Recast Layer Thickness Research Articles

Optimization of hydroxyapatite powder mixed electric discharge machining process to improve modified surface features of 316L stainless steel

Traditional methods for producing 316L steel with a desired roughness and a uniform thin coating for sufficient bioactivity and long-term durability are highly difficult and require post-processing. Electric discharge machining (EDM) is a nontraditional process that can accomplish both machining and coating the surface concurrently. This study provides a thorough examination of the impacts of EDM process parameters on the machining responses, which are significantly necessary for processing 316L steel to reach its full potential. The nano-hydroxyapatite particles added EDM method is used to improve the machining and the surface responses. EDM process parametric optimization is conducted using Taguchi's design to achieve the lowest surface roughness of 2.04 µm and a recast layer thickness of 6.11 µm. As a machinability metric, the greatest material erosion rate (MER) of 20.64 mg/min is achieved. Current intensity, discharge period and hydroxyapatite amount are identified to be important predictors for MER, surface roughness and recast layer thickness. Scanning electron microscope with the energy dispersive X-ray spectrums and atomic force microscopic images validate the machined surface morphology, elemental compositions and surface topography. Errors of the confirmatory tests are less than 10%, showing that the projected solution sets by the grey wolf optimizer are highly accurate. The study shows a thin recast layer with both nano and micro-surface roughness formation, which have a great importance in biomedical applications.

Surface Integrity Analysis of Wire Electric Discharge Machining of Nitinol Shape Memory Alloy: A Literature Review

When nitinol is machined, quantitative details regarding the surfaces such as the surface crack density, utmost peak-to-valley heights, recast layer thickness and the mean peak-to-valley height among others offer the most appropriate features to consider in the integrity of the surfaces of machined nitinol. This quantitative information directs the integrity and the projected future performance of the machined nitinol in the components. Consequently, the research question of how to achieve optimal surface integrity of the machined nitinol is important. A literature review is conducted to study the surface integrity analysis of wire electrical discharged machined nitinol. In particular, published papers between 2007 and 2021 have been reviewed. Literature is explored concerning the method of analysis, parameters of research interest and the problems/issues arising from the literature. Diverse methods were employed to evaluate the surface integrity of nitinol after machining. Commonly, both mathematical optimization and microstructural characterizations are used to suggest ideas. Mathematical optimization has been in two broad perspectives, namely, experimental design-based methods such as orthogonal arrays, signal-to-noise ratios, Taguchi's utility and quality loss function, Box-Behnken design and response surface methodology. The non-traditional optimization schemes such as the differential evolution, multi-objective optimization based on ratio analysis and teaching learning-based optimization have been applied. For microstructural characterization, tools to evaluate the surface integrity of nitinol such as field emission scanning electron microscope and energy dispersive X-ray have been deployed. Parameters such as residual stress, geometric deviation, microhardness and profile accuracy are pursued to be optimized. It is known that various literature reviews in previous years have studied the surface integrity problem of nitinol using large-scale approaches. However, in this article, a brief review is prescribed and this work reveals how the surface integrity analysis of nitinol has been tackled in the literature.

Experimental Investigation of Heat-Treated Tool on Wire Electric Discharge Machining of Titanium Alloy (Ti-6Al-4V)

Ti-6Al-4V is known for its lightweight, high tensile strength, exceptional corrosion resistance, and low thermal coefficient of expansion due to which it finds its application in precision surgical instruments and aeronautical and marine engine parts. In this study, Ti-6Al-4V has been machined using wire EDM machine with two different wire materials. The wire thickness of 0.25 mm heat-treated brass and zinc-coated brass tool and super cool oil as dielectric fluid were utilized for the machining process. Taguchi L9 orthogonal array was used. Ton (Pulse ON time), Toff (Pulse OFF time), wire feed, and Ip (Current) were the input parameters, and the response parameters such as surface roughness (Ra), material removal rate (MRR), tool wear rate (TWR), and recast layer thickness were optimized by TOPSIS optimization. From the results, higher tool wear rate was observed in brass tool. The titanium percentage after machining was found to be 55.05% for zinc-coated brass wire, whereas it was 51.58% for brass wire. Better productivity and surface integrity were observed in zinc-coated brass tool compared to heat-treated brass. MRR increased to 54.93%, surface roughness decreased to 2.40%, recast layer thickness increased to 3.44%, and tool wear rate was increased to 47.96%.

