Electrochemical machining, as a technological method, originated from the process of electrolytic polishing offered already in 1911 by a Russian chemist E.Shpitalsky.
As far back as 1929, an experimental ECM process was developed by W.Gussef.
In 1959 a commercial process was established by the Anocut Engineering Company.
The idea of using electrolytic reactions for manufacturing had been discussed since at least 1920, thanks to the Russian chemist Evgeny Shpitalsky,. It is only in 1959 that ECM became a commercialized process. The Anocut Engineering Co., Downers Grove, Ill., implemented ECM as a new way to shape high-strength alloys.
New ECM suppliers are emering and consistent innovations are occurring during the past few decades, including the development of pulsed ECM (PECM). It has become now an increasingly viable option for manufacturers across multiple industries to consider this production technology option.
https://www.americanmachinist.com/machining-cutting/media-gallery/21171164/a-guide-to-electrochemical-machining-part-1
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For each manufacturing process, industrial engineers have to identify the process parameters that have an effect on productivity and do experiments to find the relation between the parameter and productivity. The increased understanding of these relations leads to productivity engineering and more productivity.
WECM - Process Parameters and Machining Conditions
The performance, applicability, and efficiency of the WECM process greatly depend upon proper choice and control of operating parameters and their ranges, selection of suitable machining condition and desired output criteria based on the requirements.
Nature of power supply
There are two types of power supply available, normal DC and pulsed DC. Although normal DC is applicable for electrochemical machining, for producing features within micron ranges with enhanced geometric and dimensional accuracy, use of pulsed DC is preferred. WECM needs lower voltages to confine the dissolution in sub-micron dimensions and during WECM the current density remains rather high given the dimeter of the wire used. As such, reaction products must be removed efficiently in order to achieve easy renewal of fresh electrolyte and to avoid coagulation of machining products in the IEG. Moreover, the generated joule heat and changed electrolyte concentration also attribute toward worsening of machining accuracy. For this, a pulsed DC power supply is used during WECM. In pulsed DC, the pulse on time and off time is matched to obtain the required current density. Also, during pulse on time the sudden increase in current density results in enhanced machining accuracy compared to continuous DC, while during pulse off time removal of machining products, dissipation of heat, and renewal of fresh electrolyte occur with no dissolution taking place.1
The pulse power parameters consist of applied voltage, pulse on time, and frequency, all of which must be maintained individually and adequately to ensure quality machining.
Applied voltage
The applied voltage is one of the most significant process parameters in WECM. As the applied voltage is proportional to current density, the voltage must be maintained beyond a critical value to initiate machining. On the other hand, the application of very high voltage leads to very high current density and material removal rate. This, in turn, results in unstable and uncontrolled machining, which is detrimental to a machining process occuring in micron ranges. Moreover, the increased tool polarization area can result in corrosion of feature edges due to the stray current effect, which decreases the localization effect resulting in increased side gap. Also, very high voltage increases the amount of electrolysis products in the tiny IEG, which pose many challenges even for adequate flushing. Moreover, too high a voltage results in a higher concentration of machining products. This can cause frequent micro-sparks in the tiny IEG due to the accumulation of reaction products and can also form a deposit on the tool wire which then restricts uniform dissolution. The applied voltage is dependent on the pulse on time, meaning that the total voltage during each pulse period increases with an increase in pulse on time. If the voltage is too low, a longer pulse on time is required to achieve the desired current density to match the dissolution with the feed rate. This, in turn, results in a shorter pulse off time and decrease the time available for effective flushing.
The dissolution rate increases with a rise in applied voltage, leading to an increase in slit width. But at low voltage values of 8 V and below, the dissolution rate is very low. Moreover, the removal of electrolysis products from the narrower IEG becomes difficult, leading to accumulation of reaction debris and occurrence of frequent shortcircuits. This leads to unstable machining and increase in the deviation in slit width. With increase in machining voltage above 8 V, the number of short circuits decreases gradually due to the large machining gap. Therefore, machining stability enhances and slit width increases. However, at higher voltages, a higher amount of electrolysis product forms and cannot be removed quickly. As a result, shortcircuiting again becomes prevalent, decreasing the stability of the operation and the uniformity of the microslits.
