Patent Publication Number: US-2012037509-A1

Title: Method of electrochemical machining of materials

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefits from the Russian Application RU 2010133710 filed on Aug. 11, 2010. The content of this application is hereby incorporated by reference and in its entirety. 
     BACKGROUND OF THE INVENTION 
     The invention relates to the field of electrochemical pulse machining (ECM) of high alloy steels, alloys and composite conducting materials comprising components with substantially differ in electrochemical properties. In particular, the invention can be used to perform various copy-piercing operations when producing intricately shaped surfaces of machine workpieces and tools using WC—Co, WC—TiC—Co alloys. 
     U.S. Pat. No. 5,833,835, IPC B23H3/02, published on Nov. 10, 1998, discloses a method of ECM by applying bipolar electric pulses, wherein pulses of normal polarity are alternated with pulses of opposite polarity. According to said method, the pulse voltage of opposite polarity is limited based on the condition whether working surface dissolution of a tool electrode is absent. In order to achieve this, the voltage of each pulse of opposite polarity is gradually decreased during machining in a range between the value of polarization voltage (which is defined at the point when the pulses of normal polarity are stopped being applied) and the value of voltage at which the electrochemical surface dissolution of the tool electrode begins; further the instantaneous value of the pulse voltage of normal polarity at a particular point is measured; the difference between the measured instantaneous values for each of the successive and current pulses of normal polarity is measured, and whether said difference reverses from negative to positive the upper boundary is determined, while whether said difference subsequently reverses from positive to negative, the lower boundary of the pulse voltage of opposite polarity is determined; the machining process is carried out while adjusting the pulse voltage of opposite polarity within said boundaries, and said machining process is finished with a pulse of opposite polarity. 
     Disadvantageously said method has limited process control possibilities precluding producing a surface layer of a desirable chemical composition or aligning the speeds of anodic dissolution of workpiece components to provide minimal surface roughness. 
     U.S. Pat. No. 6,402,931, IPC B23H 3/02, published on Jun. 11, 2002 discloses a method for ECM using modulated reverse current], the method comprising electrochemical machining using current pulses of normal (anode) polarity and opposite (cathode) polarity. The pulse of opposite (cathode) polarity is supplied before the pulse of normal (anode) polarity which allows creating desirable surface geometry of good quality (polishing properties) during electrochemical machining of easily passivated metals and alloys. 
     Disadvantageously, the method has limited process control possibilities precluding producing a surface layer of a desirable chemical composition or aligning the speeds of anodic dissolution of workpiece components to provide minimal surface roughness. 
     U.S. Pat. No. 5,242,556, IPC B23H 3/00, published on Sep. 7, 1993 discloses a method of electrochemical pulse machining in a neutral electrolyte, the method comprising applying alternating pulses of normal polarity with pulses of opposite polarity with changing time intervals between a pulse of opposite polarity and a pulse of normal polarity. 
     Disadvantageously, the method has limited process control possibilities precluding producing the surface layer of a desirable chemical composition or aligning the speeds of anodic dissolution of workpiece components to provide minimal surface roughness. 
     U.S. Pat. No. 6,231,748, IPC B23H 3/00, published on May 15, 2001 discloses a method and apparatus for electrochemical machining of conducting materials in an electrolyte, the method comprising subjecting a tool electrode and a workpiece to machining voltage pulses and passivating voltage pulses. Amplitude values of passivating voltage pulses are gradually increased from zero to the value at which the anode dissolution of workpiece material starts. After each voltage increase step, the electrical resistance of the gap between electrodes is measured. The voltage value with the highest electrical resistance of the gap between electrodes is used for subsequent machining. 
     Disadvantageously, the method has limited process control possibilities precluding producing the surface layer of a desirable chemical composition or aligning the speeds of anodic dissolution of workpiece components to provide minimal surface roughness. 
