Method for preparing aluminum alloy superplastic plate for rail transit through asymmetrical rolling

Abstract

The invention discloses a method for preparing an aluminum alloy superplastic plate for rail transit through asymmetrical rolling, and belongs to the technical field of alloy material processing. In order to solve the problems that commercial 5083 aluminum alloy is poor in high-temperature superplasticity and low in strain rate, the method comprises the steps that a commercial hot-rolled aluminum alloy plate serves as an initial material, small-rolling-reduction conventional rolling and large-rolling-reduction small-asynchronous-ratio overturning asynchronous rolling are adopted, and the high-temperature superplasticity of the commercial 5083 aluminum alloy plate is obtained; and straightening, recrystallization annealing and the like are carried out to obtain the aluminum alloy fine-grain plate with good superplasticity for rail transit. According to the method, different shear stresses in the rolling direction are introduced through overturning asymmetrical rolling, thin and long crystal grains obtained through conventional rolling are broken for multiple times to form fine sub-grains, uniform and fine equiaxed crystal grains are obtained after recrystallization annealing, and preparation of the superplastic plate is achieved. According to the method, the cost of superplastic forming is reduced, and guidance is provided for industrial production of superplastic forming of the aluminum alloy for rail transit.

Description

Technical Field

[0001] The present invention belongs to the technical field of alloy material processing, and particularly relates to a method for preparing an aluminum alloy superplastic sheet for rail transit by asynchronous rolling. Background Art

[0002] In order to obtain high superplasticity, severe plastic deformation is increasingly used to produce ultrafine-grained materials. Recently, some bulk ultrafine-grained metals have obtained extremely high ductility through severe plastic deformation. Various processing methods of severe plastic deformation have been developed, such as high-pressure torsion, equal-channel angular pressing, and friction stir welding, etc. However, the samples used in high-pressure torsion are usually very thin disks; equal-channel angular pressing and friction stir welding can only significantly refine grains in a limited area. Obviously, it is very difficult for these technologies to be applicable to industrial-scale and larger-volume components. In addition, the relatively high cost also limits their commercial applications. At the same time, the traditional mechanical heat treatment method has cumbersome steps and insignificant grain refinement effect. However, in practice, due to the relatively slow optimal strain rate of superplasticity, the wide application of superplastic forming of aluminum alloys is usually limited to small-volume components. In order to achieve a high-productivity superplastic forming method, an aluminum alloy with superplasticity at a strain rate greater than 10 -3 s -1 is required. Summary of the Invention

[0003] Aiming at the problems of poor high-temperature superplasticity and low strain rate of commercial 5083 aluminum alloy, the present invention provides a post-annealing treatment method after asynchronous rolling, enabling the material to obtain high strain rate superplasticity and being suitable for producing larger structural materials under industrial conditions.

[0004] In order to achieve the above object, the present invention adopts the following technical solutions:

[0005] A method for preparing an aluminum alloy superplastic sheet for rail transit by asynchronous rolling, comprising the following steps:

[0006] Step 1: Using a hot-rolled aluminum alloy sheet as the raw material, performing cold rolling with a small reduction by conventional rolling to obtain a rolled specimen;

[0007] Step 2: Performing large reduction and small asynchronous ratio flip asynchronous cold rolling on the rolled specimen to obtain a cold-rolled sheet;

[0008] Step 3: Straightening the cold-rolled sheet on a straightening machine to obtain a flat cold-rolled sheet;

[0009] Step 4: Performing recrystallization annealing treatment on the flat cold-rolled sheet to eliminate residual stress and non-uniformity of the structure, and obtain a fine-grained sheet with high strain rate superplasticity.

[0010] Further, in the step 1, the rolling passes are 2 to 4 passes, and the reduction per pass is controlled at 5 to 10%. Conventional rolling causes the grains of the sheet to elongate along the rolling direction under the action of normal stress, reducing the thickness of the sheet so that the shear stress introduced in the subsequent asynchronous rolling is evenly distributed in the thickness direction.

[0011] Further, the large reduction and small asynchronous ratio reversed asynchronous cold rolling in the step 2 is specifically that at the same rotational speed, by changing the roll diameters of the upper and lower rolls, the asynchronous ratio of the upper and lower rolls is made 1.1 to 1.2. After the asynchronous ratio is greater than 1.05, the grain refinement will increase significantly. An excessive asynchronous ratio will increase the curvature of the sheet, increasing the difficulty of subsequent forming.

[0012] Further, the large reduction and small asynchronous ratio reversed asynchronous cold rolling in the step 2 is specifically that the rolling passes are 2 to 4 passes, and the reduction per pass is controlled at 10 to 35%. Using reversed asynchronous rolling can cause the elongated grains obtained by conventional rolling to be mechanically broken under the action of shear stress to form fine sub-grains, providing a high dislocation density for subsequent annealing and making the recrystallized grains fine.

