A heat treatment method for aluminum-lithium alloy forgings

Abstract

This invention relates to a heat treatment method for aluminum-lithium alloy forgings. The method includes (1) solution treatment and quenching; (2) cold pressing; and (3) aging. The cold pressing can be performed in steps, with precision cold pressing completed in two or more steps depending on the thickness of different parts. The step-by-step cold pressing first compresses the thinner parts, and then sequentially completes the cold compression of the entire forging. Forgings treated by this method exhibit high dimensional accuracy, minimal machining requirements, and excellent performance. This invention is applicable to forging production in aerospace, nuclear industry, transportation, sporting goods, and weaponry fields.

Description

Technical Field

[0001] This invention relates to a heat treatment method for aluminum-lithium alloy forgings, belonging to the field of metal materials engineering. Background Art

[0002] Al-Cu-Li-X series aluminum-lithium alloys containing the basic alloying elements Cu and Li are gradually gaining widespread application in the aerospace manufacturing field due to their excellent comprehensive properties of strength, toughness, fatigue resistance, and corrosion resistance. Significant progress has been made in the development of aluminum-lithium alloy metallurgical technology in recent years. The metallurgical development trend of aluminum alloys is towards high purity, high alloying, and micro-alloying, thereby achieving excellent comprehensive properties. High-performance new aluminum-lithium alloys in the Al-Cu-Li-X series, such as 2198, 2196, 2297, 2397, 2099, 2060, and 2050, have already been applied in aerospace. The urgent need for structural weight reduction in aircraft design necessitates the development of large-scale, high-performance materials to meet the needs of integral manufacturing of large parts. The development of aluminum-lithium alloy materials with high modulus, high strength, high toughness, and high hardenability can replace high-strength and high-toughness aluminum alloys such as 7050 and 7085 to meet the needs of aerospace manufacturing development. However, the unit cost of aluminum-lithium alloy materials is more than four times that of ordinary aluminum alloys such as 7050 and 7085. The high cost of aluminum-lithium alloy applications, stemming from traditional thick aluminum alloy plates and forgings, limits their widespread use in aerospace. There is an urgent need to develop new technologies to reduce the cost of aluminum-lithium alloy materials. Summary of the Invention

[0003] This invention addresses the aforementioned limitations of existing technologies by providing a heat treatment method for aluminum-lithium alloy forgings. Its aim is to significantly reduce the application cost of aluminum-lithium alloys while ensuring the uniformity and excellent overall performance of the products. Specifically, aluminum-lithium alloy forgings produced using this method exhibit excellent strength and fracture toughness, with a room temperature tensile strength exceeding 510 MPa, a yield strength exceeding 450 MPa, an elongation exceeding 8%, and a LT-direction KIC exceeding 34 MPa1 / 2.

[0004] The objective of this invention is achieved through the following technical solution:

[0005] This heat treatment method for aluminum-lithium alloy forgings is specifically for Al-Cu-Li-X alloy products. The steps of this method are as follows:

[0006] Step 1: Solution treatment and quenching

[0007] The forging is heated to 500℃~540℃ for solution treatment, and then quenched.

[0008] Step 2: Cold pressing

[0009] The forging is placed in a mold and cold-pressed at room temperature, requiring the compression deformation of each part of the forging along the thickness direction to reach 2.5 to 3.5%.

[0010] Step 3: Artificial Efficiency

[0011] The artificial aging process employs one of the following two methods:

[0012] Method 1: Heat the forging to 145℃~165℃ and hold for 12~42 hours;

[0013] The second method involves heating the forging to 90℃~135℃, holding it at that temperature for 10h~24h, and then continuing to heat it to 150℃~195℃, holding it at that temperature for 4h~22h.

[0014] In practice, the chemical composition and weight percentage of the Al-Cu-Li-X alloy are as follows: Cu 2.0-4.2%, Li 0.6-2.2%, Zr 0.04-0.20%, with the balance being Al.

