Comparative Study on Failure Prediction in Warm Forming Processes of Mg Alloy Sheet by the FEM and Ductile Fracture Crit
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TRODUCTION
RECENTLY, industrial applications of magnesium alloys have increased due to their advantages such as low density, high specific strength, excellent machinability, and high electrical and thermal conductivities. The efforts of the automobile industry to reduce fuel consumption through use of lightweight structural materials have accelerated the development of forming technologies, especially for skin structures with large surface areas. However, magnesium alloys exhibit low ductility at room temperature because of their hexagonal close-packed structure with limited slip systems. In order to increase the number of slip planes and obtain good formability, forming technologies between 423 K and 573 K (150 C and 300 C) where additional slip systems can be activated have been developed. Lee et al.[1] reported a comparative investigation on gas forming and conventional punch-die forming of AZ31 alloy sheets, and Yoshihara et al.[2] developed a local heating and cooling technique to enhance the formability of magnesium alloys in a drawing process. And, Kim et al.[3] and Zhang et al.[4] published a report on the formability of magnesium alloys in single-point incremental forming at elevated temperatures. More recently,
SANG-WOO KIM, Senior Researcher, and YOUNG-SEON LEE, Principal Researcher, are with the Department of Materials Deformation, Korea Institute of Materials Science, Changwon, Gyeongnam, Republic of Korea. Contact e-mail: [email protected] Manuscript submitted January 24, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
Meng et al.[5] have introduced a hybrid process, electromagnetic forming combined with warm forming techniques, to improve the formability of magnesium alloy sheets and investigate the effects of process parameters on the process. In the design of the forming process for a sheet metal component, a thorough understanding of the plastic deformation mechanism and forming limit of the material can reduce the development cost and time. Various criteria have been proposed to predict crack initiation. A forming limit diagram (FLD) has been most widely used in sheet metal forming.[6] Lee et al.[7] have employed the strain rate-dependent FLD of AZ31 alloy sheet with the thickness of 1.5 mm and applied it to predict forming failure. However, construction of strain rate-dependent FLD requires tremendous amounts of experimental data for various temperatures, strain rates, and strain modes. Furthermore, its strain path dependency often leads to an inaccurate prediction under non-proportional loading conditions.[8,9] The ductile fracture model is an alternative method to the FLD to predict the forming failure. While FLD is not applicable to thick plates over the thickness of 2 mm because it cannot consider the effect of strain in the thickness direction, ductile fracture criteria considering stress triaxiality can be applied to all kinds of bulk and sheet forming operations. Various type of ductile fracture models based on different theoretical and experimental hypotheses have been proposed.[10–17] Gen
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