[0013] Further, the large reduction and small asynchronous ratio reversed asynchronous cold rolling in the step 2 is specifically that after each pass of rolling, the sheet is longitudinally flipped 180° for the next pass of rolling. Reversing the sheet for each pass of rolling can introduce shear stresses in different directions, making the grain refinement more uniform.

[0014] Further, the total reduction in the step 1 and the step 2 is controlled at 50 to 70%. Only when the total reduction is greater than 50% will enough dislocation density be introduced to obtain fine grains in subsequent annealing.

[0015] Further, in the step 3, the straightening passes are 1 to 3 passes, and the reduction is 0.01 to 0.1 mm. Straightening can roll the bent sheet obtained by asynchronous rolling to a flat state to provide a flat sheet for subsequent forming. At the same time, more shear stress is introduced during the straightening process, making the grain crushing more uniform.

[0016] Further, in the step 4, the recrystallization annealing temperature is 250 to 350 °C, the heating rate is 10 °C / min, and the annealing time is 0.5 to 2 h. A lower annealing temperature and a shorter annealing time inhibit the growth of recrystallized grains.

[0017] Further, in the step 1, the hot-rolled aluminum alloy sheet is commercial 5083 aluminum alloy with a thickness of 2 to 10 mm. An overly thick original sheet will cause non-uniform distribution of grain refinement in the thickness direction of the sheet.

[0018] Further, the structure of the fine-grained sheet of the flat cold-rolled sheet obtained in step 4 is an equiaxed structure, the average grain size is 1-10 μm, the proportion of high-angle grain boundaries (grain boundaries with a misorientation angle of not less than 15°) is higher than 90%, and the sheet has good superplasticity at a strain rate of 10 -1 ~10 -3 s -1 . The fine equiaxed grains and the high proportion of high-angle grain boundaries are beneficial to improving the superplastic forming ability and ensuring the subsequent superplastic forming.

[0019] Compared with the prior art, the present invention has the following advantages:

[0020] Compared with commercial hot-rolled aluminum alloy sheets, the grains of commercial hot-rolled aluminum alloys are coarse, resulting in poor plasticity and low strength. The grains of the aluminum alloy sheets prepared by the present invention are fine, the elongation is increased by more than 50%, and at the same time, they have good mechanical properties. Techniques such as high-pressure torsion, equal-channel angular pressing, and friction stir welding are effective methods for refining grains, but in industrial applications, it is impossible to prepare large-size strip products. The traditional mechanical heat treatment (multiple cold rolling + intermediate annealing + multiple hot rolling + heat treatment, etc.) for preparing fine-grained aluminum alloy sheets has cumbersome steps and requires a large total reduction (>80%) to achieve. The present invention obtains fine grains smaller than 10 μm at a total reduction of 50-70%, enabling the sheet to have excellent superplasticity at a high strain rate (>10 -3 s -1 ). The present invention is suitable for preparing superplastic aluminum alloy sheets for rail transit and improving the superplasticity of the sheets. This technology has prospects for industrial application in the preparation of aluminum alloy materials for rail transit. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0022] Figure 1 It is a schematic diagram of the preparation process of aluminum alloy for rail transit;

[0023] Figure 2 It is a schematic diagram of rolling a flipped sheet;

[0024] Figure 3 It is a schematic diagram of the grain structure of the 5083 aluminum alloy superplastic sheet after recrystallization annealing in Example 1;

[0025] Figure 4 It is a schematic diagram of the superplastic tensile curve in Example 1;

[0026] Figure 5 Schematic diagram of the grain structure of Comparative Example 1;

[0027] Figure 6 Schematic diagram of the grain structure of Comparative Example 3. Detailed implementation manners

[0028] To gain a deep understanding of the present invention, a comprehensive and detailed description will be provided. However, the present invention has multiple implementation manners and is not limited to the specific examples listed herein. The presentation of these examples aims to deepen the overall understanding of the disclosed content of the present invention. The following further elaborates on the embodiments of the present invention with reference to the accompanying drawings.

[0029] Embodiment 1

[0030] A method for preparing an aluminum alloy superplastic sheet for rail transit by asynchronous rolling, the process is Figure 1 , including the following steps:

[0031] Step 1: Using a hot-rolled 5083 aluminum alloy sheet with a thickness of 8 mm as the raw material, cold rolling is carried out by conventional rolling with a small reduction ratio. After 4 passes of conventional rolling, the reduction ratio per pass is 8%, and the rolled specimen is obtained.

