Polycrystalline Cubic Boron Nitride Research Articles

Corrosion Properties in Friction Stir Welded 304 Austenitic Stainless Steel

A 304 austenitic stainless steel was friction stir welded using polycrystalline cubic boron nitride (PCBN) tool. Relationship between corrosion properties and microstructure was examined in the weld using several corrosion tests and microstructural observation techniques. The stir zone (SZ) had better corrosion properties than the base material (BM). The corrosion tests showed that the intergranular corrosion was developed in the heat-affected zone (HAZ) compared to the BM and SZ although the grain boundaries were not severely corroded. TEM/EDS analysis revealed that Cr depletion zone near grain boundary in the HAZ was shallow and narrow. Friction stir welding (FSW) suppressed sensitisation in the HAZ, which could be explained by short duration at sensitisation temperatures during welding. On the other hand, many grain boundaries were deeply corroded in the AS, where the corrosion resistance was significantly degraded. The microstructural observation revealed that sigma phase was formed in the AS during FSW. Sigma formation produced the wide and deep Cr depletion zone with the minimum Cr content less than 12 wt % in the vicinity of the grain boundary in the AS, which severely deteriorated the corrosion resistance in the AS.

STUDIES ON FACE GRINDING PROCESS OF METAL LAYER BOND WRAPPED MONORAIL GRIT CUP CBN QUILL

The grinding effects of small diameter cup CBN wheel which monorail grit is wrapped by metal layer bond (it is named monorail grit CBN quill) in face grinding of bottom of grooves are discussed. There is wheel self-dressing of monorail grit CBN quill in the grinding process. The flank wear, edge chipping, crack, breaking and fall off of CBN grits are happened in succession. The grinding force of monorail grit CBN quill changeful characteristics of low-frequent and high-frequent fluctuation, which is the integrated function of CBN grit being blunt during the flank wear and subsequently the blunt grits occurring chipping and breaking. The process is dominated by the performance of CBN grit and state of its cutting edge. High-toughness polycrystalline CBN grit (BORAZON550, GE type) will keep the sharpness of monorail grit CBN quill, prolong the serve time of single grit, has obvious group-effect when new grit with low protrusion height add to grinding process and grinding force has obvious low-frequent fluctuation. It will be processed continuously that single-crystal CBN grit (BORAZON500, GE type) is breaking and chipping and accordingly new grit cutting edges join to grinding process, as the results, grinding force will show high-frequent fluctuation obviously.

Modeling the Progression of Flank Wear on Uncoated and Ceramic-Coated Polycrystalline Cubic Boron Nitride Tools in Hard Turning

Abstract Accurate wear modeling has always been desired, but has also been difficult and elusive. Most useful wear models have relied on experimental calibration because the physical wear mechanisms are not fully understood. This is particularly true in machining, where contact stresses and temperatures can be extremely high. In machining, the two wear modes most frequently discussed are crater wear and flank wear. Flank wear receives much more attention because it is easier to measure and the mechanism of material loss is thought to be better understood for most machining situations. This work focuses on flank wear for the same reasons. In hard turning, tool life is relatively short and both crater wear and flank wear influence the cutting process substantially. Understanding the progression of flank wear at various cutting conditions is beneficial in itself, but the ability to predict this progression will be extremely valuable. This work addresses both. Experimental flank wear progression is shown for uncoated and ceramic-coated polycrystalline cubic boron nitride (PCBN) tools at a range of cutting conditions. These data are used to calibrate a proposed mechanical wear model that predicts the progression of flank wear and tool failure points based on the cutting speed, feed, and cutting depth. The model was validated by additional experiments, which show good agreement with the predictions.

Growth and characterization of thick cBN coatings on silicon and tool substrates

Recently some research groups have achieved progress in the deposition of cubic boron nitride (cBN) coatings with a thickness of 2 μm and more, which is necessary for cutting tool applications. In our laboratory, thick cBN coatings were sputter deposited on silicon substrates using a boron carbide target. Following a boron carbide interlayer (few 100 nm thick), a gradient layer with continuously increasing nitrogen content was prepared. After the cBN nucleation, the process parameters were modified for the cBN film growth to a thickness of more than 2 μm. However, the transfer of this technology to technically relevant substrates, like cemented carbide cutting inserts, required some further process modifications. At first, a titanium interlayer had to be deposited followed by a more than 1-μm-thick boron carbide layer. The next steps were identical to those on silicon substrates. The total coating thickness was in the range of 3 μm with a 0.5- to nearly 1-μm-thick cBN top layer. In spite of the enormous intrinsic stress, both the coatings on silicon and on cemented carbide exhibited a good adhesion and a prolonged stability in humid air. Oxidation experiments revealed a stability of the coating system on cemented carbide up to 700 °C and higher. Coated cutting inserts were tested in turning operations with different metallic workpiece materials. The test results will be compared to those of well-established cutting materials, like polycrystalline cubic boron nitride (PCBN) and oxide ceramics, considering the wear of coated tools.

