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.     

Experimental investigation on the performance of nanoboric acid suspensions in SAE-40 and coconut oil during turning of AISI 1040 steel

Machining is one of the most fundamental and indispensable processes in manufacturing industry. The heat generated in the cutting zone during machining is critical in deciding the workpiece quality. Though cutting fluids are widely employed to carry away the heat in machining, their usage poses threat to ecology and the health of workers. Hence, there arises a need to identify eco-friendly and user-friendly alternatives to conventional cutting fluids. The present work features a specific study on the application of nanosolid lubricant suspensions in lubricating oil in turning of AISI 1040 steel with carbide tool. SAE-40 and coconut oil are taken as base lubricants and boric acid solid lubricant of 50 nm particle size as suspensions. Variation of cutting tool temperatures, average tool flank wear and the surface roughness of the machined surface with cutting speed and feed are studied with nanosolid lubricant suspensions in lubricating oil.     

Hard machining of hardened bearing steel using cubic boron nitride tool

In many cases, hard machining remains an economic alternative for bearing parts fabrication using hardened steels. The aim of this experimental investigation is to establish the behaviour of a CBN tool during hard turning of 100Cr6-tempered steel. Initially, a series of long-duration wear tests is planned to elucidate the cutting speed effects on the various tool wear forms. Then, a second set of experiments is devoted to the study of surface roughness, cutting forces and temperature changes in both the chip and the workpiece. The results show that CBN tool offers a good wear resistance despite the aggressiveness of the 100Cr6 at 60HRC. The major part of the heat generated during machining is mainly dissipated through the chip. Beyond 280 m/min, the machining system becomes unstable and produces significant sparks and vibrations after only a few minutes of work. The optimal productivity of machined chip was recorded at a speed of 120 m/min for an acceptable tool flank wear below 0.4 mm. Beyond this limiting speed, roughness (R_a) is stabilized because of a reduction in the cutting forces at high speeds leading to a stability of the machining system. The controlling parameter over roughness, in such hard turning cases, remains tool advance although ideal models do not describe this effect rationally. Surface quality obtained with CBN tool significantly compared with that of grinding despite an increase in the advance by a factor of 2.5. A relationship between flank wear (VB) and roughness (R_a) is deduced from parametric analysis based on extensive experimental data.     

Investigation of the effects of cryogenic treatment applied at different holding times to cemented carbide inserts on tool wear

Cutting tool costs is one of the most important components of machining costs. For this reason, tool life should be improved using some methods such as cutting fluid, optimal cutting parameters, hard coatings and heat treatment. Recently, another one of the methods commonly used to improve tool life is cryogenic treatment. This study was designed to evaluate the effects of different holding times of deep cryogenic treatment on tool wear in turning of AISI 316 austenitic stainless steel. The cemented carbide inserts were cryogenically treated at −145 °C for 12, 24, 36, 48 and 60 h. Wear tests were conducted at four cutting speeds (100, 120, 140 and 160 m/min), a feed rate of 0.3 mm/rev and a 2.4 mm depth of cut under dry cutting conditions. The wear test results showed that flank wear and crater wear were present in all combinations of the cutting parameters. However, notch wear appeared only at lower cutting speeds (100 and 120 m/min). In general, the best wear resistance was obtained with cutting inserts cryogenically treated for 24 h. This case was attributed to the increased hardness and improved micro-structure of cemented carbide inserts. These improvements were confirmed through hardness, image processing, and XRD analyses.     

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.     

Ultraviolet-laser-assisted precision cutting of yttria-stabilized tetragonal zirconia polycrystal

Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) is a promising biomaterial for use in dental and femoral implants. The current method for machining Y-TZP involves grinding after sintering. However, the grinding process is time consuming and therefore costly. To resolve these issues, this paper proposes a precision cutting process that utilizes a UV-laser-assisted machining method that requires no expensive cutting tools such as diamond tools. The UV laser is used to heat the Y-TZP, which improves its machinability. First, we performed experiments to determine that the most suitable machining temperature was 600 °C. A simulation was then used to determine the optimal distance between the tool edge and the laser spot. Finally, experiments using a UV laser were conducted to confirm the effectiveness of using a UV laser for machining. In these experiments, a Y-TZP sample was cut, and grooves were generated. The carved grooves were 20 mm long, 100 μm wide, and approximately 10 μm deep. Cutting without using a UV laser was also performed as a reference experiment. The results show that the use of the laser significantly decreased the number of large cracks from 14 to 3, the specific cutting energy by 35%, and the breakage of the tool edge. These results demonstrate the possibility of enhancing the productivity of Y-TZP products.     

