Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2I7l7~l
JNL/A8424CA
Title: "Method of m~kin~ a product comprising an intermetallic compound"
Description of Invention
This invention relates to a method of m~king a product comprising an
intermetallic compound.
Intermetallic compounds, for example titanium ~luminides, have good
high temperature mechanical properties, a relatively low density and good oxidation
resistance. This renders them potential replacements for super alloys and other high
temperature materials in, for example, aerospace, industrial gas turbines, electronic
and automotive applications.
Hitherto, it has been proposed to make products comprising an
intermetallic compound by one of the following methods.
Intermetallic compound ingots may be produced by electroslag or
vacuum induction melting for further processing and shaping using hot forging
techniques. However, problems with shape deformation and resulting microstructure
and mechanical properties have restricted the use of this method from widespread use
- in manufacturing industry.
Alternatively, it has been proposed to use a lost wax investment casting
method to provide a near net shape product by melting intermetallic compound ingots
using processes based on conventional titanium alloy melting and pouring processes.
These processes require relatively high energy input and have the additional
disadvantages of the molten metal reacting with mould surfaces and with a crucible
in which the melting operation is performed, together with problems arising fromgrain growth in the casting.
Another alternative method hitherto proposed is to carry out a powder
metallurgical process to produce a initially porous structure which is then hot
isostatically pressed to allow the reaction required to produce an intermetalliccompound to take place and to fully densify the structure. Complexity and limitations
on the size and accuracy of products have restricted the application of this method.
2171~01
Objects of the present invention are to provide a method of and
apparatus for m~king a product comprising an intermetallic compound whereby the
above mentioned disadvantages are overcome or are reduced.
According to the present invention we provide a method of m~3king a
product comprising an intermetallic compound including the steps of permeating apowder metal component, having a first melting point, with a liquid metal component,
having a second melting point which is lower than the first melting point, to produce
a product comprising an intermetallic compound of said powder and liquid metal
components.
The method may be performed in a mould cavity. The mould cavity
may define the net shape of the product or near net shape of the product.
In a first more specific aspect of the invention:
Said step of permeating a powder metal component with a liquid metal
component may be performed to produce an intermediate form and the method may
include the step of heat treating the intermediate form to produce said product.Preferably in this aspect, the powder metal component comprises
titanium or may comprise iron and the liquid metal component aluminium.
Alternatively to said first more specific aspect, in a second more specific
aspect of the invention:
Said step of permeating a powder metal component with a liquid metal
component may be performed under specific conditions to produce said product
without a discrete step of heat treating an intermediate form.
Preferably in this aspect the powder metal component comprises nickel
or may comprise iron and the liquid metal component aluminium.
The method may include the step of subjecting the liquid metal
component to a pressure which is greater than a pressure to which the powder metal
component is subjected. The subjection of the components to such a pressure
difference may cause or assist said permeating of the powder metal component with
the liquid metal component.
2171~
The powder metal component and the liquid metal component may be
subjected to pressure to cause or assist permeation of the powder metal component
with the liquid metal component.
The components may be subject to pressure by the application of fluid
pressure such as gas, or by the application of mechanical pressure, such as by amechanical ram.
The powder metal component may comprise a rigid preform.
Alternatively, the powder metal component may be in flowable form.
The powder metal component may be introduced into the mould cavity
and then the liquid metal component may be introduced into the mould cavity.
The powder metal component may be degassed. That is, the powder
metal component may be treated to remove gaseous and/or volatile material, for
example by heating and evacuation.
The powder metal component may be degassed after being introduced
into the mould cavity and before introduction of the liquid metal component.
After the step of permeating the powder metal component the liquid
metal component may be allowed to solidify and the solidified intermediate form may
then be subjected to said heat treatment to effect a solid state phase transformation
to provide said product comprising an intermetallic compound.
A predetermined atomic fraction of said powder metal component and
said liquid metal component may be introduced into said mould cavity.
The mould cavity may have a wall which is impermeable as
hereindefined.
The liquid metal component may be introduced into the mould cavity
via an ingate from a reservoir.
In one more specific aspect of the invention.
The liquid metal component may be introduced into the mould cavity
from the reservoir through a passage which communicates with the ingate.
The liquid metal component may be introduced into the mould cavity
via an ingate from a reservoir.
2~7.I~
The or each ingate may comprise an aperture or a plurality of apertures
or a porous material such as a porous ceramic material which may also function as
a filter.
The reservoir may be disposed above the mould cavity and the liquid
metal component being fed downwardly into the mould cavity through the passage.
The liquid metal component may be fed do~vnwardly into the mould
cavity from the reservoir by virtue of flow under gravity.
A valve means may be provided to control the flow of molten metal
from the reservoir to the mould cavity.
The valve means may comprise a stopper rod or other valve member
moveable relative to a valve seat.
