High-speed Electrical Discharge Machining Research Articles

Geometric and Flow Characterization of Additively Manufactured Turbine Blades With Drilled Film Cooling Holes

Abstract As designers investigate new cooling technologies to advance future gas turbine engines, manufacturing methods that are fast and accurate are needed. Additive manufacturing facilitates the rapid prototyping of parts at a cost lower than conventional casting but is challenged in accurately reproducing small features such as turbulators, pin fins, and film cooling holes. This study explores the potential application of additive manufacturing and advanced hole drill methods as tools to investigate cooling technologies for future turbine blade designs. Data from computed tomography scans are used to nondestructively evaluate each of the cooling features in the blade. The resulting flow performance of these parts is further related to the manufacturing through benchtop flow testing. Results show that while total blade flow is consistent for all additively manufactured cooled blades, flow through smaller regions of the blades shows variations. Shaped film cooling holes manufactured using a high-speed electrical discharge machining method are within tolerance in the metering section but do not expand at the specified angle in the diffuser even though design tolerances are met. In contrast to high-speed EDM, conventional EDM holes are undersized throughout the length of the hole. Due to the additive manufacturing process, the surface roughness was higher on the additively manufactured parts in the current study than has been previously reported for surface roughness of commonly used cast components. The roughness results show high levels on thin walls, particularly at the trailing edge as well as on downskin surfaces. Internal surface roughness is higher than external roughness at most locations on the blade. The results of this study confirm that additive manufacturing along with advanced hole drilling techniques offers faster development of blade cooling designs.

Research on an Intelligent Control Method of a Magnetic Actuator for Micro Electrical Discharge Machining

Electrical discharge machining (EDM) is not limited to the strength and hardness of conductive materials, and is a non-contact special processing technology. In micro-EDM, there are problems such as untimely axial positioning, unstable inter-electrode machining voltage, and difficulty in discharging inter-electrode electric erosion products. This paper considers a magnetic actuator with fast response and high accuracy as the local actuator for a micro-EDM. By introducing a domain adjustment mechanism, a variable domain fuzzy PID controller was designed to control the inter-electrode voltage control system of the magnetic actuator for micro-EDM using an intelligent control strategy. During the micro-EDM machining process, the controlled magnetic actuator drives the tool electrode in the axial direction for rapid micro-positioning, thus maintaining effective inter-electrode machining voltage and achieving a high-speed and high precision EDM. Simulation and experimental results showed that compared with traditional micro-EDM, the machining efficiency of the variable domain fuzzy PID control magnetic actuator, and traditional micro-EDM cooperative control, was increased by 40%, the machining process was more stable, and the quality of the machined surface was better.

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.

Performance improvement of high-speed EDM and ECM combined process by using a helical tube electrode with matched internal and external flushing

Micro-hole fabrication at a high speed and accuracy of machining while maintaining high surface quality is challenging. A core difficulty is the removal of the products of machining from extremely narrow gaps. To solve this problem, this study proposes an approach that combines high-speed electrical discharge machining (EDM) with electrochemical machining (ECM) by using a helical tube electrode with matched internal and external flushing. During high-speed electrical discharge drilling, matching the internal flushing with the clockwise rotation of the helical electrode can help remove debris from the bottom of the blind hole. During ECM, matching the external flushing with the anticlockwise rotation of the helical electrode can improve the flow of electrolyte in the gap. First, the flow field was simulated to show that matching the internal and external flushing of the helical electrode can enhance the flow of the medium and reduce particle concentration in extremely narrow gaps. Second, a series of experiments were conducted to verify that the taper of the hole and the surface quality of its wall can be improved by using the helical tube electrode. Finally, an experiment was carried out to optimize the machining parameters and yielded a minimum taper of 0.008 at a speed of rotation of 460 rpm, and pressures of internal and external flushing of 9 MPa and 4 MPa, respectively.

