Note: Descriptions are shown in the official language in which they were submitted.
75~L3~
LA~L W ~UJ U~ Nll0N
Investment casting, also referred to as the lost wax
process, is a casting process which is particularly suited for
the production o~ small metal parts having a high degree of
dimensional accuracy. The investment casting process is
widely used for the fabrication of blades and vanes for gas
turbine engines. Articles produced by this process h2ve the
advantage of requiring only minimal processing following cast-
ing. This process is discussed in U.S. Patents to Earl,
1,831,555; Watts, 3,590,905; Horton~ 3,686,006, and Moren,
3,179,523 and 3,196,505.
Turbine efficiency is closely related to operating temp-
erature. Demands for improved efficiency have resulted in
the development of more heat resistant alloys. Surface
condition also affects high temperature life and it is
important that the finished casting have a good surface
condition, one which is free from defects which may cause
subsequent failures.
A technique which has been employed to improve the high
temperature properties of superalloys is directional solid-
ification. In this technique a molten casting slowly solidi-
fied at a controlled rate so that the interface between the
molten and solidified portions of the alloy passes slowly
along the longitudinal axis of the part. One result of this
technique may be to produce a series of columnar grains with
the longitudinal axis of the grains being oriented with the
longitudinal axis of the casting~ Improved longitudinal
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high temperature properties are obtained as a result of the
reduction in grain boundary area perpendicular to the
longitudinal axis. This technique is described in the VerSnyder
Patent 3,260,505 which is assigned to the present assignee.
In the past, a common problem with nickel base super-
alloys used at elevated temperatures was a lack o~ ductility
at intermediate temperatures such as about 1400F. This lack
of ductility was responsible for many failures of turbine parts.
It was discovered that the addition of small amounts of
hafnium to nickel base superalloys greatly improved the
intermediate temperature of these alloys. Additionally such
hafnium additions were found to improve the transverse
mechanical properties of the castings. The addition of hafnium
to superalloys is discussed in U.S. Patents 3,677,747 and
3,711,337.
A casting defect has been observed in these hafnium
containing alloys which has not previously been noted in
superalloys. This defect is a surface defect having a resem-
blance to a crack or hot tear. The defect has been given the
name chain porosity. This defect is found in castings having
a change in cross sectional area of at least 1:2 in the vicinity
where the change in cross section occurs. When the moving
solidification interface passes from a portion of th~ casting
having a small cross sectional area ~o a portion of the cast-
ing having a larger cross sectional area it is believed that
a condition arises which leads to the formation to the surface
defect in the partially solidified casting. Careful studies
~7~43~
of the defect reveal that it has a crack-like morphology and
that the root portion of the crack contains hafnium/hafnium
oxide. Although chain porosity has only been observed in
hafnium containing alloys, those skilled in the art will
appreciate that the problem may arise in the future in alloys
which do not contain hafnium as more advanced alloys are
developed.
Accordingly it is the purpose of the present invention
- to disclose a mold which may be used for the directional
solidification of nickel base superalloys which will substan-
tially eliminate the problem known as chain porosity. A
further object of the present invention is the description
of the technique useful in producing composite molds useful
for the elimination of chain porosity in nickel base superalloy
castings.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The mold of the present invention is a composite mold
having an inner component comprised predominantly of alumina
and an outer component comprised predominantly of zircon.
The inner alumina component interacts with the molten and
partially solidified metal in such a way as to reduce and
eliminate chain porosity. The outer zircon component has
desirable mechanical properties both before and after casting
and easily removed fromthe finished part. The mold of the
present invention may easily be produced using conventional
equipment. Since the mold is predominantly zircon which is the
material presently used to produce investment casting molds,
i43~
the thermal expansion characteristics will be similar to those
of the molds presently used. Accordingly, the patterns used
to produce present molds will produce dimensionally satis-
factory molds using the process of the present invention.