     RU 2271905 IPC B23H 3/00, published on Mar. 20, 2006 discloses a method of electrochemical machining of titanium and titanium alloys in electrolytes using activating adjustable high-frequency rectangular anode current pulses that are supplied in bursts which are synchronized with the moment of maximum convergence between an oscillating tool electrode and a workpiece, the method comprising performing machining at short interelectrode gaps. The duration of each pulse burst is controlled, the pulse burst delivery phase is controlled with respect to the moment of maximum convergence between electrodes, and the rate of the tool electrode delivery is also controlled, thereby maintaining a minimum value of the gap between electrodes, which provides the maximum number of decreasing voltage pulses in a burst. 
     Disadvantageously, the method has limited process control possibilities when machining highly-doped alloys and composite conducting materials precluding producing the surface layer of a desirable chemical composition or aligning the speeds of anodic dissolution of workpiece components to provide minimal surface roughness. 
     EP1714725 IPC B23H 3/00, published on Apr. 18, 2005 discloses a method for electrochemically processing high alloy steels, alloys and composite conducting materials comprising components with substantially different electrochemical properties, the method comprising changing the parameter ratio of pulses of normal and opposite polarity to provide a desirable ratio of anodic dissolution rates of a component based on acidity of the anode layer. Machining is carried out in neutral electrolytes at short interelectrode gaps using high-frequency microsecond anode current pulses supplied in bursts which are synchronized with the moments of maximum convergence between an oscillating tool electrode and a workpiece, and additional pulses supplied between bursts. 
     The above method is the most similar to the present invention and considered as being the closest prior art. 
     Disadvantageously, the known method cannot be used to regulate the machining process due to lack of criteria to promptly decide on adjustment of pulse parameters. Moreover, adjusting acidity of the interelectrode medium is not sufficient for achieving optimal operation of the machining process. For example, electrochemical anodic dissolution processes of Ni part of WC—Ni cermet hard alloy is preferably carried out at elevated temperatures in a neutral medium, while WC part is preferably machined in an alkaline medium in order to achieve minimum roughness of the workpiece surface. Obviously fast-acting impact is needed in order to perform the electrochemical machining process wherein physicochemical properties of the interelectrode medium (temperature, acidity) and physicochemical properties of the surface are adjusted optimally (in accordance with the technological objective) each time before applying a working burst of pulses of normal polarity. 
     The impact can be provided by an additional preceding current pulse of opposite polarity. However, the role thereof must be broader than that proposed in the prior art technical solutions. Namely, a chosen set of parameters for pulses of normal and opposite polarity must affect a number of factors: electrolyte acidity (pH), electrolyte and workpiece heating, charging and discharging of electric double layer capacity at the “surface-electrolyte” boundary. 
     Thus, the known methods of electrochemical machining cannot provide conditions for the efficient regulation of the ECM process or provide a way of choosing pulse parameters in order to achieve a desirable ratio of anodic dissolution rates for components of composite materials, wherein said components belong to different classes depending on electromechanical machinability thereof. Consequently, it is difficult to achieve significant decrease in surface roughness or provide a surface layer with a desirable ratio of machined material components using the above-identified methods. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a surface layer having a predetermined chemical composition, as well as surface quality, by means of electrochemical machining of conducting composite materials that include components with substantially different electrochemical properties by operatively controlling and adjusting machining process parameters that provides control over component anodic dissolution intensity. 
     Thus, in one aspect of the invention, a method of electrochemical machining of composite materials including components with substantially different electrochemical properties, is provided, wherein the machining is performed at short interelectrode gaps using anode or bipolar high-frequency current pulses supplied in bursts which are synchronized with the moments of maximum approach between an oscillating tool electrode and a workpiece, and additional singular pulses of opposite polarity, wherein the parameter ratio of pulses of normal and opposite polarity is adjusted based on the initial electrolyte acidic value, in which, in contrast to the closest prior art, the machining process is controlled in a way so that a transitional bend is formed at a peak of an additional pulse, said bend being caused by a change in electrical resistance of the interelectrode gap up to a steady value, the duration of additional pulse sections before and after the bend point being set depending on the machining mode and the composition of the material subjected to machining. 
