Electrical-thermal-mechanical coupled modeling and simulation on deformation behaviors of Ti6554 alloy in electrically-assisted micro-tension

Abstract

The electrically assisted forming (EAF) technology can reduce the load & stress and improve the formability of difficult-to-deform alloys due to its coupled thermal and athermal effects. To decouple the two effects and to elucidate each mechanism for deeply understanding and optimizing the EAF processing, we developed an electrothermalmechanical (ETM) macro-meso multiscale model by combing a modified Johnson-Cook (JC) constitutive model with the dislocation density based crystal plasticity finite element (DD-CPFE). The modified JC was used to describe the macroscopic electro-assisted deformation, while the meso-scale DD-CPFE was used to deal with the thermal and athermal effects on the slip rate. Statistically stored dislocations (SSDs) were introduced into the Orowan rate equation to explain the thermal effect part, and the athermal part was reflected through the effect of free energy change on the dislocation motion in the rate equation, due to the collision between free electrons and metallic ions. The simulated results show that the equivalent stress, strain, and dislocation densities are distributed heterogeneously in a meso-scale and sensitive to the current density. Local thermal effect due to geometrically necessary dislocations (GND) exerts no effect on the temperature gradient because of rapid heat conduction between grains. The thermal effect is dominated by the thermal expansion obviously reducing the flow stress in the single-phase and two-phase fields (0 to 70 A/mm2), while the athermal effect is effective in the two-phase field (corresponding to the current density range of 0 to 20 A/mm2) in reducing the flow stress, which is reasonably explained by the change of bonding strength in defect zones, such as grain boundaries, of the HCP structure via the first principle calculations.