Compound and Hybrid Micromachining Processes

11.05

Compound and Hybrid Micromachining Processes

M Rahman, ABMA Asad, and YS Wong, National University of Singapore, Singapore MP Jahan, Western Kentucky University, Bowling Green, KY, USA T Masaki, Masaki Giken, Osaka, Japan Ó 2014 Elsevier Ltd. All rights reserved.

11.05.1 11.05.2 11.05.3 11.05.4 11.05.4.1 11.05.4.2 11.05.4.3 11.05.4.4 11.05.4.5 11.05.4.6 11.05.4.7 11.05.4.8 11.05.4.9 11.05.5 11.05.5.1 11.05.5.2 11.05.5.3 11.05.5.4 11.05.5.5 11.05.5.6 11.05.5.7 11.05.5.8 11.05.5.9 11.05.5.10 11.05.6 References

11.05.1

Introduction Characteristics of the Compound and Hybrid Micromachining Processes Requirements of Compound and Hybrid Processes Compound Micromachining Processes and Related Applications Micro-EDM and Microturning Micro-EDM and Microgrinding Micro-EDM and Micromilling Micro-EDM and Micro-ECM X-ray Lithography, Electroplating, and Molding (LIGA) and Micro-EDM Sequential Laser and Micro-EDM Drilling Micro-EDM and Laser Welding Sequential Laser and Mechanical Microdrilling Combined Micro-EDM Milling and Laser Ablation Process Hybrid Micromachining Processes and Related Applications Combined Microgrinding and Microelectrochemical Machining Micro-EDM and Micro-USM Combined Process Vibration-Assisted Micro-EDM Powder-Mixed Micro-EDM Microelectrochemical Discharge Machining (Micro-ECDM) Magnetic-Assisted Micro-EDM Laser-Assisted Microturning Laser-Assisted Micromilling Laser-Assisted Microgrinding Laser Microdrilling with Jet Electrochemical Machining Summary

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Introduction

The trend in miniaturization of products is pervasive in areas such as information technology, biotechnology, environmental technology, and the medical industries (1). Micromachining is the key supporting technology that has to be developed to meet the challenges posed by the requirements of product miniaturization. However, in many cases, a single micromachining process cannot fulfill all of the requirements, due to the limitation of that process. For example, the machining of difficult-to-cut materials, at both the macro- and microscales, has become a challenge in the manufacturing industries. The conventional micromachining processes (e.g., microturning, micromilling, and microgrinding) cannot machine these materials easily due to their extreme hardness, brittleness, and toughness. However, several nonconventional machining processes (e.g., microelectrodischarge machining (micro-EDM), microelectrochemical machining (micro-ECM), and laser micromachining) are found to be capable of machining difficult-to-cut materials irrespective of their hardness. However, these nonconventional machining processes also have several disadvantages, such as lower machining speed, a heat-affected zone (HAZ), higher tool wear, and poor surface finish. Therefore, the development of micromachining processes combining nonconventional micromachining with conventional micromachining is of prime importance. In recent years, compound and hybrid micromachining has become the most promising technology for the production of miniaturized parts and components. This technology is becoming increasingly more important and popular because of a growing demand for industrial products, with an increased number not only of functions but also of reduced dimensions, higher dimensional accuracy, and better surface finish. Compound and hybrid machining is the combination of processes and/or machines to produce parts in a more efficient and productive way (2). Although the terms compound micromachining and hybrid micromachining are often used as if they have the same meaning, there are differences in the meanings of the two processes. Compound machining is defined as the combination of two different machining processes in a single setup applied one after another; the hybrid machining process is defined as the integrated application or combination of different physically active

Comprehensive Materials Processing, Volume 11

http://dx.doi.org/10.1016/B978-0-08-096532-1.01105-5

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principles in a single process. In a CIRP (College International pour la Recherche en Productique, or International Academy for Production Engineering) keynote presentation, the hybrid manufacturing processes are defined as follows: “Hybrid manufacturing processes are based on the simultaneous and controlled interaction of process mechanisms and/or energy sources/tools having a significant effect on the process performance” (2). The hybrid material removal processes can machine microfeatures on a wide range of materials and machined shapes, so that flat surfaces, arbitrary curvatures, and long shafts can be machined, which are required for moving parts and guiding structures (3). The hybrid micromachining techniques are able to fabricate microparts and components, with potential applications in the electronics, optics, biotechnology, automotive, communications, and avionics industries.

11.05.2

Characteristics of the Compound and Hybrid Micromachining Processes

The development of compound and hybrid micromachining processes by integrating conventional and nonconventional processes on the same machine has several advantages over a single process. Some of the important objectives of developing compound and hybrid micromachining processes are as follows (2): l

To machine materials that could not be machined earlier Lower process forces l Less tool wear l Higher productivity l Required shapes of the microfeatures l

Some of the important characteristics of the compound and hybrid machining processes are as follows (2): l

Sequential or simultaneous and controlled interaction Interaction that is more or less in the same machining zone and at the same time l Interaction of process mechanisms, energy sources, and tools l Significant effect on the process performance l Better material removal, lower process forces, less tool wear, improved surface finish, and overall improved performance l

11.05.3

Requirements of Compound and Hybrid Processes

In order to achieve effective implementation of compound and hybrid micromachining techniques, four important areas need to be addressed (1): l

Development of a machine tool capable of both conventional and nonconventional micromachining Motion and process control l Process development to achieve the necessary accuracy and quality l On-machine measurement and inspection l

Figure 1 shows the technologies required for successful development of compound and hybrid micromachining processes. One of the main difficulties in compound micromachining is the availability of an appropriate machine tool that can be used for the development of compound and hybrid micromachining processes. Most machine tools capable of nonconventional machining are not designed to perform conventional machining processes. Furthermore, most of the machine tools do not facilitate the measurement of fabricated products on machine, which has the potential to be used as feedback and to compensate tool trajectory

Figure 1

Technologies required for compound and hybrid micromachining.

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online. Another constraint is imposed by the precision required for such fabrication, which most conventional computer numerical control (CNC) machine tools cannot fulfill. Mechanical and thermal deformation, chatter vibration, tooling, and its clamping usually constrain making small, superprecise parts with large, conventional machines. Ultraprecision machines that provide a high degree of motion accuracy are extremely expensive and mostly do not include facilities for compound processes. Therefore, the first and the most important requirement for the development of compound and hybrid machining processes is the development of a multipurpose machine tool or the integration of facilities for performing two or more manufacturing processes in one single setup and platform. Attempts were made to perform compound micromachining by modifying a machine tool good for one process as an adaptation for the supportive compound process (e.g., modification of a micro-EDM machine for micromilling) (4–6). Although the capability of compound and hybrid micromachining was demonstrated, the kinds of machines used in these processes encounter performance difficulties in the full scale due to the setup being too weak to support all the processes equally. Moreover, the conversion of a machine tool, which was developed for a dedicated process, to perform another machining process requires a considerable amount of effort and investment. Therefore, for successful compound and hybrid micromachining processes, there is a need for a unique dedicated platform to be called a universal miniature machine tool for performing multiple processes at the lower boundary of the micromachining domain readily on a single platform. This would ensure that the underlying equipment hardware is capable of benefitting the multipleprocess needs for execution and realization of the art of micromachining. On a miniaturized machine tool, the thermal expansion can be minimized, and advanced sensors and instruments can be employed to compensate for positioning errors. In addition to significant contributions in accuracy, it would also enable tool-based micromachining as a completely feasible option. Besides, it would also speed up the integrated fabrication process by saving the reclamping time and relieving operators from the handling needs during the interprocess transfer of such miniaturized components. Figure 2 shows an example of such a multipurpose machine tool developed for compound and hybrid micromachining processes (1). With the development of a precision mechanical structure, a specialized precision motion control system is another major requirement for compound and hybrid micromachining processes. For conventional micromachining, the CNC system is expected to provide the function of synchronized servo feed control based on real-time monitoring of process control parameters, which are sometimes difficult to integrate into the nonconventional machine tool. For example, the accuracy and speed of micro-EDM, a nonconventional process, depend a great deal on the gap control performance of the motion controller. This might be sometimes very difficult to implement with a commercially available motion controller. Similarly, the motion required for micro-EDM

Figure 2 An example of a multipurpose machine tool developed for compound and hybrid micromachining (reproduced from Rahman, M.; Asad, A. B. M. A.; Masaki, T.; Saleh, T.; Wong, Y. S.; Kumar, A. S. A Multiprocess Machine Tool for Compound Micromachining. Int. J. Mach. Tools Manuf. 2010, 50 (4), 344–356). The machine is capable of performing conventional machining (e.g., microturning, micromilling, and microgrinding) and nonconventional machining (e.g., micro-EDM, micro-ECM, and micro-wire electrodischarge machining (micro-WEDM)) in a single setup.

