JPH06182731A - Consolidation method for preformed workpiece - Google Patents

Abstract

PURPOSE: To obtain a pressed part which is low in porosity, damageless, and has ultrafine particle structure by correcting the resistivity of conductive fluid particles by taking up a second substance having properties different from those of the particles onto the particles. CONSTITUTION: A layer of conductive fluid particles having controlled surface properties is formed to control resistivity, and the particles are arranged in a zone in a container. Next, a pre-molded processing piece is positioned in the layer. Appropriate processing is done in order to consider the proper electric contact of the layer particles. After that, an enough quantity of electric energy to heat the layer to the press temperature of the processing piece is applied to the fluid particles in the layer. Finally, enough pressure to induce the substantial pressing of the processing piece is applied to the layer of the fluid particles. In this way, in order to control electric properties such as resistivity, a reaction with a selected element is made possible.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention is directed to a method of controlling resistivity in a related method of consolidating particulate metal, ceramic materials or combinations of such materials into molded articles of very low porosity. There is.
Specifically, the present invention achieves substantially complete densification at extremely fast processing speeds as compared to the prior art,
It provides a controlled resistivity in connection with the means of applying a high compression pressure at temperatures within the sintering temperature range of the material to be consolidated. In addition, the present invention is generally directed to "charge resistors.
Since an electrothermal heating method called "esistor)" heating is used, the temperature obtainable by the method of the present invention is
Many refractories and materials that have been difficult to densify in the past, i.e., well beyond those required for a wide variety of metals and ceramic materials such as silicon carbide, boron carbide and other very high melting point materials. high.

[0002]

BACKGROUND OF THE INVENTION Methods for making dense metal and / or ceramic articles from their powders are well established in the prior art industrial practice. It is possible, for example by means of what is known as "hot pressing", to place the powder between two cylindrical pistons housed in a cylindrical chamber having a diameter slightly larger than the piston, The powder can be formed into highly dense cylindrical billets by direct application of heat and pressure. Since this method applies external pressure directly to the material at or near its sintering temperature, hot pressing results in a denser consolidated product than if the powder material was consolidated by pressureless sintering. Hot pressing is a relatively simple prior art method for achieving essentially complete densification and is now widely used to make numerous articles in commercial applications. However, because of the high temperatures required to consolidate high melting metal alloy powders and recently developed high performance ceramic materials, the dies and plungers used in hot pressing are materials that can withstand the required temperatures. Must be manufactured. Graphite is the most practical material and is commonly used in industrial hot pressing of ceramics. To avoid oxidation of graphite tools, hot pressing equipment must be housed in a controlled atmosphere chamber and shielded by an inert gas. Often, a vacuum is applied to the chamber containing the hot pressing equipment to allow for the removal of reactive gases that may occur during heating of the material.

The main drawback of this consolidation method is that the cycle time of hot pressing increases with temperature and the method slows down at very high temperatures. Another drawback is that it is necessary to assemble a series of parts into a "stack" to make effective use of the space available in the hot press furnace. This task is complicated and cannot easily be adapted for automated production.

A further disadvantage in terms of commercial production of high performance structural ceramics is the inability to use a simple uniaxial hot press to produce complex shaped parts. For example, turbine rotors with complex curved shapes and substantial changes in thickness cannot be hot pressed.

To produce articles of complex shape, commonly used processing methods "preform" the powder into a partially densified article having the desired shape of the finished part.
Ming) "and then further compacting the preform in a second step. Commonly used methods for making preforms include pressure molding, slip casting, injection molding and extrusion. Then, the preform is any one of operations including room temperature isostatic pressing, hot pressing, hot isostatic pressing (HIPing), room temperature and hot forging, hot extrusion and pressureless sintering. Or they can be compression-hardened by a combination thereof.

Hot isostatic press or HIP
Ing involves sealing the preform in an evacuated flexible container, loading the sealed part into a heating chamber, and heating to the sintering temperature. The container may be a thin metal sheath that can be welded, evacuated and sealed. With the proper choice of metallization material, it transforms into the form of a preform, transferring pressure uniformly and isostatically to the parts to be consolidated. The pressure is applied by the compression of gas in the chamber surrounding the part. Ceramic components, which require very high temperatures to achieve consolidation
For silicon carbide parts, for example, metal sheaths are not practical and means have been developed to encapsulate the parts in glass compositions.

Hot isostatic pressing provides a means for compacting metal and ceramic preforms into complex shaped, compact articles, using HIPing in the commercial production of powdered metal parts and ceramic components. Will increase. However, the main use of HIP is to compact cast metal and compact powder metal parts. HIP technology can be applied to ceramic materials including high temperature non-oxide ceramics such as silicon carbide, but the cost is very high. About 2812kg / cm ² (30000ps
i) The temperature above 2000 ° C. is required at the above pressure. As in hot pressing, HIPing requires long cycle times. The cost of high pressure equipment and systems is higher than that of hot pressing, so HIPin
g is more expensive. Therefore, there has been increased interest in other methods that allow the production of complex shaped ceramic components at low cost.

In addition, one method that has received extensive research by potters is pressureless sintering. Pressureless sintering is the process of making a preform from ultrafine sinter-active ceramic powder, for example by injection molding or cold isostatic pressing, and then making the preform close to the melting point of the material to be sintered. Heating and shrinking and compacting to the desired final size and shape. Pressureless sintering is not only capable of compacting complex, indented shaped parts, but also offers the distinct advantage of being compatible with low cost continuous and automated production of mass production.

Despite the advantages already mentioned of the pressureless sintering method,
Technical factors may limit the utility of this method for producing certain high performance structural ceramic components from certain powdered ceramic materials. This is especially true for refractory non-oxide ceramic materials such as nitrides, carbides and borides. For example, in order to obtain parts of near theoretical density by pressureless sintering of silicon carbide powder, the chemistry for oxygen, carbon and trace elements, all of which is substantially lower than the particle size of 1-2 μ. It is essential to use silicon carbide powder that approximates the characteristic values.

