Polycrystalline Cubic Boron Nitride Research Articles

Microstructure and texture of friction stir welds in low carbon steel using prior austenite reconstruction method

ABSTRACT Friction stir welding (FSW) was performed on thick plates of high-strength low-alloy (HSLA) steel with polycrystalline cubic boron nitride (PCBN) tools. To investigate the mechanism of microstructure formation in the stir zone (SZ), the prior austenite structure undergone through a thermo-mechanical history during the FSW process was investigated. The prior austenite structure was reconstructed by the local crystal orientations of prior austenite calculated from the bainite orientations by using the Kurdjumov–Sachs orientation relationship between the prior austenite and bainite. The reconstructed austenite revealed a heterogeneity in the SZ. The austenite grain size in the advancing side (AS) of SZ was larger than that of the retreating side (RS). Thus, a reheated temperature of AS was considered to be higher than that of RS. The textures of reconstructed austenite were also different in SZ. The rolling texture was observed in RS, but the shear texture was observed in the centre and AS. The heterogeneous microstructure and texture in SZ was considered to be caused by the material flow behaviour in FSW process.

Investigation on Surface Quality of a Rapidly Solidified Al-50%Si Alloy Component for Deep-Space Applications.

To meet the requirements for high-performance products, the aerospace industry increasingly needs to assess the behavior of new and advanced materials during manufacturing processes and to ensure they possess adequate machinability, as well as high performance and an extensive lifecycles. Over the years, industrial research works have focused on developing new alloys with an increased thermal conductivity as well as increased strength. High silicon content aluminum (Al–Si) alloys, due to their increased thermal conductivity, low coefficient of thermal expansion, and low density, have been identified as suitable materials for space applications. Some of these applications require the use of intricate parts with tight tolerances and surface integrity. These challenges are often tied to the machining conditions and strategies, as well as to workpiece materials. In this study, experimental milling tests were performed on a rapidly solidified (RS) Al–Si alloy with a prominent silicon content (over 50%) to address challenges linked to material expansion in deep space applications. The tests were performed using a polycrystalline cubic boron nitride (PCBN) tool coated with amorphous diamond to reduce tool wear, material adhesion, surface oxidation, and particle diffusion. The effects of cutting parameters on part surface roughness and microstructure were analyzed. A comparative analysis of the surface with a conventionally utilized Al6061-T6 alloy showed an improvement in surface roughness measurements when using the RS Al–Si alloy. The results indicated that lower cutting speed and feed rate on both conventional and RS Al–Si alloys produced a better surface finish. Reduced vibrations were also identified in the RS Al–Si alloy, which possessed a stable cutting time at low cutting speeds but only displayed notable vibrations at cutting speeds above 120 m/min.

High throughput deep-hole drilling of Inconel 718 using PCBN gun drill

Overcoming the myriad challenges in the high-speed machining of Inconel 718, especially for deep-hole drilling (DHD), has been a long-standing area of research for several manufacturers and researchers. Owning to the difficult-to-machine characteristics of Inconel 718, the traditional carbide-tipped gun drills often get worn at an accelerated pace and require repetitive re-sharpening or replacement. This lowers productivity and increases costs. Furthermore, it is also a challenging task to meet the stringent hole-straightness requirement of 1/1000 mm for DHD of such difficult-to-machine materials, especially using those carbide gun drills. Hence, this study presents a development of polycrystalline cubic boron nitride (PCBN)-tipped gun drill to address the rapid tool degradation and poor hole-straightness issues. The developed PCBN-tipped gun drill has a layer of PCBN on its cutting tip and bearing pads. Several gun drilling tests on Inconel 718 have been performed at high drilling speed conditions to assess the performance of developed gun drill. In contrast to traditional carbide-tipped gun drills, the developed PCBN-tipped gun drill has the capability of not only performing in the higher cutting speeds but also reducing drilling forces, drilling torques, and tool wear. Furthermore, the surface quality of drilled holes is also much better than those drilled by traditional carbide gun drills. With these results, it further validates the credibility of the developed PCBN gun drill in enhancing the deep-hole drilling process with high throughput productivity and superior quality of drilled holes, especially for difficult-to-machine materials.

