METHOD OF ELECTROCHEMICAL MACHINING

- PECM INDUSTRIAL, LLC

The invention relates to the metalworking field, particularly to electrochemical sizing machining, and can be used for manufacturing of machine workpieces having an intricate profile and shaping furniture from chromium-containing steels and alloys operating in aggressive environment under excessive friction. Technical effect: improving machining accuracy by forming a lustrous layer on the machined surface and reduction of concentration of hexavalent toxic chromium ions in a waste electrolyte solution. Summary of invention: in the initial step, the unipolar electrochemical machining by operating pulses of normal polarity is carried out forming a layer enriched with chromium ions in the electrolyte area adjacent to the workpiece surface, then, upon achievement of the predetermined machining depth, shape and size of the workpiece, the operational current pulses of normal polarity and the machining electrode feeding are turned off and the residual polarization voltage value at the interelectrode gap is measured using the test high-frequency pulses of normal polarity, then low voltage pulses of opposite polarity synchronized with the phase of maximal approximation of the electrodes to each other are turned on and chromium cathode deposition onto the machined workpiece surface is carried out by means of alternating the pulses of opposite polarity with test high-frequency pulses of normal polarity and controlling the chromium deposition by increment of residual polarization value relative to its value after operational pulses of normal polarity.

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Description
CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefits from the Russian Application RU 2011101550 filed on Jan. 17, 2011. The content of this application is hereby incorporated by reference and in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to the metalworking field, particularly to electrochemical sizing machining, and can be useful in manufacturing intricately profiled surfaces of machine components and shaping furniture from chromium-containing steels and alloys operating in aggressive environment under excessive friction. More particularly, the invention is directed to form a protective chromium layer on a machined surface of a workpiece within a single electrochemical machining step, wherein the protective layer will have low roughness and thus, lustrous finish and provide high corrosion resistance and low friction coefficient as well as reducing the concentration of toxic hexavalent chromium ions in a waste electrolyte solution.

A method of pulse electrochemical machining (ECM) with DC voltage supply to electrodes within a pause between operational pulses is known, in which the DC voltage value is set lower than an electrolyte decomposition potential [Russian Patent Specification No. 506484, B 23 H 3/00, Bulletin of Inventions, Iss. 10, 1976].

The drawback of the known method resides in the fact that supplying to interelectrode gap DC voltage lower than an electrolyte decomposition potential, first, does not provide improvement of precision since constant charge of double electric layer on the metal-electrolyte interface decreases localization of anodic dissolution process and, and second, does not provide improvement of the surface quality (i.e., reduction of roughness and increase of corrosion resistance) since the conditions for forming high-quality chromium layer on the surface of a workpiece being processed are not defined. Another shortcoming is the absence of information about a point of time and conditions for the chromium layer formation as well as methods to control the formation of said layer.

A method of electrochemical machining chromium-containing steels in alkali metal nitrate-based electrolytes is known, in which the amplitude of a positive half-wave of the current (of normal polarity) is more than the negative one [Electrokhimicheskaya obrabotka metallov. Moroz, I. I. et al. Moscow, Machinostroenie Pub., 1969, pp. 64-65, 130].

The drawback of the known method resides in the fact that conditions for forming on a machined surface a chromium layer with high lustre are not defined. The conditions for reducing a concentration of toxic hexavalent chromium ions in a waste electrolyte solution during machining chromium-containing steels and alloys are not defined either. Moreover, continuous alternation of normal and opposite half-waves will result in anodic dissolution of the workpiece surface; therefore, the chromium layer will be dissolved even if it is formed during previous opposite half-wave. The information about a point of time and conditions for chromium layer formation as well as methods to control formation of the layer is not presented.

A method of electrochemical machining is known [U.S. Pat. No. 4,213,834, B23H3/02; B23H3/00; Jul. 22, 1980] in which in order to carry out a process at small interelectrode gaps, a signal representative of the voltage pulse distortion (when using a current source) is used. More particularly, a signal proportional to a maximal value of the second derivative with respect to pulse voltage is used.

