EP0388235B1

EP0388235B1 - Method and apparatus for casting

Info

Publication number: EP0388235B1
Application number: EP90302880A
Authority: EP
Inventors: Arnold J. Cook
Original assignee: PCC Composites Inc
Current assignee: PCC Composites Inc
Priority date: 1989-03-17
Filing date: 1990-03-16
Publication date: 1995-07-26
Anticipated expiration: 2010-03-16
Also published as: ATE125476T1; JPH02284756A; EP0388235A2; EP0388235A3; DE69021103T2; DE69021103D1

Abstract

An apparatus (10) comprising a pressure vessel (12) and a device for evacuating and pressurizing the vessel (12). The evacuating and pressurizing device is in fluidic connection with the vessel (12). The apparatus (10) is also comprised of a crucible (14) disposed in the pressure vessel (12) within which material (16) is melted. There is a mold chamber (18) disposed in the pressure vessel (12) within which a mold (20) is held. The mold chamber (18) is in fluidic connection with the crucible (14). Additionally, the apparatus (10) is comprised of a device for heating material in the crucible (14) and the mold (20) in the mold chamber (18) such that material is melted in the crucible (14) and stays melted as it is drawn to the mold chamber (18) while the evacuating and pressurizing device acts on the vessel (12), and when it is forced into the mold (20) while the evacuating and pressurizing device pressurizes the vessel (12). The heating device is disposed in the vessel (12). Additionally, there is a method comprising the steps of placing in a mold chamber (18) of a pressure vessel (12) a fiber array mold (20); placing in a crucible (14) of the pressure vessel (12) a material; evacuating the vacuum vessel (12) through the mold chamber (18); melting the material in the crucible (14); placing the crucible (14) in fluidic connection with the mold chamber (18); and charging the vessel (12) while it is continually evacuated such that the melted material is drawn into the mold chamber (18) and forced into the mold (20). Additionally, there is a method for reducing the required mold (20) strength by controlling the pressuie at the mold (20) wall by controlling the pressurization rate.

