JP3098654B2 - Surface treatment method and apparatus by electric discharge machining - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface treatment technique by electric discharge machining, and more particularly, to a surface treatment for forming a smooth and uniform coating layer by suppressing scattering of a deposited layer caused by an explosion pressure due to electric discharge. Regarding the method and apparatus, as a primary surface treatment, or as a secondary surface treatment after an appropriate primary surface treatment, abrasion resistance, corrosion resistance or low friction on a material to be treated such as a conductive material (eg, metal). It is suitable as a surface treatment for giving a coefficient.

[0002]

2. Description of the Related Art Conventionally, as means for imparting abrasion resistance and corrosion resistance to metal surfaces, CVD (chemical vapor deposition), P
VD (vacuum deposition), electrodeposition, nitriding, electrochemical plating, electroless plating and the like are known.

However, both CVD and PVD have the drawback that the base material has a drawback of causing a dimensional change or a decrease in hardness because the coating is performed by raising the temperature of the base material from 360 ° C. or higher to about 1100 ° C. Are known. The hardened layer is as thin as several μm. In addition, nitriding also has a disadvantage in that a steel material is heated to about 500 ° C. for treatment.

[0004] It is well known that the electrodeposited surface is easy to peel off because the deposited metal simply deposits or precipitates on the base material and is not diffused, and has the disadvantage that it causes hydrogen embrittlement. is there. The same applies to electrochemical plating and electroless plating.

[0005] What has been deposited on the base material surface by thermal spraying,
It is already known that it is porous and easy to peel off. Further, even if this is to be re-melted with a laser beam, the heat input becomes non-uniform depending on the position of the spot, and streaks are generated at the boundary of the beam progress, so that a beautiful surface cannot be obtained. Further, it is difficult to apply a laser beam or the like to a three-dimensional processed shape due to its structure.

In addition, in the conventional surface treatment method, since hardly any diffusion into the base material occurs, a material such as fine ceramics, which is difficult to diffuse, has a sufficient thickness (for example, several tens μm to 100 μm).
m) is difficult to coat.

[0007]

On the other hand, the present inventors differ from these surface treatment methods in that the dimensional changes and the hardness (strength) of the base material caused by keeping the entire metal material of the base material at a high temperature.
A surface treatment method capable of forming a strong coating layer having desired surface characteristics such as corrosion resistance and heat resistance with a sufficient thickness without causing defects such as reduction in film thickness and film peeling has been previously proposed (Japanese Patent Application No. Hei. No. 329499). In this method, first, a coating of metal, ceramics, etc. is formed (primary processing) on the surface of a metal base material by a method such as discharge deposition, vapor deposition, thermal spraying, and then the material to be coated (eg, WC, TiC, TiB ₂ , Pulse discharge on a material to be surface-treated (eg, carbon steel, etc.), using a compact made of a hard material such as TiN, VC, etc. and a metal such as W, Ti, V, Mo, etc. This is a method of transferring and depositing by processing (secondary processing).

According to the method proposed above, the base material surface is coated by discharge deposition, vapor deposition, thermal spraying, etc. (primary processing)
In the case where the adhesive force is weak, the composition is not dense, but by performing pulse discharge (secondary processing) in liquid or gas,
Since the discharge point is re-melted and diffused into the base material due to the high temperature and high pressure generated at the discharge point microscopically, the entire average temperature can be strengthened without increasing the average temperature to a temperature that causes dimensional change and hardness change of the base material. A suitable surface coating can be formed.

However, this method requires several repetitions of the processing operation, and is extremely effective particularly when a thick surface coating layer is formed. It is preferable that the formation of the surface layer be completed by the primary processing and the secondary processing. In contrast to such a process, in the above-described method, since a layer deposited in the primary processing is scattered in the secondary processing, the processed layer may not be deposited to a uniform thickness.

