Method for improving surface quality and comprehensive performance of aluminum-lithium alloy sheet

Abstract

The present application belongs to the technical field of deformation heat treatment of aluminum-lithium alloy, and relates to a method for improving the comprehensive performance of aluminum-lithium alloy sheet. After solid solution quenching treatment of the aluminum-lithium alloy sheet, a certain amount of pre-stretching deformation is carried out in a short time, and then after a period of storage, a certain amount of secondary pre-stretching deformation is carried out again, and then natural or artificial aging is carried out to the stable state. The storage for a period of time after the first pre-stretching can promote the formation of coherent GP zones, improve the uniformity of sheet stretching deformation, and avoid the formation of slip bands on the surface in the second pre-stretching process with a large amount of deformation. The second pre-stretching with a large amount of deformation can further eliminate the quenching residual stress, and lay a foundation for obtaining the required types and distribution of precipitated phases in the subsequent natural aging and artificial aging processes. Finally, the natural or artificial aging is carried out to obtain the sheet with excellent comprehensive performance, which is suitable for aluminum-lithium alloy sheets used in the fields of aviation, aerospace, weapons, transportation and the like.

Description

Technical Field

[0001] This invention belongs to the field of deformation heat treatment technology for aluminum-lithium alloys, and relates to a method for improving the surface quality and overall performance of aluminum-lithium alloy thin plates. Background Technology

[0002] Aluminum-lithium alloys possess advantages such as high specific stiffness, high specific strength, excellent damage resistance, corrosion resistance, and machinability, making them a key metallic material of interest in the aerospace industry. Third-generation aluminum-lithium alloys, through compositional design methods that appropriately increase copper content, reduce lithium content, and introduce multi-element microalloying elements such as Mg, Zn, Mn, Zr, and Ag, have significantly improved the overall performance of aluminum-lithium alloys, leading to their widespread application in the aerospace field. Third-generation aluminum-lithium alloy sheets, due to their excellent stiffness and damage resistance, are primarily used in structures such as fuselage skins and rocket outer skins. Since the overall performance of third-generation aluminum-lithium alloys requires deformation heat treatment, for thin sheets, considering the performance requirements and subsequent use in skin structures (requiring cold forming followed by chemical milling or mirror milling), aluminum-lithium alloy sheets require pre-stretching treatment with a specific deformation amount (generally not less than 2.5%) after quenching to eliminate residual stress from quenching and lay the foundation for subsequent performance control.

[0003] Due to the complex composition of third-generation aluminum-lithium alloys, there are numerous types of precipitates. Among them, the Al3Li phase is a coherent precipitate. A large amount of Al3Li precipitation can easily lead to coplanar slip, resulting in strong anisotropy and low fracture toughness. Therefore, it is necessary to strictly control the quantity and distribution of Al3Li precipitates. The Al3Li phase generally precipitates rapidly after quenching. Pre-stretching deformation after quenching can effectively suppress the precipitation of Al3Li phase and promote the formation of the precursor GP region of T1 phase and θ′ phase. However, for thin plates, pre-stretching with a large amount of deformation after quenching can easily form interlocking "slip stripes" on the plate surface, affecting the surface quality of the final component. On the other hand, a small amount of deformation makes it difficult to obtain the final required performance and low quenching residual stress, which also affects the final use of the plate. Summary of the Invention

[0004] The purpose of this invention is to address the challenge of controlling the surface quality and overall performance of third-generation aluminum-lithium alloy thin plates in the application of aerospace and aviation fields, and to provide a method for achieving high surface quality and overall performance. This method can ensure that aluminum-lithium alloy thin plates have excellent overall performance while avoiding the formation of slip bands on the surface, thus guaranteeing the surface quality of the thin plates.

[0005] To solve this technical problem, the technical solution of the present invention is as follows:

[0006] A method for improving the surface quality and overall performance of aluminum-lithium alloy thin plates includes: solution-quenching the aluminum-lithium alloy thin plate, followed by pre-stretching with a certain deformation amount within a short period of time, then allowing it to stand for a period of time before undergoing a second pre-stretching treatment; after the second pre-stretching, natural or artificial aging treatment to a stable state; the first pre-stretching suppresses the formation of Al3Li coherent precipitates and lays the foundation for the formation of GP zones; the precipitation of GP zones during the standing period improves the deformation uniformity and suppresses the formation of slip bands during stretching; the second pre-stretching further eliminates residual stress and obtains the deformation amount required for deformation heat treatment; finally, natural or artificial aging treatment is used to obtain the desired microstructure and ensure the excellent overall performance of the thin plate.

