Study of electrical discharge machining technology for slicing silicon ingots

Silicon slicing technology is an undergoing process and its performance improvements meet the ever-challenging and versatile demands. A new attempt to apply the WEDM strategy to slice the semiconductor materials is studied. The barriers from unusual material characteristics are to be conquered to make this idea realizable. The existing WEDM technology is utilized to slice the heavy-doped silicon ingot and its feasibility is examined. The machining rate and surface roughness are measured under various current on times and servo voltages in both the water immersed and water flushing WEDM machines. If small current on time is collocated with proper off time and lower gap voltage sensitivity under automatic feed mode, the stable area machining rate of around 76 mm²/min can be attained, and the R_a value is 3.6 μm or so which is acceptable if the following polishing procedure is considered. The thickness of defects to be polished can be predicted from the SEM photographs of the cross-sections of the sliced wafers. If the wire diameter is 0.25 mm and the wafer thickness is 1 mm, the portion of material loss including the kerf and the amount to be polished is under 26%.     

Effect of metal coating on machinability of high purity germanium using wire electrical discharge machining

This paper reports on the research of wire electrical discharge machining (WEDM) as a cutting process for n-type high purity germanium (HP Ge). WEDM requires sufficient electrical conductivity of the work piece for discharges to occur. Owing to the very high material resistivity of HP Ge (32.8 Ω cm), the electrical conduction is too low for WEDM to be efficient. To temporarily enhance the conduction, metals (aluminum and nickel) were deposited on the HP Ge on 1 or 2 sides with various thicknesses (1.0, 2.0, and 3.0 μm) using sputter deposition. This shortens the path of conduction between the HP Ge and the WEDM ground and also serves to trigger the discharges. Machining experiments were performed to determine the correlation between the slicing rate and locally enhanced HP Ge through various discharge energies (potential voltage: 150, 200, 250 and 300 V and capacitance: 1, 3.3, 5.5, 9.9 and 21.4 nF). From the results, the obtained maximum slicing rate is 7.7 mm²/min for Al coating (2 sides, 1.0 μm thickness) at high energy (300 V, 21.4 nF), which is improved as much as 27 times over uncoated HP Ge. The fastest cutting without creating subsurface microcracks was measured as 1.12 mm²/min performed at 150 V and 9.9 nF. Additional slicing experiments at reverse polarity (positive wire and negative work piece, uncommon polarity for WEDM) were performed at 150 V and various capacitances. The experiment proved that there were rectifying contacts at the metal coating surface. It was found that under identical EDM settings, a faster slicing rate also showed a reduction in kerf size due to less lateral discharge energy. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) were used to investigate microcracks and to analyze surface impurities.     

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

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

电火花线切割加工铜钨合金切缝宽度及加工精度影响因素研究

铜钨合金是一种广泛应用于精密及难加工材料电火花成形加工的低损耗电极材料。在窄缝、清棱清角、高纵横比等结构的电极加工中,低速走丝电火花线切割加工较传统加工方法具有一定的优势。通过单因素实验研究了电火花线切割加工参数对铜钨合金粗加工切缝宽度、加工速度及表面粗糙度的影响规律,为实际加工提供理论指导。     

Material Removal Rate,Kerf, and Surface Roughness of Tungsten Carbide Machined with Wire Electrical Discharge Machining

In this article, the effects of varying seven different machining parameters in addition to varying the material thickness on the machining responses such as material removal rate, kerf, and surface roughness of tungsten carbide samples machined by wire electrical discharge machining (WEDM) were investigated. The design of experiments was based on a Taguchi orthogonal design with 8 control factors with three levels each, requiring a set of 27 experiments that were repeated three times. ANOVA was carried out after obtaining the responses to determine the significant factors. The work piece thickness was expected to have a major effect on the material removal rate but showed to be significant in the case of surface roughness only. Finally, optimization of the machining responses was carried out and models for the material removal rate, kerf, and surface roughness were created. The models were validated through confirmation experiments that showed significant improvements in machining performance for all investigated machining outcomes.     

Monitoring and self-learning fuzzy control for wire rupture prevention in wire electrical discharge machining

Wire breaking is a serious problem in the application of wire electrical discharge machining (WEDM). A WEDM sparking frequency monitoring and control system based on the characteristics of the voltage waveform of WEDM is developed. A new self-learning fuzzy controller is proposed to control the sparking frequency at a safe level by regulating the pulse off-time in real time for avoiding wire rupture and maintaining a state of high metal removal rate. The developed control strategy is tested under the conditions of cutting a workpiece with continuous sharp angles, a change in workpiece height during machining process, and machining with a high feed-rate. Experimental results show that this monitoring and control system can control the sparking frequency at a predetermined level without the risk of wire rupture.     

