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 similar to 7 dimples to similar 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.

Automated summary

This study investigates the impact of spot-welding current on the load-bearing capacity and fracture mechanism of galvannealed dual-phase steel joints. Increasing the welding current leads to a significant increase in the load-bearing capacity of the joints. Quantitative fractography demonstrates a shift in fracture mechanism and the proportion of dimples at different welding currents. The relationship between microstructure and hardness in different welding zones is analyzed using advanced microscopy techniques. Fatigue tests reveal endurance at 10% of the maximum load-bearing capacity. The study also examines the initiation of cracks and the possibility of liquid metal embrittlement cracks.