The method allows carrying out the machining at irreducible interelectrode gaps along with providing high copying precision. However, the method does not disclose conditions for forming on a machined surface a lustrous chromium layer as well as reducing a concentration of toxic hexavalent chromium ions in a waste electrolyte solution during machining chromium-containing steels and alloys. A point of time and conditions for the chromium layer formation as well as methods to control the process of formation are not defined, either.

A method of electrochemical sizing machining is also known [Russian Patent No. 2038928, Oct. 10, 1990] in which the machining is carried out using a pulse power supply with steeply-falling current-voltage characteristics and oscillation of one of the electrodes, wherein an instant present value of voltage pulses is being controlled by selecting voltage spikes at sites of electrodes approximation to each other and moving apart and therewith increasing a machining electrode feeding speed until the third local voltage extreme in the pulse midpoint is formed, and maintaining this speed while keeping the following ratio


0<(Ul.e.−Umin)/Umin≦0.2,

where Ul.e.>Umin is the voltage amplitude of the third local extreme;

Umin is the minimal voltage value.

The method allows performing the machining at irreducible interelectrode gaps along with providing high copying precision during copy-piercing process and obtaining high electrolyte pressure in the interelectrode gap. However, formation of a voltage local extreme in the voltage pulse midpoint using machining electrodes made of 0.2-0.3 mm thick plate (foil) is impossible. It is explained by low rigidity of such electrodes that does not allow increasing the electrolyte pressure in an interelectrode gap and obtaining a signal for controlling the machining process, the signal being in the form of the voltage third local extreme in the pulse midpoint. However, the method does not disclose conditions for forming on a machined surface a lustrous chromium layer as well as reducing a concentration of toxic hexavalent chromium ions in a waste electrolyte solution during machining chromium-containing steels and alloys. A point of time and conditions for the chromium layer formation as well as methods to control the process of formation are not defined, either.

A method of ECM of an electrically conductive workpiece in an electrolyte by supplying bipolar pulses between the workpiece and an electrically conductive electrode is known, wherein one or more current pulses of normal polarity are being alternated with voltage pulses of opposite polarity [U.S. Pat. No. 5,833,835, B23H3/02; B23H3/00; Nov. 10, 1998].

The method is the closest one to the inventive method, and we accept it as the closest prior art.

The drawback of the known method resides in the fact that although the method allows conducting the machining at irreducible interelectrode gaps providing high copying precision during copy-piercing process to obtain high electrolyte pressure in the interelectrode gap, it is impossible to form a voltage local extreme in a pulse midpoint by increasing feeding speed when using machining electrodes made of 0.2-0.3 mm thick plate (foil). As was indicated above, it is explained by the low rigidity of such electrodes that does not allow increasing electrolyte pressure in the interelectrode gap and obtaining a signal for controlling the machining process, the signal being in the form of the voltage third local extreme in the pulse midpoint.

Moreover, while implementing this method, a pulse of opposite polarity is supplied at relatively large interelectrode gaps when an oscillating electrode is backed out from the surface of a workpiece to a large distance thereby decreasing the efficiency of opposite polarity pulses for producing a lustrous surface by deposition of chromium from the electrolyte. Thus, at large gaps the fluid resistance of IEG is decreased, the electrolyte rate is increased and the flow becomes turbulent that hinders relatively slow cathode deposition processes. Changing the ambient pressure at the IEG entry with the frequency of 10-100 Hz is difficult enough from the viewpoint of technology. And the supply of pulses of opposite polarity just after the pulses of normal polarity when the electrodes are moved apart results in reduction of electrolyte pressure in the interelectrode gap at the instance and beginning of intense gas-filling of a medium between the electrodes due to the boiling of overheated electrolyte and increasing the volume of a gas phase accumulated in the electrolyte during the positive half-wave period. The properties of such vapor-gas electrolyte mixture become substantially heterogeneous throughout the surface being machined. Upon that, the electrolyte conductivity is dramatically decreased resulting in general increase of the process power consumption.

