Microstructural Assessment of Drilled Cross-sections on Titanium generated Under Different Cooling Strategies

Abstract High heat generation during machining of titanium alloy causes microstructural damage on machined surfaces. Different cooling strategies in machining of titanium alloy have been studied to understand their effectiveness in improving heat dissipation and machined surface microstructure. In this work, heat-sink based cooling has been used to channelize an ice water for effecting positive heat dissipation. Before drilling experiments, solution annealing was performed on all Ti-6Al-4 V samples to achieve an equiaxed microstructure. Extensive drilling experiments under dry, flood cooling and heat-sink based cooling environments were performed under different cutting parameters. Electron Backscatter Diffraction (EBSD) technique was utilized to characterize microstructure of titanium drilled top and bottom surfaces. Dry drilling showed the highest average of 38 µm of machining affected zone (MAZ), whereas the least MAZ average 3 µm which was evident in the heat-sink based cooling. Image quality (Kikuchi pattern intensity) quantification revealed the highest normalized IQ value, indicating the lowest deformation in heat-sink based cooling over dry and flood cooling at the top and bottom surfaces of the drilled samples. Misorientation maps show that low angle grain boundaries misorientation is reduced in heat-sink based cooling by maximum 75 % over dry drilling, and by 57 % over flood cooling, at the exit hole surfaces. Kernel Average Misorientation (KAM) analysis at the bottom surfaces shows that the local misorientation in heat-sink based cooling is reduced by 90 % as compared to dry drilling and by 60 % over flood cooling.