Note: Descriptions are shown in the official language in which they were submitted.
1 339 1 84
CERAMIC SHELL MOLDS AND CORES FOR CASTING OF
REACTIVE METALS
Introduction and Backqround
The present invention relates to ceramic shell molds for
the casting of reactive metals, such as titanium, and process
of making molds. In more detail, the invention pertains to a
slurry containing yttria refractory used as casting composi-
tions to form shell molds, and processes for preparing same.
More particularly, the invention pertains to a slurry com-
prising yttria refractory, an acid, and an organic solvent,
for making ceramic shell molds for casting of reactive metals
such as titanium and titanium alloys. The compositions of
the present invention are characterized by having a desirable
shelf life and improved stability. Molds produced in accor-
dance with the present invention are particularly useful
because they enable the casting of reactive metals with
minimized or essentially no alpha case.
In a further aspect, the present invention relates to
ceramic and foundry cores and the yttria slurries used in
making them.
Much effort has been devoted for the past 25 years to
providing compositions and methods for casting reactive
metals, particularly titanium and its alloys, into ceramic
molds. This development and the interest in providing this
capability was stimulated by activity in the nuclear and
aircraft industries where it was necessary to search for high
strength and lightweight metals. Titanium, because of its
~i' ~
1 339 1 84
_ - 2 -
high strength to weight ratio is sought after for use in the
aircraft industry.
The melting point of titanium metal is almost 3100~F
and, in the molten condition, reacts with most refractories.
Earlier attempts to cast titanium into ordinary foundry molds
were unsuccessful due to the undesirable chemical reactions
between the hot metal and the surfaces with which it came
into contact. For example, reduction of the silica present
in such foundry molds produced heavy reaction zones on the
casting surface. This reaction layer is known in the indus-
try as "alpha case" and the problems associated therewith
have been described in detail in the literature.
Machined and formed graphite molds have been used com-
mercially to make titanium castings. Such molds can be made
in such a way as to minimize the alpha case layer. The U.S.
Bureau of Mines has supported research for almost 25 years on
the casting of refractory metals. Thus, there have been
continuing efforts to search for new materials and methods to
reduce or eliminate alpha case.
The use of graphite in investment molds has been de-
scribed in the art in such patents as U.S. 3,241,200;
3,243,733; 3,256,574; 3,266,106; 3,296,666 and 3,321,005 all
to Lirones. Other prior art which show a carbonaceous mold
surface utilizing graphite powders and finely divided inor-
ganic powders called "stuccos" are Operhall, U.S. 3,257,692;
Zusman et al., U.S. 3,485,288 and Morozov et al., U.S.
- 1 339 1 84
_ -3-
3,389,743. These documents describe various ways of obtain-
ing a carbonaceous mold surface by incorporating graphite
powders and stuccos, various organic and inorganic binder
systems such as colloidal silica, colloidal graphite, syn-
thetic resin which are intended to reduce to carbon during
burnout, and carbon coated refractory mold surfaces. These
systems were observed to have the disadvantage of the neces-
sity for eliminating oxygen during burnout, a limitation on
the mold temperature and a titanium carbon reaction zone
formed on the casting surface.
Further developments including variations in foundry
molds are shown in Turner et al., U.S. 3,802,902 which uses
sodium silicate bonded graphite and/or olivine which was then
coated with a relatively non-reactive coating such as alumi-
na. However, this system still did not produce a casting
surface free of contamination.
Schneider, U.S. 3,815,658 shows molds which are less
reactive to steels and steel alloys containing high chromium,
titanium and aluminum contents in which a mangnesium oxide-
forsterite composition is used as the mold surface.
A number of attempts have been made in the past to coat
the graphite and the ceramic molds with materials which would
not react with the reactive metals being cast. For example,
metallic powders such as tantalum, molybdenum, columbium,
tungsten, and also thorium oxide had been used as
1 339 1 8~
_ - 4 -
non-reactive mold surfaces with some type of oxide bond. See
Brown, U.S. Patents 3,422,880; 3,537,949 and 3,994,346.
Operhall, U.S. 2,806,271 shows coating a pattern materi-
al with a continuous layer of the metal to be cast, backed up
with a high heat conductivity metal layer and investing in
mold material.
