Development of an elasto-viscoplastic constitutive equation for an al-mg alloy undergoing a tensile test during partial solidification

Abstract

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 Norton-Hoff 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.