As a response to recent developments in manufacturing processes for near net shape (NNS) manufacture of titanium alloys, cutting tool manufacturers are seeking to develop cutting tool technology aimed specifically at high productivity finishing of precision components. Polycrystalline diamond (PCD) is currently under development as a next generation cutting tool material for titanium alloy finishing applications due to its unrivalled high temperature hardness offering excellent wear resistance properties and the ability to withstand very high cutting speeds. Whilst prior research has indicated promising results regarding the wear performance of PCD tooling, the influence of high speed PCD machining processes on component surface integrity has not received significant research attention. Accurate fatigue life predictions of titanium alloy components require an understanding of how the machining affected metallurgical and micro-mechanical subsurface condition influences the nucleation and subsequent growth of fatigue cracks. To qualify finish machining methods and cutting parameters used for the production of fatigue critical components, manufacturers often define limits for machining induced defects such as surface anomalies and subsurface deformation. These practices, however, do not consider the complete surface integrity system and therefore, fail to account for the interaction between different metallurgical and micro-mechanical material characteristics.
This research presents a comprehensive study on the influence of machining induced surface integrity characteristics on the fatigue performance of Ti-6Al-4V components following high speed PCD machining and the wear behaviour of PCD tool material during finish machining of Ti-54M. The findings have been benchmarked alongside material machined employing industry standard tungsten carbide (WC-Co) tooling at more conventional cutting parameters. The effects of residual stress distribution, subsurface microstructural condition, and the presence of machining induced surface defects/anomalies have been analysed with respect to their influence on crack initiation and propagation mechanisms.
High speed PCD milling has been demonstrated to be highly effective at generating surfaces free from microstructural deformation and surface defects. Achieving such low levels of surface alteration has been made possible by machining at very high cutting speeds (up to 450 m/min) which, in turn, has allowed for low feed rates to be employed whilst still achieving productive surface generation rates. Coupled with the very sharp edge radii of PCD inserts, the resulting cutting conditions produce a highly efficient chip generation process involving very low cutting forces. In comparison, cutting conditions during WC-Co milling were shown to induce substantial levels of severe plastic deformation to the subsurface microstructure as well as surface defects, such as “pick-up” and smearing.
Despite promising surface integrity characteristics, the fatigue performance of PCD machined components exhibited significantly inferior fatigue performance to their WC-Co machined counterparts. Mechanically induced compressive residual stresses, promoted by higher feed rates and the larger cutting edge radii of WC-Co tools, were shown to provide an overriding enhancing effect on fatigue life by suppressing crack initiation and reducing the deleterious effects of microstructural deformation and surface imperfections. These results highlight that machining induced surface defects and anomalies can be tolerated provided they are combined with sufficient levels of compressive residual stress.
This research has involved fatigue assessment of Ti-6Al-4V processed by conventional uni-directional rolling, as well as material fabricated by NNS powder processes. These include selective laser melting (SLM) and selective electron beam melting (SEBM) powder bed additive manufacturing (AM) processes as well as material consolidated by field assisted sintering technology (FAST). This has made it possible to analyse the effects of finish machining on the fatigue behaviour of material susceptible to fatigue crack initiation at porosity related defects as well as material prone to failure by conventional grain faceting mechanisms. The results demonstrate that machining induced compressive residual stresses remain effective at enhancing fatigue performance even when porosity related defects are responsible for failure and can mitigate against the fatigue limiting effects of pores in AM components.