Basche, U.S. 4,135,030 shows impregnation of a standard
ceramic shell mold with a tungsten compound and firing in a
reducing atmosphere such as hydrogen to convert the tungsten
compound to metallic tungsten or tungsten oxides. These
molds are said to be less reactive to molten titanium but
they still have the oxide problems associated with them.
Brown, U.S. 4,057,433 discloses the use of fluorides and
oxyfluorides of the metals of Group IIIa and the lanthanide
and actinide series of Group IIIb of the Periodic Chart as
constituents of the mold surface to minimize reaction with
molten titanium. This reference also shows incorporation of
metal particles of one or more refractory metal powders as a
heat sink material. However, even those procedures have
resulted in some alpha case problems.
A development by General Electric has provided barrier
layers of refractory oxide in a silica bonded mold for cast-
ing alloys containing significant amounts of reactive metals;
see Gigliotti et al. U.S. 3,955,616; 3,972,367 and 4,031,945.
'~-
~,.
1 339 1 8~
-5-
Huseby, U.S. 4,240,828 shows doping a nickel and cobalt
alloy with a rare earth metal and casting into a ceramic
mold.
In the 1960's, developments at Wright Air Development
Center led to the formation of a crucible for melting titani-
um formed from a titanium enriched zirconium oxide crucible
with less reaction to molten titanium than pure zirconium
oxide.
Richerson, U.S. 4,040,845 shows a ceramic composition
for crucibles and molds containing a major amount of yttrium
oxide and a minor amount of a heavy rare earth mixed oxide.
Such methods including the making of a titanium metal en-
riched yttrium oxide were only partially successful because
of the elaborate and expensive technique which required
repetitive steps.
Molds for casting molybdenum made from zirconium acetate
bonded calcia stabilized zirconium oxide have been made by
the Bureau of Mines.
Feagin, U.S. 4,415,673 discloses a zirconia binder which
is an aqueous acidic zirconia sol used as a binder for an
active refractory including stabilized zirconia oxide thereby
causing reaction and gelation of the sols. Solid molds were
made for casting depleted uranium. A distinction is made in
this patent between "active" refractories and refractories
which are relatively inert. The compositions of Feagin are
1 339 1 84
-6-
intended to-contain at least a portion of active refractor-
ies. See also Feagin, U.S. 4,504,591.
Adhesive plasters made of a suspension of oxide powder,
such as yttrium oxide and an acid are shown in Holcombe et
al., U.S. 4,087,573. These compositions are described as
being spontaneously hardening and useful for coating surfaces
or for casting into a shape. Of particular interest is the
coating of graphite crucible used in uranium melting opera-
tions.
It is generally recognized in the industry that all
commercial processes have some alpha case on their casting.
This may range from about 0.005 inches to 0.04 inches in
thickness depending on process and casting size. The alpha
case must be milled off by chemical means or other means from
the casting before a satisfactory casting is obtained. The
extra cost imposed by the chemical milling operation is a
disadvantage and presents a serious problem from the stand-
point of accuracy of dimensions. Normally, the tooling must
take into consideration the chemical milling which results in
the removal of some of the material in order to produce a
casting that is dimensionally correct. However, since cast-
ing conditions vary, the alpha case will vary along the
surface of the casting. This means that there is a consider-
able problem with regard to dimensional variation.
Some refractory compositions have been developed that
exhibit reduced alpha case and can be used successfully to
1 339 1 84
-7-
make production castings by applying the coatings to the wax
patterns by special techniques, such as spraying. However, a
difficulty arises in that certain refractory mixes do not
have a long pot life and gel quickly, even spontaneously with
stirring in a few minutes, depending upon exact composition.
See Holcombe et al., U.S. 4,087,573.
Accordingly, it would be highly advantageous to have a
slurry that is stable at least for several days and prefera-
bly for several weeks in order that patterns of the desired
shape may be dipped into the slurry according to present
production practice.
SummarY of the Invention
The present invention pertains to yttria used in making
ceramic shell molds, and in particular to slurry compositions
comprising yttrium oxide refractory, an acid, and an organic
inert solvent. The yttria refractory can be either fused or
unfused, and can be present alone, or along with some other
refractory if desired. It is preferred that the yttria be
fused and ground to appropriate flour size, but unfused
yttria or sintered yttria can also be used. The fused re-
fractory allows for somewhat higher filler loading which
decreases the tendency toward minute cracks on firing.