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milling can be another example of a special trajectory requirement. An open-architecture motion controller can be adopted for compound micromachining, which can be programmed to provide necessary trajectory control of the machine tool required for the nonconventional machining. The open-architecture motion controller has a multiprocessor-based design with a universal asynchronous receiver–transmitter (UART)-based bus network communication to distribute processing tasks between multiple processors. Thus, programs and scheduling can be developed for real-time process control accordingly to meet the synchronized servo motion required for some of the micromachining processes. The motion controller can execute a program downloaded from the host computer independently using high-speed communication, which creates a very user-friendly environment for the operator on a standard PC. Research work needs to be undertaken toward the understanding of process physics to provide relevant background data for modeling, measurement, identification of control parameters, and application of feedback control for successful implementation of compound and hybrid manufacturing processes. The difficulties of microfabrication using available techniques need to be realized, and then compound processes need to be proposed to complement the weaknesses of different processes. Finally, an on-machine measurement system needs to be developed for on-machine inspection of machined features by one process before further machining operations by other processes.

11.05.4

Compound Micromachining Processes and Related Applications

11.05.4.1 Micro-EDM and Microturning Microturning has the capability to produce three-dimensional (3D) structures on a microscale. The major drawback of the microturning process is the limit of machinable sizes and the fact that the cutting forces influence machining accuracy (7). It is very difficult to achieve straight shaft below 100 mm diameter, and in many cases the tool either breaks or starts to wobble due to excessive radial cutting force on the microshaft. Therefore, a compound process has been developed where the commercial cutting tool is modified using the micro-EDM process to reduce the force component responsible for breaking of the shaft (7,8). This compound process is the combination of micro-EDM and microturning in a single setup. First, a commercially available polycrystalline diamond (PCD) tool is modified by the micro-EDG process to reduce the nose radius of the cutting tool, thus minimizing the force component that causes shaft deflection during microturning. Commercially available PCD inserts, designed for a light finishing cut, have a relatively large tool nose radius (e.g., 100 mm). This tool nose resolves the cutting force on the shaft into two components, namely Fx and Fy, as can be seen in Figure 3(a) (8). The Fy component of the cutting force does the actual cutting, while the Fx component causes deflection of the microshaft. A commercially available PCD insert is modified using the micro-EDG process to achieve a very sharp cutting edge, so as to reduce the Fx component of the cutting force significantly. This modification of the cutting tool makes it possible to achieve a straight shaft of much smaller diameter. The compound process combining micro-EDG and microturning is presented schematically in Figure 3(b) (8). A dual-cutter setup is arranged for microturning, one with a round nose for initial turning up to 100 mm, then a sharp tool for a final cut up to 20 mm. After the microturning process, the fabricated microelectrodes are used in machining of small and higher aspect ratio microholes by micro-EDM on the same machine (Figure 1(b)). Therefore, this compound process is in fact a combination of three steps: modifying the cutting tool using micro-EDG, fabricating microshafts using microturning, and applying fabricated shafts in microEDM drilling. Figure 4 illustrates the concept of the microturning–micro-EDM compound machining process. An electrode of required dimension is first fabricated by microturning prior to micro-EDM (7). Using this compound process, clamping error can be avoided, and deflection of electrode can be minimized; consequently, the accuracy of machining can be improved. The fabricated microelectrode and machined microholes using the microelectrode are presented in Figure 5(a) and 5(b), respectively (1).

Figure 3 (a) Modification of a conventional cutting tool using the micro-EDG (variant of micro-EDM) process; and (b) a schematic representing the compound process combining modification of a cutting tool by micro-EDM and turning of a microshaft by a modified tool tip. Reproduced from Asad, A. B. M. A.; Masaki, T.; Rahman, M.; Lim, H. S.; Wong, Y. S. Tool-based Micro-machining. J. Mater. Process. Technol. 2007, 192–193, 204–211.

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Figure 4 Compound process of fabricating a microelectrode using microturning and applying a fabricated microelectrode in the micro-EDM drilling. Reproduced from Lim, H. S.; Kumar, A. S.; Rahman, M. Improvement of Form Accuracy in Hybrid Machining of Microstructure. J. Electron. Mater. 2002, 31 (10), 1032–1038.

Figure 5 (a) A 19 mm graphite electrode of 0.5 mm length fabricated by a micro-EDG–microturning compound process; and (b) fabricated microholes with the microelectrode obtained by the microturning–micro-EDM drilling compound process. Reproduced from Rahman, M.; Asad, A. B. M. A.; Masaki, T.; Saleh, T.; Wong, Y. S.; Kumar, A. S. A Multiprocess Machine Tool for Compound Micromachining. Int. J. Mach. Tools Manuf. 2010, 50 (4), 344–356.

11.05.4.2 Micro-EDM and Microgrinding The fabrication of a miniaturized (sub-100 mm) grinding tool for difficult-to-cut materials like PCD and tungsten carbide (WC) is necessary for machining of microchannels with improved surface finish. The micro-EDM process is found to be capable of machining any difficult-to-cut materials down to the desired dimension. Therefore, a compound process is developed to solve the issues by combining the micro-EDM process with the microgrinding process. In this compound machining process, a PCD tool is fabricated on a machine in a desired shape using the block micro-EDG process. The PCD tool contains randomly distributed protrusions of diamond particles with dimensions around 1 mm that serve as the cutting edges for micromachining on glass. When the dimension of the PCD tool is reduced to the required dimension of the grinding tool by the micro-EDG process, the binder materials (usually nickel or WC) are removed because they are conductive, thus protruding the diamond particles, which are nonconductive. PCD with a cobalt binder, which can be shaped with micro-EDG, is emerging as a tool material for microgrinding of hard and brittle materials. The cobalt binder provides an electrically conductive network that can be removed with EDM (5). The diamond cutting edges are exposed as the discharges erode away the cobalt binder. In addition to microgrinding, reaming of microholes, grinding of microslots, and machining of V-grooves with a fabricated PCD tool have been reported (9). Figure 6 shows the different steps of the micro-EDM–microgrinding compound process with a machining example in BK-7 glass (8). As can be seen from Figure 6(c) and 6(d), the fabricated slot has a very fine and smooth surface, which is comparable to the surface obtained from ductile mode cutting of glass in macroscale. Figure 7 shows the same compound process for machining slots and microfeatures in ultralow-expansion (ULE) glass (5). However, the grinding tool has been fabricated by the wire electrodischarge grinding (WEDG) process instead of the block micro-EDG process used in Figure 6.

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Figure 6 (a) A schematic diagram showing the block micro-EDG process (a variant of micro-EDM); (b) a PCD tool before the micro-EDG process; (c) fabrication of a microgrinding tool with the micro-EDG process; (d) microchannels on glass machined by the microgrinding process with a fabricated PCD tool; and (e) surface finish of the microchannel in glass. Reproduced from Asad, A. B. M. A.; Masaki, T.; Rahman, M.; Lim, H. S.; Wong, Y. S. Toolbased Micro-machining. J. Mater. Process. Technol. 2007, 192–193, 204–211.

Figure 7 (a) A PCD scratch tool produced with the WEDG process; (b) a scratch in ULE glass produced with the tool shown in (a); (c) a cylindrical 50 mm PCD tool used to cut pockets in ULE glass; and (d) a slot ground in ULE glass using the tool shown in (c). Reproduced from Morgan, C. J.; Vallance, R. R.; Marsh, E. R. Micro-machining and Micro-grinding with Tools Fabricated by Micro Electro-discharge Machining. Int. J. Nanomanuf. 2006, 1 (2), 242–258.

