Ductile Mode Machining Research Articles

Effect of Radial Directional Vibration-Assisted Ductile-Mode Grinding of Al2O3 Ceramics

Effect of Radial Directional Vibration-Assisted Ductile-Mode Grinding of Al₂O₃ Ceramics

Ductile mode machining of piezoelectric single crystal by laser-assisted diamond turning process

Piezoelectric single crystals are multifunctional materials used in advanced ultrasound devices owing to their exceptional piezoelectric properties. Nevertheless, machining methods for these single crystals have been limited due to their brittle nature and material deterioration issues. Herein, we established a ductile machining process of piezoelectric single crystal based on a laser-assisted diamond turning process through comprehensive analysis. In order to achieve defect-free surfaces, we analyzed the cutting behavior of softened piezoelectric single crystals concerning crystal orientation and machining direction. The experimental studies examined the effects of processing parameters on mechanical, chemical, crystallographic, and piezoelectric properties. The cutting status was characterized by measuring acoustic emission signals and analyzing the cutting chip formation. By optimizing the process and establishing a ductile mode of diamond turning mechanisms of piezoelectric material, we machined the brittle single crystal into various shapes, such as double-sided and multi-scale microstructure, without showing any defects all over the crystal orientation. We also demonstrated the validity of the machining process for piezoelectric material through an ultrasound characteristic test, which showed that the fabricated ultrasound transducers with machined single crystal maintained ultrasonic performance without deterioration in underwater experiments.

Investigating elastic recovery of monocrystalline silicon during plunging experiments with chamfered diamond tools

This study investigates the relationship between elastic recovery and friction during ductile mode machining of monocrystalline silicon. Plunging experiments were performed on an ultra-precision CNC machining center as the silicon work material was rotated. This approach allowed for investigating the combined effect of varying crystal directions and increasing depth on elastic recovery, pressures, and friction. A diamond cutting tool with a chamfered edge that maintains a constant negative rake along the cutting edge was used in plunging tests. Considering the importance of flank face contact area on friction and pressure distributions at the cutting zone, a non-linear expression is proposed to model the elastic recovery ratio as a function of groove depth. It is then employed in an analytical machining model to calculate the variation of pressure and coefficient of friction. The influences of pressure variation and changes in pressure direction as the machining mode transitions from ductile to brittle are investigated based on resolved stresses on the identified active slip plane. The methodology proposed in this study can be extended to study other monocrystalline materials, and the findings can be utilized in the process planning of advanced optics manufacturing.

Surface integrity of monocrystalline silicon nanostructured with engineered multi-tip diamond tools

The ability to fabricate micro/nanostructures on large surface areas would enhance product performance in optics and solar energy systems, where maintaining high productivity is also critical. Recently, diamond tools structured with nanoscale features have been used to machine ductile materials such as copper and electroless nickel. This study uses engineered diamond tools featuring multi-tip cutting edges to investigate nanoscale grooving of silicon. Multi-tip cutting edges create a certain level of pressure and temperature at the cutting zone, which leads to phase transformations in silicon. Experiments were performed using an ultra-precision machining setup to identify conditions leading to nanoscale ductile-mode machining of silicon. As nanogrooves reach 300 nm depth, hexagonal-Si (Si-IV) phase formation was observed based on laser Raman spectroscopy measurements. Hexagonal allotropes of silicon are known to improve light absorption of silicon. Additional experiments with non-structured diamond tools did not yield any Si-IV phase transformation, indicating the importance of obtaining necessary pressure and temperature conditions at the cutting zone.

Shoulder Damage Model and Its Application for Single Point Diamond Machining of ZnSe Crystal.

The damaging of ZnSe crystal has a significant impact on its service performance and life. Based on the specific cutting energies for brittle and ductile mode machining, a model is proposed to evaluate the damage depth in the shoulder region of ZnSe crystal during single point diamond machining. The model considers the brittle-ductile transition and spring back of ZnSe crystal. To verify the model, the elastic modulus, hardness, spring back, and friction coefficient of ZnSe crystal are measured by nanoindentation and nanoscratch tests, and its critical undeformed chip thickness is obtained by spiral scratching. Meanwhile, orthogonal cutting experiments are conducted to obtain the different shoulder regions and cutting surfaces. The shoulder damage depth is analyzed, indicating that the effect of the feed on the damage depth at a high cutting depth is stronger than that at a low one. The model is verified to be effective with an average relative error of less than 7%. Then, the model is used to calculate the critical processing parameters and achieve a smooth ZnSe surface with a roughness Sa = 1.0 nm. The model is also extended to efficiently predict the bound of the subsurface damage depth of a cutting surface. The research would be useful for the evaluation of surface and subsurface damages during the ultra-precision machining of ZnSe crystal.

