CERAMIC MOULD AND METHOD OF PRODUCING SAME

Abstract: A ceramic mould (10) for use in the production of pow­ der metallurgy articles (22); the mould (10) has a ceramic body (12) with a mould cavity (14) therein conforming sub­ stantially to the configuration of the article (22) to be produced. The mould cavity (14) is adapted to be filled with powdered metal (20) which is then compacted to form the article (22). The improvement of the invention is the mould cavity (14) having a continuous metal coating (18) covering the entire surface thereof so that the powdered metal (20) prior to and during compaction does not come into contact with and be contaminated by the ceramic of the mould body (12).

Description: CERAMIC MOULD AND METHOD OF PRODUCING SASIE This invention relates to ceramic moulds and to a method of producing same. It is known to produce powder metallurgy shapes of various configurations by the use of a ceramic shell type mould. The ceramic mould has an interior which conforms to the desired configuration of the article to be produced. The cavity is filled with powdered metal, the mould is sealed, heated and then subjected to force sufficient to compact the powdered metal therein to form a dense article. The preferred compacting method is to place the powder filled mould at elevated temperature in an autoclave for hot isostatic pressing by the use of a gas, such as helium. One practice for producing moulds of this type involves mating a wax pattern of the configuration of the article to be produced and then casting the ceramic around the pattern so that the interior of the ceramic conforms to the surface configuration of the pattern and thus the article to b produced. An opening is provided ih the ceramic through which upon heating the wax may be melted to Pxpose the cavity. Thereafter the cavity is filled with powdered metal and the powder is compacted in the conventional manner as by hot isostatic compacting. In operations of this type, during heating of the powder within the ceramic mould the powder tends to become contaminated by the ceramic with which it is in contact within the mould cavity. Particles of the ceramic, including oxides, enter the powder and during compacting are confined within the densified article. These nonmetallic substances weaken the article such as by providing sites for crack propogation. In addition, reaction products, which may be from the ceramic, chemically react with powder metal to form nonmetallic compounds at prior particle boundaries after compacting. It is accordingly a primary object of the present invention to provide a ceramic mould adapted for use in producing powder metallurgy articles of intricate configuration that avoids contamination of the metal article by contact with the ceramic material. Broadly, according to the present invention there is provided a ceramic mould body having a mould cavity therein with the surface of the mould cavity conforming substantially to the configuration of the article to be produced. The mould cavity is generally somewhat "oversize" with respect to the final article to allow for shrinkage during compacting. An opening is provided for filling the cavity with powdered metal and thereafter sealing it prior to compacting In accordance with the invention the mould cavity has a continuous metal coating covering its surface. This prevents contact between the metal powder and the ceramic material of the mould and thus acts as a barrier to prevent contamination by impurities such as oxides from entering the powder during the heating and compacting cycle. A preferred material for this coating is nickel or nickel-base alloys as they remain continuous during the high temperatures incident to hot compacting operations such as hot isostatic pressing. In accordance with the invention, the alloy powders are generally superalloys, such as nickel, iron or cobalt-base alloys1 and titanium-base alloys for use in high temperature applications, such as jet engine components and the like. However, the mould of the invention is also suitable for use with high speed steels and tool steels. Preferably, to facilitate coating of the mould cavity surface the mould is segmented into at least two selectively separated parts. When separated the interior can be coated with the metal and then the parts can be joined to form the integral mould. A preferred method for applying the coating so that it is thin and continuous involves vapour deposition. It is known to produce powder metallurgy articles that have, after compacting, internal passages. This is conventionally achieved by providing within the mould a ceramic core or mandrel having an exterior configuration conforming to that of the internal passage desired to be produced in the final compacted article. The powder fills the mould and surrounds the mandrel. During compacting, the powder is compacted against the mandrel, and upon removal of the mandrel after compacting an internal passage is produced within the compacted article. To produce an article with an internal passage, in accordance with the present invention, the exterior of the core or mandrel is coated with metal to prevent contamination of the powder. The invention will be more particularly described with reference to the accompanying drawings, in which: Figure 1 is a cross-section of one embodiment of a mould according to the present invention, Figure 2 is a cross-section of the mould of Figure 1 having the mould cavity filled with metal powder and being ready for compacting; Figure 3 is an elevation of a powder metallurgy product resulting from compacting using the mould of Figures 1 and 2; and Figures 4, 5 and 6 are photomicrographs at a magnification of 150x showing compacted alloys produced by the use of conventional ceramic moulds and moulds according to the present invention. With respect to the drawings, Figure 1 shows two identical mould segments in cross-section identified as 10 having a ceramic body portion 12 with a mould cavity 14 conforming to the configuration of the article to be produced, which in this case is a gear wheel. The gear teeth 16 are defined in the interior surface of the body 12 This may be achieved by dipping a wax pattern conforming substantially to the shape of the gear wheel in a refractory slurry and then melting away the wax after drying the refractory material. Although this method is preferred, any other method may be used for producing the mould. The interior surfaces of the mould cavity are coated with a metal 18, as shown in Figure 2, and the segments are joined and filled with powdered metal 20. The mould is then sealed and compacted such as by hot isostatic compacting. Upon removal of the compacted article, the gear, designated as 22, is produced as shown in Figure 3. Although the terms "metal", "nickel" and "titanium" are used herein it is to be understood that these include alloys as well as metal in elemental form. The following specific examples are illustrative of the present invention: Example I A ceramic shell was internally coated with nickel using the chemical vapour deposition (CVD) process. This involved injecting nickel carbonyl gas into the heated shell, whereupon the nickel metal was deposited by thermal decomposition of the gas onto the surface of the shell cavity. A coating was produced with certain areas left bare. The coating thickness was of the order of 0.0254 to 0.254mm (1 to 10 milts). The shell was then processed in a normal manner to produce a powder metallurgy part, using a conventional nickel-base superalloy powder (Rene 95). The resulting compact was normal with an unusually bright surface appearance. Metallographic examination revealed a nickel surface coating, a boundary layer of interdiffusion products, and the Rene 95 base metal (see Figure 4). In an area unprotected by the nickel coating, the Rene 95 powder particles are outlined by a contamination layer, a normal phenomenon with the conventional ceramic mould process (see Figure 5)t. The nickel coating is thus seen to provide protection from contamination for the Rene 95 base metal from which the final compacted superalloy is to be made. Example II: A ceramic shell was split and the interior of the shell conforming to the powdermetallurgy article to be produced was coated with nickel by the ion deposition process wherein metal atoms are directly deposited on the target surface in a vacuum chamber. The coating thickness was of the order of 0.00254mm (.1 mils). The shell was then processed in a normal manner to produce a powder metallurgy part, using titanium alloy powder. This included filling the shell with titanium alloy powder, evacuating, heating and hot-isostatic pressing to compact the powder to full density. The resulting compact had an unusually smooth surface appearance. Metallographic examination revealed a surface diffusionproducts layer of the order of 0.00254mm (.1 mil) thick (see Figure 6). In addition to the metal coating techniques demonstrated in the specific examples, a metal coating may be produced on the interior surfaces of a ceramic shell such as by electroplating the metal coating, which may be nickel, on a wax pattern shape and then providing a ceramic shell around this metal-coated wax pattern, as by dipping the assembly in a refractory slurry. When the wax is removed by melting followed by chemical cleaning the metal layer remains bonded to the ceramic shell to produce the desired shell mould with a metal coated interior. A wax pattern may be coated with nickel by first spraying the surface of the pattern with graphite to provide an electrically conductive base and then electroplating onto this conductive base a metal such as nickel.

