U.S. patent application number 10/781832 was filed with the patent office on 2004-08-26 for component casting.
This patent application is currently assigned to ROLLS-ROYCE PLC. Invention is credited to Boswell, John H..
Application Number | 20040163790 10/781832 |
Document ID | / |
Family ID | 9953655 |
Filed Date | 2004-08-26 |
United States Patent
Application |
20040163790 |
Kind Code |
A1 |
Boswell, John H. |
August 26, 2004 |
Component casting
Abstract
A deflector element 3,13 is provided, in order to improve
temperature interface control with respect to directional
solidification of a component casting. This deflector element 3,13
is formed during the mould making process and is arranged to be
positioned with respect to a part 6 of a principal mould formation
2,12 within which the final cast component is produced. The
deflector element 3,13 ensures a uniform temperature throughout the
component cross-section relative to other parts of the component as
it is solidified.
Inventors: |
Boswell, John H.; (Derby,
GB) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 19928
ALEXANDRIA
VA
22320
US
|
Assignee: |
ROLLS-ROYCE PLC
London
GB
|
Family ID: |
9953655 |
Appl. No.: |
10/781832 |
Filed: |
February 20, 2004 |
Current U.S.
Class: |
164/122.2 ;
164/361 |
Current CPC
Class: |
B22C 7/02 20130101; B33Y
80/00 20141201; B22C 9/04 20130101; B22D 27/045 20130101; C30B
11/003 20130101 |
Class at
Publication: |
164/122.2 ;
164/361 |
International
Class: |
B22C 009/04; B22D
027/04 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 26, 2003 |
GB |
0304327.0 |
Claims
1. A method of component casting comprising forming a mould with a
displaced deflector element adjacent to a part of a principal mould
formation whereby the deflector element controls the rate of heat
loss from that part of the principal mould formation.
2. A method as claimed in claim 1 wherein the mould is formed by a
lost wax process or stereo lithographic process.
3. A method as claimed in claim 1 or claim 2 wherein the displaced
deflector element is secured to a downpole of the mould.
4. A method as claimed in claim 1 or claim 2 wherein the displaced
deflector element is secured to a separate upstanding leg to
present the deflector element to the principal mould formation.
5. A method as claimed in any preceding claim wherein the deflector
element is coated for improved radiation with a low emission
material.
6. A method as claimed in claim 5 wherein the defector element is
coated with a magnesium oxide coating.
7. A mould for component casting comprising a principal mould
formation and a displaced deflector element immediately adjacent a
part of the principal mould formation to control in use the rate of
heat loss from that part of the principal mould formation.
8. A mould as claimed in claim 7 wherein the mould includes a
downpipe from which the displaced deflector element is secured.
9. A mould as claimed in claim 7 wherein the mould includes an
upstanding leg to present the deflector element to the principal
mould formation.
10. A mould as claimed in any of claims 7 to 9 wherein the
displaced deflector is coated with a low emission material.
11. A mould as claimed in claim 10 wherein the low emission
material is a magnesium oxide coating.
12. A mould as claimed in any of claims 7 to 11 wherein the
displaced deflector element is approximately 2 mm to 3 mm
thick.
13. A mould as claimed in any of claims 7 to 12 wherein the
displaced deflector element has a configuration such that there is
at least a 15 mm wide overlap with the principal mould
formation.
14. A mould as claimed in any claims 7 to 13 wherein the mould is
formed from a ceramic material located about a wax perform of a
desired component casting.
15. A method of component casting substantially as hereinbefore
described with reference to the accompanying drawings.
16. A mould substantially as hereinbefore described with reference
to the accompanying drawings.
17. A component casting formed by a method as claimed in any claims
1 to 6 or claim 15.
18. A component casting formed using a mould as claimed in any of
claims 7 to 14 or claim 16.
19. An engine incorporating a component casting formed by a method
as claimed in any of claims 1 to 7 or claim 15.
20. An engine incorporating a component casting formed using a
mould as claimed in any of claims 7 to 14 or claim 16.
