Note: Descriptions are shown in the official language in which they were submitted.
:~L'7~i0~8
- 1 - 694 P/10 CA ~DIV)
A method and an apparatus for manufacturing metallic
composite material bars by unidirectional solidification
The invention relates to a method for manufacturing
metallic`composite material bars by unidirectional solidifi-
cation, and also to an apparatus ~or practicing the method.
It applies to the manufacture by directed solidifi-
cation of bars of composite materials re;nforced with fibresor flakes formed during the alloy solidification. The
expression "bars" should be here intended in its general
meaning, viz. that they are pieces of cylindrical elongated
shape the section of which, which is constant, can be
lO circular, polygonal or complex and from which are machined
the desired articles.
The invention applies in particular to the manufac-
ture of bars from the refractory composite materials proposed
by Applicant in ~dian Patent No. 928,532 issued June 19, 1973;
15 C~d~ Patent No. 1,052,597 issue~ April 17, 1979 and C~dian
Patent Application S.N. 339,708 file~ November 13, 1979 and which c-omprise
a complex matrix made of a nickel- and/or iron-, and/or
cobalt- based superalloy containing chromium as well as
eventually other elements such as tungsten and aluminum and
20 in which is present a reinforcement phase made of mono-
crystalline fibres of at least one transition metal mono-
carbide. Due to their very good mechanical properties, such
materials are particularly suitable as constituent materials
for parts subject in operation to high stresses at high
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temperatures, such as the blades of aircraft turbines.
Such parts are manufactured either from rough casts
or in~ots in which is machined the required part, for
instance a turbine blade, or directly by casting, the alloy
being solidified in a mold having substantially the shape
of the required part. In the processes used hitherto, the
alloy wh'ich has to be subjected to the unidirectional
solidification is previously introduced in one time only in
the mold, either by casting or else in the form of a pre-
10 alloyed powder, viz. a powder the grains of which are sub-
stantially identical and have the nominal composition of
the alloy. The mold is then shifted relative to a hot
source and a cold source superimposed, the distance between
the hot source and the cold source as well as the efficiency 1'
15 of the hot source on the one hand, and the displacement
speed of the mold on the other hand, being set so that
within the alloy contained in the mold is formed a rigorous-
ly planar solidification front with a high thermal gradient
at the level of said front, and so as to thereby obtain
20 grains and reinforcement flakes or fibres perpendicular to
the solidification front. For materials with monocrystalline
fibres of monocarbides like those provided by Applicant,
the thermal gradient which is established is of the order
of 120 to 200C/cm at the solidification front and the mold
25 is moved at speeds of the order of 1 cm/h.
Hitherto, it has never been possible to manufacture
parts of a great length exhibiting over their whole length
constant mechanical properties. In fact, and due to the
great height of the liquid portion contained in the mold,
30 there appears a segregation phenomenon connected to the
convection movements of the liquid, which phenomenon is well
known of metal!founders. At the level of the solide-liquid
interface, the constituent elements of the alloy are une-
qually distributed between the solid phase and the liquid
35 phase, in accordance to their respective coefficients of
parting. Thus, for instance, the chromium is incorporated
to the solid in formation in a smaller proportion than its
proportion in the liquid phase, the latter having therefore
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a tendency of becoming richer in chromium in the vicinity
of the solidification front; on the contrary, the tungsten
is preferentially incorporated to the solid in formation in
a higher proportion than its proportion in the liquid phase,
the lat-ter growing poorer in tungsten. The result is that,
when one wishes to manufacture a large size part and when
the mold contains in consequence, at the beginning of the
solidification process, a liquid portion of great height,
the solidified material which is obtained as the solidifica-
10 tion progresses unavoidably exhibits composition variationsbetween the portion which was solidified in the first place
and the portion which was solidified in the last place. As
regards the particular elements which are considered here
by way of example, the latter portion exhibits a chromium
15 content and a tungsten content which are, respectively,
substantially higher and lower than the contents present in
the portion which was first solidified. The result is that
the metallurgical structure is not constant from one end to
the other of the manufactured material, the volume fraction
20 of the reinforcement fibres within the matrix, considered in
the cross!sections of the material, altering from one end lo
the other due to the concentration evolution of the consti-
tuent elements of the fibres with, as a consequence, a varia-
tion of the mechanical properties of the solidified part.
