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 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.

Prediction and Experimental Validation of Cooling Rate Dependence of Viscoplastic Properties in a Partially Solidified State of Al–5 mass%Mg Alloy

“…However, the strain rate sensitivity coef cient m is treated as constant. Solid fraction dependence, which is con rmed experimentally 10) , is ignored. Furthermore, no validation of the equation by comparison with experimental values has been provided regarding the cooling rate dependence.…”

“…The device was developed by Chiba et al 28) based on one developed originally by Oya et al 29) . The device revealed true stress-true strain curves of an aluminum alloy and a copper alloy in a partially solidi ed state 10,16,17,30,31) .…”

“…In thermal stress analysis reported by research institutes in Japan, elastoplastic constitutive equations are used 4,7) . However, reports of the literature emphasize that stress depends on the strain rate in partially solidi ed alloys 1,9,10) . Thermal stress analysis using elasto-viscoplastic constitutive equations has been reported worldwide [1][2][3]5,8) .…”

“…From this background, the current study examines the following two points using an Al-5 mass%Mg alloy, for which mechanical behavior in a partially solidi ed state was investigated earlier by the authors 10,16,17) . I.…”

Effect of Sc and Zr on Al₆(Mn,Fe) Phase in Al–Mg–Mn Alloys

“…Hence, the Mg content should be limited to 5% or less. 4) However, in industrial production, impure elements are inevitable in the aluminium alloys, which have some negative effects on properties of products. In general, iron is an common impurity in AlMg alloys and can precipitate as the Al 13 Fe 4 phase.…”