Method for manufacturing a large-sized seamless dome, and corresponding dome

Abstract

The method comprises the following steps: • providing a semi-finished product (11); • upset hot forging of the semi-finished product (11) to form a slug (13) with a developed surface larger than that of the semi-finished product (11) and smaller than the developed surface of the wall (3) of the dome; • hot forming of the slug (13) to create a preform (15) that has a developed surface smaller than or equal to the developed surface of the wall (3), the preform (15) comprising at least two consecutive portions (19, 21) extending radially in the extension of one another away from the central axis (A-A) such that, for all of the portions (19, 21), the average thicknesses and / or the convexities of two consecutive portions (19, 21) are distinct; • hot working of the preform (15) in a press between a die and a punch to form the wall (3).

Description

[0001] DESCRIPTION

[0002] TITLE: Method for manufacturing a large seamless dome, and corresponding dome

[0003] The present invention relates to a method of manufacturing a large dome-shaped hollow part.

[0004] Some large pressure vessels consist of two domes joined by welding or with the addition of a cylindrical crown between the two. The ends consist of domed bases or covers, fixed by bolting or welding to a cylindrical body.

[0005] These pressure tanks are, for example, high-pressure cryogenic tanks for liquefied gases such as liquid hydrogen, liquid oxygen or liquid nitrogen, for applications in the field of space launchers, terrestrial or mobile storage or in the aeronautical field.

[0006] To manufacture such parts, it is known to shape several machined sheets by stamping to form petals, which are then assembled to form a corolla, then welded to each other to form the part.

[0007] This technique has the disadvantage of requiring a series of costly operations. In particular, welding assembly requires special, large equipment. In addition, rigorous ultrasonic testing of the welds must be carried out to ensure good strength and the absence of defects in the weld areas.

[0008] Single-piece dome-shaped parts can also be manufactured by die-forging. This technique has the advantage of requiring only one forming operation, but has the disadvantage of being limited to parts of small dimensions, due to the limited dimensions of existing forging presses and the very high forces required to deform the material and fill the final part.

[0009] The aim of the present invention is to overcome these drawbacks by proposing a method for manufacturing a dome-shaped part such as a cryogenic tank dome, which does not require welding, and which can be produced on available forging presses whose dimensions and / or force capacities are limited.

[0010] To this end, the invention relates to a method for manufacturing a hollow part comprising a seamless, dome-shaped wall extending around a central axis, the wall being made of a material consisting of a metal alloy, the wall having an average thickness of less than 50 mm and a largest dimension, orthogonal to the central axis, of between 1500 mm and 3700 mm, the method comprising the following consecutive steps:

[0011] • supply of a semi-finished product of said material;

[0012] • hot forging by upsetting the semi-finished product to form a wafer with a developed surface greater than that of the semi-finished product and less than the developed surface of the wall;

[0013] • hot shaping of the wafer to create a preform having a developed surface area less than or equal to the developed surface area of ​​the wall, the preform comprising at least two successive portions extending radially in the extension of each other away from the central axis, so that for all the portions, the average thicknesses and / or the convexities of two successive portions are distinct;

[0014] • hot deformation of the preform under a press between a die and a punch to form the wall.

[0015] Preferably, the method according to the invention may comprise one or more of the following characteristics, taken in isolation or in any technically possible combination:

[0016] - the portions comprise a central portion and at least one intermediate annular portion, the central portion and the intermediate annular portion being of distinct convexities.

[0017] - the portions further comprise a peripheral annular portion, the peripheral annular portion having a convexity distinct from the convexity of the adjacent intermediate annular portion;

[0018] - the developed surface area of ​​the preform is substantially equal to the developed surface area of ​​the wall; a ratio between the projected surface area of ​​the preform on a transverse plane and the developed surface area of ​​the preform is less than 90%, and the largest dimension of the projected surface area of ​​the preform on a transverse plane is less than or equal to 150% of the largest dimension of the punch orthogonal to the central axis;

[0019] - the step of deforming the preform under the press comprises unfolding the preform between the die and the punch such that at the end of the unfolding, the surfaces of the portions facing the punch are concave;

[0020] - the surface of the central portion facing the punch has a concave shape;

[0021] - the hot forming stage of the wafer includes: • A first phase of forming by hot stamping of the wafer to produce a first concave rough shape;

[0022] • A second phase of shaping by hot stamping of the first concave blank to produce the preform;

[0023] - the second shaping phase comprises a first intermediate shaping phase by hot stamping of the first concave blank to produce a second blank comprising a concave central part and a convex peripheral part extending radially in the extension of the central part, and a second intermediate shaping phase by hot stamping of the second blank to produce the preform;

[0024] - the portions comprise a central portion and at least one intermediate annular portion, the average thicknesses of the central portion and the intermediate annular portion being distinct, the developed surface of the preform being less than the developed surface of the wall;