Performance evaluation of cBN-Ti3AlC2–Al composites fabricated by HTHP method

In this paper, polycrystalline cubic boron nitride (PcBN) with Ti3AlC2–Al as the compound binder was manufactured by high-temperature and high-pressure (HTHP) method. Effects of Ti3AlC2 decomposition on PcBN microstructural features and mechanical properties at different sintering temperatures were systematically analyzed. Results revealed that sintering temperature of 1350 °C was critical temperature for complete Ti3AlC2 decomposition under HTHP conditions. Sintering driving force improved with increasing temperature, resulting in significant enhancement of PcBN compactness. In addition, decomposition products of Ti3AlC2 reacted with cBN to form TiC, TiB2, and AlN, improving the adhesion between cBN grains. At 1450 °C, as-prepared composites with the highest relative density value (99.6%), bending strength (586.22 MPa) and Vickers hardness (48.8 GPa) were attained. Moreover, at 1300 °C, fracture toughness of PcBN was 7.18 MPa m1/2.

Granularity-induced plastic deformation mechanism of pure polycrystalline cubic boron nitride

Combined X-ray diffraction(XRD) profile analysis and HRTEM observation, a method for exploration of plastic deformation in pure polycrystalline cubic boron nitride (PcBN) samples with sizes of primary cBN powders was developed. XRD profile results showed that the coarse-grained PcBN exhibited a larger micro-strain ε, a greater deformation stacking faults probability fD, which was an order of magnitude larger than that of fine-grained PcBN, but a smaller twin stacking faults probability fT. It was deduced that the plastic deformation of the coarse-grained PcBN was dominated by stacking faults and mechanical twins mode, which would result in strain strengthening and then recrystallization. While the manner of growth twins was the mainly modes of fine-grained PcBN by phase transform, especially utilized the curled SP2 structure as a basis for a cubic structure nucleation. Fundamental plastic deformation principles of the ultrafine polycrystalline cBN was crucial for the field of high-precision cutting tools.

Influence of cBN content, Al2O3 and Si3N4 additives and their morphology on microstructure, properties, and wear of PCBN with NbN binder

Polycrystalline cubic boron nitride (PCBN) tool materials with NbN binder without additives and PCBN with Al2O3 or Si3N4 micropowder and whisker additives were manufactured and compared. PCBN materials with Si3N4 whisker reinforcement have the best mechanical properties of all the evaluated materials. Composites reinforced with Al2O3 whiskers have the lowest fracture toughness. However, Al2O3 whisker-reinforced tools outperform both commercial and Si3N4 reinforced tools when machining hardened steel. Thus Al2O3 whisker-reinforced PCBN materials are promising for industrial applications, likely due to their higher resistance to oxidation and diffusional wear mechanisms during cutting operations.

In Situ Synthesis of Polycrystalline Cubic Boron Nitride Reinforced by Different Morphologic TiB2

Polycrystalline cubic boron nitride (PcBN) reinforced by different morphologic titanium diboride (TiB2) was in situ synthesized in the temperature range of 1100-1600 °C under an ultra-high pressure of 5.5 GPa in cBN-Ti-Al system. The composition and microstructure of PcBN composites were investigated by x-ray diffractometry, scanning electron microscopy and energy-dispersive spectroscopy. The mechanism and influencing factors of TiB2 microstructure formation with different morphologies were analyzed. The mechanical properties of PcBN were also tested and analyzed. The results showed that the microstructure of TiB2 evolves from whisker to plate and rod to granular as the temperature increases. Under the combined effects of phase composition, relative density and TiB2 morphology, the microhardness of PcBN increased continuously, while the flexural strength first increased and then decreased. The high flexural strength was obtained at 1300 and 1400 °C, which were 840.39 and 823.32 MPa, respectively.

