Note: Descriptions are shown in the official language in which they were submitted.
77Z
The present invention relates to a method of manu-
facturing an object of metallic or ceramic material.
In the manufacture of objects of metallic or
: ceramic material by sintering powder of the material while
using isostatic pressing, the powder is suitably preformed
into a manageable powder body. This can be done by loose
sintering, i.e. by sintering a powder, filled into a
forming cavity, in vacuum or protective gas so that a cohe-
rent body is formed but no mentionable densification takes
place. It can also be done by subjecting the powder to an
isostatic compaction, for example arranged in a sealed cap-
sule of yielding material, such as a plastic capsule. The
Gompaction can be carried out with advantage without the
; use of a binder at room temperature or any other tempera-
ture which is considerably lower than the temperature
during the compression in connection with the sintering.
The product can thereafter be given its desired shape by
; machining. For the preforming it is al~o possible to use,
among other things, conventional technique for the manu-
facture of ceramic goods. Thus, usually the powder is
mixed before the forming with a temporary binder, for
example methyl cellulose, cellulose nitrate, an acrylate
binder, a wax or a mixture oE waxes. After the preforming
the binder is driven off by heating so that the preformed
powder body in all essentials becomes free from binder.
When the preformed powder body is subjected to
the isostatic pressing at the sintering temperature, it
must, in order to give a desired dense, sintered product,
be enclosed in a casing which, during the pressing, is able
to prevent the pressure medium then used, normally a gas,
from penetrating
.~ ~_~
~, ,.
772
into the powder body. The oasing, like its contents, is liberated from un-
desirable gases during a process stage prior to the æealing. Various ways
of forming the casing are known. According to one known method, a preformed
capsule of glass is used as casing. According to another known method, the
casing i9 manufactured on the spot by dipping the preformed powder body
into a suspension of particles of glass, or surrounding it in some other
manner with a layer of particles of glass and then heating it under vacuum
at such a temperature that the particles form a tight casing around it.
As far as~silicon nitride is concerned, it is also known to use a porous
layer of a glass of a low-melting type outside a porous layer of glass of
a high-melting type. In the known case, the outer porous layer is transformed
into a layer i~permeable to the pressure medium while the powder body is
degassed. When a tight layer has been formed, pressure is applied to the
enclosed powder body by ægon or helium to counteract dissociation of the
silicon nitride when the temperature is continually raised. During the
continued temperature increase, the glass in the outer layer reacts with
the material in the inner porous layer while forming an increasingly high-
melt:ing glass and while maintaining a layer impenetrable to the pressure
med um, and finally a glass layer which i8 impenetrable to the pressure
medium is formed from the innermost part of the inner porous layer before
the glass in the outer layer has time to run off. ~his last formed glass
layer forms a casing around the powder body when the isostatic pressing
thereof is carried out at the ~intering temperature.
,''
In certain cases it has been found that there are problems in achieving
a desirably great reproducibility in the manufacture of objects of powder
material while using the known methods described above, especially when
s it is a question of objects having a complicated shape such as objects
having sharp corners or edges or having thin-walled portions such as turbine
disks with blades. If a preformed capsule of glass is used, there is a risk
that the glass, when softening, is accumulated in pockets and, because of a
relatively high viscosity, may cause damage there in thin-walled portions
of the preformed powder body in connection with a high pressure being applied
for the isostatic pressing of the powder body. If the casing is manufactured
on the spot by surrounding the powder body with layers of particles of glass,
there is a risk, which is also present when using a preformed body of glass~
that the powder body in certain places, especially at sharp corners or
edges, is not covered by any casing material when the pressing is to be
carried out, becau~e the glass is not retained there.
-2-
~877Z
According to the present invention, it has
proved to be possible to manufacture objects of powder
ma-terial with high density with greater reproducibility
than with the previously mentioned methods.