The present invention is an improvement of the surface treatment technology according to the prior proposal, and is a surface treatment technology by electric discharge machining capable of forming a smooth and uniform coating layer even with a thin surface coating layer as thin as 50 μm or less. The purpose is to provide.

[0011]

Means for Solving the Problems The present inventor has studied the cause of what is deposited in the primary processing and scatters in the secondary processing in the surface treatment technology according to the above-mentioned proposal, and as a result, immediately after the occurrence of electric discharge in the secondary processing. It has been found that the scattering pressure is increased because the discharge pressure is too high, and an effective method capable of relaxing the discharge pressure has been found, and the present invention has been completed here.

That is, according to the present invention, in a surface treatment for depositing a conductive or non-conductive material on a surface of a material to be treated by pulse discharge machining, a gas which does not generate a toxic gas by electric discharge is formed between the machining electrodes together with a machining fluid. The gist of the present invention is to provide a surface treatment method by electric discharge machining, characterized in that by supplying, a scatter of a deposited layer caused by an explosion pressure due to electric discharge is suppressed to obtain a smooth and uniform surface coating layer.

Another object of the present invention is to provide a method for depositing a conductive or non-conductive material on a surface of a material to be treated by pulsed discharge machining, by supplying a current capable of suppressing the rise of a discharge current, thereby forming a discharge mark current. By reducing the density,
The gist of the present invention is a surface treatment method by electric discharge machining, characterized in that scattering of a deposited layer caused by explosion pressure due to electric discharge is suppressed to obtain a smooth and uniform surface coating layer.

Still another object of the present invention is to provide a surface treatment apparatus for depositing a conductive or non-conductive material on a surface of a material to be treated by pulse discharge machining, wherein a gas which does not generate a toxic gas by electric discharge is machined together with a machining fluid. A gist of the present invention is a surface treatment apparatus using electric discharge machining, which has a means for supplying a gap.

Still another object of the present invention is to provide a surface treatment apparatus for depositing a conductive or non-conductive material on a surface of a material to be treated by pulse electric discharge machining, by supplying a current capable of suppressing the rise of the discharge current. The gist of the present invention is a surface treatment apparatus by electric discharge machining, which has an electric discharge machining circuit for reducing the explosion pressure.

[0016]

The present invention will be described below in more detail.

As described above, the gist of the present invention is,
At the time of pulse electric discharge machining, gas is mixed between the discharge electrodes to reduce the discharge pressure and reduce the scattering pressure, or to suppress the rise of current immediately after dielectric breakdown in the discharge circuit and gradually increase the current. The control is to reduce the scattering force. Of course, and can be used together.

(1) Mixing of gas between discharge electrodes FIG. 1 shows an example of a pulse discharge machining apparatus provided with a device for mixing gas between discharge electrodes. In the figure, 1 is a deposited layer after primary processing (eg, 60 to 80 μm), 2 is a material to be treated,
3 is an electric discharge machining electrode (eg, copper), 4 is an electric discharge machining pulse power supply, 5 is a machining liquid, 6 is a gas (eg, Ar gas) charging cylinder,
Reference numeral 7 denotes a pressure reducing valve, 8 denotes a gas diffusion nozzle, and 9 denotes a servo mechanism.

The secondary machining is performed on the deposited layer formed by the primary machining by an appropriate method by injecting argon gas into the machining fluid using this electric discharge machining apparatus. That is, the argon gas is supplied from the cylinder 6 filled with argon gas through the pressure reducing valve 7 to the vicinity of the gap by the argon gas diffusion nozzle 8. Since the electrode 3 is usually processed while moving up and down by the servo mechanism (several seconds to 10 seconds when facing the processing, each vertical movement time: several seconds), the argon gas is diffused between the electrodes, and the next discharge is caused by bubbles. It is processed in a liquid containing. Gas is mixed into the electric discharge machining liquid and supplied to the gap.To make this effective, when machining is performed by moving the electrode close to and away from the machining direction, the time between the poles is large. Preferably, a working fluid is supplied into the inside.

