U.S. patent application number 10/177658 was filed with the patent office on 2003-12-25 for directional solidification method and apparatus.
Invention is credited to Brinegar, John R..
Application Number | 20030234092 10/177658 |
Document ID | / |
Family ID | 29717871 |
Filed Date | 2003-12-25 |
United States Patent
Application |
20030234092 |
Kind Code |
A1 |
Brinegar, John R. |
December 25, 2003 |
Directional solidification method and apparatus
Abstract
Method as well as apparatus for DS casting by withdrawing a
melt-filled mold from an end of a casting furnace, spraying a
liquid cooling medium to impinge on exterior surfaces of the mold
as the mold is withdrawn, and collecting the cooling medium after
it impinges on the exterior mold surfaces.
Inventors: |
Brinegar, John R.;
(Muskegon, MI) |
Correspondence
Address: |
Mr. Edward J. Timmer
Walnut Woods Centre
5955 W. Main Street
Kalamazoo
MI
49009
US
|
Family ID: |
29717871 |
Appl. No.: |
10/177658 |
Filed: |
June 20, 2002 |
Current U.S.
Class: |
164/122.1 ;
164/338.1 |
Current CPC
Class: |
B22D 27/045
20130101 |
Class at
Publication: |
164/122.1 ;
164/338.1 |
International
Class: |
B22D 027/04 |
Claims
I Claim
1. Method of casting, comprising relatively moving a metallic
melt-filled mold and a casting furnace to withdraw said metallic
melt-filled mold from an end of a casting furnace and spraying a
liquid cooling medium to impinge on exterior surfaces of said mold
as said mold is withdrawn.
2. The method of claim 1 including disposing a plurality of spray
nozzles below a thermal baffle at said end to discharge sprays of
said cooling medium to impinge on said exterior surfaces.
3. The method of claim 1 including collecting said cooling medium
after it impinges on said exterior mold surfaces and reusing
it.
4. The method of claim 1 wherein the cooling medium is selected
from the group consisting of molten tin and molten aluminum.
5. The method of claim 1 wherein said mold is withdrawn from said
end of said casting furnace by moving a chill member on which said
mold rests.
6. Method of directional solidification, comprising withdrawing a
melt-filled mold from an end of a casting furnace disposed in a
vacuum chamber by relative movement therebetween, spraying a liquid
cooling medium to impinge on exterior surfaces of said mold as said
mold is withdrawn, and collecting said cooling medium in said
vacuum chamber after it impinges on said exterior mold
surfaces.
7. The method of claim 6 including disposing a plurality of spray
nozzles below a thermal baffle at said end to discharge sprays of
said cooling medium to impinge on said exterior surfaces.
8. The method of claim 6 including reusing the collected cooling
medium.
9. The method of claim 6 wherein the cooling medium is selected
from the group consisting of molten tin and molten aluminum.
10. Directional solidification casting apparatus, comprising a
casting furnace having an open lower end, a metallic melt-filled
mold, said furnace and said metallic melt-filled mold being
relatively movable to withdraw said melt-filled mold through said
end out of said furnace, and means for spraying a liquid cooling
medium to impinge on exterior surfaces of said mold as said mold is
moved out of said furnace.
11. The apparatus of claim 10 including a thermal baffle at said
lower end, said means being disposed below said baffle.
12. The apparatus of claim 10 wherein said means comprises a
plurality of spray nozzles disposed below a thermal baffle at said
end to discharge sprays of said cooling medium to impinge on said
exterior surfaces.
13. The apparatus of claim 10 wherein said means comprises an
annular pipe disposed below a thermal baffle at said end and having
one or more orifices to discharge one or more sprays of said
cooling medium to impinge on said exterior surfaces.
14. The apparatus of claim 10 including means for collecting said
cooling medium after it impinges on said exterior mold
surfaces.
15. The apparatus of claim 10 wherein the cooling medium is
selected from the group consisting of molten tin and molten
aluminum.
16. The apparatus of claim 10 including a chill member on which
said mold is disposed and moved out of said end of said casting
furnace by moving said chill member relative to said furnace.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to directional solidification
apparatus and processes wherein heat is removed in a unidirectional
manner from a metallic melt in a mold to form a columnar grain or
single crystal casting.
BACKGROUND OF THE INVENTION
[0002] In the manufacture of components, such as nickel base
superalloy turbine blades and vanes, for gas turbine engines,
directional solidification (DS) investment casting techniques have
been employed in the past to produce columnar grain and single
crystal casting microstructures having improved mechanical
properties at high temperatures encountered in the turbine section
of the engine.
