A method for manufacturing a double-layer, multi-directional, cross-ribbed skin

Abstract

This invention provides a method for manufacturing a double-layer multi-directional cross-ribbed skin, belonging to the field of metal material processing technology. The method includes: Step 1, accurately unfolding the blank according to a three-dimensional model; Step 2, fabricating a core layer template; Step 4, precisely applying solder resist; Step 5, resistance spot welding, followed by argon arc welding to seal the periphery and weld the air inlet pipe, forming a sealed cavity; Step 6, performing diffusion bonding; Step 7, superplastic forming to create multi-directional cross-ribs; and finally, Step 8, removing excess material to obtain the finished product. The synergistic effect of these eight steps solves the problems of long machining cycles, low efficiency, low material utilization, long time required for critical equipment such as five-axis gantry machining centers, easy deformation during thin-walled part processing, and difficulty in controlling deformation during multiple machining operations. This invention realizes the manufacturing of a double-layer multi-directional cross-ribbed skin for aluminum-lithium alloys.

Description

Technical Field

[0001] This invention relates to the field of metal material processing technology, and in particular to a method for processing and manufacturing a double-layer multi-directional cross-ribbed skin. Background Technology

[0002] With the rapid development of the aerospace industry, increasing payload has become the primary goal in the aerospace field, and lightweighting has become a development trend for aerospace components.

[0003] The presence of ribbed aluminum alloy skin parts maximizes weight reduction, while the crisscrossing ribs ensure the strength and rigidity meet usage requirements. The conventional machining method for double-layer, multi-directional ribbed skin parts involves first rounding a thick plate, then machining the outer curved surface, and finally using CNC machining to process the weight-reducing grooves and ribs. However, the weight-reducing cavity of the ribbed skin has a very thin wall, making it prone to deformation after machining and difficult to control precision. Multiple heat treatments and annealing processes are required to reduce stress and control deformation. This method is time-consuming and inefficient, taking nearly a month to process a single skin part. Furthermore, this method requires piece-by-piece machining on machine tools, which is labor-intensive and occupies critical equipment such as five-axis gantry machining centers. More importantly, most existing processes are only designed for "simple hollow double-layer structures" (such as flat plates or single-curvature parts). For complex rib patterns with double-layer multi-directional intersections (two-way and above), CNC milling faces problems such as tool interference, complex programming, and uncontrolled stress deformation. The processing cycle is as long as several months, and the yield is extremely low, which cannot meet the engineering requirements of lightweight aerospace structures. Summary of the Invention

[0004] Based on the above analysis, the present invention aims to provide a manufacturing method for a double-layer multi-directional cross-ribbed skin, in order to solve one of the problems of the prior art, such as long processing cycle, low production efficiency, high occupancy rate of key equipment, need for multiple heat treatment annealing, poor machining deformation accuracy, low pass rate, and CNC milling facing tool interference, complex programming, and uncontrolled stress deformation.

[0005] This invention provides a method for manufacturing a double-layer, multi-directional, cross-ribbed skin, comprising the following steps: Step 1: Based on the 3D model, unfold the blank of the double-layer multi-directional cross-ribbed skin, and determine the blank size of the double-layer multi-directional cross-ribbed skin according to the unfolded size; the blank is divided into the upper blank panel and the lower blank panel. Step 2: Make a core layer template according to the shape of the double-layer multi-directional cross stiffener in the three-dimensional model for subsequent application of solder resist. Step 3: Grind the edges and burrs according to the outer dimensions of the blank and the core layer template; pickle to remove surface oil. Step 4: Place the cut core layer template on the lower panel of the blank, apply solder resist to the blank areas according to the core layer template, and then apply water-based graphite lubricant to the outer surfaces of the upper panel and lower panel of the blank and the inner cavity of the mold. Step 5: According to the core layer template, the periphery of the multi-directional cross reinforcing ribs of the upper and lower layers of the blank is reinforced by resistance spot welding. The periphery of the double-layer multi-directional cross-ribbed skin is sealed by argon arc welding to form a sealed cavity, and an air inlet pipe is welded. Step 6: Place the diffusion bonding mold into the thermoforming equipment and heat the mold. Once the mold reaches the required temperature, place the spot-welded and reinforced blank into the mold for diffusion bonding. Step 7: Heat the superplastic forming mold. Once the mold reaches the set temperature, place the blank with the welded vent pipe into the mold and perform superplastic forming. Step 8: After superplastic forming is completed, remove the sample from the mold, draw lines on the surface of the sample according to the dimensions of the three-dimensional model, and cut off the excess material according to the drawn lines.

[0006] Furthermore, in step 1, the thickness of both the upper and lower panels of the blank is 1-5 mm.

[0007] Furthermore, the upper panel and lower panel of the billet are one of 5083 aluminum alloy, 2A97 aluminum-lithium alloy, and 5A90 aluminum-lithium alloy lightweight alloy.

[0008] Furthermore, one of the two positioning holes on the upper panel and the lower panel of the blank is a round hole, and the other is an oblong hole.

[0009] Furthermore, in step 5, the welding current of the resistance spot welding is adjusted according to the thickness of the upper and lower panels of the blank, with a current range of 10-50KA, a voltage range of 2-5KW, a time range of 200-1000ms, and a spot welding gap range of 10-30mm.

[0010] Furthermore, in step 5, the argon arc welding current is 60-80A, the tungsten inert gas (TIG) wire diameter is 2.0-4.0mm, and the shielding gas flow rate is 6-18L / min.

[0011] Furthermore, in step 6, the temperature range for diffusion bonding is 450-490℃, the pressure for diffusion bonding is 50-500 tons, and the diffusion bonding time is 1-6 hours.

[0012] Furthermore, in step 7, the temperature range for superplastic forming is 450-480℃.

[0013] Furthermore, in step 7, the ventilation rate of superplastic forming is 0.1MPa-0.5MPa / 20min, and the pressure is maintained for 28-32min after every 55-65min of ventilation until the pressure reaches 2-10MPa.

[0014] Furthermore, in step 7, after the superplastic forming reaches the maximum pressure, the pressure is held for 30-120 minutes.

[0015] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects: 1. This invention provides a manufacturing method for a double-layer multi-directional cross-ribbed skin, comprising steps 1-8: blank unfolding → making a core layer template based on the shape of the multi-directional cross-ribbed ribs in the three-dimensional model → removing burrs and surface oil stains → applying solder resist to the blank area (non-ribbed area) according to the core layer template + applying water-based graphite lubricant to the outer surface of the upper and lower layer panels and the inner cavity of the mold → resistance spot welding around the multi-directional cross-ribbed ribs (areas requiring diffusion connection) + sealing the periphery of the double-layer multi-directional cross-ribbed skin with argon arc welding (and welding the air inlet pipe) → diffusion connection of the spot-welded reinforced blank → superplastic forming → removing excess material.

[0016] This invention achieves near-net-shape blanking through precise blank unfolding in step 1, significantly improving material utilization compared to traditional machining and substantially reducing the raw material cost of expensive aluminum-lithium alloys. Steps 2-4 involve manually brushing solder resist onto a core layer template, replacing complex CNC coating equipment or photolithography processes with low-cost physical tooling, greatly reducing tooling investment. Steps 5-7 involve temporary fixing with resistance spot welding, sealing with argon arc welding, forming rib boundaries through diffusion bonding, and superplastic forming to bulge out the ribs in one go. The entire process is completed in the same thermal cycle, eliminating the need for multiple heat treatment annealings. Furthermore, the thermoforming equipment can form multiple parts at once, avoiding the long-term occupation of critical equipment such as five-axis gantry mills for one-by-one machining as in traditional machining. Therefore, the processing and manufacturing method of this invention can achieve efficient and low-cost forming of double-layer ribbed skins. Compared to existing thick plate integral milling techniques (first, the thick plate is circularized, the outer arc surface is machined, and then weight-reducing grooves and ribs are CNC machined, requiring multiple heat treatments and annealing to control deformation), this invention shortens the processing cycle from nearly a month to several hours or days, avoids the increased energy consumption and cycle time caused by multiple heat treatments, completely eliminates the problem of deformation during thin-walled machining, and transforms the traditional long process of "thick plate milling" into a short process of "unfolding and blanking + template definition + diffusion connection + superplastic forming," significantly shortening the manufacturing cycle. Moreover, this invention directly abandons the "milling" method for forming complex rib patterns with multiple intersecting directions (three or more directions), turning to a completely new technical route of "diffusion connection + superplastic forming," solving the problems of tool interference, complex programming, and uncontrolled stress deformation faced by CNC milling for complex rib patterns with multiple intersecting directions (three or more directions). The diffusion bonding and superplastic forming process of this invention mainly uses thermoforming equipment. After heating the mold, multiple parts can be formed at one time, avoiding the need for machining one by one on a machine tool, which is time-consuming, labor-intensive, and occupies a high proportion of critical equipment such as five-axis gantry machining centers.

