Methods for eliminating surface defects in superplastically formed diffusion-bonded parts with large thickness differences

Abstract

This invention relates to a method for eliminating surface defects in superplastically formed diffusion-bonded parts with large thickness differences, belonging to the field of metal material plastic processing technology. Without altering the existing superplastically formed diffusion-bonded process, a process compensation layer is added to the outer skin, reducing the thickness ratio of the inner and outer skins to ≤1. This improves the deformation trend of the outer skin and mitigates the degree of abrupt step changes. After forming, the process compensation layer is first removed, and the step defect area is preserved using a scribing fixture. Finally, a smooth transition is achieved through mechanical grinding, resulting in a high surface quality with small step differences and low waviness.

Description

Technical Field

[0001] This invention relates to a method for controlling surface defects after diffusion bonding of double-layer plates with large thickness differences in superplastic forming, belonging to the field of metal material plastic processing technology. Background Technology

[0002] Superplastic forming utilizes the excellent plasticity of materials under specific conditions to create parts with special shapes. Its forming process is characterized by low deformation resistance, high forming limits, large processing freedom, and no stress, thus imposing fewer constraints on product design. When combined with diffusion bonding technology, this process can also achieve the manufacturing of hollow, high-rigidity, thin-walled, lightweight structures. Specifically, double-layer sheets are diffusely bonded into a single layer in the bonding area, while trapezoidal cross-section ribs are superplastically formed in other areas requiring reinforcement.

[0003] To achieve zone control in superplastic forming of double-layer sheet metal, a release agent needs to be applied between the two layers of raw material according to the final rib contour shape, and the area should be appropriately increased. This shape should match the shape of the die cavity. For superplastic forming dies, high-temperature alloy steel is generally selected based on its long-term performance characteristics. This material has advantages such as strong high-temperature deformation resistance, good dimensional stability, long service life, and relatively low operating costs.

[0004] Due to the difference in expansion coefficients between the tooling material and the titanium alloy material, the dimensions of the mold after thermal expansion at the superplastic forming temperature are larger than the dimensions of the titanium alloy material after thermal expansion. Therefore, the mold needs to be scaled down proportionally beforehand. This brings a series of coordination problems to the positioning accuracy of the titanium alloy blank and the accuracy of the release agent contour shape. In actual production, due to uneven heating of the mold, the temperature of the casting varies in different areas, and the distribution of casting material and porosity is not entirely the same. Moreover, the forming temperature cannot be guaranteed to be consistent each time. This leads to the inability to guarantee a uniform material thickness gap between the upper and lower mold surfaces at high temperatures. At the same time, the thickness of the titanium alloy sheet raw material also has a certain error, generally exceeding the theoretical thickness. The accumulation of these combined error factors will result in the mold upper and lower mold closing gap being larger than the theoretical gap at high temperatures. Under this condition, superplastic forming is performed, with the inner skin adhering to the rib cavity. Due to the excellent plasticity of the titanium alloy sheet, when there is a gap between the outer skin and the mold surface, material will simultaneously fill the gap, eventually forming a protrusion on the outer skin surface along the edge of the rib contour. In severe cases, a step feature is formed, which in turn affects the surface quality of the outer skin. Especially for stealth aircraft with high surface quality requirements, control and solutions need to be sought. Summary of the Invention

[0005] To overcome the above-mentioned technical problems, the present invention provides a novel process method. Without changing the existing superplastic forming diffusion bonding process, the process method increases the outer skin process compensation layer, reduces the inner and outer skin thickness ratio to ≤1, improves the deformation trend of the outer skin, and slows down the degree of step difference abruptness. After forming, the process compensation layer is first removed, and the step difference defect area is retained by scribing fixture. Finally, a smooth transition is achieved by mechanical grinding, resulting in a high surface quality with small step difference and low waviness.

