Forging method and application of exfoliation-corrosion-resistant 7-series aluminum alloy

Abstract

The invention discloses a forging method and a characteristic structure of an exfoliation-corrosion-resistant 7-series aluminum alloy. The method comprises the following steps: preheating an alloy ingot to 330-350 DEG C, and carrying out multidirectional forging to obtain a primary forge piece; and the primary forge piece is sequentially subjected to annealing treatment and multi-directional forging and is air-cooled to the room temperature, and the high-strength steel is obtained. By means of the method, the recrystallization degree of the alloy can be improved, and a nested structure with large recrystallization grains and a large number of small-angle grain boundary substructures or deformed structures distributed in an intermingled mode is obtained. Due to the fact that the corrosion resistance of the small-angle grain boundary is higher than that of the large-angle grain boundary, the small-angle grain boundary penetrates into metal along with corrosion in the exfoliation corrosion process, the large-angle grain boundary is corroded preferentially, a recrystallization part is hollowed out, and when corrosion enters a substructure and the small-angle grain boundary part, the corrosion rate is reduced, and the corrosion depth is controlled. On one hand, the nested structure ensures the excellent comprehensive performance of the forge piece, and on the other hand, the forge piece under various conditions and directions presents an approximate exfoliation corrosion grade which can generally reach the EA grade.

Description

TECHNICAL FIELD

[0001] The present application relates to an aluminum alloy forging method, in particular to a method for forging a 7-series aluminum alloy with anti-exfoliation corrosion and its application, and belongs to the field of metal material processing. BACKGROUND

[0002] 7-series aluminum alloys are widely used in civil industries, especially in aerospace and transportation industries. In order to better meet the industrial demand, large-section aluminum alloy forgings are often processed as a whole to obtain large and complex workpieces. However, the existing 7-series aluminum alloy forgings have serious corrosion performance problems, which limit the application occasions and service life in design, and restrict the improvement of comprehensive application benefits.

[0003] As the most common aluminum alloy in 7-series aluminum alloys, 7085 aluminum alloy has significant strength differences in L-T and S-T directions. Patent US20190368009A1 discloses a high-strength 7XXX aluminum alloy product and a manufacturing method, and the obtained product has obvious anisotropy.

[0004] The present application discloses a method for forging a 7xxx-series aluminum alloy with anti-exfoliation corrosion. The one-stage warm forging step can make the alloy obtain a large number of substructures containing small-angle grain boundaries and a nested distribution of recrystallized structures mainly composed of large-angle grain boundaries, thereby improving the strength of the forging in all directions, reducing anisotropy as much as possible, and improving the overall anti-exfoliation corrosion resistance. SUMMARY

[0005] In view of the problems in the prior art, the first object of the present application is to provide a method for forging a 7-series aluminum alloy with anti-exfoliation corrosion. The core of the method is to insert a low-temperature warm forging process during the forging process of the forging. During the first forging process, the material accumulates a large amount of deformation energy due to the low temperature, and recrystallization occurs during the annealing and holding process. The annealing time is strictly controlled to control the recrystallized grain growth and form coarse grains, while a large number of substructures containing small-angle grain boundaries remain. Since small-angle grain boundaries have strong corrosion resistance, the corrosion rate is significantly slowed down after the corrosion enters the substructure. At the same time, the nesting of the substructure and the recrystallization provides strong toughness, ensuring the mechanical properties of the forging.

[0006] To achieve the above technical purposes, the present application provides a forging method of a 7-series aluminum alloy resistant to spalling corrosion, comprising: preheating an alloy ingot to A ℃, adjusting to B ℃ for multi-directional forging to obtain a primary forged piece; and sequentially subjecting the primary forged piece to annealing treatment and multi-directional forging, and air cooling to room temperature. 3 The value of A is 440-480, preferably 455-465, and the value of B is 330-350, preferably 335-345.

[0007] The forging method provided by the present application strictly controls the process parameters of two-stage forging, especially the temperature and deformation parameters, effectively reduces the anisotropy of the material, and improves the comprehensive mechanical properties of the material, and is especially suitable for aluminum alloy materials with large thick cross sections.

[0008] As a preferred scheme, the single-direction deformation amount of each upsetting and elongation process is 30%.

[0009] As a preferred scheme, the deformation amount of the forging direction in each multi-directional forging process is ≤30%. Further preferably, the deformation amount of the forging direction in each multi-directional forging process is 30%.