The mold of the present invention has an inner component
comprised o~ at least 80 percent alumina having a thickness
of from 2 to 20 mils. The inner component also contains
silica which serves as a binder. The sum of the alumina and
silica components must excede 90%. The outer component is
comprised predominantly of zircon and has a thickness of from
.1 to .4 inches. Because alumina and zircon have different
coefficients of thermal expansion the thickness of the inner
component must be carefully maintained within the previously
described limits. If the thickness of the inner component
varies ou-tside of these limits satisfactory performance will
not be obtained and it is quite probable that the inner
alumina component will crack or will spall away from the outer
zircon component.
The exact cause of chain porosity is not known and
therefore the way in which the alumina component eliminates
chain porosity is not known, however, it has been observed
that the alloys in question, nickel base superalloys which
contain hafnium, wet alumina to a much greater extent than
they wet zircon. It is believed that this difference in
wetting significantly changes the heat transfer coefficient from
the mold to the molten metal and this change in heat transfer
coefficient quite likely changes the shape of the interface
~543~
between the solid and molten metal. It is believed that this
change in interface shape is responsible for the elimination
of the chain porosity problem. A minor drawback which has
been encountered in the use of an alumina mold surface is
that some slight reaction occurs between the molten metal and
the alumina. This reactivity is somewhat implicit since the
fact that the alumina is wet by the molten material implies
some chemical affinity between the mold material and the molten
metal. This slight reaction between the mold and the metal
has not been found to be deleterious and can be completely
removed by a sandblasting treatment and/or etching treatment.
Such reaction is entirely confined to the outer most layer
of a casting and the maximum depth of reaction has been found
to be not greater than 0~5~ mil. Although it might seem that
the problem to which the present invention is addressed might
be solved through the use of a shell comprised completely
of alumina this is not the case. An all alumina shell becomes
extremely strong and hard after it is heated to elevated
temperatures by the molten metal. This strength is retained
at room temperature and as a result it is difficult if not
impossible to remove such a shell from the solidified casting.
In addition, since alumina has a different coefficient of
thermal expansion, use of alumina molds would require an
expensive redesign of existing patterns. The dimensional
equivalence of the mold of the present invention with the
prior art molds is of great commercial significance. Through
the use of the composite mold of the present invention a
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desirable combination of the attributes of both the zircon
shell and alumina shell may readiLy be obtained. The resultan~
shell has adequate mechanical properties for the loads and
stresses applied during the casting process but is weak
enough to be readily removed from the solidified casting.
The thickness and composition of the alumina component are
critical to the proper functioning of the mold. In its final
dried form, the alumina component of the mold must contain at
least 80% alumina if satisfactory results are to be obtained.
Silica is the preferred binder, and the sum of the alumina
and silica should excede 90%. The average thickness of
the inner component must be restricted to the range of
2 to 20 mils and this restriction can be satisfied through
the selection of the slurry components from which the alumina
component is fabricated. The thickness of the alumina
component or prime coat is largely determined by the viscosity
and density of the slurry. For proper results the viscosity
, of the slurry must be from 13 to 19 seconds as measured in a
~4 Zahn cup at 75F. Two further restrictions are that the
specific gravity of the slurry must lie in the range of 2.~5
to 2.60 and the pH must be between 9 and ~0. This is in
conjunction with forced drying, as those skilled in the art
know that with air dried shells these parameters can vary.
As an alternate embodiment, a plurality of prime coats using a
lower viscosity slurry may be employed to achieve an inner
layer of a satisfactory thickness.
Within the restrictions set forth above we have obtained
particularly satisfactory results with an alumina containing
slurry described below: In the following description all
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parts are by weight and include the water necessary to obtain
the desired viscosity. From 20 to 30 parts of a 30 percent
aqueous solution of colloidal silica, from 50 to 70 parts of
alumina having a particle size of -325 mesh, from 10 to lS
parts of aLumina having a particle size -400 mesh, from 4
to 8 parts of alumina having a particle size of -100 mesh
and from 0 to 5 parts of kyanite having a -100 mesh particle
size. We have found that the addition of from 1 to 5 parts
of cobalt al~lminate to the slurry mixture produces a signifi-
cant improvement in surface finish. The cobalt aluminateaddition is preferred, but not required. In the preceding
description the colloidal silica component is a binder which
insures strength and adherence of the alumina component. At
high temperatures the silica reacts with the alumina to form
complex aluminates which bond the alumina particles together.
