EVOLUTION OF GRAIN REFINEMENT AND PHASE TRANSFORMATION BASED ON STRESS- STRAIN INDUCED INCREMENTAL MODEL IN MACHINING OF TITANIUM ALLOY

Grain refinement occurs on the machined surface and adiabatic shear band (ASB) through severe plastic deformation (SPD) in the high-speed machining (HSM) Ti-6Al-4V alloy, resulting in the formation of desirable ultrafine grain structure. The extremely high temperature changing rate induces the transformation of the original a-phase into beta-phase, which then forms martensitic phase after cooling process. The altered microstructure has different effects on the material properties and machined surface integrity. Therefore, the established model has received the increasing attentions to accurately predict microstructural evolution. This research proposes a straininduced incremental model of grain dynamic recrystallization (DRX) evolution based on the conventional Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The model emphasizes the contribution of large strain to grain refinement during the machining of Ti-6Al-4V alloy. In addition, a stress-induced phase transformation increment model is proposed to describe the phase evolution in machining since high stress can effectively reduce the initial temperature in transiting alpha-phase to beta-phase and the martensite phase transition temperature. Subsequently, the orthogonal cutting simulations of Ti-6Al-4V alloy are carried out to validate the proposed model by comparing with the existing research results. The simulation results indicate that the average grain size in the ASB is increased from 46.2% to 54.2% and the average grain size at the machined surface decreased from 85% to 83% with the cutting speed range of 5 - 20 m/s. The average grain size at the machined surface is decreased from 83% to 78% with the tool flank face wear (VB) ranging from 0.00 mm (fresh) to 0.08 mm. The decreased temperature has an inhibitory effect on the grain refinement process of the ASB and the machined surface within the range of initial temperatures from -150 degrees to 25 degrees.