Deviations from these characteristic values are essentially pore-free, uniform, intact structures that are absolutely necessary to obtain the high strength characteristics required for aircraft or automobile engine turbine rotors. Fail to get. Thus
The performance of properly qualified powders that meet the stringent requirements for pressureless sintering involves complex and careful control, which results in a relatively high cost of the starting powder. Furthermore, even if very carefully controlled during the manufacture of the powder, pressureless sintering at high temperature allows compression of the preformed powder at the sintering temperature in order to take into account sufficient shrinkage and densification. It requires a finite amount of time to survive. Intrinsic to the sintering process is the tendency for grain growth to cause anomalies from the idealized ultrafine equi-axed microstructure that provides the highest material strength.
Powders that undergo a crystalline phase transition at or near the sintering temperature are particularly susceptible to grain growth, resulting in reduced strength properties. This effect becomes more pronounced with increasing residence time at the sintering temperature.

It can be appreciated that very high temperatures are required to sinter the materials recommended for high performance engine components. As mentioned above, silicon carbide requires temperatures above 2000 ° C. in order to achieve a high degree of densification by pressureless sintering. From the point of view of minimizing grain growth, the preform is heated rapidly to the sintering temperature,
It is highly desirable to limit this temperature to a residence time no longer than a few minutes and preferably no more than a few seconds, but there are a number of practical limits to doing this by pressureless sintering. is there. First, conventional design sintering furnaces are not easily adapted for rapid loading or unloading of the material to be sintered. But more important is the fact that during heating from the preform gas is likely to evolve due to a combination of desorption, decomposition and chemical reactions. Overly rapid heating rates and associated outgassing rates cause mechanical stresses and component failure. Third, overly rapid heating and / or cooling can also cause failure due to different expansions or contractions called "thermal shock". The range of temperature rise and cooling rates during pressureless sintering are different for different materials and are dictated by the particular material and the size and shape of the part being processed.

However, with respect to the range of temperature rise rates and sintering cycle rates, it should be appreciated that substantially higher cycle rates are possible when the material is under pressure, such as in a hot press. The pressure minimizes gassing cracking and, when maintained during the densification cycle, provides a means of achieving a substantially more reasonable theoretical density than by pressureless sintering.

The current state of the art can then be summarized as follows: a pressureless sintering process for producing advanced materials by compaction from powders, in particular high pressure ceramics for producing high performance ceramics. It is the most attractive method from the viewpoint of low cost production. However, the mechanical properties obtained for pressureless sintered materials are generally not as good as those for materials produced by the application of pressure during sintering, as is the case for hot pressed and / or HIPed products. However, these latter methods are unsuitable for the production of complex shaped parts and / or cannot be easily or completely automated; thus these methods are very expensive.

[0014]

Therefore, one important object of the present invention is to overcome many of the drawbacks of the prior art methods, in particular consolidation with a low porosity, intact and ultrafine grain structure. To provide a controlled and controllable means for sintering very rapidly under pressure to obtain a controlled part.

Another object of the invention is to be able to achieve these basic objects even for materials which require temperatures above 2100.degree.

Still another object of the present invention is 50 ° C./se.
Achieving the above objective within a few seconds using a temperature rise rate exceeding c.

It is a further object of the present invention to consolidate complex shaped preformed materials under compression pressures close to isostatic compression conditions. Furthermore, the object of the invention is to achieve these objects by using a relatively inexpensive device which is simple in construction and operation and which can be easily adapted to automated, controlled and continuous high-volume production. It is to be.

It will be appreciated that some of the above problems of the present invention have been addressed by previous researchers in a particular sect. In this respect, a description of methods intended as improvements in the production of parts by powder compaction is given in the technical literature and in previously issued patents. Examples are Lichti and Hofstater (Hofsta).
tter) patents (US Pat. Nos. 4,539,175 and 4,640,711). These researchers used a simple device similar to that used in hot presses.
In order to consolidate articles of complex shape, the article is first surrounded by a spherical granular medium with sufficient resilience to deformation under high compression pressure without undue failure, which makes the compression pressure approximately isostatic. Proved that it can actually be used by transmitting to the parts to be consolidated. Lissich Hofstatter patents (US Pat. No. 4,539,175 and US Pat. No. 4,640,711)
Is the preferred pressure-transmitting granular medium of Superior Graphite C
o. 1) describes the advantages provided by using the spheroidal graphitic carbon product (9400 series).

While the Lysich and Hofstatter consolidation methods can theoretically provide significant advantages in consolidation of preformed articles, in practice materials that require very high temperatures for consolidation, such as consolidation, are used. It has been found that it is extremely difficult to apply and control the desired method for materials that must be heated above 1500 ° C. (2732 F °) to achieve U.S. Pat. No. 4,539,175 to Lishić et al. Describes that the compaction method is carried out after placing the heated preform into a layer of preheated carbonaceous particles (col. 4, line 35 et seq.). . Furthermore, the graphite particles are 2204 ° C (4000F °)
Can be induction heated to about 426.7 ° C
It is also described that it is significant above (800 F °). Therefore, the working process at a temperature close to 2204 ° C. (4000 F °) becomes extremely difficult. U.S. Pat. No. 4,539,175 expressly states that heating to the required temperature occurs before compression.

US Pat. No. 4,640,741 again describes the consolidation method, wherein non-graphitic spherical carbon and mixtures of graphite and non-graphitic spherical carbon and spherical carbon and / or mixtures of spherical graphitic carbon and ceramics are used as pressure transmission media. It describes the use of particles. US Pat. No. 4,640,741 again clearly states that the layer of preform and particles should be at elevated temperature prior to pressing. Thus, the Lissith and Hofstatter patented method involves independently preheating both the granular media and the preform and then individually transporting the preform to the compression apparatus so that the preform can be immersed in the medium prior to compression. Request. Such transfer of material after heating is at a temperature (1990 ° -2150 ° C.) required to consolidate a non-oxide type ceramic such as silicon carbide.
Would be extremely difficult, and the equipment to accomplish the transfer would be complex, expensive and energy consuming.

