GB2024256A

GB2024256A - Isostatically hot pressing powdered material

Info

Publication number: GB2024256A
Application number: GB7915111A
Authority: GB
Original assignee: ASEA AB
Current assignee: ABB Norden Holding AB
Priority date: 1978-05-02
Filing date: 1979-05-01
Publication date: 1980-01-09
Also published as: GB2024256B; SE7804992L; SE414920C; SE414920B; JPS54146205A; DE2915831A1; FR2424783A1; JPS6232241B2; FR2424783B1; DE2915831C2

Abstract

A body preformed from a powdered material other than silicon nitride is surrounded with a casing, which is permeable to gas. The casing is transformed upon heating into a layer which is impermeable to the pressure medium which is used during isostatic pressing of the preformed body, while a pressure is maintained by a pressure gas outside the casing which is at least as high as the pressure of the gas present in the pores of the preformed product. When the casing has been made impermeable to the pressure medium and the preformed body has been enclosed therein, the isostatic pressing is carried out while simultaneously sintering the body. The casing may be a glass, metal or ceramic powder applied as a slurry or by flame spraying. The powders pressed may be Fe or Ni based alloys or Al2O3.

Description

SPECIFICATION Method of manufacturing an article of powdered material In the manufacture of articles of metallic or ceramic material by sintering together powder of the material while employing isostatic pressing, the powder is suitably preformed into a manageable powder body. This can be done by loose sintering, i.e. by sintering a powder, filled into a forming cavity, under vacuum or in a protective gas so that a coherent body is formed no mentionable densification takes place. Preforming of the powder into a manageable body can also be done by subjecting the powder to an isostatic compaction, for example arranged in a sealed capsule of yielding material, such as a plastics capsule.The compaction can be carried out with advantage without using any binder at room temperature or any other temperature which is considerably lower than the temperature employed during the compression associated with the sintering process. Thereafter, the product can be given its desired shape by machining. For the preforming it is also possible to use, among other things, conventional techniques for the manufacture of ceramic goods. Thus, the powder may be mixed before the preforming with a temporary binder, for example methyl cellulose, cellulose nitrate, an acrylate binder, a wax or a mixture of waxes. After the preforming the binder is given off by heating so that the preformed powder body in all essentials becomes binder-free.

When the preformed powder body is subjected to the isostatic pressing at the sintering temperature, it must be, in order to give a desired dense, sintered product, be enclosed in a casing which, during the pressing, is able to preventthe pressure medium used, normally a gas, from penetrating into the powder body. The casing, which like its contents is liberated from undesirable gases during a process stage prior to the sealing, must of course also have a sufficiently high strength or viscosity during the pressing operation so that it does not penetrate into the pores of the powder body.If a preformed capsule of glass is chosen as the casing, which, when it is a question of materials with sintering temperatures in excess of 1 000 C, must be of a high-melting type in order not to run off or penetrate into the powder body at the high sintering temperature, the glass, when softened, cannot be prevented from accumu lating in pockets and other cavities of the preformed powder body. This often leads to fractures at protruding portions of the sintered article when it cools down because of differences in the coefficient of thermal expansion of the powder material and the glass. The method is therefore only suitable for manufactue of articles of very simple shapes.Parti cularly, if it is a question of manufacturing an article having a very complicated shape, the casing can be allowed to form on the spot by dipping the preformed powder body into a suspension of particles of high-melting glass (for materials with a high sintering temperature), or otherwise surrounding the body with a layer of particles of such glass and then heating the powder body under vacuum at such a temperature that the particles form a dense casing around it. The last-mentioned method permits the application of a casing which can be made thin and closely follows the shape of the powder body so that accumulations of glass on the sintered article can be avoided and thus also the disadvantages connected therewith.A tight casing is only achieved at high temperatures as the glass of course has to be of high-melting type in order not to run off or penetrate into the powder body during the sintering of the powder material. It is also known, namely for silicon nitride, to use a porous layer of a glass of a low-melting type outside a porous layer of glass of a high-melting type. In this known case, the outer porous layer is transformed into a layer which cannot be penetrated by the pressure medium while the powder body is being degassed. When a dense layer has been formed, pressure is applied to the enclosed powder body with a gaseous pressure medium to counteract dissociation of the silicon nitride during the continued temperature increase.

