Evolutions of grain size and micro-hardness during chip formation and machined surface generation for Ti-6Al-4V in high-speed machining

Abstract

Machined surface with ultrafine grained microstructure can be obtained through severe plastic deformation in high-speed machining (HSM) process. The aim of this paper is to investigate the evolution of grain size and microhardness during HSM of Ti-6Al-4V alloy using the finite element method (FEM) and user subroutine VUSDFLD of Abaqus/Explicit. Firstly, a FE model to simulate the cutting process of Ti-6Al-4V is proposed. The proposed cutting simulation model is verified by high-speed machining experiments in terms of cutting force and chip morphology. Secondly, a novel user subroutine VUSDFLD based on equations of Zener-Hollomon and Hall-Petch is developed to simulate the modifications of grain size and micro-hardness in chip formation and machined surface generation under different cutting speeds. Parameters in the equations of Zener-Hollomon and Hall-Petch are modified for Ti-6Al-4V for simulation of the change of grain size and micro-hardness during HSM. Lastly, the simulation results of the microstructure evolution in chips and machined surfaces are compared with experimental results obtained by optical microscopy, scanning electron microscopy (SEM), and measurement of micro-hardness. The comparison results show that the evolution of grain size and micro-hardness of Ti-6Al-4V in HSM can be accurately predicted by the modified Zener-Hollomon and Hall-Petch equations. This research indicates that smaller grain sizes are produced into both chips and machined surfaces due to more severe deformations with increasing of the cutting speed. The findings validate that HSM is a reliable approach to generate refined grains if proper machining parameters are selected. HSM can also be applied as a novel material test method to study the relationship between microstructure evolution and deformation parameters.

A coupled in-situ measurement of temperature and kinematic fields in Ti-6Al-4V serrated chip formation at micro-scale

“…In recent years, various modelling attemps has met with the need of reliable measurements to either discriminate and/or validate models [37]. The use of post-mortem analysis such as chip morphology and SEM analysis [57,67,17,61,60] remains the most common approach. Force components measurement in real time trough Kistler dynamometer is also very popular and has become almost dogmatic in machining research [53,33].…”

Dislocation Dynamics-Based Modeling and Simulations of Subsurface Damages Microstructure of Orthogonal Cutting of Titanium Alloy

“…During the micro-cutting Ti-6Al-4V process, the workpiece in the annealed condition was adopted, and its microstructure mainly consisted of equiaxed α phase grains and a small quantity of β phase. On the basis of Wang et al [ 19 ], the effect of the β phase can be ignored because of its small volume fraction in Ti-6Al-4V alloy. Therefore, only equiaxed α phase grains were used in the present study, and the grain size was 3.4 μm × 2.95 μm.…”

The influence of cutting parameters on the defect structure of subsurface in orthogonal cutting of titanium alloy

“…It is found that the subsurface damage layer is principally accommodated by dislocation evolution and slip bands within alpha phase grains [10]. In particular, on the basic of Liu et al [28], the effect of beta-titanium on subsurface plastic deformation can be ignored in cutting simulation process due to the low volume fraction of beta phase, which is not more than 8%. Thus, it is reasonable to assume that only alpha grains were considered in present study, and indicating that the subsurface zone was composed of hexagonal close-packed (HCP) structural crystals.…”