Orbital electrode actuation to improve efficiency of drilling micro-holes by micro-EDM

This paper presents a novel machining technique for micro-EDM that actuates the EDM electrode on an orbital trajectory that is created by a 2-axis flexural micro-EDM head with a range of ±100 μm in both x- and y-directions. The orbital motion with its adjustable radius decouples the size of the hole to be drilled from the size of the electrode, allowing a range of hole sizes to be drilled. The orbital motion of the electrode increases the hole diameter proportional to the orbital radius, thereby creating a larger gap between the work piece and the electrode, which promotes increased flushing. For holes with large depth to diameter ratios, the increased flushing reduces electrode wear, creates a better surface finish, and eliminates the exponential reduction in material removal rates typical for EDM drilling.     

Micro fabrication by electrochemical process in citric acid electrolyte

This paper describes micro electrochemical machining of stainless steel in an environmentally friendly electrolyte of citric acid. Electrochemical dissolution region is minimized by applying a few hundred nanosecond duration pulses between the tungsten SPM tip and the work material. Electrochemical machining (ECM) characteristics according to citric acid concentration, feed speed and electric conditions such as pulse amplitude, pulse frequency, and tool electrode baseline potential are investigated through a series of experiments. Micro holes of 60 μm in diameter with the depth of 50 μm and 90 μm in diameter with the depth of 100 μm are perforated using citric acid electrolyte. Square and circular micro cavities are also fabricated by electrochemical milling. This research may contribute to the development of safe and eco-friendly micro manufacturing technology.     

Fabrication of a micro-pin array with high density and high hardness by combining mechanical peck-drilling and reverse-EDM

To fabricate circular cross-section micro-pin array with high hardness and high density in a fast and efficient way, a combined method of mechanical peck-drilling and reverse electrical discharge machining (reverse-EDM) is proposed in this research. First, a ball-cone-hole-magnet (BCHM) method is applied in high vibration cantilevered platform (HVCP) and quick release holder/jig to produce highly precise, fast and elastic positioning. Second, a micro-hole array with high density and different types of holes on a workpiece (brass material) is produced by a vibration-assisted mechanical peck-drilling (VAMPD), which includes the high vibration of workpiece created by HVCP and mechanical peck-drilling of micro-drill. This VAMPD can drill up to 1600 single-stage or multi-stage micro-holes, and the aspect ratio of the drilled one-stage micro-holes of Ø60 μm is up to ten. Finally, reverse-EDM is used to fabricate the micro-pin array made of tungsten carbide. In this process, the effects of the chip removal mechanism, the various micro-hole types, and the density of the micro-holes on the electrodes are investigated. The results indicate that the combination of multi-stage micro-hole electrodes and three chip removal methods (working fluid spraying, vibration-assisted electrode and shake-down type workpiece) can produce a 1600-micro-pin array with an average diameter below Ø30.00 μm, a length of 625.0 μm, and a pitch of 100 μm. Consequently, the proposed method of combining mechanical peck-drilling and reverse-EDM can fabricate a micro-pin array with high hardness, high density, high quantity, and uniform diameter in a fast and efficient way.     

TEM study on the electrical discharge machined surface of single-crystal silicon

EDM is a useful process for machining high-aspect ratio features with good accuracy in electrically conductive materials irrespective of their mechanical properties. With the ability of micro-EDM to compete with the resolution of conventional semi-conductor processing techniques, the process has attracted interest for the potential machining of single-crystal silicon. In order for the process to be feasible, the damage mechanism occurring during machining must be characterised to assess the need for secondary processing. Despite this the microstructural transformations induced by the process on the surface of the workpiece have not yet been assessed. In this study transmission electron microscopy (TEM) and laser-Raman spectroscopy are employed to characterise the microstructural changes as well as the presence of any contaminants and defects at the nano-scale. A twinned-crystalline structure created by epitaxial growth is formed in the recast layer. Some amorphous phase is also present. Findings indicate sub-surface pores between 10 nm and 200 nm diameter formed by gas expansion are observed. If the formation of such pores can be generalised for EDM processing of other materials, this phenomenon may contribute to the reduced mechanical integrity of such machined surfaces. Significant tool electrode material deposition with crystals of down to 3 nm diameter also occurred in the workpiece surface. The nano-scale of embedded material may have implications for the progress of electrical discharge machining as a coating process and the properties of such coatings.     

