RU2465993C2 - METHOD OF ELECTROMACHINING SOLID WC-Co ALLOYS - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to pulsed electromachining of WC-Co alloys and may be used in precision copying broaching in making complex-surface machine parts and tools from hard metals. Part is machined by trains of current pulses time to maximum approach between oscillating electrode and part. Here, small electrode gap filled with electrolyte is kept there between. First, in pause between trains of current pulses, measured is residual voltage u1 corresponding to clean alloy anode surface to be taken as reference parameter. Also, residual voltage u2 corresponding to alloy surface blocked by anode films is defined. Then, part is machined with measuring residual voltage u3 in pauses between current pulse trains. In case u3 is under reference u1, voltage in electrode gap is increased while electrolyte logarithmic hydrogen ion exponent pH is decreased at electrode gap inlet. In case u3 exceeds reference u1, voltage in electrode gap is decreased while electrolyte logarithmic hydrogen ion exponent pH is increased at electrode gap inlet. In case u3 exceeds u2, reverse-polarity current pulses are fed unless difference between u3 and u1 is zero.

EFFECT: higher stability and efficiency of machining.

6 cl, 2 dwg, 1 ex

Description

The invention relates to the field of pulsed electrochemical processing (ECHO) of conductive materials. In particular, the present invention can be used to perform various copy-stitching operations in the manufacture of complex-shaped surfaces of machine parts and tools from solid WC-Co alloys (based on tungsten carbide and cobalt binder).

A known method of pulsed electrochemical processing of conductive parts from alloys and composite materials containing components with substantially different electrochemical properties, where the ratio of the parameters of the pulses of direct and reverse polarity is changed to provide a certain ratio of the rates of anode dissolution of the components due to changes in the acidity of the anode layer. Processing is carried out in initially neutral electrolytes at small interelectrode gaps using anode high-frequency microsecond current pulses supplied by packets that synchronize with the moments of maximum approach of the oscillating electrode tool and the workpiece and additional pulses supplied between packets [EP 1714725, B23H 3/02, publ. 10/25/2006].

The disadvantages of this method are:

- The inability to control the actual changes in the pH in the interelectrode space (MEP) and, moreover, the pH of the electrolyte in the area adjacent to the surface of the part. The effect of the use of pulses of direct and reverse polarity for a real process is estimated indirectly by the total integral technological effect - surface roughness. In this connection, it is not possible to explain why roughness indices are improved: due to a change in the acidity index or other reasons.

- There are no informative parameters for the operational control of the process of dissolution of WC-Co hard alloys and ensuring a stable ECHO process.

- The method proposed in the analogue has limited possibilities for controlling the appearance and removal of films from cobalt salts and tungstates on the surface of the part, which have a significant effect on the dissolution of WC-Co hard alloys and even completely blocking the process.

A known method of ECHO by bipolar current pulses [5833835, IPC V23N 3/02, publ. 10.11.1998], in which the pulses of direct polarity alternate with pulses of reverse polarity. According to this method, the voltage pulses of reverse polarity current are limited from the condition that there is no dissolution of the working surface of the electrode-tool. To do this, during processing, the voltage of each pulse of reverse polarity is successively reduced in the range from the polarization voltage determined at the moment the current pulse ceases to be of direct polarity to the voltage at which the electrochemical dissolution of the working surface of the electrode-tool (EI) begins, then measure the instantaneous the voltage pulse value of the current of direct polarity at a given point, calculate the difference between the measured instantaneous values for each subsequent and previous pulses directly polarity and when the sign of this difference is changed from minus to plus, the upper limit is determined, and when the sign changes from plus to minus, the lower boundary of the voltage pulses of reverse polarity current is determined and the processing process is carried out, holding the voltage of reverse polarity current pulses at these boundaries, and complete the process of processing a current pulse with reverse polarity.

The disadvantage of this method is the limited ability to control the occurrence and removal during the anodic dissolution of WC-Co hard alloys blocking the process of films of cobalt salts and tungstates (CoO, Co (OH) 2, WO ₃ , C, CoWO ₄ ) on the surface of the part. Also, the possibility of controlling the deviation of the chemical composition of the treated surface from a predetermined reference is not considered, for example, by changing the ratio of the dissolution rates of the components and there are no means or actions for adjusting the chemical in case of deviation.

A known method of electrochemical processing [RU 2286233, IPC B23H 3/02, publ. 10.27.2006] hard alloys by applying bipolar electric pulses between the workpiece and the electrically conductive electrode, while one or more unipolar voltage pulses with the corresponding dimensional processing polarity alternate with voltage pulses of the opposite polarity, while between the workpiece and the electrode the tool maintains a gap that is filled with electrolyte. Processing according to this method includes the following stages: measuring the first value of the working parameter, which depends on the cleanliness of the front surface of the electrode-tool, measuring the second value of the working parameter in the interval between unipolar pulses and calculating the difference between the measured values and applying at least one pulse of opposite polarity only after the measured value becomes nonzero.