During the continued temperature increase, the glass in the outer layer reacts with the material in the inner porous layer while forming an increasingly high-melting glass and while maintaining a layer impermeable to the pressure medium, and finally a glass layer, impermeable to the pressure medium, is formed of the innermost part of the inner porous layer before the glass in the counter layer is able to run off. This last formed glass layer forms a tight casing around the powder body when the isostatic pressing of the preformed product is carried out at the sintering temperature.

The present invention aims to provide a method of manufacturing an article of powder material of high density with greater reproducibility than with the previously known methods.

According to the invention a method of manufacturing an article of a material in the form of a powder by isostatically pressing, with a gaseous pressure medium, a body preformed from the powder, the preformed body then being surrounded with a gas-permeable casing which is transformed into a casing which is impermeable to the pressure medium and which encloses the preformed body, the isostatic pressing thereafter being carried out while sintering the preformed body, is characterised in that the gas-permeable casing is transformed into the casing impermeable to the pressure medium while the gas-permeable casing is in contact with a pressure gas and while a pressure is maintained in the pressure gas which is at least as great as the pressure simultaneously prevailing in the gas which is present in the pores of the preformed body.The preformed body is degassed, but suitably not until the gas-permeable casing has been applied thereon.

A probable explanation of the favourable result obtained with the method according to the present invention is that the method, more efficiently than in previously known methods, ensures the formation of a tiqht casing around the preformed body. In the previously known methods, where the tight casing is formed on the body, the formation of the casing takes place during degassing of the body. The continuous departure of gases, which are present in the powder body or which are formed in the powder body by contaminations or by the powder material itself, can cause formation first of bubbles and subsequently of wounds or cracks in the casing which was intended to enclose the powder body tightly so that such a tight containment is not achieved.With the method of the present invention, however, a departure of gases from the powder body is prevented during the formation of the casing for containing the powder body. This is done by maintaining a pressure in the pressure gas which is at least as high as the pressure of the gas which is present or formed in the pores of the powder body.

This leads to a formation of a casing with no defects.

The material in the powder preferably consists of a metallic or ceramic material, and particularly such a material with a sintering temperature in excess of 1000"C, such as an iron-based alloy, for example 3 per cent Cr-Mo steel containing 0,33 per cent of C, 0.30 per cent of Si, 0.40 per cent of Mn, 0.01 per cent of P, 0.01 per cent of S, 2.8 per cent of Cr and 0.6 per cent of Mo, the remainder being Fe, or 12percent Cr-Mo-V-Nb-steel containing 0.18 per cent of C, 0.25 per cent of Si, 0.60 per cent of Mn, 0.01 per cent of P, 0.01 percent of S, 11.5 per cent of Cr, 0.5 per cent of Ni, 0.5 per cent of Mo, 0.30 per cent of V and 0.25 per cent of Nb, the remainder being Fe, or a nickel-based alloy, for example an alloy containing 0.03percent of C, 15 per cent of Cr, 17 percent of Co, 5 per cent of Mo, 3.5 per cent of Ti, 4.4per cent of Al and 0.03 per cent of B, the remainder being Ni, or an alloy containing 0.06 per cent of C, 12 per cent of Cr, 17 per cent of Co, 3 percent of Mo, 0.06 per cent of Zr, 4.7 percent of Ti, 5.3 per cent of Al, 0.01 4per cent of B and 1.0 per cent ofV, the remainder being Ni, or among other things, a metal oxide such as A1203.

The abovementioned percentage contents, as well as those mentioned hereinafter, relate to percentages by weight.

Preferred gases for use as the gaseous pressure medium during the isostatic pressing are inert gases, for example argon, helium and nitrogen.

Preferred gases for use as the pressure gas for transforming the gas-permeable casing into the casing impermeable to the pressure medium are hydrogen, especially when sintering metallic materials, and nitrogen. It is possible, however, to use as the pressure gas other gases which do not damage the powder body by forming undesirable reaction products or giving an unacceptable porosity in the powder body.

