In this article we will discuss about the electrochemical machining (ECM):- 1. Meaning and Working of Electrochemical Machining (ECM) 2. Electrochemistry of ECM Process 3. Kinematics and Dynamics 4. Effects of Heat and H2 Bubble Generation 5. Effect of ECM on Surface Finish 6. Tool Design of ECM 7. Electrolytes Used 8. Electrochemical Machining Plant 9. Effects of ECM on Materials 10. Characteristics of ECM.
Contents:
- Meaning and Working of Electrochemical Machining (ECM)
- Electrochemistry of ECM Process
- Kinematics and Dynamics of ECM
- Effects of Heat and H2 Bubble Generation in ECM
- Effect of ECM on Surface Finish
- Tool Design of ECM
- Electrolytes Used in ECM
- Electrochemical Machining Plant
- Effects of ECM on Materials
- Characteristics of ECM
1. Meaning and Working of Electrochemical Machining (ECM):
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Electrochemical machining is one of the most potential unconventional machining processes. Though it is a new process for metal working, the basic principle had been well-known for a long time. This process may be considered as the reverse of electroplating with some modifications. Further, it is based on the principle of electrolysis.
In a metal, electricity is conducted by the free electrons, but it has been established that in an electrolyte the conduction of electricity is achieved through the movement of ions. Thus, the flow of current through an electrolyte is always accompanied by the movement of matter.
The electrolysis principle has been in use for long for electroplating where the objective is to deposit metal on the work piece. But since in electrochemical machining the objective is to remove metal, the work piece is connected to the positive, and the tool to the negative, terminal. Figure 6.25 shows a work piece and a suitably-shaped tool, the gap between the tool and the work being full of a suitable electrolyte. When the current is passed, the dissolution of the anode occurs.
However, the dissolution rate is more where the gap is less and vice versa as the current density is inversely proportional to the gap. Now, if the tool is given a downward motion, the work surface tends to take the same shape as that of the tool, and at a steady state, the gap is uniform, as shown in Fig. 6.25. Thus, the shape of the tool is reproduced in the job.
In an electrochemical machining process, the tool is provided with a constant feed motion. The electrolyte is pumped at a high pressure through the tool and the small gap between the tool and the work piece. The electrolyte is so chosen that the anode is dissolved but no deposition takes place on the cathode (the tool). The order of the current and voltage are a few thousand amperes and 8-20 volts. The gap is of the order of 0.1-0.2 mm.
In a typical machine, the metal removal rate is about 1600 mm3/min for each 1000 amp. Approximately 3 kWh are needed to remove 16 x 103 mm3 of metal, which is almost 30 times the energy required in a conventional process (of course, when the metal is readily machinable). But with ECM, the rate of metal removal is independent of the work piece hardness. So, ECM becomes advantageous when either the work material possesses a very low machinability or the shape to be machined is complicated.
Unlike most other conventional and unconventional processes, here there is practically no tool wear. Though it appears that, since machining is done electrochemically, the tool experiences no force, the fact is that the tool and work are subjected to very large forces exerted by the high pressure fluid in the gap.
2. Electrochemistry of ECM Process:
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The electrolysis process is governed by the following two laws proposed by Faraday:
(i) The amount of chemical change produced by an electric current, that is, the amount of any material dissolved or deposited, is proportional to the quantity of electricity passed.
(ii) The amounts of different substances dissolved or deposited by the same
quantity of electricity are proportional to their chemical equivalent weights. In the quantitative form, Faraday’s two laws state that –
When a metallic body is submerged in an electrolyte (Fig. 6.27), the metallic atoms leave the body and become ions and the ions move to the body and become atoms. The process goes on continuously and the equilibrium is maintained. A potential difference exists between a point on the surface of the metallic body (electrode) and an adjacent point in the electrolyte.
This potential difference is known as the electrode potential. The electrode potential varies depending on the electrode-electrolyte combination. If two different electrodes (A and B) are immersed, a potential difference between these electrodes will exist since the potentials of A and B with respect to the common electrolyte are different. This potential difference is the electromotive force (emf) of the cell, generated by the electrodes and the electrolyte. This is explained in Fig. 6.27. For example, if Fe and Cu electrodes are dipped in brine (solution of kitchen salt in water) as shown in Fig. 6.28a, the electrode potentials are –
The nature of the electrolysis process depends on the electrolyte used. To understand how ECM is realized, let us consider the aqueous solution of sodium chloride as the electrolyte. When a voltage difference is applied across the electrodes (Fig 6.28b) the reactions at the anode and the cathode are-
The water gets two electrons from the electrode and, as a result, the hydrogen gas is evolved and hydroxyl ions are produced. The positive metal ions tend to move towards the cathode and the negative hydroxyl ions are attracted towards the anode. Then, the positive metal ions combine with the negatively-charged hydroxyl ions to form ferrous hydroxide as-
This ferrous hydroxide forms an insoluble precipitate. So, with this kind of electrode metal-electrolyte combination, the anode dissolves and H2 generates at the cathode, leaving the cathode shape unchanged. This is the most important characteristic of the electrochemistry of the ECM process. It should be noted that for ECM the choice of electrodes and the electrolyte must be such that no deposition at either electrode can take place.
