Effect of microstructure and deformation mechanisms on the forming limit curve of automotive steels and aluminium alloys

Increased crashworthiness and lightweighting are key factors influencing material development in the automotive sector. Aluminium alloys and advanced high-strength steels (AHSS) are extensively used in the automotive industry because of their exceptional formability and strength-ductility balance. This study investigates the influence of microstructure and deformation mechanisms on the forming limit curve (FLC) of representative automotive steels (SAPH 370, DP 780, DP 980, TRIP 780, and TWIP 940) and aluminium alloys (AA5182 and AA2024-O). The tensile uniform elongation, post-uniform elongation, and associated mechanisms such as dislocation slip, mechanical twinning, and transformation-induced plasticity are analysed to establish their relationship with FLC 0 (plane strain forming limit). Uniaxial tensile parameters are used to create a predictive model that connects FLC 0 to both uniform and post-uniform elongations. Experimental results demonstrate that post-uniform elongation plays a dominant role in governing FLC 0 , being nearly 3.5 times more influential than uniform elongation. Although TWIP and TRIP steels have a high total elongation, their limited post-uniform deformation results in moderate FLC 0 values. In contrast, SAPH and DP steels exhibit moderate total elongation with appreciable post-uniform elongation, resulting in comparatively higher FLC 0 . In aluminium alloys, limited strain-hardening and low normal anisotropy reduce FLC 0 , whereas cryogenic conditions suppress dynamic recovery and thereby improve FLC 0 . The proposed model offers a unified framework to predict formability from tensile elongations, showing strong agreement with experimental FLCs for both steels and aluminium alloys.