Hardness control of Al–Si HPDC casting alloy via microstructure refinement and tempering parameters

• Al–17%Si based alloy and selected die casting and tempering parameters were evaluated.
• Optimized T5 temper & casting process yielded hardness sufficient for engine applications.
• Control of water quenching after casting de-molding increased T5 hardness.
• Microstructure refinement allowed for T5 hardness increase while reducing processing time.
• GP zones and θ′-Al2Cu precipitates were responsible for hardening effect.

Analysis of the effect of microstructure refinement, water quenching, natural ageing as well as T5 and T6 tempers on macro-hardness development of the hypereutectic HPDC Al–Si based alloy was performed. Duration of casting holding at room temperature after its removal from the die, and prior to subsequent temper operation had a measurable effect on casting macro-hardness. Such finding underlined the need for tighter control of parts handling between casting and heat treatment operations. Water quenching after casting de-molding operation increased alloy macro-hardness in naturally aged as well as T5 temper conditions but did not have any effect on macro-hardness after T6 treatment. T5 temper carried out after casting de-molding and water quenching from 380 °C enabled on average approximately 9 to 19% macro-hardness increase as compared to the T1 condition. Microstructure refinement had a minor effect on macro-hardness increase but the measurements of the α-Al matrix micro-hardness needed to be used to demonstrate this effect. Compared with T6, the T5 temper offered up to approximately 20 to 60% shorter process duration and could offer potential cost reduction gains achieved at the expense of lower macro-hardness. Microstructure refinement did not have the effect on T6 casting macro-hardness.

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