Note: Descriptions are shown in the official language in which they were submitted.
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T~OD OF FO~U~ING ~U~TIC~E WIT~ ~iRI ~ ~E ~ ~OY
CO3oeO-QI~ION
_ _
The subject application relates to copending
applications Serial No. (Attorney Docket RD-17,399)
filed ; Serial No. (Attorney Docket RD-18,041)
filed ; and Serial No. (Attorney Docket RD-20,134)
filed The text of the related applications are
incorporated herein by reference.
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The pressnt invention relates to the formation of
structures with variable alloy composition. More
particularly, it relates to a method by which structures can
be formed having an inner portion of one composition and an
outer portion of a different composition.
It is known that superalloys including nickel base and
iron base superalloys have been employed extensively in
applications which require high strength at high temperature.
The design of jet engines has in large part been determined
by the properties which superalloys used as fabricating
materials for components of the engine can display. As the
properties of the alloys are improved the design of the jet
engine improves and greater thrust to weight ratios are
achieved. Generally, higher temperature operation results in
greater fuel efficiency for such engines and the drive for
higher operating temperatures and for superalloy materials
which can operate at such higher temperatures is a continuous
design criteria for fabrication of more and more efficient
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jet engines. The need for higher temperature capability in
high strength superalloys continues as efforts are made to
continue to improve operatlng performance for jet engines.
Many metallurgical advances have assisted in improving
high strength superalloys. These have included the increase
in the precipitate volume fraction for the gamma prime
precipitate strengthening agent of s~ch alloys. Also
improvements have been made through powder metallurgy and
through the use of isothermal forging. Improvements in the
alloy temperature capability of superalloys have been
achieved in this way. It has also been recognized that not
all components of a jet engine are subject to the same
operating conditions and that different metallurgical
compositions may be employed in different components of the
engine to best suit the needs of that component.
There are some parts where tradeoffs have been made in
properties because the part i5 large enough so that the
engine operating conditions over the full extent of the part
are not uniform. In other words, certain large pieces which
are installed in an engine encounter different temperatures
and different property requirements and service from one
portion of the component to another. Accordingly, for such
large components it is necessary to sacrifice a property in
one location of the component in order to obtain an
acceptable property at another location. Such different
properties are needed for example in engine disks which
rotate at high speeds of 12,000 revolutions per minute and
more and result in the application of high stress to portions
of the disk and particularly to the outer portions of the
~0 disk.
In order to compensate for the different property
requirements of the different portions of disks, schemes and
methods have been devised to impart desirable combinations of
properties to the inner and outer portions of such disks.
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For example, the patent 4,820,358 issued to the ~ssignee of
the sub~ect application has taught a method by which a disk
made of a single alloy can be given different properties at
its inner or core portion as contrasted with its outer or rim
portion. The attainment of different properties in the
different portions of the disk is a valuable achievement.
Other efforts have been made to form an inner portion
of a disk of one alloy and an outer portion of a different
alloy. However, problemq have arisen where efforts are made
to join the two alloys together. If oxide layers exist at
the boundary it is difficult to be sure that any welding that
has occurred overcomes the presence of the oxide and does not
leave a region of weakness in the disk. The detection of
flaws in such weldments between an inner and outer portion of
a disk is difficult.
The present method is directed toward altering the
composition of an alloy during spray processing so that one
portion of spray deposit, with the attendant properties of
that portion, is of a first composition and another portion
of the spray deposit, with its different set of alloy
properties is of a second composition.
It is, accordingly, one object of the present
invention to provide a method for forming a composite
structure having two or more different compositions at the
different portions thereof.
Another ohject is to provide a structure in which two
alloy portions are joined without a sharp demarcation
therebetween.
Still another object is to provide a method for
forming a preform of a disk having at least two differer.t
metals compositions therein.
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Another object is to provide an alloy structure in
which two different alloy~ are present in different portions
thereof.
Other objects will be in part apparent and in part
pointed out in the description which follows.