[0007] The specific steps of the method are as follows:

[0008] Step 1: Solution quenching + first pre-stretching treatment:

[0009] The aluminum-lithium alloy sheet is heated and subjected to solution quenching followed by a short-term pre-stretching treatment with a certain amount of deformation; preferably, the aluminum-lithium alloy sheet is heated in an air-circulating furnace or a salt bath furnace.

[0010] Solution treatment is performed by furnace loading at a set temperature of 495℃ to 535℃, with a holding time of 20 to 90 minutes. The quenching temperature is 18 to 40℃, and the quenching transfer time does not exceed 10 seconds. Preferably, the quenching medium is water.

[0011] The pre-stretching treatment involves pre-stretching the quenched sheet within 2 hours, with a pre-stretch deformation of 0.8% to 1.5%. Since the sheet length in this invention is at least 1500 mm, drilling, hoisting, disassembling, and transferring are required during solution treatment, and the transfer of the sheet takes at least half an hour. Therefore, for the sheet size of this invention, 2 hours is considered a relatively short time.

[0012] Step 2: Parking + Secondary Pre-stretching Treatment:

[0013] After the board material treated in step 1.1 is placed at room temperature for 12 to 22 hours, it undergoes a second pre-stretching treatment with a pre-stretching deformation of 1.7 to 4.5%.

[0014] Step 3, Aging Treatment: The boards treated in Step 1.2 are subjected to natural aging or artificial aging until they reach a stable state.

[0015] The natural aging process involves placing the thin sheet, after undergoing two pre-stretching processes, at room temperature for at least 100 hours.

[0016] The artificial aging treatment involves placing the thin sheet, after secondary pre-stretching, into an air-circulating furnace for artificial aging. The furnace is opened at the designated temperature, and the aging process is carried out at 135℃~165℃ for 15~36 hours, followed by cooling to room temperature. Preferably, the cooling method is air cooling.

[0017] The method is applicable to thin plates with a thickness ranging from 0.8 mm to 6.0 mm. Plates thicker than 6 mm are considered medium-thick plates. Since it is difficult for slip lines to appear when stretching medium-thick plates thicker than 6 mm, the amount of stretching is not considered. However, for plates with a thickness of 0.8 mm to 6.0 mm, a large amount of stretching at once can easily produce slip strips, so the method involves stretching in two stages.

[0018] The aluminum-lithium alloy to which the method is applicable has the following composition and mass percentage of each component: Cu 2.1-4.5%, Li 0.8-2.1%, Mg 0.4-1.3%, Zn 0.2-0.8%, Mn 0.2-0.6%, Zr 0.10-0.20%, Ti 0.02-0.10%, and Ag 0.2-0.8%, Sc 0.08-0.25%, any 1-2 of the following: impurity elements Si≤0.10%, Fe≤0.12%, other impurities individually≤0.05%, total ≤0.15%, and the balance is Al.

[0019] The beneficial effects of this invention are:

[0020] The present invention provides a method for achieving high surface quality and comprehensive performance. Through a first pre-stretch deformation after solution quenching, a second pre-stretch deformation after a period of rest, and a final natural or artificial aging treatment, the method can achieve excellent comprehensive performance by controlling the type and distribution of precipitated phases in thin plates, while avoiding the formation of slip bands on the surface of thin plates and improving the surface quality of thin plates.

[0021] Specifically, reasonable pre-stretching deformation within a short time after solution quenching can suppress the formation of coherent Al3Li precipitates in Al-Cu-Li-Mg-X aluminum-lithium alloys and promote the enrichment of elements such as Cu, Mg, and Ag near dislocations. Simultaneously, it prevents the formation of slip bands on the thin plate surface. A period of rest after the first pre-stretching promotes the formation of coherent GP zones, improves the uniformity of tensile deformation, and avoids the formation of slip bands on the surface during the second pre-stretching process with a larger deformation. The second pre-stretching with a larger deformation further eliminates residual quenching stress and lays the foundation for obtaining the desired type and distribution of precipitates during subsequent natural and artificial aging processes. Finally, the desired microstructure is obtained, and a thin plate with excellent comprehensive properties is obtained through natural or artificial aging. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] The features of various aspects of the embodiments of the present invention will now be described in detail. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention can also be practiced without these specific details. The following description of the embodiments is merely intended to provide a better understanding of the invention by illustrating examples. The invention is not limited to any specific setups and methods provided below, but covers all improvements, substitutions, etc., to product structures and methods without departing from the spirit of the invention.