[0025] - the portions further comprise a peripheral annular portion, and the intermediate annular portion is a thickened portion, the average thickness of the intermediate annular portion being greater than the average thickness of the central portion and the average thickness of the peripheral annular portion;

[0026] - the step of deforming the preform comprises a shaping phase during which at least a part of the intermediate annular portion is brought into contact with the die by the punch, and a stamping phase corresponding to an expansion during which at least a part of the preform is not in contact with the punch;

[0027] - during the expansion phase, the thickened intermediate annular portion of the preform is compressed between the die and the punch, at least part of the central portion and / or the peripheral annular portion not being in contact with the punch, such that the material flows plastically from the intermediate annular portion towards the central portion and the peripheral annular portion then beyond the peripheral annular portion to increase the developed surface area of ​​the preform and form the wall;

[0028] - the material is an aluminum alloy, in particular a 2XXX or 7XXX alloy.

[0029] The invention also relates to a hollow part comprising a wall in a single piece without welding in the shape of a dome, the wall extending around a central axis, the wall being made of a material consisting of a metal alloy, the wall having an average thickness of less than 50 mm and a largest dimension, orthogonal to the central axis, of between 1500 mm and 3700 mm. Preferably, the wall has a homogeneous fibering around the central axis, the orientation of the grains, also called fibers, of said metal alloy being such that in any transverse plane, the angle formed between the direction of elongation of a grain seen in section in this transverse plane and the tangent to the wall at this grain is substantially constant.

[0030] The invention also relates to a cryogenic tank comprising a part as defined above or manufactured by a method as defined above.

[0031] [Fig 1] Figure 1 is a schematic perspective view of a hollow dome-shaped part according to one embodiment;

[0032] [Fig 2] Figure 2 schematically illustrates a method according to one embodiment;

[0033] [Fig 3] Figure 3 is a schematic view of a section along an axial plane of a preform according to a first embodiment;

[0034] [Fig 4] Figure 4 schematically illustrates a step of shaping a wafer to form the preform of Figure 3;

[0035] [Fig 5] Figure 5 is a schematic perspective view of a preform according to a second embodiment, before its hot deformation under a press;

[0036] [Fig 6] Figure 6 is a schematic view of a section along an axial plane of an initial phase of a hot deformation step of the preform of Figure 3;

[0037] [Fig 7] Figure 7 illustrates the grain orientation in the hollow part of Figure 1.

[0038] Figure 1 shows a hollow part 1 in the shape of a dome, according to one embodiment, in an axial section.

[0039] The hollow part 1 comprises a wall 3 in one piece, without welding, in the shape of a dome. By wall, we mean a part in its entire thickness, as opposed to the term “surface” or “face”.

[0040] Wall 3 comprises a convex outer surface, facing outward from the dome, and a concave inner surface.

[0041] Wall 3 is without welding, and is thus made from a single piece, having come from the same material.

[0042] The hollow part 1 is for example intended to form an end dome of a pressure tank of generally cylindrical shape.

[0043] Wall 3 extends around a central axis AA, between two planes orthogonal to the central axis AA, these two planes axially delimiting wall 3.

[0044] In the following, we will consider, as is the case in the Figures, an axial direction oriented such that, seen from top to bottom, the wall is concave in shape. Furthermore, we will call "transverse" a plane orthogonal to the central axis AA and "axial" a plane including this central axis AA.

[0045] Thus, with reference to Figure 1, the wall 3 extends between a lower transverse plane 5 and an upper transverse plane 7.

[0046] For example, wall 3 has a shape of revolution, that is to say such that the section of wall 3 through any axial plane A is of constant shape.

[0047] The wall 3 is, for example, of a substantially hemispherical shape or in the form of a spherical cap.

[0048] Alternatively, the wall 3 is in the form of an ellipsoidal cap, or in the form of a Cassini dome, a Cassini cap, or more generally in the form of an ovoidal cap.

[0049] Wall 3 has a cross-section (i.e. in a plane orthogonal to the central axis AA), of curved shape, for example circular, oval, in particular Cassini oval, or elliptical shape.

[0050] According to one embodiment, the wall 3 comprises a curved portion such as an ellipsoidal cap, a Cassini dome, a Cassini cap, or an ovoidal cap, and a cylindrical or truncated cone-shaped portion in the extension of the curved portion.

[0051] Wall 3 is made of a material consisting of a metal alloy.

[0052] The metal alloy is for example an aluminum alloy, in particular a 2XXX alloy such as alloy 2050 or alloy 2219, or a 7XXX alloy such as alloy 7020.

[0053] Alternatively, the metal alloy is a titanium alloy or steel.

[0054] Wall 3 has an average thickness of less than 50 mm, and generally greater than 25 mm, in particular greater than 30 mm.