[0017] 2. This invention, through step 1, uses approximate net-shape blanking. Based on the 3D model, the double-layer skin blank is unfolded into an upper and lower panel (1-5mm thick) that approximate the final shape, providing a precise shape reference for subsequent template positioning and avoiding cumulative positioning errors caused by blank size deviations. Step 2 involves creating a core layer template. Based on the shape of the multi-directional intersecting reinforcing ribs in the 3D model, a core layer template complementary to the rib pattern is created. The hollow / solid areas of the core layer template precisely correspond to the rib positions (solid areas correspond to ribs, hollow areas correspond to non-connected areas). Step 4 involves applying solder resist. The template is stacked on the lower panel of the blank, and solder resist is applied directly to the "blank areas" (i.e., non-rib areas). In this way, the solder resist precisely covers all areas where cavities will be formed, enabling the formation of multi-directional intersecting ribs. The invention provides a direct guarantee of "avoiding adhesion in areas that should not be connected," offering a low-cost, high-reliability manual method. It precisely defines the rib positions and non-connected areas. Then, through resistance spot welding reinforcement in step 5 and argon arc welding, a dense, non-porous, and crack-free peripheral sealing weld is formed, ensuring the airtightness of the sealed cavity. This provides a precise prefabricated assembly for diffusion connection, ensuring that the rib periphery area always maintains tight contact, preventing gaps or misalignment due to thermal expansion differences during heating within the mold. Combined with the diffusion connection in step 6, this firmly metallurgically bonds the upper and lower panels of the blank around the rib periphery, forming a "rib boundary," dividing the entire double-layer structure into a "connected area" (rib periphery) and a "area to be bulged" (…). The rib position provides precise constraints for superplastic forming – gas pressure cannot penetrate the connected metallurgical bonding zone, and can only cause the unconnected zone to bulge upwards, thus ensuring the accuracy of the rib shape and dimensional stability; during step 7 superplastic forming, the material grain boundary sliding ability is the strongest and the flow stress is the lowest, which can achieve a high elongation rate and form complex multi-directional cross ribs. The pressurization process is more stable, reducing the probability of defects in the forming process. Sufficient pressure and holding time overcome the material flow resistance and make the billet completely adhere to the mold; the excess material removal in step 8 completes the final finishing; the synergistic effect of each step realizes the efficient, low-cost, and high-precision integral forming of complex double-layer multi-directional cross rib skin, improving the yield and airtightness.

[0018] 3. Regardless of whether the ribs are bidirectional, tridirectional, or more complex intersecting patterns, this invention can precisely define the weld resistance area simply by changing the template, thereby forming a complex internal cavity rib network in one step during superplastic forming. It has strong process adaptability; this method can be easily extended to tridirectional, quadridirectional, and even more complex intersecting patterns, and is applicable to curved surfaces. 4. The upper and lower panels of the blank of the present invention have a thickness of 1-5mm. Within this thickness range, superplastic forming can bulge out ribs with sufficient rigidity to meet the load-bearing requirements of aerospace.

[0019] 5. The upper and lower panels of the blank in this invention are provided with two positioning holes, one round and the other oblong. The round hole provides the main positioning reference, and the oblong hole provides a unidirectional floating degree of freedom, compensating for differences in thermal expansion at high temperatures and sheet metal cutting errors. Together, they enable rapid and safe mold insertion under high-temperature environments, avoiding panel deformation or positioning pin jamming caused by forced assembly. This ensures precise alignment of the upper and lower panels around the ribs during spot welding, improving the diffusion connection and rib forming accuracy, and avoiding poor deformation accuracy caused by extensive machining. The design of the positioning holes is clear from the blank design stage in step 1 and continues through key processes such as cutting in step 3, spot welding reinforcement in step 5, and diffusion connection in step 6. Specifically, in step 3, when cutting according to the blank's external dimensions, both round holes and oblong holes are machined simultaneously; in step 5, during resistance spot welding reinforcement, the upper and lower panels of the blank are relatively fixed through positioning holes to ensure precise alignment of the upper and lower panels around the ribs during spot welding, preventing misalignment and ensuring tight contact in the diffusion connection area; in step 6, the blank with positioning holes is placed into the preheating mold and positioned with the mold by positioning pins, achieving rapid and safe mold entry under high temperature conditions, avoiding forced assembly that could cause panel deformation or positioning pin jamming; the oblong hole end allows for slight displacement during thermal expansion, eliminating internal stress.

[0020] 6. The method of this invention systematically controls deformation accuracy through multi-dimensional means: First, in step 1, the blank is precisely unfolded according to the three-dimensional model and the upper and lower layers of the blank are cut to near net thickness, avoiding residual stress release deformation caused by the removal of a large amount of material during thick plate milling from the source; Second, in step 2, a core layer template is made and combined with step 4, a welding resist is precisely applied, defining complex rib areas with low-cost tooling, ensuring accurate rib boundaries during superplastic forming, and avoiding local stress concentration caused by positional deviations; Step 4 applies water-based graphite lubricant to reduce friction between the blank and the mold at high temperatures, ensuring uniform material flow and mold adhesion; Step 5 optimizes resistance spot welding parameters (current 10~50KA, voltage 2~5KW, time 200~1000ms, spot gap 10~30mm) and argon arc welding parameters ( The current is 60~80A, the welding wire diameter is 2.0~4.0mm, and the gas flow rate is 6~18L / min. This forms a temporary fixation and airtight seal, and avoids thermal deformation caused by excessive welding. The diffusion connection in step 6 (450~490℃, 50~500 tons, 1~6 hours) is a solid metallurgical bond with no fusion welding thermal stress, and also eliminates interface unevenness. The superplastic forming in step 7 (450~480℃, stepped air passage 0.1~0.5MPa / 20min, with pressure holding for 28~32min after every 58~62min of air passage, maximum pressure 2~10MPa, final pressure holding for 30~120min) uses high temperature dynamic recovery to release stress in time, uses uniform air pressure loading to avoid cutting force impact, and eliminates springback by holding pressure to ensure complete mold adhesion. Finally, step 8 only removes a small amount of excess material. The above-mentioned methods work together to achieve a surface contour accuracy of ±0.3~0.5 mm / m and a diffusion welding rate of ≥95% in the embodiment, without the need for multiple stress-relief annealing, thus completely solving the technical problems of thin-walled materials being easily deformed and having poor precision in traditional machining processes.

[0021] 7. In the process of resistance spot welding reinforcement of the periphery of the multi-directional cross stiffeners in this invention, by controlling the resistance spot welding parameters (such as current 10-50KA, voltage 2-5KW, time 200-1000ms, spot gap 10-30mm, and setting the resistance spot welding parameter range), sufficient temporary weld strength is formed to fix the position, while avoiding excessive welding that could lead to changes in the material's microstructure or excessive thermal stress. After resistance spot welding, the upper and lower panels of the blank form a uniform, discontinuous spot connection around the stiffeners, preventing misalignment and avoiding the formation of large-area melting zones or severe heat-affected zones at the interface. This preserves a clean surface for subsequent diffusion bonding at 450-490℃, providing a precise prefabricated assembly for diffusion bonding, ensuring that the area around the stiffeners always maintains tight contact, and preventing gaps or misalignment due to thermal expansion differences during heating in the mold.

[0022] 8. In the process of sealing the periphery of the double-layer multi-directional cross-ribbed skin using argon arc welding, controlling the argon arc welding parameters (such as current 60-80A, tungsten inert wire diameter 2.0-4.0mm, shielding gas flow rate 6-18L / min, and setting the argon arc welding parameter range) can form a dense, non-porous, and crack-free peripheral sealing weld, ensuring the airtightness of the sealed cavity (in conjunction with the airtightness check in step 7); proper heat input control avoids the thin-walled panel from burning through or warping, protects the internal spot welds from being affected, ensures full weld penetration without generating an excessively large heat-affected zone, and achieves a welding rate of ≥95%.

