Investigation of TI6AL4V microstructure effects on process material interaction during micro milling

Abstract

The interrelationship between material microstructure and process parameters must be well understood in order to improve the machinability. In micro milling, the process parameters including depth of cut, feed and cutting edge radius are comparable to grain size of the material which significantly affects the mechanics of machining. This thesis investigates the contribution of microstructural characteristics including grain size, grain morphology and phase fractions/distributions of dual phase Ti6Al4V titanium alloy in micro scale milling. Various heat treatments were performed on the Ti6Al4V samples obtaining five different microstructures including fine equiaxed plus elongated, two size enlarged equiaxed, lamellar and martensitic microstructures. The influences of microstructures on built upedge (BUE) formation, cutting forces, surface quality and burr formation were studied. It was observed that smaller grain size leads to larger BUE and burr formation and higher cutting forces. However, when feed is set properly it also yields better surface roughness. The crystallographic texture and microstructure of the machined surface of selected samples were investigated using electron backscatter diffraction (EBSD) analysis which revealed that at low feed rates can result in occurrence of dynamic recrystallization (DRX) on the microstructure of the machined surfaces. It was observed that up and down milling stages led to different crystallographic texture of the machined samples during micro scale milling. The findings of this study are important in terms of developing predictive modeling of machining based on material microstructure.