This method is the closest to the claimed and adopted by us as the closest analogue.

The disadvantage of this method is the lack of informative parameters that allow you to control the process of anodic dissolution, in particular, changing the ratio of the dissolution rates of the components and quickly make decisions about changing the parameters of the pulses. So, for example, when processing WC-Co hard alloys, it is necessary to have an information parameter that allows you to control the occurrence and development of a blocking anode film and a controlled operational external effect that allows you to destroy this film.

Thus, the known methods of electrochemical processing cannot provide conditions for the operational control of the treated surface in terms of the relative ratio of components (WC and Co), as well as determining the moment of occurrence and elimination of blocking films. Therefore, when using these methods, it is technologically difficult to achieve process stability and high productivity.

The problem to which the invention is directed, is to ensure stability and high performance of the electrochemical treatment of conductive WC-Co hard alloys, due to operational control and destruction of the anode film blocking the dissolution process.

The problem is solved by the method of electrochemical processing of solid WC-Co alloys by applying groups of electric current pulses between the part and the electrode-tool, while the supply of current pulses is synchronized with the moments of the closest approach of the oscillating electrode-tool to the part, while between the workpiece and the electrode - the tool maintains a small interelectrode gap, which is filled with an electrolyte, and which includes the stages of measuring and comparing the operating parameters depending on the state of the surface, in which, in contrast to the prototype, the residual voltage u1 corresponding to the clean anode surface of the alloy is measured in a pause between groups of current pulses and taken as a reference parameter, the residual voltage u2 corresponding to the surface blocked by the anode films is also determined, then they conduct the processing process by measuring in the pause between groups of current pulses the residual voltage u3, and in the case when u3 is less than the reference u1, the carbide dissolution rate is slowed down tungsten and increase the dissolution rate of cobalt, increasing the voltage on the MEP and decreasing the logarithmic pH of the electrolyte at the entrance to the MEP, and when u3 is more than the reference u1, increase the dissolution rate of tungsten carbide and decrease the dissolution rate of cobalt, decreasing the voltage on the MEP and increasing the value of the logarithmic pH of the electrolyte at the entrance to the MEP, and in the case when u3 is greater than u2, single pulses of reverse polarity current are applied until the difference between u3 and u1 will not be equal to zero.

The value of the logarithmic pH of the electrolyte at the entrance to the MEP is adjusted by using the catholytic or analytic component of the electrolyzer located at the entrance to the interelectrode gap.

As an electrolyte providing simultaneous dissolution of the components and minimal formation of blocking films, depending on the chemical composition of the alloy and the grain size, use:

for WC-Co hard alloys with a high cobalt content of 15-25% and a WC grain size of more than 1.5 μm, the composition is 3-5% NaOH + 8-10% NaNO ₃ at a temperature of 20 ... 25 ° C;

for WC-Co hard alloys with a low cobalt content of 5-15% and a WC grain size of more than 1.5 μm, the composition is 3-5% NaOH + 10-20% NaNO ₃ , at a temperature of 45 ... 50 ° C;

for WC-Co hard alloys with a high cobalt content of 15-25% and a WC grain size of less than 0.7 μm, the composition is 5-10% NaOH + 10-15% -NaNO ₃ at a temperature of 20 ... 25 ° C;

for WC-Co hard alloys with a low cobalt content of 5-15% and a WC grain size of less than 0.7 μm, the composition is 10-15% NaOH + 10-15% NaNO ₃ at a temperature of 40 ... 45 ° С.

The following working area of the processing mode parameters is used:

the voltage amplitude of the pulses of direct polarity is 14 ... 28 V;

the duration of the pulses of direct polarity is 50 ... 100 μs;

the number of pulses of direct polarity in the group is 5 ... 20;

the voltage amplitude of the pulses of reverse polarity is 2 ... 5 V;

pulse width of reverse polarity - 2 ... 18 ms;

the pressure of the electrolyte at the entrance to the MEP is 50 ... 300 kPa;

vibration frequency - 10-100 Hz.

The reverse polarity current pulse is supplied in the phase of maximum approximation of the electrode-tool and the part immediately after the supply of direct polarity current pulses.

The reverse polarity current pulse is supplied in the phase of maximum approximation of the electrode-tool and the part in the next period after the supply of direct current polarity pulses.

The proposed method of electrochemical processing allows the processing of parts from WC-Co hard alloys to quickly determine the occurrence of blocking anode films of cobalt and tungstate salts (CoO, Co (OH) 2, WO ₃ , C, CoWO ₄ ) and destroy them, ensuring stable and high-performance ECHO process and supporting by controlling the properties of the electrolyte at the entrance to the MEP and the parameters of the processing mode.

