Wear behaviour of cryogenically treated tungsten carbide inserts under dry and wet turning conditions

Cryogenic treatment has been acknowledged as a means of extending the life of tungsten carbide inserts but no study has been reported in open literature regarding the effect of coolant on the performance of cryogenically treated tungsten carbide inserts in turning. In order to understand the effect of coolant, a comparative investigation of the wear behaviour of cryogenically treated tungsten carbide inserts in dry and wet orthogonal turning has been carried out in this study. The commercially available uncoated square-shaped tungsten carbide inserts with chip breakers were procured and cryogenically treated at ?196 °C and the cutting tests were executed in accordance to the International Standard Organisation, ISO 3685-1993 for continuous and interrupted machining mode. The criterion selected for determining the tool life was based on the maximum flank wear (0.6 mm) and the selection of cutting conditions was made to ensure the significant wear at a suitable time interval. The results showed that cryogenically treated tungsten carbide inserts performed significantly better in wet turning conditions under both continuous and interrupted machining modes especially at higher cutting speeds. A considerable increase in tool life was also recorded in interrupted machining mode as compared with continuous machining mode.     

Investigation on wear behaviour of cryogenically treated TiAlN coated tungsten carbide inserts in turning

Cryogenic treatment has been ascribed as a way of improving the cutting life of tungsten carbide turning inserts. Most of the research conducted till date has not reported any effort to excavate the effect of cryogenic treatment on the performance of coated tungsten carbide inserts in terms of adhesion strength of coatings deposited on tungsten carbide substrate. In order to understand the effect of cryogenic treatment on the adhesion strength of coatings, a comparative investigation of the wear behaviour and machining performance of cryogenically treated coated tungsten carbide inserts in orthogonal turning has been carried out in this study. The commercially available TiAlN coated square shaped tungsten carbide inserts (P25) were procured and subjected to cryogenic treatment at two levels −110 °C (shallow treatment) and −196 °C (deep treatment) of temperature independently. The criterion selected for determining the turning performance was based on the maximum flank wear (0.6 mm) as recommended in ISO 3685-1993. The results showed that shallow cryogenically treated coated tungsten carbide inserts performed significantly better as compared with deep cryogenically treated and untreated inserts. Major outcome of the present study includes a substantial decrease in tool life of deep cryogenically treated inserts as compared to untreated inserts indicating the destructive effect of deep cryogenic temperature (−196 °C) on TiAlN coated inserts which is further supported by VDI-3198 indentation test.     

Machinability improvement of titanium alloy (Ti–6Al–4V) via LAM and hybrid machining

Titanium alloy (Ti–6Al–4V) is one of the materials extensively used in the aerospace industry due to its excellent properties of high specific strength and corrosion resistance, but it also presents problems wherein it is an extremely difficult material to machine. The cost associated with titanium machining is also high due to lower cutting speeds (<60 m/min) and shorter tool life. Laser-assisted machining (LAM) and consequently hybrid machining is utilized to improve the tool life and the material removal rate. The effectiveness of the two processes is studied by varying the tool material and material removal temperature while measuring the cutting forces, specific cutting energy, surface roughness, microstructure and tool wear. Laser-assisted machining improved the machinability of titanium from low (60 m/min) to medium-high (107 m/min) cutting speeds; while hybrid machining improved the machinability from low to high (150–200 m/min) cutting speeds. The optimum material removal temperature was established as 250 °C. Two to three fold tool life improvement over conventional machining is achieved for hybrid machining up to cutting speeds of 200 m/min with a TiAlN coated carbide cutting tool. Tool wear predictions based on 3-D FEM simulation show good agreement with experimental tool wear measurements. Post-machining microstructure and microhardness profiles showed no change from pre-machining conditions. An economic analysis, based on estimated tooling and labor costs, shows that LAM and the hybrid machining process with a TiAlN coated tool can yield an overall cost savings of ～30% and ～40%, respectively.     

