Research On Process And Mechanism Of Self-reacting Friction Stir Welding For AZ61 Magnesium Alloy

Self-reacting friction stir welding is a new type of friction stir welding method.It not only retains the advantages of solid-state joining,but also solves the typical problems of conventional friction stir welding like high demand for fixtures,root defect of joint due to lack of penetration.In this paper,we designed the welding tool,and carried out friction stir welding experiment on 5mm thick AZ61 Mg alloy.The influence of rotational speed and welding speed on mechanical properties and microstructure of joints and process parameters have been analyzed.The welding experimental results showed that the welding parameters of self-reacting friction stir welding of AZ61 magnesium alloy have a relatively narrow range,and defect-free joints can be obtained within that small range of parameters;The process parameters during the welding process were measured.The peak temperature in the retreating side of the weld is higher than that of the advancing side.With the increasing of the rotation speed,the interaction between the pin tool and material increased and the material were softened more,so the peak temperature and peak torque increased.The peak temperature and torque decreased with increasing welding speed due to less heat input and the harder material.The microstructure analyze shows that the cross section of the joint is dumbbell-shaped and divided into four different areas: weld nugget zone(WNZ),thermo-mechanically affected zone(TMAZ),heat-affected zone(HAZ)and base metal(BM),among which,weld nugget zone is composed of fine equiaxed grains;Unlike the aluminum alloy,the thermo-mechanically affected zone of AZ61 magnesium alloy is also mainly composed of equiaxed grains;The average grain size in WNZ,TMAZ and HAZ all increased with the increasing of the rotation speed or decreasing of the welding speed.The micro-hardness of welded joints is uniform.It can be seen that the weld nugget zone had a higher hardness value obtained under low heat input,but there is no significant difference in other zones obtained under different welding parameters;In the tensile test,the joints with pore defects shows a poor tensile property and fractured in the WNZ,and the fracture surface showed a mixed fracture characteristic.And the tensile property changes with welding parameters.The joint without defect shows a good tensile property,the largest tensile strength is 2 13 MPa,corresponding to 79% of base metal.The impact experiment shows that,the impact energy of weld nugget zone is higher than the base metal,and it changed with different parameters.The fracture surface of the impact sample of HAZ shows a mixed fracture characteristic,while the fracture surface of the impact sample of WNZ shows a ductility characteristic.