Blends of yttria, fused or unfused, with other refractories
such as zirconium oxide can be employed in making a mold
coating composition.
- 1 3391 84
-8-
The invention further relates to the reactive composi-
tions of the yttria refractory, optionally with other refrac-
tory materials, and the mold coatings and cast shapes pre-
pared therefrom. The invention also pertains to the method
of making the coating compositions and methods of making
molds. The invention is of particular interest with respect
to the casting of titanium and titanium alloys such as
Ti6A14V.
In a further aspect of the invention, the invention
pertains particularly to coatings and molds, ceramic and
- foundry cores for metal casting, and to the use of a slurry
of yttria that has good stability and can contain a variety
of other refractories, metal powders, and fibers for various
applications and purposes.
An object of the invention is to provide a stable
composition suitable for making a mold coating which is
less reactive with titanium and titanium alloys during
casting.
Another object of the invention is to provide a low
reactivity mold coating which is characterized by good sta-
bility for reactive metal casting.
A further object of the invention is to provide a ceram-
ic mold having a casting surface having low reactivity with
reactive metals.
~,~
~,
1 3391 84
g
A still further object of the present invention is to
provide a process for making an investment casting mold
having low reactivity with reactive metals.
A further object of the invention is to provide a ceram-
ic shell mold with a relatively reaction-free surface against
which metals may be poured.
Yet another object of the invention is to provide a
castable refractory composition of good stability, using a
slurry of yttria, and the resulting ceramic body.
A still further object of the present invention is to
provide a process for making an investment casting mold
having low reactivity with molten titanium, titanium alloys,
zirconium and zirconium alloys.
A further object of the present invention is to provide
a coating composition suitable for spraying or painting of a
foundry mold, foundry and ceramic cores, melting crucible,
ladle, or pouring basin to make it more resistant to reactive
metals.
A still further object of the present invention is to
provide ceramic and foundry cores utilizing a slurry of
yttria.
As used herein, the term "reactive metals" means those
metals and alloys which may react with or produce a relative-
ly rough mold surface when poured into ordinary investment
casting molds having mold surfaces containing one or more of
the following refractories: silica, aluminà, aluminosili-
1 339 1 84
-- -10-
cates, zirconium silicate (zircon) or other oxides and mixed
oxides normally used in investment casting molds. Examples
of these reactive metals are titanium, titanium alloys such
as Ti6A14V, zirconium, zirconium alloys, high carbon steels,
eutectic alloys (containing appreciable amounts of tungsten,
hafnium, carbon, nickel, cobalt, etc.), aluminum-lithium
alloys, nickel base alloys containing appreciable amounts of
titanium or aluminum or hafnium or tungsten. The reactive
metals are well known in the art and one of the most reactive
of all of these metals is titanium.
In accordance with the present invention, there is
provided a slurry or suspension type composition containing
yttria in finely divided form, either fused or unfused, as
the main component. Any additional compatible finely divided
refractory can be blended with the yttrium oxide. In partic-
ular, suitable refractory materials for this purpose include
zirconium oxide, fused zirconium oxide, monoclinic zirconium
oxide, cubic zirconium oxide, fused stabilized zirconium
oxide having as the stabilizing agent a member selected from
the group consisting of calcium oxide, magnesium oxide,
yttrium oxide, lanthanum oxide, dysprosium oxide, and other
rare earth oxides, blends of zirconium and other constituents
in this group, and fused blends of zirconium with other rare
earth oxides, and mixtures of any of the above. Generally,
the yttria is present as the major refractory component when
blended with other refractory materials as mentioned above.
1 339 1 8~
- 1 1
The term ~rare earth" is used herein to denote a member of
the Periodic Table of Elements with an atomic number of 57
to 71.
In a further aspect, the invention resides in a process
for making an investment casting mold having low reactivity
with reactive metals comprising providing a stable slurry
composition or suspension of yttria refractory, organic
solvent and acid, applying the slurry to a pattern, exposing
the coated pattern to appropriate conditions of moisture to
facilitate the formation of a "green bond" and thereafter
firing to produce a fired bond.