Description

This invention relates to a method for casting, for instance to produce fibre-reinforced materials.
Fibre-reinforced materials, where the material is commonly a metal, provide a level of strength to a structure that otherwise cannot be attained with just the material itself. The fibres in the material increase the stiffness of the material such that for a given applied load, the material deflects less than an unreinforced material. In order to produce fibre-reinforced materials, the fibres are typically bundled together in some fashion and form corresponding to the ultimate desired shape of the fibre-reinforced material. Then the material must somehow be forced into the fibre bundle so it completely fills the interstices of the fibre bundle. This is normally accomplished by liquifying the material and then forcing it into the fibre bundle. US-A-4573517 discloses an apparatus and method for forcing melted metal into a fibre bundle. It requires the apparatus wherein this method is performed to first be evacuated through a furnace thereof. Then melted metal is forced into a fibre bundle under pressure of gas pumped into the vessel. However, this technique requires, as do all other techniques heretofore known, that the chamber within which the production of the fibre-reinforced material occurs be strong enough to withstand the necessary pressures so it does not explode either from the inside or the outside.
Additionally, in the aforementioned U.S. patent, the level of melted metal within the structure must be carefully controlled or cooling thereto causes it to solidify making the process useless.
An apparatus is disclosed by Masur, L.J., Mortensen, A., Cornie, J.A. and Flemings, M.C.; "Pressure casting of Fiber-Reinforced Metals" Proceedings of the Sixth International Conference on Composite Materials ICCM-VI, F.L. Matthews, N.L.R. Bushell, and J.M. Hodginson, Eds. London, 1987, Elsevier Applied Science, pp. 2.320-2.329 wherein a furnace is disposed inside a pressure vessel, and pressure alone is used to fill a fibre preform with melted material. However, since this apparatus is for the purpose of determining the pressure necessary to penetrate a fibre preform, usable products therefrom are questionable.
DE-A-3416132 discloses an apparatus for casting, comprising:
a pressure vessel;
means for pressurizing the vessel, the pressurizing means being fluidly connected to the vessel;
a crucible disposed in the pressure vessel within which material is in use melted;
a mould chamber within which a mould is held, the mould chamber being in fluid connection with the crucible; and
means fluidly connected to the mould chamber for evacuating the same; whereby melted material in the crucible can be drawn to the mould chamber as the evacuating means acts on the mould chamber.
According to the present invention, there is provided a method for producing a reinforced part, characterized by the steps of:
placing a reinforcement inside a mould chamber which is disposed in a pressure vessel;
sealing the pressure vessel with the mould chamber having the reinforcement therein;
heating a meltable material in the pressure vessel, which material is to be infiltrated into the reinforcement, to a melted state;
pressurizing the pressure vessel;
infiltrating the reinforcement in the mould chamber with the melted material; and
solidifying the melted material surrounding and infiltrating the reinforcement:
wherein, after the pressurizing step but before the solidifying step, there is a step of surrounding the reinforcement in the mould chamber with the material in the melted state under the action of the pressure in the vessel; and wherein the infiltrating step includes the step of infiltrating the reinforcement in the mould chamber after the melted material has surrounded the reinforcement.
Preferably, after the solidifying step, there is the step of removing the reinforcement with the solidified material infiltrating it and surrounding it with a skin of the material, from the pressure vessel.
Preferably the heating step includes the step of heating the mould chamber to such a temperature that the material is in a melted state.
Preferably the skin of the material is a pure skin of material, the material preferably being a metal, preferably aluminium.
The present invention provides a reinforced part characterized by being formed by forcing with a gas a melted material into a reinforcement contained in a mould chamber inside a pressure vessel.
The present invention also provides a reinforced part with a pure metal skin characterized by being formed by forcing a melted metal into a mould chamber having a reinforcement such that the melted metal travels along the wall of the mould chamber and then infiltrates into the reinforcement.
The present invention requires that a pressure vessel be strong enough to withstand the necessary pressures inside the vessel. As a result, a mould chamber holding a fibre array mould inside the vessel is subject only to small pressure differences, thus allowing much thinner or low strength mould chamber structures. This enables the present invention to be easier to use and quicker to use and enables a more uniform heating of the mould chamber with a single furnace design. Moreover, the level of melted material is not of great concern as long as it fills the mould chamber since cooling of the melted material is actually used for the benefit of the process as opposed to a detriment of the process.
It is to be appreciated that, with the present invention, the mould chamber is disposed in the pressure vessel, within which a mould is held. The method of the present invention preferably employs furnace means for heating material in the crucible and the mould in the mould chamber such that material is melted in the crucible and stays melted as it is drawn up to the mould chamber while the evacuating and pressurizing means evacuates the vessel, and when it is forced into the mould while the evacuating and pressurizing means charges the vessel, with the furnace means being disposed in the vessel.
Preferably, the evacuating means includes a snorkel fluidly connected to the mould chamber and extending therefrom out of the vessel, and means for solidifying the melted material in the snorkel to form a solidification plug when the melted material is drawn therein.
A preferred embodiment is one wherein the mould chamber includes a feeder tube fluidly connected thereto and extending out therefrom, the feeder tube being disposed in the vessel; and wherein the apparatus includes a crucible lifter connected to the crucible for lifting the crucible such that it is in fluid connection with the feeder tube that extends from the mould chamber, the mould chamber being fluidly isolated from direct communication with the vessel interior.
For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:-