The gram equivalent weight of the metal is given by ԑ = A / Z, where A is the atomic weight and Z is the valency of the ions produced. Using this in equation (6.20), we get the rate of mass removal in the form –
When the anode is made of an alloy instead of a pure metal, the removal rate can be found out by considering the charge required to remove a unit volume of each element. If the atomic weights and the valencies (of the corresponding ions entering the electrolyte) are A1, A2, A3,… and Z1, Z2, Z3,…, respectively, and the composition (by weight) of the alloy is x1% of element 1, x2% of element 2,…, then a volume v cm3 of the alloy contains vρxi /100 gram of the i-th element, where ρ is the overall density of the alloy in g / cm3.
The charge required to remove all of the i-th element in volume v is given by –
3. Kinematics and Dynamics of ECM:
Figure 6.31 shows a set of electrodes with plane and parallel surfaces. The work (the upper electrode) is being fed with a constant velocity ƒ in the direction -y (normal to the electrode surfaces).
The problem is considered to be one dimensional and the instantaneous distance of the work surface from the tool surface is taken to be y. Considering the work piece to be of pure metal, the removal rate of the work piece metal is given by equation (6.23). If the overvoltage is ΔV, the density of the current flow through the electrolyte is given by –
Where K is the conductivity of the electrolyte. Now, the removal of work material causes the surface of the work piece to recede (in the y-direction) with respect to the original surface with a velocity given by Q’, where Q’ is the volume rate of work piece metal removal per unit area of the work piece surface. Thus, the rate at which the gap between the work and the tool surface changes is –
We shall now investigate a few basic cases:
Zero Feed:
Constant Feed:
An ever increasing gap is not desirable in an ECM process. So, in practice, the electrode is provided with a constant feed velocity of suitable magnitude. Thus, in equation (6.28), ƒ is constant. Obviously, when the feed rate ƒ equals the velocity of recession of the electrode surface due to metal removal, the gap remains constant. This gap (which depends on the feed velocity) is called the equilibrium gap (ye). Thus, for the equilibrium gap, equation (6.28) yields –
Figure 6.32b shows the plot of y̅ versus t̅ for different values of the initial gap. It is seen that the gap always approaches the equilibrium value irrespective of the initial condition.
Feed Motion Inclined to Surface:
When the feed velocity vector is inclined to the surface (Fig. 6.33), the component of the feed normal to the surface is ƒ cos θ. In this case, the equilibrium gap is given by λ / (ƒ cos θ).
Machining Uneven Surface:
When an uneven work surface is subjected to ECM, the metal is removed from all portions of the surface (unlike other machining operations). The portion projecting outwards (the hills) is nearer the tool surface and gets machined more quickly than that projecting inwards (the cavities). Thus, the ECM process has the effect of smoothening out the unevenness.
As shown in Fig. 6.34, the equilibrium work surface position (y̅ = 1) can be regarded as the desired final work piece surface. The deviations from this desired surface are the defects characterized by the non-dimensional depth or height (δ̅), depending on whether the defect is a valley or a hill. Since δ = y – ye,
Theoretically, it would take an infinite time to remove a defect completely; in practice however, as soon as δ̅ goes below a pre-assigned allowable value, the process is finished. Figure 6.35 shows how the hills and the valleys are smoothened out.
4. Effects of Heat and H2 Bubble Generation in ECM:
The different parameters and properties were assumed to be uniform throughout the face of the electrodes. But, in practice, it is not true. A variation in these properties affects the machining process. Also, the electrolyte conductivity changes as the electrolyte passes along the gap due to – (i) the increase in electrolyte temperature, (ii) the evolution of hydrogen bubbles, and (iii) the formation of precipitates, the last effect being small.
Because of the flow of electricity, the electrolyte temperature gradually increases and the conductivity changes, resulting in non-uniformity in the current density along the direction of electrolyte flow. Apart from this, bubbles are formed since hydrogen is generated during machining. These bubbles are swept by the electrolyte, and the concentration of such bubbles tends to increase along the direction of electrolyte flow. As a result, the overall conductivity and the current density vary along the same direction. The resultant effect of these causes the equilibrium gap between the electrodes to vary.
5. Effect of ECM on Surface Finish:
Since, in general, a very good surface finish is desired in the parts machined by ECM, a study of the possibilities that may result in a bad finish is important.