The operation of the gas-mixed working fluid in the gap is as follows.

Electric discharge machining starts when the gaps are close to each other. However, where the mixed gas exists, since the dielectric strength is higher than in the liquid, no electric discharge occurs, and the electric discharge occurs in a place where only the liquid exists. Occur. In that case, three phenomena occur.

(A) The discharge that has occurred in the place where only the liquid is present is performed at a lower pressure because the surroundings are surrounded by gas, and the discharge is alleviated as compared with when there is no gas. Therefore, although the processing speed is reduced, the amount of scattering of the material deposited on the surface of the material to be processed is significantly reduced. (B) Since the gas between the electrodes moves, the next discharge point is generated at another place. Eventually, the discharge is not concentrated locally but is dispersed, so that the so-called arc discharge is not generated. That is, the surface is smooth. (C) In this case, if a gas is mixed using an oil-based electric discharge machining liquid, highly active carbon generated by high-temperature decomposition of the oil is generated. Therefore, if oxygen and air are used as the mixed gas, carbon monoxide (CO) is generated, and if hydrogen gas is used, carbohydrate (CnHm) is generated. Therefore, it is preferable to use a chemically stable gas such as Ar or He as the gas. Of course, nitrogen gas and carbon dioxide gas can also be used stably and effectively.

(2) Suppression of Rise of Discharge Power The method of suppressing the rise of discharge power is not particularly limited. For example, a method in which an inductance is inserted in a discharge circuit in series, and a plurality of transistors are used as switching elements. And the like.

FIG. 2 shows a method of inserting an inductance in series in a discharge circuit. As shown in FIG. 3, the current waveform when no inductance is inserted has a rectangular shape as shown in FIG. 3, but the discharge current at the time when the discharge occurs, as in the case where the inductance is inserted, is suppressed and reduced.

If the current is large at the time of generation of the discharge current, the current density of the discharge mark increases as shown in FIG. 4 (that is, since the diameter of the discharge mark is small at the beginning of discharge and increases with the lapse of discharge time). In the case of a current having a fast rise time, the current density becomes high.) Therefore, the explosion pressure due to the discharge becomes high, so that the deposited layer on the surface is blown off. Therefore, when the discharge current at the time of generation of the discharge current due to the insertion of the inductance is suppressed, the explosion pressure due to the discharge is reduced, so that the surface deposited layer is less likely to be blown off.

Instead of inserting an inductance in series, a method of using a plurality of transistors as switching elements as shown in FIG. 5 is also possible. The slope control may be performed such that the switching transistors are sequentially turned on as the discharge time elapses and a triangular wave as shown in FIG. 6A or a sawtooth wave as shown in FIG. Even with such a slope control, an effect similar to the method of inserting an inductance in series in a discharge circuit is obtained, and the explosion pressure due to discharge is reduced, so that the deposited layer on the surface is less likely to be blown off.

Under the above conditions, the pulse discharge is applied in the liquid by the electric discharge machining technique, but other electric discharge machining conditions are not particularly limited. For example, the procedure is as follows.

At the time of pulse electric discharge machining, it is preferable to use an electrode (copper) which is not easily consumed. In addition, it is preferable to use an electrode having a composition close to the deposit as an electrode in the case of the discharge deposition method as the secondary processing, and to use a conductive powder (e.g., W
C, TiC, TiB ₂ , TiN, VC or W, Ti, V
Or two or more kinds of metals such as
o, one or more of Ni, etc.) for the primary processing of electrodes formed by compression molding or low-temperature sintering, in secondary processing, for example, when WC is mainly deposited on the surface of a metal material,
A material obtained by sintering WC-Co (eg, a chip material for a bite) is used for the electrode.

