Investigation On The Friction Stir Welding Of LZ91 Dual-phase Mg-Li Alloys

In this study,4 mm-thick LZ91 dual-phase Mg-Li alloy was joined by the friction stir welding(FSW).The welding process was optimized,and the relationship among the welding process parameters,formation and properties of joints was established.Moreover,the microstructure evolution of LZ91 Mg-Li alloy during FSW welding was revealed.The good bonding of FSW LZ91 joint was difficult to achieve at the low rotational speeds(200 rpm,400 rpm,and 800 rpm).The weld formatiom was improved with increasing the rotational speed(1200 rpm,1600 rpm and 2000 rpm).Especially,the rotational speed was 2000 rpm,the good joints were obtained at welding speeds of 400 mm/min and 800 mm/min.However,when the welding speed was further increased(1200 mm/min and 2000 mm/min),the holes or tunnel defects were found at the bottom of the joint due to insufficient heat input.The hardness test results of FSW LZ91 joints showed that the hardness distribution trend of the joints under different welding speeds(400 mm/min,800 mm/min and 1200 mm/min)were similar.The value of hardness at the nugget zone(NZ)and the thermo-mechanically affected zone(TMAZ)were higher than that at the heat affected zone(HAZ).Tensile test results showed that the properties of jonts decreased as the welding speed increased.When the rotational speed was 2000 rpm and the welding speed was 400 mm/min,the value of tensile strength and elongation of the joint were the largest(204.6 MPa and 50.3%).Moreover,the value of tensile strength and elongation of the joint were the least(125.6 MPa and 33.8%)when the welding speed was 1200 mm/min.The fracture of the joint occured in the area where the HAZ/TMAZ at the retreating side was located.The distance between the fracture location and the NZ of the joint increased gradually as the welding speed increased.The fracture morphology showed that the joint exhibited mixed fracture characteristics.The analysis of the microstructure of the joint showed that the base materials(BM)of the LZ91 alloy was mainly composed of ?-Mg phase and ?-Li phase grains,and the dislocation density in the ?-Mg phase grains was higher than that of the ?-Li phase grains.The TMAZ were dominated by deformed grains,and there were higher density dislocation densities in the ?-Mg phase and ?-Li phase grains.The grain at the HAZ was coarsening compared with TMAZ.In addtion,the dislocation density of HAZ was lower than that of BM.While the grains at the NZ were completely dynamic recrystallized,the grains at the upper and lower of NZ were refinement,and amounts of grains at the middle part were slightly grown.And in the NZ,the dislocation density in the ?-Mg phase grains was higher than that of ?-Li phase grains,and a few MgZn phases were found in this region.