The surface finish is adversely affected by the:
(i) Selective Dissolution:
In alloys, the different constituents have varying electrode potentials. In pure metals too, the dissolution potentials at the grain boundaries are different from those inside the grains. Let us consider the work surface (with two constituents A and B) shown in Fig. 6.38a. In this figure, the voltage profile across the gap has also been shown. Let the dissolution potential of the constituent B (VdB) be greater than the dissolution potential of the constituent A (VdA).
So, the required potential difference between a point on the surface and the adjacent electrolyte for ECM to start must be either VdA or VdB, depending on the local constituent. Since the whole anode surface is equipotential and the electrolyte potential varies across the gap as shown, the surface of a grain of B must project away from the surface of the constituent A (to meet the electrolyte with a lower potential) so that a larger difference, VdB is achieved. Thus, in the steady state, the work surface will be uneven and not very smooth.
When the potential gradient is higher, the unevenness is less. Figure 6.38b shows two situations with different potential gradients, the other parameters remaining the same. It is obvious from this figure that the height of the projection of a grain of the constituent B is less when the potential gradient is higher. An approximate expression of the projection height can also be derived as follows. From Fig. 6.38b,
(ii) Sporadic Breakdown of Anodic Film:
The main reason for the sporadic breakdown of the anodic film is the gradual fall in the potential difference between the work surface and the electrolyte in the region away from the machining area. Figure 6.39 shows the variation of the surface potential of the anode in this region. Here, till the point P1, the potential is enough to cause the dissolution of all the phases. At P1, the available potential falls below the dissolution potential of one phase, and so the anode stops dissolving.
Beyond P1, the anode surface potential continues to drop and an increasing number of phases stop dissolving, resulting in an uneven surface. Ultimately, when only a few phases remain active and dissolve a concentration of the electric field results since the active phases occupy a small proportion of the anode surface. This field concentration causes these phases to dissolve very rapidly, forming deep pits as shown in Fig. 6.39. Beyond the point P2, the anode surface potential drops to such a low value that no dissolution takes place.
(iii) Flow Separation and Formation of Eddies:
The presence of hills and valleys on the anode surface may cause a separation of electrolyte flow and eddy formation. In these eddies, separated from the main stream, a large concentration of the metal ions may build up, resulting in a high concentration over potential in the eddies.
This introduces a localized variation in the removal rates, and consequently an uneven finished surface. Apart from the presence of hills and valleys, the flow separation may be caused by an improper design of the tool and the electrolyte flow path. So, a great care has to be taken in designing the electrolyte flow path in a tool.
(iv) Evolution of H2 Gas:
The flowing electrolyte collects the evolving hydrogen gas generated at the cathode. The presence of H2 in the electrolyte reduces the specific conductivity of the solution. This effect increases as the H2 concentration goes on increasing downstream, and the overall effect is a deterioration of the surface finish.
Apart from the foregoing four mechanisms, there are some other sources of surface deterioration. But since their importance is of a lower magnitude, we shall not discuss them.
6. Tool Design of ECM:
There are two major aspects of tool design.
These are:
(i) Determining the tool shape so that the desired shape of the job is achieved for the given machining conditions.
(ii) Designing the tool for considerations other than (i), e.g., electrolyte flow, insulation, strength, and fixing arrangements.
Theoretical Determination of Tool Shape:
When the desired shape of the machined work piece surface is known, it is possible to theoretically determine the required geometry of the tool surface for a given set of machining conditions.
Let the applied potential, the overvoltage, and the feed rate be V, ΔV, and ƒ, respectively. The equilibrium gap between the anode and the cathode surfaces can be expressed as –
Design for Electrolyte Flow:
A sufficient electrolyte flow between the tool and the work piece is necessary to carry away the heat and the products of machining and to assist the machining process at the required feed rate, producing a satisfactory surface finish. Cavitation, stagnation, and vortex formation should be avoided since these lead to a bad surface finish. One basic rule is that there should be no sharp comers in the flow path. All corners in the flow path should have a radius of at least 0.7-0.8 mm.
The initial shape of a component generally does not comply with the tool shape and only a small fraction of the area is close to the tool surface at the beginning. The problem of supplying the electrolyte over such an area is usually solved by the flow restriction techniques.
In many situations, when the initial work shape conforms to the tool shape,
A tool with an electrolyte supply slot is simple to manufacture, but such a slot leaves small ridges on the work. However, the ridges can be made very small by making the slot sufficiently narrow. Of course, the slot width should be enough to provide an adequate flow. The flow from a slot takes place in a direction perpendicular to the slot and the flow at the end is poor. Therefore, the slot should be terminated near the corners of the work piece surface as shown in Fig. 6.43a.
The distance between the tip of the slot and the corners should be at least 1.5 mm, whereas a slot with a width 0.7-0.8 mm is recommended. When a work piece comer is rounded, the slot end should be made larger as shown in Fig. 6.43b. The shape and location of the slot should be such that every portion of the surface is supplied with electrolyte flow and no passive area exists. Figure 6.44 shows two situations where the passive areas exist since the slot design is faulty.