[0006] 1. Titanium alloy materials exhibit strong plastic deformation capabilities under superplastic forming temperature conditions. In a double-layer superplastic forming / diffusion bonding structure, after diffusion bonding is completed, when the ribs are formed using air pressure inflation, the outer skin is simultaneously subjected to the same forming pressure. If the material thickness is thin, it will preferentially fill the mold gap. To avoid this phenomenon, a thickness compensation layer is added for thin skins, ensuring the outer skin thickness is greater than or equal to the inner skin thickness. During air inflation forming, the inner skin preferentially undergoes plastic elongation deformation to fill the mold gap. Even if surface protrusions and inner surface ribs appear simultaneously, they are avoided on the outer skin, improving the smoothness of the original outer skin surface of the formed part. Figure 4 The lower forming mold 3 and the upper forming mold 4 are aligned by the guide pillars. The outer skin material 11 and the inner skin material 2 are aligned and positioned according to the edge contour. Then, the ear piece at the allowance is positioned with the lower forming mold 3 and the upper forming mold 4. The area of ​​the anti-welding flux 5 is obtained by adding the circumferential process compensation width to the normal projection of the cavity area of ​​the upper forming mold 4. Air passage A 6 is the upper mold inlet / outlet air hole, and air passage B 7 is the lower mold near / outlet air hole, which is located at the lowest point of the cavity.

[0007] 2. Utilizing the consistency of sheet metal forming limits, although titanium alloy sheets have high deformation limits and strong plastic deformation capabilities at superplastic forming temperatures, the bending limits of sheets of the same material under the same external conditions are consistent, approximately 1 to 1.5 times the material thickness. Using this principle, the original thickness of the outer skin before forming is increased, that is, thickness compensation is added to the thinner outer skin so that the outer skin thickness is not less than that of the inner skin. During high-temperature superplastic forming, under the same mold gap, the bulging deformation characteristics obtained by plastic forming of the thick skin are slower than those obtained by plastic forming of the thin skin. Although the initial defect step depth L of the outer skin is consistent, the transition width H is much larger than that of the thin skin after forming.

[0008] 3. After adding a compensation layer to the outer skin, the overall thickness is greater than or equal to the thickness of the inner layer. Under high temperature conditions, the force applied to the sheet metal in the mold wall by the air pressure superplastic forming is uniform. Therefore, the deformation resistance of the thick skin is relatively high, the plastic deformation time is relatively late, and the thin material is more likely to fill the mold gap first. This reduces the probability of deformation of the outer skin. Finally, after the air pressure stabilizes, the bulging degree of the outer skin will also tend to slow down, the step difference depth L is small, and the transition zone width H is wide.

[0009] 4. The superplastic forming diffusion connection process of double-layer sheet metal is optimized to have the outer skin on the bottom and the inner skin on the top. During the process, the diffusion connection of the double-layer sheet metal is carried out first. If there is a gap in the mold, argon gas is first injected into the upper mold before superplastic forming to force the sheet metal to adhere to the lower mold. Taking into account the influence of gravity, the flatness of the outer skin can be guaranteed first. Then, the rib shape at the superplastic forming point is carried out. The mold gap is distributed as much as possible between the upper mold and the inner skin material, thereby avoiding the outer skin material from filling the gap and ensuring its surface quality.

[0010] 5. The outer skin compensation layer is removed by chemical milling. Taking advantage of the uniformity and consistency of the strong acid corrosion bath, the material thickness can be reduced at a uniform corrosion rate without being limited by the curved surface of the skin. No special clamping milling fixture is required, and batch processing can be carried out with high production efficiency. During processing, protective glue is sprayed on the skin surface, and then the transition area is retained according to the boundary. The protective glue is removed in the remaining areas to expose the metal surface. The compensation layer is thinned and removed by chemical milling until the original theoretical thickness is reached.

