ELECTROCHEMICAL MACHINING (ECM)

As machining proceeds, and with the simultaneous movement of the cathode at a typical rate, for example, 0.02 millimeter /second toward the anode, the gap width along the electrode len[r]

ELECTROCHEMICAL MACHINING (ECM) Joseph McGeough

Institute for Integrated Micro and Nano Systems University of Edinburgh

Edinburgh, EH9 3JL, United Kingdom (July, 2005)

Michael Faraday�s early metallurgic researches, from 1818 to 1824, anticipated the developments which have led to widespread use today of alloy steels Much effort has been expended to improve their performance for their service as cutting tools in machining The aim has always been to yield higher rates of machining and to tackle recently developed harder materials on the principle that the tool material must be harder than the workpiece which is to be machined Much progress has been made; however, in recent years some alloys, which are exceedingly difficult to machine by the conventional methods, have been produced to meet a demand for very high-strength, heat resistant materials Moreover, these new materials often have to take a complex shape A search has had to be made for alternative methods of machining since the evolution of suitable tooling has not kept pace with these advances

Electrochemical machining (ECM) has been developed initially to machine these hard to machine alloys, although any metal can so be machined ECM is an electrolytic process and its basis is the phenomenon of electrolysis, whose laws were established by Faraday in 1833 The first significant developments occurred in the 1950s, when ECM was investigated as a method for shaping high strength alloys As of the 1990s, ECM is employed in many ways, for example, by automotive, offshore petroleum, and medical engineering industries, as well as by aerospace firms, which are its principal user Metal removal is achieved by electrochemical dissolution of an anodicallypolarized workpiece which is one part of an electrolytic cell in ECM Hard metals can be shaped electrolytically by using ECM and the rate of machining does not depend on their hardness The tool electrode used in the process does not wear, and therefore soft metals can be used as tools to form shapes on harder workpieces, unlike conventional machining methods The process is used to smooth surfaces, drill holes, form complex shapes, and remove fatigue cracks in steel structures Its combination with other techniques yields fresh applications in diverse industries Recent advances lie in computer-aided tool design, and the use of pulsed power, which has led to greater accuracy for ECM-produced components

Theoretical background

Since electrolysis is the basis of ECM, it must be understood before going further through the characteristics and other details of the process

Electrolysis is the name given to the chemical process which occurs, for example, when an electric current is passed between two conductors dipped into a liquid solution A typical example is that of two copper wires connected to a source of direct current and immersed in a solution of copper sulphate in water, as shown in Figure An ammeter, placed in the circuit, will register a flow of current; from this indication, the electric circuit can be deduced to be complete A significant conclusion that can be made from an experiment of this sort is that the copper sulphate solution obviously has the property that it could conduct electricity Such solution is termed an electrolyte The wires are called electrodes, the one with positive polarity being the anode, and the one with negative polarity the cathode The system of electrodes and electrolyte is referred to as the electrolytic cell, whilst the chemical reactions which occur at the electrodes are called the anodic or cathodic reactions or processes

Electrolytes are different from metallic conductors of electricity in that the current is carried not by electrons but by atoms, or group of atoms, which have either lost or gained electrons, thus acquiring either positive or negative charges Such atoms are called ions Ions which carry positive charges move through the electrolyte in the direction of the positive current, that is, toward the cathode, and are called cations Similarly, the negatively charged ions travel toward the anode and are called anions The Fig Electrolysis of copper sulphate solution

movement of the ions is accompanied by the flow of electrons, in the opposite sense to the positive current in the electrolyte, outside the cell, as shown also in Figure and both reactions are a consequence of the applied potential difference, that is, voltage, from the electric source

A cation reaching the cathode is neutralized, or discharged, by the negative electrons on the cathode Since the cation is usually the positively charged atom of a metal, the result of this reaction is the deposition of metal atoms

To maintain the cathodic reaction, electrons are required to pass around the external circuit These are obtained from the atoms of the metal anode, and these atoms thus become the positively charged cations which pass into solution In this case, the reaction is the reverse of the cathodic reaction

