The technical field of the invention is the manufacture of parts made of metal or metal alloy, in particular aluminum or aluminum alloy.
Aluminum is an element present in many industrial fields, for example the building, food, automobile and aeronautic industries. Because of its lightness, corrosion behavior and ability to be shaped, it is commonly used for the production of structural elements, for example frameworks in buildings or frames or panels for vehicles, for example automobiles or airplanes.
Some structural elements are fuselage panels, having a flat or curved surface, forming a skin, from which protrude stringers, these increasing the rigidity of the panel. In general, the skin and the stringers are manufactured separately, and then the latter are assembled, for example using rivets or bolts. This represents a significant labor cost, as a large number of rivets or bolts needs to be used.
Alternatively, to form such an aluminum structural element, a solid plate, for example of thickness between 30 and 100 mm, can be machined. Machining makes it possible to form both the skin and the stringers. However, such a method, called mass machining, involves removing a large amount of material. The ratio between the quantity of material purchased over the quantity of material actually used is unfavorable, such a ratio being designated in aeronautics by the term buy-to-fly ratio.
Since the 1980s, additive manufacturing techniques have developed, which consist of forming a part by adding material, as opposed to machining techniques, which remove material. Formerly confined to prototyping, additive manufacturing is now operational for manufacturing industrial products in series, including metal parts.
Patent application US20160136891 describes the use of an additive manufacturing process to form the skin of a fuselage. According to this method, a composite skin is formed consisting of two aluminum layers between which is arranged a layer of titanium. After cooling, the skin takes on a curved shape.
Patent application FR1458451 describes a method of manufacturing an airplane fuselage panel, comprising a skin from which stringers extend, the skin and the stringers being made simultaneously by additive manufacturing. Such a method is, however, expensive in terms of material deposition time and hardly compatible with the requirements of a fuselage skin, especially from the standpoint of fatigue behavior and damage tolerance.
In sensitive applications of the aeronautical type, the material composing certain parts of pieces, called critical parts, must be controlled in order to meet precise requirements in terms of mechanical properties. Critical parts include parts of a piece that may be exposed to significant mechanical stress. Additive manufacturing techniques may not meet the mechanical property requirements for critical parts of certain pieces.
The invention meets the limitations of the methods of prior art, and proposes an optimized manufacturing process.
The first subject of the invention is a method of manufacturing a structural element from a so-called metal base piece, the structural element comprising a skin, extending between an inner surface and an outer surface, and at least one stringer, the stringer extending from the skin, to a distal end, the method comprising the following steps:
In this way, in step c), the addition of material displaces the distal end of the stringer blank relative to the inner surface to obtain the stringer.
Skin is taken to mean a slab of flat or curved material delimited by an inner surface and an outer surface, the thickness of the skin being less than 2 cm, or even 1 cm. The inner surface and the outer surface preferably extend parallel to, or substantially parallel to, each other. The term substantially parallel means parallel to a given tolerance, which may be an angular tolerance of ±10° or ±20°.
Extending from the skin means protruding from the skin.
Preferably, at the end of step c), the second extension distance is greater than twice or even three times the thickness of the skin.
Preferably, the first extension distance d1 may be, for example, between 1 cm and 10 cm or between 1 cm and 5 cm or between 2 cm and 4 cm.
Another subject of the invention is a method of manufacturing a structural element from a so-called metal base piece, the structural element comprising a skin and at least one stringer, the stringer extending from the skin, to a distal end, the method comprising the following steps:
Preferably, the difference between the second extension distance and the first extension distance (d2−d1) may be, for example, between 1 cm and 10 cm or between 1 cm and 5 cm or between 2 cm and 4 cm.
The common concept behind these methods is the combination of two techniques for forming a skin, from which one or more stringers extend: a conventional machining technique, of recognized efficiency, making it possible to form a part of each stringer, called proximal, arranged near the skin. Each process combines machining and an additive manufacturing technique, machining being performed either before or after additive manufacturing. The distal end of each stringer is obtained by adding material, prior to machining or after machining. The method can be employed from a thinner base piece than in prior art, in particular by optimizing the amount of material forming the structural element relative to the amount of material forming the base piece. The use of a thinner base piece also allows better control of the mechanical properties, particularly at the parts of the structural element exposed to the greatest stresses, as described below.