Conceptual Design of a High-Speed Wire EDM Robotic End-Effector Based on a Systematic Review Followed by TRIZ

Exotic materials such as titanium offer superior characteristics that, paradoxically, make them hard-to-cut by conventional machining. As a solution, electric discharge machining (EDM) stands out as a non-conventional process able to cut complex profiles from hard-to-cut materials, delivering dimensional accuracy and a superior surface. However, EDM is embodied in CNC machines with a reduced axis and machining envelope, which constrains design freedom in terms of size and shape. To overcome these CNC constraints, traditional machining using six-axis industrial robots have become a prominent research field, and some applications have achieved cost efficiency, an improved envelope, and high flexibility. However, due to the lack of stiffness and strength of the robot arm, accuracy, material rate removal, and surface finishing are not comparable to CNC machining. Therefore, the present study investigates the design of a novel WEDM combined with six-axis robotic machining to overcome the limitations of traditional robotic machining and enhance EDM applications. This study extends the work of a conference paper to confirm potential outcomes, quantifying and ranking undesired interactions to map technical problems and applying the TRIZ approach to trigger solutions. Finally, an effective robotic end-effector design is proposed to free EDM from CNC and deliver robotic machining as a flexible and accurate machining system for exotic materials.

Rotating short arc EDM milling method under composite energy field

• A new EDM milling method with rotating short arcs under composite energy field was proposed. • A mathematical model to describe the trajectory of particles in the composite field was established. • Rotating short arc was generated by the Lorentz force, the electrical force and the high-speed rotation of electrode. • The MRR of rotating short arc machining is increased by 46 % compared with traditional EDM. • An increase in the strength of the magnetic field increases the MRR and decreases the TEWR. Electrical discharge machining (EDM) is a rapidly developing technique amongst nontraditional techniques. However, low processing efficiency significantly hinders its application in industry. This study proposes a new high-speed EDM milling method using rotating short arcs under a composite energy field. By the combined effect of the Lorentz force, electric field force, and high-speed rotation of the tool electrode, rotating short arcs are generated between the tool electrode and the workpiece, which in turn provide constant erosion of materials. The proposed method can increase the discharge energy density by converging the electric arcs through the composite energy field, and increase the energy obtained by the workpiece. By this means, it improves the material removal rate. During processing, a portion of debris is attracted by the magnetic field and adheres to the end surface of the electrode, which can protect the electrode and reduce the electrode wear rate. Compared with traditional EDM, the material removal rate of the rotating short arc milling was increased by 46 %, while the tool electrode wear rate decreased by 62 %. Furthermore, an increase in the strength of the magnetic field increases the material removal rate of the rotating short arc milling and decreases the tool electrode wear rate and surface roughness. Therefore, high-efficiency and high-quality EDM milling can be achieved by using the proposed method.

Investigation of Advanced Control Schemes for High-speed Small-Hole EDM Drilling

High-speed small-hole electrical discharge machining (EDM) drilling is extensively applied to machine cooling film holes of turbine blades and diesel nozzle spray holes due to its non-contact characteristics and dedicated features, e.g. high-speed rotation and high-frequency pulse power generator. However, with increasing drilling depth, the process stability could be easily deteriorated by frequent debris due to restricted flushing conditions. Consequently, the machining efficiency and surface finish of high-aspect-ratio holes cannot be guaranteed. To address this problem, this paper proposes a sliding mode and self-tuning control scheme to regulate the discharging process. An integral function is presented as the switch mode to avoid chattering, and a direct algorithm that reparameterizes the model in terms of controller parameters is considered to reduce computation. The presented comparative drilling experiments with high aspect ratio (nearly 100) show the benefits of the adaptive controller.

Intelligent pulse analysis of high-speed electrical discharge machining using different RNNs

High-speed electrical discharge machining (EDM) is a nontraditional machining method using high electric energy to efficiently remove materials. In this paper, a novel pulse classification method was proposed based on the recurrent neural network (RNN) for high-speed EDM pulse analysis. This study is the first time that an RNN has been applied in high-speed EDM pulse analysis. Different from traditional EDM, discharge pulses of high-speed EDM were classified into five types during the machining process: open, spark, arc, partially short and short. Models based on three different RNNs including the traditional RNN, LSTM (long short-term memory) and IndRNN (independently recurrent neural network) with different activation functions were built to analyze the discharge pulses in the research. A new input data structure based on the minimum signal change period was proposed in the classification method to simplify the model structure and improve accuracy at the same time. Without setting thresholds, the highest classification accuracy of the proposed model is up to 97.85%, which can simultaneously classify discharge pulses based on 10,000 orders of magnitude including various current values. The proposed method was effectively adapted to the complicated machining conditions and the compound power source of the high-speed EDM. The optimal model was used to analyze the distribution of discharge pulses during the machining process under different currents, fluxes and feeding speeds. The proportion of the discharge pulses was clearly predicted. Through analyzing the discharge pulses of long machining time, the regulation of discharge under different machining parameters was revealed more reliably, providing valuable information for the improvement of high-speed EDM servo systems.