[0032] Step 2: The rolled specimen is subjected to large reduction ratio and small asynchronous ratio flip asynchronous cold rolling. It is rolled using an asynchronous rolling mill. The upper and lower roll diameters of the asynchronous rolling mill are 320 mm and 280 mm respectively, and the upper and lower roll asynchronous ratio is 1.14. After 2 passes of asynchronous rolling with reduction ratios of 25% and 20% respectively, and as Figure 2 shown, the sheet is longitudinally flipped 180° after each pass of rolling for the next pass of rolling, and the sheet is rolled to 3.4 mm, with a total rolling reduction ratio of 57%.

[0033] Step 3: The cold-rolled sheet obtained by asynchronous rolling is straightened on a straightening machine. The number of straightening passes is 3 passes, and the reduction amount is 0.01 mm, obtaining a flat cold-rolled sheet.

[0034] Step 4: The flat cold-rolled sheet is subjected to recrystallization annealing at 300 °C, with a heating rate of 10 °C / min. After holding for 1 h, the superplastic fine-grained sheet obtained, Figure 3 for its EBSD, the proportion of high-angle grain boundaries is 95%, and the average grain size is 8 μm. As Figure 4 shown in the stress-strain curve diagram, at a temperature of 540 °C and a strain rate of 10 -3 s -1 conditions, the maximum superplastic elongation rate reaches 400%. At the same time, at 10 -1 s -1 and 10 -2 s -1Obtain an elongation greater than 150% at a high strain rate to achieve high strain rate superplasticity.

[0035] Example 2

[0036] A method for preparing an aluminum alloy superplastic sheet for rail transit by asynchronous rolling, the process is as follows Figure 1 , including the following steps:

[0037] Step 1: Using a hot-rolled 5083 aluminum alloy sheet with a thickness of 8 mm as the raw material, perform cold rolling by conventional rolling with a small reduction amount. After 4 passes of conventional rolling, the reduction amount per pass is 5%, and the rolled specimen is obtained.

[0038] Step 2: Perform large reduction amount and small asynchronous ratio flipping asynchronous cold rolling on the rolled specimen. Use an asynchronous rolling mill to roll it. The upper and lower roll diameters of the asynchronous rolling mill are 320 mm and 280 mm respectively, and the upper and lower roll asynchronous ratio is 1.14. After 2 passes of asynchronous rolling with reduction amounts of 25% and 20% respectively, and as Figure 2 shown, the sheet is longitudinally flipped 180° after each pass of rolling for the next pass of rolling, and the sheet is rolled to 3.9 mm, with a total rolling reduction amount of 51%.

[0039] Step 3: Straighten the cold-rolled sheet obtained by asynchronous rolling on a straightening machine. The number of straightening passes is 1 to 3 passes, and the reduction amount is 0.01 mm to obtain a flat cold-rolled sheet.

[0040] Step 4: Perform recrystallization annealing on the flat cold-rolled sheet at 300 °C, with a heating rate of 10 °C / min, and the superplastic fine-grained sheet obtained after holding for 1 h.

[0041] Example 3

[0042] A method for preparing an aluminum alloy superplastic sheet for rail transit by asynchronous rolling, the process is as follows Figure 1 , including the following steps:

[0043] Step 1: Using a hot-rolled 5083 aluminum alloy sheet with a thickness of 8 mm as the raw material, perform cold rolling by conventional rolling with a small reduction amount. After 4 passes of conventional rolling, the reduction rate per pass is 10%, and the rolled specimen is obtained.

[0044] Step 2: Perform large reduction amount and small asynchronous ratio flipping asynchronous cold rolling on the rolled specimen. Use an asynchronous rolling mill to roll it. The upper and lower roll diameters of the asynchronous rolling mill are 320 mm and 280 mm respectively, and the upper and lower roll asynchronous ratio is 1.14. After 2 passes of asynchronous rolling with reduction amounts of 25% and 20% respectively, and as Figure 2 shown, the sheet is longitudinally flipped 180° after each pass of rolling for the next pass of rolling, and the sheet is rolled to 2.6 mm, with a total rolling reduction amount of 68%.

[0045] Step 3: Straighten the cold-rolled sheet obtained by asynchronous rolling on a straightening machine. The number of straightening passes is 1 - 3, and the reduction is 0.01 mm to obtain a flat cold-rolled sheet.

[0046] Step 4: Perform recrystallization annealing on the flat cold-rolled sheet at 300 °C with a heating rate of 10 °C / min and hold for 1 h to obtain a superplastic fine-grained sheet.

[0047] Comparative Example 1

[0048] Compared with Example 1, the difference lies in: In Step 2, the reduction per pass of asynchronous rolling is 5%, and the total rolling reduction after 2 passes is 35%.

[0049] Figure 5 For its EBSD, the proportion of high-angle grain boundaries is 72%, and the average grain size is 11.7 μm, not meeting the requirement of superplastic grain size (<10 μm).