In one of its broader aspects objects of the invention
can be achieved by providing a spray forming apparatus in
which a first metal is flowed as a ~tream from a dispensing
crucible to an atomization zone. The first metal is atomized
in said zone and is spray deposited onto a recei~ing surface
to form a first layer of a preform on the surface. The spray
of said first metal to form a preform on said surface is
continued. When a significant portion of the first metal is
present in the atomization crucible a small portion of an
additional metal is added to the atomization crucible to mix
the first and second metals therein to form a modification of
the first composition. The spray deposit of metal from the
atom dispensing crucible onto the preform is continued to
enlarge the preform but no more metal is added to the
dispensing crucible. The result is the formation of a
preform which has the first metal disposed on the receiving
surface and wh~ch has the modified metal bonded to the first
metal and forming the outer portions of the preform.
The description which follows will be understood with
greater clarity if reference is made to the accompanying
drawings in which:
FIG~R~ 1 is a schematic illustration of the
arrangement of a mandrel, preform, and dispensing crucible;
FIG~R~ 2 is a schematic illustration similar to that
of Figure 1 but illustrating the addition of alloying agents
to the dispensing crucible; and
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FlGnRE 3 is a graph in which experimental data is
displayed showing the change in concentration in an alloying
element against depth in a preform.
~
There are certain alloy systems which are altered
significantly depending on the additives which are present in
the alloy. It is recognized for example, that certain
additive elements au~nent the strength of an alloy system so
that an alloy which has less of the strengthening additive
has lower strength and the same alloy which has the
strengthening additives in higher concentration display a
higher level of strength.
Similarly, there are modifiers which alter the
ductility of an alloy system so that an alloy with lower
concentrations of the ductilizing additive have lower
ductility and the same alloy with a higher concentration of
the ductilizing additive has a higher level of ductility.
The subject invention takes advantage of the alloy
systems which are subjec~ to such alteration in desired
propertie~ depending on the level of alloy property modifying
elements such a~ strengthening agents or ductilizing agents
or other property modifying agents. One such agent is
mentioned in the background statement of the invention. In
particular, it has been found that the high strength of
superalloys has been increased and improved by an increase in
the precipitate volume fraction for the gamma prime
precipitate strengthened superalloys. In other words, it was
found that by specific addition of the precipitate forming
elements to cause an increase in the volume content of gamma
prime precipitate, that a strengthening of the alloy itself
resulted.
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Pursuant to the present invention, a spray forming
processing of an alloy is started employing an alloy
composition which is free of a modifier or which has a lower
level of a modifier element of elements present and the spray
processing with the alloy is continued after a modifier is
included in the alloy composition so that the latter part of
the spray forming is accomplished with the modified alloy
rather than with the unmodified alloy which was used at the
start of the spray processin~. Unique advantages are derived
from the use of a single alloy base in two different parts of
a spray formed article where different properties are needed
in the two different portions of the article.
For example, in the case of a disk, it is known that
higher strength is needed in the rim portions of the disk
than is needed in the core or central portions of the disk.
Accordingly, one appropriate use for the subject invention is
the spray forming of a disk preform in which the core of the
preform has a base alloy content which has a lower
concentration of certain additive or modifier elements and in
which the outer portion of the preform has the higher
concentration of the additive or modifier where this modifier
or additive imparts additional strength to the outer portion
of the disk eventually produced from the preform.
The manner in which this may be accomplished is now
described with reference to the Figures 1 and 2. The results
of this trial o:E the invention are then described with
respect to Figure 3 which is a plot of data obtained from a
measurement of the results of carrying out the invention on a
particular alloy system.
Referring now first to Figure 1, the figure is a
schematic elevational view in part in section of a spray
forming operation sucA as may be used in connection with the
present invention. In this regard, a crucible 10 is provided
as a dispensing crucible. The crucible is used to hold a
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body of liquid metal and to dispenYe it ac a stream to an
atomization zone located beneath the crucible. The crucible
is preferably a segmented crucible made up of a number of
water cooled metal segments which fit around and form the
walls of the crucible much as the staves of a barrel. Each
segment is electrically isolated from its adjoining segments
and each segment is individually water cooled. A benefit of
the water cooling is to permit a skull of the metal of the
melt to form on the inside of the crucible to avoid the
entrapment of ceramic particles in the melt. However, the
melt is kept at its operating tempera~ure by the action of a
set of induction coils 14. The segmented character of such a
crucible permits the electric flux to penetrate the crucible
12 to act on the liquid metal content- of the crucible 16. In
this way, some of the disadvantages of ceramic particle
inclusion as discussed in the background of the invention are
avoided and overcome.
