Investigation of failure mechanism and mechanical properties of resistance spot welded magnesium alloy joints

Resistance spot welded magnesium alloy joints can fail in two markedly different failure modes(interfacial failure and button pullout failure)under tensile shear loading conditions.For button pullout failure,the crack first propagates along cellular dendritic structure of the nugget circumference,and then passes through heat-affected zone(HAZ)and base metal in sequence.The tensile shear load has smaller values under the interracial failure occurring in a small weld nugget as compared to the button pullout failure appearing in a large weld nugget.The tensile shear load increases with the increasing nugget diameter for expulsion free joints.However,for joints which experienced expulsion,the tensile shear load decreases in spite of nugget diameter increasing.Under the equivalent nugget diameter(5.8 mm),the tensile shear load of joints with 9 × 10-4 g KBF4 addition was increased by around 20% as compared to that of joints without KBF4 addition.