[0003] In the manufacture of turbine blades and vanes using the
well known DS casting "withdrawal" technique where a melt-filled
investment mold residing on a chill plate is withdrawn from a
casting furnace, a stationary thermal baffle has been used
proximate the bottom of the casting furnace to improve the
unidirectional thermal gradient present in the molten metal or
alloy as the investment mold is withdrawn from the casting furnace.
The baffle reduces heat loss by radiation from the furnace and the
melt-filled mold as the mold is withdrawn from the casting
furnace.
[0004] Attempts to improve the thermal gradient have included
introducing a cooling gas below the stationary thermal baffle of
the casting furnace to extract heat from the mold and/or
withdrawing the hot melt-filled investment mold from the furnace
into a bath of liquid cooling metal, such as liquid tin, positioned
below the furnace.
[0005] The low heat capacity of cooling gas creates a disadvantage
in that large gas quantities are needed to effect cooling and in
that the presence of the cooling gas below the baffle has a
negative impact on the thermal profile of the mold heater in the
casting furnace due to a chimney effect. The large quantities of
cooling gas require complex and expensive vacuum pumping and
recycling systems associated with the casting apparatus as well as
more complex heat shielding and cooling of the casting furnace
equipment.
[0006] Use of a liquid metal cooling bath adds significantly to
complexity of the mold design and withdrawal apparatus since the
mold must be lowered into a hot circulating cooling media. Complex
bath circulation and level control systems are needed. In addition,
the liquid metal cooling bath can be subject to contamination and
reactions should a prior cast investment mold experience a leak or
significant run-out of molten metal into the bath.
SUMMARY OF THE INVENTION
[0007] The present invention provides in an embodiment a method and
apparatus for DS casting wherein a liquid cooling medium is sprayed
directly on exterior surfaces of a melt-filled ceramic investment
mold as it is withdrawn from an end of a DS casting furnace by
relative movement therebetween so as to extract heat from the mold
and improve the thermal gradient in the melt residing in the mold.
The liquid cooling medium preferably is collected for reuse after
it impinges on the exterior mold surfaces.
[0008] In an illustrative embodiment of the invention, a plurality
of spray nozzles are disposed beneath a thermal baffle at a lower
end of the DS casting furnace. The nozzles are spaced about a
baffle opening through which the investment mold is withdrawn
downwardly out of the casting furnace. The spray nozzles are
oriented to spray a liquid metallic cooling medium in directions
transverse to the path of mold withdrawal through the opening so
that the sprays impinge directly on the exterior mold surfaces as
the mold is withdrawn from the casting furnace by relative movement
therebetween.
[0009] The invention provides a high heat extraction capability
from the melt-filled mold without the disadvantages described above
associated with use of a cooling gas and/or liquid metal cooling
bath in which a mold is immersed. The invention will be described
in more detail below in connection with the following drawings.
DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a schematic cross-sectional view of a DS casting
apparatus in accordance with an embodiment of the invention.
[0011] FIG. 1A is a partial schematic cross-sectional view of the
DS casting furnace of another embodiment where the nozzles are
oriented at an upward angle relative to horizontal.
[0012] FIG. 2 is a schematic view taken along lines 2-2 of FIG. 1
of the bottom of the thermal baffle showing spray nozzles disposed
about the baffle opening as well as liquid metal supply piping. The
mold is schematically and partially shown in cross-section for
convenience.
[0013] FIG. 3 and 4 are a schematic cross-sectional views of DS
casting apparatus in accordance with other embodiment of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0014] The present invention provides a DS casting method and
apparatus especially useful, although not limited, to casting of
nickel, cobalt and iron base superalloys to produce a columnar
grain or a single crystal cast microstructure. Referring to FIG. 1,
casting apparatus in accordance with an illustrative embodiment of
the invention for DS casting nickel, cobalt and iron base
superalloys to produce columnar grain or a single crystal cast
microstructure includes a vacuum casting chamber 10 having a
casting furnace 12 disposed therein in conventional manner. Thermal
insulation members 13a, 13b form a furnace enclosure. Positioned
within the tubular thermal insulation member 13a is an inner solid
graphite tubular member 15 forming a susceptor that is heated by
energization of the induction coil 18. The thermal insulation
member 13b includes an aperture 13c through which molten metal or
alloy (metallic melt), such as a molten superalloy, can be
introduced into the mold 20 from a crucible (not shown) residing in
the chamber 10 above the casting furnace 12 in conventional
manner.
[0015] Induction coil 18 disposed about the susceptor 15 is
energized by a conventional electrical power source (not shown).
The induction coil 18 heats tubular graphite susceptor 15 disposed
interiorly thereof. After the empty mold 20 is positioned in the
furnace 12, the mold is preheated to a suitable casting temperature
for receiving the metallic melt by the heat provided by the
susceptor 15.