Gas-assisted green and efficient compound sinking machining of nickel-based alloy

Electrical arc machining is a promising method to process nickel-based alloy. Electrical arc machining commonly adopts water-based dielectric because water has no fire hazards and good cooling capacity. However, water-based dielectric has high electric conductivity, which will lead to unstable machining, including short circuit and electrochemical reactions. The gas-based dielectric is considered a more environmental and economical dielectric than the water-based dielectric. Nevertheless, the gas-based dielectric is mainly used in electrical arc machining where the machining gap is relatively open, including milling and turning. This paper proposes a gas-assisted green and efficient compound sinking machining. The compound machining combines electrical discharge machining (EDM) and electrical arc machining. The gas-liquid two-phase flow formed by dispersing gas into the deionized water is used as dielectric, whose continuous phase is liquid. The performance of the proposed method is systemically investigated by comparing with that using pure deionized water. The effects of gas type (air, oxygen, nitrogen, and argon) and machining parameters (peak current and pulse duration) on the material removal rate (MRR) and relative electrode wear rate (REWR) are studied. The characteristics of the machined surface are also analyzed. The results demonstrate that dispersing gas into the water can improve the insulation of the dielectric and reduce the unstable discharge phenomena. Besides, it can cause electric field distortion where the gas bubble locates, thereby enlarging the machining gap, improving the gap flushing environment, and enhancing the machining stability. The combustion supporting characteristic of oxygen is also helpful for improving the machining performance; thus, oxygen has the best promoting effect. Compared with the water dielectric, the proposed dielectric can improve the machining efficiency, decrease REWR, and reduce the thickness of recast layer of the machined surface in the compound sinking machining of nickel-based alloy. The proposed method is promising to meet the increasing requirement of high efficient and sustainable machining.

The evaluation of machining performances and recast layer properties of AISI H13 steel processed by tungsten carbide powder mixed EDM process in the semi-finishing process

In the literature, investigations on the electrical discharge machining (EDM) process with Tungsten carbide powder mixed into dielectric fluid, also called PMEDM, are still limited. In this study, machining performances of PMEDM, including material removal rate (MRR) of AISI H13 steel and tool wear rate (TWR), were investigated in the semi-finishing process. Thereafter, recast layer properties of AISI H13 steel, including composition and content of chemical elements, recast layer thickness (RLT), and acreage percentage of micro-cracks on surfaces (APCS), were evaluated and discussed. Quantitative methods of MRR, TWR, RLT, APCS, and the composition and content of chemical elements on machined surfaces have been proposed. The results revealed that machining performances and recast layer properties were influenced by peak current (Ip), pulse on time (Ton), and powder concentration (Cp). MRR and TWR have been changed in an uptrend when Ip, Ton, and Cp increased. The chemical composition of machined surfaces was modified. APCS and RLT formed by PMEDM are notably decreased as compared with EDM, with a reduction of 41.093% and 48.982%, respectively. In addition, the changing trend of machining performances and recast layer properties were also explored.

Multi-Response Optimization of Al2O3 Nanopowder-Mixed Wire Electrical Discharge Machining Process Parameters of Nitinol Shape Memory Alloy