Influence of pulse on time
Pulse on time is another important factor during WECM and can be represented by duty ratio, which is the ratio of a pulse on time to total pulse period. An increase in pulse on time means an increase in duty ratio under the same pulse period. However, with an increase in pulse period, duty ratio decreases under the same pulse on time. During pulse on time, material dissolution occurs whereas during pulse off time cleaning of electrolysis products happens, making the machining zone clean and ready for subsequent dissolution. As such, for effective removal of reaction products a lower pulse on time and higher pulse off time is preferable, which in turn also decreases the IEG and enhances the accuracy. However, with too great a decrease in the duty ratio and pulse on time, the current efficiency decreases below what is required. This results in no dissolution and the occurance of short circuiting. However, a very high duty ratio can also result in stray current effect and frequent sparking as well as short-circuiting due to the accumulation of reaction products in the machining gap. Finally, if the pulse on time is shorter even than the charging time constant, the double layer will remain undercharged and no machining will occur at all.63 Figure 11 shows the influence of duty ratio on the average width of the fabricated microslit during WECM carried out with 50 μm tungsten wire, 5V, 1MHz frequency, and 0.1 M concentration of electrolyte (H2SO4). Here, the slit width increases with an increase in the duty ratio. Increase in the duty ratio means more time has been made available for carrying out machining because material removal takes place only during pulse on time. However, after 40% duty ratio, the increase in slit width is much more rapid. This is due to the increased accumulation of reaction products which adhere to the wire surface and increase the effective diameter of the wire under constant flushing conditions.19
Applied frequency
During machining, if the frequency is very high, then pulse on time will be lower under a constant duty ratio. As noted above, too low a pulse on time hinders the machining process. Moreover, the surplus time excluding that required for charging the double layer is also very small, which can decrease the dissolution rate. However, at higher frequency values, there are chances for successful dissolution at very low values of IEG. Such confined dissolution contributes to the betterment of surface quality with increased dimensional accuracy. On the other hand, if the frequency is low enough, then the total time per pulse required for charging the double layer will be less as the charging time is constant for each time period. As a result, the machining current will be higher per pulse period and can significantly increase the material removal and side gap of the feature.38 Figure 12 shows the influence of frequency on average width of the microslit during machining with 50 μm tungsten wire, 5V, 30% duty ratio, and 0.1 M concentration of electrolyte (H2SO4). The graph shows that with an increase in applied frequency, the average width of the microfeature decreases and accuracy increases. Increasing the frequency means that less time is made available for effective machining; hence, dissolution efficiency decreases. However, if the gap becomes too small due to the application of very high frequency beyond 1.5 MHz, flushing of electrolysis products may become difficult, leading to deterioration of machining stability. On the other hand, the application of frequency values lower than 0.25MHz has resulted in decreased accuracy and increased side gap.
Influences of electrolyte types, concentration and flow rate
In any machining process, the effectiveness depends upon the type of machining medium. Similarly, depending upon the type of work material, various types of electrolytes such as acidic, alkali, salt-based, and mixed are used during WECM. Acidic electrolytes like H2SO4 and HCl are extensively used for machining stainless steel, nickel, and cobalt based alloys, etc.64,22,26 This is because acid electrolytes donot produce any insoluble reaction products during machining which can accumulate in the IEG, decrease the conductivity, and hinder the dissolution process.65 As such, the use of acidic electrolytes results in the generation of microfeatures with high aspect ratio and smoother surfaces. Here, better surface quality and localization have been achieved by using H2SO4 due to the growth in the passive film within the stable passivation regime contributing to electrochemical polishing. However, the use of HCl has resulted in high material removal and non-uniform surfaces due to the effect of chloride induced pitting corrosion which disintegrates the passive film.34 On the other hand, the amount of reaction debris produced during machining in salt-based electrolytes is much larger,which can clog the machining gap if a sufficient flushing strategy is not implemented. Also, in contrast to acidic electrolytes, salt-based electrolytes produce a thick layer of corrosion products in the machined surface of iron-based work materials and they are herefore not suitable for machining high aspect ratio microstructures.66,67 Because of this, the application of salt-based electrolytes like NaCl and NaNO3 is limited to machining stainless steel substrate with tungsten, copper and molybdenum wire of somewhat larger diameter i.e, 20 μm, 100 μm, or beyond68,23,69 by implementing better flushing strategies like axial electrolyte flow and monodirectional travelling of wire.69,24 Apart from these, mixed salt-based electrolytes such as the combination of NaCl and NaNO3 have been used to machine titanium alloys (TC1) and ɣTiAl alloys. However, the maximum feed rate that can be achieved with NaNO3 is greater than NaCl electrolyte.38,70 On the other hand, as mentioned above, alkali electrolytes like NaOH and KOH are used for in situ fabrication of tungsten wire electrodes for WECM.22,40,41 However, during machining with an aqueous electrolyte, the presence of water may contribute to passive film growth, which can be suppressed using a nonaqueous electrolyte or a solution with low water content.71,72 During machining of Ni-based metallic glass, methanol has been used as an electrolyte solvent that results in higher reactivity, a better feed rate, and also makes the passive film less protective.73 However, the application of nonaqueous solvent in WECM is still not popular and requires further investigation to ensure its usability and effectiveness. Apart from the selection of proper electrolyte, there are two other associating parameters which significantly affects the machining, the concentration of electrolyte and the flow rate.
