Effect of Cooling Rate and Heat Treatment on the Microstructure and Impact Resistance of Recycled Aluminium Sand Cast Alloy

Abstract

In this study the effect of cooling rate and heat treatment on the microstructure and impact resistance of aluminium alloy A356 were investigated. The alloy was obtained from recycled aluminium alloy wheels that were melted, modified and cast in carbon silicate bonded sand moulds. An external chill was placed in the mould and thermocouples attached to a digital data logger were used to capture the cooling rates across the length of the cast after pouring of the metal. Three sections with different cooling rates were identified with the first being adjacent to the chill, followed by a second section and the third farthest from the chill. In the experimental process 5 sets of specimens from each of the sections were solution treated for periods of 30 minutes, 1 hour, 3 hours, 6 hours and one set retained in the as-cast state, respectively. The solution treatment temperature was set at 5400C and was followed by quenching in water at 600C and precipitation ageing at 1700C for 3 hours. All samples were then prepared for microstructural analysis and charpy impact testing. Results showed the section with the fast cooling rate had finer microstructure and had higher impact resistance energy even after short heat treatment processes. The slow cooling section showed marginal gain in impact resistance even when exposed to solution treatment of 6 hours. Therefore shortened heat treatment processes may be applied on castings with up to 65% improvement in impact resistance energy. This technique can enhance the quality demands of products manufactured in the foundry industry dominated locally by the jua kali practitioners.