Material Removal Mode Research Articles

A feeding-directional cutting force model for end surface grinding of CFRP composites using rotary ultrasonic machining with elliptical ultrasonic vibration

End surface grinding of carbon fiber reinforced plastic (CFRP) composites using RUM with elliptical ultrasonic vibration has been proven to be effective in improving surface quality and simultaneously decreasing cutting forces. The cutting force is considered as one of the key output variables to evaluate the machining performance of the cutting process. Investigating cutting force and its modeling development provides great help to understand the effects of input variables and material removal mechanisms of RUM end surface grinding of CFRPs with elliptical ultrasonic vibration. However, there is no investigation on modeling cutting force for this process. This investigation will, for the first time, present a mechanistic feeding-directional cutting force model for such a process. This model is developed based on the material removal mode of brittle fracture. The approaches of the modeling development start from the analysis of one single abrasive grain, including the material removal volume, the effective cutting time, the average indentation depth, and the impact grain force in one ultrasonic vibration cycle. The designed pilot experiments are performed to verify this mechanistic model. The trends of predicted cutting forces are consistent well with those of experimental results. In addition, it can be also applied for predicting the effects of input variables (including depth of cut, feedrate, tool rotation speed, ultrasonic amplitude, abrasive size, and abrasive concentration) on feeding-directional cutting forces.

Material removal behaviour in axial ultrasonic assisted scratching of Zerodur and ULE with a Vickers indenter

Compared to conventional grinding, axial ultrasonic vibration-assisted peripheral grinding (AUPG) has advantages in terms of the improved grinding quality, higher efficiency, as well as lowered brittle damages. However, the present studies on material removal mechanism of AUPG are still not sufficient which thereafter limit its application potential. This paper aims to investigate the material removal mechanism of Zerodur and ULE through scratching by using a Vickers indenter under two conditions, with (ultrasonic vibration-assisted scratching (UVS)) and without (conventional scratching (CS)) axial ultrasonic vibration-assisted scratching while the other scratching parameters are identical. The indenter's kinematic characteristic, scratched morphology, scratched groove dimensions and critical depth of cut are compared in between UVS and CS. The experimental results indicate that the dynamic contact length between indenter and workpiece for UVS is remarkable longer than that for CS, which is helpful to promote the crack interference and hence to increase the proportion of ductile material removal mode, resulting in the diminishment of brittle fracture size in UVS. The propagation direction of median cracks in CS are relatively consistent, while in UVS the periodic varied contact zone and contact force in between the indenter and workpiece could promote the cracks propagating in different orientations, and then to improve the material removals rate in UVS. Moreover, both the critical brittle-ductile transition depth of cut and critical brittle depth of cut in UVS are bigger than that in CS, meaning the material removed in UVS with a bigger ductile ratio leading to a decreased brittle damage depth. In addition, the different material removal behaviours between Zerodur and ULE are also investigated. This fundamental work lays a theoretical foundation for the technological development and broad application of hard/brittle material oriented AUPG, as well as poses a meaningful guidance for the ultrasonic vibration assisted grinding on low-expansion optical glasses.

Influence of Wax Lubrication on Cutting Performance of Single-Crystal Silicon in Ultraprecision Microgrooving

Lubrication can improve the tool-workpiece interface condition in ultraprecision machining, especially in microgrooving processes. The aim of this study was to compare the effects of dry, liquid (cutting oil), solid (wax) and hybrid (cutting oil and wax) lubrication conditions, on the creation of arc-sectioned microgrooves on single-crystal silicon wafers using a single-crystal diamond tool. Micro cutting forces, chip morphology, and surface quality at different locations of the grooves were investigated. Rose-shaped continuous chips were observed in dry cutting, indicating stable ductile mode material removal. The entrance side of the groove was always smoother than the exit side with a higher degree of phase transformation. Chip adhesions on the tool edge were found in dry and oil lubricated cutting, whereas wax coating could prevent the adhesion. Wax coating played the dual roles of a chip breaker as well as a high-performance lubricant, thus improved the surface integrity. This study demonstrated the capability of using wax coating as an environmentally friendly solid lubricant for microgrooving of single-crystal silicon and possibly for other hard brittle materials.

