Study On Microstructure And Properties Of Al/Mg Dissimilar Alloy Fsw Joint With Low Heat Input

Lightweight and energy saving technology has become the inevitable trend of the future development of rail transportation equipment manufacturing industry.The bogie and car shell of permanent magnet rail transit system are made of aluminum alloy.In order to further reduce the weight of permanent magnetic levitation train and reduce energy consumption,it is a reliable way to achieve energy saving and emission reduction in rail transit industry to use lightweight magnesium alloy material to partially replace aluminum alloy.Friction Stir Welding(FSW)is a solid phase joining technology invented by the Welding Institute(TWI)in 1991.When welding Mg/Al dissimilar metals,Low welding temperature reduces IMC generation in joints,and provide a potential way to obtain high quality joints.In this paper,FSW docking technology with low heat input is used for 3 mm AZ31magnesium alloy and AA2024-T3 aluminum alloy were welded,and the effects of shoulder diameter,welding speed and rotation speed on joint forming were studied.The distribution of intermetallic compound(IMC)in Mg/Al dissimilar alloy FSW joint,the formation mechanism of IMC and fracture behavior of joint were analyzed,the effect of Ni as interlayer on IMC distribution and properties of Mg/Al FSW joint was analyzed.The details are as follows:The results show that the optimum parameters for FSW process of Mg/Al dissimilar alloy with low heat input are as follows:stirring head diameter is 10 mm,stirring needle top diameter is 2.8 mm,cone angle is 14°and needle length is 2.8 mm.The optimum process parameters are rotation speed 500 rpm,welding speed 500 mm/min,maximum tensile strength 68 MPa and maximum yield strength 62 MPa.The IMC with thickness of 1.4?m was observed by scanning electron microscope(SEM),and the main component was Mg₁₇Al₁₂.The numerical simulation results show that the peak temperature of the welding process is 430?and the time to keep above 400?is only 5 s.It can be determined that Mg₁₇Al₁₂ is formed by mg Al eutectic.The fracture analysis shows that the fracture mode is brittle fracture,which is mainly caused by the brittle IMC in the joint.The effect of bias on FSW under low heat input was studied by biasing both sides of magnesium and aluminum respectively.It was found that the mechanical properties of the joint increased the most when biasing 0.5 mm to the aluminum side.The tensile strength reached 82.8 MPa,increased by 21.9%,and the yield strength was 73.1 MP,increased by16.8%.The thickness of IMC layer is about 1.1?m and the main component is Mg₁₇Al₁₂.The fracture of the joint is on the interface of nugget zone,and the fracture mode is brittle fracture.The effect of Ni on FSW joint of Mg/Al dissimilar alloy was studied by adding Ni as the intermediate layer.The discontinuous distribution of Ni in the joint was on the Al Mg interface,IMC Mg₁₇Al₁₂in the nondistributed Ni layer,no mg Al IMC and micro Ni-Al IMC in the position of Ni layer.