Difference in properties between chill zone and inner parts of ADC12 alloy die casting

Studies on solidified structure and properties of Al-Si-Cu alloy die castings (2nd report)

Abstract

The chill zone must be kept intact in order to prevent the lowering of properties of die castings. However, the differences in properties between the chill zone and inner parts have not yet been explained thoroughly.
This paper describes physical, mechanical, and chemical properties of both of the chill zone and inner parts of Al-12 Si-Cu-alloy (JIS ADC 12) die castings having thickness of 10mm.
The results obtained were as follows:
(1) Specific gravity of the chill zone was found to be higher by 6% than the center parts and coefficient of thermal expansion of the former was lower by 7% than the latter. Whereas, the lattice constant was found to be smaller with the increase of depth up to 1mm, and then abruptly larger approximate to the center. The cause of the above facts were due to the segregation of principal components, Si, Cu and Fe.
(2) Each of ultimate tensile strength, 0.2% yield strength, elongation, bending strength, impact strength, and fatigue strength showed a markedly higher value on the chill zone and lower by 3065% in the center part.
The values of hardness and wear resistance were found to be relatively lowered by the formation of primary α-crystals on the chilled surface. However, the maximum values were obtained on the chill zone of the die castings and the values were also lower in the inner parts. An evident correlation was found between these properties.
(3) The rates of corrosion and penetration in alkaline solutions were lower on the chilled surface rather than in the inner parts. Whereas, these rates in acidic solutions were higher on the chilled surface rather than the inner parts.