2010 年 51 巻 8 号 p. 1420-1427
The mechanism of shell mold cracking and its prediction during casting of aluminum alloy were elucidated. A cylindrical shell mold made of silica sand fractures easily when filled with aluminum alloy melt. The cracking mechanism can be considered as follows. The immediate inner surface of a shell mold undergoes a sudden temperature rise from heating by the melt and attempts to expand. This thermal expansion is restrained by the other part of the mold that is still low in temperature. Consequently, compressive stress in the area near the inner surface and tensile stress in the area near the outer surface develop respectively, causing the shell mold to fracture when the tensile stress exceeds the tensile strength of the shell mold. With some part of a cylindrical shell mold cut to a thinner thickness, a higher tensile stress acts on the outer surface of the thinner part and a crack is formed in a shorter time after the mold has been filled with aluminum alloy melt. The criterion for shell mold cracking can be described by the relation of fracture stress and effective volume based on the Weibull’s statistical method, which is utilized for evaluating the strength of brittle materials. The relation of fracture stress and effective volume enabling us to predict the shell mold cracking was obtained from the statistical properties of the tensile strength of the shell mold material.