An evaluation of the evolution of workpiece surface integrity in hole making operations for a nickel-based superalloy

White layers and extensive material drag introduced during rough machining are regarded as detrimental to surface integrity. As such a sensible method for determining the amount of material to be removed in a roughing process would be to understand the relationship and interaction between roughing (i.e. drilling) and finishing (i.e. plunge milling) operations. Within this work non-standard cutting parameters were employed during the roughing process to generate a white layer and material drag up to a depth of 20 μm. Various plunge milling cutting strategies followed, with radius removal ranging from 25 μm to 250 μm in order to identify the amount of material removal necessary to eliminate the anomalies previously generated from mistreated surface history. The results show that finishing with a depth of cut between 50 μm and 125 μm removes all anomalies from the roughing process, leaving behind a negligible amount of material drag (3–4 μm). X-ray diffraction demonstrates significant tensile residual stresses (1000–2000 MPa) were generated in the axial and hoop direction by abusive hole drilling while subsequent plunge milling operation leaves compressive surface stresses in the region of ?500 MPa in both the axial and hoop directions; in both cases the depth of the surface stresses extended to around 125 μm from the drilled surface. It was also found that a depth of cut of 25 μm was not sufficient to recover the abused surface; this was due to intense material drag accompanied by surface cracking (i.e. 2 μm depth). The research shows that understanding the interaction between successive cutting operations can provide a suitable machining route to fulfil the industrial quality requirements in terms of the machined surface mechanical/metallurgical properties.     

Development of an ultrathin BD-PCD wheel-tool for in situ microgroove generation on NAK80 mold steel

This study presents the development and application of an ultrathin diamond wheel-tool that combines in situ reverse RWEDM (rotary wire electrical discharge machining) with in situ HSFSG (high-speed and fast-shallow grinding) technique for directly generating precision microgrooves onto NAK80 mold steel. The mechanism for reverse RWEDM involves the brass wire being located beneath the disk workpiece to upwardly machine it by spark erosion. The method is employed for thinning a diamond wheel-blank made of boron-doped polycrystalline composite diamond (BD-PCD). The good electrical conductivity of BD-PCD allows for an ultra-thin grinding-edge of 5-μm thickness to be produced on the wheel blank. The HSFSG technique is used to successfully grind micro-grooves in NAK80 mold steel. The method overcomes the traditional obstacles to using diamond to machine steel by having excellent metal removal rates. Experimental results prove the precision of the microgrooves generated. Nanometric grinding depth results in much less friction allowing for the cold machining conditions and preservation of diamond's sp³ bond structure. The grinding-edge has an extremely low wear rate of just 0.1 μm per 280 mm of grinding. The factors influencing formability, thermal machinability, graphitizing of diamond and the wear process of the BD-PCD wheel-tool are discussed in detail.     

Effect of surface roughness of tool electrode materials in ECDM performance

Electrochemical discharge machining (ECDM) is an emerging non-traditional machining process that involves high-temperature melting assisted by accelerated chemical etching. In this study, the tool electrode (200 μm in diameter) is fabricated by wire electrical discharge grinding (WEDG). After the tool electrode is machined, the surface roughness of tool electrode materials (stainless steel, tungsten carbide, and tungsten) is different because of the physical properties. However, the surface roughness affects the wettability on tool electrode, and also changed the coalesce status of gas film in ECDM. Hence, this study explores the wettability and machining characteristics of different tool electrode materials and their impact on gas film formation. Their machining performance and extent of wear under gravity-feed micro-hole drilling are also examined. Experimental results show that the optimal voltage of different tool electrode can shed light on the machining performance. Moreover, wettability of tool electrode is determined by surface roughness of tool material, which in turn affects the coalesce status of gas film, machining stability and micro-hole diameter achieved. In addition, differences in tool material also results in variations in machining speed. Significant tool wear is observed after repeated gravity-feed machining of 50 micro-holes.     

