Wear behaviour of coated carbide tools during machining of Ti6Al4V aerospace alloy associated with strong built up edge formation

Abstract

In the machining of Ti alloys, it is challenging to optimise the cutting tool life and process productivity. It is not a trivial task to find an efficient strategy to improve the tool life during machining of Ti alloys using surface engineered tooling. In the case of rough turning operation with strong built up edge formation we establish that it can be achieved through the application of self-lubricating TiB2 PVD coating. It has been shown that the application of a TiB2 coating results in tool life improvement by over 60% compared to the uncoated tool and over 70% compared to the TiAlN coated tool. Comprehensive characterization of the coated vs. uncoated cutting tool wear performance was performed using optical 3D imaging, SEM/EDX and XPS methods. Various micro-mechanical characteristics of the TiB2 coating were evaluated. It was determined that tool life improvement using the TiB2 coating is mostly related to the ability of the coating layer to provide self-lubrication effect and in this way, very efficiently dissipate frictional energy. The coating also exhibited less substrate exposure as it fails indicating better protection of the coated tool surface. This is of particular importance for machining of materials like Ti, which have strong sticking intensity. It has been demonstrated that the TiB2 coating combines beneficial micro-mechanical characteristics and self-lubricating properties due to the formation of B-O tribo-films on the tool surface under operation. These tribo-films serve as a liquid lubricant formed in-situ on the tool surface under the elevated temperature of cutting. The formation of liquid tribo-films is an effective way to address intensive adhesive interaction followed by built up edge formation at the tool/chip interface, which is typical for roughing operations during Ti machining.