According to thè present invention, there is
provided a method of manufacturing an object of metallic
or ceramic material, characterized in: embedding a pre-
formed powder body of metallic or ceramic material in an
embedding material, placing said preformed body and the
; 10 embedding material in an open vessel which is resistant
to temperatures at which sintering of the metallic or
` ceramic material is carried out, transforming the embed-
ding material into a melt having a substantially horizontal
surface limited by the walls of the vessel with the pre-
formed body being located below the surface of the melt,
and applying directly on the melt a pressure by a gaseous
pressure medium for isostatic pressing while sintering the
preformed powder body.
The word melt in the description and the claims
refers to a gas-impermeable mass, which at least partly and
preferably at least for the main part consists of molten
phase. It is thus not necessary that all the constituents
of the embedding material have melted in their entirety in
order for a functioning gas-impermeable mass, here included
in the concept melt, to have formed.
It is essential that the melt is subjected to
pressure by a gaseous pressure medium, which takes place
in a heatable high-pressure chamber, and not by a piston in
a mould cavity, in which the melt is enclosed while making
contact with the walls of the mould cavity. In the latter
case it is not possible, or at any rate there are extre-
mely great difficulties, to avoid damage to weak portions
of the powder body, because it is not possible to maintain
a sufficiently low viscosity of the glass melt, since in
that case it tends to penetrate out between the piston
., ,
;fi, . - 3-
~8772
and the mould cavity.
The invention is oC extremely great importance
in connection with isostatic pressing of objects of silicon
nitride or of materials built up with silicon nitride as
the main constituent. The invention will therefore first
be described with respect to its application for the manu-
facture of objects of silicon nitride.
As pressure medium for carrying out the present
invention, there are preferred inert gases such as argon
and helium, as well as nitrogen gas. The pressure during
the sintering of a preformed silicon nitride body is depen-
dent on whether a sintering-promoting additive, such as
magnesium oxide or yttrium oxide, has been added to the
silicon nitride or not. If no such additive is used, the
pressure preferably amounts to at least 100 MPa, preferably
to 200-300 MPa. When using a sintering-promoting additive,
a lower pressure may be used, however suitably at least
`~ 20 MPa. The sintering of the preformed body is suitably
.- carried out a-t 1600-1900C, preferably at 1700-1800C.
As material in the vessel which is resistant to
the sintering temperature there is preferred graphite,
but also other materials such as nitride or molybdenum
may be used.
The embedding material may with particular advan-
tage consist of particles of glass or of material forming
glass upon heating. The preformed silicon nitride body is
then embedded into the particles in the resis-tant vessel
and the particles are transformed into a melt in the vessel,
that is, the embedding material is made gas-impermeable in
the vessel. It is also possible to use larger pieces of
the glass or of the glass-forming material, such as pre-
formed pieces which at least substantially follow the
shape of the preformed body. For example, it is possible
- to use a preformed piece on the lower side of the pre-
formed body and another preformed piece on the upper side
~ -4-
;
7~2
of the body, the edges of the pieces then suitably making
contact with each other. Depending on the shape of the
~- ob]ect, it is also possible in principle to use a capsule
of glass formed in one piece and having an opening which
is adapted so that the prèformed body may be inserted into
the capsule. When the embedding material is made from one
or a few glass pieces, it is possible to make it gas~
impermeable either before or after it has been placed in
the vessel. In the first-mentioned case this can be done
with advantage in a separate process in a furnace which is
;~ suited for this pourpose and in the latter case it may be
done with advantage in connection with the embedding
material being transformed into a melt in the high-pressure
furnace in which the isostatic pressing is carried out.
The embedding material may advantageously be made
gas-impermeable while maintaining vacuum around it. To
avoid a dissociation of the silicon nitride, a glass or a
glass-forming material should be used in this case, which
makes the embedding material gas-impermeable at a relati-
vely low temperature. Thus, if the embedding material
consists of particles of glass or glass-forming material
~; ` -4a-
.;. .
'72
which are made gas-impermeable by being transformed into a melt in the
resistant vessel while maintaining vacuum across it, a glass should be used
which results in a melt with a low viscosity, suitably at most 106 poises at
a temperature of about 1150 C, so that a high pressure may be applied at this
temperature without risks of damage to the pre~ormed body arising.