The essence of the method is as follows.

In industry, WC-Co-type hard alloys with a cobalt content in the range from 12 to 25% and tungsten carbide grain sizes from 0.2 to 4 μm are most common.

The electrochemical anode behavior of WC-Co type hard alloys changes significantly with a change in the percentage of cobalt binder and with a change in grain size.

Based on the results of polarization studies, studies of current efficiency and volumetric dissolution rates, it was found that during the electrochemical dissolution of hard alloys, blocking films of cobalt and tungstate salts (CoO, Co (OH) ₂ , WO ₃ , C, CoWO ₄ ) are formed on the anode surface , preventing the anodic dissolution of the entire alloy.

It is known that hydrogen bubbles formed on the cathode have a powerful mechanical effect that can destroy salt-blocking surface films [Removing cathode deposits during pulsed electrochemical processing / Amirkhanova N.A., Gimaev N.E., Zaitsev A.N. and others // Technology of metals, 2001, No. 7, p.25-31].

Electrochemical and chemical studies have shown that the electrochemical treatment of hard alloys occurs selectively both in sodium nitrate solutions and in alkali. In a solution of 8% NaNO _3, cobalt from a cobalt bond is mainly ionized; therefore, tungsten accumulation in the surface layer should be expected. In alkali solutions in the surface layer, one should expect the accumulation of cobalt, which is passivated in alkali with the formation of oxides and basic salts of cobalt.

With a combination of ions that provide ionization of both cobalt and tungsten, in combined electrolytes based on aqueous NaNO ₃ solutions and small percentages of NaOH, comparable rates of cobalt and tungsten ionization can be achieved. In particular, the regulation of the ratio of the rates of dissolution of the components can be achieved by adjusting the pH of the solution at the entrance to the MEP.

It is also known that the appearance of various films and deposits on the electrode surface changes the residual polarization (residual voltage on the MEP after the current pulse ceases) on the surface [Zaitsev AN, Zhitnikov VP, Idrisov TR et al. High-speed anodic dissolution under conditions of non-stationary electrode potentials. - Ufa: Gilem, 2005. - P.180-185], also the residual polarization depends on the chemical composition of the surface.

Thus, when processing solid WC-Co alloys according to the proposed method, by measuring the residual polarization, it becomes possible to conduct the process in a stable mode, comparing it with the reference value of the clean surface, and also to determine the appearance of a blocking film on the anode surface that can be destroyed by hydrogen bubbles, giving single pulses of reverse polarity.

The electrolyte compositions are selected based on the composition of the alloy being processed:

for WC-Co hard alloys with a high cobalt content of 15-25% and a WC grain size of more than 1.5 μm, electrolytes of the composition 3-5% NaOH + 8-10% NaNO ₃ can be recommended,

for WC-Co hard alloys with a low cobalt content of 5-15% and a WC grain size of more than 1.5 μm, electrolytes of the composition 3-5% NaOH + 10-20% NaNO ₃ can be recommended,

for WC-Co hard alloys with a high cobalt content of 15-25% and a WC grain size of less than 0.7 μm, electrolytes of the composition 5-10% NaOH + 10-15% NaNO ₃ can be recommended,

For WC-Co hard alloys with a low cobalt content of 5-15% and a WC grain size of less than 0.7 μm, electrolytes of the composition 10-15% NaOH + 10-15% NaNO ₃ can be recommended.

In the future, the invention is illustrated by specific examples of its implementation and the accompanying drawings, confirming the possibility of its implementation, which depict:

Figure 1 Harvesting and sample details of WC-Co carbide,

Figure 2 The pulse supply circuit.

Concrete implementation example

The proposed method of electrochemical processing is implemented on a modernized copy-firmware machine model ET500. The electrode tool is made of 12X18H10T material, and the workpiece is made of WC-Co alloy containing 8% cobalt.

Under optimal processing conditions, while maintaining the mode when the measured value of the residual voltage in the pause between groups of pulses of direct polarity was maintained equal to the reference value, in the manufacture of a part with a hexagonal recess (Figure 1) from WC-Co carbide containing 8% cobalt with a grain size of 1 ... 4 μm in an electrolyte based on an aqueous solution of 8% NaNO ₃ + 0.5% NOH with a change in pH at the entrance to the MEP, a linear electrochemical processing capacity of 50 μm / min was ensured.