[0055] The wall 3 has a largest dimension, orthogonal to the central axis AA, between 1500 mm and 3700 mm. By largest dimension is meant the largest dimension in any transverse plane, that is to say in the example shown in Figure 1 the largest dimension in the upper transverse plane 7.

[0056] For example, if wall 3 has a circular cross-section, the largest dimension of wall 3 is the diameter of the outer circle in the upper transverse plane 7. If wall 3 has an elliptical cross-section, the largest dimension of wall 3 is the major diameter of the outer ellipse in the upper transverse plane 7.

[0057] The wall 3 optionally comprises features arranged on the wall 3. These features are for example intended for the connection of equipment such as pipes. These features include for example openings, reliefs on the outer surface of the wall 3, appendages, bulges, bosses, tappings and / or ribs. These features are preferably integrated into the wall 3, having come in one piece with the wall, therefore produced during the manufacture of the wall 3.

[0058] In particular, the wall 3 preferably comprises, in its lower part, a feature such as a tubular opening 9 centered on the central axis AA. The opening 9 is for example of circular, elliptical or ovoidal cross-section.

[0059] The opening 9 is for example delimited by edges of thickness greater than the average thickness of the wall 3.

[0060] We will now describe, with reference to Figures 2 to 6, a method of manufacturing such a hollow part 1 according to an embodiment of the invention.

[0061] The method firstly comprises a step of providing a semi-finished product 11 of the material from which the hollow part 1 is made.

[0062] The semi-finished product 11 (is for example a billet or a plate of said material (as seen from above in Figure 2). The semi-finished product 11 is for example obtained by casting said material.

[0063] The method then comprises a hot forging step by upsetting the semi-finished product 11 to form a wafer 13. For example, in the example of Figure 2, the substantially circular wafer 13 is formed from the semi-finished product 11 by exerting pressure between the lower and upper faces of the semi-finished product 11.

[0064] The wafer 13 has a developed surface area greater than that of the semi-finished product 11 and less than the developed surface area of ​​the wall 3 to be manufactured.

[0065] The hot forging step is performed at a temperature that depends on the material used. For example, if the material is an aluminum alloy, the hot forging step is usually performed at a temperature between 300°C and 500°C. If the material is a titanium alloy, the hot forging step is usually performed at a temperature between 700°C and 1300°C.

[0066] If the hollow part 1 to be manufactured includes an opening 9 as described above, during this step, a circular opening 14 is made in the wafer 13.

[0067] The wafer 13 is then hot-shaped to create a preform 15.

[0068] The hot forming step is performed at a temperature that depends on the material used. For example, if the material is an aluminum alloy, the hot forming step is generally performed at a temperature between 300°C and 500°C. If the material is a titanium alloy, the hot forming step is generally performed at a temperature between 700°C and 1300°C.

[0069] The preform 15 has a developed surface area which is either less than or equal to the developed surface area of ​​the wall 3. The preform 15 comprises at least two successive portions extending radially in the extension of one another away from the central axis AA, such that for all of the portions, the average thicknesses of a first and a second successive portion are distinct and / or the first portion is convex, concave or planar, while the second portion is, respectively, concave or planar, convex or planar, or convex or concave (i.e. if the first portion is convex, the second portion is concave or planar; if the first portion is concave, the second portion is convex or planar; if the first portion is planar, the second portion is convex or concave).

[0070] As described below, the preform 15 is then hot deformed under a press between a die and a punch to form the wall 3 of the hollow part 1.

[0071] By convex or concave, we mean a portion such that the surface of the portion considered intended to face the punch during hot deformation is convex, respectively concave, the opposite surface being concave, respectively convex.

[0072] Furthermore, we will subsequently consider that two portions are of distinct convexities if one of the portions is concave and the other is convex, or if one of the portions is plane and the other is convex, or one of the portions is concave and the other is plane.

[0073] The preform 15 therefore comprises at least two successive portions, extending radially in the extension of one another away from the central axis AA, so that for all of these portions, the average thicknesses and / or the convexities of two successive portions are distinct.

[0074] By distinct average thicknesses, it is meant that the average thickness of a first portion according to at least one axial section and the average thickness of a second portion according to this axial section are such that the ratio between the greatest average thickness and the smallest thickness is greater than or equal to 1.2, preferably greater than or equal to 1.3, better still greater than or equal to 1.5, or even greater than or equal to 2. Generally, this ratio is less than or equal to 5. In other words, the greatest average thickness is greater than or equal to 120%, preferably 130%, better still 150% or even 200% of the smallest thickness, but remains less than or equal to 500% of the smallest thickness.

[0075] Thus, the preform 15 comprises a central portion 19 and at least one annular portion 21 extending radially in the extension of one another away from the central axis AA, the central portion 19 and the annular portion 21 having distinct convexities and / or distinct average thicknesses.

