EP3119544B1 - Lined centrifugal mold with controlled thermal inertia - Google Patents

Description

The present invention relates to the molding by centrifugal casting of metal parts, in particular turbomachine blades, and in particular turbojet or aircraft turboprop turbine blades.

Specifically concerned are a mold, its use, and the assembly comprising the mold and the molten metal alloy cast therein.

It is known to produce turbomachine blades by machining a blank obtained from a foundry. The blank is typically a bar, that is to say a block of material generally of elongated shape.

One of the techniques for obtaining the blank is lost wax casting with casting of the metal alloy in a preheated ceramic mold, centrifuged or not. In this case the roughing can approach the final machined geometry. This mold is single use. In addition, chemical interactions between the molten alloy and the ceramic can generate defects on the surface of the blank,

Another solution is to cast bars in a permanent centrifuged metal mold. This appears promising, in particular for manufacturing TiAl-based metal alloy blades, more particularly with β/α solidification.

However, these bars are segregated, particularly in aluminum, and the chemical heterogeneity can locally lead to properties that do not comply with the expected requirements.

It is therefore important to control (limit/remove) these segregations.

• un moule rotatif autour d'un axe, pour une coulée centrifugée,
• et un alliage métallique de TiAI coulé dans le moule,

tel que défini dans la préambule de la revendication 1.In EP992305 , is also known a set comprising:

• a rotating mold around an axis, for centrifugal casting,
• and a TiAI metal alloy cast in the mold,

as defined in the preamble to claim 1.

A problem to be solved concerns the control of the cooling rate of the molten metal alloy after casting to promote obtaining a homogeneous level of aluminum in the bar, and therefore a controlled microstructure. It should be noted that the problem can therefore arise with metal alloys containing aluminum, other than the aforementioned TiAl.

The bars cast in a centrifugal foundry are currently mainly intended to be remelted. Segregations are then not a problem since they are then re-homogenized during remelting. If we wish to use these cast bars directly, the simplest solution would consist of heating the molds to restrict the thermal gradients and thus limit segregation. However, centrifugal foundry installations are not suitable or only slightly suitable for preheating metal molds. This solution is therefore imperfect.

Thus, the technical solutions described below result from an effort to understand and explain the phenomena involved in the solidification of alloy bars, in particular TiAl.

• un moule rotatif autour d'un axe, pour une coulée centrifugée,
• et un alliage métallique de TiAI coulé dans le moule,
  • le moule comprenant :
    • des logements de réception, en acier doux, en acier, alliage métallique et/ou en céramique, adaptés pour qu'y soit coulé de façon centrifugée ledit alliage métallique de TiAl en fusion, lesdits logements s'étendant radialement autour de l'axe précité, et
    • des chemises définissant toutes ensembles lesdits logements et qui sont logées dans au moins un exosquelette creux, en acier doux, acier ou alliage métallique, et
  • le moule présentant une capacité thermique surfacique inférieure à celle de l'alliage métallique coulé, à l'endroit de plusieurs plans de coupe passant chacun par le moule et l'alliage métallique coulé,
  • ledit ensemble étant caractérisé en ce que :
    • le ou les exosquelettes est/sont à claire-voie, ou
    • un espace vide existe périphériquement entre chaque chemise et l'exosquelette qui l'entoure, ou,
    • une structure alvéolaire s'étend périphériquement entre chaque chemise et l'exosquelette qui l'entoure.

According to a first definition, the solution proposed here consists of recommending the use of a set comprising:

• a rotating mold around an axis, for centrifugal casting,
• and a TiAI metal alloy cast in the mold,
  • the mold comprising:
    • receiving housings, made of mild steel, steel, metal alloy and/or ceramic, adapted so that said molten TiAl metal alloy is cast in a centrifugal manner, said housings extending radially around the aforementioned axis , And
    • shirts all together defining said housings and which are housed in at least one hollow exoskeleton, made of mild steel, steel or metal alloy, and
  • the mold having a surface heat capacity lower than that of the cast metal alloy, at the location of several cutting planes each passing through the mold and the cast metal alloy,
  • said assembly being characterized in that:
    • the exoskeleton(s) is/are slatted, or
    • an empty space exists peripherally between each shirt and the exoskeleton that surrounds it, or,
    • an alveolar structure extends peripherally between each shirt and the exoskeleton that surrounds it.