[0023] 9. In this invention, the spot-welded reinforced billet is placed in a mold for diffusion bonding. The diffusion bonding parameters are controlled (e.g., temperature 450-490℃, pressure 50-500 tons, time 1-6 hours). Within this temperature range, the selected aluminum alloy / aluminum-lithium alloy exhibits high atomic diffusion activity while maintaining a solid state and fine-grained structure, forming a metallurgical bond with properties close to the base material. Sufficient pressure eliminates interfacial micro-irregularities, achieving close contact; sufficient time ensures full atomic diffusion. This avoids excessively low temperatures leading to incomplete oxide film breakage (weak bonding), and excessively high temperatures causing material softening, deformation, or adhesion. A closed, high-strength "rib boundary" is constructed inside the skin, dividing the entire double-layer structure into a "connected area" (rib perimeter) and a "bulging area" (rib location), providing precise constraints for superplastic forming—gas pressure cannot penetrate the connected metallurgical bond area, only causing the unconnected area to bulge upwards, thus ensuring the accuracy of the rib shape and dimensional stability. Ultimately, a diffusion bonding rate of ≥95% is achieved, improving the yield rate and production efficiency. The diffusion bonding of the present invention is carried out at 450-490°C, and atomic diffusion forms a metallurgical bond without melting. This avoids the need for multiple heat treatment annealings to reduce stress and control deformation, and eliminates the need for additional stress-relieving annealing.

[0024] 10. This invention involves heating the superplastic forming mold. Once the mold reaches the set temperature, a blank with welded ventilation pipes is placed into the mold. During the superplastic forming process, the superplastic forming parameters are controlled (temperature 450-480℃, ventilation rate 0.1-0.5MPa / 20min, pressure holding for 28-32min after every 58-62min of ventilation, reaching a pressure of 2-10MPa). This 450-480℃ temperature range maximizes the material's grain boundary sliding ability and minimizes flow stress, achieving high elongation and facilitating the filling of complex ribs. A ventilation rate of 0.1MPa-0.5MPa / 20min ensures uniform material deformation and reduces the likelihood of localized necking or voids. Gradual pressure increase through 55-65min of ventilation followed by 28-32min, rather than a sudden pressure increase to 2MPa, results in a smoother pressurization process, reduces the probability of defects during forming, and lowers the requirements for the forming equipment. The maximum air pressure is 2-10 MPa. After reaching the maximum pressure, it is held for 30-120 minutes. Sufficient pressure and holding time overcome the material flow resistance and ensure that the blank is completely attached to the mold. Ultimately, a welding rate of ≥95% is achieved, improving the pass rate and production efficiency.

[0025] The superplastic forming of this invention is performed at a high temperature of 450-480℃, which is close to the annealing temperature of the material. The effect is that the material undergoes dynamic recovery and recrystallization simultaneously during the forming process, and the small amount of stress generated during forming is released immediately at the high temperature. After forming, the part is in a stress-free or low-stress state, eliminating the need for additional stress-relieving annealing to reduce stress and control deformation.

[0026] This invention employs gas pressure (2-10 MPa) to drive the mold application in superplastic forming, providing a uniform and flexible loading method rather than the concentrated cutting force of a tool. The effect is that it avoids the localized cutting force impacts seen in milling, preventing localized deformation in thin-walled areas due to uneven stress distribution. Therefore, additional stress-relief annealing is unnecessary to reduce stress and control deformation.

[0027] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0028] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0029] Figure 1 This is a sample of the core layer of the lower skin of a double-layer cross-bidirectional skin; Figure 2 It is a straight-plate double-layer cross-shaped upper skin core layer template; Figure 3 It is a curved, double-layered, bidirectional upper skin core layer template; Figure 4 It is a straight-plate, double-layer, bidirectional, cross-layer upper skin core sample. Detailed Implementation

[0030] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0031] With the rapid development of the aerospace industry, increasing payload capacity has become an important aspect of the aerospace field, and lightweighting has become a development trend for aerospace components.

[0032] The presence of ribbed aluminum alloy skin parts maximizes weight reduction, while the crisscrossing ribs ensure the strength and rigidity meet usage requirements. The conventional machining method for double-layer, multi-directional ribbed skin parts involves first rounding a thick plate, then machining the outer curved surface, and finally using CNC machining to create the weight-reducing grooves and ribs. However, the weight-reducing cavities in the ribbed skin have very thin walls, making them prone to deformation and poor precision after machining. Multiple heat treatments and annealing processes are necessary to reduce stress and control deformation. This method is time-consuming and inefficient, requiring nearly a month to process a single skin part. Furthermore, it necessitates piece-by-piece machining on machine tools, which is labor-intensive and time-consuming, and occupies critical equipment such as five-axis gantry machining centers. More importantly, most existing processes are only designed for "simple hollow double-layer structures" (such as flat plates or single-curvature parts). For complex rib patterns with double-layer multi-directional intersections (three or more directions), CNC milling faces problems such as tool interference, complex programming, and uncontrolled stress deformation. The processing cycle is as long as several months, and the yield is extremely low, which cannot meet the engineering requirements of lightweight aerospace structures.

[0033] Therefore, the present invention provides a method for manufacturing a double-layer multi-directional cross-ribbed skin, comprising the following steps: Step 1: Based on the 3D model, unfold the blank of the double-layer multi-directional cross-ribbed skin, and determine the blank size of the double-layer multi-directional cross-ribbed skin according to the unfolded size; the blank is divided into the upper blank panel and the lower blank panel. Step 2: Make a core layer template according to the shape of the double-layer multi-directional cross stiffener in the three-dimensional model for subsequent application of solder resist. Step 3: Grind the edges and burrs according to the outer dimensions of the blank and the core layer template; pickle to remove surface oil. Step 4: Place the cut core layer template on the lower panel of the blank, apply solder resist to the blank areas according to the core layer template, and then apply water-based graphite lubricant to the outer surfaces of the upper panel and lower panel of the blank and the inner cavity of the mold. Step 5: According to the core layer template, the periphery of the multi-directional cross reinforcing ribs of the upper and lower layers of the blank is reinforced by resistance spot welding. The periphery of the double-layer multi-directional cross-ribbed skin is sealed by argon arc welding to form a sealed cavity, and an air inlet pipe is welded. Step 6: Place the diffusion bonding mold into the thermoforming equipment and heat the mold. Once the mold reaches the required temperature, place the spot-welded and reinforced blank into the mold for diffusion bonding. Step 7: Heat the superplastic forming mold. Once the mold reaches the set temperature, place the blank with the welded vent pipe into the mold and perform superplastic forming. Step 8: After superplastic forming is completed, remove the sample from the mold, draw lines on the surface of the sample according to the dimensions of the three-dimensional model, and cut off the excess material according to the drawn lines.

[0034] Furthermore, the thickness of both the upper and lower panels of the blank in step 1 is 1-5mm.

[0035] By unfolding the 3D model into a 2D blank, the required material dimensions can be accurately calculated. Cutting the blank to approximately net shape avoids the waste caused by traditional "thick plate milling" or large amounts of material removal due to dimensional deviations, significantly improving material utilization. The skin blank is divided into upper and lower panels, allowing for the placement of cross-reinforcing ribs in the middle area; a cavity is formed between the upper and lower panels, providing space for gas expansion during superplastic forming. Based on the 3D model, the double-layer skin blank is unfolded into a thin plate (1-5mm thick) close to the final shape, providing a precise external reference for subsequent template positioning and avoiding cumulative positioning errors caused by blank dimensional deviations. This is a prerequisite for realizing the "double-layer multi-directional cross-ribbed structure," ensuring accurate rib placement and reasonable distribution.

[0036] The thickness of the upper and lower panels of the billet is 1-5mm. This range is typical for thin-walled structures. Within this thickness range, superplastic forming can create ribs with sufficient rigidity to meet aerospace load-bearing requirements. Within this thickness range, the cross-reinforcing ribs created through subsequent superplastic forming provide sufficient rigidity and load-bearing capacity to meet the requirements of aerospace structural components. If the thickness of the upper and lower panels is below the lower limit (<1mm), the panel's own rigidity may be insufficient, limiting the rib height and affecting overall performance. If the thickness of the upper and lower panels exceeds the upper limit (>5mm), the weight reduction effect is significantly reduced, deviating from the initial goal of lightweighting. Examples include thicknesses of 1mm, 2mm, 3mm, 4mm, and 5mm.

[0037] Furthermore, the upper and lower panels of the billet are made of one of the following lightweight alloys: 5083 aluminum alloy, 2A97 aluminum-lithium alloy, or 5A90 aluminum-lithium alloy.

