JP4630374B2 - Method for evaluating powder molded body electrode - Google Patents

Description

  The present invention uses a powder molded body formed from a metal powder as an electrode, generates a pulsed discharge between the electrode and the workpiece in the machining fluid or in the air, and uses the energy to generate an electrode on the surface of the workpiece. The present invention relates to a discharge surface treatment for forming a film made of a material in which a material or an electrode material reacts with discharge energy.

It is necessary to coat or build up a material having strength and lubricity in a high temperature environment on the surface of an aircraft gas turbine engine such as a turbine blade. Since it has been known that Cr (chromium) and Mo (molybdenum) are oxidized to form an oxide in a high-temperature environment, it exhibits lubricity, so it is based on Co (cobalt) and contains Cr and Mo. The coating is thickened by welding or spraying the material.
Here, welding is a method in which the material of the welding rod is melted and adhered to the workpiece by electric discharge between the workpiece and the welding rod, and thermal spraying is a state in which a metal material is melted and sprayed onto the workpiece in a spray form. Is a method of forming
However, both the welding and thermal spraying methods are manual operations and require skill, so that there is a problem that it is difficult to line the operations and the cost is increased.
In particular, welding is a method in which heat concentrates and enters the workpiece. Therefore, when processing thin materials or materials that are easily broken such as directional control alloys such as single crystal alloys and unidirectionally solidified alloys, There is also a problem that cracking is likely to occur and the yield is low.

  On the other hand, although different from surface treatment methods such as welding and thermal spraying having strength and lubricity in a high temperature environment, other surface treatment techniques include, for example, electrical discharge machining as disclosed in International Publication WO99 / 58744. Surface treatment has also been established.

  Further, although it has been difficult to form a thick film until now, as disclosed in Japanese Patent Application Laid-Open No. 2004-60013, an alloy containing 50% by weight or more of a metal material that does not form carbide or is difficult to form is used as an electrode material. This makes it possible to form a thick film.

International Publication No. WO99 / 58744 (pages 7-8)
Japanese Patent Laid-Open No. 2004-60013

In the formation of the thick film by the discharge surface treatment, the method of supplying the material from the electrode side, melting the supplied material on the workpiece surface, and bonding with the workpiece material has the greatest influence on the coating performance.
If all the material supplied from the electrode side is melted and combined with the workpiece material, a dense and high-strength film can be formed. However, the material supplied from the electrode side was supplied excessively. If all of the materials cannot be melted, the coating will contain materials that are not sufficiently melted, and as a result, a coating with high strength cannot be formed.

In order to determine the melting state of the material supplied from the electrode side, the interelectrode voltage and current waveform during the discharge surface treatment were observed. As a result, the material supplied from the electrode side was well melted to form a high-strength film. At that time, there was almost no immediate discharge, and conversely, when the material supplied from the electrode side could not be melted and a high-strength film could not be formed, immediate discharge was frequently caused. Found by the inventors' experiments.
Here, the immediate discharge is a phenomenon in which the floating material between the electrode and the workpiece approaches the electrode, and the floating material is discharged before the voltage between the electrodes rises. Specifically, the discharge occurs within 2 μsec after the voltage is applied.
When immediate discharge occurs, the discharge concentrates at one point, and it becomes difficult to supply discharge energy to the electrode material. Therefore, it is difficult to melt all the electrode material and it is difficult to form a coating film having high strength.

  The object of the present invention is to establish a technology for forming a dense and high-strength film by discharge surface treatment, and in particular, a pulsed discharge is generated in the material supplied from the electrode side, and the electrode material is formed by the energy. The object is to establish an evaluation of a powder molded body electrode for melting and bonding with a work material to form a dense and high-strength film.

  The method for evaluating a powder molded body electrode according to the present invention uses a powder molded body electrode formed by molding a powder of a metal or a metal compound, and has a pulse shape between the electrode and the workpiece in a processing liquid or air. In the method for evaluating a powder molded body used in a discharge surface treatment method for generating an electric discharge and forming a coating made of an electrode material or a material obtained by reacting the electrode material with the discharge energy on the surface of the workpiece by the energy. Four electrode-like electrodes are installed on the electrodes in the order of A, B, C, and D, and when a current is passed through the needles A and D, the potential difference generated between the needles B and C is measured. The short needle method is measured, the electrical resistance of the electrode of the powder compact is measured, and the propriety of the powder compacted electrode is judged.

