Fabrication of porous structure vitrified bond diamond grinding wheel via direct ink writing

Porous vitrified bond grinding wheels with complex structure, high porosity, controllable pore size have fundamental application in high efficiency and precision grinding of hard and brittle materials. In this paper, direct ink writing (DIW) is proposed to fabricate three kinds of grinding wheels, including solid structure, triangle structure, and lattice structure. Moreover, the rheological property of ceramic ink with different doses of xanthan gum (XG) solution was investigated to ensure printability, demonstrating 3% XG solution can meet requirements. Additionally, the effect of sintering temperature and pore former (PMMA) contents on size shrinkage rate, morphology, mechanical strength, and porosity et al. were studied. The results indicate that the diamond grinding wheel with 30 vol% PMMA and sintered at 670 °C possess the best comprehensive performance. Besides, grinding performance was evaluated by surface morphology, surface roughness, and material removal rate. Among the DIW-fabricated wheels, triangle structure grinding wheel and lattice structure grinding wheel possess a higher material removal rate than solid structure grinding wheel. Therefore, the porous structure grinding wheels fabricated by DIW present the advantage of controllable porosity, excellent self-sharpening ability, and higher bond strength, which may pave the way for designing a new generation vitrified bond diamond grinding wheel.     

Morphological evolution and grinding performance of vitrified bonded microcrystal alumina abrasive wheel dressed with a single-grit diamond

Dressing experiments under different conditions were carried out on a vitrified bonded microcrystal alumina abrasive wheel with a single-grit diamond dresser. The grinding performance of the as-dressed abrasive wheels was investigated. The dressing force, grinding force and the surface morphology of abrasive wheel and machined workpiece were studied to shed light on the relationship among the dressing processing vectors, morphology of abrasive wheel and the grinding performance. The results obtained show that the dressing forces increase with the increasing volume of the abrasive wheel material removed per unit time. The sensitive analysis reveals that the dressing feed speed take a greater effect than the single dressing depth on the dressing force. The self-sharpness of vitrified bonded microcrystal alumina abrasive wheel brings into some functions under certain dressing conditions, but a deep dressing depth would lead to an excessive abrasive self-sharpness, i.e. abrasive grits fall off and embed into the workpiece surface.     

Grinding of alumina ceramic with microtextured brazed diamond end grinding wheels

Brazed monolayer diamond grinding wheels have advantages of a high abrasive bonding strength, high protrusion, and a large chip disposal space. However, it is difficult to prepare ordered and fine-grained brazed diamond grinding wheels. This study presents a new method for grain-arranged, brazed diamond grinding wheels with microtextures with similar performance to ordered and fine-grained brazed diamond grinding wheels. First, coarse diamond grains (18/20 mesh) were orderly brazed to fabricate the end grinding wheels. Next, a series of microtextures were ablated on the diamond grains using a pulsed laser, and two types of textured end grinding wheels—TG-G (ablated microgrooves only) and TG-GH (ablated microgrooves and microholes)—were prepared. Then, an experiment involving the grinding of alumina ceramics was performed, and the grinding characteristics and grinding mechanism were analyzed. The results indicated that compared with untextured diamond end grinding wheels (TG), the textured diamond grinding wheels (TG-G and TG-GH) significantly reduced the grinding force and the roughness of the machined surface. The local stress concentration at the microtextures promoted the formation of microcracks in the diamond grains of TG-G and TG-GH, and the self-sharpness of the grinding wheel was significantly improved. The brittle fracture mode of ceramic materials in grinding included intergranular fracture and transgranular fracture. Ironing pressure action was a key material-removal mechanism. It had an important influence on the cutting force and plasticity characteristics of the TG machined surface. For the surfaces processed by TG-G and TG-GH, the effect of ironing was weakened, while shearing played a more important role. The TG-GH grinding wheel ablated with microgrooves and microholes was superior to the TG-G grinding wheel ablated with only microgrooves, with regard to the grinding force, roughness, and self-sharpening.     

Investigation on preparation of composite vitrified bonding ultrafine diamond grinding wheel and machining of PcBN inserts

The grinding of polycrystalline cubic boron nitride (PcBN) is hard owing to its high hardness and superior wear-resistance capacity. Machining of PcBN tools via vitrified diamond grinding wheels with a size above 10 μm may lead to brittle fracture instead of a ductile machining because of the poor toughness of cubic boron nitride. A uniformly dispersed M0.5/1.5 diamond grinding wheel with a composite vitrified bonding was fabricated to improve the surface roughness of PcBN inserts. It is demonstrated that the preparation of composite vitrified bonding with various additions of vitrified bonding produced by the melting-quenching technique (VB-MQ) has little effect on the performance of vitrified bonding, such as bending strength, CTE and phase and achieves the uniform dispersion of M0.5/1.5 diamond as the addition of VB-MQ is no greater than 50%. Both the grinding ratios and the surface roughness of PcBN inserts are enhanced.     