:.
The embedding material may also advantageously be made gas-impermeable while
keeping it in contact with a gas which at least for the main part consists of
nitrogen gas and in which a pressure is maintained which is at least 2S great
as in the nitrogen ~as in the pores of the preformed silicon nitride at the
temperature in question. ~hen using this method there may be used a glass or
a glass-forming material which makes the embedding material gas-impermeable
at a considerably higher temperature than with the vacuum method. If the
embedding material consists of particles of glass or a glass-forming material
which is made gas-impermeahle by being transformed into a melt in the resis-
tant vessel while maintaining nitrogen gas pressure, a glass may be used
which gives a melt with low viscosity, suitably at most 106 poises, at con-
siderably higher temperaturesthan the glass materials which may be used in
the vacuum method. The high pressure required for the isostatic pressing
is then applied, as in the preceding case, when the melt has acquired a low
viscosity, which, depending on the type of glass, may be done in an interval
of from around 1150C to around 1700C.
The density of the glass which is used for silicon nitride should be at the
most 2.4 g/cm3 in order not to risk that the powder body is raised to such
an extent from the melt that certain parts of it will not be covered by
molten glass.
If a limited superficial penetration of glass into the pores of the powder
body is allowed, it is possible to use a plurality of different glass types
which provide melts with such low viæcosity, possibly while applying nitrogen
gas pressure during the formation of the melt, that the preformed body of
silicon nitride is not damaged when the high pressure required for the iso-
static pressing is applied. Among other things, different types of lead
silicate glass and aluminium silicate glasæ may be used, as well as quartz
and mixtures of different glass-forming oxides. In certain cases it may then
be necessary to remove the surface layer on the pressed silicon nitride body,
for example by blasting.
-5-
72
~owever. according to the present invention it has been found to be possible
to avoid a penetration of glass melt into the prefoxmed silicon nitride body
by using a glass containing ~23 around the silicon nitride
body and a sufficiently small grain size of the silicon nitride, pre-
ferably a grain size of less than 5 microns. A plausible explanation
why a boron-containing glass does not penetrate into the silicon nitride
body is that a boron nitrogen compound, probably boron nitride, is formed
at the boundary surface between the glass and the silicon nitride before
the glass foxms a low-viscous melt and that this boron nitrogen compound
counteracts the penetration of the glass into the pores of the powder body.
- The content of ~23 in the glass may advantageously amount to between 2 per
cent by weight and 70 per cent by weight. As examples of applicable boron-
containing glasses may be mentioned a glass containing 80.3 per cent by
weight SiO2, 12.2 per cent by weight ~23 2.8 per cent by weight Al203,
4.0 per cent by weight ~a20, 0.4 per cent by weight K20 and 0.3 per cent
by weight CaO (Pyrex ~ , a glass containing 58 per cent by weight SiO2,
9 per cent by weight ~?03~ 20 per cent by weight Al203, 5 per cent by weight
CaO and 8 per cent by weight MgO, a glass containing 96.7 per cent by weight
SiO2, 2.9 per cent by weight ~23 and 0.4 per cent by weight Al20~ (Vycor ~)
and a glass containing 38 per cent by weight SiO2, 60 per cent by weight
B203, and 2 per cent by weight Al203. It is also possible to use mixtures
of particles of substances, for example SiO2, Al203, ~23 as well as alkali -
and earth alkali oxides, which form glass when being heated.
After the pressing and the sintering, the finished object of silicon nitride
is embedded in the glass. According to an advantageous embodiment of ths
invention, a glass is used which has approximately the same coefficient of
thexmal expansion as silicon nitride within a considerable part of the area
between the solidification temperature of the glass and room temperatuxe,
preferably a coefficient of thermal expansion of 3.0 - 3.8 x 10 6 per C
within the temperature range 500C - 20C. ~his prevents damage to the
object caused by cracks or rupture during the cooling. A suitable glass
i8 the previously mentioned glass containing 80.3 per cent by weight SiO2,
12.2 per cent by weight ~23' 2.8 per cent by weight Al203, 4.0 per cent
by weight Na20, 0.4 per cent by weight K20, and 0.3 per cent by weight CaO,
which has a coefficient of thermal expansion of 3.2 x 10 per C between
500 C and 20 C. For silicon nitride the corresponding value is 3.2 x 10 6
per C. It is possible, although it complicates the manufacture of
the object to a considerable extent, to manage the exposure of the
~- finished object when using a glass the coefficient of thermal expansion
:
--6--
'77Z
of which is considerably different from that of the silicon nitride. An
example of such a glass is the previously mentioned glass containing 96.7
per cent by weight SiO2, 2.~ per cent by weight 323 and 0.4 per cent by
weight A1203. Such a glass can be removed substantially completely from
the silicon nitride object, for example by reducing the outer pressure
below the dissociation pressure of the silicon nitride, for example at 1600C,
the glass then lifting from the silicon nitride body and the object being
allowed to cool without being damaged by the glass. In certain cases it
may be suitable to remove the glass by increasing the temperature above
the temperature used during the sintering in order~for the glass to become
sufficiently low-viscous to leave only a sufficiently thin film which may not
damage the body and which, if necessary, may be removed by blasting.
In the same way as has been described for silicon nitride, the invention
may be used for manufacturing objects of materials built up with silicon
nitride as main constituent, such as silicon aluminium, oxynitride and the
said oxynitride in which aluminium has been at least partly replaced with
yttrium, as well as other silicon metal oxynitrides and further mixtures of
silicon nitride and silicon metal oxynitride.
Examples of other materials for which the present invention may be used
are further iron and nickel-based alloys such as e.g. an iron-based alloy
containing 0.33 % C, 0.30 % Si, 0.40 % Mn, 0.01 % P, 0.01 % S, 2.8 % Cr, o.-6
% Mo, the remainder being Fe (3 % Cr-Mo steel) or an iron-based alloy
containing 0.18 ~ C, 0.25 % Si, o.60 % Mn, 0.01 % P, 0.01 % S, 11.5 % Cr,
0.5 % Ni, 0.5 % Mo, 0.30 % V, 0.25 % Nb, the remainder being Fe (1~ % Cr-Mo-
-V-Nb-steel), a nickel-based alloy containing 0.03 % C, 15 % Cr~ 17 % Co,
5 % Mo, 3.5 ~ Ti, 4.4 % Al, 0.03 % ~, the remainider being Ni, or a nickel-
based alloy ccntaining o.o6 % c, 12 % Cr, 17 % Co, 3 % Mo, o.o6 % Zr, 4.7 %
~i, 5.3 % Al, 0.014 % B, 1.0 % V, the remainder being Ni, and further,among
other things, a metal oxide such as Al203 and a carbide such as silicon
carbide. The above contents expressed in per cent, as well as the contents
mentioned hereinafter and expressed in per cent, refer to percentage by weight.
In addition to the gases previously mentioned, hydrogen gas is suited as
pressure medium when pressing metallic materials, particularly if the sealing
of the embedding material takes place while supplying ga6. The pressure and
the temperature during the sintering of the preformed body are9 of course,
dependent on the properties of the metallic or ceramic material. Normally,
the pressure should amount to at least 50 MPa, preferably to at least 100 MPa
--7--
8'772
If the material consists of an iron-based alloy'
the temperature should be at least 1000C, preferably
1100 - 1200C, and if the material consists of a nickel-
based alloy the temperature should be at least 1050C,
preferably 1100 - 1250C.` If the material is aluminium
- oxide the temperature should be at least 1200C, prefe-
rably 1300 - 1500C, and if the material is silicon
carbide the temperature should at least 1700C and
preferably 1800 - 2000C.
Among the glass types mentioned previously for
silicon nitride, for materials with particularly high
sintering temperatures, such as silicon carbide, quartz
glass may suitably be used in the embedding material.
