Development of High-Ductility and Low-Hot-Tearing-Susceptibility Non-heat Treatment Al–Mg–Mn-Based Die Casting Alloy for Automotive Structural Parts

Non-heat-treated Al–Mg-based die casting alloys have been developed for the structural parts of automobiles. In previous studies, alloy compositions with at least 1.0%Si have been proposed to reduce the hot tearing susceptibility (HTS). On the other hand, the increase in the Si content reduces the ductility. For some automotive body structures, Al–Mg alloy die castings with Si content exceeding 1.0% should not have the required ductility. This study aims to develop an Al–Mg alloy with both high ductility and low HTS by investigating the following three characteristics of an Al–4.5Mg–1.0Mn alloy with 0.2%Si added: (1) additional elements to reduce the HTS, (2) the associated mechanical properties (requirements for the automotive company: 0.2% proof stress ≥ 140 MPa and fracture elongation ≥ 15%), and (3) the mechanism of decreasing the HTS. It was revealed that the co-addition of 0.025%Sr, 0.08%Ti, and 0.016%B reduced the HTS when the hydrogen content of the melt was 0.5–1.0 mL/100 g Al. Furthermore, the 0.2% proof stress and fracture elongation of a lower link arm produced via HPDC with the above composition were found to achieve the requirements. It was also indicated that the mechanism of decrease in the HTS by the Sr addition should result from the decrease in the thermal tensile load due to the formation of hydrogen porosity at a lower solid fraction than that without Sr. It is suggested that when 0.025%Sr, 0.08%Ti, and 0.016%B are added to the Al–4.5Mg–1.0Mn–0.2Si alloy, non-heat-treated automotive structural parts with both high ductility and low HTS will be obtained by high-pressure die casting.