[0016] The mold 20 typically comprises a conventional ceramic
investment shell gang or cluster mold formed by the well know lost
wax process to include a pour cup 20a that receives the melt from
the crucible and that communicates via sprues 20b to a plurality of
shell molds 21 each having a mold cavity 22 in the shape of the
article to be cast. Although two shell molds 21 are illustrated in
FIG. 1, four or more shell molds 21 can be disposed around the
center post 20d. Each mold cavity 22 communicates to a chill plate
26 at an open bottom end of each mold cavity in conventional manner
to provide unidirectional heat removal from the metallic melt
residing in the mold and thus a thermal gradient in the metallic
melt M in the mold extending along the longitudinal axis of the
mold. In casting single crystal components, a seed or crystal
selector (not shown), such as pigtail passage, will be incorporated
into the mold above the open lower end thereof to select a single
crystal for propagation through the metallic melt, all as is well
known. The invention is not limited to use with a gang or cluster
mold 20 having a plurality of shell molds 21 and can be practiced
with any type of refractory shell mold having one or more mold
cavities.
[0017] In one embodiment of the invention, the mold 20 is formed
with an integral mold base 20c and central support post 20d that
rest on the chill plate 26 as shown. The base 20c can be clamped
thereto in conventional manner if desired. The chill plate resides
on a ram 28 raised and lowered by a fluid actuator (not shown) to
move the mold 20 into and out of the casting furnace 12.
Alternately, the invention envisions using any relative movement
between mold 20 and casting furnace 12 to effect withdrawal of the
melt-filled mold 20 from the end of the furnace 12. For example, in
another embodiment, the mold 20 on chill plate 26 and collection
vessel 50 can be disposed in a fixed position, while the casting
furnace 12 and nozzles 40 are moved together to effect withdrawal
of the melt-filled mold 20 from the end of the furnace 12.
[0018] In the DS casting of gas turbine engine blades or vanes,
each mold cavity 22 will have a root region 22a corresponding to a
root of the blade or vane and a relatively large platform cavity
22b corresponding to the platform portion of the blade or vane to
be cast. Each mold cavity 22 also will have a relatively smaller or
narrower airfoil cavity region 22c corresponding to the airfoil
portion of the blade or vane to be cast.
[0019] A stationary thermal baffle 30 is disposed at the lower end
of the casting furnace 12 and is connected in conventional manner
to the walls W of the vacuum chamber 10. The baffle 30 includes an
opening 30a oriented perpendicular to the mold withdrawal direction
(vertical direction in FIG. 1) and having a cross-sectional
configuration selected to accommodate movement of the relatively
large platform region or profile of the melt-filled molds 21
therepast with only a small gap (e.g. 1/2 inch) present between the
platform region 22b and the inner periphery of the baffle 30. The
baffle 30 typically is made of graphite material, although other
refractory materials can be used.
[0020] In accordance with an illustrative embodiment of the
invention, a plurality of spray nozzles 40 are disposed beneath
thermal baffle 30 at the lower end of the DS casting furnace 12.
The nozzles 40 are spaced about the periphery of baffle opening 30a
through which the metallic melt-filled investment mold 20 is
withdrawn downwardly out of the casting furnace. The spray nozzles
40 are oriented to spray a liquid cooling medium as a plurality of
liquid cooling sprays S in generally horizontal directions
transverse to the downward vertical path of mold withdrawal through
the opening 30a so that the sprays S impinge directly on and around
the exterior surfaces of shell molds 21 as the mold 20 is withdrawn
from the casting furnace. In FIG. 1, the nozzles 40 are shown
horizontally oriented to provide spray coverage of the exterior
surfaces of the mold. Alternately, each nozzle 40 can be mounted on
a bracket 41 to angle the nozzle 40 at an upward angle relative to
horizontal as shown in FIG. 1A to this end. The nozzles 40 are
mounted by any suitable mounting means on the structural frame F
that supports the casting furnace 12 in vacuum casting chamber
10.
[0021] The spray nozzles 40 preferably are of the type that produce
a flat fan-shaped spray of the liquid cooling medium to provide the
sharpest temperature transition and to minimize the number of
nozzles needed to cool the mold surfaces. The nozzles can be made
of any suitable material that can withstand prolonged contact with
the cooling medium, such as for example a liquid tin cooling
medium.
[0022] The sprays S each can comprise a relatively low melting
point, relatively high heat capacity liquid metal or alloy, or
other liquid material such as a molten salt or oxide, that is
compatible with the shell mold material so as not to react
adversely therewith. For purposes of illustration and not
limitation, molten tin, molten aluminum, or any other suitable
liquid cooling metal or alloy may be used. The relatively low
melting point of the liquid cooling metal or alloy is with respect
to the melting point of the metal or alloy to be cast and
directionally solidified in the molds 21. The relatively high heat
capacity of the liquid cooling metal or alloy is with respect to
the heat capacity provided by a cooling gas, such as Ar or other
inert gas, used in the past to cool molds.
