US3451467A - Centrifugal casting apparatus - Google Patents

Description

June 24, 1969' J. FRANKENY ET AL 3,451,467

CENTRIFUGAL CASTING APPARATUS Filed Aug. 5, 1967 VARIABLE POWER SUPPLY OOOO JUlius'L. Fronkeny KennerhE. Voiles IN VEN TORS BY v United States Patent 3,451,467 CENTRIFUGAL CASTING APPARATUS Julius L. Frankeny, Arvada, and Kenneth E. Voiles, Denver, Colo., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed Aug. 3, 1967, Ser. No. 658,289 Int. Cl. B22d 13/10, 13/00 US. Cl. 164297 5 Claims ABSTRACT OF THE DISCLOSURE A centrifugal casting assembly comprising a bottom discharge crucible, heated by an electrical furnace and mounted above an open top rotary mold encompassed by a heating means to control outside wall cooling; a top entering centrally disposed rod plug for controlling molten metal flow from the crucible; and a radiant heat sink extending concentrically into the rotary mold, said heat sink having means to control the rate of heat removal therefrom.

Background of invention Molten materials such as beryllium which contract on solidification are especially difiicult to cast as cracks, voids, and defects may be formed in the casting on freezing. In a solid stationary casting the side walls may be insulated and a cooling means installed at the bottom of the mold such that freezing proceeds upward uniformly across the casting. Excess melt may be provided in a bottom discharge reservoir above the casting. Thus cast contraction which is confined to a longitudinally downwardly direction, is immediately refilled by the excess melt.

This method is inadequate in centrifugal castings where all the melt initially strikes the bottom of the spinning mold and is impelled to the vertical mold walls by centrifugal force. A bottom chill plate in such an application would prematurely cool the melt. Excess melt in a bottom discharge reservoir above the mold would immediately fall within the normally vacant center of the centrifugal casting. Side wall cooling alone has resulted in cracking or hot tearing due to the temperature gradient between the mold wall and the interior cast surface.

Furthermore, cross radiation within the internal surface of the centrifugal casting delays freezing. This slow cooling and freezing increases the grain size and consequently reduces the casting strength. Also delayed cooling frequently results in hot tearing and subsequent metallurgical failures during fabrication operations. Many metals, such as aluminum, regardless of whether they contract on freezing may suffer large grain size and hot tearing at the internal centrifugal cast surfaces if a prompt and uniform rate of cooling is not maintained during casting operations. Centrifugal casting is used to form hollow open ended circular shapes such as cylinders, cones, frustocones etc. This operation is particularly useful where desired wall thickness are too thin for still casting or where non-ferrous materials such as beryllium are used which are difficult to fabricate from sheet metal form. Other advantages are the ability to produce seamless ferrules and pipes as well as precise circular shapes.

Summary of invention It is therefore an object of this invention to provide an improved centrifugal casting apparatus having controlled and uniform cooling of the casting.

It is a further object to provide a centrifugal casting assembly which will minimize grain size and defects in the internal surface of a casting.

Various other objects and advantages will appear from the following description of one embodiment of the invention, and the most novel features will be pointed out in the appended claims.

The invention is an improved centrifugal casting apparatus comprising a rotary mold disposed beneath a bottom discharge crucible, a radiant heat sink projecting concentrically into the rotary mold and having means for heat transfer control, and means for controlling the heat transfer rate through the mold walls.

Description of drawings The drawing is a cross-sectional elevation view of a centrifugal casting apparatus in accordance with the invention.

Detailed description The centrifugal casting assembly shown in the drawing includes a crucible 11 for holding molten metal having a lid 12 which may be composed of a refractory layer 14 and a graphite layer 13. A centrally located rod plug 15 may be used to control the melt flow through the bottom discharge port 16. The crucible may be surrounded by refractory fire brick 17 supported laterally by a castable refractory cylinder 18. The crucible may be heated by an electrical heater 19 such as a part of a vacuum induction furnace.

A metal or otherwise housing 20, which may be the walls of a vacuum induction furnace, having suitable lateral projections 21 supports the crucible 11 and stationary portion of the casting assembly. A rigid base ring 22 may adapt from the crucible diameter to the spacing between the lateral projections 21 and be insulated from the crucible bottom and remaining top portion of the assembly by a refractory ring 23.

A radiant heat sink 24 may be supported by a metal or otherwise ring 25 which is attached to the crucible bottom but thermally insulated therefrom by a layer of refractory material 26. The heat sink may have double vertical cylindrical walls 27 and 28 separated by a passageway 29. During operation the heat transfer rate through the heat sink may be controlled by pumping fluid such as water through passageway 29 by route of suitable ports 30 and 31. Suitable piping and a valve 41 may be used to control the flow of fluid. Bafiles (not shown) may be installed in passageway 29 to provide eflicient fluid circulation.

A rotary mold 32, having a lid 33 with a center cutout 34 admitting the radiant heat sink 24, may be rotated by drive 35 through shaft 36 and mounting plate 37 such that centrifugal casting is formed on the vertical walls of mold 32. The mold and heat sink may be arranged with coincident longitudinal axes so that all points on the internal cast surface will be equal distant from the heat sink promoting uniform cooling and reducing cross-radiation between internal surfaces of casting 40.

A stationary resistance or induction electric heater 38 which is controllable between ambient and about 1000 C. with a variable power supply 42 may surround the rotary mold and may be supported and protected by a suitable metal or otherwise shield 39. A nonelectric heater may also be used, but temperature control would be made more diificult. The internal surface of the shield 39 may be a black heat radiating body to efiiciently control the flow of heat from the mold 32 walls.