Theoretical approach on the critical depth of cut of single crystal MgF2 and application to a microcavity

Optical microcavity, which can localize light at a certain spot for a short period of time, has a wide range of applications, such as optical signal processing and optical frequency combs. Single crystal CaF2 has excellent optical properties and many research on the manufacturing process to create a microcavity has been done. However, due to its unstable thermal conditions, stable oscillation of Kerr comb could not be done. Single crystal MgF2, which also has excellent optical properties, has stable thermal conditions suitable to create a microcavity. Like CaF2, MgF2 microcavity can only be manufactured by ultra-precision cylindrical turning, yet its appropriate processing conditions is not known. In this study, a method to derive the critical depth of cut theoretically in ductile mode machining was proposed in consideration of specific cutting energy, mechanical properties and slip system of workpiece, and tool geometry. The critical depth of cut calculated by the proposed method roughly agreed with the experimental value on the MgF2 plane. Unlike CaF2, MgF2 has an optical anisotropy; therefore, in order to create a microcavity where the refractive angle is constant, the resonance part needs to be made on a cylinder with (001) plane as end face. Therefore, a microcavity was fabricated according to the critical depth of cut on a cylinder with (001) as end face derived from the proposed method, and the surface integrity was evaluated from the viewpoint of microcavity performance.

Investigation of surface integrity and material removal mechanism of single-crystal Si subjected to micro-laser-assisted machining

Single-crystal Si, which is one of the best infrared optics materials and an important semiconductor substrate, is difficult to process owing to its inherent brittleness and hardness. Micro-laser-assisted machining (μ-LAM) has the potential for improving surface integrity by combining the advantages of LAM and conventional ultra-precision machining. However, its material removal mechanism differs from that of conventional machining (CM) technology owing to the interaction between LAM and the diamond turning action. In this study, a three-dimensional transient thermal model based on the finite element method was developed to investigate the temperature responses under different machining conditions. To examine the material removal mechanism and feasibility of high-efficiency machining with μ-LAM, the machining parameters, including laser power and rotation speed, were considered. Furthermore, the surface integrity was systematically characterised and analysed. It was observed that the dislocation motion velocity increased to approximately 2.57 × 109 times that of CM, thereby facilitating ductile deformation of single-crystal Si in μ-LAM. Based on the X-ray diffraction (XRD) pattern analysis, the residual stress significantly decreased with μ-LAM. Furthermore, it was revealed that increasing the rotation speed facilitated ductile mode machining and suppressed the occurrence of micro-pits, and the surface roughness and amorphous layer thickness decreased with increasing rotation speed. Thus, this study provides an important reference for μ-LAM of hard and brittle materials.

Ultra-precision diamond turning of ZnSe ceramics: Surface integrity and ductile regime machining mechanism

ZnSe ceramics has extensive applications in infrared imaging and laser systems. However, as a soft–brittle optical material, the mechanism of material removal and surface defects formation in ultra-precision diamond turning have not been clarified. In the present study, novel taper cutting tests were conducted to determine the ductile–brittle transition depth. Furthermore, the precondition for achieving ductile mode machining was revealed by comparing the maximum undeformed chip thickness and ductile–brittle transition depth. The effect of feed rate on surface roughness, surface quality, phase transition, and subsurface damage were systematically investigated. With the aid of cross-sectional TEM, the material removal mechanism in the ductile regime was revealed first, and the results indicated the formation of high-density dislocations, stacking faults, and deformation twins coordinating to promote ductile deformation. The study has provided insight into ductile regime machining mechanisms of ZnSe ceramics in the ultra-precision diamond turning process.

