Effect of welding current on the microstructures and mechanical properties of GTAW joints for ZLa22 alloy

The effect of welding current on the microstructures and mechanical properties of gas tungsten arc welding (GTAW) joints for ZLa22 (Mg-2Zn-2La, wt%) alloy was investigated. The results show that increasing the welding current gradually increases the average grain size in the fusion zone (FZ) and gradually decreases the welded joints strength. However, incomplete penetration is formed in the FZ at a low welding current of 130A, resulting in a decrease in tensile strength. The ultimate tensile strength (UTS) of the welded joint reaches the maximum value of 117 MPa at a welding current of 150A, which is only 72.22% of the base metal (BM). The rupture location occurs in the transition zone (TZ), due to the stress concentration at the junction of large and small grains, which may lead to cracks. The Mg-Zn-La phase in the welded joints precipitates at the grain boundaries in a coarse semi-continuous network, which reduces the tensile strength of the welded joints. In addition, with the increase of the welding current, the microhardness of the FZ decreases. In the welded joints, the FZ exhibits as-cast quench structure with fine grains, offering the highest hardness in this zone.