Mechanism of ductility change in Al-10.44Si-0.25Mg-0.71Mn-0.25Fe die-cast alloy by short time heat treatment

Mechanical joining methods such as self-piercing riveting (SPR) are used to joint iron-based and aluminum alloy parts. However, low-ductility aluminum die-cast alloy often results in cracking and insufficient quality. Therefore, a method has been proposed to locally recover ductility and prevent cracking while maintaining overall strength by rapidly heating and briefly heat-treating the joint area using a laser or similar tool. This study aims to investigate the mechanical properties and microstructural changes in Al-Si-Mg-Mn-Fe die-cast alloys subjected to rapid heating and short-time heat-treatment, focusing on the mechanism of ductility changes. After rapid heating and short-duration heat treatment, the eutectic Si particles became more spherical as the heat treatment temperature increased. An exothermic peak at 510 K in DSC disappeared after the heat-treated samples. SEM and TEM observations showed no precipitates in the as-cast α-Al phase, but precipitates of Si were confirmed after the heat treatment. This suggests that Si was supersaturated in the α-Al phase in the as-cast state. The precipitation of Si in the α-Al phase contributed little to strength but suggested improvement of ductility. Synchrotron multi-scale tomography showed no significant changes in casting defects or intermetallic compounds due to the heat treatment, suggesting that they have a little effect on change of ductility. Therefore, the primary factors contributing to increased ductility of aluminum die-cast alloy after the heat treatment are stress concentration relaxation due to the spheroidization of eutectic Si particles and reduced connectivity of Si particles.