On the analysis of the electrochemical spark machining process

The electrochemical spark machining (ECSM) process has been proved as a potential process for machining of low machinability high-strength electrically non-conducting materials, but the mechanism of material removal during the process, by and large, is not yet understood. In the present work, the electrochemical discharge is modelled as a phenomenon similar to that which occurs in arc discharge valves. This phenomenon is used to explain various experimental results, on the basis of circuit and arc discharge valve characteristics. The spark energy and the approximate order of hydrogen gas bubble diameter are computed by the proposed valve theory. Material removal rate is evaluated by modelling the problem as a 3-D unsteady state heat conduction problem. The problem is solved by the finite element method to compute the temperature distribution which is post-processed for estimating material removal per spark, overcut obtained in the machined cavity, and attainable maximum penetration depth. The conclusion drawn is that the application of valve theory to the ECSM process seems to be realistic. Estimated material removal rate, overcut and maximum penetration depth show a good agreement with experimental findings.     

Improving machining performance of wire electrochemical discharge machining by adding SiC abrasive to electrolyte

The use of wire electrochemical discharge machining (WECDM) to slice hard brittle materials has recently been studied because its effectiveness is independent of the mechanical characteristics of the machined materials. Therefore, materials with high hardness, brittleness, strength and electrical insulation, which are difficult-to-cut, can be machined. In ECDM, the electrochemical reaction produces hydrogen bubbles, which accumulate around the cathode. A thin gas layer forms on the surface of the electrode and isolates the electrode from the electrolyte. When a voltage that exceeds the critical voltage is applied, continuous discharge occurs. The material near the electrode is removed by the discharge erosion and chemical etching. The use of WECDM to cut electrically insulating materials has only recently been investigated. However, the breakdown of the gas in the bubbles and the vibration of the wire in WECDM strongly affect the shape accuracy. This work aims to improve the over cut quality by adding SiC abrasive to the electrolyte. A mechanism that combines discharge, chemical etching and abrasive cutting is studied. The effects on expansion, roughness and material removal rate (MRR) are discussed. The experimental results reveal that adding abrasive reduces the slit expansion because it increases the critical voltage. The particles disrupt the bubble accumulation to form an isolating layer around the wire, increasing the critical voltage and reducing the discharge energy. The surface roughness is improved because the abrasive helps to refine the micro-cracks and melted zone that is formed by discharge heat erosion. Meanwhile, smaller grit produces lower roughness. The quality of the slit can be controlled; its expansion and roughness of the slit are 0.024 mm and 0.84 um Ra, respectively.     

Fixed abrasive machining of non-metallic materials

This paper summaries advancements in fixed abrasive machining of non-metallic materials, which include reinforced concretes, stones, rocks, carbon fiber reinforced plastic, metal and ceramic matrix composites, wood, wood-fiber plastic composite, biomaterials (bone, plaque, and enamel), and structural and electronic ceramics. The broad impacts, diverse applications, and innovations of fixed abrasive machining processes are presented. Benefits of the engineered deterministic distribution of abrasive grain grinding tools are demonstrated. Industrial perspectives and future research on innovative fixed abrasive machining technologies that enable new processes and improve the productivity are highlighted.     

Water-assisted laser thermal shock machining of alumina

A combined experimental and computational approach was undertaken to investigate water-assisted laser cutting of 96% pure alumina specimens through controlled thermal shock fracture mechanism. A low-power CO₂ laser (<300 W) was used for localized heating and scribing of alumina samples followed by water quenching to induce thermal stress cracking. In order to elucidate the cutting mechanisms and identify the regime of processing conditions suitable for controlled fracture, laser cutting experiments were performed under two different environments: water-assisted laser cutting and dry laser cutting. Temperature profiles of the heat-affected zones were obtained using thermocouples and data acquisition system. Finite element analysis was applied to predict the temperature and thermal stress distributions developed during both water-assisted and dry laser cutting operations. Temperature histories of the samples recorded during cutting were compared with numerical model predictions to determine heat transfer parameters associated with wet and dry laser cutting of alumina samples. Both experimental data and numerical analysis indicate that water quenching makes a substantial difference in thermal stress distribution, which governs the ability to control the fracturing of alumina. This in turn, resulted in better control of cutting and higher feed rates than previously reported in laser machining of alumina.     