The electrolyte in its bulk must be electrically neutral; that is, there must be equal numbers of opposite charges within it, and thus there must be equal amounts of reaction at both electrodes Therefore, in the electrolysis of copper sulphate solution with copper electrodes, the overall cell reaction is simply the transfer of copper metal from the anode to the cathode When the wires are weighted at the end of the experiment, the anodic wire will be found to have lost weight, whilst the cathodic wire will have increased in weight by an amount equal to that lost by the other wire Some examples of the reactions occurring in these processes are shown in the Appendix

These results are embodied in Faraday� s two laws of electrolysis:

1 The amount of any substance dissolved or deposited is directly proportional to the amount of electricity which has flowed

2 The amounts of different substances deposited or dissolved by the same quantity of electricity are proportional to their chemical equivalent weights

A popular application of electrolysis is the electroplating process in which metal coatings are deposited upon the surface of a cathodically polarized metal An example of an anodic dissolution operation is electropolishing Here, the item which is to be polished is made the anode in an electrolytic cell Irregularities on its surface are dissolved

preferentially so that, on their removal, the surface becomes flat and polished

ECM is similar to electropolishing in that it also is an anodic dissolution process But the rates of metal removal offered by the polishing process are considerably less than those needed in metal machining practice

Some observations relevant to ECM can be made:

 Since the anode metal dissolves electrochemically, its rate of dissolution depends

 Since only hydrogen gas is evolved at the cathode, the shape that electrode

remains unaltered during the electrolysis This feature is perhaps the most relevant in the use of ECM as a metal shaping process

Characteristics of ECM

In ECM, electrolytes serve as conductors of electricity and Ohm� s law also applies to this type of conductor The resistance of electrolytes may amount to hundreds of ohms Accumulation within the small machining gap of the metallic and gaseous products of the electrolysis is undesirable If growth were left uncontrolled, eventually a short circuit would occur between the two electrodes To avoid this crisis, the electrolyte is pumped through the interelectrode gap so that the products of the electrolysis are carried away The forced movement of the electrolyte is also essential in diminishing the effects both of electrical heating of the electrolyte, resulting from the passage of current and hydrogen gas, which respectively increase and decrease the effective conductivity

Working principles

Electrochemical

machining is founded on the principles outlined As shown in Figure 3, the workpiece and tool are the anode and cathode, respectively, of an electrolytic cell, and a constant potential

heating The rate at which metal is then removed from the anode is approximately in inverse proportion to the distance between the electrodes As machining proceeds, and with the simultaneous movement of the cathode at a typical rate, for example, 0.02 millimeter/second toward the anode, the gap width along the electrode length will gradually tend to a steady-state value Under these conditions, a shape, roughly complementary to that of the cathode, will be reproduced on the anode A typical gap width then should be about 0.4 millimeter Being understood the characteristics and working principles of ECM, its advantages should be stated in short before going further through machining processes:

• the rate of metal machining does not depend on the hardness of the material, • complicated shapes can be machined on hard metals,

• there is no tool wear

The schematic of an industrial �electrochemical machine� is shown in Figure 4, and an actual example of a cathode tool and anode workpiece are shown in Figure

Electrochemical machining Machine components

Industrial electrochemical machines work on the principles outlined Particular attention has to be paid to the stability of the electrochemical machine tool frame, and to the machining table which should also be stable and firm The electrolyte has to be filtered carefully to remove the products of machining and often has to be heated in its reservoir to a fixed temperature, for instance 30oC (86oF),

before entering the machining apparatus This procedure is used to provide constant operating

conditions During machining the electrolyte heats up from the passage of current Precautions must be taken to avoid a high electrolyte

temperature which can cause changes in the electrolyte specific

conductivity and subsequent undesirable effects on machining accuracy

Rates of machining

The rates at which metals can be electrochemically machined is in proportion to the current passed through the electrolyte and the elapsed time for that operation, and is in inverse proportion to the electrochemical equivalent of the anode-metal which

corresponds to the atomic weight of the dissolving ions over the ionic charge times the Faraday� s constant See the Appendix for more details

