BACKGROUND OF THE INVENTION
The present invention relates to the field of
5 casting, and more particularly to a mold for casting, and
also to methods of fabricating shell molds, and to
methods of casting using such a pattern.
In the description below, the terms "high", "low",
"horizontal", and "vertical" are defined by the normal
10 orientation of such a mold while metal is being cast into
it.
So-called "lost-wax" or "lost-pattern" casting
methods have been known since antiquity. They are
particularly suitable for producing metal parts that are
15 complex in shape. Thus, lost-pattern casting is used in
particular for producing turbine engine blades.
In lost-pattern casting, the first step normally
comprises making a pattern out of a material having a
melting temperature that is comparatively low, such as
20 for example out of wax or resin. The pattern is itself
coated in refractory material in order to form a mold,
and in particular a mold of the shell mold type. After
removing or eliminating the material of the pattern from
the inside of the mold, which is why such methods are
25 referred to as lost pattern casting methods, molten metal
is cast into the mold in order to fill the cavity that
the pattern has formed inside the mold by being removed
or eliminated therefrom. Once the metal has cooled and
solidified, the mold may be opened or destroyed in order
30 to recover a metal part having the shape of the pattern.
In the present context, the term "metal" should be
understood to cover not only pure metals but also, and
above all, metal alloys.
In order to be able to make a plurality of parts
35 simultaneously, it is possible to unite a plurality of
patterns in a single assembly in which they are connected
together by a tree that forms casting channels in the
mold for the molten metal.
Among the various types of mold that can be used in
lost-pattern casting, so-called "shell" molds are known
that are formed by dipping the pattern or the assembly of
5 patterns into a slip, and then dusting refractory sand
onto the pattern or the assembly of patterns coated in
the slip in order to form a shell around the pattern or
the assembly, and then baking the shell in order to
sinter it and thus consolidate the slip and the sand.
10 Several successive operations of dipping and dusting may
be envisaged in order to obtain a shell of sufficient
thickness prior to baking it. The term "refractory sand"
is used in the present context to designate any granular
material of grain size that is sufficiently small to
15 satisfy the desired production tolerances, that is
capable, while in the solid state, of withstanding the
temperature of the molten metal, and that is capable of
being consolidated into a single piece during baking of
the shell.
20 In order to obtain particularly advantageous
thermomechanical properties in the part produced by
casting, it may be desirable to ensure that the metal
undergoes directional solidification in the mold. The
term "directional solidification" is used in the present
25 context to mean that control is exerted over the
nucleation and the growth of solid crystals in the molten
metal as it passes from the liquid state to the solid
state. The purpose of such directional solidification is
to avoid the negative effects of grain boundaries within
30 the part. Thus, the directional solidification may be
columnar or monocrystalline. Columnar directional
solidification consists in orienting all of the grain
boundaries in the same direction so that they cannot
contribute to propagating cracks. Monocrystalline
35 directional solidification consists in ensuring that the
part solidifies as a single crystal, so as to eliminate
all grain boundaries.
In order to obtain such monocrystalline directional
solidification, the mold typically presents, beneath the
molding cavity, a starter cavity that is connected to the
molding cavity by a selector channel, as disclosed by way
5 of example in French patent FR 2 734 189 and US patent
4 548 255. While the metal is solidifying in the mold,
the mold is cooled progressively starting from the
starter cavity so as to cause crystals to nucleate
therein. The role of the selector channel is firstly to
10 favor a single grain, and secondly to enable the single
grain to advance towards the molding cavity from the
crystallization front of this grain that nucleated in the
starter cavity.
A drawback of that configuration is nevertheless
15 that of ensuring that the mold has mechanical strength,
in particular when the mold is of the so-called "shell
mold" type, made up of relatively thin walls around the
cavities and channels that are to receive the molten
metal, since the molding cavity occupies a high position
20 above a starter cavity that is normally smaller. For
this purpose, it is common practice, as shown in patent
US 4 940 073, to incorporate support rods in the mold.
Nevertheless, such support rods, which penetrate
into the starter and molding cavities can interfere with
25 grain nucleation and propagation.
