FR2978075A1 - Method for assembling metal shells for high pressure compressor casing of turboshaft engine, involves heating titanium alloy shell to deform shell such that face of shell follows face of another titanium alloy shell - Google Patents

Abstract

The method involves providing a shell (10) of fire resistant titanium alloy in a shape close to its final shape, and providing another shell (20) of titanium alloy. A face (21) of the latter shell is placed on a face (12) of the former shell, and the temperature of the former shell is maintained below 200 degrees Celsius. The latter shell is heated to a temperature higher than 500 degrees Celsius. The latter shell is deformed such that the face of the latter shell follows the face of the former shell. The latter shell integral with the former shell is cooled to room temperature. The fire resistant titanium alloy is steel or an alloy of INCO 909 or INCO 783.

Description

The present invention relates to a method of assembling metal shells. Some parts are made of titanium alloy because of the particular properties of these alloys, in particular mechanical strength, temperature withstand, and corrosion resistance for a density less than that of a steel or that of a steel. other alloys such as those based on nickel or cobalt. This is particularly the case of aeronautical parts, for example parts of turbomachines such as high pressure compressor housings. In this case the titanium piece is a shell. In the description which follows, a shell is understood to mean a part of which one of the three dimensions (its thickness) in the space is small (at least five times smaller) with respect to the two other dimensions (its length and its width) perpendicular to this thickness. A shell thus includes a plate, a tube, a shell, a housing. The term titanium is used hereinafter to mean an alloy in which titanium is the major element. Such a piece of titanium must be able to withstand fire titanium, that is to say a catastrophic ignition of titanium in case of sudden rise in temperature. Various solutions are currently employed to prevent such titanium ignition from a part that is used in a high temperature environment. These solutions all consist in fixing on the titanium part a piece made of another alloy (that is to say an alloy other than a titanium alloy), this part made of another alloy being intended to be exposed to the higher temperatures. high and forming screen between these high temperatures and the titanium piece. One solution is to attach with sockets a shell of another alloy (steel, nickel-base superalloy or cobalt, or other alloy) on the surface of the titanium part that is exposed to the highest temperatures. Another solution is to carry out a hot roll of a blank of another alloy on the titanium blank. Yet another solution is to plate a shell of another alloy on the titanium shell, by hydraulic plating or by explosion plating.

All these solutions, however, have disadvantages. On the one hand it is difficult to control the exact position of the interface between the titanium piece and the piece of another alloy. In addition, depending on the tolerances of implementation, machining tolerances and machining strategies, the thickness of one or the other part is not always optimized. For example, it is often impossible for this interface to follow the definitive shape throughout the piece as close to the ribs as the geometry of the titanium piece is in three dimensions.

It is also impossible to control the thickness ratio between the two materials (within tolerances). In addition, when one of the above methods is used, the shear or peel strength between the titanium piece and the other alloy piece is quite low. This shear strength is even lower than the difference between the expansion coefficients of titanium and the other alloy is important. Each of the above methods are expensive because of the complexity and the number of steps in the process, and the need to machine the formed titanium piece assembly and the workpiece into another one. alloy after assembly so as to put this set to the final ribs. The present invention aims to remedy these disadvantages. The aim of the invention is to propose a method of assembling metal shells which makes it possible to assemble a titanium shell and a shell made of a fire-resistant titanium alloy in an efficient manner and at a lower cost. This object is achieved by virtue of the fact that this method comprises the following steps: (a) A first shell made of a titanium fire-resistant alloy is provided in a shape close to its final shape, this first shell having a first face and a second face opposite to the first face, (b) providing at least one second titanium alloy shell, this second shell having a first face and a second face opposite to the first face, (c) placing the first face of the second shell on the second face of the first shell, (d) maintaining the first shell at a temperature below a first temperature T1, (e) heating said second shell to a temperature greater than a second temperature T2 greater than the first temperature T1 (f) the second shell on the first shell is deformed superplastically at this second temperature T2, so that the first face 21 of the second shell shell hugs the second face 12 of the first hull, the second hull thus being secured to the first hull, (g) the assembly formed of the first hull and the second hull is cooled to ambient temperature. With these provisions, the assembly consisting of two metal shells assembled according to the method of the invention, while being resistant to fire titanium, has a better dimensional accuracy because the titanium fire-resistant alloy shell has been maintained at a higher temperature. T1 temperature lower than T2 superplastic deformation temperature of the titanium alloy, and has therefore little deformed. Thus, less subsequent machining and in particular less machining of the titanium alloy is necessary compared to the methods according to the prior art, and therefore the manufacturing cost is lower. In addition, this assembly has a better shear strength because due to the superplastic deformation, the titanium alloy is distributed better in all the interstices of the titanium fire-resistant alloy shell and better suits its shape. Advantageously, the first face of the first shell rests on a rigid core with which it is in contact. Thus, the deformation of the first shell during the superplastic deformation is minimized, which makes it possible to ensure that the assembly consisting of the two metal shells is even closer to the desired final shape of this set. Thus, even less subsequent machining and in particular less machining of the titanium alloy is necessary compared to the methods according to the prior art, and therefore the manufacturing cost is further minimized.