Development of micro-plasma transferred arc (μ-PTA) wire deposition process for additive layer manufacturing applications

Micro-plasma transferred arc (μ-PTA) deposition process has potential to meet requirements of the meso-sized fabrication and repair of the high value components. This paper reports on the development of μ-PTA as cost effective and energy efficient alternative process for small sized deposition with an overall objective to repair and/or remanufacture the defective dies and molds. An experimental setup was developed to deposit 300 μm diameter wire of AISI P20 tool steel on the substrate of the same material which is one of the most commonly used materials for making the dies and molds used for various applications. Two stage experiments were conducted to indentify the important process parameters generating regular and smooth single bead geometry. The process was further explored for highest possible deposition rate for fabrication of straight walls through multi-layer deposition. The μ-PTA deposition process was found to be capable of fabricating straight walls having total wall width of 2.45 mm and effective wall width of 2.11 mm. The deposition efficiency was found to be 87% for the maximum deposition rate of 42 g/h. The microscopic examination and micro-hardness measurements revealed that the deposited wall is free from cracks, porosity, and inclusions. This study confirms the capability of μ-PTA for ALM in comparison to the existing high energy deposition processes used for meso-scale fabrication and repair applications of the dies and molds. This work confirms that μ-PTA wire deposition process offers the advantages of the laser based processes at much lower cost and more energy efficiency thus making it potential alternative process for repair and remanufacturing of the defective dies and molds. Use of finer wire can further reduce the deposition size enabling μ-PTA wire deposition process to fabricate the miniaturized parts.     

Processing and dry sliding wear performance of spray deposited hyper-eutectic aluminum–silicon alloys

The influence of spray deposition process on the refinement of silicon phase and the tribological performance of hyper-eutectic Al–Si alloys is reported in this work. Due to the rapid solidification conditions that prevail during the spray deposition process, both primary and eutectic silicon were found to be refined resulting in equi-axed morphology of the silicon phase across the matrix. The average silicon particle size increased from 7 μm to 17 μm with increase in the silicon content of the spray deposited alloys used for the present study. Transmission electron microscopy of the spray deposited samples exhibited sub-micron sized silicon particles of both equi-axed and acicular morphology in the aluminum matrix. Pin-on-disc wear tests were performed on the spray deposited samples, by sliding samples against hardened steel counterface for about 1000 m at a speed of 0.3 m/s under varied loading conditions ranging from 0.17 MPa to 1 MPa. Scanning electron microscopy of the wear tracks and wear debris was carried out to understand the wear mechanism. The wear performance was improved with increase in the silicon content of the alloy. The wear performance of the alloys was compared with similar alloys produced through various processing routes reported in the literature. The spray deposited alloys were observed to exhibit relatively better wear performance for the range of composition and loading conditions employed.     

A new tool wear compensation method based on real-time estimation of material removal volume in micro-EDM

Owing to the reduced tool area and poor flushing conditions in deep holes, tool wear in micro-electrical discharge machining (EDM) is more significant than in macro-EDM. In micro-EDM drilling, the z-axis of the tool position is monitored as machining progresses. However, due to significant electrode wear, the machined hole depth is not identical to the programmed depth of the hole, and thus this will result in geometrical inaccuracy. This paper presents a new micro-EDM drilling method, in which the material removal volume is estimated as machining progresses. Compensation length is calculated and adjustment is made repeatedly along the tool path until the targeted material removal volume is reached. A real-time material removal volume estimator is developed based on the theoretical electro-thermal model, number of discharge pulse and pulse discrimination system. Under various energy input and machining depth settings, the experimental and estimated results are found to be in satisfactory agreement with average error lower than 14.3% for stainless steel, titanium, and nickel alloy work materials. The proposed drilling method can compensate the tool wear and produce more accurate micro-holes as compared to other methods. Experimental work also shows that the proposed method is more reliable as compared to the uniform wear method. In drilling micro-holes of 900 μm depth, the depth error can be reduced to 4% using the proposed method.     