  According to the present invention, it is possible to determine whether or not the electrode can stably form a dense and high-strength film during discharge surface treatment.

FIG. 4 is a voltage and current waveform diagram during normal discharge. It is a voltage and current waveform diagram at the time of immediate discharge. It is a characteristic diagram which shows the relationship between the ratio for which an immediate discharge accounts to all the discharges, and the intensity | strength of the film formed at that time. It is a characteristic diagram which shows the relationship between the electrical resistance value of the electrode measured by the four-short needle method, and the probability of occurrence of immediate discharge when Co alloy is used as the electrode. It is a simplified block diagram which shows the measurement principle of the four short needle method. It is a characteristic diagram which shows the relationship between the electrical resistance value of the electrode measured by the four-short needle method when Co alloy is used as an electrode, and when the rest time is 4, 6, and 8 μs and the likelihood of immediate discharge.

Embodiment 1 FIG.
In the formation of the thick film by the discharge surface treatment, as described above, the supply of the material from the electrode side, the melting of the supplied material on the workpiece surface, and the bonding with the workpiece material have the greatest influence on the coating performance.
Therefore, the inventors observed the interelectrode voltage and current waveform during the discharge surface treatment with a waveform measuring instrument in order to determine the molten state of the material supplied from the electrode side.
When the material supplied from the electrode side is well melted to form a high-strength film, the interpolar waveform shown in FIG. 1 is observed, and conversely, the material supplied from the electrode side cannot be melted. When a high-strength film could not be formed, many interpolar waveforms shown in FIG. 2 were observed.
The waveform shown in FIG. 2 is a waveform observed during immediate discharge.

FIG. 3 is a diagram showing the relationship between the frequency of occurrence of immediate discharge and the strength of the coating film formed at that time.
In showing this relationship, a Co (cobalt) alloy was used as the electrode material.
In the figure, the horizontal axis indicates the ratio of the immediate discharge to the total discharge, and the vertical axis indicates the strength of the formed film.
As is clear from the figure, it can be confirmed that the greater the rate at which immediate discharge occurs, the lower the strength of the formed film.
When an immediate discharge occurs, the discharge concentrates at one point, making it difficult for the discharge energy to be transmitted to the material supplied from the electrode. Therefore, the electrode material cannot be completely melted, and unmelted material is caught in the film. Therefore, it is assumed that a high-strength film cannot be formed.
That is, in this embodiment, in order to form a high-strength film, the optimum processing conditions are found from the relationship between the ratio of immediate discharge in discharge surface treatment and the film strength, and electrodes necessary to satisfy the conditions are found. Is what you get.

First, considering FIG. 3, when the proportion of immediate discharge is 50% or less, the intensity decreases gradually even if the proportion of immediate discharge increases, but the proportion of immediate discharge It has been found that the strength rapidly decreases when the content exceeds 50%.
This is presumably because when the occurrence of immediate discharge is larger than normal discharge, most of the formed coating is filled with unmelted powder, and the adhesion between the powders is significantly reduced.

Although it depends on the required coating strength, in the aircraft field where a high-strength coating with a strength of 30 MPa or more is required, the demand can be met by setting the frequency of immediate discharge within 50% of the total discharge. A high-strength film can be formed.
Needless to say, it is better if the ratio of immediate discharge is within 40% and within 30%.
This relationship has been explained by taking the case where Co (cobalt) alloy is used as an electrode material as an example, but the same tendency is shown when a powder molded body electrode formed from a powder of another metal or metal compound is used as an electrode. It has been found that when the proportion of immediate discharge exceeds 50%, the strength rapidly decreases.

