Experimental study on fracture mechanism transformation in chip segmentation of Ti-6Al-4V alloys during high-speed machining

Ti6Al4V alloy is one of the typically difficult-to-machine materials which is widely used in aerospace industry. During the machining process of Ti6Al4V alloys, serrated chips are easily formed and would influence cutting forces significantly. In this study, the variation of cutting forces of Ti-6Al-4V alloys during high-speed milling from 50-500 m/min is investigated, and the fracture mechanisms and microstructure evolution of chips are analyzed by optical microscope (OM), scan electron microscope (SEM) and transmission electron microscope (TEM). The results show that cutting forces and serrated degree of chips both increase first and then decrease with the increase of cutting speeds, especially when adiabatic shear bands appear. The microstructure evolution inside adiabatic shear bands also show a significant difference with different chip morphology at different cutting speeds, which finally cause the variation of cutting forces due to the change of fracture mechanisms between chip segmentations. As a result, the evolution of adiabatic shear bands is analyzed, and replica method by polydimethylsiloxane (PDMS) is used to reveal the relationship between cutting forces variation and fracture mechanism transformation in chip segmentation. It shows the relationship between cutting forces and microstructure evolution in adiabatic shear bands, and the transformation of fracture mechanism between type-I and type-II would occur around the gap between chip segmentations, where cutting forces are higher under type-I fracture.