It is an object of the invention to provide a method
for manufacturing alloy parts by unidirectional solidifica-
tion, which allows preventing the advent of the segregation
phenomenon or at least which strongly limits its effects.
A further object of the invention is to provide a
30 method for manufacturing bars of any length by unidirectional
solidification of an alloy, said bars exhibiting nevertheless
good mechanical properties remaining constant over all their
length.
The method of the invention, according to which the
35 mold is displaced in a downward movement relative to a hot
source and a cold source superimposed, the relative position
of the two sources as well as their efficiency being set so
that a rigorously planar solidification front is established
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within the alloy as well as a high thermal gradient at the
level of said front, whereby the starting alloy is in the
form of an alloy powder of the required nominal composition,
is characterized in that the mold is permanently supplied
5 with alloy powder at a given flow rate chosen in order to
maintain a quantity of liquid alloy substantially constant
during the whole solidification process.
Such a method allows maintaining at substantially
constant values the concentrations of the various constituents
10 of the alloy at the level of the solidification front, the
uneven distribution of the constituents between the solid
phase and the liquid phase at the level of the solid-liquid
interface being compensated at every moment by the addition
of alloy at the nominal composition as the solidification
15 front progresses. Thereby is o~t:ained a part which exhibits,
in all its cross-sections, whatever its length, a metallic
composition as weel as a metallurgical structure substantially
constant.
Generally, according to the method of the invention,
20 there is maintained therefore in the mold a liquid alloy phase
exhibiting a composition substantially constant during the
whole solidification period.
The mold shape and the solidification speed allow
establishing the flow rate of the all~ which is to be intro-
? 5 duced in the mold.
An apparatus for carrying out the method of the inven-
tion comprises means for introducing the alloy powder in the
mold, as the solidification progresses, at a constant flow
rate corresponding to the quantity of alloy solidified by
30 unit of time.
Advantageously, said means comprises a distribution
device for the alloy powder having the required nominal compo-
sition.
Further characteristics and advantages of the inven-
35 tion will become more apparent from the following descriptionwhich is given by way of example, reference being made to the
accompanying drawing wherein :
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Figure 1 is a schematic view in lonyitudinal section
of a portion of an apparatus according to the invention;
Figure 2 is a cross-sectional view along line 2-2 of
Figure l;
Figure 3 is a sectional schematic view of a feed
device for the powder to the apparatus of the invention;
Figure 4 is a sectional view along line 4-4 of
Figure 3; and
Figure 5 is a diagram of the composition of a bar
made of a composite super-alloy obtained by the method of
the invention.
For the manufacture by casting of alloy parts of any
length, with an oriented structure, particularly of the type
comprising a complex matrix made of a nickel and/or cobalt-
and/or iron-based super-alloy containing chromium as well as
other elements such as tungsten and aluminum, and monocrys-
talline reinforcement fibres made out in situ in monocarbides
of transition metals, one uses a unidirectional solidifica-
tiOIl process of a starting alloy in a mold surrounded by a
hot source and a cold source placed under the hot source, by
displacing the mold relative to the two sources at a constant
and appropriate speed, the relative positions of said sources
and their efficiency being set so as to establish within the
alloy a planar so:Lidification front and a high thermal grade
?5 at said front, and by feeding continuously the mold with alloy
powder in a quantity which is function of the solidification
progress-.
Figure 1 shows a part of the apparatus according to
the invention, viz. the solidification oven as such.
Said oven comprises an upper chamber 10 and a lower
chamber 11, the lower end of the wall 12 of the upper chamber
10 being inserted into a plate or frame 13 which is rigid with
the lower chamber 11, the latter having two walls between
which flows the cooling water. The wall 12 is mounted around
a cooling block 14 which is also solid with frame 13, said
cooling block being formed by a copper sleeve thro~gh which
extend water flow passages, the water arriving through a
tubing 15 and being discharged through a tubing 16.