[0038] These three alloys possess two properties simultaneously within the process window (temperature 450-490℃) described in the invention. Excellent superplasticity: They achieve extremely high elongation and extremely low flow stress, allowing thin sheets to smoothly and precisely fill molds under air pressure, forming complex multi-directional intersecting ribs. Good solid-state diffusion bonding: Under high temperature and pressure, their high atomic diffusion activity enables the formation of high-quality metallurgical joints, avoiding welding defects.

[0039] Furthermore, the two positioning holes on the upper and lower panels of the blank are circular holes on one side and oblong holes on the other side, facilitating gap adjustment during mold insertion. The difference in radius between the coaxial hole and the oblong hole on the upper and lower panels is not less than 5mm.

[0040] The upper and lower panel of the billet have two positioning holes, one of which is a round hole and the other is an oblong hole (e.g., Figure 1 (As shown), this facilitates gap adjustment during mold placement. Precision alignment in high-temperature environments is both difficult and dangerous; striving for a "perfect fit" in assembly significantly reduces efficiency and increases operational complexity. Round hole end: Provides the primary positioning reference, defining a precise coordinate origin. Oblong hole end: Provides a unidirectional floating degree of freedom. At the instant the blank is placed into the high-temperature mold, it allows for a slight thermal expansion displacement or position adjustment along the oblong hole direction to eliminate the aforementioned differences. The round hole end provides the primary positioning reference, and the oblong hole end provides a unidirectional floating degree of freedom, compensating for thermal expansion differences and sheet metal cutting errors at high temperatures. This enables rapid and safe mold placement in high-temperature environments, avoiding forced assembly that could lead to panel deformation or jamming of the positioning pins. It ensures precise alignment of the upper and lower panels around the ribs during spot welding, improving the diffusion connection and rib forming accuracy.

[0041] This allows for a "spot welding first, then mold placement" process sequence. Step 5 involves resistance spot welding for reinforcement before placing the panels into the mold for diffusion bonding. This means that during spot welding, the upper and lower panels are already relatively fixed using the positioning holes. The difference in radius between the coaxial / oblong holes of the upper and lower panels is no less than 5mm, taking into account the difference in thermal expansion between the two layers when heated to 450-490℃ and the sheet metal cutting error. If the radius is less than 5mm, it may lead to difficulties in high-temperature mold placement.

[0042] Further, in step 2, a core layer template is made according to the shape of the multi-directional cross stiffeners in the 3D model for subsequent application of solder resist.

[0043] The solder resist pattern was ensured to match the design of the reinforcing ribs, thus precisely "reserving" channels for the ribs that would later bulge out on the lower panel. Based on the shape of the multi-directional intersecting reinforcing ribs in the 3D model, a "negative" template complementary to the rib pattern was created. The hollow / solid areas of the template precisely corresponded to the rib positions.

[0044] Further, in step 3, according to the dimensions of the blank and core layer template, grind the edges and burrs, requiring white gauze to check for any snagging threads; pickle to remove surface dirt, debris, water stains, oil stains, etc.

[0045] Specifically, in step 3, when cutting according to the blank's external dimensions, both round and oblong holes are machined simultaneously. The core layer template undergoes the same cutting and grinding process to ensure accurate dimensions and smooth edges. Pickling thoroughly removes grease, dirt, and natural oxide film from the blank surface, ensuring a clean surface.

[0046] Further, in step 4, the cut templates are stacked on the lower layer panel of the blank, and solder resist is applied to the blank areas in accordance with the core layer template. Water-based graphite lubricant is applied to the outer surfaces of the upper and lower layer panels of the blank and the inner cavity of the mold.

[0047] The template itself is "negative-dimensional" (the ribs are solid, and the blank areas are hollow or areas requiring solder resist). It is stacked on the lower panel, and solder resist is directly applied to the "blank areas" (i.e., non-rib areas). This ensures precise coverage of all areas where cavities will be formed, directly guaranteeing the formation of multi-directional intersecting ribs and preventing adhesion in areas that shouldn't be connected. It's a low-cost, highly reliable manual method.

[0048] Applying water-based graphite lubricant to the outer surfaces of the upper and lower panels of the billet lubricates the surface, significantly reducing friction between the billet and the mold cavity during subsequent superplastic forming. This ensures that the material flows evenly and smoothly to all parts of the mold under gas pressure, avoiding localized thinning, tearing, or incomplete forming caused by uneven friction. Mold cavity lubrication: Applying water-based graphite lubricant to the mold cavity primarily prevents adhesion or welding between the aluminum alloy and the mold steel at high temperatures, ensuring smooth demolding of the part after forming and protecting the expensive mold surface. Good lubrication is a crucial auxiliary factor for successful superplastic forming, directly affecting the quality, consistency, and mold life of the formed part. If solder resist is not applied to the blank areas of the reference core layer template, and water-based graphite lubricant is not applied to the outer surfaces of the upper and lower layer panels and the inner cavity of the mold, the non-rib areas inside will be welded together during diffusion bonding, resulting in no formed cavity. At high temperatures, the blank will stick to the mold, making demolding difficult. High friction will lead to uneven wall thickness and insufficient filling, which may result in the inability to form reinforcing ribs, scrapping of parts, and ultimately low diffusion bonding rate and low pass rate.

[0049] Further, in step 5, according to the template, resistance spot welding is performed to reinforce the periphery (i.e. the area that needs to be diffused and connected) of the multi-directional cross reinforcing ribs of the upper and lower panels of the blank. Argon arc welding is used to seal the periphery of the double-layer multi-directional cross-ribbed skin to form a sealed cavity, and an air inlet pipe is welded.

[0050] Furthermore, in step 5, the welding current of the resistance spot welding is adjusted according to the thickness of the skin panel, with a current range of 10-50KA, a voltage range of 2-5KW, a time range of 200-1000ms, and a spot welding gap range of 10-30mm.

[0051] Furthermore, in step 5, the argon arc welding current is 60-80A, the tungsten inert gas (TIG) wire diameter is 2.0-4.0mm, and the shielding gas flow rate is 6-18L / min.

[0052] Furthermore, in step 5, after the sealing of the welded vent pipe is completed, an airtightness check is performed to ensure that the sealing weld is leak-proof.

[0053] In step 5, during resistance spot welding reinforcement, the upper and lower panels of the billet are relatively fixed through positioning holes to ensure precise alignment of the upper and lower panels around the ribs during spot welding, preventing misalignment and ensuring tight contact in the diffusion connection area. Resistance spot welding is then applied to the periphery of the multi-directional cross-ribs (i.e., the area requiring diffusion connection). This temporarily fixes the upper and lower panels around the ribs, preventing misalignment during mold placement and heating, and ensuring tight contact in the diffusion connection area. Argon arc welding is used to seal the periphery of the double-layer multi-directional cross-ribbed skin, forming a sealed cavity. An air inlet pipe is then welded to create a sealed cavity, which is necessary for subsequent superplastic forming when gas is introduced to expand the material and form the ribs.

[0054] The welding current for resistance spot welding is adjusted according to the thickness of the skin panel, with a current range of 10-50 kA, a voltage range of 2-5 kW, a time range of 200-1000 ms, and a spot weld gap range of 10-30 mm. For argon arc welding, the current is 60-80 A, the tungsten inert gas (TIG) wire diameter is 2.0-4.0 mm, and the shielding gas flow rate is 6-18 L / min. Setting the parameter ranges for resistance spot welding and TIG welding aims to create sufficient temporary weld strength to fix the position, while avoiding over-welding that could alter the material's microstructure or generate excessive thermal stress. After resistance spot welding, the upper and lower panels of the blank form a uniform, discontinuous spot weld connection around the reinforcing ribs. This prevents misalignment and avoids large molten zones or severe heat-affected zones at the interface, preserving a clean surface for subsequent diffusion bonding at 450-490℃. ，This provides a precise prefabricated assembly for subsequent diffusion bonding, ensuring tight contact around the ribs and preventing gaps or misalignment due to thermal expansion differences during heating in the mold. The resistance spot welding current range is 10-50kA, and the voltage range is 2-5kW. Different currents and voltages are required for panels of different thicknesses (1-5mm). The time is 200-1000ms, with both time and current determining the total heat. Shorter times (200-400ms) with higher current are suitable for thinner plates, resulting in concentrated heat and a small heat-affected zone; longer times (600-1000ms) with lower current are suitable for thicker plates, ensuring sufficient heat transfer. The spot gap is 10-30mm; spot bonding according to these parameters reduces the difficulty of subsequent diffusion welding.

