Abstract:
A method for assembling metal shells, including: (a) providing a first shell made of a titanium fire-resistant alloy with a shape similar to a final shape thereof and having first and second opposite surfaces, the second surface including raised portions acting as anchoring points between shells; (b) providing at least one second shell made of a titanium alloy and having first and second opposite surfaces; (c) placing the first surface of the second shell on the second surface of the first shell; (d) heating the second shell to a temperature higher than a second temperature; (e) deforming the second shell at the second temperature on the first shell, such that the first surface of the second shell matches the second surface of the first shell, the second shell thus being secured to the first shell; and (f) cooling an assembly of the first and second shells to ambient temperature.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method of assembling metal shells. 
       BACKGROUND OF THE INVENTION 
       [0002]    Certain parts are made of titanium alloy because of the special properties of such alloys, in particular mechanical strength, ability to withstand high temperatures, and corrosion resistance for density that is less than of steel or that of other alloys such as alloys based on nickel or based on cobalt. 
         [0003]    This applies in particular to aviation parts, e.g. parts of turbine engines such as high pressure compressor casings. Under such circumstances, the titanium part is in the form of a shell. 
         [0004]    In the description below, the term “shell” is used to designate a part for which one of its three dimensions (its thickness) in three-dimensional space is small (at least five times smaller) than its other two dimensions (its length and its width) perpendicular to its thickness. The term “shell” thus covers a plate, a tube, a collar, or a casing. 
         [0005]    The term “titanium” is used below to mean an alloy in which titanium is the majority element. 
         [0006]    Such a titanium part must be capable of withstanding titanium fire, i.e. a catastrophic fire to which titanium can be subjected in the event of a sudden rise in temperature. 
         [0007]    Various solutions are presently in use for preventing the titanium of a part that is used in a high temperature environment from catching fire. All of such solutions consist in fastening a part made of some other alloy (i.e. an alloy other than a titanium alloy) on the part made of titanium, the part made of some other alloy being positioned so as to be exposed to the highest temperatures and serving to form a shield between those high temperatures and the titanium part. 
         [0008]    One solution consists in using bushings to fasten a shell made of some other alloy (steel, a superalloy based on nickel or cobalt, or some other alloy) on the surface of the titanium part that is exposed to the highest temperatures. 
         [0009]    Another solution consists in hot-rolling a blank made of another alloy together with a blank made of titanium. 
         [0010]    Yet another solution consists in pressing a shell made of some other alloy against a titanium shell, by hydraulic pressing or by explosive pressing. 
         [0011]    Nevertheless, all of those solutions present drawbacks. 
         [0012]    Firstly, it is difficult to control the exact position of the interface between the part made of titanium and the part made of some other alloy. 
         [0013]    Furthermore, depending on implementation tolerances, on machining tolerances, and on machining strategies, the thickness of one or the other of the parts is not always optimized. For example, it is often impossible for the interface to follow the final shape all over the part as closely as possible to its design dimensions when the shape of the titanium part is three-dimensional. 
         [0014]    It can also be impossible to ensure that the thickness ratio between the two materials lies within tolerances. 
         [0015]    Furthermore, when one of the above-mentioned methods is used, the shear or separation strength between the part made of titanium and the part made of another alloy is quite low. Such shear strength is particularly low when there is a large difference between the coefficients of thermal expansion of titanium and of the other alloy. 
         [0016]    Each of the above methods is expensive because of the complexity and the number of steps in the method, and because of the need to machine an assembly made up of the titanium part and the part of another alloy after they have been assembled together in order to ensure that the assembly has the required final dimensions. 
       Object and Summary of the Invention 
       [0017]    The present invention seeks to remedy those drawbacks. 
         [0018]    The invention seeks to provide a method of assembling metal shells together that enables a titanium shell to be assembled with a titanium fire resistant alloy shell in a manner that is effective, with excellent bonding between them, and at reduced cost. 
         [0019]    This object is achieved by the fact that the method comprises the following steps: 
         [0020]    a) providing a first shell made of titanium fire resistant alloy in a shape close to its final shape, the first shell presenting a first face and a second face opposite from the first face, the second face presenting portions in relief that act as anchor points for anchoring the second shell on the first shell; 
         [0021]    b) providing at least one second shell of titanium alloy, the second shell presenting a first face and a second face opposite from the first face; 
         [0022]    c) placing the first face of the second shell on the second face of the first shell; 
         [0023]    d) heating said second shell to a temperature higher than a second temperature T 2 ; 
         [0024]    e) deforming the second shell on the first shell at the second temperature T 2 , such that the first face of the second shell fits closely to the second face of the first shell, the second shell thus being secured to the first shell; and 
         [0025]    f) cooling the assembly formed by the first shell and the second shell to ambient temperature. 