The mould and the reservoir may be mounted for movement relative to
a horizontal axis so that in a first position the mould may be disposed above the
reservoir and in a second position the mould may be disposed below the reservoir so
that metal can flow from the reservoir into the mould cavity under gravity.
The reservoir and mould may be mounted for rotation about said axis
or may be mounted for orbital or any other desired movement in which said changeof orientation may take place.
The reservoir may be disposed below the mould cavity and the liquid
metal may be fed upwardly into the mould cavity through the passage.
Alternatively to said one more specific aspect in another more specific
aspect of the invention.
The liquid metal component may be introduced into the mould cavity
na an ingate by immersing the ingate in a bath of said liquid metal component held
m a reservolr.
Preferably the whole of the mould is immersed in the bath.
The ingate may be at the top of the mould cavity or may be at any other
desired position.
Where the ingate is not at the top of the mould cavity only a part of the
mould may be immersed in the bath of liquid metal component and the mould cavity
2~ 71 70~
being filled by applying pressure to the liquid metal bath to force it into the mould
cavity.
The introduction operation may be performed so that the temperature
of the metal in the bath is above the melting point of the liquid metal component but
below the melting point of the intermetallic compound so that the intermetallic
compound will solidify within the mould cavity whilst the mould cavity is immersed
or partly immersed in the bath, and the mould cont71ining the solidified intermetallic
compound can be removed from the bath.
The mould cavity may be provided with a plurality of ing~tes.
The or each ingate may be positioned at any desired location in a wall
of the mould.
Where the wall of the mould defines a tubular mould cavity, at least one
ingate may be provided in the part of the wall of the mould which defines the internal
surface of the tubular mould cavity.
In each more specific aspect:
The liquid metal component in the reservoir may be subject to pressure.
The pressure may be mechanical pressure, for example, by advancing
a piston in said reservoir to apply pressure to a surface of the liquid metal.
The pressure may be applied by subjecting a surface of the metal in the
reservoir to fluid pressure, for example, gaseous pressure.
Said application of pressure may feed or aid the feed of metal from the
reservoir into the mould cavity and/or subject the metal in the mould cavity to
pressure.
The mould cavity may be disposed in an enclosure, the interior of which
may be placed in communication with the reservoir, for example through the ingate
and the interior of the enclosure may be subject to said fluid pressure.
The mould cavity may be connected in communication with a degassing
means through the ingate or, alternatively, through a conduit separate from the ingate.
Where the mould and the reservoir are mounted for said movement
relative to said axis, conduit means, which may comprise a passage, are provided
2.~7l7~l
continuously to connect the interior of the enclosure to the source of fluid pressure.
Flow of fluid pressure through the passage may be controlled by suitable valve means.
The conduit means may comprise a passage extending lengthwise of a
pivot member by which the mould and reservoir are mounted for rotation about said
axis.
The pressure may be m~int~ined for a predetermined time, for example,
until the liquid metal in the mould cavity has solidified, or, alternatively, for a time
which is predetermined so as to ensure complete or substantially complete permeation
of the liquid metal component into the powder metal component.
The powder metal component may comprise titanium, iron or nickel.
The liquid metal component may comprise aluminium.
The intermetallic compound may comprise titanium aluminide, which
may be TiAl3, TiAI or Ti3Al, nickel aluminide which may be NiAl3, Ni2Al3, NiAl,
Ni3Al or iron aluminide which may be Fe Al3 Fe2Als, FeAl or Fe3Al.
The intermetallic compound may comprise (expressed in Atomic %):
2~7170L
Powder Al Others Whenpresent
Ti + usual Optionally
impurities
TiAl3 Balance 75 ) 1 - 4
) 1-25
TiAI Balance 47 - 56 ) 1 - 4
Ti3AI Balance 20 - 38 ) 7 - 25
Ni + usual Optionally
impurities
NiAl3 Balance 75 ) 1 - 2
Ni2Al3 Balance 58 - 63 ) 1 - 2
1- 15
NiAl Balance 45 - 59 ) 1 - 2
Ni3Al Balance 25 - 28 ) 7 - 14
Fe + usual Optionally
impurities
FeAl3 Balance 75 ) 1 - 4
Fe2Als Balance 71 - 73 ) 1 - 4
1- 15
FeAl Balance 36 - 55 ) 1 - 4
Fe3Al Balance 22 - 36 ) 5 - 6
The "other" components, if and when present, may comprise chromium,
m~n~nese or niobium and may be present in the range specified above.
The intermetallic compound alternatively may comprise molybdenum
silicide or niobium beryllides or chromides.
At least one additional component may be introduced into the mould
cavity prior to permeating the powder metal component with the liquid metal
~ 71 ~ I
component. The non-metallic reinforcement may be added to the intermetallic
compound to create a metal matrix composite (MMC) having modified thermo-
physical properties. The additional component may comprise ceramic particles such
as silicon carbide or alumina particles, fibres such as silicon carbide or ~ min~ fibres,
whiskers such as silicon carbide whiskers or monofilament, multifilament or random
fibres.
