Microstructural Analysis of Machined Surface Integrity in Drilling a Titanium Alloy

Severe mechanical deformation coupled with high heat generation prevails during drilling. Establishing correlations between microstructure and surface integrity has always been a challenge, which is the main focus of this work. High-speed drilling experiments were performed by varying speed, feed rate and machining environments (dry and wet). The changes in microhardness, residual stresses and microstructure on the drilled surfaces were analyzed. A dominant mechanical deformation is found to lower grain size and increase grain boundary misorientation angle, whereas under a dominant thermal deformation higher grain size and lower grain boundary misorientation angle was evident. In dry drilling, a combined effect of temperature and mechanical deformation, the deformed and then recrystallized grains are observed to have orientation. The drilling parameters that increase strain rate aggravate machining-affected zone, whereas heat accumulation increases heat-affected zone, only in dry drilling. An empirical model for predicting grain size has been developed.