• plusieurs logements de réception s'étendent radialement autour de l'axe de rotation du moule,
• et au moins un exosquelette creux où sont logées des chemises définissant toutes ensembles lesdits logements, le ou les exosquelettes étant en acier doux, aciers ou alliages métalliques et les chemises étant en acier doux, aciers ou alliages métalliques et/ou céramique, avec comme spécificités additionnelles:
  • -- que le ou les exosquelettes sont à claire-voie, ou
  • -- qu'un espace vide existe périphériquement entre chaque chemise et l'exosquelette qui l'entoure, ou
  • -- qu'une structure alvéolaire s'étend périphériquement entre chaque chemise et l'exosquelette qui l'entoure.

To promote cooling control, it is also proposed that the mold, rotating around an axis, include:

• several receiving housings extend radially around the axis of rotation of the mold,
• and at least one hollow exoskeleton in which are housed liners all together defining said housings, the exoskeleton(s) being made of mild steel, steels or metal alloys and the liners being made of mild steel, steels or metal alloys and/or ceramics, with the specific features additional:
  • -- that the exoskeleton(s) are slatted, or
  • -- that an empty space exists peripherally between each shirt and the exoskeleton which surrounds it, or
  • -- that an alveolar structure extends peripherally between each shirt and the exoskeleton that surrounds it.

• proposer des parois de chemises fines à inertie thermique faible,
• et/ou proposer des parois de chemises d'épaisseur variable à inertie thermique contrôlée,
• tendre vers un refroidissement contrôlé efficace de l'alliage coulé,
• faire assurer la résistance mécanique de l'ensemble essentiellement via la structure extérieure (le(s) exosquelette(s)).

By providing such an insert mold, we will in particular be able to:

• offer thin liner walls with low thermal inertia,
• and/or offer liner walls of variable thickness with controlled thermal inertia,
• tend towards effective controlled cooling of the cast alloy,
• ensure the mechanical resistance of the whole essentially via the external structure (the exoskeleton(s)).

It is also recommended that the shirts exhibit, with respect to the molten metal alloy, a predominant thermal behavior compared to that of the exoskeleton(s).

Concerning the mold itself, it is planned, in addition to its rotary nature, adapted to centrifugal casting, therefore with liners defining all together said housings and which are housed in at least one hollow exoskeleton, the fact that the exoskeleton(s) is/are made of mild steel, steels or metal alloys and that the liners are made of mild steel, steels or metal alloys and/or ceramic .

• qu'ensemble, une chemise et l'exosquelette (ou la partie d'exosquelette) qui l'entoure présentent une première capacité thermique surfacique,
• que l'alliage en fusion coulé dans cette chemise présente une seconde capacité thermique surfacique,
• et que le couple formé par chaque chemise et son exosquelette, d'une part, et l'alliage, d'autre part, soit choisi pour que le rapport entre les première et seconde capacités thermiques surfaciques soit inférieur à 1.

Thus (and this is therefore valid at the location of each of several distant cutting planes along any shirt), we recommend:

• that together, a shirt and the exoskeleton (or the part of an exoskeleton) which surrounds it present a first surface thermal capacity,
• that the molten alloy cast in this jacket has a second surface thermal capacity,
• and that the couple formed by each shirt and its exoskeleton, on the one hand, and the alloy, on the other hand, is chosen so that the ratio between the first and second surface thermal capacities is less than 1.

Thus, the present invention will make it possible to remedy at least part of the aforementioned drawbacks in a simple, effective and economical manner.

In the case of a TiAl block, maintaining the aforementioned ratio makes it possible to obtain aluminum segregations between the periphery and the core of the order of 0.2% by mass.

• certaines caractéristiques physiques du ou des exosquelettes, le(s)quel(s) pourront participer à la dissipation plus ou moins rapide de la chaleur née de la coulée, tout en reprenant une partie importante des efforts lors de la centrifugation (on conseille une tenue permettant de supporter une accélération centrifuge de plus de 10 g),
• par rapport aux caractéristiques physiques des chemises qui pourront donc être de faible épaisseur et/ou dans un matériau différent de celui du/des exosquelettes.

