US 8062452 B2
The invention relates to a method of making a casing-and-diverging-portion unit, the method comprising the following steps:
1. A method of making a casing-and-diverging-portion unit, the method comprising the following steps:
forming a fiber reinforcement by making a first fiber preform with a plurality of annular fiber plies formed as rings with a first type of fiber, the plies being stacked in a direction along a longitudinal axis of the unit, said first preform corresponding to the diverging portion of the casing-and-diverging-portion unit that is to be made, and making a second fiber preform with a second type of fiber, different from the first type of fiber, said second preform corresponding to the casing portion of the casing-and-diverging-portion unit that is to be made, and being placed on the first preform;
maintaining the fiber reinforcement in a shape identical to that of the casing-and-diverging-portion unit that is to be made; and
impregnating said fiber reinforcement with a thermosetting resin and polymerizing the resin.
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This application claims priority to French application No. 06 55553 filed Dec. 15, 2006.
The invention relates to making a casing-and-diverging-portion unit for use in particular in making components for rocket exhaust nozzles or outlet cones.
That type of part comprises an internal portion referred to as the “diverging portion”, that is required essentially to present good ability at withstanding temperature and ablation since it is subjected directly to the flow of hot gas. The part also has an external portion, referred to as its “casing”, having the main function of imparting good stiffness to the part.
Particularly because of the specific properties that the diverging portion and the casing need to present, these two portions are made separately. Thereafter they are united to form a casing-and-diverging-portion unit.
A method presently in use for forming composite diverging portions or casings consists in draping and molding plies of carbon fabric preimpregnated with a resin, e.g. a phenolic resin. More precisely, the method comprises draping plies of preimpregnated fabric in a rosette pattern on a mold presenting the shape of the unit to be made, compacting the plies in a vacuum by means of a diaphragm, installing a female mold if necessary, polymerizing the unit in an autoclave or a press, and unmolding the part. This produces a rigid part with fiber reinforcement.
There also exist other known methods of making composite diverging portions or casings such as methods comprising, for example, filamentary winding, braiding, or knitting sleeves.
Nevertheless, with those methods of fabrication, the number of components needed for making the casing-and-diverging-portion unit is relatively large, as is the time required, thus leading to a manufacturing cost that is high.
Furthermore, since the diverging portion and the casing are made separately, assembling these two elements together can be complex and can present difficulties of implementation. For example, aligning the contact surfaces between the casing and the diverging portion requires additional machining, thereby increasing the time and the cost of fabrication accordingly. In addition, bonding the two elements together requires the assembly surfaces to be prepared and good control over the adhesive used in order to obtain good adhesion at all points.
In addition, the above methods are difficult to implement. In particular, draping fiber plies to form the fiber reinforcement is a manual operation that is performed directly on a shaping support and that does not enable accurate control to be maintained over the orientation and the quantity of fibers at all points in the reinforcement.
Furthermore, most existing methods use preimpregnated fibers that are less flexible than dry fibers, thereby making it even more difficult to shape the plies to match the shape of a body of revolution such as a diverging portion and the associated casing.
The present invention seeks to remedy the drawbacks of prior art methods by proposing a method of making a casing-and-diverging-portion unit that enables the number of components and operations required for making it to be reduced.
This object is achieved by a method of making a casing-and-diverging-portion unit, the method comprising the following steps:
Thus, by making fiber reinforcement that combines the fiber preform for the diverging portion and the fiber preform for the casing, the method of the invention makes it possible to make a composite casing-and-diverging-portion unit with a minimum of operations. Once made, the fiber reinforcement is impregnated on a single occasion with a resin that, once polymerized, constitutes a matrix that is common both to the diverging portion and to the casing, so as to make both of those two structures within a single part. The number of operations needed for making the casing-and-diverging-portion unit is considerably reduced. In addition, by making common fiber reinforcement using two “dry” fiber preforms corresponding respectively to the diverging portion and to the casing portion, it is relatively easy to fit those two portions to each other since fiber preforms made of dry fibers lend themselves to being shaped.