[0055] The purpose of argon arc welding is to completely seal the perimeter of the double-layer skin and weld the gas inlet pipe to form a sealed cavity for subsequent superplastic forming. The weld must be dense, leak-proof, and have a minimal heat-affected zone to prevent warping of the surrounding panels. The welding current for argon arc welding is 60-80A, providing the heat to melt the base material and welding wire. The diameter of the tungsten inert gas (TIG) wire is 2.0-4.0mm, which determines the wire feed rate and the cross-section of the weld filler metal. The shielding gas flow rate is 6-18 L / min, with argon gas protecting the weld area. This process creates a dense, non-porous, and crack-free peripheral sealing weld, ensuring the airtightness of the sealed cavity (in conjunction with the airtightness check in step 7). Proper heat input control prevents the thin-walled panel from burning through or warping, protects internal spot welds from impact, ensures full weld penetration without an excessively large heat-affected zone, and achieves a weld success rate of ≥95%.

[0056] If the welding current, voltage, or time is too high in resistance spot welding, or if the current, tungsten inert gas (TIG) wire diameter, or shielding gas flow rate is too high in TIG welding, it may cause significant welding deformation around the skin, affecting the fit with the mold cavity, resulting in poor forming effect, failure to form reinforcing ribs, and wasted energy. Ultimately, this leads to a low diffusion weld rate and a low pass rate. Conversely, if the current, voltage, or time is too low in resistance spot welding, or if the current, tungsten inert gas (TIG) wire diameter, or shielding gas flow rate is too low in TIG welding, the weld may not form a dense bond across the entire plate thickness, resulting in incomplete fusion or internal defects. This reduces production efficiency and ultimately leads to a low diffusion weld rate and a low pass rate.

[0057] Therefore, the welding current for resistance spot welding is adjusted according to the thickness of the skin panel, with a current range of 10-50KA, a voltage range of 2-5KW, a time range of 200-1000ms, and a spot weld gap range of 10-30mm; the argon arc welding current is 60-80A, the tungsten inert gas (TIG) wire diameter is 2.0-4.0mm, and the shielding gas flow rate is 6-18L / min. For example, the welding current for resistance spot welding is 10KA, 15KA, 20KA, 25KA, 30KA, 35KA, 40KA, 45KA, and 50KA; the voltage is 2KW, 2.5KW, 3KW, 4KW, 4.5KW, and 5KW; and the time is 200ms, 300ms, 400ms, 500ms, 600ms, 700ms, 800ms, and 900ms. ms, 1000ms; tack weld gaps of 10mm, 20mm, 30mm; TIG welding current of 60 A, 61 A, 62 A, 63 A, 64 A, 65 A, 66 A, 67 A, 68 A, 69 A, 70 A, 71 A, 72 A, 73 A, 74 A, 75 A, 76 A, 77 A, 78 A, 79 A, ​​80A; tungsten inert gas (TIG) wire diameter of 2.0mm, 3.0mm, 4.0mm; shielding gas flow rate of 6 L / min, 7 L / min, 8 L / min, 9 L / min, 10 L / min, 11 L / min, 12 L / min, 13 L / min, 14 L / min, 15 L / min, 16 L / min, 17 L / min, 18 L / min.

[0058] Specifically, in step 5, resistance spot welding and argon arc welding form a dense, non-porous, and crack-free peripheral sealing weld, ensuring the airtightness of the sealed cavity. This provides a precise prefabricated assembly for diffusion connection, ensuring that the area around the ribs always maintains close contact and preventing gaps or misalignments due to thermal expansion differences during heating in the mold. Together with the diffusion connection in the subsequent step 6, the upper and lower panels are firmly metallurgically bonded to the area around the ribs, forming the "rib boundary".

[0059] Furthermore, in step 6, the temperature range for diffusion bonding is 450-490℃, the pressure for diffusion bonding is 50-500 tons, and the diffusion bonding time is 1-6 hours.

[0060] Furthermore, in step 6, for planar structures, a self-made steel bag needs to be placed inside and laid flat on the blank.

[0061] Preheating the mold ensures that the billet is placed in a uniform and stable thermal environment. The mold's thermal radiation and conduction allow the billet to be heated quickly and evenly to the target temperature. For planar structures, a self-made steel bag is placed flat on the billet to make the surface smoother and facilitate diffusion bonding. In step 6, the billet with positioning holes is placed into the preheated mold and positioned with the mold by positioning pins, achieving rapid and safe mold entry under high temperature conditions. This avoids forced assembly that could cause panel deformation or jamming of the positioning pins. The elongated hole allows for slight displacement during thermal expansion, eliminating internal stress.

[0062] In step 6, the diffusion bonding temperature is 450-490℃, the pressure is 50-500 tons, and the time is 1-6 hours. Within this temperature range, the selected aluminum alloy / aluminum-lithium alloy exhibits high atomic diffusion activity while maintaining a solid state and fine-grained structure, forming a metallurgical bond with properties close to the base material. Sufficient pressure eliminates interfacial micro-irregularities, achieving close contact; sufficient time ensures full atomic diffusion. This avoids excessively low temperatures that could lead to incomplete oxide film breakage (weak bonding), and excessively high temperatures that could cause the material to become too soft, deform, or adhere. Ultimately, a diffusion bonding rate of ≥95% is achieved, improving the yield and production efficiency. The diffusion bonding of this invention is performed at 450-490℃, where atomic diffusion forms a metallurgical bond without melting, avoiding the need for multiple heat treatment annealing processes to reduce stress and control deformation. No additional stress-relief annealing is required to reduce stress and control deformation.

[0063] Step 6 firmly metallurgically bonds the upper and lower panels around the ribs, forming a "rib boundary." This divides the entire double-layer structure into a "connected area" (around the ribs) and a "bulging area" (at the rib location), providing precise constraints for superplastic forming. Gas pressure cannot penetrate the connected metallurgically bonded area, only causing the unconnected area to bulge upwards, thus ensuring the accuracy of the rib shape and dimensional stability. Diffusion bonding ensures sufficient atomic diffusion to form a metallurgical bond, creating a joint with properties and microstructure very close to the base material. The temperature range of 450-490℃ fully activates atomic diffusion in the selected aluminum / aluminum-lithium alloy while preventing material melting or the formation of harmful liquid phases. The temperature is high enough to overcome the diffusion barrier, yet low enough to maintain the material's solid state and fine grain structure. Sufficient pressure (50-500 tons) eliminates microscopic irregularities at the bonding interface, ensuring close contact between the upper and lower panels. The diffusion bonding time is 1-6 hours, ensuring sufficient time for atoms to cross the interface and form a sufficiently thick diffusion layer. If the diffusion bonding temperature, pressure, or time is too high, or the material is too soft, unexpected overall deformation may occur under pressure, or the material may stick to the mold, resulting in poor forming effect, inability to form reinforcing ribs, and ultimately a low diffusion bonding rate and low pass rate. If the diffusion bonding temperature is too low, pressure too low, or time too short, the oxide film may not be broken down, and the surface oxide film may hinder bonding, forming a weak bonding interface, failing to achieve effective metallurgical bonding, and ultimately resulting in a low diffusion bonding rate and low pass rate.

[0064] For example, the diffusion connection temperature is 450℃, 455℃, 460℃, 465℃, 470℃, 475℃, 480℃, 485℃, and 490℃, the diffusion connection pressure is 50 tons, 100 tons, 150 tons, 200 tons, 250 tons, 300 tons, 350 tons, 400 tons, 450 tons, and 500 tons, and the diffusion connection time is 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, and 6 hours.

[0065] Furthermore, in step 7, the temperature range for superplastic forming is 450-480℃, the air flow rate is 0.1MPa-0.5MPa / 20min, the pressure is held for 28-32min after every 55-65min of air flow, and the maximum air pressure is 2-10MPa.

[0066] Furthermore, in step 7, after the superplastic forming reaches the maximum pressure, maintain the pressure for 30-120 minutes.

