WO1990001385A1

WO1990001385A1 - Process for making a consolidated body

Info

Publication number: WO1990001385A1
Application number: PCT/SE1988/000391
Authority: WO
Inventors: Bengt SJÖBERG; Jan Nybom
Original assignee: Uddeholms AB
Current assignee: Uddeholms AB
Priority date: 1988-08-02
Filing date: 1988-08-02
Publication date: 1990-02-22
Anticipated expiration: 1991-02-02

Abstract

In a process for making a consolidated body having a near net shape, a ceramic mold (13) is produced by casting a ceramic material against a mold (10), which has been made of a polymeric material, in order to obtain a mold cavity (17), the shape of which with a contraction equals the corresponding portion of the wanted body. The mold cavity (17) is filled with a metal powder (15) and/or other solid finely divided material and that said ceramic is placed in a deformable, gas-tight container (16, 16'), the walls of which directly or via a solid particle-type filling medium (19) are caused to lie close to the ceramic mold, and the container and its contents are heated and are subjected to an external isostatic pressure so that the ceramic mold and the finely divided material are compressed to a complete tightness.

Description

PROCESS FOR MAKING A CONSOLIDATED BODY

THE TECHNICAL FIELD The present invention relates to a process for producing a consoli¬ dated body having a near net shape, i.e. a body, the shape of which is almost identical with the required final shape of the product, made of a metal powder and/or one or several other solid finely divided materials, including fibers and finely divided ceramic materials, graphite and composites.

THE PRIOR ART

It is known in the art to produce objects having a near net shape using powder metallurgy, a so called near net shape technique by means of a process including a hot isostatic pressing (HIP) to achieve full density. This technique allows a combination of the quality advantages obtainable by starting with a metal powder having a homogenous compo¬ sition and without segregations amounting to the advantages which result from a high product yield and low machining costs. It is also known in the art, using said technique, to start from other finely divided materials than metal powders or from mixtures of metal powders and other finely divided materials, e.g. fibers and finely divided ceramic materials, graphite and compositions. The near net shape technique has followed two lines of development. According to one of these lines of development there is first produced a green body of a metal powder or the like, which green body is sintered, until the communicating porosity of the green body has been eliminated, and subsequently the sintered body can be consolidated to a full density by means of a hot isostatic pressing. Examples of this technique are described in e.g. Swedish patent specification SE-B-442486. According to the other line of development the powder is placed in a shell, the internal side of which has a shape, which substantially equals the shape of the wanted object, the shell-mold is closed, the shell-mold and its contents are heated to a temperature suited for a hot isosta- tic pressing and the shell-mold and its contents are subjected to a hot isostatic pressing in order to compress the metal powder or a similar material into a body having full density. Another embodiment of this technique is to seal the surface of the powder body by means of a glass material or a melting of the surface layer, the suface layer forming a shell having the desired shape. Examples of these techniques are described in Swedish patent specifications SE-B-382929 and SE-B-435 026.

BRIEF DESCRIPTION OF THE INVENTION The object of the present invention is to suggest a process along a new line of development for powder metallurgic production of bodies having a near net shape. A special object of the invention is to suggest a comparatively inexpensive shaping process for a near net shape production using hot isostatic pressing. These and other objects can be achieved by the following steps:

a) producing a mold of a polymeric material having a mold surface, which with a contraction equals the shape of a corresponding surface of the body to be produced;

b) producing a ceramic mold obtained by casting a ceramic material against the mold, which has been produced from the polymeric material, in order to obtain a mold cavity, the shape of which with a contrac¬ tion equals the corresponding portion of the wanted body;

c) filling the mold cavity with a metal powder and/or another solid finely divided material, including fibers, finely divided ceramic materials, graphite and ceramic materials;

d) placing the ceramic mold - before or after its filling with the metal powder and/or with said other finely divided materials - in a deformable, gas-tight container, the wall of which directly or via a solid particle type filling medium are caused to lie close to the ceramic mold; e) evacuating the air from the container, which subsequently is hermetically closed;

f) heating the container and its contents to a temperature suited for hot isostatic pressing of said ceramic material and of the used finely divided material; and

g) subjecting the container to an external isostatic pressure for such a long time at said temperature, that the ceramic mold as well as the finely divided material in the mold cavity are compressed to full density, such that the finely divided material, while shrinking, is consolidated to a body having substantially the desired shape.

The above-mentioned mold of a polymeric material is produced from an elastomer, e.g. a silicone-rubber. The mold of silicone-rubber or a similar elastomer can e.g. be produced by casting from another mold, which in its turn can be produced by a polymeric casting from a model, which with a contraction has the same shape as the body to be produced. This second polymetic material, which can be used for the initial casting from a model, is a rigid or an elastic polymeric material, e.g. a polyurethan, an epoxy resin, a siloxane resin, a polyester, a copolymer thereof or a similar plastic or rubber material, which preferably is a thermosetting resin.