Concentration of electrolyte
The concentration of the electrolyte is another significant factor during WECM. A higher concentration of electrolyte leads to the generation of a higher number of charge carriers or ions that increases the flow of current and current density during WECM operations. This results in faster removal of material but also increases the amount of electrolysis products in the IEG. Also, if the flushing is inadequate, the accumulation of electrolysis product may lead to the occurrence of micro sparks and deteriorate the surface quality of the desired feature. As per the double layer theory, a higher concentration of electrolyte decreases the electrolyte resistivity. As a result, the charging time constant of the electrical double layer is lowered. Although small pulse on time is required for machining with higher concentration electrolyte, more area of electrical double layer remains fully charged, resulting in the side gap of the generated feature increasing. This contributes to the fact that low side gap or higher machining accuracy can also be achieved when electrolyte concentration is very low. However, lowering the electrolyte concentration beyond a certain value results in a decrease in the number of charge carriers, leading to a decrease in dissolution efficiency. Moreover, the anodic film generated during implementation of tungsten wire doesnot dissolve properly in an electrolyte with very low concentration, also hindering the dissolution process.
The side gap and average current increases with an increased concentration of electrolyte and the number of charge carriers. At very low electrolyte concentration, the material removal rate becomes lower than the employed feed rate. As a result, physical contact between the tool and workpiece may occur and shortcircuiting and sparking become prevalent. Increasing the electrolyte concentration results in an increase in the machining current which increases the side gap, lessens shortcircuiting and sparking, and makes the machining more stable. But machining with an excessively high electrolyte concentration can also increase the dissolution of sharp edges which can deteriorate the feature resolution.38
Flow rate of electrolyte
The flow rate of electrolyte needs to be chosen and maintained carefully during WECM. Increase in flow rate can effectively increase the feed rate of the tool wire, machining efficiency, and machining stability by enhancing the flushing efficiency during operations. This is achieved by easy dispersion of reaction products as well as generating uniform flow field and subsequent renewal of fresh electrolytes. However, very high flow rate can result in a radial swing of the wire and deteriorate machining quality. Also, the feed rate can only be increased by increasing the flow rate up to a certain extent. This is because with an increase in feed rate, the side gap reduces to such an extent that the increase in flow rate has no further effect in flushing and can result in frequent short-circuiting and sparking. The flow rate is therefore effective only up to a critical value, beyond which the flow rate in the IEG doesnot increase further and remains unchanged where the maximum amount of electrolyte flows through the outside of machining area. Accordingly, the flow rate of aqueous NaNO3 has been kept as 0.75 m/sec during machining with 20 μm tungsten wire on 5 mm stainless steel workpiece with a feed rate of 0.5 μ/sec.69
Feed rate of the wire electrode
The feed rate of the wire is another important parameter for machining intricate microfeatures. Wire feed rate should be correctly adjusted and matched with the material removal rate to maintain a constant IEG. If the feed rate is too low, this will result in increased material removal, side gap, and stray current effect.23 With an increase in feed rate, both the width of the microfeature and deviation in width decreases, due to shortening of the machining time at a given position and a decrease in material removal. Also, an increase in feed rate increases the homogeneity of the microfeature and improves edge radius by minimizing secondary corrosion.74 However, if the feed rate is higher than the material removal rate, this will lead to too large a decrease in the machining gap. This results in improper flushing of electrolyte and inadequate cleaning of electrolysis products, which may lead to short-circuiting and degrade the final shape of the micro product. However, better flushing techniques, such as the implementation of a 200 μm diameter monodirectional traveling tungsten wire with a velocity of 0.02 m/sec, can increase the working feed rate to 1.2 μ/sec during machining micro-slits on a 5 mm stainless steel plate.