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The wall 12 of the ~pper chamber 10 is internally
coated, above the cooling block 14, with a thermal insulation
lining, for example made of a graphite felt, for reducing
the lateral thermal losses and comprising a first sleeve 17
5 of smaller diameter than that of the wall 12 and placed on
the cooling block 14, a second sleeve 172 spaced apart from
sleeve 171 by a graphite sleeve 32 and a sheath 173 surrounding
sleeves 171 and 172. The graphite felt lining 17, the sleeve
32 and the cooling block 14 are formed with a bore having a
10 s ze adapted for movement of a mold 19 with a minimum clea-
rance.
A piston 20 is slidably mounted longitudinally in the
lower chamber 11, with interposition of a sealing joint 23,
24, the mold 19, which is open at its two ends, being fixed
15 by being fitted by its lower end onto the core 21 of a support
22 formed on the upper frontal face of piston 20.
The upper portio~ of wall 12 of the upper chamber 10
is covered by a cap 25 fixed on a flange 26 and having a side
wall 27, a bottom 28 and a cover 29 having at its center a
20 tubular coupling 30 provided for communicating with an al.loy
feed device for the mold which will be described hereinbelow
with reference to Figure 3.
An inductor 31 surrounding wall 12 of the upper chamber
forms the hot source of the apparatus, the graphite sleeve 32
25 interposed between the sleeves 171 and 172 f the insulating
lining 17 having the function of a resistor.
The mold 19 is formed by a thin walled tube, of thick-
ness from 0.5 to 1.5 mm, made of a refractory oxide such as a
very high purity alumina (over 99.5%) very slightly porous
30 (less than 10%) and manufactured for example by spraying with
an oxy-acethylene torch or a plasma gun a model of the bar to
be solidified. A mold thus formed resists chemically to high
carbon content materials of those of Applicant when they are
in the li~uid state and offers a low thermal resistance which
35 does not prevent the obtention of a high thermal gradient (of
the order of 120 to 200DC/cm). Since such a mold does not have
a sufficient mechanical strength for being used without pre-
vious.cooking, it runs the risk of being deformed by creeping
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at a high temperature. It is foreseen to surround the mold
with banding elements 33 (see Figure 2), which may be made
as graphite rings stuck onto each other around the mold, or
by a sleeve made of a carbon-carbon composite material.
A powder distribution device 34 (Figure 3), connected
to the solidification oven through the junction 30, comprises
a casing 34 with a bottotn 35 on which is fixed a support
bracket 36 for a powder tank 37 provided with an interchan-
geable nozzle 38 discharging the powder, an opening 39 for
feeding the powder tank 37 being provided in the cover 40 of
casing 34. The cover 40 carries a driving motor 41 of adjus-
table speed driving a vertical axis 42 housed inside casing
34 and carrying at its other end a rotating horizontal plate
43.
lS In operation of the distributor, a calibrated bead
or ribbon of pre-alloyed powder 44 flows through the nozzle 3~3
of the powder tank 37 orito the plate 43, powde~ ,~7nich, while
the plaLe ~3 roia,es (Figure 4), is forwaxded by a i)l3 :32 ~5
carried by an arm fixedly mounted onto a pin 47 into the
20 introduction opening of a funnel 48 the discharge channel 49
of which is mounted in the bore of the tubular connection 30
and emerges inside the mold 19. The flow of powder is regula-
ted by choosing the diameter of the nozzle 38 and by setting
the rotation speed of plate 43.
.25 ~ The inner free volume of the oven can be degased and
placed in a reducing atmosphere, for example argon containing
5% of hydrogen, introduced by an input channel 51 extending
across the powder distributor cover 34, and discharged by a
channel 51' arranged at the lower portion of the lower chamber
30 11 of the oven. With the same aim of "capturing" the oxygen
of the oven atmosphere, it is foreseen to place in the thermal
insulation lining 17 of the oven upper chamher 10 a crucible
52 filled with a deoxidizing agent, such as Ti-Zr chips (50/50),
said crucible being in a location such that when the oven is
35 set in operation, the temperature to which it is exposed is
that corresponding to the deoxidizing activity of the material
it contains.