Edge Grinding Characteristics of Display Glass Substrate

Display glass substrate as a brittle material is very challenging to machine due to its excellent physical, mechanical, electrical, and optical properties such as high hardness, high strength, high wear resistance, good fracture toughness, good chemical stability, and good thermal stability. On the basis of Griffith fracture mechanics, our theoretical analysis indicated that edge grinding of the display glass substrate is under brittle mode when grinding with the given conditions, which was verified by the experimental studies of ground glass edge surface topography and fractured surface obtained. Grinding force (Fy) in the vertical direction was much larger than grinding force (Fx) in the horizontal direction, causing a large compressive stress acting on the grinding glass edge. Grinding torque was slightly increased with the increase of grinding speed. Grinding temperature was very high when measured under dry grinding compared with measurement under high-pressure coolant. Grinding of glass substrate edge was performed partially under ductile mode machining in the experimental conditions, which can be attributed to and contributed by those micro cutting edges generated by the fractured diamond grit on the grinding wheel surface.

The importance of wavelength for tight temperature control during μ-laser-assisted machining

The area of single point diamond turning of brittle materials like semiconductors and ceramics is significantly benefitted by incorporation of laser assistance. In a new developmental technology that is now recognized as micro-laser-assisted machining (μ-LAM), a laser is shone through a diamond tool to soften the high-pressure phase transformed ductile machining phases that in turn allows thermal softening and thereby enables a higher material removal rate during ductile mode machining. One of the lasers currently used in μ-LAM is the neodymium-doped yttrium aluminum garnet (Nd:YAG) laser operating at 100 W (continuous wave) at the wavelength of 1064 nm. Although this configuration has worked to the benefit of the technology, here we report futuristic developments that will significantly enhance temperature control by selecting a laser wavelength according to the material being machined, allowing tunable machining properties. The concept is illustrated with sample calculations for μ-LAM of silicon, and it appears to offer better target temperatures, thus enhancing the performance of the μ-LAM process.

Experimental and numerical investigation of cutting forces in micro-milling of polycarbonate glass

Polycarbonates have found important applications in various types of industries including optical, automotive, aerospace, biomedical, and defense manufacturing industries. Conventional mechanical machining has the capability to create complex multi-scale parts and components for various materials including polymeric materials. This study investigates the cutting forces generated during the machining of polycarbonate glass using the micro-milling process. The goal of this research is to machine high quality micro-channels in polycarbonates for microfluidic applications. Both experimental investigation and numerical simulations using the Finite Element Method (FEM) have been carried out to assess the cutting forces generated in three directions during machining of polycarbonate. The effectiveness of tool coating on the reduction of cutting forces has been investigated. It was found that with the careful combination of depth of cut and feed rate, the ductile mode machining of polycarbonate can be achieved, which produces lower cutting forces, that could result in improved surface finish and low tool wear. Both lower and higher of depths of cut were found to generate higher cutting forces due to dragging action and higher tool-workpiece contact area respectively. The Finite Element Method (FEM) was found to be effective in simulating the cutting forces with acceptable range of errors, and thus, could be used to predict cutting forces at the parametric combinations beyond the capacity of the machine or without carrying out further expensive experimentation, for which the chances of tool failure are higher.

Damage evolution in vibration assisted nano impact machining by loose abrasives at elevated temperatures: A numerical study

In this research, a numerical study is conducted to investigate the effects of different machining parameters at elevated operating temperatures in a novel nanomachining process, Vibration Assisted Nano Impact machining by Loose Abrasives (VANILA). In this novel VANILA machining process, an atomic force microscope (AFM) is used as a platform and the nano abrasives slurry is injected between the vibrating AFM probe and the silicon workpiece. These nano abrasives are accelerated by vibration of the AFM probe and hit the workpiece and finally generate nano cavities on the top surface of workpiece. In this study, diamond particles are used as loose abrasives and the ductile mode machining is employed to describe the behavior of the brittle workpiece. The commercial finite element method (FEM) software package ABAQUS is employed to create numerical models for simulations. In addition to experimental validation, the FEM model is validated by comparing the numerical solutions against the analytic solutions to the impact problem between a semi-infinite stationary workpiece and a rigid particle at the impact speeds of 25 and 12.5 m/s. The percentage error is under 4.4%. This validated FEM model is then used to investigate the influence of the machining parameters (impact speed, impact angle, and friction coefficient between the nano particles and silicon workpiece) and the number of impact hits at elevated operating temperatures. It is found that the impact speed, impact angle, and friction coefficient between the silicon workpiece and nano particles have big influence on the damage volume and damage value (D) of the silicon workpiece at elevated temperatures. The research results show that by increasing the level of operating temperature, the machinability of silicon in the VANILA process can be improved substantially.

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.

