RU2264886C1 - Method for making parts of powder materials and apparatus for performing the same - Google Patents

Abstract

FIELD: powder metallurgy, namely processes for hot pressing of parts with inner cavities, mainly of powders of refractory ceramic materials.

SUBSTANCE: method comprises steps of shaping blank; placing it in die of graphite press-mold; forming layer of separation material between blank and shaping members of press-mold; hot pressing of blank; after termination of hot pressing fixing part in cavity of die and releasing punches at temperature of hot pressing. Graphite press-mold includes heater in the form of yoke having bandage ring of high-strength carbon composition material. Upper punch is mechanically joined with holder and it is mounted with possibility of fixation in limit upper position. Lower punch and die are mounted with possibility of mutual motion.

EFFECT: enhanced quality of parts.

4 cl, 1 dwg, 1 ex

Description

The invention relates to powder metallurgy, in particular to a method for hot pressing parts with internal cavities, mainly from powders of refractory ceramic materials.

A known method of manufacturing products from refractory compounds, including the preliminary shaping of the workpiece of complex shape in the mold, and before hot pressing the workpiece of complex shape is supplemented to a simple form with auxiliary material, which is inert to the main one (A.S. No. 1676755, IPC B 22 F 3 / 14, "A method of manufacturing products from refractory compounds", publ. 09/15/91. BI No. 34).

The closest in technical essence to the claimed method is a method of manufacturing compacted sintered parts with uniform density, which consists in the fact that powders of refractory materials are pre-compacted into a workpiece, which is placed in the cavity of the press matrix. The residual free space of the matrix cavity is filled with another solid material and then pressed by directed loading at the sintering temperature. A solid release agent is applied to the contact surface of the preforms (Germany, application No. 21585876, IPC B 22 F 3/14, "Method for the manufacture of compacted sintered parts with uniform density", published 02.06.1977).

The disadvantages of these methods include the lack of the possibility of mutual movement of the forming elements of the mold, since in the manufacture of parts with holes or in the form of shells, due to the difference in the coefficients of thermal expansion of the material of the mold and the material of the part, the latter will be subject to significant stresses, which may cause cracks or destruction of the part in the mold during co-cooling.

A device for hot pressing products containing a matrix, upper and lower punches and an additional punch made in the form of a sleeve and placed between the matrix and the upper punch. It also contains a replaceable element located between the die and the lower punch, and is made of a chemically inert material that is compacted under load (A.S. No. 1731439, IPC B 22 F 3/14, "Device for hot pressing of products, publ. 07.05.92 . BI No. 17).

A device of molds for hot pressing from graphite or other heat-resistant materials is known, which ensures the formation of holes or cavities in a part due to rods or other mold elements corresponding in shape and size to the cavity formed (S.S. Kiparisov, G.A. Libenson, Powder Metallurgy, M., Metallurgy, 1991, p.302).

The disadvantage of such devices is that the forming element remains inside the cavity until the part cools down. As a result, when the product is cooled inside the mold due to the difference in the thermal expansion coefficients of the material of the part and the material of the mold, internal stresses (mainly tensile) accumulate in the part, which can reach significant values. This problem is especially relevant in the hot pressing of ceramic parts that do not have significant tensile strength. As a result, parts are often destroyed, it is necessary to increase the thickness of parts or to completely abandon the formation of cavities or holes and produce a workpiece with large allowances, which can then be processed mechanically, and, as you know, machining solid ceramics is very laborious, requires special expensive tools and highly skilled workers, employed during processing.

The problem to which the invention is directed is to reduce rejects, save material by reducing allowances for machining and reduce the complexity of manufacturing parts of complex shapes from ceramic materials.

The technical result achieved using the proposed method is as follows:

- rejects during hot pressing are excluded;

- the utilization of the material increases to 90%;

- reduces the complexity of manufacturing parts due to a significant reduction in the volume of machining.

To solve this problem, it is proposed to pre-form the workpiece from a mixture of powders. If necessary, cavities and / or holes are formed in the blank. On the entire surface of the workpiece, including the inner surface of the cavities or holes, a layer of separation material is formed, providing a good transfer of load to the part. This material should be inert with respect to the material of the part and have a melting point (decomposition) significantly higher than the sintering temperature of the material of the manufactured part. The workpiece is placed in the cavity of the matrix developed mold between the upper and lower punches. In this case, the elements forming the cavity and / or hole are placed in the respective cavities and / or holes of the workpiece. Hot pressing of the part is carried out in a known manner. After the hot pressing process is completed, it is proposed to release the pressed part from the forming elements of the mold and thereby prevent significant stresses from appearing in the part during cooling due to the difference in the thermophysical characteristics of the material of the part and the mold.

