Tool life and wear mechanisms in high speed machining of Ti–6Al–4V alloy with PCD tools under various coolant pressures

Usage of titanium alloys has increased since the past 50 years despite difficulties encountered during machining. Many studies involving different tool materials, cutting parameters, tool geometry and cutting fluids when machining this important aerospace material have been published. However, there are relatively few literatures available on the application of ultra hard tools in the machining of titanium-alloys. The primary objective of this study is to investigate the behaviour of Polycrystalline Diamond (PCD) tools when machining Ti–6Al–4V alloy at high speed conditions using high pressure coolant supplies. Tool performance under different tribological conditions and the dominant wear mechanisms were investigated. Increase in coolant pressure tends to improve tool life and reduce the adhesion tendency, accelerated by the susceptibility of titanium alloy to gall during machining. Adhesion and attrition are the dominant wear mechanisms when machining at the cutting conditions investigated.     

Tool wear mechanism of WC–5TiC–10Co ultrafine cemented carbide during AISI 1045 carbon steel cutting process

WC–5TiC–10Co ultrafine and conventional cemented carbides were prepared and used for AISI 1045 carbon steel cutting tool inserts. The microstructure and mechanical properties were characterized, and cutting tests were conducted with different cutting parameters. Tool wear mechanism was analyzed by SEM and EDS. Ultrafine inserts possess higher hardness and transverse rupture strength. There were adhesive wear on the rake and slight abrasive wear on the flank of ultrafine inserts. As for conventional inserts with the same composition but with medium grain size, there were combinations of more serious abrasive and adhesive wear on the rake and flank.     

Viscosity of WC–Co compacts during sintering

This paper describes experiments performed on WC–Co compacts in order to measure the viscosities of a Newtonian constitutive law commonly used to simulate sintering. An intermittent loading method is used during two series of experiments. The first series are dedicated to determining the axial viscosity and takes place in a loading-dilatometer. The second one takes advantage of a video-extensometry device and provides results about the viscous Poisson's ratio. The axial viscosity is obtained as a function of relative density and temperature. Viscosity shows strong exponential increase with increasing density during isothermal conditions but decreases from 10 to 1 GPa·s between 1100°C and 1325°C during a conventional sintering cycle. Viscous Poisson's ratio shows low values at low densities and increases to 0.5 at full density.     

Cutting mechanics in high speed dry machining of biomedical magnesium–calcium alloy using internal state variable plasticity model

Magnesium–Calcium (MgCa) alloys become attractive orthopedic biomaterials due to their biodegradability, biocompatibility, and congruent mechanical properties with bone tissues. However, process mechanics of cutting biomedical MgCa alloys is poorly understood. Mechanical properties of the biomedical magnesium alloy at high strain rates and large strains are determined using the split-Hopkinson pressure bar testing method. Internal state variable (ISV) plasticity model is implemented to model the material behavior under cutting regimes. A finite element analysis (FEA) model has been developed to study the chip formation during high speed dry cutting of MgCa0.8 (wt%) alloy. Continuous chip formation predicted by finite element simulation is verified by high speed dry face milling of MgCa0.8 using polycrystalline diamond (PCD) inserts. Chip ignition as the most hazardous aspect in machining Mg alloys does not occur in high-speed dry cutting with sharp PCD tools. The predicted temperature distribution well explains the reason for the absence of chip ignition in high speed dry cutting of MgCa0.8 alloy. In addition, sporadic surface deterioration and void marks on the back face of chips are explained.     

Mechanical Properties and Microstructural Behaviour of Microwave Sintered WC–Co

Metals and Materials International - Cemented carbides have been of great interest within industrially manufacturable hard materials for their mechanical properties. Microwave sintering is known...     

Correlation between residual stress and abrasive wear of WC–17Co coatings

This investigation had been conducted to determine the influence of residual stresses on the abrasive wear resistance of HVOF thermal spray WC–17 wt.% Co coatings, as well as to derive stress relaxation after cutting by wire electric discharge machining (EDM). The abrasive wear properties of the coatings were characterised using an ASTM-G65 three body abrasive wear machine with silica sand as the abrasive. The residual stress was measured by means of X-ray diffraction techniques, on the coated samples before and after the abrasive wear tests. Compressive residual stresses were observed in the surface layer of the large coated samples. However, stress relaxation results after cutting into small sizes were distinctly different. There was strong correlation between residual stresses in the surface layer and abrasive wear resistance, as well as yield strength of a material.     