11.05.4.3 Micro-EDM and Micromilling This compound process is a combination of the micro-WEDG process and micromilling. In this process, first the micromilling tool is fabricated out of the difficult-to-cut tool materials with the help of the micro-WEDG process. After that, the fabricated cutting tool is used for a conventional micromilling operation in the same setup. The use of the micro-WEDG for the production of milling tools has several advantages. The geometry can be changed quite easily, and the potential of scaling down the size of the milling tools is very high (6). In comparison to other contactless machining technologies, micro-EDM has an acceptable machining time, and the resulting costs for the machining are tolerable. An advantage of using micro-EDM with the milling process is the prevention of inaccuracy by rechucking processes (6). Figure 8(a) shows the schematic diagram of the micro-WEDG process of fabricating a milling cutting tool. Figure 8(b) and 8(c) shows the fabricated cutting tool and machined slot, respectively (6). Figure 9(a) shows a scanning electron micrograph (SEM) of a 100 mm diameter WC microtool (5). The fabricated microslot and surface roughness of the microslot after milling are shown in Figure 9(b) and 9(c), respectively (5). WC was chosen as the tool material in this compound process because of its high hardness and low wear rate. Three-fourths of the cylinder was removed to provide a single cutting edge, and then a 45 slice was also removed from the nose of the tool to provide clearance for various micromilling applications. Microtools fabricated by WEDG have been used to remove material by mechanical cutting, rather than with electrical discharges, to achieve better surface finishes and a higher material removal rate (MRR).

11.05.4.4 Micro-EDM and Micro-ECM This compound process is the combination of micro-EDM and micro-ECM processes in a single setup. The objective is to improve the surface finish generated by micro-EDM using micro-ECM as a postprocessing step. The surface machined by micro-EDM is

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Figure 8 (a) A schematic diagram of the micro-WEDG process used to fabricate a micromilling tool; (b) a micromilling tool fabricated by micro-WEDG; and (c) a microslot machined by micromilling using a fabricated microtool. Reproduced from Fleischer, J.; Masuzawa, T.; Schmidt, J.; Knoll, M. New Applications for Micro-EDM. J. Mater. Process. Technol. 2004, 149 (249), 246.

Figure 9 (a) WC microtool by micro-WEDG of 100 mm diameter; (b) a micrograph of a square groove machined in AA3003 aluminum using the fabricated microtool in (a); and (c) Ra of 121 nm on the bottom of the groove. Reproduced from Morgan, C. J.; Vallance, R. R.; Marsh, E. R. Micromachining and Micro-grinding with Tools Fabricated by Micro Electro-discharge Machining. Int. J. Nanomanuf. 2006, 1 (2), 242–258.

relatively rough due to microcraters and microcracks produced by the microdischarges. Hence, the process consisting of microEDM followed by micro-ECM can be a suitable solution to improve the machined surface (10). The deionized water used in the micro-EDM process can serve as an electrolyte medium for micro-ECM at certain machining conditions (11). The surface after applying micro-ECM becomes much smoother compared to that of micro-EDM, and the peak-to-valley distances of craters (Rmax) reduce significantly. Micro-ECM can also be applied for finishing the slot machined by micro-EDM milling. In addition to a sequential micro-EDM and micro-ECM process, a combined or concurrent micro-EDM and micro-ECM process has also been reported (11,12) that could be considered as a hybrid process. The difference is that in a hybrid process, the discharging of dissolution takes place in the same cycle during machining, thus applying micro-EDM and micro-ECM concurrently. Figure 10 shows the steps of the micro-EDM–micro-ECM compound process (11) and the change in gap and current during the EDM and ECM processes of the compound system (10). The improvements of surface finish with both the compound and hybrid processes are presented in Figure 11 (12).

(a)

(b)

(c)

Figure 10 (a) A schematic diagram showing a micro-EDM and micro-ECM compound or hybrid process (reproduced from Nguyen, M. D.; Rahman, M.; Wong, Y. S. Simultaneous Micro-EDM and Micro-ECM in Low-resistivity Deionized Water. Int. J. Mach. Tools Manuf. 2012, 54–55, 55–65); (b) the variation of gap distance with time for micro-EDM and micro-ECM in the compound process; and (c) the change of current due to discharge and dissolution in two stages of EDM and ECM. Reproduced from Campana, S.; Miyazawa, S. Micro-EDM and ECM in DI Water. In Proceedings of Annual Meeting of American Society of Precision Engineering (ASPE); 1999.

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Figure 11 (a) Surface generated by micro-EDM; (b) surface generated by micro-ECM followed by micro-EDM (a compound process); (c) surface generated by the micro-EDM milling process; and (d) surface generated by the micro-EDM and micro-ECM combined milling process. Reproduced from Zeng, Z.; Wang, Y.; Wang, Z.; Shan, D.; He, X. A Study of Micro-EDM and Micro-ECM Combined Milling for 3D Metallic Micro-structures. Precis. Eng. 2012, 36 (3), 500–509.

11.05.4.5 X-ray Lithography, Electroplating, and Molding (LIGA) and Micro-EDM Despite its excellent capability in terms of precision, surface quality, and complex 3D formation, micro-EDM has not achieved widespread use in product manufacturing primarily because of its productivity drawbacks (13). For example, in order to machine arrays of microholes, a single electrode is used to machine one hole each time. However, if the numbers of holes are even higher, then the electrodes need to be changed in the middle of the machining process due to the electrode wear. On the other hand, photolithographic methods offer various paths to the fabrication of such arrays with arbitrary patterns on a substrate. The arrays of electrodes fabricated by the photolithography process are precisely arranged on the substrate and have high structural uniformity across the arrays, offering high precision and uniformity in the machined products. Moreover, only one electrode is used for machining one hole, in contrast to the conventional serial-processing method of micro-EDM (13). Therefore, a compound micromachining process has been developed that combines micro-EDM and LIGA (from the German “Lithographie, Galvanoformung, Abformung”). In this process, the LIGA process fabricates arrays of microelectrodes, and then those microelectrodes are applied by machining high-aspect-ratio microholes or microstructures using micro-EDM. The LIGA process uses X-ray lithography to form high-aspect-ratio molds for electroplated structures. Micro-EDM produces 3D microstructures in any electrically conductive materials. The steps of the LIGA process are presented in Figure 12 (13). In this compound process, first an array of negative-type electrodes was fabricated in nickel using the LIGA process. After that, a positive-type patterned structure is produced by micro-EDM using the arrays of microelectrodes. Figure 13(a) shows an example of a 20  20 array of high-aspect-ratio electrodes of electroplated copper with a 20 mm diameter, 60 mm pitch, and 300 mm structural height. The LIGA-fabricated array of 400 Cu electrodes with 20 mm diameter was used to machine through-holes in 50 mm thick stainless steel using micro-EDM as shown in Figure 13(b) (13). The machining time was about 5 min, which is 600 times less than that required for serial machining by a single electrode. Arrayed electrodes of even complicated cross-section shapes, like hexagonal and gear shapes, can be fabricated using LIGA with a high aspect ratio.

Figure 12 Schematic representation showing the step-by-step mechanism of the LIGA–micro-EDM compound process. Reproduced from Takahata, K.; Gianchandani, Y. B. Batch Mode Micro-electro-discharge Machining. IEEE/ASME J. Microelectromech. Syst. 2002, 11, 102–110.

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Figure 13 (a) A 20  20 array of LIGA-fabricated copper electrodes; (b) through-holes batch machined in 50 mm thick stainless steel by the micro-EDM process using the array electrodes shown in (a). Reproduced from Takahata, K.; Gianchandani, Y. B. Batch Mode Micro-electro-discharge Machining. IEEE/ASME J. Microelectromech. Syst. 2002, 11, 102–110.