Also note that by using a solution with shirts and exoskeleton(s), we will be able to dissociate:

• certain physical characteristics of the exoskeleton(s), which(s) will be able to participate in the more or less rapid dissipation of the heat generated by the casting, while taking up a significant part of the efforts during the centrifugation (we recommend an outfit capable of supporting a centrifugal acceleration of more than 10 g),
• in relation to the physical characteristics of the shirts which may therefore be of low thickness and/or in a material different from that of the exoskeleton(s).

We will also be able to work more appropriately on the shapes (particularly interior) of the shirts, without necessarily working on those of the exoskeleton(s) in the same way.

Note also that providing a cellular structure extending peripherally between each shirt and the exoskeleton which surrounds it will typically make it possible, due to its box structure, to promote thermal control and/or mechanical strength, as permitted by a production of the slatted exoskeleton(s): the thermal inertia will then be lower than if the same exoskeleton(s) had a solid wall(s).

• que le ou les exosquelettes soi(en)t en acier et que les chemises soient en un matériau métallique de plus faible inertie thermique, ou en matériau(x) réfractaire(s), et /ou
• qu'individuellement les chemises présentent une paroi périphérique ; entre deux extrémités libres opposées, une longueur selon la direction radiale suivant laquelle chacune s'étend et, sur cette longueur,
  • -- un conduit central de coulage de l'alliage ayant une section moyenne et,
  • -- une épaisseur moyenne de paroi périphérique inférieure à 1/8, et de préférence 1/10, de l'une des dimensions de ladite section

Same consideration if, as advised, we plan:

• that the exoskeleton(s) are made of steel and that the jackets are made of a metallic material of lower thermal inertia, or of refractory material(s), and/or
• that individually the shirts have a peripheral wall; between two opposite free ends, a length in the radial direction in which each extends and, over this length,
  • -- a central alloy pouring conduit having an average section and,
  • -- an average peripheral wall thickness less than 1/8, and preferably 1/10, of one of the dimensions of said section

• que le ou les exosquelettes puisse(nt) être à claire-voie,
• qu'un espace puisse exister périphériquement entre chaque chemise et l'exosquelette qui entoure,
• qu'une structure alvéolaire s'étende périphériquement entre chaque chemise et l'exosquelette qui l'entoure.

Furthermore, it is planned, independently or in combination:

• that the exoskeleton(s) can be slatted,
• that a space can exist peripherally between each shirt and the surrounding exoskeleton,
• that an alveolar structure extends peripherally between each shirt and the exoskeleton which surrounds it.

This will be favorable for controlling thermal gradients, controlling solidification and therefore controlling segregation.

With metal liners, we will help to limit the contamination of the material of the blank by that of the mold.

In addition, we will tend towards the above by using a rotating mold as above, with housings, made of steel, metal alloy and/or ceramic, for receiving a TiAl metal alloy, and respecting the ratio of aforementioned surface thermal capacities.

• la figure 1 est une vue schématique de face d'un barreau plein de la technique antérieure, dans lequel au moins une aube de turbine de turbomachine est destiné à être usinée,
• la figure 2 est une vue schématique d'un moule de la technique antérieure,
• la figure 3 est une vue schématique de dessus d'un moule à chemises et exosquelettes, dans lequel seront moulés des barreaux ayant moins de ségrégation,
• les figures 4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21 schématisent des chemises et exosquelettes suivant divers modes de réalisation, en vue de face (figures 4,6), coupes schématiques longitudinales (l'un des axes radiaux B ; figures 12,14,17,18,20) ou transversale (figure 7, coupe VII-VII, figure 11, coupe XI-XI, figures 15,16,19,21), vues de côté (figure 5 -vue suivant V- et figures 8,9,10,) ; la figure 13 étant un détail d'une variante de réalisation de zones identiques à celle référencée XIII,
• la figure 22 est un graphique qui montre des résultats de tests obtenus sur plusieurs coulées de TiAl dans des moules en acier de différentes formes et épaisseurs, avec en abscisse le rapport (Ci/Ci') et en ordonnée le taux de ségrégation constaté dans l'alliage coulé (en poids d'Al),
• et la figure 23 schématise les emplacements des quatre sections carrées (54, 56, 58 et 61 mm montrées figure 22) choisies pour caractériser un unique moule de section variable.