Microstructural changes in CVD κ-Al2O3 coated cutting tools during turning operations

This work is concerned with the microstructural evolution in CVD κ-Al₂O₃ coatings during high speed metal cutting. The wear characteristics and the κ-Al₂O₃ to α-Al₂O₃ phase transformation during metal cutting have been studied in detail by means of transmission electron microscopy and scanning electron microscopy. Based on the results of these investigations, the different wear mechanisms of κ-Al₂O₃ coated cemented carbide tools in metal cutting are discussed.Under the cutting conditions studied, flank wear depth developed at about twice the speed of rake face wear. No phase transformation occurred on the flank face, leading to wear of the non-transformed κ-Al₂O₃ of a more abrasive nature. On the rake face, a region of the κ-Al₂O₃ coating was transformed to α-Al₂O₃, and the wear crater was positioned inside the transformed α-Al₂O₃ area. The surface zone in the crater displayed a higher dislocation density, an indication of the occurrence of plastic deformation. It is concluded that the transformation from κ-Al₂O₃ to α-Al₂O₃, and the wear of the transformed α-Al₂O₃ by plastic deformation, are characteristic of the rake face wear.     

Variation of WC grain shape with carbon content in the WC–Co alloys during liquid-phase sintering

WC–Co hard metals have faceted WC grains dispersed in a ductile cobalt-rich matrix. The effect of carbon (C) content on the shape of WC grain in the WC–Co metals during liquid-phase sintering is investigated in this work. The WC grain shape varies with the C content and, more importantly, the shape change occurs reversibly with the C content.     

Microstructural evolution during high temperature deformation of lamellar Ti48Al–2Nb–2Cr

The microstructural evolution of lamellar Ti48Al–2Nb–2Cr during deformation at temperatures between 1000 and 1200 K was investigated by light optical and electron microscopy. The lamellar structure which initially covers more than 95% of the volume transforms during deformation into a globular structure consisting of equiaxed subgrains with a significant amount of large angle boundaries (<30%). The steady state subgrain size and the spacing of dislocations in the subgrain interior vary in inverse proportion to shear modulus normalized stress. Based on the steady state data the evolution of the volume fraction with globular (subgrain) structure and the characteristic spacings is quantified.     

Parametric optimization of dry sliding wear loss of copper–MWCNT composites

The wear behavior of multi-walled carbon nano-tubes (MWCNTs) reinforced copper metal matrix composites (MMCs) processed through powder metallurgy (PM) route was focused on and further investigated for varying MWCNT quantity via experimental, statistical and artificial neural network (ANN) techniques. Microhardness increases with increment in MWCNT quantity. Wear loss against varying load and sliding distance was analyzed as per L16 orthogonal array using a pin-on-disc tribometer. Process parameter optimization by Taguchi's method revealed that wear loss was affected to a greater extent by the introduction of MWCNT; this wear resistant property of newer composite was further analyzed and confirmed through analysis of variance (ANOVA). MWCNT content (76.48%) is the most influencing factor on wear loss followed by applied load (12.18%) and sliding distance (9.91%). ANN model simulations for varying hidden nodes were tried out and the model yielding lower MAE value with 3-7-1 network topology is identified to be reliable. ANN model predictions with R value of 99.5% which highly correlated with the outcomes of ANOVA were successfully employed to investigate individual parameter's effect on wear loss of Cu–MWCNT MMCs.     

Effect of nanostructuring and Al alloying on friction and wear behaviour of thermal sprayed WC–Co coatings

Nanostructured WC–Co and WC–Co–Al coatings, with about 300-μm as-deposited coating thickness, were deposited by high velocity oxy-fuel (HVOF) spraying. Agglomerated nanostructured cermet powders produced by the Mechanomade® process was used for HVOF spraying. Dense and well-adherent coatings with crystal sizes below 30 nm were deposited on stainless steel 304 substrate. Porosity was less than 5% and the bond strength with the substrate was around 60 MPa. Experimental data on friction, wear, and abrasion resistance revealed that nanostructured WC–Co based coatings containing some Al as alloying element, exhibit improved tribological characteristics in comparison to nanostructured and micron-sized WC–Co coatings. This was attributed to a carbide particle distribution within the coating revealed by SEM, the absence of brittle W₂C-like phases revealed by XRD, and the presence of Al at particle/matrix boundaries revealed by TEM.     