21. Any novel subject matter or combination including novel subject
matter disclosed herein, whether or not within the scope of or
relating to the same invention as an.about. of the preceding
claims.
Description
[0001] The present invention relates to component casting and more
particularly but not exclusively to component casting of
directional solidification or single crystal components for engines
such as blades, seal segments and nozzle guide vanes.
[0002] Component casting is used in order to produce a wide range
of components and members. Essentially, the component is cast in a
mould from a molten liquid and then allowed to cool in order to
leave a solidified component. Some components such as turbine
blades for jet engines require structural abilities such as high
temperature creep resistant. This is achieved with turbine blades
through forming a single crystal. At high temperatures, typically
above half the absolute melting temperature of the metal, the grain
boundaries become weaker than the grain bodies such that the
absence of such grain boundaries in a single crystal provides
resistance to creep.
[0003] Techniques for producing single crystal components are well
known. Essentially, the component is cast in a mould and then
gradually withdrawn from the furnace in an appropriate manner such
that propagation of a single crystal is achieved. Typically, a
so-called "pig-tail" selector is used in order to initiate a single
grain or crystal growth. The most important consideration with
respect to continued propagation of a single crystal within the
component is to ensure so-called directional solidification. This
is achieved by gradual withdrawal of the component from the furnace
such that the temperature gradient is effectively controlled.
Generally, the interface temperature between the solid and liquid
must be slightly lower than the melting point of the solid and the
liquid temperature must increase beyond the interface. To achieve
this temperature gradient, the latent heat of solidification must
be conducted through the solidifying solid crystal. In any event,
ideally the temperature interface should be flat and gradually
progress through the component in order to ensure a uniform single
crystal is provided with few, if any, defects at the interface. It
should also be understood that the solidus/liquidus mix or mushy
zone between the solid component and the liquid material should be
rendered as stagnant as possible. Unfortunately, most components by
their nature are shaped and so provide differing radiation heat
effects due to the varying thickness of the component at particular
points. These changes render it difficult to fully control the
temperature gradient and therefore an unacceptable proportion of
components are rejected due to defects formed during casting.
[0004] A preferred method of component casting is that known as the
lost wax process. This is a traditional technique in which a
component is initially formed as a wax structure and then a ceramic
coat placed upon that wax structure and allowed to harden. The wax
is then removed typically by heating in order to leave the ceramic
as a mould for the component. As indicated above, the component is
cast from a molten liquid and then allowed to cool and
solidify.
[0005] In accordance with the present invention there is provided a
method of component casting comprising forming a mould with a
displaced deflector element adjacent to a part of a principal mould
formation whereby that deflector element controls the rate of heat
loss from that part of the principal mould formation for more
equalised solidification through the component.
[0006] Preferably, forming of the mould is by a lost wax
technique.
[0007] Normally, the displaced deflector element is horizontal.
Possibly, the displaced deflector element is secured to a downpole
of the mould.
[0008] Possibly, the deflector element is coated for improved
radiation with a low emission reflective material. Typically, such
low emission material is a magnesium oxide.
[0009] Also in accordance with the present invention there is
provided a mould for component casting comprising a principal mould
formation and a displaced deflector element immediately adjacent a
part of the principal mould formation to control in use the rate of
heat loss from that part of the principal mould formation for more
equalised solidification in use through the component.
[0010] Possibly, the mould includes a downpole from which the
displaced deflector element is secured. Possibly, the displaced
deflector element is coated with a low, emission material such as a
magnesium oxide. Typically, the displaced deflector element is
located with sufficient gap to ensure there is sufficient mould
thickness thus there is a gap of approximately 2 mm to 3 mm from
the part of the principal mould formation. Generally, the displaced
deflector element has a configuration such that there is at least a
15 mm wide overlap with the principal mould formation.
[0011] Alternatively, the mould may be formed by stereo
lithography.