     In another aspect of the invention, an apparatus for electrochemical machining of composite materials with components having substantially different electrochemical properties, is proposed, wherein the apparatus comprises an oscillating tool electrode, and further
         a current pulse generator, generating anode or bipolar high-frequency current pulses supplied in bursts, and further generating additional singular pulses of opposite polarity supplied during pauses between the bursts of pulses, wherein a ratio of parameters for the pulses of normal and opposite polarity is adjusted based on the initial electrolyte acidic value;   a synchronizing unit for synchronizing the said current pulses with the moments of maximum approach between the oscillating tool electrode and workpiece, and   a pulse regulator, the pulse regulator adjusting the pulses so that a transitional bend is formed at a peak of an additional pulse in response to the change in electrical resistance of the interelectrode gap to a preset value, wherein the duration of additional pulse sections before and after the bend point are preset depending on the machining mode and the composition of the material subjected to machining.       

     Alternatively, an example embodiment of the apparatus comprises a first current pulse generator generating anode or bipolar high-frequency current pulses supplied in bursts, and further a second current pulse generator generating additional singular pulses of opposite polarity supplied during pauses between the bursts of pulses, wherein a ratio of parameters for the pulses of normal and opposite polarity is adjusted based on the initial electrolyte acidic value. 
     In still another aspect of the invention, an article of manufacture is provided, obtained by a process according to the first aspect of the invention, wherein the article of manufacture has a surface layer having a predetermined chemical composition and surface roughness. The article of manufacture can be chosen from workpieces and tools made of WC—Co, WC—TiC—Co or other transitional metal of IV-VI group and alloys thereof. 
     Further, according to the invention, the process comprises a control step performed as follows: when the initial electrolyte acidity value is increased, the duration of additional pulse section after the transitional bend point, which characterizes the established process, is decreased, and when the initial electrolyte acidity value is decreased, said duration is increased. 
     Further, according to the invention, the process comprises a control step performed as follows: when the number of pulses in a burst is increased, the duration of additional pulse section after the transitional bend point, which characterizes the established process, is increased, and when the number of pulses in a burst is decreased, said duration is decreased. 
     Further, according to the invention, the process comprises a control step performed as follows: when the duration of anode pulses in a burst is increased, the duration of additional pulse section after the transitional bend point, which characterizes the established process, is increased, and when the duration of anode pulses in a burst is decreased, said duration is decreased. 
     Further, according to the invention, the process comprises a control step performed as follows: when the amplitude of anode pulses in a burst is increased, the duration of additional pulse section after the transitional bend point, which characterizes the established process, is increased, and when the amplitude of anode pulses in a burst is decreased, said duration is decreased. 
     Further, according to the invention, the process comprises a control step performed as follows: when the amount of binding component (e.g. cobalt) in the workpiece composite material is increased, the duration of additional pulse section after the transitional bend point, which characterizes the established process, is decreased, and when the amount of binding component in the workpiece composite material is decreased, said duration is increased. 
     Further, according to the invention, the process comprises a control step performed as follows: the ratio between durations of additional pulse sections before and after the transitional bend point is adjusted by changing the amplitude of the additional pulse. 
     Furthermore, according to the invention, the process comprises a control step performed as follows: the ratio between durations of additional pulse sections before and after the transitional bend point is adjusted by changing the amplitude of the pulses of opposite polarity in the burst of bipolar pulses. 
     Advantageously, the present method of electrochemical machining allows to achieve the low surface roughness and a predetermined chemical composition of the surface layer at varying component ratios when machining workpieces made of conducting composite materials comprising components with substantially different electrochemical properties. 
     The technical effect is achieved by regulating the machining process based on a selected criterion, which is the shape of an additional pulse peak. 
     Advantageously, the step of applying additional current pulse of opposite polarity during a pause between pulse bursts provides cleaning a workpiece surface from films and residues with hydrogen bubbles, and further obtaining a highly alkaline medium with high pH value in the interelectrode gap prior to supplying a burst of short pulses. 