[0011] 6. Use scribing fixtures to define the boundaries and retain the step defect area. After the double-layer structure is superplastically formed / diffused, the thickened outer skin and inner skin are combined into a whole. In order to obtain the expected material thickness, the outer skin compensation layer is removed as a whole by secondary processing. However, the compensation layer material in the transition area is retained separately. The method for confirming its edge position is as follows: expand outward according to the edge of the transition area and increase the width by d = t × e, where d represents the transverse corrosion amount, t represents the longitudinal corrosion amount, and e represents the etching ratio. The etching ratio can be obtained by processing test pieces of the same material grade. Then the width of the retained compensation layer should be w = H + 2d. The edge contour of the retained area is projected with the maximum edge contour of the inner skin rib. The scribing fixture 8 is designed according to this contour. The surface of the fixture 8 is consistent with the outer skin rough 11. It is made of rigid metal sheet. The scribing area is designed with a hollow to express the contour. The hollow edge is used for scribing.

[0012] 7. For example Figure 11 As shown, according to the surface finish requirements, the height-to-width ratio of the abrupt change in surface gradient is smoothed according to a 1:k ratio. Considering technical requirements, the following should be ensured:

[0013] L:H=1:k

[0014] After the main area of ​​the outer skin reaches the theoretical thickness, the remaining area is a transition zone with protrusions. To ensure a continuous and consistent outer skin surface, mechanical grinding is used to remove the protrusions. Due to the height difference between the two sides of the protrusion feature, a beveled feature should be ground out, transforming the abrupt step difference into a gradual bevel. This optimizes the step difference feature into a waviness feature, where waviness λ = a / b, where a represents the maximum waviness depth and b represents the waviness length, i.e., the distance between two peaks or troughs. Based on this, it is calculated that in the extreme case, protrusions appear on both sides of the inner skin rib. Given the rib width B, b represents the trough width as B + 2H. The waviness calculation method after grinding the outer skin should simultaneously ensure:

[0015] λ = L / (B + 2H)

[0016] Where k and λ are determined by the surface quality requirements, the final calculation derivation of the reserved grinding width on the outer surface is as follows:

[0017] H≥B kλ / (1-2kλ)

[0018] This is to reserve the minimum width. When the width of the H-processing is greater than this value, the surface quality will be better than the design requirements.

[0019] For the corresponding position, the width of the milled template W≥B kλ / (1-2kλ)+2te, that is, when the thickness of the outer skin is t, if the unmachined width reserved before milling is greater than this value, the surface quality will be better than the design requirements.

[0020] 8. The remaining stepped areas on the outer skin surface are removed by mechanical grinding. The scribing fixture 8 is designed according to all transition areas of the part. The fixture forms a positioning relationship with the blank according to the outline. When using it, the grinding width area is determined by the length of the actual surface defects of the part with the help of part 8. During the grinding process, avoid damaging both sides of the stepped features to form a smooth material transition, which can avoid the risk of the material being too thin due to direct grinding. The actual operation process depends on the specific surface condition. The scribing fixture expresses the edges of all retained areas according to the outline of all ribs. After the part is formed, observe the surface of the outer skin. If there are raised stepped features, the scribing fixture is used to retain the material in that area. Other areas without defects are simultaneously thinned to the theoretical material thickness.

[0021] The beneficial effects of this invention are as follows: By using this novel process technology, based on the theory that the ultimate bending radius of the same material plates is consistent, the surface protrusion and step features are initially mitigated by increasing the compensation thickness of the outer skin and placing it in the lower layer during forming. Then, by using a scriber fixture, material is retained at the locations where protrusion and step features appear. After the remaining areas are all milled and thinned to the theoretical outer skin thickness, a wider and gentler protrusion feature is obtained by mechanical grinding. This transforms the local protrusion and step features into surface waviness features, ultimately meeting the high standard requirements of continuous, smooth, and stepless aerodynamic surfaces for stealth aircraft, while avoiding the risk of the material being too thin after local grinding. Attached Figure Description

[0022] Figure 1 Schematic diagram of forming parts with large thickness differences

