Note: Descriptions are shown in the official language in which they were submitted.
l~a647~
METHOD OF REDUCING CASTING TIME
Baclcqround of the Invention
The present invention relates to a method oE making
directîonally solidified (DS) castings and more
specifically to a method which reduces the t;me required
to cast a directionally solidified article without
reducin~ the quality of the cast product.
In casting directionally solidiEied articles rom
nickel~base superalloys, a mold is commonly positioned on
a chill plate which is slowly withdrawn from a furnace to
provide for controlled solidification of molten metal in
the mold in a manner similar to that disclosed in U.S.
Patent Nos. 3,700,023 and 3,714,977. During various DS
castin~ oper~tions, it has been observed that the mold can
be withdrawn ~rom the furnace at speeds oE up to about 20
in./hr. to generate acceptable columnar grain structures.
The speciic speeds at which a particular article is
withdrawn from the furnace are yoverned by the geometry of
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the article. If it is attempted to use higher speeds,
such as 30 in./hr., it has previously been determined that
a substan~ial and very objectionable coarsening of the
columnar grains occurs. Attempts to change the
temperatures and temperature distribution in the furnace
hot zone have proven to be ineffective in permitting the
use of faster withdrawal speeds for the production of gas
`turbine articles such as blades and vanes~
In an efort to increase withdrawal speeds to a rate
oE 25 in./h.r. or faster, U.S. Patent No. 3,S32,155
discloses an apparatus in which the mold and cooling plate
are moved through a heat sink which is disposed
immediately beneath the furnace. As a still Eurther
effort to reduce the time required to form a casting, U.S~
Patent No. ~,19~,094 suggests varying the rate of
withdrawal of the mold from a furnace as a Eunct.ion o the
geometry oE the article to be cast and other factors.
Sumrnary of the Present Invent;on
The present invention decreases the time required to
~orm a directionally solidiEied ~DS) casting without
substantial coarsening of the columnar grains of the
casting. This is accomplished by initially withdrawing a
mold from a furnace at relatively slow speeds. As the
mold is slowly withdrawn from the furnace, a dendritic
structure CJrOWS upwardly toward the upper end of the mold
.
cavity~ The uppermost interstices oE tlliS dendritic
structure are filled with molten metal. In the art this
region of the casting in which a skeleton of solid
dendrite and liquid metal coexist is called the mushy
~one. When the dendritic structure reaches the upper end
of the mold cavity, the rate of withdrawal of the mold
from the furnace is increased to increase the rate of
solidification o the molten metal in the interstices of
the dendritic structure, that is, to complete
solidification of the mushy zone.
Although it is contemplated that the present invention
may be used to cast many different types of articles, the
invention is advantageously used during the casting of
airfoils having relatively thick bases and thin airfoils.
During the solidification o the metal in the relatively
thick base and a lower portion of the airfoil, the mold is
slowly withdrawn from the furnace. ~lowever, as soon as
the dendritic structure in the relatively thin airfoil
reaches the upper end oE the mold cavity, that is the tip
of the airfoil, the rate of withdrawal of the mold from
the Eurnace is increased to increase the speed oE
solidiication of the molten metal remaining in the
dendritic skructure.
Accordingly, the present invention provides an
improved method o casting an article by initially
solidifying molten metal at a relatively slow rate and
i
then increasing the rate of solidification of the molten
metal after a dendritic structure has been extended to.an
upper end oE the mold.
Brief Description_of he Drawinqs
The foregoing and other features and advantages of the
present invention will become more apparent upo~ a
consideration of the following description taken in
conjunction with the accompanying drawings wherein:
~ ig. 1 is a schematic illustration depicting the.
relationship between a mold containing molten metal and a
urnace immediately after pouring of the molten metal in-to
the mold;
Fig~ 2 is a schematic illustration depicting the
relationship between the mold and the Eurnace after the
mold has been partially withdrawn from the furnace at a
relatively slow speed and the molten metal in the mold
cavity has partially solidiEied;
Fig. 3 is an enlarged Eragmentary schematic
illustration depictiny the relationship hetween a portion
of the mold of Fig. 2, the solidified metal at a lower end
of the mold cavity, and a schematically illustrated
dendritic structure extending upwardly Erom the solidified
metal to an upper end of the mold cavity;
Fig. 4 .is a schematic illustration depictin~ the
columnar ~rain appearance of a blade cast with the mold o.E
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1.