Grinding behavior of biomimetic fractal-branched silicon carbide ceramic inspired from leaf-vein structure

Silicon carbide ceramics are widely used in many industrial fields owing to their outstanding physical and chemical characteristics. However, their inherent hardness and brittleness make the grinding process very difficult compared to that involving ductile materials. In the present study, the effects of the biomimetic fractal-branched structure, inspired from the leaf-vein, on the grinding behavior of silicon carbide were investigated. Two biomimetic fractal-branched structures with different densities of micro-channels were designed and compared with the non-structured silicon carbide surface. The surface of the silicon carbide ceramic was textured through pulsed-laser ablation. Thereafter, the grinding experiment was conducted on the biomimetic fractal-branched and non-structured workpieces. The surface topography, subsurface damage, grinding force, grinding force ratio, surface roughness and grinding wheel wear were examined throughout the experiment. The experimental results indicated that the normal and tangential grinding forces for the fractal-branched structure surface are 7.61–18.21% and 8.34–26.13% lower than those for the non-structured surface. The grinding force ratio also increased significantly with an increase in the micro-channel density. In addition, a larger volume of coolant was transported through the grinding zone of the fractal-branched structure. The research results confirmed that the biomimetic fractal-branched structure on the silicon carbide surface enhanced the grinding performance and improved the grinding quality.     

Grinding performance improvement of laser micro-structured silicon nitride ceramics by laser macro-structured diamond wheels

Silicon nitride ceramics are widely used in various industrial fields because of their excellent characteristics: high hardness, high elastic modulus, abrasion resistance, and high heat resistance. Diamond wheel grinding is the predominant and most productive method to machine silicon nitride ceramics. However, a lot of heat is generated due to high friction between a diamond grinding wheel and extremely rigid silicon nitride during grinding. This causes surface/subsurface damage, wheel wear, etc., which impairs the surface quality of silicon nitride. This impairment can restrict the use of silicon nitride ceramic components. To improve the surface quality and service life of grinding wheels, a laser macro-micro combination structured grinding (LMMCSG) method was presented. The results indicated that the grinding force ratio and surface roughness when using LMMCSG were respectively 31% and 40% lower than the grinding force ratio and surface roughness when using conventional grinding. Moreover, the LMMCSG method effectively reduced the wheel wear and workpiece subsurface damage.     

Self-sharpening property of porous metal-bonded aggregated cBN wheels during the grinding of Ti–6Al–4V alloys

Grinding process with cubic boron nitride (cBN) superabrasive wheels has been the subject of extensive research during high efficiency and precision machining difficult-to-cut materials in aerospace and aviation industries. However, the grinding performance and tool-life of conventional cBN abrasive wheels are severely affected by the probable macro-fracture and pull-out of cBN grains owing to their anisotropic crystalline structure. In this case, porous metal-bonded grinding wheels coupled with high-performance aggregated cBN abrasive grains were developed to improve tool performance and machined surface integrity. Characterisation of morphologies, including as-sintered aggregated cBN abrasive grains, pore structures and grain wear evolutions, was performed. The grinding ratio, grinding forces, force ratio and ground surface roughness were evaluated through single-grain grinding of Ti–6Al–4V alloys. Experimental results indicated that the porous aggregated cBN wheels had abundant chip storage space and excellent wear resistance. A stable grinding force ratio and small ground surface roughness were obtained during the tool wear tests due to the combined characteristics of microfracture and partial macrofracture of multi-layer cBN particles.     

Numerical investigation of crack initiation,propagation and suppression in robot-assisted abrasive belt grinding of zirconia ceramics via an improved chip-thickness model