[0023] Figure 2 Schematic diagram of typical defects in the forming of parts with large thickness differences

[0024] Figure 3 Schematic diagram of typical surface defects in parts with large thickness differences

[0025] Figure 4 Schematic diagram of forming parts with uniform thickness

[0026] Figure 5 Typical defects in the forming of parts with uniform thickness Figure 1

[0027] Figure 6 Typical defects in the forming of parts with uniform thickness Figure 2

[0028] Figure 7 Schematic diagram of typical surface defects of parts with uniform thickness

[0029] Figure 8 Schematic diagram of typical defect areas on the surface of parts of equal thickness

[0030] Figure 9 Schematic diagram of material removal for parts of equal thickness

[0031] Figure 10 Schematic diagram after surface defect removal of parts with large thickness differences

[0032] Figure 11 Schematic diagram of surface convexity waviness characteristics

[0033] In the diagram: 1 Outer skin; 2 Inner skin material; 3 Lower forming mold; 4 Upper forming mold; 5 Solder resist; 6 Air path A; 7 Air path B; 8 Grating fixture; 9 Residual boss area to be polished; 11 Outer skin material. Detailed Implementation

[0034] 1. Calculate the blanking diagram according to the theoretical numerical model to obtain the inner and outer skin planar blanking diagrams. At the same time, calculate the anti-welding flux unfolding diagram with reference to the rib protrusion contour, and select the scribing fixture. The inner skin, outer skin, and scribing fixture are positioned according to the outer contour.

[0035] 2. For example Figure 6 As shown, the outer skin raw material is thickened and then laser-cut according to the unfolded pattern to obtain outer skin material 11 and inner skin material 2, and the thickness of outer skin material 11 is not less than the thickness of inner skin material 2.

[0036] 3. Using a scribing fixture, while the outer skin material 11 is in a flat state, according to the shape, scribing the anti-weld flux and ventilation groove patterns on the material;

[0037] 4. Perform laser cleaning on the surface of the raw material to remove oil and impurities, resulting in an exposed metal surface;

[0038] 5. Apply anti-welding agent to the surface of the outer skin material 11 according to the engraved lines. After drying, align it with the inner skin material 2. Seal the perimeter with argon arc welding. Weld a vent pipe to the edge of the vent slot. After completion, perform an airtightness check to confirm that there is no air leakage.

[0039] 6. Coat the entire outer surface of the welded blank with anti-welding agent, place it in the lower forming mold 3 and the upper forming mold 4, fix it according to the shape, and after the mold is closed, place it in the superplastic forming machine for pressurization and heating. Low-pressure argon gas is introduced into the lower forming mold 3 and the upper forming mold 4 through gas passage A6 and gas passage B7 respectively for protection, while the blank is evacuated at the same time.

[0040] 7. When the forming temperature reaches about 920℃, diffusion connection is first carried out. High-pressure argon gas is filled into the rib cavity of the upper forming mold 4 through gas passage B7. Under the dual action of external gas pressure and internal negative pressure, the outer skin material 11 and the inner skin material 2 are completely welded together as a whole in the area except for the area coated with anti-weld flux 5. The high-pressure gas in the upper forming mold 4 is replaced with low-pressure protective argon gas to complete the diffusion connection.

[0041] 8. Superplastic forming is performed at high temperature. The vacuum negative pressure is removed through the process gas path between the outer skin material 11 and the inner skin material 2, and high-pressure argon gas is injected into the middle layer of the material to perform superplastic forming of the inner skin material 2, so that it is filled into the mold cavity of the forming mold 4. Rib features are obtained in the corresponding anti-welding agent 5 coating area. After completion, the high-pressure gas between the materials is released, and low-pressure protective gas is injected. Then the mold is cooled down with the furnace.