~ig. 1 by withdrawing the mold from the furnace at a
relatively 510w speed until the dendritic structure
extends to the upper end of the mold cavity as shown in
Fig. 3 and then rapidly withdrawing the mold from the
furnace; and
Fig~ 5 is a schematic illustration, generally similar
to Fig. 4, of the columnar grain appearance oE an airEoil
formed by rapidly withdrawing a mold from a furnace.
Description of One Specific
Pre~erred Embodiment of the Inventi.on
A mold 10 (Fig. 1) is preheated in a known furnace
assembly 12 prior to pouring of molten metal into the
mold. The known furnace assembly 12 is provided with a
refractory outer wall 16 which is surrounded b~ an
induction heatiny coil 18. A graphite susceptor wall 20
is enclosed by the outer wall 16 and is heated by the
induction effect of the coil 18. The furnace assembly 12
has a top plate 22 with an opening which may be provided
with a funnel 24 through which molten metal is poured into
the mold 10. It is contemplated that the entire furnace
assembly 12 will be disposed within a vacuum.
The mold 10 has a pouring basin 32 through which
molten metal enters a plurality of runners or passages 34
which are connected with a plurality of mold cavities 3~
which are ~isposed in a circular array around the pouring
.. . .... ,, _ _
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basin 32. A cylindrical heat shield 40 may be provided on
the inside of the circular array of mold cavities 38.
The mold 10 is disposed on a copper chill plate 42.
The chill plate 42 promotes the directional solidification
of molten metal in the mold cavities to provide a casting
having a columnar grain structure with a grain orientation
extending generally parallel to the longitudinal central
axes ~vertical axes) of the mold cavities 38. The furnace
12 is of a known construction and may be constructed in
accordance with U.SO Patent ~os. 3,376r915; 3~700/023
and/or 3,714,977.
~ len molten metal is poured into the basin 22 and the
runners 34 to mold cavity 38, the molten metal flows
downwardly and solidifies against the chill plate 42. A
large number of randomly oriented crystals are nucleated
at the chill plate. As this is occurring, a dendritic
structure starts to extend upwardly from the metal which
is solidified against the chill plate into a competitive
growth zone. The chill plate is then slowly lowered from
the furnace. As the chill plate is lowered, the most
favorably oriented grains or crystals emerge from the
competitive ~rowkh zone and the dendritic structure
contillues to grow upwardly into the mold cavity 38.
Although ~t is contemplated that the chill plate could be
lowered at many difEerent rates, relatively slow initial
lowering rates below about 20 in./hr. are presently
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preferred. It is contemplated that for some parts it will
be preferred to maintain the relatively slow initial
withdrawal rate substantially constant. However, for
other parts, it may be preferred to vary the initial
withdrawal somewhat between speeds which are less than
about 20 in./hr.
In accordance with a feature of the present invention,
once the dendritic structure has reached tlle.upper end 5~
of the mold cavity 38, the rate of withdrawal of the mold
is substantially increased. This results in relatively
rapid solidification of the molten metal remaining in the
interstices of the dendritic structure. However, since
the basic dendritic structure has already been established
throughout the length of the mold cavit~, the rapid
solidification of the molten metal that remains in the
so-called mush~ zone does not lead to coarsening of the
grain structure.
In practicing the invention, the mold 10 is ini~ially
lowered from the position shown in Fig. 1 to the position
shown in Fig. 2 at relatively slow speeds, that is speeds
of approximately 20 in./hr. or less. Once the mold 10 has
been moved to the partially withdrawn position shown in
Fi~. 2 and a dendritic structure 56 (Fig. 3) extends from
a fully solidified body 58 oE metal at the lower end
portion oE the mold cavity 38 to the upper end 54 of the
mold cavity, the rate of downward movement of the chill
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plate is increased. The interstices of the uppermost
portions of the dendritic structure, the so~called mushy
zone, are Eilled with molten metal 60.
It is contemplated that the rate of downward movement
of the chill plate will be increased by a substantial
extent when the dendritic structure 56 reaches the top of
the mold cavity. However, it is believed that the amount
by which the rate of withdrawal can be increased will
depend upon the article being cast and the specific alloy
of which it is formed. However, in one specific instance,
the rate of withdrawal of the mold was increased Erom a
speed of less than 20 in./hr. to a speed of more than 34
in./hr. in casting a turbine blade Eormed oE a nickel-base
superalloy. Even though the rate oE withdrawal of the
mold 10 from the furnace assembly 12 was substantially
increased, there was no objectionable coarsening of the
yrains oE the cast article. It is believed that this is
because the molten metal 60 in the interstices of the
dendritic structure solidified without altering the basic
dendritic structure which had been established throughout
the molten metal in the mold cavity 3~ prior to the
increased rate of withdrawal of the mold 10 from the
Eurnace assembly 12.
