Coupled Mechanisms of Tool Wear and Subsurface Strain Hardening in Milling of Ti-6Al 4V Alloy Under Various Cutting Environments

Abstract Degradations of machined surface integrity due to thermal softening and strain hardening are prevalently found in machining of titanium alloys due to their low thermal conductivity (~ 6.7 W/m·K). However, these contradictory effects can alternatively occur due to interactions in cutting environments and tool wear patterns. This study investigates the influences of cutting environments on the contradictory physical effects of thermal softening and strain hardening in cutting Ti-6Al-4 V parts under dry, flooding, subcooled and cryogenic conditions. By inversely correlating the cutting forces against the cutting lengths, tool wear and cutting environments, the altered cutting force coefficients ( $$\:{K}_{t}$$ , $$\:{K}_{r}$$ , and $$\:{K}_{a}$$ ) incorporating the engaged shear areas in the tangential, radial and axial directions can be approximated. In dry cutting, the tool wear produced grain refinement and fragmentation-induced hardening (~ 344.1 HV 0.025 ) thereby significantly overrode those of thermal softening on the subsurface, resulting in increased cutting forces. In contrast, cutting with flooding provided better cooling, lubrication and heat dissipation, thereby reducing tool wear and suppressing strain hardening (~ 313.8 HV 0.025 ). Although cryogenic cooling in gas or liquid conditions offered the lowest cutting-zone temperatures, severe abrasive wear and flaking fractures were adversely encountered at the tool edges. The coupled effects of tool wear resulting in pronounced grain distortion, fragmentation, and refinement on the machined subsurface were proven with EBSD analysis. Spectroscopic analysis showed that flooding, subcooled, and cryogenic environments did not sufficiently reduce the cutting-zone temperature, resulting in graphitisation of the DLC coating, while diffusion wear occurred in all cutting environments.