In addition, the University of Texas
and the collaborators, while applying compression pressure to the preform, rather than through the bed, rather directly.
High-energy, high-speed consolidation method using high-energy discharge for preforms (High-Energy,
It is also acknowledged that a method called High-Rate Consolidation) has been reported. In this way, electrical energy is transferred directly through the materials to be consolidated and thus can only be used to compact those materials with sufficient conductivity. The present invention, without the use of such limitations and the ability to consolidate non-conductive materials by the method of the present invention, by using a suitably conductive particulate medium. Still further, the method described by Elitzer et al. Is performed in a manner similar to hot pressing and is thus less suitable than hot pressing for producing complex shaped components.

Still another object of the present invention, taking into account the methods of Lichtsch-Hoffstatter and Elitzer, utilizes the principle of consolidation by compressing a free-flowing solid granular medium as an approximate isostatic pressure transmitter. However, it is to achieve heating and compression substantially simultaneously by rapid, advantageous and energetically effective means. Another object of the invention is to include implementations that utilize layer materials that are not limited to the strictly specialized layer materials of the prior art.

The electric consolidation method (El according to the present invention is based on the following principle.
It is also an object of the present invention to control the resistivity of the layer material used in the electrocondensation.

An important objective of developing means to control the resistivity of the medium is to optimize the conditions for any particular application. The electrical system in a particular implementation is a substantially relatively low resistance system. That is, for a given power application (ie, 10 kW, 100 kW, etc.), the applied voltage is low and the current is high. The reason for this phenomenon is that current-carrying structures made of very low resistance materials (e.g. graphite, copper, etc.), especially in order to avoid resistive heating of the device and the resulting loss of temperature control and external power loss. This is because it is desirable to have the upper and lower plunger materials in. So the main electrical resistance of the system is that of the pressurized medium.

One preferred pressurizing medium is the Superior Graphite Company of Chicago, Illinois.
Graphite Co. ), A graphitic carbon such as Superiyl Graphite Special Graphite Carbon Grade 9400. This material possesses the desired excellent flow and elastic properties (resilience) and inert properties to act as a similar isostatic pressure transfer medium. However, the resistivity of graphitic carbon is relatively low (0.03
~ 0.05 ohm-cm). The low resistance of the system has the disadvantage that a relatively high current is required to produce the necessary electrothermal conversion of electrical energy into heat by resistive heating of electricity. Therefore, the equipment must be designed so that it can be operated at high currents requiring connection cables with high current ratings or preferably using water cooling. This significantly increases the cost of the device. From the established electrical system relations, the amount of current that must pass through the medium to achieve the electrothermal conversion energy release necessary to raise the temperature of the preform to the sintering temperature within the desired time period. Is easily calculated. For example, in order to achieve a 50 kW energy release in a system that only produces a resistance of 2.0 × 10 ^-3 ohms, it is necessary to pass 5000 amps through the device,
The required current can be reduced to 2500 amps by correspondingly reducing the current rating of the auxiliary equipment busbars and connecting cables. Therefore, it is generally desirable to have a passage medium that has a relatively high resistivity.
In fact, the resistance of a packed bed of particulate material cannot be measured directly, as is the case for solid materials that can be cut into special shapes, but is measured at the total or theoretical density of the particulate material.
The resistivity of the packed bed of granules depends strongly on the applied pressure. The reported resistivity of the granular medium is then defined in particular by the applied pressure and the method of measuring the particle size distribution of the material (because both conditions influence the packing degree of the material).

Various methods for measuring the electrical resistance of granular carbon materials are known in the prior art. It can also be seen in the prior art that the resistivity of granular carbon is strongly dependent on the pressure applied to the layer of carbon, "Carbon", 24, 337-341 (1986).
Year) “Electrical properties of carbon --- resistance of powder materials”).
Thus, in the electroconsolidation method, the resistance of the system can be adjusted somewhat by the applied pressure. However, in order to achieve the highest degree of densification, it is desirable to apply as high a pressure as possible, and therefore the use of pressure as a means of increasing the resistance of the system does indeed form a possible method for that purpose. I can't. Therefore, in accordance with the practice and principles of the present invention, it is necessary to increase the resistivity of particulate media materials.

The resistivity of the filled material depends on the degree of particle-to-particle contact, which determines the actual interparticle contact area. Thus, the application of pressure to the packed bed serves primarily to increase the interparticle contact surface by compressing the particles together. This is especially true for softer, more elastic materials such as graphitic carbon.

In addition, surface hardness is also an important factor in particle-to-particle electrical resistance in a packed bed of particles, just as in contact resistance between bulky solid materials. For example, the contact resistance between solid graphite and highly conductive solid copper is almost twice that between solid graphite and graphite even if the resistivity of graphite is 1000 times or more that of copper. is there.
This is because graphite is a relatively soft material that deforms much more easily under pressure, providing a more highly effective contact surface. In addition, surface cleanliness and surface coatings can also have a significant impact on contact resistance.

It is therefore based on the above that it is possible to increase the contact resistance, and thus the resistivity, of a packed bed of graphitic carbon by introducing a film-forming material which tends to coat the surface of the particles. This is another aspect. In detail, as described in the present invention, when a film can be formed on the surface of the graphitic carbon particles which is harder than graphite, the electric resistance of the packed bed of this product is thereby reduced to that of the graphite carbon particles It is substantially higher than the electrical resistance of the layer. further,
Materials that react with the surface of the graphitic carbon to form a hard formulation that is stable at high temperatures have particular advantages for use in the electroconsolidation process of the present invention, particularly for consolidation processes that require high temperatures. Thus, the class of film forming compounds of the present invention includes hard, high temperature stable compounds such as carbides, borides, nitrides and related chemical complex compounds such as carbon nitrides, the use and composition of which is Details are given below.