         [0026]    By means of these provisions, the assembly constituted by the two metal shells assembled together by the method of the invention is not only capable of withstanding titanium fire, but also presents better dimensional accuracy, since the titanium alloy fits closely to the shell of titanium fire resistant alloy. Thus, less subsequent machining and in particular less machining of the titanium alloy is needed compared with prior art methods, and consequently the fabrication cost is smaller. 
         [0027]    In addition, the assembly presents better shear strength since, because of the deformation, the titanium alloy is better distributed in and around the portions in relief of the titanium fire resistant alloy shell, and fits more closely to its shape. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The invention can be well understood and its advantages appear better on reading the following detailed description of an implementation given by way of non-limiting example. The description refers to the accompanying drawings, in which: 
           [0029]      FIG. 1A  is a perspective view of a first shell and a second shell prior to being assembled together by a first implementation of the method of the invention; 
           [0030]      FIG. 1B  is a perspective view of a first shell and of a second shell after they have been assembled together by a first implementation of the method of the invention; 
           [0031]      FIG. 2  is a section view of the first shell and of the second shell on plane II-II of  FIG. 1B ; 
           [0032]      FIG. 3  is a perspective view of a first shell and of a second shell before being assembled together by a variant of the method of the invention; 
           [0033]      FIG. 4A  is a perspective view of a first shell and of a second shell prior to being assembled together by a second implementation of the method of the invention; and 
           [0034]      FIG. 4B  is a perspective view of a first shell and of a second shell after they have been assembled together by a second implementation of the method of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    In the method of the invention, a first shell  10  is provided that is made of a titanium fire resistant alloy. 
         [0036]    By way of example, the alloy may be a steel, or an alloy of the INCO 909 or the INCO 783 type. 
         [0037]    By way of example, this first shell  10  is obtained by forging a slug or by stamping a sheet. The shell as formed in this way may optionally be subjected subsequently to machining in order to be transformed into this first shell  10 , so that the first shell  10  has a shape close to its final shape. 
         [0038]    The term “shell” is used to mean a part in which one of its three dimensions (its thickness) in three-dimensional space is smaller (at least five times smaller) than its other two dimensions (its length and its width) perpendicular to said thickness. 
         [0039]    As shown in  FIG. 1A , the first shell  10  presents a median plane in three-dimensional space. The first shell  10  presents a first face  11  and a second face  12  opposite from the first face  11  such that these two faces are situated on either side of the median plane. 
         [0040]    The length and the width of the first shell  10  are measured along the median plane. 
         [0041]    The thickness of the first shell  10  at a point M of the shell is measured in a direction perpendicular to the median plane passing through the point M, and it is equal to the distance between the first face  11  and the second face  12 . 
         [0042]    A second shell  20  is also provided, which second shell is made of titanium. The term “titanium” is used herein to mean an alloy in which titanium is the majority element, such that the terms “titanium” or “titanium alloy” cover both titanium that is almost pure and also an alloy of titanium. 
         [0043]    By way of example, the titanium is TA6V or Ti 6242. 
         [0044]    The second shell  20  is defined in similar manner to the first shell  10 , and it presents a first face  21  and a second face  22  opposite from the first face  21 , these faces being on either side of a median plane of the second shell  20 . 
         [0045]    As shown in  FIG. 1A , the first face  21  of the second shell  20  is placed against the second face  12  of the first shell  10  (step c)). The first faces are the concave faces of each of the shells, while the second faces are the convex faces of each of the shells. 
         [0046]    In step a), the first shell  10  presents on its second face  12  portions in relief over which the titanium alloy is suitable for flowing so that the portions in relief act as anchor points  19  for anchoring the second shell  20  on the first shell  10 . 
         [0047]    By flowing, the titanium alloy is suitable for fitting closely over these portions in relief, which can thus act as anchor points  19 . 
         [0048]    By way of example, these portions in relief are indentations in the second face  12  of the first shell  10 . Alternatively, these portions in relief are protuberances. By way of example, these protuberances may be hook-shaped. These portions in relief may also be a mixture of indentations and of protuberances. 
         [0049]      FIG. 2  is a section view of the first shell  10  and of the second shell  20  after they have been assembled together, this figure showing these portions in relief. 