[0076] If the central portion 19 and the annular portion 21 are of distinct convexities, the central portion 19 is generally concave, the adjacent annular portion 21 being convex. If the central portion 19 and the annular portion 21 are of distinct average thicknesses, the central portion 19 is generally of average thickness less than the average thickness of the adjacent annular portion 21.

[0077] If the hollow part 1 to be manufactured comprises an opening 9, the central portion 19 is an annular portion. The central portion 19 thus comprises a recess 22. This recess 22 is intended to form the opening 9 of the hollow part 1.

[0078] Preferably, the preform 15 comprises a plurality of successive portions 19, 21, 23... extending radially in the extension of one another away from the central axis AA, so that for all of the portions, the average thicknesses and / or the convexities of two successive portions are distinct.

[0079] The central portion 19 generally has a developed surface area of ​​between 5% and 90% of the developed surface area of ​​the preform 15.

[0080] If the preform comprises only two portions, the annular portion 21 therefore has a developed surface area of ​​between 10% and 95% of the developed surface area of ​​the preform 15.

[0081] Furthermore, if N is the number of portions constituting the preform 15, with N>3, each annular portion has a developed surface area of ​​between 10% / (N-1) and 100% / (N-1) of the developed surface area of ​​the preform 15.

[0082] Each portion under consideration differs from each adjacent portion in that the average thickness of the portion under consideration is distinct from the average thickness of that adjacent portion and / or in that the convexity of the portion under consideration is distinct from the convexity of that adjacent portion.

[0083] For example, the portions have distinct successive convexities.

[0084] Preferably, the preform 15 then comprises a succession of at least three portions 19, 21, 23 alternately concave and convex, which form a pattern in the form of waves or wavelets. In this case, the central portion preferably has a developed surface area of ​​between 10% and 90% of the developed surface area of ​​the preform 15, and each annular portion has a developed surface area preferably of between 10% / (N-1) and 50% / (N-1) of the developed surface area of ​​the preform 15.

[0085] In another example, the portions have distinct successive average thicknesses.

[0086] According to yet another example, at least one portion has a convexity identical to the convexity of a first adjacent portion, but an average thickness distinct from the average thickness of the first adjacent portion, and has a convexity distinct from the convexity of a second adjacent portion, but an average thickness identical to the average thickness of the second adjacent portion.In particular, the preform comprises a central portion 19, an intermediate annular portion 21 extending radially in the extension of the central portion 19 away from the central axis AA, and a peripheral annular portion 23 extending radially in the extension of the intermediate annular portion 21 away from the central axis AA, the intermediate annular portion 21 having an average thickness different from the average thicknesses of the central portion 19 and of the peripheral annular portion 23, and / or a convexity different from the convexity of the central portion 19 and of the peripheral annular portion 23.

[0087] In a first embodiment, of which Figure 3 illustrates an example, the preform 15 is such that the portions comprise a central portion 19 and at least one intermediate annular portion 21, the central portion 19 and the intermediate annular portion 21 being of distinct convexities.

[0088] Preferably, as illustrated in Figure 3, the portions further comprise a peripheral annular portion 23, the peripheral annular portion 23 having a convexity distinct from the convexity of the intermediate annular portion 21 adjacent thereto. The preform 15 then comprises a succession of at least three alternately concave and convex portions, which form a wave-like or wavelet-like pattern.

[0089] In the example illustrated in Figure 3, the preform 15 comprises a single intermediate annular portion 21.

[0090] Thus, the central portion 19, the intermediate annular portion 21 and the peripheral annular portion 23 extend radially in line with each other, away from the central axis AA, the central portion 19 being adjacent to the intermediate annular portion 21 and the intermediate annular portion 21 being adjacent to the central portion 19 and to the peripheral annular portion 23.

[0091] The central portion 19 has, for example, a developed surface area of ​​between 50% and 90% of the developed surface area of ​​the preform 15.

[0092] The intermediate annular portion 21 has, for example, a developed surface area of ​​between 5% and 25% of the developed surface area of ​​the preform 15.

[0093] The peripheral annular portion 23 has, for example, a developed surface area of ​​between 5% and 25% of the developed surface area of ​​the preform 15.

[0094] In this example, the central portion 19 is an annular portion and includes a recess 22.

[0095] In one variant, the preform comprises at least two intermediate annular portions. In this first embodiment, for all of the portions, the convexities of two successive portions are distinct. For example, if the preform comprises a central portion, a first intermediate annular portion, a second intermediate annular portion and a successive peripheral annular portion extending radially in the extension of one another away from the central axis AA, the first intermediate annular portion has a convexity distinct from the convexity of the central portion and the second intermediate annular portion, and the peripheral annular portion has a convexity distinct from that of the second intermediate annular portion.