A still further feature of the invention resides in a
process for making a ceramic shell mold comprising coating
the outside of a pattern with the afore-described slurry
comprising yttria, acid, solvent and any of the above-
mentioned additional refractory materials, drying and heating
and then firing to form the intended shell mold. In an
alternative procedure, before the coating is dry, there is
applied thereto a finely divided refractory called a "stuc-
co'~. The art of stuccoing is well understood. After the
stucco is applied, the coating is allowed to dry, the shell
is removed from the pattern and fired at a sufficiently high
temperature to bond the refractory together.
Also, a further feature of the invention resides in a
process for casting a reactive metal in a mold having low
reactivity with reactive metals comprising providing a stable
1339~84
~ -12-
suspension of yttria refractory, organic solvent and acid,
optionally with other refractory material and/or some water
to form a coating composition, applying said coating composi-
tion to a pattern shaped in the desired configuration, allow-
ing the coating react with moisture to gel, heating the
resulting coated pattern to a sufficiently high temperature
to fire and thereby fusing said coating composition into the
desired shape, and thereafter casting said metal into the
said desired mold.
A still further feature of the invention resides in a
process for making a core wherein a suitable core mold is
coated with a slurry comprising yttria, acid and organic
solvent. After coating and while the coating is still wet,
it can be stuccoed by application of a suitable refractory
such as for example fused yttrium oxide and thereafter per-
mitted to dry. After drying a castable mix of any compatible
refractory is added to the core mold having the coating
applied thereto and the mix is permitted to gel. After
gelation, the core can be removed from the mold and fired to
bond the coating to the cast back up to thereby form a fin-
ished core.
Detailed Description of the Invention
The compositions of the present invention can be charac-
terized as suspensions comprising yttria refractory, an inert
organic solvent and a suitable acid. It has been found that
- 1 339 1 84
_ -13-
a small amount of water can be tolerated in these composi-
tions. Typically, the acid used for purposes of the inven-
tion, for example, HCl, HNO3, etc., will contain an apprecia-
ble amount of water even though the acids are used in small
amounts in relation to the total amount of slurry. In carry-
ing out the invention, care should be taken to control the
amount of water as the more water that is present, the short-
er will be the shelf life ("pot" life) of the slurry. Under
ideal conditions, no water should be added to obtain longtime
shelf life of the slurry; i.e. up to several months of sta-
bility. However, the slurries of the invention can tolerate
some water especially if it is desired to use the slurry
quickly. Also, such factors as amount of exposure to the
atmosphere, particle size of the yttrium oxide refractory,
amount of acid used, and whether one wishes a fast setting
slurry will influence the water content. If the slurry is to
be used quickly as for spraying or drying, then more water
should be added so that some setting takes place rapidly
during the drying operation and to be less dependent on the
humidity conditions of the atmosphere. These are matters that
will be apparent to persons skilled in the art after a start-
ing of this specification.
The yttria slurry is prepared by mixing the components
thereof in any convenient manner using conventional equip-
ment. The yttria is in finely divided form, sometimes called
"flour". The term "flour" is commonly used in the foundry
X
1 339 1 84
_ -14-
industry to signify the finely ground refractory materials
that are commonly used to prepare slurry compositions.
Particle sizes can vary considerably and still be suitable
for the intended purposes. In general, however, in the
investment casting industry, it means particle sizes below
150 microns can designate sizes down to 1 to 10 microns. A
common flour size used in the industry is a flour containing
particles essentially 75~ finer than 325 mesh (44 microns)
and usually has a wide distribution range. The "mesh" sizes
refer to U.S. Standard Screen Series.
Another commonly used size in the industry is generally
referred to as 325 mesh flour and is understood to mean that
at least 95~ of the particles pass through a 325 U.S. Stan-
dard Screen mesh. In this instance, 95~ of the flour parti-
cles are finer than 44 microns. Commonly used flours are
-150 mesh, -200 mesh and -325 and this is understood to mean
that the particles are sufficiently ground so that at least
96~ of the particles pass through the designated screen. The
distribution of particle size ranges is not generally provid-
ed but can be determined by means known in the art. Produc-
ers of such flours have been known to provide what is called
a "typical" screen analysis of a particular grade with no
guarantee that each lot will be the same or conform to the
analysis of the "typical" batch. Essential for purposes of
the invention is that the flour be of sufficiently fine
~ .