An apparatus according to the invention is operated
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as follows :
On the core 21 of support 22 made of a refractory
steel, the cross-section of which is identical to the inner
cross-section of the mold 19, -Laking into account the
dilatation clearances-, one places a block of alloy, or
bar-foot 53, of same composition as the powder and also of
same cross-section as the inside of the mold-taking into
account the dilatation clearances. The mold 19 is then
inserted onto the block 53 and then on the core 21 and after
the banding elements 33 are placed around the mold. The
piston 20 carrying the mold is moved by means of a rack 61
and a gear 62 coupled with a motor 63, so that the upper
face of the bar-foot 53 is at the level of the upper portion
of the resistor 32. The chamber is then closed, degased,
filled with an argon and hydrogen mixture at a slight over-
pressure, the resistor 32 being then progressively raisedto its operational temperature by high frequency induction,
provided by the inductor 31 fed by a source 64. The distance
between the resistor 32 and the cooling block 14, as well as
the heating conditions, are set so that the isotherm corres-
ponding to the melting point of the alloy to be solidifiedis placed in the unidirectional and vertical thermal flux
zone, that is in the zone where the heat exchanges through
the side wall of the apparatus are non-existent due to the
presence of the insulating lining 17, and more precisely so
that the solid-liquid interface is at a few millimcters
below the lower level of resistor 32, the position of the
solidification front being indica-ted by a thermocouple 55
the sensing joint 56 of which is positioned immediately
above the solidification front level, in the liquid portion
of the alloy. The thermocouple 52 is connected to a tempera-
ture monitor 65 which adjusts the energy feeding the inductor
31 in order to maintain constant the temperature in the area
of the sensin~ joint 56 of the thermocouple 55.
The powder feed channel 49 is placed in the mold
with its outlet opening 57 positioned at a distance from the
free surface of the liquid alloy contained in the mold such
that, on the one hand, the temperature of the area in which
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is placed said outlet opening 57 is not sufficient for the
powder grains to melt therein, and, on the other hand, the
latter are spread finely shower-wise and are distributed
- on all the surface of the liquid alloy.
The mold is then fed with powder by driving in
rotation the rotating plate 43, and the mold is simultane-
ously moved downwards. The powder feed rate is set so that
the upper level of the liquid alloy remains in the same
position relative to the fixed elements of the solidifica-
tion oven, or in other words so that the volume of the
liquid a~loy contained by the mold remains constant.
Taking into account the sclidification speed and
the size of the mold, one calculates the quantity of soli-
dified alloy by unit of time and one sets accordingly the
powder feed rate of the mold by a suitable choice of the
size of the nozzle 38, by setting the position of its
opening relative to the plate ~3 and the rotation speed of
said plate.
After a duration of a few minutes of displacement
of the mold, one reaches stationary thermal conditions as
well as stationary diffusion conditions of the elements
of the alloy from the solid phase to the liguid phase. The
small volume of the liquid phase and the constant feed in
the liquid portion of constant volume of powder having the
!?5 nominal composition of the alloy to be solidified allow
maintaining a constant average composition in the liquid
in spite of the fact that the distribution of the elements
is different in the liquid phase and in the solid phase at
the solidification front and in spite of the stirring of
the liquid due to convection. The solidified alloy according
to the invention thus exhibits much more reduced segregations
than an alloy of same nominal composition which would have
been solidified by any other known method which does not
comprise the metered alloy feed to the mold as the solidifi-
35 cation progresses and in which all the alloy mass would beinitially in the liquid s-tate.
The movement of the mold, the powder feed and the
heating are interrupted when the surface of the liquid bath
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reaches the upper end of the mold. The length of the latter
and the space available for its displacement are therefore
the only limits to the length of the part obtained by uni-
directional solidification.
EXAMPLE 1
A bar in a nickel-based super-alloy of the following
composition (by weight) : base Ni - 10~ Cr - 10% W - 20~ Co -
4~ Al - 4.~% Nb - 0;55~ C, is manufactured by using the
method of the invention in a mold having a cross-section
shown in Figure 2, viz. a cross-section in the shape of a
parallelogram of 22 mm x 35 mm with rounded angles of 75.
The mold is placed in the oven and fed with the
alloy so that the height of the melted zone, at the maximum
operating temperature, viz. 1600~CI is of the order of 50 mm.