Furthermore, reducing the number of components and operations required for making the casing-and-diverging-portion unit obtained by the method of the invention serves to reduce manufacturing costs considerably, in particular because it is possible to use single tooling for making the fiber reinforcement and single tooling for performing resin impregnation and polymerization using a single injection and polymerization cycle. With the method of the invention there is no need for any intermediate machining and adhesive bonding.
The first type of preform that is to form the diverging portion is made using fibers that present low thermal conductivity, fibers presenting thermal conductivity of less than 50 watts per meter per kelvin (W·m−1·K−1), and preferably less than 20 W·m−1·K−1. Such fibers may be constituted in particular by pitch-precursor carbon fibers, by polyacrylonitrile-precursor (PAN-precursor) carbon fibers, or by rayon-precursor carbon fibers.
The second fiber preform, that is to form the casing portion, is made with fibers that present traction strength of at least 3000 megapascals (MPa) and preferably strength that is greater than or equal to 3400 MPa. Such fibers also present a Young's modulus or elasticity modulus of at least 200 gigapascals (GPa). Such fibers may be constituted in particular by PAN-precursor carbon fibers or by pitch-precursor carbon fibers.
In an aspect of the invention, the first fiber reinforcement is made from a plurality of annular fiber plies, each comprising at least a first series of fibers oriented at an angle α and a second series of fibers oriented at an angle −α. The annular fiber plies are stacked on tooling of conical shape or egg-cup shape, each ply being placed on said tooling at an angle β relative to the axis of the tooling.
The first and second series of fibers in each annular fiber ply are made by automatically placing and stitching fibers between two canvases that define an annular space, each ply being removed from the canvases by being cut out after a circular connection has been made in said ply. In this way, the fiber plies for forming the preform of the diverging portion can be made flat between two canvases, thus making it possible to control accurately the orientation of the fibers. In addition, by using an automatic embroidery machine for placing the fibers between the two canvases, it is possible to automate fiber placement and to form fiber plies that are identical, in particular in terms of the orientations and quantities of fibers used.
In an implementation, the second preform is made from at least first and second superposed fiber plies, the first ply having fibers oriented in a determined direction, the second ply being made by disposing on the first ply a ply of fibers that are oriented perpendicularly to the fibers of the first ply. The first ply is made by automatically placing and stitching fibers between two canvases defining an annular space, the ply being removed from the canvases by being cut out after a circular connection has been made in said ply, and the second ply being made by filamentary winding on the first ply while it is held in shape on tooling that is egg-cup shaped or conical.
In another implementation, the second fiber preform is made from at least one fiber ply comprising at least first and second series of superposed fibers, the first series having fibers oriented at an angle α and a second series having fibers oriented at an angle −α. The first and second series of fibers in each fiber ply are made by automatically placing and stitching fibers between two canvases defining an annular space, each ply being removed from the canvases by being cut out after a circular connection has been made in said ply.
The second fiber preform may be made either independently of the first preform, or else directly on the first fiber preform.
The fiber reinforcement made up of the superposed first and second fiber preforms is then placed in tooling, the tooling comprising a mold core and an outer mold defining an internal space in which the reinforcement is held. The thermosetting resin is then injected into the base of the reinforcement with a pressure gradient being established inside said internal space so as to cause the resin injected into the base of the reinforcement to flow up towards the top of said reinforcement. Injection and polymerization are thus performed using the same tooling and in a single cycle.
The thermosetting resin is a polyaddition resin presenting structural characteristics and having a coke fraction of at least 50%.
Other characteristics and advantages of the invention appear from the following description of particular implementations of the invention given as non-limiting examples, with reference to the accompanying drawings, in which:
The present invention proposes a method of making a casing-and-diverging-portion unit that mainly comprises forming a fiber reinforcement from two types of fiber and impregnating the reinforcement with a resin to form a matrix. The fiber reinforcement is constituted by two fiber preforms corresponding respectively to the diverging portion and to the casing of the unit that is to be made.
The first preform, which is to form the fiber reinforcement corresponding to the diverging portion, is made with a first type of refractory fibers that present in particular low thermal conductivity, i.e. fibers presenting thermal conductivity of less than about 50 W·m−1·K−1, and preferably less than 20 W·m−1·K−1. By way of example, this first type of fiber may correspond to pitch-precursor, PAN-precursor, or rayon-precursor carbon fibers.