Effects of deep cryogenic treatment on the microstructure and properties of WCFeNi cemented carbides

The effects of deep cryogenic treatment on microstructure and properties of WCFeNi cemented carbides were investigated in this paper. The WCFeNi cemented carbides were deep cryogenically treated at about − 196 °C for 2, 12 and 24 h. In order to study the phase composition and quantitative analysis of the deep cryogenically treated specimens, a selective electrolytic corrosion was used. The internal friction, mechanical properties, wear properties and corrosion behavior of the deep cryogenically treated WCFeNi cemented carbides were measured. The results show that with an increase in deep cryogenic treatment time, martensitic phase transformation of binder phase from γ → α has taken place, the α-(Fe,Ni) phase content in the binder phase gradually increases from 12.7% to 86.8% (wt.%) and W particles precipitate from the binder phase. Compared with the as-sintered specimens, the maximum hardness and transverse rupture strength of the specimens treated by cryogenic environment increase by almost 20% and 7.7%, respectively. The fracture toughness decreases from 25.7 MPam^½ for as-sintered specimens to 19.6 MPam^½ for the alloy after deep cryogenic treatment. The wear rate and friction coefficient decrease by almost 56% and 17.2%, while the corrosion resistance slightly decreases compared with the alloys without deep cryogenic treatment. The change of the properties can be primarily attributed to the fact that the martensite phase transformation from γ-(Fe,Ni) to α-(Fe,Ni) and precipitation of W particles in the binder phase improve the hardness and strengthen the binder phase during the deep cryogenic treatment.     

Wear,cutting forces and chip characteristics when dry turning ASTM Grade 2 austempered ductile iron with PcBN cutting tools under finishing conditions

Experimental studies of wear, cutting forces and chip characteristics when dry turning ASTM Grade 2 austempered ductile iron (ADI) with polycrystalline cubic boron nitride (PcBN) 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. Flank wear and crater wear were the main wear modes within this range of cutting speeds. Abrasion wear and thermally activated wear were the main wear mechanisms. At cutting speeds greater than 150 m/min, shear localization within the primary and secondary shear zones of chips appeared to be the key-phenomenon that controlled the wear rate, the static cutting forces as well as 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.     

Microstructural analysis of wear micromechanisms of WC–6Co cutting tools during high speed dry machining

This original study investigates the damages of WC–6Co uncoated carbide tools during dry turning of AISI 1045 steel at mean and high speeds. The different wear micromechanisms are explained on the basis of different microstructural observations and analyses made by different techniques: (i) optical microscopy (OM) at macro-scale, (ii) scanning electron microscopy (SEM), with back-scattered electron imaging (BSE) at micro-scale, (iii) energy dispersive spectroscopy (EDS), X ray mapping with wavelength dispersive spectroscopy (WDS) for the chemical analyses and (iv) temperature evolution during machining. We noted that at conventional cutting speed Vc ≤ 250 m/min, normal cutting tool wear types (adhesion, abrasion and built up edge) are clearly observed. However, for cutting speed Vc > 250 m/min a severe wear is observed because the behavior of the WC–6Co grade completely changes due to a severe thermomechanical loading. Through all SEM micrographs, it is observed that this severe wear consists of several steps as: excessive deformation of WC–6Co bulk material and binder phase (Co), deformation and intragranular microcracking of WC, WC grain fragmentation and production of WC fragments in the tool/chip contact. Thus, the WC fragments accumulated at the tool/chip interface cause abrasion phenomena and pullout WC from tool surface. WC fragments contribute also to the microcutting and microploughing of chips, which lead to form a transferred layer at the tool rake face. Finally, based on the observations of the different wear micromechanisms, a scenario of WC–6Co damages is proposed through to a phenomenological model.     

