Towards intrinsically safe microstructures in resistant spot welded advanced and ultra high strength automotive steels

The potential for weight reduction, ease of manufacturing and improved crashworthiness makes advanced and ultra high strength steels attractive for automotive applications. Resistance spot welding is by far the most widely used joining method in the automotive industry due to the high operating speeds, the reliability of the process and the suitability for automation. Safe microstructures in resistance spot welds in AHSS and UHSS have to be assured to promote acceptance of these steels in the automotive industry. However, the higher alloying contents of AHSS/UHSS steels limit their weldability and unfavourable modes of weld failure are frequently observed. The main aim of this research is to identify and understand the unfavourable failure of the AHSS welds and to modify the microstructure and thus the mechanical response of the welds. In this PhD thesis the results of alternative welding schedules to modify the microstructure and mechanical performance of the AHSS resistance spot welds are reported. The effects of a paint bake cycle on the microstructure of the welds have also been investigated and the predominant mechanisms involved were studied. The residual stress within these welds were measured and simulated to facilitate the residual stress prediction before welding. Double pulse resistance spot welding with different second pulse current levels was applied to improve the microstructure of the weld edge. The second current pulse equal to the first pulse anneals the weld edge and modifies the weld edge microstructure. Microstructural analysis was performed using optical microscopy, scanning electron microscope, electron probe microanalysis (EPMA) and electron back scattered diffraction (EBSD). The double pulse weld showed a reduction in segregation of alloying elements such as phosphorous and a change in grain morphology from dendritic to a more equi-axed shape and smaller grain size. The results obtained from the mechanical testing i.e. cross tension strength test (CTS) and tensile shear strength test (TSS) showed enhanced cross-tension strength and energy absorption capability of the weld for the double pulse welds.