01-10-2016, 03:51 PM
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Introduction
Material removal techniques have a pivotal role to play in component fabrication. In recent years many high strength alloys such as copper beryllium and titanium alloys were produced that are extremely difficult to machine using the traditional processes. These alloys were developed for a variety of industries ranging from aerospace to medical engineering. Machining these alloys with conventional tools results in subsurface damage of the workpiece and in tool damage. The tool size and geometry limit the final component shape that can be machined. Another problem with these tools is that they tend to leave burrs on the machined surface. These burrs are undesirable in many applications. For example, in the medical industry the presence of even very small burrs will damage living tissues where these machined parts are used as implants. In electronic devices where a number of components are in close contact, the burrs may lead to short circuits. In mechanical components burrs may result in a misfit [1].
Electrochemical machining (ECM) can machine these alloys. Devices are becoming smaller as time progresses but their features are increasing at the same time. Machining materials on micro and sub-micro scale is considered a key technology for miniaturizing mechanical parts and complete machines. Micro manufacturing techniques find application in various industries such as electro-communications, semi-conductors, medicine, and ultra-precision machinery. A suitable manufacturing technique for mass production of these micro scale components needs to be established. The current techniques used for machining these components are mainly the dry vacuum process and wet chemical etching. These techniques come under the non-conventional machining processes category [2].
The major difference between conventional and non-conventional machining processes is that conventional processes use a sharp tool for material removal by physical means where as the non-conventional techniques remove material by utilizing chemical, thermal, or electrical energy or a combination of these energies. These processes suffer from several inherent problems. Dry-etching techniques require high cost equipment and do not offer good selectivity in material removal. The chemicals used in wet etching processes are commonly toxic and extreme care has to be taken to dispose of them. These techniques can precisely perform 2D machining at the micro level, that is, they can machine thin films extremely well. However, they are unable to produce 3D components and components with high aspect ratio. Most of these techniques were developed for the electronics industry specifically silicon. Silicon does not find applications in fields other than the electronics industry because it is toxic. High exposure to silicon dust causes chronic respiratory problems. These techniques also suffer from limitations such as restricted materials choice, inability to produce complex profiles, and huge investment for facilities and equipment.
Electrochemical machining is a non-conventional process that found wide-spread applications because of these advantages:
1. It can machine difficult to cut materials, generate complex contours, produce a stress free surface, and have no tool wear.
2. It has been used in various industries at macro level.
3. Electro-chemical machining can be used effectively for micro machining components by suitable tool design and process control.
4. Electrochemical machining uses direct current with the current applied continuously.
This project proposes a new approach of μECM, which uses pulsed current and a feedback loop. The advantages of pulsed current are that it aids in the effective removal of metal ions between anode and cathode and it offers good control of the etched surface. The feedback loop is to be designed in such a way that the system detects variations in the current in machining zone and automatically compensates for them [1].
1.1. ELECTROLYSIS
Electrolysis is the chemical reaction that occurs when an electric current is passed between two conductors dipped in a liquid solution. The completeness of this electric circuit is found by attaching an ammeter to the system and ammeter displays a reading. The liquid solution conducts electricity because otherwise the circuit would be incomplete.
The chemical reactions are named anodic reactions or cathodic reactions depending on whether they occur at the anode or cathode, respectively. The major difference between electrolytes and metallic conductors of electricity is that current is carried by electrons in metals whereas it is carried by ions in electrolytes. Ions are nothing but atoms that have either lost or gained electrons and thereby acquired a positive or negative charge. The positively charged ions travel towards the cathode and the negatively charged ions travel towards the anode. Since the electrolyte must be neutral, there must be a balance between the total positive charge and the negative charge. At the end of the reaction, the amount of material lost by one of the electrodes is equal to the amount of material gained by the other. Hence, this process can be used for both material removal and addition. The major applications of electrolysis are electroplating and electro-polishing [1].
1.2. ELECTROCHEMICAL MACHINING
Electrochemical machining is a material removal process similar to electro polishing. In this process the workpiece to be machined is made the anode and the tool is made the cathode of an electrolytic cell with a salt solution being used as an electrolyte. The tool is normally made of copper, brass, or stainless steel. The tool and the workpiece are located so there is a gap between 0.1mm to 0.6mm between them. The tool is designed so that it is the exact inverse of the feature to be machined. On application of a potential difference between the electrodes and subsequently when adequate electrical energy is available between the tool and the workpiece, positive metal ions leave the workpiece. Since electrons are removed from the workpiece, oxidation reaction occurs at the anode which can be represented as,
M → Mn+ + ne− (1)
where n is the valence of the workpiece metal. The electrolyte accepts these electrons
resulting in a reduction reaction which can be represented as,
nH 2O + ne¯ → n/2 H2 + nOH− (2)
Hence the positive ions from the metal react with the negative ions in the electrolyte forming hydroxides and thus the metal is dissoluted forming a precipitate. The electrolyte is constantly flushed in the gap between the tool and the workpiece to remove the unwanted machining products which otherwise would grow to create a short circuit between the electrodes. The electrolyte also carries away heat and hydrogen bubbles. The tool is advanced into the workpiece to aid in material removal.
A pump system must filter the electrolyte and circulate it because the electrolyte carries away machining waste. A schematic of material removal as the tool advances into the workpiece is shown in Figure 1.1.2.
There are several process configurations that can be selected based on the requirements and the capabilities of the machine. The various configurations are:
1. Both the tool and the workpiece are stationary.
2. The tool is given linear and rotary motion while the workpiece is stationary.
3. Both the tool and the workpiece move
Advantages of Electrochemical Machining
Electrochemical machining offers several advantages over other competing technologies. These advantages have made ECM the best choice for a variety of applications.