The discharge is generated several hundred to several tens of thousands of times per second. The machined surface is a surface on which small microscopic discharge traces are accumulated. The discharge mark current density is a very small area,
High tens of thousands A / cm ² , high temperature and high pressure of several tens μs to 1000 μ
It occurs in a short time of about s. The surface temperature at the discharge point is about the boiling point of the material, and the pressure at that point is several thousand kgf / c.
m ² and the melted portion is re-melted and diffuses into the base material.
Since the discharge time is short, the discharge point is immediately cooled, and the average temperature of the base material does not rise.

The preferred conditions of the pulse discharge machining are as follows: power supply voltage: 60 to 100 V, pulse discharge current value (Ip): 1 to 1
00A, pulse width (τp): 5-2000 μs, pause time
(τr): 5 to 2000 μs. Generally, when the pulse discharge current value Ip is small, for example, when Ip = 3 A, τp
= 16 .mu.s, and when Ip is large, for example, .tau.p = 2000.tau. At Ip = 50 A, etc., when Ip is small, .tau.p is short, and when Ip is large, .tau.p is long.

In the pulse discharge machining according to the present invention, the surface of the material to be treated may be coated by itself. However, as a primary machining, a non-conductive or conductive material is applied to the surface of the material to be treated in advance. After coating, it is desirable to carry out as a secondary processing.

When the pulse discharge machining is performed as a secondary machining, an appropriate primary machining method can be used, and examples thereof include thermal spraying, electrodeposition, discharge deposition, and slurry coating. The discharge deposition method is a surface treatment method previously proposed by the present inventors ("Presentation Papers of Spring Meeting of the 1991 Precision Engineering Society Spring Meeting" (March 26, 1991)).
p.463), by forming the conductive material to be deposited as a green compact and using it as an electrode for electrical discharge machining,
This is a method in which a green compact material is deposited on a mating metal. However, since these deposits do not diffuse into the base material, the adhesion strength is low. Either of these methods is possible,
Judging from the relationship with pulse electric discharge machining performed as a post process,
The discharge deposition method is preferred.

As the coating material, various metal materials or non-metal materials can be used.
Non-metallic elements, ceramics, carbides, nitrides, borides and the like. Specifically, as hard materials, WC, Ti
Carbides such as C, TaC, ZrC, SiC, TiB ₂ , ZrB
₂ or a nitride such as TiN or ZrN (fine ceramics) can be coated alone or with a sintering aid added. Also, metal materials such as W and Mo, Al,
Corrosion resistant materials such as Ti, Ni, Cr, and Co can also be used. Further, iron powder, cobalt powder, nickel powder, chromium powder, etc., even if they are not conductive like diamond, Al ₂ O ₃ and Si ₃ N ₄
It may be coated by mixing with a conductive material such as copper powder. In short, the material may be selected in relation to the surface characteristics to be provided.

According to the surface treatment method of the present invention, a dense layer having desired properties such as heat resistance, corrosion resistance, abrasion resistance, and hardness is formed on the surface of an inexpensive metal material such as a steel material such as carbon steel. can do. Even if the material does not easily diffuse into the steel material such as fine ceramics, the diffusion and adhesion to the base material can be strengthened by remelting. Further, even if a material such as Al, Ti, Ni, Cr, and Co that is easily dissolved in a steel material is subjected to the pulse discharge treatment, a more robust surface treatment can be performed. That is, when high-speed discharge deposition is performed using a large current in order to increase the rate of discharge deposition, even if a material such as Al, Ti, Ni, Cr, and Co is easily dissolved in a steel material, Although the diffusion into the metal is insufficient and the precipitation state becomes severe, the pulse discharge treatment promotes the diffusion by remelting. In addition, when the plating speed is increased at a large current density by electrodeposition or electroplating, only a rough plating layer having a small adhesive force can be obtained, but when pulse discharge machining is performed, a surface layer having a large adhesive force can be formed. it can. Pulse discharge to non-conductive hard material such as diamond, Al ₂ O ₃ , Si ₃ N ₄ mixed with conductive metal such as iron powder, cobalt powder, nickel powder, chromium powder, copper powder, etc. When the treatment is performed, the conductive metal is re-melted, and the non-conductive hard material is firmly fixed to the surface of the base material.

