A study of heat treatment of aluminium wheel castings

Continual attempts are made to improve production techniques in modern manufacturing industries to cater for value added products and optimised processing time. Southern Aluminium Pty Ltd (SAPL), a subsidiary of Comalco Aluminium Ltd, established in 1989, is a manufacturer of automobile wheels, having to meet production in excess of fifty thousand wheels per month for the world automobile industry. SAPL produces a variety of wheel types and offer a complete package in design right from inception stage through development, casting, heat treatment, machining and finishing of aluminium alloy wheels. All wheels at SAPL are produced using commercial aluminium alloy 601 which is predominantly an aluminium-silicon magnesium material. A detailed analysis of product flow at SAPL has shown that heat treatment occupies the majority of value added wheel processing time. Heat treatment at SAPL is the controlled process of heating and cooling the alloy wheels in order to improve their mechanical properties and enhance their performance. It is essentially a three stage manufacturing process involving solution treatment, quenching and aging. The improvement in mechanical properties of the alloy during heat treatment is significantly influenced by the degree of heat treatment time and temperature used. A preliminary investigation carried out at SAPL has encouraging results to reduce heat treatment time without affecting the mechanical properties of the alloy. This work proposes a modified heat treatment process and the associated product flow which results is substantial time savings. The current processing techniques in use at SAPL could not be sufficiently adapted to accommodate the proposed changes to the heat treatment process. Hence, it was necessary to develop a system that would provide a means of incorporating the proposed changes into the alloy wheel manufacturing process. The development of this system involved a preliminary design of an experimental heat treatment cell followed by numerous experimental investigations to study the functioning of the cell. A number of experimental investigations were completed in order to investigate the behaviour of aluminium alloy 601 during heat treatment, and in particular, the effect of varying solution treatment time and temperature on the mechanical properties of the alloy. The procedure leading towards the experimental investigation has necessitated the development of various testing rigs, temperature analysis and mechanical tests. The process is simulated on a smaller scale to check the proposed changes. A significant outcome of the experimental investigation completed was that solution treatment for the alloy could be reduced from the standard condition of four and a half hours at 540°C to a significantly improved condition of twenty two minutes at 570°C. A comparison of the proposed optimised method with the customer specifications and the existing heat treatment method is carried out using statistical routines. A quantitative substantiation using statistical methods has shown that the optimised method is not significantly different to the existing method of heat treatment. The optimum solution treatment developed did not affect the hardness, yield strength, ultimate tensile strength, percentage elongation and impact resistance of the alloy. In addition, the machinability and painted finish of the alloy wheels using the optimised method is also found to be extremely satisfactory. The project has shown that productivity improvements at SAPL were possible through a substantial reduction in processing time of a major manufacturing stage in the production cycle without affecting the quality of the final product. The product cycle when implemented would result in significant cost savings for the company.