[0012] An embodiment of the present invention will now be described
by way of example only with reference to the accompanying drawings
in which:
[0013] FIG. 1 is a side illustration of a quarter section of a wax
prefrom of a cast component in accordance with the present
invention;.
[0014] FIG. 2 is a front elevation of the wax perform depicted in
FIG. 1;
[0015] FIG. 3 is a plan view in the direction A-A of the wax
perform depicted in FIGS. 1 and 2;
[0016] FIG. 4 is a front view of a part of a mould in accordance
with the present invention;
[0017] FIG. 5 is a plan view in the direction B-B of the part of
the mould depicted in FIG. 4; and,
[0018] FIG. 6 is a three quarter view, one component removed, of a
mould in accordance with the present invention.
[0019] As indicated previously, it is necessary to provide single
crystal components for use in certain environments. For example, a
turbine blade must be highly resistant to creep at high
temperatures and so a single crystal with its lack of grain
boundaries is the preferred structure. In order to achieve such a
single crystal structure close control of solidification after
casting is required. This close control of temperature ensures as
the component is removed from a furnace that single crystal growth
is propagated. Generally a calm flat. temperature boundary or
interface is required in order to achieve a single crystal
structure with no discontinuities.
[0020] One technique for forming a mould suitable for casting of a
single crystal component is using a lost wax process.
Alternatively, the mould could be formed by a stereo lithography
process. A one part mould is produced by coating or investing a wax
replica structure of the desired final cast component with a
refractory slurry which then sets at room temperature. The wax is
then removed generally by melting in order to leave a cavity in the
refractory slurry which is a mould of exactly the same shape as the
desired component, that is to say the initial wax. structure. The
cavity is then filled with a molten liquid material to cast the
final component. In accordance with the desired single crystal
structure it is necessary then to appropriately arrange cooling of
the liquid, that is to say solidification, in order to create that
desired single crystal structure.
[0021] FIGS. 1, 2 and 3 illustrate a wax form utilised in
accordance with the present invention. Thus, the form 1 comprises a
principal mould formation 2 equivalent to the shape of the desired
cast component and a displaced deflector element 3. The displaced
deflector element 3 is secured to a downpole 4 which is also part
of the form 1 and the eventual mould. As described previously, this
mould structure is formed in wax and a quarter section is shown in
FIGS. 1 to 3. Thus, the. deflector 3 will generally be presented
all round the principal mould formation 2 at a desired
position.
[0022] As indicated previously, temperature interface control with
respect to the solidification of the component is necessary in
order to achieve the desired single crystal structure. A technique
known as directional solidification is used whereby the mould
incorporating the cooling cast component is gradually removed from
a furnace in order to precipitate the desired single crystal
structure. Generally, a pig-tail selector 5 is used in order to
initiate such single crystal structure propagation. Unfortunately,
as can be seen in the mould form 1 formed of wax shown in FIGS. 1
to 3 and in particular the principal mould formation 2 which is a
replica of the final cast component, the dimensions and thicknesses
of that cast component vary along its length. Such variations make
it very difficult to achieve the desired flat temperature gradient
interface in order to achieve the desired single crystal structure.
In particular as depicted in FIGS. 1 to 3 root portions of the
principal mould formation 2 are generally narrower than other parts
of that formation 2 such that there can be a variable temperature
gradient through the cross-section of the formation 2 and therefore
cast component in these areas.
[0023] In accordance with the present invention the deflector
element 3 is positioned about the root part of the formation 2
whereby the temperature gradient across the component at that
position is maintained as substantially flat. Thus, central parts
of the cast component will be at approximately the same temperature
as surface parts of that component despite the relatively thin
cross-section of the component. In such circumstances, the whole
width and cross-section of the component is substantially at the
same temperature and will cool at the same rate to ensure better
single crystal propagation or directional solidification without
defects.