The proposed method for the manufacture of parts with cavities and / or holes is that the parts are obtained by sintering from powder, placing a workpiece with a release material in the matrix between the lower and upper punches, and is subjected to hot pressing in the mold of the developed design. The formation of cavities and / or holes is carried out by one of the punches or rods associated with the lower puncheon. After the hot pressing is completed, the mold lowers down until its upper element (clamp) connected with the upper punch comes into contact with a fixed element of the plant structure (for example, with thermal protection of the inductor). The die with the pressed part, the cage and the lower punch moves further until the upper punch is fixed in the extreme upper position by the stop element associated with the matrix. The pressed part is fixed inside the matrix, resting on the intended stop, and the lower punch falls further, separating from the part and removing the forming elements (rods) from the cavities and / or holes of the part. In the future, the part may remain on the stop inside the matrix until it is completely cooled or withdrawn from the matrix at elevated temperature.

The drawing shows a mold for hot pressing.

The graphite mold for hot pressing includes a matrix (5), two punches (1, 4) coaxially mounted and a heater (holder) (6). In this case, one of the punches is mechanically connected (connected) to the clamp (8) and installed with the possibility of fixing in the highest position, and the second punch (pallet) (1) is interconnected with the base (9) and the holder (6). Mold parts are installed with the possibility of joint movement. At the same time, at the moment of fixing the upper punch, the part is separated from one or both punches and the forming cavities and / or holes of the elements (rods). The pressed part (3) remains in the matrix (5) and no longer experiences the influence of the forming elements of the mold.

The mold can be made of heat-resistant material that can withstand the pressure that is necessary for the manufacture of a part, for example, graphite, at a hot pressing temperature.

To prevent the interaction between the material of the part and the material of the punches and the forming elements of the mold, before pressing begins, a layer of release material (2) is formed between their surfaces, which transfers the load well and does not substantially react with the mold material and material of the part.

The holder (6) into which the matrix (5) is mounted is a heating element and may contain a retaining ring (7) made of high-strength carbon composite material, which serves to prevent the destruction of the matrix during hot pressing.

EXAMPLE.

An experimental batch of shells was made from the composite material "boron carbide - boron nitride" by hot quasi-isostatic pressing in a graphite mold.

Initially, molding was made from a mixture of powders of the starting components of the composite material. The molding was placed in a graphite mold of the described construction. A graphite tray (1) was used to form the cavity. As the separation material (2) at the border "pallet-molding" used powders of boron nitride or casting graphite.

The resulting molding was subjected to hot pressing at a temperature exceeding 2000 ° C.

After completion of the hot pressing process at a temperature of 1980-2000 ° C, the shell (3) was released by removing the tray and moving the upper punch (4) to the extreme position. In this case, the shell (3) remained in the matrix (5), resting its conical part on the annular protrusion, and in this form cooled to room temperature.

The shells made were a hollow cone passing at the top into a sphere. The outer diameter of the cone is 190 mm, the wall thickness is 18 mm. Cracks in the finished shell were absent. The yield of the shells is 100%.

Claims (4)

1. A method of manufacturing parts from ceramic powder materials, including the formation of a workpiece, placing the workpiece in the cavity of the matrix of a graphite mold between the upper and lower punches, forming a layer of separation material between the workpiece and the molding elements of the mold and hot pressing, characterized in that after After hot pressing, the part is fixed in the cavity of the matrix and freed from punches at the temperature of hot pressing. 2. The method according to claim 1, characterized in that the manufacture of a part with cavities and / or holes. 3. The method according to claim 1, characterized in that the part is made of refractory ceramic material, mainly boron carbide - boron nitride. 4. A graphite mold for hot pressing of parts containing a matrix, coaxially mounted upper and lower punches and a heater, characterized in that the heater is made in the form of a holder into which the matrix is installed and contains a retaining ring of a high-strength carbon composite material, the upper punch is mechanically connected to the clamp and installed with the possibility of fixing in the highest position, and the lower punch and the matrix are installed with the possibility of mutual movement.