In one embodiment of the method in accordance with the invention, the gas-permeable casing consists of a porous layer which, at least substantially, completely surrounds the preformed body, which layer then being transformed into a gas-tight layer around the preformed body. The porous layer, which suitably has a thickness in the range of from 0.05 to 1 mm, may, among other things, be applied by dipping the preformed body into a suspension of a particulate material which is to form the casing, or by flame spraying or other thermal spraying. The particles may suitably have a size of from 0.1 to 100 microns.

In another suitable embodiment of the method in accordance with the invention, the gas-permeable casing consists of one or more elements in the form of plates or the like, arranged on the preformed body, which elements soften and change their shape when heated and are transformed into a casing impermeable to the pressure medium by the element parts, which are brought into contact with each other when changing their shape, sintering together.

If the powder has a sintering temperature above 1000"C, the material of the casing, i.e. the material in the particles and the elements, respectively, in the embodiments just described, may advantageously consist of a high-melting glass such as "Vycor" (Trade Mark) glass containing 96.7 per cent of SiO2, 2.9 per cent of B203 and 0.4per cent of Al 203, quartz glass and mixtures of particles (for the firstmentioned embodiment), for example SiO2 and B203, which when heated form a gas-tight glass layer. It is also possible to use a high-melting metallic material having the ability to form a layer impermeable to the pressure medium, for example molybdenum, tungsten and other refractory metals.

When using a high-melting glass in the casing, a temperature of from 12000 to 1 6500C is suitably used when the casing is made impermeable to the pressure medium.

If the powder has a sintering temperature in excess of 1000"C, the material of the casing, i.e. the material in the particles and the elements, respectively, in the above-described embodiments, may under certain circumstances consist of a low-melting glass, namely if the preformed powder body is coated with a fine-grained layer of the material included in the powder body, the fine-grained layer thus preventing the glass, in molten state, from penetrating into the powder body, or if a porous layer of a high-melting material, such as highmelting glass or high-melting metallic material, is arranged inside the casing and transformed into a layer impermeable to the pressure medium after the casing has been made impermeable to the pressure medium.Also when using more than one porous layer, each porous layer may be applied in the previously described manner by dipping in a suspension of the particulate material, flame spraying or other thermal spraying. Each porous layer may suitably have a thickness of from 0.05 to 1 mm and the particles a grain size of from 0.1 to 100 microns.

As examples of material which may be used for casings of low-melting glass may be mentioned "Pyrex" (Trade Mark) glass containing 80.3 percent of SiO2, 12.2 per cent of B203, 2.8 per cent of Awl 203,, 4.0 per cent of Na2O, 0.4 per cent of K20 and 0.3 per cent of CaO, an aluminium silicate containing 58 per cent of SiO2, 9per cent of B203, 20 per cent of a1293 Sper cent of CaO and 8per cent of MgO, as well as mixtures of particles of substances, for example SiO2, B203, A1203, and alkali metal oxides and alkaline earth metal oxides, which when heated form a gas-impermeable glass layer.

When using a low-melting glass in the casing and a porous layer positioned thereinside, a temperature of from 600" to 1 000 C is suitably used to make the casing impermeable to the pressure medium. The inner layer is thereafter suitably densified under isostatic pressure, which may be achieved after the outer layer has become gas-tight, at temperatures of from 1000" to 1200"C. For such a densification, a pressure of the order of magnitude of from 20 to 300 MPa is required.

The pressure and the temperature during the sintering of the preformed body are, of course, dependent on the properties of the powder material.

Normally, the pressure should amount to at least 100 MPa, preferably to at least 150 MPa. If the material consists of an iron-based alloy, the temperature should be at least 1 000 C, preferably from 1100 to 1200"C, and if the material consists of a nickel-based alloy, the temperature should be at least 1050"C, preferably from 1100 to 1 2500C. If the material consists of aluminium oxide, the temperature should be at least 1200"C, preferably from 1300 to 1500"C.