Analytical prediction of subsurface microcrack damage depth in diamond wire sawing silicon crystal

Subsurface microcrack damage depth (SSD) in silicon wafer processed by diamond wire sawing has a great influence on the subsequent processing process and fracture strength of silicon wafer. Based on the indentation fracture mechanics, a new numerical model for predicting the SSD of silicon wafer on the basis of half-penny crack system is proposed in this paper. In this model, effect of the random distribution characteristics of size, position and tip half angle of abrasives and different material removal modes to the contour of workpiece cutting groove are all considered. Median cracks and radial cracks are regarded that propagating into one body, that is half-penny crack system. Validity of this model is verified by comparing with experimental results in the literature. From this model, the relationship between SSD and feed rate, wire speed, size of abrasives and density of abrasives on the surface of saw wire is analyzed, the critical speed ratio of feed rate to wire speed that can achieve pure ductile processing is discussed as well. The results show that using the maximum extended length of half-penny crack system as the SSD of silicon wafer is more reasonable than using the maximum extended length of median cracks as the SSD of silicon wafer. As wire speed increases, feed rate decreases, or density of abrasives on the surface of saw wire increases, the SSD will decrease. Keeping other parameters of saw wire unchanged, the size of abrasives has no significant effect on the change of SSD. Keeping the ratio of feed rate to wire speed unchanged, the SSD varies within a small range. When the speed ratio is in the range of 10.43–16, the slice surface tends to be formed by pure ductile removal of material. The results can be used to guide the optimization of diamond wire sawing process.

A mechanistic cutting force model based on ductile and brittle fracture material removal modes for edge surface grinding of CFRP composites using rotary ultrasonic machining

Rotary ultrasonic machining (RUM) has been proven to be an effective method for surface grinding of carbon fiber reinforced plastic (CFRP) composites. Cutting force is considered as the main criterion to evaluate the performance of RUM surface grinding process. The cutting force modeling is essential to better understand such a process. All reported cutting force models for RUM of CFRP are developed based on brittle fracture material removal mode (brittle mode). However, it is recently found that both ductile material removal mode (ductile mode) and brittle mode exist in RUM of CFRP. Among surface grinding processes, edge surface grinding is the mandatory and primary process to remove an amount of composite material from the workpiece edge to achieve the desired workpiece with high precision. In edge surface grinding process, cutting forces in both feeding direction and in depth-of-cut direction (being perpendicular to feeding direction) play important roles in material removal. In addition, the understandings of material removal mechanisms will greatly benefit the modeling development of cutting forces and improve the RUM process. In this study, a mechanistic model based on both ductile mode and brittle mode is developed to predict cutting forces in both feeding and depth-of-cut directions for RUM edge surface grinding of CFRP composites. A series of experiments are conducted to verify this mechanistic model. The model prediction agrees well with the experimental results under different groups of input variables.

Effect to the Surface Composition in Ultrasonic Vibration-Assisted Grinding of BK7 Optical Glass

Because of the changes in cutting conditions and ultrasonic vibration status, the proportion of multiple material removal modes are of uncertainty and complexity in ultrasonic vibration-assisted grinding of optical glass. Knowledge of the effect of machined surface composition is the basis for better understanding the influence mechanisms of surface roughness, and also is the key to control the surface composition and surface quality. In the present work, 32 sets of experiments of ultrasonic vibration-assisted grinding of BK7 optical glass were carried out, the machined surface morphologies were observed, and the influence law of machining parameters on the proportion of different material removal was investigated. Based on the above research, the effect of surface composition was briefly summarized. The results indicated that the increasing of spindle rotation speed, the decreasing of feed rate and grinding depth can improve the proportion of ductile removal. The introduction of ultrasonic vibration can highly restrain the powdering removal, and increase the proportion of ductile removal. Grinding depth has a dominant positive effect on the surface roughness, whereas the spindle rotation speed and ultrasonic amplitude both have negative effect, which was caused by the reduction of brittle fracture removal.

