Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
1999-07-19
2002-09-10
Elve, M. Alexandra (Department: 1725)
Metal founding
Process
Shaping liquid metal against a forming surface
C128S122100, C128S122100
Reexamination Certificate
active
06446700
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to high temperature gradient casting apparatus utilising a molten metal crystallizer and more particularly to floating heat insulating baffles for shielding the cooling bath from high temperature radiating from such casting furnace, and to a method of directional casting of articles using such ceramic baffles.
The casting of single crystal and directionally solidified superalloys requires the cooling of the superalloy in a temperature gradient. Typically, the temperature gradient is achieved by melting the superalloy in a high temperature furnace and then slowly withdrawing the superalloy from the furnace and lowering it into a liquid metal cooling bath, commonly referred to a crystallizer bath, which causes corresponding gradual solidification of the cast article. The best superalloy mechanical properties are achieved when the highest temperature gradients are used. Common metals for use as a crystallizer bath include tin and aluminium.
A typical apparatus for casting directionally solidified metals is illustrated in U.S. Pat. No. 4,108,236 (Salkeld). Salkeld shows an apparatus having a high temperature furnace suspended over a molten metal bath. Beneath the heating chamber is a floating insulating baffle. Openings through the bottom of the furnace enable a mold to be lowered therethrough.
Materials used for floating baffles must be chemically stable with respect to the liquid crystallizer, have low thermal conductivity, and a density low enough (and a displaced volume high enough) to allow the material to float on the liquid crystallizer. The most stable ceramic materials with respect to liquid aluminum crystallizers are alumina (aluminum oxide) and zirconia (zirconium oxide). Different methods of lowering the density of alumina, zirconia or other chemically compatible materials have been used. For example, the desired material has been formed into integral hollow ceramic bubbles. Unfortunately, the method of manufacturing substantially spherical, hollow ceramic bubbles produces a very thin wall that is easily broken. This makes the substantially spherical bubbles unsuitable as a floating baffle material because whenever a bubble is broken, the ceramic will sink to the bottom of the crystallizer vessel. Bubbles with a thicker, stronger wall are not currently available and a manufacturing technique to make a thicker, stronger wall on the bubbles is not known.
The Salkeld reference discussed above teaches a discshaped, heat insulating baffle constructed of a fibrous zirconia insulating core bonded in a sandwich-type arrangement between heat resistant graphite sheets as being suitable for applications in molten tin at 500° F. (250° C.).
Published EPO 0631832A1 illustrates a floating thermal insulating layer
13
which floats on a cooling bath used in a directional solidification casting process (see
FIG. 4
thereof). It is disclosed that the thermal layer “consists of a material capable of flowing” and that the mold when lowered into the cooling bath “penetrates” the thermal layer. The material of the insulating layer is disclosed to be prepared from granules of graphite, ceramic, or aluminum oxide with a coating that prevents wetting, such as boron nitrate. Alternately, solids of boron nitrate or spherules of Si AlO
2
N are disclosed as being capable of being used. Undesirably, however, as mentioned above, where aluminum is used as the molten metal cooling bath, dense solids (including boron nitrate) will typically be too dense to float on such cooling bath. In addition, integral hollow spherules of SiAlO
2
N, are generally unsuitable since the method of manufacture produces, as mentioned above, a thin wall that is easily broken or cracked with a resultant of loss of insulating capability.
Each of the above references provides that the insulating baffle disclosed in such references possess an opening for lowering the mold into the cooling bath. This configuration has the drawback that there exists an uninsulated portion of the cooling bath through which conduction of heat can readily occur.
Likewise, Russian patent No. 1401715 teaches a floating thermal baffle
5
for a cooling bath used in a directional casting furnace. Such baffle
5
is constructed of alternating layers of heat resistant material
6
(graphite). Again, due to the provision of an opening in such baffle
5
to allow for the lowering of the cast metal and mold into the cooling bath, such thermal layer and casting method suffers from the same disadvantages of Salkeld and EPO 0631832A1, namely an opening in the floating thermal baffle which results in undesirable convection of heat, thus undesirably reducing the thermal gradient between the cooling bath and the mold being lowered into such bath.
SUMMARY OF THE INVENTION
In order to overcome the aforesaid disadvantages of the prior art, the present invention in one broad aspect thereof provides a method for providing a floating thermal insulating layer over a liquid metal cooling bath used in a directional casting furnace, which avoids having to provide an opening in the thermal layer. A plurality of hollow ceramic baffles are provided, which float in the surface of the crystallizer cooling bath. Such plurality of baffle members provide an insulating layer and advantageously are displaced only to the extent caused by the lowering of the mold into the crystallizer during the directional solidification process. Due to their floating and displaceable characteristics, the individual baffle members surround and conform to the exterior periphery of the mold when it is lowered into the crystallizer bath, advantageously maintaining the thermal insulating layer as close to the mold as possible.
Moreover, upon removal of the mold and cast article from the cooling bath, the floating baffles, due to their displaceable nature, then float to their original position substantially re-covering all of the surface of the cooling bath.
In order to ensure floatation of such baffles in cooling mediums such as molten aluminum, the ceramic members of the present invention are each constructed in a particular manner to create a hollow hermetically sealed chamber therein. Accordingly, in one of the broadest aspects of the present invention, a method of providing an insulative heat barrier over a surface of a cooling bath during directional casting of superalloys is provided, comprising the steps of substantially covering said surface with a plurality of ceramic baffle members, such ceramic baffle members each comprising a hollow ceramic member and ceramic seal means sintered to said hollow member so as to form a hermetically sealed chamber within said hollow member, such baffle members floatable on the surface of the cooling bath and substantially non-reactive with the cooling bath.
The ceramic baffle members are each assembled from the hollow ceramic member and seal means when each are in a green (unsintered) state, and subsequently both sintered to each other to form the sealed chamber. It is provided, in a preferred embodiment, that the ceramic seal means be situate within the hollow member and adapted to shrink less than the hollow member during the sintering process, so that the hollow member is “shrink-fit” around the seal means so as to assist in the seal means hermetically sealing the chamber of the baffle member.
It is preferred that the ceramic baffle member be formed as an extruded ceramic tube (of any shape such as cylindrical, rectangular, square, or triangular), and having a pair of respectively opposite distal ends, and that the seal means comprise a pair of diepressed thin flat end members adapted for respective insertion in the distal ends of the ceramic tube member. When the end members are inserted in a green state into the opposite distal ends of the hollow ceramic member (also in a green state), and the assembly fired, the end members become sintered to the hollow member creating a hermetically sealed chamber sealed at both ends by the respective end members. Where the end members are of a ceramic material
Elve M. Alexandra
Johnson Noreen C.
McHenry Kevin
Santandrea Robert P.
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