Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Pretreatment of consolidated powders
Patent
1992-02-10
1994-02-08
Walsh, Donald P.
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Pretreatment of consolidated powders
419 49, 419 38, 427294, 264 62, 264313, 264317, B22F 314, C04B 3500, C04B 3564
Patent
active
052846164
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method for the manufacture of objects from a powdered material by isostatic or pseudo-isostatic pressing of a porous body preformed in the powdered material.
When a preformed porous body is compacted and consolidated by isostatic pressing and sintering, the desired density is obtained by surrounding the powder body by a dense deformable casing. The casing prevents the pressure medium used, normally a gas, from penetrating into the powder body. As casing there is often used a glass which is applied in the form of particles. Similarly, in connection with certain pseudo-isostatic compaction methods, a preformed body is surrounded by a dense, deformable casing, which is also able to function as a pressure-transferring medium.
After the glass has been applied around the body, the body and the glass powder are heated, usually under vacuum, to a temperature such that the glass particles form a dense, coherent casing around the preformed porous body. Problems will arise if glass penetrates into the pores of the porous body and/or reacts with superficially located portions of the body during the subsequent isostatic or pseudo-isostatic pressing.
More particularly, the invention relates to a method in the application of layers, by means of dipping. The layers are primarily intended to function as barrier and/or protective layers and prevent glass, from the casing, from penetrating into the pores of the preformed porous body and/or reacting with superficially located portions of the body. In the following these layers are referred to as intermediate layers, but the invention also relates to the application of other types of layers that supplement the intermediate layers, e.g. layers comprising materials intended to improve the release of the intermediate layer(s) and glass casing from the compacted and sintered body, in the following referred to as supplementary layer(s).
BACKGROUND ART
In connection with isostatic pressing into a fully dense body starting from a powdered material, a porous body is preformed by injection moulding, die pressing, slip casting or some other suitable forming method. Alternatively, the powder is filled into a container which, after the isostatic pressing, will give a body of the desired shape and size. However, the present invention relates to a method in the isostatic pressing of a preformed porous body. The above-mentioned preforming methods may be used in connection with pseudo-isostatic pressing.
To obtain an essentially dense body during isostatic pressing of a porous body, the body is enclosed in a coherent dense casing or enclosure to prevent the pressure medium used from penetrating into the pore system of the body. Usually, a glass or a material forming glass is used, which is applied in powdered state. Prior to compaction by isostatic pressing, the porous body and the glass powder are heated, the powdered glass or glass-forming material thus forming a dense coherent casing around the porous body. Problems will arise if glass from the casing penetrates, to varying extent, into the porous body and/or reacts with superficially located portions of the porous body in connection with the porous body being compacted or while the porous body is compacted.
During pseudo-isostatic pressing, a mechanical pressure is applied by means of one or more punches onto a powdered or liquid medium which transfers the press force from the punches into an essentially isostatic pressure acting on the preformed body.
From Swedish patent specification SE-B-456 651, it is known to apply one or more layers comprising powdered material onto the porous body. The layers are applied to the external surfaces of the porous body that are to be exposed to glass. In the following these surfaces are referred to as external surfaces. External surfaces in this context are all surfaces that define the shape of the body, i.e. also surfaces within open cavities, depressions and larger holes but not internal surfaces such as surfaces within the pore system. The layers
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Collin Marianne
Mattsson Bertil
Nilsson Jan
ABB Cerama AB
Chi Anthony R.
Sandvik AB
Walsh Donald P.
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