Figure 1 is a cross-sectional schematic view of an apparatus for casting.
Figure 2 is a cross-sectional side view of an apparatus for casting having a crucible in a first position.
Figure 3 is a cross-sectional schematic view of an apparatus for casting wherein the crucible is in fluidic connection with a mold chamber.
Figure 4 is a cross-sectional schematic representation of an apparatus for casting being loaded.
Figure 5 is a cross-sectional schematic view of the apparatus for casting wherein material in the crucible is melted in a vessel wherein the mold and vessel is evacuated through the mold.
Figure 6 is a cross-sectional schematic view of the apparatus for casting wherein the crucible is placed in fluidic communication with the mold chamber isolating the mold chamber from the vessel interior.
Figure 7 is a cross-sectional schematic view of the apparatus for casting wherein the melted material in the crucible is being drawn into the mold chamber.
Figure 8 is a cross-sectional schematic view of the apparatus for casting wherein a solidification plug is formed.
Figure 9 is a cross-sectional schematic view of the apparatus for casting wherein controlled increasing pressure is provided to the vessel to force the melted material into the mold.
Figure 10 is a cross-sectional schematic view of the apparatus for casting wherein the melted material is undergoing directional solidification.
Figure 11 is a cross-sectional schematic view of the apparatus for casting being unloaded.
Figure 12 is a side view of the apparatus for casting wherein a support frame holds the vessel with quick release heads.
Figure 13 is a partial cross-sectional side view of the apparatus for casting wherein a foil seal is in place over the fill tube.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings wherein like reference numerals refer to similar or identical parts throughout the several views and more specifically to Figure 1 thereof, there is shown a cross-sectional schematic view of an apparatus 10 for casting. The apparatus 10 comprises a pressure vessel 12 and means for evacuating and pressurizing the vessel 12. The vessel 12 is preferably made of 316 stainless steel. The evacuating and pressurizing means is in fluidic connection with the vessel 12. The apparatus 10 is also comprised of a crucible 14 disposed in the pressure vessel 12 within which material 16, such as aluminum, is melted. There is a mold chamber 18 disposed in the pressure vessel 12 within which a mold 20, such as preferably a fiber array mold, is held although the invention is not in any way limited to fiber array molds. The mold chamber 18 is preferably made of 304 stainless steel however, other materials can also be used and the crucible 14 is preferably graphite. The fibers used are preferably made of silicon carbide. When the mold chamber 18 is in fluidic connection with the crucible 14, melted material 16 in the crucible 14 can be drawn into the mold chamber 18 as the evacuating and the pressurizing means pressurizes the vessel 12. Typical pressures for use with silicon carbide fibers, and melted aluminum are 1450-2900 Pa (1000 PSI-2000 PSI) and preferably 1885-2175 Pa (1300 PSI-1500 PSI). The pressure required is related to the volume fraction of fibers. In general, the more fibers per given unit of volume, the greater pressure is required to force the melted material between the fibers.
The apparatus also includes means for controlling the rate at which pressurization occurs such that the pressure in the mould chamber 18 is able to be driven towards instantaneous equilibrium with the pressure outside the mould chamber 18.
The apparatus is also comprised of means for heating material 16 in the crucible 14 and the fiber array mold 20 in the mold chamber 18 such that material 16 is melted in the crucible 14 and stays melted as it is drawn up to the mold chamber 18 while the evacuating and pressurizing means evacuates the vessel 12, and when it is forced into the fiber array mold 20 while the evacuating and pressurizing means pressurizes the vessel 12. The heating means is preferably disposed in the vessel 12. The heating means should provide enough heat to maintain the material in a melted state. For instance, with aluminum, the temperature should be over 600°C and preferably between 650°C and 700°C.
Preferably, the evacuating and charging means includes means 22 for evacuating the vessel and means 24 for pressurizing the vessel, such as a vacuum pump 23 and a pressure pump 25, respectively. The evacuating means 22 is fluidically connected to the mold chamber 18 in the vessel 12 and the pressurising means is fluidically connected to the vessel 12. The heating means preferably includes a mold furnace 26 for heating the mold chamber 18 and a crucible furnace 28 for heating the crucible 14. The mold furnace 26 is preferably positioned about the mold chamber 18 and the crucible furnace 28 is preferably positioned about the crucible 14 to provide essentially uniform heating to the mold chamber 18 and crucible 14, respectively. It should be noted however that the mold furnace 26 is not necessary for the effective operation of the apparatus.
The evacuating means 22 preferably includes a snorkel 30 fluidically connected to the mold chamber 18 and extending therefrom out of the vessel 12. The evacuating means 22 also preferably includes means for solidifying the melted material 16 and the snorkel 30 to form a solidification plug when the melted material 16 is drawn therein. Preferably, the solidifying means includes a water cooled head 34 in thermal communication with the snorkel 30. In such an embodiment, the melted material solidifies to form the solidification plug as it moves adjacent the head 34. The head 34 can then be used, as above, to start a directional solidification down the mold such that, for instance, metal shrinkage during the solidification can be fed with additional metal from the crucible.
The mold chamber 18 preferably includes a feeder tube 38 fluidically connected thereto and extending out therefrom. The feeder tube is disposed in the vessel 12. The crucible 14 is fluidically connected to the mold chamber 18 through the feeder tube 38 such that melted material 16 in the crucible 14 can be drawn to the mold chamber 18 as the vacuum pump 23 evacuates the vessel 12 and the mold chamber 18 interior is fluidically isolated from direct communication with the vessel 12 interior. Note the pressure pump can also be used to aid in filling the mold chamber 18 with melted material 16 by pressurizing the vessel 12 and forcing the melted material 16 in the crucible 14 into the mold chamber 18.
In an alternative embodiment, fiber array molds 20 can also be heated and evacuated prior to insertion in the vessel with the use of a foil seal 41 disposed at the bottom of the snorkel 30, as shown in Figure 13. The foil seal 41 is, for instance, made out of the same material 16 as the material 16 in the crucible 14. The foil seal 41 melts when placed in the crucible 14 filled with melted material 16, thus fluidically isolating the interior of the mold chamber 18 from direct communication with the interior of the vessel and allowing material 16 to move into the feeder tube 38. In such an embodiment, the material can be melted subsequent to insertion of the fill tube 30 therein, thus eliminating the need for any furnaces or a crucible lifter 40.
Preferably, the apparatus 10 includes a crucible lifter 40 connected to the crucible 14 for lifting the crucible 14 such that it is placed in fluidic connection with the feeder tube 38 that extends from the mold chamber 18, as shown in Figures 2 and 3. Figures 2 and 3 are cross-sectional schematic views of an apparatus 10 with the crucible 12 out of fluidic connection with the feeder tube 38, and in fluidic connection with the feeder tube 38 after the crucible lifter 40 has lifted the crucible 14, respectively. (Note Figures 1, 2 and 3 are drawn to scale so that the relationship of the various elements and structures thereof are defined regardless of the actual size chosen therefore.)