[0030]

SUMMARY OF THE INVENTION The present invention consolidates a wide variety of metals, ceramics and mixed work pieces such as granular, powdered, sintered, fibrous, sponge-like materials or other solid-containing materials that can be consolidated. The improved method of controlling the resistivity in the method described above. The steps of such an improved method include the step of providing a layer of electrically conductive, flowable particles in a zone within the container. The preformed work piece is then positioned in such a layer of electrically conductive flowable particles having a controlled resistivity. Then, the conductive medium is compressed sufficiently to have good conductivity. The electrically conductive, flowable particles within the layer are then subjected to an amount of electrical energy sufficient to heat the layer to the consolidation temperature of the work piece. Finally, pressure is applied to the layer of electrically conductive, flowable particles in an amount sufficient to cause substantial consolidation of the work piece.

The resistivity of such conductive flowable particles is controlled by altering the conductivity and / or hardness properties of its surface and / or internal porosity, as detailed below. In particular, the contact properties of such particles are modified in preferred embodiments of the invention by coating the surface or pores of the electrically conductive flowable particles with various substances.

Additional details of the improved method of the present invention are better understood with reference to the following extensive description of the drawings, detailed description of the preferred embodiments, the claims and the accompanying drawings.

The present invention generally controls the resistivity of electrically conductive flowable particles for use in compacting work pieces preformed from granular, sintered, fibrous, sponge or other solid encapsulating compactible materials. Includes improved methods. The steps of such an improved method provide a layer of electrically conductive, flowable particles having controlled surface properties to control resistivity, and then place such particles in a zone within the container. The preformed work piece is then positioned in a layer of such electrically conductive flowable particles. Appropriate processing is performed to allow for proper electrical contact of the layer particles. Thereafter, a sufficient amount of electrical energy is applied to the electrically conductive, flowable particles within the layer to heat the layer to the consolidation temperature of the work piece. Finally, a sufficient amount of pressure is applied to the layer of electrically conductive, flowable particles to cause substantial consolidation of the work piece. Such pressure may be applied uniaxially in certain desirable implementations.

Some embodiments of the general method of using such electrically conductive flowable particles with modified surface and / or internal porosity characteristics were first described. In a preferred embodiment of the general method, the conductive, flowable particles of the layer desirably consist of first of all, flowable, resiliently compressible carbonaceous particles in the form of beads, the beads protruding to the outer surface. It may have bumps and at least some of the beads may have surface cracks.

The special work pieces may include combinations of silicon carbide, boron carbide, and / or other carbides, and / or in combination with the metal or the metal itself.

In a preferred embodiment, the amount of electrical energy applied to the layer of electrically conductive flowable particles is such that the temperature of such layer is about 2200 for certain uses, such as ferrous materials.
Sufficient to raise the temperature to above 0 ° C, and lower temperatures (eg, about 1000 ° C) to oxides, powder metal,
Suitable for other materials such as non-ferrous materials and the like. Further, such preferred implementations define such temperature rise within about 60 seconds.

The electrically conductive layer is electrically isolated from its surroundings in the preferred embodiment. Also in such an embodiment, a supply of inert gas is input and maintained in the layer prior to and during the application of electrical energy to the layer.

Such particles of the layer may contain silicon carbide spheroidal graphitic carbon. Such silicon carbide spheroidal graphitic carbon can include spherical particles having a diameter of substantially about 0.1 mm to 0.8 mm. Such silicon carbide spheroidal graphitic carbon may contain about 50% silicon carbide with substantially the balance consisting of free carbon. However, such silicon carbide spheroidal graphitic carbon has some tendency to sinter, especially upon consolidation of silicon carbide work pieces. However, nevertheless, such silicon carbide materials can provide more advantageous properties in certain applications, including more precise resistivity control, especially in low temperature applications.

The electrically conductive, flowable particles are, in some preferred embodiments, about 0.01 for some special uses.
It may have an electrical resistivity of 2 ohms / 2.5 cm (1 in). Such particles may also have a resilience of at least about 50%.

In addition to other materials, the preformed work pieces may contain as binder a phenolic resin binder in an amount equal to yield about 2% free carbon after pyrolysis. About 7
Compression pressures of 0.3 kg / cm ² (1000 psi) can be used, but higher pressures (eg 14
0.6 kg / cm ² (2000 psi) is also advantageously used in other implementations.

The inert gas utilized may include nitrogen, argon or noble gases.

The electrical energy applied depends on the equipment and workpiece size, workpiece geometry and material, and in some systems can include 15 volts (DC) at 800 amps for about 30 seconds. These parameters and the like can be adjusted by one of ordinary skill in the art without undue experimentation.

Such consolidation methods can, in the preferred embodiment, form a densification of approximately at least 80%. The preformed work piece may desirably have a 60% densification prior to applying electrical energy and pressure thereto. The density of such preformed work pieces, in preferred embodiments, is from about 2.01 g / cc to about 2.80.
It can be increased to g / cc.

Boron can be introduced into the structure of the particles of the electrically conductive fluidized bed and, in a preferred embodiment, can form 1.75% by weight of the electrically conductive, fluidized particles.

The preformed work piece can be formed by various techniques, including cold compression, in a preferred embodiment, and such cold compression can be applied uniaxially.

Prior to pyrolysis, such preformed workpieces contained approximately 1% of a 5% solution of phenolic resin dissolved in a solvent.
It may contain wt%.

Electrical energy, preferably at least about 20 kW, can be applied to the conductive flowable particles during compaction of such work pieces.

