Microstructural evolution, dislocation density and tensile properties of Al–6.5Si–2.1Cu–0.35Mg alloy produced by different casting processes

Abstract

Al–Si–Cu–Mg foundry alloys are used in casting process technologies. However, their strength properties remain low due to their microstructural characteristics and porosity. In this work, the microstructural characteristics, dislocation densities, and mechanical properties of Al–Si–Cu–Mg cast alloys prepared through different casting methods were studied experimentally. Four casting processes, namely, gravity casting (GC), rheocasting (RC), thixoforming (Thixo), and Thixo with heat treatment, were used. The GC and RC samples had mainly dendritic α-Al phase microstructures and exhibited coarse Si particles and intermetallic compounds in their interdendritic regions. By contrast, the Thixo and heat-treated Thixo (HT-Thixo) samples exhibited microstructural refinement with uniformly distributed α-Al globules, fine fibrous Si particles, and fragmented intermetallic compounds among α-Al globules. The accumulation of dislocation densities increased in the Thixo sample as the strain was increased due to plastic deformation. Furthermore, the ultimate tensile strength and yield strength of the HT-Thixo sample increased by 87% and 63%, respectively, relative to those of the GC sample. The cleavage fracture displayed by the GC and RC samples led to brittle failure. Meanwhile, the Thixo and HT-Thixo samples presented dimple-based ductile fracture.