Research on the key techniques of composite processing of EDM and vibration ultrasonic drilling

Abstract

Purpose

Utilizing electrical discharge machining (EDM) to process micro-holes in superalloys may lead to the formation of remelting layers and micro-cracks on the machined surface. This work proposes a method of composite processing of EDM and ultrasonic vibration drilling for machining precision micro-holes in complex positions of superalloys.

Design/methodology/approach

A six-axis computer numerical control (CNC) machine tool was developed, whose software control system adopted a real-time control architecture that integrates electrical discharge and ultrasonic vibration drilling. Among them, the CNC system software was developed based on Windows + RTX architecture, which could process the real-time processing state received by the hardware terminal and adjust the processing state. Based on the SoC (System on Chip) technology, an architecture for a pulse generator was developed. The circuit of the pulse generator was designed and implemented. Additionally, a composite mechanical system was engineered for both drilling and EDM. Two sets of control boards were designed for the hardware terminal. One set was the EDM discharge control board, which detected the discharge state and provided the pulse waveform for turning on the transistor. The other was a relay control card based on STM32, which could meet the switch between EDM and ultrasonic vibration, and used the Modbus protocol to communicate with the machining control software.

Findings

The mechanical structure of the designed composite machine tool can effectively avoid interference between the EDM spindle and the drilling spindle. The removal rate of the remelting layer on 1.5 mm single crystal superalloys after composite processing can reach over 90%. The average processing time per millimeter was 55 s, and the measured inner surface roughness of the hole was less than 1.6 µm, which realized the  micro-hole machining without remelting layer, heat affected zone and micro-cracks in the single crystal superalloy.

Originality/value

The test results proved that the key techniques developed in this paper were suite for micro-hole machining of special materials.

Citation

Liu, J., Luo, X., Li, L., Liu, F., Qiu, C., Fan, X., Dong, H., Li, R. and Liu, J. (2024), "Research on the key techniques of composite processing of EDM and vibration ultrasonic drilling", Journal of Intelligent Manufacturing and Special Equipment, Vol. 5 No. 3, pp. 273-286. https://doi.org/10.1108/JIMSE-06-2024-0014

1. Introduction

The micro electrical discharge machining (EDM) drilling process generates micro-holes in conductive materials through electronic discharge. Tools and workpieces are not in contact during processing, and workpiece materials are melted and evaporated due to the high temperature generated by electronic discharge. Kaushik et al. (2023) EDM is the mainstream machining method for micro holes in aerospace critical components. However, many studies have shown that in each discharge process, 1–10% of the molten material will be recondensed on the surface of the workpiece, forming a remelting layer and causing defects such as pores and micro-cracks Wang et al. (2024), Chu and Quan (2020). These defects seriously affect the reliability, performance, and service life of the workpiece. To solve the problem of residual remelting layer after EDM micro-hole machining, extensive research has been conducted by scholars both domestically and internationally.

Kliuev et al. (2016) researched reducing the thickness of EDM remelting layers by combining EDM drilling with surface dressing techniques. Electrodes with a diameter of 1 mm were used in the machining, and nonlinear regression and response surfaces were used to optimize the process parameters. This method achieved a material removal rate of 77 mm³/min, reduced relative tool wear to 20%, and an average remelting layer thickness of 8 µm. To reduce the thickness of the remelting layer of the gas film holes in turbine blades, (Tong et al., 2021) optimized the discharge parameters by orthogonal test. After machining until the holes were penetrated entirely, the discharge polarity was changed to repair the penetrating holes. When the hole repair time was 3–4 s, the thickness of the remelting layer was reduced from 10.472 µm to 2.416 µm. Although reducing the thickness of EDM remelting layers through surface dressing can effectively decrease the thickness, it cannot eliminate the remelting layer.