Effect of micro-textured tools on machining of Ti–6Al–4V alloy: An experimental and numerical approach

In this work, an attempt is made to reduce the detrimental effects that occurred during machining of Ti–6Al–4V by employing surface textures on the rake faces of the cutting tools. Numerical simulation of machining of Ti–6Al–4V alloy with surface textured tools was employed, taking the work piece as elasto-plastic material and the tool as rigid body. Deform 3D software with updated Lagrangian formulation was used for numerical simulation of machining process. Coupled thermo-mechanical analysis was carried out using Johnson-cook material model to predict the temperature distribution, machining forces, tool wear and chip morphology during machining. Turning experiments on Ti–6Al–4V alloy were carried out using surface textured tungsten carbide tools with micro-scaled grooves in preferred orientation such as, parallel, perpendicular and cross pattern to that of chip flow. A mixture of molybdenum disulfide with SAE 40 oil (80:20) was used as semi-solid lubricant during machining process. Temperature distribution at tool–chip interface was measured using an infrared thermal imager camera. Feed, thrust and cutting forces were measured by a three component-dynamometer. Tool wear and chip morphology were captured and analyzed using optical microscopic images. Experimental results such as cutting temperature, machining forces and chip morphology were used for validating numerical simulation results. Cutting tools with surface textures produced in a direction perpendicular to that of chip flow exhibit a larger reduction in cutting force, temperature generation and reduced tool wear.     

Cladding of pre-blended Ti–6Al–4V and WC powder for wear resistant applications

In this paper, a cladding investigation to achieve uniform distribution of WC particles which is crack-free, non-porous and without delamination using a 2 kW IPG Ytterbium doped, continuous wave, fibre laser with 1070 nm wavelength was reported. The single track deposition of a pre-blended powder, 27 wt.% Ti–6Al–4V/73 wt.% WC with a particle size range of 40–120 μm was made on Ti–15V–3Cr–3Sn–3Al substrate using a co-axial nozzle and a standard powder feeding system. The laser cladding samples were subjected to various microstructure examinations, microhardness and micro-abrasion tests. The results revealed that the best clad layers were achieved at an energy density of 111.10 J.mm^?2, 15–18.3 mm.s^?1 traverse speed; (583–667) mg.s^?1 powder feed rate with substrate surface irradiated by laser beam raising its temperature to about 200 °C. This resulted in a uniform distribution of WC within the clad and the results obtained from SEM, EDS and XRD revealed that the WC particles experienced surface melting with some diffusion into the matrix, thus promoting excellent bonding with the matrix and the formation of titanium and tungsten carbides, which include TiC and W₂C. The emergence of β-Ti, TiC and W in the clad resulted in enhanced hardness values. The mean value of microhardness in clad matrix is 678 HV when measured from the top of a transverse cross section of the clad sample into the interface region with the Ti substrate which has a hardness of 396 HV. Wear tests indicated the wear resistance of the clad was seven times that of the Ti alloy substrate.     

Comparison between oxidation kinetics of HVOF sprayed WC–12Co and WC–10Co–4Cr coatings

In this study, the high temperature oxidation behavior of HVOF-sprayed WC–12Co and WC–10Co–4Cr coatings were investigated. To explore the oxidation mechanism, thermo-gravimetric analysis (TGA) was applied for isothermal treatments in the range of 500–800 °C for 3 h. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to evaluate the structural changes and microstructural evolutions during oxidation tests. The TGA experiments showed negligible oxidation mass gains at 500 °C for both coatings. At higher temperatures, i.e. 700 and 800 °C, the oxidation mass gains of WC–12Co were found to be much higher than those for WC–10Co–4Cr coating, respectively. The higher oxidation resistance of WC–10Co–4Cr coating probably results from the formation of compact chromium oxide layers and higher MWO₄ type tungstate (M: Co and/or Cr) to tungsten trioxide (WO₃) ratios which provide lower porosity and consequently more efficient passivation effect against oxidation. The time dependent mass gain of WC–12Co coating obeys the linear law within temperature range of 600–800 °C with apparent oxidation activation energy of ~ 104 kJ/mol. As for the oxidation of WC–10Co–4Cr coating, a negligible deviation from linear law was observed possibly due to the presence of chromium oxide and higher tungstate to tungsten trioxide ratio which hinders the diffusion process through the scales compared with WC–12Co coating. The apparent activation energy for oxidation of the WC–10Co–4Cr coating was found to be ~ 121 kJ/mol.     

Co evolutions for WC–Co with different Co contents during pretreatment and HFCVD diamond film growth processes

Systematical researches were accomplished on WC–Co with different Co contents (6%, 10% and 12%, mass fraction). Based on the XPS and EDX, from orthogonal pretreatment experiments, it is indicated that the acid concentration, the time of the acid pretreatment and the original Co content have significant influences on the Co-removal depth (D). Moreover, the specimen temperature, original Co content and Co-removal depth dependences of the Co evolution in nucleation, heating (in pure H₂ atmosphere) and growth experiments were discussed, and mechanisms of Co evolutions were summarized, providing sufficient theoretical bases for the deposition of high-quality diamond films on WC–Co substrates, especially Co-rich WC–Co substrates. It is proven that the Co-rich substrate often presents rapid Co diffusion. The high substrate temperature can promote the Co diffusion in the pretreated substrate, while acts as a Co-etching process for the untreated substrates. It is finally found that the appropriate Co-removal depth for the WC–12%Co substrate is 8–9 μm.     

