The Research On Welding Process And Mechanical Properties Of Friction Stir Lap Welding For Magnesium Alloy

In this paper, friction stir lap welding experiments were used for2mm thickness of extruded magnesium alloy AZ61B, the effects of different welding parameters on the performance of the joints were studied, the main selected welding parameters included the rotation speed, the welding speed, the pressure amount of shoulder, the tool shape and the under-load methods of lap joint. By combining with research status and mechanism of friction stir welding, the optimal welding parameters for AZ61B magnesium alloy were explored, and the migration of the weld interface morphology under different tool shapes effect was also researched; through the reasonable control of the welding parameters, friction stir welding technology was applied to achieve a valid connection to the magnesium alloy material.The main analysis of the experiment under different welding parameters were the weld forming appearance characteristic, the welding macroscopic morphology of the nugget zone, the internal microstructure, the interface morphology, the distribution of hardness, the tensile shear performance characteristics, and other aspects of the fracture morphology. The experimental results showed that:when choosing a reasonable welding parameters, it was able to get the fine appearance of lap joints; otherwise, the joints had flash, burning, inside tunnels, trenches and other external weld defects. The tool shape played a crucial influence on welded nugget morphology; The results show that weld nugget zone appears to be stratification with the reverse conical tool; while, the stratification of weld nugget is not obvious with conical tool. The welding parameters (such as rotation speed, welding speed, tool shape, etc.) also have a direct relationship with the microstructure characteristics of weld, which directly controlled the heat input in the welding process to affect microstructure morphology in various districts. Interface morphology was the most important factors for tensile shear strength of lap joints, and welding process parameters have an important impact on the interface morphology of lap joints, in the same experiment condition, the interface migration heights of lap joints with reverse conical tool were lower than that of lap joints with conical tool; Furthermore, with reducing rotation speed or increasing welding speed, the interface migration heights reducing. Tensile shear experiment analysis showed that the fracture location originated in the migration interface, tensile shear strength of the joints with advancing side (AS) under load were lower than that with retreating side (RS) under load. morphology analysis showed that crack propagation area had three fracture forms, these were ductile fracture, brittle plastic mixed fracture and brittle fracture. Hardness test analysis showed that high hardness was discovered in nugget area, even higher than that of the base material; minimum hardness was in heat affected zone (HAZ); and hardness of thermo-mechanically affected zone (TMAZ) is between both of them; and the both hardness of HAZ and TMAZ were lower than that of the base material; with the rotation speed increasing the hardness of the weld also decreasing; Upper and lower plate hardness analysis showed that the overall hardness of the upper plate slightly lower than the lower plate.Experimental results show that the welding process parameters determine the connection performance of lap joints in the above welding process. Go deep into explore the influence of process parameters on the joint performance for magnesium alloy, it could provide important theoretical basis for friction stir lap welding experiments. The experimental subjects have good research value, whose content can provide important theoretical reference for subsequent experimental studies.