The apparatus 10 preferably is water cooled, for instance, with pipes 42 positioned about the vessel 12. The apparatus 10 also preferably includes insulation 44 disposed in the vessel 12 and positioned about the mold furnace 26 and the crucible furnace 28 for maintaining heat therein.
The present invention pertains to a method for producing a fiber reinforced material. The method comprises the steps of placing in the mold chamber 18 of the pressure vessel 12 the fiber array mold 20. Then the step of placing in the crucible 14 of the pressure vessel 12 the material 16, as shown in Figure 4, is performed. Next, the step of evacuating the pressure vessel 12 through the mold chamber 18, and melting the material 16 in the crucible 14, as shown in Figure 5, is performed. Then the step of placing the crucible 14 in fluidic communication with the mold chamber 18, as shown in Figure 6, is performed. Next, the step of pressurizing the vessel 12 while it is continually evacuated such that the melted material 16 is drawn into the mold chamber 18 and forced into the fiber array mold 20, as shown in Figures 7 and 8, is performed. The pressurizing step preferably includes the step of controlling the rate at which pressurization of the vessel 12 occurs such that the pressure in the mold chamber 18 is able to have time to be driven toward instantaneous equilibrium with the pressure outside the mold chamber 18.
Preferably, before the pressurizing step, there is included the step of forming a solidification plug such that further evacuation is prevented from the pressure vessel 12, as shown in Figure 8, and the melted material is prevented from leaving the vessel 12 via the snorkel 30. After the solidification plug forms, the continuous pressurizing from the pressure pump 25 of the vessel 12 forces the melted material 16 through the feeder tube 38 into the mold chamber 18. This continuous pressure causes the melted material 16 to penetrate into the fiber array mold 20 and eventually fill the fiber array mold 20, as shown in Figures 9 and 10.
The fact that the mold chamber 18 and crucible 14 are disposed inside the vessel 12 as are the mold furnace 26 and crucible furnace 28 requires that only the pressure vessel 12 be strong enough to withstand the internal forces that build up under the pressure from the pressure pump 25. But since the pressure in the pressure vessel 12 is uniform essentially throughout, by controlling the pressurization rate of the vessel such that the pressure in the mold chamber is able to have time to be driven toward instantaneous equilibrium with the vessel pressure, only a small differential pressure between the interior of the mold chamber 18 and the interior of the pressure vessel 12 exists at any give moment. Consequently, strength concerns of the mold chamber structure and mold seals are minimized. This small pressure differential between the interior of the mold chamber 18 and the interior of the pressure vessel 12 is kept to a minimum due to the action of the melted material 16 being forced into the mold chamber 18 by the pressure pump 25 in an amount corresponding to the pressure on the melted material 16.
Once the fiber array mold 20 is saturated with melted material 16, the temperature of the mold chamber 18 is lowered to allow the melted material 16 to solidify. Then pressure is released and the mold 20 is removed from the pressure vessel 12.
In the operation of the preferred embodiment, the crucible 14 is loaded with aluminum and placed in the vessel 12 which is then sealed, as shown in Figure 4, with high temperature VITON^® seals. The crucible furnace 28 and mold furnace 26 is then activated to melt the aluminum in the crucible 14. At the same time, the vacuum pump 23 is activated which evacuates the vessel 12 through the mold chamber 18, as shown in Figure 5. As can be more clearly realized with respect to Figure 1, the vacuum pump 23 evacuates the pressure vessel 12 through the mold chamber 18 by drawing fluid present in the vessel 12 first through the feeder tube and then through the mold chamber 18 and then through the snorkel 30. By evacuating the mold 20, there is less chance of voids being formed in the fiber reinforced material after the melted material has infiltrated the mold 20.
When the aluminum in the crucible 14 is melted, the crucible lifter 40 is activated causing tile crucible 14 to be moved in fluidic connection with the snorkel 30 fluidically isolating the interior of the mold chamber from direct communication with the vessel interior such that the melted aluminum in the crucible 14 can be drawn up to the mold chamber 18 through the snorkel 30 under the action of the vacuum pump, as shown in Figures 6 and 7. At the same time, the vessel pressurization is started with nitrogen gas to assist in forcing the aluminum into the feeder tube. As the vacuum pump 23 continues to draw fluid out of the vessel 12, the melted aluminum is drawn up through the feeder tube 38 into the mold chamber 18 where it surrounds the fiber array mold 20 and comes into contact with the water cooled head in the snorkel 30, a solidification plug is formed.
Once the melted aluminum has filled the mold chamber 18 as much as possible under the action of the vacuum pump 23, the pressure pump 25 pumps nitrogen gas into the vessel 12, as shown in Figure 8. The pressure in the vessel 12 is consequently increased throughout the vessel 12 and specifically at the surface of the melted aluminum in the crucible 14. As the melted aluminum in the crucible 14 prevents the pressurized gas in the vessel 12 from entering the feeder tuber 38 and reaching the mold chamber 18 since the interior of the mold chamber 18 is fluidically isolated from direct communication with the interior of the pressure vessel, a pressure differential is created between the interior of the vessel 12 and the interior of the mold chamber 18. This pressure differential results in the melted aluminum being forced up through the feeder tube 38 and, into the mold chamber 18, as shown in Figure 9. The amount of melted aluminum that is forced into the mold chamber 18 and consequently the fiber array mold 20 corresponds to the amount of pressure in the vessel 12 at the surface 48 of the melted aluminum in the crucible 14. The more pressure in the vessel, the more fluid is forced into the mold chamber 18 and fiber array mold 20 to compensate for the difference in the pressure between the inside of the mold chamber 20 and the inside of the vessel 12, as shown in Figure 9. As the aluminum is forced into the fiber array mold 20, the pressure is equalized between the inside of the fiber array mold 20 and the inside of the vessel 12 itself. By controlling the pressurization rate, it is possible to control the difference between the pressure on the inside and outside of the fiber array mold 20. The slower the rate, the lower the pressure exerted on the outside of the mold and so a thinner or lower stength wall thereof is required. Quick pressurization rates require heavy walls to withstand the pressures exerted on the walls of the mold chamber.
As the melted aluminum fills the fiber array mold 20, the melted aluminum is also forced into the snorkel 36. When the melted aluminum reaches the top of the snorkel, the water cooled head 34 causes the melted aluminum in the snorkel 36 nearest the water cooled head 34 to solidify. This solidification of the melted aluminum forms the solidification plug. This solidification of the melted aluminum propagates as a wave down from the solidification plug, fiber array mold 20 and to the crucible 14, as shown in Figure 10. The pressure pump 25 is then turned off and the mold chamber 18 with the fiber array mold 20 therein is removed from the pressure vessel 12. The vessel 12 is maintained in a support frame 46, as shown in Figure 12, through the process.

