Correlation between microstructures and mechanical properties of super-sized new-energy automobile structural component formed by vacuum HPDC process

Abstract

Correlation between microstructures and mechanical properties of a super-sized new-energy automobile rear floor component (three-dimensional size: 1842 mm × 1549 mm × 741 mm, projected area: 2.85 m², largest projected area in available reports) manufactured by vacuum high-pressure die casting (HPDC) process using a non-heat treated aluminum alloy was clarified. Effects of filling behavior and solidification sequence during HPDC on microstructures and mechanical properties of various regions for the HPDC component were unraveled. According to filling and solidification characteristic, 5 regions of the HPDC component were selected for evaluation. Rear floor platform region exhibits high comprehensive mechanical properties. The yield strength (YS) and ultimate tensile strength (UTS) for various locations are higher than 146 MPa and 252 MPa, respectively, while average elongation (EL) reaches 8.60 %. High YS in this region is attributed to fine-grained structure formed at rapid solidification condition. Utilizing local loading and feeding strategy, fine and dense microstructure was successfully obtained in longitudinal beam region, which guarantees excellent strength and plasticity (UTS and EL near local loading and feeding area reach 226.86 MPa and 10.41 %). Low filling velocity in wheel housing region increases the residence time of the melt in die cavity and promotes nucleation and growth of Fe-rich phase, while sluggish solidification causes the further coarsening of Fe-rich phase, resulting in degradation of elongation. Agglomeration of grains under turbulent flow condition during die filling and slow cooling condition in horizontal support column region cause abnormal growth of externally solidified crystals (ESC) grains, which is detrimental to ductility.