Method for manufacturing a metal structure for use in an aircraft

Abstract

The invention relates to a method for manufacturing aerospace metal structures using an integrated high-energy hydroforming (HEHF) process. Combining press forming and HEHF, the method ensures uniform metal displacement, reduces residual stresses, and enhances material properties while improving precision.

Description

Method for manufacturing a metal structure for use in an aircraftTechnical field

[0001] The invention relates to a method for manufacturing a metal structure for use in an aircraft. The invention also relates to a metal structure of an aircraft and to an aircraft comprising the metal structure.Prior art

[0002] In the context of advanced metal forming techniques for aerospace and aeronautic applications, the demand for lightweight yet robust metal structures have led to the adoption of high-energy hydroforming (HEHF) process. This process uses the force generated under water explosion to shape metal into complex geometries within a die. However, conventional HEHF methods face challenges related to the non-uniform displacement of metal during forming, leading to uneven springback and residual stresses that compromise shape accuracy and fatigue resistance. Moreover, ductility reductions in the formed components can adversely affect their impact resistance, such as during a bird strike event. To mitigate these issues, a pre-forming step using press forming tools is often employed to improve metal flow during HEHF. Traditionally, this pre-forming is carried out separately from the HEHF process, requiring the metal to be transferred and repositioned in the HEHF die. This transfer introduces inaccuracies, further affecting the uniformity of metal displacement and reducing overall process efficiency. Thus, there is a need for an improved method for manufacturing a metal structure for use in an aircraft.Disclosure of the invention

[0003] An objective of the invention is to provide a streamlined, precise, and efficient method for high-energy hydroforming that addresses the challenges of non-uniform metal displacement, positioning inaccuracies, and residual stresses.

[0004] For that, the invention relates to a method for manufacturing a metal structure for use in an aircraft, and comprising:- positioning a preliminary-shaped metal sheet in a high-energy hydroforming (HEHF) die;- forming the preliminary-shaped metal sheet into a partially-shaped metal sheet using a press forming process on the HEHF die, wherein the metal sheet is formed to achieve a predefined distance from a forming surface of the HEHF die; and- forming the partially-shaped metal sheet into a finally-shaped metal sheet using a HEHF process.

[0005] The invention offers multiple advantages. By combining press forming and HEHF in a single operation, the metal sheet is first press formed into the HEHF die in a controllable, defined and precise manner. The metal sheet is not removed from the HEHF die after the press forming process. This ensures more controlled and uniform displacement of the metal during the HEHF process, significantly reducing springback and residual stresses. This integration eliminates the inaccuracies associated with transferring the metal between separate operations, thereby improving the precision of the final shape. Additionally, the process enhances ductility and fatigue resistance, ensuring better structural performance. The invention also simplifies the manufacturing process, reducing setup times and operational complexity, making it both cost-effective and efficient for aerospace applications.

[0006] Furthermore, the combined press forming and HEHF process is typically performed without lubricants, which is standard practice for a single press forming process. This lubricant-free approach simplifies process execution, and minimizes post-processing requirements. Additionally, the edges of the metal sheet are firmly clamped onto the HEHF die during this combined process, effectively preventing flange draw-in into the die — a behavior typically required in single press forming operations. This precise clamping mechanism improves material flow control, enhances forming accuracy, leading to higher-quality components.

[0007] As understood by a person skilled in the art, High-Energy Hydroforming (HEHF), also referred to as explosive forming, dynamic hydroforming, orshockwave forming, is an advanced metal forming process that utilizes high-energy shock waves to shape a metal sheet into the desired geometry. This method typically involves the detonation of an explosive charge within a water-filled tank, generating a shock wave that propagates through the water and applies uniform high-pressure forces to the metal sheet. The sheet is plastically deformed, conforming to the contours of a die cavity with high force. HEHF is particularly effective for forming complex, large-scale, or high-strength components that are challenging to manufacture using conventional methods. This process is widely used in aerospace and defense applications.

[0008] As understood by a person skilled in the art, the term "sheet" in the present disclosure is intended to encompass both thin metal sheets and thicker metal plates, depending on the specific application and material requirements. The distinction between a sheet and a plate is generally based on thickness, with sheets typically being thinner and more flexible, while plates are thicker and more rigid. However, for the purposes of this invention, both terms are used interchangeably to describe the metal workpiece subjected to the described forming processes.

