The Research On Microstructure And Properties Of Joints Of AZ31Mg/6061Al Alloy By Friction Stir Lap Welding

Friction stir lap welding?FSLW?,as a solid phase welding technique,was widely used to joint Mg/Al dissimilar metals.Intermetallic compounds?IMCs?was inevitably formed at the interface of Mg/Al dissimilar metals.The content and distribution of IMCs were the key to effect the microstrcture and properties of Mg/Al lap joints.Therefore,based on the optimization of AZ31Mg/6061Al FSLW process parameters in air,the mechanism of the formation and evolution of Mg/Al IMCs were analyzed.Addtionnally,to inhibit the formation of IMCs and improve the microstructure and mechanical properties of welded joints,this experiment was done by adding intermediate Zn layer between Mg alloy and Al alloy and welded underwater.The main findings are as follows:According to the process parameters of Mg/Al FSLW in the air,it was found that it was easier to obtain the better lap joints,when the magnesium alloy plate was placed on the upper and located on the advancing side and the reduction was 0.2 mm⁰.25 mm.Additionally,the stirring pin was the conical tape right-threaded.When the length of the stirring pin was greater than the thickness of the upper plate?5.5 mm?,Mg/Al lap joints have good moldability and the best shear performance by a lower rotation speed and welding speed.When the length of the stirring pin was less than the thickness of the upper plate?3.8 mm?,Mg/Al lap joints have good moldability and the best shear performance by a higher rotation speed and welding speed.The analysis found that the transition layer of the weld zone was composed mainly of Mg,Al,Al₁₂Mg₁₇???and Al₃Mg₂???.Among them,?was discontinuously distributed near the Mg side,and?was distributed in bands near Al side.In addition,fracture occurred in the interface transition layer IMCs of Mg/Al lap joints.When a short stirring needle?3.8 mm?was selected,the addition of an intermediate layer Zn can effectively restrict the formation of Mg/Al IMCs during the Mg/Al FSLW.A eutectic structure such as MgZn₂ and MgZn were formed on the interface near the Mg side and Al-Zn solid solution structure formed near the Al side.Furthermore,when using a long stirring needle at welding parameters 750 r/min,23.5 mm/min and using a short stirring needle at 1180 r/min and 47.5 mm/min in a circulating water medium,generation of interfacial Mg/Al IMCs reduced,distribution of IMCs had been improved and shear strength increased significantly.Finally,by coupling the double factor of underwater and intermediate layer Zn,using the welding parameters of 1180/47.5 and short stirring needle during SFSLW,The Mg/Al joints had a good formability and the shear strengh could be improved again,about 5.6 kN.