The Role of Microstructural Modifications in Improving the Mechanical Properties of Resistance Spot Welded Automotive Steels

Abstract

The advent of advanced high-strength steel (AHSS) in the automotive industry has evolved in recent years to satisfy the global demands for lightweight and safer vehicles. These AHSSs offer an attractive combination of strength and ductility, making them ideal for use in vehicle body-in-white structures. However, their weldability and joint performance in-service are crucial to remain competitive for selection in the automotive industry. Joining AHSS is mostly achieved by resistance spot welding (RSW), and it is envisaged to continue for the foreseeable future. Despite its advantages, the rapid cooling rates during the RSW process result in the formation of martensite in the weld fusion zone, which is known to be hard and brittle, thereby resulting in low energy absorption that is undesirable in case of a crash event.

This thesis explores various metallurgical pathways that can be employed during in-situ RSW process to modify the joint microstructure and enhance the energy absorption capability of the weld. For each technique, an understanding of how different in-situ post-weld heat treatment (PWHT) parameters induce microstructural changes was investigated in this work. Furthermore, this research elucidates the microstructural evolution occurring during the non-equilibrium in-situ PWHT process and correlates these changes with the resulting mechanical properties. Grain refinement was found to be the most effective approach to improve the energy absorption capability of the weld compared to tempering, strain hardening, and paint baking processes. The refined prior austenite grain (PAG) structure was accompanied by a refinement of the substructure with high-angle grain boundary that poses more resistance to crack propagation thereby resulting in 89% improvement in energy absorption capability to failure compared to the baseline welds. It was found that the grain refinement is achieved after applying a PWHT current pulse when the edge of the FZ is solidified and in the austenitic region, rather than when the region has transformed martensite – the latter being preferred for tempering. The recrystallization schedule that induces the grain refinement was also achieved at a relatively shorter process time compared to the other PWHT techniques, which is an important criterion for industrial applicability.

Additionally, the PWHT schedules adopted in this research altered the weld failure mode, causing crack propagation to deviate at the edge of the FZ during cross-tension tests. For the welds with grain refinement, the improved energy absorption capability was majorly attributed to the new equiaxed prior austenite grain structure and the change in crystallographic texture from the cleavage (001) plane in the baseline welds to the (101) plane that supports plastic deformation ahead of the crack tip, thereby retarding the crack propagation. These changes led to ductile failure, in contrast to the brittle failure observed in baseline schedules where cracks propagated into the fully martensitic FZ along the columnar structure. The findings of this research provide a unique perspective on the metallurgical transformations during in-situ RSW PWHT, offering valuable insights to the scientific community. Furthermore, these results inform the automotive industry of the optimal PWHT technique that can be employed in their manufacturing lines, enhancing the performance and safety of AHSS joints in vehicles.

Keywords: Resistance spot welding (RSW) Advanced high strength steels (AHSSs) Post weld heat-treatment (PWHT) Microstructure Mechanical properties.