The advantages:
1. No tool wear as non contact working mode avoiding problems such as elastic deformation, vibration and breakage.
2. High material removal rate.
3. Ability to machine a wide variety of materials without affecting microstructure or surface properties.
4. No heat generated during machining.
5. Cutting, drilling, deburring and shaping possible.
6. Ease of machining complex features.
7. Stress free machined surface.
8. Environmentally acceptable.
Literature Review
Development of electrochemical micro machining for manufacturing material to micron level by varying various process parameters. This study aims at developing a novel μECM utilizing high frequency voltage pulses and close loop control. Stainless steel SS-316L and copper alloy CA-173 were chosen as the workpiece materials. A model was developed for material removal rate. The research studied the effect of various parameters such as voltage, frequency, pulse ON/OFF time, and delay between pulses of the stepper motor on the machined profiles. The conclusion of this study is that Micro burrs can be effectively removed by optimal μECM setup and using high frequency pulses [1].
Influence of tool vibration on machining performance in electrochemical micro-machining of copper. In this paper a suitable micro-tool vibration system has been developed, which consists of tool-holding unit, micro-tool vibrating unit, etc. This system was used successfully to control material removal rate (MRR) and machining accuracy to meet the micro-machining requirements. Micro-holes have been produced on thin copper workpiece by EMM with stainless-steel micro-tool. Experiments have been carried out to investigate the most effective values of process parameters such as micro-tool vibration frequency, amplitude and electrolyte concentration for producing micro-hole with high accuracy and appreciable amount of MRR. The results of this study are that the introduction of micro-tool vibration improves EMM performance characteristics. Lower electrolyte concentration in the range of 15–20 g/l reduces stray current effects. Hertz (Hz) range of tool vibration frequency improves the removal of sludge and precipitates from very small interelectrode gap. The 150–200 Hz range of tool vibration frequency can be recommended for EMM, which provides a better electrochemical machining in the narrow end gap. Compared to kHz range, Hz range micro-tool’s vibration improves the MRR and accuracy in EMM[2].
Advancement in Electrochemical Micro Machining, In this paper, a review is presented on current research, development and industrial practice in micro-ECM. This paper highlights the influence of various predominant factors of EMM such as controlled material removal, machining accuracy, power supply, design and development of microtool, role of inter-electrode gap and electrolyte, etc. EMM can be effectively used for high precision machining operations, that is, for accuracies of the order of ±1 µm on 50 µm. Also some industrial applications of EMM have also been reported this paper. Extensive research efforts and continuing advancements in the area of EMM for effective utilization in microfabrication require improvements in microtool design and development, monitoring and control of the IEG, control of material removal and accuracy, power supply, and elimination of microsparks generation in IEG and selection of electrolyte, are expected to enhance the application of EMM technology in modern industries[3].
Influence of electrochemical machining parameters on machining rate and accuracy in micromachining domain. In this paper an attempt has been made to develop an EMM experimental set-up for carrying out in depth research for achieving a satisfactory control of the EMM process parameters to meet the micromachining requirements. Keeping in view these requirements, sets of experiments have been carried out by Munda et al they investigated the influence of some of the predominant electrochemical process parameters such as machining voltage, electrolyte concentration, pulse on time and frequency of pulsed power supply on the material removal rate (MRR) and accuracy to fulfil the effective utilization of electrochemical machining system for micromachining [5].
Investigation on Complex Structures Machining by Electrochemical Micromachining. In this paper Electrochemical micromachining (EMM) technology for fabricating micro structures is presented. By applying ultra short pulses, dissolution of a workpiece can be restricted to the region very close to the electrode. First, an EMM system for meeting the requirements of the EMM process is established. Second, sets of experiments is carried out to investigate the influence of some of the predominant electrochemical process parameters such as electrical parameters, feed rate, electrode geometry features and electrolyte composition on machining quality, especially the influences of pulse on time on shape precision and working end shape of electrode on machined surface quality. Finally, after the preliminary experiments, a complex microstructure with good shape precision and surface quality is successfully obtained [6].
CHAPTER 3
Electrochemical Micromachining
Micromachining may literally mean the machining of the dimension between 1 to 999 µm. However, as a technical term, it also means the smaller amount of machining which cannot be achieved directly by a conventional technique. Chemical micromachining is widely used in the present day electronics industry for a variety of applications including fabrication of metallic parts, printed circuit board, and semiconductor devices. When this ECM process is applied to the micromachining range for manufacturing ultra precision shape, it is called electrochemical micromachining (EMM). EMM appears to be promising as a future micromachining technique since in many areas of applications, it offers several advantages that include higher machining rate, better precision and control, and a wider range of materials that can be machined. Alternately, it can be thought of as a material removal process maintaining micron range tolerances. The removal of material occurs atom by atom from the workpiece surface. The semi-conductor industry requires the machining of components of complex shape in high strength alloys.
Electrochemical micro machining is the key technology for the semiconductor, electro communication, optics, medicine, bio technology; automotive, avionics, and ultra precision machinery industries. This technique has replaced the chemical etching process which was predominantly being used in these industries because of the many advantages it offered. This process does not induce any stress into the workpiece or form micro cracks and ridges which are inevitable in other thermal processes. Micro fabrication by μECM can be done through mask or mask less techniques. This technique requires a better degree of tooling and process control compared to the conventional ECM technique. The selection of electrolyte is very critical because of the extremely small gap between the tool and the workpiece. Electrochemical micro machining is still in its initial stages and lot of research needs to be done to improve material removal, surface quality, and accuracy by optimizing the various process parameters. Though ECM has a lot of scope for micro machining there are a number of technical issues that need to be addressed such as stray material removal, tool structure, and machining gap. Surface finishing can also be controlled by μECM because it removes material at the micro level. This technique was employed in the manufacturing of micro nozzles