Identification of cutting errors in precision hard turning process

During past 10 years, the availability of polycrystalline cubic boron nitride (PCBN) tools has made application of hard turning increase remarkably in a range of industrial areas, especially in automotive, roller bearing and hydraulic industry. Among the applications of CBN cutting tools, precision hard turning attracts great interests since its environmentally friendly and flexibility, which ultimately has potential to replace grinding. Using today’s high precision hard turning with high precision motion control, high static and dynamic stiffness and thermal stability CNC machine, the dimensional accuracy can reach IT5. However, this accuracy still does not meet the demands from most industries. In addition, precision hard turning has been hampered by uncertainties with respect to part quality and process reliability. Many factors affect the machining accuracy and process stability. The presented work analysis the possible error driver factor and error sources in a precision hard turning and the strategy of on-line compensation of the dimensional errors in the process, based on the monitoring of tool wear and prediction of thermal expansion. Test results indicate that within the certain range of flank wear the developed method can improve the geometric accuracy significantly.

The effect of chamfer angle of polycrystalline cubic boron nitride cutting tool on the cutting forces and the tool stresses in finishing hard turning of AISI 52100 steel

Abstract Cutting experiments and finite element analysis (FEA) using ANSYS software have been carried out in this paper in order to investigate the effects of chamfer angle on the cutting forces and the cutting tool stresses, principal and von Mises stress, of polycrystalline cubic boron nitride (PCBN) cutting tools in finishing hard turning of AISI 52100 bearing steel. The obtained cutting force measurement tests and FEA results showed that the chamfer angle has a great influence on the cutting forces and tool stresses, especially in the passive force and von Mises stress distribution. From the present work, it can be drawn that critical chamfer angle is 20° in finishing hard turning of AISI 52100 bearing steel

Chip formation, cutting forces, and tool wear in turning of Zr-based bulk metallic glass

The chip light emission and morphology, cutting forces, surface roughness, and tool wear in turning of Zr-based bulk metallic glass (BMG) material are investigated. Machining results are compared with those of aluminum 6061-T6 and AISI 304 stainless steel under the same cutting conditions. This study demonstrates that the high cutting speeds and tools with low thermal conductivity and rake angle activate the light emission and chip oxidation in BMG machining. For the BMG chip without light emission, serrated chip formation with adiabatic shear band and void formation is observed. The cutting force analysis further correlates the chip oxidation and specific cutting energy and shows the significant reduction of cutting forces for machining BMG at high cutting speeds. The machined surface of BMG has better surface roughness than that of the other two work materials. Some tool wear features, including the welding of chip to the tool tip and chipping of the polycrystalline cubic boron nitride (PCBN) tool edge, are reported for turning of BMG. This study concludes that BMG can be machined with good surface roughness using conventional cutting tools.

Chip formation, cutting forces, and tool wear in turning of Zr-based bulk metallic glass

The chip light emission and morphology, cutting forces, surface roughness, and tool wear in turning of Zr-based bulk metallic glass (BMG) material are investigated. Machining results are compared with those of aluminum 6061-T6 and AISI 304 stainless steel under the same cutting conditions. This study demonstrates that the high cutting speeds and tools with low thermal conductivity and rake angle activate the light emission and chip oxidation in BMG machining. For the BMG chip without light emission, serrated chip formation with adiabatic shear band and void formation is observed. The cutting force analysis further correlates the chip oxidation and specific cutting energy and shows the significant reduction of cutting forces for machining BMG at high cutting speeds. The machined surface of BMG has better surface roughness than that of the other two work materials. Some tool wear features, including the welding of chip to the tool tip and chipping of the polycrystalline cubic boron nitride (PCBN) tool edge, are reported for turning of BMG. This study concludes that BMG can be machined with good surface roughness using conventional cutting tools.

The effect of temperature on the hardness of polycrystalline cubic boron nitride cutting tool materials

Polycrystalline cubic boron nitride (PcBN) cutting tools are used for the machining of ferrous alloys at high speeds and temperatures where the use of diamond is precluded due to graphitisation. To accurately predict the life of these tool materials it is necessary to understand inter alia the deformation and relevant mechanisms that occur at the temperatures and pressures associated with cutting. This paper uses Vickers indentation as a means of assessing the role of cBN content, binder phase and cBN grain size on the mechanical properties of a number of polycrystalline cubic boron nitride materials. It will be shown that as experimental temperatures increase, a change in deformation mechanism occurs in the tool materials, confirming that the indentation method is useful in the identification of such changes.

