Microstructure Characteristics And Mechanical Properties Of Dissimilar Friction Stir Welded 7003/6060 Aluminum Alloy Joints

Aluminum alloys have been widely used in various industrial fields due to their advantages,such as light weight,high specific strength,ease of processing and recycling.6xxx and 7xxx aluminum alloys are two series of precipitation hardening alloys with broad application.With the development of lightweight design in automotive industry,there are many potential applications in welding of 7xxx/6xxx aluminum alloys,such as bumper beams and battery bracket.Since the weldability are relatively poor,7xxx aluminum alloys are generally considered as un-weldable in fusion welding processes.Therefore,applications of assemblies containing 7xxx series high-strength aluminum alloys are restricted.As a solid-state welding technique,friction stir welding(FSW)has high energy efficiency and the temperature during processing is below the melting point of alloys.Hence many fusion welding defects,including gas pores,segregation and hot cracking can be avoided.Due to these advantages,FSW has provided effective method for the welding of unweldable metals and dissimilar alloys.In this study,7003-T4 and 6060-T4 Al alloys were used as base materials,and 7003 FSW joints and 7003/6060 dissimilar FSW joints were produced in air and underwater.Microstructure evolution and mechanical properties were investigated and effect of post weld heat treatment(PWHT)on joint misstructure and properties was studied,which were aiming to provide theoretical base and technology for improving joint properties by microstructure control.Firstly,7003 Al alloy was FSWed in air by using different process parameters and the optimum parameter was obtained.The highest tensile strength of the joint was achieved at rotational speed of 800 rpm and welding speed of 40 mm/min(designated as 800-40),which was 340.4 MPa,82.5 % of the BM strength.Under the 800-40 condition,the peak temperature was 504 ℃,and the temperature of the region 30 mm away from welding center was still high enough to dissolve GP zone.The temperature on the advancing side(AS)was a little higher than corresponding locations on the retreating side(RS).In heat affected zone(HAZ),in the interior of the grains,GP had dissolved first and then η’ phase and GP were formed during subsequent processing and cooling stages.On the grain boundaries,pre-existing η’ phase was transformed into η phase.In the stir zone(SZ),elongated grains of BM were replaced by newly-formed equiaxed grains due to dynamic recrystallization(DRX);the precipitation was dissolved completely and then the reprecipitation of η’ and η phases occurred.After direct artificial ageing(AA)and T6 heat treatment,the joints strength was both improved.The tensile strength of joints were 352.3 MPa and 371.2 MPa,and joint strength efficiency were 85.3% and 89.9%,respectively.The joints fractured on HAZ and SZ,respectively.AA heat treatment promoted the precipitation of GP and transformation of GP to η’ phase,and the pre-existing η’ phase transformed into η phase.Coarse precipitates on grain boundary and fine precipitates in interior of the grain coexisted.During T6,the precipitates were completely dissolved and then fine and uniformly-distributed η’ phase was reprecipitated.Secondly,based on the study of the 7003 FSW,7003/6060 dissimilar Al alloys were FSWed(Dissimilar Friction Stir Welding,DFSW).The highest tensile strength of 7003/6060 DFSW joint was 159.2 MPa,78.3 % of BM,and the elongation was 10.4 %.In SZ,the intermixing of two BMs was limited,and this region was mainly composed of 7003,which was placed on AS.During welding,the precipitates were completely dissolved and then η’ and η phases were reprecipitated during subsequent cooling,while the number of these precipitates was small.In HAZ on AS,GP zone dissolved and η’ and η phases were formed.In HAZ on RS,the transformation of GP and β’’ to β’ phase and the density of precipitates decreased.Fracture took placed at the soft region of HAZ on RS,which was formed due to the coarsening and reducing of precipitation and grain growth.In the thermo-mechanical affected zone(TMAZ),equiaxed fine grains and elongated grains coexisted due to incomplete DRX.Thirdly,7003/6060 Al alloys were FSWed underwater(underwater dissimilar FSW,UDFSW).Under parameter of 100-120,the highest joint strength was achieved,which was 183.9 MPa and the elongation was 12.6 %.Water cooling made soft region closer to weld center,where more and finer precipitates were formed as compared with air cooling DFSW joint,and this was the main reason for the improvement of the joint strength.In SZ,fine grains were formed by continuous dynamic recrystallization(CDRX).In TMAZ,dynamic recovery and partial DRX took place on the advancing side of 7003;dynamic recovery was the dominant evolution mechanism on the retreating side of 6060,and the fraction of low angle grain boundary was increased.The textural components of BM 7003 were weak Cube {100}<001>,moderate E {111}<110> and strong texture {223}<122>,while 6060 had Cube {100}<001> texture.Simple shear texture B/B?{112}<110> was formed in the SZ.In HAZ,texture components were kept similar as compared with BM.Cooling condition had an obvious effect on grain structure and texture evolution.In UFSW joint,the effect of geometric strain and grain subdivision became more obvious.Recovery after welding in SZ of air cooling joint took place,and fiber texture with <110> parallel to the shearing direction was formed.Local strain distribution had an important effect on grain evolution,which had obvious relationship with grain size and determined the grain evolution mechanism to a certain extent.Finally,PWHT was conducted for the UDFSW 7003/6060 joints to further improve the mechanical properties.As to solution treatment and artificial aging(STA),the highest tensile strength was increased to 254.0 MPa,which was 124.9 % of BM 6060,and the elongation was 12.7%.High-density and uniformly-dispersed β’’ and GP,which reprecipitated during STA,offered the highest strength.Moreover,since the soft region was eliminated,the deformation was more uniform on RS during tensile test,which was the main reason for the large elongation.For AA joints,the highest tensile was 218.9 MPa,which was 107.6 % of BM,and elongation was 4.4%.During AA heat treatment,although pre-existing β’ phase grown coarse to some extent,more GP and β’’ were formed in the soft region.Therefore,AA joint strength was improved,while elongation was deteriorated.