Optimizing the Heat Treatment Process of Cast Aluminium Alloys

The influence of heat treatment process on the microstructure and mechanical properties, and the distortion of a low-pressure die-cast AlSi7Mg0.3 alloy is reported. This work is aimed at optimizing the whole heat treatment process, including solution heat treatment, quenching and artificial ageing, of A356 alloy wheels, produced by low pressure die casting. In particular, the optimization process is focused on reducing time and temperature of solution and ageing heat treatments, and on selecting the right quenching medium, reducing the distortion of A356 alloy 17-in and 18-in wheels and obtaining the required mechanical properties. The time and temperature of solution treatment are normally chosen to dissolve coarse β-Mg2Si phase coming from the solidification process, especially where the solidification rate is slow as in the thickest zones of wheel, e.g. the hub and the spoke regions. Generally, the time required is about 45 minutes at the temperature of 540°C, which is the typical solution temperature used for A356-type alloy. More time is however required at this temperature to change the morphology of eutectic silicon particles. This is aimed to improve the mechanical properties, especially ductility and fatigue properties, of wheels after further artificial ageing. The solutionising temperature is here selected to reduce the time of the heat treatment and to avoid any type of incipient melting of the material within an industrial tunnel furnace. The quenching is the most critical step in the sequence of heat-treating operations. The objective of quenching is to preserve the solid solution formed at the solution heat-treating temperature, by rapidly cooling to some lower temperature, usually near the room temperature. The most rapid quench rate, giving the best mechanical properties, can also cause unacceptable amounts of distortion or cracking in components. This is particularly true for aluminium wheels where the different thickness throughout the casting can produce distortion higher than 2-3 mm, critics for subsequent machining operations. The optimization process is here focused on the quenching rate, which is varied by changing the temperature of the quenchant, in order to reduce the wheels’ distortion and to guarantee the appropriated supersaturation level of atoms for subsequent ageing treatment. Therefore, the temperature of forced water, used as quenchant, has been varied in the range of 50 to 95°C, and the distortion level and the hardness of the wheels systematically measured. A temperature of 95°C is observed to be the optimum solution, but useless from an industrial point of view. The extreme vapour produced by the boiling water can compromise the automatism for handling of wheels and delay too much the cooling of the wheels after solutioning. Therefore, the best compromise between distortion and mechanical properties and productivity has been revealed to be at a temperature of 75°C. The time and temperature of ageing are considered in the present work in order to reach an underageing temper of the material, which is typical for the manufacture of wheels to improve impact and fatigue properties. The influence of the different painting processes, generally carried out at a temperature of 180÷190°C for several minutes after ageing and subsequent machining operations, have to be considered to determine the final mechanical properties. These temperatures are typical temperature for ageing treatment of cast aluminium alloys, such as the A356-type alloys. Therefore, the present study takes into account the painting temperatures and times to optimize the ageing treatment of the wheels, reaching the desired mechanical properties. This study develop an optimization approach of the whole heat treatment process of cast aluminium alloy wheels. The work evidences the typical problems and targets of wheels’ producers, suggesting an integrated approach to improve the productivity and the quality of castings. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes occurring during heat treatment; while hardness and tensile testing measurements have been carried out to monitor the evolution of the mechanical properties after each step of T6 heat treatment.