[0096] Preferably, in this first embodiment, and as illustrated in Figure 3, the surface of the central portion 19 facing the punch (i.e. seen from top to bottom) has a concave shape.

[0097] In the example illustrated by Figure 3, the intermediate annular portion 21 is convex, while the central portion 19 and the peripheral annular portion 23 are concave.

[0098] In this example, the average thicknesses of the central portion 19, of the intermediate annular portion 21 and of the peripheral annular portion 23 are substantially identical, that is to say such that the ratio between the greatest average thickness and the smallest thickness is less than 1.2.

[0099] In this first embodiment, the developed surface of the preform 15 is generally substantially equal to the developed surface of the wall 3.

[0100] Thus, the hot deformation of the preform 15 to form the wall 3 is carried out without variation in thickness, and therefore requires limited efforts.

[0101] Furthermore, due to the alternation of concave and convex portions, the projected surface area of ​​the preform 15 on a transverse plane is less than the developed surface area of ​​the preform 15.

[0102] The ratio between the projected surface of the preform 15 on a transverse plane and the developed surface is generally less than 90%, preferably less than 80%, preferably less than 50%, and the largest dimension of the projected surface of the preform on a transverse plane is less than or equal to 150% of the largest dimension of the punch orthogonal to the central axis AA, in particular less than or equal to 135% of the largest dimension of the punch orthogonal to the central axis AA, preferably less than or equal to 120% of the largest dimension of the punch orthogonal to the central axis AA.

[0103] Therefore, it is possible to produce a large hollow part 1 by means of a press of limited size, in particular at least 10%, preferably at least 20%, even better at least 50% smaller than the size which would have been necessary to stamp a flat preform with a developed surface equal to the developed surface of the wall 3.

[0104] Figure 4 illustrates a step of shaping the wafer 13 to form the preform 15 according to this first embodiment.

[0105] In the example illustrated by Figure 4, the step of hot shaping the wafer to form the preform 15 comprises a first phase of shaping by hot stamping or die-stamping the wafer 13 to produce a first concave blank 27.

[0106] This first shaping phase is preferably a shaping phase by pushing the material towards the periphery of the wafer 13, such that the developed surface of the first concave blank 27 is greater than the developed surface of the wafer 13.

[0107] The hot forming step then comprises a second phase of forming by hot stamping or die-stamping the first concave blank 27 to produce the preform 15.

[0108] During this step, a peripheral portion of the first concave blank 27 is deformed to form the intermediate annular portion 21 and the peripheral annular portion 23.

[0109] Preferably, during this second shaping phase, the material is again pushed back towards the periphery of the first concave blank 27 so that the developed surface of the preform 15 is greater than the developed surface of the first concave blank 27.

[0110] Preferably, the first and second shaping phases are carried out by means of a first auxiliary press 29 and a second auxiliary press 37 respectively.

[0111] As illustrated in Figure 4, the first auxiliary press 29 comprises a first die 33 and a first punch 31.

[0112] The first shaping phase is preferably implemented by die-stamping or hot stamping of the wafer 13 between the first die 33 and the first punch 31.

[0113] The second shaping phase is implemented in the second auxiliary press 37 between a second die 35 and a second punch 39.

[0114] The shapes of the second die 35 and the second punch 39 are adapted for shaping the preform 15, in particular the intermediate 21 and peripheral 23 annular portions. The second shaping phase is then implemented by stamping or die-stamping the first concave blank 27 between the second die 35 and the second punch 39.

[0115] According to a variant, the second shaping phase comprises a first intermediate shaping phase by die-stamping or hot stamping of the first concave blank to produce a second blank comprising a concave central portion and a convex peripheral portion extending in the extension of the central portion, and a second intermediate shaping phase by die-stamping or hot stamping of the second blank to produce the preform.

[0116] Figure 5 illustrates an example of a preform 15 according to a second embodiment. This figure represents only a portion of the preform 15, in a perspective view. Furthermore, in this Figure 5, the press 47 is represented, including the die 49 and the punch 51, just before the preform 15 is hot deformed.

[0117] In this second embodiment, the preform 15 is such that the portions comprise a central portion 19 and at least one intermediate annular portion 21, the average thicknesses of the central portion 19 and of the intermediate annular portion 21 being distinct.

[0118] The developed surface of the preform 15 is less than the developed surface of the wall 3.

[0119] Furthermore, a ratio between the projected surface area of ​​the preform 15 on a transverse plane and the developed surface area of ​​the preform 15 is generally greater than 90%. Furthermore, the largest dimension of the projected surface area of ​​the preform 15 on a transverse plane is less than or equal to 150% of the largest dimension of the punch 51, preferably less than or equal to 120% of the largest dimension of the punch 51 orthogonal to the central axis AA.

[0120] The portions comprise at least one thickened portion intended to provide excess material which will be used, during the hot deformation step, to increase by plastic flow the developed surface area of ​​the preform 15.

