2010 Volume 82 Issue 2 Pages 79-85
The occurrence mechanism and the occurrence criterion of cracks in shell molds during aluminum alloy casting were elucidated. Cylindrical shell molds made of silica sand break easily when filled with aluminum alloy melt. The breaking mechanism can be considered as follows. The immediate inner surface of the shell mold undergoes rapid temperature rise from sudden 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 break when the tensile stress exceeds the tensile strength of the shell mold. With some parts of the cylindrical shell mold cut to a thinner thickness, a large tensile stress occurs at the outer surface of the thinner part and a crack is formed in a shorter time after aluminum alloy melt has filled. The criterion for the occurrence of cracks in shell molds can be described by the fracture stress-effective volume relation based on the Weibull's statistical method, which is utilized for the strength evaluation of brittle materials. The fracture stress-effective volume relation of the present shell mold was derived from tensile test results to predict the occurrence of cracks in shell molds.
