Insights on Microstructure and Failure Characteristics of Resistance Spot Welds of Galvannealed Dual Phase Steel

This work examines the effect of spot-welding current on the load-bearing capacity and the fracture mechanism of galvannealed dual-phase steel (DP600) joints. A greater than three-fold increase in the load-bearing capacity of the joint was achieved by increasing the welding current from 6 to 9 kA. Quantitative fractography showed a transition from predominantly transgranular cleavage fracture at 6 kA current with only ~7 dimples to ~17% ductile dimples at 7 kA current and eventually 100% ductile dimples for 9 kA current. Detailed microstructure-hardness relationship across the different welding zones is presented using scanning electron microscopy and electron backscattered diffraction techniques. At the optimized welding parameter, load-controlled fatigue tests revealed a decrease in the number of cycles to failure with increasing load amplitude, and endurance was achieved for 10% of the maximum tensile-shear load-bearing capacity. Investigation of failed specimens revealed that while under tensile loading conditions, crack initiated from the base metal, fatigue crack initiated from the heat-affected zone. Also, the possibility of different categories of liquid metal embrittlement cracks are discussed. Graphical Abstract