[0024] In the following description, well-known structures and techniques are not shown in order to avoid unnecessarily obscuring the invention.

[0025] Example 1

[0026] The aluminum-lithium alloy sheet used in this invention is processed to treat the aluminum-lithium alloy sheet for skin. First, an aluminum-lithium alloy sheet with the following alloy composition (Cu 4.0%, Mg 0.9%, Li 0.85%, Zn 0.42%, Ag 0.35%, Mn 0.25%, Zr 0.12%, Ti 0.03%, impurity elements Si 0.04%, Fe 0.06%, balance Al) and a thickness of 2.5 mm is loaded into a material frame and placed in an air-circulating heating furnace. The furnace is opened at 500°C and held for 10 minutes, then heated to 505°C and held for 50 minutes. After removal from the furnace, the sheet is quenched in water at room temperature. The quenched sheet is then pre-stretched within 60 minutes, with a pre-stretch deformation of 1.0%.

[0027] After the first pre-stretching treatment, the board was placed at room temperature for 16 hours and then placed on a pre-stretching machine for a second pre-stretching treatment. The deformation amount of the second pre-stretching was 2.0%.

[0028] The pre-stretched sheet was placed in an air-circulating furnace for artificial aging treatment. The furnace was opened at 143°C, and timing began after the material reached this temperature. The holding time was 30–38 hours, after which the sheet was air-cooled to room temperature. The surface of the sheet was observed, and tensile strength, fracture toughness, fatigue crack propagation rate, and corrosion resistance were measured.

[0029] Comparative Example 1: A comparison was made between a sheet of the above alloy composition that was quenched and then subjected to 2.8% pre-stretch deformation (existing technology) and then subjected to the same aging process. The results are shown in Table 1.

[0030] Table 1

[0031]

[0032] The results show that the aluminum-lithium alloy thin plates prepared by the method of the present invention have significantly improved surface quality, fracture toughness and fatigue crack propagation rate while maintaining comparable tensile and corrosion properties.

[0033] Comparative Example 2: Under the same test conditions, the quenched sheet was pre-stretched within 60 minutes, with a pre-stretch deformation of 1.0%. After the sheet underwent the first pre-stretching treatment, it was placed at room temperature for 10 hours (shortening the resting time) and then placed on a pre-stretching machine for a second pre-stretching treatment, with a second pre-stretch deformation of 2.0%.

[0034] Comparative Example 3: Under the same test conditions, the quenched sheet was pre-stretched within 60 minutes, with a pre-stretch deformation of 1.0%. After the sheet underwent the first pre-stretching treatment, it was placed at room temperature for 30 hours (the resting time was extended) and then placed on a pre-stretching machine for a second pre-stretching treatment, with a second pre-stretch deformation of 2.0%.

[0035] The test results for Comparative Example 2 and Comparative Example 3 are shown in Table 2 below.

[0036] Table 2

[0037]

[0038] The results show that in Comparative Example 2, the thin plate was left to rest for a short time after the first pre-stretch, resulting in slip strips on the surface of the final plate and a decrease in surface quality. In Comparative Example 3, the thin plate was left to rest for too long after the first pre-stretch. Although the surface quality of the thin plate was normal, the tensile strength of the thin plate increased, the plasticity decreased, and the fatigue crack propagation rate increased.

[0039] Example 2

[0040] The aluminum-lithium alloy sheet processing method of this invention is used to process a 1.5mm thick aluminum-lithium alloy sheet for skin. First, an aluminum-lithium alloy sheet with the following alloy composition (Cu 3.65%, Li 1.52%, Mg 0.45%, Zn 0.32%, Mn 0.40%, Zr 0.11%, Sc 0.09%, Ti 0.04%, impurity elements Si 0.05%, Fe 0.07%, balance Al) and a thickness of 1.5mm is loaded into a material frame and placed in an air-circulating heating furnace. The furnace is opened at 510℃ and held for 5 minutes, then heated to 520℃ and held for 30 minutes. After removal from the furnace, the sheet is quenched in water at room temperature. The quenched sheet undergoes a pre-stretching treatment within 60 minutes, with a pre-stretch deformation of 1.3%.

[0041] After the first pre-stretching treatment, the board was placed at room temperature for 20 hours and then placed on a pre-stretching machine for a second pre-stretching treatment. The deformation amount of the second pre-stretching was 2.0%.