Analysis of kerf width in micro-WEDM

In micro-wire electrical discharge machining (micro-WEDM), the machined kerf width varies with different machining parameters, which greatly influences the machining precision. In order to study the kerf variations in micro-WEDM, the mathematical model of wire lateral vibration in machining process is established and its analytical solution is obtained in this paper. The model is practically verified on a self-developed micro-WEDM machine. Under this model, a 30.8 μm width slot is achieved on a stainless steel workpiece with ?30 μm wire-tool.     

Microstructural characterization and hardness evaluation of HVOF sprayed Ni–5Al coatings on Ni- and Fe-based superalloys

HVOF sprayed Ni–5Al coatings on Ni- and Fe-based superalloy substrates were characterized to assess the microstructural features and strength in the as deposition condition for their applications in high-temperature corrosive environment of gas turbine. X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDAX), and X-ray mapping analysis are used to characterize the Ni–5Al coatings. The dense coatings with less porosity and inclusions were produced using HVOF process. The deposited Ni–5Al coatings exhibited splat like layered morphologies due to deposition and resolidification of successive molten and semi molten powder particles. The hardness of coatings on three different superalloy substrates was measured and it was in the range of 210–272 Hv. The average bond strength and surface roughness of the as-sprayed coatings were 42.62 MPa and 9.22–9.45 μm, respectively. Diffusion of alloying elements from the substrate into the coating has occurred in all the three superalloy substrates as observed from the X-ray mapping analysis.     

Machinability improvement of titanium alloy (Ti–6Al–4V) via LAM and hybrid machining

Titanium alloy (Ti–6Al–4V) is one of the materials extensively used in the aerospace industry due to its excellent properties of high specific strength and corrosion resistance, but it also presents problems wherein it is an extremely difficult material to machine. The cost associated with titanium machining is also high due to lower cutting speeds (<60 m/min) and shorter tool life. Laser-assisted machining (LAM) and consequently hybrid machining is utilized to improve the tool life and the material removal rate. The effectiveness of the two processes is studied by varying the tool material and material removal temperature while measuring the cutting forces, specific cutting energy, surface roughness, microstructure and tool wear. Laser-assisted machining improved the machinability of titanium from low (60 m/min) to medium-high (107 m/min) cutting speeds; while hybrid machining improved the machinability from low to high (150–200 m/min) cutting speeds. The optimum material removal temperature was established as 250 °C. Two to three fold tool life improvement over conventional machining is achieved for hybrid machining up to cutting speeds of 200 m/min with a TiAlN coated carbide cutting tool. Tool wear predictions based on 3-D FEM simulation show good agreement with experimental tool wear measurements. Post-machining microstructure and microhardness profiles showed no change from pre-machining conditions. An economic analysis, based on estimated tooling and labor costs, shows that LAM and the hybrid machining process with a TiAlN coated tool can yield an overall cost savings of ～30% and ～40%, respectively.     

Femtosecond laser machining of cooling holes in thermal barrier coated CMSX4 superalloy

Machining of cooling holes on thermal barrier coated superalloy components using a nanosecond (ns) laser generates considerable collateral damage such as recast layer, spatter and delamination of the ceramic coating. However, recent studies have suggested that these damages can be virtually eliminated by machining with femtosecond (fs) lasers. A detailed study on the microstructural characteristics of fs laser machined holes with diameters of 300 μm and 600 μm, generated on thermal barrier coated superalloy CMSX4 under various processing conditions has been conducted. Features examined include the shape, size and the surface finish of the hole wall. Femtosecond laser machined holes with a surface roughness of less than 2 μm and no major collateral damage could be generated in coated samples up to a thickness of 1.5 mm. The machining was found to cause minor ablative material removal from the top ceramic layer within 100 μm of the outer edge of the hole. The presence of machined holes did not affect the thermal cycling life at 1100 °C of the coated samples.     