Thus, none of the known methods of ECM when applied to machining of workpieces made of chromium-containing steels does not provide achievement of high copying precision and formation of a lustrous chromium layer on the machined surface as well as reducing a concentration of toxic hexavalent chromium ions in a waste electrolyte solution within a single technological operation.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to improve the quality of machining by forming a lustrous layer on a machined surface, and reduce a concentration of toxic hexavalent chromium ions in a waste electrolyte solution by means of machining at small interelectrode gaps using pulses of normal polarity at high current density as to form a polished surface followed by depositing chromium onto the machined surface using pulses of opposite polarity as to provide high lustre.

The object is attained by providing, in one aspect of the invention, a method of electrochemical machining of chromium-containing steels and alloys in electrolytes comprising aqueous solutions of alkali metals nitrates, wherein oscillation is being imparted to a machining electrode while bipolar current pulses synchronized with machining electrode oscillations are supplied to an interelectrode gap, and the machining is carried out at minimal gaps with controlled feeding speed. In accordance with the invention, in the initial step, unipolar electrochemical machining by operating pulses of normal polarity is carried out to form a layer enriched with chromium ions in the electrolyte area adjacent to a surface of the workpiece. Then, upon achievement of a predetermined machined depth, shape and size of the workpiece, operating current pulses of normal polarity and the machining electrode feeding are turned off and a value of residual polarization voltage at the interelectrode gap is measured using test high-frequency pulses of normal polarity. Then low voltage pulses of opposite polarity are turned on while synchronizing the pulses supply with a phase of maximal approximation of the electrodes to each other, and cathode deposition of chromium onto the machined surface of the workpiece is carried out by means of alternating the pulses of opposite polarity with test high-frequency current pulses of normal polarity along with controlling a process of chromium deposition by determining an increment of the residual polarization value with respect to the corresponding value after operating pulses of normal polarity.

In addition, in accordance with one embodiment of the invention, the upper limit of amplitude and duration of pulses of opposite polarity is bounded proviso that the etching of machining electrode operational surface is absent while the lower limit of amplitude and duration of pulses of opposite polarity is bounded proviso that continuous chromium layer is formed onto the machined workpiece surface.

In addition, in accordance with one embodiment of the invention, the duration of test voltage pulses of normal polarity is set within the range of 10-50 μs with frequency of 5-10 kHz while the amplitude—within 6-8 V.

In addition, in accordance with one embodiment of the invention, the value of increment of residual polarization relative to its value after operational pulses of normal polarity is empirically set during first 2-3 workpieces from the batch.

In addition, in accordance with one embodiment of the invention, at supply of current pulses of normal polarity the electrolyte pressure at the entrance of interelectrode gap is reduced to 50-150 kPa, and chromium deposition is carried out under the resulted electrolyte rate in the interelectrode gap.

In addition, in accordance with one embodiment of the invention, at the machining by operational pulses of normal polarity the size of interelectrode gap is reduced by gradual increase of machining electrode feeding speed upon the first breakdown of the interelectrode gap thereafter the feeding speed is reduced by 3-10% relative to the speed at which the breakdown occurred, and the machining is continued repeating this action if desired.

In addition, in accordance with one embodiment of the invention, the machining by operational current pulses of normal polarity is carried out in following modes: voltage on IEG is 5-15 V, electrolyte pressure at the IEG entrance is 50-500 kPa, electrolyte concentrations are 7-15%, and electrolyte temperatures are 18-40° C., ensuring current density within 50-1000 A/cm2.

In addition, in accordance with one embodiment of the invention, the polarization voltage is measured at the end of the last test pulse in the initial point of residual polarization decay curve, the duration of test pulse group being selected proviso that the polarization voltage achieves a steady-state value.

