Tensile shear fracture load bearing capability, softening of HAZ and microstructural characteristics of resistance spot welded DP-1000 steel joints

The main objective of this investigation is to enhance the tensile shear fracture load (TSFL) bearing capability and minimize softening in heat affected zone (HAZ) of resistance spot welded DP-1000 steel spot joints for automotive applications. The lap tensile and cross tensile shear fracture load tests (LTSFL and CTSFL) were conducted. The process parameters were optimized using numerical and graphical optimization techniques to maximize the TSFL capability of spot joints. The microstructure of spot joints was studied using optical microscopy and correlated to TSFL and hardness of spot joints. Results showed that DP-1000 steel spot joints made using the welding power of 70 W, welding time of 1.0 s, electrode pressure of 4.25 MPa showed maximum LTSFL of 22 kN and CTSFL of 9.1 kN. The parametric optimization showed 46.66, 45.77, 22.33 and 9.87% increase in LTSFL, CTSFL, nugget zone hardness and HAZ hardness of DP-1000 steel spot joints. The higher TSFL and nugget hardness of spot joints is mainly attributed to the evolution of finer martensitic sandwiching ferrite phases in nugget zone and lower softening of HAZ than other joints. Welding power showed significant influence on TSFL of spot joints followed by welding time and electrode pressure.