J-Integral Analysis of PCBN Tool Crack Propagation in Hybrid Machining of Advanced Materials

Failure of polycrystalline cubic boron nitride (PCBN) cutting tools, resulting from microcrack development, is one of the major causes of tool replacement in the machining of advanced materials. In this research, the J-integral method is applied to study the relationship between crack growth in a cutting tool and tool life in a hybrid machining process. Results show that with the decrease of the cutting temperature and its gradient in the cutting tool, the J value decreases and the tendency for micro crack propagation in the tool declines. Therefore, tool wear and early fracture rates are remarkably reduced.

Reactivity of Cubic Boron Nitride with Metal at High Temperature

Reaction between cubic boron nitride(cBN) and metal has been studied in this paper. Cubic BN grains immersed in metal powders : iron, chromium, nickel, manganese and molybdenum were heated up to 1100°C in argon or vacuum. Hollows observed on the heated cBN grains reveals that cBN should react with metal. The shapes etched depend on the metal, the crystallographic polarity of the cBN surface and the oxygen remaining in the chamber. Nickel powder made particularly large and clear hollows.Moreover, the reaction was applied to microprocessing of cBN. A polycrystalline cBN on which nickel thin films had been deposited was heated up to 1100°C in vacuum. Removing the films exposed discolored marks. Raman spectroscopy revealed that the discolored areas contain Ni3B. The areas were easily turned to be shallow holes by lapping.

Evaluation of the wear mechanisms and surface parameters when machining internal combustion engine valve seats using PCBN tools

This research aims to investigate the influence of cutting speed ( v c), depth of cut (doc) and feed rate ( f) on the machining of valve seats of internal combustion engines using polycrystalline cubic boron nitride (PCBN) tools. The workpiece material was Fe–C–Cu–Ni–Mo alloy obtained from powder metallurgy technique. The quality of the resulting surface is fundamental for the performance of engines and was estimated by R a, R q, R z, R t and W t surface roughness parameters, while relations between surface and cutting parameters were performed. Among the cutting parameters the cutting speed was the most influent on the wear of the tools. The predominant types of wear were flank and crater, while mechanisms as attrition prevailed at low v c and diffusion at medium v c. The dynamic instability identified by accelerometers at the machine was decisive in leading to discontinuous chip flow and chipping phenomena due to low fracture toughness of tool material.

Burr formation in face milling of cast iron with different milling cutter systems

Two face milling cutter systems, both with PCBN (polycrystalline cubic boron nitride) tools, were used to study burr formation in high-speed machining of grey cast iron under various cutting conditions. Surface roughness parameters Ra and Ri, tool life (based on flank wear, VBmax) and burr formation (length of the burr, h) were recorded and used for comparing machining performance. The best performance in terms of tool life and surface roughness was obtained with the milling cutter system consisting of 24 teeth and 24 square wiper inserts. Machining with this cutter configuration produced acceptable surface roughness values, well below the rejection criterion, after machining a batch of 3000 motor blocks in addition to achieving a significant reduction in the burr length.

Evaluation of machining performance of STAVAX with PCBN tools

A study was undertaken to investigate the wear characteristics of polycrystalline cubic boron nitride (PCBN) cutting tools and surface integrity during machining of STAVAX (specialised stainless steel) with and without coolant. Plastic deformation and formation of overtempered martensite and white layer (untempered martensite) were the dominant subsurface and surface defects. It was found that decreasing the hardness of the STAVAX from 55 HRC to 40 HRC could result in fracture on the flank face, leading to a deterioration of the surface finish. It was observed that low CBN content tools (60%CBN/40%TiN) exhibited greater fracture resistance than high CBN content tools (85%CBN/15%TiN, 90%CBN/10%Co). Although coolant could not bring about a reduction in the flank wear, it was effective in reducing the subsurface and surface defects, and in preventing chipping of the tool edge, leading to an improved surface finish. A superior surface finish (Ra<0.3 μm) was obtained with cutting fluid using a tool with a radius of 0.8 mm, depth of cut of 0.05 mm and feed rate of 0.05 mm/rev.

Turning of hardened 100Cr6 bearing steel with ceramic and PCBN cutting tools

This work deals with the machinability of hardened 100Cr6 bearing steel (62–64 HRC) when continuous dry turning using mixed alumina, whisker reinforced alumina and polycrystalline cubic boron nitride (PCBN) inserts. A full factorial experimental design was employed and the cutting range tested was as follows: cutting speed varying from 70 to 210 m/min and feed rate from 0.08 to 0.28 mm/rev. Tool life and surface finish were evaluated. As far as tool life is concerned, best results were obtained with the PCBN compact, followed by the mixed alumina tool at low feed rates and by the whisker reinforced alumina when feed rate was increased. Comparable surface roughness values were produced, with R a values as low as 0.25 μm.