Grindability of plasma sprayed thermal barrier coating using super abrasive wheel

ABSTRACTThermal Barrier Coatings (TBC) are commonly used on exhaust muffle of engines and gas turbine blades to protect the component from generated heat and increase the operational temperature. Machining of ceramic coatings is comparatively more challenging than sintered ceramics. In this study, a commonly used TBC coating, namely yttria stabilised zirconia coating was subjected to surface grinding using a mono layer nickel bonded diamond wheel. The grinding forces and specific grinding energy were studied experimentally. The normal force obtained experimentally was compared with a force model and the correlation was studied. The forces obtained were significantly low as compared to sintered YSZ and a close relationship was observed with the existing model. In addition, the specific grinding energy was also found to be minimum. This indicates that most of the energy is expended in ploughing, micro-brittle fracture and rubbing. This was ascertained through scanning electron images of the ground surface and the fragmented chips. X-Ray diffraction images of the ground sample and as-sprayed sample showed an insignificant change in material structure owing to the material removal mechanism. The surface roughness of the ground specimen was reduced significantly. The fracture toughness obtained using the Evans and Charles method was used to calculate the critical load for crack initiation and a significantly low value was observed for ductile to brittle transition. SEM images of ground chips of YSZ sample showed fragmented and broken chips indicating that the primary mode of material removal is owing to micro-brittle fracture.

Ductile-brittle transition detection in scratching of single crystal silicon using charged particle emissions

During machining of single-crystal silicon, material removal involves two modes ductile shear-based removal and brittle fracture-based removal. Ductile shear-based chip removal occurs when fracture is suppressed due to local stress conditions along with reduced chances of defect involvement and is desirable for achieving better surface integrity of the machined silicon wafer. In this work, we use charged particle emissions to identify mode of material removal (ductile or brittle) during scratching of a silicon wafer. Scratching tests were performed using a pin-on-disc tribometer setup with a conical diamond tip indenter, in which the wafer was held at an inclined position to achieve a varying-depth tapered scratch. The varying-depth scratch test was performed in such a manner that both ductile-to-brittle and brittle-to-ductile modes occur in a single scratch test. The charged particles emitted during the material deformation were collected using a Faraday plate mounted in the vicinity of the indenter and the intensity of the charged particles were measured using a sensitive femto/picoammeter. The scratch depth was measured using a 3D surface profiler and the mode of fracture was identified by examining crack density per unit length in a scanning electron microscope. These results were then correlated with the emission intensity signals. From the experimental results, a positive current intensity was observed for ductile mode of scratching and highly varying current intensity signal is observed during brittle mode of scratching. The results obtained were consistent over time and exhibited good repeatability. The present work indicates suitability of employing charge emission signals to detect mode of material removal during scratching of silicon. This work can be field-tested by conducting diamond turning experiments of silicon in real-time machining environment further testing the scope of use of charged particle emission to monitor real-time machining process.

Experiment and theoretical prediction for surface roughness of PV polycrystalline silicon wafer in electroplated diamond wire sawing

The electroplated diamond wire sawing technology is the mainstream processing method of cutting PV polycrystalline silicon ingots. Surface roughness is one of the most significant evaluation indexes for wafers surface quality, and has an important influence on subsequent processes such as surface texturization, screen printing, subsurface damage layer prediction and so on. In this paper, a mathematical model, considering the random distribution characteristic of abrasive parameters and its location on saw wire surface, was developed. The influence of brittle and ductile material removal modes on the wafers surface morphology was taken into account, the model can obtain the cutting depths of abrasives by discretizing the cutting groove and wire profile during the sawing process, and then the material removal modes of abrasives are judged by cutting depth. According to the calculation nodes closest to the sawn surface, the wafer surface profile and roughness along the feed speed direction were calculated. The correctness and reasonability of the model were verified by experiments. Then, the model was used to predict the variation trend of surface roughness under different process parameters. The results showed that the relationships between surface roughness and feed speed Vf, wire speed Vs and their ratio Vf /Vs satisfied power functions. Finally, the calculation result of surface roughness was used to generate a 3D surface morphology of wafer.

