METHOD OF MANUFACTURING COMPOUND PRODUCTS

Abstract: A method of manufacturing a compound product from powder material, at least one powder material (10) being preformed (1) and loose-sintered (2) into a porous, manageable body with a density of 75-85 % of T.D., starting from a powder material with a filling density of 65-75 % of T.D. The preformed, loose-sintered body is placed (3) in a capsule (11). The capsule is filled (4) with at least one additional powder material, evacuated (5), sealed (6), essentially gas-tightly, and compacted (7), by means of isostatic pressing, into an essentially fully dense body (10), preferably with a metallic bond, atomic bond, between the materials included.

Description: Method of manufacturing compound products TECHNICAL FIELD The invention relates to a method of manufacturing products of compound type starting from powder material. At least two powder materials are compacted into products with essentially full density while at the same time obtaining a metallic bond between the materials included. The invention is particularly applicable to the manufacture of products in which property combinations are required which are normally not obtained with the same material, for example good erosion and corrosion properties or great hardness combined with toughness, good thermal conductivity or thermal shock resistance. BACKGROUND ART In selecting the material, undesirable compromises and reconsiderations of priorities must often be made regarding performance requirements made or desired. These compromises - are made in order to adequately satisfy the requirements made with one material. Alternatively, the product may be assembled from several parts, which usually both increases the cost of and decreases the quality of the product . The improvements within the powder metallurgy have also provided the possibility of manufacturing products by a combination of a plurality of materials. By selection of suitable mate¬ rials, these so-called compound products exhibit the very property combinations required by the product. In the manufacture of compound products by means of powder metallurgical compound technique, there are a plurality of variants as to how to locate the different materials in relation to each other. The different powder materials may be filled into a capsule by means of separate pieces of equipment. However, this usually requires that the capsule be provided with some form of partitions, which may have to be removed before the body is compacted. One of or all the materials may in certain cases be pre¬ formed into solid bodies, which are placed in a capsule and compressed. The powder may be filled around the solid preformed bodies, whereafter the powder is compacted together with the solid bodies. Differences in density between the solid bodies and the loose powder lead to powder and body being compacted to varying degrees during the pressing operation. In certain cases this gives rise to internal stresses or other imperfections in the compound product, which limits the applications of the compound technique since for certain fields of application, material combinations or product geometries, for which these internal imperfections cannot be tolerated, the optimum solutions which can be obtained with compound technique must be dispensed with. The problems with the removal of partitions as well as the problem with different degrees of compaction in different parts of the compound product are avoided by preforming bodies, according to the invention, which are compacted and sintered into essentially the same density as that of the powder used. SUMMARY OF THE INVENTION According to the invention, at least one of the materials included in the compound product is preformed and loose- sintered from powder into a body of the desired shape. The density during preforming and loose sintering preferably increases to a density of the loose-sintered body of 75-85% of the theoretical density (hereinafter abbreviated "T.D.") from an apparent density of the filled powder (hereinafter referred to as "filling density") of 65 - 75% of T.D. The preformed, loose-sintered body is thereafter placed, together with at least one additional powder material, in a capsule. The capsule is sealed essentially gas-tightly and isostatically compressed into an essentially fully dense body with an essentially perfect metallic bond, atomic bond, between the materials included. After the isostatic pressing, the capsule is removed and the product is after- treated in conventional manner by machining and heat treat¬ ment. By preforming, according to the invention, at least one of the materials included in the compound product into at least one porous, loose-sintered body with a density insig¬ nificantly higher than the filling density of the powder used, problems with varying degrees of compaction between those parts of the compound product which consist of pre¬ formed bodies and those parts which during the final com¬ paction are formed from loose powder, are avoided. It is, of course, possible to use more than one material for the preformed body, it then being possible to mix these materials into a composite by intermixing a material into a matrix of another material, or else mixing the materials also in the preformed body into a compound body, in which case the interface between different materials may be distinct or be made diffuse, to create more favourable properties, for example regarding stresses in a transition zone. A body preformed according to the foregoing description is sintered according to the invention into a porous body; preferably the body is sintered at a temperature corre¬ sponding to 30 - 60% of the melting point