Note: Descriptions are shown in the official language in which they were submitted.
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COMPOSITE CORES AND METAL CASTING THEREWITH
In the preparation of castings of beryllium alloys, particularly beryllium-
aluminum alloys, many of the alloys have been found to be corrosive to the core
(and in some cases the mold) materials leading to degradation of the core (and
mold) and poor quality castings. It has been found possible to protect the core
(and if necessa,y, the mold) with selected coatings which do not react sufficiently
during the casling process to allow the core (and mold) to be detrimentally
affected.
Aluminum and magnesium castings have desirable properties for many
applications where light weight, good corrosion resistance and reasonable strength
are important. Various alloys of these metals have been developed to improve the ei)ylh and high temperature properties.
Certain beryllium alloys, particularly beryllium-aluminum alloys, have high
stiffness, low density and high melting points giving a desirable combination ofproperties. In applying the casting techniques to these alloys, it was found that the
higher casting temperatures and corrosiveness of these alloys caused substantialand unacceptable degradation of the core as well as poor metallurgical integrity of
the casting. Reaction products formed from cast alloy and core become
detrimental defects in the casting.
Beryllium-aluminum alloys are difficult to cast due to mutual insolubility and
wide solidification temperature range leading to undue microporosity and coarse
microstructure causing reduced strength and ductility. To reduce these effects
various ternary, quaternary and higher order aiioys have been developed including
additives such as silicon, silver, copper, nickel or cobalt. Alternatively or additionally
powder metallurgy techniques have been applied to these alloys in efforts to reduce
these difficulties. However, the problem of reaction with core (and some mold)
materials during melt casting still is present with the various higher order alloys.
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Typical ternary and higher order alloys are described in U.S. Patents 5,417,778,May 23, 1995 and 5,421,916, June 6, 1995, both issued to Nachtrab et al. In PCT
Application Publication No. WO 95/27088, October 12,1995, Grensing et al., certain
aluminum alloys containing beryllium, and formation and investment casting of these
alloys, are disclosed.
It would be desirable to form cores able to withstand the effect of these
molten alloys during ca~ling, giving high quality castings and allowing removal, and
in some cases, reuse of the cores.
We have found that core materials (and susceptible mold materials) can be
protected during melt casting of beryllium alloys by providing on the core (and
optionally mold) a coating selected to be substantially inert or non-reactive during
the casting process. By substantially inert or non-reactive is meant not reacting
during the casting process sufficiently to detrimentally affect either core or casting.
One aspect of the present invention includes the provision, in a process of
casting beryllium alloys in which mold or core materials are reactive to a significant
extent with the molten alloys, of the improvement comprising: utilizing at least one
20of a mold and a core coated with a protective coating which is selected to be
suL,sld"lially non-reactive during the casting process.
The invention further includes a shaped mold or core or combination forberyllium alloy casting, the mold and/or core having a protective coating selected
to be suL,slal ,lially non-reactive with the beryllium alloy being cast throughout the
casting process.
The invention includes, more particularly, a process of casting beryllium-
aluminum alloys having from about 20 to about 80% by weight beryllium,
30comprising: providing a casting mold and at least one core yielding the desired
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shape; coating the core, and optionally the mold, with at least one of the groupcGnsis~i~,y of oxides, borides, nitrides and cermets selected to be inert to th
molten alloy; casting the alloy while molten, into the mold and about the core;
cooling, removing the mold, and recovering the cast part.
In some cases, the mold may be used to form further castings. In cases
where the core is removed intact from the casting, the core can be re-used also.Some of the core coatings may serve as parting agents which facilitate core
removal.
Another aspect of the present invention is to provide a process for casting
beryllium alloys wherein mold surfaces which contact and which react with the
molten metal are coated similarly to the core surfaces. The benefits of this arereduced reactivity with molten metal and improved quality of the casting. A
preferred embodiment is where the coating serves as a parting agent as well as aprotective barrier, thus facilitating removal of both mold and core.
Those coatings found to have parting agent properties (on casting beryllium
alloys) are MgO, ZrO2, TiN and Al203. Particularly preferred as combined protective
chemical barrier coating plus parting agent is ZrO2 or Al203.
