Effects of different holding pressures on the microstructure and mechanical properties of Al-5.9Zn-2.2Mg-1.8Cu alloy

Abstract

In this study, a low-pressure casting process was used to examine the influence of the holding pressure on the Al–5.9Zn–2.2Mg–1.8Cu alloy. The study aimed to refine the grain structure and optimize the microstructural homogeneity and mechanical properties of the alloy. The phase composition, microstructure, chemical composition, and fracture morphology of the alloy were determined using optical microscopy, SEM, inductively coupled plasma spectroscopy, and XRD. As the holding pressure increased, the theoretical crystallisation temperature of the alloy increased, which increased the degree of undercooling during solidification and the effective number of nuclei, resulting in grain refinement. In addition, increase in the holding pressure strengthened the feeding ability of the alloy melt and reduced the occurrence of shrinkage porosity defects. When the optimal pressure was exceeded, the nucleation rate as well as the denseness of the alloy decreases. When the holding pressure was 0.035 MPa, the grain refining effect was the best and the microstructure was the most uniform. The yield strength, tensile strength, and elongation of the alloy reached the maximum values of 264.6 MPa, 290.8 MPa, and 7.7%, respectively. When the holding pressure was >0.035 MPa, the structure was not further refined; in contrast, the grains were larger, and the mechanical properties were inferior.