Method of electrochemical machining

Abstract

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.

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/cm 2 . 
     
     
         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.