In a further more specific aspect of the invention:
Said step of permeating a powder metal component with a liquid metal
component may be performed to provide a product having a first portion which hasa melting point which is higher than the melting point of a second portion of the
product.
Said step of permeating a powder metal component with a liquid metal
component may be performed with different atomic proportions by atomic weight ofpowder metal component and liquid metal component in at least one pre-determinedpart of the mould.
The method may comprise a step of introducing said powder metal
component into a first part of the mould cavity and permeating the powder metal
component in said first part with said liquid metal component and introducing another
component, selected from a powder metal component and a liquid metal component,
in a second part of the mould cavity not occupied by the powder metal component. The other component may be a liquid metal component.
Alternatively, the other component may be at least one powder metal
component the or each of which provides a different atomic proportion of powder
metal component to liquid metal component to the proportion provided in the first
part of the cavity and the method including a further step of permeating the or each
other powder metal component with a liquid metal component.
In the latter case, the mould cavity may comprise a third part
unoccupied by a powder metal component and into which a liquid metal component
is introduced.
2~7~70~
The compositions and proportions by atomic weight in each part may
be in accordance with the values tabulated in Table 1 hereof.
In consequence a product, in accordance with the further more specific
aspect of the invention, may be provided with different physical properties such as
melting points, in different regions of the product. For example, in the case of an
automotive piston or an aerospace heat shield a part of the product adjacent a surface
which, in use, is subjected to high temperature such as the crown of an automotive
piston may be provided with a higher melting point than the remainder of the product.
In the regions of the boundaries between said parts there may be an
abrupt change in composition or a gradual change of composition. It is believed that
providing the powder metal components in a flowable form facilitates an extendedtransition zone between the compositions of the parts.
For example, where the powder metal component comprises a surface
part having an atomic proportion to provide a surface part of NiAl, there may betransition regions of a Ni2Al3 and then NiAl3 as an integral part of the product, as the
composition approaches that of aluminium or an aluminium alloy the remainder of
which comprises aluminium or an aluminium alloy. In between these parts, for
example between NiA1 and Ni2A13, there may be a boundary layer consisting of a
mixture of precipitates of NiA1 and Ni2A13.
Embodiments of the invention will now be described by way of example
with reference to the accompanying drawings, wherein:
FIGURES 1 - 8 illustrate, diagr~mm~tically, stages in a method
embodying the invention;
FIGURE 9 is a diagr~mm~tic illustration of one form of apparatus
which may be used in the method illustrated with reference to Figures 1 - 8;
FIGURE 10 is a diagrammatic illustration of an alternative form of
apparatus for use in the method of Figures 1 - 8;
FIGURE 11 is a diagr~mm~tic illustration of a further alternative form
of apparatus for use in the method of Figures 1 - 8;
21717~1
FIGURE 12 is a diagr~mm~tic illustration of a further alternative form
of apparatus for use in the method of Figures 1 - 8;
FIGURE 13 is a diagr~mm~tic illustration of a further alternative form
of apparatus for use in the method of Figures 1 to 8;
FIGURE 14 is a diagr~mm~tic illustration of a yet further alternative
form of apparatus for use in the method of Figures 1 to 8, and
FIGURE 15 is a diagr~mm~tic illustration of a yet further embodiment
of the invention.
Referring to Figure 1 of the drawings, a mould cavity 10 of a desired net
shape, that is to say, a shape which is the same as, or similar to, a desired final shape
of a product, and having an ingate I is provided in a mould or die 11 by suitable
means such a shaped mould can be distinguished from, for example, an ingot mouldby virtue of the product having a desired three dimensional shape and generally being
not formed to a final shape by further forming . In the present example the cavity is
made using a lost wax disposable pattern in a refractory material which in the present
example is silica, but which may be zirconia and/or alumina.
The refractory material of the mould in the present example comprises
silica bonded with silicate formulated so as to make the mould impervious, or
substantially impervious, to liquid or gas.
The mould is then fired in conventional manner to a temperature
sufficient to remove volatile material and to stabilise the mould.
If desired, the cavity may be made in any other suitable manner and
need not be made to net shape, although such is preferred. For example, the mould
may be made in one of said refractory materials, or in plaster or any suitable
moulding material using removable patterns. For example, if, alternatively, the mould
is made of alumina it may be made, for example, of alumina silicate which renders
the mould, in itself, impermeable. Further alternatively, the mould may be made by
m~çllining a suitable mould material and where, for example, the mould material is
metal, then the cavity is generally referred to as being made in a die.
2l7l7al
In all cases described herein the mould is made in such a way as to be
impermeable. By "impermeable" we mean herein that the mould is at least sufficiently
impermeable a) for liquid metal to be cast into the mould so as to ~revell~
penetration of the mould surface by the liquid metal surface, and b) to prevent
passage of gas from the interior of the wall of the mould to prevent ingress of gas into
the interstices between the powder particles, of the powder metal component, from
within the interior of the mould wall prior to being permeated with the liquid metal
component.
