Predicting hot tearing during direct chill casting using thermal stress analysis requires constitutive equations in both semi-solid state and below the solidus of the alloy. However, numerous difficulties have been hindered constitutive equations used heretofore for hot tearing predictions. (1) Testing methods for obtaining material constants were inappropriate. First, the elastic strain reversibility was unconfirmed. Second, a flat distribution of temperature in the specimen gauge length was not guaranteed. Third, strain was measured not from local strain but from cross-head displacement. Fourth, the melt-back phenomenon was unavoidable in test during partial remelting because of homogenization of the segregation structure. (2) Temperature dependence of the strain-rate sensitivity of stress was not considered. (3) Some material constants were inferred, not obtained experimentally. This study developed elasto-viscoplastic constitutive equations (Hooke's and viscoplastic NortonHoff laws) for partially solidified state and below the solidus. To obtain material constants experimentally, two tensile tests for which issue (1) was addressed were conducted using Al-5 mass%Mg alloy. They were a tensile test after partial solidification and high-temperature tensile test with high-frequency induction coil. After the temperature dependence of elastic and viscoplastic properties was investigated, material constants were obtained and were compared with those obtained using earlier testing methods.
In order to predict the hot tearing during DC casting by using thermal stress analysis, constitutive equations in both solid-liquid coexisting and below solidus of alloy are inevitable. However, previous constitutive equations used in hot tearing predictions have no less than one problem as follows. 1 Testing methods of obtaining material constants weren t appropriate. Firstly, elastic strain reversibility was unconfirmed. Secondary, flat distribution of temperature in gauge length of specimen wasn t guaranteed. Thirdly, strain was measured from not local strain but cross head displacement. Fourth, the melt-back phenomenon was unavoidable in partial melting method which was caused by homogenization of segregation structure. 2 Temperature dependence of strain rate sensitivity of stress wasn t considered. 3 Some material constants were not obtained experimentally but guessed. In this study, elastoviscoplastic constitutive equations Hooke s and Norton s laws for both partial solidification and below solidus were developed. In order to obtain material constants experimentally, two tensile tests which the problem 1 was solved were conducted using Al-5mass%Mg alloy. They were partial solidification tensile tests above solidus and high temperature tensile test with high frequency induction below solidus. Then, temperature dependence of elastic, viscoplastic properties were investigated and material constants were obtained. Furthermore, they were compared with other previous testing methods.
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