of the material. The purpose of the sintering is only to hold together the preformed body during the handling up to the point where it is placed in the capsule. At the same time, any organic binders used during the forming are driven off. The sintering is preferably performed under vacuum or in a protective gas atmosphere to avoid reactions on the surfaces of the powder grains . The temperature is chosen such that the powder grains are bonded together at the poits of contact, only an insignificant increase of the density then being obtained. Preferably, the density during preforming and loose sintering increases from a filling density of 65- 75% of T.D. to a density of the loose-sintered body of 75- 85% of T.D. More specifically, the preformed body is sintered, according to the invention, when using spherical powder, for example obtained by gas atomization of metal melts, into a porous body with a density of 75-80% starting from a filling density of about 70%. At least one loose-sintered body is placed in a capsule together with at least one additional powder material. The capsule, which at least at the pressing temperature is deformable, is thereafter sealed essentially gas-tightly and is compacted by means of isostatic pressing, preferably hot- isostatic pressing, the compound product in its entirety being compacted into essentially full density, while at the same time the materials are bonded together with a metallic bond. After compaction and sintering, the capsule is removed mechanically or chemically and the product is after- treated in conventional manner, for example by mechanical treatment and/or heat treatment . By manufacturing compound products from powder according to the present invention, products of essentially full density, with good metallic bond, atomic bond, between the materials included and a body which, in one compaction step, is com¬ pressed with essentially the same degree of compaction in all materials are obtained. By means of the invention, products with unique material combinations can be obtained, which have been optimized from the point of view of both cost and function. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates the manufacture of compound products according to the invention and Figure 2 shows how the invention is applied when a diffuse transition is desired between the materials united in the compound product. DESCRIPTION OF THE PREFERRED EMBODIMENTS In the manufacture of compound products according to the invention, powdered material is formed at 1 and loose- sintered at 2 into a body 10, the density of which insig¬ nificantly exceeds the filling density of the used powder. The loose-sintered body 1 may consist of only one powder material or be a composite of a plurality of powder materials. In addition, it is possible to intermix, for example, reinforced fibre materials or other materials which, in the strictest sense, are not completely powdered. The loose sintering at 2 takes place under inert conditions for the powder, preferably under vacuum or in a protective gas atmosphere, to avoid reactions on the surfaces of the powder grains. Thereafter, at 3, the loose-sintered body 10 is placed in a capsule 11 which is filled, at 4, with at least one additional powder material before it is evacuated, at 5, and gas-tightly sealed, at 6. In case of powder- metallurgical manufacture, the loose-sintered body 10 of course undergoes the usual steps such as checks and analyses of a quality-assuring nature, dimensional adjustments and other conventional operations before being placed in the capsule 11. The capsule 11 containing at least one loose-sintered body 10 produced from powder material and additional powder material, which has been supplied to the capsule, at 4, in loose form, is then compacted, at 7, into an essentially dense compound product 12, the materials included in the compound product 12 having been compacted to essentially the same degree. In this way, internal stresses and other imperfections, which easily arise as a result of varying compaction degrees in different parts of the compound product 12, are avoided. According to the invention, the compaction, at 7, preferably takes place in a hot-isostatic press. Of course, the manufacture of a compound product 12, according to the invention, comprises conventional opera¬ tions in powdermetallurgical manufacture, carried out both on the loose-sintered body 10 and on the finished compound product 12, to ensure quality, dimensions, and material structure. To further reduce internal stresses, certain material combinations may require a transition zone between the materials included, where the interface between the materials is made diffuse. According to the invention, this is achieved by forming a body, in Figure 2 exemplified by the sleeve 100 which will be described below, with a diffuse transition from one of the materials A, in Figure 2 exemplified as a surface layer 101 on a compound roll 120, to the material B, which in Figure 2 constitutes the material in the core 105 of the roll. The preformed sleeve 100 comprises the surface layer 101, consisting of the material A, and a transition zone 102 where the content of the material B is gradually increased to achieve a diffuse transition between the materials. According to the invention, the sleeve is preformed according to the steps shown in Figure 1, that is, preforming 1 and loose sintering 2 whereupon the sleeve 100 consisting of the surface layer 101 and the transition zone 102 is placed in a capsule. Thereafter, at 4, the capsule is filled with material B in . the form of a powder material. The capsule is evacuated, at 5, and gas-tightly sealed, at 6, before it is isostatically