[0042] After the thin plate underwent secondary pre-stretching, it was placed at room temperature for 120 hours. The surface of the plate was then observed and its tensile strength, fracture toughness, fatigue crack propagation rate, and corrosion performance were measured.

[0043] Comparative Example 5 compares a sheet of the above alloy composition that has been quenched and then subjected to 1.0% pre-stretch deformation (existing technology) and the same natural aging treatment. The 1% deformation was chosen in Comparative Example 5 to compare the sheet performance under the condition that neither the present invention nor the existing process exhibits slip lines. The results are shown in Table 3.

[0044] Table 3

[0045]

[0046]

[0047] The results show that the aluminum-lithium alloy thin plates prepared by the method of the present invention, with the same surface quality and slightly lower strength, have significantly improved elongation, fracture toughness, fatigue crack propagation rate and corrosion resistance compared with plates prepared by traditional processes.

[0048] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should be covered within the protection scope of the present invention.

Claims

1. A method for improving the surface quality and overall performance of aluminum-lithium alloy thin plates, characterized in that: The method involves solution-quenching an aluminum-lithium alloy sheet, followed by pre-stretching with a certain deformation amount within a short period, and then allowing it to stand for a period of time before undergoing a second pre-stretching treatment. After the second pre-stretching, the sheet is subjected to natural or artificial aging treatment until it reaches a stable state. The first pre-stretching inhibits the formation of coherent Al3Li precipitates and lays the foundation for the formation of GP zones. During the standing period, the precipitation of GP zones can improve the deformation uniformity and inhibit the formation of slip bands during stretching. The second pre-stretching can further eliminate residual stress and obtain the deformation amount required for deformation heat treatment. Finally, natural or artificial aging treatment is used to obtain the expected microstructure and ensure the excellent comprehensive performance of the sheet.

2. The method according to claim 1, characterized in that: The thin plate refers to a sheet material with a thickness ranging from 0.8 mm to 6.0 mm.

3. The method according to claim 1, characterized in that: The method steps are as follows: Step 1: Solution quenching + first pre-stretching treatment: The aluminum-lithium alloy sheet is heated, and after solution quenching, it is subjected to a pre-stretching treatment with a certain amount of deformation within a short period of time. Solution treatment is carried out by furnace entry at a certain temperature. The solution treatment process is 495℃~535℃, and the holding time is 20min~90min. The quenching temperature is 18~40℃, and the quenching transfer time does not exceed 10s. Pre-stretching treatment involves pre-stretching the quenched sheet within 2 hours, with a pre-stretch deformation of 0.8-1.5%. Step 2: Parking + Secondary Pre-stretching Treatment: After the board material treated in step 1.1 is placed at room temperature for 12 to 22 hours, it undergoes a second pre-stretching treatment with a pre-stretching deformation of 1.7 to 4.5%. Step 3, Aging Treatment: The boards treated in Step 1.2 are subjected to natural aging or artificial aging until they reach a stable state.

4. The method according to claim 3, characterized in that: Step 1: The quenching medium is water.

5. The method according to claim 3, characterized in that: The heating of the aluminum-lithium alloy sheet in step one is carried out in an air-circulating furnace.

6. The method according to claim 3, characterized in that: The heating of the aluminum-lithium alloy sheet described in step one is carried out in a salt bath furnace.

7. The method according to claim 3, characterized in that: The natural aging process described in step three involves placing the sheet metal, after undergoing two pre-stretching processes, at room temperature for at least 100 hours.

8. The method according to claim 3, characterized in that: The artificial aging process described in step one involves placing the thin plate, after secondary pre-stretching, into an air-circulating furnace for artificial aging. The furnace is opened at the specified temperature, and the aging process is carried out at 135℃~165℃ for 15~36 hours, after which the plate is removed from the furnace and cooled to room temperature.

9. The method according to claim 3, characterized in that: The aluminum-lithium alloy to which the method is applicable has the following composition and mass percentage of each component: Cu 2.1-4.5%, Li 0.8-2.1%, Mg 0.4-1.3%, Zn 0.2-0.8%, Mn 0.2-0.6%, Zr 0.10-0.20%, Ti 0.02-0.10%, and Ag 0.2-0.8%, Sc 0.08-0.25%, any 1-2 of the following: impurity elements Si≤0.10%, Fe≤0.12%, other impurities individually≤0.05%, total ≤0.15%, and the balance is Al.

10. The method according to claim 8, characterized in that: The cooling method is air cooling.

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