Effect of ER5356 and AlMg66 Welding Wires on the Microstructure and Mechanical Properties of 6061 Al-Alloy MIG-Welded Joints

The 6xxx series Al alloys undergo severe softening in the fusion zone (FZ) and heat-affected zone (HAZ) after welding. To improve the mechanical properties of 6061 Al-alloy-welded joints, ER5356 and AlMg66 wires, selected for their differing Mg content, were used to weld 3-mm-thick 6061-T6 Al-alloy plates. Post-weld heat treatments were subsequently applied. Experimental results showed that the mechanical properties of the welded specimens were substantially inferior to those of the base material. However, after solution treatment at 535 °C for 4 h followed by aging at 180 °C, the strength of the welded joints was restored. The recovery of HAZ strength in the ER5356 and AlMg66-welded joints was 53% and 56%, respectively, with hardness values exceeding those of the base material. The superior recovery observed in joints made with the higher-Mg AlMg66 wire highlights the critical role of Mg in facilitating precipitation strengthening. When subjected to a heat-treatment regime consisting of solution treatment at 535 °C for 2 h followed by aging at 180 °C for 8 h, the plasticity of the ER5356-welded joints improved. In contrast, the plasticity of the AlMg66 joints decreased with increasing holding time. Overall, the performance of the AlMg66-welded joints was superior to that of the ER5356 joints in the as-welded condition, whereas the ER5356 joints exhibited better performance after heat treatment. These findings confirm the pivotal importance of filler wire Mg content and subsequent post-weld heat treatments in restoring the performance of 6xxx series Al-alloy-welded joints.