Experimental investigation of fatigue performance of spot welded dual phase sheet steels

Spot weld fatigue performance of dual phase steels is of great interest to worldwide automotive manufacturers due to their expanding use in automotive industry. Given that the majority of the fatigue life is spent propagating the crack through the microstructure, changes in microstructure of welding steels could result in significant changes in spot weld fatigue life compared to that of low carbon steel spot weld. In the present study, tensile shear spot welds of DP600GI, DP780GI with different thickness were used for fatigue test under various amplitude tensile loads. For comparison tensile shear fatigue test was also conducted for spot welds of low carbon steel. Microhardness was measured for spot weld of dual phase steels to study the hardness change across the weld nugget. Failure mechanism and crack propagation path of dual phase steel spot weld was studied. For spot welds of low thickness, the fatigue strength of dual phase steel spot weld is higher than that of low carbon steel specimen in high cycle fatigue test. When thickness increases, the fatigue strength of dual phase steel spot weld and low carbon steel spot weld are almost the same, and fatigue strength of dual phase spot welds also increases with the thickness. And it is also shown in the present study that static tensile interfacial fracture of spot weld does not necessarily lead to interfacial fracture of high cycle fatigue tests. Crack propagation path of tensile shear spot weld of dual phase sheet steels under fatigue test was examined. Crack initiates at the faying surface around the weld nugget, and then passes through the thickness of sheet, which leads to the failure of specimens.