Each manufacturing method may include any of the following characteristics, taken separately or in technically feasible combinations:
Another subject of the invention is a structural element, intended for a structure of a construction, characterized in that it is made by using the method described in this application. Another subject of the invention is a vehicle for land, sea, air or even space transport, whose metal structure comprises at least one structural element made according to the method described in this application.
Other advantages and features will emerge more clearly from the description of particular embodiments of the invention, given by way of non-limiting examples, and shown in the figures listed below.
In this example, the structural element 1 is intended for the construction of an aircraft, in particular an airplane. This is a panel intended to form a structural element such as a fuselage, a door, a door frame, a floor, a hatch, a partition, an airplane wing or any other wing element of the tail fin, tail unit, flap or rudder type; this list is not exhaustive. Stringers 12 are intended to endow panel 1 with a certain rigidity and/or a certain mechanical resistance. Stringers 12 may also be designated by the terms spars or ribs. The thickness of skin 11, along a transverse axis Z shown, may range between 0.5 mm, or even 0.8 cm and 1 cm, or even 1.5 cm or 4 cm, while the height of a stringer 12, along the same axis, may range between 1 cm and 10 cm or between 1 cm and 20 cm. In this example, the stringer is rectilinear and extends along the longitudinal axis Y. The dimensions of panel 1, in a plane XY, may be up to 5 m×20 m or more. The example of a rectilinear stringer is a particular, non-limiting example and the invention also applies to obtaining a non-rectilinear stringer. The main steps of the method are as follows:
Step 100 (
Step 110: obtaining a workpiece 10′ from the base piece 10. During this optional step the base piece 10 obtained after the previous step can be formed and/or have its dimensions adjusted, so as to obtain a workpiece 10′. This step may also comprise a thermomechanical treatment, i.e. a thermal and/or mechanical treatment, comprising in particular, for alloys with age hardening, a solution heat treatment followed by quenching, before a step involving stress relieving by stretching and then aging. Thermomechanical treatment can be applied before or after shaping the base piece 10 or adjusting its dimensions.
Solution heat treatment is a thermomechanical treatment known to those skilled in the art. This treatment involves heating an alloy for a sufficient length of time to allow the precipitates present in the alloy to dissolve so that alloying elements, present in the alloy, are in a solid solution. The solution heat treatment temperature depends on the alloy. A solution heat treatment is followed by quenching, during which the alloy is cooled rapidly, so as to maintain a homogeneous distribution of the alloying elements. Precipitation hardening can be obtained during a subsequent natural or artificial aging or tempering step. The conditions for solution heat treatment, quenching, stress relief and aging are determined by those skilled in the art depending on the alloy and the dimensions of the piece.
The thermomechanical treatment can be performed in successive or separate steps. Advantageously, therefore, the solution heat treatment and quenching steps are carried out during step 110 while aging is carried out at a later stage of the method, as described below.
The method may include no deformation, thermomechanical treatment or dimensional adjustment, in which case the workpiece 10′ corresponds to the base piece 10.
Step 120 (
The thickness of the skin is preferably less than 4 cm, 2 cm, or even 1.5 cm or 1 cm. Machining also makes it possible to form at least one stringer blank 12′, extending, along the transverse axis Z, between the inner surface 11i and an end called the distal end 12′d. The distal end 12′d of the stringer blank 12′ extends to a distance d1, called the first extension distance, from the inner surface 11i. The first extension distance d1 may for example be between 1 cm and 5 cm, or between 1 cm and 10 cm. In this example, the stringer blank 12′ extends in an direction of extension corresponding to the transverse axis Z, the latter being perpendicular to the inner surface 11i. Another direction of extension secant to the inner surface 11i is possible. Preferably, several stringer blank 12′ are simultaneously formed during this step.