In Fig. 6.44a, the passive area is not getting the supply because of the presence of outside space between the slot and this area, whereas in Fig. 6.44b, the passive area is created since there is a sharp bend in the slot (and the fact that the flow is normal to the slot). The correct designs are as shown in Fig. 6.45. Sometimes, a reverse flow tool is used to cut accurately and produce superior surfaces, but this process is more complex and expensive and is not generally recommended.
The techniques for controlling the electrolyte flow when the initial work surface does not conform to the tool shape are illustrated in Fig. 6.46. The general rules for putting a flow restrictor can be stated as follows. The flow restrictor must be adjacent to the area of initial close proximity (between the tool and the work surface) and should not increase the flow path appreciably. Also, it must be at the electrolyte entry or exit position.
Design for Insulation:
The areas on a tool where electrochemical machining is not desirable have to be insulated. In die sinking also, the tool should be properly insulated to minimize stray machining. Figure 6.47 shows the ECM process without and with a proper insulation. Figure 6.48 illustrates die sinking without and with a proper insulation.
The insulation must be tough and securely bonded to the tool surface. It can be provided by securing the reinforced solid plastic material to the toll with epoxy resin cement and plastic screws. Sometimes, the insulation can also be done by applying a synthetic rubber coating on the artificially oxidized copper tool surface. For this, a hot chemical oxidizing solution is used. The boundaries of the insulation layer should not be exposed to a high velocity electrolyte flow as this may tend to tear up the glued layer.
7. Electrolytes Used in ECM:
An electrolyte in ECM performs three basic functions, viz.:
(i) Completing the electrical circuit and allowing the large currents to pass,
(ii) Sustaining the required electrochemical reactions,
(iii) Carrying away the heat generated and the waste product.
The first function requires the electrolyte, ideally, to have a large electrical conductivity. The second function requires the electrolyte to be such that at the anode the work piece material is continuously dissolved, and a discharge of the metal ion on the cathode should not occur. Generally, the cationic constituent of the electrolyte is hydrogen, ammonia, or alkali metals. The dissolution of the anode should be sustained at a high level of efficiency.
Also, the electrolyte must have a good chemical stability. Apart from all these, the electrolyte should be inexpensive, safe, and as noncorrosive as possible. Generally, an aqueous solution of the inorganic compounds is used. Table 6.4 lists the electrolytes used for various types of alloys.
8. Electrochemical Machining Plant:
A few important points should be kept in mind when designing an electrochemical machine. These include the stiffness and the material of the components. Though, at a first glance, it appears that the machining force is negligible as there is no physical contact between the tool and the work piece surface, very large forces may develop between them due to the high pressure of the electrolyte required to maintain an adequate flow velocity through the narrow gap.
So, the machine must possess enough rigidity to avoid any significant deflection of the tool which may destroy the accuracy of the parts being machined. A change of temperature may also cause some relative displacement between the tool and the work piece, and the design should take care of it.
To avoid corrosion, wherever possible, the nonmetallic materials should be used. When strength and stiffness are required, the plastic coated metals should be used. The material used to hold the work piece is exposed to anodic attack, and Ti appears to be most suitable because of its passivity. When different metals are in contact in the presence of the electrolyte, especially when the machine is idle, corrosion may occur.
To minimize this, the metals in contact should be so chosen that they do not differ much in their electrochemical behaviour. The slide ways cannot be protected permanently, and so they are heavily coated with grease. Sometimes, a corrosion protection may be provided by applying a small electrical potential in such a direction that the whole structure becomes more noble electrochemically. This is commonly known as cathodic protection.
The pump is the most important element of the ancillary plant. Generally, the positive displacement pumps (similar to gear pumps) made of stainless steel are used. The tank for the electrolyte, the pipeline, and the valves are normally made of PVC.
9. Effects of ECM on Materials:
In contrast with the conventional machining processes, the material removal during ECM is smooth and gentle. As a result, the maximum residual compressive stress is very low in the work piece surface. Moreover, the depth of the work hardened surface layer is negligible. When the depth of the work hardened surface layer is about 0.5 mm and 1.5 mm for turning and milling, respectively, that in ECM is only about 0.001 mm. Similarly, the order of magnitude of the residual stress in a surface machined by a conventional process is about 50 kg / mm2, whereas that with ECM is almost zero.
This results in a 10-25% lower fatigue strength of the parts produced by ECM. This is because the micro crack tips are exposed at the surface produced by ECM and also because the process leaves a stress-free surface. For increasing the fatigue strength, some mechanical processes (e.g., mechanical polishing, glass bead blasting, and vapour blasting) can be used.
10. Characteristics of ECM:
