U.S. patent application number 12/385206 was filed with the patent office on 2010-06-24 for manufacture of an article by hot isostatic pressing.
This patent application is currently assigned to ROLLS-ROYCE PLC. Invention is credited to Junfa Mei, Wayne E. Voice, Nicholas Wain, Xinhua Wu.
Application Number | 20100158742 12/385206 |
Document ID | / |
Family ID | 39522680 |
Filed Date | 2010-06-24 |
United States Patent
Application |
20100158742 |
Kind Code |
A1 |
Voice; Wayne E. ; et
al. |
June 24, 2010 |
Manufacture of an article by hot isostatic pressing
Abstract
In a hot isostatic pressing (HIP) process, for example for
securing blades (5) to a disc (3) to form a blisk (1), the
temperature of the powder (6) to be consolidated is raised to a
predetermined temperature, which may be the eventual required
pressing temperature, before the pressure rise to the pressing
pressure is initiated. If the powder is a titanium alloy and the
process employs steel tooling pieces, the application of pressure
occurs only when the temperature has risen to the extent that the
steel is harder than the titanium alloy.
Inventors: |
Voice; Wayne E.;
(Nottingham, GB) ; Mei; Junfa; (Birmingham,
GB) ; Wu; Xinhua; (Birmingham, GB) ; Wain;
Nicholas; (Worcester, GB) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 320850
ALEXANDRIA
VA
22320-4850
US
|
Assignee: |
ROLLS-ROYCE PLC
LONDON
GB
|
Family ID: |
39522680 |
Appl. No.: |
12/385206 |
Filed: |
April 1, 2009 |
Current U.S.
Class: |
419/49 |
Current CPC
Class: |
B22F 3/15 20130101; B23K
20/021 20130101; B23K 2101/001 20180801; B22F 2998/00 20130101;
B22F 3/003 20130101; B22F 2998/00 20130101; F01D 5/34 20130101 |
Class at
Publication: |
419/49 |
International
Class: |
B22F 3/15 20060101
B22F003/15 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 29, 2008 |
GB |
0807703.4 |
Claims
1. A method of manufacturing an article by hot isostatic pressing
of a powder using a tooling piece, wherein the powder comprises a
first material and the tooling piece comprises a second material,
the powder material being harder at ambient temperature than the
tooling piece material, the method comprising: raising the
temperature of the powder to a pressing temperature; and, applying
a pressing pressure to the powder by the tooling piece,
characterised in that the relative hardness of the first and second
materials varies with temperature and the pressure applied to the
powder is maintained below the yield stress of the second material
until the temperature of the powder is greater than a predetermined
temperature at which the hardness of the two materials is
equal.
2. A method as claimed in claim 1, wherein the pressing temperature
is greater than the predetermined temperature such that the first
material is softer than the second temperature upon pressing.
3. A method as claimed in claim 1, wherein the powder comprises a
metallic material.
4. A method as claimed in claim 3, wherein the powder comprises a
titanium alloy.
5. A method as claimed in claim 1, wherein the tooling piece
comprises a metallic material.
6. A method as claimed in claim 5, wherein the tooling piece
comprises steel.
7. A method as claimed in claim 5, wherein the tooling piece
comprises a steel body provided with a ceramic coating.
8. A method as claimed in claim 1, wherein the predetermined
temperature is greater than 400.degree. C.
9. A method as claimed in claim 8, wherein the predetermined
temperature is not less than 100.degree. C. lower than the pressing
temperature.
10. A method as claimed in claim 9, wherein the predetermined
temperature is approximately equal to the pressing temperature.
11. A method as claimed in claim 1, wherein the temperature of the
powder is raised from ambient temperature to the pressing
temperature over a first time period, the applied pressure is
raised from a holding pressure to the pressing pressure over a
second time period which begins when the temperature has reached
the predetermined temperature, and the pressing temperature and
pressing pressure are maintained for a third time period from the
end of the second time period.
12. A method as claimed in claim 11, wherein the first time period
is not less than 2.5 and not more than 3.5 hours.
13. A method as claimed in claim 11, wherein the second time period
is not less than 1.5 hours and not more than 2.5 hours.
14. A method as claimed in claim 11, wherein the third time period
is not less than 2.5 hours and not more than 3.5 hours.
15. A method as claimed in claim 1, wherein the article is a bladed
disc for a gas turbine engine.
Description
[0001] This invention relates to the manufacture of an article by
hot isostatic pressing (HIP), and is particularly, although not
exclusively, concerned with the manufacture of a component of a gas
turbine engine such as a bladed disc or "blisk".
[0002] It is common for bladed rotors of gas turbine engines to be
assembled from a disc and individual blades which are separately
fitted to the disc and secured by mechanical fixings. More
recently, the disc and blades have been formed integrally with one
another, the resulting components being referred to as
"blisks".
[0003] One method of manufacturing a component such as a blisk is
disclosed in EP1702709A. In the process disclosed in that document
the connection between each of the blades and the disc is
established by a union piece which is formed by hot isostatic
pressing of a powder, such as a titanium alloy. The powder is
formed into a respective preform for each blade and the preform is
set on the disc in the appropriate position and receives the blade.
When all blades have been assembled on the disc with their union
pieces, tooling pieces are fitted between adjacent blades, to
contact the union pieces. The union pieces are thus situated in
cavities defined by the disc, the blades and the union pieces.
[0004] To perform the hot isostatic pressing operation, the
assembly is heated to a pressing temperature between 920 and
930.degree. C. in an inert gas environment (for example, argon) at
a pressing pressure between 100 and 150 MPa. The combined effect of
the temperature and pressure is to force the pressing pieces
radially inwardly of the assembly to consolidate the powder of the
union piece preforms so that they form fully dense union pieces
with "wrought" properties. At the same time, the union pieces are
connected by diffusion bonds to the surface of the disc and to the
blades.
[0005] The shape of each union piece over its surface extending
from the respective blade to the disc is formed by the shape of
respective tooling pieces on each side of the blade. The tooling
pieces are made from steel, and may be coated, for example with a
ceramic coating such as boron nitride, to prevent adherence between
the tooling pieces and the union piece so that the tooling pieces
can be re-used.
[0006] In the process disclosed in EP1702709A, the hot isostatic
pressing process is performed by placing the assembly in an oven
which is then filled with the inert gas. The pressure and
temperature in the oven are raised simultaneously to the required
pressing temperature and pressure referred to above, and the
assembly is then maintained at those conditions for a prolonged
period, such as about 4 hours.
[0007] In many applications of hot isostatic pressing processes,
particularly those used for the manufacture of blisks, the material
of the powder is much harder at ambient temperatures than the steel
from which the tooling pieces are made. Consequently, elevation of
the pressure while the temperature remains relatively low causes
the tooling pieces to be pressed with great force against the union
piece preform (or loose powder if the cavities are filled with
loose powder instead of preforms), which deforms the surface of the
tooling pieces so that they receive impressions of the individual
particles of the powder.
[0008] This can result in a poor finish on the union pieces, and
also damage the surface quality of the tooling piece for future
forming operations.
[0009] According to the present invention there is provided a
method of manufacturing an article by hot isostatic pressing of a
powder in which method the temperature of the powder is raised to a
pressing temperature and a pressing pressure is applied to the
powder by a tooling piece, characterised in that the pressure
applied to the powder is maintained below the yield stress of the
material of the tool piece until the temperature of the powder has
reached a predetermined temperature.
[0010] In one application of the method, the powder is of a first
material and the tooling piece is of a second material, the powder
material being harder at ambient temperature than the tooling piece
material. The relative hardness of the tooling piece material and
powder material may vary with temperature. The hardness of the
tooling piece material and powder material may be equal at a
predetermined temperature, which is greater than ambient. The
hardness of the tooling piece material may be greater than the
powder material at the pressing temperature.
[0011] The pressing temperature is typically greater than or equal
to the predetermined temperature such that the powder is softer
than the tooling material upon pressing. The pressing pressure may
be greater than the yield stress of the powder material but less
than the yield stress of the tooling material at the pressing
temperature.
[0012] By the "hardness" of a material, in the context of the
present invention, is meant the stress at which plastic deformation
of the material occurs.
[0013] The powder may be formed of, or consist of, a metallic
material, for example a titanium alloy. The tooling piece may be
made from, or consist of, a metallic material, for example steel.
The tooling piece may comprise a steel body provided with a ceramic
coating, for example of boron nitride.
[0014] In some embodiments of the present invention, for example
when a titanium alloy powder is subjected to a hot isostatic
pressing operation using a steel tooling piece, the predetermined
temperature is greater than 400.degree. C. The predetermined
temperature may be within 100.degree. C. of the pressing
temperature, and may be approximately equal to the pressing
temperature.
[0015] In a particular method in accordance with the present
invention, the temperature of the powder may be raised from ambient
to the pressing temperature over a first time period, the applied
pressure may be raised from a holding pressure to the pressing
pressure over a second time period which begins when the
temperature has reached the predetermined temperature, and the
pressing temperature and pressing pressure may be maintained for a
third time period from the end of the second time period. In a
particular embodiment, the first time period may be not less than
2.5 and not more than 3.5 hours, for example 3 hours, the second
time period may be not less than 1.5 hours and not more than 2.5
hours, for example 2 hours, and the third time period may be not
less than 2.5 hours and not more than 3.5 hours, for example 3
hours.
[0016] By "ambient temperature" is meant the temperature of the
ambient atmosphere around the equipment in which the hot isostatic
pressing operation takes place. By "holding pressure" is meant an
initial pressure to which the powder, or a preform made from the
powder, is subjected by the tooling piece before the assembly is
subjected to the increased temperature and pressure.
[0017] In general, the holding pressure is sufficient to maintain
the components of the assembly (for example a disc, an array of
blades, and preforms formed from the powder) in their correct
relationships with each other, but lower than that required to
cause any significant compaction of the powder. Thus, application
of the holding pressure is not considered to be initiation of the
application of the pressing pressure.
[0018] In one application of the method in accordance of the
present invention, the article is a bladed disc for a gas turbine
engine.
[0019] For a better understanding of the present invention, and to
show more clearly how it may be carried into effect, reference will
now be made by way of example, to the accompanying drawings, in
which:
[0020] FIG. 1 shows a schematic cross section of a bladed disc
arrangement for manufacture in accordance with the present
invention;
[0021] FIG. 2 shows a schematic cross section of the bladed disc
and tooling during manufacture;
[0022] FIG. 3 is a graph of stress plotted against strain for a
titanium alloy (Ti-6-4) against mild steel at ambient
temperature;
[0023] FIG. 4 corresponds to FIG. 3 but represents stress plotted
against strain at 800.degree. C.;
[0024] FIG. 5 is a graph plotting temperature in .degree. C. and
pressure in bar against time for a known hot isostatic pressing
process;
[0025] FIG. 6 corresponds to FIG. 3 but represents a process in
accordance with the present invention;
[0026] FIG. 7 is a photo-micrograph of the interface between a
compacted titanium alloy powder and a tooling piece following
completion of a known hot isotatic pressing process; and
[0027] FIG. 8 corresponds to FIG. 5, but represents the condition
of the components following completion of a hot isostatic pressing
process in accordance with the present invention.
[0028] Turning to FIG. 1, a compressor blisk 1 comprises a central
workpiece, cylindrical disc 3, manufactured from a titanium alloy
forging. A number of second workpieces, aerofoil blades 5 forged
from the same titanium alloy, are attached to the disc 3,
distributed evenly about the radially outer surface 7 of the disc
and projecting radially therefrom. Each blade 5 is attached to the
disc 3 via a union piece 6, consolidated from a powdered titanium
alloy, in the present case Ti6/4, with particle sizes, before
consolidation, of about 250 micron or less. Each union piece 6
provides a blended, radiussed fillet joint between the disc 3 and
each blade 5.
[0029] In the present example, this joint has a mean radius of
about 5 mm. The union piece 6 extends around the entire blade
section and each union piece 6 comprises a piece of titanium alloy
with similar properties to a forged item of the same material, the
base of which defines a lower surface which conforms with the outer
surface 7 of the disc 3, and defines a footprint of similar shape
to the compressor blade 5 cross-section but of larger area, by at
least 100%. The union 6 tapers upwards and inwards from this
footprint to a height of about 5 mm from the base, at which height
it blends with the external gas-washed surface of the aerofoil 5,
with minimal step. The taper is shaped to form the radiussed fillet
joint between each blade 5 and the disc 3. Such joints serve to
reduce stresses at the blade/disc interface.
[0030] The union material 6 is centrally relieved to provide a
blind socket which forms a close-fitting location feature to
receive the blade 5, which is diffusion bonded to the union 6.
During manufacture, the base of each blade is diffusion bonded to
the socket base and the blade gas-washed surfaces are diffusion
bonded to the socket sidewalls. The base of the union piece 6 is
diffusion bonded to the disc surface 7.
[0031] FIG. 2 shows a cross section through part of a blisk
assembly used in the manufacture of the blisk 1. The titanium alloy
powder which provides the union material is supplied to cavity 9
formed between each blade 5 and the disc 3. Tooling pieces 4 are
inserted into the region between adjacent blades 5 and form a close
fit therewith. The assembly is enclosed within a mild steel
surround 11.
[0032] Each tooling piece 4 comprises a block of mild steel,
provided with a boron nitride coating, which occupies substantially
the whole volume between adjacent blades 5. With the tooling pieces
4 in place, the assembly forms a substantially solid disc with
outer diameter equal to the blade tip diameter.
[0033] Each tooling piece has faces shaped to closely conform to
the surface of an adjacent blade 5. Accordingly, the tooling piece
comprises a convex blade engagement surface and an opposing a
concave blade engagement surface, those surfaces being shaped to
conform to respective pressure and suction surfaces of the adjacent
blades.
[0034] The opposing side faces of each tooling piece 4 are tapered
to form a wedge which narrows in the direction of the disc surface
7. The radially inner surface 15 of this wedge is truncated and
relieved to provide one or more forging surfaces shaped to conform
to the opposing fillet joints of the adjacent blades 5 and the
region of the disc surface 7 which lies between the blade fillet
joints. The tooling pieces 4 are pressed radially inwards during
formation of the blisk to apply the desired shape to the union
pieces 6.
[0035] In a preferred embodiment of the present invention, the
metal powder for the union 6 is supplied as a
`partially-consolidated` preform, with solid-like properties, but
with a material density of typically about 67%. The preform is
shaped to define a socket which facilitates location of the preform
about the base of the corresponding blade 5.
[0036] The mild steel surround 11 comprises a drum-like structure
which fully encloses the assembly therein and which wraps around,
and abuts the blade tips and outer surfaces of the tooling pieces
4. The surround 11 is evacuated prior to sealing about the
components of the blisk assembly.
[0037] It will be appreciated from FIGS. 3 and 4 that, at ambient
temperature, titanium alloy, and particularly Ti-6-4, is
substantially harder than mild steel. The elastic limit of mild
steel is reached at approximately 300 MPa, while that for Ti-6-4 is
reached at approximately 900 MPa. As shown in FIG. 4, it has been
discovered that the position is reversed at 800.degree. C., with
the elastic limit for Ti-6-4 being reached below 20 MPa, while that
for mild steel is reached at approximately 30 MPa.
[0038] In a known hot isostatic pressing process, the sequence of
operation is shown in FIG. 5. For convenience of illustration, a
single y-axis scale is shown, so that the same scale can represent
both the temperature in .degree. C. and the pressure in bar.
[0039] FIG. 5 relates to the manufacture of a bladed disc for a gas
turbine engine. It will be appreciated that from time 0 (ie the
time at which the assembled disc, union piece, preforms and blades)
are placed in an oven, that both the temperature and pressure begin
to rise immediately as power is supplied to the heating means of
the oven and inert gas (such as argon) is admitted to the oven and
pressurised, both by means of a pump and by means of the increasing
temperature. The temperature rises to approximately 930.degree. C.
over a period of approximately 3 hours, and the pressure rises to
100 MPa (100 bar) over the same time period. When the pressure and
temperature reach these pressing values, they are maintained for 4
hours, following which heating is terminated and the temperature
and pressure are allowed to return to the ambient values.
[0040] In the early stages of the process, the pressure increases
significantly while the temperature rises from the ambient level to
the level at which the hardnesses of Ti-6-4 and mild steel become
equal (i.e. a condition between the two conditions represented in
FIGS. 3 and 4). The predetermined temperature at which this happens
will vary with the precise composition of the titanium alloy and
the mild steel, but in many cases will fall in the range 400 to
500.degree. C.
[0041] The result of this is that the titanium alloy powder and the
steel are subjected to substantial pressures, of the order of 40 to
50 MPa (400 to 500 bar) while the titanium alloy is harder than the
mild steel. Consequently, the mild steel of the tooling pieces 4 is
deformed, under the applied pressure, by the particles of the
titanium alloy. This gives rise to an interface between the two
materials as shown in FIG. 7, in which indentations 2 in the
tooling piece 4 can be seen, which are formed by individual
particles of the titanium alloy powder of the union piece 6.
[0042] The consequence of this is, firstly, that the compacted
titanium alloy union piece 6 at the end of the pressing operation
does not have a smooth surface finish, with the result that a
finishing operation may be required to eliminate surface roughness.
Secondly, the surface of the steel cooling piece is deformed from
its initial smooth configuration to the rough, indented
configuration shown in FIG. 7. Consequently, the surface of the
tooling piece 4 is degraded, possibly making it unsuitable for
re-use in a subsequent hot isostatic pressing operation. If the
tooling piece has a ceramic coating, the indenting of the surface
can cause cracking of the coating
[0043] FIG. 6 corresponds to FIG. 5, but represents a hot isostatic
pressing operation in accordance with the present invention. In the
process FIG. 6, the temperature is increased, as before, to
930.degree. C. over a first time period of three hours. However,
unlike the process represented in FIG. 5, the increase in pressure
is deferred until the temperature has reached the pressing
temperature of 930.degree. C. Instead, a relatively low holding
pressure of the order of 5 MPa is maintained in the oven as the
temperature is raised.
[0044] At the end of the first time period, ie when the pressing
temperature of 930.degree. C. is reached, the pressure is increased
to the pressing pressure of 100 MPa over a second time period of
approximately two hours. As will be appreciated from FIG. 4, the
steel tooling pieces will be harder at the pressing temperature
than the titanium alloy of the powder. As the pressure rises, it is
the particles of the titanium alloy powder that are deformed by
contact with the steel tooling pieces 4, so that the steel tooling
pieces maintain their surface smoothness and the same smoothness is
imparted to the surfaces of the union pieces. Consequently, as
shown in FIG. 8, the interface between the compacted titanium
powder and the steel tooling piece 4 is smooth, by comparison with
the roughened interface shown in FIG. 7.
[0045] The pressing temperature and pressure are maintained for a
third time period of 3 hours from the end of the second time period
(ie when the pressing pressure has been reached), in order to
complete compaction of the titanium alloy powder. At the end of the
third time period, the temperature and pressure are returned to
ambient in the same manner as for the known process presented in
FIG. 5.
[0046] The resulting surface of the union piece 6 formed from the
titanium alloy powder, as shown in FIG. 8, means that the surface
definition and surface quality of the union piece 6 is improved
over that achieved with the known process. Consequently, the union
piece 6 does not need subsequent processing prior to use, and the
process can be regarded as a true net-shape process which achieves
the desired finish surface directly from the hot isostatic pressing
operation. The use of a ceramic coating on the steel tooling piece
4 assists with the removal of the tooling piece 4 from the formed
titanium powder union piece 6 or other component, and prevents any
reaction, or undesired bonding, between the titanium alloy and the
tooling piece 4.
[0047] It will be appreciated from FIGS. 3 and 4 that the increase
in pressure need not be deferred until the increase in temperature
to the pressing temperature of 930.degree. C. is complete. Instead,
the second time period during which the pressure is increased can
begin earlier, provided that the pressure applied to the powder
remains below the yield stress of the material of the tooling piece
until a temperature has been reached at which the hardness of the
material of the tooling piece 4 (for example steel) is at or above
the hardness of the material of the powder, (for example titanium
alloy). Preferably, until that temperature is reached, the applied
pressure is kept at an appropriate level blow the yield stress of
the tooling piece material to take account of the higher
temperature of the tool as heating progresses, and to take account
of stress concentrations at the surface of the tooling piece as a
result of uneven contact between the tooling piece and the powder
in the early stages of pressure application.
[0048] By increasing the pressure at a controlled rate as the
temperature rises, it is possible to maintain contact between the
tooling and the powder to ensure consistent compression of the
powder, without the formation of voids which can occur if the
powder should sinter and consolidate lower in the mould cavity.
[0049] While the invention has been described with reference to a
titanium alloy powder and mild steel tooling pieces, it will be
appreciated that the present invention can be applied to hot
isostatic pressing processes involving different materials,
provided that the relative hardnesses of the materials invert with
increasing temperature, as discussed with reference to FIGS. 3 and
4.
* * * * *