Abstract
For alloys that are quenched to 25℃ room temperature, there are Mg-Mg, Si-Si and Mg-Si clusters shown by exothermic peaks in a A357 alloy. However, there are no clear Mg-Mg, Si-Si and Mg-Si cluster exothermic peaks in a A356 alloy. Hence, the low Mg content in a A356 alloy has less impact with natural ageing. The natural ageing impact on the mechanical properties of a A357 alloy is higher than a A356 alloy due to the precipitation of Mg-Si clusters for which the nucleation size does not reach the critical size. The 90℃ pre-ageing process could promote Mg-Si clusters to a critical size to become the nucleation site for of β''. This then increases the artificial ageing strength and mitigates the impact of natural ageing.
Highlights
A356 and A357 aluminum casting alloys are widely used in the automotive and aerospace industries due to their good castability, wearability, corrosion resistance, weldability, and good strength to weight ratio etc
For alloys that are quenched to 25 ̊C room temperature, there are Mg-Mg, Si-Si and Mg-Si clusters shown by exothermic peaks in a A357 alloy
The natural ageing impact on the mechanical properties of a A357 alloy is higher than a A356 alloy due to the precipitation of Mg-Si clusters for which the nucleation size does not reach the critical size
Summary
A356 and A357 aluminum casting alloys are widely used in the automotive and aerospace industries due to their good castability, wearability, corrosion resistance, weldability, and good strength to weight ratio etc. A356 and A357 alloys are heat treatable Al-Si-Mg hypoeutectic alloys. T6 artificial ageing can enhance their strength and hardness. This major heat treatment procedure includes solution treatment, quenching and artificial ageing. There is usually a waiting time between quenching and artificial ageing due to production schedule arrangements that will impact the follow up artificial mechanical strength. Natural ageing is pre-ageing at room tem-
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