Many factors other than current influence the rate of machining These involve

electrolyte type, rate of electrolyte flow, and some other process conditions For example current efficiency decreases when current density is increased for a certain metal, for example, for nickel

If the ECM of titanium is attempted in sodium chloride electrolyte, usually very low (10�20%) current efficiencies are obtained When this solution is replaced by some mixture of fluoride-based electrolytes, to achieve greater efficiencies (>60%), a higher voltage is used

If the rates of the flow are kept too low, the current efficiency of even the most easily electrochemically machined metal is reduced Insufficient flow does not allow the products of machining to be so readily flushed from the machining gap When complex shapes have to be produced the design of tooling incorporating the right kind of flow ports becomes a considerable problem

Surface finish

Type of electrolytes used in the process affects the quality of surface finish obtained in ECM Depending on the material, some electrolytes leave an etched finish This finish results from the nonspecular reflection of light from crystal faces electrochemically dissolved at different rates Sodium chloride electrolyte tends to produce an etched, matte finish with steels and nickel alloys

The production of an electrochemically-polished surface is usually associated with the random removal of atoms from the anode workpiece, whose surface has become covered with an oxide film This is governed by the metal-electrolyte combination used

Occasionally, metals that have undergone ECM have a pitted surface while the remaining area is polished or matte Pitting normally stems from gas evolution at the anode; the gas bubbles rupture the oxide film

Process variables also affect surface finish For example, as the current density is raised the finish generally becomes smoother on the workpiece surface A similar effect is achieved when the electrolyte velocity is increased In tests with nickel machined in hydrochloric acid solution the surface finish has been noted to improve from an etched to a polished appearance when the current density is increased from about to 19 A/square centimeter with constant flow velocity

Accuracy and dimensional control

Electrolyte selection plays an important role in ECM Sodium chloride, for example, yields much less accurate components than sodium nitrate The latter electrolyte has far better dimensional control owing to its current efficiency - current density characteristics Using sodium nitrate electrolyte, the current efficiency is greatest at the highest current densities In hole drilling these high current densities occur between the leading edge of the drilling tool and the workpiece In the side gap there is no direct movement between the tool and workpiece surface, so the gap widens and the current densities are lower The current efficiencies are consequently lower in the side gap and much less metal than predicted from Faraday� s law is removed Thus the overcut in the side gap is reduced with this type of electrolyte If another electrolyte such as sodium chloride solution was used instead, then the overcut could be much greater Using sodium chloride solutions, its current efficiency remains steady at almost 100% for a wide range of current densities Thus, even in the side gap, metal removal proceeds at a rate which is mainly determined by current density, in accordance with Faraday�s law A wider overcut then ensues

Shaping

Most metal-shaping operations in ECM utilize the same inherent feature of the process whereby one electrode, generally the cathode tool, is driven toward the other at a constant rate when a fixed voltage is applied between them Under these conditions, the gap width between the tool and the workpiece becomes constant The rate of forward movement between the tool and the workpiece becomes constant The rate of forward movement of the tool is matched by the rate of recession of the workpiece surface resulting from electrochemical dissolution

Three practical cases are of interest in considering some equations derived for the variation of the interelectrode gap width:

2 When the tool is moved mechanically at a fixed rate toward the workpiece, the gap width tends to a steady value This inherent feature of ECM, whereby an equilibrium gap width is obtained, is used widely in ECM for reproducing the shape of the cathode tool on the workpiece

3 Under short-circuit conditions the gap width goes to zero If some process conditions, such as too small equilibrium gap width caused by a too high movement of the tool toward the workpiece, occur, contact between the two electrodes ensues This causes a short circuit between the electrodes and hence premature termination of machining

The equilibrium gap is applied widely in the shaping process Studies of ECM shaping are usually concerned with three distinct problems:

1 The design of the cathode tool shape needed to produce a required profile geometry of the anode workpiece

2 For a given cathode tool shape, prediction of the resultant anode workpiece geometry, for example, hole drilling by ECM

3 Specification of the shape of the anode workpiece, as machining proceeds This is most readily predicted for the smoothing of surfaces, although for actual shaping of components by ECM, estimates of the machining times as the shape develops provide useful information about the process

