Evolving-plasticity-based forming limit prediction for magnesium alloys at elevated temperature

초록

In this study, the formability of magnesium alloys is predicted using a macroscopic evolving yield function at elevated temperature. At room temperature, plastic deformation in magnesium alloys is limited due to the activation of only a few slip systems; therefore, sheet forming is often performed at elevated temperatures. Owing to the complex activation of slip/twinning systems and the strong texture effects, the plastic anisotropy of magnesium alloys evolves significantly during deformation, exhibiting isotropic-distortional hardening behavior. Consequently, the formability of magnesium alloys, which strongly depends on their plastic properties, is evaluated using the Marciniak-Kuczynski (M-K) approach extended with an evolving non-quadratic yield function. The directional anisotropy of two magnesium alloys, E-form and AZ31B, is calibrated from uniaxial tension data. Their biaxial properties, which are difficult to measure experimentally, are indirectly determined from force-displacement curves of circular specimens during Nakajima tests. The results show that the forming limits of both materials are accurately predicted by the evolving Yld2000-2d model with exponent a = 8, whereas models with lower exponents or quadratic yield functions fail to match the experimental data. Furthermore, non-evolving yield functions, derived at a specific plastic work from the best-fit parameters, yields predictions that deviate from the experimental results. The proposed approach highlights the necessity of using an evolving non-quadratic yield function for accurate plasticity modeling and forming limit prediction of magnesium alloys, and introduces a concise, practical methodology applicable when experimental biaxial data are limited.

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