11.05.4.6 Sequential Laser and Micro-EDM Drilling The current practice of using micro-EDM drilling for fabricating fuel injection nozzles is limited in terms of the hole size that it can produce effectively and the length of time needed to drill. In addition, the tooling cost is also high (14). Attempts have been made by a number of research groups and companies to use nanosecond pulsed laser drilling instead of EDM. However, the hole quality cannot meet the industrial standard, as laser drilling typically produces larger recast layers and HAZs (15). Therefore, a compound micromachining process combining laser machining and micro-EDM drilling one after another has been developed (16). In this technique, pilot holes are first drilled by a nanosecond pulsed laser beam. Micro-EDM drilling then finishes the holes. The reason for using laser machining for drilling pilot holes has two advantages. First, it will enhance the machining speed by reducing the overall machining time, as laser machining is faster than the micro-EDM process. Second, the recast layer formation is higher in laser machining, and hence it should not be used as the final process. Moreover, finish machining through micro-EDM could reduce the HAZ generated by laser machining. This hybrid process can be applied for faster machining of high-aspect-ratio microholes in difficult-to-cut materials with the same dimensional accuracy and surface finish obtained by the micro-EDM drilling process. Using this hybrid process, the cycle time of EDM could be reduced from 40 to 20 s by using a laser-drilled pilot hole. The initial laser hole was approximately 60 mm in diameter, and the final EDM hole was 140 mm in diameter. It was found that this hybrid process has eliminated the problems of recast and HAZs that are typically associated with the laser drilling process. The new process has enabled a 70% reduction in total drilling time compared to standard EDM drilling, as less material is removed by the EDM. The quality of the holes is as good as with direct EDM drilling, thus eliminating the need for recertification of the drilling process. The microholes fabricated using this compound process can be used for the fuel injection nozzle, giving the minimum total drilling time and the best quality holes. The technique has enabled valuable cost savings and increases in production capacity for next-generation fuel injection nozzle manufacture. Figure 14 shows a schematic presentation of the sequential process and inner surface of the micro-EDM after laser machining and the compound process (16). The improvement in surface finish after implementing the compound process is seen in Figure 14(c).

11.05.4.7 Micro-EDM and Laser Welding Microassembly is a feasible method to make 3D micrometal parts. However, the first challenge is how precisely to position the microparts assembled. Online assembly may be one possible solution. The micro-EDM process can fabricate microparts and components with desired accuracy. Also, laser welding is a very effective technique of microassembly. Therefore, for the online assembly of parts, it is possible to integrate two processes in a single setup. A novel process has been developed to solve the issues by combining the micro-EDM and Nd-YAG laser welding workstation in a single setup (17–19). The micro-EDM process fabricates the assembled parts, and the Nd-YAG laser performs microjoining to produce the assembly, so the whole processes from microfabrication to microassembly can be completed on the same system. The Nd-YAG laser possesses the advantageous features of heating concentration and rapid cooling and is well suited to microwelding (17). This can overcome the problem of smalldimensional assembly. The system, having precision stages and ultraprecision motion control technology, can control the positional accuracy precisely between the assembled parts. In this compound process, parts can be machined and fuse-welded in an online process, and, at the same time, precise positioning can be attained by controlling the ultraprecise motion of stages on the machine. Furthermore, the fusion-welded process can attain sufficient bonding strength. A laser can also be utilized to separate the assembled parts, so some redundant forces can be avoided during the separation process. Therefore, an adequately high aspect ratio can be easily attained. Figure 15 shows the steps of the laser–micro-EDM combined process (18). Figure 16 shows examples of assembled microfeature parts using this compound process (17,19).

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Figure 14 (a) Schematic representation of the concept of a sequential laser and micro-EDM process; (b) the inner surface of the microhole generated after the pilot hole by laser machining; and (c) the inner surface of the microhole after sequential a laser and micro-EDM process. Reproduced from Lin, L.; Diver, C.; Atkinson, J.; Giedl-Wagner, R.; Helm, H. J. Sequential Laser and EDM Micro-drilling for Next Generation Fuel Injection Nozzle Manufacture. Ann. CIRP 2006, 55 (1), 179–182.

Figure 15 The process of pin–plate microassembly. (a) Making the pin with WEDG; (b) the pin held by the spindle for micro-EDM; (c) making a hole under micro-EDM; (d) reworking the pin end to eliminate wear during hole machining; (e) the first spot at point ‘a’; and (f) fusing and separating the pin from the back via only one laser beam emission. Reproduced from Kuo, C.-L.; Huang, J.-D.; Liang, H.-Y. Precise Micro-assembly through an Integration of Micro-EDM and Nd-YAG. Int. J. Adv. Manuf. Technol. 2002, 20, 454–458.

Figure 16 (a) Pin–plate microassembly (microjoining of two microrods that has been fabricated by micro-WEDG) (reproduced from Huang, J.-D.; Kuo, C.-L. Pin-plate Micro Assembly by Integrating Micro-EDM and Nd-YAG Laser. Int. J. Mach. Tools Manuf. 2002, 42, 1455–1464); and (b) a microscopic view of a tungsten pin assembled into an SUS 304 plate. Reproduced from Kuo, C.-L.; Huang, J.-D.; Liang, H.-Y. Fabrication of 3D Metal Microstructures Using a Hybrid Process of Micro-EDM and Laser Assembly. Int. J. Adv. Manuf. Technol. 2003, 21, 796–800.

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11.05.4.8 Sequential Laser and Mechanical Microdrilling Laser percussion drilling is inherently associated with poor geometry and thermal defects. The challenge for laser drilling is the control of drilling quality in terms of minimizing the recast layer and HAZ (20). In addition, drilling hollow parts such as airfoil blades and fuel injector nozzles without damage to the back wall is one of the biggest challenges in laser drilling. Although mechanical microdrilling produces good-quality holes, premature drill breakage and drilling holes at acute angles are the areas of major concern. During the drilling of microholes at an acute angle, the microdrill tip is subjected to high lateral force at the contact point with the workpiece surface. This lateral force makes the tip diverge from the required position and causes the microdrill to bend and eventually break the tool. In addition, burr formation in mechanical drilling is another unavoidable issue, which affects workpiece accuracy and quality (21). Therefore, a compound process combining both laser drilling and mechanical microdrilling has been developed in order to reduce the challenges with both the processes (22). The compound process can overcome the limitations of tip divergence and low tool stiffness in pure mechanical microdrilling, especially for drilling at acute angles. It also can solve the issues of poor geometry, HAZs, recast layer formation, and back-wall damage problems associated with laser microdrilling. In this compound process, a laser beam first drills a pilot hole, then an end mill is used to machine the diffuser portion of the hole and provide a flat surface for the drill entrance side. Micromechanical drilling then finishes the holes. During the machining of the pilot hole, the laser beam was focused to a beam diameter of 240 mm at the workpiece surface. In order to drill inclined holes at an inclination angle of 30 to the surface, the nozzle and the axis of the laser beam were inclined, while the workpiece remained in a horizontal position. For subsequent drilling, the centers of the two holes were accurately aligned. The workpiece was mounted on a fixed jig on the machine, and then four alignment holes were drilled on the workpiece corners. The workpiece was removed and fixed on the laser machine CNC table. A visible laser guide was then used to locate the four holes to be the reference points for the CNC table. A matrix of laser holes was drilled in the workpiece. After that, the workpiece was mounted on the same place on the fixed jig on a drilling machine for the mechanical drilling process. The recorded eccentricity between the laser hole and the drill was less than 10 mm. Figure 17 clearly shows the reduction of the HAZ and improvement of surface finish from the single laser drilling process to the compound process (22). The improvement of tool life in the compound process can be realized from Figure 18 (22).

Figure 17 Comparison of HAZ and burr formation during the machining of an inclined hole using (a) laser drilling; (b) mechanical microdrilling; and (c) sequential laser and mechanical microdrilling. Reproduced from Okasha, M. M.; Mativenga, P. T.; Driver, N.; Li, L. Sequential Laser and Mechanical Micro-drilling of Ni Superalloy for Aerospace Application. CIRP Ann. – Manuf. Technol. 2010, 59, 199–202.

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Figure 18 Comparison of the tool life between the mechanical microdrilling process and the sequential laser and mechanical microdrilling process. Reproduced from Okasha, M. M.; Mativenga, P. T.; Driver, N.; Li, L. Sequential Laser and Mechanical Micro-drilling of Ni Superalloy for Aerospace Application. CIRP Ann. – Manuf. Technol. 2010, 59, 199–202.