Other advantages and characteristics of the invention will appear on reading the following description given by way of non-limiting example and with reference to the appended drawings in which:

• there figure 1 is a schematic front view of a solid bar of the prior technique, in which at least one turbomachine turbine blade is intended to be machined,
• there figure 2 is a schematic view of a mold of the prior art,
• there Figure 3 is a schematic top view of a mold for shirts and exoskeletons, in which bars with less segregation will be molded,
• THE figures 4,5,6,7 , 8.9 , 10 , 11,12,13 , 14,15,16,17,18,19,20,21 schematize shirts and exoskeletons according to various embodiments, in front view ( figures 4,6 ), schematic longitudinal sections (one of the radial axes B; figures 12 , 14,17,18,20 ) or transverse ( Figure 7 , cut VII-VII, Figure 11 , cut XI-XI, figures 15,16,19,21 ), side views ( Figure 5 -view following V- and figures 8,9,10 ,); there Figure 13 being a detail of an alternative embodiment of zones identical to that referenced XIII,
• there Figure 22 is a graph which shows test results obtained on several castings of TiAl in steel molds of different shapes and thicknesses, with the ratio (Ci/Ci') on the abscissa and the segregation rate observed in the cast alloy on the ordinate. (in weight of Al),
• and the Figure 23 diagrams the locations of the four square sections (54, 56, 58 and 61 mm shown Figure 22 ) chosen to characterize a single mold of variable section.

Figure 1 we see a metal bar 11 made from a foundry and in which at least one, here two, turbine blades 12 of a turbomachine are intended to be machined.

The bar 11 can have a cylindrical shape and is solid. It is obtained by casting a metal alloy into a mold.

There figure 2 represents a conventional device 10 for manufacturing bars or blanks 11, by successive melting and casting operations.

The device 10 comprises a closed and sealed enclosure 20 in which a partial vacuum is applied. An ingot 16 of metal alloy, in this case based on TiAl, is first melted in a crucible 14. While melting, it is poured into a permanent metal mold 13.

The mold 13 allows the alloy to be cast by centrifugation, in order to obtain bars 11. For this, it is rotated around an axis A vertical, preferably via a motor 18. The mold 13 comprises several housings or cavities 17, which extend radially (axes B1, B2; figures 2,3 ) around axis A. These cavities are preferably regularly spaced angularly around axis A which is here vertical. The alloy to be cast, brought towards the center of the mold, is distributed towards the cavities.

After cooling, the mold 13 is dismantled and the molded bars 11 are extracted. The thicknesses of the mold can lead to strong thermal inertia and create significant thermal gradients during the cooling of the cast metal, generating radial segregations, particularly in aluminum, from the center to the periphery of the bars. The aluminum segregation results during the solidification of the alloy by a progressive depletion of the residual liquid during the growth of the dendrites from the walls of the mold. The parts from the bars 11 can therefore have differences in microstructure.

Furthermore, in the event of wear, it is necessary to change the part of the mold surrounding the radial housing 17 concerned.

The invention makes it possible to provide a solution to the cited problem of segregations and, if necessary, to satisfy the requirements of resistance to centrifugal forces and rapid and frequent change of at least part of the mold.

To this end, it is planned that the mold 13 selected be designed, then the centrifugal casting of the blocks 11 carried out, so that at the location of several cutting planes (such as P1, P2 figure 4 , P3, P4 Figure 14 , P5, P6 Figure 18 ) passing through the mold and the molten metal alloy 11 contained (in contact with it; see Figure 14 ), this mold has a surface thermal capacity (Ci) lower than that (Ci') of the molten metal alloy contained. Although this is not limiting, the cutting planes illustrated are perpendicular to axis B.

• p1 et p1', les masses volumiques respectivement du matériau constituant le moule, et de cet alliage métallique,
• S1 et S1', les sections respectivement du moule (chemise 135 entourée de l'exosquelette 137), et de cet alliage métallique, et
• c1 et c1' les chaleurs spécifiques du moule (chemise 135 entourée de l'exosquelette 137), et de l'alliage métallique,

il est prévu :

• que C1 / C1'< 1
• et que ceci soit aussi vérifié de la même manière à l'endroit d'autres plans de coupe, tels ceux référencés P2,P3,P4,P5,P6.

