Effects of ultrasonic melt treatment and solution treatment on the microstructure and mechanical properties of low-density multicomponent Al70Mg10Si10Cu5Zn5 alloy

The effects of ultrasonic melt treatment (UST) and solution treatment on the microstructure and mechanical properties of a low-density multicomponent Al70Mg10Si10Cu5Zn5 alloy at room and elevated temperatures were investigated. The as-cast Al70Mg10Si10Cu5Zn5 alloy was composed of an fcc Al solid-solution, coarse secondary phases (5-30 μm), fine Cu-containing precipitates (<1 μm), and Zn clusters (<20 nm). The UST significantly reduced the porosity and size of primary Mg2Si particles and grains, resulting in an improvement in the strength and ductility of the as-cast alloy at 25 and 350 °C. The solution treatment at 440 °C caused the dissolution of hcp Zn and fine Cu-containing precipitates. It also resulted in the formation of Zn and Cu clusters in the supersaturated Al solid-solution at 25 °C, which led to an improved room-temperature strength. A phase transformation from Mg2Si to Q-Al5Cu2Mg8Si6 occurred during the solution treatment along with the spheroidization of coarse secondary phases and grain growth, which deteriorated the ultimate compressive strengths at 25 and 350 °C and improved the ductility. Unlike the commercial Al alloys, the Al70Mg10Si10Cu5Zn5 alloy showed excellent strengths at temperatures below 200 °C. This is attributed to the combined strengthening by coarse secondary phases, fine precipitates, and clusters.