Some researchers combine EDM with other processing methods to improve the machining quality of micro-holes. Ding et al. (2017) has designed a method for electric spark electrolysis composite machining, using sodium nitrate salt solution as the machining fluid. The formation of the remelting layer on the electrolytic hole wall is controlled by adjusting the electrode dwell time. Using this method, they successfully machined large deep-to-diameter ratio micro-holes without remelting layers. Liu et al. (2023) machined micro-holes with a diameter of 500 µm on GH2312 superalloy and polished them with abrasive water flow. The alumina abrasive combined with the bidirectional fluid can effectively remove the remelting layer and reduce the surface roughness from an initial Ra of 5.7 μm–0.6 μm. However, using electrochemical assistance to remove the remelting layer is more challenging in terms of precision control. Zhu et al. (2011) proposed a processing method of electrical discharge machining-electrochemical-grinding composite processing. By using the nickel-diamond co-deposition method, a processing electrode with a diameter of 350 μm was prepared, ultimately achieving micro-holes with a diameter of 0.6 mm and a depth-to-diameter ratio of 16.25. This method successfully removed the remelting layer on the surface of a nickel-based alloy and achieved a hole wall surface quality of Ra = 1.25 µm. However, the fabrication process of this electrode is relatively complex. Okasha et al. (2010) proposed a laser-mechanical micro-drilling composite processing method, which reduces defects and decreases burr size. However, the roundness of the holes produced by laser processing is poor, and the taper is large.

Furthermore, ultrasonic vibration–assisted EDM (UEDM) can significantly improve the machining quality of micro-holes. Sabyrov et al. (2019) found that compared to conventional EDM, UEDM improved material removal rate (MRR), surface roughness (SR), and tool wear rate (TWR). Li et al. (2024) introduced longitudinal torsional ultrasonic vibration into EDM, significantly improving the machining performance. When the optimal peak current increased from 2 A to 3 A, the pulse width increased from 125 μs to 150 μs, the material removal rate increased by 83.8%, and the relative electrode wear rate, hole taper, and surface roughness decreased by 33.7%, 22.3%, and 16.4%, respectively. Wang et al. (2020) established a model for the single-pulse micro-removal quantity in UEDM and fundamentally studied the material removal mechanism based on heat transfer theory. Experimental results also confirmed that the interference of ultrasonic vibration reduced the deposition area of carbon and decreased the surface roughness. (Xing et al., 2021) analyzed the advantages and disadvantages of parameter combinations by changing ultrasonic amplitude, frequency parameters, and matching discharge parameters, significantly improving the processing efficiency of high depth-to-diameter ratio micro-holes in titanium alloy, reducing the taper of the holes and electrode wear. Domestic and foreign scholars have conducted in-depth research on the processing technology and mechanisms of EDM and ultrasonic vibration machining. (Fan et al., 2023) These studies have laid a solid foundation for applying electrical discharge ultrasonic vibration drilling composite processing technology. With the increasing precision requirements for manufacturing critical components in the aerospace industry, the demand for special material micro-hole processing without remelting layer, heat-affected zone, and micro-cracks is becoming increasingly urgent (Ying et al., 2020).

Regarding the EDM control system, Zhang et al. (2024a) developed a desktop EDM machine tool numerical control system hardware platform based on ARM and field programmable gate array (FPGA), which can adapt to various pulse power supplies. (Zhang et al., 2004) developed a micro-EDM device with a four-axis and three-linkage, which can process a micro-shaft with a minimum diameter of 12 µm and a micro-hole with a minimum diameter of 25 µm. Song et al. (2024) developed a multi-electrode and multi-variable adaptive control machining device for EDM and processed closed integral impeller through a step-by-step machining method. Zhang et al. (2024b) also developed an EDM pinhole machining numerical control system based on embedded ARM + FPGA architecture. Zhao et al. (2020) designed an open architecture CNC platform for EDM machine tools, using the Generalized Unit Arc Length Increment (GUALI) interpolation method and digital instrument/player system architecture. This platform incorporates the most advanced technologies in both hardware and software, thus avoiding limitations caused by outdated technologies. Ye et al. (2022) integrated a color confocal sensor into a micro-EDM machine, enabling the system to achieve sub-micron accuracy in in-machine area measurement and reducing inspection time by 64%.