[0009] As understood by a person skilled in the art, the press forming process, also known as mechanical forming or stamping, is a metal forming technique that employs a punch to shape a metal sheet under controlled force. The punch applies force to the sheet, pressing it into the die cavity to achieve the desired intermediate shape.

[0010] In one embodiment, the method ensures that the predefined distance between the preliminary-shaped metal sheet and the forming surface of the HEHF die is less than or equal to three times the thickness of the metal sheet. This precise spacing is essential for maintaining uniform deformation during the press forming process, minimizing stress concentrations, and preventing material thinning. By carefully controlling this distance, the process enhances the dimensional accuracy of the partially-shaped sheet and ensures its structural integrity, which is critical for high-performance aerospace components.

[0011] In one embodiment, the preliminary-shaped metal sheet comprises aluminum or an aluminum alloy. These materials are widely used in aerospaceapplications due to their lightweight properties, high strength-to-weight ratio, and excellent corrosion resistance.

[0012] In one embodiment, the preliminary-shaped metal sheet includes at least a friction stir weld. Friction stir welding (FSW) is a solid-state joining process that creates seamless, high-strength welds without melting the base materials. This ensures that the sheet retains its mechanical properties even at the weld joint, improving the overall integrity of the material. The incorporation of FSW into the method allows for the production of larger, complex components with superior durability.

[0013] In one embodiment, the preliminary-shaped metal sheet can be flat, curved, cylindrical, or double-curved in shape, depending on the design requirements. This versatility ensures that the method can accommodate a wide range of geometries, enabling the production of components tailored to specific applications.

[0014] In one embodiment, the step of forming the preliminary-shaped metal sheet into the partially-shaped metal sheet comprises:- advancing a press-forming punch through the preliminary-shaped metal sheet to form the partially-shaped metal sheet; and- withdrawing the press-forming punch from the partially-shaped metal sheet. The use of a press-forming punch allows for precise control over the initial deformation of the metal sheet, ensuring uniform distribution of stresses and minimizing the risk of localized thinning or defects. By accurately guiding the metal sheet into the desired intermediate shape, this process enhances the overall accuracy and quality of the final structure.

[0015] In one embodiment, the step of forming the partially-shaped metal sheet into the finally-shaped metal sheet, comprises:- positioning an explosive charge above the partially-shaped metal sheet in a water basin; and- detonating the explosive charge to form the finally-shaped metal sheet.Advantageously, the use of an explosive charge provides a highly localized and controlled release of energy, which enables the partially-shaped metal sheet to conform accurately to the contours of the forming surface within the HEHF die.

[0016] In one embodiment, the step of forming the partially-shaped metal sheet into the finally-shaped metal sheet, further comprises:positioning the HEHF die containing the partially-shaped metal sheet in the water basin.

[0017] Alternatively, the step of forming the partially-shaped metal sheet into the finally-shaped metal sheet comprises positioning a water basin around the HEHF die containing the partially-shaped metal sheet. In this configuration, the HEHF die and the metal sheet remain stationary, while the water is introduced around them to create the required hydrostatic environment. Once the water has filled the basin, an explosive charge is positioned above the metal sheet and detonated to initiate the forming process. This alternative approach offers some advantages, including greater flexibility in accommodating larger or more complex HEHF die configurations that may not be easily submerged. Additionally, it reduces the need for extensive handling and repositioning of the HEHF die and metal sheet, which can enhance process repeatability.

[0018] Additionally, in some embodiments, the step of forming the preliminaryshaped metal sheet using the press forming process can be performed within a water basin, which is commonly integrated into the HEHF die. This integration allows the same water basin to be utilized in the subsequent HEHF process, optimizing workflow efficiency and minimizing unnecessary operations. Advantageously, a press forming process conducted within a water basin could mitigate thermal and mechanical stresses by acting as a stabilizing medium that absorbs and distributes mechanical loads more uniformly, preventing localized stress concentrations that could lead to material weakening or deformation defects.

[0019] In one embodiment, the method further comprises processing a raw metal sheet into the preliminary shape through machining, heat treating, or pre-forming. Each of these processes provides distinct advantages. Machining ensures precise dimensional control, heat treating improves material properties such as strengthand ductility, and pre-forming provides an initial geometric configuration that facilitates subsequent forming steps. This preparation ensures that the sheet is optimized for the high-energy forming processes that follow.