[0010] As a preferred scheme, the preheating process of the alloy ingot is: heating to A ℃ at a rate of 5-10 ℃ / min, and maintaining for 0.5-1 h. 3 For the alloy ingot, the holding time is increased by 1 min for each 10% increase in volume, with 1 dm

[0011] As a preferred scheme, the alloy ingot is maintained at B ℃ for 0.5-1 h. 3 For the alloy ingot, the holding time is increased by 1 min for each 10% increase in volume, with 1 dm

[0012] In the present application, the values of A and B must be strictly in accordance with the above requirements, because when the deformation temperature is lower than the recrystallization temperature, dynamic recrystallization is inhibited, the recovery effect is strong, and a large amount of deformation energy is introduced into the forged piece at this stage, which facilitates the generation of coarse recrystallized structure in the subsequent solid solution process.

[0013] As a preferred scheme, the annealing conditions of the alloy ingot are: maintaining at 440-480 ℃ for 0.5-1 h. 3As a reference, the holding time is extended by 1 min for every 10% increase in volume. The annealing process needs to be strictly controlled in terms of temperature and holding time. During the annealing process, the deformation energy accumulated in the material is preliminarily released, a large number of fine recrystallized grains are nucleated, and the recrystallized grains are not fully grown in a short time, thereby forming fine-grained and uniformly dispersed in the material.

[0014] As a preferred solution, the temperature difference between the surface and the core of the alloy ingot during the multi-directional forging process is ≤25℃.

[0015] When the obtained primary forged piece is subjected to multi-directional forging again, the temperature is higher than the recrystallization temperature, and after the forging is completed, in the solid solution stage, static recrystallization promotes grain growth to form coarse recrystallized grains, and the grains with insufficient deformation energy still have a large number of small-angle grain boundaries to form substructures. At this time, the substructure improves the corrosion resistance and strength, and the recrystallization provides toughness, thereby realizing the strengthening and toughening of the forged piece and the anti-exfoliation corrosion characteristics.

[0016] As a preferred solution, the product is further subjected to heat treatment and aging treatment.

[0017] As a preferred solution, the heat treatment is performed at a temperature of 460-480℃ and a holding time of 0.5-1 h, and the alloy ingot is heated to 1 dm 3 As a reference, the holding time is extended by 1 min for every 1% increase in volume.

[0018] The 7-series aluminum alloy includes a 7085 aluminum alloy.

[0019] As a preferred solution, after the multi-directional forging is performed at B℃, the L direction is forged from 200 mm to 140 mm.

[0020] As a further preferred solution, after the multi-directional forging is performed at B℃, the L direction is forged from 200 mm to 140 mm. Then, the forged piece is heated to 460℃ and held for 0.5 h, and free forging is performed at 460℃. After the forging is completed, the S direction is forged from 140 mm to 100 mm, and cooling liquid is sprayed on the upper and lower surfaces during the free forging at 460℃ until the surface temperature reaches 380℃. After the forging is completed, air cooling is performed.

[0021] As a preferred scheme, the aging treatment is performed at 110-130℃ for 4-10h, then heated to 145-160℃, and held for 8-12h, and the alloy ingot is forged at 1dm 3 For reference, the holding time is prolonged by 1min for each 1% increase in volume. The aging treatment adopted in the present application is T74 aging treatment, which can introduce a strengthening dispersion precipitate phase, effectively improving the strength of the alloy.

[0022] The present application can improve the recrystallization degree of the alloy and obtain large recrystallized grains and a nested structure with a large number of intercrystalline boundaries and a distribution of substructures or deformed structures. Since the intercrystalline boundaries have stronger corrosion resistance than the large-angle grain boundaries, during the exfoliation corrosion process, the large-angle grain boundaries are preferentially corroded, and the recrystallized part is hollowed out. When the corrosion enters the substructure and the small-angle grain boundary part, the corrosion rate decreases and the corrosion depth is controlled. The nested structure ensures the excellent comprehensive performance of the forged piece, and the forged piece under various conditions and directions shows a close exfoliation corrosion grade, usually reaching EA level.

[0023] The present application also provides an application of a forging method of an exfoliation corrosion-resistant 7-series aluminum alloy, which is used for forging aluminum alloy materials with a thickness of ≥50mm and a large-thickness cross-section specification.

[0024] Compared with the prior art, the technical scheme of the present application has the following beneficial technical effects:

[0025] 1) In the forging method provided by the present application, a segmented forging process is adopted. In the first forging process, a large amount of deformation energy is accumulated in the material due to the low temperature, and static recrystallization occurs in the solid solution process, thereby controlling the recrystallization coarsening and retaining part of the structure as a substructure full of small-angle grain boundaries, so as to obtain an aluminum alloy forged piece with a nested structure characteristic.