Effects of dielectric fluids on surface integrity for the recast layer in high speed EDM drilling of nickel alloy

Dielectric fluids affect not only the discharge status but also the structure and composition of recast layer in the electrical discharge machining (EDM) process. In order to reduce the thickness of recast layer and improve the surface quality of film cooling holes of nickel alloy, in this paper, the effects of four different types of dielectric fluids on surface integrity of the recast layer formed during high speed EDM drilling process were investigated; the formation mechanism of the recast layer on nickel alloy was discussed by comparing the surface characters of the bulk and those of the recast layer using various characterization methods. The dielectric fluids with larger electric conductivity (emulsion and water solution) were found to have higher sensitivity to the single pulse discharge energy. With the increase in electric conductivity of dielectric fluids, the recast layer became thicker accordingly. The thickness of the recast layer when using kerosene fluid was the smallest (4.8 μm), which was nearly one third of the that of using water-based fluid. The cooling rate of the dielectric fluid affected the microstructure of the recast layer, where the surface of recast layer was apt to form fine grain zone and the bottom formed columnar grain.

Study of anti-corrosion and anti-wear properties on superhydrophobic aluminium alloy surfaces

ABSTRACTTo improve the wear and corrosion resistance of aluminium alloys in salt environments, we produced superhydrophobic surfaces via high-speed electrical discharge machining. The specimens were characterised using a scanning electron microscope and a laser scanning confocal microscope and through contact angle (CA) measurement, measurement of electrochemical corrosion at an electrochemical workstation and wear testing. Results showed that the superhydrophobic specimens had a water CA of 151.5°. The corrosion of the as-obtained superhydrophobic specimens was significantly reduced by more than 50%, indicating their excellent anti-corrosion properties. The wear of as-obtained superhydrophobic specimens were reduced by approximately 10–35% in dry conditions and by approximately 45–85% in 3.5% NaCl solution, revealing their excellent anti-wear properties in both dry and salt environments.

Study on the characteristics of plasma channel based on multi-spark pulse discharge machining effect

Plasma channel characteristics and energy distribution in electrical discharge machining (EDM) were mostly studied by analyzing the geometry parameters of craters caused by a single pulse discharge in previous studies. However, single pulse experiments cannot provide us insights into superposition, migration, abruption, interruption, and other phenomena of the plasma channel which have significant effects on EDM. Besides, EDM itself is a consecutive pulse discharge process. Thus, this paper focuses on the characteristics of plasma channel and the mechanism of material removal based on experimental data from multi-spark pulse discharge machining. The contrastive milling experiments of different parameters in multi-spark pulse discharge machining in high-speed dry EDM by using nickel-based superalloy as workpiece were conducted. The effects of peak current, dielectric type, breakdown voltage, air pressure, and electrode rotation speed on the crater number, crater distance, crater depth, and crater removal volume were studied. The plasma channel characteristics and material removal mechanism in continuous machining of high-speed dry EDM were revealed.

Machining characteristics and removal mechanisms of moving electric arcs in high-speed EDM milling

Low machining efficiency is the major challenges of conventional electrical discharge machining (EDM). To resolve this problem, a novel machining method of high-speed EDM milling with moving electric arcs was developed, which can remove material continuously without discharge intervals of pulse power supply and achieve extremely low tool-electrode wear for the direct-current power supply employed. An EDM milling equipment was devised to explore machining characteristics and removal mechanisms of moving electric arcs in high-speed EDM milling process. Firstly of all, the motion characteristics of moving electric arcs were investigated. The theoretical analysis shows that the arcs maintain an independent movement relative to the tool-electrode and workpiece. Then, the plasma channels were captured by high-speed video camera, which contributed to develop the model of removal mechanisms of moving electric arcs. Finally, the machining experiments were carried out based on titanium alloy, which verified that a much higher material removal rate could be easily achieved by moving electric arc. Moreover, the tool-electrode wear in high-speed EDM milling is much lower than conventional EDM due to the protective layer formation.