Next, the electrode conditions for discharge surface treatment for suppressing immediate discharge will be considered.
As described above, the immediate discharge is a phenomenon in which the floating material existing between the electrode and the workpiece approaches the electrode and discharges to the floating material before the voltage between the electrodes rises. By controlling the amount of suspended matter in between, the rate of immediate discharge can be changed.
In the present embodiment, a case where a powder compact formed from a metal powder is used as an electrode for discharge surface treatment will be described.

As is well known, when a powder molded body is used as an electrode for discharge surface treatment, the metal powder forming the electrode collapses from the electrode due to discharge and is deposited on the workpiece in a molten state to form a film.
Here, the amount of electrode powder that collapses from the electrode and floats between the electrodes is closely related to the occurrence of immediate discharge.
That is, by using the ease of electrode collapse as an index, it is possible to discriminate between an electrode that is likely to cause an immediate discharge and an electrode that is less likely to occur.

Next, the ease of collapse of the electrode will be described.
Whether or not the electrode powder easily collapses in the electrode of the powder compact is affected by the adhesion strength between the powders in the electrode.
That is, in an electrode compressed or sintered at a high pressure, the adhesion strength between the powders is high, and the electrode powder is not easily broken, but similarly, the electrode powder supplied to the work piece is small, and electric discharge machining was performed. In this case, digging is performed without forming a film in which the electrode powder is melted.
On the other hand, when the electrode powder is insufficiently compressed, the electrode tends to collapse, and a large amount of electrode powder is supplied by the discharge. The film strength cannot be obtained.

Therefore, the present inventors have repeated experiments to obtain an electrode in which the fragility of the electrode is optimal.
Since the electrode used for the discharge surface treatment is composed of a powder compact, the adhesion strength of the electrode is closely related to the electrical resistance of the electrode.
That is, when the adhesion force between the electrode powders is small, the electrode easily collapses, and the resistance value of the electrode increases. When the adhesion force between the electrode powders is large, the electrode is less likely to collapse and the resistance value of the electrode. Becomes smaller.
Therefore, using electrodes with different electrode forming conditions (electrodes having different electrode resistance values measured by the four-short needle method), the electrical resistance of the electrode and the ratio of immediate discharge, which is one index for forming a high-strength film, Sought the relationship. (Fig. 4)
The conditions in FIG. 4 are as follows: the discharge surface treatment electrode used was a mixed Co alloy of 25 wt% Cr (chromium) powder and 75 wt% Co (cobalt) powder. In FIG. 1, the peak current value ie = 4 A, the discharge duration (discharge pulse width) te = 4 μs, and the rest time to = 4 μs.

As shown in FIG. 4, the smaller the resistance value of the electrode, the higher the adhesion strength of the powder and the less likely to cause immediate discharge, and the greater the resistance value of the electrode, the lower the adhesion strength of the powder and the more likely to cause immediate discharge.
In addition, as the electrical resistance of the electrode decreases, the film formation rate tends to be slow.If the electrical resistance is less than 4.0E-4Ω, the adhesion strength between the powders is too high and the discharge surface treatment conditions Even if it was set, a film could not be formed.
In this case, it is not the process of depositing the electrode material on the workpiece side as described above, but the process of digging the workpiece side.
On the other hand, as the electrical resistance of the electrode increases, the electrode tends to collapse, and when the electrical resistance exceeds 3.0E-2Ω, a phenomenon that the electrode material adheres to the work without being sufficiently melted is observed, and the formed film is not dense It becomes very porous.

  From the above, when 25% by weight of Cr (chrome) powder and 75% by weight of Co (cobalt) powder are mixed and the molded Co alloy is used as an electrode material, the electrical resistance of the discharge surface treatment electrode is 4.0E-4Ω or more. The range of less than 3.0E-2Ω is the optimum value for forming a dense and high strength film.

  Table 1 shows the optimum values of the electrical resistance of the electrodes when other materials are used for the electrodes.