11.05.4.9 Combined Micro-EDM Milling and Laser Ablation Process One of the issues with laser beam micromachining (LBM) with ultrashort laser pulses is that the machining times are slow if goodsurface qualities are required. In contrast, micro-EDM is capable of removing material nearly without process forces, but with a higher processing speed if structure sizes exceed a certain value. With the combination of both the processes, specific advantages can be realized while disadvantages can be partially eliminated. Therefore, a compound machine tool was developed that combines the two processes of ultrashort-pulsed laser ablation and micro-EDM milling (23). No reclamping is necessary in this compound process. The limitations and challenges of the single processes have been reduced in the proposed combined process (23). The developed process can be used for the efficient and economic manufacturing of microstructures in materials that are difficult to machine by conventional processes. Both processes have different requirements for the kinematics. Micro-EDM needs only very low velocities of the axes. Due to the higher pulse frequency, the LBM process requires axes with considerably higher dynamics. Furthermore, the control of the axes for the micro-EDM process is relatively complex because the distance of the electrode to the workpiece has to be readjusted constantly. Therefore, control of the vertical axe and the generator from the EDM device has been integrated into the hybrid machine. The developed novel hybrid machine tool is shown in Figure 19 (23). To calculate the optimal sequence of the two processes, an algorithm was developed that evaluates all possible combinations of manufacturing micro-EDM with the different electrode diameters and pulse shapes: LBM with different pulse energies extracts the sequence with the lowest manufacturing costs. A given structure is therefore divided into different parts that can be machined only by a particular process (e.g., if the structure contains details that are smaller than 100 mm, it can be machined only by micro-EDM with a 50 mm electrode or by LBM). A test structure that consists of a square-shaped part of 300  300  40 mm3 and a channel of 100  200  40 mm3 was machined by LBM only and the combined process of microEDM and LBM, as shown in Figure 20 (23). Although there is no significant change in the surface finish, the manufacturing cost has been reduced from V22.6 to V9.75 (57%), and machining time has been reduced from 1344 to 578 s (23) in the combined process.

Figure 19 CAD schema (left) and machine compartment (right) for the developed combined micro-EDM and LBM process. Reproduced from Weber, P.; Haupt, S.; Schulze, V. Hybrid Machining of Microstructures Using a Combination of Electrical Discharge Machining Milling and Laser Ablation. J. Vac. Sci. Technol. B 2009, 27 (3), 1327–1329.

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Figure 20 Test structure in TSF44 machined (a) only by LBM and (b) by the micro-EDM and LBM combined process. Reproduced from Weber, P.; Haupt, S.; Schulze, V. Hybrid Machining of Microstructures Using a Combination of Electrical Discharge Machining Milling and Laser Ablation. J. Vac. Sci. Technol. B 2009, 27 (3), 1327–1329.

11.05.5

Hybrid Micromachining Processes and Related Applications

11.05.5.1 Combined Microgrinding and Microelectrochemical Machining This is a hybrid process of grinding and electrochemical removal for machining of precise small holes in hard-to-machine materials (24). In the process, a spherical metal rod with coated diamond abrasives is used as a cathode tool, which rotates at high speed and removes material electrochemically and mechanically for a premachined pilot hole. The diameter of the pilot hole is smaller than the required diameter and is usually machined by micro-EDM drilling. This process has been developed for machining difficultto-cut materials. However, the process can be applied to other materials also as long as the material is electrically conductive. The tool core should be electrically conductive, and the abrasive needs to be electrically nonconductive. During the machining process, the tool acts as the cathode and the workpiece performs as the anode. The abrasive diamond particles in the tool protrude beyond the conductive bond surface (i.e., the nickel layer). This establishes a small gap between the tool’s nickel layer and the hole’s side wall. Figure 21 shows a schematic representation of the hybrid process of microelectrochemical machining and mechanical microgrinding (24). In this hybrid process, material removal occurs in two phases. The electrolytic action begins when the gap is filled with an electrolyte and the tool is electrically charged, and it results in electrochemical dissolution. Therefore, phase 1 is entirely electrochemical action. In this phase, passivation film occurs on the microhole surface due to the application of passive NaNO3 as an electrolyte. Phase 2 is a combination of electrochemical action and mechanical grinding. As the abrasive tool goes downward, the gap decreases until the abrasives on the tool base come into contact with the workpiece. The abrasive grains remove the soft, nonreactive passivation layer by mechanical grinding action, thus exposing fresh metal for an electrolytic reaction. Simultaneously, the electrolyte trapped between the protruding abrasive grains and the workpiece forms tiny electrolytic cells, thus electrochemical dissolution of workpiece materials occurs. Phase 2 ends at the point of maximum tool diameter. For obtaining sharp edges and high-dimensional accuracy of holes, the tool should be insulated except for the first half of the sphere. This is done in phase 3, and

Figure 21 Schematic representation of the hybrid process combining electrochemical removal and mechanical grinding. Reproduced from Zhu, D.; Zeng, Y. B.; Xu, Z. Y.; Zhang, X. Y. Precision Machining of Small Holes by the Hybrid Process of Electrochemical Removal and Grinding. CIRP Ann. – Manuf. Technol. 2011, 60, 247–250.

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there is no material removal in phase 3. If the tool is not insulated, the electrochemical dissolution will continue for the machined hole surface (in phase 3) because of the electrical field between the machined hole surface and the tool, resulting in a taper hole. This hybrid process is able to machine microholes with sharp edges and without any burrs in difficult-to-cut materials. In addition, the aspect ratio of the microholes can also be increased due to the combined action of electrochemical dissolution and mechanical material removal. The proposed processing is also employed to remove the recast layer of an engine component in Ni-based superalloys, which has an unclosed internal cylindrical surface with a high aspect ratio of 16.25 premachined by wire-cut electrodischarge machining (24). After being machined by the proposed hybrid process, the cylindrical hole was enlarged, and the machined surface finish was improved. Therefore, the recast layer produced by WEDM could be totally removed with this hybrid process. Figure 22 shows the entrance side and cross-section of a microhole machined using the hybrid process (24). The improved surface finish at the inside surface of the microhole can be realized from the image.

11.05.5.2 Micro-EDM and Micro-USM Combined Process This hybrid machining process combines micro-EDM and micro-ultrasonic vibration machining (micro-USM) (25). In this process, the holes and cavities are machined in hard or difficult-to-cut materials using high-frequency mechanical motion abrasive slurry in association with EDM. The abrasive used in this process should be harder than the material being machined. USM can be used alone for any hard materials; however, for the combined process, the workpiece must be conductive. The material removal is from the combined action of the electrical discharging and mechanical polishing of the abrasive slurry. As a result, the MRR of the hybrid process is higher than that of the single USM or micro-EDM process. The high-frequency pumping action of the vibrating surface of the electrode accelerates the slurry circulation, giving smaller machining times. The pressure variations in the gap lead to more efficient discharges, which remove more melted metal. The affected layer is reduced, thermal residual stresses are modified, fewer microcracks are observed, and fatigue resistance is increased due to the abrasive action of slurries. The MRR and surface finish of the

Figure 22 SEM images of the hole machined by a combined electrochemical and mechanical grinding hybrid process: (a) the entrance side of the hole; and (b) a cross-sectional view of the hole showing an improved surface finish around the edge. Reproduced from Zhu, D.; Zeng, Y. B.; Xu, Z. Y.; Zhang, X. Y. Precision Machining of Small Holes by the Hybrid Process of Electrochemical Removal and Grinding. CIRP Ann. – Manuf. Technol. 2011, 60, 247–250.

Figure 23 (a) Schematic diagram showing the setup for the micro-EDM–USM hybrid process; and (b) magnified view showing the working principle of the hybrid process. Reproduced from Lin, Y. C.; Yan, B. H.; Chang, Y. S. Machining Characteristics of Titanium Alloy (Ti-6Al-4V) Using a Combination Process of EDM with USM. J. Mater. Process. Technol. 2000, 104 (3), 171–177.