The materials of construction of the assembly are not critical and may be selected by one skilled in the art of high temperature molten metal casting. The crucible, mold, and lids may be of graphite which will withstand the high temperature but, when beryllium is cast a protective coating of BeO-BeSO should be sprayed and baked on the exposed graphite surface to prevent reaction therewith. The rod plug may be alumina (A1 0 while any hard castable refractory material such as titanium dioxide (TiO and heat insulating firebrick, such as silica (SiO), may be used which is compatable with the furnace ternperature (i.e., about 1500" C. for beryllium casting). The heat sink may be of graphite, copper, or other materials but high thermal conductivity and ease of fabrication make copper the better choice.

The heat sink is preferably circular in conformity to the mold shape chosen so that the inside surfaces of the heat sink may be disposed as closely as possible to the internal cast surfaces thus minimizing cross-radiation between the inside cast surfaces. Generally both the mold and heat sink are cylindrical but may be hemispherical, conical, frusto-conical etc. It may be practical to have only cylindrically shaped heat sinks of various sizes which could be used in conjunction with any mold shape. Noncircular heat sinks might also be used accompanied by reduced efficiency.

An example of the centrifugal casting operation may proceed as follows. Raw or scrap beryllium metal may be placed in crucible 11 disposed in a vacuum induction furnace with rod plug 16 in place. The lid 12 and firebrick 17 may then be arranged in position as shown in the drawing. It is preferable to maintain a vacuum about the complete assembly to prevent oxidation of the melted beryllium metal. The furnace heater may then be activated and the metal raised to a superheated melt temperature of about 1450 C. to insure melting of the entire charge. The temperature may be dropped to about 1350 C. just prior to pouring to eliminate superheat in the casting. Prior to introducing the melt into rotary mold 32, drive 35 should be started at a suitable rate such as about 1000 r.p.m. The rod plug may then be slowly lifted admitting melt into the rotating mold at a rate which minimizes splatter on the radiant heat sink 24. Casting 40 will consequently be formed. When the casting 40 cools to about 1000 C. an inert gas such as helium gas may be introduced into the vacuum furnace to promote cooling. All of the above mentioned temperatures may be measured with a radiation pyrometer through suitable ports (not shown) in the furnace.

The cast cooling rate may be uniformly controlled by regulating current flow in heater 38 by-means of the variable power supply 42. which will consequently control the heat transferred through the walls of rotary mold 32. Also water or another suitable fluid may be circulated at a controlled rate and accordingly at a controlled temperature through passageway 29 Within the double walls of radiant heat sink 24 to control the cooling rate at the inner surface of casting 40.

Various modifications may be made to the centrifugal casting assembly within the scope of the invention. If desired a solid wall radiant heat sink may be used without means for controlling and promoting heat transfer therefrom, however, uniform cooling may be more difiicult to achieve as the radiant heat sink may tend to excessively heat up. An alternate method for controlling cooling at the outside wall of rotary mold 32 is by setting its position in relation to furnace heater 19. This may be done by longitudinally positioning drive 35 along its base plate (not shown) to a particular location for a complete casting operation. If this is done in lieu of using heater 38 the cooling rate could not be readily varied during operation. More precise control of the rotary mold wall temperature can thus be achieved with this additional heating unit 38 and therefore cracking of the casting will be less likely to occur.

The rotary mold of the improved centrifugal casting assembly may be rotated about a vertical axis as illustrated or if desired rotation may be about a horizontal axis. In a horizontal centrifugal casting assembly the crucible may be located to the side of the rotary mold and the melt may gravitate onto the inside cylindrical walls of the rotating mold. The heat sink may be attached to and rotate with the rotary mold drive rather than being held by a stationary supporting member as has been illustrated.

The improved centrifugal casting assembly described maybe used to prepare hollow circular castings of beryllium with wall thickness up to 1 /2" thick. Prior castings prepared in conventional centrifugal molds sustained cracking caused by the large temperature gradient if Wall thickness exceeded 4". Castings up to 13 inches long and 11 inches in diameter have been prepared, but it is logical to assume larger castings could be produced in larger equipment.

By controlling the freezing rate at the surface of the casting and by blocking internal cross-radiation, the improved centrifugal casting assembly provides a casting having small grain size and high strength. Furthermore, hot tearing or cracking does not occur during casting which facilitates subsequent fabrication operations. These advantages make the improved centrifugal casting assembly useful in forming other metals in addition to beryllium such as aluminum or any metal having critical heat transfer characteristics on solidification.

It will be understood that various changes in the details, materials and arrangements of the parts, which have been herein described and illustrated may be made by those skilled in the art within the scope of the invention expressed in the appended claims.

What is claimed is:

1. A centrifugal casting apparatus comprising a crucible for containing molten metal, a heating means encompassing said crucible for melting such metal, a rotary mold disposed adjacent said crucible for receiving molten metal therefrom, means for rotating said mold to form a tubular casting from such molten metal, a radiant heat sink means extending concentrically into and spaced from said casting for removing heat from the interior thereof, and a cast cooling control means surrounding said rotary mold and casting.

2. The apparatus according to claim 1 wherein said radiant heat sink means includes means for varying the flow of a fluid circulating therein to control removal of heat from the casting.

3. The apparatus according to claim 2 wherein said radiant heat sink means comprises an open ended hollow cylinder having double walls defining an annular passageway therebetween, and ports for fluid flow to and from said passageway.

4. The apparatus according to claim 1 wherein said cast cooling control means comprises a heater encompassing said rotary mold and means for controlling the temperature of said heater.

5. The apparatus according to claim 1 wherein said crucible contains molten beryllium metal.

References Cited UNITED STATES PATENTS 1,623,997 4/1927 Catlin 164297 X 2,148,802 2/1939 Bunke 164297 2,555,998 6/1951 Rankin et al 164297 X J. SPENCER OVERHOLSER, Primary Examiner.

US. Cl. X.R. 164-286

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