Author

George Y. Liu

Date of Award

11-27-2009

Degree Type

Thesis

Degree Name

Master of Applied Science (MASc)

Department

Mechanical Engineering

Supervisor

Sumanth Shankar

Language

English

Comments

The aluminium A356.2 casting alloy is one of most popular alloys for shaped casting of automotive components because of its high strength to weight ratio and the ability to heat treat to attain fairly high toughness. Besides Al, the alloy has Si and Mg as the principal alloying additions to aid in the precipitation of the Mg2Si phase in the primary Al matrix to strengthen the alloy during heat treatment. Presently, the T6 heat treatment temper is most used on cast components from these alloys. Commercially, in the T6 temper, the component is solution heat treated at around 540C for about 10 to 12 hours immediately followed by quenching in water maintained at 80C. The component is then left to natural age at room temperature for about 8 to 10 hours followed by artificial ageing at around 155C for about 6 to 10 hours. This T6 temper treatment process has been followed for many years and adopted from the 6xxx series Al wrought alloy where the strengthening precipitate is Mg2Si as well. No significant research has been carried out to evaluate to optimize the T6 heat treatment in A356.2 casting alloys, specifically, the natural ageing phenomenon adopted from the 6xxx series alloy where the Si to Mg ratio is between 1 and 3, has not been optimized in A356.2 alloy, wherein, the ratio Si to Mg is between 4 and 6. Hence, the mechanism of redistribution of Mg and Si atoms during the natural and artificial ageing process would have to be studied for the A356.2 alloy and the process optimized to attain favourable mechanical properties.

In this study, the mechanism of Mg and Si atoms redistribution during the process of natural ageing has been proposed. Additionally, the effect of natural ageing combined with the artificial ageing process on the mechanical properties of the cast component has been quantified. The results of this study propose that the extent of natural ageing in A356.2 alloy depends on the desired combination of strength and elongation in the cast component. A high strength (~230 MPa) and low elongation (~2%) could be achieved if the natural ageing process at room temperature is limited to less than one hour after solution heat treatment and quenching in water at 80°C, and low strength (~195 MPa) and high elongation (~8%) could be achieved for natural ageing of about 6 hours at room temperature prior to the artificial ageing treatment. The recommendations are valid for solutionizing the components at 540C for 12 hours, followed by quenching in water at 80C and an artificial ageing at 155C for various times.

McMaster University Library

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