Preferably, the beryllium alloys are beryllium-aluminum alloys containing from
about 20 to about 80% by weight beryllium and having additives to improve the
microstructure, strength and ductility. More pre~eraL)ly, the alloys will have from
about 50 to about 70% beryllium.
The casting alloys may contain up to about 80% by weight beryllium. The
beryllium alloys, which are amenable to casting, include those containing from
about 20 to about 80% by weight beryllium; from about 20 to about 75% aluminum,
and the balance additives selected from silicon, silver, magnesium, copper, nickel,
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cobalt and impurities. All of these alloys melt at temperatures above about 1 1 50~C
(beryllium melts at 1277~C).
Any casting technique involving molten beryllium alloys and the use of
reaction-susceptible molds or cores, including investment casting, shape casting,
sand casting, permanent mold and die casting, can be improved by the invention.
Normally, a vacuum or an inert gas atmosphere (e.g. argon, helium) is maintainedduring casting.
Mold materials commonly used in such casting techniques include sand-
plus-binder, ceramics such as alumina (with binder), silica, alumina-silicate mixtures,
zircon, sodium and potassium silicates, zirconia (with binder), gypsum, graphite,
and magnesium/iron silicates. Any of the known processes for shaping the mold
may be used. Many of these mold materials will react with molten beryllium alloys
and can be coated similarly to the cores to protect against reaction.
The cores may be formed of a) suitable metals (or alloys) which melt above
the casting temperatures, b) suitable ceramics, for example alumina/binder or silica-
base ceramics, and c) mixtures thereof. Such mixtures may comprise e.g. stainless
steels + silica-base ceramics; titanium + A1203 + binder and mixed ceramics.
Frequently, alumina is used with some form of binder and the binder has been
found to be reactive with the molten alloy. Examples of metals useful in formingcores are various stainless steels, e.g. 304, 316 and 321; titanium and Ti-base alloys
such as Ti6AI4V; nickel and Ni-base alloys such as IN-100. Such cores have been
found to react with the beryllium alloys during casting.
The core base may be shaped by any known metallurgical technique (in the
case of metals) or ceramic molding technique (in the case of ceramics and
mixtures). The cores may be hollow, e.g. as metal tube or slip-cast fired ceramic;
or s~b:jlal ,lially solid, e.g. as metal rod or sintered ceramic powder. If the cores are
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to be removed from the finished part, the core material should be susceptible tochemical dissolution or mechanical disruptions. These mechanical removal
processes might include vibration, drilling, abrasion, and/or grinding. Depending
on the process, residual core coating material might remain in the casting without
del,ime"l. If the protective coating also acts as a parting agent, it may be possible
to remove the core as a unit or in several pieces.
Selected coatings have been found which are substantially non-reactive
during casting and able to protect the mold and/or core from molten beryllium
alloys. The coatings are selected from oxides, e.g. alumina, magnesia, beryllia,thoria, titania and zirconia; and borides, e.g. beryllium boride, aluminum boride,
titanium boride; as well as nitrides, e.g. beryllium nitride, boron nitride, aluminum
nitride and titanium nitride. Cermets may also be used e.g. Be + beryllia; Be +
alumina; Be + zirconia; Mo + alumina; Ta + alumina and Ta + zirconia.
Intermetallic oxides or borides or nitrides or cermets may be used e.g. Be-Ti boride;
B-A1 nitride; beryllia-zirconia; alumina-thoria.
The coating is formed on the core by any suitable technique, e.g. plasma
spraying, vapour deposition, dipping, electro-deposition, injection around core
body, brushing, spraying, impregnation, painting, and flow or gravity or cascadecoating. Vaporization, melt or sintering temperatures will be reached in forming the
coating, as required.
The thickness of the coating should be selected to constitute an effective
diffusion barrier during the entire casting process. Usually the thickness will be
within the range of about 20 to 1000 microns, preferably about 50 to 200 microns.
Multi-layer coatings may be used: examples include Al203 under ZrO2 and Al203
under TiO2.