On the other hand, the feed rate has a different effect on the roughness of the machined surface. With a lower feed rate, surface roughness decreases, whereas surface quality improves with increasing feed rate due to low material dissolution and stray current effect. However, if the feed rate is excessive, then surface quality deteriorates due to low mass transfer rates and deformation of wire leading to short-circuiting and degradation of machining stability. Accordingly, an optimum feed rate is preferable for generation of a smoother surface. In addition, different process parameters, such as voltage, electrolyte pressure, frequency, machining conditions, and nozzle diameter also influence the feed rate. Higher feed rates can be achieved by employing higher voltages. However, the application of higher voltage also results in increased concentration of electrolyte products and can reach a certain point where electrolyte boils. Feed rate also increases by a proportion of one-quarter of electrolyte pressure during machining with copper coated steel wire and copper wire, but reaches a maximum value and remains at a critical electrolyte pressure. Nozzle diameter also affects the feed rate. Here, the critical electrolyte pressure at which maximum feed is achievable increases with increasing feed rate and decreasing nozzle diameter. Also, the feed rate is inversely proportional to the square root of workpiece thickness, however, the experimental values fall slightly behind that.
Other process parameters and machining conditions
During machining with a reciprocating traveling wire electrode, the reciprocating amplitude as well as frequency and workpiece vibration also affect the machining quality. Figure 15 shows the influence of anode vibration frequency and amplitude on side gap during machining.
With an increase of anode vibration frequency to 60 Hz, the side gap decreases during machining with a 4 μm diameter tungsten wire on a 80 μm cobalt alloy sheet. Here, the anode vibration amplitude was 5 μm and cathode traveling frequency and amplitude were 2 Hz and 70 μm, respectively. With an increase in anode vibration beyond 100 Hz, the slit width increases rapidly due to the radial swing of the wire.25 The deviation at 100 Hz is the lowest and the homogeneity the best. It has also been suggested that low-frequency vibration aids toward an increase in machining accuracy where high frequency doesnot have much effect.3 On the other hand, anode vibration amplitude has very little effect on slit width but a larger effect on deviation of the slit width. For this, the anode vibration amplitude of 5 μm has been taken with 100 Hz vibration frequency, ensuring better homogeneity. Cathode traveling amplitude has a significant effect on slit width, which increases almost linearly with amplitude, whereas an increase in cathode traveling frequency results in a very small increase in slit width. Apart from these, when cathode vibration frequency and amplitude is very low, electrolysis products accumulate in the machining gap and short-circuits occur. An increase in vibration frequency and amplitude to a certain extent results in increase of hydrodynamic effects on mass transport75 and better flushing, which lead to better machining stability. However, too high a vibration frequency and amplitude is detrimental, and results in radial swing of wire. Side gap is dependent of workpiece thickness, which highly effects machining stability because with an increase in aspect ratio of the workpiece, the occurrence of short-circuit increases as the implemented flushing technique tends to become improper.46 Moreover, during WECM the nozzle diameter also affects the flow pattern and slit width. Here, the side gap increases with an increase in nozzle diameter. When nozzle diameter increases, more scattering of flow occurs, resulting in dissolution over an increased area. Also, with an increase in nozzle diameter, the pressure and flow rate of electrolyte decreases, allowing electrolyte to envelop the tool loosely. Accordingly, a smaller nozzle diameter is needed to obtain machining with high resolution.
Apart from the solo influence of WECM parameters, there may be a coupling influence of process parameters like the combined effect of voltage, duty ratio, and concentration of electrolyte which may increase the effective feed rate drastically and make this process much more applicable for batch scale manufacturing. Moreover, the coupling effects of axial flushing and piezoelectric vibration or others have a very positive influence on product homogeneity and quality, as discussed above. Extensive research is required to further enhance process capability. Nonetheless, except discussing the influences of different operating parameters and their selection criteria for making WECM much more capable and effective, this new anodic dissolution technique also comes with its own challenges which need to be addressed as well as considered early.
https://iopscience.iop.org/article/10.1149/2.0391910jes
Machining high-quality metal components using pulsed electrochemical machining (PECM)
PECM is a non-contact, non-thermal advanced material removal process capable of small features, superfinished surfaces, and high repeatability for a range of metallic parts.
Voxel - Pulsed electrochemical machining (PECM) process
https://www.voxelinnovations.com/
Voxel has developed a pulsed electrochemical machining (PECM) process that enables unique geometries, better tolerances, and faster machining times than competing manufacturing technologies.
https://www.voxelinnovations.com/our-technology
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Electro-chemical Machining (ECM) for Producing Dies
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Selection of Electrolyte
Electrolytes can be classified into four main categories depending upon their nature and physical form. They are neutral aqueous salts, aqueous acids, aqueous alkalis, and nonaqueous electrolytes, respectively. Neutral aqueous salts are mostly used electrolytes since they are relatively cheaper and are generally harmless to the working tool and machinery.
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