Also, a material having a gradient can be manufactured. The graded material is, for example, a material in which the base material is a metal material, the content ratio of fine ceramics gradually increases from the base material side, and the content ratio of the fine ceramics is significantly increased on the material surface. Such a graded material is less likely to generate shear stress and bending stress at the joint surface due to a remarkable difference in expansion coefficient even if there is a temperature rise, compared to a material obtained by simply joining or coating a metal material and fine ceramics. Therefore, breakage during use at high temperature is unlikely to occur. This is because even if thermal expansion occurs due to a rise in temperature, stress is reduced.

Next, examples of the present invention will be described.

[0037]

Embodiment 1

This embodiment is an example in which pulse electric discharge machining in which a gas is mixed is performed as a secondary machining method. The primary processing method and the secondary processing method were performed under the following conditions. The base material is carbon steel
(S55C), and the apparatus of FIG. 1 was used.

Primary processing (discharge deposition) conditions : Electrode: green compact electrode (WC: Co = 8: 2), pulse discharge current value (Ip): 25 A (minus electrode), pulse width (τp): 1
6 μs, rest time (τr): 512 μs, processing time: 15
Minutes.

Secondary machining (pulse discharge machining) conditions : Electrode: copper electrode (16 mmφ), pulse discharge current value (Ip): 1
5A (electrode minus), pulse width (τp): 1024 μs, rest time (τr): 1024 μs, processing time: 15 minutes, type of gas: argon gas, gas injection pressure: 0.1 kg / cm ² .

As a result, the cross-sectional thickness of the surface-hardened layer after processing was about 50 μm and the micro-Vickers hardness was about 1800, which was sufficient even after the second processing was performed only once. The fact that the thickness was about 60 to 80 μm in the primary processing, but a uniform surface hardened layer of about 50 μm is considered that the scattering due to the secondary processing was reduced by mixing of argon gas. FIGS. 7 to 9 show electron micrographs of the processed cross section at the same location. 7 shows a case where the magnification is 50 times, FIG. 8 shows a case where the magnification is 200 times, and FIG. 9 shows a case where the magnification is 300 times. It can be seen that even if the surface hardened layer is thick, it is smooth and uniform.

[0042]

Embodiment 2

This embodiment is an example in which pulse discharge machining for suppressing the rise of discharge power is performed as a secondary machining method. The primary processing method and the secondary processing method were performed under the following conditions. The base material was carbon steel (S55C), and the circuit shown in FIG. 2 was used.

Primary processing (discharge deposition) conditions : Electrode: green compact electrode (WC: Co = 8: 2), pulse discharge current value (Ip): 25 A (minus electrode), pulse width (τp): 1
6 μs, rest time (τr): 512 μs, processing time: 15
Minutes.

Secondary machining (pulse discharge machining) conditions : Electrode: copper electrode (16 mmφ), pulse discharge current value (Ip): 1
0A (electrode plus), pulse width (τp): 1024 μs, rest time (τr): 1024 μs, processing time: 4.5 minutes, discharge circuit configuration: FIG. 2 (coil diameter 36 mm, 22 turns, inductance L ≒ 12 μH) .

FIG. 10 shows a micrograph of the processed cross section. 1
A dense processing layer with a thickness of about 30 μm by multiple processing (secondary processing)
It can be seen that (WC-Co) was obtained. FIG. 11 is a photograph showing the surface layer observed at a magnification of 1100.
It can be seen that the hard particles are uniformly dispersed in the processed layer. FIG. 12 shows the micro Vickers hardness (load 1) of the processed layer.
0g), where about 25 μm is mHv from the surface.
1710 and sufficient hardness as a WC-Co sintered body. FIG. 13 shows the result of X-ray diffraction on the processed surface, and it can be seen that WC is strongly precipitated. Good results are obtained when the magnitude of the insertion inductance is about 4 to 200 μH.

