Method for Manufacturing Billets of Complicated Shape

A method for manufacturing billets of metal with a metal powder as the starting material is disclosed in U.S. patent application Ser. No. 123,731, filed Feb. 22, 1980. According to this method the metal powder is enclosed as a charge in a capsule, the capsule with the powder charge therein is heated, the heated capsule is inserted into a press chamber and surrounded by a readily deformable layer of a thermally stable powder whose powder grains have a layer structure, and thus slide easily against one another, and which layer has good heat-insulating properties. A piston is inserted into the press chamber and brings about compression of the powder charge and bonding between the powder grains of the charge so that a homogeneous body with the full (or substantially full) theoretical density is obtained. From considerations of price and availability, the readily deformable powder is suitably talcum powder, but other substances having similar properties, such as pyrophyllite, may also be used. The billets manufactured by this method have so far been intended for further machining into end products having a shape and size which are different from those of the billet. The method has not been suitable for the manufacture of billets of a shape and size which are virtually the same as those of the desired end product.

The present invention relates to a method which makes possible the manufacture of a body of a complicated shape and of such a size that only slight machining is required for the final shape and dimensions to be obtained. The material is a powder which is enclosed in a sealed metal capsule and which is subjected to a high, substantially all-sided pressure at the bonding temperature of the powder to be compacted. According to the invention, the powder to be compacted is enclosed in a first capsule having the same shape as that of the end product, but being somewhat larger. The powder is degassed and the capsule is then sealed. This first capsule is then placed in the center of a second larger capsule. This second capsule may have any convenient shape, e.g., a simple shape. The space between the first and second capsules is filled with a pressure-transmitting medium which is viscous at the chosen compacting temperature. The second capsule with its contents is heated to a temperature at which the powder grains within the first capsule may be bonded under pressure, is placed in a press chamber and is surrounded by an easily deformable substance such as talcum powder or pyrophyllite, whereafter a piston is inserted into the press chamber to exert a pressure on the contents. The pressure-transmitting medium surrounding the first capsule is so viscous that it will exert an all-sided isostatic pressure on the first capsule and compress the first capsule, without changing the proportions of the first capsule to any noticable degree. In connection with this compression, at a high temperature, the powder grains within the first capsule are bonded together into a solid body.

The viscous material between the first and second capsules may consist of a salt, a metal or a species of glass, having a melting temperature or a softening temperature at or below the compacting temperature chosen. The compacting pressure should normally be above 1 kilobar, suitably between 3 and 10 kilobar. The compacting temperature is dependent on the material being fabricated. Suitable compacting temperatures are: for steel, high-speed tool steel, 1050.degree.-1100.degree. C., for superalloys 1100.degree.-1250.degree., for ceramics 1000.degree.-1700.degree. C. and for hard metals 1400.degree.-1500.degree. C. A high temperature results in a high compression even at a relatively low pressure and a short compression time. If the powder temperature is lowered, the same high compression may be obtained by increasing the pressure and/or the compression time. Below a certain temperature, no bonding and compression at all may occur.

The viscous material between the first and the second capsules also constitutes a heat store which surrounds the first capsule and delays the cooling down thereof. This prevents small, projecting portions having a large surface in relation to the enclosed powder volume from being cooled down preferentially. Therefore, all portions of the powder within the capsule will be held at their bonding temperature for a long time, so that a pressure may be applied while bonding conditions still exist. It is therefore possible to press articles having very thin, protruding portions .

The invention will now be described, by way of example, in greater detail with reference to the accompanying drawing, the sole FIGURE of which is a schematic sectional view through a pressure chamber carrying out the method of the invention.

In the drawing 1 designates a press table and 2 a movable piston in a press stand, the rest of which is not shown. On the press table 1 there is placed a press cylinder 3 with a loose inner bottom 4. An inner or first capsule 5 having a shape corresponding to the shape of a finished product is filled with charge 6 of powder. The inner capsule 5 is placed in the center of an outer or second capsule 7 so that it is surrounded on all sides by a pressure-transmitting medium 8 which is so viscous at the compression temperature that it behaves in all essentials as a fluid, thus exerting an all-sided pressure on the inner capsule 5 and compressing the powder charge 6 without changing the shape of the capsule 5 to any mentionable degree. The outer capsule 7 is placed in a press chamber 9 which is formed by the cylinder 3, the bottom 4 and the piston 2. The outer capsule 7 is surrounded on all sides by a layer of talcum powder 10. When the piston 2 is inserted into the cylinder 3, a pressure is exerted on the talcum powder 10. This is propagated to the outer capsule 7. Talcum powder is not an ideal pressure-transmitting medium since it changes the shape of the outer capsule 7 to a certain extent. This imperfection is a considerable disadvantage when it is desired to press an article having a complicated shape in a single operation into near-final shape and dimensions. Because of the pressure-transmitting viscous medium 8, the inner capsule 5 will be subjected to an all-sided pressure so that no deformation, or only an insignificant deformation, of the shape of the capsule 5 occurs during the compression. The medium 8 may be a powder at room temperature or blocks pressed or cast from a powder which together define a cavity adapted to receive the capsule 5. When the medium 8 acquires its viscous properties during the heating, there is a risk that the capsule 5 may sink down or float up. To prevent this, supports 11 may be placed between the outer capsule 7 and the inner capsule 5.

Various modifications may be made to the method as detailed above and all such modifications, within the scope of the following claims, are intended to be included in the invention.