In another aspect of the invention, an apparatus for electrochemical machining of chromium-containing steels and alloys in electrolytes based on aqueous solutions of nitrate of alkali metals is provided, wherein the apparatus comprises an oscillating machining electrode, a speed regulator for regulating the speed of feeding the instrument to maintain the minimal interelectrode gap, and a current pulse generator for generating pulses of bipolar current synchronized with machining electrode oscillations for supplying to interelectrode gap,

wherein, the current pulse generator in the initial step of unipolar electrochemical machining generates operating pulses of normal polarity to form a layer enriched with chromium ions in the electrolyte area adjacent to the workpiece surface, the apparatus further comprising

a measurement unit for measuring a residual polarization voltage at the interelectrode gap using test high-frequency pulses of normal polarity, in the state once the predetermined machining depth, shape and size of the workpiece are achieved and the operational current pulses of normal polarity are switched off and the machining electrode feeding is stopped, and

wherein the current pulse generator generates low voltage pulses of opposite polarity synchronized with the phase of maximal approximation of the electrodes with each other to enable chromium cathode deposition onto the machined workpiece surface by alternating pulses of opposite polarity with test high-frequency pulses of normal polarity, and wherein the chromium deposition is controlled by increment of residual polarization value relative to its value after operational pulses of normal polarity.

In still another aspect of the invention, an article of manufacture obtained by a method according to the first aspect is provided, wherein the article has a protective chromium layer on a machined surface, wherein the protective layer provides at least one of low roughness, lustrous finish, high corrosion resistance and low friction coefficient.

The inventive method of electrochemical machining of chromium-containing steels and alloys allows improving the machining accuracy and producing lustrous finish as well as reducing the concentration of hexavalent toxic chromium ions in a waste electrolyte solution.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Furthermore, the proposed invention is illustrated by non-limiting examples of realization and accompanying figures confirming the possibility of its realization on which:

FIG. 1 illustrates the process flow diagram in accordance with the invention;

FIG. 2 illustrates oscillograms of voltage and current in IEG at the step of shaping by pulses of normal polarity and at final step during chromium deposition onto the machined surface in accordance with the invention, where S is a track of oscillating machining electrode, mm depending on time t, s, U is voltage of pulses of normal polarity, V, UA is voltage of residual polarization after test pulses succeeding the shaping step, V; UB is voltage of residual polarization determined by test high-frequency pulses after final step of chromium deposition, V; j is technological current density of pulses of normal polarity, A/cm2.

FIG. 3a illustrates oscillograms of residual polarization voltage after pulses of normal polarity determined by high-frequency pulses of normal polarity (curve 1), and oscillograms of current of high-frequency pulses (curve 2) in IEG at the step of shaping by pulses of normal polarity,

FIG. 3b illustrates oscillograms of residual polarization voltage after pulses of opposite polarity determined by high-frequency pulses of normal polarity (curve 1), and oscillograms of current of high-frequency pulses (curve 2) in IEG in the final step during machining by pulses of opposite polarity at chromium deposition onto the machined surface,

FIG. 4 illustrates the control structure of technological current generator and source of current of opposite polarity of electrochemical machine in accordance with the invention, where: 3 is a controllable technological current source; 4 is a controllable source of technological current of opposite polarity; 5 is a controllable source of current (test pulses) of normal polarity; 6 is an electronic switch of the technological current source; 7 is an electronic switch of the source of current of opposite polarity; 8 is an electronic switch of the source of test pulses; 9 is an automatic control system of the technological process; 10 is a control block of the generator.

FIG. 5 illustrates an appearance of the machined workpiece surfaces with corresponding profilograms after ECM using unipolar current pulses of normal polarity (A) and after ECM pulses of opposite polarity (B) in accordance to the provided method;

FIG. 6 illustrates ratio of chromium to iron concentration in the surface layer after ECM by unipolar pulses of normal polarity (A) and at ECM by pulses of opposite polarity (B) in accordance to the provided method derived from the method of secondary ion mass spectroscopy.