The ceramic mold is preferably made of a ceramic material having a filling ratio, which essentially is as large as the filling ratio of the powder in the mold cavity before the hot isostatic pressing. In case a spherical gas atomized metal powder is to be used, the filling ratio of the powder in the mold cavity may be as large as about 70%, provided the powder has been maximally close-packed by a vibration or a slight mechanical compressing. Consequently, in this case a ceramic mold of a ceramic material having a filling ratio of 65-75% is produced and used. In case a metal powder, atomized by a liquid, is to be used, then this material is very un-smooth, which results in a filling ratio of only about 35-40%. Consequently, in this case a ceramic material having a filling ratio of 35-55%, preferably 35-45%, is used. However, the filling ratio of the liquid-atomized metal powder can be improved, provided the powder is subjected to a machin¬ ing operation, before it is charged into the mold, in order to round the edges. In this case a filling ratio of about 60% can be attained. Consequently, the ceramic material in the ceramic mold ought in this case have a filling ratio of 55-65%. If instead of the metal powder entirely or partly different finely divided material are used, which result in a different filling ratio, a filling ratio for the ceramic material adapted to this is chosen according to the principles mentioned above.

The ceramic mold can be produced in the following manner. A model of the body which is to be produced is first made of e.g. wood or plastic material. The shape of the model is identical, with a contraction, with the shape of the finished product. The term contraction means in this context that the dimensions of the model exceed the dimensions of the product to be produced with what equals the shrinkage of the metal powder from compressing to full density, while it is also necessary to keep in mind the contractibility of the polymeric materials as well as the ceramic material. Thus, one has to take into consideration the properties of the metal powder or a similar finely divided material, i.e. whether the powder or the like is spherical, very uneven, fibrous, or rounded as well as to the properties of the polymeric materials and the ceramic material in these respects. The particle size distribution of course also affects the close-packing ratio and consequently also the contraction. From the model, made in this way, a rigid polymer casting is made and from the polymer casting a casting of said first elastic polymeric material, preferably silicone-rubber, is made. In the elastic mold of silicone-rubber or the like, made in this way, the ceramic mold or a portion of it subsequently is formed.

A number of ceramic materials can be used in accordance with the invention. In order to be able to easily scour the ceramic material from the finished product a ceramic material, which after the hot isostatic pressing easily can be removed by dissolving in a solvent, e.g. in water or in an acid, ought to be used. It is suitable to blend a binding agent in the ceramic material, which binding agent is soluable in a suitable solvent, e.g. a sodium hydroxide solution. A ceramic material can also be selected, which can be made brittle by chemical reactions. It is also possible to use a combustible ceramic material, e.g. a material which contains such a large amount of graphite or pulverized coal, that the final mold can be removed by burning it off from the metal body, which has been consolidated by the hot isostatic pressing.

The definition of the term ceramic material used in this specification is a solid material, which neither is a metal nor an alloy, nor an organic material, but which readily can include or be composed of compounds with metals or organic materials. Also, mixtures of various materials, defined in this manner, are to be included in this defini¬ tion of the term ceramic material.

Additional characterizing features and aspects of the invention are set forth in the following description of two preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following description of a preferred embodiment reference will be made to the attached drawings, in which:

Fig. 1 A-E schematically shows the different steps of the process according to the first preferred embodiment of the invention; and

Fig. 2 shows an alternative embodiment of the hot isostatic consolidation of the body to be produced.

The drawings are of course solely schematic and equipment details, which are not needed in order to understand the principles of the invention, have been left out. DESCRIPTION OF PREFERRED EMBODIMENTS

Initially a model 1 of e.g. wood or a plastic material of the body to be produced is made, Fig. 1A. The shape of the model is identical with the shape of the finished product with contraction. Model 1 is placed on a plate 2 and on this plate also a plastic tube 3 is placed in order to obtain a first mold cavity 4. This mold cavity 4 is filled with a liquid thermosetting material, e.g. polyurethane, in order to obtain a casting 5 of model 1. Model 1 is removed from polymeric casting 5, which is provided with a cavity 7 obtained from medel 1 and subsequently is placed on a plate 6. On the same plate 6 a wider tube 8 is also placed. Cavity 7 and a space 9 between tubes 8 and 3 well as a space above the upper edge of polymeric casting 5 are filled with a silicone-rubber. When the silicone-rubber has hardened, the silicone- rubber mold 10, obtained in this way, is pulled off from polymeric casting 5 and from the enclosing tube 8 and is placed on another plate 11, the obtained new cavity 12 thus being turned upwards. The silicone-rubber mold is supported on its outer side by a tube 20. Cavity 12 is filled with a mass of a ceramic material, which is hardened, after which silicone-rubber mold 10 is pulled off from ceramic mold 13, obtained in this way.

Jointly with a ceramic cover 14, made of the same ceramic material as mold 13 is made of, ceramic mold 13 forms a mold cavity 17. The geo¬ metrical shape of mold cavity 17 Is identical with the wanted product with contraction. Mold cavity 17 is filled with a metal powder 15, preferably -a gas-atomized spherical metal powder, before ceramic parts 13 and 14 are placed in a container 16. Metal powder 15 can e.g. consist of a gas-atomized spherical steel powder of a steel alloy suited for producing molding tools or cutting tools. In the latter case the steel powder can consist of a high speed steel powder.

