Abstract
In the present study, experimental investigations on microstructures and tensile properties of an counter-pressure cast (CPC) A356 aluminum alloy under different T5 heat treatment conditions were conducted in the temperature range of 160–200 ∘C for 1–48 h. As the T5 heat treatment time increased, both tensile and yield strength of the CPC A356 alloy either continuously increased at 160 ∘C until 48 h of heat treatment time or increased until the maximum strength values were achieved and then decreased, showing peak aging behavior at 180 and 200 ∘C. Changes in microstructural aspects, such as size and aspect ratio, of the eutectic Si, Mg and Si distribution in the α-Al grain and the stability of intermetallic compounds were found to be negligible during the T5 heat treatments employed in the present study. From high resolution-transmission electron microscope (HR-TEM) analysis, nanosized needle-like β″ precipitates were identified in the specimens, showing a significant increase in strength after the T5 heat treatment. Based on the measured tensile properties and observed microstructure changes, a yield strength model was proposed to predict yield strengths of CPC A356 alloys at arbitrary T5 heat treatment conditions. The calculation results of the model showed good agreement with the experimental data obtained in the present study. From the model calculations, the optimal T5 heat treatment time or temperature conditions were suggested.
Highlights
The consumption of aluminum alloys in the automotive industry has been increased for weight reduction in order to improve mileage or reduce emissions of vehicles
The yield strength of the T5 heat-treated counter-pressure cast (CPC) A356 alloy was calculated based on several strengthening mechanisms, including precipitation hardening by β
The time required to obtain the maximum strength of the CPC A356 alloy after the T5 heat treatment at 160 ◦C might be longer than 48 h
Summary
The consumption of aluminum alloys in the automotive industry has been increased for weight reduction in order to improve mileage or reduce emissions of vehicles. Mass productions of automobile parts using the A356 aluminum alloy are based on various casting processes, such as gravity die casting, low-pressure die casting, and counter-pressure die casting. The counter-pressure die casting process pressurizes both the lower chamber containing molten aluminum and the upper chamber containing casting molds so as to fill the mold under a working pressure higher than the atmospheric pressure. This filling process is beneficial in terms of less melt turbulence, higher heat transfer efficiency between mold and casting, less gas porosity, etc. Undercooling of the molten metal could occur during the solidification under the increasing external pressure, resulting in further refinement of the microstructure [3]
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