[0026] 2) In the forging method provided by the present application, the nested grain structure is formed by control, the stress concentration in the deformation process is relieved by the coarse recrystallized structure, the toughness is improved, and the strength is provided by the remaining high dislocation density substructure, thereby realizing the strengthening and toughening of the forged piece.

[0027] 3) In the technical scheme provided by the present application, the process combination is simple and easy to implement in industrialization. By adding the warm forging process at a lower temperature, the exfoliation corrosion resistance of different forged pieces is effectively improved. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 Exfoliation corrosion morphology in Example 1 of the present application;

[0029] Figure 2 Intergranular corrosion morphology of the product obtained in Example 1 of the present application;

[0030] Figure 3 EBSD morphology of the corrosion site of the product obtained in Example 1.

[0031] Figure 4 Schematic diagram of L, T, and S directions of the forging DETAILED DESCRIPTION

[0032] The following comparative examples and examples are intended to further illustrate the present application and are not intended to limit the present application.

[0033] The room temperature tensile test was performed according to the standard GB / T228-2002 to produce the relevant standard tensile sample.

[0034] Example 1

[0035] The blank was 200x200x200mm 7085 aluminum alloy. The blank was heated to 460℃, and kept for 1h, then placed at 340℃, and kept for 1h. Multidirectional forging was performed at 340℃ (the temperature difference between the surface and the center of the alloy ingot during the multidirectional forging was about 15℃). After the forging, the L direction was forged from 200mm to 140mm. Then the forging was heated to 460℃, and kept for 0.5h. Free forging was performed at 460℃. After the forging, the S direction (thickness direction) was forged from 140mm to 100mm. During the free forging at 460℃, the upper and lower surfaces were sprayed with cooling liquid until the surface temperature reached 380℃. After the forging, air cooling was performed. At this time, the size of the forging was 280x240x100mm. The center part of the forging with a size of 30x15x2mm was cut off for spalling corrosion test.

[0036] Example 2

[0037] The blank was 200x200x200mm 7085 aluminum alloy. The blank was heated to 460℃, and kept for 1h, then placed at 340℃, and kept for 1h. Multidirectional forging was performed at 340℃ (the temperature difference between the surface and the center of the alloy ingot during the multidirectional forging was about 15℃). After the forging, the L direction was forged from 200mm to 140mm. Then the forging was heated to 460℃, and kept for 0.5h. Free forging was performed at 460℃. After the forging, the S direction (thickness direction) was forged from 140mm to 100mm. During the free forging at 460℃, the upper and lower surfaces were sprayed with cooling liquid until the surface temperature reached 300℃. After the forging, air cooling was performed. At this time, the size of the forging was 280x240x100mm.

[0038] Comparative Example 1

[0039] The blank is 200x200x200mm 7085 aluminum alloy. The blank is heated to 460℃, and kept for 1h, then put to 340℃, and kept for 1h, and then multi-directional forging is carried out at 340℃ (the temperature difference between the surface and the core of the alloy ingot during the multi-directional forging process is about 15℃), the single upsetting and elongation process has a unidirectional deformation of 30%, and after the forging is finished, the L direction is forged from 200mm to 140mm, and a first forging piece is obtained; then the forging piece is heated to 460℃, kept for 0.5h, and then multi-directional forging is carried out at 460℃, the single upsetting and elongation process has a unidirectional deformation of 30%, and after the forging is finished, the S direction is forged from 140mm to 100mm, and then air cooling is carried out, and finally the forging piece is obtained.

[0040] The final size of the comparative examples and the examples after processing is 280x240x100mm, the materials prepared in the comparative examples and the examples are heat treated in T74 state, the heat treatment process parameters are 470℃ / 2h, and water quenching. The performance comparison and corrosion performance of the materials prepared according to the conventional forging process and the process of the application in three directions are shown in Table 1.

[0041]

[0042] In Table 1, the intergranular corrosion is a mixed solution of sodium chloride (NaCl) and hydrogen peroxide (H2O2). The specific ratio is: 30g / L NaCl + 10 mL / L H2O2 (30%), and the pH value is controlled in the acidic range (about 1.0-3.0), and the ratio of the sample surface area to the solution volume should be ≤20 mm 2 / mL (i.e. ≥5 mL / cm²)

[0043] The exfoliation corrosion is 3.5% NaCl solution at 25℃ constant temperature immersion for 48h, and the ratio of the sample surface area to the solution volume should be ≤20 mm 2 / mL (i.e. ≥5 mL / cm 2 ).

[0044] From the data in Table 1, it can be seen that the example 1 and the two comparative examples of the application all apply a warm forging step at a lower temperature during forging, although the differences in forging processes cause differences in the mechanical properties and intergranular corrosion properties of the forgings, but in the case of forming the microstructure characteristics of recrystallization and substructure nesting distribution, all the forgings reach the exfoliation corrosion level of EA grade.

