Strain Behavior of JIS ADC12 Alloy Die Castings during Heat Treatment and Its Mechanism

Abstract

This study investigated the strain variation of JIS ADC12 aluminum alloy die castings by heat treatment and its relation with the strain arising from the precipitation of silicon, copper, and magnesium so that the most effective measures can be adopted to ensure dimensional precision in various service environments.

Expansion strain of over 0.1% was produced in JIS ADC12 aluminum alloy die castings by heat treatment, which resulted in a simultaneous decrease in the half-width angles of the X-ray diffraction (XRD) peaks of the aluminum phase in the die castings. Moreover silicon, copper, and magnesium concentrated zones appeared in the aluminum phase after heat treatment. Thus, the decrease in the half-width angles can be regarded as a result of the improved crystallinity of the aluminum phase because of the relaxation of the lattice strain by the precipitation of silicon, copper, and magnesium. Hence, the strain variation in the ADC12 alloy die castings due to the heat treatment can be attributed to the precipitation of silicon, copper, and magnesium from the supersaturated aluminum phase. For quantitative verification of the above relation, growth attributable to the precipitation of silicon, copper, and magnesium from the aluminum phase and the transformation of the precipitated metastable Cu-Al compounds was estimated theoretically and compared with the measured strain variation caused by the heat treatment of the ADC12 alloy die castings. The results confirmed that the strain variation of the ADC12 alloy die castings by heat treatment corresponds well to the growth resulting from the precipitation of silicon, copper, and magnesium from the aluminum phase and the transformation of the precipitated metastable Cu-Al compounds. The results also revealed that silicon, copper, and magnesium precipitated at the early stage of heat treatment and that silicon precipitation contributed the most to the growth. The precipitated metastable Cu-Al compound, θ”, transformed into another metastable compound, θ’, and finally into the stable compound, θ, thereby resulting in expansion and contraction growth.

 

This Paper was Originally Published in Japanese in J. JFS. 89 (2017) 111–118.