Unlike the method described in application WO2004/056501, the thermomechanical treatments, described in connection with step 110, are not carried out following step 120, i.e. after machining, but nonetheless exhibit the advantage of being made on a thin base piece. In the method according to the invention, therefore, it is not necessary to carry out a stress relieving step on a machined product of complex shape, as described in WO2004/056501, which may prove difficult industrially.
Step 130 (
Material is added by additive manufacturing, as shown in
Whatever the additive manufacturing method applied, the thickness of each elemental layer added is typically at least of the order of 10 microns and may also range up to few millimeters, for example 5 mm. An additional piece 12+ is then gradually formed, layer by layer, on the stringer blank 12′.
The different elementary layers may consist of the same material. According to one variant, the elementary layers do not consist of the same material; the material constituting an elementary layer may vary from one elementary layer to another. Each elemental layer may be of the same thickness. According to one variant, elementary layers may be of different thicknesses.
Step 130 may also include thermomechanical treatment following the addition of material. This may be, for example, deformation by compression, for example hot isostatic compression. This makes it possible to fill pores formed during the addition of material.
Step 140 (
The method does not require the use of a very thick base piece 10, compared to the methods of prior art in which the skin 11 and the stringers 12 are mass machined from the same base piece. According to the invention, the base piece must have a thickness, along the transverse axis Z, greater than or equal to the thickness of skin 11 plus the thickness of the stringer blank 12′, i.e., the thickness of skin 11 and of the first extension distance d1. This gives an improvement to the buy-to-fly ratio mentioned previously. The thickness of the base piece 10 may be reduced by a factor greater than or equal to 2. For example, to manufacture a panel whose total thickness is 70 mm, using a mass-machining method from prior art, a base piece with a thickness of about 75 mm is used. By using the invention, the thickness of the base piece can be reduced to 30 mm, instead of 75 mm, a gain in thickness of 45 mm. The total thickness of 70 mm corresponds to the thickness E of skin 11 plus the height of the stringers, i.e. the second extension distance d2 described above.
Another advantage of using a thinner base piece is related to the influence of the thermomechanical treatment on the mechanical properties. The thermomechanical treatment, regardless of whether it is applied during step 110 or following step 120, or during step 140, is performed on a workpiece whose thickness is reduced with respect to prior art. It is known that the efficiency of thermomechanical treatment is limited by the thickness of the part treated, insofar as lower quenching speeds are obtained on thicker parts. The application of a thermomechanical treatment on a thinner part makes it easier to obtain more advantageous mechanical properties. These mechanical properties are also more predictable because they are less subject to fluctuations related to thermal diffusion. In addition, this reduces the length of the thermomechanical treatment, which increases the efficiency of the method and reduces the related costs. The mechanical properties are thus controlled, in particular at the most critical parts of the piece, corresponding to skin 10 or the interface between skin 10 and stringers 12 (i.e. at the proximal end 12p of the stringers), on which mechanical stresses may be great. If material was deposited directly on the skin, the junction between the skin and the added stringer would be located at a critical part from the standpoint of fatigue behavior, in particular.
Another consequence of reducing the thickness of the base piece is that the latter is easier to shape, as illustrated in a variant of the first embodiment, described below, with reference to
From a base piece 20 (step 100), a deformation step is applied (step 110—
The method may include finishing machining, so as to improve the surface condition of the piece obtained. It may also undergo aging before or after finishing machining.
The method may include finishing machining, so as to improve the surface condition of the piece obtained. It may also undergo aging before or after finishing machining.
The invention can be used to produce structural elements for buildings or vehicles, for land, sea or air transport, and in particular in the manufacture of structural elements for fuselages or aircraft wings. It is in particular intended to be used to produce structural elements made of aluminum or aluminum alloy, from an aluminum base piece, the additional material added during the addition step being also aluminum or an aluminum alloy.
Number | Date | Country | Kind |
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1662068 | Dec 2016 | FR | national |
Filing Document | Filing Date | Country | Kind |
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PCT/FR2017/053445 | 12/7/2017 | WO | 00 |