Wear damage of cemented carbides with different combinations of WC mean grain size and Co content. Part I: ASTM wear tests

In the present work we made and examined cemented carbides characterized by very different WC grain sizes varying from near-nano with a WC mean grain size of about 200 nm to coarse-grain with a WC mean grain size of about 4.5 μm and Co contents varying from 3 to 24 wt.%. The major objective of the present work was to examine the wear damage, wear behavior and wear mechanisms of cemented carbides having nearly the same hardness but greatly varying with respect to their WC grain size and Co content in the high-load ASTM B611 test and low-load G65 test.Both the hardness and resistance to fracture and micro-fatigue of cemented carbides play an important role in the wear damage by use of the high-stress ASTM B611 test when the carbide surface is subjected to alumina particles at high loads. In this case, the wear-resistance increases with increasing the WC mean grain size and decreasing the Co content at nearly the same hardness of the different cemented carbides. The submicron and near-nano cemented carbides are characterized by lower wear-resistance in comparison with the coarse-grain grade due to their reduced fracture toughness, fracture resistance and resistance to micro-fatigue.The Co mean free path in the carbide microstructure plays an important role with respect to wear-resistance in the low-stress ASTM G65 test when the carbide surface is subjected to gentle scratching by abrasive silica particles. The predominant wear of the thick Co interlayers leaving unsupported WC grains plays the decisive role in the wear behavior of the coarse-grain grade resulting in its low wear-resistance. In contrast to the ASTM B611 test the wear rate decreases with decreasing the WC mean grain size and increasing the Co content due to the corresponding reduction of Co mean free path in the carbide microstructure. As a result, the wear-resistance of the near-nano grade in the ASTM G65 test is the best of all in spite of its reduced fracture toughness.Phenomena of micro-fatigue, micro-fracturing and micro-chipping are found to play a decisive role in the wear damage of cemented carbides if they are subjected to abrasion wear, high loads and severe fatigue.     

Dry sliding wear of cryotreated multiple tempered D-3 tool steel

Wear test using pin-on-disc machine was used to investigate the role of multiple tempering after cryogenic treatment of D-3 tool steel. Conventional quenching (950 °C) and tempering (150 °C) treatments were given along with intermediate cryogenic treatment (?185 °C). Specimens were subjected to wear tests on pin-on-disc machine in dry sliding condition for sliding distance of 6000 m at 5.5 kg load and for sliding speed of 3.0 m/s. Hardness data, microstructures, wear loss and SEM analysis of worn surface throw light on the underlying metallurgical mechanism responsible in improving wear resistance property of the D-3 tool steel.     

Studies on drilling processes of intermetallic compounds

Intermetallic compounds have high-specific strength, good creep resistance and excellent oxidation and corrosion resistance at high temperatures. Those good mechanical properties make intermetallic compounds very promising structure materials in industries. Ni₃Al and Fe₃Al are two typical intermetallic compounds. This paper focuses on machinability and tool wear mechanism of PVD TiAlN coated cemented carbide tool in drilling of Ni₃Al and Fe₃Al. The influences of cutting speeds on tool wear and machined surface roughness were investigated. The tool wear mechanism was analyzed using scanning electron microscopy (SEM) and EDX analysis. Chips were analyzed by SEM and the machined surfaces were examined by surface roughness measurement instrument. Tool life when drilling of Ni₃Al is shorter than that of Fe₃Al. The built-up-edge (BUE) could be formed on the cutting edge in drilling of Fe₃Al, and no BUE was observed of Ni₃Al at the cutting speed of 29 m/min.     

Elevated temperature repetitive micro-scratch testing of AlCrN,TiAlN and AlTiN PVD coatings

