Effect of welding time on microstructure and tensile shear load in resistance spot welded joints of AZ31 Mg alloy

Magnesium alloys are becoming widely recognised as the candidates to replace steel and aluminium alloys, especially in automotive industries and thereby they have potential to be widely used. In the present paper, resistance spot welding (RSW) was performed on 1 mm thick AZ31B magnesium alloy sheets. The effect of welding time on the microstructure and tensile shear load was investigated. Some interesting characteristics for nugget growth were observed during the RSW of Mg alloy. The nuggets were initiated in the first cycle, grown rapidly in the following 2-4 cycles and reached a plateau after around 6 cycles. The microstructure of the nugget consisted of non-equilibrium alpha-phase dendrites surrounded by a certain amount of beta-Mg17Al12 precipitated in the grain boundaries. With increased welding time, the size and shape of grains in the nugget would be more uniform and the width of corona bonds would be larger. Tensile shear tests showed that the tensile shear load of the joints increased with increasing welding time when the welding time was,10 cycles. The maximum tensile shear load is up to 1980 N.