Motivated by the prevailing assisted techniques in wheel grinding of brittle and hard materials, an attempt is made in this paper to identify the feasibility of robot-assisted abrasive belt grinding of zirconia ceramics. Owing to the flexible machining system, the challenge of this attempt resides in achieving the required profile accuracy and surface quality, in which the evolution of grinding-induced micro-cracks is prioritized. The single-grit scratching simulation based on an improved chip-thickness model that incorporates elastic modules of tool-workpiece engagement is employed to explore the damage mechanism in terms of the initiation, propagation and suppression of micro-cracks. The simulation results demonstrate that the critical depth of cut for brittle-to-ductile transition of zirconia ceramics is determined as 0.42 μm according to the tentative maximum undeformed chip thickness (UCT) values. In ductile-regime grinding, the zirconia surface morphologies are independent of the abrasive particle velocity. Lateral cracks begin to initiate especially when the maximum UCT exceeds 0.42 μm, and the brittle removal becomes dominant. In brittle-regime grinding, high abrasive particle velocity could help substantially enhance the workpiece surface integrity by suppressing the median/radial cracks that initiate once the maximum UCT approaches 0.8 μm. Experiment concerning the force-controlled robotic belt grinding of zirconia ceramics is conducted to verify the simulation results via the microscope observation of ground surface morphologies. The findings are likely to provide experimental evidence on the feasibility of belt grinding of brittle and hard materials with a flexible industrial robot.     

Effects of open pores on grinding performance of porous metal-bonded aggregated cBN wheels during grinding Ti–6Al–4V alloys

Porous metal-bonded wheels coupled with the aggregated cubic boron nitride (cBN) grains and water-soluble carbamide particles as pore-forming agents were fabricated, aiming to the improvement of grinding wheel performance and ground surface quality in grinding of Ti–6Al–4V alloys. Grinding forces and force ratio, ground surface roughness and microhardness were investigated to evaluate wheel performance. In addition, the wear evolutions of cBN grains and macropores were performed as the material removal volume increases during wheel wear tests. Findings show that the dynamic changing behavior of the coverage and exposure of open pores attributes to the improvement of grinding performance and ground surface quality of porous AcBN wheels. Meanwhile, the promising self-sharpening property of wheels can be guaranteed in basis of dynamic wear variations of multiple cBN abrasive grains layer by layer.     

High resolution optical microprobe investigation of surface grinding stresses in Al2O3 and Al2O3/SiC nanocomposites

It has previously been suggested that Al₂O₃/SiC nanocomposites develop higher surface residual stresses than Al₂O₃ on grinding and polishing. In this work, high spatial resolution measurements of residual stresses in ground surfaces of alumina and nanocomposites were made by Cr³⁺ fluorescence microspectroscopy. The residual stresses from grinding were highly inhomogeneous in alumina and 2 vol.% SiC nanocomposites, with stresses ranging from ～ ?2 GPa within the plastically deformed surface layers to ～ +0.8 GPa in the material beneath them. Out of plane tensile stresses were also present. The stresses were much more uniform in 5 and 10 vol% SiC nanocomposites; no significant tensile stresses were present and the compressive stresses in the surface were ～ ?2.7 GPa. The depth and extent of plastic deformation were similar in all the materials (depth ～ 0.7–0.85 μm); the greater uniformity and compressive stress in the nanocomposites with 5 and 10 vol% SiC was primarily a consequence of the lack of surface fracture and pullout during grinding. The results help to explain the improved strength and resistance to severe wear of the nanocomposites.     

Residual Stresses in Machined MgO Crystals

The residual stresses introduced in MgO crystals by grinding on {100} surfaces in 〈100〉 directions were measured using photoelastic techniques. Grinding was conducted with two wheels; a 100-grit diamond wheel removed material by brittle fracture, and a 46-grit alumina wheel caused plastic flow and burnishing. Both wheels introduced a discrete, highly deformed layer adjacent the machined surface. In all cases the machined surfaces were under a residual tensile stress which became compressive within the deformed region. Beneath the deformed layer the residual stress patterns were distinctly different. In crystals ground with the alumina wheel the stresses became tensile again within 0.5 mm of the ground surface, whereas the subsurface stresses in crystals ground with the diamond wheel remained compressive to distances ≥1 mm. These residual stress distributions are discussed in terms of a simple model based on the superposition of mechanically and thermally induced stresses.     

Preparation of a novel vitrified bond CBN grinding wheel and study on the grinding performance

The vitrified bond CBN grinding wheels are characterized by high efficiency, high precision, and low environmental pollution. In recent years, the vitrified bond CBN grinding wheel has been widely used in manufacturing industries such as aerospace, automotive, and machine tools. In this study, a novel vitrified bond formulation containing nano SiO₂ and nano CeO₂ is selected to prepare the grinding wheel. The grinding experiments on 45# steel and YG20 alloy indicate that the grinding performance of the nano vitrified bond grinding wheel is significantly better than that of the conventional vitrified bond grinding wheel. The introduction of nano SiO₂ and nano CeO₂ greatly improves the machining performance of the vitrified bond CBN grinding wheel.     

