Superabrasive Grains Research Articles

Process limits in high-performance peel grinding of hardened steel components with coarse CBN grinding wheels

Recent developments in the production processes for cubic boron nitride (CBN) abrasive grains have led to commercially available grain sizes larger than lg > 300 µm. These superabrasive grains allow higher material removal rates during grinding of hardened steel components. Currently, these components are pre-machined by turning processes before being hardened and eventually finished by grinding. However, the turning process can be substituted by grinding with coarse CBN-grains since higher depths of cut are achievable when machining hardened components. This paper investigates the process behaviour of vitrified and electroplated grinding wheels with large grain sizes during the machining of hardened steel components. Process forces, wear behaviour and workpiece surface roughness are investigated for three different grain sizes, and the process limits of both bond types are examined. The investigations show that vitrified tools do not fully suit the demands for peel grinding process with high material removal rates since wear by bond breakage occurs. The electroplated tools on the other hand are capable of very high material removal rates. Their wear behaviour is characterized by clogging of the chip space if the process limit is reached. Even so, both tools outperform a standard hard-turning process in terms of process time by 74% and 94% respectively. This process time reduction in combination with the possibility to use the same (machine) tool to machine both soft and hard sections of a workpiece adds flexibility to current process chains.

Improvement of Removal Rate of Tape Lapping by Applying Fluid with Ultrasonic Excited Cavitation

Lapping with a lapping tape refers to a finishing process in which a new tape is continuously supplied to its processing area and pressed down with a pressure roller during its relative motions. In response to the growing demand for lapping high-hardness materials with high efficiency, lapping tapes with superabrasive grains formed in a textured structure with a bond have been recently utilized. In this study, we supply a working fluid that has passed through the slit between ultrasonically oscillating blades to processing points. Variations of sound pressure generate cavitation in the working fluid. Impulse waves due to the collapse of cavitation bubbles inhibit chips from getting adhered to the chip pockets of the lapping tape. We have lapped hardened SUJ2 with plural lapping tapes of different characteristics. In lapping with a lapping tape using diamond abrasive grains, we have improved the time constant for the changes in surface roughness in relation to the lapping time from 18.9 s to 7.6 s by superimposing ultrasonic oscillations; consequently, the lapping speed improved. We have compared the effects of ultrasonic oscillations for three types of lapping tapes and discovered that fewer abrasive grains and lower chip adhesion firmness due to the abrasive grains formed in a textured structure increased the effects of ultrasonic oscillations.

Additive manufacturing of metal-bonded grinding tools

Grinding tools with superabrasive grains can be manufactured from different bond materials. In several industrial applications, metallic bond systems are used. In general, these show good grain retention and offer a high thermal conductivity, when compared to the other widely used bond types such as vitrified and resin bonds. One drawback of the metallic bond is the lack of pores in the grinding layer. This is caused by the manufacturing processes that are typically used, like brazing or hot pressing. These generally produce very dense layers. The high density and low porosity lead to comparatively little space for the transport of lubricant, coolant, and chips. One approach to eliminate this disadvantage is to introduce cavities into the grinding layer, using the laser powder bed fusion technique (LPBF). In order to evaluate the general suitability of LPBF in combination with the bond material and diamond grains, grinding layer samples with a nickel-titanium bond were produced. The abrasive behavior of these samples was tested in scratch tests on cemented carbide to verify the applicability as grinding tools. While the diamond grains in the powder mixture are not part of the fusion process, they also did not interfere with the manufacturing process, and the scratch tests showed promising abrasive capabilities. The grinding layer itself withstood the process forces, and no grain breakout could be observed. This indicates that the grain retention forces are high enough for the grinding process and that NiTi has a high potential as a bonding material for the manufacturing of grinding tools via LPBF.

Micro-fracture variation and grinding performance of PCBN superabrasive grains in high-speed grinding

In this work, a finite element analysis model of micro-fracture of polycrystalline cubic boron nitride (PCBN) superabrasive grains based on Voronoi diagrams was established to study the fracture behaviour during the single-grain high-speed grinding process of Inconel 718 nickel-based superalloys. The profile evolution of grain was predicted by iterating the profile of fractured grain and analysed by combination with the experimental results. The effects of internal stresses and microstructure of PCBN grain on grain fracture were investigated. The profile evolution and chip formation process of PCBN grain due to micro-fracture were analysed, and the effect of micro-fracture on the grinding performance of PCBN grain was studied. Results showed that the main fracture modes of PCBN grain were micro-fracture near cutting edges and crack propagation inside the grain. Tensile stress was the main internal affecting factor of the fracture behaviour and led to more consumed volume than compressive stress. The microstructure of PCBN grain hindered the stress transfer and made the grain susceptible to cracks and micro-fractures. The number of cutting edges involved in the cutting process varied due to micro-fractures. The balance between the difficulty in chip formation and energy loss could only be reached when the percentage of fractured regions ranged from 40% to 50%. Micro-fracture changed the profile and sharpness of PCBN grain, thereby forming a ‘passivation-sharpening’ cycle.

