마그네슘 합금 판재의 변형률, 변형률 속도 및 온도 환경을 고려한 유동응력 모델에 대한 연구

Abstract

The formability of magnesium alloy sheets at room temperature is generally low because of the inherently limited number of slip systems, but higher at temperatures over <TEX>$150^{\circ}C$</TEX>. Therefore, prior to the practical application of these materials, the forming limits should be evaluated as a function of the temperature and strain rate. This can be achieved experimentally by performing a series of tests or analytically by deriving the corresponding modeling approaches. However, before the formability analysis can be conducted, a model of flow stress, which includes the effects of strain, strain rate and temperature, should be carefully identified. In this paper, such procedure is carried out for Mg alloy AZ31 and the concept of flow stress surface is proposed. Experimental flow stresses at four temperature levels (<TEX>$150^{\circ}C$</TEX>, <TEX>$200^{\circ}C$</TEX>, <TEX>$250^{\circ}C$</TEX>, <TEX>$300^{\circ}C$</TEX>) each with the pre-assigned strain rate levels of <TEX>$0.01s^{-1}$</TEX>, <TEX>$0.1s^{-1}$</TEX> and <TEX>$1.0s^{-1}$</TEX> are collected in order to establish the relationships between these variables. The temperature-compensated strain rate parameter which combines, in a single variable, the effects of temperature and strain rate, is introduced to capture these relationships in a compact manner. This study shows that the proposed concept of flow stress surface is practically relevant for the evaluation of temperature and strain dependent formability.