So, either, at the location for example of the cutting plane P1 of the figure 4 :
C1 = ρ 1.S1.c1 (kJ.K ^-1 .m ^-1 ), the surface thermal capacity of the mold (here therefore of the liner 135 surrounded by the exoskeleton 137), and C1' = ρ 1' .S1 '.c1' (kJ.K ^{-1.m -1} ), the surface thermal capacity of the molten metal alloy 11 contained, with:

• p1 and p1', the densities respectively of the material constituting the mold, and of this metal alloy,
• S1 and S1', the sections respectively of the mold (shirt 135 surrounded by the exoskeleton 137), and of this metal alloy, and
• c1 and c1' the specific heats of the mold (shirt 135 surrounded by the exoskeleton 137), and of the metal alloy,

it is provided :

• that C1 / C1'< 1
• and that this is also verified in the same way at the location of other cutting planes, such as those referenced P2,P3,P4,P5,P6.

The limit value (surface thermal capacity of the mold / surface thermal capacity of the metal alloy cast in contact with it < 1) was established from results obtained in particular on several castings of TiAl, in steel molds of different shapes and thicknesses . For each bar, segregation was obtained by carrying out precise measurements of the aluminum content (uncertainty less than 0.06% by weight of AI - wt Al) on the surface and in the heart of the bar. The measured deviation defines the radial macro-segregation. The results are reported in the Figure 22 where the shapes of the liners tested and their conformations in radial vertical section compared to the example of axes B1 or B2 figure 2 (solid peripheral wall, relative dimensions, etc.) have been figured, with specified dimensional values. The square sections (54, 56, 58 and 61 mm) come from a single mold of variable section ( Figure 23 ) ; radial segregation was measured for these four slices and compared to the specific ratio of each section.

The three sections with a ratio less than 1 (within 10%) have radial aluminum periphery-core segregations less than or equal to 0.2% wt (to within 10%). Conversely, all sections giving a ratio greater than 1 present a greater aluminum segregation, the greater the greater the ratio.

THE figures 3 to 21 represent embodiments of a mold 130 according to the invention, it being specified that the figures 5 and following diagrams of variants. As for all the functional means with which these mold productions are preferably provided, they have not been illustrated, nor systematically included in all the variants described below. The particularities of these embodiments can be combined and applied from one mode to another.

The mold 130 differs from the mold 13 in the production of certain of its structural means.

Around the central block 131, through the bent internal conduits 132 from which the alloy is caused to distribute itself radially around the vertical central axis A, are regularly spaced liners 135 (or for example 135a, 135b figure 4 ). The conduits 132 open respectively into radial conduits 133 which receive the alloy through an opening 133a and each extend inside one of the liners, in a radial direction B. The opening 133a of each liner is thus located in the radially interior end part 134a of the conduit concerned.

The shirts, which are therefore hollow, are arranged in at least one exoskeleton 137, and preferably in as many exoskeletons as there are shirts, each exoskeleton then containing a shirt 135 defining one of said housings.

The exoskeleton(s) hold the liners against the centrifugal forces generated by the rotation of the mold.

Figure 4 , the central axis A of rotation of the mold is vertical and both the liners 135 and the exoskeletons 137 each extend along a horizontal longitudinal axis (axis B).

At its radially outer end (extreme part 134b), each conduit 133 has a solid bottom 135c.

Comparably, each exoskeleton 137 has, at its radially inner end, an opening 137a (see for example figures 12 , 17 ) through which, for example, a liner 135 can pass and, at its radially outer end, a bottom 137b which can participate in the radial retention of the liner.

Figure 6 , we notice in 139a, 139b fixings, here removable, established between the shirt illustrated, here 135a, and the exoskeleton, here 137a, which surrounds it, so as to allow replacement of the shirt. Screwed fixings may be suitable.

We also note, see Figure 4 , that removable fixings, such as 141a, 141b, are provided between each shirt (and/or the exoskeleton which surrounds it, references 142a, 142b) and the central block 131.

Thus it will be possible to separate the shirts from the exoskeletons and from the central block 131, in particular to replace these shirts. Again, screwed fixings may be suitable.

The removable fasteners established between shirts and exoskeleton(s) and/or between the central block 131 and shirts and/or exoskeleton(s) may form thermal bridge break zones.