~ lthough it is contemplated that the present invention
can be used during the casting of many difeerent articles,
the present invention is advantageously used during the
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castir.g oE a directionally solidified airfoil. Thus, each
of the mold cavi~ies 38 has a lower portion with a
configuration corresponding to the configuration oE a
starter block and the base of the blade. ~ach of the mold
cavities 38 also has a portion which extends upwardly from
the base portion of the mold cavity and has a
configuration corresponding to the confîguration of the
airfoil of the blade. The airEoil oE the blade has a
substantiall~ uniform thickness throughout its axial
extent.
A mold cavity 38 with a partially cast blade 62 is
shown schematically in Fig. 3. The mold cavity 38
includes a lower end portion 64 which extends upwardly
from an upper surface 66 of the chill plate 42. This
lower end portion 64 of the mold cavity has a generally
rect:angular configuration. Directly above the lower end
portion 64 of the mold cavity 38 is an intermediate
portion 68 having a configuration corresponding to the
con~iguration of the base 70 of the blade 62. An upper
portion 72 of the mold cavity 38 extends upwardly from the
intermediate portion 68 and has a configuration
corresponding to the con;~uration of an airfoil portion
74 oE the blade 62. The mold cavity terminates at the
upper ~nd surface 5~ which is connected with a runner 34
khrough which molten metal enters the mold cavity 38.
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When the blade 62 is to be cast in the mold cavity 38,
molten metal enters the mold cavity through the runner
34. Molten metal flows downwardly through the mold cavity
38 into engagement with the upper surface 66 of the chill
plate 42. ~he molten metal immediately soldifies in the
lower end portion 64 of the mold cavity 38. The initially
solidified molten metal has a random columnar grain
structure next to the chill plate 42. However, the more
favorably oxiented grains grow rapidly upwardly from the
chill plate 42 through a competitive growth zone from
which the most favorably oriented grains emerge. These
most avorably oriented grains enter the intermediate
portion 68 of the mold cavity and solidify to initiate
formation o a base portion 70 of the blade 62.
As the molten metal is solidifying, an upwardly
extending dendritic structure 56 is formed. This
dendritic structure consists of a plurality of most
favorably oriented grains which form ~ plural.ity of
upwardly extending dendrites. As the upward formation of
the dendritic structure 56 continues, the molten metal in
the interstices of the uppermost portions of dendritic
structure solidifies to continue the formation o the base
portion 70 o.E the blade 62.
~ s the upward growth of the dendritic structure 56
extends illtO the upper portion 72 oE the mold cavity 38,
the formation of the base portion 70 of the blade 62 is
~.
completed and continued sol.idification of the molten metal
in the interstices oE the dendritic structure initiates
the formation of the airfoil 74. As the molten metal
solidifies to form ~he lower end portion of the airfoil
74, the dendritic structure grows upwardly to the tip oE
the airfoil at the surface 54.
At this time, the base 70 of the air~oil has
solidified and the lower portion of the a;rfoil 74 of the
bl.ade is solidified~ However, the upper portion of the
airfoil of the blade has not fully solidified. Thus,
there is a basic dendritic structure 56 extending from the
solidified lower portion of the airfoil 74 to the tip of
the airfoil at the upper end surface 54 of the mold cavity
38. The interstices of the uppermost portions o the
basic dendritic structure 56 are filled with molten metal
60. This uppermost portion, containing both solid
dendrites and interstices filled with molten metal is
known in the art as the mushy zone. The height of the
mushy zone can be several inches, with the specific
distance being related to the alloy being cast and how
sharp the vertical temperature drop or thermal gradient is
in the solidifyincJ metal.
When the derldritic structure 56 has reachecl the upper
end 54 o~ the mold cavity 38, the rate of withdrawal oE
the mold 10 from the furnace assembly 12 is substantially
increased~ Since the dendritic structure has been Eormed
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throughout the length of the airfoil 74 of the blade 62,
there is no coarsening of the gr~ins at the upper end
portion or tip of the airfoil due to the increased speed
of withdrawal of the mold 10 from the furnace 12. This is
true even though the airfoil 74 has a substantially
uniform thickness throughout its l~ngth.