[0067] The superplastic forming mold is heated to the specified temperature. Once heated, a blank with welded ventilation pipes is placed into the mold for superplastic forming. This ensures the material instantly enters and maintains its optimal superplastic state, achieving precise and uniform forming. The superplastic forming temperature range in step 7 is 450-480℃. Within this range, the material's grain boundary sliding ability is strongest, flow stress is lowest, and high elongation can be achieved, enabling the forming of complex multi-directional cross ribs. Superplastic forming is performed at a high temperature of 450-480℃, which is close to the material's annealing temperature. Effects: The material undergoes dynamic recovery and recrystallization simultaneously during forming, and the small amount of stress generated during forming is instantly released at high temperatures. After forming, the part is in a stress-free or low-stress state, eliminating the need for additional stress-relieving annealing to reduce stress and control deformation. The ventilation rate is 0.1MPa-0.5MPa / 20min. Forming at this rate results in uniform material deformation and reduces the likelihood of localized necking or voids. The pressure is gradually increased by holding the gas for 28-32 minutes after each 55-65 minutes of ventilation, rather than increasing the pressure to 2 MPa all at once. This ensures a smoother pressurization process, reduces the probability of defects during forming, and lowers the requirements for forming equipment. The maximum gas pressure is 2-10 MPa, and after reaching the maximum pressure, it is held for 30-120 minutes. Sufficient pressure and holding time overcome material flow resistance and ensure complete mold adhesion of the blank. This invention uses gas pressure (2-10 MPa) to drive mold adhesion, which is a uniform and flexible loading method, rather than the concentrated cutting force of a tool. The effect is that it avoids the localized cutting force impacts seen in milling, preventing localized deformation in thin-walled areas due to uneven stress. Therefore, additional stress-relieving annealing is unnecessary to reduce stress and control deformation. Ultimately, a diffusion bonding rate of ≥95% is achieved, improving the yield rate and production efficiency.

[0068] If the superplastic forming temperature, air flow rate, air pressure, or holding time is too high, the superplasticity of the material may actually degrade, accompanied by high-temperature brittleness, making it prone to cracking in the later stages of deformation, resulting in a low diffusion weld rate and a low pass rate. Alternatively, if the material is too soft, demolding may be difficult, potentially leading to unexpected overall deformation under pressure or adhesion to the mold, resulting in poor forming effects and the inability to form reinforcing ribs. If the superplastic forming temperature is too low, the air flow rate is too low, the air pressure is too low, or the holding time is too short, the superplasticity may not be fully activated, resulting in high material flow stress and insufficient elongation, leading to incomplete filling of ribs, incomplete filling at corners, or brittle fracture in the middle of deformation, ultimately resulting in a low diffusion weld rate and low production efficiency.

[0069] For example, the superplastic forming temperatures are 450℃, 455℃, 460℃, 465℃, 470℃, 475℃, and 480℃, the air permeation rates are 0.1 MPa / 20min, 0.2 MPa / 20min, 0.3 MPa / 20min, 0.4 MPa / 20min, and 0.5 MPa / 20min, and the maximum air pressures are 2 MPa, 3 MPa, 4 MPa, 5 MPa, 6 MPa, 7 MPa, 8 MPa, 9 MPa, and 10 MPa, and the pressure is held for 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, 100 min, 110 min, and 120 min after reaching the maximum pressure.

[0070] The diffusion bonding in step 6 and the superplastic forming in step 7 of this invention mainly use thermoforming equipment. After heating the mold, multiple parts can be formed at one time, avoiding the need for machining one by one on a machine tool, which is time-consuming and labor-intensive, and also avoids the technical problem of high occupancy of key equipment such as five-axis gantry machining centers.

[0071] Further, in step 8, after superplastic forming is completed, the sample is removed from the mold, lines are drawn on the surface of the sample according to the dimensions of the three-dimensional model, and the excess material is cut off according to the drawn lines.

[0072] Step 8, the removal of excess material, completes the final finishing process. This step, through drawing lines based on the 3D model, removes only a small amount of excess material, providing precise dimensional basis for subsequent cutting. This process transforms the near-net-shape part with excess material into a "net part" that fully conforms to the design dimensions, meeting the stringent dimensional requirements of aerospace components.

[0073] This invention employs the following steps: Step 1 involves cutting the material to approximately net shape, unfolding the double-layer skin blank into a thin plate (1-5mm thick) close to the final shape based on a 3D model. This provides a precise external reference for subsequent template positioning, avoiding cumulative positioning errors caused by blank size deviations. Step 2 involves creating a core layer template. Based on the shape of the multi-directional intersecting reinforcing ribs in the 3D model, a "negative" template complementary to the rib pattern is created. The hollow / solid areas of the template precisely correspond to the rib positions (solid areas correspond to ribs, hollow areas correspond to non-connected areas). Step 4 involves applying solder resist, stacking it on the lower layer panel, and directly applying the solder resist to the "blank areas" (i.e., non-rib areas). In this way, the solder resist precisely covers all areas where cavities will be formed, enabling the formation of multi-directional intersecting ribs. The invention provides a direct guarantee of "avoiding adhesion in areas that should not be connected," offering a low-cost, high-reliability manual method. It precisely defines the rib positions and non-connected areas. Then, through resistance spot welding reinforcement in step 5 and argon arc welding to form a dense, non-porous, and crack-free peripheral sealing weld, the invention ensures the airtightness of the sealed cavity, providing a precise prefabricated assembly for diffusion connection. This ensures that the rib periphery area always maintains tight contact, preventing gaps or misalignment due to thermal expansion differences during heating within the mold. Combined with the diffusion connection in step 6, this firmly metallurgically bonds the upper and lower panels around the rib periphery, forming the "rib boundary," dividing the entire double-layer structure into the "connected area" (rib periphery) and the "to be shaped" area. The "expansion zone" (rib location) provides precise constraints for superplastic forming—gas pressure cannot penetrate the connected metallurgical bonding zone, but can only cause the unconnected zone to bulge upwards, thus ensuring the accuracy of the rib shape and dimensional stability; during step 7 superplastic forming, the material grain boundary sliding ability is strongest and the flow stress is lowest, enabling high elongation and forming complex multi-directional cross ribs. The pressurization process is smoother, reducing the probability of defects in the forming process. Sufficient pressure and holding time overcome the material flow resistance and allow the billet to completely adhere to the mold; the excess material removal in step 8 completes the final finishing; the synergistic effect of each step achieves efficient, low-cost, and high-precision integral forming of complex double-layer multi-directional cross rib skin.

[0074] This invention, through the synergistic effect of the above eight steps, solves the technical problems of long machining cycles, low efficiency, low material utilization, occupation of critical equipment such as five-axis gantry milling for thick plates, and the need for multiple heat treatments and poor machining deformation accuracy for thin-walled plates that are prone to deformation. It transforms the traditional long process of "thick plate milling" into a shorter process including "unfolding and blanking + template definition + diffusion bonding + superplastic forming," significantly shortening the manufacturing cycle. 。First, step 1 involves precisely unfolding the billet based on the 3D model to achieve near-net-shape cutting and significantly improve material utilization. Step 2 involves creating a core layer template, and then, in conjunction with step 4, precisely applying solder resist to the template stack, defining the area of ​​complex multi-directional cross ribs using low-cost tooling. Step 5 involves temporarily fixing the upper and lower layers around the ribs using resistance spot welding to prevent misalignment, followed by sealing the perimeter with argon arc welding and welding an air inlet pipe to form a sealed cavity. Step 6 involves diffusion bonding under high temperature and pressure to ensure a reliable metallurgical bond around the ribs, while preventing adhesion in the non-connected areas protected by the solder resist. Step 7 involves introducing high-pressure gas into the sealed cavity through superplastic forming, causing the unconnected areas to bulge upwards, forming multi-directional cross reinforcing ribs in one step. Finally, step 8 involves removing excess material to obtain the finished product. The above steps are interconnected: precise blanking and template definition ensure the accuracy of rib position; spot welding and argon arc welding provide temporary fixation and airtight conditions; diffusion bonding constructs the boundary constraints of the ribs; and superplastic forming utilizes the superplasticity of the material to efficiently bulge the ribs as a whole. Together, they achieve the overall technical effect of "high efficiency, low cost, and integral forming of complex double-layer multi-directional cross-ribbed skin," significantly shortening the manufacturing cycle, improving production efficiency (welding rate ≥95%), reducing the occupation of key equipment, and solving the technical problems of needing multiple heat treatment annealing and poor machining deformation accuracy.

[0075] To more clearly describe the present invention, the following embodiments and comparative examples are provided for further illustration.

