Effectiveness of uncoated WC–Co and PCD inserts in end milling of titanium alloy—Ti–6Al–4V

Titanium and its alloys have found wide application in the aerospace, biomedical and automotive industries owing to their good strength-to-weight ratio and high corrosion resistance. However, these alloys have very poor machinability, which is attributed to their inherent high strength maintained at elevated temperature and low thermal conductivity leading to high cutting temperatures. Chips formed are serrated in nature as a result of a cycle of compression and adiabatic plastic shear phases in the chip formation process, causing high fluctuations of cutting force acting over a small chip tool contact area (about one-third that in the case of steel). High chemical reactivity of titanium at high-elevated temperatures, especially with titanium-based tools or coatings limits their application during machining. So the strategy of titanium machining is to use tools which show less reactivity, has higher thermal conductivity to increase the chip–tool contact length and effectively take away the generated heat and to use tougher and harder tools grades which could withstand the dynamic action of the cutting force. The recommended tools for many years had been the uncoated tungsten carbide grade K. But these tools cannot be used at high cutting speed since they too fail due to dynamic normal component of cutting force that acts on the very small contact area of the tool and high cutting temperature, leading to intensive diffusion and superficial plastic deformation, causing catastrophic failure of the tool. In this work, the effectiveness of polycrystalline diamond inserts (PCD) has been compared to that of uncoated tungsten carbide–cobalt inserts in machining titanium alloy Ti–6Al–4V. The comparison has been made with respect to the applicable cutting speed ranges, metal removal per tool life and tool wear rates, tool wear morphology, surface finish, chip segmentation and chatter phenomena.