Shape memory alloy (SMA), particularly those having a nickel–titanium combination, can memorize and regain original shape after heating. The superior properties of these alloys, such as better corrosion resistance, inherent shape memory effect, better wear resistance, and adequate superelasticity, as well as biocompatibility, make them a preferable alloy to be used in automotive, aerospace, actuators, robotics, medical, and many other engineering fields. Precise machining of such materials requires inputs of intellectual machining approaches, such as wire electrical discharge machining (WEDM). Machining capabilities of the process can further be enhanced by the addition of Al2O3 nanopowder in the dielectric fluid. Selected input machining process parameters include the following: pulse-on time (Ton), pulse-off time (Toff), and Al2O3 nanopowder concentration. Surface roughness (SR), material removal rate (MRR), and recast layer thickness (RLT) were identified as the response variables. In this study, Taguchi’s three levels L9 approach was used to conduct experimental trials. The analysis of variance (ANOVA) technique was implemented to reaffirm the significance and adequacy of the regression model. Al2O3 nanopowder was found to have the highest contributing effect of 76.13% contribution, Ton was found to be the highest contributing factor for SR and RLT having 91.88% and 88.3% contribution, respectively. Single-objective optimization analysis generated the lowest MRR value of 0.3228 g/min (at Ton of 90 µs, Toff of 5 µs, and powder concentration of 2 g/L), the lowest SR value of 3.13 µm, and the lowest RLT value of 10.24 (both responses at Ton of 30 µs, Toff of 25 µs, and powder concentration of 2 g/L). A specific multi-objective Teaching–Learning-Based Optimization (TLBO) algorithm was implemented to generate optimal points which highlight the non-dominant feasible solutions. The least error between predicted and actual values suggests the effectiveness of both the regression model and the TLBO algorithms. Confirmatory trials have shown an extremely close relation which shows the suitability of both the regression model and the TLBO algorithm for the machining of the nanopowder-mixed WEDM process for Nitinol SMA. A considerable reduction in surface defects owing to the addition of Al2O3 powder was observed in surface morphology analysis.

Experimental study on surface characteristics in μED milling of Al6063%-5%B4C-5%ZrSiO4 composite using TOPSIS method

Al6063 composites are widely used in automobile, aerospace and biomedical industries due to their excellent mechanical properties. Machining of micro channels in composites is difficult for conventional machining process due to presence of hard reinforcement. Materials with base metal. Micro Electrical discharge (μED) milling is popular micromachining technique for machining simple, intricate shapes and microchannels on any conductive material. However, it a slow machining process, the identification of optimum condition has become wide research area in μEDM. The present work aims to study the influence of process variables namely voltage, spindle speed and threshold on machining characteristics of μED milling of Hybrid Metal Matrix Composites (HMMCs). Experimental trials are carried out with copper electrode at different parametric condition. Al6063%-5%B4C-5%ZrSiO4 composite was fabricated using stir casting method and experimental runs were designed using general full factorial method. The significant parameters are identified using Analysis of Variance (ANOVA) and the ideal machining conditions for multi-response are determined using Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) method. The morphology of machined surface with best and worst conditions is examined using SEM. Results indicated that the voltage and threshold are the influencing parameters for considered response indicators. Recast layer thickness seems to be low with best machining conditions as compared to worst conditions. Increase in voltage and threshold increases the Material Removal Rate (MRR) and decreases the Electrode Wear Rate (EWR). Surface finish is better when the lower order of capacitance and voltage is used. MRR is increased by 48% with best machining conditions compared to worst machining condition.

Multiobjective optimization of electric discharge machining of an Al–SiCp composite using the Taguchi–PCA method as well as the firefly and cuckoo search algorithms

Electric discharge machining (EDM) processes are extensively used in industries to machine materials and geometries that are complex and are not machinable by conventional methods. In our study, we focused on identifying the optimal process parameters for EDM during the machining of an aluminum alloy 6061 (matrix) –10% silicon carbide (particle) composite. The novel aspect of this work is the use of a copper electrode with different geometries (circular, triangular, square) for machining, together with input variables such as discharge current density (A) as well as pulse on- and off-timing (Ton and Toff). We used the L27 (313) Taguchi orthogonal array for our experimental layout and the responses we measured were recast layer thickness (RCT), electrode tool wear rate (TWR), and material removal rate (MRR). Taguchi’s approach of signal-to-noise (S:N) ratio was integrated with principal component analysis (PCA) for multicriterion optimization. Also, the nature inspired cuckoo search (CS) and firefly (FA) algorithms were used to identify the optimal conditions and to predict the outputs for maximum MRR and minimum TWR and RCT. From S:N + PC analyses, the optimal conditions we identified were: circle (12 A, 65 μs, 2 μs); triangle (12 A, 95 μs, 6 μs); and square (12 A, 65 μs, 8 μs). Under all of the conditions, the influence of discharge current was the most significant. Metallurgical examination conducted through micrographs of the machined surface clearly supported the predicted results. The optimized conditions we identified are appropriate for use in the automobile and aerospace industries to obtain holes of specific geometries with good surface integrity and reduced wear of tools.

