Effect of post-weld heat treatment on mechanical characteristics of AZ31 magnesium alloy welded joints

Tungsten inert gas (TIG) welding was performed on 2.7 mm thick commercial extruded AZ31B magnesium alloy plates. We investigated the effect of post-weld heat treatment (PWHT) on the microstructure, mechanical properties and precipitated phase of the weld joints. The results showed that during the annealing treatment (200 °C-1 h, 250 °C-1 h, 300 °C-1 h, 350 °C-1 h, 400 °C-1 h, and 450 °C-1 h), the average grain size in the weld seam was the minimum after annealing at 400 °C for 1 hour, and then abnormally grew up after annealing at 450 °C for 1 hour. The mechanical properties enhanced when the joints were processed from 200 °C-1 h to 400 °C-1 h but sharply decreased with increasing annealing temperature. In contrast to the annealing treatment, solution treatment (250 °C-10 h, 300 °C-10 h, 350 °C-10 h, 400 °C-10 h, and 450 °C-10 h) exhibited a better ductility but a slight deterioration in tensile strength. Especially speaking, no eutectic compounds (such as Mg17Al12) were observed in the weld seam. The supersaturated Al atoms were precipitated in a coarse spherical shape dispersed in the weld seam. The precipitated Al atoms dissolved in the matrix substances at the condition (400 °C-1 h) or (250 °C-10 h). The solution treatment caused grain coarsening and precipitated Al atoms dissolved in the weld seam substantially, which resulted in a drop in micro-hardness at the weld seam compared to the area of the annealed joints.