To prevent glass from penetrating into pores
in the powder body of the metallic or ceramic material, it
is suitable to surround the powder body with a blocking
layer, for example, a layer of finely-divided boron nitride
or of finely divided glass having a higher melting tempe-
rature than the glass in the embedding material.
Embodiment of the invention will be explained in
greater detail by describing examples, in no limitative
manner, with reference to the accompanying schematic
drawings, in which:
Figure 1 shows a body preformed from silicon
nitride powder in the form of a turbine
wheel for a gas turbine motor seen from
above;
Figure 2 shows the same body in axial cross-
section;
Figure 3 shows the body placed in a temperature-
resistant vessel and embe~ded in a mass
of glass par-ticles; and,
Figure 4 shows a high-pressure furnace in which
the isostatic pressing and the sintering
of the preformed powder body are carried
out.
8-
.. ; .
. - .
~1~8'772
.
Example 1
Silicon nitride powder having a powder grain size of less
than 5 microns and containing about 0.5 per cent by weight
free silicon and about o.i per cent by weight magnesium
ox.ide is placed in a capsule of plastic, for example plasti-
cized polyvinyl chloride, or of rubber, having approxima-
tely the same shape as the preformed powder body to be
manufactured, whereafter the capsule is sealed and placed
in a press device, for example the device shown in Figures
1 and 2 of British Patent 1,522,705. The powder is sub-
jected to a compaction at 600 MPa for a period of 5 minutes.After completed compaction the capsule is removed and
the preformed powder body thus manufacture~ is machined
into the desired shape. The powder body has a density
of 60~ of the theoretical density.
,. ~
~,'; '''
-8a-
~B'~72
The preformed powder body 10, which is shown in Figures 1 and 2, consists
of a turbine wheel having hub 11, web 12, edge 13 and blades 14.
The powder body 10 is placed, as i9 clear from ~igure 3, in a vessel 15which is open at the top, said vessel being resistant to the sintering
temperature used, the powder body then being embedded in glass powder 16.
The vessel in the exemplified case consists of graphite and is internally
provided with a release layer 17 of boron nitride. The glass powder consists
of particles of a glass containing 80.3 per cent by weight SiO2, 12.2 per
cent by weight B203, 2.8 per cent by weight Al203, 4.0 per cent by weight
Na20, 0.4 per cent by weight K20 and 0.3 per oent by weight CaO. Thus, in
this case the embedding material consists of particles of glass
One or more ves6els 15 are then placed in a high-pressure furnace according
to Figure 4. For the sake of clarity, only one vessel is shown in this
figure. In Figure 4, ~2 designates a press stand which is supported by
wheels 23 and is displaceable on rails 24 on the floor 25 between the posi-
tion shown in the figure and a position where the stand surrounds the high-
pressure chamber 42. The press stand is of the type which consists of an
upper yoke 26, a lower yoke 27 and a pair of spacers 28, which are held
together by a prestressed strip sheath 29. The high-pressure chamber 42
is supported by a column 49 and comprises a high-pressure cylinder which
is built up of an inner tube 50, a æurrounding prestressed strip sheath 51
and end rings which axially secure the strip sheath and constitute
suspension means by which the high-pressure chamber is attached to the
column 49. The chamber 42 has a lower end clos~lre 53 projecting into the
tube 50 of the high-pressure cylinder. The end closure is provided with
a groove, in which a sealing ring 54 is inserted, a channel 55 for degassing
the products to be pressed and for supplying pressure medium, suitably argon,
helium or nitrogen gas, and a channel 56 for cables for feeding heating
elements 57 for the heating of the furnace. The elements 57 are supported by
a cylinder 58 which rests on an insulating bottom 59, which projects into
an insulating mantle 60. The upper end closure comprises an annular portion
61 with a sealing ring 62 sealing against the tube 50. The mantle 60 is
suspended from the portion 61 and is gas-tightly connected thereto. The end
closure also comprises a lid 63 for sealing the opening of the portion 61,
which is usually permanently applied in the high-pressure cylinder. The lid
is provided with a 6ealing ring 64 sealing against the inner surface of the
portion 61, and with an insulating lid 65 which, when the high- pressure
chamber is closed, projects into the cylinder 60 and constitutes part of the
_9_
.