Tool Wear Rate and Surface Integrity Studies in Wire Electric Discharge Machining of NiTiNOL Shape Memory Alloy Using Diffusion Annealed Coated Electrode Materials

Electrode material used in wire electric discharge machining (WEDM/wire EDM) plays a vital role in determining the machined component quality. In particular, when machining hard materials like nickel titanium/NiTi (NiTiNOL) shape memory alloy, the quality of electrode material is important as it may have adverse effects on the surface properties of the alloy. Different electrode materials give different performances, as each electrode material is made up of different conductivity, compositions and tensile strength. Therefore, detailed experimental studies have been carried out to understand the effect of diffusion annealed coated wires (X-type and A-type) on NiTiNOL SMA during the wire EDM process. The tool wear rate and surface roughness responses have been studied for both the electrode materials against different wire EDM variables such as pulse time, pause time, wire feed and spark gap set voltage. The impact of these process parameters on the stated output responses has been analyzed and further surface and subsurface analysis of the machined component has been carried out to understand the impact of diffusion annealed electrode materials during the wire EDM process. The investigation reveals that an A-type diffusion annealed coated wire is found to be most suitable in terms of tool wear rate, surface roughness and surface integrity during machining of NiTiNOL shape memory alloy compared to X-type and traditional brass-based electrode materials. Surface topographical properties were studied using confocal microscopic analysis and scanning electron microscope (SEM) with energy-dispersive spectroscopy (EDS) analysis. The subsurface analysis like microhardness and recast layer thickness was also studied for both the wires against different machining conditions.

A comprehensive review on wire EDM performance evaluation

Wire electric discharge machining (WEDM) technique is a fast-growing, advanced manufacturing process which is used for machining of hard materials including various alloys of steel, nickel, tungsten, cubic boron nitride, metal matrix composites (MMC's), superalloys, and ceramic materials. Moreover, it is employed for machining complex profiles and micromachining of hardened materials which are not feasible by various conventional machining processes. Generally, this process is employed to machine electrically conductive materials but during this scenario, it is also being used for machining of electrically low conducting and non-conducting ceramics including silicon carbide, aluminium oxide, silicon nitrate, and zirconium oxide ceramics by modifying the WEDM method with assisted electrode technique or forming composite of conductive and non-conductive materials. This paper presents a comprehensive review on the influence of several WEDM control factors including pulse-on time, pulse-off time, wire feed rate, peak current, wire tension, type of wire (tool) materials, types of base materials and types of powder mixed dielectric fluids on the output responses such as surface roughness (Ra), material removal rate (MRR), width of cut (kerf), wire wear ratio (WWR), wire breakage, microstructural defects, recast layer thickness, and thermal deformation. Moreover, various simulation methods, post processing methods and optimization of WEDM parameters using various techniques were discussed and at last this paper shapes the important recommendations and upcoming developments in WEDM.

A Soft Computing-Based Analysis of Cutting Rate and Recast Layer Thickness for AZ31 Alloy on WEDM Using RSM-MOPSO.