Study on influence of grinding depth and grain shape on grinding damage of K9 glass by SPH simulation

This work use the SPH method to study the relationship between grinding depth and grinding force of K9 glass in ultra-precision grinding, and the effect of grain shape on material removal. The relationship between the force and the depth in the stable scratching stage is FR = 0.55078ap1.15356, which provides a basis for controlling the grinding depth by force. The material removal modes at different grinding depths were obtained, that is, plastic removal occurs below 0.2 μm, brittle transition occurs between 0.2 and 0.4 μm, and brittle removal occurs at more than 0.5 μm. It is found that the cutting resultant force of sharp grains is smaller and the normal force is smaller than the tangential force. By analyzing the plastic deformation depth and residual stress depth, it is found that the material removal mode with small deformation and little removal has the lowest cutting force fluctuation and the highest grinding quality. It provides a reference for the choosing of grain shape in rough grinding and ultra-precision grinding. The correctness and reliability of the simulation results were verified by the comparison of the simulation findings with the results obtained from varied-depth scratching experiments and the multi-shape indenter scratch experiments.

Study on the three-dimensional topography of the machined surface in laser-assisted machining of Si3N4 ceramics under different material removal modes

The surface topography directly impacts the component performance. However, the two-dimensional (2D) surface roughness parameters in the previous studies have been inefficient and inadequate to describe a surface. In this study, with the help of a confocal laser scanning microscope and a scanning electron microscope, the three-dimensional (3D) topography of a machined Si3N4 surface using laser assisted machining (LAM) under different material removal modes was comprehensively characterised and compared using the standard ISO 25178–2:2012 surface texture parameters. The relationship between the 3D topography parameters and service properties was analysed. LAM is effective for machining ceramics with a high hardness and high brittleness. Different material removal modes were obtained by changing the power of the laser and were inferred by comprehensively analysing the macroscopic and microscopic topography of the chips, tool wear, and machined surface. In ductile material removal mode, the surfaces are anisotropic and cutting marks are clearly visible. Decreasing the power of the laser increases the value of the height and functions parameters. Decreasing the power of the laser also decreases the value of the hybrid and feature parameters.

Mechanism of Unstable Material Removal Modes in Micro Cutting of Silicon Carbide.

This study conducts large-scale molecular dynamics (MD) simulations of micro cutting of single crystal 6H silicon carbide (SiC) with up to 19 million atoms to investigate the mechanism of unstable material removal modes within the transitional range of undeformed chip thickness in which either brittle or ductile mode of cutting might occur. Under this transitional range, cracks are always formed in the cutting zone, but the stress states cannot guarantee their propagation. The cutting mode is brittle when the cracks can propagate and otherwise ductile mode cutting happens. Plunge cutting experiment is conducted to produce a taper groove on a 6H SiC wafer. There is a transitional zone between the brittle-cut and ductile-cut regions, which has a mostly smooth surface with a few brittle craters on it. This study contributes to the understanding of the detailed process of brittle-ductile cutting mode transition (BDCMT) as it shows that a transitional range can occur even for single crystals without internal defects and provides guidance for the determination of tcritical from taper grooves made by various techniques, e.g., to adopt larger tcritical around the end of the transitional range to increase machining efficiency for grinding or turning as long as the cracks do not extend below the machined surface.

Mechanism of material removal by fixed abrasive lapping of fused quartz glass

In this paper, a novel fixed-abrasive lapping process to finish off-axis aspheric surfaces efficiently is proposed. The lapping mechanism of fused quartz glass was evaluated using the novel fixed-abrasive pad tool. The applied load and not the rotation speed and feed rate is the key factor affecting the material removal mode of fused quartz glass in the fixed-abrasive lapping process. Analysis of surface characteristic and subsurface damage indicates that lower applied load produces more ductile mode removal and that the removal on fused quartz glass can be changed from brittle fracture to ductile regime if the applied load is less than 25 N. The material removal rate (MMR) increases as the applied load is increased, which further increases the surface roughness. In addition, the MMR increases and the surface roughness decreases as the rotation speed is increased or the feed rate is deceased. A theoretical model for predicting the maximum SSD depth of the specimens during lapping was developed. The model can predict well that the maximum SSD depth increases with increasing applied load.

