Resistance spot welding of advanced high strength steels: Mechanical properties and failure mechanisms

The safety of passenger cars and lorries is determined significantly by the performance of resistance spot welds (RSW) that keep the components of steel together. Advanced high strength steels (AHSS) are known to be more susceptible to weld failure than conventional mild steels. A key issue encountered in the current development of the AHSS and next-generation automotive steels is the catastrophic failure of the welds, accompanied by considerably reduced strengths and toughness. Problems arise mainly in the cross-tension tests of spot welds in AHSS steels of strength > 800 MPa, where welds are subjected to a mode I type loading. The lowered cross-tension strength and poor failure mode of spot welds form a direct hurdle for a successful implementation of these new advanced steels in the automotive industry. Therefore, improvement of the mechanical performance of AHSS resistance spot welds is of vital importance to ensure the safety and crashworthiness of the cars. The present work aims at identifying the process-structure-property relationship in the RSW of dual phase (DP) steels and third generation AHSS steels. Special attention is paid to the microstructural evolution of the welds as a function of welding scheme and parameters. The effect of the precise welding scheme (i.e. single and double pulse) on the microstructural evolution and mechanical response of DP1000 resistance spot welds were studied