[0121] Preferably, as illustrated in Figure 5, the portions further comprise a peripheral annular portion 23, the peripheral annular portion 23 having an average thickness distinct from the average thickness of the intermediate annular portion 21 which is adjacent to it. The preform 15 then comprises a succession of at least three portions of distinct successive average thicknesses.

[0122] In the example illustrated in Figure 5, the preform 15 comprises a single intermediate annular portion 21. Thus, the central portion 19, the intermediate annular portion 21 and the peripheral annular portion 23 extend radially in line with each other, away from the central axis AA, the central portion 19 being adjacent to the intermediate annular portion 21 and the intermediate annular portion 21 being adjacent to the central portion 19 and to the peripheral annular portion 23.

[0123] The central portion 19 has, for example, a developed surface area of ​​between 5% and 90% of the developed surface area of ​​the preform 15.

[0124] The intermediate annular portion 21 has, for example, a developed surface area of ​​between 5% and 50% of the developed surface area of ​​the preform 15.

[0125] The peripheral annular portion 23 has, for example, a developed surface area of ​​between 5% and 50% of the developed surface area of ​​the preform 15.

[0126] In this example, the central portion 19 is an annular portion and includes a recess 22.

[0127] Furthermore, in this example, the intermediate annular portion 21 has an average thickness distinct from the average thickness of the central portion 19 and from the average thickness of the peripheral annular portion 23.

[0128] In particular, the intermediate annular portion 21 is an overthickened portion, that is to say of average thickness greater than the average thickness of the central portion 19 and the average thickness of the peripheral annular portion 23.

[0129] In this example, the average thicknesses of the central portion 19, of the intermediate annular portion 21 and of the peripheral annular portion 23 are such that the ratio between the greatest average thickness and the smallest thickness is greater than or equal to 1.2, preferably greater than or equal to 1.3, better still greater than or equal to 1.5, or even greater than or equal to 2. Preferably, this ratio is less than or equal to 5.

[0130] The shaping of the wafer 13 to form the preform 15 is for example carried out by upsetting forming, using flat bands or discs.

[0131] After the step of hot shaping the wafer 13 to create the preform 15, the method comprises a step of hot deformation of the preform 15 under a press 47 between a die 49 and a punch 51 to form the wall 3.

[0132] The hot deformation step is carried out at a temperature that depends on the material used. For example, if the material is an aluminum alloy, the hot deformation step is generally carried out at a temperature between 300°C and 500°C. If the material is a titanium alloy, the hot deformation step is generally carried out at a temperature between 700°C and 1300°C.

[0133] Generally, the die 49 has a largest diameter less than the largest diameter of the developed surface of the wall 3. The hot deformation step includes stamping and optionally die-stamping the preform 15 to form the wall 3.

[0134] During stamping, the shape of the preform 15 is modified to match the contours of the die 49 and / or the punch 51 without significant variation in thickness.

[0135] During the stamping, wall 3 is modeled, the local and average thicknesses being modified.

[0136] In the first embodiment described above, in which the preform 15 comprises at least two portions of distinct convexities, the hot deformation step comprises stamping.

[0137] In particular, if the developed surface of the preform 15 is equal to the developed surface of the wall 3, the hot deformation step consists of stamping.

[0138] In particular, the hot deformation step comprises unfolding the preform 15 between the die 49 and the punch 51 such that at the end of the unfolding, the surfaces of the convex portions of the preform facing the punch 51 become concave.

[0139] In particular, in the example illustrated in Figure 3, the preform 15 is unfolded during the hot deformation step so that the surface of the intermediate annular portion 21 facing the punch 51 becomes concave.

[0140] If the preform 15 has a developed surface area equal to the developed surface area of ​​the wall 3, this hot deformation step does not involve any change in thickness. Thus, such a step requires reduced press forces.

[0141] Furthermore, the projected surface of the preform 15 on a transverse plane being less than the developed surface of the preform 15, it is possible, during this step, to produce a large hollow part 1 by means of a press of limited size, in particular at least 50% less than the size which would have been necessary to stamp a flat preform with a developed surface equal to the developed surface of the wall 3.

[0142] Figure 6 schematically shows, as an example, an initial phase of a hot deformation step of the preform 15 of Figure 3.

[0143] As illustrated in this Figure, the deformation step comprises an initial phase of bringing the punch 51 into contact with the preform 15, in particular with the central portion 19.

[0144] The punch 51 then continues its travel, causing the preform 15 to unfold, such that the convexity of the intermediate annular portion 21 is reversed, the intermediate annular portion 21 becoming concave. At the end of the travel of the punch, the surfaces of the wall 3 in contact with the punch are concave.

[0145] In the second embodiment described above, in which the preform 15 comprises at least two portions of distinct average thicknesses, the hot deformation step comprises, for example, stamping and die-stamping of the preform 15 between the die 49 and the punch 51.