Object and summary of the invention
The invention thus seeks to remedy those drawbacks
by proposing a mold for monocrystalline cavity with a
30 molding cavity, a support rod, a starter cavity of shape
enabling grains to nucleate, and providing a support
appropriate for the support rod, and a selector channel
connected to the top of the starter cavity for
propagating a single grain to the molding cavity.
3 5 In at least one embodiment, this object is achieved
by the fact that the starter cavity comprises at least a
first volume in the form of upside-down funnel, and a
distinct second volume forming a plinth, located at the
bottom of the first volume and projecting perceptibly
relative to said first volume in at least one horizontal
direction, and in that the support rod is laterally
5 offset relative to the selector channel and is connects
the second volume of the starter cavity to the molding
cavity. The term "in the form of an upside-down funnel"
is used to mean a shape having a converging profile such
that the greatest cross-section of the first volume is
10 located adjacent to the second volume and the smallest
cross-section of the first volume is located remote from
the second volume. This shape is not necessarily conical
nor axisymmetric. The term "projecting perceptibly" is
used to mean that the horizontal difference between the
15 bottom edge of the first volume and the top edge of the
second volume can easily be detected by conventional
measuring means. This horizontal projection of the
second volume thus makes it possible to provide a stable
base for the support rod in spite of being laterally
20 offset, thus making it possible to avoid interfering with
grains being selected in the transition between the
starter cavity and the selector channel via the funnelshaped
first volume.
In particular, the second volume may project
25 horizontally around the entire perimeter of said first
volume, thereby creating a discontinuity between the
first and second volumes, which discontinuity contributes
to selecting grains.
In addition, said first volume may be axisymmetric
30 about a vertical axis, thus facilitating the transition
towards a selector channel of round section, thereby
reducing the risk of interfering grains nucleating, and
also reducing the risk of weak points in the walls of the
mold.
3 5 Furthermore, said second volume may be nonaxisymmetric
about a vertical axis, in particular in
order to facilitate positioning the meltable pattern for
the starter cavity when assembling an assembly of
patterns for fabricating the mold. Nevertheless, the
second volume may in particular be symmetrical relative
to a vertical plane, thereby facilitating the use of
5 injection molding to produce the meltable pattern that is
to be used for forming this cavity, by making the pattern
easier to unmold.
In order to obtain temperature conditions that are
particularly uniform in said first volume and in the
10 selector channel, the lateral offset of the support rod
relative to the selector channel may be such that a
minimum distance between the support rod and the first
volume is greater than the sum of a thickness of the mold
around the support rod plus a thickness of the mold
15 around the first volume. The mold may in particular be a
mold of the "shell" mold type, produced by a "lost wax"
or "lost pattern" method, thereby making it possible to
obtain a mold with walls that are relatively thin.
In particular, said selector channel may be a
20 baffle-forming selector channel, specifically for the
purpose of reliably ensuing that a single crystal grain
is selected. In addition, said selector channel may
present a cross-section that is round, in particular for
ensuring the integrity of the mold walls around the
25 selector channel, and also for avoiding interfering
grains nucleating in the sharp corners of the selector
channel.
The invention also provides a casting method
comprising at least fabricating such a mold, e.g. by the
30 "lost wax" or "lost pattern" method, casting molten metal
into the mold, cooling the metal with directional
solidification of the metal starting from the starter
cavity, and knocking out the mold in order to recover the
raw metal casting. By way of example, this method may
35 also include an additional step of finishing the raw
metal casting.
s BRIEF DESCRIPTION OF THE DRAWINGS
The invention can be well understood and its
advantages appear better on reading the following
detailed description of an embodiment given by way of
5 non-limiting example. The description refers to the
accompanying drawings, in which:
Figure 1 is a diagram showing the implementation
of a directional solidification casting method;
Figure 2 is a diagram showing an assembly of
10 casting patterns;
Figure 3 is a side view of the starter cavity in
an embodiment of the invention, with the corresponding
selector channel, and also a portion of the corresponding
molding cavity, and a ceramic support rod; and
15 Figure 4 is a plan view of a meltable pattern for
the starter cavity of Figure 3.
DETAILED DESCRIPTION OF THE INVENTION
Figure 1 shows how progressive cooling of the molten
20 metal in order to obtain directional solidification can
typically be performed in a casting method.
The shell mold 1 used in this method comprises a
central descender 4 extending along the main axis X
between a casting cup 5 and a plate-shaped base 6. While
25 the shell mold 1 is being extracted from the heater
chamber 3, the base 6 is directly in contact with a
soleplate 2. The shell mold 1 also has a plurality of
molding cavities 7 arranged as an assembly around the
central descender 4. Each molding cavity 7 is connected
30 to the casting cup 5 by a feed channel 8 through which
the molten metal is inserted while it is being cast.
Each molding cavity 7 is also connected at the bottom via
a baffle-selector channel 9 to a smaller starter cavity
10 adjacent to the base 6.
35 The shell mold 1 may be produced by the so-called
"lost-wax" or "lost-pattern" method. A first step of
such a method is creating a non-permanent assembly 11
comprising a plurality of patterns 12 connected together
by a tree 13, as shown in Figure 2. The patterns 12 and
the tree 13 are for forming hollow volumes in the shell
mold 1. They are obtained using a material having a low
5 melting temperature, such as a suitable resin or wax.
When it is intended to produce large numbers of parts, it
is possible in particular to produce these elements by
injecting the resin or wax into a permanent mold. In
order to support each pattern 12, a support rod 20 made
10 of refractory material, e.g. of ceramic, connects each of
the models 12 to the base of the assembly 11.
In this implementation, in order to produce the
shell mold 1 from the non-permanent assembly 11, the
assembly 11 is dipped in a slip, and then dusted with
15 refractory sand. These dipping and dusting steps may be
repeated several times, until a shell of slip-impregnated
sand of desired thickness has been formed around the
assembly 11.
The assembly 11 covered in this shell can then be
20 heated so as to melt the low melting-temperature material
of the assembly 11 and remove it from the inside of the
shell. Thereafter, in a higher temperature baking step,
the shell is sintered so as to consolidate the refractory
sand and form the shell mold 1.
25 The metal or metal alloy used in this casting method
is cast while molten into the shell mold 1 via the
casting cup 5, and it fills the molding cavities 7 via
the feed channels 8. During this casting, the shell mold
1 is kept in a heater chamber 3, as shown in Figure 1.
30 Thereafter, in order to cause the molten metal to cool
progressively, the shell mold 1 supported by a cooled and
movable support 2 is extracted from the heater chamber 3
downwards along the main axis X. Since the shell mold 1
is cooled via its base 6 by the support 2, the
35 solidification of the molten metal is triggered in the
starters 10 and it propagates upwards during the
progressive downward extraction of the shell mold 1 from
the heater chamber 3, along the arrow shown in Figure 1.
The constriction formed by each selector 9, and also its
baffle shape, nevertheless serve to ensure that only one
of the grains that nucleates initially in each of the
5 starter cavities 10 is capable of continuing so as to
extend to the corresponding molding cavity 7.
Arnong the metal alloys that are suitable for use in
this method, there are to be found in particular
monocrystalline nickel alloys such as in particular AM1
10 and AM3 from Snecma, and also other alloys such as
CMSX-28, CMSX-48, CMSX-6@, and CMSX-10~3 from C-M Group,
Ren& N5 and N6 from General Electric, RR2000 and SRR99
from Rolls-Royce, and PWA 1480, 1484, and 1487 from Pratt
& Whitney, amongst others. Table 1 summarizes the
15 compositions of these alloys:
Table 1: Monocrystalline nickel alloys in weight
wercentaaes
After the metal has cooled and solidified in the
20 shell mold, the mold can be knocked out so as to release
the metal parts, which can then be finished by machining
and/or surface treatment methods.
Figure 3 shows more specifically the shape of one of
the starter cavities 10, with the corresponding selector
5 channel 9, together with a portion of the molding cavity
7 connected to the starter cavity 10 via the selector
channel 9. It can thus be seen how the starter cavity 10
comprises a first volume 10a of the type comprising an
upwardly converging profile, i.e. in the form of an
10 upside-down funnel, and a distinct second volume lob at
the bottom of the first volume 10a. The upward
convergence of the first volume is such that the greatest
cross-section of the first volume is located adjacent to
the second volume and the smallest cross-section of the
15 first volume is located remote Erom the second volume.
In other words, the greatest cross-section of the first
volume 10a is in a position that is lower than its
smallest cross-section in the orientation of Figure 3.
Advantageously, this second volume lob presents a
20 horizontally arranged cross-section that is substantially
constant, projecting laterally relative to the first
volume 10a all around the first volume 10a, but to a
greater extent in a main direction. The bottom end of
the rod 20 is received in this lateral projection of the
25 second volume lob.
In the embodiment shown, the second volume lob
presents a height h, of at least 5 millimeters (mm) in
order to provide sufficient anchoring for the rod 20.
The top edge of the second volume lob is rounded in order
30 to avoid stress concentrations and consequently cracks at
this location in the shell mold 1. Such cracks could
lead to fine leaks of metal contained in the wall of the
shell mold 1, which could constitute sites for nucleating
interfering grains. The radius of this rounded portion
35 may be about 0.5 mm, for example.
The transitions between the first volume 10a and the
second volume lob, and also between the first volume 10a
and the selector channel 9 are likewise rounded for the
same reasons. The angle of inclination o: (ALPHA)
relative to the horizontal of one or more walls of the
first volume 10a in a plane that is assumed to be
5 vertical may lie in the range 40' to 70°, for example.
This angle of inclination enables a first grain selection
operation to be performed and avoids shrink marks at the
end of solidification that could generate sites for
nucleating interfering grains. Nevertheless, other
10 angles of inclination could be envisaged, depending on
the shape of the first volume IOa.
Although in the embodiment shown this first volume
10a is frustoconical in shape, other shapes of upwardly
decreasing horizontal section, and more particularly but
15 not exclusively shapes that are axisymmetric, could
equally well be envisaged. For example, a hemispherical
shape with its convex side pointing upwards could also be
envisaged. Independently of its shape, the height h, of
the first volume 10a may for example lie in the range
20 2 mm to 20 mm.
The selector channel 9 is in the form of a baffle
having five successive elements 9a to 9e, of
substantially constant round cross-section with a
diameter d, of at least 5 nun, and preferably lying in the
25 range 6 mm to 8 mm, for example. This range of diameters
makes it possible to achieve single grain selection while
avoiding too small a diameter for the selector channel 9,
which could lead to cracks forming in the walls of the
shell mold 1, which would encourage the nucleation of
30 interfering grains. For the same reason, the connections
between the successive segments 9a to 9e are rounded,
e.g. with a radius of about 7 nun. These five successive
segments 9a to 9e comprise first and fifth segments 9a
and 9e that are substantially vertical, a third segment
35 9c that is substantially vertical as well, but that is
laterally offset relative to the first and fifth segments
9a and 9e, and sloping second and fourth segments 9b and
9d that connect the ends of the third segment 9c
respectively to said first and fifth segments 9a and 9e.
The angle of inclination (BETA) of said second and
fourth segments 9b and 9d relative to the horizontal may
5 lie in the range 5" to 45', for example. The overall
height -h of the entire starter cavity 10 plus the
selector channel 9 may lie in the range 30 mm to 40 mrn,
for example.
The bottom portion of the molding cavity 7 can also
10 be seen in Figure 3. In order to provide the transition
between the selector channel 9 and the molding cavity 7,
so as to avoid creating interfering grains at this
critical location of the mold 1, the bottom edges of the
molding cavity 7 are sloping and rounded. The angle of
15 inclination y (GAMMA) of these edges relative to the
horizontal may likewise lie in the range 5" to 45", for
example. A canonical curve connects these rounded edges
to the selector channel 9. This canonical curve is
constituted by rounded portions with radii close to those
20 of the edges, in order to avoid changes of shape that
would contribute to nucleating interfering grains.
