As transportation-related greenhouse gas emissions continue to rise, largely driven by automobiles, achieving more sustainable mobility has become an increasingly complex challenge. In this scenario, materials with high specific strength, such as certain aluminum alloys, have gained attention and are often combined with high-strength steels in multi-material joints that balance weight reduction with structural performance. However, these materials pose challenges in terms of cold plastic deformation, which can lead to failure. A promising approach to overcoming this issue involves localised heat treatments, which enhance ductility and facilitate forming while maintaining structural integrity. In this regard, a numerical study was conducted on self-piercing riveting applied to same-material and multi-material joints. Initially, a same-material joint composed of two sheets of equal thickness of 6061-T6 aluminum alloy was analyzed, considering the material in its initial state and after three localised heat treatments. The results indicate that although heat treatments do not prevent the failure of the lower sheet, applying the treatment in regions farther from the area of highest stress concentration increases load-bearing capacity and delays failure, significantly improving ductility. Based on the conclusions drawn from the same-material joint, a numerical analysis was then carried out on a multi-material joint composed of 6061-T6 aluminum alloy and DP600 steel. Once again, it was observed that localised softening in an area farther from the highest stress concentration in the lower sheet delays its failure, although it does not entirely prevent it.