir welded Inconel 600 and SS 400 lap joints were evaluated in this study. Friction stir welding was carried out at a tool rotation speed of 200 rpm and a welding speed of 100 mm/min. Application of friction stir welding was notably effective in reducing the grain size of the stir zone, as a result, the average grain size of Inconel 600 was reduced from 20 μm in the base material to 8.5 μm in the stir zone. The joint interface between Inconel 600 and SS 400 was soundly welded without voids and cracks, and MC carbides with a size of 50 nm were partially formed in the region of the lap joint interface in Inconel 600. In addition, a hook from SS 400 was formed on the advancing side of the Inconel 600 alloy, which directly affected an increase in the peel strength of the weld. In this study, we systematically discussed the effect of friction stir welding on the evolution of the microstructures and mechanical properties of friction stir lap jointed Inconel 600 and SS 400.     

工艺参数对AA2524回填式搅拌摩擦点焊接头成形及力学性能的影响

目的 改善2 mm厚AA2524回填式搅拌摩擦点焊（RFSSW）接头的成形和力学性能。方法 采用不同扎入深度和不同转速进行焊接,焊后对接头的孔洞大小、套筒退出线上的裂纹长度和拉剪强度进行对比,获得扎入深度和转速对接头成形和拉剪强度的影响规律。结果 扎入深度为2.6 mm时,接头孔洞较小,断裂载荷较高。随着转速的增加,接头孔洞逐渐减小,套筒退出线的裂纹减少,拉剪强度逐渐升高。扎入深度2.6 mm,焊接时间6 s,转速为2600 r/min时,获得了无孔洞和无退出线裂纹的接头,断裂载荷最大,为9.4 kN。结论 合适的扎深和转速匹配,可以获得成形良好,高断裂载荷的接头。     

Influence of probe offset distance on interfacial microstructure and mechanical properties of friction stir butt welded joint of Ti6Al4V and A6061 dissimilar alloys

Friction stir butt welding of titanium alloy Ti6Al4V and aluminum alloy A6061-T6 with 2 mm thickness was conducted by offsetting probe edge into the titanium alloy at rotation speed of 750 rpm and 1000 rpm and welding speed of 120 mm/min. The effect of probe offset distance on the interfacial microstructure and mechanical properties of the butt joint was investigated. When the probe offset distance is not sufficient, the two alloys cannot be completely joined together, i.e. there exists no bonding or kissing bonding at the root part of joint interface. However, when the probe offset distance is too large, a great amount of intermetallic compounds are formed at the joint interface and its adjacency, leading to fracturing roughly along the joint interface during a tensile test. In a proper range of probe offset distance, sound dissimilar butt joints are produced, which have comparatively high tensile strength and fracture in heat affected zone of the aluminum alloy during a tensile test.     

Improving mechanical properties of pinless friction stir spot welded joints by eliminating hook defect

Hook defect (HD) seriously decreases the mechanical properties of friction stir spot welded (FSSW) joints. In this study, two methods were therefore used to eliminate the HD in pinless FSSW joints. The one is changing welding parameters such as rotating speed and dwell time. The other one is FSSW plus subsequent friction stir welding (FSSW-FSW), which is an innovative method proposed in this study. Experimental results showed that the HD in pinless FSSWed AA2024 joints was successfully eliminated by using FSSW-FSW, not by changing process parameters. The joints without HD exhibited a tensile–shear load of as much as 12 kN, which was higher than that of 6.9 kN in the joints with HD. Furthermore, it was proved that the tensile–shear load is not greatly improved only by increasing the nugget zone when HD still existed in the FSSW joints. In addition, the fracture morphology analysis demonstrated that the shear fracture of the FSSW-FSW joints took place along the boundary between the upper and lower sheets through the weld nugget, and the faying surface between the two sheets was completely sheared off.     

Tensile behavior of friction stir welded AA 6061-T4 aluminum alloy joints

In this investigation response surface methodology based on a central composite rotatable design with three parameters, five levels and 20 runs, was used to develop a mathematical model predicting the tensile properties of friction stir welded AA 6061-T4 aluminum alloy joints at 95% confidence level. The three welding parameters considered were tool rotational speed, welding speed and axial force. Analysis of variance was applied to validate the predicted model. Microstructural characterization and fractography of joints were examined using optical and scanning electron microscopes. Also, the effects of the welding parameters on tensile properties of friction stir welded joints were analyzed in detail. The results showed that the optimum parameters to get a maximum of tensile strength were 920 rev/min, 78 mm/min and 7.2 kN, where the maximum of tensile elongation was obtained at 1300 rev/min, 60 mm/min and 8 kN.