To achieve the objects of the present invention, the particulate medium not only provides the desired degree of compression or resilience characteristics and temperature stability as described in the Lichsch-Hoffstatter (supra), but the current causes the compression assembly to It is a suitable electrically conductive material that allows passage therethrough, and is selected to allow heating by direct electrical resistance heating of the medium during the compression cycle. Such direct electrothermal heating occurs from within the device, specifically within the zone between the compression plungers in which the preform is placed. Therefore, there is no need to either preheat either the preform or the granular media or transfer the hot material from a separate auxiliary heater to the compression device. It is therefore clear that with a proper design of the compression chamber and a suitable choice of electrically conductive and resilient granular medium, very high energy release rates are possible. In fact, temperatures of the invention above 2200 ° C. are achieved with a current flow of less than 60 seconds in a device of the type described.

[0049]

【Example】

EXAMPLE I In one application of the present invention, a device such as that generally shown in FIG. 1 has an outer diameter of about 0.06 to 0.95 cm (2/8 to 3
/ 8 in), inner diameter of about 2.54 to 2.38 cm (1 to 15)
/ 16 in) and was assembled to include a cylindrical vertical sleeve 10 having a height of about 5.08 cm (2 in). The sleeve 10 was machined from a solid graphite rod. To minimize electrical conduction through the inner surface 12 of the sleeve 10, a ceramic coating 14 was applied by painting the inner surface with a silicon carbide containing paint obtained from ZYP Coatings. Sleeve 10 in plenum chamber 1
Placed on a machined graphite support 16 having eight, into the chamber an inert gas (see arrow A) is through a gas inlet 20 consisting of an array of vertically perforated stoma and from the top thereof a plenum 18. And was able to shield the vertical outer surface 22 of the sleeve 10 from attack by the ambient air, as indicated by arrows B and C.

A vertical graphite piston 24 having a total height of about 10.16 cm (4 in) has a height of about 5 to 1.9 cm (2 to 3/4 i).
n) cylindrical rods, as shown in FIG. 1, have a diameter of approximately 2.5 for insertion into the vertical sleeve 10 from above.
Machined to 4-1.9 cm (1-3 / 4 in).

Approximately 1.27 cm (1/2 in) thick upper and lower copper plates 26, 28 with connected perforations used as cooling water passages are in direct contact with piston 24 and lower support 34. Located and used as an electrical contactor. Further, these copper plates were connected by electric cables attached to tap hole connectors 30 and 32 to the DC power supply lines having a capacity of 600 Kw indicated by 36 and 38, respectively.

The assembly is a hydraulic press plate 40,
Held between 42 and "Glastic (glasti
c) ″ electrically insulated by insulating plates 44, 46 of a composition referred to below.
The holes 50 are used to provide compressive force to the upper and lower assemblies.

Example II A series of experiments were performed to determine the ability to compact silicon carbide by the improved method. The experiment was carried out by introducing into the sleeve 75 g of a granular medium charge of silicon carbide spherical carbon. This substance has a diameter of 0.1
0.8 mm spheres containing approximately 50% SiC with a balance consisting of free carbon. Silicon carbide spheroidal carbon is available from Superior Graphite Co., Chicago, Illinois, and has a resistivity of 0.012 ohm-in (2.54 cm) measured and a resilience determined to be 52%. Had. Resilience is about 703kg / cm
It is defined as the rate of volume increase of the sample upon compression pressure release of ² (10000 psi).

Silicon carbide diameter 2.5 in the form of a work piece 56
A 4 cm (1 in) by 0.3 cm (1/8 in) thick preform was placed in the granulate layer so that it was aligned horizontally. The preforms were made using Superior Graphite Silicon Carbide (Type HSC 059) containing about 2% by weight of B ₄ C and a substantial amount of phenolic resin binder giving about 2% of free carbon after pyrolysis. Was molded by cold compression.

After charging the silicon carbide pellets, the plunger was advanced into the sleeve, and about 70.3 kg / cm ²
A compression pressure of (1000 psi) was applied by a hydraulic cylinder (not shown) to raise the lower plate 42 of the press (see large arrow D in Figure 1).

The nitrogen flow was started (arrow A) and a 15 volt DC and 800 amps power was maintained for 30 seconds.
The sleeve 10 begins to glow red, similar to the lower portion 52 of the plunger 24 extending above the cylindrical sleeve 10.

After cooling and removing the pellets, it was found that the silicon carbide spheroidal graphitic carbon was partially sintered around the silicon carbide pellets. 80% from 60% dense material, taking out pellets, weighing and measuring
Was found to have been consolidated.

Example III Prepared as described in Example II, but 150 in a laboratory furnace for complete pyrolysis of the phenolic resin binder.
Another experiment was performed using a 2.54 cm (1 in) diameter silicon carbide preform preheated to 0 ° C. In this experiment, about 70.3 kg / cm (100
0 psi) compression pressure is maintained and current is applied, input 12
It was maintained at kW for 1 minute. After the experiment, the pellets have a density of 2.
It was found to increase from 01 g / cc to 2.80 g / cc, which corresponds to an increase in the theoretical density of silicon carbide from 60% to 87.2%.

Thus, while these initial experiments of Examples II and III did not show complete consolidation of silicon carbide, these experiments demonstrated the feasibility of the rapid high energy method for consolidation of high temperature materials. .

Example IV The silicon carbide used in some examples of the invention was:
It is available under the trade name "HSC" from Superior Graphite, Inc. of Chicago, Illinois. Such HSC silicon carbide has been developed for a wide range of uses, including metallurgical additives, refractories, fine abrasive grit and for high performance ceramics and special refractory mix materials.