In a preferred embodiment, the exoskeleton(s) will be made of steel (such as mild steel) and the jackets will be made of a metallic material of lower thermal inertia, or of refractory material(s).

Figure 7 , the peripheral wall is referenced 135d and we can see, in the center, the molded bar (blank) 110 resulting from the casting.

There figure 8 illustrates a solution where the exoskeleton 137a shown schematically is provided with a movable door 143a which, in the open position, releases an opening 145 allowing the shirt considered, here 135a, to pass through it. Hinges 147a can facilitate the operation of each mobile door.

On the figures 4 to 8 , it will also be noted that the exoskeletons are transparent.

They thus appear as sort of mesh cages.

To promote low thermal inertia, it is provided here that a space 155 exists peripherally (around axis B) between each shirt, such as 135a, and the exoskeleton, such as 137a, which surrounds it.

Centering means 157 position, in a fixed manner at least during centrifugation, the shirt considered relative to the exoskeleton, for casting (see Figure 5 ).

Figures 9,10 the liners are formed individually from several shells, such as 150a, 150b.

These shells open and close along a surface (here with joint plane 152) which is generally transverse to the axis (A) around which the mold rotates.

In addition, a separable fastening 153, such as a lock, is established between the shells in order, once the shells are separated, to be able to remove the bar 110 from the inside of the jacket, here 135a, considered, through the released opening 154.

In the solution of figures 11,12 , a cellular structure 159, which extends peripherally between each shirt, such as 135a, and the exoskeleton which surrounds it, such as 137a, plays this role and therefore defines at least part of said aforementioned centering means 157.

The cellular structure 159 can be annular. It can occupy a space between the bottom 135c of the shirts and that 137b of the exoskeleton considered ( figure12 ).

Including for the desired thermal transfers, the Figure 13 shows that the shirt considered and the cellular structure, such as 159, are in contact through discrete zones, such as 159a, 159b,
Rather than in separate pieces, we could plan to make the liner and the cellular structure in a single piece ( Figure 13 ), so that they meet through these discrete zones located at the end radially interior of the walls 161 separating the cavities 163 of the cells in pairs.

Alternatively, it will be possible to produce each shirt, such as 135a, said structure 159 which surrounds it and the exoskeleton, such as 137a, which surrounds this structure, in three distinct elements, dissociable from each other, the shirt and the structure being engaged in the exoskeleton, concentrically, therefore following a radial B to axis A.

Figures 14 to 21 , but this can apply to the previous cases, the exoskeleton(s), such as 137a, individually comprise(s) a radially outer end 137c ( figures 17,18 ) towards where the formed shirt 135a rests radially against a transverse surface 165 of the exoskeleton.

The radially outer end 137c can be opened.

An attached plug 167 (which can be removable) will then block this radially outer end 137c.

The exterior structure, in particular that of exoskeleton(s), will preferably be made of mild steel, more or less refractory steels or alloys. There will therefore be slid axially an insert (the aforementioned shirt) made of metallic material as mentioned above and/or ceramic.

• que l'insert assure l'obtention de la géométrie souhaitée pour la pièce coulée et permette d'en contrôler la solidification, par maîtrise des contraintes thermiques,
• et que la structure extérieure assure le positionnement du moule sur le montage de coulée centrifuge ainsi que la résistance mécanique de l'ensemble.

We will understand that this allows:

• that the insert ensures that the desired geometry is obtained for the casting part and makes it possible to control its solidification, by controlling thermal constraints,
• and that the external structure ensures the positioning of the mold on the centrifugal casting assembly as well as the mechanical resistance of the assembly.

To persevere in thermal control, preferably in combination with that of the forces, it is recommended that, transversely to the radial direction in which they extend (axis B of the liner considered), the liners each have at least a thickness which varies along said radial direction (length L) and which is, at least overall, weaker towards at least one of the radially inner and outer ends, 134a, 134b, than in the intermediate part, as shown figures 17,18,20 ; see also thicknesses e1,e2 and e3 Figure 20 . In other words, we can then find, along an axis B, a shape 133 first narrowing in section from the end 133a, towards an intermediate zone, then possibly ( figures 18,20 ) widening towards the opposite end 133b.