[0015] In practice, the mold targeted by the technical solution of this invention is a multi-H-shaped cross-section combination structure for aircraft beams and frames. The dimensions of each part of the forging are inconsistent. The thickness of the upper and lower edge strips is 12mm, the thickness of the rib strips is 10mm, the height of the rib strips is 21mm, the thickness of the web plate is 4mm and 8mm, and the dimensional tolerance requirement is ±0.2mm.

[0016] During implementation, the transfer time for quenching after solution treatment in step one shall not exceed 30 seconds, and the temperature of the quenching medium shall not exceed 60°C.

[0017] In practice, the cold pressing in step two is carried out within 4 hours after the solution treatment and quenching in step one.

[0018] In implementation, step two adopts a step-by-step cold pressing method. First, the thicker part of the die forging is cold pressed, and then the die forging is cold pressed as a whole.

[0019] In implementation, the mold used for cold pressing in step two consists of two symmetrical structures, an upper mold base (1), an upper mold mating plate (2), and an upper mold (3), and a lower mold base (7), a lower mold mating plate (6), and a lower mold (5). A pad (4) is provided between the upper mold base (1) and the lower mold base (7) to control the opening and closing gap between the upper mold base (1) and the lower mold base (7) in conjunction with the upper mold mating plate (2) and the lower mold mating plate (6).

[0020] During implementation, the aging temperature for the first type of artificial aging in step three is 165℃, and the aging time is 12 to 20 hours.

[0021] During implementation, the aging temperature for the first type of artificial aging in step three is 145℃, and the aging time is 28 to 42 hours.

[0022] In implementation, the second type of artificial aging in step three has a two-stage aging system: 120℃ / 10h + 160℃ / 20~22h.

[0023] The features and beneficial effects of the technical solution of this invention are as follows:

[0024] I. The technical solution of this invention employs a room temperature precision cold pressing process to ensure accurate and controllable final dimensions of the formed parts. The precision die-pressing cold deformation is controlled between 2.5% and 3.5%, ensuring dimensional deviations in the forgings. Since the amount of cold deformation after quenching significantly affects the aging properties of Al-Cu-Li-X series aluminum-lithium alloys, the uniformity of cold deformation is crucial for controlling performance uniformity. This invention employs a dedicated precision cold pressing die to complete precision cold pressing at room temperature. The precision cold pressing deformation should be pre-designed to ensure the uniformity of cold pressing deformation, thereby guaranteeing the uniformity of performance in different parts of the forging. For the above requirements, the die in this invention is more advantageous than conventional forging fixtures. Conventional forging fixtures have gaps between the upper and lower dies, and the upper and lower dies are not completely closed, belonging to open die forging. This structure makes it impossible to accurately and stably control the amount of deformation during forging, leading to excessive or insufficient deformation in various parts of complex forgings with inconsistent flange, rib, and web dimensions. This can result in low dimensional accuracy of the forgings, uneven reduction of residual stress, and unstable performance. However, the mold of this invention, by adding a backing plate, ensures the stability of the Δh dimension during forging. This also guarantees the stability of the overall height of the tooling after forging, thus avoiding the instability of the forging dimensions caused by the instability of Δh in open die forging. The mold of this invention ensures the dimensional stability of each forging while maintaining dimensional accuracy. When used in conjunction with adjustable upper and lower die fitting plates, it closes the gap between the upper and lower dies, constituting closed die forging. This structure allows for precise and stable control of the compression deformation during forging, resulting in forged parts with high dimensional accuracy and uniform, qualified performance.

[0025] Furthermore, the cold pressing using this mold can be performed in stages, completing precision cold pressing in two or more steps according to the thickness of different parts of the forging. The design and use of appropriate shims based on the different thicknesses of each part ensures that the cold deformation after compression is 2-4%. During staged compression, shims of a certain thickness are first placed on the flanges and ribs of the forging to induce a certain degree of cold deformation in these areas. Then, the shims are removed, and the overall compression is completed, causing cold deformation of the web. This staged compression method ensures sufficient deformation in the difficult-to-deform flanges and ribs, and also guarantees the uniformity of cold compression deformation in the flanges, ribs, and web, thereby effectively reducing residual stress in various parts of the forging.