Applications

Smoothing of rough surfaces

Deburring, or

smoothing, of surfaces (Figure 6), is the

simplest and a common use of ECM A plane-faced cathode tool is placed opposite a workpiece that has an irregular surface The current densities at the peaks of the surface irregularities are higher than those in the valleys The former are,

therefore, removed preferentially and the workpiece becomes smooth, admittedly at the expense of stock metal (which is still

machined from the valleys of the irregularities, even though at a lower rate)

Electrochemical smoothing is the only type of ECM in which the final anode shape may match precisely that of the cathode tool

Electrochemical deburring is a fast process; typical times for smoothing the surfaces of manufactured components are to 30 seconds Owing to its speed and simplicity of operation, electrochemical deburring can be performed with a fixed, stationary cathode tool The process is used in many industries

Hole drilling

Hole drilling is another principal way of using ECM (Figure 7) The cathode-tool is usually made in the form of a tubular electrode Electrolyte is pumped down the central bore of the tool, across the main machining gap, and out between the sidegap that forms between the wall of the tool and the hole Reversal of the

electrolyte flow can often produce considerable improvement in machining accuracy The main machining action is carried out in the gap formed between the leading edge of the drill tool and the base of the hole in the workpiece ECM also proceeds laterally between the side walls of the tool and component, where the current density is lower than at the leading edge of the

advancing tool Since the lateral gap width becomes progressively larger than that at the leading edge, the side-ECM rate is lower The overall effect of the side-ECM is to increase the diameter of the hole produced The distance between the side wall of the workpiece and the central axis of the cathode tool is larger than the external radius of the cathode This difference is known as the "overcut" The amount of overcut can be

at the highest current densities In hole drilling these high current densities occur between the leading edge of the drill and the base of the workpiece If another electrolyte such as sodium chloride were used the overcut could be much greater The current efficiency for sodium chloride remains steady at almost 100% for a wide range of current densities Thus, even in the side gap, metal removal proceeds at a rate that is mainly determined by the current density, in accordance with Faraday's law

Holes with diameters of 0.05 to 75 millimeter have been achieved with ECM For holes of 0.5 to 1.0 millimeter diameter, depths of up to 110 millimeter have been produced Drilling by ECM is not restricted to round holes; the shape of the workpiece is determined by that of the tool electrode

Full-form shaping

Full-form shaping utilizes a constant gap across the entire workpiece and the tool is moved mechanically at a fixed rate toward the workpiece in order to produce the type of shape used for the production of compressor and turbine blades In this procedure, current densities as high as 100 A/square centimeter are used, and across the entire face of the workpiece, the current density remains high

Electrolyte flow plays an even more influential role in full-form shaping than in drilling and smoothing of surfaces The entire large cross-sectional area of the workpiece has to be supplied by the electrolyte as it flows between electrodes The larger areas of

electrodes involved mean that comparatively higher pumping pressures and volumetric flow rates are needed

Electrochemical grinding

The main feature of electrochemical grinding (ECG) is the use of a grinding wheel in which an insulating abrasive, such as diamond particles, is set in a conducting material This wheel becomes the cathode tool The nonconducting particles act as a spacer

between the wheel and workpiece, providing a constant interelectrode gap, through which electrolyte is flushed

Accuracies achieved by ECG are usually about 0.125 millimeter A drawback of ECG is the loss of accuracy when inside corners are ground Because of the electric field effects, radii better than 0.25 � 0.375 millimeter can seldom be achieved

A wide application of electrochemical grinding is the production of tungsten carbide cutting tools ECG is also useful in the grinding of fragile parts such as hypodermic needles

Electrochemical arc machining

on the onset of arcs rather than sparks, it has been named electrochemical arc machining (ECAM) A spark has been defined as a sudden transient and noisy discharge between two electrodes; an arc is a stable thermionic phenomenon Duration discharges of

approximately second to millisecond are described as sparks, whereas for durations of about 0.1 second said discharges can be considered arcs Because in the ECAM process duration, energy, and time of ignition of sparks are under control, it is valid to regard them as arcs

