Electrochemical Processes 4.1 Electrochemical Machining 4.1.1 Introduction Electrochemical machining (ECM) is a modern machining process that relies on the removal of workpiece atoms by electrochemical dissolution (ECD) in accordance with the principles of Faraday (1833). Gusseff introduced the first patent on ECM in 1929, and the first significant development occurred in the 1950s, when the process was used for machining high-strength and heat-resistant alloys. 4.1.2 Principles of electrolysis Electrolysis occurs when an electric current passes between two electrodes dipped into an electrolyte solution. The system of the electrodes and the electrolyte is referred to as the electrolytic cell. The chemical reactions, which occur at the electrodes, are called the anodic or cathodic reactions. ED of the anodic workpiece forms the basis for ECM of metals. The amount of metal dissolved (removed by machining) or deposited is calculated from Faradays laws of electrolysis, which state that

1. The amount of mass dissolved (removed by machining), m, is directly proportional to the amount of electricity.

m It

2. The amount of different substances dissolved, m, by the same quantity of electricity (It) is proportional to the substances chemical equivalent weight .

m and

= A/Z where I = electrolyzing current, A t = machining time, min = chemical equivalent weight, g A = atomic weight Z = workpiece valence 4.1.3 Theory of ECM ECM uses a direct current at a high density of 0.5 to 5 A/mm2 and a low voltage of 10 to 30 V. The machining current passes through the electrolytic solution that fills the gap between an anodic workpiece and a preshaped cathodic tool. The electrolyte is forced to flow through the interelectrode gap at high velocity, usually more than 5 m/s, to intensify the mass and charge transfer through the sublayer near the anode. The electrolyte removes the dissolution products, such as metal hydroxides, heat, and gas bubbles, generated in the interelectrode gap. McGeough (1988) claimed that when a potential difference is applied across the electrodes, several possible reactions occur at the anode and the cathode. Figure 4.1 illustrates the dissolution reaction of iron in a sodium chloride (NaCl) water solution as an electrolyte. The result of electrolyte dissociation and NaCl dissolution leads to

The negatively charged anions OH- and Cl- move toward the anode, and the positively charged cations of H+ and Na+ are directed to the cathode. At the anode, Fe changes to Fe++ by losing two electrons. At the cathode, the reaction involves the generation of hydrogen gas and the hydroxyl ions. The outcome of these electrochemical reactions is that iron ions combine with other ones to precipitate out as iron hydroxide, Fe(OH)2. The ferrous hydroxide may react further with water and oxygen to form ferric hydroxide, Fe(OH)3. With this metal-electrolyte combination, electrolysis has involved the dissolution of iron, from the anode, and the generation of hydrogen, at the cathode (McGeough, 1974). 4.1.4 ECM equipment Figure 4.2 shows the main components of the ECM machine: the feed control system, electrolyte supply system, power supply unit, and workpiece holding device. As shown in Fig. 4.3, the feed control system is responsible for feeding the tool at a constant rate during equilibrium machining. The power supply drives the machining current at a constant dc (continuous or pulsed) voltage. The electrolyte-feeding unit supplies the electrolyte solution at a given rate, pressure, and temperature.

Figure 4.1 Electrochemical reactions during ECM of iron.

Facilities for electrolyte filtration, temperature control, and sludge removal are also included. ECM machines are capable of performing a wide range of operations such as duplicating, sinking, and drilling. Semiautomatic and fully automated facilities are used for large-size machining, such as deburring in the automotive industry. ECM machines, in contrast to conventional machine tools, are designed to stand up to corrosion attack by using nonmetallic materials. For high strength or rigidity, metals with nonmetallic coatings are recommended at www.unl.edu/.

4.1.4.1 Power supply. The dc power supply for ECM has the following features: 1. Voltage of 2 to 30 volts (V) (pulsed or continuous) 2. Current ranges from 50 to 10,000 amperes (A), which allow current densities of 5 to 500 A/cm2 3. Continuous adjustment of the gap voltage 4. Control of the machining current in case of emergency 5. Short circuit protection in a matter of 0.001 s 6. High power factor, high efficiency, small size and weight, and low cost 4.1.4.2 Electrolytes. The main functions of the electrolytes in ECM are to 1. Create conditions for anodic dissolution of workpiece material 2. Conduct the machining current 3. Remove the debris of the electrochemical reactions from the gap 4. Carry away the heat generated by the machining process 5. Maintain a constant temperature in the machining region The electrolyte solution should, therefore, be able to (www.unl.edu/) 1. Ensure a uniform and high-speed anodic dissolution 2. Avoid the formation of a passive film on the anodic surface (electrolytes containing anions of Cl, SO4, NO3, ClO3, and OH are often recommended) 3. Not deposit on the cathode surface, so that the cathode shape remains unchanged (potassium and sodium electrolytes are used) 4. Have a high electrical conductivity and low viscosity to reduce the power loss due to electrolyte resistance and heat generation and to ensure good flow conditions in the extremely narrow interelectrode gap 5. Be safe, nontoxic, and less erosive to the machine body 6. Maintain its stable ingredients and pH value, during the machining period

7. Have small variation in its conductivity and viscosity due to temperature rise 8. Be inexpensive and easily available The most common electrolytes used are sodium chloride (NaCl), sodium nitrate (NaNO3), and, sodium hydroxide. Industrial ECM operations usually involve using mixed electrolytes to meet multiple requirements as shown in Table 4.1. The selection of the ECM electrolyte depends on the workpiece material, the desired dimensional tolerance, the surface finish required, and the machining productivity. During ECM, the electrolyte plays an important role in dimensional control. As shown in Fig. 4.4, sodium nitrate solution is preferable, because the local metal removal rate is high at the small gap locations where both the current density and the current efficiency are high. Additionally, the local removal rate is low at the larger gap locations where both the current density and current efficiency are low. This results in the gap distribution tending toward uniformity.

Figure 4.4 Effect of current density on current efficiency for different electrolytes.

The current efficiency in ECM depends on the anodic material and the electrolyte. When the pulsed voltage is applied instead of the commonly used continuous voltage, proper use of pulse parameters (e.g., pulse on-times) can significantly improve the current efficiency and surface quality. Depending on the tool shape and type of the machining operation, several methods of supplying electrolyte to the machining gap are shown in Fig. 4.5. The choice of the electrolyte supply method depends on the part geometry, machining method, required accuracy, and surface finish. Typical electrolyte conditions include a temperature of 22 to 45C, a pressure between 100 to 200 kPa, and a velocity of 25 to 50 m/s. 4.1.4.3 Tools. The design of a suitable tool for a desired workpiece shape, and dimension forms a major problem. The workpiece shape is expected to be greater than the tool size by an oversize. In determining the geometry of the tool to be used under steady-state conditions, many variables should be specified in advance such as the required shape of the surface to be machined, tool feed rate, gap voltage, electrochemical machinability of the work material, electrolyte conductivity, and anodic and cathodic polarization voltages. With computer integrated manufacturing, cathodes are produced at a lower cost and greater accuracy. Computer-aided design (CAD) systems are used first to design a cathodic tool. This design is programmed for CNC production by milling and turning. After ECM, the coordinate measuring machine inspects the workpiece produced by this tool and sends data back to the CAD computer- aided manufacturing (CAM) unit for analysis of the results. Iterations of the cathodic tool are made so that the optimum tool design is selected. The material used for ECM tools should be electrically conductive and easily machinable to the required geometry. The various materials used for this purpose include copper, brass, stainless steel, titanium, and copper tungsten. Tool insulation controls the side electrolyzing current and hence the amount of oversize. Spraying or dipping is generally the simplest method of applying insulation. Durable insulation should ensure a high electrical receptivity, uniformity, smoothness, heat resistance, chemical resistance to the electrolyte, low water absorption, and resistance against wear in the machine guides and fixtures. Teflon, urethane, phenol, epoxy, and powder coatings are commonly used for tool insulation (Metals Handbook, 1989).

4.1.5 Basic working principles The simplest case to consider is that of plane-parallel electrodes normal to the feed direction as described by Tipton (1971) and shown in Fig. 4.6. Consider an electrolyte of conductivity k and density e that flows at a mean velocity u, in the direction of increasing x, in a channel. The channel is assumed to extend to the left of the origin x = 0 where the tool and workpiece start, so that the flow has reached a steady state and the inlet conditions can be neglected. All properties of the system are assumed to be independent of the z direction. The position of the workpiece surface relative to the tool and hence the gap thickness is represented by the coordinate y. The workpiece surface moves away from the tool surface in the direction of increasing y at a rate proportional to the current density J and equal to At the feed rate a, in the direction of decreasing y, the workpiece rate of change of position dy/dt can be written as

where = current efficiency of the dissolution process, % F = Faradays constant, 96,500 C/g per ion k = electrolyte conductivity, -1mm-1 = density of anode material, g/mm3 = chemical equivalent weight v = applied voltage, V

v = overvoltage, V

Figure 4.6 Working gap with plane-parallel electrodes at a constant gap voltage.

The current efficiency is defined as the ratio of the observed amount of metal dissolved to the theoretical amount predicted from Faradays laws for the same specified conditions of electrochemical equivalence, current, etc. Apparent current efficiency values may be due to 1. The choice of wrong valence 2. Passivation of the anodic surface 3. Grain boundary attack, which causes the removal of metal grains by electrolyte forces 4. Gas evolution at the anode surface It is convenient to write the machining constant C for the particular workpiece-electrolyte combination (m2.min-1) as Then, Integrating