         [0050]    These anchor points  19  thus contribute to securing the first shell  10  and the second shell  20  together more firmly, and consequently they contribute to increasing the shear strength of the assembly constituted by these two shells. 
         [0051]    A first implementation of the invention is described below, in which: 
         [0052]    in step d), said first shell  10  is maintained at a temperature lower than the first temperature T 1 , the second temperature T 2  being greater than the first temperature T 1 ; and 
         [0053]    in step e), the second shell  20  is deformed superplastically at the temperature T 2  on said first shell  10 . 
         [0054]    Thus, in step d), the hottest point of the first shell  10  is at a temperature lower than the first temperature T 1 . 
         [0055]    This first temperature T 1  is advantageously a temperature at which the titanium fire resistant alloy deforms little and does not deform superplastically. 
         [0056]    For example, at the first temperature T 1 , the deformation ratio of the titanium fire resistant alloy is less than 1% when T 1  is less than 50° C., less than 3% when T 1  is less than 600° C., or less than 10% when T 1  is less than 950° C. 
         [0057]    Advantageously, a calculation is performed beforehand to simulate the plastic deformation of the titanium alloy in order to assist in developing the fabrication range as a function of the final dimensions desired for the second shell  20 . 
         [0058]    For example, the first temperature T 1  is less than 200° C. 
         [0059]    For example, this first temperature T 1  is equal to ambient temperature, so the first shell  10  then needs to be cooled during the superplastic deformation of the second shell  20 . 
         [0060]    By way of example, this cooling is obtained by causing a fluid at a temperature lower than the first temperature T 1  to flow in a circuit inside the first shell  10  along its first face  11 . 
         [0061]    Thereafter, the second shell  20  is heated to a temperature higher than a second temperature T 2 , which is higher than the first temperature T 1  (step d)), i.e. the coldest point of the second shell is at a temperature higher than the second temperature T 2 . 
         [0062]    Advantageously, the titanium fire resistant alloy is selected so that its flow stresses at the first temperature T 1  are clearly greater than those of the titanium alloy at the second temperature T 2 . The term “clearly greater” is used to mean at least twice. 
         [0063]    Thus, the fact that the first shell  10  deforms only very little while performing the method of the invention contributes to ensuring that an assembly made up of the first shell  10  and of the second shell  20  is obtained that is very close to its final dimensions and shape, thereby minimizing or avoiding subsequent machining of the assembly. Above-described steps c) and d) are performed in that order. Alternatively, it is possible initially to maintain the first shell  10  at a temperature lower than the first temperature T 1  and/or to heat the second shell  20  to a temperature higher than a second temperature T 2 , and then place the second shell  20  against the first shell  10 . 
         [0064]    The second shell  20  is deformed superplastically at this second temperature T 2  against the first shell  10  so that the first face  21  of the second shell  20  fits closely to the second face  12  of the first shell  10  (step e)), as shown in  FIG. 1B . 
         [0065]    The second shell  20  is thus secured to the first shell  10 , in particular because of the portions in relief  19 . The consequence of being secured in this way is that the two shells then form a single-piece unit. 
         [0066]    Thereafter the assembly made up of said first shell  10  and said second shell  20  is cooled to ambient temperature (step f)). 
         [0067]    By way of example, the titanium second shell  20  is deformed superplastically by using one or more dies placed against the second face  22  of the second shell  20  and moved to deform the second shell  20  and press it against the first shell  10 . 
         [0068]    The fact of deforming the second shell  20  in superplastic manner enables the second shell  20  to fit more closely to dies of any shape, thereby obtaining the desired distribution of thickness over the entire surface area of the second shell  20 , and thus coming even closer to the desired dimensions. 
         [0069]    The superplastic range of titanium alloys is optimized when the microstructure is biphasic alpha and beta and when the grain size of the alloys is as small as possible. The second temperature T 2  is thus ideally situated in a temperature range where both of those conditions apply. 
         [0070]    Thus, the second temperature T 2  is lower than the boundary temperature Tb, which is the temperature above which the titanium alloy has a β microstructure. 
         [0071]    Thus, the second temperature T 2  is then low enough to avoid any chemical reaction taking place at the interface between the titanium alloy and the titanium fire resistant alloy. Consequently, no weakening phase is formed at this interface, and the bonding between the first shell  10  and the second shell  20  is thus better. 
         [0072]    For example, for the TA6V titanium alloy, this boundary temperature Tb is equal to about 1050° C. 