Simulation of erosive smoothing in the abrasive jet micro-machining of glass

Abrasive jet micro-machining (AJM) is a promising technique to machine micro-features in brittle and ductile materials. However, the roughness of micro-channels machined using AJM is generally greater than that from other methods of micro-machining such as wet etching. Previous investigators have suggested that the surface roughness resulting from AJM can be reduced by post-blasting with abrasive particles at a relatively low kinetic energy. This approach was investigated in the present work by measuring the roughness reduction of a reference unmasked channel in borosilicate glass as a function of post-blasting particle size, velocity, dose, and impact angle. Post-blasting the reference channels reduced the roughness by up to 60%. It was observed that post-blasting at shallower angles was more efficient, probably due to the increased amount of edge chipping as opposed to cratering, which contributed to the enhanced removal of profile peaks, leaving a smoother surface. Moreover, post-blasting with smaller particles ultimately resulted in smoother surfaces, but at the penalty of requiring a relatively large particle dose, and consequently a significantly increased channel depth, before reaching the steady-state roughness. Hence, finishing with smaller particles until reaching the steady-state roughness may not be practical when a shallow channel is desired. A previously developed numerical model was modified and used to simulate the post-blasting process leading to the creation of smooth channels as a function of particle size, velocity, dose, impact angle, and target material properties. The model simulated both crater formation (due to growth of lateral cracks) and the chipping of facet edges. Comparisons with centerline roughness measurements for channels in borosilicate glass showed that the model can predict the transient roughness reduction with post-blasting particle dose with a 7% average error.     

Wear behavior of the abrasive grains used in optical glass polishing

In this work the wear behavior of cerium oxide abrasive grains during the glass polishing was studied. Polishing tests have been done by different types of cerium oxide abrasive grains. The grains have been recovered and examined during the operation. The morphology, the granulometric distribution, the chemical composition and the agglomeration phenomenon of recovered grains have been studied.     

Influence of alumina particles on the mechanics of machining metal matrix composites

A plane-strain thermo-elasto-plastic finite element model has been developed and used to simulate orthogonal machining of alumina/aluminium 6061 metal matrix composite using a tungsten carbide tool. Simulations were carried out employing temperature-dependent material physical properties. The interface failure mode between the aluminium matrix and alumina particles was incorporated in this model. The model is used to investigate the effective and shear stresses on the alumina particles. Detailed results of the cutting forces generated during the machining process are presented and a comparison has been made with the experimental results for a range of feeds. Of particular interest are the contact stress distributions and alumina particle's interface failure. Normal and shear stresses and cutting temperatures were investigated.     

Abrasive water jet machining of glass with stagnation effect

Abrasive water jet processes of glass are presented for crack-free machining of micro grooves and fluid polishing of micro channels with CFD analysis. In machining of the micro grooves, the abrasive is supplied to flow through intended machining area using the tapered masks. Stagnation under the jet and the horizontal flow on the machining area are controlled to generate crack-free surfaces by the mask shape. The same effect can be applied to polishing of the micro channels pre-machined by milling. Stagnation controlled by the inner wall of the channel changes the flow direction while keeping high fluid velocities.     

Abrasive jet micro-machining of planar areas and transitional slopes in glass using target oscillation

Analytical models are presented which allow the prediction of the shape, sidewall slope, and depth of abrasive jet micro-machined planar areas and transitional slopes in glass using a novel technique in which the target is oscillated transversely to the overall scan direction. A criterion was developed to establish the minimum oscillation velocity to ensure negligible surface profile waviness in the scanning direction. If the oscillation velocity is sufficiently greater than the scanning velocity, the target receives an approximately uniform energy flux, resulting in a high degree of flatness for both masked and unmasked planar areas micro-machined in glass. It was also found that particle scattering from the mask edge caused the sidewalls of a planar area to be very shallow, on the order of a few degrees. Two methods were investigated to machine planar areas with increased sidewall slope using target oscillation: (i) machining micro-channels adjacent to the planned planar area, and (ii) target oscillation with an obliquely oriented nozzle. Among these two methods, target oscillation with an obliquely oriented nozzle created steeper sidewalls and was easier to implement, but it also caused appreciable mask under-etching. A major distinction between the target oscillation approach and a previously published method that was based on the superposition of the erosion profiles of adjacent nozzle scans, is that the latter is capable of machining an arbitrary surface profile over a large area, whereas the present target oscillation technique is intended only for the machining of flat planar areas at a single elevation. For such applications it is the preferred approach.     