In situ synthesis of polycrystalline cubic boron nitride in cBN–Al–Ti system with different Gd2O3 additions

Polycrystalline cubic boron nitride was sintered under high temperature (1400 °C) and high pressure (5.5 GPa) using an in situ synthesis method. The composition, microstructure, relative density and mechanical properties of the polycrystalline cubic boron nitride (PcBN) with different contents of Gd2O3 were studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The results show that the phases of TiB2, TiN, AlN, AlTi2N and GdB6 are formed through in situ synthesis of cBN–Al–Ti–Gd2O3 system. With the Gd2O3 content increasing, the relative density increases and mechanical properties of PcBN begin to increase and then decrease. Proper amount of Gd2O3 can advance the formation of TiB2 and improve the aspect ratio of TiB2. When Gd2O3 content is 1.0 wt%, the PcBN exhibits optimal comprehensive mechanical properties, with a microhardness of 33.35 GPa and a flexural strength of 972.37 MPa.

Influence of the cutting direction angle on the tool wear behavior in face plunge grinding of PcBN

Polycrystalline cubic boron nitride (PcBN) is a highly wear resistant material. Due to its high hardness this material is typically machined with diamond grinding tools. The high hardness and high-temperature hardness of PcBN leads to a significant grinding tool wear. The applied cutting direction angle during face plunge grinding offers the possibility to influence the geometry of the contact area between the grinding tool and the PcBN workpiece. However, the underlying principal mechanisms and influences of parameters are not fully understood today. The contact zone geometry is described by the width of cut and the geometric contact length. The paper provides a mathematical description of these two parameters for S-shapes PcBN cutting inserts depending on the workpiece geometry and the cutting direction angle. It is shown that the contact length significantly determines the wear mechanism.

Experimental investigations on machining force in turning of AZ91D magnesium alloy using PCBN tools

The present work aims to study the behavior of AZ91D Magnesium alloy during turning with Polycrystalline cubic boron nitride (PCBN) tools via investigation on machining force with variable speed, feed, depth of cut, and nose radius. Experimentation is planned and executed as per L16 orthogonal array. The parametric effects and their interactions were discussed using main effects plot and interaction plot. Results concluded that Machining force is significantly influenced by depth of cut followed by feed, speed and nose radius.

Tool wear progression of PCD and PCBN cutting tools in high speed machining of NiTi shape memory alloy under various cutting speeds

NiTi shape memory alloys are coming into prominence in various industrial applications owing to their unique shape memory effect, super-elasticity and mechanical properties. However, this material is characterized by its very poor machinability rates. One of the major reasons of this problem is the high tool wear rate, which primarily occurs due to nature of this material. The previous work reported in the literature has been limited to investigate low to medium speed machining characteristics of this material especially with carbide and coated carbide cutting tools. This paper presents the results of experimental investigation on the progressive wear behaviors of Polycrystalline Diamond (PCD) and Polycrystalline Cubic Boron Nitride (PCBN) cutting tools in turning of equi-atomic NiTi shape memory alloy. Effects of cutting speed, which varied between Vc = 70 and 250 m/min, were investigated. Periodical tool wear measurements were conducted in order to find tool life and evaluate wear mechanisms. The results show that tool wear mechanism is highly cutting speed dependent. PCD cutting tool performed better wear resistance at all of the investigated cutting speeds. The optimized cutting speed and cutting tool couple is suggested to increase productivity regarding longer tool life.

Hard Turning Cutting Tool Materials used in Automotive and Bearing Manufacturing Applications – A review

Hard turning for the machining of hardened steels by using super abrasive cutting tool materials is a comparatively newer manufacturing technology. Due to phenomenal advancements in super abrasive tool materials and machine tool technologies, hard turning becoming a potential alternate machining process to the traditional grinding. Hard turning offers many distinguished benefits over grinding process including fewer machining setups, shorter manufacturing time, lower manufacturing cost, achieving grinding comparable product quality. Fewer machining setups bring higher manufacturing flexibility, and lower the investment cost in machine tools in comparison to grinding. It is possible to carryout hard turning without coolant that eliminates the detrimental effect of coolants on the human health and environment. These benefits made hard turning a popular alternative for the automotive and bearing manufacturing industries for machining of cam shafts, gears, crank shafts, bearings, dies and mould. This paper reviews the researches that were performed on hard turning using cutting tool materials; cemented tungsten carbide (WC), poly-crystalline cubic-boron-nitride (PCBN) and ceramics. The research findings related to the effect of cutting parameters, tool geometry and surface coating on the tool life and tool wear phenomena of these hard turning tool materials in the hard turning of major steels used in automotive and bearing manufacturing industries have been discussed.

