WO1988005701A1

WO1988005701A1 - Method relating to powder metallurgical manufacturing of articles and apparatus for carrying out the method

Info

Publication number: WO1988005701A1
Application number: PCT/SE1988/000025
Authority: WO
Inventors: Per Hasselström
Original assignee: Uddeholm Tooling Aktiebolag
Priority date: 1987-02-03
Filing date: 1988-01-25
Publication date: 1988-08-11
Also published as: SE8700394D0; SE8700394L; AU1296488A

Abstract

A method and a device for powder metallurgical manufacturing of an object, at which one or more thermoplastic films (3) are shaped against a die to a shaping barrier surface between a first fine particulate metallic or metallic and ceramic material, which after pressing and/or sintering is to constitute the shaped object, and a powder filling (12), consisting of a second fine particulate ceramic material or a mixture of a ceramic material and other materials, at which method the first fine particulate material (13) as well as the second fine particulate material (12) are placed in a tight container (8, 14, 16), said tight container (8, 14, 16) being heated to a temperature, at which the film or films (3, 11) are gasified, and the container (8, 14, 16) being evacuated so that the gasified film (3) is sucked out of the container. Preferably the first fine particulate material consists of substantially spherical and/or irregularly shaped particles.

Description

METHOD RELATING TO POWDER METALLURGICAL MANUFACTURING OF ARTICLES AND APPARATUS FOR CARRYING OUT THE METHOD

TECHNICAL FIELD

The invention relates to a method of powder metallurgically manu¬ facturing an object. The invention also relates to a device for carrying out the metod.

BACKGROUND OF THE INVENTION

Powder metallurgical manufacturing of objects has many advantages: homogenous micro-structure, small losses of material, close gauge limits, reduced machining and possibilities e.g. to impregnate. The cost for the manufacturing of conventional press form tools, however, is considerable and consequently powder metallurgical manufacturing according to known technology can be profitable only when small parts and large series are concerned.

When practising hot isostatic pressing, HIP-technique, in connection with powder metallurgical manufacturing of objects a metal container is filled with a suitable mixture of powders. The container consists of a metal casing, usually a low alloyed, relatively inexpensive steel. The container is welded tight and provided with a device for the evacuation of gases. The pressing and shaping is carried out at an increased temperature and at a raised isostatic pressure, usually a gas pressure. The method is, therefore, only suited for simple shaping where the objects also must be of relatively large dimensions. Thus the utility of the method is limited.

As the rule the method is not used for the manufacturing of objects to a completely finished shape but it has been used for the manufacturing of objects of near net shape. Examples of this technique and variations of the technique are described in SE-B-382 929, SE-B-435 026 and SE-B-442 486. DESCRIPTION OF THE INVENTION

The purpose of the invention is to follow a new trend in powder metallurgical manufacturing of objects. One purpose in particular is to offer a comparatively inexpensive method of shaping, applicable at sintering with or without pressing, which method preferably can be used in connection with hot isostatic pressing. These and other purposes can be achieved by pressing one or more thermoplastic films against a die to form a shaping barrier surface between a first fine particulate, metallic or metallic and ceramic material, which after pressing and/or sintering is to constitute the shaped article, and a powder filling consisting of a second fine particulate, ceramic material or a mixture of ceramic material and other materials, the first fine particulate material as well as the second fine particulate material being placed in a tight container, which tight container is heated to a temperature at which the film or films are gasified, the container is evacuated so that the gasified film is exhausted out of the container. The first fine particulate material preferably consists of substantially spherical and/or irregluarly shaped particles.

The powder filling serves as a secondary pressure medium when using the HIP-technique. The shaping film can be prepared by using vacuum forming in a way known per se. In connection with the vacuum forming of the film a frame can be placed over the die.

The film having been made into a shaping barrier surface by vacuum forming the frame can be filled either with said first fine particulate material or with said powder filling. The frame is then covered with a thermoplastic film which is heated and at the same time the frame is connected to an evacuating pump. When gas has thus been evacuated from the powder filling, or from the first fine particulate material in the frame, the contents of the frame solidifies to a solid managable form, owing to inner friction within the system consisting of thermoplastic films, frame and either powder filling of said second fine particulate material or said first fine particulate material. Through turning the frame and filling the space in the frame limited by the vacuum-formed thermoplastic film with the first fine particulate material or with said filling the vacuum formed film can be made into a barrier layer between the first fine particulate material and the filling. The first material as well as the filling can now be enclosed in a tight container through the welding of a flat plate into the upper side as well as into the bottom side of the frame, sealing against the frame. The part of the frame enclosing the powder filling can then be connected to an evacuating pump. Gas is then evacuated from the tight container formed by the frame and the welded plates. This evacuation takes place at a raised temperature whereby the plastic film is gasified and evacuated together with other contents of gas in the container formed by the frame and the covering plates.