The stream 18 of liquid metal which pours from the
bottom spout 20 of crucible 10 passes into an atomization
zone 20 where it is acted on by a jet or jets of atomizing
gas delivered from a source which is not shown to the nozzle
22.
The result of the atomization of the liquid stream 18
by the atomizing gas in zone 21 is a production of a cone 24
of droplets of liquid metal. The formation of such a cone is
consistent with the normal practice of the art of spray
forming. The droplets impact on a receiving surface 26 which
is shown in the form of an annular band formed on the hollow
mandrel 28. The mandrel 28 is given both a rotary and a
reciprocating motion indicated by the arrows by drive means
which are not shown. Such motion causes a uniform layer to
form on the receiving surface. The first part, and in the
illustration shown, the inner part of the preform 26 is
formed of an alloy which is subject to being modified by
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addition of a modifying agent such as a strengthening or
ductilizing or other type of modifying agent.
The second part of the preform, and in the
illustration shown the outer portion of the preform, i~
formed with an alloy which has been modified. There are
numerous ways in which modification of the alloy can be
accomplished. Essentially, the modification is accomplished
by the addition of a modifying agent to the remaining liquid
in the dispensing crucible.
With reference now specifically to Figure 2, the
apparatus as illustrated in Figure 1 is again shown with the
preform deposit 26 about half completed. Further, about half
of the liquid metal 16 has been exhausted from the dispensing
crucible 10. At this point, a charge of a modifying agent 32-
is added by any convenien~ means such as a scoop or probe 30.
The modifying agent may be added in a liquid or a solid
powdered form or in any form which will readily dissolve in
~; the liquid body 16 within the dispensing crucible 10. The
~ heating coil 14 acts on the liquid within the crucible to not
only heat in but also to stir it so that there is a
continuous stirring action goi~g on and the modifier material
32 which is added is quickly stirred into and mixed with and
dissolved in the molten metal liquid 16 within the crucible
10 .
Reference is next made to Figure 3. Figure 3 is a
graph in which data is recorded on an experimental run which
was made employing an alloy which could be modified by the
addition of aluminum in a relatively small weight percent.
The expcriment was carried out by first forming the
first half or the inner half of the preform 26 employing an
alloy which exhibited a low volume ~ of a gamma prime
microstructure. At a predetermined time during atomization,
a known amount of aluminum was immersed into the remaining
bath of the molten metal. Within seconds the composition of
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the bath had changed from a low gamma prime alloy to that of
an alloy with a high gamma prime content. Figure 3 is a plot
of the change in aluminum concentration of the alloy in
relation to the thickness of the preform set forth on the
abscissa. As is evident from the figure, the initial weight
concentration of aluminum was approximately 2.2 znd this
concentration of aluminum was pre-qent in the preform to a
thickness of about 1.5 inches. At this point, an addition of
aluminum was made and, as is evident from the graph, a
dramatic increase in the aluminum content of the alloy at
about 1.70 inches is the result of the addition. For the
alloy at a depth or at a thickness of about 2.0 inches the
aluminum content had risen to about 4 weight percent.
Concentration of the aluminum in the alloy remained at about
4 weight percent for the remainder of the run.
Two runs are represented in Figure 3 and the two runs
are respectivelv mar~ed as run 1 and run 2.
Although in the illustrative example given, aluminum
was added to a melt to vary the gamma prime content of the
preform during the atomization, it will be understood that
other modifier elements may also be added to an alloy system
to modify the properties of a spray deposit in one portion
relative to another portion. For example, it is contemplated
that other elements such as tantalum or master alloys with or
without aluminum may be dissolved in a dispensing crucible as
the atomization is in progress to provide a desired
microstructure and a tailored set of properties at different
portions of the preform being formed.