In the present research, the AZ31 alloy is machined by wire-cut electric discharge machining (WEDM). The experiments were designed according to the Box-Behnken design (BBD) of response surface methodology (RSM). The input process variables, namely servo feed (SF), pulse on-time (Ton), servo voltage (SV), and pulse off-time (Toff), were planned by BBD, and experiments were performed to investigate the cutting rate (CR) and recast layer thickness (RCL). The analysis of variance (ANOVA) was performed to determine the influence of machining variables on response characteristics. The empirical models developed for CR and RCL were solved using Multi-Objective Particle Swarm Optimization (MOPSO). Pareto optimal front is used for the collective optimization of CR and RCL. The optimal solution suggested by the hybrid approach of RSM-MOPSO is further verified using a confirmation test on the random setting indicated by the hybrid algorithm. It is found that the minimum RCL (6.34 µm) is obtained at SF: 1700; SV: 51 V; Toff: 10.5 µs; and Ton: 0.5 µs. However, maximum CR (3.18 m/min) is predicted at SF: 1900; SV: 40 V; Toff: 7 µs; and Ton: 0.9 µs. The error percentage of ±5.3% between the experimental results and predicted solutions confirms the suitability of the proposed hybrid approach for WEDM of AZ31.

Influence of powder-mixed EDM on surface morphology and metallurgical alterations of Inconel 625

ABSTRACT Current research work aims at improving the surface integrity characteristics of Inconel 625 (a nickel-based super alloy) during electric discharge machining (EDM) process. Powder particles such as graphite, aluminium and silicon were added to kerosene dielectric during the EDM process to enhance the surface integrity characteristics. A specially designed and fabricated dielectric recirculating system was attached to the EDM setup to carry the experiments. Design of experiments was carried out using response surface methodology. Influence of process variables such as powder material, particle concentration in dielectric, peak current, pulse on time, duty cycle and gap voltage was investigated on surface integrity characteristics of machined specimens. Mixing of powder particles with dielectric resulted in reduced surface cracking, recast layer thickness and surface microhardness. While EDM with conventional dielectric primarily demonstrated FCC structure of γ-phase of Inconel 625 along with some δ(Ni3Nb) phase, powder-mixed EDM has promoted formation of carbides of Ni, Cr and Mo along with Ni3Nb from Inconel 625. Addition of powder with dielectric also caused reduction in crystallite size as well as lattice strain due to enlargement of crater while maintaining uniform discharge over a large area.

Effect of pulse duration and vibration tumbling on re-cast layer thickness and surface roughness of super alloy undergoing EDM

Many geometrically complex functional parts of turbo-pumps of Liquid Oxygen (LOX)-Hydrocarbon based rocket engines (Semi-Cryogenic Engines) which are difficult to be realized through conventional machining processes, are realized through Electric Discharge Machining (EDM). Every EDM process, deposits a re-cast layer on the spark eroded surface due to melting and subsequent quenching of the eroded surface. Studies show that structure and hardness of the re cast layer, differs from the parent material. Energy Dispersive Spectroscopic (EDS) analysis reveal that the chemical composition of the re-cast layer matches very closely to that of the parent metal itself. Re-cast layer has inherent micro cracks due to both thermal and tensile stresses during cooling. Even though re-cast layer is a superficial phenomenon, cracks once initiated on the re-cast layer, penetrate deep down to the parent metal as well, under fatigue loads causing fractures. In order to reduce the micro cracks on such spark eroded surfaces, thickness of the re-cast layer is to be reduced to the extent possible by fine tuning the EDM parameters. Failure analysis carried out on the fractured turbine blades of a LOX-Hydrocarbon based engine, realized through 4-axis EDM, which failed during the functional tests, indicated the propagation of such micro cracks from spark eroded surface into parent material causing fatigue fracture at turbine blade tips. Amongst the significant parameters governing the formation of re-cast layer in the EDM process, the effect of pulse duration is studied for its effect on re-cast layer thickness and surface roughness on super Nickel alloy turbine blades. Additionally, a vibration tumbling process is introduced and its effect on re-cast layer thickness and surface roughness is demonstrated. Turbine blades realized from the fine-tuned pulse duration, followed by vibration tumbling, show great improvements in surface finish, reduction of micro cracks, surface porosities and thickness of recast layer. This turbine successfully passes through the functional tests without any fractures.