The range of 1.0E-4Ω to 3.0E-3Ω is optimal when using Mo (molybdenum) as the main component, and 0.8E when using Cu (copper) as the main component. The range of -3Ω to 1.0E-2Ω was optimal.
Thus, as a result of experiments using an electrode mainly composed of Mo, Cu, Co, etc., an electrode having an electric resistance of less than 1.0E-4Ω or 3.0E-2Ω is a dense and high-strength film. It was found that the optimum value for forming a dense and high-strength film is in the range of 1.0E-4Ω or more and less than 3.0E-2Ω.

Next, a method for measuring the resistance value of the discharge surface treatment electrode in the present invention will be described.
When measuring the resistance value of metals, it is common to measure with a tester, but when measuring with a tester, only the value depending on the size of the sample to be measured is measured. The measured electrical resistance increases, and the electrical resistance decreases if the sample to be measured is small.
Here, in the discharge surface treatment according to the present invention, it is necessary to always detect under constant conditions because of the relationship with the index of the ease of collapse of the electrode, regardless of the size of the electrode (sample to be measured). It is.

Therefore, the electrical resistance measurement of the discharge surface treatment electrode in the present invention is characterized by using a four-short needle method.
The measurement principle of the four-short needle method is as follows. When four needle-shaped electrodes are installed in a straight line on the test piece and current (I) is passed between the short hand A and the short hand D as shown in FIG. The potential difference (V) generated between B and the short hand C is measured, and the resistance (V / I) is obtained.
Although the value of resistance calculated | required changes with the space | interval of a short hand, in this Embodiment, the electrical resistance is measured using the probe whose space | interval of a short hand is 1.5 mm.
However, this method only measures the potential difference generated between the short hand B and the short hand C, and is not precisely the measurement of the electrical resistance of the electrode itself.
In the tester method, when measuring the electrical resistance of the electrode, it was necessary to cut the electrode into a certain shape, but by using the four-short needle method, the electrode resistance was measured when measuring the electrical resistance. It is not necessary to make the shape uniform, and an electrode whose electrical resistance is measured can be used for the discharge surface treatment, so that the electrode is not wasted and is general-purpose.

According to the present embodiment, since the electrical resistance of the electrode can be used as an index indicating the adhesion strength between the powders in the electrode, by examining the electrode resistance based on the four-short needle method that can be quantitatively measured, It can be tested whether a strong coating can be formed.
In addition, an electrode whose electric resistance by the four-short needle method is in the range of 1.0E-4Ω or more and less than 3.0E-2Ω can form a dense and high-strength film.
In addition, there is an index for measuring the hardness of the electrode directly as an index of the tendency of the electrode to collapse, but it is difficult to evaluate, for example, when the electrode surface is hard and the inside is a soft electrode, and the electrode is easily collapsed. These problems can also be solved by obtaining the index indicating the value by the electrode resistance by the four-short needle method.

Embodiment 2. FIG.
In the present embodiment, attention is paid to the relationship between the occurrence frequency of immediate discharge, the electrical resistance of the electrode, and the discharge pause time.
FIG. 6 is a diagram showing a ratio of immediate discharge with respect to total discharge when performing discharge surface treatment using several electrodes having different electric resistances.
The discharge surface treatment electrode used was a mixture of 25% by weight of Cr (chromium) powder, 50% by weight of Co (cobalt) powder, and 25% by weight of Ni (nickel) powder. In FIG. 1, the pulse conditions were set such that the peak current value ie = 5 A, the discharge duration (discharge pulse width) te = 4 μs, and the rest time to = 4, 6, 8 μs.

As in FIG. 4, the greater the electrical resistance, the easier the immediate discharge occurs, and the smaller the electrical resistance, the less likely the immediate discharge occurs.
When the rest time to = 4 μs, there was a range in which the ratio of immediate discharge did not become 50% or less when the electrical resistance of the electrode was 4.0E-4Ω or more and less than 3.0E-2Ω.
On the other hand, in the case of the rest time to = 6 μs, the occurrence of immediate discharge was 50% or less in all the ranges where the electrical resistance of the electrode was 4.0E-4Ω or more and less than 3.0E-2Ω.
This relationship was unchanged even when the experiment was performed by changing the peak current value ie and the discharge duration te.