[0146] In particular, the stamping of the preform 15 comprises a shaping phase during which the preform 15 is brought into contact with the die 49 by the punch 51.

[0147] At the end of this phase, for example, the lower surfaces of at least one portion of the preform 15, preferably of all the portions of the preform, are in contact with the die 49, and the upper surface of at least one portion is not in contact with the punch 51. Preferably, at the end of this phase, at least a part of the intermediate annular portion 21 is in contact with the die 49 and the punch 51.

[0148] Alternatively, at the end of this phase, the upper surfaces of at least one portion of the preform 15, preferably of all the portions of the preform 15, is / are in contact with the punch 51, and the lower surface of at least one portion is not in contact with the die 49.

[0149] Stamping is an expansion phase, during which the material is forced back until the material fits the surfaces of the punch 51 and the die 49.

[0150] According to one embodiment, during this phase, as long as the expansion is not finished, at least a portion of the preform 15 is not in contact with the punch 51.

[0151] Thus, the forces to be exerted by the press are reduced. Furthermore, the projected surface of the preform 15 on a transverse plane being less than the developed surface of the preform 15, it is possible to produce a hollow part 1 of large dimension by means of a press of limited size, in particular at least 50% less than the size which would have been necessary to stamp a flat preform of developed surface equal to the developed surface of the wall 3.

[0152] In the example illustrated in Figure 5, during the expansion phase, the thickened intermediate annular portion 21 of the preform 15 is compressed between the die 49 and the punch 51, at least a part of the central portion 19 and / or of the peripheral annular portion 23 not being in contact with the punch 51. Thus, the material flows plastically from the intermediate annular portion 21 towards the central portion 19 and the peripheral annular portion 23 then beyond the peripheral annular portion 23 to increase the developed surface area of ​​the preform 15 and form the wall 3.

[0153] The method according to the invention thus makes it possible to manufacture a large dome-shaped part, such as a cryogenic tank dome, which does not require welding, which can be produced on available forging presses whose dimensions are limited and in particular whose largest diameter is smaller than the largest diameter of the developed surface of the wall 3, and / or whose capacities in terms of forces are limited. Furthermore, it is possible to produce by this method a part whose fibering is homogeneous, such that the mechanical characteristics of the part are homogeneous around the central axis AA and along the wall 3, from the lower transverse plane 5 to the upper transverse plane 7, as illustrated in Figure 7.

[0154] In particular, the orientation of the grains is homogeneous, both around the central axis, in any axial plane including the AA axis and in any plane transverse to the AA axis.

[0155] Generally, in any transverse plane, the angle a formed between the direction of elongation of the grains (long direction) seen in section in this transverse plane and the tangent to the surface of the wall in this grain, as illustrated in Figure 7, is substantially constant (for example + / - 10°).

[0156] In fact, the grains elongate in a preferred direction due to the deformations applied by the method according to the invention.

[0157] By direction of elongation of a grain seen in section in the transverse plane, we mean the direction in which the greatest dimension (length) of a grain seen in section in this transverse plane extends.

[0158] The thesis “Behavior and damage of 6061-T6 aluminum alloys: Micromechanical approach”, Yang Shen, 2012, illustrates in particular the microstructural characterization of grains, in particular the direction of elongation.

[0159] With regard to the graining, the tangential direction is preferably more pronounced than the axial direction, and the axial direction is more pronounced than the radial direction, which means that in general, the grains are more elongated in the tangential direction than in the axial direction, and more elongated in the axial direction than in the radial direction. By "radial direction" is generally meant here a direction which extends through the thickness of the wall, or in other words, depending on the thickness, perpendicular to a plane tangent to the wall (and therefore not necessarily perpendicular to the AA axis).

[0160] Generally, if we consider the grains seen in three dimensions, the greatest direction of elongation of the grains is in the tangential direction, included in the transverse plane and tangent to the surface of the wall 3 (that is to say the angle a formed between the direction of elongation of the grains seen in section in this transverse plane and the tangent to the surface of the wall in this grain is substantially zero).

[0161] The mechanical characteristics of the resulting part are higher in the tangential direction, which is the highest stress mode for this type of part.

[0162] The grains are preferably axisymmetric around the central axis AA.

[0163] Generally speaking, the person skilled in the art will be able to adapt the geometry of the preform and the dimensions of the different portions thereof, in particular the diameters and thicknesses, as well as the number of portions, depending on the shape and dimensions of the wall to be manufactured, and depending on the maximum dimensions and capacities of the press.

[0164] Furthermore, the process is not limited to the manufacture of large domes for pressure vessels. It can be adapted to any large part that can be manufactured by stamping / die-forging.