Prediction of tool and chip temperature in continuous and interrupted machining

In this paper, a numerical model based on the finite difference method is presented to predict tool and chip temperature fields in continuous machining and time varying milling processes. Continuous or steady state machining operations like orthogonal cutting are studied by modeling the heat transfer between the tool and chip at the tool—rake face contact zone. The shear energy created in the primary zone, the friction energy produced at the rake face—chip contact zone and the heat balance between the moving chip and stationary tool are considered. The temperature distribution is solved using the finite difference method. Later, the model is extended to milling where the cutting is interrupted and the chip thickness varies with time. The time varying chip is digitized into small elements with differential cutter rotation angles which are defined by the product of spindle speed and discrete time intervals. The temperature field in each differential element is modeled as a first-order dynamic system, whose time constant is identified based on the thermal properties of the tool and work material, and the initial temperature at the previous chip segment. The transient temperature variation is evaluated by recursively solving the first order heat transfer problem at successive chip elements. The proposed model combines the steady-state temperature prediction in continuous machining with transient temperature evaluation in interrupted cutting operations where the chip and the process change in a discontinuous manner. The mathematical models and simulation results are in satisfactory agreement with experimental temperature measurements reported in the literature.     

Measurement and finite element simulation of micro-cutting temperatures of tool tip and workpiece

Temperature generates in micro-scale cutting process has a great effect on cutting performance due to centralized heat generation. In this study, a set of micro-cutting experiments (8 μm/r≤f≤50 μm/r) were carried out to measure temperatures in micro-cutting process with high accuracy. A fast-response thermocouple with a property of self-renewing was installed in a cylinder workpiece to measure the temperatures of workpiece and tool tip simultaneously. In each test, temperature of the workpiece surface is obtained just before the hot junction of thermocouple is machined. When the hot junction is machined, the tested maximum temperature is recognized as the temperature of tool tip. In parallel, an energy density-based ductile failure material model is developed to simulate the micro-cutting process by finite element method. In simulation, when mesh distribution is changed, the predicted forces with same energy density G_ε are closer to the forces at original mesh distribution than those predicted by same energy G_f. Consequently, the energy density-based ductile failure material model can reduce mesh dependence in different mesh distribution conditions. Under new mesh distribution, Temperatures of the workpiece surface and tool tip are identified in the predicted micro-cutting temperature field. The predicted micro-cutting temperatures of workpiece surface and tool tip are very close to the experimental results. Further, the variation of temperature and its relationship with chip curling are also discussed.     

Innovative use of biologically produced ferric sulfate for machining of copper metal and study of specific metal removal rate and surface roughness during the process

The biologically produced ferric sulfate (in the form of bacterial culture supernatant) was used for machining of copper metal workpiece. A 27.04 mg/h cm² average specific metal removal rate was achieved during oxidation of copper workpiece. The leaching performance of culture supernatant was comparable to that of microbial cells indicating that an indirect non-contact leaching mechanism is predominant for metal solubilization. The surface of copper workpiece was analyzed by scanning electron microscopy before and after oxidation. The changes in surface appearance were found during oxidation of copper. Also the change in surface roughness was observed during machining process. The quality of the surface produced is a very important aspect of the performance of the manufacturing process. Therefore the present study characterizes an effect of various physicochemical parameters on specific metal removal rate and surface roughness. An increasing concentration of FeSO₄, shaking speed and volume of culture supernatant showed pronounced effect on metal removal and surface roughness. At the same time an application of varying temperatures showed little effect.     

Adhesion phenomena in the secondary shear zone in turning of austenitic stainless steel and carbon steel

This paper aims to increase the understanding of the adhesion between chip and tool rake face by studying the initial material transfer to the tool during orthogonal machining at 150 m/min. Two types of work material were tested, an austenitic stainless steel, 316L, and a carbon steel, UHB 11. The tools used were cemented carbide inserts coated with hard ceramic coatings. Two different CVD coatings, TiN and Al₂O₃, produced with two different surface roughnesses, polished and rough, were tested. The influences of both tool surface topography and chemistry on the adhesion phenomena in the secondary shear zone were thus evaluated. Extensive surface analyses of the inserts after cutting were made using techniques such as Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS), and Transmission Electron Microscopy (TEM). As expected, cutting in the stainless steel resulted in a higher amount of adhered material, compared to cutting in the carbon steel. Remnants of built-up layers were found on the surfaces of the 316L chips but not on the UHB 11 chips. Moreover, it was shown that for both materials the tool roughness had a profound effect, with the rougher surfaces comprising much higher amounts of adhered material than the polished ones. Non-metallic inclusions from both types of workpiece steels accumulate in the high temperature area on the inserts. The general tendency was that higher amounts of transferred material were found on the TiN coating than on the Al₂O₃ coating after cutting.     

