Plasticity and Damage Modeling of the AA7075 Aluminium Alloy for Hot Stamping

LS-DYNA® has several plane stress material models available for isothermal aluminum sheet stamping, most notably *MAT_3-PARAMETER_BARLAT, *MAT_BARLAT_YLD2000, and *MAT_KINEMATIC_HARDENING_BARLAT89. Recent models such as *MAT_BARLAT_YLD2000 based on Barlat’s YLD2000 yield surface accurately capture plastic flow and yield anisotropy of most aluminum sheet alloys. Some of these models are also applicable for non-isothermal forming. However, there are no general stress state models available for solid elements that can describe aluminum anisotropy and support temperature and rate depended parameters and hardening. Another problem is failure prediction as there are no temperature and rate sensitive failure criteria available for hot forming. This paper presents the development, implementation and validation of a user defined material model (UMAT) for the AA7075 aluminium hot stamping process which supports both shell and solid elements. It includes Hill plasticity with a non-associated flow rule and a damage model similar to GISSMO but extended to cover non-isothermal conditions. All simulations were performed using the implicit thermal and mechanical solvers in LS-DYNA which has several features for hot stamping modeling.