If the frame is then subjected to a further raised temperature the first fine particulate material can be sintered and if the frame is simultaneously subjected to pressure and heating it can be consolidated to complete density through pressing. The filling then acts as a secondary pressure medium.

BRIEF DESCRIPTION OF DRAWINGS

In the following descriptions of some preferred embodiments reference will be made to the accompanying drawings, of which

Fig. 1 schematically shows a vertical section through an equipment for vacuum forming, which is used to shape a thermoplastic film to be used as a barrier wall between a first fine particulate material and a powder filling of a second fine particulate material,

Fig. 2 shows the vacuum forming device on which a frame-like structure has been placed and which, after being filled with powder filling, is covered by a thermoplastic film, Fig. 3 shows the frame-like structure which after evacuating contains powder filling between gas-tight films, which owing to vacuum have solidified to be managable,

Fig. 4 shows the frame-like structure turned upside-down and in its concave part filled with the first fine particulate material and on its upper and bottom sides provided with welded plates,

Fig. 5 shows how two frame-like structures with solidified filling material can be placed together to form a hollow space to be filled with powder material, and

Fig. 6 shows an embodiment where it is desired that the first fine particulate material is placed on the concave side of the barring film between said fine particulate material and the filling material.

In the drawings only such details have been shown which are essential for the understanding of the idea of the invention whereas other parts have been left out. Thus, it has not been shown, for instance, how evacuating pumps are arranged, nor details of connecting tubes to taps and valves, as these parts belong to known technology and do not form part of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

With reference first to Fig. 1 a heating device with adherent vacuum • forming equipment for thermoplastic films or plates is designated numeral 1, a perforated plate belonging to the same vacuum forming equipment numeral 4, and a gas permeable die numeral 2. The gas permeability of the die 2 can be achieved either by using a suitable gas permeable material when making the die or by supplying the die with a great number of essentially vertical through holes 5. Connected to and below the perforated plate 4 there is a vacuum chamber 28 from which gas, usually air, can be evacuated by an evacuating device 7. This may consist of a vacuum pump 31 and a valve 30. At the initial stage a film of thermoplastic material, is placed over the die 2, which film well covers the die 2 and the perforated plate 4, and which from practical reasons can be so arranged that it is unrolled from a reeling device 10. At heating by the heating device 1, emitting radiation heat, and simultaneous evacuation of air from the vacuum chamber 28, the film 3 is sucked downwards against the perforated plate 4 and the die 2. The heating is then arranged so that the temperature of the plastic film is within the limits between thermoelastic and thermoplastic temperatures. The film 3 will thus be brought to complete adaptness and contact towards the perforated plate 4 and the die 2. The heating device 1 and the evacuation by means of the evacuation equipment 7 being switched off the film 3 will cool, thereby keeping its shape, which represents a true copy of the shaped surface of the die 2.

In Fig. 2 is shown a frame 8 having been placed around the die 2 and on the film 3 at the parts of the film 3 which are resting on the perforated plate 4. The frame 8 is filled from the top with a power filling material 12, consisting of fine particulate, firm material, e.g. sand. This material has in the preamble been referred to as second fine particulate material. It must then be made sure that the frame is completely filled so that a plane surface of filling material 12 is achieved at the same level as the upper edge of the frame 8.

From the reeling device 10 a new piece of plastic film is unrolled so as to cover the frame 8. The frame 8 is provided with an evacuation equipment 9. This may consist of a valve 32 and a vacuum pump 33. Connected to the evacuation opening leading to the evacuation equipment 9 there is a screening device, not shown in the drawing, which prevents the filling material 12 from passing to the evacuation opening 9. The screening device concerned is made according to known technology and does not form part of the invention. Therefore, it has not been shown in the drawing. Through renewed heating by the heating device 1 the temperature of the film 11 is raised to the limit between thermoelastic and thermoplastic temperatures and simultaneously gas is evacuated from the space defined by the film 11, the film 3 and the frame 8. The film 11 is then sucked against the surface of the filling material 12, simultaneously sealing against the edge of the frame 8. Also the film 3 is sucked against the filling material and seals against the frame 8. The edges around the openings of the frame 8 are designed to promote good sealing against the films 3 and 11. The edges are further provided with adhesive to ensure the sealing. The valve 32 is closed while vacuum is maintained in the space filled with the filling material 12. The valve device concerned can be of different designs according to known technology. The vacuum in the space between the plastic films 11 and 3 subjects the particles of the powder filling 12 to a compression leading to high friction between the particles as well as^" high friction between the particles and the films 3 and 11 respectively, and the frame 8. Through this high inner friction in the system consisting of the frame 8, the films 3 and 11 and the filling material 12, the filling material solidifies in the shape it has assumed before the evacuation via the evacuation device 9. The following elimination of the heating from the heating device 1 does not affect the state of inner friction of the system consisting of the frame 8, the films 3 and 11, and the filling material 12.

