Towards an understanding of high strength metastable β titanium alloy machinability

Metastable β titanium alloys are used within the aerospace industry due to their high strength, fracture toughness, hardenability and low density. Utilisation of these alloys is expensive due to their poor machinability. To improve the machinability cutting tool wear and machined surface quality must be better understood. To this end, drilling, turning and orthogonal cutting studies have been conducted on the metastable β titanium alloy, Ti-5Al-5Mo-5V-3Cr. The titanium alloys; Ti-6Al-4V, Ti-6Al-2Sn-4Zr-6Mo and Ti-5Al-2Sn-2Zr-4Cr-4Mo were also investigated to provide comparability. Emphasis was placed on; 1) Characterising microstructural deformation in the workpiece. 2) Analysing force and torque response. 3) Assessing machined surface quality. 4) Understanding how the chemical interaction between titanium and WC-Co influences crater wear. In this work, a novel surface response methodology enabled subsurface damage to be characterised for multiple alloys, the results showed how subsurface damage depth decreased with increasing alloy strength and how the depth varied for PVD-TiAlN coated and uncoated tools. Uncoated tools resulted in a more efficient cutting process compared to coated tools at the start of drilling: but as the tools started to wear, the efficiency of uncoated tools decreased, while the efficiency of coated tools increased. Built up edge (BUE) was prominent in Ti-5Al-5Mo-5V-3Cr machining, resulting in poor machined surface quality and reducing process stability. Analysis of Ti-5Al-5Mo-5V-3Cr and WC-Co interfaces using transmission electron microscopy showed that during crater wear Co diffused rapidly from the tool substrate once titanium had adhered and this was followed by decarburisation and subsequent breakdown of the WC. The methods developed in this thesis can help speed up the tool-design process efficiency while the knowledge can help tool designers to make intelligent design decisions for tools used to machine titanium alloys, both of which can contribute to improving the cost effectiveness of using metastable β alloys within industry.