 DETAILED DESCRIPTION OF THE INVENTION

The invention is illustrated for better understanding by way of non-limiting example embodiments thereof, which are discussed in more detail below

The process flow diagram of pulse bipolar electrochemical machining (ECM) by an oscillating EDM electrode at ET series machines in its most general form is illustrated in FIG. 1. As shown in FIG. 1, a machining electrode 1 performs periodic oscillations S(t) relative to the machined surface of the blank electrode 2 uniaxial to the feeding direction Vk.

At the beginning, near the phase of maximal approximation of the electrodes to each other, an operational current pulse or group of pulses of normal polarity and high density (within the range of 50-1000 A/cm2) is supplied, then, upon the upon achievement the predetermined machining depth at final step of the process, the current pulses of normal polarity and the machine feeding are turned off, the total residual polarization voltage value UA after supply of group of test low voltage pulses is determined (FIG. 2), and low voltage pulses of opposite polarity are turned on, the instant of the pulses of opposite polarity supply being synchronized also with the phase of maximal approximation of the electrodes, and chromium cathode deposition onto the machined workpiece surface is carried out from the electrolyte at small interelectrode gaps. Hereupon, the test high-frequency pulses are again turned on in order to determine residual polarization total voltage value after pulses of opposite polarization UB. Then the low voltage pulses of opposite polarity are turned on, the instant of the pulses of opposite polarity supply being synchronized also with the phase of maximal approximation of the electrodes to each other, and chromium cathode deposition onto the machined workpiece surface is carried out from the electrolyte at small interelectrode gaps. Hereupon, the test high-frequency pulses are again turned on in order to determine residual polarization total voltage value after pulses of opposite polarization UB.

Changes in polarization values UA and UB are determined after the current being turned off, thereby excluding the resistive component from the measuring voltage value and improving the reliability of measurement of difference between UA and UB that is defined as a representative parameter of enrichment of the machined surface by chromium (FIG. 3).

The test pulses ensure recharging of double electric layer capacity, establishment of the value of polarization that consists from anode and cathode potential. Hereupon, cathode potential is established more rapidly than the anode one, and its steady-state value at fixed current density is stable while the anode potential value depends on properties of the machined surface and makes a major contribution to the increment of residual polarization value UA and UB. The size of group of test pulses being selected proviso that the polarization voltage achieves a steady-state value.

Moreover, the test pulses alternated with the pulses of opposite polarity can provide more favorable conditions for chromium deposition onto the machined surface since the electrolytic brightening ensuring large amount of nuclei and good adhesion with the support is considered to be the best method of preparation of the surface for metal deposition [Povetkin, V. V. Structure of electrolytic coatings (in Russian)/Povetkin, V. V., Kovenskij, I. M. Moscow. Metallurgy Pub. 1989. 136 P.].

The proposed method of electrochemical machining of chromium-containing steels in electrolytes based on aqueous solution of alkali metal nitrates is carried out in electrolyte flow with superposition of oscillations to one of the electrodes (FIG. 1). A source with steep current-voltage characteristic (FIG. 4) which is periodically connected to IEG by electronic switch of technological current source 6 near the phase of maximal approximation of electrodes to each other is used as power supply 5 of normal polarity (Ip). Time of closed condition of the electronic switch of technological current source 6 dictates the duration of current pulse (ti) of normal polarity (Ip).

The current pulses of opposite polarity (In) flow through IEG is ensured by turning on the electronic switch of source of current of opposite polarity 7 (see FIG. 4).

A source with steep current-voltage characteristic (FIG. 4) which is periodically connected to IEG by electronic switch of test pulse source 8 is used as the test pulse generator.

Increase of chromium amount onto the machined surface after bipolar electrochemical machining is confirmed by results of determination of surface layer composition carried out using different methods.

Studies of surface of the machining electrode made of 40×13 steel were carried out after unipolar ECM and bipolar ECM with an additional pulse of opposite polarity after operational current pulse when the significant changes in machined surface quality are taking place (FIG. 5).