[0050] Comparative Example 2

[0051] Compared with Example 1, the difference lies in: In Step 2, the reduction per pass of asynchronous rolling is 9%, and the total rolling reduction after 3 passes is 44%. After detection, the average grain size is 11.3 μm, not meeting the requirement of superplastic grain size (<10 μm).

[0052] It can be seen from Comparative Example 1 and Comparative Example 2 that when the minimum total reduction of 50% is not reached, the grain crushing is insufficient and the dislocation accumulation is not enough to form fine and complete recrystallization during annealing, and the grain refinement is not sufficient to meet the requirement of superplastic grain size (<10 μm).

[0053] Comparative Example 3

[0054] Compared with Example 1, the difference lies in: In Step 2, the sheet is not longitudinally flipped after each pass of rolling, and then the next pass of rolling is continued. Figure 6 This is the EBSD map of the sheet made in this comparative example. The proportion of high-angle grain boundaries is 91%, and the average grain size is 10.1 μm. Compared with Example 1, the grain refinement is not obvious and uniform, not meeting the requirement of superplastic grain size (<10 μm). It can be seen that rolling with sheet flipping in each pass can introduce shear stresses in different directions, making the grain refinement more uniform.

[0055] The content not described in detail in the specification of the present invention belongs to the prior art well-known to those skilled in the art. Although the illustrative specific embodiments of the present invention are described above for the convenience of those skilled in the art to understand the present invention, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention defined and determined by the appended claims, these changes are obvious, and all inventions made using the concept of the present invention are within the scope of protection.

Claims

1. A method for preparing aluminum alloy superplastic sheet for rail transportation by asynchronous rolling, characterized in that: The following steps are involved: Step 1, using a hot-rolled aluminum alloy plate as a raw material, cold rolling is performed by conventional rolling with a small reduction to obtain a rolled sample; Step 2, subjecting the rolled sample to large reduction and small asynchronous ratio reversing asynchronous cold rolling to obtain a cold-rolled sheet; Step 3, straightening the cold-rolled sheet on a straightening machine to obtain a straight cold-rolled sheet; Step 4: Perform recrystallization annealing on the straight cold-rolled sheet to eliminate residual stress and structural inhomogeneity, thereby obtaining a fine-grained sheet with high strain rate superplasticity.

2. The method for preparing aluminum alloy superplastic sheet for rail transportation by asynchronous rolling according to claim 1, characterized in that: In step 1, the rolling passes are 2 to 4, and the rolling reduction in a single pass is controlled at 5 to 10%.

3. The method for preparing aluminum alloy superplastic sheet for rail transportation by asynchronous rolling according to claim 1, characterized in that: The large reduction and small asynchronous ratio reversing asynchronous cold rolling in step 2 is specifically to achieve an upper and lower roller speed ratio of 1.1 to 1.2 by changing the roller diameters of the upper and lower rollers at the same rotation speed.

4. The method for preparing aluminum alloy superplastic sheet for rail transportation by asynchronous rolling according to claim 1, characterized in that: The large reduction and small asynchronous ratio reversing asynchronous cold rolling in step 2 specifically comprises 2 to 4 rolling passes, and the reduction of a single pass is controlled at 10 to 35%.

5. The method for preparing aluminum alloy superplastic sheet for rail transportation by asynchronous rolling according to claim 1, characterized in that: The large reduction and small asynchronous ratio reversal asynchronous cold rolling in step 2 specifically involves longitudinally reversing the plate 180° after each rolling pass for the next rolling pass.

6. The method for preparing aluminum alloy superplastic sheet for rail transportation by asynchronous rolling according to claim 1, characterized in that: The total rolling reduction in step 1 and step 2 is controlled within a range of 50-70%.

7. The method for preparing aluminum alloy superplastic sheet for rail transportation by asynchronous rolling according to claim 1, characterized in that: In step 3, the straightening passes are 1 to 3 passes, and the pressing amount is 0.01 to 0.1 mm.

8. The method for preparing aluminum alloy superplastic sheet for rail transportation by asynchronous rolling according to claim 1, characterized in that: In step 4, the recrystallization annealing temperature is 250-350° C., the heating rate is 10° C. / min, and the annealing time is 0.5-2 h.

9. The method for preparing aluminum alloy superplastic sheet for rail transportation by asynchronous rolling according to claim 1, characterized in that: The hot-rolled aluminum alloy plate in step 1 is commercial 5083 aluminum alloy with a thickness of 2 to 10 mm.

10. The method for preparing aluminum alloy superplastic sheet for rail transportation by asynchronous rolling according to claim 1, characterized in that: The microstructure of the fine-grained plate obtained in step 4 is equiaxed, the average grain size is 1-10 μm, the proportion of high-angle grain boundaries is higher than 90%, the grain boundaries with mismatch angles of not less than 15°, and the plate is 10 -1 ~10 -3 s -1 It has good superplasticity at low strain rates.

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