[0020] In one embodiment, the method further comprises processing the finally-shaped metal sheet using a machining process, and / or a surface-treating process, and / or a heat-treating process. These finishing steps ensure that the final metal sheet not only meets performance specifications but also exhibits enhanced resistance to environmental factors, making it ideal for demanding aerospace applications.

[0021] The invention also relates to a metal structure obtainable according to a method of any of the preceding embodiments. The invention further relates to an aircraft comprising the metal structure.Brief description of the figures

[0022] Other characteristics and advantages of the present invention will appear on reading the following detailed description, for the understanding of which, it is referred to the attached figures where:- Figure 1 illustrates a flowchart of a method according to the invention;- Figure 2 illustrates a flowchart of a method according to an embodiment of the invention; and- Figures 3A-3E illustrate cross-sectional side-views of a metal sheet progressing through steps of a method according to an embodiment of the invention.

[0023] The drawings in the figures are not scaled. Similar elements can be assigned by similar references in the figures. In the framework of the present document, identical or analogous elements may have the same references. The presence of reference numbers in the drawings cannot be considered to be limiting, in particular if these numbers are indicated in the claims.Description of specific embodiments of the invention

[0024] Description of preferred embodiments of the present invention are hereafter described with references to figures, but the invention is not limited by thesereferences. In particular, the drawings or figures described below are only schematic and are not limiting in any way.

[0025] Figure 1 illustrates a flowchart detailing the method 100 for manufacturing a metal structure designed for use in aircraft applications. The method 100 begins with step a. of positioning of a preliminary-shaped metal sheet into a high-energy hydroforming (HEHF) die. This step ensures that the metal sheet is accurately aligned within the die, setting the foundation for subsequent shaping stages. Following this, the preliminary-shaped metal sheet undergoes step b. of press forming process, where controlled force is applied to transform it into a partially-shaped metal sheet. During this stage, the metal sheet is strategically displaced to a predefined distance from the forming surface of the HEHF die, a critical adjustment that ensures uniformity and precision in the final structure. The final transformation is achieved through the application of step c. of high-energy hydroforming (HEHF) process, where the partially-shaped metal sheet is subjected to high-energy pressure, allowing it to take on the complex contours of the die with remarkable accuracy. This multi-stage method 100 combines the advantages of mechanical forming and hydroforming to produce a finely-tuned metal structure with high strength and dimensional precision.

[0026] Figure 2 illustrates an extended flowchart that elaborates on additional steps to refine the method 100 described in Figure 1. The method 100 introduces preferably, an initial preparation phase, labeled as step aO, where a raw metal sheet undergoes processing to achieve its preliminary shape. This preparation may involve machining, heat treating, or pre-forming processes, either individually or in combination, depending on the requirements. Machining can be employed to adjust the metal sheet to a specific thickness profile, ensuring precise dimensional accuracy. Heat treatment, such as annealing or solutionizing followed by quenching, can be applied to improve the material's mechanical properties and workability. Additionally, a pre-forming process may be utilized to impart an initial geometric configuration to the metal sheet, facilitating subsequent forming steps. These preparatory steps ensure that the metal is optimized for dimensional precision, and readiness for the next stages of the manufacturing process.Preferably, the flowchart further incorporates step c1 , which occurs after the final shaping of the metal sheet. At this stage, the finally-shaped metal sheet undergoes additional processing to meet stringent aerospace quality standards. This processing may include machining to refine dimensions and achieve precise tolerances. The sheet may also be subjected to surface treatments such as anodizing, polishing, and / or painting to enhance its resistance to environmental factors and improve durability. Heat treatment, including aging, may also be performed to increase the material’s strength and ductility. Each of these postprocessing steps, whether applied individually or in combination, ensures that the finally-shaped metal sheet meets the mechanical performance and quality requirements essential for aerospace applications.

[0027] Figures 3A through 3E provide a series of cross-sectional side views illustrating the progressive transformation of a metal sheet as it advances through the key steps of the method 100.