An attraction of the ECAM technique is the very fast rates of metal removal attainable by the combined effects of sparking and ECM The ECAM technique can be applied in all the ways discussed for ECM, thus surfaces can be smoothed and drilled Turning is also possible, as is wire machining

One form of this process relies on a pulsed direct current, that is, full-wave rectifiedac power supply that is locked in phase with a vibrating tool head The oscillation of the tool gives rise to a set of conditions whereby ECM occurs over each wave cycle The

interelectrode gap narrows as the tool vibrates over one cycle During the same period the current rises until sparking takes place by breakdown of the electrolyte and/or generation of electrolytic gas or steam bubbles in the gap, the production of which aids the discharge process

For drilling, the discharge action occurs at the leading edge of the tool, whereas ECM takes place on the side walls between the tool and the workpiece The combined spark erosion and ECM action yields fast rates of metal removal Because ECM is still possible, any metallurgical damage to the components caused by the sparking action can be

removed by a short period of ECM after the main ECAM action Currents of 250 A at 30 V are typically used in the process

Economic aspects

The industrial sectors utilizing ECM technology fall into five main categories: tool and die, automotive, aerospace, power generation, and oil and gas industries Leading the world�s principle machine tool manufacturing nations in production and export of tools in the 1980s were Japan followed by the former West Germany The United States led in imports and consumption; consumption was high for both Japan and W Germany as well Unconventional machine tools including ECM are generally considered to account for only 1% of total production Electrodischarge machining (EDM) holds the largest share, possibly as much as 50% and ECM about 15% lagging behind laser processes which are 20%

Computer-controlled equipment and sensors are available for electrochemical machining systems However in the 1990s practical ECM systems are often favored because the amount of control and/or monitoring of the process is far less than that which was required in the 1970s Thus machines are used successfully in which electrical spark detection is eliminated and machining products control, for example, pH monitoring, is nonexistent

The present and future status of ECM

High-rate anodic electrochemical dissolution is a practical method of smoothing and shaping hard metals by employment of simple aqueous electrolyte solutions without wear of the cathodic tool ECM can offer substantial advantages in a wide range of cavity-sinking and shaped-hole production operations

Control of the ECM process is improving all the time, with more sophisticated servo-systems, and better insulating coatings However there is still a need for basic

information on electrode phenomena at both high current densities and electrolyte flow-rates

Tool design continues to be of paramount importance in any ECM operation The use of computer-aided design to predict cathode tool profiles will continue to advance

Recently developments in ECM practice have dwelt on the replacement of constant dc by pulsed currents (PECM) Significant improvements in surface quality have been claimed Much smaller electrode gaps may be obtained, for example, below 0.1 millimeter leading to improved control of accuracy, for example to 0.02 to 0.10 millimeter, with dies, turbine blades, and precision electronic components The key to further advancement in PECM lies in development of a low cost power supply Successful development of technique will enable on-line monitoring of the gap size, enabling closer process control Despite these attractions, PECM should be regarded as complementary to, and not a substitute for, established ECM technology; the former is expensive and metal removal rates can be lower than these of the latter

The advent of new technology for controlling the ECM process and the development of new and improved metal alloys, which are difficult to machine by conventional means, will assure the future of electrochemical machining

Appendix Electrolysis

Reactions that occur during the electrolysis of copper sulphate (Figure 1) are as follows The anodic reaction is ionizing of copper:

-While at the cathode the copper ions are discharged to form copper metal: Cu2+(aq) + 2e- ==> Cu

Reactions that occur during the electrolysis of iron (Figure 2) are as follows The anodic reaction is ionizing of iron:

Fe ==> Fe2+(aq) + 2e

-At the cathode, the reaction is likely to be the generation of hydrogen gas and the production of hydroxyl ions:

H2O + 2e- ==> H2 + 2OH

-The net reaction is thus:

Fe + 2H2O ==> Fe(OH)2(s) + H2

The ferrous hydroxide may react to form ferric hydroxide: 4Fe(OH)2 + 2H2O + O2 ==> 4Fe(OH)3

