Microstructure And Properties For Lap Weld Of Friction Stir Welding For Mg/Al Dissimilar Alloys

As the light-alloy materials,Mg and Al alloy have many advantages such as lowdensity and high specific strength stiffness.They are widely applied in engineering,aviation,aerospace and automotive manufacturing.It's very easy to form defects such as Mg/A1 intermetallic compounds,cracks and air vents in the traditional fusion welding process of Mg and Al alloy,which will deteriorate the performance of joints,difficult to get a satisfactory joint.As a new type of solid state welding method,friction stir weld can effectively improve performance of joint.In this paper,3mm AZ31B Mg and 6061 Al alloy were welded by the method of friction stir lap welding and the mechanical properties and microstructure of welded joints were studied.The influence of different welding parameters?rotation speed,welding speed,stirring pin lenth?on the microstructure and mechanical properties of Mg/Al joints was studied.The microstructure and mechanical properties of Mg/Al joint after adding copper foil or zinc foil to the interface of magnesium/aluminum welding joint were studied.The conclusion is as follows:?1?The length of the stirring pin and the selection of welding parameters have great influence on the surface forming and mechanical properties of 3mm AZ31B Mg and6061 Al alloy friction stir lap welding.When the rotation speed and welding speed are too low,such defects as surface furrow,air vents and crack are easy to form.When selecting the appropriate stirring pin lenth and welding parameters,there is no obvious defect on the weld surface.When the length of the stirring pin is 2.8mm,the rotation speed is 1350r/min,and the welding speed is 50mm/min,the maximum tensile shear strength of the joint can reach 5648.364N.The junction of Mg/Al welding interface produces a transition zone,which is different from two kinds of parent materials.The transition zone is mainly composed of Al₁₂Mg₁₇ and Al₃Mg₂,and the content of Mg element in the transition zone is greater than that of Al element.?2?After adding 0.01mm copper foil to lap interface of friction stir lap welding of3mm AZ31B Mg and 6061 Al alloy,the mechanical properties of the joints improved.The maximum tensile shear strength of the welded joint can reach 6022.078N when the stirring pin length is 3.0mm,the rotation speed is 900r/min,and the welding speed is 50mm/min.The added Cu elements are dispersed in the whole interface of the joint,and the joints detect Al₂Mg,Al₃Mg₂,Al₆CuMg₄,Al₂Cu,Mg₂Cu,etc.,which obviously limits the produce of Mg/Al intermetallic compounds.?3?After adding 0.02mm zinc foil to lap interface of friction stir lap welding of 3mm AZ31B Mg and 6061 Al alloy,the mechanical property of the joint was further improved.When the rotation speed is 900r/min¹300r/min,the maximum tensile shear strength decreases with the increase of rotation speed.The maximum tensile shear strength of the welded joint can reach 6425.466N when the stirring pin length is 3.0mm,the rotation speed is 900r/min,and the welding speed is 50mm/min.At the lap interface,the Zn element is free diffusion to Mg side and Al side,and the highest content zone is in the transition zone.The joints detected the diffraction peaks of Al₃Mg₂,Al₁₂Mg₁₇,Mg-Zn eutectic and Mg-Zn-Al compounds.