The thermal gradient established is of the order of 140C/cm
and the solidification speed, in other wDrds, the displace-
ment speed of the mold, is of 1.2 cm/h.
Figure 5 shows the representative curves of the
content of the bar for each of the elements others than that
forming the base, namely the nickel, by plotting in abscissas
the solidified length in mm and in ordinate the content in %
by weight of each of the elements. E`rom said curves, it is
clear that if one suppresses at the beginning of the bar a
length portion equivalent to the height of the liquid zone,
viz. about 50 mm, the variation of the contents of the bar
obtained in each of the elements is less than 5~ (as a
relative value) along the length the bar, and is less than 2
~as a relative value) if one leaves out the chromium element.
The oriented fibrous structure, which i5 formed by the matrix
reinforced by the nicbium monocarbide fibres, is perfect
along its whole length and no evolution of the volume frac-
tion of the niobium monocarbide fibres is noticeable.
The representative curves of the element concentra-
tion as a function of the solidified length all tend towards
an asymptote the ordinate of which represents the content of
the corresponding alement in the parent alloy. This fact
confirms that one can obtain bars of solidified alloy of any
length exhibiting no segregation over their length as regards
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their metallic composition and their metallurgical struc-
ture, and this, while always putting aside the first portion
of the solidified zone, that is that corresponding to the
height of the liquid ~one.
EXAMPLE 2
One solidifies under the same conditions as those
of Example 1 a bar made of an alloy of following composition
by weight : base Ni - 10% Co - 4% Cr - 10~ W - 16~ Al -
4.2~ Nb - 0.46~ C - 2% Mo.
The variations of the contents in each of the
elements, including the addition elements, of the bar
obtained after solidification are similar to those of
Example 1. The fibrous structure of the composite material
is without defect through the whole solidified length and
no evolution of the volume fraction of the fibres is noticed.
EXEMPLE 3
Bars made of an alloy as in Example 1 on the one
hand, and of an alloy as in Example 2 on the other hand, of
rectangular cross-section of 12 mm x 65 mm, with rounded
ends, are obtained by unidirectional solidification under
the same conditions as those of Examples 1 and 2.
The contents variations of the addition elements in
the bars obtained are similar to those of Examples 1 and 2.
The structure of the obtained composite is also perfectly
fibrous, with a constant volume fraction over the whole
solidified portion.
COMPARISON EXAMPLE
As a comparison, a bar has been prepared by uni-
directional solidification with the same alloy as that
indicated in Exarnple 1, having the same cross-section as in
said Example and a total length of 250 mm corresponding to a
length which can be obtained by using the prior art methods.
A bar has been prepared by unidirectional solidifi-
cation according to a usual method without continuous feed
~method A), the total length of the initially melted portion
being equal to 250 mm.
On the other hand, a bar of same length has been
prepared by unidirectional solidification, but with the
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method with continuous feed of a pre-alloyed powder
according to the invention (method B), the height of the
melted zone being of 50 mm.
In one and the other method, the thermal gradient
established was of 140~C/cm, and the solidification speed
of 1.2 cm/h.
Cross-sectional cuts have been effected on the bars
obtained, and the proportion in weight of some of the
elements in each of said cuts has been established.
The results of these tests are shown in the
following Table.
TABLE
Comparison of the compositions of alloy bars obtained
by unidirectional solidif~cation~ of same total length
A- by a standard method without continuous feed (total
length of the initially melted portion : 250 mm)
B- by the method with continuous feed of a pre-alloyed
powder (height of the melted zone : 50 mm~
. _
Distance from the Cr % W % C %
beginning of the
solidification A s A s A s
(mm)
8,89,1 11,8 9,51 0,61 0,55
_
9,01 9,54 11,65 9,38 0,57 0,53
100 9,45 1 9,83 1],4 9,50 0,54 0,53
155 10,49,95 10,8 9,38 0,535 0,53
200 10,89,99 10,5 9,40 0,525 0,53
relative compo-
sition difference 18% 4,5% 11% 1% 10% 0%
50 and 200 mm (*) (X)
.
~ Except for the titration precision