Effects of Different Electrode Materials on High-speed Electrical Discharge Machining of W9Mo3Cr4V

In order to break through the limit in material removal rate (MRR) of electrical discharge machining (EDM), electric arc machining (EAM), as a new and high-speed processing method, is used to machine difficult-to-machine materials. This method has an exciting high MRR with tens times increase in EDM and a low relative electrode wear ratio (REWR) when machining W9Mo3Cr4V. The electrode material is an important factor that affects the processing cost and quality of EAM. In this paper, traditionally conductive metal materials were used as electrode materials to process W9Mo3Cr4V in EAM, including brass (H62), copper (T2), stainless steel 304, aluminum alloy 6061 and graphite (C). Experimental results show that tubular graphite (C) is the best electrode material for EAM. Additionally, when tubular graphite with 16mm external diameter and 12mm internal diameter is used as tool electrode, the effects of processing parameters (peak current, electrode polarity, electrode speed and flushing pressure) are also investigated in present study. MRR, REWR, white layer thickness (WLT) and surface roughness (Rz) are selected as output responses.

Improvement of EDM Machining Speed by Using Magnetic/Piezoelectric Hybrid Drive Actuator

The objective of this research is to achieve high-speed electrical discharge machining (EDM) of small holes using a high-speed, high-precision, 3-DOF controlled, magnetic/piezoelectric hybrid drive actuator. In this paper, the proposed actuator was attached to the conventional electrical discharge machine, and the increase in the machining speed using the actuator was experimentally confirmed. The relationships between the machining speed, the amplitude and the frequency of the electrode vibration were discussed. Experimental results show that the use of the proposed actuator can speedily adjust the distance between the electrode and the workpiece, and the machining speed was increased by 138% compared with the conventional EDM. Moreover, the machining speed using the actuator also was increased as the changes of the amplitude and the frequency of the electrode vibration, and it was increased by 76% compared to without the electrode vibration.

Improvement in machining speed with working gap control in EDM milling

High-speed electrical discharge machining (EDM) milling is investigated with working gap control. EDM milling has a merit that does not need complex electrode fabrication; however, it is necessary to improve the removal rate in EDM milling. Rotation of the electrode improves the removal rate, but this rotation causes a periodic change in the working voltage. As a consequence, the periodic change causes a disturbance that decreases the control performance. EDM milling realizes a high machining precision by the scanning method. A small working area allows for fast scanning. If the working gap is appropriate, it becomes longer because the machined surface of the workpiece is removed. Therefore, the conventional controller remains at the steady offset and lowers the machining efficiency. The proposed controller introduces a notch filter, which eliminates the frequency component due to rotation of the electrode. It also has a feed-forward compensator to eliminate the steady offset. The controller for each machining condition is derived from a coordinate transformation and the least-squares approximation. It is confirmed that the proposed controller achieves machining speeds that are 2–6 times faster in a straight line and greater than 30% higher in the profile machining than in the conventional one.

Surface integrity of Inconel 718 in high-speed electrical discharge machining milling using air dielectric

This paper proposes a novel, efficient, and environmental friendly machining method named high-speed dry electrical discharge machining (EDM) milling. The material removal rate of this method is much higher than that of conventional EDM and is also higher than that of traditional mechanical machining method for materials that are “difficult to machine.” Nickel-based superalloy Inconel 718 is widely used in the aerospace field as its surface integrity exerts significant effects. In this study, the microscopic characteristics, such as surface roughness, re-solidified layer, micro-hardness, micro-crack, and micro-void, of the machined Inconel 718 in high-speed EDM milling using air dielectric were studied compared with those using liquid dielectric. The formation mechanism of the microscopic characteristics was revealed. Experimental results showed that the machined Inconel 718 in high-speed EDM milling using air dielectric exhibited smaller surface roughness, thinner thickness of re-solidified layer, and smaller and less micro-cracks compared with those using liquid dielectric. Theoretical and technical foundations were laid for the industrial application of this novel machining method.