Mathematical modeling of surface roughness for evaluating the effects of cutting parameters and coating material

The work presented in this paper examines the effects of cutting parameters (cutting speed, feed rate and depth of cut) onto the surface roughness through the mathematical model developed by using the data gathered from a series of turning experiments performed. An additional investigation was carried out in order to evaluate the influence of two well-known coating layers onto the surface roughness. For this purpose, the experiments were repeated for two CNMG 120408 (with an ISO designation) carbide inserts having completely the same geometry and substrate but different coating layers, in a manner that identical cutting conditions would be ensured. The workpiece material machined was cold-work tool steel AISI P20. Of the two types of inserts employed; Insert 1 possesses a coating consisting of a TiCN underlayer, an intermediate layer of Al₂O₃ and a TiN outlayer, all deposited by CVD; whilst Insert 2 is PVD coated with a thin TiAlN layer (3 ± 1 μm). The total average error of the model was determined to be 4.2% and 5.2% for Insert 1 and Insert 2, respectively; which proves the reliability of the equations established.     

Tool wear control in single-crystal diamond cutting of steel by using the ultra-intermittent cutting method

Excessive tool wear is a major drawback to the ultraprecision cutting of steel with geometrically defined single-crystal diamond tools. This paper presents a new approach to reduce this wear. In general, the wear of the diamond tool is due to chemical reactions such as diffusion into the steel, oxidation, graphitization, and carbide formation under cutting conditions of high temperature and high pressure. To suppress these types of chemical reactions, the contact time between the diamond tool and the steel in the cutting process was controlled by intermittent cutting method such as fly-cutting or milling. A series of intermittent cutting experiments were carried out to control the tool–workpiece contact time in one cutting cycle by changing the cutting speed and cutting length in each cutting cycle. The experimental results showed that the diamond tool wear was highly dependent on the tool–workpiece contact time, regardless of the cutting speed, and that the wear was greatly reduced by decreasing the contact time to less than 0.3 ms under these cutting conditions. It is expected that steel can be successfully cut with a single-crystal diamond tool by controlling the tool–workpiece contact time.     

Micromachining of ceramic surfaces: Hydroxyapatite and zirconia

Computerised Numerical Control (CNC) precision machining can be employed as a fast and reproducible method for surface micropatterning. For biomedical applications an efficient and reproducible micropatterning of zirconia and calcium phosphate based materials is highly sought in order to guide implant interactions with surrounding biological tissues for a better osseointegration. Therefore, CNC precision machining of zirconia and hydroxyapatite substrates is investigated in this study and optimised process parameters are reported. By microgrinding and micromilling microgrooves with a minimum width of 100 μm were obtained and process parameters such as cutting tool diameter and feed velocity discussed. As all samples were sintered prior to the micropatterning process, the influence of sintering temperature on the pattern quality, size and hardness of the obtained samples are studied. Vickers hardness of the different sintered ceramic surfaces was measured to correlate the possible wear impact on the tip of the cutting tools. The stiffness and the hardness of the used cutting tools were measured and their effect on the cutting results was discussed. The pattern quality and the average roughness in the machined microgrooves were analysed by 3D-profilometry and imaged by SEM. Comparison of the two machining techniques yielded more defined and less fractured micropatterns for microgrinding. The process efficiency for both methods was limited by the economic life time of the tool tips. For CNC grinding the life time was downsized due to more pronounced abrasive wear. For both materials the hardness was the crucial process parameter, which was adjusted by the sintering temperature. For milling of zirconia the sintering should not exceed a temperature of 1100 °C to minimize tool wear. A temperature of lower 1200 °C is suggested for the milling of HA. For sintering temperatures higher than 1200 °C the machining of both ceramic surfaces was hardly possible. The feed velocity was found not having a significant influence on the obtained micropattern width. The preset line pitch of 100 μm was excellently reached for both applied machining processes. It was found that lower feed velocities and smaller tool diameters caused deeper micropatterns.     

Virtual compensation of deflection errors in ball end milling of flexible blades

The deflections of highly flexible turbine blades and slender end mills lead to tolerance violations during milling. This paper presents a digital simulation and compensation model for blade machining operations. Stiffness of the blade at the cutting zone is updated as the metal is removed without re-meshing using a computationally efficient sub-structuring technique. The cutter–workpiece engagement is evaluated by considering the deformations of both end mill and the blade under the cutting loads. The estimated deformations are compensated by modifying the tool path coordinates. The model has been experimentally verified in ball-end milling of a blade whose dimensional errors have been reduced from ～70 μm to ～10 μm.     