[0024] The deflector element 3 is formed by providing upon the
downpole 4, during formation of the wax replica for the finished
component, a shaped protrusion which extends towards the principal
mould formation 2 such that a surface 6 is adjacent but displaced
from an overlap. zone of the principal mould formation 2. In
accordance with the usual lost wax process the structure shown in
FIGS. 1 to 3 is then coated with a ceramic slurry in order to
provide the structure as shown in FIGS. 4 and 5. This comprises
coating the wax with several layers of ceramic slurry and stucco
and allowing those layers to solidify. The wax is then removed to
leave the mould behind. Molten metal is then placed in the mould
and allowed to solidify in accordance with the control regime as
described previously in order to create a single crystal. This
technique is known as single crystal or directional solidification
(CDS). Directional solidification furnaces usually comprise one or
two heater zones, an insulation ring or baffle and a lower cooling
zone. The mould in which the casting is solidifying is slowly
withdrawn from the heater zone into the cooling zone at a
predetermined rate in order to produce uni-axial heat flow in the
opposite direction to the desired crystal growth. In such
circumstances, it will be appreciated that variations in the
cross-sectional area of the component can create convective
instabilities, local thermal gradients and other problems which in
turn can create defects within the crystal structure.
[0025] In accordance with the present invention and as depicted in
FIGS. 4 and 5, the deflector element 13 is designed to control the
radiative heat flux from the relatively narrow component part of
the principal mould formation 12 cavity during solidification. This
control is achieved by modifying the radiation view factors of
local overlap areas of the principal mould formation 12. In
particular, local areas specifically on the inner or downpole 14
side of the principal mould formation 12 where a traditional
furnace baffle would be ineffective. As can be seen in the Figures
the deflector element 3; 13 essentially comprises a radiation
deflector in the form of a horizontal plate mounted upon the
downpole 4; 14 at a position below an identified temperature
gradient problem area of the casting. The deflector 3; 13 is added
during the wax form arrangement stage as a thin wax plate attached
to the down pole 4. Alternatively, the deflector 3; 13 may be a
ceramic plate instead of a wax plate provided a sufficient mould
thickness is created and it has the correct shape. The deflector 3;
13 ideally will follow the contour of the opposed overlap part of
the component principal mould formation 2, 12 to avoid higher heat
loss around the surface of that part. It is possible to use a
simple disc as the deflector 3; 13 but this 30 would provide a
lower benefit. Generally, the deflector 3; 13 is formed by the wax
protrusion or plate will. be processed or modelled as a 2 mm to 3
mm thick plate of wax with a nominal reflector thickness and a
coating with the layer of ceramic slurry and stucco in the order of
15mm. The thickness can be between 1 to 3 mm, a vertically thin
feature is required, but thickness is governed by the strength of
the wax used, again a gap of 15 mm is not specific to the present
invention. When the preferred loss wax technique is used such a gap
allows a mould shell thickness to form about the wax pattern of
sufficient strength for casting.
[0026] As indicated previously,. the effect of the deflector 3; 13
is to reduce the radiation view factor for the principal moulding
formation 2; 12 on the inner downpole, 4: 14 side of the casting
within that principal moulding formation 2; 12. In such
circumstance, there is a heat flux from these protruding features
which in turn balances the temperature gradient at the critical
solid/liquid interface. The deflector 3, 13 reflects heat back onto
the protruding feature of the casting (principal mould feature) so
reducing the radiative heat loss from these features. A more
controlled temperature gradient at this solid liquid interface
reduces the risk of secondary grain formation caused by convective
flow instabilities and other problems within the still fluid
casting material. The deflector 3, 13 also reduces other defects
caused by low thermal gradients, such as linear eutectics in DS
castings and shrinkage porosity. Thus, defects as a result of a
number of problems are relieved.
[0027] It will be appreciated by allowing more complex component
structures to be used additional features can be cast into the
component rather than requiring those features to be achieved by
subsequent machining of the cast component. Clearly, such machining
also requires removal of material which is wasteful and expensive.
Such machining may also introduce stress fracturing which may be
exploited by temperature cycling in use of the component and so
cause premature failure.