High-speed near dry electrical discharge machining

High-speed near dry electrical discharge machining (EDM) is a novel and promising machining method, which obtains higher material removal rate, lower surface roughness and thinner width of overcut compared with that of high-speed dry EDM. Moreover, debris deposition on the machined surface and electrodes are significantly reduced and an obviously thin heat affected zone is obtained using the near dry dielectric (mist). In this study, a low-flux pump is utilized to supply deionized water at a controlled rate to the high-speed air pipe, which affords a mist dielectric to the machining process. Compared with dry EDM, the high-speed near dry EDM improves the cooling capacity and increases the dielectric strength of the dielectric. Furthermore, electrode injection flushing assisted by side flushing of high-speed mist was used to reduce debris deposition and improve cooling capacity. Comparative milling experiments of Ti6Al4V were conducted under the dry and near dry conditions using the different flushing ways. The effects of current, pressure, droplet size, electrode rotation speed, and droplet density on the material removal rate, surface roughness, width of overcut in continuous machining and material removal weight, trace length, trace width in single pulse machining were studied. In addition, scanning electron microscope, X-ray diffractometer, energy dispersive spectrometer, and micro-hardness tester were also used to study the micro-characters of the machined materials.

Interelectrode discharge mechanism in high-speed wire electrical discharge machining

Different from conventional electrical discharge machining (EDM), given that the dielectric fluid utilized in high-speed wire EDM (HSWEDM) has a certain conductivity, a passive film can form on the workpiece surface because of the electrochemical reaction during processing. Such formation affects the interelectrode discharge characteristics. The formation mechanism of the passive film was analyzed in this study. The effect of the passive film on the discharge gap was evaluated through single-pulse discharge experiments. Results show that the density and thickness of the passive film increase with the increase in dielectric fluid conductivity or the accumulation of leakage energy; the discharge gap is thus reduced. However, in actual processing, the existence probability of open-circuit voltages is very small, and the high temperature produced by spark discharge has an erosion effect on the passive film. Consequently, forming a dense passive film on a workpiece surface is difficult, and spark discharge still occurs in an interelectrode gap in HSWEDM. A multi-cutting experiment was conducted to study the effect of the passive film on dimensional accuracy. Results indicate that the passive film affects the discharge gap because of the difference in dielectric fluid conductivity. A size error of approximately 2 μm was generated when the initial conductivity difference was roughly 2000 μS/cm.

High-speed dry electrical discharge machining

A novel high-speed dry electrical discharge machining (EDM) method was proposed in this study. Using this method, the material can be rapidly melted by extremely high discharge energy and flushed out of the discharge gap by high-pressure and high-speed air flow. The material removal rate (MRR) of dry EDM was significantly improved by the proposed method. The MRR of dry EDM is usually in tens mm3/min, whereas the MRR of the proposed method can be as high as 5162mm3/min, which improves the MRR by 2nd to 3rd order of magnitude. Investigation was conducted systemically. The influences of work piece polarity, discharge current, pulse duration time, gas pressure, and electrode rotation speed on machining performance were studied. The machining mechanism of this method was thoroughly analyzed. Moreover, the re-solidified layer, surface morphology, elementary composition, and phase of AISI 304 stainless steel for high-speed dry EDM were also investigated. Theoretical and technical foundations were laid for the industry application of dry EDM.

Compound machining of titanium alloy by super high speed EDM milling and arc machining

A novel compound machining of titanium alloy (Ti6Al4V) by super high speed electrical discharge machining (EDM) milling and arc machining was proposed in this paper. The power supply consisted of a pulse generator and a DC power source which were isolated from each other. A rotating pipe graphite electrode was connected to the negative pole of the power supply. The plasma channel was able to deionize, and maximum material removal rate (MRR) reached 21,494mm3/min with a relative electrode wear ratio (REWR) of 1.7% because of high current and efficient flushing. Compared with traditional EDM, the compound machining achieved a significantly higher MRR but a similar REWR. To investigate the characteristics of the compound machining, the effects of electrode polarity, peak voltage, peak current, and flushing pressure on the performance of the process, including its MRR, REWR, and radius of overcut (ROC), were determined. In addition, scanning electron microscopy, X-ray diffraction, and microhardness analysis were conducted. Result shows that the proposed method can machine difficult-to-machine materials efficiently.