compacted, at 7, with preformed sleeve " and powder material, into an essentially dense compound roll 120. According to the invention, the compaction 7 is preferably performed in a hot-isostatic press. The manufacture of a compound roll, according to the invention, of course comprises conventional operations for powdermetallurgical manufacture, carried out on the loose-sintered sleeve 100 as well as on the finished compound roll 120, to assure quality, dimensions and material structure. EXAMPLES OF APPLICATIONS The invention is applicable, for example, to the manufacture of valve seats and valve cones in which parts exposed to wear are formed as loose-sintered bodies in hard wear- resistant materials, for example Co-based alloys, loose- sintered at 500-700°C in a vacuum furnace. The loose- sintered body is then placed in a capsule made of steel plate, whereupon a powder of a high temperature material, such as a 12% Cr steel, is added. Thereafter, the capsule is evacuated and sealed before being compacted, with its contents of porous loose-sintered bodies of a Co base alloy and a steel powder, by means of hot-isostatic pressing at a temperature of 1000-1250°C and a pressure of 50-300 MPa, into a valve cone or a valve seat. As described above, the invention can be applied to the manufacture of compound rolls, in which case a relatively thick surface layer of a hard material is united with a tough core. According to the invention, a porous loose- sintered sleeve is manufactured of the hard material. Possibly, the hard material constitutes only an outer shell, which is provided with an inner zone where the content of the tough material, which is to constitute the core mateial in the frame of the roll, is gradually increased to create a diffuse transition zone between the hard outer layer and the tough frame and hence considerably reduce the tendency to crack. The porous sleeve is loose-sintered under the same conditions as the bodies in the manufacture of valve parts and, like these, is compacted with a steel powder in a steel plate capsule under equivalent conditions. Alternatively, the loose sintering may take place in a protective gas furnace with Ar atmosphere at the same temperatures as above and the compaction take place using cold-isostatic pressing followed by sintering. In the manufacture of valve housings and other armature parts, which are exposed to corrosion, for the offshore, chemical or power industry, the corrosion resistance can be considerably increased if the armature part can be made with a relatively thick surface layer of a material with a high corrosion resistance in those surfaces which are in contact with the corroding medium. In accordance with the invention, porous bodies are preformed and loose-sintered in a corrosion-resistant material, for example an Ni base alloy. The porous bodies are placed in a capsule whereupon the capsule is filled with a powder, for example in the form of a tempering steel, evacuated, and gas-tightly sealed. The capsule with its contents of at least one loose- sintered, porous body as well as steel powder is compacted into an essentially tight armature part by hot-isostatic pressing. Loose sintering and hot-isostatic pressing are carried out under conditions essentially equivalent to those described in previous examples.

Claims: CLAIMS 1. A method of manufacturing a compound product from powder material wherein at least one body (10) is preformed (1), sintered (2) and placed (3) in a capsule (11) , and wherein at least one additional powder material is filled (4) into the capsule, the capsule, containing at least one preformed body and at least one powder material, being evacuated (5) , sealed (6), and compacted (7), characterized in that at least one body (10) is preformed (1) and loose-sintered (2) into a porous, manageable body with a density of 75-85% of T.D., starting from a powder material with a filling density of 65-75% of T.D., that the preformed loose-sintered body is placed (3) in a capsule (11) , that at least one additional material is filled (4) into the capsule, and that the capsule is evacuated (5), sealed (6), essentially gas-tightly, and compacted (7) , by means of isostatic pressing, into an essentially fully dense body (10) with preferably a metallic bond, atomic bond, between the materials included. 2. A method according to claim 1, characterized in that the body (10) is preformed and loose-sintered (2) into a porous body with a density after loose-sintering of 75-80% starting from a spherical gas-atomized metal powder with a filling density of about 70%. 3. A method according to claim 1 or 2, characterized in that the body (10) is loose-sintered (2) at a temperature corresponding to 40-60% of the melting temperature of the powder material in question, preferably in a vacuum or protective gas furnace. 4. A method according to any of the preceding claims, characterized in that the preformed sintered body (10) is formed (1) from at least two powder materials. 5. A method according to claim 4, characterized in that the powder materials in the preformed sintered body (10) are mixed so as to achieve a boundless, continuous transition between the powder materials. 6. A method according to any of the preceding claims, characterized in that the capsule containing the preformed sintered body (10) and a powder material is compacted (7) by means of hot-isostatic pressing at a temperature of between 1000 nd 1250°C and a pressure of between 50 and 300 MPa.

Inventors: FRIBORG SIGURD, LILL KURT, TORSSELL KRISTER, FRIBORG, SIGURD, LILL, KURT, TORSSELL, KRISTER
Applicants: ASEA BROWN BOVERI, ASEA BROWN BOVERI AB
Application year: 1991
Main classification: B22F7
IPC codes: B22F7