The experiments were carried out in 8% sodium nitrate NaNO3 solution at current density of operational pulse of ˜100 A/cm2 and pulse duration of 1.5 ms, current density and duration of pulse of opposite polarity of ˜5 A/cm2 and 2 ms, respectively. The instant of supply of the operational pulses and pulses of opposite polarity was synchronized with the phase of maximal approximation of EDM electrode to the machined surface. The duration of test pulses was of 50 μs while the voltage amplitude was selected to be not more than 8 V.

The results of assay of the surface layer by method of secondary ion mass spectroscopy showed increase of chromium relative to iron concentration after bipolar ECM compared to unipolar ECM.

When using such type of surfaces in mating pairs, shaping tooling (dies, mandrels) etc. a friction coefficient is reduced and fatigue strength, wear-resisting properties and corrosion resistance are improved. For example, durability of a die from instrumental steel for production of “Torx” type hollows in steel screws increased more than twice compared to the analogous die manufactured using conventional technology (by means of mechanical benchwork) and coated by titanium nitride. The similar results are expected with dies for tablet formation (drug industry).

It shall be appreciated that the prior art methods of unipolar machining are usually connected with depletion of the surface layers of chromium-containing steels in chromium. Advantageously, a method of bipolar machining in accordance with the present invention provides the formation of chromium-containing layers on a wide range of chromium-containing steels where the process is automatically controlled.

In view of the high requirements for personnel protection and environmental protection from pollutants derived from electrochemical machining (ECM) of chromium-containing steels and alloys, study of changes in amount of bichromate ions in an electrolyte gains an important role. It becomes even more important due to need in reducing the wastes, from the economical standpoint (adoption of recirculation scheme) results in dramatic increase of electrolyte solution utilization time and, thereby, to increase of bichromate ions content in the solution that will require the solution regeneration or replacement operations.

However, the amount of bichromate ions in the solution could rather be reduced by means of their deposition onto the pre-machined surface at high current density (e.g., larger that 100 A/cm2) by pulses of opposite polarity in accordance with the provided method. Since the metal atoms in bichromate ions Cr2O72− have the maximal oxidation number they can not oxidize on the positively charged anode, therefore they possess high standard potential ((φ°Cr2O72−/Cr3+=+1.33 V) and reduce on the machined workpiece surface upon to metal chromium. If the workpiece polarity will be changed to negative and the corresponding conditions (e.g., small interelectrode gaps of 10-100 μm and voltage pulses of opposite polarity preventing etching of the operational surface of machining electrode but sufficient for discharge of chromium ions on the machined surface) will preliminary be provided, chromium deposition will occur in accordance with the reaction:


Cr2O72−+14H++12e→2Cr+7H2O.

EXAMPLE

A particular embodiment of the inventive method of electrochemical machining in accordance with the invention.

The inventive method of ECM by bipolar current pulses was carried out at ET500 model electrochemical copy-piercing machine produced by “ECM” Ltd, Ufa, Russia, using 40×13 steel as the material of sample and machining electrode. The machining was carried out in aqueous 9.5% sodium nitrate solution upon the depth of 5 mm with area of 200 mm2.

Before the machining step, an oscillating machining electrode 1 (FIG. 1) and machined blank 2 were approximated to each other upon the mutual contact with the absence of technological voltage on them and moved apart to predetermined value of minimal interelectrode gap St=20 μm (FIG. 1).

Then, at the first machining step, the following mode of machining by pulses of normal polarity was set:

    • the frequency of rectangular current pulses and machining electrode oscillations, Hz—49;
    • the duration of voltage pulse, ms—1.5;
    • the amplitude of machining electrode oscillations, mm—0.15;

The amplitude of rectangular voltage pulse at instant of minimal distance between the electrodes, V—10.5;

The electrolyte pressure at the entrance of interelectrode gap, kPa—100;

The electrolyte temperature, ° C.—20.