A route to highly porous grinding wheels by selective extraction of pore inducers with dense carbon dioxide

A novel route for the production of highly porous vitrified grinding wheels was developed via selective extraction of pore inducers with dense CO₂. The extraction was performed with liquid and supercritical CO₂ (scCO₂) at temperatures ranging from 295 to 338 K, pressures from 8.8 to 27.6 MPa and flow rates of 3.4×10⁻⁵ and 7.5×10⁻⁵ kg s⁻¹ CO₂. The extraction rate was a strong function of temperature, flow rate, and flow direction, while unaffected by particle size of the pore inducer and pressure. The extraction had no detrimental effect on the green wheel’s microstructure. Grinding tests were performed on the CO₂ extracted pore induced wheels and results were compared to those from a conventionally manufactured pore induced grinding wheel. The extracted grinding wheels performed similarly to the conventional wheels. At high metal removal rates, the extracted wheels with large pore sizes outperformed the wheels with smaller particle sizes as well as the conventional wheel. This may be due to the larger pore sizes increasing lubrication at the surface and increasing the wheel strength.     

Enhancing ductile removal of Cf/SiC composites during abrasive belt grinding using low-hardness rubber contact wheels

C_f/SiC composites are used as advanced thermal protection and friction materials. However, machining these materials is difficult because of their hard, brittle, anisotropic, and heterogeneous characteristics. This study investigated the removal behavior and surface integrity of C_f/SiC composites during abrasive belt grinding using rubber contact wheels of various hardness. Additionally, detailed analysis was performed on their thermal-mechanical coupling characteristics, surface integrity (that is, surface roughness, surface micro morphology, and subsurface damages), and the grinding chips produced. Results revealed that with decreasing hardness of the contact wheel, the surface roughness in all directions, grinding force, and temperature decreased significantly. Moreover, the surface removal morphology of the C_f/SiC composites changed from macro-fracture to micro-fracture, and the subsurface morphology changed from SiC matrix cracking and carbon fibers pull-out to matrix plastic flow and fiber micro-fracture, respectively. Furthermore, strip chips with plastically squeezed and cut surfaces were visible in the grinding chips obtained under the 40-HA contact wheel. Therefore, the ductile removal behavior of the C_f/SiC composites was enhanced, and the surface quality in abrasive belt grinding with low-hardness contact wheels was markedly improved.     

Cr3+ microspectroscopy measurements and modelling of local variations in surface grinding stresses in polycrystalline alumina

Surface residual stresses caused by grinding and polishing of alumina are thought to influence materials properties but have previously been measured only by low spatial resolution techniques which sample average stresses. In this work confocal Cr³⁺ fluorescence microscopy has been used to investigate the spatial distribution of the residual stresses. A model for the residual stresses, accounting for both surface plastic deformation and “pullout” of material from the surface by brittle fracture, was developed to help in analysing the results. After coarse diamond grinding, the results showed that the residual stresses fluctuate greatly with position. Large tensile stresses (up to ～600 MPa) were found below the plastically deformed surface layer in regions between the “pullouts”. These tensile stresses are expected to aid crack propagation and further surface pullout. They arise because pullout removes parts of the plastically deformed surface layer. The stresses beneath the pullout sites themselves were compressive, but the largest compressive stresses (≈?1.5 GPa) were within the plastically deformed surface regions and extended to a depth of 1.3 μm. The plastically deformed surface layer was much shallower following polishing with 3 μm diamond paste but the compressive stress within it was of similar magnitude to that in the plastically deformed surface layer caused by grinding.     

Evolution of Surface Morphology in Laser-Dressed Alumina Grinding Wheel Material

An approach to laser dressing of alumina grinding wheels is proposed based on solidification microstructures associated with rapid cooling rates obtained in laser surface processing. Laser dressing of alumina grinding wheels forms surface microstructures characterized by multifaceted grains that are expected to facilitate the micro-scale material removal during precision machining. A detailed investigation of variation of grain size and melt depth with laser fluence is conducted. The results are correlated with calculated cooling rates derived from a thermal model. In addition, based on microscopic observations, the formation of surface grains by stacking of individual multifaceted grains formed during laser dressing is suggested.     

Effect of grinding parameters on surface integrity and flexural strength of 3Y-TZP ceramic

Grinding parameter effects on grinding forces, the surface phase transformation of tetragonal to monoclinic (t-m), surface residual stress, surface roughness, and flexural strength of 3 mol% Yttrium stabilized Tetragonal Zirconia Polycrystal (3Y-TZP) were investigated. The results have shown that the grinding force, compressive residual stress, and surface roughness increase with the increase in depth of cut and the feed speed. On the other hand, they decreased as the wheel speed increased. Additionally, grinding improved the 3Y-TZP surface t-m phase transformation. The m-phase content increased with the increase in the depth of cut and decreased as the wheel speed increased. Finally, the flexural strength of ground 3Y-TZP diminished with the increase in the cut depth. The fracture and smear were observed on the dry ground surface since it has a higher surface roughness compared to that of wet grinding.     