[0042] 9. When the overall temperature of the mold drops to about 500℃, cut off the air supply, remove the mold from the equipment, open the upper forming mold 4, take out the processed part blank, clean the anti-weld flux and oxide scale on the surface of the blank, and then perform ultrasonic welding rate test to confirm the internal quality.

[0043] 10. For example Figure 7 As shown, the outer skin material 11 has a partial protrusion defect at the edge of the local anti-weld flux transition zone, with a width of H and a height of L. After marking, the entire surface of the material is coated with glue.

[0044] 11. For example Figure 8 As shown, the scribing fixture 8 is positioned according to the shape and the raw material. Then, the horizontal width line of the grinding area is drawn according to the fixture in the area where the defect occurs, and the longitudinal line is extended appropriately to determine the grinding area. The protective glue of the area to be chemically milled is removed, and only the grinding area is not chemically milled to reduce its thickness.

[0045] 12. Immerse the entire raw material in the chemical milling tank for thickness reduction. Stop the thinning process when the thickness of the outer skin raw material 11 reaches the theoretical required thickness of the outer skin 1. Leave any remaining bosses in the grinding area, such as... Figure 9 As shown;

[0046] 13. Remove any remaining protective adhesive from the surface, then use mechanical grinding to remove the boss, smoothing both sides of the transition step. The finished product should look like this. Figure 10 As shown, this process ultimately eliminates surface abrupt defects, resulting in a continuous, smooth, and stepless aerodynamic surface.

Claims

1. A method for eliminating surface defects in superplastically formed diffusion-bonded parts with large thickness differences of double-layer plates, characterized in that, The steps are as follows: 1) Calculate the blanking diagram according to the theoretical numerical model to obtain the inner and outer skin planar blanking diagrams. At the same time, calculate the anti-welding flux unfolding diagram with reference to the rib protrusion contour, and select the scribing fixture. The inner skin, outer skin, and scribing fixture are positioned according to the outline. 2) Thicken the outer skin raw material, and cut it into pieces by laser cutting according to the unfolded pattern to obtain outer skin material (11) and inner skin material (2), and the thickness of outer skin material (11) is not less than the thickness of inner skin material (2). 3) Using a scribing fixture, the anti-welding flux and ventilation groove patterns are engraved on the outer skin material (11) in a flat state according to the shape. 4) Perform laser cleaning on the surface of the raw material to remove surface oil and impurities, resulting in an exposed metal surface; 5) Apply anti-welding agent (5) to the surface of the outer skin material (11) according to the engraved lines. After drying, it is aligned with the inner skin material (2). The perimeter is sealed by argon arc welding. A vent pipe is welded at the edge of the vent slot. After completion, an airtightness check is performed to confirm that there is no air leakage. 6) Coat the entire outer surface of the welded blank with anti-welding agent, place it in the lower forming mold (3) and the upper forming mold (4), fix it according to the shape, and after closing the mold, put it into the superplastic forming machine for pressurization and heating. Through gas path A (6) and gas path B (7), low-pressure argon gas is introduced into the lower forming mold (3) and the upper forming mold (4) respectively for protection, while the blank is evacuated. 7) When the temperature reaches the forming temperature, diffusion connection is first carried out. High-pressure argon gas is filled into the rib cavity of the upper forming mold (4) through the gas passage B (7). Under the dual action of external gas pressure and internal negative pressure, the outer skin material (11) and the inner skin material (2) are completely welded together in the area except for the area coated with anti-welding agent. The high-pressure gas in the upper forming mold (4) is replaced with low-pressure protective argon gas to complete the diffusion connection. 8) Superplastic forming is performed at high temperature. The vacuum negative pressure is removed through the process gas path between the outer skin material (11) and the inner skin material (2), and high pressure argon is injected into the middle layer of the material to perform superplastic forming of the inner skin material (2), so that it fills the rib cavity of the forming mold (4) and obtains the rib feature in the corresponding anti-welding agent (5) coating area. After completion, the high pressure gas between the materials is released, and low pressure protective gas is injected. Then the mold is cooled down with the furnace. 9) When the overall temperature of the mold drops below 500℃, cut off the gas supply, remove the mold from the equipment, open the upper forming mold (4), take out the finished part blank, clean the surface of the blank of the anti-welding agent and oxide scale, and then perform ultrasonic welding rate test to confirm the internal quality. 10) Outer skin material (11) Some protrusion defects are generated at the edge of the local anti-welding flux transition zone, with a width of H and a height of L. After marking, the entire surface of the material is coated with glue. 11) Position the scribing fixture (8) according to the shape and the raw material, then retain the transition area according to the boundary, draw the horizontal width line of the grinding area according to the fixture in the area where the defect occurs, and extend it longitudinally to determine the grinding area, remove the protective glue of the area to be chemically milled, and only retain the grinding area without chemically milling and thinning. 12) The entire raw material is placed into the chemical milling tank for material removal and thickness reduction. When the thickness of the outer skin raw material (11) reaches the theoretical required thickness of the outer skin (1), the thinning process is stopped, and the remaining grinding area bosses are removed. 13) Remove the remaining protective adhesive from the surface, and then use mechanical grinding to remove the bosses and smooth the transition steps on both sides, thus finally eliminating the surface abrupt defects and obtaining a continuous, smooth, and stepless aerodynamic surface.