,;.~ .
1 339 1 84
-15-
particle size so that a smooth surface will be provided on
the mold.
During the slurry preparation, the acid present will
react with some of the yttrium oxide to form an yttrium salt
of the acid. This salt then reacts in a manner that is
similar to that of calcium sulfate when exposed to moisture;
namely, it forms a hydrate of the salt which constitutes the
"green bond" when the coated pattern is exposed to a moist
atmosphere. This hydrate then dehydrates upon firing to form
yttrium oxide which'becomes the fired bond.
For purposes of the present invention, refractory pow-
ders or aggregate suitable for use in addition to the yttrium
oxide for the mold compositions and coatings are those of the
following group: monoclinic zirconium oxide, yttrium oxide,
cubic zirconium oxide, fused yttrium oxide, fused zirconium
oxide, fused stabilized zirconium oxide having as the stabi-
lizing agent a member from the group of calcium oxide, magne-
sium oxide, yttrium oxide, scandium oxide, the oxides of the
lanthanides of the Periodic Table; e.g. lanthanum, cerium,
dysprosium, praseudymium, neodymium, samarium, and other rare
earth oxides, blends of zirconium and yttrium oxides or with
other constituents in this group and fused blends of zirconi-
um and/or yttrium oxide with other rare earth oxides. These
may be used in with the yttria in the slurry, or mixtures of
one or more thereof may be used.
133918~
-16-
In accordance with the invention, the slurry also con-
tains as an essential ingredient an acid. The acid used may
be any acid that will react with the yttria under normal room
temperature conditions to form a salt that will, in turn,
form a hydrate. The salt preferably should be totally solu-
ble in the solvent, but partial solubility is satisfactory.
The acid also should be essentially soluble in the solvent
and should be relatively stable under ambient conditions
except for any reaction with the yttria refractory. Examples
of suitable acids are organic mono- and polycarboxcylic
saturated acids such as formic, acetic, propionic, citric,
succinic, oxalic, tricarballyllic, phthalic, maleic and
tartaric. Inorganic acids include sulfuric, nitric, hydro-
chloric and sulfamic. Others may also be used. The concen-
tration of the acid must not be too great so as to apprecia-
bly dissolve the yttrium oxide or other refractory substanc-
es; it must also not be dilute with water as that will con-
tribute too much water to the system.
The solvent that is present in the slurry in accordance
with the invention should preferably be one that does not
have a high vapor pressure at room temperature which may
cause cooling and possible cracking of the wax pattern upon
applying the slurry. It should also not have an extremely
low vapor pressure, taking days to dry on the wax pattern.
The solvent preferably should be water soluble or partially
so to facilitate the hardening of the- coating. Solvents
1 3391 84
,. ,
-17-
that can be used in accordance with the invention are gener-
ally inert, organic solvents including but not limited to
ketones, lower alkanols and esters such as acetone, methyl
isobutyl ketone, methanol, ethanol, butanol, isobutanol, n-
propanol, isopropanol, hexanol, methyl ethyl ketone, ethyl
acetate, methyl acetate, isopropyl acetate, 1,4-dioxane,
ethoxy ethanol (cellosolve), methoxy ethanol (methyl cello-
solve), methoxy isopropanol, and others. Blends of solvents
can also be used. Higher boiling organic solvents, such as
propylene glycol monomethyl ether, carbitol, may also be used
in whole or in part but drying time of the coating is extend-
ed.
Yttrium salts have the characteristic of forming a
hydrate or hydroxysalt in the presence of water. (New Yttria
Plasters, C.E. Holcombe, et al., U.S. Department of Energy,
Report Y-2104, January 1978). For example, yttrium oxide,
when mixed with a dilute acid, such as nitric acid, to form a
slurry will gel up into a hardened condition within a short
period of time, generally within several minutes.
Because of the reaction of the yttrium salt with water
and resulting in a short pot life a slurry can be prepared
using a solvent containing no H2O or having the H2O at a
minimum so that the slurry will be stable Lor days and weeks.