The film 11 is cut off between the frame 8 and the reeling device 10. Thereafter the frame 8 can be lifted away from the perforated plate 4 and the die 2, and then the system, consisting of the frame 8, the films 3 and 11, the filling material 12 and the valve 32, constitutes a solid unit, which is managable owing to the inner friction within the system. Fig. 3 shows said system which may be lifted, turned and reversed without the powder filling altering its shape, which shape along the surface being in contact with the film 3 represents an exact copy of the outlines of the shaped part of the die 2, if, you disregard the in this context irrelevant discrepancies arising from the thickness of the film 3. The films 3 and 11 can be made of ethyl-polyvinyl acetate plastic. Other films, too, may be chosen. As the evaporation temperature of plastic films as a rule is lower than the sintering temperature of the powder material there is a vide range of materials which can be used for the plastic films.

In Fig. 4 is shown that a base plate 16 has been welded into the frame by means of a weld joint 15 on the side of the frame which presents a essentially plane surface of the filling material 12. In Fig. 4 the frame 8 is shown in an upside-down position in relation to the position shown in Figs. 1-3. The space which before was occupied by the die 2 is then filled with a powder material 13, which may consist of substantially spherical and/or irregularly shaped metallic material or metallic and ceramic material, preferably in the form of granulate. In the preamble this material has been termed first fine particulate material. It must then be made sure that the space is completely filled so that the open surface of the powder material 13 forms a plane surface level with the edge surface of the frame 8. Thereafter, a plate 14 is welded into the frame 8 by -means of a weld joint 15. The filling material 12 and the powder material 13 is then tightly enclosed in the capsule formed by the frame 8, the base plate 16 and the plate 14. At this, it is essential that the weld joints 15, by which the plates 14 and 16 respectively are fixed to the frame 8, are carefully executed in order to achieve good sealing. The we*ld joints must also be executed in such a way that cracks and brittleness in the material around the weld joints will be avoided.

The device shown in Fig. 4 is then again connected to a vacuum pump 33 which via the evacuation opening 9 evacuates gas from the frame 8. Simultaneously the device according to Fig. 4 is subjected to influence by heat through, for example, being placed in a furnace. The frame and its contents is then heated to a temperature where the plastic films 11 and 3 are gasified. The gaseous remainders of the plastic films 3 and 11 are then evacuated through the evacuation equipment 9. At the same time gas, as a rule air, is evacuated from the space filled with the powder material 13. The powder material 13 meanwhile keeps its shape which on the demarcation surface between the powder material 13 and the filling 12 represents a true copy of the shaped surface of the die 2.

Through a further raise in the temperature of the device shown in Fig. 4, after the evacuation of the gasified film 3, the powder material 13 can be sintered. The powder filling 12 should then be chosen in order that this does not also sinter at the melting temperature of the powder material. Through exposure of the device shown in Fig. 4 to a raised temperature as well as to pressure, after the evacuation of the gasified film, the powder material 13 can be compressed to complete density. In this connection it is essential to chose a filling material 12 which can stand the pressure needed for the compression of the powder material 13. The powder filling material 12 can be chosen so that also the filling material 12 is subjected to a reduction in volume in connection with the compression. The filling material 12 may consist of a mixture of different components giving the volume of filling material 12 characteristics which are suited to the change in density of the powder material 13 related to the control program in question, for time, pressure and temperature. Said components usually consist of ceramic material but may include additives of a different material. The filling material 12 may consist of a mixture of different materials or a mixture of, for example, ceramic material with particles of various shapes and/or sizes.

It is not always desirable to continue the compressing of the powder material 13 to complete density. When merely sintering whithout compressing as well as with compressing to a certain increased density it may be desirable to impregnate the sintered and/or compressed powder body obtained from the powder material 13. This is done, for instance, when sintered material for bearings is concerned. For the infiltration molten metal may also be used which is brought to solidify once the sintered powder body is completely inpregnated.

In Fig. 5 is shown that the shaped surface of the powder material 13 also can be made double-sided. Two dies are manufactured according to Fig. 3, each provided with a base plate 16. Both base plates 16 are thus welded into a frame 8a, 8b. One of the frames 8b is turned upside-down and placed on top of the other frame 8a. The doubly shaped hollow space formed by the two frames 8a, 8b is filled with powder material 13 and thereafter the frames are welded together by means of one or more weld joints 15.