         [0073]    The second temperature T 2  must not be too low, since the lower the temperature at which the titanium is deformed, the more difficult it is to deform it (a more powerful press is needed). 
         [0074]    For example, the second temperature T 2  is higher than 500° C. 
         [0075]    Advantageously, the second temperature T 2  is higher than 700° C. 
         [0076]    Ideally, the second temperature T 2  is about 900° C. 
         [0077]    Advantageously, the dies for deforming the second shell  20  are heated so that the titanium remains at the temperature T 2  throughout its superplastic deformation. 
         [0078]    Tests performed by the inventors show that the deformation rate to be used for superplastically deforming the titanium preferably lies in the range 10 −1  per second (s −1 ) to  10   −5  s −1 . Ideally, this deformation rate is about  10   −3  s −1 . 
         [0079]    This superplastic deformation of the second shell  20  serves to optimize the pressing of the second shell  20  against the first face  12  of the first shell  10 , even in the zones of the second face  12  that do not present any machining (such as holes), or zones in which the radius of curvature is small (i.e. relative to the thickness of the first shell  10 ). 
         [0080]    Advantageously, in use, the difference Δα 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. 
         [0081]    Thus, for a variation in external temperature of a given amplitude, the stresses generated at the interface between the two shells are smaller. Consequently, the shear strength of the assembly constituted by these two shells is greater. 
         [0082]    After superplastically deforming the second shell  20  (step e)), it is possible under certain circumstances to perform heat treatment with machining in order to impart its final shape to the assembly constituted by the first shell  10  and the second shell  20 . 
         [0083]    In a variant, and as shown in  FIG. 3 , prior to step c), the first face  11  of the first shell  10  rests against a core  30  with which it is in contact. 
         [0084]    The core  30  is substantially a close fit against the first face  11  of the first shell  10 , and the surface of the core  30  in contact with the first face  11  presents a shape as close as possible to the final dimensions desired for the first shell  10 . 
         [0085]    Thus, during the method of the invention, the deformation of the first shell  10  is minimized, since the first shell  10  is restricted in its deformation by the core  30 . Consequently, the assembly constituted by the metal shells is even closer to the final shape desired for this assembly, and even less subsequent machining and in particular less machining of the titanium alloy is needed in comparison with prior art methods. This serves to further reduce fabrication costs. 
         [0086]    Advantageously, the core  30  is maintained at a temperature lower than the first temperature T 1  so that it is easier to maintain the first shell  10  at this temperature lower than the first temperature T 1  during the superplastic forging of the second shell  20 , thereby imparting greater rigidity to the core  30 . 
         [0087]    Advantageously, the core  30  possesses a heating and/or cooling system in order to be maintained at said temperature lower than the first temperature T 1 . For example, the core  30  possesses an internal system for circulating liquid. 
         [0088]    The second shell  20  is a single shell or an assembly of a plurality of shells. The first faces are the concave (respectively convex) faces of each of the shells, and the second faces are the convex (respectively concave) faces of each of the shells. 
         [0089]    In a particular configuration, the first shell  10  and the second shell  20  form a collar, e.g. of tubular or conical shape. The first faces are then the concave faces of each of the shells, and the second faces are the convex faces of each of the shells. 
         [0090]    Under such circumstances, prior to step a), the first shell  10  may be subjected to pre-rolling or pre-forging in order to be given the shape of a collar. 
         [0091]    There follows a description of a second implementation of the invention, in which: 
         [0092]    in step c), the shells  10  and  20  are placed in a hermetically sealed enclosure  80 ; 
         [0093]    in step d), the first shell  10  is heated to a temperature higher than the second temperature T 2 ; and 
         [0094]    in step e), pressure is established inside the enclosure  80  that is high enough to deform the second shell  20  at the second temperature T 2 . 
         [0095]    As shown in  FIG. 4A , the first face  21  of the second shell  20  is placed against the second face  12  of the first shell  10  (step c)), and these two shells are placed in a hermetically closed enclosure  80  that is filled with a fluid (a preferably inert gas, or a liquid). 
         [0096]    Thereafter, the first shell  10  and the second shell  20  are heated to a temperature higher than the second temperature T 2  and pressure is established inside the enclosure  80  that is high enough for the second shell  20  (and possibly also to a smaller extent the first shell  10 ) to deform at this second temperature T 2  (hot isostatic compression). 
         [0097]    Thus, the second shell  20  is deformed on the first shell  10  in such a manner that the first face  21  of the second shell  20  fits closely to the second face  12  of the first shell  10  (step e)), as shown in  FIG. 4B . 