When water is used as the suspending medium the slurries gel
or "set" very quickly within minutes or a few hours making it
unsuitable for some purposes. The wax pattèrns may be dipped
1 339 1 84
-18-
into the slurry and exposed to the atmosphere which contains
some moisture. The moisture in the atmosphere will enter
into the reaction with the yttrium salt formed to form a
hydrated salt and cause hardening of the coating on the
pattern. It may be preferable to expose the moist dipped
pattern instead to a controlled high humidity atmosphere and
then drying to produce stronger coatings and to have better
production control. This technique enables a stable slurry
to be used for dipping patterns and then to harden the coat-
ing on the pattern after dipping and treatment with stucco.
It may be advantageous to include in the slurry certain
other materials such as plastic latexes, film-formers, solu-
ble plastic materials, organic or inorganic fibers, other
refractory fillers, etc. The inclusion of certain film
formers or plastic materials may be desirable to minimize or
prevent penetration of backup slurry media through the first
coat or to minimize any spalling or cracking during dewaxing
of the finished ceramic sheil. Such additives and adjuvants
are well known in the art for this purpose.
In another embodiment of the invention, it is desirable
to include an yttrium salt in the slurry for greater control
of the setting action of the coating. The yttrium salt
should be soluble in the solvent and can be used in place of
the yttrium oxide and acid combination. Thus, the yttrium
salt can replace all, or part of, the yttrium oxide and acid
in the slurry composition. The mixture of the yttrium salt
1 339 1 84
- 1 9 -
is set forth above and one that is capable of forming a
hydrate with moisture. Examples of such salts are the ace-
tate, nitrate, chloride, sulfates and any other salt capable
of forming a hydrate with water.
A number of experimental slurries were made to determine
shelf life of the slurries. These are shown in Tables 1 and
2. These tables illustrate a number of examples of types of
solvents and acids used. Unfused Y203 powder (made by pre-
cipitation and drying) and sintered Y203 powders were also
used. The influence of water additions is noted on the shelf
life of the slurry. The presence of water reduces the shelf
life and therefore the amount of water should be controlled
as previously indicated.
With regard to the yttria powder used in the examples,
two types of powder were used. One was obtained from the
producer and used as such. The other is a highly sintered
powder adjacent to a fused yttrium oxide ingot. The sintered
powder is densified from that received from the producer, but
is not densified completely to a fused product. Fused ingots
were made which were crushed and ground to appropriate sizes
for later experiments. Any suitable yttria powder can be
used for purposes of the invention such as are commercially
available.
1 33ql 84
-20-
TABLE 1
R08402
Formula No. -37 -38 -39 -41 -42 -48 -53 -17
Composition:
Y2O3 Powd. 25 25 25 25 25 25 25 40
Isopropanol
99.9~ 25 25 25 25 25 25 25 25
Citric Mono-
Hydrate Acid 2.5 1.25 15.0 17.5
85~ Lactic Acid 2.5
Stability 6 mo. 6 mo. 6 mo. 6 mo. 6 mo. 6 mo. 6 mo. 6 mo.
Glac. Acetic 10.0
Conc. Nitric 10.0 5.0
1 339~ 84
-21 -
~ o ~ ~, o o ~
__ ~ B ~ ~ ~
o o ~ ~ o :~
o ~ ~ o o
o ~ o
' O ~i
F~ ~ B
~ o ~ ~ o ~ ~
_ _ _
_ _ ~
~ ~ ~ ~ ~ t
_ _ ~
~ ~ ~ ",
0~,~ ¢ ~ ¢ ~¢ _~ ~9 ¢ ~ e-
. 1 33ql 84
-22-
A number of the slurries were deposited on a sheet of
wax, the excess slurry drained off, and exposed to the atmo-
sphere having a humidity above 50~ and in most cases as high
as 75-80~. These were dried overnight and examined for film
strength. Slurries 37, 38, 39, 41, 17, gave some films that
were abrasion resistant with 17, 25, 39, 41 being quite hard
and strong. Films from -60 and -61 required extensive drying
times to acquire strength. Film strengths on 75, 76 and -77
were very strong and abrasion resistant as were 81 and 82.
