Weldability assessment of Mg alloy (AZ31B) sheets by an ultrasonic spot welding method

A series of ultrasonic spot welding experiments with similar Mg alloy (AZ31B) were performed to determine the process parameters and their effect on weld quality, including weld strength and fracture morphologies. Two dominant welding parameters including vibration amplitude and welding time were evaluated independently to obtain good weld quality. A horn and an anvil tip surfaces were designed with pyramidal patterns to prevent slippage of lap-structured Mg alloy sheets among tool tips during ultrasonic spot welding process. The lap joint thinning was significant at higher vibration amplitudes and longer welding times and resulted in the variation of fracture types at the weld interface. Lap-shear tests on the ultrasonic spot welded Mg alloy lap joints yielded two fracture types: shear and pullout fracture. Metallographic examinations of the fracture surfaces provided insights on the fracture characteristics of the ultrasonic spot welded Mg alloys. Variations in the fracture morphologies were the results of the actual weld nugget development and closely related to weld quality. Higher weld strength was obtained at a low welding energy range of 100-140 J.