         [0098]    Advantageously, the titanium fire resistant alloy is selected so that the first shell  10  deforms only very little during the method of the invention, thereby contributing to obtaining an assembly formed by the first shell  10  and the second shell  20  that is as close as possible to its final dimensions and shape, and the distribution of thickness in the second shell  20  over its entire surface area is as desired. This minimizes or avoids any need for subsequent machining of the assembly. 
         [0099]    The second temperature T 2  is lower than the overheating or burning temperature of each of the materials constituting the two shells so as to avoid damaging the shells. 
         [0100]    By way of example, the second temperature T 2  is less than 1200° C. 
         [0101]    The second shell  20  is thus secured to the first shell  10 , in particular because of the portions in relief  19 . As a consequence of being secured in this way, the two shells then form a single-piece unit. 
         [0102]    The assembly constituted by the first shell  10  and the second shell  20  is then cooled to ambient temperature (step f)). 
         [0103]    Because of the deformation of the second shell  20 , the second shell  20  is pressed against the second face  12  of the first shell  10  in optimum manner, even in zones of the second face  12  that present machining (such as holes) or that present a radius of curvature that it small (i.e. relative to the thickness of the first shell  10 ). 
         [0104]    Advantageously, the assembly comprising the first shell  10  and the second shell  20  is covered in a metal sheath (e.g. made up of welded-together metal sheaths) prior to raising the pressure and the temperature inside the enclosure  80 . The inside of the sheath is evacuated (to a pressure lower than atmospheric pressure). 
         [0105]    Thus, the distribution of pressure on the two shells is made more uniform. 
         [0106]    Advantageously, a layer of a material such as a thermal barrier (alloy based on Ni, Co, or Mo) or a mineral is placed at the interface between the first shell  10  and the second shell  20 . By way of example, this layer is placed (e.g. deposited) on the first shell  10 . This measure is particularly useful when the temperature T 2  is higher than about 1000° C. in order to avoid forming weakening phases at the interface between the two shells. 
         [0107]    This ensures that greater shear strength is obtained at the interface. 
         [0108]    Tests performed by the inventors show that the deformation rate to be used for deforming the titanium preferably lies in the range  10   −1  s −1  to  10   −5  s −1 . Ideally, this deformation rate is about  10   −3  s −1 . 
         [0109]    Advantageously, in use, the difference Δα 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. 
         [0110]    Thus, for a variation in external temperature of a given amplitude, the stresses generated at the interface between the two shells are smaller. Consequently, the shear strength of the assembly constituted by these two shells is greater. 
         [0111]    Advantageously, the second temperature T 2  is less than the boundary temperature Tb, which is the temperature above which the titanium alloy has a β microstructure. 
         [0112]    Below the boundary temperature Tb, there is the optimum range of superplasticity in titanium alloys where the microstructure is biphasic alpha and beta and the grain size of these alloys is as small as possible. 
         [0113]    Thus, the second shell  20  is deformed in the superplastic deformation range of titanium and at a temperature that is low enough to avoid a chemical reaction taking place at the interface between the titanium alloy and the titanium fire resistant alloy. Consequently, no weakening phase is formed at the interface, and the bonding between the first shell  10  and the second shell  20  is therefore better. 
         [0114]    By way of example, for the TA6V titanium alloy, this boundary temperature Tb is equal to about 1050° C. 
         [0115]    The second temperature T 2  must not be too low, since the lower the titanium at which the titanium is deformed, the more difficult it is to deform (greater pressure is required). 
         [0116]    For example, the second temperature T 2  is higher than 500° C. 
         [0117]    Advantageously, the second temperature T 2  is higher than 700° C. 
         [0118]    Ideally, the second temperature T 2  is about 900° C. 
         [0119]    After deforming the second shell  20  (step e)), it is possible in certain circumstances to perform heat treatment with machining in order to impart its final shape to the assembly constituted by the first shell  10  and the second shell  20 . 
         [0120]    The second shell  20  is a single shell or an assembly of a plurality of shells. The first faces are the concave (respectively convex) faces of each of the shells, and the second faces are the convex (respectively concave) faces of each of the shells. 
         [0121]    In a particular configuration, the first shell  10  and the second shell  20  form a collar, e.g. of tubular or conical shape. The first faces are then the concave faces of each of the shells, and the second faces are the convex faces of each of the shells. 
         [0122]    Under such circumstances, prior to step a), the first shell  10  may be subjected to pre-rolling or pre-forging in order to be given the shape of a collar.