In developing advanced wear-resistant coatings for tribologically extreme highly loaded applications such as high speed metal cutting a critical requirement is to investigate their behaviour at elevated temperature since the cutting process generates frictional heat which can raise the temperature in the cutting zone to 700–900 °C or more. High temperature micro-tribological tests provide severe tests for coatings that can simulate high contact pressure sliding/abrasive contacts at elevated temperature. In this study ramped load micro-scratch tests and repetitive micro-scratch tests were performed at 25 and 500 °C on commercial monolayer coatings (AlCrN, TiAlN and AlTiN) deposited on cemented carbide cutting tool inserts. AlCrN exhibited the highest critical load for film failure in front of the moving scratch probe at both temperatures but it was prone to an unloading failure behind the moving probe. Scanning electron microscopy showed significant chipping outside the scratch track which was more extensive for AlCrN at both room and elevated temperature. Chipping was more localised on TiAlN although this coating showed the lowest critical loads at both test temperatures. EDX analysis of scratch tracks after coating failure showed tribo-oxidation of the cemented carbide substrate. AlTiN showed improved scratch resistance at higher temperature. The von Mises, tensile and shear stresses acting on the coating and substrate sides of the interface were evaluated analytically to determine the main stresses acting on the interface. At 1 N there are high stresses near the coating-substrate interface. Repetitive scratch tests at this load can be considered as a sub-critical load micro-scale wear test which is more sensitive to adhesion differences than the ramped load scratch test. The analytical modelling showed that a dramatic improvement in the performance of AlTiN in the 1 N test at 500 °C could be explained by the stress distribution in contact resulting in a change in yield location due to the high temperature mechanical properties. The increase in critical load with temperature on AlTiN and AlCrN is primarily a result of the changing stress distribution in the highly loaded sliding contact rather than an improvement in adhesion strength.     

Analytical modeling and experimental investigation of tool and workpiece temperatures for interrupted cutting 1045 steel by inverse heat conduction method

Cutting temperature is a key factor which directly affects tool wear, workpiece integrity, and machining precision in high speed machining process. The interrupted cutting process consists of several periodical characteristics, such as cutting force and time varying heat source. Induced cutting temperature models with time varying heat flux are developed in this paper to predict temperature distribution at tool inserts and workpiece during interrupted cutting process. A set of interrupted cutting experimental installation is designed to verify the proposed models. The comparison of predicted and measured results for 1045 steel in interrupted cutting processes shows reasonable agreement. The measured temperature of both the tool inserts and workpiece increase firstly and then decrease as the cutting speed increases. The peak temperature of the workpiece appears at 1500 m/min, while the peak tool inserts temperature appears at 1250 m/min approximately. Heat flux is calculated by the inverse heat conduction method. The applicability of Salomon's hypothesis to the temperature of tool inserts and workpiece is discussed during the interrupted cutting process. The dropped temperature at high cutting speed is mainly caused by that heat flux into tool inserts decreases and heat transfer time is not enough after the critical cutting speed.     

Effect of ultra-fine TiC0.5N0.5 on the microstructure and properties of gradient cemented carbide

To increase the performance of cemented carbide, it was common to coat the wear surfaces with thin layers of hard materials. Cemented carbides with surface zone depleted of hard cubic phase and enriched in ductile binder phase were commonly used as the substrates. At the present, the so-called “gradient cemented carbides” were usually prepared by adding TiN or medium-sized TiCN, and a two-step sintering process including pre-sintering in nitrogen and gradient sintering in vacuum was usually adopted. In this paper, gradient cemented carbide based on WC–5.19 wt.%Ti–9.2 wt.%Co was prepared by simple vacuum sintering with the addition of 0.13 μm ultra-fine TiC_0.5N_0.5, compared with medium-sized TiN or TiCN. The result showed that gradient cemented carbide prepared with 0.13 μm ultra-fine TiC_0.5N_0.5 had better properties. With the increase of ultra-fine TiC_0.5N_0.5 content, the thickness of gradient surface zone decreased. TiCN coatings were deposited on gradient substrate and conventional substrate by moderate temperature chemical vapor deposition (MTCVD), respectively. Gradient substrate resulted in better adhesion and cutting performance.     

Face milling of the EN AB-43300 aluminum alloy by PVD- and CVD-coated cemented carbide inserts