Characterization of electroplated diamond wires and the resulting workpiece quality in silicon sawing

Abstract The aim of this research is to introduce a fast characterization method for diamond wires and the resultant silicon workpiece surface quality at high wire cutting speeds. The work is presented in two parts. Initially, four electroplated diamond wires with varying properties are characterized with the proprietary wire analysis algorithm and experimentally investigated at wire speeds up to 75 m/s. Following, the proprietary surface analysis algorithm which evaluates the brittle and ductile material removal characteristics of silicon workpiece surfaces is introduced and experimentally validated. Influence of the wire speed and feed rate on the material removal regime, surface roughness and process forces are studied on the Wire Wrapped Grinding Wheel setup, a novel-design experiment test rig designed to emulate the wire sawing process. The aspects and kinematics of the experiment setup are discussed in detail. Primary findings show that (1) better surface finish is achievable at higher wire speeds with ductile material removal as the dominant material removal mode (2) for wires of different grain sizes, at the cutting speed threshold of around 30 m/s, cutting force behavior changes and the difference between normal forces starts diminishing over this limit.

Mechanisms of ductile mode machining for AlON ceramics

This paper aims to reveal the mechanisms of ductile mode machining for AlON ceramics. The removal characteristics during machining were studied through ultra-precision grinding experiments. The machined surface consists of fractured and smooth areas, which were generated by brittle and ductile removal, respectively, of the individual AlON grains. The material removal mode has a determining effect on the surface/subsurface quality. The proportion of the fractured areas on the ground surface decreased gradually with a decrease in the depth of cut. The crystal indices of the grains most prone to brittle removal on the workpiece surface were determined using micro-area X-ray diffraction (μXRD) analysis performed using a beam with 50 μm diameter. The results showed that the ductile removal of the {111} planes is critical for the ductile mode machining of AlON. Nanoindentation tests based on electron back-scattered diffraction (EBSD) indicated that AlON shows strong anisotropy in its mechanical properties and machinability. The (111) plane has the highest hardness and lowest fracture toughness, at 22.91 GPa and 1.8 MPa m1/2, respectively. The material removal mechanism during the grinding of AlON was discussed in detail. The minimum and maximum dc(hkl) values must be known for classifying whether the removal mode of the workpiece is brittle or ductile. A damage-free surface could be obtained during ductile mode grinding by controlling hmax to be less than dc(111). The subsurface deformation mechanism during ductile mode grinding was analysed. An amorphous layer was observed close to the ground surface. Further, dense dislocations with no particular orientation were present beneath this amorphous layer. As the crystal structure became clearer with an increase in the depth, the plastic deformation shifted to stacking faults parallel to the {111} planes.

Investigation on the Edge Chipping in Ultrasonic Assisted Sawing of Monocrystalline Silicon

Monocrystalline silicon is an important semiconductor material and occupies a large part of the market demand. However, as a hard-brittle material, monocrystalline silicon is extremely prone to happen edge chipping during sawing processing. The edge chipping seriously affects the quality and performance of silicon wafers. In this paper, both conventional and ultrasonicassisted sawing tests were carried out on monocrystalline silicon to study the formation mechanism of edge chipping. The shape and size of edge chipping after sawing process were observed and measured. The experimental results demonstrated that different sawing processes present different material removal modes and edge quality. The mode of crack propagation was continuous cracks in conventional sawing process, while the expansion mode in ultrasonic assisted sawing was blasting microcracks. This results in the cutting force of ultrasonic assisted sawing becomes much smaller than that of conventional sawing process, which can reduce the size of edge chipping and improve the quality of machined surface.

AlNiCrFeMn Equiatomic High Entropy Alloy: A Further Insight in Its Microstructural Evolution, Mechanical and Surface Degradation Response

The microstructure and properties of an equiatomic AlCrFeMnNi high entropy alloy (HEA) in the as-cast condition are presented. The microstructure consists of two phases (NiAl-rich B2 and FeCr-rich A2) featuring various eutectic morphologies, ranging from regular to anomalous development. Solidification phenomena (undercooling and recalescense) are used in order to assess the various morphological features along with theories of anomalous eutectic growth. Sliding wear testing shows an oxidation-delamination mode of material removal along with some partial abrasive mode character. Modulus of elasticity and yield point are significantly high and elongation presents at a satisfying level, whereas fracture surface examination reveals the characteristics of a brittle fracture mode. High temperature oxidation tests show multiple oxidation phenomena of different kinetics, formed oxide phases and material deterioration phenomena.