The second preform, which is to form the fiber reinforcement corresponding to the casing, is formed with a second type of fibers that present in particular great mechanical strength, i.e. fibers presenting traction strength of at least 3000 MPa, preferably greater than or equal to 3400 MPa, and also presenting a Young's modulus or elasticity modulus of not less than 200 GPa. The fibers of this second type may be constituted in particular by PAN-precursor or pitch-precursor carbon fibers.
The first fiber preform corresponding to the diverging portion of the casing-and-diverging-portion unit that is to be made is itself formed from a stack of fiber plies.
In an implementation of the invention, each fiber ply is initially made flat on canvasses. In this implementation shown in
The fiber ply is made from two canvases 20 and 21 defining an annular space 27. The canvases 20 and 21 define respectively the inner periphery and the outer periphery of the annular space 27, which space is of a width l that is selected to be slightly greater than the dimensions required for the ply.
The series of fibers 22 and 23 are put into position by using the so-called tailored fiber placement (TFP) technique, i.e. by programming an automatic embroidery machine so that it places and stitches the fibers 22 and 23 on the canvas at respective angles α and −α. The angles α and −α may correspond for example respectively to about +45° and −45°. The TFP technique consists in placing and fixing fibers by means of stitches at precise locations on a backing (canvas) by using an automatic embroidery machine.
Nevertheless, in the present invention, the TFP technique is used in a different manner. As described for example in document US 2004/0074589, the TFP technique is used for placing and stitching fibers on a backing that forms an integral portion of the ply. In the present invention, backings (canvases) are used differently, merely to define the shape and the dimensions of the fiber ply that is to be made. The backing is not to be found in the final fiber ply. If the usual TFP technique were to be used, then the fiber ply embroidery on the backing would present excessive stiffness and would not be suitable for being deformed so as to fit closely, e.g. to a three-dimensional surface of revolution that is not capable of being developed.
Concretely, and as shown by way of example in
Because of the annular shape of the space 27 and the radial placing of the fibers therein, there remains gaps between the fibers placed by the machine, which gaps become greater on approaching the outer periphery of the annular space. In order to conserve a fiber density that is substantially identical at all points of the ply, additional fibers 22 b of various lengths are added in the gaps left between the fibers 22 a, as can be seen in
As shown in
These steps may optionally be repeated to form a plurality of series of superposed fibers that are disposed respectively at the angles −α and α.
Once all these spaces have been filled in, the annular space 27 is filled by a ply 30 containing at least two series of fibers 22 and 23 oriented in two different directions. A circle of stitching 31 is then made (
This provides a fiber ply 30 (
The fiber preform corresponding to the diverging portion of the casing-and-diverging-portion unit is made from a stack comprising a plurality of fiber preforms all made in the manner described above for the fiber ply 30. More precisely, as shown in
In a particular implementation of the invention, the second fiber preform corresponding to the “casing” portion of the casing-and-diverging-portion unit that is to be made is formed by superposing at least one fiber ply having fibers oriented in a first direction corresponding to the axis of the weave and referred to as the “ply at 0°” and another ply in a second direction perpendicular to the axis of the weave and to the fibers of the ply at 0°, and referred to as a “ply at 90°”. This superposition of these two plies can be repeated a plurality of times as a function of the stiffness required for the casing.
Each ply at 0° is made in the same manner as that described above for making the plies constituting the diverging portion of the fiber reinforcement for the casing-and-diverging-portion unit, i.e., and as shown in
As described above, in order to conserve a substantially identical fiber density at all points of the ply, the automatic embroidery machine is programmed to fill the empty gaps between the fibers 122 a by placing additional fibers 122 b in these gaps and by stitching them to the adjacent fibers (
Once all the gaps have been filled in, the annular space 127 contains a ply with fibers at a density that is substantially constant at all points. A circle of stitching is then made in the vicinity of the inner periphery of the annular space so as to hold the fibers together before removing the fiber ply from the canvases.