Understanding the self-sharpening characteristics of polycrystalline cubic boron nitride super-abrasive in high-speed grinding of Inconel 718

This study aims to understand the self-sharpening characteristics of the polycrystalline cubic boron nitride (PCBN) super-abrasive grains in high-speed grinding of Inconel 718. Comparative single grain grinding operations by separately using the brazed PCBN and conventional CBN abrasive grains are carried out. An in-depth analysis on the grain fracture morphologies and the workpiece scratch profiles during grinding are provided. The cutting mechanism and the material removal difference induced by the PCBN and conventional CBN grain fracture behaviour during grinding are revealed. The obtained results indicate that, the PCBN abrasive grain tends to intergranular fracture during grinding, which not only leads to the grain micro-fracture behaviour but also provides plenty of micro cutting-edges of microcrystalline CBN particles to participate in the grain cutting process. The surface roughness of the workpiece scratch grooves produced by the PCBN abrasive grain is 0.30 μm in average, which is 0.10 μm higher than that produced by the conventional CBN abrasive grain. This is mainly attributed to the additional cutting effects of the outcropped microcrystalline CBN particles on the PCBN grain fracture surface in the actual grinding process, and the additional grain cutting effects on the workpiece materials contribute to the self-sharpening characteristics of the PCBN. Finally, a mechanical analysis model of the single abrasive grain in grinding is also developed to provide a more comprehensive understanding on the self-sharpening mechanism of PCBN, and the established mechanical model can be correctly demonstrated by the calculated grinding force ratios in the single grain grinding operations.

Investigation on stress distribution and wear behavior of brazed polycrystalline cubic boron nitride superabrasive grains: Numerical simulation and experimental study

The polycrystalline cubic boron nitride (PCBN) superabrasive grains, which are comprised of microcrystalline CBN particles and AlN ceramic binder, have a distinguished advantage in self-sharpness during grinding by means of controllable fracture wear. The present work intends to clarify the mechanism of grain fracture based on the analysis of brazing-induced residual stress and the resultant stress in grinding; as such, the grain fracture wear could be predicted and controlled effectively. A finite element model based on Voronoi tessellation method has been first established for PCBN grains. The effects of embedding depth, volume fraction, and grinding loads on the stress distribution within PCBN grains are discussed. It is found that the distribution patterns of microcrystalline CBN particles generally have less influence on grain fracture. Large tensile stress is produced at the interfaces of microcrystalline CBN particles, AlN ceramic binder and Ag-Cu-Ti filler. Particularly, the largest tensile stress is generated near the grain vertex in the case of the embedding depth of 50%, volume fraction of 80%, and uncut chip thickness of 0.6μm. The simulation results are verified experimentally through characterizing the wear topography evolution of brazed PCBN grains in grinding.

Understanding the temperature distribution and influencing factors during high-frequency induction brazing of CBN super-abrasive grains

The temperature distribution characteristics in high-frequency induction brazing of cubic boron nitride (CBN) super-abrasive grains is investigated based on finite element simulation in this article. Influences of the following factors, i.e., current frequency, current magnitude, heating gap, and scanning speed, on the resultant heating temperature are discussed. The results obtained display that, during the rotating induction heating process, the effective heating zone is concentrated on the top surface of the metallic wheel substrate (i.e., AISI 1045 steel) below the coil. The highest value of the resultant heating temperature becomes larger when heating with a higher current frequency, a higher current magnitude, and smaller heating gap values; however, a higher peripheral scanning speed results in a decrease of the resultant highest temperature. The optimum parameters are determined as the heating gap of 2 mm, current frequency of 1 MHz, current magnitude of 20 A, and peripheral scanning speed of 0.5 mm/s. Finally, the simulation results are testified and validated in the high-frequency induction brazing experiment of CBN grains.