High efficiency slicing of low resistance silicon ingot by wire electrolytic-spark hybrid machining

As the semiconductor industry requires the cutting of silicon ingots into wafers, the slicing of large, ultra thin wafers is one of the main technologies to prevent wastage. Recently, apart from conventional inner diameter (ID) blade and multi-wire saw methods, wire electrical discharge machining (WEDM), which has no cutting force, has been introduced to this area and low resistance silicon may be sliced by WEDM. In this paper, a novel approach, based on wire electrolytic-spark slicing strategy using hybrid oil/aqueous electrolyte, combining electric discharge and anodic etching into a single process, is investigated experimentally. Some improvements, such as a new wire winding system, hybrid electrolyte and high efficiency pulse generator, have been adopted in a kind of high speed (HS)-WEDM machine. Experiments have been conducted to evaluate the machining rate, surface quality and wafer thickness of low resistance (0.5–3 Ω cm) mono-crystalline silicon. It has been demonstrated that with properly selected electrical parameters and electrolyte, a maximum machining rate of 600 mm²/min can be obtained and with a wafer thickness less than 120 μm. Furthermore, in comparison with WEDM, heat affected zone and harmful metal residues are considerably diminished, which provides significant theoretical and experimental support for future applications.     

太阳能硅片切割技术的研究

简述了太阳能硅片常用的切割方法及特点,指出电火花线切割由于其适合大厚度、超薄切割等特点,将成为太阳能硅片切割的一个研究热点.但因半导体特殊的电特性,目前常规电火花线切割工艺还不能胜任太阳能硅的切割.研究了太阳能硅材料的放电加工模型,提出了电火花线切割太阳能硅片的思路和工艺方法,并采用改进的脉冲电源首次对电阻率为2.1 Ω*cm的P型太阳能硅进行了切割试验,切割效率大于100 mm2/min.     

稳定电极丝空间形位的一种有效方法

高速走丝电火花线切割加工时电极丝的形位变化较大，严重影响线切割加工的稳定性和加工精度。如何稳定电极丝的空间形位，是高速走丝线切割加工亟待解决的课题。作者在大量试验研究的基础上，开发了一种高耐磨性的导向装置，能有效地稳定电极丝的空间形位，提高线切割加工稳定性和加工精度。     

Optimization of the WEDM process of particle-reinforced material with multiple performance characteristics using grey relational analysis

This paper presents an effective approach for the optimization of the wire electric discharge machining (WEDM) process of Al₂O₃ particle-reinforced material (6061 alloy) with multiple performance characteristics based on the grey relational analysis. The machining information for the difficult-cutting particle-reinforced material is inadequate and complicated. The response table and response graph for each level of the machining parameters are obtained form the grey relational grade, and select the optimal levels of machining parameters. In this study, the machining parameters namely the cutting radius of working piece, the on time of discharging, the off time of discharging, the arc on time of discharging, the arc off time of discharging, the servo voltage, the wire feed and water flow are optimized with considerations of multiple performance characteristics, such as the surface removal rate and the maximum surface roughness. It is clearly shown that the above performance characteristics in the WEDM process are great improved together through this approach.     

Experimental determination of convective heat transfer coefficient in WEDM

An experimental determination method of the convective heat transfer coefficient in wire electro-discharge machining (WEDM) is introduced. A special device is developed to measure the average temperature increment of the wire after a period of short circuit discharges, and the thermal load imposed on the wire is also tracked and recorded in advance. Then, based on the thermal model of the wire, the convective coefficient can be calculated accurately. Some tuning experiments are carried out inside and outside a previously cut profile to examine the influence of the kerf on the convective coefficient. As soon as the wire cuts into the workpiece, the convective coefficient will decrease more than 30%. With this method, the effect of the coolant flushing pressure on the convective coefficient is also estimated. If the pressure is raised from 0.1 to 0.8 Mpa, the convective coefficient will increase more than 20%, and thus ameliorate the cooling condition of the WEDM process.     