By "interior of the mould wall" we mean gas which has either permeated
into the mould wall and been held there within the mould wall or gas which has
passed through the interior of the mould wall from the exterior of the mould wall into
the mould cavity. The degree of "prevention" can vary with the desired application
and/or m~nllf~cturing technique including the mould material of the product
concerned. For example, for some applications a relatively rough surface is desirable.
The mould may be made of material which is itself impermeable as well
as a mould which is coated or otherwise treated so as to be impermeable. The
porosity, for example the pore size of the mould, may vary with temperature within
the above-mentioned limits. An important requirement is that the mould is, in
practice, sufficiently impermeable to liquid metal to be cast into the mould as well as
to passage of gas from the interior of the wall of the mould to produce s~ticf~ctory
results.
Referring to Figure 2.
The mould cavity then has a powder metal component introduced
thereinto, through the ingate I as shown at 12 in Figure 2.
The morphology and the packing density of the material is adjusted to
provide the required volume fraction. In the present example the powder is titanium
metal powder, but may comprise other suitable material for producing intermetallic
compounds, such as nickel or iron.
2 1 ~
If desired, the powder metal component, instead of comprising metal
powder in flowable form, may comprise a rigid preform made by conventional powder
metallurgical techniques to produce a preform with interstices between the powder
metal particles.
In the present example the titanium powder had an average particle size
of 70 ,um and a purity of about 99.75%. The particle size may lie in the range, for
example 10 - 150 ,um or, for example, 30 - 250 ,um. The volume fraction introduced
in the present example was about 50% but the volume fraction may lie in the range
35% to 75~.
Where the powder metallurgical component comprises a preform the
preform may be pressed as necessary to achieve the desired volume fraction and the
mould cavity is configured so as to provide a predetermined clearance with the
preform. The clearance may, for example, be the minimllm necessary to allow
introduction of the preform into the cavity.
Referring now to Figure 3,
The mould 11 is disposed in the chamber 13. The chamber 13, in the
present example, comprises a lower part 14 in which the mould 11 is disposed and an
upper part 15 in which a crucible 16 for receiving liquid metal component is received.
The mould 11 may be introduced into a lower part 14 of the chamber
either before or after filling the mould 11 with the powder metal component.
The crucible 16 is filled with the desired amount of liquid metal 17
which in the present example comprises aluminium, but which, if desired, may
comprise other suitable material for any of the alternative powder metal components.
Suitable means are provided to maintain the metal in the crucible 16.
Such means may comprise a magnetic field, a mechanical plug or other means as
described hereinafter with reference to Figures 9 - 15.
The chamber 13 is then sealed from atmosphere and is evacuated to
remove air from the interior of the chamber and consequently from the interstices
between the powder particles 12 in the mould 11.
~71~01
If desired, the inside of the chamber 13 and or the mould 11 may be
heated to facilitate the above described degassing process and the removal of volatile
materials from the mould and the chamber.
Referring now to Figure 4.
When degassing has been completed the liquid metal component 17 is
poured from the bottom of the crucible 16 through a tap hole 18 into a feeder system
19 of the mould 11 and, hence, via the ingate I, into the mould 11. Initially the
molten metal 17 rests temporarily in the feeder system 19 on top of the powder
component 12 so as to seal the evacuated interstices inside the impermeable mould
11.
Referring now to Figure 5.
The powder and liquid components within the mould 11 are and then
subjected to pressure to ensure that the powder metal component is permeated with
the liquid metal component so that all, or substantially all, of the interstices between
the particles of the powder metal components are filled with liquid metal. The
application of this pressure overcomes flow resistance and surface tension effects and
ensures complete permeation.
The pressure may be applied using any one of the casting apparatus
described hereinafter and illustrated with reference to Figures 9 - 15.
When the liquid metal component 17 has solidified, the pressure in the
chamber 13 is reduced to atmospheric pressure and the upper and lower parts 15, 14
are separated and the mould 11 is removed, as shown at 21 in Figure 6.
Mould material and excess metal 20 in the feeder system 19 are
removed to leave a near net shape intermediate product, shown at 22 in Figure 7.The intermediate product 22 contains the correct ratio of finely distributed
components necessary to produce the intermetallic compound.
The thus produced intermediate product is then subject to a solid state
phase transformation heat treatment to create an intermetallic compound.
In the present example, where the liquid metal and powder metal
components comprise approximately SO~o volume fraction titanium and ~ minium~
2~ ~I 7~
14
the heat treatment to produce a titanium ~ minide intermetallic compound of desired
microstructure and mechanical properties comprises a two stage solid state phasetransformation in the range 520 - 600C, typically 570C and in the range 1200C up
to the limit of stability, typically 1350C.
If desired, the powder metal component may comprise titanium, iron or
nickel.
The liquid metal component may comprise alnminillm.