Two commercially available WC-6Co cemented carbide substrates (Extramet EMT100 and Pramet H10), were industrially coated with PVD TiB₂ or CVD diamond. Subsequently, the coated inserts were submitted to dry sliding tests (slider on cylinder contact geometry) against the aluminum alloy EN AB-43300, for preliminary performance ranking and identification of basic wear mechanisms. The best substrate/coating combination (CVD-Diamond coated Extramet EMT100) was then tested in face milling EN AB-43300 with milling tool characterized by two different geometries (A and B), using PCD inserts as a reference for comparison. In milling tests, the influence of both insert geometry and cutting fluid feed rate were taken into account. The geometry of the tool was identified as the main parameter in influencing the tool performance. In particular, in the case of the A geometry, the relative flank wear of CVD coated tools increased abruptly during the test due coating detachment, whilst with the B geometry no catastrophic failure of the CVD coated insert was observed. The influence of Cutting Fluid Feed Rate (CFFR) also changed with tool geometry: in particular, with the B geometry, which allowed to obtain the best results with the CVD coated inserts, a decrease of CFFR from 100 to 25% did not affect significantly the wear resistance of CVD-coated inserts and allowed to maintain the roughness of the workpiece (R_a) below 0.6 μm, notwithstanding a slightly increased tendency towards the formation of Al-based transfer layers.     

Magnetic abrasive finishing of cutting tools for machining of titanium alloys

Uncoated carbide tool surfaces are conditioned using magnetic abrasive finishing (MAF) to improve the tool wear characteristics by reducing friction between the tool and chip. The configuration of the magnetic particle chains that drive the abrasives plays an important role in surface finishing with minimal damage to the tool cutting edges. Roughnesses of less than 25 nm Ra on the flank and nose and less than 50 nm Ra on the rake can be achieved. In turning of Ti–6Al–4V alloy rods (at 100 m/min cutting speed), MAF-processed tools exhibited tool lives of up to twice as long as unprocessed tools.     

Chromium carbide–CNT nanocomposites with enhanced mechanical properties

Chromium carbide is widely used as a tribological coating material in high-temperature applications requiring high wear resistance and hardness. Herein, an attempt has been made to further enhance the mechanical and wear properties of chromium carbide coatings by reinforcing carbon nanotubes (CNTs) as a potential replacement of soft binder matrix using plasma spraying. The microstructures of the sprayed CNT-reinforced Cr₃C₂ coatings were characterized using transmission electron microscopy and scanning electron microscopy. The mechanical properties were assessed using micro-Vickers hardness, nanoindentation and wear measurements. CNT reinforcement improved the hardness of the coating by 40% and decreased the wear rate of the coating by almost 45–50%. Cr₃C₂ reinforced with 2 wt.% CNT had an elastic modulus 304.5 ± 29.2 GPa, hardness of 1175 ± 60 VH_0.300 and a coefficient of friction of 0.654. It was concluded that the CNT reinforcement increased the wear resistance by forming intersplat bridges while the improvement in the hardness was attributed to the deformation resistance of CNTs under indentation.     

Performance evaluation of cryogenically treated tungsten carbide tools in turning

This paper describes a study on the effects of cryogenic treatment of tungsten carbide. Cryogenic treatment has been acknowledged by some as a means of extending the tool life of many cutting tool materials, but little is known about the mechanism behind it. Thus far, detailed studies pertaining to cryogenic treatment have been conducted only on tool steels. However, tungsten carbide cutting tools are now in common use. The main aim of this study is to analyze the differences in tool performance between cryogenically treated and untreated tool inserts during orthogonal turning of steel. This will aid in the quest for optimal cutting conditions for the turning of steel using these inserts, and will also enhance the understanding of the mechanism behind the cryogenic treatment of tungsten carbide, and the changes in its properties after cryogenic treatment. In the process of ascertaining these findings, it was shown in this study that under certain conditions, cryogenic treatment can be detrimental to tool life and performance. It was also shown that cryogenically treated tools perform better while performing intermittent cutting operations.     

Preparation of polycrystalline cubic boron nitride compact by high-pressure infiltration using cemented carbide

Polycrystalline cubic boron nitride (PcBN) compacts, using the infiltrating method in situ by cemented carbide (WC–Co) substrate, were sintered under high temperature and high pressure (HPHT, 5.2 GPa, 1450 °C for 6 min). The microstructure morphology, phase composition and hardness of PcBN compacts were investigated by using scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). The experimental results show that the WC and Co from WC–Co substrate spread into cubic boron nitride (cBN) layer through melting permeability under HPHT. The binder phases of WC, MoCoB and Co₃W₃C realized the interface compound of PcBN compact, and the PcBN layer formed a dense concrete microstructure. Additionally the Vickers hardness of 29.3 GPa and cutting test were performed when sintered by using cBN grain size of 10–14 μm.     

