Study On Microstructure And Mechanical Properties Of Aluminum And Magnesium Dissimilar Metals Welded By Friction Stir Lap Welding

Conventional fusion welding was used to weld aluminum and magnesium dissimilar metals,the joint has a large number of microstructure defects,resulting in a low joint strength.In order to avoid the microstructure defects caused by fusion welding,Friction stir lap welding?FSLW?was used to weld aluminum and magnesium dissimilar metals in air and water in this paper,respectively.The macroscopic morphology,grain size and intermetallic compound?IMC?of FSLW joints of magnesium and aluminum alloys in air and water media were characterized by optical microscopy,scanning electronic microscopy,electron back scattering diffractometer and X ray diffractometer,respectively.The microhardness and shear properties of welded joints were evaluated.The main results of this paper are as follows:While Al/Mg dissimilar metals FSLW in the air,the joint welded by the taper threaded stir pin had excellent formability.The high rotational speed resulted in poor formability of the welded joint,and the forming quality was improved in the condition of lower rotation speed.Compared with base metal,the grain of the stir zone?SZ?in the side of Al and Mg was refined from 39?m and 27?m to 1?m and 6.5?m,respectively.The grain size was refined due to the increase of rotation speed.IMC layer generates due to element interdiffusion in the Al/Mg interface of joint.IMC layer was aggregated by the particulate phase of Al₁₂Mg₁₇,Al₃Mg₂,MgZn₂ and Al₂Cu.The phase content of IMC increased due to the increase of rotation speed.IMC caused the hardness of the joint to rise sharply.IMC layer distributed at the joint interface deteriorated the shear performance.The shear fracture broken along the IMC layer,and a large number of lamellar IMC were distributed on the fracture surface.The temperature of Al/Mg joint was reduced by Subermedged friction stir lap welding?SFSLW?,and the forming quality of high rotation speed welded joints in water medium was high.Compared with the joint welded in the air,the grain was more refined by SFSLW,the grain of the SZ in the side of Al and Mg were 0.5?m and 2.9?m,respectively.The increase in rotational speed increased the grain refinement of the SFSLW joint.The water cooling effect inhibited the element diffusion at the interface of the joint,which reduced the reaction rate of the element at interface,therefore the Al/Mg interface did not form an IMC layer.The fine granular Al₁₂Mg₁₇,Al₃Mg₂,MgZn₂and Al₂Cu phases distributed at the Al/Mg interface.Grain refinement promoted an increase in the hardness of the SZ of the Mg side of the joint.Compared with the joint welded in the air,the shear performance of the SFSLW joint was increased by 127%,which is mainly attributed to the grain refinement and no brittle IMC layer of joint.The SFSLW joint shear broken along the Al/Mg interface,and a small amount of fine IMC was distributed on the fracture surface.