If necessary in connection with this aspect, the figures 17,18,20 show the advantage of having a mold where, individually, the open radially interior end 133a of the central conduit 133 for casting the alloy of all or part of the liners 135 would have a shape 169 therefore narrowing in section towards the center of the liner, along the radial direction, B, along which the corresponding liner extends. It should be noted that shape 169 can thus be single or double funnel (head-to-tail). A truncated cone could be suitable.

As for the radially outer extreme part of this conduit, near the end 134b ( figures 18,20 ), it can be shouldered, to present an enlarged terminal part 133b.

Figure 20 , we see that longitudinal reinforcements 171 can be provided to ensure the rigidity, centering and/or guiding of the liner 135 concerned in the peripheral structure 137. The reinforcements are radially projecting relative to the rest of the liner concerned.

Positioning the reinforcements 171 towards the radial ends 134a, 134b will make it possible to clear the intermediate zones along the length of the mold, such as at least one favorable space 155 for controlling thermal stresses and inertia, the objective always being to achieve low thermal inertia to allow homogeneous cooling of the metal casting form.

Figure 21 , the reinforcements 171 are radial to the axis of the schematic jacket and define between them several free spaces, or secondary cavities, such as 155a, 155b.

For vacuum use of the mold, this free space(s) and secondary cavities 155a, 155b established between the peripheral structure 137 and the exterior face of the jacket 135 considered, including if it these are the exterior surfaces of the machined (half-)shells, it is recommended to "vent" the exterior of space 155.

For this, it is proposed that this space 155 be in fluid communication with the external environment of the mold via at least one orifice 175. In a particular embodiment, each bar 110 may have a length or axial dimension between 10 and 50cm, an external section (such as a diameter) between 5 and 20cm, an internal section (such as a diameter) between 4 and 10cm and a radial thickness between 1 and 10cm, on average at the location of a given section.

Claims (4)

1. Unit comprising :

   - a mould rotating around an axis (A), for centrifuge casting,

   - and a TiAl metal alloy poured thereinto,

   the mould including:

   - receiving recesses (135) made of soft steel, steel, metal alloy and/or ceramic, adapted to receive the centrifuge casting of the said molten TiAl metal alloy, the said recesses (135) extending radially (B) around the axis (A), and

   - liners (135) defining all the said recesses and which are housed in at least one hollow soft steel, steel or metal alloy exoskeleton (137), and

   at the location of a plurality of section planes (P1, P2, P3, etc.) each passing through the mould and the poured metal alloy, the mould having a surface heat capacity (C1, Ci) less than that (C1 ',Ci') of the poured metal alloy,

   characterised in that:

   - the exoskeleton or exoskeletons (137) is/are open-worked, or

   - an empty space (155) exists around the periphery between each liner and the exoskeleton (137) that surrounds it, or

   - an alveolar structure (163) exists around the periphery between each liner and the exoskeleton that surrounds it.
2. A centrifuge casting mould, specific for the unit of claim 1, the said mould being rotational around an axis (A) and comprising:

   - several receiving recesses (135) to receive a molten TiAl alloy extending radially (B) around the axis (A), and

   - liners (135) defining all the said recesses and which are housed in at least one hollow exoskeleton (137),

   - the exoskeleton or exoskeletons being made of soft steel, steel or metal alloy, and the liner (135) being made of soft steel, steel or metal alloy and/or ceramic, and

   - the exoskeleton or exoskeletons (137) is/are open worked, or

   - an empty space exists around the periphery between each liner and the exoskeleton (137) that surrounds it, or

   - an alveolar structure (163) exists around the periphery between each liner and the exoskeleton that surrounds it.
3. The mould of claim 2, characterised in that it comprises a central block (131) having ducts (132) through which the alloy is poured and which are connected to the interior of the liners, and at least one fixing, preferably removable, (141a, 142a) is implemented:

   - between each liner and the exoskeleton that surrounds it, and/or

   - between each liner and/or the exoskeleton that surrounds it and the central block.
4. The mould of either claim 2 or 3, characterised in that the liners (135) being filled with molten metal alloy, the said mould has a surface heat capacity (Ci, C1) less than that (Ci', C1') of the contained metal alloy, this occurring at a plurality of section planes (P1, P2, P3, etc.) passing respectively through at least one of the said liners (135) and the exoskeleton (137) that surrounds it and the contained metal alloy.

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