Claims: Claims: 1. A mould (10) for use in the production of a powder metallurgy article (22), comprising a ceramic mould body (12) having a mould cavity (14) therein with a surface conforming substantially to the configuration of a said article to be produced and adapted for filling with a powdered metal (20) for compaction to form said article, characterised in that said mould cavity (14) has a continuous metal coating (18) covering said surface. 2. A mould according to claim 1, characterised in that said metal coating (18) is nickel. 3. A mould according to claim 1 or 2, characterised in that said ceramic mould body (12) is segmented into at least two selectively separable parts. 4. A mould according to claim 1, 2 or 3, characterised in that said mould (10) has a ceramic core within said mould cavity (14) and said ceramic core has a continuous metal coating thereon. 5. A method of producing a mould (10) for use in the production of a powder metallurgy article (22), said method comprising producing a ceramic mould body (12) having a mould cavity (14) therein with a surface conforming substantially to the configuration of said article (22) and adapted for filling with powdered metal (20) for compaction to form said article (22), charactersied in that the method further comprises applying a continuous metal coating (18) onto said surface. 6. A method according to claim 5, characterised in that said ceramic mould body (12) is produced by forming at least two separable segments (10) and uniting said segments to result in said mould cavity (14). 7. A method according to claim 6, characterised in that said segments (10) each have a surface portion adapted to form said mould cavity (14) when said segments (10) are united and a continuous metal coating (18) is applied to said surface portion prior to uniting said segments (10) to result in said mould cavity (14). 8. A method according to claim 5, 6 or 7, characterised in that said metal coating (18) comprises nickel. 9. A method according to any one of the preceding claims 5 to 8, characterised in that said coating (18) is applied by chemical vapour deposition. 10. A method according to any one of the preceding claims 5 to 9, characterised in that a ceramic core having a continuous metal coating thereon is provided within said mould cavity.

Inventors: BROSIUS, KARL SIMMS, CHANDHOK, VIJAY KUMAR, MCCOLLOUGH, WILLIAM LEE
Applicants: CRUCIBLE MATERIALS CORPORATION
Application year: 1982
Main classification: B22C3
IPC codes: B22F3