Exploring the possibilities of extending the application of PcBN cutting tools to the machining of ADI through the understanding of wear mechanisms

Abstract

Experimental studies of wear, cutting forces and chip characteristics when dry turning ASTM Grade 2 ADI with cBN-TiC cutting tools under finishing conditions were carried out. A depth of cut of 0.2 mm, a feed of 0.05 mm/rev and cutting speeds ranging from 50 to 800 m/min were used. Oxidation experiments of the cBN-TiC cutting tools were carried out in the temperature range 500-1110 °C for a constant dwell time of 60 min. Static interaction experiments were done with ADI/cBN-TiC/ADI and Si/cBNTiC/ Si sandwiches in the temperature range 1000-1100 °C under argon for a constant dwell time of 60 min and a pressure of 200 MPa. An X-ray diffractometer, optical, scanning and transmission electron microscopes as well as an energy dispersive spectroscope (EDS) were appropriately used for characterization of ADI workpieces, PcBN cutting tools, chips, wear scars on PcBN cutting tools, scale on PcBN cutting tools after oxidation experiments and interaction interfaces after static interaction experiments. Flank wear and crater wear were the main wear modes within the range of cutting speeds investigated. At cutting speeds greater than 150 m/min, shear localization within the primary and secondary shear zones of chips was the key-process that controlled the wear rate indirectly, the static cutting forces and the dynamic cutting forces. Cutting speeds between 150 and 500 m/min were found to be optimum for the production of workpieces with acceptable cutting tool life, flank wear rate and lower dynamic cutting forces. Adhesion and adhesion induced abrasion were the main wear mechanisms at cutting speeds less than 150 m/min. Abrasion and wear by thermally activated diffusion and oxidation / chemical reaction wear were the main wear mechanisms at cutting speeds greater than 150 m/min. At cutting speeds greater than 150 m/min, the superficial melting of the BUL on the chip underside produced a lubricating film that maintained more or less constant tribological conditions at the toolchip interface, thus reducing shear localization in the secondary shear zone of chips. This lubricating film also played a role in the reduction of the tool-chip contact length, the increase of the shear angle and, consequently in the reduction in average chip thickness. The BUL on the crater wear scar was a sandwich of approximately 3 layers: a very thin C rich layer, an intermediate layer containing mainly Si, Mg, and O, slight amounts of B, C and N and very low amounts of Al and Ti as well as a Fe layer in preferential contact with the TiC binder. It became apparent that clues of diffusion wear should be sought in the BUL on the wear scars rather than in the secondary shear zone of chips. Constituents in the BUL on contact zones and non-contact zones of the tools were the products of thermally activated chemical reactions between constituents of the cutting tool, constituents of ADI (particularly Mg and Si) and atmospheric O2. Accordingly, the superficial melting of the BUL increased the wear rate of cBN-TiC cutting tools for cutting speeds greater than 150 m/min. IV Although a minor phase in cBN-TiC cutting tools, TiB2 emerged as a critical phase with regard to their wear behaviour. Its thermally activated dissolution in Fe and its thermally activated reactions with O2 or O2 and Mg as well as its thermally induced cracks might be expected to have synergistic effects on cBN grain pull-out and thus on the wear rate. Blistering and cracking of the non-contact zones of cBN-TiC cutting tools characterized the oxidation behaviour of cBN-TiC cutting tools. The cBN-TiC cutting tool material is not oxidation-resistant in air above 550 °C and the scale that formed on cBN-TiC cutting tools is not able to provide effective protection against oxidation. The TiC binder of the cutting tool material showed extensive oxidation, producing brittle titanium oxide crystallites. Oxidation experiments on cBN-TiC cutting tools showed that oxidation involved intense outward diffusion of elements such as Ti and Al. In addition to the grain coarsening of TiO2 and increased segregation of Al2O3 in the outer scale layer, it was clear that the formation and evaporation of B2O3 affected the morphology of the inner oxygen-affected zone significantly in terms of porosity. Mutual dissolution or reaction between B2O3 and TiO2 which could be expected to reduce such porosity was not evident. The same was true for TiO2 and Al2O3. From the static interaction experiments, strong indications of diffusion of Fe in the TiC binder as well as reprecipitation of Fe in TiC were evident. Strong indications of dissolution of TiC and diffusion of Ti and C as well as reprecipitation of TiC in Fe were also evident. The reprecipitation in Fe of this C in the form of Fe3C or Ti(C,N) is evidence of depletion in Si of the ADI close to the interface and thus evidence of diffusion of Si towards the cBN-TiC side. The evidence of diffusion of N in Fe and its reprecipitation in the form of Ti(C,N) is a strong indication of the dissolution of N-bearing phases such as Ti(C,N), AlN and BN in the cBN-TiC cutting tool material. Strong indications of diffusion of Si in the TiC binder as well as its reprecipitation as SixC or TiSi4-x were evident. All this correlates with the penetration of Si and Fe observed approximately 0.5 μm below the crater scar of the cBN-TiC cutting tools, at the boundaries between cBN grains and the TiC binder. The diffusion of Si in the TiC binder and its eventual reprecipitation as SixC or TiSi4-x give an indication of how the degradation of the wear resistance of the cBN-TiC cutting tool can occur during the machining of ADI. The same applies to the diffusion of Fe in the TiC binder. The strong evidence of interaction of Si, Fe and TiC obtained from the static interaction couples clearly indicates that during the machining of ADI with cBN-TiC cutting tools, bonding of ADI to the tool occurs preferentially through the TiC binder. Indication is also given of how the depletion in Si of the ADI can occur at the tool-chip and tool-workpiece interfaces. The knowledge generated in this work should contribute towards improved design and processing of PcBN cutting tools for use in the machining of ADI.