Sub-nanosecond monitoring of micro-hole electrical discharge machining pulses and modeling of discharge ringing

Monitoring the gap voltage and current in micro-hole electrical discharge machining (EDM) using high-speed data acquisition with 0.5 ns sampling period is conducted. The spark and arc pulses at three stages, namely electrode dressing, drilling, and penetration, of the micro-hole EDM are recorded. The EDM process parameters are setup to use negative polarity to blunt the electrode tip and positive polarity for micro-hole drilling and penetration. A new phenomenon of pre-discharging current is discovered. In the first 20–30 ns of spark and arc pulses, the current starts to rise while the voltage remains the same. Effects of EDM process parameters, including the open voltage, electrode diameter, and polarity, on the rate of spark and arc pulses and electrode feed rate are investigated. A model based on the RLC circuit is developed to study the ringing effect at the end of a discharge. The intrinsic parasitic capacitance and resistance of a RLC circuit are calculated from the decaying voltage signal and compared under two sets of experiments with varying wire electrode diameter and gap voltage to validate the ringing model. The calculation and experimental results validate the proposed RLC model for ringing phenomenon. The model shows the electrode diameter has negligible effect on ringing and high open voltage increases the parasitic resistance and damping in ringing. The monitoring technique and ringing model developed in this research can assist in the selection and optimization of micro-hole EDM process parameters.     

Influence of kinematic operating parameters on kerf geometry in abrasive waterjet machining of silicon carbide ceramics

Silicon carbide (SiC) is extensively used for manufacturing of highly engineered parts due to its high hardness, low coefficient of friction, wear resistance and high decomposition temperature. However, generating 3D surfaces (e.g. pockets) in such structural ceramics by conventional machining is a difficult task. In this context, abrasive waterjet (AWJ) machining, with its capability to cut any material with low specific cutting forces, seems to be the “ideal” processing technique for such materials; nevertheless machining 3D shapes by AWJ milling is still in its infancy. 3D shapes can be generated by “enveloping” them with successions of jet footprints (kerf geometries) generated by varying the process operating parameters. To enable this, the present work investigates the influence of key kinematic operating parameters (i.e. α-jet impingement angle and v-jet feed rate) on the kerf geometry and its dimensional characteristics. Furthermore, the kerf generation mechanism under multi-pass jet erosion was analysed to get control over erosion depth in multi-pass machining. It was found that by varying α (90°–40°), the symmetric/asymmetric kerf geometry is intimately dependent on the variation of standoff distance (SOD), abrasive particle velocity distributions and their local impact angles accounted across the jet footprint. Variation in v influences the exposure time of material to jet and enhances the erosion capability of abrasives impacting at shallow angle that results in different erosion rates along the kerf profile; this combined effect leads to departure of kerf geometry from simple cosine profile approximation to more elliptical type with the decrease of feed speed. Further, at lower jet feed rates, the depth of erosion increased and the low energy abrasive particles along trailing edge of jet plume get enough time to erode the material that results in variation of slope of kerf walls and hence, overall geometry. Based on these observations, the multi-pass trials showed that the successive passes have to account for both the local impact angles of abrasive particles as well as the actual SOD (SOD+initial kerf depth). In this way, by understanding the influence of key kinematic operating parameters (α and v) on the kerf geometry and its dimensional characteristics, the paper establishes a good basis for developing strategies for controlled 3D AWJ machining of complex shapes.     

State of the art in wire electrical discharge machining (WEDM)

Wire electrical discharge machining (WEDM) is a specialised thermal machining process capable of accurately machining parts with varying hardness or complex shapes, which have sharp edges that are very difficult to be machined by the main stream machining processes. This practical technology of the WEDM process is based on the conventional EDM sparking phenomenon utilising the widely accepted non-contact technique of material removal. Since the introduction of the process, WEDM has evolved from a simple means of making tools and dies to the best alternative of producing micro-scale parts with the highest degree of dimensional accuracy and surface finish quality.Over the years, the WEDM process has remained as a competitive and economical machining option fulfilling the demanding machining requirements imposed by the short product development cycles and the growing cost pressures. However, the risk of wire breakage and bending has undermined the full potential of the process drastically reducing the efficiency and accuracy of the WEDM operation. A significant amount of research has explored the different methodologies of achieving the ultimate WEDM goals of optimising the numerous process parameters analytically with the total elimination of the wire breakages thereby also improving the overall machining reliability.This paper reviews the vast array of research work carried out from the spin-off from the EDM process to the development of the WEDM. It reports on the WEDM research involving the optimisation of the process parameters surveying the influence of the various factors affecting the machining performance and productivity. The paper also highlights the adaptive monitoring and control of the process investigating the feasibility of the different control strategies of obtaining the optimal machining conditions. A wide range of WEDM industrial applications are reported together with the development of the hybrid machining processes. The final part of the paper discusses these developments and outlines the possible trends for future WEDM research.     