[0028] The present deflector 3, 13 is created during the wax
formation stage and so unlike fixed baffles within a furnace can be
placed at any location where there is an identified need. It will
also be understood that the deflector 3, 13 need not follow the
extreme outer profile of the mould so the reflector can be placed
anywhere that there is a consistent repeatable defect identified
with variable overlap and spacing determined as necessary for
temperature control. The deflector 3; 13 can be easily produced
upon the downpole 4 during the wax form fabrication 3; 13 stage or
as an additional piece added to the downpole 4 during such wax
structure fabrication. Essentially, the deflector is formed by the
already present wax and ceramic slurry/stucco combination and in
such circumstances does not significantly add to cost particularly
in comparison with baffles and more sophisticated twin zone heaters
used in existing furnace arrangements in order to overcome the
described defect problems due to solid liquid interface temperature
variations. The deflector could be made from any material as long
as it either does not react with the cast alloy, or can be removed
during dewax or pre-firing of the mould, can be shelled or coated
to form the mould, or can be produced as a. feature in a stereo
lithographic ceramic mould.
[0029] FIG. 6 illustrates a three quarter part front perspective of
a practical mould 100 in accordance with the present invention,
that is to say with one component mould removed. Thus, it can be
seen in this embodiment that the mould in use presents four
principal mould formations 102 located substantially perpendicular
to each other on a base plinth. However, the deflector can be used
with any number of components on a mould but may be limited by the
size of the component, or the size of the furnace used to cast the
component. The deflector must follow the outer profile of the
component and is positioned in a desired location to prevent
defects. The deflectors 103 are formed as a downpole 104 to be
presented at appropriate positions for temperature gradient
stabilisation as described above.
[0030] It will be appreciated a number of deflectors in accordance
with the present invention could be utilised at different positions
relative to the principal moulding formation in order to ensure
appropriate temperature control during solidification of the
casting. The specific shape, size and location of the reflector is
determined through a radiative modelling procedure in order to best
maintain temperature to achieve the consistency of temperature
across the solid-liquid interface through the component as it is
gradually withdrawn from the furnace.
[0031] With a turbine blade, the present deflector will generally
be used with respect to the thermal gradient in the root fir tree
or shank neck regions of the casting which forms the turbine blade.
As indicated previously, these are the parts of the turbine blade
where the reduced cross-sectional area can result in variable
thermal gradient and convection flow induced defects. Generally,
root and shank parts are the biggest problem areas on a turbine
blade, but a deflector could be used on any directional
solidification or single crystal formed component. Deflector being
located below any feature which is the source of a solidification
defect.
[0032] In order to enhance the performance of the deflector it will
be appreciated that once the layer of ceramic slurry and stucco has
dried and hardened, an appropriate high emission material such as a
magnesium oxide paint may be applied to the deflector in order to
further enhance its radiative abilities. It will be understood that
it is heat from the casting within the principal mould formation
which is reflected back to that moulding in order to control
temperature as required to resist grain boundary formation. It will
be understood that radiative heat loss is the main manner of
cooling of the casting within the principal moulding formation as
the mould is generally within a vacuum furnace whereby convective
airflows about the mould are removed to again avoid differential
cooling along the length of the casting within the principal mould
formation. Nevertheless, generally the upper part of the furnace
and therefore the principal mould formation and casting will be
hotter than the lower part due to positioning of the heaters, etc.
Thus, the mould is gradually removed from the furnace by a downward
motion thus gradually cooling the casting in the principal moulding
formation.
[0033] Although mainly described with reference to use of a
downmpole it will be understood that separate upstanding legs may
be used in the mould where space is available in order to
appropriately present the deflector element to the principal mould
formation. Similarly, although described with regard to a turbine
blade other components such as a seal or nozzle vane could be made
using the present method and/or mould.
[0034] Whilst endeavouring in the foregoing specification to draw
attention to those features of the invention believed to be of
particular importance it should be understood that the Applicant
claims protection in respect of any patentable feature or
combination of features hereinbefore referred to and/or shown in
the drawings whether or not particular emphasis has been placed
thereon.
* * * * *