The rod 20 penetrates into the molding cavity 7
through one of its rounded bottom edges. In order to
avoid forming gaps that might constitute sites for
25 nucleating interfering metal grains, the connection 21
between the rod 20 and the molding cavity 7 presents the
smallest possible angle radius, or none, with this
applying all around the rod 20. The support rod 20 may
be made of a refractory material such as a ceramic, in
30 particular alumina, and it may present a cross-section of
diameter d, of 3 mm, for example.
Figure 4 is a plan view of the meltable pattern 10'
used for forming the starter cavity 10. The shapes of
the first and second volumes 10'a and 10'b of this
35 meltable pattern 10' correspond to the shapes of the
first and second volumes 1Oa and lob of the starter
cavity 10. As can be seen in the figure, the second
volume 10'b of this meltable pattern 10' presents a
symmetrical horizontal section formed by two circular
arcs of different radii, having their ends connected
together by straight lines. This shape serves in
5 particular to ensure that the pattern 10' is properly
oriented when assembling the assembly 11. One of said
circular arcs, of radius R, is centered on the central
axis of the first volume 10'a of the meltable pattern
lo', while the other circular arc, of radius -r
10 substantially smaller than the radius R, is centered on
the central axis of the rod 20. The minimum distance S
between the rod 20 and the first volume 10a of the
starter cavity 10 is greater than the sum of the
thicknesses e, and e, of the walls of the mold 1
15 respectively around said rod 20 and around said first
volume 10a, so as to avoid these walls overlapping, since
that would be harmful to temperature uniformity within
said first volume 10a of the starter cavity 10.
Although the present invention is described with
20 reference to a specific embodiment, it is clear that
various modifications and changes may be made thereto
without going beyond the general ambit of the invention
as defined by the claims. In addition, individual
characteristics of the various embodiments mentioned may
25 be combined in additional embodiments. Consequently, the
description and the drawings should be considered in an
illustrative sense rather than in a restrictive sense.

CLAIMS
1. A mold (1) for monocrystalline casting, the mold
comprising at least:
a molding cavity (7);
5 a starter cavity (10) having at least:
a first volume (10a) in the form of an upsidedown
funnel; and
a distinct second volume (lob) forming a plinth
at the bottom of the first volume and projecting
10 perceptibly relative to said first volume in at least one
horizontal direction; and
a selector channel (9) connecting said starter
cavity (10) to said molding cavity (7) ; and
the mold being characterized in that it further
15 comprises a support rod (20) that is laterally offset
relative to said selector channel, and that connects the
second volume (lob) of the starter cavity (10) to the
molding cavity (7).
20 2. A mold (1) according to claim 1, wherein a bottom end
of the support rod (20) is received in said lateral
projection of the second volume (lob).
3. A mold (1) according to claim 2, wherein a minimum
25 distance betx-teen the support rod (20) and the first
volume (10a) is greater than the sum of a thickness of
the mold (1) around the support rod (20) plus a thickness
of the mold (1) around the first volume (10a).
30 4. A mold (1) according to any preceding claim, wherein
the second volume (lob) projects horizontally around the
entire perimeter of said first volume.
5. A mold (1) according to any preceding claim, wherein
35 said first volume (10a) is axisymmetric about a vertical
axis.
6. A mold (1) acco 4 ding to any preceding claim, wherein
said second volume (lob) is not axisymmetric about a
vertical axis.
5 7. A mold (1) according to any preceding claim, wherein
said selector channel (9) is a baffle-shaped selector
channel.
8. A mold (1) according to any preceding, wherein said
10 selector channel (9) presents a round cross-section.
9. A casting method comprising at least the following
steps: -3
fabricating a mold (1) according to any preceding
15 claim;
casting molten metal into the mold (1);
cooling the metal with directional solidification of
the metal startingtfrom the starter cavity (10); and
knocking out the mold (1) in order to recover the
20 raw metal casting.