High temperature mechanical properties of WC–Co hard metals

Understanding of the load situation and consequently the lifetime of cutting tools made of WC–Co hard metal requires quantitative data for thermo-mechanical properties. For the elevated temperatures present in application, these data are currently rather rare. The present work does discuss elastic material properties up to 1100 °C and compressive yield strength up to 900 °C, both as a function of Co content. The fracture toughness was determined as a function of the WC grain size and Co content up to 800 °C. Young's modulus and yield strength decrease with increasing temperature. A significant rise in fracture toughness was observed at 800 °C with increasing Co content and decreasing WC grain size. A possible reason for this increase is an increase in the plastic zone size at elevated temperatures.     

Microstructure and sliding wear assessment of Co–TiC composite materials

Monolithic Co and Co based composites reinforced by TiC precipitates were fabricated by vacuum arc melting process. The ratio of hexagonal to cubic phase cobalt was affected by the presence of the TiC precipitates. The microstructure of the produced materials was also associated with the presence of TiC crystals. The TiC primary precipitates are characterised by a strong faceted morphology which is explained in terms of Jackson’s theory of crystal growth. The solidification progress was altered by the presence of the TiC crystals. The sliding wear behaviour of the produced composites was assessed in terms of wear track and debris examination and compared to that of Co29Cr5Mo alloy.     

Processing and dry sliding wear performance of spray deposited hyper-eutectic aluminum–silicon alloys

The influence of spray deposition process on the refinement of silicon phase and the tribological performance of hyper-eutectic Al–Si alloys is reported in this work. Due to the rapid solidification conditions that prevail during the spray deposition process, both primary and eutectic silicon were found to be refined resulting in equi-axed morphology of the silicon phase across the matrix. The average silicon particle size increased from 7 μm to 17 μm with increase in the silicon content of the spray deposited alloys used for the present study. Transmission electron microscopy of the spray deposited samples exhibited sub-micron sized silicon particles of both equi-axed and acicular morphology in the aluminum matrix. Pin-on-disc wear tests were performed on the spray deposited samples, by sliding samples against hardened steel counterface for about 1000 m at a speed of 0.3 m/s under varied loading conditions ranging from 0.17 MPa to 1 MPa. Scanning electron microscopy of the wear tracks and wear debris was carried out to understand the wear mechanism. The wear performance was improved with increase in the silicon content of the alloy. The wear performance of the alloys was compared with similar alloys produced through various processing routes reported in the literature. The spray deposited alloys were observed to exhibit relatively better wear performance for the range of composition and loading conditions employed.     

Microstructural evolution during containerless rapid solidification of Ni–Mo eutectic alloys

Ni–45%Mo hypoeutectic, Ni–47.7%Mo eutectic and Ni–50%Mo hypereutectic alloys are rapidly solidified during containerless processing in drop tube. The microstructures of Ni–47.7%Mo eutectic alloy are composed of lamellar eutectic plus anomalous eutectic of Ni and NiMo phases. When the droplet size decreases, the volume fraction of anomalous eutectic becomes larger. The structural morphology transforms into Ni dendrite plus lamellar eutectic in very small droplets which are highly undercooled. The microstructures of Ni–45%Mo hypoeutectic alloy are characterized by primary Ni dendrite plus lamellar eutectic, whereas those of Ni–50%Mo hypereutectic alloy consist of NiMo dendrite plus lamellar eutectic. For both off-eutectic alloys, the experimental results show that the microstructure evolution depends mainly on droplet size. In the case of Ni–45%Mo hypoeutectic alloy, with the decrease of droplet size, the primary Ni phase transforms from dendrites to equiaxed grains. As for Ni–50%Mo hypereutectic alloy, when droplets become smaller and smaller the microstructural transition proceeds from primary NiMo dendrite plus lamellar eutectic to anomalous eutectic. The calculated highest undercoolings of the three alloys are 226, 182 and 135 K, respectively. By classical nucleation theory, Ni phase is the primary phase to nucleate for Ni–47.7%Mo eutectic alloy. The TMK eutectic growth and LKT/BCT dendritic growth theories are applied to analyze the rapid solidification process and investigate the microstructural transition mechanisms. The coupled zone of Ni–Mo eutectic alloy has also been calculated on the basis of TMK and LKT/BCT models, which covers a composition range from 45.7% to 57.1% Mo.