Enhancing Ductile-mode Cutting of Calcium Fluoride Single Crystals with Solidified Coating

Positive improvements have been observed during machining brittle materials under high hydrostatic pressure, but the techniques to achieve such desirable effects often utilize complex and expensive equipment or tools. This work presents a cost-efficient method to achieve ductile-mode machining of brittle materials at higher uncut chip thicknesses, by the application of a solidified coating on workpiece surface before a machining process. Orthogonal microcutting experiments were conducted on calcium fluoride single crystals oriented with the (111) plane and an increase in critical uncut chip thickness was observed with the solidified coating. The primary cause has been resolved to be mechanical-related and results in a stabilized microcutting process. Transmission electron microscopy provided evidence of slip deformation occurring in the machined subsurface regions and a layer thickness of subsurface damages reduced by ~ 45% under the influence of the solidified coating. In addition, erratic fluctuations in direction of the resultant machining force were subdued with the applied coating, which is proved to be caused by the compressive stresses induced from the sandwiching of the CaF2 material between the tool and the solidified coating. The proposed technique successfully reduces the cost and pollution in the fabrication process of optical components from the use of coolant in an ultraprecision machining process to the time consumed by eliminating the subsurface damage with abrasive slurries in post-machining polishing.

Characterization of Zirconia-Based Ceramics After Microgrinding

Despite the recent developments of ductile mode machining, microgrinding of bioceramics can cause an insufficient surface and subsurface integrity due to the inherent hardness and brittleness of such materials. This work aims to determine the influence of a two-step grinding operation on zirconia-based ceramics. In this regard, zirconia (ZrO2) and zirconia toughened alumina (ZTA) specimens are ground with ultrasonic vibration assistance within a variation of the machining parameters using two grinding steps and different diamond grain sizes of the tools in each of the machining procedure. White light interferometry, scanning electron microscope, X-ray diffraction (XRD), and four-point bending tests are performed to evaluate surface roughness, microstructure, residual stresses, and flexural strength, respectively. The strategy applied suggests that the finished parts are suitable for certain biomedical uses like dental implants due to their optimum surface roughness. Moreover, concerning the mechanical properties, an increase of the flexural strength and compressive residual stresses of ground ZrO2 and ZTA workpieces were observed in comparison to the as-received specimens. These results, as well as the methodology proposed to investigate the surface integrity of the ground workpieces, are helpful to understand the bioceramic materials response under microgrinding conditions and to set further machining investigations.

Structuring of Bioceramics by Micro-Grinding for Dental Implant Applications.

Metallic implants were the only option for both medical and dental applications for decades. However, it has been reported that patients with metal implants can show allergic reactions. Consequently, technical ceramics have become an accessible material alternative due to their combination of biocompatibility and mechanical properties. Despite the recent developments in ductile mode machining, the micro-grinding of bioceramics can cause insufficient surface and subsurface integrity due to the inherent hardness and brittleness of these materials. This work aims to determine the influence on the surface and subsurface damage (SSD) of zirconia-based ceramics ground with diamond wheels of 10 mm diameter with a diamond grain size (dg) of 75 μm within eight grinding operations using a variation of the machining parameters, i.e., peripheral speed (vc), feed speed (vf), and depth of cut (ae). In this regard, dental thread structures were machined on fully sintered zirconia (ZrO2), alumina toughened zirconia (ATZ), and zirconia toughened alumina (ZTA) bioceramics. The ground workpieces were analysed through a scanning electron microscope (SEM), X-ray diffraction (XRD), and white light interferometry (WLI) to evaluate the microstructure, residual stresses, and surface roughness, respectively. Moreover, the grinding processes were monitored through forces measurement. Based on the machining parameters tested, the results showed that low peripheral speed (vc) and low depth of cut (ae) were the main conditions investigated to achieve the optimum surface integrity and the desired low grinding forces. Finally, the methodology proposed to investigate the surface integrity of the ground workpieces was helpful to understand the zirconia-based ceramics response under micro-grinding processes, as well as to set further machining parameters for dental implant threads.

