Effect of Solution Treatment Temperature on the Microstructure and Mechanical Properties of Al-5.1Zn-1.9Mg Alloy Produced by Squeeze Casting

Abstract

Aluminum alloys are widely used in aviation industries, especially for the body and wings of aircraft. The heat treatment for 7xxx series aluminum alloys is precipitation hardening which consists of solution treatment, quenching, and ageing. One key for successful ageing process is the amount of solute elements and vacancies dissolve in the matrix during solution treatment. Therefore, this research is aimed to study the effects of solution treatment temperature on the hardness and microstructure of Al-5.1Zn-1.9Mg (wt. %) alloy which produced by squeeze casting. Solution treatment temperatures were varied to 220, 420, and 490 °C for 60 minutes, followed by quenching. The samples were then stored at -10 °C to prevent natural ageing. Characterization included hardness testing and microstructural observation by using OM and SEM-EDS, XRD, and STA. Ageing was conducted at 130 °C for 48 h followed by hardness testing and microstructural observation.The results showed that increasing solution treatment temperature induced enhancement of second phase dissolution and the amount of trapped vacancies in the matrix. However dissolution of second phase was hardly detected at solution treatment temperature of 220 °C. It was shown by the volume fraction of the second phase found after homogenizing was 7.13 % and decreased to 7.06, 4.80, and 4.19 % after solution treatment at temperatures 220, 420, and 490 °C respectively. Therefore the increase in hardness after ageing at 130 °C for 48 hours was 11.54, 42,1, and 66.7 HRB for solution treatment temperatures of 220, 420, and 490 °C respectively. This can be concluded that inceasing solution treatment temperature enhanced the hardness of alloy after ageing.