The intermetallic compound may comprise titanium aluminide, which
may be TiAI3, TiAl or Ti3Al, nickel ~ minide which may be NiAl3, Ni2Al3, NiAl,
Ni3Al or iron aluminide which may be FeAl3 Fe2Als, FeAl or Fe3AI.
The liquid metal and powder metal components may comprise
compositions selected from the ranges set out below (expressed in Atomic ~):
2~ 71 7~1
TABLE 1
Powder Al Others Whenpresnet
Ti + usual Optionally
hllpulilies
TiAl3 Balance 75 ) 1 - 4
) 1-25
TiAl Balance 47 - 56 ) 1 - 4
Ti3Al Balance 20 - 38 ) 7 - 25
Ni + usual Optionally
impurities
NiAl3 Balance 75 ) 1 - 2
Ni2Al3 Balance 58 - 63 ) 1 - 2
1 - 15
NiAl Balance 45 - 59 ) 1 - 2
Ni3Al Balance 25 - 28 ) 7 -14
Fe + usual Optionally
impurities
FeAl3 Balance 75 ) 1 - 4
)
Fe2Als Balance 71 - 73 ) 1 - 4
1 - 15
FeAl Balance 36 - 55 ) 1 - 4
Fe3Al Balance 22 - 36 ) 5 - 6
For example, the resultant intermetallic components may be as set out
in Table 2.
~ 71~
16
TABLE 2
Intermetallic Density, Aluminium, Melting
g/cm3 wt% point, C
Ni3Al 7.293 13.28 1395
NiAl 5.91 31.49 1639
Ni2Al3 4.787 40.81 1133
NiAl3 3.957 57.96 854
Fe3Al 6.648 13.87 1502
FeAl 5.585 32.57 1215
Fe2Als 3.963 54.70 1171
Ti3Al 4.216 15.81 1600
TiAl 3.63 36.03 1460
TiAl3 3.371 62.82 1340
Where the powder metal component comprises Nickel, due to the heat
energy during the exothermic transformation from separate Ni and A1 into NiA1
intermetallic, the respective intermetallic component may be formed during the
permeation operation without the need for a subsequent heat treatment to cause asolid state transformation to the respective intermetallic component. When the
powder metal component comprises iron or titanium a combination of intermetallicformation during permeation and during a subsequent solid state heat treatment
occurs. This condition is possible but it is not necessarily always like this. In fact, Ni,
Ti and Fe have an essentially similar nature, that is, during infiltration of analnminillm melt a more or less partial reaction may occur depending upon the powder
volume fraction, the melt temperature, the applied pressure, the particle morphology
and hence, generally, the velocity of infiltration.
The "other" component if and when present may comprise chromium,
m~ng;~nese or niobium, or any desired combination of two or more of the above.
~171 ~o:~
The intermetallic compound alternatively may comprise molybdenum
silicide or niobium beryllides or chromides depending on the powder and liquid metal
components.
At least one additional component may be introduced into the mould
cavity prior to permeating the powder metal component with the liquid metal
component. The non-metallic reinforcement may be added to the intermetallic
compound to create a metal matrix composite (MMC) having modified thermo-
physical properties. The additional component may comprise ceramic particles such
as silicon carbide or alumina particles, fibres such as silicon carbide or alumina fibres,
or whiskers such as silicon carbide whiskers.
Other examples are relatively large diameter monofilaments, typically
120-140,um diameter or relatively small diameter multifilaments, typically 3-20,um
diameter. Random fibres are usually about 3,um and over in diameter and have a
typical fibre length of about 50,um. Whiskers are single crystals, so rarely grow larger
than 0.5~4m in diameter. Said additional component does not form an intermetallic
compound with the liquid metal component.
The resultant net shape product comprising titanium aluminide
intermetallic compound is illustrated at 23 in Figure 8 and may comprise a single
product or may comprise a plurality of products depending on the configuration of the
mould cavity 10.
One form of apparatus for applying pressure to the components is
illustrated in Figure 9.
In this embodiment the mould cavity 10 is defined in a die 25, having
an ingate I, and which in the present example is made by machining in a suitable die
steel. The wall of the mould cavity is impermeable since the steel is impermeable
itself and split lines in the die are provided with suitable seals. Above the die 25 is
provided a melting chamber 26 in which a crucible 27 is positioned, surrounded by a
heating coil 28 such as an induction or resistance heating coil. The crucible 27 is
made of a suitable material to withstand the temperature and mechanical stresses and
has a generally cylindrical interior 30.
2~ 7~1
18
A generally cylindrical ram 29 is positioned above the crucible 27. The
ram 29 is adapted to be advanced into the interior 30 of the crucible 27 by a suitable
mechanical means such as a hydraulic piston, illustrated diagr:~mm~tically at 31. The
ram 29 in the present case is of generally cylindrical configuration and closelyconforms to an internal surface 32 of the interior 30 of the crucible 27.