From this, it was found that the electrical resistance and rest time of the electrode were the dominant factors with respect to the amount of powder floating between the electrode and the workpiece.
From the above, when the main component of the electrode is Co (cobalt), in order to form a dense and high-strength film in the range where the electric resistance of the electrode is 4.0E-4Ω or more and less than 3.0E-2Ω, It has been found that it is sufficient to set the current to 6 μs or more.
Here, it is clear from FIG. 6 that it is even better if the pause time is 8 μs or more.
However, from the standpoint of processing efficiency, the shorter the downtime, the better, and the optimum value is determined by the required coating strength.

  Table 2 shows the optimum values of the downtime when other materials are used for the electrodes.

When the electrode is composed mainly of Mo (molybdenum) and the electrical resistance is in the range of 1.0E-4Ω or more and 3.0E-3Ω or less, a dense and high-strength film can be formed by setting the rest time to = 250μs or more. When the electrode is composed mainly of Cu (copper) and the electrical resistance is in the range of 0.8E-3Ω or more and 1.0E-2Ω or less, a dense and high-strength film is formed by setting the rest time to = 16μs or more. It turns out that you can.

As described above, as a result of experiments using an electrode mainly composed of Mo, Cu, Co or the like, in order to form a dense and high-strength film, the electric resistance is 1.0E-4Ω or more and 3.0E- It has been found that it is necessary to make the rest time to = 6 μs or more by using an electrode having a range of less than 2Ω.
In addition, as other electrode components, an alloy having Cr, Ni, Fe, W as a main component or a powder mixed electrode is substantially the same.

  According to the present embodiment, in order to form a film using the powder molded body electrode, the electrode resistance by the four-short needle method is an electrode in the range of 1.0E-4Ω or more and less than 3.0E-2Ω, and the rest time By processing at 6 μs or more, a high-strength film can be formed.

  In the above-described embodiment, the formation of a high-strength film that suppresses immediate discharge by using an electrode in which the electrode resistance based on the four-short needle method is in an appropriate range has been described. It is also possible to suppress the immediate discharge by servo control by detecting whether or not an immediate discharge has occurred between the electrodes by a device that detects the time taken until the occurrence of the discharge, or the ratio of the immediate discharge.

Claims (5)

Using a powder molded body electrode formed by molding a metal or metal compound powder, a pulsed discharge is generated between the electrode and the workpiece in the machining fluid or in the air, and the energy is applied to the surface of the workpiece. In the evaluation method of the powder molded body electrode used in the discharge surface treatment method for forming a coating comprising an electrode material or a material in which the electrode material reacts with discharge energy,
Four needle-like electrodes are installed on the powder molded body electrode in the order of A, B, C, and D, and a potential difference is generated between the needles B and C when a current is passed through the needles A and D. And measuring the electric resistance of the electrode of the powder molded body, and using the measured value as an index indicating the adhesion strength between the powders in the powder molded body electrode, the powder molded body electrode A method for evaluating a powder molded body electrode, comprising: Based on the electrical resistance of the electrode measured by the four-short needle method, the smaller the resistance value, the higher the adhesion strength between the powders in the electrode, and the larger the resistance value, the lower the adhesion strength between the powders in the electrode. The method for evaluating a powder molded body electrode according to claim 1. If the composition of the electrodes is Co alloy, when measuring range electrical resistance following 4.0E-4ohm more 3.0E-2 [Omega electrode by a four hour hand method, the adhesion strength of the Co alloy powder is an optimum range The method for evaluating a powder molded body electrode according to claim 1, characterized in that: If the composition of the electrodes is Mo alloy, when measuring range electrical resistance following 1.0E-4ohm more 3.0E-3 [Omega] electrodes by a four hour hand method, the adhesion strength of the Co alloy powder is an optimum range The method for evaluating a powder molded body electrode according to claim 1, characterized in that: If the composition of the electrode is Cu alloy, when measuring range electrical resistance following 0.8E-3Ω or 1.0E-2 [Omega electrode by a four hour hand method, the adhesion strength of the Co alloy powder is an optimum range The method for evaluating a powder molded body electrode according to claim 1, characterized in that:

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