Wear characteristics of PCBN tools in the ultra-precision machining of stainless steel at low speeds

Polycrystalline cubic boron nitride (PCBN) cutting tools are widely used in the ultra-precision machining of stainless steel mould inserts for the injection moulding of optical lenses. During the machining of a spherical or an aspherical profile on a mould insert, the cutting speed reduces significantly to approximately 0 as the cutting tool is fed towards the centre of the machined profile. This paper will report on experiments carried out to investigate the wear of various grades of PCBN tool in the ultra-precision machining of STAVAX (modified AISI 420 stainless steel) at low speeds. In the initial stage of machining, fine-scale cavities were formed on the rake face and as such, the damaged surface acted like a chip breaker and thus as a preferential site for crack initiation. Once a crack was initiated, it propagated along the grain boundaries leading to intergranular fracture. The experimental results show that the formation and extent of the surface fracture are greatly dependent on the cutting forces and the severity of abrasion on the rake face which are governed by the cutting temperature. The porosity, ductility and the bonding strength of the grains in the tool, apart from its thermal conductivity appear to have major influences on the fracture resistance of the tool.

次世代超硬質材料の超高圧合成とナノ組織制御 高純度ｃＢＮ多結晶体の微細組織制御による特性向上

Properties of polycrystalline materials strongly depend on their microstructural features such as the grain size distribution, the boundary condition and so on. In this paper, we provide our works concerning microstructural features and some properties of high-purity polycrystalline cBN synthesized from hBN by the direct conversion method. A fine-grained ( 99.9%) with the highest mechanical properties can be obtained by controlling temperature conditions within 2200-2400°C at 7.7 GPa. Many cBN grains rapidly become larger at temperatures above these conditions, and a considerably inhomogeneous microstructure which causes the deterioration of mechanical properties is formed.

The Effect of Workpiece Hardness and Cutting Speed on the Machinability of AISI H13 Hot Work Die Steel When Using PCBN Tooling

When machining hardened steel (⩾45 HRC) with polycrystalline cubic boron nitride (PCBN) tooling, the cutting speeds used produce high temperatures in the primary shear zone, which are sufficient to plasticize the workpiece. The paper initially reviews the effect of workpiece hardness and cutting speed on chip formation, workpiece surface integrity and cutting forces. Equations are detailed for determining the primary shear zone temperature, the proportion of heat conducted into the workpiece and the shear flow stress. Following on from this, experimental work is presented involving the orthogonal machining of AISI H13 hot work die steel with PCBN tooling. Tests were carried out over a range of cutting speeds with workpieces of different hardness, in order to provide cutting force, shear angle, chip morphology and primary shear zone thickness data. The shear flow stress decreased with increasing cutting speed and/or workpiece hardness. With the AISI H13 heat treated to 49±1 HRC, the stress magnitude changed more significantly with cutting speed and the proportion of heat conducted away from the workpiece approached 99 percent at 200 m/min. Shear localized chips were produced with white unetched layers due to intense heat generation followed by rapid cooling.

AN EXPERIMENTAL EVALUATION OF CUTTING TEMPERATURES DURING HIGH SPEED MACHINING OF HARDENED D2 TOOL STEEL

This paper investigates experimentally the effects of different process parameters on the cutting edge temperature during high speed machining of D2 tool steel using polycrystalline cubic boron nitride (PCBN) tools. The cutting edge temperature is measured using thermocouples. The process parameters considered are cutting speed, feed rate, nose radius, rake angle, and tool wear. The effects of different edge preparations including sharp, honed and chamfered are also investigated. The results show that increasing cutting speed and feed rate increases the cutting temperature while increasing nose radius reduces the cutting edge temperature. In addition, there is an optimum rake angle value at which minimum cutting temperature is generated.

Modelling of hard part machining

The paper initially reviews the use of finite element modelling (FEM) to determine orthogonal machining performance. Over the last decade, the development and use of FEM to evaluate the effect of tool coatings, cutting environment and chip formation on cutting forces and temperatures, etc., has increased dramatically. The different chip separation criteria and FEM software used by various researchers are detailed. An FE model is presented using ABAQUS/Explicit™ to simulate continuous and segmental chip formation when machining AISI H13 tool/die steel heat treated to ∼49 HRC with polycrystalline cubic boron nitride (PCBN) tools. The work utilised the shear failure criteria and element deletion/adaptive remeshing modules found in ABAQUS/Explicit. The assignment of tool/chip interface friction was dependent on the magnitude of the direct stress acting on the rake face of the tool. Experimental data involving chip morphology and cutting forces were used to validate the model. The temperature generated in the shear zone was higher with the segmented chip (up to 700 °C) than with the continuous chip (up to 250 °C), for the same machining parameters.