The invention will now be illustrated by the following non-limitative Examples given with reference to the accompanying schematic drawing, in which Figure 1 is a sectional view of a preformed body of a nickel-based alloy, the casing of which before heat-treatment consists of a porous layer of a high-melting glass, Figure 2 is a sectional view of a preformed body of the same alloy, the casing of which before heattreatment consists of elements in the form of plates of a high-melting glass, Figure 3 is a sectional view of the body of Figure 2 after heat-treatment, Figure 4 is a sectional view of a preformed body of an iron-based alloy, the casing of which before heat-treatment consists of a porous layer of a low-melting glass, Figure 5 is a sectional view of a preformed body of an iron-based alloy, the casing of which consists of elements in the form of plates of a low-melting glass, and Figure 6 is a sectional view of the body of Figure 5 after heat-treatment.

EXAMPLE 1 A divisible form of an aluminium-silicate-based material (for example of the same type as is normally used in cores for investment casting of turbine blades having cooling channels) and having a forming cavity shaped as a turbine disc, is filled with spherical powder of a nickel-based alloy containing 0.03 per cent of C, 15 per cent of Cr, 17 per cent of CO, 5 per cent of Mo, 3.5 per cent of Ti, 4.4 per cent of Al and 0.03 per cent of B, the remainder being Ni, and having a grain size of < 250 microns.

The powder is vibrated together by striking lightly on the form, and is sintered in vacuum at 1 200 C for 2 hours. After cooling, the form, which is reusable, is divided, and the porous turbine disc having essentially the same dimensions as the forming cavity is removed. The porous turbine disc is thereafter coated with a fine-grained powder, having a grain size of < 1 micron, of the same alloy as the turbine disc, to a thickness of about 1 mm.

The turbine disc 10, which is shown schematically in Figure 1, is provided with a gas-permeable casing in the form of a porous layer 11 by being sprayed with an aqueous suspension of a powder of a glass consisting of 80.3 per cent of SiO2, 12.2 per cent of B2O3, 2.8 per cent of A12O3, 4.0 per cent of Na2O, 0.4 percentofK2O and 0.3percentofCaO, and is then dried.

The preformed powder body, thus treated, is then placed in a high-pressure furnace which is provided with a conduit through which gas can be discharged for degassing the powder body and gas can be supplied for generating the pressure required for the isostatic pressing, and which is provided with heating devices.

The preformed powder body 10 with the applied casing 11 is first degassed in a high-pressure furnace at room temperature for approximately 2 hours.

Thereafter the furnace is filled with hydrogen gas at atmospheric pressure and the temperature of the furnace is raised to 7500C while maintaining the pressure, which may take approximately 5 hours.

The temperature is then raised successively during the course of 2 hours from 750"C to 900"C while simultaneously successively leading in hydrogen gas to a pressure of 0.7 MPa, the pressure outside the casing of the preformed body being all the time maintained at at least the same pressure as that which prevails in the remaining gas in the pores of the preformed body. When the temperature has reached 900"C, a casing has been formed of the layer 11 which is impermeable to the hydrogen gas.

Thereafter, additional hydrogen gas, or argon or helium, is supplied to a pressure level which gives a pressure of 160 MPa in the pressure medium at the final sintering temperature. The temperature is then raised to 1 250"C, i.e. to a sintering temperature suitable for the nickel-based alloy. A suitable time for the sintering under the conditions mentioned is at least 30 minutes. After the sintering process, the furnace is allowed to cool to a suitable discharging temperature and the sintered article is cleaned of glass by blasting.

When manufacturing articles of powder, for example aluminium oxide, having a higher sintering temperature than the above-exemplified alloy, a high-melting glass can be used in the porous layer 11 instead of the above-exemplified low-melting glass and without any fine-grained layer being applied on the powder body.

EXAMPLE2 The same preformed body 10 as in Example 1 is surrounded, in accordance with Figure 2, with a gas-permeable casing consisting of two elements, positioned perpendicularly to the axis of the turbine disc, in the form of plates 12 and 13 of the same glass as that used in the powder in Example 1. When the body with its casing has been placed in a high-pressure furnace, it is subjected to the same treatment as the body in Example 1, i.e. it is degassed, heat-treated and isostatically pressed under the conditions states there.During the temperature increase, from 750"C to 950"C in this case instead of to 900"C as in Example 1, and the simultaneous pressure increase, the plate 12 softens and changes its shape, as is clear from Figure 3, and the end portions 1 2a and 1 3a sinter together so that the casing becomes tight.