The evacuation of the gasified film 3 and the subsequent compressing and/or sintering are performed as has been described regarding the device according to Fig. 4.

Through the earlier described method, when, according to Fig. 4, the powder material 13 in only one frame 8 was sintered and/or compressed, the sintered and/or compressed powder body gets an essentially convex shape.

Following the vacuum forming of the film 3, as shown in Fig. 1, it is also possible to fill the powder material 13 over the die, instead of the filling material 12, as shown in Fig. 2. After turning the frame 8 powder material 13 is filled into the space which previously was filled by the die 2, which space has the same shape as the die 2. The frame 8 is then joined to the base plate 16 and the plate 14 by means of welding. Unlike what is shown in Fig. 4, in this case the essentially rectangular-shaped volume of the frame 8 is filled with powder material 13 (first fine particulate material) and the space constituting the hollow space in said rectangular-shaped volume is filled with filling material 12 (second fine particulate material), as shown in Fig. 6. As it s most convenient to evacuate the gasified film through 'the filling material, the plate 14 in Fig. 6 has been provided with an evacuation equipment 24. Thus the frame 8, in this case does not have to be provided with an evacuation opening. When manufacturing a tool for e.g. press casting or injection moulding it may be needed to make a rectangular-shaped volume with one or more hollow spaces. The method according to Fig. 6 may then be adequate. Other modifications, too, than the ones disclosed above are possible whithout diverging from the general principles of the invention and without said modifications causing the modified device to come outside the limitations of the following claims.

Claims

1. Method of powder metallurgically manufacturing an object, c h a r a c t e r i ze d in that one or more thermoplastic films (3) are shaped against a die to a shaping barrier surface between a first fine particulate, metallic or metallic and ceramic material, which after pressing and/or sintering is to constitute the shaped object, and a powder filling (12) consisting of a second fine particulate ceramic material or a mixture of ceramic and other materials, at which method the first fine particulate material (13) as well as the second fine particulate material (12) are placed in a tight container (8, 14, 16), that said tight container (8, 14, 16) is heated to a temperature at which the film or films (3) are evaporated, and that the container (8, 14, 16) is evacuated so that the evaporated film (3) is exhausted out of the container.

2. Method according to claim 1, c h a r a c t e r i z e d in that the tight container (8, 14, 16) is formed by covering two end openings of a casing (8) or of a number of joint casing sections (8a, 8b) with covering plates (14, 16).

3. Method according to claim 2, c h a r a c t e r i z e d in that the film (3) is formed through suction of a thermoplastic film (3) against a die (5) the shaping surface of which is provided with a number of sμction openings.

4. Method according to claim 3, c h a r a c t e r i z e d in that the casing (8) or casing sections (8a, 8b) are so placed in relation to the shaping film (3) that the film seals against a first end opening of the casing (8) or of the casing sections (8a, 8b), that a second film (11) is shaped to bear against said first fine particulate material (13) or the powder filling (12) of said second fine particulate material, which has been inserted into the casing, and to bear against the edge of the second end opening av the casing, that gas is evacuated from the tight space defined by said films (3, 11) and the casing (8), while the first fine particulate material (13) or the powder filling (12) solidifies to become a managable unit together with the casing and said films (3,11).

5. Method according to claim 4, c h a r a c t e r i z e d in that the covering plates (14, 16) are welded into the casing (8) and that the casing sections (8a, 8b) are joined together by welding.

6. Method according to claim 5, c h a r a c t e r i z e d in that said casing (8) is shaped as a frame.

7. Method according to claim 5, c h a r a c t e r i z e d in that the first fine particulate material (13) is sintered.

8. Method according to claim 1, c h a r a c t e r i z e d in that the first particulate material consists of particles of mostly spherical and/or irregular shape.

9. Device to carry out the method according to claim 1, c h a r a c ¬ t e r i z e d in that it comprises a die (2) and means (1, 3, 10) for the shaping of at least one thermoplastic film to a shaping barrier surface, a container (18, 14, 16) arranged to enclose the die and the thermoplastic film, means (15) to tighten the container, means (19) to evacuate gas from the tightened container and means for pressing and/or sintering.

10. Device according to claim 9, c h a r a c t e r i z e d in that it comprises vacuum forming equipment (1, 4, 28) and means (9) for the evacuation of gas from a first fine particulate material (13) or a powder filling (12) of a second fine particulate material situated between two sealing thermoplastic films (3, 11) and a casing (8).

11. Device according to claim 10, c h a r a c t e r i z e d in that said casing (8) consists of a frame.

12. Device according to any of claims 9-12, c h a r a c t e r i z e d in that said casing (8) is provided with evacuating means (9).