HSC 059 grade silicon carbide is
Exhaust grade (run-) to remove excess free carbon
It was manufactured by processing a material of of-furnace). This moist, milled, chemically refined, free-flowing powder is directly applied to metal or ceramic matrix composites, metallurgical and refractory applications, lapping and finely ground grits, high purity silicon carbide reactively bonded, free It can be used in pressed or HIP sintered ceramics. Table 1 lists the characteristic values of such desired materials.

[0062]

[Table 1]

EXAMPLE V Superior Graphite Ltd. is suitable for other implementations.
We also manufacture 400 series spheroidal graphitic carbon. This free-flowing, high-purity spherical graphitic carbon product exhibits extremely high mechanical strength under compressive loading.

The 9400 graphitic carbon is highly resistant to oxidation and chemical corrosion and has good forming properties. Fields of use include refractory mixtures, hydraulically broken proppants, core moldings (and as a special shell blasting medium). In addition, antioxidants can be incorporated into the structure to achieve a very high degree of oxidation resistance.

Table 2 Properties of 9400 series spheroidal graphitic carbon Typical range Carbon content,% 99.9-99.9 + Sulfur content,% 0.01-0.03 Particle size range, mm 0.10-0 .80 ¹ particle density ^2, g / cc ^1.70~1.85 electrical resistivity, home · cm × 10 ² 3.80~4.60 resilience,% ³ 35-42 ¹ size range are Tyler standard sieve - 20 + 150 mesh.

^Two liquid pycnometers were used.

³ Resilience is about 703.7 kg / c
Percentage increase in sample volume after release of m ² (10000 psi compression pressure).

Example VI 40 × 140 mesh size mixed metal powder consisting of a blend of neodymium (Nd), iron (Fe) and boron (B) sufficient phenol resin (“Varcum”) to act as a binder. ). Next, a mixture of the metal powder and the resin is formed to a thickness of about 1.27 cm (1/2 in).
2.5 with a green density of about 5.0 g / cc at
Compressed into cylinders of less than 4 cm (1 in). The samples were dried and heated to about 250 ° C. to give a metal preform to cure the resin and give good strength and hardness. The variously compressed pellets then appeared as shown in the following table: Table 2A Density of powder metal preforms Sample number Applied power (Kw) Time (min) Final density (g / cc) 1 7. 5 1 5.70 2 10.0 1 5.59 3 12.8 1 7.14 4 10.0 2 7.44 The theoretical density of the above composition is 7.55 g / cc.

Example VII A series of other experiments were conducted to illustrate the consolidation of silicon carbide preforms. The equipment used is described in FIG. The particulate medium in such experiments was 9400RG grade spherical carbon particles manufactured by Superior Graphite of Chicago, Illinois and as described in the Examples above. This material contains 1.75% boron incorporated into the grain structure and is labeled "Metal Bearing Graphite Carbon".
Carbons) "US Patent No. 45604
No. 02, the invention of which is incorporated herein by reference.

The 9400RG product is a desirable material for compacting silicon carbide because the boron therein acts as a sintering aid and its presence in the surrounding particles enhances the densification process. However, other carbide materials that are stable at the reaction temperature (eg, carbides such as titanium, tungsten and rare earth elements) can be used.

The preform was premixed with 110% by weight of 5μ boron carbide and 5% by weight of a phenolic resin dissolved in acetone, HSC 05 from Superior Graphite.
Nine grades of silicon carbide powder were produced by cold-axial compression. Pellets are approximately 1406 kg / cm ² (20,000 ps) in a 2.54 cm (1 in) diameter steel die.
i) to obtain a green density of 2.14 g / cc (corresponding to a theoretical density of 66.6% with respect to silicon carbide). These pellets are oven dried at 100 ° C. for 8 hours and then 250 to reduce volatile formation during electroconsolidation.
It was baked at 30 ° C. for 30 minutes. A slight shrinkage occurred during the pyrolysis process, but the pellet density was largely unchanged due to the corresponding small weight loss.

A tabulation of the experimental conditions and experimental results is given in Table 3 and graphically shown in FIG. It is concluded that at applied powers below about 20 kW there was little or no densification. However, the densification was rapid at an applied electric power of about 20 Kw or more, and a pellet density close to the theoretical density of silicon carbide was obtained within a few minutes.

It is clear that these data are present, first of all, to explain the rapid compaction and densification of the silicon carbide powder preforms achieved by the process of the invention. The best consolidation conditions for silicon carbide and other materials to achieve minimal porosity and excellent microstructure require optimization of key process variables, which is well known to those skilled in the art. It can be carried out according to the invention without undue experimentation. These variables include: dimensions of the compaction chamber, level of applied power,
The table of pressures to be applied, consolidation cycle times and types and electrical properties of the granular pressure transmission medium, optimized process conditions also depend on the type of material to be consolidated and the size and shape of the preform. Also, certain types of work piece shapes require changes in process sequence pressure and / or compression pressure.

Table 3 Electroconsolidation of Silicon Carbide Powder Preforms Final Density Experiment Number Medium Amount E. (Volts) I. (Amps) Kw (g / cc) (%) Applied for 2 minutes experiment time Power 1 100 8 960 7.68 2.14 66.7 2 125 10 1040 10.40 2.14 66.7 3 125 12 1360 16.32 2.14 66.7 4 125 12 1600 19.20 2.30 71.75 125 14 1840 25.76 3.00 93.5 Power applied for 4 minutes of experiment time 8 120 8 1040 8.32 2.14 66.7 9 120 10 1200 12.00 2.14 66.7 10 120 11 1360 14.96 2.14 66.7 11 120 12 1520 18.24 2.24 69.8 12 120 13 1720 22.36 3.06 95.0 Note (1) The preform is H of Superior Graphite.
Manufactured using SC 059 silicon carbide powder.

(2) Dimension of preform diameter: about 2.54
cm (1 in) x thickness 0.6 cm (1/4 in). Prior to compression curing, it was pyrolyzed to a density of 2.14 g / cc at 1500 ° C. for 30 minutes.