Films from 88 and 89 were very hard and strong after complete
drying.
A number of slurries were used to deposit films on wax
and immediately after deposition the wax sample with film was
placed into a container above water which produced a high
humidity atmosphere. These films were quite hard after
drying.
In preparing molds for casting in Ti6A14V alloy a number
of pattern wax bars approximately 1/2" to 5/8" in diameter
were cut into lengths (fingers) about 4" long. The applica-
tion of the coating was by dipping the finger into the slurry
or spraying the coating on to the wax finger leaving about
1/2 inch of the finger free. After dipping and draining the
moist coating was stuccoed with a coarse fused yttrium oxide
of approximate particle size of -40 + 140 mesh. The coating
was then allowed to dry in the atmosphere which was above 50
relative humidity. After drying the pattern was then sealed
1 339 1 8 ~
-23-
to a central wax sprue along with several other experimental
coated fingers. The uncoated wax sprue was then painted with
a fused pure yttrium oxide slurry in colloidal yttria and
stuccoed with the fused yttria stucco. It was allowed to dry
and then dipped into a backup slurry and stuccoed. The back-
up slurry is composed of 30~ aqueous silica sol mixed with
Remasil 60, 325 mesh to a viscosity of 25 seconds #4 Zahn
cup. Seven back-up coats were used. This coating was al-
lowed to harden and was then recoated with the same slurry,
and then stuccoing with Remasil 60 having a 50 mesh grain
size. This was repeated with the 50 mesh grain size stucco
until the total number of coats had been applied to the
pattern. The last coating was not stuccoed. After all coats
were applied, the molds were allowed to dry thoroughly for
several days before dewaxing, although a long dry time is not
essential. Remasil 60 is an alumino-silfcate refractory of
approximately 60~ A12O3 content supplied by Remet Corporation
of Chadwicks, New York.
After the final coat was applied, usually 6-8 coats, no
stucco was applied to the final coat, the mold was allowed to
dry at room temperature. It was then immersed in hot motor
oil to remove the wax leaving a shell mold. The resulting
shell mold was fired to 2500~F for two hours and cooled to
room temperature.
As a variation of each of the facecoats, a small amount
of an acrylic vinyl latex could be added to the slurry to aid
~,,
1 339 1 84
-24-
in continuous film formation, better adhesion to the wax bar,
and to prevent possible penetration of the silica binder to
the mold surface during dipping of the backup coats. Several
known types of latex can be used if compatible with the
slurry.
The refractories and other materials that can be used in
preparing the slurries used in accordance with the invention
are described as follows:
1. Vinyl-acrylic latex, a commercial product available
from several sources; e.g. Air Products Co., used to provide
improved film forming properties to the slurry and to prevent
penetration of liquids from subsequent coats to the mold
surface.
2. A low-foaming wetting agent such as Sterox NJ a
product of Monsanto Chemical Co. can be used.
3. 2-ethyl hexanol, a commercial chemical, can be used
as a defoaming agent.
4. Glacial acetic acid, a standard commercial product.
5. Fused yttrium oxide, made by electrically fusing a
99.9~ Y2O3 powder and grinding to 200 mesh powder showing
1.9~ + 200 mesh.
6. Yttria stucco is the same product as 5. but -40 +
100 mesh particle size.
Each mold was then attached to a sprue connector in the
casting box to a centrifugal casting machine. Foundry sand
with a minor sodium silicate bond was rammed around the molds
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in the box. After drying, the box was put under vacuum and
degassed in a large chamber and molten commercial titanium
alloy 6A14V was poured into the molds under vacuum.
After casting, the molds were cooled, fingers were cut
off, sectioned, embedded in plastic and metallographically
polished, etched to show up the alpha case, and examined
microscopically for alpha case.
Table 3 shows the slurry formulations used for the mold
surfaces for each pattern cast.
Table 4 shows the average alpha case measurements on
each finger casting. These are very low alpha case values.
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_.