[0165] The part that can be manufactured by this method is generally, but not necessarily, a part of revolution. The wall may also include additional features, including openings, reliefs on the surface, including bosses, stitching, ribs. Furthermore, the two embodiments described above may be combined.

[0166] In particular, with reference to the first embodiment, at least one portion has an average thickness distinct from the average thickness of another portion.

Claims

CLAIMS Method for manufacturing a hollow part (1) comprising a wall (3) in a single seamless piece in the shape of a dome extending around a central axis (AA), the wall (3) being made of a material consisting of a metal alloy, the wall (3) having an average thickness of less than 50 mm and a largest dimension, orthogonal to the central axis (AA), of between 1500 mm and 3700 mm, the method comprising the following consecutive steps: • supply of a semi-finished product (11) of said material; • hot forging by upsetting the semi-finished product (11) to form a wafer (13) with a developed surface area greater than that of the semi-finished product (11) and less than the developed surface area of ​​the wall (3); • hot shaping of the wafer (13) to create a preform (15) having a developed surface area less than or equal to the developed surface area of ​​the wall (3), the preform (15) comprising at least two successive portions (19, 21) extending radially in the extension of each other away from the central axis (AA), so that for all of the portions (19, 21), the average thicknesses and / or the convexities of two successive portions (19, 21) are distinct; • hot deformation of the preform (15) under a press (47) between a die (49) and a punch (51) to form the wall (3). Manufacturing method according to claim 1, characterized in that the portions comprise a central portion (19) and at least one intermediate annular portion (21), the central portion (19) and the intermediate annular portion (21) having distinct convexities. Manufacturing method according to claim 2, characterized in that the portions further comprise a peripheral annular portion (23), the peripheral annular portion (23) having a convexity distinct from the convexity of the adjacent intermediate annular portion (21).Manufacturing method according to any one of claims 2 to 3, characterized in that a ratio between the projected surface of the preform (15) on a transverse plane and the developed surface of the preform (15) is less than 90%, and the largest dimension of the projected surface of the preform (15) on a transverse plane is less than or equal to 150% of the. largest dimension of the punch (51) orthogonal to the central axis (A- A) Manufacturing method according to any one of claims 2 to 4, characterized in that the step of deforming the preform (15) under the press comprises unfolding the preform (15) between the die (49) and the punch (51) so that at the end of the unfolding, the surfaces of the portions facing the punch (51) are concave. Manufacturing method according to any one of claims 2 to 5, characterized in that the surface of the central portion (19) facing the punch (51) has a concave shape.Manufacturing method according to claim 1, characterized in that the portions comprise a central portion (19) and at least one intermediate annular portion (21), the average thicknesses of the central portion (19) and of the intermediate annular portion (21) being distinct, the developed surface of the preform (15) being less than the developed surface of the wall (3). Manufacturing method according to claim 7, characterized in that the portions further comprise a peripheral annular portion (23), and in that the intermediate annular portion (21) is an overthickened portion, the average thickness of the intermediate annular portion (21) being greater than the average thickness of the central portion (19) and the average thickness of the peripheral annular portion (23).Manufacturing method according to any one of claims 7 or 8, characterized in that the step of deforming the preform (15) comprises a shaping phase during which at least a part of the intermediate annular portion (21) is brought into contact with the die (49) by the punch (51), and a stamping phase corresponding to an expansion during which at least a part of the preform (15) is not in contact with the punch (51).Manufacturing method according to claim 9, characterized in that during the expansion phase, the thickened intermediate annular portion (21) of the preform (15) is compressed between the die (49) and the punch (51), at least part of the central portion (19) and / or the peripheral annular portion (23) not being in contact with the punch (51), such that the material flows plastically from the intermediate annular portion (21) towards the central portion (19) and the annular portion. peripheral (23) then beyond the peripheral annular portion (23) to increase the developed surface area of ​​the preform and form the wall (3). Method according to any one of claims 1 to 10, characterized in that the material is an aluminum alloy, in particular a 2XXX or 7XXX alloy. Hollow part (1) comprising a wall (3) in a single piece without welding in the shape of a dome, the wall (3) extending around a central axis (AA), the wall (3) being made of a material consisting of a metal alloy, the wall having an average thickness of less than 50 mm and a largest dimension, orthogonal to the central axis (AA), of between 1500 mm and 3700 mm.Hollow part according to claim 12, characterized in that the wall (3) has a homogeneous fibering around the central axis (AA), the orientation of the grains of said metal alloy being such that in any transverse plane, the angle formed between the direction of elongation of a grain seen in section in this transverse plane, and the tangent to the wall (3) in this grain is substantially constant. Part according to any one of claims 12 or 13, characterized in that the material is an aluminum alloy, in particular a 2XXX or 7XXX alloy. Cryogenic tank comprising a part (1) according to any one of claims 12 to 14 or manufactured by a method according to any one of claims 1 to 11.

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