87~2
insulating sheel which surrounds the furnace space proper 66. The lid 63
is attached to a bracket 67, which is supported by a raisable, lowerable
and turnable operating rod 68. The yokes 26 and 27 absorb the compressive
forces actin~ on the end closures 53 and the lid 63 when pressure is applied
to the furnace space.
When the vessel 15 with its contents has been placed in the furnace space
66, the powder body 10 with the surrounding glass powder 16 is degassed at
room temperature for approximately 2 hours. During continued evacuation,
the temperature i8 raised to approximately 1150C. The temperature increase
is carried out so slowly that the pressure does not exceed 0.1 torr for any
part of the time. At approximately 1150C the temperature is maintained con-
stant for about 1 hour, whereafter the final degassing takes place and the
glass powder forms a melt with low viscosity, which completely surrounds the
powder body 10 and has a hori~ontal surface. Thereafter, argon or helium is
supplied at the same temperature to a pressure level which provides a pressure
of 200 - 300 M~a during final ~intering temperature. The temperature i8 then
raised to 1700 - 1800C, that is, to a suitable sintering temperature for
the silicon nitride,for a period of 1 hour. At the same time the pressure
then rises. A suitable time for sintering under the conditions mentioned
i8 at least 2 hours. After a finished cycle, the furnace is allowed to
cool to a suitable discharging temperature. The vessel 15 then contains
a blank cake~ in which the powder body is visible through the solidified
and clear glass. The powder body is completely embedded in the glass and
has therefore been locaLed below the surface of the melt in its entirety
during the pressing. ~ecause it has been possible to apply the-high pressure
required for the pressing when the glass has been low-viscous and the glass
in solidified form has the same coefficient of thermal expansion as the
silicon nitride, flawless obJects can be produced with a good reproducibility.
The cake is easily released from the vessel because of the presence of the
release layer 17. The glass can then be removed from the object by blasting.
The density of the ob~ect as finished exceeds 99.5 ~ of the theoretical
density.
Example 2
In an slternative embodiment, there is used a glass containing 96.7 per cent
by weight SiO2, 2.9 per cent by weight ~23 and 0.4 per cent by weight Al203.
~ith this ~lass a sufficiently low-viscous melt may be achieved only at 1600C.
-10-
~'' .
~8~72
To counteract the dissociation of silicon nitride that occurs at this
temperature, the glass mass 16 is transformed into a melt while main-
taining a pressure with nitrogen gas in the furnace space 66.
When the vessel with the powder body 10 and surrounding glass powder has
been degassed in the furnace space 66 at room temperature for approximately
2 hours, the furnace space is filled with nitrogen gas of atmospheric
pressure and the temperature of the furnace is raised to 1600C while
successi~ely supplying nitrogen gas to a pressure of 0.1 MPa. When the
temperature has reached 1600 C, the glass powder forms a melt with low vis-
cosity which completely surrounds the powder body 10. Thereafter argon or
helium is supplied at the same temperature and the pressing and the sintering
are carried out under the conditions described in the previous example.
Since the glass in this case has a coefficient of thermal expansion which
differs considerably from that of the silicon nitride, only a limited cooling
of the furnace may be allowed before the vessel 15 with its contents is removed.The pressed object is then heated to a temperature of about 1800C so that the
glass runs off the object and only leaves a thin film on the object. After
cooling to room temperature, the film is suitably removed by blasting.