However, other alloys can of course be used, e.g. stainless steels, nickel base alloys and other super alloys. The latter types of alloys are particularly suitable when pump bodies, impellers and other machine members are to be produced. Other finely divided materials can also be selected, instead of or as supplement to a metal powder, which has been discussed in the introductory part of the specification. Before ceramic cover 14 is applied, powder 15 is close-packed by vibration.

Ceramic mold 13 and cover 14 and the contained metal powder 15 are placed in said container 16, which can consist of a metal can or of other deformable material. Container 16 is evacuated to a pressure of

_3 not more than about 10 mbar, while heating it slowly to a tempera¬ ture of between about 300 and 400°C, and is closed by an external cover 18. Subsequently the aggregate is heated to a temperature suited for hot isostatic pressing of a tool steel alloy. The aggregate suitably is heated to a temperature of between 1000 and 1225°C, preferably to a temperature of between 1100 and 1200°C. Subsequently container 16 and its contents are subjected to an isostatic pressing at said temperature, until ceramic mold 13 and cover 14 as well as metal powder 15 have been compressed to a 100% density.

The hot isostatic pressing suitably is carried out in an autoclave press with argon as a pressure medium. Finally the compressed product is removed from the autoclave press and subsequently ceramic mold 13, 14 is removed from the consolidated dense metal body.

Fig. IE shows that ceramic mold parts 13 and 14 are cast in such a way, that they can contact the internal side of container 16 with a close fit. However, it is also possible, as has been illustrated in Fig. 2, to embed ceramic mold parts 13 and 14, which contain metal powder 14, .into a pressure transmitting medium 19, a solid particle material, e.g. sand, which therefore is applied between ceramic mold parts 13, 14 and the internal side of container 16' . Consequently, container 16' in this case is larger than according to the embodiment shown in Fig. IE, provided a production of a product having the same dimensions as according to the first embodiment is contemplated.

Claims

PATENT CLAIMS

1. A process for making a consolidated body having a near net shape, i.e. a body, the shape of which is almost identical with the required final shape of the product, c h a r a c t e r i z e d by the following steps:

a) producing a mold (10) of a polymeric material having a mold surface, which with contraction is identical with the shape of a corresponding surface of the body to be produced;

b) producing a ceramic mold (13) obtained by casting a ceramic material against said mold (10), which has been produced from the polymeric material, in order to obtain a mold cavity (17), the shape of which with contraction is identical with the corresponding portion of the wanted body;

c) filling said mold cavity (17) with a metal powder (15) and/or another solid finely divided material, including fibers, finely divided ceramic materials, graphite and ceramic materials;

d) placing the ceramic mold (17) - before or after filling it with said metal powder (15) and/or with said other finely divided materials - in a deformable, gas-tight container (16, 16'), the walls of which directly or via a solid particle-type filling medium are caused to lie close to said ceramic mold;

e) evacuating the air from said container (16, 16'), which subsequent¬ ly is closed hermetically;

f) heating said container and its contents to a temperature, which is suited for hot isostatic pressing of said ceramic material and of the used finely divided material (15); and

g) subjecting said container (16, 16') to an external isostatic pressure for such a long time at said temperature, that said ceramic mold (13) as well as said finely divided material (15) in said mold cavity (17) are compressed to full density, in order with a contraction to consolidate said finely divided material to a body substantially having the desired shape.

5 2. A process according to claim 1, c h a r a c t e r i z e d in that said mold (10) of a polymeric material is made of an elastomer, preferably silicone-rubber.

3. A process according to any of claims 1-2, c h a r a c t e r i z e d ⁰ in that at.least two ceramic mold part (13, 14) are made, which joint¬ ly form said mold cavity (17), which is filled with a metal powder (15) or a similar finely divided material or material- mixture.

4. A process according to claim 1, c h a r a c t e r i z e d in that ⁵ said ceramic mold (13, 14) is produced having a filling ratio, which substantially equals the filling ratio of said powder (15) in said mold cavity (17) before the hot isostatic pressing.

5. A process according to claim 4, c h a r a c t e r i z e d in that ° a gas-atomized, spherical metal powder is used and in that a ceramic mold having a filling ratio of 65-75% is made and used.

6. A process according to claim 4, c h a r a c t e r i z e d in that a liquid-atomized, un-smooth metal powder is used and in that a 5 ceramic mold having a filling ratio of 35-55% is made and used.

7. A process according to claim 4, c h a r a c t e r i z e d in that a liquid-atomized metal powder is used, the edges of which have been rounded, and in that a ceramic mold having a filling ratio of 55-65% is made and used.

8. A process according to any of claims 2-7, c h a r a c t e r i z e d in that said mold (10) of an elastomeric material is made as a casting from a mold (5) of a rigid or an elastic polymeric material, which mold (5) consists of a casting from a model, which with a contraction equals the body to be produced.