Processing and properties of PcBN composites fabricated by HPHT using PSN and Al as sintering additive

Dense polycrystalline cubic boron nitride (PcBN) composites were fabricated by high-pressure and high-temperature (HPHT) sintering using polysilazane (PSN) and Al as sintering additive. After high-energy ball milling, the cBN fine particles were uniformly coated with PSN by ultrasonic treatment. After thermocuring and pyrolysis, the cBN–SiCN particles were mixed with Al. The PcBN composites were prepared after sintering at 1450 °C for 10 min with a pressure of 5 GPa. The refining effect of high-energy ball milling on the cBN particles was studied by scanning electron microscopy (SEM) and laser particle size analyzer. The oxidation of the cBN particles after milling was investigated by nitrogen–oxygen analyzer. The phase composition and microstructure of the sintered PcBN composites were investigated by X-ray diffractometer (XRD) and SEM. The main phases of the sintered PcBN composites are cBN, AlN, SiC and Si3N4. The conversion of cBN to hBN was inhibited by the formation of AlN. The mechanical properties of the sintered PcBN composites were improved by the appearance of SiC and Si3N4. The density and mechanical properties of the PcBN composites both increased with the content of the cBN particles increasing. The sintered sample with 60 wt% cBN, 30 wt% PSN and 10 wt% Al showed the best results: density of 99.7%, Vickers’ hardness of (25.2 ± 0.8) GPa and flexural strength of (602 ± 15) MPa.

Study of the proportioning design method and mechanical properties of a cBN–TiN composite

This study provides a proportioning design method for the polycrystalline cubic boron nitride (PcBN) composite based on dense packing theory with the use of the Horsfield dense filling theory and Dinger–Funk equation. Subsequently, three proportioning schemes of the cBN–TiN system were designed with different titanium nitride content. The samples were analysed through X-ray diffraction, scanning electron microscopy and transmission electron microscopy for density, hardness and actual cutting performance, respectively. The predicted optimal values were consistent with the experimental results, which proved the validity of the method. Three samples designed from the method had sufficient sintering reaction under high pressure and high temperature to form a uniform and compact sintering structure, which had excellent mechanical properties to meet the industrial application. A surface reaction zone was formed on the W grains and the substitution reaction between cBN and TiN occurs and directly combines with each other to form an obvious grain boundary. It also indicated that appropriate PcBN proportioning scheme, workpiece characteristics, and cutting parameters should be sufficiently considered and reasonably adjusted in order to achieve a higher tool life and excellent workpiece surface roughness in the actual cutting application.

Effect of sintering temperature on synthesis of PCBN in cBN-Ti-Al-W system

Polycrystalline cubic boron nitride (PCBN) was prepared under high-temperature and ultra high-pressure. The influence of sintering temperature on composition, micro-structure, relative density, porosity, micro-hardness and flexural strength of the PCBN was investigated by means of X-ray diffraction (XRD), field scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results show that the phase components of PCBN have no significant difference, which are composed of BN, TiN, TiB2, W, AlN, W2B and Al3Ti when the sintering temperature at 1500 °C–1600 °C. The cBN crystals connect with each other by the reaction product.With the increase of sintering temperature, the porosity of PCBN decreased from 1.05% to 0.53%, and the relative density increased from 94.5% to 99%. When the sintering temperature increasing to 1600 °C, the PCBN exhibit optimal comprehensive mechanical properties with a micro-hardness of 34.64GPa and a flexural strength of 1005.23 MPa.

Pulsed laser micro ablation of polycrystalline cubic boron nitride

Polycrystalline cubic boron nitride (PcBN) is the second hardest material after diamond. Because of its unique properties, PcBN is extensively used as a cutting tool material for the machining of hardened steels. Pulsed laser ablation (PLA) offers the possibility to generate tailored microgeometries on PcBN tools and can thereby increase tool performance. However, the thermal response of PcBN materials to the laser ablation and the laser effeciency in laser machining is not known in dependency of certain PcBN grade characteristics. In this paper the influence of different laser sources in the short and ultrashort pulse duration regime is investigated for a wide number of PcBN grades. Thermally induced surface alterations are examined in detail and connected with the functional performance of the material. It could be shown that a careful parameter definition is crucial for enhancing the performance of laser prepared PcBN cutting tools.

