Effect of Zn Concentration on the Microstructure and Mechanical Properties of Al-Mg-Si-Zn Alloys Processed by Gravity Die Casting

The microstructure and mechanical properties of Al-8.1Mg-2.6Si-(0.08 to 4.62)Zn alloys (in wt pct) have been investigated by the permanent mold casting process. X-ray diffraction analysis shows that the τ-Mg₃₂(Al, Zn)₄₉ phase forms when the Zn content is 1.01 wt pct. With higher Zn contents of 2.37 and 3.59 wt pct, the η-MgZn₂ and τ-Mg₃₂(Al, Zn)₄₉ phases precipitate in the microstructure, and the η-MgZn₂ phase forms when the Zn content is 4.62 wt pct. Metallurgical analysis shows that the η-MgZn₂ and τ-Mg₃₂(Al, Zn)₄₉ phases strengthen the Al-8.1Mg-2.6Si-(0.08 to 4.62)Zn alloys. After solutionizing at 510 °C for 180 minutes and aging at 180 °C for 90 minutes, the η′-MgZn₂ phase precipitates in the α-Al matrix, which significantly enhances the mechanical properties. Addition of 3.59 wt pct Zn to the Al-8.1Mg-2.6Si alloy with heat treatment increases the yield strength from 96 to 280 MPa, increases the ultimate tensile strength from 267 to 310 MPa, and decreases the elongation from 9.97 to 1.74 pct.