Experimentation on MR fluid using a 2-axis wheel tool

This paper is focused on magnetorheological (MR) fluid assistive polishing of optical aspheric components. MR fluid is a functional mixture of non-colloidal magnetic particle of micrometer size suspended in a host fluid, with the special property that its viscosity can be varied by the application of a magnetic field. This paper introduces the basic principles of the methodology and presents experiment results on MR fluids using a 2-axis wheel-shaped tool supporting dual magnetic fields. Mathematical models taking into account the pressure and the tool velocity are derived. The experiments serve to evaluate the effects of process parameters on material removal and performance using a K9 glass parabolic lens of 60 mm diameter as work-piece. It is shown that surface roughness can be reduced from an initial value of 3.8–1.2 nm after 10 min of polishing. The form errors can also be improved from an initial 2.27 μm rms and 7.89 μm peak-to-valley to become 0.36 μm rms and 2.01 μm peak-to-valley after 60 min of polishing.     

Slicing,cleaning and kerf analysis of germanium wafers machined by wire electrical discharge machining

This paper investigates the slicing of germanium wafers from single crystal, gallium-doped ingots using wire electrical discharge machining. Wafers with a thickness of 350 μm and a diameter of 66 mm were cut using 75 and 100 μm molybdenum wire. Wafer characteristics resulting from the process such as the surface profile and texture are analyzed using a surface profiler and scanning electron microscopy. Detailed experimental investigation of the kerf measurement was performed to demonstrate minimization of material wastage during the slicing process using WEDM in combination with thin wire diameters. A series of timed etches using two different chemical etchants were performed on the machined surfaces to measure the thickness of the recast layer. Cleaning of germanium wafers along with its quality after slicing is demonstrated by using Raman spectroscopy.     

Loss of pseudoelasticity in nickel–titanium sub-micron compression pillars

Nickel–titanium (NiTi) is capable of undergoing pseudoelastic deformation wherein relatively large amounts of inelastic deformation are recovered upon load removal due to a martensitic phase transformation. This study investigates pseudoelastic as well as plastic deformation in sub-micron diameter NiTi compression pillars. Pillars ranging in diameter from approximately 2 μm to 200 nm were prepared using focused ion beam micro-machining of aged [1 1 1] single crystal NiTi. Results reveal pseudoelasticity in all samples tested with diameters between 2 μm and 400 nm, although permanent strain was introduced at relatively low strains compared to bulk. Decreased sample size generally showed a smaller stress–strain hysteresis, with a full loss of recoverable pseudoelastic strain for samples with a diameter smaller than 200 nm. In addition, plastic flow stress of the martensite was shown to be independent of sample diameter for the aged NiTi material. Lastly, it is observed that crystallographic orientation has a stronger influence on martensite plastic flow strength than pillar size.     