Numerical control technology and numerical control machine tools are the foundation of the modernization of China's manufacturing industry, and the numerical control system is the core key component of numerical control machine tool equipment. (Martinov et al., 2020) The development of both complements each other and is indispensable. In this paper, a composite energy field six-axis linkage machining CNC system is designed, combining the advantages of EDM technology with ultrasonic vibration drilling technology. Adopting the coaxial coupling scheme of EDM and vibration drilling aims to achieve efficient and precise machining of micro-holes in special materials without remelting layer, thermal affected zone, or micro-cracks.

2. The development of CNC system for the composite prcocessing equipment

In order to meet the openness and real-time requirements of the combined machining of EDM and ultrasonic vibration drilling, a six-axis linked CNC system for complex energy field machining based on Windows + RTX architecture was designed. This CNC system can ensure the efficient synergy of motion control, electrospindle motion control, ultrasonic vibration module control, and electrical discharge machining control.

2.1 General scheme of CNC system

The overall architectural form of the CNC system based on Windows + RTX architecture consists of four main parts: application layer, message passing layer, kernel layer, and hardware terminal.

First, the application layer is the topmost layer of the system, responsible for the user interface and the operation of the application programs. The user uses the graphical interface to learn how to program and manage CNC tasks. The application layer directly interacts with the user and is the human-computer interaction window of the CNC system, providing a friendly user experience and a convenient operating environment.

Secondly, the message-passing layer plays a bridge role between the application layer and the kernel layer. It is responsible for transferring information and instructions to ensure accurate data transmission and effective execution of system instructions. An efficient message-passing layer is crucial to the real-time and reliability of the system, which ensures that the commands from the application layer can reach the kernel layer quickly and accurately and that the feedback from the kernel layer can be sent back to the application layer promptly.

Third, the kernel layer is the core part of the system, running on the RTX real-time operating system. The kernel layer manages system resources, schedules tasks, processes real-time operations, and interacts directly with hardware terminals. The RTX architecture is a valid extension because it is not an encapsulation of Windows and does not affect or modify the underlying Windows architecture. Because of this separation, the RTX real-time system ensures that RTX-based applications are not affected by Windows crashes or blue screens.

Finally, at the core of the CNC system, the hardware terminal, with its array of physical components, including processors, memory, input/output devices, sensors, and actuators, plays a pivotal role. It directly executes the commands from the kernel layer, bringing the CNC machining operations to life.

The Windows + RTX architecture-based CNC system realizes efficient, stable, and real-time CNC machining operations through the collaborative work of these four layers. The system has a high degree of real-time stability and user-friendly operation experience, and it is suitable for various complex CNC machining tasks, as shown in Figure 1.

2.2 Efficient instruction set synchronized parsing CNC program interpreter design

CNC program interpreter converts the high-level CNC programming language into specific machine actions to control the precise operation of the machine tool. (Hatem et al., 2021) Based on the needs of composite machining, this paper customizes the development of G-code instruction parsers for EDM spindles and ultrasonic vibratory drilling spindles. The parser reads and parses the G-code commands, immediately generates the corresponding machine commands, and sends them to the machine control system, realizing the efficient connection and synergy of the control codes of EDM and ultrasonic vibratory drilling machining.

G code has both the language characteristics of ordinary programming and specialization. The interpretation and execution of G code is a complex and precise process. (Schmitt et al., 2024) Firstly, the compiler translates the G-code into the machine language that the machine tool can recognize; then, the interpreter controls the movement of the machine tool according to the machine language; in addition, the electrode loss compensation algorithm automatically compensates the electrode loss to ensure the machining accuracy; finally, the data management system records and analyzes the machining data to provide reference and optimization for subsequent machining experiments. The basic strategy of the G-code parsing is shown in Figure 2.

