Effect of Tool Material and Lubrication Condition on Machinability of Ti64 Alloy

Abstract

Undoubtedly, extensive application of Ti64 alloy in numerous application areas (such as defense and aerospace; petrochemical refineries, marine industries, chemical and biomedical applications, etc.) invites many challenges both for the manufacturing industries as well as the research community. This alloy is experienced as ‘difficult-to-cut’ mostly during conventional machining. The poor thermal property of this alloy is, however, responsible for premature failure of cutting tools as the cutting zone instigates evolution of huge amount of heat; although strength of the alloy remains unaltered. Therefore, application of coolants or Metal Working Fluids (MWFs) comes into picture to reduce cutting zone temperature thereby improving tool life and machined surface integrity. Keeping this in mind, initial machining trials are carried out employing MWFs by different delivery strategies where conventional uncoated carbide tool is used. First set of experiments incorporate dry machining (longitudinal turning), machining under supply of pressurized air and distilled water based Minimum Quantity Lubrication (MQL). Along with tool-tip temperatures, characteristic features of spatial temperature distribution profiles at the vicinity of tool-tip, severity of induced-vibrations (at varying cutting speeds as well as cooling media) and wear progression in cutting tools are studied. The results obtained from the experiments witness beneficial effects of water-MQL medium for machining of Ti64 as noticeable reductions in tool-tip temperature, amplitude (absolute value) of acceleration of vibrations and tool wear are observed. Though water possesses higher heat capacity than oils; supplied water (under MQL) is likely to be vaporized easily at the machining induced temperatures causing poor heat removal from the cutting zone. In order to take care of several alarming issues (including vaporization of MWFs, environmental protection and occupational health hazards), the second set of experiments explores application feasibility of vegetable oil (Jatropha oil) as MWF for machining of Ti64. Non-edible Jatropha oil is used as the base fluid both for MQL and nanofluid MQL (NFMQL) conditions due to its biodegradability. Graphene nanoplatelets are dispersed within Jatropha oil to prepare nanofluids. During machining, assisted by MQL and NFMQL both, tool wear morphology reveals existence of ‘unaffected zones’ which clearly indicate sustenance of strong hydrodynamic tribo-film thus protecting localized portions of tool surface against wear. Apart from this, cutting force magnitude, tool-tip temperature, chip’s macro/ micro-morphology and surface roughness of the machined work part, etc., are studied in detail. Inadequate penetration of MWFs to the core machining sites, especially at higher cutting speeds (due to faster pace of evolved chips), and sedimentation/ local agglomeration of nano-additives (due to lack of suitable surfactant addition and improper dispersion within base fluid) are some of the major causes for which application of MWFs gradually becomes to be unpopular for industrial practices. Therefore, emphasis is given towards dry machining of Ti64 which requires momentous attention for improvement in tooling system. Selection of compatible tool material and coatings, appropriate tool geometrical parameters and precise control of cutting parameters are of vital importance towards achieving satisfactory machining yield under dry cutting condition. In this context, performances of MT CVD TiCN-Al2O3 bi-layered coated carbide, PVD TiN-TiCN-TiN multi-layered coated cermet and CVD TiCN-Al2O3 bi-layered coated SiAlON inserts are studied first during dry machining of Ti64 within cutting speed range of 50-130 m/min; at constant feed ~ 0.1 mm/rev and depth-of-cut ~ 0.35 mm. It is experienced that cermet tool performs better than remaining two counterparts in purview of lower tool-tip temperature, reduced tool flank wear and better machined surface integrity. Secondly, performances of MT CVD TiCN-Al2O3-TiOCN multi-layered coated carbide and PVD TiN single layered coated composite ceramic (Al2O3/ TiCN) inserts are compared during dry machining of Ti64. Though coating peel-off and tool flaking are witnessed when using ceramic insert; it exhibits lower tool flank wear than carbide tool up to ~ v = 130 m/min (v refers to the cutting speed). Finally, application feasibility of HSN2 coated carbide tool (as compared to uncoated counterpart) is unfolded during dry machining of Ti64 by analyzing cutting force components, tool-tip temperature, tool wear morphology, morphology of evolved chips and machined surface quality. With lesser machining forces, HSN2 (TiAlxN supernitride) coated tool endorses about 20.45 % reduction in tool-tip temperature at the highest cutting speed (v = 146 m/min) which causes reduced severity of wear modes of the cutting tool. This helps to produce superior machined surface associated with better surface integrity when compared to that of using uncoated counterpart.