Constitutive relationship of fusion zone in the spot welds of advance high strength steels

Constitutive relationship of fusion zone is essential for a practical numerical model to analyze the fracture behavior of resistance spot welded advance high strength steels. In this paper, the constitutive relationship was directly measured by developing a simple but effective experimental setup with the new small tensile specimen that could be available in typical universal tensile machines. This new specimen was used to characterize the key properties of fusion zones in the spot welds of DP590 steels, Q&P980 steels and 22MnB5 steels. Compared to the ultimate tensile strength in the fusion zone of DP590 steels and 22MnB5 steels with the value of 1100 MPa, the strength was up to 1700 MPa in the fusion zone of Q&P980 steels due to the formation of retained austenite. Strain distribution results indicate that the heterogenous microstructure with smaller grain misorientation in the fusion zone of Q&P980 steels leads to the larger fracture strain in a comparison to other fusion zones. These results revealed that the fusion zone of Q&P980 steels exhibited a superior strength and ductility compared to that of DP590 steels and 22MnB5 steels.