[0026] II. The technical solution of this invention enables the machining of a precision forging with a multi-H-shaped cross-section composite structure. This product is used as a main structural component in aircraft beams and frames, such as... Figure 2 , 3 As shown, most of the surface area of ​​this product part is unmachined, requiring high control over the forging process. Uneven stress distribution during the forging process can lead to deformation in subsequent processes. The fabrication of multi-H-shaped cross-section forgings is more difficult than that of conventional Π-shaped or T-shaped cross-section forgings, easily resulting in large dimensional deviations and performance defects. Multi-H-shaped forgings integrate the complex structure of the part, reducing the number of parts, eliminating redundant weight from connecting parts, and removing connection defects and processes, thereby effectively improving weapon performance, increasing manufacturing efficiency, reducing assembly costs, and simplifying maintenance. Currently, such parts are mostly manufactured using subtractive manufacturing methods, machined from thick plates or free forgings. However, this invention uses precision die forging for integral forming, with only the upper and lower "H"-shaped surfaces machined, resulting in less machining and improving the utilization rate of expensive aluminum-lithium alloy materials while reducing raw material and overall manufacturing costs. Therefore, the Al-Cu-Li-X alloy used in this invention has a density of 2.71 g / cm³, lower than the density of the traditional 7050 alloy (2.83 g / cm³), giving the product superior overall strength and toughness, as well as a lighter weight. Replacing traditional 7050 aluminum alloy thick plates or free forgings with this alloy can generate significant weight reduction and economic benefits. This invention increases the utilization rate of expensive aluminum-lithium alloy materials from less than 10% to over 70%, reduces raw material and overall manufacturing costs by over 50%, and increases parts manufacturing efficiency by over 5 times. Replacing traditional 7050 aluminum alloy thick plates or free forgings with Al-Cu-Li-X series aluminum-lithium alloy precision forgings reduces costs by over 20%. Products made from this material can be used in parts for aerospace, nuclear industry, transportation, sporting goods, and weaponry. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the mold structure used in the process of this invention;

[0028] Figure 2 This is a schematic diagram of a forging product with a multi-"H" cross-section combination structure, for which the process of this invention is applied.

[0029] Figure 3 This is a three-dimensional schematic diagram of a forging product with a multi-H-shaped cross-section combination structure, which is the target of the process of this invention.

[0030] Figure 4 Photographs of forging products with multiple "H"-shaped cross-section composite structures for which the process of this invention is applied. Detailed Implementation

[0031] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments:

[0032] This embodiment describes the preparation of a qualified Al-Li-Cu-X series aluminum-lithium alloy forging, the alloy composition of which is shown in Table 1. The product has a multi-H-shaped cross-section combination structure, and its structure, shape, and dimensions are as follows... Figure 2-4 As shown.

[0033] Table 1 Chemical composition of Al-Cu-Li-X series aluminum-lithium alloys

[0034] Cu / wt% Li / wt% Mg / wt% Mn / wt% Zr / wt% Ag / wt% Zn / wt% 3.6 1.1 0.4 0.4 0.12 0.4 0.15

[0035] The cold pressing mold used in this embodiment is as follows: Figure 1 As shown, the mold consists of two symmetrical parts, an upper mold base (1), an upper mold fitting plate (2), and an upper mold (3). The lower mold base (7), a lower mold fitting plate (6), and a lower mold (5) are included. A pad (4) is provided between the upper mold base (1) and the lower mold base (7) to control the opening and closing gap between the upper mold base (1) and the lower mold base (7) in conjunction with the upper mold fitting plate (2) and the lower mold fitting plate (6).

[0036] The process steps in this embodiment are as follows:

[0037] Step 1: Solution treatment and quenching

[0038] The final forging of the die forging is solution-treated at 520℃, and then quenched. The transfer time is not higher than 30 seconds, and the temperature of the quenching medium is not higher than 60℃.