After execution begins, the program is deposited into the automatically created temporary file Tempfle (TemporaryFile), with the current line counter set to 0. The program writes some data to the temporary file and initializes the line counter to 0. The program checks to see if the file is empty. If it is, the program reports an error or an alarm indicating it is empty. If not, continue to the next step. Read One Line from Tempfile to Buffer: The program reads one data line from the temporary file into the buffer. The program scans the code in the buffer from left to right and compares it with a predefined keyword. The keyword is checked to see if it matches a standard name in an array or list. If it does not match, the program reports an error or an alarm indicating that the keyword does not match. If it matches, continue to the next step. The program calls the appropriate subroutine or function based on the matched keywords. Determine if the executable parsing is complete: The program checks to see if the executable parsing is complete. If so, end: the program ends. If not, the loop returns to reading the following line from Tempfile. By processing the code, checking for specific keywords, calling subroutines, or issuing alerts accordingly.

Because of the characteristics of CNC code, the design and development of code parsing structure type ISO_CODE _ENTRY:

• typedef struct {

• char ISO_CODE _NAME (Ding et al., 2017);

• int ISO_CODE _ID;

• int ISO_CODE _GROUP;

• int ISO_CODE _PARAM_TYPE;

• } ISO_CODE_ENTRY;

ISO_CODE _NAME stores the name of the code instruction; ISO_CODE _ID is used to identify the number of each instruction; ISO_CODE _GROUP is used to indicate the group to which the instruction belongs; and ISO_CODE _PARAM_TYPE characterizes the subsequent parameters.

Under the instruction set synchronous parsing method designed based on this paper, the full program pre-parsing before code processing is realized to ensure the correctness and completeness of the code program; the efficient information interaction between the application layer and the kernel layer during processing is realized to ensure the timely parsing and execution of the CNC program instructions. To help you visualize this, Figure 3 below shows the interface of code execution, providing a clear representation of the process.

3. The key tecniques of the composite processing's electrical system

3.1 Design of discharge control system based on SoC chip

For the EDM control parameters, EDM high real-time control, and intelligent adaptive adjustment needs, this topic uses Xilinx's new generation of fully programmable system-on-chip (APSoC) Zynq-7000 for EDM control. Zynq-700 is a single device that integrates a dual-core or single-core ARM Cortex-A9-based processing system (PS) and 28 nm Xilinx programmable logic (PL). The combination of the two can achieve a variety of functions. PS and PL can be independent of custom logic in the PL to implement custom software, and the combination of the two can achieve a variety of functions. The Zynq-700 integrates into a single device a dual-core or single-core ARM Cortex-A9-based processing system (PS) and a 28 nm Xilinx programmable logic (PL), which implements customized logic in the PL and customized software in the PS and combines the two to achieve a variety of functions. Both the PS and the PL can work independently, or an operating system can be run on the PS, e.g. Linux. Other standard operating systems are also available from Xilinx. (Tambara et al., 2015)

Based on the intelligent and efficient synergy and adaptive needs of discharge machining, the ARM core is responsible for processing the serial port data and the feedback signals from the detection circuit in the developed discharge control system. Adaptive control of the parameters in the discharge process is realized through the fuzzy adjustment algorithm integrated with the intelligent discharge parameters. The hardware circuit end control is realized, and the discharge parameters can be adjusted timely and efficiently according to the processing situation, environmental changes in the discharge processing composition, and adaptive processing state.

The intelligent discharge module, as shown in Figure 4, consists of communication module, pulse power control module, differential isolation module, and electrode voltage detection module.

Communication module: responsible for communicating with the PC, receiving discharge control parameters sent by the processing software, and sending the electrode voltage detected by the detection circuit board back to the processing software.

The pulse power control module, a key player, converts the processing parameters received from the communication module into control quantities. It is responsible for regulating the on/off of IGBT and other high-voltage switches, as well as the number of pulses of the center knife. These controls directly influence the advantages and disadvantages of the discharge state, making the module's role crucial in the intelligent discharge module's performance.

Differential isolation module: This module isolates the low-voltage control signal from the high-voltage control module to improve the anti-interference performance of data transmission.

The electrode Voltage Detection Module, with its ability to detect the voltage between two discharge electrodes and convert the analog voltage into a digital signal, is a reliable component for any electronic system. Three threshold comparison circuits and optoelectronic isolation circuits are embedded in the module, ensuring the highest level of safety. The threshold comparison signal, after optoelectronic isolation, is carefully input to the Soc chip, which plays a crucial role in controlling the triggering of the monostable main oscillator module, further enhancing the module's reliability.

