The objective of this study is to investigate the microstructural changes and phase transformation of chips and workpiece during slot milling on Ti-6Al-4V alloy in dry, flood coolant, and MQL conditions using uncoated carbide tools. The experiments were performed at varying feed rate and depth of cut with a fixed cutting speed of 50 m/min. The microstructures of the machined chips indicate that dry machining had the highest percentage of β-phase indicating higher phase transformation owing to higher tool temperature. The β-phase was found to be the least in MQL machining chips, which is very similar to the microstructural composition of un-machined surface. Although there were signs of phase transformation, there were very minimal changes in phases in the workpiece for all three machining conditions. In many cases, no change or slight decrease in β-phase was observed at the sub-surface, indicating thermal softening of the workpiece, especially in dry machining. The findings from this study confirm the fact that, high temperature close to beta transition temperature is generated during dry machining of titanium alloy, and most of heat is carried away by the chips resulting in phase transformation from alpha to beta phase in chips. However, no significant phase changes occurred into the microstructure of the workpiece in any condition, although minor thermal softening was found at the sub-surface of dry machined workpiece.

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