Microstructure and mechanical properties and of pulse plasma compacted WC - Co

Tungsten carbides-based inserts have been considered as one of the dominant hard materials in the cutting industry, receiving great interest for their excellent combination of mechanical properties. Pulse plasma compaction (PPC) process has been applied to a series of WC-Co samples with varying sintering temperature, initial particle size and sintering pressure in order to study the mechanical and microstructural behaviour. The quality of the products, as well as the mechanical properties and microstructural features this process yields, are commendable and worth looking into. A high hardness of more than 2000 HV has been achieved while a maximum fracture toughness of 15.3 MPa √ m was recorded in samples that were sintered at 1100 °C and 100 MPa. Microstructural features like grain growth and other properties are discussed with respect to the varying parameters. While grain size shows an incremental pattern with increasing temperature, it was still possible to limit them to a great extent ensuring high mechanical properties. The effect of sintering pressure in the range of 60–100 MPa, while keeping sintering temperature constant, was found to be almost negligible.     

Investigations of yield stress,fracture toughness,and energy distribution in high speed orthogonal cutting

This paper presents a novel prediction method of the yield stress and fracture toughness for ductile metal materials through the metal cutting process based on Williams' Model [38]. The fracture toughness of the separation between the segments in serrated chips in high speed machining is then deduced. In addition, an energy conservation equation for high speed machining process, which considers the energy of new created workpiece surfaces, is established. The fracture energy of serrated chips is taken into the developed energy conservation equation. Five groups of experiments are carried out under the cutting speeds of 100, 200, 400, 800 and 1500 m/min. The cutting forces are measured using three-dimensional dynamometer and the relevant geometrical parameters of chips are measured with the aid of optical microscope. The experiment results show that the yield stress of machined ductile metal material presents an obviously increasing trend with the cutting speed increasing from 100 to 800 m/min while it decreases when the cutting speed increases to 1500 m/min further. Meanwhile, the fracture toughness between the chip and bulk material displays a slightly increasing tendency. In high speed machining, the fracture toughness of the separation between the segments in serrated chips also presents increasing trend with the increasing cutting speed, whose value is much greater than that between the chip and bulk material. In the end, the distribution of energy spent in cutting process is analyzed which mainly includes such four portions as plastic deformation, friction on the tool–chip interface, new generated surface and chip fracture. The results show that the proportion of plastic deformation is the largest one while it decreases with the cutting speed increasing. However, the proportions of energy spent on new created surface and chip fracture increase due to the increasing of both the chip's fracture area and the fracture toughness.     

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.     

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.     

Cutting conditions for finish turning process aiming: the use of dry cutting

To avoid the use of cutting fluids in machining operations is one goal that has been searched for by many people in industrial companies, due to ecological and human health problems caused by the cutting fluid. However, cutting fluids still provide a longer tool life for many machining operations. This is the case of the turning operation of steel using coated carbide inserts. Therefore, the objective of this work is to find cutting conditions more suitable for dry cutting, i.e., conditions which make tool life in dry cutting, closer to that obtained with cutting with fluid, without damaging the workpiece surface roughness and without increasing cutting power consumed by the process. To reach these goals several finish turning experiments were carried out, varying cutting speed, feed and tool nose radius, with and without the use of cutting fluid. The main conclusion of this work was that to remove the fluid from a finish turning process, without harming tool life and cutting time and improving surface roughness and power consumed, it is necessary to increase feed and tool nose radius and decrease cutting speed.     

Effect of heat treatment on green machinability of SiAlON compacts

SiAlON based ceramics are promising materials for wear applications such as wire extrusion dies, pipe bending rollers, etc. due to their outstanding mechanical properties both at high and low temperatures. To be able to utilize these materials for such applications, they should have specific geometrical details as holes, threads, groves, etc. for the ease of fixation. Machining of a ceramic component in its green state is one of the most common techniques which enable producing SiAlON wear parts with desired geometries. It is a prerequisite for green machining that the compact should have a sufficient green strength to withstand against stresses at the cutting zone during machining. In this study, the required green strength was obtained by a simple heat treatment step performed between 1100 and 1400 °C. The effect of this process on the microstructure, phase development, strength and machinability of SiAlON green compacts were investigated. It was observed that the compacts heat treated at 1400 °C provide a sufficient strength against damage formation on the machined part and a relatively low tool wear as a result of the formation of fragmented chips during the cutting process. Although these fragmented chips have beneficial effects on the tool wear, they resulted in a relatively poor surface quality in the machined parts.