The strengthening mechanism of Al-Mg-Si-Cu alloy laser-arc hybrid welds with Ni alloying

The strategic incorporation of nickel alloying in weld seams was investigated as a means to improve both the static and dynamic mechanical properties of high-performance Al-Mg-Si-Cu alloys laser-arc welds. Microstructural characterization revealed that Ni addition induced dual strengthening mechanisms: precipitation of Al₃Ni particles and solid solution strengthening through Ni dissolution, accompanied by significant grain refinement. However, microstructural analysis revelaed a critical threshold effect: at Ni concentrations exceeding 1.8 wt%, Al₃Ni particles formed continuous networks that initiated microcrack propagation under stress, ultimately leading to macrocrack formation in weld seam containing 3.6 wt% and 6.0 wt% Ni. Optimal mechanical performance was achieved at a 1.8 wt% Ni addition, where the weld seam exhibited a 23 % increase in yield strength (239 ± 2 MPa) and an 11 % enhancement in tensile strength (281 ± 3 MPa) compared to non-alloyed counterparts. The high temperature baking process further improved joint efficiency through precipitation hardening, with nano-scale βʹ particles, increasing the welded joint coefficient to 0.78. The synergistic combination of grain boundary strengthening, precipitation hardening, and solid solution strengthening resulted in exceptional fatigue performance, demonstrating a fatigue limit of 147 MPa (R = 0.1, N = 10⁷) and a fatigue ratio of 0.46 exceeding those reported in previous studies on similar alloy systems.