The cavity 10 is connected by a conduit 33 and a valve 34 to a vacuum
system 35. A clamp 38 is provided to hold the die 25 in engagement with the melting
chamber 26.
In use, the cavity 10 is filled with powder metal component as described
hereinbefore, and the crucible 27 is filled with a desired amount of liquid metal
component.
In this example the liquid metal is, substantially, retained in the crucible
30 simply by virtue of the liquid metal flowing through the opening 18 until it rests
on the upper surface of the powder metal component in the cavity 10. If desired,however, liquid metal may be retained wholly within the crucible 27 by any othersuitable means such as those described hereinbefore.
A degassing operation is carried out as described previously using the
vacuum system 35 and then liquid metal is permitted to leave the crucible through the
tap hole 18. The hydraulic piston 31 is operated to advance the ram 29 into the
interior 30 of the crucible 27 and thus apply mechanical pressure to the molten metal
in the crucible 27 and maintain this pressure in the range 10 - 30 bar as the molten
metal is caused to permeate the powder metal component. The pressure may be
maintained until the liquid metal component has solidified, or may be removed orreduced prior to complete solidification of the liquid metal component. The die 25
is then separated from the melting chamber 26 and is opened and the method
continued as described hereinbefore.
Referring now to Figure 10, in this example the apparatus comprises a
mould chamber 50 in which is housed an impermeable disposable ceramic mould 11
having an ingate I and made from plaster or refractory material such as silica, zirconia
and/or alumina as described hereinbefore and defining a mould cavity 10. The mould
~ 7~ 7~
19
cavity 10 in this embodiment has a feeder portion 19 similar to that described
previously and illustrated in Figure 3. The mould 11 is rendered impervious by virtue
of a silicate coating, as described hereinbefore. The interior of the mould chamber
50 is connected by a manifold 52 and valves 53, 54 to a vacuum system 55 or a
pressure source 56 respectively.
Removably sealed to the mould chamber 50 is a melting chamber 57
which houses a crucible 58 surrounded by a heating coil 51. The crucible 58 has a
hollow interior 59 into which molten metal is fed via a gate valve 61. A stopper rod
62 is provided to close a pouring hole 63 of the crucible 58.
A metal loading port 60 above the crucible 58 is connectable by a
manifold 64 and valves 65, 66 to a pressure source 56 or a vacuum system 55
respectively.
The method is performed as described hereinbefore with reference to
Figures 1 - 8, feed of liquid metal into the feed portion 19 being controlled by the
stopper rod 62. Prior to feeding the molten metal into the feeder portion 19, the
above described degassing operation is performed using the vacuum system 55.
When the metal pouring operation is completed the valves 53, 66 are
closed and the valves 54, 65 opened so as to apply gaseous pressure to the top of the
metal in the feeder portion 19 and thus pressuring the interior of the cavity 10.
In other respects the method is performed as described previously.
Referring now to Figure 11, a mould chamber 70 is releasably mounted
on and sealed to a melting chamber 71 which is mounted for rotation about a
horizontal axis 72, by virtue of a stub axle 73 received in a bearing 74 and a hollow
stub axle 75 being received in a bearing 76. The bearings 74, 76 are supported by
bearing frames 77. The hollow interior of the stub axle 75 is connected at one end
by an ~nmll~r aperture 78 to the interior of the mould chamber 70 and by virtue of
a transverse aperture 79 at the other end and a valve 80 to a pressure source 81 and
by a second, diametrically opposite transverse aperture 82 and a valve 83 to a vacuum
system 84.
2~17,~ 1
A crucible 85 is disposed within the melting chamber 71 and surrounded
by a heating coil 86. An impermeable disposable ceramic mould 11, made as
described hereinbefore, with at least one passage 89 thereof, is provided within the
mould chamber 70 and is held in place by a clamp means 87.
Initially the liquid metal component 17 is introduced into the crucible
85 and then the mould chamber 70 is sealingly mounted on the melting chamber 71.A deg~c.cing operation is then performed using the vacuum system 84 with the valve
83 open and the valve 80 closed.
When the degassing operation has been completed and the melt
temperature measured by a pyrometer 88, the assembly of melting chamber and
mould chamber 71, 70 is rotated through 180 about the axis 72 so that liquid metal
from the crucible 85 is poured into the feeder portion 19 and through the ingate I into
the mould 11 and then the interior of the assembly 70, 71 is pressurised from the
source 81 through the valve 80, the valve 83 being closed, and so gaseous pressure is
applied to the surface of metal in the feeder portion 19 via the passage 89. Theremainder of the process is performed as described hereinbefore.
In this version the powder material is either in the form of a rigid or
semi-rigid preform, or is introduced into the mould cavity in flowable form held in
place by a filter or is subsequently compacted to sufficient extent so as to retain its
shape and position in the mould 11 whilst in an inverted position.
Referring now to Figure 12.