The electrolyte feeding is direct through the central vent of machining electrode.

During penetration of machining electrode 1 (FIG. 1) into the blank 2 upon the depth of 0.1-0.3 mm the feeding speed was of 0.1 mm/min. Then, with further penetration of the machining electrode 1 (FIG. 1) into the blank 2 the electrolyte pressure was gradually increased upon 350 kPa. During the machining by pulses of normal polarity the feeding speed was gradually increased upon the first breakdown that corresponded to EDM electrode feeding speed of 0.16 mm/min, then the feeding speed was reduced by about 7% and the machining was continued upon the predetermined depth.

Upon achievement of the predetermined depth of 5 mm the current pulses of normal polarity and the machine feeding were turned off, and the residual polarization total voltage was determined by turning on the test high-frequency pulses with voltage amplitude of 7 V and pulse duration of 100 μs. Then the machining was turned on, in the initial step low voltage rectangular pulses of opposite polarity with amplitude of 3 V and duration 2 ms were set, the instant of supply of pulses of opposite polarity being synchronized also with the phase of maximal approximation of the electrodes to each other, and cathode deposition of chromium from the electrolyte onto the machined surface at small interelectrode gaps was carried out with periodical measurements of voltage by test high-frequency pulses. Upon that, at the step of chromium deposition, the pulses of opposite polarity were alternated with test high-frequency pulses of normal polarity ensuring the control of chromium deposition upon the necessary increment of residual polarization level relatively to its value after pulses of normal polarity, the value of necessary increment being predetermined on 2-3 workpieces within the batch.

When supplying pulses of opposite polarity, the electrolyte pressure was reduced upon 100 kPa thereby forming laminar stream within the interelectrode gap providing favorable conditions for deposition of chromium from the electrolyte onto the machined surface. Hereupon, the amplitude and duration of pulses of opposite polarity were bounded proviso that the etching of machining electrode operational surface is absent while the pulse amplitude and duration are sufficient for discharge of chromium ions on the machined workpiece surface.

The analysis of machining results showed that upon the use of the provided method a significant reduction of hexavalent chromium in the waste electrolyte and producing a lustrous finish on the machined surface (Ra<0.15 μm) occurred, the EDM electrode copying error did not exceed 0.01 mm while the feeding speed value during the machining by pulses of normal polarity was of 0.15 mm/min.

Claims

1. A method of electrochemical machining of chromium-containing steels and alloys in alkali metal nitrate aqueous solutions-based electrolytes, wherein a machining electrode is subjected to oscillations, and the pulses of bipolar current synchronized with machining electrode oscillations, are supplied to an interelectrode gap,

the method further comprising the step of controlling the speed of feeding the machining electrode to maintain minimal gap between the machining electrode and the machined workpiece, wherein,
in the initial step, the unipolar electrochemical machining by operating current pulses of normal polarity is carried out to form a layer enriched with chromium ions in the electrolyte area adjacent to the workpiece surface;
then, upon achievement of the predetermined depth of machining, shape and size of the workpiece, the operating current pulses of normal polarity and the machining electrode feeding are turned off, and the residual polarization voltage value in the interelectrode gap is measured using test high-frequency pulses of normal polarity;
then low voltage pulses of opposite polarity are turned on while synchronizing feeding the pulses of opposite polarity with the phase of maximal approximation of the electrodes to each other; and chromium cathode deposition onto the machined workpiece surface is carried out by alternating the pulses of opposite polarity with the test high-frequency current pulses of normal polarity while controlling the chromium deposition by increment of a residual polarization value relative to the value obtained after operating pulses of normal polarity are applied.

2. A method as claimed in claim 1, wherein the upper limit of amplitude and duration of the pulses of opposite polarity is bounded as to avoid etching of the machining electrode operating surface, while the lower limit of amplitude and duration of pulses of opposite polarity is bounded as to provide formation of a continuous chromium layer onto the machined workpiece surface.

