Effect of Post-weld Heat Treatment Methods on Microstructure and Fatigue Behavior of Al-Mg-Si Alloy Oscillating Laser Welding

To address the softening issue in aluminum alloy weld joints, post-weld heat treatment(PWHT) techniques were employed to enhance the mechanical properties of Al-Mg-Si alloy weld joints joined by oscillating laser welding(OLW). A systematic investigation was conducted on the impact of two different PWHT methods, artificial aging(AA) and solution treatment followed by aging(STA), on the static mechanical properties, fatigue performance, and microstructure of the weld joints. The results indicated that both AA and STA could improve the tensile strength and fracture strain of the welded joints. Compared to AA, STA did not show a significant improvement in increasing tensile strength and elongation at break. Notably, AA significantly increases the fatigue life of the joints, while STA also contributes to enhanced fatigue life, albeit to a lesser degree. At the lowest stress level, the fatigue life of both STA and as-welded(AW) joints exceeded 106 cycles, with the STA joints exhibiting approximately 57.9% higher fatigue life than the AW joints. Microstructural analysis of the AW joints revealed the presence of dislocations and a few large-sized precipitate phases, primarily Mg2Si and AlFeSi phases. After AA, nano-scale second phases, mainly the AlFeSi phase, appeared in the weld joints, accompanied by a reduction in dislocation density. Following STA, there was a significant increase in the density and size of precipitate phases, with the coarse AlFeSi phase becoming more prominent and large-sized grain boundary precipitates appearing. The AA treatment, due to grain refinement and second phase precipitation, enhanced the hardness, tensile strength, and fatigue strength of the joints. As the STA joints exhibited more β-AlFeSi precipitates and grain boundary precipitates, the second phase strengthening effect was reduced. Although the mechanical properties of STA joints were better than those of AW joints, they were inferior to those of AA joints.