On the mechanism of material removal in electrochemical spark machining of quartz under different polarity conditions

Electrochemical spark machining (ECSM) process has been successfully applied for cutting of quartz using a controlled feed and a wedge edged tool. Contrary to the common belief that only cathode works as a tool, both cathode and anode have been used as a tool, i.e. ECSM with reverse polarity (ECSMWRP) as well as ECSM with direct polarity (ECSWDP) have been used to machine quartz plates. In ECSMWRP, deep crater on the anode (as a tool) and work-piece interface is formed because of chemical reaction. Chemical analysis of electrolyte solution after the ECSM experiments, also agrees with the feasibility of dissolution of quartz into solution due to chemical reaction. Reverse polarity cuts quartz plate at a faster rate as compared to the direct polarity. But in reverse polarity overcut, tool wear and surface roughness are higher as compared to the direct polarity machining. Magnified view of the machined surface also shows a difference in the mode of material removal in ECSMWDP and ECSMWRP. The cutting is possible even if we make auxiliary electrode of small size. In conclusion, experiments have revealed that cutting can be performed simultaneously at both the electrodes (anode and cathode) during ECSM.     

Modification of hot glass surface with alumina by combustion CVD

A combustion CVD process with aluminium acetylacetonate as a precursor was used to apply alumina on a float glass surface at a temperature of 600 °C in order to improve its hydrolytic resistance. The generated surfaces were investigated by XPS, SEM, AFM, and leaching experiments. It was found that fine alumina particles were generated, which cover the surface, and that the percentage of alumina on the surface determined its hydrolytic resistance. While there was no diffusion of aluminium into the glass matrix detected, deposited alumina demonstrated a measurable influence on the sodium-silicium-ratio at the glass surface.     

Precision machining of microstructures on electroless-plated NiP surface for molding glass components

Micro-cutting experiments were conducted on electroless-plated nickel phosphorous (NiP) surfaces to fabricate microstructures such as microgrooves and micropyramid arrays. Burr formation behavior and cutting force characteristics were investigated experimentally and simulated by the finite element method (FEM) under various conditions. A simple two-step cutting process was proposed to improve the surface quality. The machined microstructure arrays were used as molds for hot-press glass molding experiments and good geometrical transferability was confirmed. The results from this study verify that diamond-machined NiP microstructure molds are applicable to glass molding processes for mass production of precision micro-mechanical and optical components.     

Wear behavior of Al2O3/Ti(C,N)/SiC new ceramic tool material when machining tool steel and cast iron

Design, fabrication and application of ceramic cutting tools are one of the important research topics in the field of metal cutting and advanced ceramic materials. In the present study, wear resistance of an advanced Al₂O₃/Ti(C,N)/SiC multiphase composite ceramic tool material have been studied when dry machining hardened tool steel and cast iron under different cutting conditions. Microstructures of the worn materials were observed with scanning electronic microscope to help analyze wear mechanisms. It is shown that when machining hardened tool steel at low speed wear mode of the kind of ceramic tool material is mainly flank wear with slight crater wear. The adhesion between tool and work piece is relatively weak. With the increase of cutting speed, cutting temperature increases consequently. As a result, the adhesion is intensified both in the crater area and flank face. The ceramic tool material has good wear resistance when machining grey cast iron with uniform flank wear. Wear mechanism is mainly abrasive wear at low cutting speed, while adhesion is intensified in the wear area at high cutting speed. Wear modes are dominantly rake face wear and flank wear in this case.     

High speed versus conventional grinding in high removal rate machining of alumina and alumina–titania

High removal rate (up to 16.6 mm³/s per mm) grinding of alumina and alumina–titania was investigated with respect to material removal and basic grinding parameters using a resin-bond 160 μm grit diamond wheel at the speeds of 40 and 160 m/s, respectively. The results show that the material removal for the single-phase polycrystalline alumina and the two-phase alumina–titania composite revealed identical mechanisms of microfracture and grain dislodgement under the grinding conditioned selected. There were no distinct differences in surface roughness and morphology for both materials ground at either conventional or high speed. An increase in material removal rate did not necessarily worsen the surface roughness for the two materials at both speeds. Also the grinding forces for the two ceramics demonstrated similar characteristics at any grinding speeds and specific removal rates. Both normal and tangential grinding forces and their force ratios at the high speed were lower than those at the conventional speed, regardless of removal rates. An increase in specific removal rate caused more rapid increases in normal and tangential forces obtained at the conventional grinding speed than those at the high speed. Furthermore, it is found that the high speed grinding at all the removal rates exerted a great amount of coolant-induced normal forces in grinding zone, which were 4–6 times higher than the pure normal grinding forces.     