[0028] Figure 3A depicts a friction stir welding process, which preferably, serves as a preliminary step for preparing a preliminary-shaped metal sheet 10. An FSW tool 5, featuring a rotating pin and shoulder, is applied along the interface of two metal pieces 50. The FSW tool 5 generates frictional heat that softens the metal without melting it. This softened material is stirred and forged under high pressure to form a defect-free weld with superior mechanical properties. Unlike conventional welding techniques such as TIG welding, where aluminum reaches its melting point, the Friction Stir Welding (FSW) process occurs in a solid state, preventing melting. As a result, the material’s microstructure remains intact, enhancing its mechanical properties. This step is particularly significant for aerospace applications, as it ensures the structural integrity of the preliminary-shaped metal sheet 10 while maintaining minimal distortion. The resulting preliminary-shaped metal sheet 10 is now ready for subsequent forming steps, providing a robust foundation for further transformation.

[0029] Figure 3B illustrates primarily, a High-Energy Hydroforming setup, including a HEHF die 110. The preliminary-shaped metal sheet 10, created in the previous step, is carefully positioned within the HEHF die 110. The HEHF die 110 comprisesa forming cavity 6 defined by a precisely machined forming surface 1. This forming surface 1 is specifically engineered to ensure the metal sheet conforms to the desired geometry during the subsequent forming steps. Additionally, the HEHF die 110 is equipped with features to stabilize and secure the metal sheet, such as holddown rings or similar retaining mechanisms. Preferably, a vacuum line extends from the forming cavity 6 and connects to a vacuum system, which removes air pockets from the forming cavity 6 to facilitate uniform contact between the sheet and the HEHF die 110 during forming. The HEHF setup commonly features a tank (not shown) filled with water, which serves multiple critical roles during the HEHF forming process, including the uniform distribution of pressure, impact cushioning, and thermal management.

[0030] Figure 3C illustrates a press forming process, which constitutes the first active forming stage implemented within the HEHF setup. This process is critical for initiating the transformation of the preliminary-shaped metal sheet 10 into an intermediate form while maintaining precise control over the deformation. In this step, a press-forming punch 3 applies controlled pressure to the preliminary-shaped metal sheet 10, forcing it to partially conform to the contours of the forming surface 1 and the forming cavity 6 within the HEHF die 110. The HEHF die 110, with its specifically designed forming cavity 6, guides the metal sheet's deformation and ensures accuracy in shape. The movement of the press-forming punch 3 is carefully calibrated to achieve uniform deformation, ensuring that the structural integrity of the sheet is preserved throughout the process. The result of this stage is a partially-shaped metal sheet 20, an intermediate form essential for preparing the material for subsequent high-energy forming stages. A defining feature of this step is the predefined distance maintained between the partially-shaped metal sheet 20 and the forming surface 1 of the HEHF die 110. Preferably, this distance is less than or equal to three times the thickness of the metal sheet. This precise spacing minimizes stress concentrations and prevents excessive thinning, enhancing the material’s mechanical properties and preserving its structural stability. Once the metal sheet achieves the desired intermediate contours, the press-forming punch 3 is removed, leaving the sheet in position within the HEHF die 110. This stepprepares the system for the HEHF process, which will further shape the sheet into its final form. The press forming process can be carried out in either a single pass or multiple passes, depending on the complexity of the required intermediate shape and the characteristics of the material. In cases where a single pass is insufficient to achieve the desired deformation, additional passes may be performed. These subsequent passes involve re-engaging the press-forming punch 3 to progressively shape the metal sheet in smaller increments. This incremental approach reduces the risk of overloading the material, prevents defects such as cracks or excessive thinning, and ensures more precise conformity to the forming surface 1.

[0031] Figure 3D illustrates the placement of an explosive charge 4 above the partially-shaped metal sheet 20, which is held firmly in place by a hold-down ring or similar retaining mechanism to prevent displacement during the explosive process. The explosive charge 4 is connected to a detonation line, which links it to a remote detonator for precise activation. The water in the tank (not shown) plays a vital role in the process by moderating the shock wave, ensuring the explosive energy is transmitted uniformly across the sheet's surface, cushioning the impact to prevent localized stress or cracking, and absorbing heat generated during the explosion to protect both the HEHF die 110 and the sheet. The positioning of the explosive charge 4 is calculated to ensure that the energy released during detonation is directed efficiently toward the sheet. This careful arrangement prepares the system for the subsequent high-energy deformation stage, ensuring precise and controlled results.