2. The method for eliminating surface defects in superplastically formed diffusion-bonded parts with large thickness differences of double-layer plates according to claim 1, characterized in that, In step 11): According to the outer expansion of the transition zone edge, the width is increased by d = t × e, where d represents the transverse corrosion amount, t represents the longitudinal corrosion amount, and e represents the etching ratio. The etching ratio can be obtained by processing test pieces of the same material grade. Then the width of the remaining compensation layer should be w = H + 2d.

3. The method for eliminating surface defects in superplastically formed diffusion-bonded parts with large thickness differences of double-layer plates according to claim 1, characterized in that, In step 13): according to the surface finish requirements of the machine body, the height-to-width ratio of the abrupt step difference is smoothed according to a 1:k ratio. Considering technical requirements, the following should be ensured: L:H=1:k After the main area of ​​the outer skin reaches the theoretical thickness, the remaining area is a transition zone with protrusions. To ensure a continuous and consistent outer skin surface, mechanical grinding is used to remove the protrusions. Due to the height difference between the two sides of the protrusion feature, a beveled feature should be ground out, transforming the abrupt step difference into a gradual bevel. This optimizes the step difference feature into a waviness feature, where waviness λ = a / b, where a represents the maximum waviness depth and b represents the waviness length, i.e., the distance between two peaks or troughs. Based on this, it is calculated that in the extreme case, protrusions appear on both sides of the inner skin rib. Given the rib width B, b represents the trough width as B + 2H. The waviness calculation method after grinding the outer skin should simultaneously ensure: λ = L / (B + 2H) Where k and λ are determined by the surface quality requirements, the final calculation derivation of the reserved grinding width on the outer surface is as follows: H≥Bkλ / (1-2kλ) This is to reserve the minimum width. When the width of the H-processing is greater than this value, the surface quality will be better than the design requirements. For the corresponding position, the width of the milled template W≥Bkλ / (1-2kλ)+2te, that is, when the thickness of the outer skin is t, if the unmachined width reserved before milling is greater than this value, the surface quality will be better than the design requirements.

4. The method for eliminating surface defects in superplastically formed diffusion-bonded parts with large thickness differences of double-layer plates according to claim 1, characterized in that, The surface of the engraving fixture (8) is in close contact with the outer skin material (11). It is made of rigid metal sheet. The area to be engraved is designed with a hollowed-out shape to express the outline by engraving the lines using the hollowed-out edges.

5. The method for eliminating surface defects in superplastically formed diffusion-bonded parts with large thickness differences of double-layer plates according to claim 1, characterized in that, The molding temperature is 920℃.

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