The modules cooperate to ensure the stability and continuity of the processing state. In this system, an internal timer is used as the pulse square wave source, and its triggering condition is that the voltage between the two discharge electrodes reaches the set threshold. Once the voltage reaches the set threshold, a pulse trigger is performed. After triggering, the output of the monostable device will generate a square wave voltage waveform with a preset frequency, which is connected to the signal isolation device. The signal isolation device outputs a ±15 V control voltage, which is connected to the field effect tube (IGBT) that controls the switching of the discharge circuit, thus enabling the discharge circuit to output a square wave signal with the same frequency as the monostable circuit. The threshold voltage sets up a three-channel voltage detection circuit to detect the discharge voltage, the arc pulling voltage, and the short-circuit three kinds of inter-electrode states. The monostable device is triggered only under average discharge voltage. This design effectively reduces the occurrence of situations such as arc discharge and short-circuit discharge, thus improving processing efficiency.

Based on the anti-interference requirements of discharge control in a complex electromagnetic field environment, the communication between the Soc chip and the host selects RS422 level transmission. It uses the RS422 level conversion chip to convert the serial port output level of the Soc to RS422 level, ensuring that the link computer transmits the discharge state in real-time and dramatically reducing the discharge circuit's electromagnetic interference on the communication line.

3.2 Seamless circuit design for EDM and ultrasonic vibration machining

In order to realize the synergy and functional mutual exclusion of EDM and ultrasonic vibration drilling, and meet the requirements of automatic small-hole composite machining, the redundant function design method is adopted in this paper. This method combines the mutually exclusive and synergy redundant functions of software and hardware to ensure the stability and reliability of the machining process. The microcontroller STM32 is selected for the control system. STM32 microcontroller using the Modbus communication protocol to communicate with the computer host through the protocol data to determine the control GPIO output pins. Modbus communication protocol to communicate with the computer host, through the protocol data to determine the control of the GPIO output pins.

The software is executed in the CNC programming software to write the NG code to control the relay, and the Windows system sends MODBUS protocol data to the microcontroller through the serial port. After the microcontroller receives the Windows data, it controls the pin output at high and low levels according to the program settings.

Building on the previous setup, the processing mode is altered by writing the NC code. After the EDM processing, the program is executed, opening the two function modules of drilling and ultrasonic to initiate compound processing. The process concludes with the execution of the closing code, ensuring a seamless transition between the two modes.

The specific hardware control circuit principle is shown in Figure 5. STM32 microcontroller pin configuration for push-pull output, initialization configuration pin output low level. When the need to open, through the received instructions, will be the corresponding pin output for 3.3 V voltage high level, control transistor base voltage high and low, high-level state, transistor conduction, relay coil end of the ground, so that the relay coil has a current flow so that the normally open contacts by the electromagnet magnetic suction conduction so that the ultrasonic control terminal ends of the control voltage drop of 24 V control ultrasound to open. This method has a slight chance of error and good stability.

4. Design the mechanical system of the composite processing equipment

Aiming at the processing requirements of aviation turbine blades without remelting layer, micro-cracks and rear wall damage, the solution of coaxial coupling EDM-vibration drilling is adopted in this paper. This solution modularizes the ultrasonic vibratory drilling system for microscopic holes and integrates it into the host test platform of the composite machining machine. In addition, the spindle table is equipped with a high-precision double-indexing rotary table based on the machining requirements of small holes in complex positions in special materials. The architecture of the aerospace blade without a remelting layer of EDM ultrasonic drilling machining equipment is shown in Figure 6.

Based on the requirements of high surface quality and high precision preparation of air film holes in aerospace blades, the rotary axis movement using the indexing rotary table to achieve, machine tool in addition to the X, Y, and Z three linear axes, in addition to the installation of a hole machining particular linear axis W axis, responsible for air film hole machining of the electrode wire trajectory, and the separation of the axis of motion Z axis reduces the positioning accuracy of the error. The repeatability of the linear axes in this paper reaches ±4 µm. The EDM medium and the drilling coolant use deionized water, and the deionized water recycling system in this machining equipment realizes the self-circulation and continuous supply of the machining medium.