':
. Example ~
A divisible form or mould having a cavity shaped as a turbine disk and
composed of an aluminium-silicate based material, for example of the same
type as is normally used in cores for the investment casting of turbine
blades having cooling channels, i8 filled with spherical powder of an iron-
based alloy containing 0.18 % C, 11.5 % Cr, 0.25 % Si, 0.5 % MQ, o.60 % Mn,
4.4 % Al, 0.01 % P, 0.01 % S, 0.5 % ~i, 0.30 % V, 0.25 % Nb, the remainder beingFe and having a grain size of less than 250 microns. The powder is vibrated
together by striking lightly on the form and the powder is sintered in a
vacuum at about 1200C for about 2 hours. After cooling, the reusable form
is divided and the porous turbine disk having essentially the same dimensions
as the formin~ cavity is removed. The porous turbine disk is thereafter
provided with a blocking layer by being coated with a fine-grained powder
of a grain size less than 1 micron of a high-melting glas~ to a thickness
of about 0.3 mm. The glass consists of 96.7 per cent by weight SiO2, 2.9
per cent by weight ~23 and 0.4 per cent by weight A1203.
The turbine disk is completely embedded in a compound of glass particles in
a graphite crucible. The glass in this compound consists of 80.3 per cent
-11-
8'77Z
by weight SiO2, 12.2 per cent by weight ~23' 2.8 per cent by weight A1203,
4.0 per cent by weight Na20, 0.4 per cent by weight K20 and 0.3 per cent by
weight CaO.
The grPphite crucible with its contents is thereafter placed in a high-
pressure furnace of the type described in Example 1.
The preformed powder body with the glass wrapping is first degassed in the
high-pressure furnace for approximately 2 hours at room temperature. There-
after the furnace is heated to 950C. When the temperature has reached
950C and the embedding glass has sintered densely, the pressure is raised
by pumping in argon gas. The temperature is also raised to 1200C9 the em-
bedding glass then forming a melt with a horizontal surface. At the pres~ure
100 MPa this temperature is maintained for 2 hours, the powder body then
being completely densified. After a finished cycle, the furnace is allowed
to cool to a suitable discharging temperature. The vessel then contains a
blank cake in which the powder body is visible through the solidified and
clear glass. The powder body is completely embedded in the glass and has
thus been located below the surface of the melt in its entirety during the
pressing. The glass may be removed in the manner described previously.
Example 4
A divisible form or mould having a cavity shaped as a turbine disk and
composed of an aluminium-silicate based material, for example of the same
type as is normally used in cores for the investment casting of turbine
blades having cooling channels, is filled with spherical powder of an iron-
based alloy containing 0.18 % C, 11.5 % Cr, 0.25 % Si, 0.5 % Mo~ 0.60 % Mn,
4.4 % Al, 0.01 % P, 0.01 ~o S, 0.5 % ~i, 0.30 % V, 0.25 ~ Nb, the remainder being
Fe and having a grain size of less than 250 microns. The powder is vibrated
together by striking lightly on the form and the powder is sintered in a
vacuum at about 1200C for 2 hours. After cooling, the reusable form
is divided and the porous turbine disk having essentially the same dimensions
as the forming cavity is removed. The porous turbine disk is thereafter
provided with a blocking layer by being coated with a fine-grained powder
of a grain size less than 1 micron of the same high-melting glass as in
Example 1 or of boron nitride powder to a thickness of 0.3 mm.
-12-
.''
~, - - . .~,
,
:
8~772
The turbine disk is completely embedded in a mass of glass particles
of the same kind as in Example 1 in a graphite crucible.
The graphite crucible with its contents is placed thereafter in a high-pressure furnace of the kind described in Example 1.
The preformed body with the glass wrapping is first degassed in a high-pressure furnace for 2 hours in room temperature. Thereafter, the furnace
is heated to 750C. At this temperature the pressure is increased by supp-
lying hydrogen gas with about 109 mbar/min. while the temperature is raised
by 5C/min.
When the temperature has reached 950C and the embedding glass has sintered
densely, tne pressure is raised further by pumping in argon gas. The tempe-
rature is also raised to 1200C, the glass then forming a melt. At the
pressure 100 MPa this temperature is maintained for 2 hours, the powder body
then being densified completely. The pressure and the temperature are then
reduced and the crucible may be removed from the hot press. The sintered
powder body is then obtained from a blank cake of glass, as described in
Example 1.