The invention will be better understood and its advantages will appear better, on reading the detailed description which follows, of an embodiment of the invention.

represented by way of non-limiting example. The description refers to the accompanying drawings in which: FIG. 1A is a perspective view of a first shell and a second shell before assembly by the method according to the invention, FIG. 1B is a perspective view of a first shell and a second shell after assembly by the method according to the invention, Figure 2 is a sectional view of the first shell and the second shell according to the plane II-II of Figure 1B, Figure 3 is a perspective view of a first shell and a second shell before assembly by a variant of the method according to the invention. In the method according to the invention, there is provided a first shell 10 of a titanium fire-resistant alloy. This alloy is for example a steel, or an alloy of INCO type 909 or INCO 783. This first shell 10 is obtained for example by forging a billet or by stamping a sheet. Optionally, the shell thus formed then undergoes machining to be transformed into this first shell 10, so that the first shell 10 is in a shape close to its final shape. A shell is understood to mean a part of which one of the three dimensions (its thickness) in space is small (at least five times smaller) than the other two dimensions (its length and its width) perpendicular to this thickness. Thus, as shown in FIG. 1A, the first shell 10 has a median surface in three-dimensional space. The first shell 10 has a first face 11 and a second face 12 opposite to this first face 11 so that these two faces are located on either side of this median surface. The length and width of the first shell 10 are measured along this medial surface. The thickness of the first shell 10 at a point M of this shell is measured in a direction perpendicular to this median surface passing through this point M, and is equal to the distance between the first face 11 and the second face 12.

A second shell 20, which is made of titanium, is also provided. The term titanium is used hereinafter to mean an alloy where titanium is the major element, so the terms "titanium" or "titanium alloy" refer to both near-pure titanium or a titanium alloy.

This titanium is, for example, TA6V or Ti 6242. The second shell 20 is defined similarly to the first shell 10, and has a first face 21 and a second face 22 opposite this first face 21, from both sides. other of the median surface of the second hull 20.

As represented in FIG. 1A, the first face 21 of the second shell 20 is placed on the second face 12 of the first shell 10 (step (c)). The first faces are the concave face of each of these hulls, the second faces are the convex face of each of these hulls.

Then the first shell 10 is maintained at a temperature below a first temperature T1 (step (d)), that is to say that the hottest point of the first shell 10 is at a temperature below the first temperature T1. This first temperature Ti is advantageously a temperature at which the fire-resistant titanium alloy does not deform superplastically and deforms little. For example, at the first temperature T1, the deformation rate of the titanium fire-resistant alloy is less than 1% when T1 is less than 50 ° C, less than 3% when T1 is less than 600 ° C or less than 10% when Ti is less than 950 ° C. Advantageously, a simulation calculation of the plastic deformations of the titanium alloy is carried out beforehand in order to help the development of the production range as a function of the final ribs desired for the second shell 20.

For example, this first temperature T1 is less than 200 ° C. For example, this first temperature T1 is equal to the ambient temperature, in this case the first shell 10 must be cooled during the superplastic deformation of the second shell 20. This cooling is for example obtained by circulating in a circuit inside. of the first shell 10 along its first face 11 a fluid at a temperature below the first temperature Ti.

The second shell 20 is then heated to a temperature greater than a second temperature T2 which is greater than the first temperature T1 (step (e)), that is to say that the coldest point of the second shell is at a temperature higher than the second temperature T2. Advantageously, the titanium fire-resistant alloy is chosen such that its flow stresses at the first temperature Ti are significantly greater than that of the titanium alloy at this second temperature T2. By "significantly higher" is meant at least more than twice. Thus, the fact that the first shell 10 deforms only very slightly during the process according to the invention contributes to ensuring that we obtain an assembly formed of the first shell 10 and the second shell 20 which is at most near its final size and shape, which minimizes or avoids subsequent machining of this assembly. Steps (c), (d), and (e) above are performed in this order. Alternatively, one can first maintain the first shell 10 at a temperature lower than the first temperature T1 (step (d)) and / or heat the second shell 20 at a temperature greater than a second temperature T2 (step (e)) , then place the second shell 20 on the first shell 10 (step (c)). This second temperature T2 is superplastically deformed the second shell 20 on the first shell 10, so that this first face 21 of the second shell 20 matches the second face 12 of the first shell 10 (step (f)), as shown in Figure 1B. The second shell (20) is thus secured to the first shell (10). This joining has the consequence that the two shells then form a monobloc assembly. The fastening between the two shells results from a mechanical connection by micro-asperities clashes present on the first face 21 of the second shell 20 and on the second face 12 of the first shell 10. Then cooled to room temperature l assembly formed of said first shell 10 and said second shell 20 (step (g)). The superplastic deformation of the second titanium shell 20 is effected for example by using one or more dies which are placed on the second face 22 of the second shell 20 and are

displaced to deform the second shell 20 and press against the first shell 10. Deforming the second shell 20 superplastically allows the second shell 20 to better match all the shapes of these matrices, and thus to obtain the distribution desired thickness across the entire surface of this second shell 20, and therefore to be even closer to the desired ribs. The field of superplasticity of titanium alloys is optimal when the microstructure is biphasic alpha and beta and when the grain size of these alloys is the lowest possible. The second temperature T2 is therefore ideally located in a temperature range where these conditions are met. Thus, the second temperature T2 is lower than the boundary temperature Tb which is the temperature above which the titanium alloy has a microstructure R. Thus, the second temperature T2 is then low enough so that it does not occur. chemical reaction at the interface between the titanium alloy and the titanium fire-resistant alloy. As a result, no embrittling phase is formed at this interface, and the connection between the first shell 10 and the second shell 20 is then better. For example, for the titanium alloy TA6V, this boundary temperature Tb is approximately equal to 1050 ° C. The second temperature T2 should not be too low, because the lower the temperature at which the titanium is deformed, the more difficult it is to deform (a more powerful press is needed). For example, the second temperature T2 is greater than 500 ° C. Advantageously, the second temperature T2 is greater than 700 ° C. Ideally, the second temperature T2 is of the order of 900 ° C.

Advantageously, the matrices which deform the second shell 20 are heated in such a way that the titanium remains at the temperature T2 during its superplastic deformation. The tests carried out by the inventors show that the rate of deformation to be used for the superplastic deformation of titanium is preferably in the range 10-1 s-1 and 10-5 s-1. Ideally, this rate of deformation is of the order of 10-3 s-1.

Thanks to the superplastic deformation of the second shell 20, the plating of this second shell 20 on the second face 12 of the first shell 10 is optimal, even in the areas of this second face 12 which have machining operations (such as holes). or whose radius of curvature is small (that is to say, the thickness of this first shell 10). Advantageously, in use, the difference Da between the coefficient of expansion of the titanium of the second shell 20 and the coefficient of expansion of the alloy of the first shell 10 is less than 3.10-6 / ° C.