Potential for improving efficiency of the internal cylindrical grinding process by modification of the grinding wheel structure—Part II: Grinding wheels made of superabrasive grains

This article offers an overview of 11 grinding wheel construction modifications used in the peripheral grinding of flat, shaped, internal, and external cylindrical surfaces, when grinding wheels made of superabrasive grains are used (natural and synthetic diamonds, as well as mono- and microcrystalline cubic boron nitride). The text contains characteristics of grinding wheels with: bubble corundum grains, glass-crystalline bond, conic chamfer, zones of different diameters, a centrifugal provision of the coolant into the grinding zone, aggregate grains, zone-diversified structure, as well as impregnated (self-lubricating), multiporous, segment and “intelligent” grinding wheels. Each of the presented structural modifications were described by giving construction scheme, used abrasive grains, range of applications, advantages as well as disadvantages. Modifications of the grinding wheel construction allow for effective improvement of both the conditions and the results of the grinding process. A wide range of the known modifications allow for their proper selection depending on the required criteria of effective evaluation and taking into account the specific characteristics of superabrasive grains. As a result, it is possible to obtain positive influence on a number of technological factors of the grinding process. The described modifications of the grinding wheel structure can be also an inspiration and the basis for creating new solutions in this field.

Stress characteristics and fracture wear of brazed CBN grains in monolayer grinding wheels

Compared to electroplated cubic boron nitride (CBN) grinding wheels, monolayer-brazed CBN wheels have a distinguished advantage in high bonding strength to super-abrasive grains. However, fracture-induced grain wear can occur during grinding. This study aims to understand the mechanism of such fracture, and hence to predict the fracture based on a stress analysis. A two-dimensional finite element (FE) model was developed to investigate the stress characteristics and fracture wear of the grains. The effects of grain embedding depth, grain wear, bond wear, grain size and grinding load on the stress distribution were investigated. It was found that grain macro-fracture wear is dominated by the brazing stresses in the junction region of a CBN grain, and that the peak brazing stress reduces with the increase in the grain embedding depth, grain size and undeformed chip thickness. This suppresses the possibility of grain macro-fracture wear in grinding.

Picosecond laser treatment of metal-bonded CBN and diamond superabrasive surfaces

Laser treatment is a novel and promising non-conventional method to reduce the conditioning costs and time of superabrasive tools. In this study, the thermal effects of picosecond laser radiation on metal-bonded cubic boron nitride (CBN) and diamond superabrasive tool surfaces have been analytically and experimentally investigated. The analytical approach is intended to find threshold process parameters for the selective ablation of the superabrasive grains and the bond material. The laser radiation experiments have been conducted with a picosecond Yb:YAG laser on metal-bonded CBN and diamond honing tools. It has been analytically and experimentally demonstrated that, using suitable parameters, the selective and controlled treatment of superabrasive tool components is possible.

Joining of Diamond Grains to Medium Carbon Steel with Ni-Base Powder during Laser Brazing

In order to develop new generation brazed Diamond grinding wheels, the joining experiments of Diamond super abrasive grains and medium carbon steel using the powder mixture of Ni-Cr alloy as active brazing alloy are carried out via laser in an argon atmosphere. The relevant characteristics of the special powder mixture, the microstructure of the interfacial region, which are both the key factors for determining the joining strength among the Diamond grains, the filler layer and the steel substrate, are investigated extensively by means of scanning electron microscope (SEM) and energy dispersion spectrometer (EDS), as well X-ray diffraction (XRD) analysis. The formation mechanism of carbide layers was discussed. All the results indicate that high strength bonding between diamond grits and the steel substrate has been successfully realized because the chromium in the Ni-based alloy segregated preferentially to the surface of the diamond to form a chromium-rich reaction product Cr3C2, and the bond between the alloy and the Diamond was established through the reaction product.

Modelling the abrasive flow machining process on advanced ceramic materials

Abrasive flow machining (AFM) is a unique machining method used to achieve high surface quality on inner, difficult-to-access and on outside contours. Using AFM, it is possible to realise predefined edge rounding on any brittle or hard material. AFM is easy to integrate in an automated manufacturing environment. The abrasive medium applied during AFM is a fluid consisting of a polymer which carries silicon carbide or super-abrasive grains. With a specified pressure and temperature, this fluid flows in alternating directions along the contours of the workpiece resulting in an abrasive effect. AFM is also well suited to process advanced ceramic materials. Especially advanced ceramics are playing increasingly a significant role as a substitute for metals. However the high costs for the inevitable finishing process on ceramics prevent a more frequent use. This paper represents the technological results of a research-project discovering the fundamental principles of AFM on advanced ceramic materials such as a correlation between flow processes, surface formation and edge rounding. Furthermore an insight into a process model is given, which was developed using modern simulation techniques. The overall objective of this approach is to anticipate work results like surface quality and edge rounding on any user-defined geometry.