Material removal and surface generation mechanisms in diamond wire sawing of silicon crystal

Silicon crystal is usually first cut into wafers in the applications of electronic devices and photovoltaic solar cells. At present, fixed abrasive wire sawing technology has been widely used in slicing. The mechanism of material removal and surface generation during wire sawing is the key basic problem to study the surface quality of sliced wafers, which is important to realize high-precision slicing. This paper presents theoretical analysis on the relationship between the average cutting depth of abrasives distributing in different positions along the wire circumferential surface and the process parameters. The wire sawing experiments of single crystal silicon were carried out, then the morphologies of sawn surface and swarf were observed, the surface roughness was measured and analyzed, the material removal and surface generation mechanism of single crystal silicon cut by electroplated diamond sawing wire were addressed. Research results show that there is an approximate non-linear increasing relationship between the average cutting depth of the abrasives and the ratio of the workpiece feed speed to the wire speed, and the abrasives cutting depth and surface roughness of sliced wafers increases with the increasing of ratio value. It is difficult to obtain the sliced surface in a fully ductile mode of material removal in the actual wire sawing process because there are always a small amount of abrasives with high protrusion on the wire surface whose cutting depths are greater than the critical ductile regime cutting depth, but adopting a low ratio can make the generation of the sliced wafer surface in near-ductile or local-ductile material removal mode which can produce high quality wafer surface. This research is expected to be useful in optimizing the combination of process parameters in fixed abrasive diamond wire saw precision slicing of silicon wafers.

Specific cutting energy index (SCEI)-based process signature for high-performance milling of hardened steel

The specific cutting energy can characterize the machinability of the cutter for a material, and its change is a reflection of the size effect in the metal cutting process. It is critical to study the cutting process using an energy-based processing signature method with the purpose of improving the machining performance. In this paper, a specific cutting energy calculation method is presented based on a mechanistic model, and an exponential function is used to describe the trend in the specific cutting energy with the average undeformed chip thickness. A dimensionless index with value of greater than 1, referred to as the Specific Cutting Energy Index (SCEI), is proposed to address the energy efficiency of the milling process and reflect the machining performance for different machining parameters. Machining experiments with 300 M steel are conducted to validate the effectiveness of the proposed model. Analyses are performed to determine the relationships between SCEI and the material removal mode, chip morphology, tool wear rate, and surface roughness. Based on these results, a feed rate scheduling method is proposed to obtain optimal machining strategies using SCEI, which is an effective way to achieve high-performance machining.

Micro-ultrasonic drilling of monocrystalline silicon: An experimental investigation on machined surface topography and optimization using User's preference rating based TOPSIS

In the current scenario, silicon is extensively used as raw material in microelectromechanical systems and microelectronic devices. But, it has low fracture toughness that renders it difficult to process by conventional machining techniques. Thus, to overcome the barriers of conventional machining, rotary tool micro-ultrasonic drilling (RT-MUSD) process has been attempted for machining of silicon in this research endeavour. Taguchi's L9 orthogonal array was adopted as experimental design. The influence of variable parameters viz. rotation speed, power rating and abrasive size were analysed on performance characteristics (i.e. hole overcut (HOC), taper angle (TA) and material removal rate (MRR)) of RT-MUSD process. Analysis of variance (ANOVA) was applied to determine the significant parameters affecting the RT-MUSD process. Subsequently, optimal parametric setting for minimum HOC and TA and maximum MRR was obtained through user's preference rating based technique for order preference by similarity to ideal solution method. The ANOVA results showed that rotation speed was the most influencing parameter affecting the HOC, TA and MRR. The optimal parametric combination resulted in an average improvement of 26% in the performance of RT-MUSD process. The machined surface topography revealed that two types of material removal mode (i.e. pure brittle fracture and ductile + brittle fracture) occurred during RT-MUSD of silicon.