In this case, before the start of processing, after the interelectrode gap S was set and the electrolyte was supplied to the MEP, groups of pulses of direct polarity were applied and the value of the residual polarization in the pause between the groups of pulses was recorded. This value was the parameter u1 on the voltage waveform (Figure 2). Next, conditions were created for blocking the treated surface with anode films (by increasing the pulse duration), while the supply of the electrode-tool was stopped. The value of the residual polarization in the pause between groups of pulses was measured, this value was recorded as parameter u2 (Figure 2). After the formation of the reference parameters, a new blank of the same material was installed or the surface was cleaned of a blocking film. For this, the phase of the supply of groups of pulses of direct polarity was shifted to the region up to the phase of the closest approach of the EI and the workpiece, where single pulses of reverse polarity were supplied. Then, the processing process was carried out, measuring the residual stresses u3 in a pause between groups of pulses, and in the case where u3 was less than the reference u1, the dissolution rate of tungsten carbide was slowed down and the dissolution rate of cobalt was increased, decreasing the logarithmic pH of the electrolyte at the entrance to the MEP, and when u3 became more than the reference u1, the dissolution rate of tungsten carbide was increased and the dissolution rate of cobalt was decreased, increasing the value of the logarithmic pH Olita inlet IEP, and when it became more u3 u2, fed individual pulses of opposite polarity to until the difference between u3 and u1 becomes equal to zero.

Thus, a stable process of ECHO WC-Co carbide with a high productivity was ensured due to the operational control of the occurrence of a blocking anode film and its timely removal.

Claims (6)

1. The method of electrochemical processing of solid WC-Co alloys, comprising treating the product with groups of electric current pulses supplied synchronously with the moments of maximum convergence of the oscillating electrode-tool and product, while a small interelectrode gap filled with electrolyte is maintained between the product and the electrode-tool, and including the stage of measuring and comparing the operating parameters depending on the state of the surface of the product, characterized in that it is pre-paused between groups of current pulses and measure the residual voltage u1 corresponding to the clean anode surface of the alloy and take its value as a reference parameter, and also determine the value of the residual voltage u2 corresponding to the surface of the alloy blocked by the anode films, then process the product, measure the residual voltage in a pause between groups of current pulses u3, and in the case when u3 is less than the reference u1, they slow down the dissolution rate of tungsten carbide and increase the dissolution rate of cobalt, increasing the voltage between electrode gap (MEP) and decreasing the value of the logarithmic hydrogen pH of the electrolyte at the entrance to the MEP, and when u3 is higher than the reference u1, increase the dissolution rate of tungsten carbide and decrease the dissolution rate of cobalt, decreasing the voltage on the MEP and increasing the value of the logarithmic hydrogen pH electrolyte at the entrance to the MEP, and in the case when u3 is greater than u2, current pulses of reverse polarity are applied until the difference between u3 and u1 becomes equal to zero. 2. The method according to claim 1, characterized in that the value of the logarithmic pH of the electrolyte at the entrance to the MEP is adjusted by using the catholyte or analytic component of the electrolyzer located at the entrance to the interelectrode gap. 3. The method according to claim 1, characterized in that as an electrolyte that provides simultaneous dissolution of the components and minimal formation of blocking films, depending on the chemical composition of the alloy and the grain size, use:
for WC-Co hard alloys with a high cobalt content of 15-25% and a WC grain size of more than 1.5 μm, the composition is 3-5% NaOH + 8-10% NaNO ₃ at a temperature of 20 ... 25 ° C;
for WC-Co hard alloys with a low cobalt content of 5-15% and a WC grain size of more than 1.5 μm, the composition is 3-5% NaOH + 10-20% NaNO ₃ at a temperature of 45 ... 50 ° C;
for WC-Co hard alloys with a high cobalt content of 15-25% and a WC grain size of less than 0.7 μm, the composition is 5-10% NaOH + 10-15% NaNO ₃ at a temperature of 20 ... 25 ° C;
for WC-Co hard alloys with a low cobalt content of 5-15% and a WC grain size of less than 0.7 μm, the composition is 10-15% NaOH + 10-15% NaNO ₃ at a temperature of 40 ... 45 ° С. 4. The method according to claim 1, characterized in that use the following working area of the parameters of the processing modes:
the voltage amplitude of the pulses of direct polarity is 14 ... 28 V;
the duration of the pulses of direct polarity is 50 ... 100 μs;
the number of pulses in the group is 5 ... 20;
the voltage amplitude of the pulses of reverse polarity is 2 ... 5 V;
pulse width of reverse polarity - 2 ... 18 ms;
the pressure of the electrolyte at the entrance to the MEP is 50 ... 300 kPa;
vibration frequency - 10-100 Hz. 5. The method according to claim 1, characterized in that the reverse polarity current pulse is supplied in the phase of maximum approximation of the tool electrode and the component immediately after applying a group of current polarity pulses. 6. The method according to claim 1, characterized in that the reverse polarity current pulse is supplied in the phase of maximum approximation of the tool electrode and the part in the next period after applying a group of current polarity pulses.

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