Machinability of copper-based metallic glass in micro EDM and its optimisation by grey fuzzy logic

The present study investigates the machinability of metallic glass by drilling micro holes using the micro-electrical discharge machining process. The variation of microhole features such as material removal rate (MRR), overcut (OC), circularity error (CE), recast layer thickness (RLT) and edge deviation (ED) with respect to variable process parameters such as voltage, capacitance and tool rotation speed has been investigated by Taguchi method and analysis of variance. Multi-objective optimisation approaches such as grey relational analysis (GRA) and grey-based fuzzy logic (GFL) are implemented to optimise the process parameters to attain microholes having quality features. The studies exhibit that microholes of higher MRR and lower OC, CE, RLT, and ED can be machined by controlling the process parameters. GFL exhibits a lower error of 7.16% as compared to GRA's 11.40% in between the experimental and predicted results.

High power water jet guided laser drilling of angular holes through nickel superalloy

This paper investigates the basic characteristics of water jet guided laser drilling process using a high power laser with a maximum average power of 400 W to drill 0.7 mm diameter angular holes (60° to the surface) through 1.6 mm thick nickel superalloy (resultant hole length of 1.85 mm). Experimental trials were performed using average powers of up to 148 W to understand the effect of different process parameters on the hole quality and cycle time. Process parameters investigated included shielding gas flow rate, water jet pressure, trepanning speed, pulse frequency, and average power. The results found water jet pressure and average power to be significant in controlling the recast layer thickness and drilling cycle time. At higher power and throughput, average recast layer thicknesses of ∼30 μm could be obtained with a cycle time of 3 seconds. This surpasses the quality of holes, which can be produced via state-of-the-art laser drilling techniques without a significant reduction in productivity.

Optimizing machining responses of homologous TiNiCu shape memory alloys using hybrid ANN-GA approach

Fabrication of shape memory alloys using wire electro discharge machining (WEDM) has gained popularity over the last few years. Most widely used machining parameters of WEDM process are pulse on time (Øon), pulse off time (Øoff), servo voltage (σ) and wire feed (ω). WEDM responses like material removal rate (MR), surface roughness (SR), kerf width (KW) and recast layer thickness (LT) have been evaluated by researchers to determine machining characteristics and are also considered for this study. These machining responses determine the quality of machining and are majorly influenced by thermal conductivity and melting temperature of the WEDM workpiece. Actuation behavior of shape memory alloys is a function of phase transformation characteristics which in turn depends on elemental composition of the selected alloys. Therefore, dissimilar machining responses of Ti50Ni40Cu10 and Ti50Ni25Cu25 have been observed even though similar machining input values were used. This study utilized artificial neural network (ANN) mapping to establish WEDM response function – which was used as fitness function to perform multi objective optimization using genetic algorithm (GA). It was found that ANN successfully predicted machining responses of selected homologous alloys and GA helped in identifying suitable input parameter values to optimize machining responses.

Surface integrity analysis in powder mixed EDM of Nimonic 263

Powder mixed Electrical Discharge machining (PMEDM) is an effective process in improving rate of material removal (MRR) and surface finish. But influence of powder mixed in the dielectric on the surface quality of the machined profile is an essential aspect of machining effectiveness. Nimonic 263 though imbibed with many advanced properties, is regarded as a difficult to machine materials by conventional methods. Spark erosion machining process is independent of the workpiece hardness and can machine any electrically conductive material. Hence EDM can be an effective alternative for machining Nimonic 263. But machining performance may vary depending upon the working medium. In this work, performance of spark erosion process in machining of Nimonic 263 has been evaluated using three different dielectric conditions. Kerosene has been taken as the dielectric fluid. Graphite micro powder as a conductive powder and alumina micro powder as a non-conductive powder have been added to the dielectric fluids for assaying their influence on the responses. Only the dielectric condition has been varied and all other parameters viz. pulse on time, peak current, gap voltage, duty factor and spark time have been kept constant at 20 µs, 6 A, 30 V, 42% and 3 s respectively. Dielectric breakdown voltage (DBV), surface topography, surface crack density (SCD), recast-layer thickness (RLT), microhardness have been considered as the performance measures in this study. Apart from these responses, 3D surface analysis has been carried out for the machined surfaces obtained by each of the dielectric conditions. Graphite powder mixed kerosene has been found to be the most suitable dielectric condition with best surface finish.

