Fatigue and deformation mechanisms of ultrasonic spot-welded dissimilar joints of a magnesium alloy to a clad aluminum alloy

A low rare-earth containing ZEK100-O magnesium alloy was welded to AA1230-clad high-strength AA2024-T3 aluminum alloy via solid-state ultrasonic spot welding (USW) to evaluate the microstructure, tensile lap shear strength, and fatigue properties. The tensile strength increased with increasing welding energy, peaked at a welding energy of 1000 J, and then decreased due to the formation of an increasingly thick diffusion layer mainly containing Al₁₂Mg₁₇ intermetallic compound at higher energy levels. The peak tensile lap shear strength attained at 1000 J was attributed to the optimal inter-diffusion between the magnesium alloy and softer AA1230-clad Al layer along with the presence of ‘fishhook’-like mechanical interlocks at the weld interface and the formation of an indistinguishable intermetallic layer. The dissimilar joints welded at 1000 J also exhibited a longer fatigue life than other Mg-Al dissimilar joints, suggesting the beneficial role of the softer clad layer with a better intermingling capacity during USW. While the transverse-through-thickness (TTT) failure mode prevailed at lower cyclic loading levels, interfacial failure was the predominant mode of fatigue failure at higher cyclic loads, where distinctive fatigue striations were also observed on the fracture surface of the softer clad Al layer. This was associated with the presence of opening stress and bending moment near the nugget edge despite the tension-tension lap shear cyclic loading applied.