A Review of Partial Ductile Mode Machining for Brittle Materials

There is an increased trend in the usage of brittle materials for industrial applications. Most of these applications demand for a precise and accurate finishing which calls for new machining methods that can give a very good surface finish with proper control over surface integrity. These properties are significantly influenced by the material removal during machining. The mode of material removal during machining for brittle and ductile materials is completely different. Ductile materials plastically deform a significant amount before fracture when deformed in bulk. But in brittle materials there is no plastic deformation before fracture and the material removal is achieved by crack propagation. This results in poor surface finish and loss of surface integrity. To overcome these, a new mode of machining is developed. The application for this machining mode can be highly useful while machining glass, metallic carbides and nitrides etc. In this study, an overview of ductile mode machining mechanism in brittle materials with reference to grinding operation is presented.

Effect of tool inclination on surface quality of KDP crystal processed by micro ball-end milling

Micro-milling has been considered as the most promising method to repair the micro-defects on the surface of KH2PO4 (KDP) crystal. However, acquiring an ultra-smooth repaired surface by ball-end milling remains a longstanding challenge for KDP crystal due to its soft-brittle properties. In micro ball-end milling of KDP crystal, tool inclination angle has a remarkable effect on the quality of machined surface. Therefore, picking out an optimal tool inclination angle plays a great role in guaranteeing the ductile-mode machining and improving the surface quality of brittle KDP crystal. In this work, the effect of tool inclination on the brittle–ductile transition and surface quality of micro-milled KDP crystal were investigated. A theoretical model considering the tool inclination direction and angle was proposed to calculate the undeformed chip thickness (UCT) and cutting speed involved in the micro ball-end milling process. Besides, micro groove experiments were conducted to evaluate the change rule of the brittle–ductile transition and surface quality related to the tool inclination. The experimental results agree well with the theoretical results, which shows that the evolution of surface quality with respect to the tool inclination depends on the competitive mechanisms between UCT and cutting speed. Inclining the cutter in the tool feed direction (positive inclination angle) and increasing the tool inclination angle both contribute to the ductile cutting of KDP crystal. A + 45° inclination angle is the optimal angle for the micro ball-end milling of KDP crystal and the best surface roughness value achieved could be up to 35.3 nm.

Experimental Investigation on Ductile Mode Micro-Milling of ZrO₂ Ceramics with Diamond-Coated End Mills.

ZrO2 ceramics are currently used in a broad range of industrial applications. However, the machining of post-sintered ZrO2 ceramic is a difficult task, due to its high hardness and brittleness. In this study, micro-milling of ZrO2 with two kinds of diamond-coated end mills has been conducted on a Kern MMP 2522 micro-milling center (Kern Microtechnik GmbH, Eschenlohe, Germany). To achieve a ductile mode machining of ZrO2, the feed per tooth and depth of cut was set in the range of a few micrometers. Cutting force and machined surface roughness have been measured by a Kistler MiniDynamometer (Kistler Group, Winterthur, Switzerland) and a Talysurf 120 L profilometer (Taylor Hobson Ltd., Leicester, UK), respectively. Machined surface topography and tool wear have been examined under SEM. Experiment results show that the material can be removed in ductile mode, and mirror quality surface with Ra low as 0.02 μm can be achieved. Curled and smooth chips have been collected and observed. The axial cutting force Fz is always bigger than Fx and Fy, and presents a rising trend with increasing of milling length. Tool wear includes delamination of diamond coating and wear of tungsten carbide substrate. Without the protection of diamond coating, the tungsten carbide substrate was worn out quickly, resulting a change of tool tip geometry.

Prediction and experimental validation of temperature rise in ductile mode end milling of soda-lime glass

The suitable thermal, chemical, and corrosion resistance properties of glass make it possible to be used in a wide variety of product manufacturing, like lenses, mirrors, mold, semiconductor, biomedical, optical, and micro-electronics. However, machining of glass like any brittle material has big challenges owing to its inherent brittleness. Ductile mode machining is known to promote the material removal from a brittle material in ductile manner rather than by brittle fracture. In high-speed machining, the thermal softening effects can enhance flexibility in ductile machining of brittle materials. In this paper, an analytical model is developed to predict the amount of temperature generated in the immediate next removable layer (INRL) of the soda-lime glass work piece per unit depth of cut \( \Delta {\overline{T}}_{\mathrm{INRL}} \) based on fundamental micro-machining principle and material physical properties. The model incorporates the effects of cutting speed, feed rate, strain rate, and thermal softening effect. The simulation and experimental results showed that at high cutting speed, glass softening can be achieved by adiabatic heating in order to facilitate ductile machining. The amount of adiabatic heating can be controlled by predicting the amount of the \( \Delta {\overline{T}}_{\mathrm{INRL}} \).