Machining of micro features through µ-ECSM process and evaluation of surface integrity

In the present experiment, micro features were fabricated on titanium alloy by means of the micro-electrochemical spark machining (µ-ECSM) process and the micro-electro discharge machining (µ-EDM) process, using similar electrical parameters. However, in case of the µ-ECSM, the types of electrolyte (KOH, NaOH, KOH+NaOH) and their concentration (0.5 M to 2.5 M) varied from that of the µ-EDM process. In course of the study, different machining output parameters, such as, material removal rate (MRR), recast layer (RCL) thickness, heat affected zone (HAZ) and surface topography, were analysed and compared. The results indicated that the µ-ECSM process produces higher MRR (7.29 times), RCL thickness (0.436 times) and HAZ (0.595 times) as compared to the µ-EDM process. Moreover, the mixed (KOH+NaOH) electrolyte at 0.5 M concentration, 25 V machining voltage, 15% duty factor and 10 kHz of frequency showed better machining performance as compared to the parent electrolyte and produced lower RCL thickness (3.54 µm) and HAZ (26.83 µm), respectively. The surface topography of the work piece, machined by the µ-EDM process, showed rough surface whereas the NaOH electrolyte-based µ-ECSM process produced smoother and clean surface.

Experimental study of EDM milling of 3D-shaped diffuser for film cooling holes on turbine blades

Cylindrical and diffuser-shaped film cooling holes on aero-engine turbine blades are usually machined by electrical discharge machining (EDM) by means of drilling and die-sinking approaches in a sequence. However, this sequential machining on different types of EDM machines is prone to deteriorate the machining accuracy since the inevitable secondary clamping error. Besides, since the nature of electrode wear, the shape of the tip of tool electrode is constantly changing during machining. To ensure the geometry accuracy of the diffuser, the worn tool electrode has to be replaced or dressed online frequently, resulting in a low machining efficiency. To address these problems, this paper proposes a machining method for diffuser shaped film cooling holes by a compound approach in combination of high speed EDM drilling and milling implemented on an EDM drilling machine with tubular electrodes and high-pressure inner flush. To obtain the optimal machining parameters, experimental investigation is carried out based on the experimental design method. Machining performance in terms of longitudinal tool electrode wear ratio, machining time, and machining stability under different machining parameters, such as pulse duration, pulse interval, peak current, capacitance, spindle speed, and flushing pressure, were taken into account. And the range analysis method is then adopted to obtain an optimal set of machining parameters. By applying the obtained optimal machining parameters, the proper compensation parameter and tool path overlapping ratio of the EDM milling process can be obtained. In the milling of an inversed pyramid, the material removal rate can reach as high as 14.02 mm3/min, the relative geometrical error can be controlled within 3.58%, and the maximum recast layer thickness was 6.8μm. Machining experiments of diffuser shaped film cooling holes verify the capability and effectiveness of the proposed method.

Tolerance-Based Optimization of Sinking Electrical Discharge Machining for Industrial Seal Slot Manufacture

Abstract Seal plates for turbine vanes significantly reduce gap losses and thus play a major role in increasing the efficiency of turbines. The industrial production of seal slots, which position the seal plates in the turbine vanes, is driven by the need for high productivity in combination with a reliable processing of necessary geometrical and surface integrity features. A machining technology that is able to machine hard-to-cut materials such as nickel-based alloys is electrical discharge machining. Due to its electrothermal working principle, it is able to machine materials independently from their mechanical properties even at high aspect ratios. Achievable removal and wear rates as well as the resulting surface properties strongly depend on the discharge energy. Furthermore, the discharge energy affects the working gap sizes and therefore flushing efficiencies when machining high aspect ratio cavities. This relationship is investigated taking into account various contemporary generator technologies and graphite grades from both published literature and own experimental investigations. Their effect on machining performance focusing on productivity, recast layer thickness, and crack formation is quantified. Based on these data, a novel empirical model for tolerance-based optimization is developed. The model is used to perform an optimization on an existing serial production, and implementation has been proven successful.