On the mechanism of crater wear in a high strength metastable β titanium alloy

Abstract

The primary wear mechanism of WC-Co tools when machining aerospace titanium alloy components is crater wear. The diffusion controlled dissolution and thus the crater wear rate is more apparent when machining the high strength, fatigue resistant metastable β titanium alloys: an expensive class of titanium alloy used in aerostructural components such as twin aisle aircraft landing gear parts. To characterise the mechanistic process of crater wear progression, outer diameter turning trials were conducted on the metastable β alloy, Ti-5553. Uncoated WC-Co inserts were used for time steps of 30, 60 and 900 s at a cutting speed of 70 m/min. The crater region was analysed using high-resolution SEM, X-EDS, WDS and STEM/TEM. Such targeted characterisation enabled the detailed mechanism of progressive crater wear in titanium machining to be further understood. In the crater wear process, rapid diffusion of Co into adhered Ti-5553 occurs within the crater zone, proceeded by decarburisation of the WC (hcp). Thermodynamic modelling and high-resolution imaging of the interface suggests Co facilitates the formation of Ti2Co, and possibly a liquid, within prior Co rich regions at WC grain boundaries. Subsequently, this creates regions that exhibit similar characteristics to a Kirkendall morphology due to accelerated C depletion and W–Ti dissolution. The formation of abrasive titanium carbides in the workpiece material increases as C is made available from the WC transforming to W (bcc) and through binder regions. Once the W (bcc) layer is formed the rapid decarburisation process is concentrated at prior Co-rich (heavily β stabilised) Ti regions, this degradation dictates the crater wear rate during the machining of Ti-5553. Now an understanding of the crater wear mechanism has been characterised for Ti-5553, tooling manufacturers can engineer the Co-binder volume fraction and distribution, to improve tool life and provide significant cost savings when machining metastable β titanium alloy components.