A ply at 90° 80 is then made directly on the ply at 0° 60 by filamentary winding (
When winding has finished, the ply at 0° 60 is completely covered by a ply at 90° 80 (
A fiber preform 100 corresponding to the casing portion of the casing-and-diverging-portion unit that is to be made is thus itself made by alternating at least one fiber ply at 0° 60 with at least one ply at 90° 80 made by winding on the tooling 70. Each ply at 0° is made using the above-described method, possibly on each occasion slightly increasing the width of the layer 60 so as to take account of the increasing volume of the stack.
In a variant implementation, the fiber preform corresponding to the casing portion may also be made from one or more plies similar to the above-described ply 30 for the diverging portion, i.e. plies each comprising two series of fibers disposed respectively at angles α and −α that may possibly correspond to +45° and −45°, for example. The method of fabricating these plies is identical to that described above for fabricating the ply 30 and is consequently not described again. The ply(ies) fabricated in this way for forming the fiber preform of the casing differ(s) from those made for the preform of the diverging portion in that they present greater width in order to enable each of them to cover the working surface of the tooling 70 corresponding to the surface of the casing that is to be made (the surface covered by the ply 60 in
As shown in
In a variant implementation, the second preform for the casing may be formed directly on the first preform for the diverging portion. With a casing preform built up from plies at 0° and at 90°, the fiber preform corresponding to the diverging portion is made on a tool similar to the tool 70 that also includes a rim at its base for enabling the plies to be stacked at an angle of β of about 10°, as described above. The preform corresponding to the casing is then formed by placing on the preform for the diverging portion at least one ply at 0° and one ply at 90° made by winding on the ply at 0°, as explained above. With a casing preform made of plies each comprising two series of fibers at angles α and −α, these plies are placed directly on the preform of the diverging portion.
The following step of the method of making the casing-and-diverging-portion unit in accordance with the present invention consists in densifying the fiber reinforcement 150 with a common matrix in a single operation. For this purpose, the reinforcement 150 is impregnated with a thermosetting resin that is polymerized by heat treatment.
For this purpose, the well-known method of resin transfer molding (RTM) is used that consists in impregnating the fiber reinforcement with a resin and in causing it to polymerize in the same mold. To implement the RTM method in the context of the present invention, special tooling 200 as shown in
More precisely, as shown in
Once assembled together as shown in
In order to initiate polymerization of the resin injected into the fiber reinforcement, the tooling needs to be raised to and maintained at a temperature corresponding to the polymerization temperature of the resin used. For this purpose, the tooling 200 includes heater means that may be of various types (resistive, inductive, etc.). In the embodiment of the tooling shown herein, the core of the tooling is heated by a flow circuit 210 formed inside the mold core 201 and in which hot oil 211 is caused to circulate. The resin is injected into the tooling at an appropriate injection temperature by heating the injection head (not shown in the figures) that is connected to the feed orifice 205. An additional advantage of using circulating oil as heater means is that the oil also serves to regulate the temperature in the reinforcement, since once polymerization has been triggered it can lead to an exothermal reaction.
The resin used in the present invention needs to ensure that the final part, corresponding to the casing-and-diverging-portion unit, has formed therein a matrix that presents the ability to withstand both mechanical forces and ablation. For this purpose, the resin used is a polyaddition resin that presents, once polymerized, structural characteristics that are useful in particular for the casing portion of the final part. This resin also presents a coke fraction of at least 50%, which makes it possible to ensure that the matrix formed after polymerization withstands ablation well, as is necessary in particular for the diverging portion whose inside wall is subjected to the flow of combustion gases.
In addition, the resin must be suitable for RTM methods, i.e. it should preferably present low viscosity so as to make it easier to inject into the fibers of the reinforcement. By way of example, the resin used may be a cyanate ester resin of the phenolic triazine type.
After injection and polymerization, the part is unmolded. It may optionally be subjected to a post-baking cycle to improve its thermomechanical characteristics (increasing its glass transition temperature), such as, for example, a cycle of 2 hours at 180° C. In the end, the part is trimmed to remove excess resin and its chamfers are machined. No other machining is needed since, because the part is molded, it complies with the required dimensions.
As shown in