Characteristics of ECM

By use of Faraday� s laws, if �md� (kg) is the mass of metal dissolved, and because

�md = vd� where �v� (m3) is the corresponding volume and �d� (kg/m3) the

density of the anode metal, the volumetric removal rate of anode metal (m3/second) is

given by:

Where �a� (kg/mol) is the atomic weight of the anode metal, �I� (ampere) is the current flowing, �z� is the ionic charge of the anode metal, and the Faraday constant �F� equals 96,487 coulombs/mol If a machining operation has to be carried out on an iron workpiece at a typical rate of 2.6 × 10-8 kg/C, for this removal rate to be achieved by

ECM, the current in the cell must be about 700 A, because �a/zF� = 29 × 10-8 and

�d�= 7,860 kg/m3 for iron

Rates of machining

Where �md� (kg) is the mass of metal electrochemically machined by current �I�

(ampere) passed for a time �t� (second) The quantity �a/zF� is called the electrochemical equivalent of the anode metal as mentioned before

Table I shows the metal machining rates that can be obtained when a current of 1000 A is used in ECM Metal removal rates in terms of volumetric machining are often more useful than mass removal rates, and both quantities are included (It is assumed that the anodic current efficiency is 100%, that is all the current is used to remove metal, which is not always the case.)

Table I Metal machining rates

Metal Atomic Ionic Density Removal rate

weight charge 103 kg/m3 10-3 kg/s 10-6 m3/s

Aluminum 26.97 2.67 0.093 0.035

Beryllium 9.0 1.85 0.047 0.025

Chromium 51.99 7.19 0.269 0.038

3 0.180 0.025

6 0.090 0.013

Cobalt 58.93 8.85 0.306 0.035

3 0.204 0.023

Niobium 92.91 9.57 0.321 0.034

(Columbium) 0.241 0.025

5 0.193 0.020

Copper 63.57 8.96 0.660 0.074

2 0.329 0.037

Iron 55.85 7.86 0.289 0.037

3 0.193 0.025

Magnesium 24.31 1.74 0.126 0.072

Manganese 54.94 7.43 0.285 0.038

4 0.142 0.019

6 0.095 0.013

7 0.081 0.011

Molybdenum 95.94 10.22 0.331 0.032

4 0.248 0.024

6 0.166 0.016

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PRECISION ELECTROCHEMICAL MACHINING AND ROST4000 MACHINE

For machining punches end surfaces for rotary pill presses

DESCRIPTION

Machining of a punch is carried out under effect of DC pulses from the MTCS manufacturing current source using the EME electrode tool by means of varying F frequency and A amplitude

adjusted spark gap is provided with the ACS adaptive monitoring system During machining a hardened end surface of the punch, the EME is embedded into the slug surface with a speed of V = 0.2-0.45 mm/min, and copies its own geometry with 0.010 microns accuracy on the slug surface

The H preprogrammed manufacturing spark gap between the EME and a processed surface of the punch is filled with electrolyte using a pumping station Through the thin layer of electrolyte filled into the gap, there is carried out feeding a pulsed manufacturing current and high-productive local directional dilution of metal of the slug punch is performed Products of electrochemical dilution of the slug metal are washed out from inside the gap by means of electrolyte flows and poured together with them into the

container

Roughness of the processed surface after electrochemical manufacturing is 0.2-0.6 microns Surfaces are easily polished up to the high finish

non of electrolysis laws were established by Faraday by electrochemical dissolution n anodically polarized n electrolytic cell electrode electric current i o conductors source of direct ammeter, pl electrolyte polarity cathode electrons but by charges ions Ion cations anions potential , voltage neutralized , or discharged Appendix chemical equivalent weights electropolishing atomic weight y hydrogen gas is evolved Ohm resistance of ohms conductivity 50 V meter/ millimeter/ specific electrochemical equivalent Faraday Appendix current efficiency de n current density of 0.2 µm saturated s o passive acid 700 A centimeter w n insulating electric field rectified ac pH dc (Figure 1 ron (Figure 2 (kg 7,860 kg/m (kg/mol (ampere 96,487 coulombs kg/C