Study On The Dissimilar Friction Stir Welded Joint Of Al/Mg Thick Plates

Joining aluminum to magnesium is of great application in lightweight design.However,due to poor weldability,it is difficult to obtain good joints by conventional fusion welding.In this paper,friction stir welding?FSW?was used to join 12mm thick AA6061/AZ31B dissimilar alloy.The weld formation and interface morphology were observed by optical microscope and scanning electron microscope.The microstructure,composition and phase structure were analyzed by the energy spectrum analysis and micro-area XRD.The micro-hardness and tensile properties of the joint were tested.The influence of welding parameters on weld formation was studied.The relationship between weld structure and mechanical properties was analyzed.The results show that the weld forming better when placing the magnesium alloy on the return side of the joint.When the magnesium alloy is placed on the advancing side of the joint,a large amount of Al₁₂Mg₁₇ phase formed at the magnesium side interface of the weld nugget zone,it makes the joint crack;With the content of aluminum alloy in the weld metal decreases,the area of the low-melting eutectic region in the weld increases,the plasticity of the weld metal decreases,and holes and grooves are formed in the upper layer of the weld;With the welding speed increased,the cooling speed of the joint after welding is increased,and the interface on both sides of the joint weld nugget zone is cracked;when the rotation speed of the stirring head is low,cracking occurs at the interface of the nugget zone magnesium side,when the rotation speed of the stirring head is too high,A large-area low-melting eutectic region is formed at the magnesium-side interface of the nugget zone,and a large thickness diffusion layer is formed,it reduce the tensile strength of the joint.When the friction stir welding of thick plate AA6061/AZ31B dissimilar alloy,the peak temperature of the Al side of the joint is 24.5°C higher than that of Mg,and the peak temperature of the upper metal of the weld is 48.4°C higher than that of the underlying metal.The AA6061/AZ31B dissimilar alloy FSW weld formed well when the aluminum alloy is placed on the advancing side of the joint,the weld composition is 80%of the aluminum alloy,the welding speed is 23.5mm/min,and the rotation speed of the tool is 750r/min.The microstructure of the weld is obviously layered.There is a low-melting eutectic and elemental diffusion layer at the magnesium-side interface of the nugget zone.The eutectic region is??Mg?+Al₁₂Mg₁₇ daisy-like eutectic structure,and the element diffusion layer is made of Al-rich.The element diffusion layer structure is composed of??Al?+Al₃Mg₂ Al-rich structure and??Mg?+Al₁₂Mg₁₇ Mg-rich structure.The micro-hardness distribution of the joint is"W"shaped along the cross section,the micro-hardness of the weld nugget zone is the highest,and the micro-hardness of the heat affected zone is the lowest.The micro-hardness of the weld nugget is layered.The micro-hardness of the upper layer of the nugget zone is higher than that of the lower layers of the nugget zone.The tensile strength of the joint is 53.4MPa.The joint break-off position during tension is the low-melting eutectic region of the upper layer of the weld nugget,and the fracture reaction layer along the magnesium-side interface of the weld nugget breaks.The fracture morphology is river-like and is characterized by typical brittle fracture.