The micro-hole ultrasonic vibration drilling system consists of a drilling spindle, a frequency converter, an ultrasonic generator, and an amplitude-change rod. In the six-axis linkage EDM numerical control system, the drilling spindle control and ultrasonic parameter and drilling parameter control function and interface are added by the same control of the numerical control system and combined with the special instructions of the CNC system and the modular power supply system to expand the functions, to realize the fully automatic switching and seamless connection of the EDM micro-hole processing and high-speed micro-hole drilling processing, and to provide the conditions for the realization of no remelting layer blade processing. Ultrasonic vibration drilling can completely remove the surface remelting layer and improve the surface quality on the basis of realizing the micro-hole machining by EDM. The burring of drilled holes can be reduced or even avoided with ultrasonic vibration assistance.

5. Drilling test on the composite machining of the EDM and ultrasonic virbration drilling

Before machining, select the appropriate electrode wire for clamping according to the size of the target hole and the drill size with sufficient removal volume for clamping while checking the status of the center punch. The workpiece to be machined is then clamped and checked to make sure it is secure. The relative positions of the EDM spindle and the ultrasonic drilling spindle are corrected according to the standard ball placed on the table. The single hole without remelt layer processing is divided into two processes: (1) machining the EDM bottom hole on the workpiece; (2) ultrasonic-assisted hole reaming with a drilling tool in the same position; firstly, the surface of the hole wall is trimmed on the EDM bottom hole, and then the leading cutting edge of the tool is utilized to remove the remelt layer from the wall of the hole, to complete the ultrasonic drilling process of the EDM without remelt layer, and the machining process is shown in the schematic diagram of Figure 7.

Two 1.5 mm thick single crystal superalloys were used for comparative experiments. During the slant hole machining, the wire electrode forms a 30° angle with the workpiece surface, the average processing time is 55 s per millimeter, and the surface roughness after processing is less than 1.6 µm. Figure 8a shows the electron microscope display of the slanted hole entrance of the single crystal superalloy processed by EDM only, while Figure 8b shows the morphology of the small holes after composite processing. By comparing the two figures, it can be observed that EDM produces more solidified materials on the hole wall before composite machining. After ultrasonic vibration-assisted drilling, the quality of the small hole wall is significantly improved. To better observe the morphology of the hole wall after composite machining, we dissected the small holes along the axis after composite machining, as shown in Figure 8g. After a roughness comparison, it was found that the roughness of the composite-machined hole wall reached Ra = 0.8 μm. In addition, we polished and sanded the dissected holes and corrupted them with Kaller's solution. After etching, the matrix material was yellowish, while the remelted layer was whitish, so the remelted layer was also called the white layer. Figure 8c, 8d, 8e, and 8f shows a view of the wall of the hole after etching, and no residual remelted layer was found.

6. Conclusions

For the demand of no remelting layer, no heat-affected zone, and no microcrack machining of complex bit-positioned micro holes in special materials, the composite machining six-axis CNC system of EDM ultrasonic vibration machining, the discharge control module based on the SoC chip, the composite machining synergistic and mutual-exclusion circuits, the composite machining mechanical system, as well as the composite machining process technology of EDM ultrasonic vibratory drilling are developed, which achieves efficient precision machining of inclined holes in typical special materials superalloys. The research results show that EDM-ultrasonic vibratory drilling composite machining is feasible for manufacturing special materials without remelting layers, heat-affected zones, and microcracks.

Figures

Figure 1

Composite machining CNC system framework

Figure 2

Flowchart of CNC program analysis

Figure 3

Code execution within the interface

Figure 4

Intelligent discharge control module based on SoC chip

Figure 5

EDM ultrasonic drilling machining circuit control schematic diagram

Figure 6

EDM ultrasonic drilling machining equipment architecture

Figure 7

EDM ultrasonic drilling process without remelting layer

Figure 8

Experimental machining of holes