A New Damage Constitutive Model for Thermal Deformation of AA6111 Sheet

Hot tensile tests were conducted using a Gleeble 1500, at the temperature range of 623 K to 823 K (350 A degrees C to 550 A degrees C) and strain rate range of 0.1 to 10 s(-1). Flow stress is significantly affected by temperature and strain rate. As strain increases; the flow stress first rapidly increases, subsequently maintains a steady state, and finally drops sharply because of damage evolution. The features and mechanism of the damage were studied utilizing a scanning electron microscope. Micro-void nucleation, growth, and coalescence result in the failure of the hot-formed specimen. A damage equation based on continuum damage mechanics and damage mechanism in hot metal forming was proposed. A unified viscoplastic damage model coupled with strain, strain rate, temperature, dislocation, hardening, damage, damage rate, and so on was developed and calibrated for AA6111 using Genetic Algorism Tool in three steps. This model can be used to describe viscoplastic flow behavior and damage evolution at various temperatures and strain rates. The model was implemented into the finite element (FE) model in ABAQUS platform via the variable user material subroutine. Thus, the FE model could be employed to study the damage distribution and the effects of blank holder force (BHF) and forming velocity on hot cylindrical deep drawing. It is revealed that lower BHF and higher velocity are beneficial for drawability. A good agreement between simulated and experimental results has been achieved.