EXAMPLE3 Powder of 3per cent Cr-Mo-V-N b-steel containing 0.33 per cent of C, 0.25 per cent of Si, 0.40 per cent of Mn, 0.01 percent of P, 0.01 percentof S, 2.8percent of Cr and 0.6 per cent of Mo, the remainder being Fe, and having a grain size of < 800 microns, is placed in a capsule of plastics material, for example plasticised polyvinyl chloride, which has a forming cavity in the shape of a turbine disc. After sealing of the capsule, it is placed in a high-pressure press where the powder is isostatically compacted at room temperatureata pressure of 400 MPa with oil as the pressure medium. After this compaction operation, the capsule is removed and the preformed powder body thus manufactured is machined into the desired shape.

The preformed body in the form of a turbine disc 14 is surrounded with a gas-permeable casing in the form of a porous layer 15 of a low-melting glass and a porous layer 16 of a high-melting glass located inside the porous layer 15. This is achieved by first dipping the body in an aqueous suspension of powder of a high-melting glass consisting of 96.7 per cent of Si02, 2.9 per cent of B203 and 0.4 per cent of A1203 and then, after drying this layer, in an aqueous suspension of a powder of a low-melting glass consisting of 80.3 per cent of Si02, 12.2 per cent of B203, 2.0 per cent of Awl 203,4.0 per per cent of Na20, 0.4per cent of K20 and 0.3 per cent of CaO, followed by further drying.

The preformed body with the porous layers is placed in the high-pressure furnace and is degassed as in Example 1 at room temperature. After filling with nitrogen gas at atmospheric pressure, the temperature of the furnace is raised to 7500C while maintaining the pressure, which takes about 5 hours.

The temperature is then successively raised during the course of 2 hours from 750"C to 9000C while simultaneously successively leading in nitrogen gas to a pressure of 0.7 MPa, the pressure outside the casings of the preformed body being all the time maintained at at least the same pressure as that which prevails in the remaining gas in the pores of the preformed body. When the temperature has reached 900"C a casing has been formed of the layer 15which is impermeable to the nitrogen gas.

Thereafter, additional nitrogen gas, or argon or helium, is added to a pressure level giving a pressure of 50 MPa at 1 1509C. The temperature is then slowly raised to 11 50 C. The pressure then increases simultaneously. This temperature increase is achieved sufficiently slowly for the glass in the inner layer 16 to be able to form a gas-impermeable layer before the glass in the casing 15 has time to run off. The pressure and the temperature are then raised to 100 MPa and 1200"C, respectively, for sintering the preformed body.

EXAMPLE4 The same preformed body 14 as in Example 3 is surrounded in accordance with Figure 5 with a gas-permeable casing consisting of two elements, placed perpendicular to the axis of the turbine disc, in the form of plates 17 and 18 of the same glass as that used in the layer 15 in Example 3 and with a porous layer 19, placed inside said casing, of the same kind as the layer 16 in Example 3. When the body with its casing has been placed in a highpressure furnace, it is subjected to the same treatment as the body in Example 3, i.e. it is degassed, heat-treated and isostatically pressed under the conditions stated there.During the temperature increase, from 750"C to 950"C in this case instead of to 900"C as in Example 3, and the simultaneous pressure increase, the plate 17 softens and changes its shape, as is clear from Figure 6, and the end portions 1 7a and 1 8a sinter together so that the casing becomes tight.

The elements 12, 13,17 and 18 in Figures 2 and 5 may, of course, have a shape other than plate-shape, and may, for example, consist of more or less bent elements, or of a container or bottle provided with an opening. Of course, they should suitably be shaped with respect to the shape of the preformed body.

When the preformed body has been sintered as described in Examples 1 to 4, it is subject to a first heat-treatment in a protective gas, for example argon, or under vacuum at 1 1000C (Examples 1 and 2 and at 11 50 C (Examples 3 and 4), respectively, for about 2 hours. This leads to diffusion of hydrogen gas and nitrogen gas from the material, and at the same time the remaining pieces of the glass layer loosen so that they may easily be removed entirely.