3. A method as claimed in claim 1, wherein the duration of test voltage pulses of normal polarity is set within the range of 10-50 μs with frequency of 5-10 kHz while the amplitude thereof is set within 6-8 V.

4. A method as claimed in claim 1, wherein the increment value of residual polarization relative to the value obtained after operational pulses of normal polarity are applied is set empirically using the first 2-3 workpieces from the batch.

5. A method as claimed in claim 1, wherein when feeding current pulses of opposite polarity, the electrolyte pressure in the entrance into the interelectrode gap is reduced to 50-150 kPa, and chromium deposition is carried out with the resulted electrolyte rate in the interelectrode gap.

6. A method as claimed in claim 1, wherein when the machining by operating pulses of normal polarity is being performed, the size of the interelectrode gap is reduced by increasing gradually the machining electrode feeding speed until the first breakdown of the interelectrode gap is occurred; then the feeding speed is reduced by 3-10% relative to the speed at which the breakdown occurred, and the machining is continued while repeating this action, if necessary.

7. A method as claimed in claim 1, wherein the machining by operating current pulses of normal polarity is carried out in the following modes: voltage on IEG is 5-15 V, electrolyte pressure at the IEG entrance is 50-500 kPa, electrolyte concentrations are 7-15%, and electrolyte temperatures are 18-40° C. as to provide the current density within 50-1000 A/cm2.

8. A method as claimed in claim 1, wherein the polarization voltage is measured at the end of the last test pulse in the initial point of residual polarization decay curve, wherein the duration of test pulse group is selected such that a steady-state value of the polarization voltage is achieved.

9. An apparatus for electrochemical machining of chromium-containing steels and alloys in electrolytes based on aqueous solutions of nitrate of alkali metals, wherein the apparatus comprises an oscillating machining electrode, a speed regulator for regulating the speed of feeding the instrument to maintain the minimal interelectrode gap, and a current pulse generator for generating pulses of bipolar current synchronized with machining electrode oscillations for supplying to interelectrode gap,

wherein, the current pulse generator in the initial step of unipolar electrochemical machining generates operating pulses of normal polarity to form a layer enriched with chromium ions in the electrolyte area adjacent to the workpiece surface, the apparatus further comprising
a measurement unit for measuring a residual polarization voltage at the interelectrode gap using test high-frequency pulses of normal polarity, in the state once the predetermined machining depth, shape and size of the workpiece are achieved and the operational current pulses of normal polarity are switched off and the machining electrode feeding is stopped, and
wherein the current pulse generator generates low voltage pulses of opposite polarity synchronized with the phase of maximal approximation of the electrodes with each other to enable chromium cathode deposition onto the machined workpiece surface by alternating pulses of opposite polarity with test high-frequency pulses of normal polarity, and wherein the chromium deposition is controlled by increment of residual polarization value relative to its value after operational pulses of normal polarity.

10. An article of manufacture obtained by a method of claim 1, having a protective chromium layer on a machined surface, wherein the protective layer provides at least one of low roughness, lustrous finish, high corrosion resistance and low friction coefficient.

11. The article of manufacture of claim 10, wherein roughness of the machined surface is less than 0.15 μm.

Patent History
Publication number: 20120181179
Type: Application
Filed: Apr 29, 2011
Publication Date: Jul 19, 2012
Applicant: PECM INDUSTRIAL, LLC (Ufa)
Inventors: Vyacheslav Alexandrovich ZAYTSEV (Ufa), Nasih Ziyatdinovich GIMAEV (Ufa), Timur Rashitovich IDRISOV (Ufa)
Application Number: 13/097,734
Classifications
Current U.S. Class: Involving Measuring, Analyzing, Or Testing (205/81); Nonreversing Pulsed Current Or Voltage (205/104); With Current Control (204/223)
International Classification: C25D 5/18 (20060101); C25F 3/06 (20060101); C25D 17/00 (20060101); C25D 21/12 (20060101);