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Figure 24 Microhole fabricated by the micro-EDM–USM hybrid process: (a) top view of the fabricated microhole; (b) a cross-section of the hole shown in (a); (c) the inner surface of the hole machined by micro-EDM only; and (d) the inner surface of the hole machined by the micro-EDM and USM hybrid process. Reproduced from Lin, Y. C.; Yan, B. H.; Chang, Y. S. Machining Characteristics of Titanium Alloy (Ti-6Al-4V) Using a Combination Process of EDM with USM. J. Mater. Process. Technol. 2000, 104 (3), 171–177.

process depend on the size of the abrasive particles used in micro-USM (25). Figure 23 presents the working principle of the microEDM and USM hybrid process (25). Comparison of the microhole inner surfaces produced by micro-EDM and the microEDM–USM hybrid process is presented in Figure 24 (25).

11.05.5.3 Vibration-Assisted Micro-EDM The application of micro-EDM in deep-hole drilling or in the fabrication of high-aspect-ratio microfeatures is still limited due to improper flushing out of the debris and unstable machining (26). The process is a combination of micro-EDM and vibration to the workpiece or electrode at the same time. The process improves the flushing conditions, the removal of debris, and the machining stability, thus reducing the machining time significantly. Unlike the micro-EDM–USM combined process, the material removal in this process is by the micro-EDM process only. The workpiece or electrode vibration just assists in enhancing the material removal from the workpiece. The process is suitable for deep-hole drilling in hard and difficult-to-cut materials. Depending on the experimental design and objective, the vibration can be applied to the tool electrode (27) or workpiece (26,28). The vibration can be a low-frequency vibration or ultrasonic vibration. Tool vibration is comparatively more difficult to apply in micro-EDM, as the tool electrode is only several microns in diameter; hence, there is a chance for tool deflection. Therefore, in recent years, research has been

Figure 25 (a) Mechanism of applying vibration to the tool electrode (reproduced from Endo, T.; Tsujimoto, T.; Mitsui, K. Study of Vibration-assisted Micro-EDM – The Effect of Vibration on Machining Time and Stability of Discharge. Precis. Eng. 2008, 32 (4), 269–277) and (b) to the workpiece (reproduced from Tong, H.; Li, Y.; Wang, Y. Experimental Research on Vibration Assisted EDM of Micro-structures with Non-circular Crosssection.J. Mater. Process. Technol. 2008, 208 (1–3), 289–298).

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Figure 26 Application of electrode vibration-assisted micro-EDM: (a) fabrication of a square shaft without vibration; (b) fabrication of a microshaft with vibration; and (c) comparison of the machining times for fabricating a square shaft without and with vibration. Reproduced from Endo, T.; Tsujimoto, T.; Mitsui, K. Study of Vibration-assisted Micro-EDM – The Effect of Vibration on Machining Time and Stability of Discharge. Precis. Eng. 2008, 32 (4), 269–277.

Figure 27 Application of workpiece vibration-assisted micro-EDM: (a) a microhole (diameter 60 mm, depth 0.5 mm) without vibration; (b) a microhole (diameter 60 mm, depth 1.0 mm) with vibration; and (c) a comparison of machining time without and with vibration. Reproduced from Jahan, M. P.; Saleh, T.; Rahman, M.; Wong, Y. S. Development, Modeling, and Experimental Investigation of Low Frequency Workpiece Vibration-assisted Micro-EDM of Tungsten Carbide. J. Manuf. Sci. Eng. 2010, 132 (5), 054503 (8 pp).

carried out on the feasibility of workpiece vibration-assisted EDM for the fabrication of microparts (28) and high-aspect-ratio microholes (26). Figure 25 shows the schematic representation of the developed devices for generating tool vibration (26) and workpiece vibration (28) during machining. The application of vibration-assisted micro-EDM in the fabrication of small and highaspect-ratio microstructures and microholes is presented in Figures 26 (28) and 27 (26), respectively. In addition, Figures 26 and 27 indicate a significant amount of reduction in machining time in vibration-assisted micro-EDM compared to that of the micro-EDM process without vibration.

11.05.5.4 Powder-Mixed Micro-EDM In recent years, to improve the quality of the micro-EDM machined surface and to reduce surface defects, several investigators have found the addition of powder particles in the dielectric as an effective process (29). In this hybrid process, the electrically conductive or semiconductive powder is mixed in the dielectric, which reduces the insulating strength of the dielectric fluid and increases the spark gap between the tool and workpiece. The enlarged spark gap makes the flushing of debris easier. As a result, the process becomes stable, improving the MRR and surface finish (30). The sparking is uniformly distributed among the powder particles in the spark gap, reducing the intensity of a single spark, which results in uniform shallow craters instead of a single broader crater.

Figure 28 Comparison of the material removal mechanism for micro-EDM (left) without powder and (right) with powder. For micro-EDM without powder: a lower spark gap, higher gas explosive pressure, and higher single crater size; and for powder-mixed micro-EDM: a larger spark gap, lower gas explosive pressure, and lower single crater size. Reproduced from Tzeng, Y.-F.; Chen, F.-C. Investigation into Some Surface Characteristics of Electrical Discharge Machined SKD-11 Using Powder-suspension Dielectric Oil. J. Mater. Process. Technol. 2005, 170, 385–391.

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Figure 29 Comparison of surface topography and crater height for micro-EDM (a) without and (b) with the addition of powder in the dielectric. The improvement of surface finish and reduction of crater height are visible in powder-mixed micro-EDM (right-side images). Reproduced from Jahan, M. P.; Anwar, M. M.; Wong, Y. S.; Rahman, M. Nanofinishing of Hard Materials Using Micro-EDM. Proc. Inst. Mech. Eng., Part B 2009, 223, 1127–1142.

Thus, the surface finish improves. There may be some abrasive actions of the powder particles during the finishing that reduce the crater boundary heights, making the surface shinier. Figure 28 shows the differences in working principle between the conventional micro-EDM and powder-mixed micro-EDM processes (31). It can be seen from Figure 28(a) that in powder-mixed micro-EDM, instead of a single spark with more energy, the sparking is distributed among the powder particles, thus reducing the strength of a single spark and making uniform discharge and a surface with uniform craters. Figure 28(b) explains that the addition of powders leads to an increase in gap size that subsequently results in a reduction in electrical discharge power density and in gas explosive pressure for a single power pulse (Figure 28(b)). The improvement of surface finish and reduction of crater heights in powdermixed micro-EDM are presented in Figure 29 (30).

11.05.5.5 Microelectrochemical Discharge Machining (Micro-ECDM) The micro-ECDM process involves a complex combination of the electrochemical (EC) reaction and electrodischarge (ED) action. The combined process provides lower electrode wear and a higher MRR compared to the single micro-EDM process (32). Although

Figure 30 (a) Working principle of the micro-ECDM process (reproduced from Bhattacharyya, B.; Doloi, B. N.; Sorkhel, S. K. Experimental Investigations into Electrochemical Discharge Machining (ECDM) of Non-conductive Ceramic Materials. J. Mater. Process. Technol. 1999, 95, 145–154); (b) machining of nonconductive Pyrex glass using the micro-ECDM process (Ra: 1.8 mm); and (c) machining of the same Pyrex glass using the microECDM process with SiC powder (Ra: 1.0 mm). (reproduced from Yang, C. T.; Song, S. L.; Yan, B. H.; Huang, F. Y. Improving Machining Performance of Wire Electrochemical Discharge Machining by Adding SiC Abrasive to Electrolyte. Int. J. Mach. Tools Manuf. 2006, 46, 2044–2050).