The particular selection of mesh sizes given above has
produced extremely satisfactory results in that an alumina
component produced with this slurry has an e~ceptional com-
bination of mechanical properties and good surface finish.
It will be appreciated by those skilled in the art that the
exact combination of particle size and distribution of par-
ticle sizes in a slurry has a significant effect on viscosity
and density. A slurry made according to the previous descrip-
tion has a density and viscosity falling withln the l;mits
previously described.
Alumina occurs in a wide variety of crystal structures
and particle sizes. Examples of different types of alumina
include tabular alumina, fused alumina and calcined alumina.
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As a general rule these types of alumina are broadly inter-
changeable in the production of the present invention.
~lowever, a particularly preferred embodiment is set forth
below in Example III.
The present invention will be made more clear through
consideration of the following illustrative examples which
deal with the production of experimental nickel base superalloy
castings in a variety of types of shell molds. The alloy
used has a nominal composition of 9% chromium, 10% cobalt,
12.5% tungsten, 1% columbium, 2% titanium, 5% aluminum, 2%
hafnium, .015% boron, .1% carbon, balance essentially nickel.
This alloy was cast from a temperature of about 2800F into
a variety of molds described below having an internal shape
corresponding to that of a turbine blade. In the molds the
blade section which had a smaller cross sectional area than
the root section as located beneath the root section so that -
during directional solidification the solidification front
passed from the small cross sectional area of the blade to
the larger cross sectional area of the root. When chain
porosity was encountered it was located in the near area of
change in cross section. The castings were solidified at a
rate of approximately 8 inches per hour using a withdrawal
technique in which the mold was withdrawn from the furnace at
a rate of about 8 inches per hour. All molds were produced
using a conventional shell mold preparation technique which
basically involves dipping a wa~ pattern into a ceramic slurry
and applying a relatively coarse dry ceramic material (called
stucco) to the wet slurry surface. This procedure is followed
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a number of times with intermediate drying steps until a
desired mold thickness is built up.
Example I
Conventionally all zircon molds were produced using the
technique described above. The details of the compositions
of the different slurries and dry ceramic mi~tures are given
in Table I. Several molds were produced and test castings
were made using these molds. A very high incidence of chain
porosity was observed and a representative photomicrograph
showing this chain porosity is shown in Fig. 1. Fig. 1 sho~7s
a chain pososity having a depth of approgimately 10 mils into
the body of the blade. Such a defect is obviously detrimental
to the strength and useful life of an article such as the tur-
bine blade which is subject to high stresses at elevated
temperatures.
Example II
Several composite molds were produced according to
the present invention using the sLurry and stucco sequence
shown in Table II. Identical castings were made in these
molds. These castings were found to be characterized by a
complete absence of chain porosity. The molds were easily
removed from the castings following solidification and the
surface of the castings was easily cleaned using conventional
abrasive techniques. The alumina slurry composition described
in Table II has given excellent results and is a preferred
embodiment.
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Example III
An all alumina shell was produced using the stucco
and slurry sequence shown in Table III. Castings were
successfully made in these molds and showed no evidence
of chain porosity, however, the molds were extremely
difficult to remove from the solidiEied casting and the
dimensional accuracy of the final castings was not as
good as those produced by the mold of the present
example in Table II.
Although the invention has been shown and described
with respect to a preferred embodiment thereof, it should
be understood by those skilled in the art that various
changes and omissions in the form and detail thereof may
be made therein without depar~ing from the spirit and the
scope of the invention.
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