Key technologies for laser-assisted precision grinding of 3D C/C-SiC composites

Due to their exceptional and distinctive qualities, 3D C/C-SiC composites are widely utilized in producing high-end equipment and the aerospace national defense industries. However, the hard and pseudo plastic nature of the material and its anisotropies make it challenging to process. To improve the processing quality of 3D C/C-SiC composites, laser-assisted precision grinding technology is introduced in this paper, which innovatively controls the depth of the thermally induced damage layer by adjusting the laser process parameters to reduce the hard brittleness of the material, and then the surface is created by precision grinding with a grinding wheel on this basis. Experiments on laser-induced damage, laser-assisted grinding, and diamond scratching were carried out to investigate the effect of laser parameters on material damage and the effect of laser-assisted grinding processes, with an emphasis on revealing the mechanism of material removal. The results show that laser irradiation causes complex reactions such as sublimation, decomposition, and oxidation of 3D C/C-SiC composites, resulting in SiO2 and Si and recondensed SiC, causing surface/subsurface damage. A maximum reduction in normal grinding force, tangential grinding force, specific grinding energy, and surface roughness of 35.6%, 43.6%, 43.58%, and 24.22%, respectively, compared to conventional grinding processes with laser-assisted grinding. After laser irradiation, the degree of brittle fracture in the precision grinding of workpieces is significantly reduced due to the degradation of matrix and fiber damage caused by laser irradiation, which reduces the hard and pseudo plastic properties of the material. The removal mechanism shows a trend of ductile domain removal in the grinding of thermally damaged layers, which reduces the grinding force and improves the surface quality.     

Material removal mechanism and grinding force modelling of ultrasonic vibration assisted grinding for SiC ceramics

In this paper, a varied-depth nano-scratch test of single grain is carried out on a nano indentation system. The critical depth of the elastic-plastic transition for SiC ceramics is 7.27 nm, as calculated by Hertz contact theory, and the critical depth of the brittle-to-ductile transition is 76.304 nm, as measured by AFM and SEM. Based on the varied-depth nano scratch test and the grain trajectory of ultrasonic vibration assisted grinding (UVAG), a theoretical model of the normal grinding force is acquired using the material removal in unit time as a bridge. The single factor experiment illustrates that the grinding force increases with the increase of the grinding depth, feed rate, and amplitude, while it decreases with the increase of the spindle speed. The contrast experiment results show that UVAG is beneficial for improving the surface quality and reducing the subsurface damage depth compared with common grinding (CG). A four-level and four-factor orthogonal experiment is designed, on the basis of which theoretical model of the normal grinding force for SiC ceramics is obtained using genetic algorithm. The tangential grinding force is obtained from the normal grinding force using the least square method. The experimental results show that the theoretical model is reliable.     

Controlled material removal mode and depth of micro cracks in precision grinding of fused silica – A theoretical model and experimental verification

A critical function for crack propagation for the single grit scratching of fused silica is developed based on the fracture mechanics. The effects of original crack density on the surface, strain rate and grinding coolant are considered in the function. A theoretical model for controlled material removal mode and depth of micro cracks precision grinding is presented based on the critical function for crack propagation. It can be predicted by the model that the material removal mode in the grinding of fused silica with original cracks damage will change from a ductile mode to a semi-brittle mode, a full-brittle mode and a semi-brittle mode in sequence with the increasing single grit scratching depth. It was found that the micro crack damage depth of fused silica does not increase with the single grit scratching depth after a full brittle mode grinding and it is always smaller than that after a semi brittle mode grinding even with a smaller single grit scratching depth. These interesting results are explained by the fracture mechanics. The ductile mode grinding is a recognized desirable process of fabricating fused silica while the full-brittle grinding is also a feasible process for its shallow subsurface damage, high efficiency, low grinding force and energy consumption. Therefore, the depth of micro cracks after grinding can be controlled by choosing suitable grinding parameters. Grinding experiments are conducted on fused silica. The undeformed chip thickness of randomly distributed effective grits is simulated based on 3D reconstruction of wheel topography to reveal the relationship between the grinding parameters and the single grit scratching depth. Ground surface roughness, sub-surface damage (SSD) depth and grinding force are measured and discussed. It is shown that the model predictions correlate well with the experimental trend of grinding modes.