In this embodiment the mould cavity 10 having an ingate I is defined in
an impermeable permanent die 90 which in the present example is made by
machining in a suitable die steel. Below the die 90 is provided a melting chamber 91
in which a crucible 92 is positioned, surrounded by a heating coil 93.
The lower end of a generally cylindrical feed tube 94 is immersed in the
liquid metal component contained in the crucible 92. The upper end of the feed tube
94 is removably sealed to the inlet to the die 90.
2 ~ 71 7~ `~
The cavity 10 is connected by a conduit 95 and a valve 96 to a vacuum
system 97. A valve 99 similarly connects the melting chamber 91 to the vacuum
system 97.
In use, the cavity 10 is filled with powder metal component as described
hereinbefore, and the crucible 92 is filled with a desired amount of liquid metal
component.
A degassing operation is carried out as described previously using the
vacuum system 97. Simultaneously vacuum valves 96 and 99 are closed and pressurevalve 101 is opened applying pressure to the molten metal in the crucible. The
molten metal is caused to rise up feed tube 94 and permeate the powder metal
component. The pressure may be maintained until the liquid metal component has
solidified whereupon the die 90 separated from the melting chamber 91 is opened and
the method continued as described hereinbefore.
Referring now to Figure 13.
In this embodiment the mould cavity 10 is defined in an impermeable
disposable ceramic mould 11 made from plaster or refractory material and which is
rendered impervious by virtue of a silicate coating as described hereinbefore. The
mould cavity is provided with an ingate I at the entrance to which is provided a filter
103 and the filter 103 also retains the powder metal component within the mould
cavity. If desired the mould cavity may be provided with a plurality of ingates at
desired positions to ensure proper filling of the mould cavity depending upon the
mould cavity configuration. A filter or a plurality of filters 103 being provided for
each ingate. The or each filter may comprise a porous ceramic material.
A crucible 106 provides a reservoir which is charged with liquid metal
component to a level L and the temperature of the liquid metal component is
controlled by an electrical heating coil 107. The mould 11 cont~inin~ the powdermetal component is mounted on a rod 104 which can be moved vertically through anopening 104a in a lid 105 of an enclosure or melting chamber 102. The lid 105 isremovably sealed to the chamber 102 so as to provide a fluid tight enclosure.
2t717~
The chamber 102 is provided with a degassing system 108 connectable
with the interior of the chamber 102 by a valve 109 and with a gaseous pressure
system 110 connectable to the interior of the chamber 102 by a valve 111.
A degassing operation is then performed as described previously using
the vacuum 108 with the 109 open and the valve 111 closed.
When the degassing operation has been completed the mould 11 is
immersed in the liquid metal in the crucible 106 by operation of the hydraulic piston
112 to lower the rod 104.
The valve 109 is closed and the valve 111 is open so as to apply gaseous
pressure from the pressure system 110 to the liquid metal contained in the crucible
106. The liquid metal is thus caused to enter the mould cavity via the or each ingate
and associated filter or filters 103 and thus permeates the powder metal component.
Fluid pressure is m~int~ined until the two components have intimately
combined or the reaction process is complete to form the intermetallic compound.Thereafter the mould 11 is withdrawn from the liquid metal by reversing the
movement of the hydraulic piston 112. The valve 111 is closed and pressure in the
chamber 102 reduced to atmospheric pressure and the lid 105 is then unsealed from
the chamber 102 and the mould 11 removed. The remainder of the process is
performed as described hereinbefore.
If desired, a plurality of moulds may be immersed in the crucible 106
by a suitable mounting on the rod 104 or by providing a plurality of rods 104
moveable simultaneously or independently by appropriate hydraulic pistons similar to
the piston 112.
The temperature of the metal in the crucible 106 is maintained above
its melting point but below the melting point of the intermetallic compound. Forexample, when the intermetallic compound comprises y titanium aluminide, meltingpoint 1450C, and the liquid metal component is aluminium, melting point 660C, the
~hlminium may be maintained at a temperature lying in the range 700C to 900C.
A similar temperature differential, lying in the range 400C to 1000C,
may be provided between the melting point of the liquid metal component and of the
2~7170l
intermetallic compound where these differ from the particular example described
hereinbefore.
Referring now to Figure 14, there is shown apparatus for use in a
method similar to that described with reference to Figure 13 but Figure 14 shows an
impermeable disposable ceramic mould 11 which defines a tubular or part tubular
product from which it will be seen that a plurality of ingates I are provided disposed
at desired positions over the whole of the wall of the mould 11 which defines the
internal surface of the tubular product, or tubular part of the product, to be made.
As previously mentioned, each ingate could comprise a single aperture or comprise
a porous material or indeed a single ingate covering the whole internal surface could
be provided. In this case, the ingate may have a plurality of discrete apertures or
comprise a porous ceramic body.
Such a disposition of ingates on the internal surface facilitates the
casting of products incorporating a tubular configuration.