日本放电加工的新方法和新技术

综述了日本在放电加工领域的最新研究动态，包括成形放电加工、微细放电加工和线切割放电加工三部分内容。在成形放电加工方面，主要介绍一项新技术——气中放电高速三维空腔铣的原理、实验装置及加工效果；在微细放电加工方面，主要介绍一种新近开发的可同时加工微细轴和微孔的自动微孔加工装置和目前已加工出的微细轴的最小尺寸；在线切割放电加工方面，主要介绍了提高圆角加工精度的新方法、提高放电位置可控性的新方法和新的硅锭切片方法。     

电火花线切割加工Si3N4/TiN纳米复相陶瓷工艺规律的实验研究

分析研究了电火花线切割加工Si3N4/TiN纳米复相陶瓷材料时,影响加工速度和表面粗糙度值的主要因素,得出了各加工参数对加工工艺指标的影响规律,为加工时参数的选择提供了依据.     

电火花线切割伺服控制系统特性实验研究

为探究电火花线切割加工的伺服控制系统特性,在门槛电压阈值控制方法的基础上,基于中走丝线切割机床,搭建电火花线切割伺服控制系统平台。将放电时采集到的频率信号通过FPGA芯片进行分析处理,并根据处理后的信号进行伺服运动。探索了电火花线切割加工效率与伺服控制系统特性之间的关系,得出阈值和检测周期对加工效率的影响;通过验证实验以及分析放电状态占比,探索最优的阈值。结果表明:当检测周期为0.5 s时,加工效率较高;当加速区占15%、匀速区占62%时,加工效率较高。     

Design of a fuzzy controller for the adaptive control of WEDM process

This paper addresses the design of a fuzzy controller for the control of the wire electrical discharge machining (WEDM) process. The power consumption and short circuit ratio are chosen as control parameters. Incorporated with pulse trains analysis and experience, fuzzy rules for the control of the WEDM process are formulated. A DSP-based on-line pulse monitoring system is developed, and the fuzzy control system is implemented on a PC-486. In the developed control system, a reference power level is specified beforehand, and feed is adjusted according to the computed actual machining power. Off time is regulated automatically so that short circuit ratio will not exceed a specific value. Fine tuning of feed and pulse off-time is conducted if the variation in the average ignition delay time is too large. Experiments show that the developed control strategy results in very satisfactory transient and steady state responses as compared with a conventional control scheme.     

Thermo-mechanical analysis and optimal tension control of micro wire electrode

The tension control of the micro wire electrode is a key technology for the micro wire electro-discharge machining (WEDM). Based on the coupled thermo-mechanical analysis, both the three-dimensional temperature and the stress distribution in the micro wire are determined. As a result, the tension of the micro wire electrode during the WEDM process can be optimized in accordance with the discharge energy, which is sampled and fed back to the tension control system in real time. Then the development of an optimal tension control system characterized by the form of master–slaver structure makes it possible to keep the wire tension optimal in the process of WEDM. The results of the machining experiments show that the optimal wire tension control is effective on the improvement of the machining accuracy with the prevention of wire breakage for the micro WEDM.     

微磨料空气射流加工技术的发展

微磨料空气射流加工(MAJM)技术是对硬脆材料进行微细加工的一种非常有潜力的技术，特别是对复杂的三维微细结构的加工。微磨料空气射流加工技术是基于传统的喷砂技术发展起来的，通过由空气喷射磨料微粒形成高速气流冲击工件表面而去除工件材料。与其它的加工技术相比，微磨料空气射流加工具有环境友好、易于控制、无热影响区、切口质量好等优点。本文介绍了此技术的基本加工原理、特点以及加工过程中的影响因素，论述了微磨料空气射流加工的材料冲蚀机理和切口特征。重点分析了一些主要的加工参数，例如空气压力、磨料材料、尺寸、供给率、喷嘴的形状和尺寸、喷射距离以及工件材料，对切削性能和切口特征的影响。并提出了微磨料空气射流加工技术中有待进一步深入开展的研究工作。