A study of abrasive water jet machining process on glass/epoxy composite laminate

Surface roughness (R_a) and kerf taper ratio (T_R) characteristics of an abrasive water jet machined surfaces of glass/epoxy composite laminate were studied. Taguchi's design of experiments and analysis of variance were used to determine the effect of machining parameters on R_a and T_R. Hydraulic pressure and type of abrasive materials were considered as the most significant control factor in influencing R_a and T_R, respectively. Due to hardness of aluminium oxide type of abrasive materials, it performs better than garnet in terms of both machining characteristics. Increasing the hydraulic pressure and abrasive mass flow rate may result in a better machining performance for both criteria. Meanwhile, decreasing the standoff distance and traverse rate may improve both criteria of machining performance. Cutting orientation does not influence the machining performance in both cases. So, it was confirmed that increasing the kinetic energy of abrasive water jet machining (AWJM) process may produce a better quality of cuts.     

玻壳屏凸模型面数控加工刀具运动轨迹研究

数控加工刀具运动轨迹是确定数控加工工艺的重要环节,刀具运动轨迹设计质量的好坏,直接影响零件的加工质量及加工成本。为此,进行了基于切削力波动控制的球头刀铣削刀具路径优选实验研究,提出了针对玻壳屏凸模表面及其相似形状特征的模具型面编程策略。     

Sintering of translucent alumina

Alumina (Al₂O₃) specimens were made via powder injection moulding and then sintered, respectively, at three different temperatures, 1800 °C, 1850 °C and 1900 °C, for 30 min in a vacuum condition to achieve a desirable translucent level. The Al₂O₃ samples with different transparency were fabricated. The sintered polycrystalline alumina samples with desirable and undesirable transparency were characterized using an X-ray diffractometer (XRD) and a scanning electron microscope (SEM) attached with an energy dispersive X-ray microanalyzer (EDX). The relationship between the degree of transparency and the microstructure was investigated. It was found that the undesirable transparency was due to the presence of second phase along the grain boundaries, porosity and the range of pore sizes. Other properties of the samples, such as density, porosity, and Vickers hardness, were also measured. The optimum sintering temperature and time, that is 1850 °C for 30 min, were given for the fabrication of translucent alumina to achieve the highest density and minimum porosity.     

Modelling of high velocity, loose abrasive machining processes

In the recent years the interest in loose abrasive machining processes as efficient, flexible processes is rising. This paper describes the development of a ‘coherent set of models’ for a category of these processes, namely those which use high velocity of the particles to obtain the necessary energy to machine a workpiece surface. The usability of this ‘coherent set of models’ will be explained with its application in the field of high-pressure abrasive waterjet cutting. At the end of this paper a forecast to the application of this modelling technique to other loose abrasive machining processes as Micro-Abrasive Air Jet Machining is given.     

Wear and oxidation resistance of combustion CVD grown alumina films

Alumina thin films were synthesized on Si (100) and Ni–20Cr substrates using combustion chemical vapor deposition. Aluminum acetylacetonate (0.005 M) dissolved in ethanol was used as the precursor solution. The films deposited at 900 and 1000 °C are found to be θ-alumina and those deposited at 1100 °C to be α-alumina. The scanning electron micrographs showed the films to be made up of crystallites of two different sizes and shapes. The coefficient of friction, of alumina coated Si samples, measured using a tribometer showed high value (~ 0.7) at initial passes and subsequently saturated to low value (~ 0.5) at higher sliding distances. It was observed that the tribological properties, of the films, are not affected significantly by the crystal structure and crystallite size. Oxidation resistance of alumina-coated Ni–20Cr specimens were studied using a thermo-gravimetric analyzer by exposing them to isothermal heating at 1000 °C in 20% O₂–Ar gas mixture. The results indicated that the coated specimens are 18 times more oxidation resistant, at initial stages, compared to uncoated specimens. The resistance slowly dropped to six times, where it reached a saturation value.     

Development of magneto abrasive flow machining process

Abrasive flow machining (AFM) is a relatively new process among non-conventional machining processes. Low material removal rate happens to be one serious limitation of almost all such processes. Limited efforts have hitherto been directed towards improving the efficiency of these processes so as to achieve higher material removal rates by applying different techniques. This paper discusses the possible improvement in surface roughness and material removal rate by applying a magnetic field around the workpiece in AFM. A set-up has been developed for a composite process termed magneto abrasive flow machining (MAFM), and the effect of key parameters on the performance of the process has been studied. Relationships are developed between the material removal rate and the percentage improvement in surface roughness of brass components when finish-machined by this process. Analysis of variance has been applied to identify significant parameters and to test the adequacy of the models. Experimental results indicate significantly improved performance of MAFM over AFM.