It was confirmed that almost the same results as in Example 2 (inductance insertion) were obtained when current waveform control was performed using a large number of switching elements.
Moreover, the method using the switching element is industrially advantageous because the degree of the rising slope of the current can be easily changed arbitrarily (in the case of inserting an inductance, it is necessary to reconnect different inductances).

[0048]

As described in detail above, according to the present invention,
Since pulse electric discharge machining is performed under conditions that alleviate the discharge pressure,
The scattering force of the coating layer can be reduced, and therefore, even with a thin surface coating layer having a thickness of 50 μm or less, a smooth and uniform coating layer can be formed. It is particularly suitable when performing secondary processing after appropriate primary processing.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a pulse electric discharge machining apparatus by mixing gas.

FIG. 2 is a diagram showing an example of a pulse electric discharge machining device using inductance insertion.

FIG. 3 is a diagram showing a current waveform obtained by the circuit of FIG. 2;

FIG. 4 is a diagram showing an increase in the area of one discharge mark and a change in a discharge mark current density.

FIG. 5 is a diagram showing an example of a pulse discharge machining control circuit by inserting a switching element.

6A and 6B are diagrams showing current waveforms obtained by the circuit of FIG.

FIG. 7 is an electron micrograph (magnification: 50) of a processed cross section (metal structure) of a processed layer obtained in Example 1.

8 is an electron micrograph (magnification: 200) of a processed cross section (metal structure) of the processed layer obtained in Example 1. FIG.

FIG. 9 is an electron micrograph (magnification: 300) of a processed cross section (metal structure) of the processed layer obtained in Example 1.

FIG. 10 is a photograph of a processed cross section (metal structure) of a processed layer obtained in Example 2.

11 is a photograph (× 1100) of a processed cross section (metal structure) of a processed layer obtained in Example 2. FIG.

FIG. 12 is a diagram showing a hardness distribution of a processed section of a processed layer obtained in Example 2.

13 is an X-ray diffraction diagram of a processed surface of a processed layer obtained in Example 2. FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Deposition layer after primary processing 2 Surface treatment material 3 Electrode machining electrode 4 Electric discharge pulse power supply 5 Machining fluid 6 Gas injection cylinder 7 Pressure reducing valve 8 Gas diffusion nozzle 9 Servo mechanism

Claims (5)

(57) [Claims] 1. A material to be surface-treated is immersed in an insulating working fluid.
And, wherein are opposed to machining electrode to be surface treated material, wherein the gas which does not generate toxic gas by electric discharge with working fluid
While supplying between the surface treatment material and the processing electrode,
The conductive or non-conductive material is referred to as the surface treated material.
A surface treatment method by electric discharge machining, wherein the surface is deposited by pulse electric discharge machining . 2. A gas that does not generate a toxic gas by electric discharge, such as argon, helium, and carbon dioxide in an oily machining fluid.
Gas or nitrogen gas.
The surface treatment method according to claim 1, which is used . 3. A material to be surface-treated is immersed in an insulating working fluid.
Then, a processing electrode is opposed to the surface-treated material,
Suppress the supply current at the start to make the discharge current rise slowly
A conductive or non-conductive material,
A surface treatment method by electric discharge machining, wherein the surface is deposited by pulse electric discharge machining . 4. A container containing an insulating working fluid,
A machining electrode facing the surface treatment material immersed in the machining fluid
Between the electrode and the surface treatment material and the processing electrode.
Pulse discharge power source to generate
Gas that does not generate toxic gas
Means for supplying between the processing electrode.
And a surface treatment apparatus by electric discharge machining . 5. A container containing an insulating working fluid,
A machining electrode facing the surface treatment material immersed in the machining fluid
Between the electrode and the surface treatment material and the processing electrode.
EDM pulse power supply for generating loose discharge and discharge current
That adjusts the supply current so that the rise of the power supply becomes gentle
Table by electric discharge machining characterized by having a circuit
Surface treatment equipment.