     A method of electrochemical machining according to the present method, is controlled to keep the dissolution rates of the workpiece material components uniform. The transitional bend at the additional pulse peak characterizes two ongoing processes when said pulse of opposite polarity is supplied. The first process (from the beginning of the pulse to the bend point) is transitional, and is associated with charging of the electric double layer, and the second process (from the bend point to the end of the pulse) is steady, during which the energy of the additional pulse starts to get spent on changing pH of the near-electrode layer of the electrolyte. 
     When the anode pulses are supplied, oxidization processes occur on the workpiece surface. pH of the near-electrode layer is thus shifted towards the acidic medium (pH is decreased). When the cathode pulses (i.e. the additional pulses of opposite polarity) are supplied, reduction processes occur on the workpiece surface. pH of the near-electrode layer is thus shifted towards the alkaline medium (pH is increased). Therefore, the pH value of the electrolyte of the near-electrode area depends on amplitude-time pulse parameters. Therefore, the pH value can be adjusted by means of changing amplitude-time parameters of both working (anode) and additional pulses. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Further the present invention is explained by means of an example embodiment thereof alongside with the accompanying drawings serving to prove the possibility of implementation thereof, wherein: 
         FIG. 1  illustrates (a) voltage and current oscillograms taken during electrochemical machining of WC8 alloy by pulse bursts of bipolar current with additional pulse current strength being 2 A; (b) a photo image of the workpiece machined in said mode. 
         FIG. 2  illustrates (a) voltage and current oscillograms taken during electrochemical machining of WC8 alloy by pulse bursts of bipolar current with additional pulse current strength being 4 A; (b) a photo image of the workpiece machined in said mode. 
         FIG. 3  image (a) voltage and current oscillograms taken during electrochemical machining of WC8 alloy by pulse bursts of bipolar current with additional pulse current strength being 8 A; (b) a photo image of a workpiece machined in said mode. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present method for electrochemical machining can be implemented using a modified version of a copy-piercing machine, called Apparatus for Electrochemical Treatment with Microprocessor, such as for example SEP-905 produced by ECM, Ufa, Russia. A tool electrode in the present example embodiment was made of 12X18H10T material, and a workpiece subjected to machining was made of WC—Co alloy WC8 containing 8% cobalt. 
     According to the present example embodiment, the initial gap is adjusted to 20 μm, whereupon, bursts of working anode or bipolar pulses generated by current pulse generator are supplied in synchrony with oscillations of a tool electrode. During pauses between the working pulse bursts, singular additional pulses of opposite polarity generated by the same current pulse generator or another current pulse generator are supplied to achieve the initial acidic value of an electrolyte. The machining process is controlled by a control unit as follows: the amplitude of the additional pulse of opposite polarity is gradually increased until the formation of a transitional bend at the peak of the pulse.  FIG. 1  shows that the additional pulse current strength of 2 A is insufficient, the transitional bend at the additional pulse peak is poorly defined, and the workpiece surface is covered with an oxide film. 
     Further the amplitude of the additional pulse is increased until the moment when the ratio between durations of the time periods before and after the transitional bend point corresponds to an experimentally predetermined value. For the machined WC8 alloy, the duration ratio of time periods shall be within the range of 0.9-1 ( FIG. 2 ) so that the alloy components are dissolved uniformly, and the required surface quality is obtained. 
     When the machining is performed at current strength of 4 A ( FIG. 2 ), the transitional bend at the additional pulse peak is well-defined, the workpiece surface is clean. 
     When the current strength is further increased up to 8 A ( FIG. 3 ), the transitional bend is well-defined, but shifted towards the beginning of the additional pulse, i.e. the required duration ratio of sections before and after the bend point is not fulfilled. Parts of the workpiece surface are covered with oxide film. 
     Thus, the present invention provides the achievement of low surface roughness and a predetermined composition of the surface layer of workpieces made of conducting composite materials which include components with substantially different electrochemical properties and varying component proportions by using a method according to the invention including a control step for regulating the machining process