[0076] Example 1: This example provides a manufacturing method for a straight-plate double-layer cross-shaped bidirectional skin: Step 1: Based on the UG 3D model provided in the design, perform 2D unfolding of the blank for the double-layer ribbed curved skin. Determine the blank dimensions according to the unfolded dimensions. The blank is divided into an upper panel and a lower panel, both made of 5083 aluminum alloy. The difference in radius between the coaxial hole and the oblong hole of the upper and lower panels should not be less than 5mm.

[0077] Step 2: Create a core layer template based on the shape of the reinforcing ribs in the 3D model. The template should look like this: Figure 2 As shown, this is for subsequent application of solder resist.

[0078] Step 3: Grind the edges and corners according to the dimensions of the blank and core layer template, ensuring that there are no protruding slags on the surface and around it; grind the burrs, ensuring that no filaments are caught when checked with white gauze; pickle to remove surface dirt, debris, water stains, oil stains, etc., to ensure that the surface is clean.

[0079] Step 4: Place the cut templates on the lower layer of the blank, apply solder resist to the blank areas according to the core template, and apply water-based graphite lubricant to the outer side of the upper and lower skin panels of the blank.

[0080] Step 5: According to the template, perform resistance spot welding to reinforce the periphery of the reinforcing ribs (i.e., the area requiring diffusion connection). The current is 15KA, the voltage is 2.4KW, the time is 400ms, and the gap between spot welds is 20mm. After laser welding, seal the periphery of the double-layer plate with argon arc welding to form a sealed cavity, and weld the gas inlet pipe. The argon arc welding current is 70A, the tungsten inert gas wire diameter is 3.0mm, and the shielding gas flow rate is 20L / min.

[0081] Step 6: Place the diffusion bonding mold into the thermoforming equipment and heat the mold. The diffusion bonding temperature is 460℃. After reaching the temperature, place the spot-welded and reinforced blank into the mold. For planar structures, a self-made steel bag is also needed. Place the bag flat on the blank and perform diffusion bonding. The diffusion bonding pressure is 80 tons and the time is 3.5 hours.

[0082] Step 7: Heat the superplastic forming mold to 470℃. After reaching the temperature, place the blank with the welded vent pipe into the mold for superplastic forming. The venting rate is 0.2MPa / 20min. Hold the pressure for 30min after every 60min of venting until the pressure reaches 5MPa, and then hold the pressure for another 60min.

[0083] Step 8: After superplastic forming is completed, remove the sample from the mold and draw lines on the surface of the sample according to the dimensions of the three-dimensional model. For double-layer planar skin, cut off the excess material and grind the edges according to the drawn lines.

[0084] Example 2: This example provides a method for manufacturing a curved double-layer cross-shaped bidirectional skin: Step 1: Based on the UG 3D model provided in the design, perform 2D unfolding of the blank for the double-layer ribbed curved skin. Determine the blank dimensions of the double-layer ribbed skin according to the unfolded dimensions. The blank is divided into an upper panel and a lower panel, both made of 2A97 aluminum-lithium alloy. The difference in radius between the coaxial hole and the oblong hole of the upper and lower panels should not be less than 5mm.

[0085] Step 2: Create a core layer template based on the shape of the reinforcing ribs in the 3D model. The template should look like this: Figure 3 As shown, this is for subsequent application of solder resist.

[0086] Step 3: Grind the edges and corners according to the dimensions of the blank and core layer template, ensuring that there are no protruding slags on the surface and around it; grind the burrs, ensuring that no filaments are caught when checked with white gauze; pickle to remove surface dirt, debris, water stains, oil stains, etc., to ensure that the surface is clean.

[0087] Step 4: Place the cut templates on the lower layer of the blank, apply solder resist to the blank areas according to the core template, and apply water-based graphite lubricant to the outer side of the upper and lower layers of the blank.

[0088] Step 5: According to the template, perform resistance spot welding to reinforce the periphery of the reinforcing ribs (i.e., the area requiring diffusion connection). The current is 30KA, the voltage range is 4KW, the time range is 600ms, and the spot welding gap is 30mm. After laser welding, seal the periphery of the double-layer plate with argon arc welding to form a sealed cavity, and weld the gas inlet pipe. The argon arc welding current is 80A, the tungsten inert gas wire diameter is 4.0mm, and the shielding gas flow rate is 18L / min.

[0089] Step 6: Place the diffusion bonding mold into the thermoforming equipment and heat the mold to the set temperature. Then, place the spot-welded and reinforced blank into the mold for diffusion bonding. The diffusion bonding temperature is 480℃, the diffusion bonding pressure is 300 tons, and the time is 5 hours. For planar structures, a self-made steel bag needs to be placed on top of the blank.

[0090] Step 7: Heat the superplastic forming mold to 480℃. After reaching the temperature, place the blank with the welded vent pipe into the mold for superplastic forming. The superplastic forming rate is 0.5MPa / 20min. After venting for 62min, hold the pressure for 32min until the pressure reaches 7MPa, and then hold the pressure for 80min.

[0091] Step 8: After superplastic forming is completed, remove the sample from the mold, draw lines on the surface of the sample according to the dimensions of the three-dimensional model, and cut off the excess material and grind the edges according to the drawn lines.

[0092] Example 3 This example provides a manufacturing method for a straight-plate double-layer cross-shaped bidirectional skin: Step 1: Based on the UG 3D model provided in the design, perform 2D unfolding of the blank for the double-layer ribbed curved skin. Determine the blank dimensions according to the unfolded dimensions. The blank consists of an upper panel and a lower panel. The thickness of the upper panel is 1mm, and the thickness of the lower panel is 5mm. Both the upper and lower panels are made of 5A90 aluminum-lithium alloy. The difference in radius between the coaxial holes and oblong holes in the upper and lower panels is not less than 5mm.

[0093] Step 2: Create a core layer template based on the shape of the reinforcing ribs in the 3D model. The template should look like this: Figure 4 As shown, this is for subsequent application of solder resist.

[0094] Step 3: Grind the edges and corners according to the dimensions of the blank and core layer template, ensuring that there are no protruding slags on the surface and around it; grind the burrs, ensuring that no filaments are caught when checked with white gauze; pickle to remove surface dirt, debris, water stains, oil stains, etc., to ensure that the surface is clean.

[0095] Step 4: Place the cut templates on the lower layer of the blank, apply solder resist to the blank areas according to the core template, and apply water-based graphite lubricant to the outer sides of the upper and lower layers of the blank.

[0096] Step 5: According to the template, perform resistance spot welding to reinforce the periphery of the reinforcing ribs (i.e., the area requiring diffusion connection). The current is 45KA, the voltage is 4.8KW, the time is 800ms, and the gap between spot welds is 30mm. After laser welding, seal the periphery of the double-layer plate with argon arc welding to form a sealed cavity, and weld the gas inlet pipe. The argon arc welding current is 80A, the tungsten inert gas wire diameter is 4.0mm, and the shielding gas flow rate is 18L / min.

[0097] Step 6: Place the diffusion bonding mold into the thermoforming equipment and heat the mold. The diffusion bonding temperature is 490℃. After reaching the temperature, place the spot-welded and reinforced blank into the mold. For planar structures, a self-made steel bag is also needed. Place the bag flat on the blank and perform diffusion bonding. The diffusion bonding pressure is 400 tons and the time is 5.5 hours.

[0098] Step 7: Heat the superplastic forming mold to 475℃. After reaching the temperature, place the blank with the welded vent pipe into the mold for superplastic forming. The venting rate is 0.45MPa / 20min. Hold the pressure for 31min after every 61min of venting, until the pressure reaches 9MPa, and then hold the pressure for 100min.

[0099] Step 8: After superplastic forming is completed, remove the sample from the mold and draw lines on the surface of the sample according to the dimensions of the three-dimensional model. For double-layer planar skin, cut off the excess material and grind the edges according to the drawn lines.

[0100] Example 4 This example provides a manufacturing method for a straight-plate double-layer cross-shaped bidirectional skin: Step 1: Based on the UG 3D model provided in the design, perform 2D unfolding of the blank for the double-layer ribbed curved skin. Determine the blank dimensions according to the unfolded dimensions. The blank consists of an upper panel and a lower panel. The thickness of the upper panel is 1mm, and the thickness of the lower panel is 5mm. Both the upper and lower panels are made of 5A90 aluminum-lithium alloy. The difference in radius between the coaxial holes and oblong holes in the upper and lower panels is not less than 5mm.

[0101] Step 2: Create a core layer template based on the shape of the reinforcing ribs in the 3D model. The template should look like this: Figure 4 As shown, this is for subsequent application of solder resist.