Another example of the invention will now be described with reference
to Figure 15 which is a view of the apparatus shown in Figure 9 and the same
reference numerals have been used to refer to corresponding parts. The cavity 10 is
again defined in an impermeable permanent die and, in the present example, is
configured to produce an automotive engine piston. In this example, instead of the
cavity 10 being filled with powder metal component as was the case of the
embodiment previously described with reference to Figure 9, the cavity has the
powder metal component introduced into a first part P1 of the mould cavity adjacent
the bottom end thereof. The powder metal component may comprise, for example,
a layer of nickel powder in flowable form, or alternately, if desired, nickel powder in
the form of a rigid preform. In either case the powder metal component may be, for
example, approxim~tely 1-2mm thick and may be of shallow cup shape as shown in
Figure 15 or of any other desired configuration to provide a desired part of the final
product with desired properties.
Liquid metal component such as aluminium or a suitable aluminium
alloy is then introduced into the mould cavity as described in connection with the
- 2 ~
24
previous embodiment described with reference to Figure 9 so that the powder metal
component disposed in the first part P1 of the mould cavity is permeated with the
liquid metal component and a second part P2 of the mould cavity, which does not
contain a powder metal component is occupied with the desired amount of liquid
metal component.
If desired, the first part of the mould cavity may have a powder metal
component of a volume fraction introduced thereinto and a second part of the mould
cavity adjacent the first part may have another powder metal component introduced
thereinto of different volume fraction to the powder metal component in the first part.
Liquid metal component is introduced into the mould cavity to permeate the powder
metal components disposed in the first and second parts of the mould cavity and to
occupy to a desired extent a third part of the mould cavity, not occupied by thepowder metal components, with liquid metal component.
Where, for example, the powder metal component in the first part
comprises nickel and the liquid metal component in the second part comprises
lminillm or an aluminium alloy, the resultant product may have a region such as a
surface part which comprises Ni Al, then a region comprising Ni2 Al3 and then a
region comprising Ni Al3 all as integral regions of the product, as the composition
approaches that of aluminium or an aluminium alloy, the remainder of which
comprises aluminium or aluminium alloy. The intermetallic compound, for example
NiA1, Ni2Al3, or NiA13 may be of constant composition throughout the respective
region. Alternatively, the composition of the intermetallic may vary in accordance
with the phase diagram, according to the range of volume fraction provided in the
region and/or according to the range of volume fraction which results from
introduction of the liquid metal component into the region.
There may be transition regions between these parts where the
composition comprises a mixture of the intermetallic components of the adjacent
regions. For example, the transition region between the NiAl region and the Ni2 Al3
region may comprise a mixture of the two intermetallic compounds which varies from
lOO~o NiAl progressively to lOO~o Ni2 Al3. Similarly for other transition regions.
21 71 7~ 1
If desired, a powder metal component or components may be disposed
in any desired part or parts of the mould cavity and powder metal components of
different volume fraction may be provided at different parts of the mould cavity.
If desired, the above mentioned partial occupation of the mould cavity
by at least one powder metal component described hereinbefore with reference to
Figure 15 may be applied in any of the other examples described hereinbefore andthe parameters described hereinbefore in connection therewith are applicable thereto.
In all cases the volume fraction can be varied by powder metallurgical
techniques for example by virtue of using different particle morphology, and/or size
and/or by putting a different amount of powder in the or a desired region.
In all cases the reaction of the powder metal component and the liquid
metal component to produce the intermetallic compound may occur either as the
liquid metal part advances through the powder metal component or generally within
the product, for example, after a short delay. When the reaction occurs in an
intermediate body the reaction may again occur as described above. The composition
of the intermetallic compound may vary without falling outside the scope of thispatent application for example by a few percent although those stated are preferred
compositions.
In all the embodiments described hereinbefore the pressure applied to
the liquid component lies in the range 10 - 30 bar and is typically 20 bar.
However, higher pressures may be used, for example, up to 100-500 bar
or even higher pressures eg 1000 bar may be used where, for example, conventional
squeeze casting is used to apply a mechanical pressure directly to the liquid metal.
If desired a pressure below atmospheric pressure may be applied to the
powder component and atmospheric pressure applied to the liquid metal component
to cause or assist the permeation. Indeed, any suitable means of providing a pressure
difference between the liquid and powder components in which the pressure actingon the liquid component is relatively greater than that acting on the powder
component may be provided.
- 2171 7Dl
26
In this specification by powder metal we mean discrete particles of metal
with a m~rimllm dimension of less than lmm (1,000 ~m). Generally, the particles of
the powder metal component are dry prior to introduction of the liquid metal
component.
The features disclosed in the foregoing description, or the accompanying
drawings, expressed in their specific forms or in terms of a means for performing the
disclosed function, or a method or process for ~tt~ining the disclosed result, or a class
or group of substances or compositions, as a~ropliate, may, separately or in anycombination of such features, be utilised for re~ ing the invention in diverse forms
thereo
