Understanding the deformation and fracture mechanisms in backward flow-forming process of Ti-6Al-4V alloy via a shear modified continuous damage model

• Published in: Journal of materials processing technology Vol. 292; p. 117060
• Main Authors: Singh, Abhishek Kumar, Kumar, Abhishek, Narasimhan, K., Singh, Ramesh
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.06.2021
• Subjects: Continuous damage model; Finite element analysis; Flow-forming process; Shear; Ti-6Al-4V alloy; Triaxiality; Triaxiality; Flow-forming process; Shear; Finite element analysis; Ti-6Al-4V alloy; Continuous damage model
• ISSN: 0924-0136
• DOI: 10.1016/j.jmatprotec.2021.117060

•Studied three different roller arrangements in backward flow-forming via user defined sub-routines in a FE framework.•Created 3D fracture strain maps for stress triaxiality/Lode parameter and stress triaxiality/thermal softening factor.•Highligted the limitations of the wedge flow-forming test to evaluate the flow-formability.•Characterized the fracture locus to evaluate the flow-formability.•Developed a summary chart correlating the process parameters and flow-formability. A shear modified coupled damage criterion based on continuum damage mechanics has been proposed in this study. Khan-Huang-Liang (KHL) model is implemented to predict the constitutive behavior of Ti-6Al-4 V (Ti64) alloy. A VUMAT subroutine has been developed for the damage model using a stress integration algorithm. Multiple simulations with tensile, compression and shear geometries are carried out in Abaqus/Explicit and compared with the experimental results to benchmark the hardening and damage criteria. The flow forming process involves a complex triaxial state of stress. This study is focused on understanding the contribution of triaxiality and shear during deformation and fracture in the flow-forming process with different roller arrangements. For this purpose, the flow-forming processes with three different roller arrangements, single roller, three roller, and wedge roller, are modeled, and their formability is compared by implementing a shear modified continuous damage model. A single roller flow-forming (SRF) arrangement undergoes high triaxiality due to excessive material displacement by the roller and high strain gradient in the axial direction. The fracture occurs near the interface of the reducing and thinning zone. The three-rollers flow-forming (TRF) provided maximum reduction till fracture and material fails due to excessive strain in the thinning zone at a relatively large percentage reduction. The wedge roller flow-forming (WRF) suffers a lack of uniform material softening, and fracture occurs near the interface of the uplift and reducing zone under high triaxiality. 3-D process maps have been developed for predicting fracture strain as a function of stress triaxiality/Lode parameter and stress triaxiality/thermal softening factor (E/σ¯2). Finally, a summary chart correlating the parameters, and flow-formability has been developed.