The blade 62 which results from this casting process
has been illustrated schematically in Fig. 4. The grains
of the directionally solidified blade extend to the tip
end oE the airfoil without coarsening of the grains. By
way of experimentation, a mold of the same general
construction as the mold 10 was withdrawn at a constant
relatively high speed from the furnace assembly 12. The
resulting blade 80 (see Fig. 5) had a relatively fine
grain structure adjacent to its base 82 and at the lower
end portion 84 of the airfoil 86. However, the upper or
airEoil tip portion 88 oE the airfoil was very coarse
grained and consisted of two or three crystals. The
coarse grained outer end portion of the airfoil 86 of the
blade ~0 makes the casting unacceptable for use in most
circumstances. However, the continuous fine grained
structure of the blade 62 (Fig. 4) ;s quite acceptable for
most purposes.
The fine grained structure of the blade 62 could have
been obtained by withdrawin~ the mold Erom the furnace
assemhly 12 at a constant and relatively low speed. Thus,
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an airfoil with the same fine grained structure as has
been illustrated schematically in Fig. 4 for the blade 62
could have been obtained by withdrawing the mold 10 from
the furnace 12 at a relatively low speeds of approximately
28 in./hr or less. However, this results in a relatively
long casting process.
The present invention substantially decreases the
amount of time required to cast the fine yrained blade 62
by increasing the rate of withdrawal of the mold 10 from
the furnace 12 when the dendritic structure 56 has grown
from the solidified body of metal 58 at the lower end
portion of the mold 10 to the upper end surface 54 of the
mold. Thus, in accordance with the present invention the
mold 10 is initially withdrawn at a relatively slow speeds
from the Eurnace 12, that is at a speeds of less than
about 20 in./hour. Once the dendritic structure has
extended throughout the molten metal in the mold cavity
38, the speed of withdrawal of the mold 10 from the
furnace 12 is increasecl to, for example, a speed of 34
in./hour.
It should be understood that the speciEic mold
withdrawal rates previously set forth have been for
purposes of clarity of illustration and it is contemplated
that these withdrawal rates may vary. The specific mold
withdrawal rates of less than about 20 in./hour before the
dendritic structure extends throughout the molten metal in
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L7~.
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the mold cavity 34 and the relatively rapid mold
withdrawal rate of 34 in./hour after the dendritic
structure has grown to the end surface 54 of the mold
cavity were used with a nickel base superalloy,
specifically PWA 1422 (trademark) alloy, with a mold which
was preheated to approximately 2700F. The time saved
during the process of casting one specific airfoil by
using the previously described low and then high speed
withdrawal after the dendrites had extended through the
molten metal was approximately 26 minutes. Of course, the
maximum rate of withdrawal and the time saved on a casting
cycle will vary with the characteristics of the article
being cast and the specific, relatively slow, speeds at
which it is cast using conventional practice.
In view of the Eoregoing it is apparent that the
present invention decreases the time required to form a
directionally solidified (DS) cast'ng without substantial
coarsening of the columnar grains of the casting. This is
accomplished by initially withdrawing a mold 10 from a
Eurnace 12 at relatively slow speeds. ~s the mold 10 is
slowly withdrawn from the furnace 12, a dendritic
structure 56 grows upwardly toward the upper end 54 of the
mold cavity 38. The interstices oE this dendritic
structure 56 are Eilled with molten metal. When the
dendritic structure 56 reaches the upper end 54 of the
mold cavity 3~, the rate of withdrawal of the mold 10 from
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the furnace 12 is increased to increase the rate of
solidification of the molten metal in the interstices of
the dendritic s~ructure 56.
Although it is contemplated that the present invention
may be used to cast many different types of articles, the
invention is advantageously used during the casting of
blades 62 having relatively thick bases 70 and thin
airfoils 74. During the solidificat.ion of the metal in
the relatively thick base 70 and a lower portion of the
airfoil 74, the mold 10 is slowly withdrawn from the
furnace 12. ~Iowever, as soon as the dendritic structure
in the relatively thin airfoil 74 reaches the upper end 54
of the mold cavity, that is the tip of the airfoil, the
rate oE withdrawal of the mold 10 from the furnace 12 is
increased to increase the speed oE solidification of the
molten metal in the dendritic structure.