Microstructure and texture of friction stir welds in low carbon steel using prior austenite reconstruction method

Friction stir welding (FSW) was performed on thick plates of high-strength low-alloy (HSLA) steel with polycrystalline cubic boron nitride (PCBN) tools. To investigate the mechanism of microstructure formation in the stir zone (SZ), the prior austenite structure undergone through a thermo-mechanical history during the FSW process was investigated. The prior austenite structure was reconstructed by the local crystal orientations of prior austenite calculated from the bainite orientations by using the Kurdjumov–Sachs orientation relationship between the prior austenite and bainite. The reconstructed austenite revealed a heterogeneity in the SZ. The austenite grain size in the advancing side (AS) of SZ was larger than that of the retreating side (RS). Thus, a reheated temperature of AS was considered to be higher than that of RS. The textures of reconstructed austenite were also different in SZ. The rolling texture was observed in RS, but the shear texture was observed in the center and AS. The heterogeneous microstructure and texture in SZ was considered to be caused by the material flow behavior in FSW process.

High-Pressure Synthesis of cBN Nanoparticles with High-Density Nanotwin Substructures.

Cubic boron nitride (cBN) exhibits superior hardness and strength as compared with other ceramics that are commonly used as abrasives and cutting tools. The recently synthesized polycrystalline cBN bulk material with ultrafine nanotwin substructures possesses a remarkable combination of high hardness, fracture toughness, and thermal stability. The twin substructure has been demonstrated to exert dislocation-blocking effect similar to grain boundary, leading to strengthening of materials. So far, the synthesis of cBN nanoparticles with ultrafine nanotwin substructures has not yet been realized. Herein, we report on the synthesis of cBN nanoparticles from onion-like boron nitride (oBN) under high pressure and high temperature. Multiple characterization methods, including X-ray diffraction, Raman spectroscopy, and high-resolution transmission electron microscopy, revealed that the as-prepared cBN nanoparticles contained high-density nanotwin substructures. The use of the highly wrinkled oBN precursor and well-designed synthetic method were the key to obtain these unique ultrafine nanotwinned cBN nanoparticles.

Optimizing cutting parameters in hard turning of AISI 52100 steel using TOPSIS approach

In the present work optimization of cutting parameters are performed while hard turning of AISI 52100 steel with polycrystalline cubic boron nitride (PCBN) tools using Technique for Order Preference by Similarity to Ideal Solution (TOPSIS). Experiments are planned and conducted based on Center Composite Rotatable Design (CCD) of the Response Surface Method (RSM). Cutting speed, feed, depth of cut, nose radius and negative rake angle are considered as input parameters. In this study machining force (F) and surface roughness (Ra) are measured during the experiment. Analysis of variance (ANOVA) is deployed to determine the influence of process parameters. Obtained optimal parameters are speed 200 rpm, feed 0.1 mm/rev, depth of cut 0.8 mm, nose radius 1.2 mm and negative rake angle 45o.

Micro-sized polycrystalline cubic boron nitride with properties comparable to nanocrystalline counterparts

High-performance polycrystalline cubic boron nitride (PcBN) was sintered without binders at 1500 °C in a pressure range from 11 to 15 GPa using commercial micrometre cubic boron nitride (cBN) with a diameter of approximately 2–4 μm. The results demonstrated that the sample sintered at 12 GPa and 1500 °C had the best mechanical properties and thermal stability. Its average Vickers hardness, fracture toughness, and thermal stability was 63 GPa, 15 MPa m1/2, and 1315 °C, respectively. The considerable improvement in the mechanical properties was mostly attributed to the high compactness, close bonding between grains, and the sample's internal defect structures. The relatively small specific surface area of the micron grains provides an advantage due to its high thermal stability. The amorphous regions observed in the sample's local areas may provide a new strengthening mechanism under high pressure and temperature.