[0045] The embodiment 1 provided by the present application shows better coupling of strength and toughness and intergranular corrosion performance compared to the comparative example 1 and the embodiment 2 by adjusting the forging process. However, the exfoliation corrosion performance of the three forgings is EA, because during the exfoliation corrosion process, the corrosion path preferentially develops along the high-angle grain boundary, and tends to preferentially corrode the recrystallized region. When the corrosion progresses to the substructure, a large number of low-angle grain boundaries scatter the corrosion path, thereby reducing the overall corrosion rate. Therefore, although the maximum depth of intergranular corrosion is affected by the grain structure configuration, the corrosion inhibition effect of the substructure makes each forging maintain a high exfoliation corrosion grade.

Claims

1. A method of forging an exfoliation corrosion resistant 7xxx aluminum alloy, characterized by, Comprise: The alloy ingot is preheated to A ℃, adjusted to B ℃ for multi-directional forging, and a primary forged piece is obtained; the primary forged piece is sequentially subjected to annealing treatment and multi-directional forging, and air-cooled to room temperature, thereby obtaining the forged piece; the value of A is 440-480, and the value of B is 330-350; the multi-directional forging process is to sequentially upset each surface of the alloy ingot, and elongate between the upsets of different surfaces of the alloy ingot; the single-direction deformation amount of each upset and elongation process is ≤30%; the volume of the alloy ingot is ≥1 dm 3 , and the length in each direction is ≥50 mm.

2. The method of claim 1, wherein the wrought method of producing an exfoliation corrosion resistant 7xxx aluminum alloy is characterized by: The preheating process of the alloy ingot is: increasing temperature to A ℃ at 5-10 ℃ / min, keeping for 0.5-1 h, and the alloy ingot is kept at 1 dm 3 Taking the volume as the reference, the keeping time is prolonged by 1 min for every 10% increase of the volume.

3. The method of claim 1, wherein the wrought method of producing an exfoliation corrosion resistant 7xxx aluminum alloy is characterized by: The alloy ingot is kept at B ℃ for 0.5-1 h, and the alloy ingot is cooled at a rate of 1 dm 3 As a reference, the holding time is prolonged by 1 min for each 10% increase in volume.

4. The method of claim 1, wherein the wrought method of producing an exfoliation corrosion resistant 7xxx aluminum alloy is characterized by: After multi-directional forging at B ℃, the L direction is forged from 200 mm to 140 mm.

5. The method of claim 1, wherein the wrought 7000-series aluminum alloy is a corrosion- resistant exfoliation 7000-series aluminum alloy. After multi-directional forging at B ℃, the L direction is forged from 140 mm to 140 mm; then the forging is heated to 460 ℃, kept for 0.5 h, free forging is carried out at 460 ℃, after the forging, the S direction is forged from 140 mm to 100 mm, during the free forging at 460 ℃, the upper and lower surfaces are sprayed with cooling liquid until the surface temperature reaches 380 ℃, and after the forging, air cooling is carried out.

6. The method of claim 1, wherein the wrought 7000-series aluminum alloy is a corrosion- resistant exfoliation 7000-series aluminum alloy. The annealing condition of the alloy ingot is: keeping at 440-480℃ for 0.5-1h, and the alloy ingot is cooled at 1dm 3 As a reference, the holding time is prolonged by 1min for every 10% increase in volume.

7. The method of claim 1, wherein the wrought 7000-series aluminum alloy is a corrosion- resistant exfoliation 7000-series aluminum alloy. The alloy ingot is heated to 460-480℃ for 0.5-1h, and then cooled to room temperature 3 The heat treatment is performed at 460-480℃ for 0.5-1h, and then cooled to room temperature The aging treatment is performed at 110-130℃ for 4-10h, and then heated to 150-170℃ for 8-12h.

8. The method of claim 7, wherein the wrought method is selected from the group consisting of extrusion, rolling, and combinations thereof. When the alloy ingot is 7085 aluminum alloy, after aging treatment, a product is obtained; The difference between the elongation rates of any two directions of the product in the L, LS and ST directions is less than or equal to 3%.

9. The forging method of the exfoliation corrosion resistant 7-series aluminum alloy according to claim 8, characterized in that: The product forged by the process can reach the EA grade of exfoliation corrosion regardless of the intergranular corrosion performance.

10. Use of the forging method of the exfoliation corrosion resistant 7xxx aluminum alloy according to any one of claims 1 to 9, characterized in that: The method is used for forging aluminum alloy materials with a thickness of ≥50 mm thick section specifications.

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