Novel approach to enhance formability in Ti–6Al–4V alloy via stress relaxation prediction with a dislocation-based finite element model

Abstract

Abstract Pulsating forming techniques are advanced metal forming processes that apply cyclic or pulsed mechanical or hydraulic loads to the workpiece, enhancing formability and reducing defects compared to conventional static forming. Pulsating forming emerges as a transformative solution to overcome the intrinsic limitations of room-temperature formability in Ti–6Al–4V alloys. However, the lack of predictive, physics-based models hinders their industrial application. To address this gap in the literature, a novel hybrid finite element model was developed, which integrates a time-dependent Bergström dislocation model with the Gurson-Tvergaard-Needleman (GTN) damage criterion. The integration of stress relaxation, a pivotal mechanism in pulsating forming, was achieved through the modification of the Bergström model with a time-dependent differential equation. A correlation was established between this relaxation time and the GTN void nucleation parameter f N in order to directly simulate damage delay. The model was calibrated with monotonic tensile and relaxation tests, after which it was thoroughly validated against pulsating tensile and cylindrical cup hydroforming experiments. The accuracy of this method was subsequently demonstrated. The simulation showed an average error of 7.92 % for rupture pressures across nine cylindrical cup hydroforming scenarios. The present work provides a robust, physically based method that quantitatively links dislocation mechanisms, stress relaxation, and damage evolution, thereby establishing a foundation for model-driven design of advanced forming processes.