TABLE 3
SLURRY FORMULATIONS
Pattern SA SB SC SD SJ SK SL
Slurry No. 422 423 424 425 460 461 462
Composition:
Citric Acid
Monohydrate 3.125 5.8 3.125 5.8 5.8 5.8 5.8
99~ Isopropanol 12.5 12.5 12.5 12.5
5~ Nirez Resin 12.5
10~ Nirez Resin 12.5
15~ Nirez Resin 12.5
Unfused Y2O3 Powder 13.4 13.4
Approx. 325 Mesh
Fused Y2O3 75 75 40.2 40.2 70 70 70
TABLE 4
AVERAGE ALPHA CASE DEPTH MEASUREMENTS
SampleCase Depth x 0.0001"
SA g
SB 13
SC 12
SD 21
SJ 12
SK g
SL 9
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w
On Table 4, it is noted that samples SA, SK and SL had
very low alpha case. Samples SB, SC and SJ were also good
but contained somewhat higher alpha case. Sample SD is
higher and may be due to a process defect unknown at this
time.
Relative to the proportions of refractory in yttria
slurry, in the case of relatively "inactive" refractories
such as monoclinic zirconia, tabular alumina, fused silica
and zircon, these can vary widely depending upon their parti-
cle size distributions, the specific gravity of the refracto-
ry, the manner of processing such as injection molding,
casting, pressing or dipping, and the application of the mix.
In general, when slurries are made for dipping investment
casting patterns with fused yttria refractory flour of about
325 mesh, a ratio of one part yttria flour to one part vehi-
cle is about the minimum. As much as two parts of refractory
can be used to produce a thick slurry coat. Variations in
these proportions may be made depending upon the particular
results desired.
It is further advantageous to include refractory fibers
with the yttria slurry to produce high temperature refrac-
tories. Such fibers include silicon carbide, silicon ni-
tride, carbon fibers, alumina fibers and the like.
Many of the compositions produced from the yttria slurry
refractory fiber system may be used for special coatings and
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for casting shapes when a gelling agent or active refractory
is used with the yttria slurry.
The separate slurries made with yttria powder, zircon,
alumina and fused silica were deposited on a wax pattern and
allowed to dry. The resulting coating was strong and resist-
ed scrapping with a knife. Protective coatings may be ap-
plied to many types of surfaces such as ceramics, metals,
foundry molds, and for electronic applications. For example,
a slurry of citric acid, isopropanol and fused yttria can be
used to spray paint or coat a refractory melting crucible to
minimize metal crucible reaction. It may also coat a pouring
basin or ladle to minimize reaction. In particular, a thin
surface layer may be sprayed on to a cope and drag foundry
mold and dried to form a strong non-reactive coating for
protection when reactive metals, such as titanium are poured
into the mold. A single layer is usually sufficient to give
good protection.
A preformed ceramic casting core suitable for casting
molds for titanium casting was made by coating the inside of
the core mold with, for example, a slurry composed of fused
yttrium oxide, citric acid and isopropanol to a viscosity of
about 20 seconds #4 Zahn cup. A few drops of a non-ionic
wetting agent, Sterox NJ, can be added to facilitate wetting
of the mold surface. After coating and while the coating is
wet, it can be stuccoed with a -40 + 100 mesh fused yttrium
oxide grain. The coating can then be allowed to dry at room
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temperature. After drying a heavy castable mix of a fused
silica refractory of varying particle size distribution from
20 mesh and down and a prehydrolyzed ethyl silicate binder
containing 20~ SiO2 and some ammonium carbonate gelling agent
can be added to the core mold with the above coating and the
mix allowed to gel. After gelation, the core can be removed
from the mold. It can then be fired sufficiently high to
bond both the coating and the cast backup to form a finished
core ready for casting.
The refractory mix suitable for the bulk of the core may
be any mix that will be compatible with the coating and is
primarily used to provide a backing or support for the coat-
ing or core surface. If titanium or other reactive metal is
poured against the core, the refractory might be yttria. If
the metal is less reactive than titanium, it may be alumina,
zirconia or some other refractory.
A ceramic core suitable for reactive metal casting and
particularly titanium can be made by making a yttria type
refractory or other and casting or injection molding or
pressing into a suitable mold and allowing the slurry to gel.
The gelled body can then be dried and fired and used as a
casting core.
Further variations and modifications will be apparent to
those skilled in the art from a reading of the foregoing and
are intended to be encompassed by the claims appended hereto.
. .
