Effect of short cycle heat treatment and cooling rate on microstructure and chemical properties of recycled aluminium sand casting Author: Elahetia, Shem Maube

Abstract

Heat treatment is an essential part of manufacture of aluminium castings. This is because it improves the mechanical properties of the casting such as hardness, ductility and tensile strength. T6 is one of the most popular heat treatment procedures used in heat treating aluminium alloys and consists of three stages; the first is solutionising of the cast part in a furnace at 540oC for 6 hours, then quenching in water at 60oC before aging in an oven for 3 hours at 170oC. This study investigated the effect of heat treatment and cooling rate on the microstructure and mechanical properties of aluminium alloy A356 with a view to determine if a short solution heat treatment could be achieved without adversely affecting the final mechanical properties of the material. The aluminum alloy A356 was obtained from used motor vehicle wheels that were melted and cast into ingots. During melting a sample was collected and analysed to determine the metal’s precise chemical composition. The ingots were then melted and poured into sand molds assembled with a metal chill in one of the walls. On the opposite end, the mold was designed to have the sprue, so that across the casting there would be regions of distinct cooling rates. Thermocouples mounted in the mold captured the varying temperatures across the casting which allowed the casting to be cut along sections having distinct cooling rates. These sections were further divided into five sets of specimens that were solution heat treated for periods of 30 minutes, 1 hour, 3 hours and 6 hours while one was retained in as-cast state. Microstructural analysis and mechanical tests including tensile, ductility, hardness and impact tests were then carried out on these specimens. Results showed that a casting of 2.11oC responded best to short heat treatment as it xvi achieved 92% of the maximum yield strength when solution treated for 30 minutes. Over the same period it achieved maximum Vickers hardness and ductility values of 60 and 5% respectively. This was attributed to the fully spheroidised and fragmented structure that was observed after only 30 minutes of solution treatment. A study of the local foundries established that there was no application of standard melt treatment procedures; foundry men lacked relevant industry training and the level of education of the majority was low. This concluded that for a short heat treatment process to benefit local foundries a holistic training approach needed to be conducted.