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micro-EDM only machines electrically conductive materials, this hybrid process can be applied to conductive metals as well as nonconductive ceramic (32). In this hybrid process, the electrochemical action helps in the generation of the positively charged ionic gas bubbles (e.g., hydrogen). The electrical discharge action takes place between the tool and the workpiece due to the breakdown of the insulating layer of the gas bubbles. The DC power supply voltage is applied between the tool (or cathode) and the anode, resulting in material removal due to melting, vaporization of the workpiece material, and mechanical erosion (33). Figure 30(a) shows the schematic representation of the material removal mechanism for the ECDM process (34). The electrolyte cell used in the ECDM process is similar to that used in electrochemical machining (ECM). In ECDM, the anode is made up of inert material, while the cathode normally is made of copper. Dilute hydrochloric acid (HCl) is generally used as the electrolyte. When a voltage is applied to the cell in proper polarity (i.e., a positive terminal to the anode and a negative terminal to the cathode), reduction of electrolyte with liberation of hydrogen gas takes place at the cathode tip. When the applied voltage is increased beyond a threshold value, hydrogen gas bubbles evolve in large numbers at the tip of the cathode and grow in size. Their nucleation site density increases, and the current path gets restricted between the cathode and electrolyte interface, causing discharge to occur at this interface instantly. Thus, discharge in ECDM always occurs when the voltage in an electrolytic cell is increased beyond a threshold value (35). Application of ECDM gives the possibility to get a machined surface with good quality, when there is high efficiency and a lack of electrode wear. However, the accuracy can be lower than with the single micro-EDM process (32). An expanded version of micro-ECDM with conductive powder-mixed electrolyte has been found to produce improved surface finish and integrity compared to the single micro-ECDM process. Figure 30(b) and (c) shows a comparison of the surface finish between ECDM and abrasive-mixed ECDM (36).

11.05.5.6 Magnetic-Assisted Micro-EDM This hybrid process involves a complex combination of micro-EDM and magnetic field assistance in order to improve machining performance by enhancing removal of debris and increasing the MRR. One of the problems associated with micro-EDM of highaspect-ratio and/or blind features is that the flushing of debris from the machined zone becomes difficult. Those debris particles result in unstable machining by generating arcing and short-circuiting, and they reduce the MRR and surface quality. In order to solve those issues, a magnetic field has been introduced in the micro-EDM process to improve debris circulation (37,38). Implementing magnetic force perpendicular to the electrode’s rotational force produces a resultant force that is efficient in transporting debris out of the hole during machining. A debris particle in a magnetic field-assisted micro-EDM is subjected to two kinds of forces: the magnetic force and the centrifugal force. The resultant force on the debris particle is given by the vector addition of the magnetic force and centrifugal force, which helps to flush out the debris particles from the machine zone, thus improving the machining stability and MRR, reducing tool wear, and overall improving micro-EDM performance (37). Magnetic field-assisted micro-EDM can produce higher aspect ratio holes compared with the conventional micro-EDM process under similar working conditions (38). The application of magnetic fields helps in gap cleaning in micro-EDM due to increased debris transport out of the gap. The enhanced debris removal due to the application of magnetic fields leads to an increase in MRR. It has been reported that for a magnetic material, the MRR was nearly three times higher than that of a hole cut without the magnetic field (37). Figure 31(a) shows the schematic representation of magnetic field-assisted micro-EDM (37). The increment in the aspect ratio of a microhole in magnetic field-assisted micro-EDM can be understood from Figure 31(b) and (c) (38).

11.05.5.7 Laser-Assisted Microturning Laser-assisted mechanical microturning offers the ability to machine difficult-to-cut materials like superalloys and ceramics more efficiently and economically by providing the local heating of the workpiece prior to material removal by a cutting tool. Laserassisted machining has an edge over conventional machining methods due to various advantages, which include a lower cutting

Figure 31 (a) Working principle of magnetic field-assisted micro-EDM (reproduced from Heinz, K.; Kapoor, S. G.; DeVor, R. E.; Surla, V. An Investigation of Magnetic-field-assisted Material Removal in Micro-EDM for Nonmagnetic Materials. J. Manuf. Sci. Eng. 2011, 133, 021002 (9 pp)); (b) the cross-section of a microhole machined by conventional micro-EDM; and (c) the cross-section of a microhole machined by magnetic field-assisted micro-EDM using the same machining conditions as in (b). (reproduced from Yeo, S. H.; Murali, M.; Cheah, H. T. Magnetic Field Assisted Micro Electro-discharge Machining. J. Micromech. Microeng. 2004, 14, 1526–1529).

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Figure 32 (a) Schematic representation and (b) photograph of the laser-assisted microturning system. Reproduced from Shin, Yung C. Laser Assisted Machining. In Industrial Laser Solutions for Manufacturing; 01/01/2011 (last accessed on 21 February 2013), www.industrial-lasers.com/articles/print/ volume-26/issue-1/features/laser-assisted-machining.html.

Figure 33 Comparison of (a) cutting force and (b) residual stress generated during the machining of Inconel 625 with conventional microturning and laser-assisted microturning. Reproduced from Samanta, A.; Teli, M.; Singh, R. K. Surface Integrity in Laser Assisted Mechanical Micro-machining of (LAMM) of Inconel 625. In Proceedings of the 7th International Conference on Micromanufacturing (ICOMM 2012) [CD ROM proceedings].

Figure 34 Surface profile for (a) conventional and (b) laser-assisted microturning (25 mm uncut chip thickness and 30 mm min 1 cutting speed). Reproduced from Samanta, A.; Teli, M.; Singh, R. K. Surface Integrity in Laser Assisted Mechanical Micro-machining of (LAMM) of Inconel 625. In Proceedings of the 7th International Conference on Micromanufacturing (ICOMM 2012) [CD ROM proceedings].

force at higher cutting speed, low specific cutting energy, a smooth surface finish, less tool wear, and increased compressive surface residual stress (39). Figure 32 shows a schematic representation and photograph of the laser-assisted microturning hybrid process (39). In laserassisted microturning processes, the workpiece is heated locally by a laser past its thermal softening point, and then ductile mode machining is carried out on the thermally softened surface (40). The laser beam can pass through the diamond tool, thus heating the surface just below the tool tip in the chip formation zone (41). The laser beam can be applied from a separate source at an angle, but

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Figure 35 The hybrid laser-assisted micromilling setup: 1 – the rotary stage for orienting the laser; 2 – the stacked linear stages – X, Y, and Z; 3 – spindle assembly; 4 – a fiber-optic cable; and 5 – collimator and micrometer assembly. Reproduced from Melkote, S.; Kumar, M.; Hashimoto, F.; Lahoti, G. Laser Assisted Micro-milling of Hard-to-machine Materials. CIRP Ann. – Manuf. Technol. 2009, 58, 45–48.

care should be taken to ensure that the laser beam does not interact with the cutting tool tip. The heating effect produced is at the microscopic scale, and hence the laser power required to heat the workpiece is less than in macro-laser-assisted machining (LAM) processes. During LAM processing, the workpiece is deformed below the fracture strength, thereby enabling a viscoplastic flow rather than a brittle fracture (42). The reduction of cutting forces and residual stresses in laser-assisted microturning compared to conventional turning can be understood from Figure 33 (43). However, the machined surface obtained by laser-assisted microturning has slightly higher roughness than that of conventional microturning, as can be seen from Figure 34 (43).

11.05.5.8 Laser-Assisted Micromilling The part feature accuracy rate and MRR in micromilling of difficult-to-machine materials are limited by the machine-tool system stiffness (especially for small-footprint machines), and the low flexural stiffness and strength of the microtools normally used. Rapid tool wear is another issue during the machining of hard and difficult-to-cut materials, since it negatively impacts part feature accuracy and finish (44). Those shortcomings can be overcome by introducing laser assistance during the machining process. The laser heating will induce localized thermal softening of the materials to be machined, which helps to reduce the cutting forces and tool wear. Tool deflection due to the extreme hardness of the materials can be reduced, which will enhance the dimensional accuracy. The hybrid laser-assisted micromilling process is able to machine freeform 3D microscale features in hard materials (45). Figure 35 shows the setup for the laser-assisted micromilling process (45). In this hybrid process, the laser nozzle is set at an angle, so that the laser radiation hits the surface to be machined just before it is machined by the micromilling process. The objective is the thermal softening of the materials by laser irradiation before machining with the milling tool. A relatively low-power, Ytterbium-doped, continuous-wave, near-infrared (1.06 mm) fiber laser is used to achieve highly localized thermal softening of the material immediately in front of the cutting tool during the micromilling process.

Figure 36 Comparison of the surface quality of the microgrooves produced by the (a) micromilling process and (b) laser-assisted micromilling process. The discontinuity along the grooves can be noticed when using the micromilling process alone. Reproduced from Melkote, S.; Kumar, M.; Hashimoto, F.; Lahoti, G. Laser Assisted Micro-milling of Hard-to-machine Materials. CIRP Ann. – Manuf. Technol. 2009, 58, 45–48.