[0032] Figure 3E illustrates the detonation of the explosive charge 4 and the resulting transformation of the partially-shaped metal sheet 20 into its final form as the finally-shaped metal sheet 30. When the explosive charge 4 is detonated, it generates a powerful shock wave that propagates radially through the water medium. This shock wave applies high-energy pressure uniformly across the metal sheet, which remains securely positioned within the forming cavity 6 of the HEHF die 110. The water medium ensures the pressure is distributed evenly across the surface of the sheet, forcing it to plastically deform and conform precisely to the precise contours of the forming surface 1 within the HEHF die 110. The process ispreferably, enhanced by a vacuum line connected to the forming cavity 6, which removes trapped air and ensures complete contact between the sheet and the HEHF die 110, eliminating voids and imperfections. This stage concludes with the metal sheet achieving its final shape, characterized by high dimensional accuracy and enhanced mechanical properties. This method, as illustrated, enables the production of complex, high-performance components that meet the rigorous demands of aerospace applications.

[0033] The present invention has been described in relation to the specific embodiments which have a value that is purely illustrative and should not be considered to be limiting. The skilled person will notice that the invention is not limited to the examples that are illustrated and / or described here above.

[0034] To sum up, the invention relates to a method for manufacturing aerospace metal structures using an integrated high-energy hydroforming (HEHF) process. Combining press forming and HEHF, the method ensures uniform metal displacement, reduces residual stresses, and enhances material properties while improving precision.

Claims

Claims1. A method (100) for manufacturing a metal structure for use in an aircraft, and comprising:a. positioning a preliminary-shaped (10) metal sheet in a high-energy hydroforming (HEHF) die (110);b. forming the preliminary-shaped (10) metal sheet into a partially- shaped (20) metal sheet using a press forming process on the HEHF die (110), wherein the preliminary-shaped metal (10) sheet is formed to achieve a predefined distance from a forming surface (1) of the HEHF die (110); andc. forming the partially-shaped (20) metal sheet into a finally-shaped (30) metal sheet using a HEHF process.

2. The method (100) according to the preceding claim, wherein the predefined distance is less than or equal to three times a thickness of the preliminaryshaped (10) metal sheet.

3. The method (100) according to any of the preceding claims, wherein the preliminary-shaped (10) metal sheet comprises aluminum or an aluminum alloy.

4. The method (100) according to any of the preceding claims, wherein the preliminary-shaped (10) metal sheet comprises at least one friction stir weld.

5. The method (100) according to any of the preceding claims, wherein the preliminary-shaped (10) metal sheet is flat, curved, cylindrical or double curved in shape.

6. The method (100) according to any of the preceding claims, wherein the step of forming the preliminary-shaped (10) metal sheet into the partially-shaped (20) metal sheet, comprises:- advancing a press-forming punch (3) through the preliminary-shaped (10) metal sheet to form the partially-shaped (20) metal sheet; and - withdrawing the press-forming punch (3) from the partially-shaped (20) metal sheet.

7. The method (100) according to any of the preceding claims, wherein the step of forming the partially-shaped (20) metal sheet into the finally-shaped (30) metal sheet, comprises:- positioning an explosive charge (4) above the partially-shaped (20) metal sheet in a water basin; and- detonating the explosive charge (4) to form the finally-shaped (30) metal sheet.

8. The method (100) according to the preceding claim, wherein the step of forming the partially-shaped (20) metal sheet into the finally-shaped (30) metal sheet, further comprises:- positioning the HEHF die (110) containing the partially-shaped (20) metal sheet in the water basin.

9. The method (100) according to any of the preceding claims, further comprising:aO. processing a raw metal sheet into the preliminary-shaped (10) metal sheet, using a machining process, and / or a heat-treating process, and / or a pre-forming process.

10. The method (100) according the preceding claim, wherein the heat-treating process comprises solutionizing and quenching.

11. The method (100) according to any of the preceding claims, further comprising:c1. processing the finally-shaped (30) metal sheet using a machining process, and / or a surface-treating process, and / or a heat-treating process.

12. A metal structure obtainable by a method according to any of the preceding claims.

13. An aircraft comprising a metal structure according to the preceding claim.

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