The pressure during this treatment should suitably not exceed 0.1 MPa.

If a binder, such as the previously exemplified methyl cellulose, cellulose nitrate, an acrylate binder, a wax or a mixture of waxes having different melting points, is used when manufacturing the preformed powder body, the binder is removed prior to or after the application of the porous layers, suitably by heating the powder body to a temperature of from 400 to 700"C under vacuum. Thereafter degassing and further treatment can be performed, as described for a preformed powder body with no binder.

In the specification of our co-pending Patent Application No. 79 filed on the same day as the present application there is claimed a method of manufacturing an article of silicon nitride by isostatically pressing, with a gaseous pressure medium, a body preformed from silicon nitride powder, the preformed body being surrounded with a gaspermeable casing which is transformed into a casing which is impermeable to the pressure medium and which encloses the preformed body, the isostatic pressing thereafter being carried out while sintering the preformed body, which is characterised in that the gas-permeable casing is transformed into the casing impermeable to the pressure medium while the gas-permeable casing is in contact with a pressure gas and while a pressure is maintained in the pressure gas which is at least as great as the pressure simultaneously prevailing in the gas which is present in the pores of the preformed body. We make no claim to this method in the present application.

Claims

1. A method of manufacturing an article of a material in the form of a powder by isostatically pressing, with a gaseous pressure medium, a body preformed from the powder, the preformed body being surrounded with a gas-permeable casing which is transformed into a casing which is impermeable to the pressure medium and which encloses the preformed body, the isostatic pressing thereafter being carried out while sintering the preformed, body characterised in that the gaspermeable casing is transformed into the casing impermeable to the pressure medium while the gas-permeable casing is in contact with a pressure gas and while a pressure is maintained in the pressure gas which is at least as great as the pressure simultaneously prevailing in the gas which is present in the pores of the preformed body.

2. A method according to claim 1, in which there is used as the pressure gas, until the casing impermeable to the pressure medium has been formed, a gas which at least substantially consists of hydrogen or nitrogen.

3. A method according to claim 1 or 2, in which the gas-permeable casing consists of a porous layer which, at least substantially, completely surrounds the preformed body, said porous layer being transformed into e casing impermeable to the pressure medium and enclosing the preformed body.

4. A method according to claim 1 or 2, in which the gas-permeable casing consists of one or more elements arranged outside the preformed body in the form of plates or the like which soften and change their shape when heated, and which are transformed into a casing impermeable to the pressure medium by the element parts, which when heated are brought into contact with each other, sintering together.

5. A method according to any of claims 1 to 4, in which the sintering temperature of the powder exceeds 1000"C and the material of the casing consists of a high-melting glass.

6. A method according to claim 5, in which the preformed body is heated to a temperature of from 1200 to 1 6500C for the formation of the casing impermeable to the pressure medium.

7. A method according to any of claims 1 to 4, in which the sintering temperature of the powder exceeds 1 000 C and the material in the casing consists of a low-melting glass and a porous layer of a high-melting material is arranged inside the casing and is transformed into a layer impermeable to the pressure medium after the casing has been made impermeable to the pressure medium.

8. A method according to claim 7, in which the preformed body is heated to a temperature of from 600" to 10000C for the formation of the casing impermeable to the pressure medium and thereafter to a temperature of from 1000" to 1 200 C under simultaneous isostatic compaction, for the formation of the layer, impermeable to the pressure medium, of the porous layer arranged inside the casing.

9. A method according to any of claims 1 to 4, in which the sintering temperature of the powder exceeds 1 000 C and the material in the casing consists of a low-melting glass and the preformed body is coated with a fine-grained layer of the same materials as in the powder body.

10. A method according to any of claims 1 to 8, in which the article after the sintering is heat-treated at low pressure and at such a temperature that dissolved pressure gas can diffuse out of the material.

11. A method of manufacturing an article of a material in the form of a powder substantially as described in a any of the foregoing Examples.

12. An article when produced by the method claimed in any of the preceding claim.