Claims

A method for producing a reinforced part, characterized by the steps of:
placing a reinforcement 20 inside a mould chamber 18 which is disposed in a pressure vessel 12;
sealing the pressure vessel 12 with the mould chamber 18 having the reinforcement 20 therein;
heating a meltable material 16 in the pressure vessel 12, which material 16 is to be infiltrated into the reinforcement 20, to a melted state;
pressurizing the pressure vessel 12;
infiltrating the reinforcement 20 in the mould chamber with the melted material 16; and
solidifying the melted material 16 surrounding and infiltrating the reinforcement 20:
wherein, after the pressurizing step but before the solidifying step, there is a step of surrounding the reinforcement 20 in the mould chamber 18 with the material 16 in the melted state under the action of the pressure in the vessel 12; and wherein the infiltrating step includes the step of infiltrating the reinforcement 20 in the mould chamber 18 after the melted material 16 has surrounded the reinforcement 20.
A method as claimed in claim 1 characterized in that, after the solidifying step, there is the step of removing the reinforcement 20 with the solidified material 16 infiltrating it and surrounding it with a skin of the material 16, from the pressure vessel 20.
A method as claimed in claim 1 or 2, characterized by the heating step including the step of heating the mould chamber 18 to such a temperature that the material 16 is in a melted state.
A method as claimed in claim 3, characterized by the skin of the material 16 being a pure skin of material 16.
A method as claimed in claim 4, characterized by the material 16 being a metal.
A method as claimed in claim 5, characterized by the metal being aluminium, and the skin being a pure aluminium skin.