Laser-Assisted Truing for Metal-Bonded Diamond Wheel

Truing and dressing are time-consuming processes in precise finishing with super-abrasive wheels because of the hard super-abrasive grains which cause extensive wear of truer. In this paper, laser-assisted truing method for super-abrasive wheels is introduced. Feasibility of the proposed method is confirmed by fundamental experiments conducted with metal-bonded diamond wheel and pulsed Nd:YAG laser. On the surface of wheel irradiated with laser beam, it is found that the abrasive grains are degraded and the machinability of surface layer is remarkably improved.

Development and performance of monolayer brazed CBN grinding tools

CBN grinding tools have been broadly utilized in machining difficult-to-cut materials in recent years. Grains of the conventional grinding tools, however, are held in the tool matrix just through the mechanical incrustation effect induced by the electroplated or sintered metal, which results in the stochastic grain distribution and limited grain protrusion, in addition to the easy grain pullout and premature tool failure by the strong impact forces generated during machining. These properties and shortcomings of the electroplated or sintered tools have restricted the potential of CBN superabrasive grains. Therefore, a new technology has been developed and introduced in this paper to fabricate successfully monolayer CBN grinding tools, in which the highly protruding grains could be planted in the required uniform pattern through the brazing effect among CBN grains, filler alloy and tool substrate at elevated temperature. Finally, comparative grinding tests performed with the conventional electroplated and newly-developed brazed CBN tools have indicated that highly increased efficiency and prolonged tool lives, as well as low fabrication and use cost could be achieved by applying the brazed CBN grinding tools.

Análise do comportamento da microestrutura de materiais endurecidos retificados com rebolos de CBN

CBN grinding wheels are superabrasive tools, with cutting ability higher than the conventional ones, specially used for grinding DTG (Difficult to Grind) materials. Due to the higher hardness and thermal conductivity of the CBN superabrasive grains, when compared with conventional abrasive grains, the use of CBN tools reduce the grinding temperatures decreasing the probability of workipece’s thermal damage. These proprieties of CBN grains permit lowering the energy partition because less heat is generated during grinding and much heat is dissipated through the grinding wheel instead of through the workpiece. This paper shows a research about the microstructural changes in the ground subsurface and the differences in the superficial integrity when using two types of superabrasive grinding wheels, manufactured with vitrified and resin bonds, analyzed in different cutting conditions. The ground material was the VC 131 steel, hardened and tempered. Were not observed microstructural changes in the subsurface ground with the two types of tools. As a result, the use of different types of the bonding do not affect the surface integrity neither resulted in microstructural changes, because the maximum temperatures measured in the grinding zone were lower than 125 °C, much less than the required level to cause microstructural changes.

5511379 Process for the preservation of products at low temperature in an insulated chamber, installation for practicing the process, insulated chamber and container for such a chamber

The polycrystalline cubic boron nitride (PCBN) superabrasive grains, which are comprised of microcrystalline CBN particles and AlN ceramic binder, have a distinguished advantage in self-sharpness during grinding by means of controllable fracture wear. The present work intends to clarify the mechanism of grain fracture based on the analysis of brazing-induced residual stress and the resultant stress in grinding; as such, the grain fracture wear could be predicted and controlled effectively. A finite element model based on Voronoi tessellation method has been first established for PCBN grains. The effects of embedding depth, volume fraction, and grinding loads on the stress distribution within PCBN grains are discussed. It is found that the distribution patterns of microcrystalline CBN particles generally have less influence on grain fracture. Large tensile stress is produced at the interfaces of microcrystalline CBN particles, AlN ceramic binder and Ag-Cu-Ti filler. Particularly, the largest tensile stress is generated near the grain vertex in the case of the embedding depth of 50%, volume fraction of 80%, and uncut chip thickness of 0.6 μm. The simulation results are verified experimentally through characterizing the wear topography evolution of brazed PCBN grains in grinding.