[0023] If liquid tin is used as the sprayed liquid cooing medium,
the liquid tin temperature is typically in the range of 300 to 500
degrees F., whereas for purposes of illustration, the temperature
of a molten nickel base superalloy residing in the molds 21 is
typically in the range of 2800 to 2300 degrees F. If liquid
aluminum is used as the sprayed liquid cooing medium, the liquid
aluminum temperature is typically in the range of 900 to 1400
degrees F.
[0024] The nozzles 40 are supplied with the liquid cooling medium
via a common distribution or manifold pipe 42 connected to a pump
44, FIG. 2. The pump 44 is connected to a heated storage tank 46
containing the liquid cooling metal or alloy. The tank 46 can
comprise a conventional metal (e.g. steel) or ceramic tank used to
heat the liquid metal or alloy, such as molten tin or aluminum, to
desired use temperature and store it for use. The pump 46 provides
the liquid cooling metal or alloy under pressure to the nozzles 40
to generate sprays S. A typical pressure range of the liquid
cooling metal or alloy supplied to the nozzles 40 will depend on
the type of nozzle 40 selected for use and typically can be in the
range of 40 to 250 psi. The pump 44 and tank 46 are disposed
outside the vacuum chamber 10 and are connected to the nozzles 40
by the piping 43 that supplies the liquid cooling metal or alloy to
the distribution or manifold pipe 42.
[0025] After the liquid cooling medium impinges on the shell molds
21 as they are withdrawn from baffle opening 30a, the still liquid
cooling medium, such as still liquid tin or aluminum, falls by
gravity from the mold 20 into a collection vessel 50 in vacuum
chamber 10. The liquid cooling medium then is returned via piping
51 and pump 52 back to tank 46 for conditioning and reuse in closed
loop manner. The tank 46 can be effective to thermally condition
the liquid cooling medium to an appropriate temperature as well as
filter or separate out any contaminates therein that might plug the
nozzles 40.
[0026] In operation, an empty mold 20 is positioned in the furnace
12 by upward movement of the ram 28. The induction coil 18 is
energized to preheat via susceptor 15 the mold 20 to a suitable
casting temperature, such as above 2500 degrees F. for casting
nickel base superalloys. The mold is filled with molten metal or
alloy to be cast from a crucible above the furnace. Then, the
metallic melt-filled mold 20 is withdrawn downwardly past baffle 30
out of the furnace 12 for example by lowering of the ram 28 (or any
relative movement between furnace 12 and mold 20) at a controlled
withdrawal rate to establish a thermal gradient in the melt to
achieve a solidification front that progresses upwardly through the
melt residing in the shell molds 21 during withdrawal to form
either a columnar grain or a single crystal microstructure, if a
single crystal selector or seed is present in the mold.
[0027] Shortly after the mold 20 begins downward withdrawal, the
nozzles 40 are supplied with the liquid cooling medium, such as a
liquid metallic cooling medium, to generate cooling sprays S that
impinge on the exterior surfaces of the shell molds 21 as they past
the baffle 30 out of the furnace 12. The liquid cooling metal or
alloy of relatively high heat capacity and relatively low
temperature impinges on and around the hot mold exterior surfaces
to extract heat and improve the thermal gradient in the melt
residing in the molds 21 above the solidification front progressing
through the melt. After the liquid cooling metal or alloy impinges
on the shell molds 21, the still liquid cooling metal or alloy is
collected in vacuum chamber 10 in vessel 50 and returned by pump 56
to tank 46 for conditioning and reuse.
[0028] In lieu of using a plurality of individual nozzles 40 as
described above, the invention envisions using other nozzle or
orifice arrangements to generate one or more sprays S of the liquid
cooling medium to impinge on the exterior mold surfaces. For
example, as illustrated in FIG. 3, the invention envisions using a
manifold pipe 42' that has a plurality of individual orifices 42a'
spaced about its inner periphery facing the mold 20 to generate the
sprays S. Also, as shown FIG. 4, a manifold pipe 42" may be
provided with an annular slit or slot orifice 42a" on its inner
periphery facing the mold 20 to generate a spray S in the form an
annulus or other suitable shape to impinge on the exterior mold
surfaces.
[0029] Moreover, it is to be understood that the invention has been
described with respect to certain specific embodiments thereof for
purposes of illustration and not limitation. The present invention
envisions that modifications, changes, and the like can be made
therein without departing from the spirit and scope of the
invention as set forth in the following claims.
* * * * *