(3) The pressure transmission medium is 9400RG, boron-containing spherical graphite particles manufactured by Superior Graphite.

(4) Graphite cylinder diameter about 5.08 cm
(2 in) (5) Applied pressure 281.2 kg / cm ² (40
00 psi) Example VIII Controlling the resistivity of flowable particles that are conductive during compaction is an important feature of the present invention. Regarding the modification of the surface characteristics of the method of the present invention, the idea of forming a hard carbide surface on the graphitic carbon particles in order to increase the electrical resistivity is to use graphite carbon particles with silica sand and increase the amount of silica sand with respect to carbon. It was investigated by reacting at elevated temperature (1950 ° C.) in the series of experiments used. The results given in Table 4 give the (a) silicon carbide content (determined as ash by the ignition loss test of the reaction product according to test methods known in the art) and (b) the corresponding measured electrical resistivity. Show. These data are plotted in FIG.

Such data show that below about 30% silicon carbide, the resistivity of the carbon material is almost linear with respect to the silicon carbide content, and accordingly, on the basis of this, about 0.
It is possible to create a pressure transmission medium with a resistivity in the range of 03 ohm-cm to 0.075 ohm-cm. Above about 30% silicon carbide content, electrical resistivity increases rapidly with silicon carbide content, and precise control of particle resistivity is more difficult to achieve due to the non-linear relationship between them. .

Example IX The change in resistivity of 9400 spheroidal graphitic carbon particles can also be increased by reaction of the particles with boron at elevated temperature (2000 ° C.), which is introduced as boron oxide (B ₂ O ₃ ). sell. Only 3.0% boron due to the reaction of oxide and carbon
The following additions, as shown in Table 5, result in a product with an almost 10-fold increase in electrical resistivity compared to the non-boron added starting material.

Table 5 Measured Changes in Properties of Boron-Added Graphitic Carbon Particles B (%) Resistivity [Ohm-cm (Ohm-in)] Resilience (%) 0 0.0383 (0.0151) 79 1.75 0.2972 (0.117) 52 When boron is used as a means to increase the electrical resistivity of carbon particles, it is soluble and very mobile within the graphite crystal lattice. Yes; if the amount of boron added is less than the solubility limit of about 2.3%, a film with a low boron content is formed on the carbon surface, causing a small change in the electrical properties of the graphitic carbon. In fact, the low efficiency is slightly reduced. However, Lowell, CE, "Solid Solution of Boron in Graphite,""Journ. Am. Ce.
r. Soc. , 50, 3142 (1967
)], And when boron is added in an amount exceeding the solubility limit into graphite, a separate phase is formed, and as shown in Table 5, the electrical resistivity is remarkably increased, and it is very hard. A surface is created. The boron content of 1.75% described in the examples is below the solubility limit of 2.35%, while the concentration localized on the surface of the particles exceeds the solubility limit.

It is important to note that the addition of elements that form hard carbides, carbon nitrides and other reaction products on the surface of graphitic carbon has a significant effect on the resilience as well as the resistivity of the material. Is. Resilience is an important property for effective use of particles in controlled electroconsolidation. As shown in Table 5, in the case of boron addition, an increase in resistivity is achieved by increasing the resilience of the material. This phenomenon becomes more brittle when the carbon surface is coated with hard carbides (such as carbonitride), the contact points are more easily broken when subjected to compression pressure, and the volume after the compression pressure is released. It occurs because the degree of expansion decreases. Loss of resilience is detrimental to the development of the ideal isotactic type pressure transfer desired during electroconsolidation and is therefore a hard film-forming element to increase or modify the resistivity properties of graphitic carbon media. There is an actual upper bound on the uptake of Generally, the resilience of the medium should be greater than 50%. Factors that influence the resilience of graphitic carbon are Goldberger (WM) and Thrower (PA), "Resilientity of Spherical Graphitic Carbon" (published by the American Carbon Society (1
987), minutes of the 18th Biennial Conference on Carbon,
July 19-24, 1987, Worcester, Mass.).

Some of the elements important in forming hard, high temperature stable coatings on graphitic carbon include those which form stable carbides, nitrides and carbonitrides. They are,
Above all, it includes silicon, boron, titanium, tungsten and rare earth elements. Many of these products are known from US Pat. No. 4,560,409 (named “Metal Bear”).
ing Graphic Carbons ", owned by the assignee of Superior Graphite of Chicago, Illinois).

The above discussion and US Pat. No. 4,560,040.
The description in No. 9 describes means of adding these elements by carbothermic reduction of their oxides, but suitable means of adding such elements, eg vapors of their volatile compounds and graphitic carbon. Addition by phase reaction can be utilized. One example is the reaction of silicon tetrachloride with hydrogen in the presence of granular graphitic carbon at temperatures above 1400 ° C., which produces a suitable silicon carbide coating. Similarly, chlorides such as boron, titanium, tungsten, etc. can be used to react and introduce the film-forming elements with the carbon surface. Vapor phase pyrolysis of organometallic compounds, such as the pyrolysis of silicones, is a feasible means for obtaining silicon carbide coatings.

It is generally desirable to have a pressure-transmitting medium with graphitic carbon and a high resistivity, but in certain cases it is possible to choose a medium with low resilience. This can be accomplished by introducing the metal element internally as a metallic phase that forms within the pore structure of the graphitic carbon rather than on the surface of the particles. For example, the reaction of iron oxide with graphitic carbon at about the melting point of iron (1500 to 1600 ° C.) produces a metallic iron-encapsulating particle structure (see FIG. 4). The X-ray diffraction diagram proves that they are included as metallic iron. The properties of this iron-indented graphitic carbon are given in Table 6.