[0102] Step 3: Grind the edges and corners according to the dimensions of the blank and core layer template, ensuring that there are no protruding slags on the surface and around it; grind the burrs, ensuring that no filaments are caught when checked with white gauze; pickle to remove surface dirt, debris, water stains, oil stains, etc., to ensure that the surface is clean.

[0103] Step 4: Place the cut templates on the lower layer of the blank, apply solder resist to the blank areas according to the core template, and apply water-based graphite lubricant to the outer sides of the upper and lower layers of the blank.

[0104] Step 5: According to the template, perform resistance spot welding to reinforce the periphery of the reinforcing ribs (i.e., the area requiring diffusion connection). The current is 42KA, the voltage is 4.6KW, the time is 500ms, and the gap between spot welds is 26mm. After laser welding, seal the periphery of the double-layer plate with argon arc welding to form a sealed cavity, and weld the gas inlet pipe. The argon arc welding current is 72A, the tungsten inert gas wire diameter is 3.8mm, and the shielding gas flow rate is 17L / min.

[0105] Step 6: Place the diffusion bonding mold into the thermoforming equipment and heat the mold. The diffusion bonding temperature is 450℃. After reaching the temperature, place the spot-welded and reinforced blank into the mold. For planar structures, a self-made steel bag is also needed. Place the bag flat on the blank and perform diffusion bonding. The diffusion bonding pressure is 50 tons and the time is 1.5 hours.

[0106] Step 7: Heat the superplastic forming mold to 450℃. After reaching the temperature, place the blank with the welded vent pipe into the mold for superplastic forming. The venting rate is 0.41MPa / 20min. Hold the pressure for 30min after every 60min of venting until the pressure reaches 2MPa, and then hold the pressure for another 35min.

[0107] Step 8: After superplastic forming is completed, remove the sample from the mold and draw lines on the surface of the sample according to the dimensions of the three-dimensional model. For double-layer planar skin, cut off the excess material and grind the edges according to the drawn lines.

[0108] Comparative Example 1 Step 5: Based on the core layer template, perform resistance spot welding to reinforce the periphery of the reinforcing ribs (i.e., the area requiring diffusion connection). The current is 60KA, the voltage is 6KW, the time is 1100ms, and the spot welding gap is 40mm. The rest is the same as in Example 1.

[0109] Comparative Example 2 Step 5: Seal the perimeter of the laser-welded double-layer panel using argon arc welding to form a sealed cavity, and weld an inlet pipe. The argon arc welding current is 90A, the tungsten inert wire diameter is 5.0mm, and the shielding gas flow rate is 20L / min. The rest is the same as in Example 1.

[0110] Comparative Example 3 Step 6: Place the diffusion bonding mold into the thermoforming equipment and heat the mold. The diffusion bonding temperature is 500℃. After reaching the temperature, place the spot-welded and reinforced blank into the mold. For planar structures, a self-made steel bag is also needed. Place the bag flat on the blank and perform diffusion bonding. The diffusion bonding pressure is 600 tons and the time is 7 hours.

[0111] Comparative Example 4 Step 7: Heat the superplastic forming mold to 500℃. Once the temperature is reached, place the blank with the welded vent pipe into the mold for superplastic forming. The superplastic forming rate is 0.6MPa / 20min, with venting for 65min followed by pressure holding for 35min, until the pressure reaches 11MPa, then hold for 130min. The rest is the same as in Example 1.

[0112] Table 1 Performance comparison of Examples 1-4 and Comparative Examples 1-4

[0113] The comparison data of Examples 1-4 and Comparative Examples 1-4 in Table 1 clearly show that when using the process parameter range defined by the present invention (diffusion bonding temperature 450-490℃, pressure 50-500 tons, time 1-6 hours; superplastic forming temperature 450-480℃, pressure 2-10MPa, holding pressure 30-120min), Examples 1-4 can stably achieve highly reliable metallurgical bonding (welding rate ≥95%) and high dimensional accuracy (within ±0.5 mm / m).

[0114] In Comparative Examples 1-2, excessive welding current, voltage, and time in step 5 of resistance spot welding, as well as excessive argon arc welding current, tungsten inert gas (TIG) wire diameter, and shielding gas flow rate, can cause significant welding deformation around the skin, affecting its fit with the mold cavity, resulting in poor forming and failure to form reinforcing ribs. Ultimately, this leads to a low diffusion weld rate and a low pass rate.

[0115] In Comparative Example 3, step 6 involved excessively high diffusion bonding temperature, pressure, and time, and the material was too soft. This could lead to unexpected overall deformation under pressure or adhesion to the mold, resulting in poor forming and failure to form reinforcing ribs. Ultimately, the diffusion bonding rate was low, and the pass rate was low.

[0116] In Comparative Example 4, step 7 of the superplastic forming diffusion bonding process involved excessively high temperature, pressure, and time, and the material was too soft. This could lead to unexpected overall deformation under pressure or adhesion to the mold, resulting in poor forming performance and the inability to form reinforcing ribs. Ultimately, the diffusion bonding rate was low, and the pass rate was low.

[0117] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method of manufacturing a double-skin multi-directional cross-ribbed skin, characterized by, Includes the following steps: Step 1: Based on the 3D model, unfold the blank of the double-layer multi-directional cross-ribbed skin, and determine the blank size of the double-layer multi-directional cross-ribbed skin according to the unfolded size; the blank is divided into the upper blank panel and the lower blank panel. Step 2: Make a core layer template according to the shape of the double-layer multi-directional cross stiffener in the three-dimensional model for subsequent application of solder resist. Step 3: Grind the edges and burrs according to the outer dimensions of the blank and the core layer template; pickle to remove surface oil. Step 4: Place the cut core layer template on the lower panel of the blank, apply solder resist to the blank areas according to the core layer template, and then apply water-based graphite lubricant to the outer surfaces of the upper panel and lower panel of the blank and the inner cavity of the mold. Step 5: According to the core layer template, the periphery of the multi-directional cross reinforcing ribs of the upper and lower layers of the blank is reinforced by resistance spot welding. The periphery of the double-layer multi-directional cross-ribbed skin is sealed by argon arc welding to form a sealed cavity, and an air inlet pipe is welded. Step 6: Place the diffusion bonding mold into the thermoforming equipment and heat the mold. Once the mold reaches the required temperature, place the spot-welded and reinforced blank into the mold for diffusion bonding. Step 7: Heat the superplastic forming mold. Once the mold reaches the set temperature, place the blank with the welded vent pipe into the mold and perform superplastic forming. Step 8: After superplastic forming is completed, remove the sample from the mold, draw lines on the surface of the sample according to the dimensions of the three-dimensional model, and cut off the excess material according to the drawn lines.

2. The preparation method according to claim 1, characterized in that, In step 1, the thickness of the upper panel and the lower panel of the blank is 1-5mm.

3. The preparation method according to claim 2, characterized in that, The upper and lower panels of the billet are made of one of the following lightweight alloys: 5083 aluminum alloy, 2A97 aluminum-lithium alloy, and 5A90 aluminum-lithium alloy.

4. The preparation method according to claim 2, characterized in that, The two positioning holes on the upper panel and lower panel of the blank are round holes on one side and oblong holes on the other side.

5. The preparation method according to claim 1, characterized in that, In step 5, the welding current of resistance spot welding is adjusted according to the thickness of the upper and lower panels of the blank. The current range is 10-50KA, the voltage range is 2-5KW, the time range is 200-1000ms, and the spot welding gap range is 10-30mm.

6. The preparation method according to claim 1, characterized in that, In step 5, the argon arc welding current is 60-80A, the tungsten inert gas (TIG) wire diameter is 2.0-4.0mm, and the shielding gas flow rate is 6-18L / min.

7. The preparation method according to claim 1, characterized in that, In step 6, the temperature range for diffusion bonding is 450-490℃, the pressure for diffusion bonding is 50-500 tons, and the diffusion bonding time is 1-6 hours.

8. The preparation method according to claim 1, characterized in that, In step 7, the temperature range for superplastic forming is 450-480℃.

9. The preparation method according to claim 1, characterized in that, In step 7, the ventilation rate of superplastic forming is 0.1MPa-0.5MPa / 20min, and the pressure is maintained for 28-32min after every 55-65min of ventilation until the pressure reaches 2-10MPa.

10. The preparation method according to claim 1, characterized in that, In step 7, after the superplastic forming reaches the maximum pressure, the pressure is held for 30-120 minutes.

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