The effect of volume fraction of WC particles on wear behavior of in-situ WC/Fe composites by spark plasma sintering

Tungsten carbide (WC) particles have been in-situ synthesized through the reaction between tungsten particles and carbide particles by spark plasma sintering (SPS). The composites with different WC content were comparatively observed by the techniques of scanning electron microscopy (SEM), high-resolution transmission electron microscope (HRTEM), X-ray diffraction, hardness and pin-to-disc abrasive wear test. The results showed that the formed WC particles were homogenously distributed in the iron matrix with the size of smaller than 25 μm. Additionally, with the increasing of the WC content, the hardness of composites, the microhardness of matrix and the wear resistance increased, but there was no change significantly between 32 vol% WC/Fe composites and 42 vol% WC/Fe composites. The composites possessed excellent wear resistance comparing the specific wear rate determined in the present work to the martensitic wear-resistant steel under the load of 80 N after a sliding distance of ~ 950 m. The specific wear rate of the martensitic wear-resistant steel was a factor of 24 and 48 times higher than WC/Fe composites, when the content of WC was 32 vol% and 42 vol% in WC/Fe composites, respectively. The main wear mechanism was synthetic of abrasion wear and oxidation wear. The wear performance of 32 vol% WC/Fe composites didn't appear to be much different from 42 vol% WC/Fe composites, due to the WC particles in the 42 vol% composites produced stress concentration easily, which could ultimately induce the creak initiation around WC particles in the subsurface (near wear surface) and propagation to wear surface promoting the breakup of surface film.     

Microstructures and properties of Ni based composite coatings prepared by plasma spray welding with mixed powders

The Ni based composite coatings have been obtained by using the plasma spray welding process and mixed powders (NiCrBSi + NiCr-Cr₃C₂ + WC). Their microstructures and properties were studied. The results showed that the coatings consist mainly of γ-Ni, WC, Cr₂₃C₆, Cr₇C₃, Ni₃Si, Cr₅B₃, CrB and FeNi₃ phases, and the Ni₃Si, Cr₅B₃, CrB and FeNi₃ phases mainly segregated between the carbide grains. The carbide contents in the coatings increased with increasing the mass fractions of NiCr-Cr₃C₂ and WC powders in the mixed powders, which results in enhancing the coating hardness. The abrasive wear resistance of the coatings depends on their hardness. The higher the coating hardness, the stronger the wear resistance is. When the mixed powder (15wt%WC + 30 wt% NiCr-Cr₃C₂ + 55wt%NiCrBSi) was used, the composite coating has higher hardness and more excellent wear resistance, and the coating hardness and weight loss after wear tests are 991 HV and 8.6 mg, respectively.     

Friction and wear properties of TiN,TiAlN, AlTiN and CrAlN PVD nitride coatings

Four nitride coatings, TiN, TiAlN, AlTiN and CrAlN were deposited on YG6 (WC + 6 wt.% Co) cemented carbide by cathodic arc-evaporation technique. The friction and wear properties were investigated and compared using ball-on-disc method at high speed with SiC ball as a counter material. The tests were evaluated by scanning electron microscopy, X-ray diffractometer, energy dispersive X-ray, micro hardness tester and an optical profilometer. The results showed that TiN and TiAlN coatings presented lower friction coefficient and lower wear rate, and that high Al content AlTiN and CrAlN coatings didn't present better anti-wear properties in this test. Oxidation and abrasive wear were the main wear mechanism of TiN coating. In spite of the observation of micro-grooves and partial fractures, TiAlN possessed perfect tribological properties compared with the other coatings. High Al content increased the chemical reactivity and aroused severe adhesive wear of AlTiN coating. CrAlN coating presented better properties of anti-spalling and anti-adhesion, but abundant accumulated debris accelerated wear of the coating under this enclosed wear environment.