[0039] Step 2: Cold pressing

[0040] After solution treatment and quenching, the forging is placed in a mold within 4 hours to complete precision cold pressing at room temperature. A step-by-step cold pressing method is adopted. First, the thicker part of the forging is cold pressed, and then the forging is cold pressed as a whole. The cold pressing deformation is 2.5-3.5%.

[0041] Step 3: Artificial Efficiency

[0042] Artificial aging process parameters: aging temperature is 160℃, and aging time is 22-24h.

[0043] The room temperature tensile and fracture toughness properties of the forging are shown in Table 2. The room temperature tensile test method of the forging is in accordance with GB / T228.1 "Metallic materials - Tensile testing - Part 1: Room temperature test method", and the fracture toughness test method is in accordance with HB 5487 "Metallic materials - Plane strain fracture toughness KIC test method".

[0044] Table 2. Room temperature tensile and fracture toughness properties of Al-Cu-Li-X alloy forgings under different heat treatment regimes.

[0045]

[0046]

[0047] The dimensional inspection results of the forging are shown in Table 3. As can be seen from Table 3, the cold pressing deformation is controlled within 2.5% to 3.5%, and the dimensional accuracy of the forging is high.

[0048] Table 3 Dimensional Inspection Results of Forgings

[0049]

Claims

1. A heat treatment method for aluminum-lithium alloy forgings, characterized in that: This method is for Al-Cu-Li-X alloys, and the steps are as follows: Step 1: Solution treatment and quenching The forging is heated to 500℃~540℃ for solution treatment, and then quenched. Step 2: Cold pressing The forging is placed in a mold and cold-pressed at room temperature, requiring the compression deformation along the thickness direction of each part of the forging to reach 2.5% to 3.5%. Step 3: Artificial Efficiency The artificial aging process employs one of the following two methods: Method 1: Heat the forging to 145℃~165℃ and hold for 12~42 hours; The second method: Heat the die forging to 90℃~135℃, hold for 10h~24h, then continue heating to 150℃~195℃, hold for 4h~22h. The chemical composition and weight percentage of the Al-Cu-Li-X alloy are as follows: Cu 2.0-4.2%, Li 0.6-2.2%, Zr 0.04-0.20%, with the balance being Al; The forging is a multi-"H" cross-section combination structure used for aircraft beams and frames. The dimensions of each part of the forging are inconsistent. The thickness of the upper and lower edge strips is 12mm, the thickness of the rib strips is 10mm, the height of the rib strips is 21mm, and the thickness of the web is 4mm and 8mm. The dimensional tolerance requirement is ±0.2mm. Step two adopts a step-by-step cold pressing method. First, the thicker part of the die forging is cold pressed, and then the die forging is cold pressed as a whole. The mold used for cold pressing in step two consists of two symmetrical parts, an upper mold base (1), an upper mold mating plate (2), and an upper mold (3). The lower mold base (7), a lower mold mating plate (6), and a lower mold (5) are included. A pad (4) is provided between the upper mold base (1) and the lower mold base (7) to control the opening and closing gap between the upper mold base (1) and the lower mold base (7) in conjunction with the upper mold mating plate (2) and the lower mold mating plate (6).

2. The heat treatment method for aluminum-lithium alloy forgings according to claim 1, characterized in that: The transfer time for quenching after solution treatment in step one shall not exceed 30 seconds, and the temperature of the quenching medium shall not exceed 60℃.

3. The heat treatment method for aluminum-lithium alloy forgings according to claim 1, characterized in that: The cold pressing in step two is carried out within 4 hours after the solution treatment and quenching in step one.

4. The heat treatment method for aluminum-lithium alloy forgings according to claim 1, characterized in that: In step three, the first type of artificial aging involves an aging temperature of 165℃ and an aging time of 12–20 hours.

5. The heat treatment method for aluminum-lithium alloy forgings according to claim 1, characterized in that: The first type of artificial aging in step three involves an aging temperature of 145℃ and an aging time of 28–42 hours.

6. The heat treatment method for aluminum-lithium alloy forgings according to claim 1, characterized in that: The second artificial aging system in step three is a two-stage aging process: 120℃ / 10h + 160℃ / 20~22h.

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