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A preferred coating is plasma-sprayed or physical vapour deposited alumina
having a thickness of about 50-100 microns. Another preferred coating is Zr02.
The cast products have been found to be improved (when these coated
molds and/or cores were used) in aspects such as smooth and defect free detailedp~s~ges, pockets and cavities.
Coated molds and cores, when able to be removed intact, can be re-used.
If necessary a coating layer can be re-applied.
The following examples are typical and illustrative and are not intended to be
limiting or exhaustive.
EXAMPLES
Example 1
A core constructed from a stainless steel tube (321) was plasma coated with
100 microns thickness of Al203. Using technology known to the art of investment
casting, the core was located inside the internal cavity of a ceramic shell mold. The
ceramic shell was preheated in the range of 900~C-1250~C (preferably 1200-1250~C)
and then molten aluminum-beryllium 40:60 alloy in the range of 1200~C-1470~C
(preferably 1400-1450~C) was poured into the shell, filling the internal cavity and
surrounding the Al203-coated tube. During casting and cooling, an argon gas
atmosphere was maintained. Once the casting was cool, it was cleaned and
prepared in a manner similar to the usual procedure for aluminum and magnesium
castings. The Al2O3-coated tube was found to be resi~lanl to the molten alloy and
to result in high quality castings.
Example 2
All processing was the same as Example 1 except the core was coated by
dipping in a ceramic slurry (a water-base slurry of beryllium oxide) followed by
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sintering. The beryllia-coated tube was found to be resistant to the molten alloy
and to result in high quality castings.
Example 3
All processing was similar to Example 1 except the core was formed from
a tube of titanium base metal and coated with aluminum boride by plasma
spraying. The boride-coated tube was resistant to the molten alloy and resulted in
high quality castings.
Example 4
All processing was similar to Example 1 except the coating was plasma-
sprayed thoria. Good quality castings resulted.
Example 5
All processing was similar to Example 1 except the coating was vapour-
deposited alumina-zirconia. The alumina-zirconia coated core was resistant to the
molten alloy and resulted in high quality castings.
Example 6
The procedures in Example 1 were repeated except the ceramic shell mold
also was coated with 100 microns of plasma-sprayed alumina on all surfaces
exposed to the molten alloy. Very high quality castings resulted when mold and
core were removed.
Example 7
A SiO2-based ceran,ic core was coated with Al203 to a thickness of 50
microns. rlasma spraying which produced a sound and chemically inert barrier,
was used to provide the layer. The coated ceramic core was located inside the
internal cavity of an investment casting ceramic shell so that part of the core would
be exposed in the casting. The ceramic shell was preheated in the range of 900~C-
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1 250~C and then molten aluminum-beryllium alloy of 65% beryllium at a temperature
in the range of 1200~C-1450~C was poured into the shell, filling the internal cavity
and surrounding the Al203-coated tube. Once the casting was cool, the casting was
cleaned and prepared in a manner similar to known aluminum and magnesium
casting procedures. The exposed ceramic core was then removed by leaching in
a solution of hydrofluoric acid. A high quality casting resulted.
Example 8
All processing was similar to that in Example 7 except the coating was
plasma-sprayed magnesia. The magnesia-coated core was resistant to the molten
alloy and yielded a high quality casting.
Example 9
The procedures were similar to those in Example 7 except the core coatings
were formed from the following ceramics: Th02, Zr02, MgO, Ti02, AIN, BeN, BN, TiN.
In each case, the coated cores were resistant to the molten alloy and yielded high
quality castings.
Example 10
The procedures were similar to those in Example 7 except the coating was
derived from at least two layers of the different ceramic materials alumina and
zirconia. Superior quality castings resulted.
Example 1 1
The procedures were similar to those in Example 7 except the coating was
derived from the cermet Be + alumina. Superior quality castings resulted.
Although embodiments of the invention have been described above, it is not
limited thereto and it will be apparent to those skilled in the art that numerous
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modifications form part of the present invention insofar as they do not depart from
the spirit, nature and scope of the claimed and described invention.
