A microstructure sensitive grain boundary sliding and slip based constitutive model for machining of Ti-6Al-4V. (June 2017)

• Record Type: Journal Article
• Authors: Fernandez-Zelaia, Patxi; Melkote, Shreyes; Marusich, Troy; Usui, Shuji
• Is Part Of: Mechanics of materials. Volume 109(2017)
• Page Start: 67
• Page End: 81
• Publication Date: 2017-06
• Subjects: Machining -- Constitutive modeling -- Titanium -- Chip segmentation -- Grain boundary sliding -- Microstructure sensitive; Strength of materials -- Periodicals; Mechanics, Applied -- Periodicals; Résistance des matériaux -- Périodiques; Mécanique appliquée -- Périodiques; Mechanics, Applied; Strength of materials; Periodicals; Electronic journals; 620.11
• DOI: 10.1016/j.mechmat.2017.03.018
• Languages: English
• ISSNs: 0167-6636

Abstract

Highlights: A composite (α+β) Ti-6Al-4V constitutive model was developed for large-strain high-rate machining applications. The model includes diffusion assisted grain boundary sliding (GBS) based physics in addition to a traditional slip-based plasticity formulation. Microstructure sensitive internal state variables are included to capture the evolution of internal structure due to dynamic recrystallization (DRX) and dislocation nucleation/annihilation. The model is calibrated to uniaxial data, implemented as a finite element subroutine, and validated against machining data. The model accurately captures the machining force response as well as chip morphology characteristics. Periodic adiabatic shear banding in the machined chip is captured by the model, which is driven by a competition between slip and grain boundary sliding based deformation modes. Grain boundary sliding dominates deformation following DRX induced grain refinement. Engagement of GBS results in flow softening as lower stresses accommodate deformation in the refined microstructure. The process repeats as less refined material enters the chip deformation zone. Abstract: A composite dual phase internal state variable constitutive model was developed for Ti-6Al-4V. The proposed model includes diffusion assisted grain boundary sliding based physics in addition to a traditional slip-based plasticity. Influence of microstructure on the flow stress is introduced via dislocation density and mean grain size internal state variables. The dislocation density evolves according to a physics based law that considers dislocation nucleation and annihilation processes. Grain refinement is driven by dynamic recrystallization, which is modeled phenomenologically. The model is calibrated with uniaxial stress–strain data that ranges between quasi-static and dynamic rates across a wide range of temperatures. Validation against machining data shows that the model predicts chip segmentation frequency, machining forces, and tool temperatures reasonably well. The newly introduced grain boundary sliding physics was found to dominate deformation following sufficient grain refinement. This deformation mode provides softening at the constitutive level without the need for invoking damage based softening mechanisms. This physical interpretation is something that has not previously been explored in the machining literature.