Thus, for an external temperature variation of a given amplitude, the stresses generated at the interface between these two shells are lower. As a result, the shear strength of the assembly consisting of these two shells is higher. As a variant, the first shell 10 has in step (a), on its second face 12, reliefs on which the titanium alloy is able to flow superplastically, so that these reliefs act as dots. anchoring 19 of the second shell 20 with the first shell 10. By superplastically flowing, the titanium alloy is able to marry these reliefs, which can therefore act as anchoring points 19. For example, these reliefs are depressions in the second face 12 of the first shell 10. Alternatively, these reliefs are protuberances. These protuberances have for example the form of hooks. These reliefs can also be a mixture of depressions and protuberances. Figure 2 is a sectional view of the first shell 10 and the second shell 20 after assembly, which show these reliefs. These anchoring points 19 thus contribute to a better bonding between the first shell 10 and the second shell 20, and therefore contribute to increasing the shear strength of the assembly consisting of these two shells. This variant may in particular be used when the deviation σa between the titanium expansion coefficients of the second shell 20 and the alloy of the first shell 10 is greater than 3.10-6 / ° C.

After the superplastic deformation of the second shell 20 (step (f)), it is possible in some cases to perform a heat treatment with

machining to give the assembly consisting of the first shell 10 and the second shell 20 its final shape. Alternatively, as shown in Figure 3, before step (c), the first face 11 of the first shell 10 rests on a core 30 with which it is in contact. The core 30 substantially marries the first face 11 of the first shell 10, and the surface of the core 30 in contact with this first face 11 has a shape at most of the desired final ribs of the first shell 10.

Thus, during the process according to the invention, the deformation of the first shell 10 is then minimized because this first shell 10 is restricted in its deformations by the core 30. Advantageously the core 30 is maintained at a temperature below the first temperature T1 so that it is easier to maintain the first shell 10 at this temperature lower than the first temperature T1 during the superplastic forging of the second shell 20, which confers more rigidity to the core 30. Advantageously, the core 30 has a heating and / or cooling system to be maintained at this temperature below the first temperature T1. For example, the core 30 has an internal liquid circulation system. The second shell 20 is a single shell or a set of several shells. The first faces are the concave (respectively convex) face of each of these shells, the second faces are the convex (respectively concave) face of each of these shells. In a particular case, the first shell 10 and the second shell 20 are a shell, for example in the form of tube or cone. The first faces are then the concave face of each of these hulls, the second faces are the convex face of each of these hulls.

In this case the first shell 10 may undergo, before step (a), a pre-rolling or pre-forging to be formed into a shell.

Claims (8)

REVENDICATIONS1. A process for assembling metal shells, characterized in that it comprises the following steps: (a) A first shell (10) is made of a titanium fire-resistant alloy in a shape close to its final shape, this first shell ( 10) having a first face (11) and a second face (12) opposite said first face (11), (b) providing at least one second titanium alloy shell (20), said second shell (20) having a first face (21) and a second face (22) opposite to said first face (21), (c) placing said first face (21) of the second shell (20) on said second face (12) of the first hull (10), (d) maintaining said first shell (10) at a temperature below a first temperature T1, (e) heating said second shell (20) to a temperature greater than a second temperature T2 greater than said first temperature T1, (f) Superplastically deforms this second temperature T2 said second shell (20) on said first shell (10), such that said first face (21) of the second shell (20) matches said second face (12) of the first shell (10), said second shell (20) thus being secured to said first shell (10), (g) The assembly formed of said first shell (10) and said second shell (20) is cooled to room temperature. 2. Method according to claim 1 characterized in that said temperature T1 is less than 200 ° C. 3. Method according to claim 1 or 2 characterized in that said second temperature T2 is lower than the temperature Tb, said temperature Tb being the temperature above which the titanium alloy has a microstructure R. 4. Method according to claim 3 characterized in that said second temperature T2 is greater than 500 ° C. 5. Method according to any one of claims 1 to 4 characterized in that, before step (c), the first face (11) of the first shell (10) rests on a rigid core (30) with which it is in touch. 6. Method according to any one of claims 1 to 5 characterized in that said first shell (10) has in step (a), on its said second face (12), reliefs on which said titanium alloy is able to flow superplastically, so that these reliefs act as anchor points (19) of said second shell (20) with said first shell (10). 7. Method according to any one of claims 1 to 6 characterized in that the difference Aa between the titanium expansion coefficient of the second shell 20 and the expansion coefficient of the alloy of the first shell 10 is less than 3.10-6 / ° C. 8. Method according to any one of claims 1 to 7 characterized in that the superplastic deformation of said second shell (20) made of titanium using at least one matrix which is placed on said second face (22) of the second shell (20) and is moved to deform said second shell (20) and press against said first shell (10).