Table 6 Physical properties of iron-encapsulated graphitic carbon Fe-encapsulation Bulk density Particle density Surface area Resistivity Resilience (%) (kg / m ³ ) (g / cc) (m ² / g) (Ohm · cm) (%) 0 686 1.59 0.44 0.0241 125 1.3 731 1.61 0.47 0.0211 116 4.7 789 1.72 0.57 0.0191 109 12.1 828 2.03 0.67 0.0145 95 In this case, the presence of metal ions reduces the resistivity. It is noted that the resistivity is reduced by the presence of iron. Similar results are observed with the use of nickel.

In summary, the highly resilient graphitic carbon particles used as the preferred pressure transmission medium for the electroconsolidation method of the present invention are selected because they allow control of their electrical properties such as resistivity. Can react with the element. Reaction with elements that form high temperature stable carbide and carbon nitride structures on the surface of carbon is desired as one means of increasing resistivity. Boron is a highly effective hard film-forming element used in amounts of greater than or equal to about 1.5% and less than or equal to about 5% in preferred embodiments to limit resilience reduction. In the range of addition of silicon carbide to graphitic carbon, addition of up to 30% silicon carbide gives a material with highly controllable resilience. Various means of forming the coating include carbothermic reduction in an inert or nitrogen atmosphere or by chemical reaction of the element-containing gas in the presence of graphitic carbon.

The resilience of graphitic carbon is the reaction of iron oxide and / or other compounds with graphitic carbon at temperatures approaching the melting point of the metal, such as iron and / or nickel into the carbon structure. It can be reduced by pushing metal.

The basic and novel features of the improved method of the present invention will be readily appreciated by those skilled in the art from the foregoing description.
The steps, ranges, parameters and conditions of the improved method of the present invention may be varied in various ways without departing from the spirit and scope of the present invention, inter alia, taking into account the nature of the material to be treated as described above. And it will be readily apparent that modifications can be made. Therefore, the preferred alternative implementations of the invention described above are not intended to limit such ideas and scopes in any way.

[Brief description of drawings]

1 shows a cylindrical vertical sleeve with a ceramic coating on its inner surface, which sleeve exerts the compressive force of a graphite piston arranged for longitudinal movement in order to compress the granular contents of such sleeve. A partial longitudinal section through a device suitable for carrying out the method according to the invention

FIG. 2 is a plot showing pellet density (g / cc) and% theoretical density versus applied power (kW).

FIG. 3 is a diagram showing changes in resistivity (home / cm) of graphitic carbon according to silicon carbide content (SiO%).

FIG. 4 is a magnification 5 of graphitic carbon particles having metallic iron inclusions.
00x micrograph

[Explanation of symbols]

 9 10 sleeve 12 inner surface 14 ceramic coating 16 support base 18 plenum chamber 20 gas inlet 22 outer surface 24 plunger 26, 28 copper plate 30, 32 tap hole connector 36, 38 DC power supply line 40, 42 platen 44, 46 insulating plate 48 downward Through bolt 50 Open hole 52 Lower part of plunger 54 Granular medium 56 Work piece A, B, C, D Arrow

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location H05B 3/14 B 7913-3K

Claims (20)

[Claims] 1. A method of compacting a preformed work piece in a layer of electrically conductive fluid particles, the method comprising the step of: adjusting the resistivity of such electrically conductive fluid particles to have a characteristic different from that of such particles. A method of consolidation of preformed workpieces, characterized in that the material is modified by incorporating it onto the particles. 2. The method of claim 1, wherein the second substance is incorporated on the surface of the electrically conductive, flowable particles. 3. The method according to claim 1, wherein the conductive fluid particles are made of a carbonaceous material having pores on the surface. 4. The method according to claim 3, wherein the second substance is injected into the pores of the carbonaceous material. 5. The method of claim 3, wherein the second material has greater surface hardness properties than the carbonaceous material. 6. The method of claim 3, wherein the second material is selected from the group consisting of carbides, nitrides and carbonitrides. 7. The carbide, nitride and carbonitride
7. The method of claim 6, wherein the method is formed from at least one of the group consisting of silicon, boron, titanium, tungsten and rare earth elements. 8. The method of claim 3, wherein the second substance is introduced to the surface of the carbonaceous material by reaction with at least one reactant selected from the group consisting of boron, titanium and rare earth elements. the method of. 9. The method according to claim 3, wherein the second substance is introduced onto the surface of the carbonaceous material by vapor phase pyrolysis of an organometallic compound. 10. The method of claim 9 wherein the organometallic compound that is pyrolyzed is silicone. 11. The method of claim 3, wherein the carbonaceous material is graphitic carbon. 12. The second substance is silicon carbide, which is introduced into the carbonaceous material by the reaction of silicon carbide and hydrogen at a temperature of about 1400 ° C. or higher in the presence of graphitic carbon,
The method according to claim 11, wherein silicon carbide is provided on the surface of the graphitic carbon. 13. The method of claim 3, wherein the second material is a material that provides less surface hardness properties than the carbonaceous material. 14. The second material for imparting low surface hardness properties is selected from at least one of the group consisting of iron and nickel compounds.
The method described. 15. The method of claim 3, wherein the second material is a metal oxide, which is reacted with the carbonaceous material at about the melting temperature of the metal. 16. The second material is iron oxide, which is reacted at a temperature of about 1500 to 1600 ° C. to form a granular structure having metallic iron inclusions in the pores of the carbonaceous material. 14. The method of claim 13 characterized. 17. The method of claim 3, wherein the second material is boron and is about 1.5-5% by weight of the carbonaceous material. 18. The method according to claim 3, wherein the second substance is reacted by a carbothermic reaction in an atmosphere selected from the group consisting of an inert gas and nitrogen. 19. The method of claim 3, wherein the second material effectively wets the conductive particles. 20. The method of claim 19, wherein the second material is forced into the pores of the conductive particles to form a metallic phase in intimate contact therewith to reduce conductivity. .