Enhancement of mechanical properties of GTAW joints for ZC63 magnesium alloy by post-weld heat treatment

To further improve the microstructure and mechanical properties of gas tungsten arc welded (GTAW) welded joints for ZC63 magnesium alloy, post-weld heat treatment is carried out. It is found that the majority of the MgZnCu phase in the fusion zone (FZ) is dissolved in the α-Mg matrix under suitable heat treatment conditions. The remainder is diffusely distributed as rods or granules at the grain boundaries. The excessive heat treatment temperature (460°C) leads to abnormal grain growth (AGG) in the FZ. The substructure gradient between the abnormally grown grains and the surrounding small grains provides the driving force for AGG. Meanwhile, the dissolution of the MgZnCu phase weakens the hindering effect of the second phase on grain boundary migration, setting the stage for AGG. In addition, the detrimental impact of the continuous MgZnCu phase on the mechanical properties of the welded joint is also lessened by its dissolution. The ultimate tensile strength (UTS), yield strength (YS) and elongation (EL) of the welded joints are 255 MPa,119 MPa and 27.0%, respectively, under the post-weld heat treatment process of 440°C × 12 h. The welding coefficient of the welded joint reaches 97.0%, satisfying the service criteria set forth by the mechanical properties of the welded joints.