Insight into the evolution of lamellar and block LPSO phase of Mg-Gd-Y-Zn-Zr alloy with laser welding after post-weld solution and aging treatment

The paper systematically investigated the effect of laser welding parameters and post-weld heat treatment on morphology microstructure evolution and mechanical properties of weld joint of Mg-13Gd-4Y-2Zn-0.6Zr alloy consisting of lamellar and block long-period stacking ordered (LPSO) phases. Experimental results demonstrate that laser power and welding speed significantly influence weld geometry, with optimal parameters (600 W, 25 mm/s) achieving full penetration, stable keyhole dynamics, and minimal defects, while resulting in a narrow heat-affected zone (HAZ)(<100 µm) dominated by fine grains. The eutectic (Mg,Zn)₃(Gd,Y) phase formed at fusion zone (FZ) and HAZ after laser welding, which was attributed to remelting process during laser welding. Furthermore, the post-weld solution and aging treatments were conducted on Mg-Gd-Y-Zn-Zr alloy with laser welding. The rare earth (RE) atoms from eutectic (Mg,Zn)₃(Gd,Y) phases and block LPSO phases dissolved into α-Mg grains in the course of solution treatment, thus lamellar 14H-LPSO phase precipitated from grain boundaries into α-Mg grains along basal planes and equilibrium β phases precipitated at grain boundaries with reduced block LPSO. The lamellar 14H-LPSO phase precipitated across α-Mg grains as solution time further increased, and only the equilibrium β phase remained at grain boundaries. The finely dispersed β′ phase precipitated inside α-Mg grains after aging treatment, thus nano-sized and densely β′ phase contributed to dislocation pinning, which significantly improves microhardness. The ellipsoidal β′ phases precipitated perpendicular to 14H-LPSO lamellae within the α-Mg grains consisting of lamellar 14H-LPSO phase. However, the β′ phases precipitated in three directions with an angle of 120° to each other within α-Mg grain that is devoid of 14H-LPSO lamellae. The equilibrium β phases gradually declined with prolonged aging treatment. The precipitation of lamellar 14H-LPSO and nano-sized β′ phase contributed to grain boundary migration of α-Mg grains. The results provide fundamental principle for optimizing weld integrity and mechanical response in Mg-RE-Zn alloys with LPSO phase.