Abrasive slurry jet micro-machining of holes in brittle and ductile materials

This paper investigated the effects of elasticity and viscosity, induced by a dilute high-molecular-weight polymer solution, on the shape, depth, and diameter of micro-holes drilled in borosilicate glass and in plates of 6061-T6 aluminum alloy, 110 copper, and 316 stainless steel using low-pressure abrasive slurry jet micro-machining (ASJM). Holes were machined using aqueous jets with 1 wt% 10 μm Al₂O₃ particles. The 180 μm sapphire orifice produced a 140 μm diameter jet at pressures of 4 and 7 MPa. When the jet contained 50 wppm of dissolved 8 million molecular weight polyethylene oxide (PEO), the blind holes in glass were approximately 20% narrower and 30% shallower than holes drilled without the polymer, using the same abrasive concentration and pressure. The addition of PEO led to hole cross-sectional profiles that had a sharper edge at the glass surface and were more V-shaped compared with the U-shape of the holes produced without PEO. Hole symmetry in glass was maintained over depths ranging from about 80–900 μm by ensuring that the jets were aligned perpendicularly to within 0.2°. The changes in shape and size were brought about by normal stresses generated by the polymer. Jets containing this dissolved polymer were observed to oscillate laterally and non-periodically, with an amplitude reaching a value of 20 μm. For the first time, symmetric ASJM through-holes were drilled in a 3-mm-thick borosilicate glass plate without chipping around the exit edge.The depth of symmetric blind holes in metals was restricted to approximately 150 μm for jets with and without PEO. At greater depths, the holes became highly asymmetric, eroding in a specific direction to create a sub-surface slot. The asymmetry appeared to be caused by the extreme sensitivity of ductile materials to jet alignment. This sensitivity also caused the holes in metals to be less circular when PEO was included, apparently caused by the random jet oscillations induced by the polymer. Under identical conditions, hole depths increased in the order: borosilicate glass > 6061-T6 aluminum > 110 copper > 316 stainless steel. The edges of the holes in glass could be made sharper by machining through a sacrificial layer of glass or epoxy.     

Effect of initial particulate and sintering temperature on mechanical properties and microstructure of WC–ZrO2–VC ceramic composites

Owing to improving the mechanical properties of cemented carbides in high speed machining fields, a new composite tool material WC–ZrO₂–VC (WZV) is prepared from a mixture of yttria stabilized zirconia (YSZ) and micrometer VC particles by hot-press-sintering in nitrogenous atmosphere. Commercial WC, of which the initial particle sizes are 0.2 μm, 0.4 μm, 0.6 μm and 0.8 μm, is mixed with zirconia and VC powder in aqueous medium by following a ball mill process. The sintering behavior is investigated by isostatic pressing under different sintering temperature. The relative density and bending strength are measured by Archimedes methods and three-point bending mode, respectively. Hardness and fracture toughness are performed by Vickers indentation method. Microstructure of the composite is characterized by scanning electron microscopy (SEM). The correlations between initial particles, densification mechanism, sintering temperature, microstructure and mechanical properties are studied. Experimental results show that maximum densification 99.5% is achieved at 1650 °C and the initial particle size is 0.8 μm. When temperature is 1550 °C and particle size is 0.4 μm, the optimized bending strength (943 MPa) is obtained. The best hardness record is 19.2 GPa when sintering temperature is 1650 and particle size is 0.8 μm. The indention cracks propagate around the grain boundaries and the WC particles fracture, which is associated with particle and microcrack toughening mechanism.     

Electrically conductive ZTA–TiC ceramics: Influence of TiC particle size on material properties and electrical discharge machining

Processing of highly abrasive materials via powder injection molding or extrusion requires mold materials with high wear resistance to increase the durability of the tools and to sustain a high quality of the manufactured products. High performance ceramics which exhibit high hardness, bending strength and toughness show the perfect combination of properties for these applications. However they also have the usual drawback that they cannot be economically customized in complex shapes and low quantities, as they are required for tool and mold design. Recent material development enabled EDM of electrically conductive oxide ceramics, the most widespread machining process for machining of hard materials, as an alternative to conventional ceramic manufacturing and hard machining technologies.This study focuses on the influence of TiC particle sizes on material properties and EDM machinability of ZTA–TiC ceramics with 24 vol.% TiC, 17 vol.% ZrO₂ and 59 vol.% Al₂O₃. Fracture toughness, bending strength and electrical conductivity were analyzed for samples produced from TiC powders with particle sizes varying from 0.43 μm to 2.54 μm. Surface integrity of wire cut samples and feed rate during machining were investigated. It was shown that reducing the size of electrical conductive grains strongly increases the electrical conductivity and slightly decreases mechanical properties. Therefore also the machining characteristics are influenced by TiC grain size. The feed rate increases with decreasing particle size to a maximum at d₅₀ = 1–1.3 μm. Reduction of TiC particle size also leads to significantly decreasing surface roughness after the main cut. Additionally the necessary number of trimming steps to achieve a distinct surface roughness is also minimized for low particle sizes.