Unveiling the influence mechanism of splat cooling on the microstructure evolution and mechanical properties of friction stir welded ZK61M magnesium alloy

The application of cooling assistance during the friction stir welding (FSW) process can effectively improve the yield strength of the joint, thereby causing more strain to occur in the base material on both sides of the weld seam, inhibiting strain localization in the joint, and achieving the effect of simultaneously improving the strength and toughness of the joint. However, there are currently limited studies on cooling-assisted FSW of magnesium alloys, especially for ZK series magnesium alloys. Therefore, conventional FSW and novel splat cooling assisted friction stir welding (SCaFSW) were carried out on ZK61M magnesium alloy in this study. It was found that the application of splat cooling slightly increases the traverse force, reduces the area of the stir zone (SZ) and refines the grain size. After applying splat cooling at a welding speed of 60 mm/min, the ultimate tensile strength (UTS) of the joint increased from 210.4 MPa to 257.1 MPa, achieving a joint efficiency of 90.7 %. The yield strength (YS) of the joint also improved, rising from 138.6 MPa to 185.0 MPa, representing a 33.4 % improvement, which can be attributed to the reduction in grain size and basal slip Schmid factor (SF). When splat cooling is applied, due to the weakened deformation effect of the newly formed shear band on the deposited shear band, a large number of recrystallized grains are preserved. At the same time, the reduced annealing effect leads to the retention of more dislocation structures. Ultimately, both the degree of recrystallization and dislocation density can be simultaneously increased in the SCaFSW joint. It revealed that the application of splat cooling is an effective method for improving the joint properties of FSWed ZK series Mg alloy.