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Figure 37 Comparison of groove depths for the micromilling process without and with the assistance of laser. Higher depth of grooves results from the laser-assisted micromilling process. Reproduced from Melkote, S.; Kumar, M.; Hashimoto, F.; Lahoti, G. Laser Assisted Micro-milling of Hardto-machine Materials. CIRP Ann. – Manuf. Technol. 2009, 58, 45–48.

No assist gas is used. The laser spot size can be adjusted depending on the dimensions of the machined feature. The laser, spindle, and four axes are controlled simultaneously via a common computer interface. The dry micromilling is performed by TiAlN-coated WC four-flute ball end mills of 250 mm diameter. The laser-assisted micromilling process can increase the MRR by increasing the depth of groove or depth of cut during the micromilling process. In addition, it can provide a comparatively better surface finish with less discontinuity and burr around the edge of the slots, as can be seen from Figure 36 (45). This is due to the fact that the workpiece materials become softer after laser irradiation before the final machining by the milling cutter. Moreover, the MRR and depth of groove also increase in the hybrid process due to the softening action of laser processing (Figure 37).

11.05.5.9 Laser-Assisted Microgrinding The grinding of difficult-to-cut materials (e.g., structural ceramics) is a major challenge due to their extreme hardness, toughness, and wear resistance. The material removal mechanism in the grinding of hard ceramics involves brittle fracture, which often results in the formation of surface microcracks because of the low fracture toughness of ceramics (46). In general, high surface cutting speeds and low depths of cut are recommended to minimize subsurface damage in ceramics, which leads to an increase in machining time (46). This limitation can be addressed by adopting a hybrid process, including a two-step strategy to reduce the grinding forces by first locally weakening the ceramic material through the introduction of thermal cracks and subsequently removing it mechanically. The thermal cracks are induced in ceramic by laser irradiation. By suitably adjusting the laser power, spot size, and speed, it is possible to create and confine the cracks to a controlled volume of material and then remove those materials by a mechanical microgrinding process (47).

Figure 38 Schematic diagram showing the mechanism of the laser-assisted microgrinding process. Reproduced from Kumar, M.; Melkote, S.; Lahoti, G. Laser-assisted Microgrinding of Ceramics. CIRP Ann. – Manuf. Technol. 2011, 60, 367–370.

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Figure 39 Reduction of average and maximum grinding forces in laser-assisted microgrinding compared to the microgrinding process alone. Reproduced from Kumar, M.; Melkote, S.; Lahoti, G. Laser-assisted Microgrinding of Ceramics. CIRP Ann. – Manuf. Technol. 2011, 60, 367–370.

In this hybrid process of laser-assisted microgrinding, the to-be-machined surface is scanned through the laser radiation in order to induce localized thermal stresses in the materials. The purpose of the laser irradiation is to induce and confine thermal cracks in the regions of interest. The thermal cracks are produced in the irradiated region due to tensile stresses generated by rapid heating and cooling of the ceramic. After that, a microgrinding tool is used to mechanically remove the laser-affected (weakened) region at higher MRRs than possible in conventional microgrinding. This step is followed by a series of finish grinding steps to improve the surface finish. This two-step strategy is particularly advantageous since coolants can be used without occlusion of the laser beam. Figure 38 shows a schematic representation of the material removal mechanism in the laser-assisted microgrinding process (47). The reduction in the grinding forces in laser-assisted microgrinding compared to mechanical microgrinding can be understood from Figure 39 (47).

11.05.5.10

Laser Microdrilling with Jet Electrochemical Machining

A novel hybrid process incorporating laser drilling with jet electrochemical machining (JECM-LD) has been developed in order to overcome the drawbacks accompanied intrinsically by conventional laser drilling, such as the formation of an HAZ and recast layer (48). Integrating the merits of both the laser drilling and jet electrochemical machining, the hybrid method is implemented by directing an electrolyte jet coaxially aligned with a focused laser beam onto the workpiece surface. The laser beam needs to transmit in the jet electrolyte before being focused on the machining area. Electrolyte is a neutral salt solution, which is attenuable to laser energy by absorption and scattering. The property of laser attenuation in electrolyte is the key factor of JECM-LD.

Figure 40 Schematic diagram showing the principles of hybrid laser drilling with jet electrochemical machining. Reproduced from Zhang, H. Laser Drilling Assisted with Jet Electrochemical Machining. In Nd YAG Laser; Dumitras, Dan C., Ed.; In Tech, 2012; pp 299–318; Chapter 15.

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Figure 41 Micrograph of the peripheral surface of a penetrated hole (a) laser-drilled in air (200 mJ, 10 s); and (b) drilled with the JECM-LD hybrid process (200 mJ, 40 V, 20 s). Reproduced from Zhang, H. Laser Drilling Assisted with Jet Electrochemical Machining. In Nd YAG Laser; Dumitras, Dan C., Ed.; In Tech, 2012; pp 299–318; Chapter 15.

JECM-LD combines two different sources of energy simultaneously: energy of photons (laser drilling) and energy of ions (ECM). The main aim of combining a jet electrolyte with a laser beam is to obtain high process quality by reducing the recast layer and spatter produced in laser drilling. The jet electrolyte is aligned coaxially with a focused laser beam and creates a noncontact toolelectrode. The focused laser beam and the jet electrolyte are acting on the same surface of workpiece synchronously. In the course of JECM-LD, mainly laser drilling removes material. The defects are overcome by the effects of the jet electrolyte, which consist of effective cooling to the workpiece, transporting of debris, and electrochemical reaction with materials in the interpulse of the laser. Figure 40 illustrates the principles of JECM-LD (49). The JECM-LD hybrid process provides high machining quality with reduced recast layers and spatters. The laser beam takes the chief responsibility of removing the scrap material during JECM-LD, while the jet electrochemical machining serves the auxiliary function of eliminating the recast layers and spatters. As can be seen in Figure 41(a), a large irregular area comprising spattering deposits and resolidified molten layers encircles the surfaces at the edge of the microhole that was machined by laser drilling. In contrast, the hybrid process provides microholes with a comparatively lower HAZ and fewer spatters, as can be seen from Figure 41(b). This may be due to the high-speed electrolyte jet, which effectively cools the material to be processed and discharges scraps. Figure 41(b) exhibits better surface quality and a smoother hole periphery for the hybrid process compared to a single lasermachining process (49). However, an obvious annular electrochemical overcut can be observed at the entrance surface for the JECM-LD process.

11.05.6

Summary

Today’s manufacturing industry is facing challenges from advanced difficult-to-machine materials (tough superalloys, ceramics, composites, etc.), stringent design requirements (high precision, complex shapes, high surface quality, etc.) for microparts and components, and machining costs associated with the difficult-to-cut materials. As a single conventional or nonconventional process is unable to solve all of the issues faced during the machining of these difficult-to-cut materials due to their improved thermal, chemical, and mechanical properties, the necessity of developing innovative compound and hybrid processes is becoming more important. Hybrid micromachining has the potential to combine the strengths and complement the weaknesses of different processes. In this chapter, a comprehensive overview on recently developed compound and hybrid micromachining processes has been provided. In addition to a description of the process mechanisms, applications of different compound and hybrid machining processes have been discussed. Most of the compound and hybrid machining processes include one conventional machining and one nonconventional machining process. The compound machining processes are mainly a combination of sequential processes, whereas the hybrid machining offers two or more simultaneous actions that are responsible for the material removal. Among the nonconventional machining processes, micro-EDM and laser machining are used more widely with different conventional machining processes (e.g., turning, milling, and grinding) to develop compound and/or hybrid processes. These compound and hybrid micromachining processes have enormous potential for the fabrication of microstructures. They can be used for the fabrication of components for microelectromechanical systems, micromolds, microfluidic channels, microprobes, and patterns in glass substrates for lab-on-chip devices or biomedical arrays. Finally, due to the effectiveness of hybrid processes at the macro- and microscales, further research is ongoing to develop innovative hybrid machining processes at the nanoscale.

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