A method for preparing high-strength corrosion-resistant rivets using 7050 aluminum alloy

Abstract

The present invention belongs to the technical field of aluminum alloy rivet processing and discloses a method for preparing high-strength, corrosion-resistant rivets using 7050 aluminum alloy. The method comprises five process steps: homogenization heat treatment, equal channel angular extrusion, cold heading, aging heat treatment, and nano-coating spraying. The method improves the mechanical properties of the rivets after forming, integrates and simplifies the process flow in the rivet manufacturing process, and shortens the production cycle. The method not only ensures that the rivets after forming have a yield strength of 550 MPa, a hardness of 190 HV, and an elongation of 8-10%, and have good corrosion resistance, but also greatly shortens the aging time after forming.

Description

Technical Field

[0001] The invention belongs to the technical field of aluminum alloy rivet processing, and particularly relates to a method for preparing high-strength corrosion-resistant rivets by using 7050 aluminum alloy. Background Art

[0002] Aluminum alloys are the most widely used nonferrous structural materials in industry due to their excellent corrosion resistance, versatile forming methods, high recycling rate, excellent thermal and electrical conductivity, high strength, and low density (only one-third that of steel). Their lightweight properties surpass those of steel, along with excellent impact resistance and significant weight reduction, make them widely used in aviation, aerospace, automotive, machinery manufacturing, shipbuilding, and the chemical industry. Rivets connect riveted parts by deforming or interfering with each other. High strength and lightweight are the development trends of rivet materials, making the forming and heat treatment of high-strength aluminum alloy rivets a research hotspot in the manufacturing industry.

[0003] 7XXX series aluminum alloys feature high strength, low density, good processing properties, and resistance to stress corrosion cracking. They are typical precipitation-strengthened aluminum alloys. Utilizing metastable precipitates such as Mg-Zn or Al-Mg-Zn precipitated during aging, the strength of the alloy can be significantly improved, with a yield strength as high as 500MPa or even above 600MPa, resulting in an ultra-high-strength aluminum alloy. Therefore, they have become a common material for high-strength aluminum alloy rivets.

[0004] The general process of manufacturing high-strength rivets using 7XXX series aluminum alloys is as follows: first, 7XXX series aluminum alloy ingots (cast aluminum alloys) are processed into high-strength 7XXX series aluminum alloy profiles through a series of processing techniques; then the high-strength profiles are formed into rivets through processes such as cold heading, hot rolling, cutting, and cold drawing. Since the heat and force generated during processing will reduce the mechanical properties of the material, the performance of the formed rivets is unsatisfactory. Therefore, the formed rivets need to be further heat treated to eliminate material defects and improve the mechanical properties of the rivets.

[0005] Among them, the processing technology of 7XXX series aluminum alloy profiles includes: homogenization treatment → hot working (hot extrusion or hot rolling) → solution treatment → aging treatment. The homogenization treatment before hot working is usually kept at a temperature above 450℃ for more than 20 hours, the solution treatment after hot working is usually kept at a temperature above 450℃ for 1-2 hours, and the aging treatment is usually kept at 100-200℃ for more than 10 hours. Its production process is cumbersome and the processing cycle is relatively long. In addition, the conventional homogenization and solution treatment have a general solid solution effect. There will still be a large number of second phase particles remaining in the grain boundary area of ​​the alloy, and these particles will further grow during hot working or aging, promoting the formation of subsequent grain boundary precipitation phase (GBP) and intergranular precipitation-free zone (PFZ), thereby causing the alloy's strength, toughness and stress corrosion resistance to decrease. In recent years, a series of solid solution processes such as multi-stage solid solution, enhanced solid solution, high temperature pre-precipitation, and solid solution cooling treatment have been developed to reduce the size and volume fraction of the grain boundary precipitate phase. Although the above solution process has a certain effect on improving the solid solubility of 7XXX series alloys, it is still difficult to shorten the production cycle of such alloys.

[0006] The heat treatment process after forming high-strength rivets made of 7XXX series aluminum alloy generally includes: solution treatment, quenching, and aging treatment. The processing steps and time of solution treatment and aging treatment are similar to those of 7XXX series aluminum alloy profile processing. The main purpose of quenching is to increase the surface hardness of the rivet after forming.

[0007] In the prior art, the method of manufacturing high-strength rivets using 7XXX series aluminum alloys, due to the two heat treatment processes after profile forming and rivet forming, not only reduces energy efficiency and production efficiency, increases the production cycle, but is also not conducive to improving the mechanical properties of the rivets. In the prior art, there are some studies on shortening the processing cycle by improving the profile processing technology or the forming process. For example, patent CN110408868B discloses a method for preparing a high-strength and tough aluminum alloy without aging based on large deformation homogenization. This method first uses a multi-pass continuous equal channel angular extrusion process at a solution temperature (450℃~480℃) to achieve homogenization, and then hot extrude the alloy rod at a relatively low temperature (300℃~350℃). The yield strength of the formed aluminum alloy profile reaches 500~600MPa or more and the elongation is greater than 10%. In addition, no subsequent aging treatment is required after hot working, further shortening the production cycle. However, this method performs large equal-channel rotational deformation at high temperature, which reduces the dislocation density and reduces the work hardening effect. As a result, the hardness of the rivets cannot be guaranteed when the high-strength profiles obtained by this method are used to manufacture rivets.

[0008] Currently, there is no research on integrating the high-strength 7XXX series aluminum alloy profile forming process with the post-forming heat treatment process of rivets to simplify the processing technology, shorten the process flow, and improve production efficiency while ensuring the mechanical properties and corrosion resistance of the rivets after forming, such as strength and hardness. Summary of the Invention

[0009] In response to the shortcomings of the existing technology, the present invention provides a method for preparing high-strength, corrosion-resistant rivets using 7050 aluminum alloy. This method not only improves the mechanical properties of the rivets after forming, but also integrates and simplifies the process flow of material strengthening, rivet forming, and post-forming heat treatment during the rivet manufacturing process, thereby shortening the production cycle. This method ensures that the rivets after forming have a yield strength of 550 MPa, a hardness of 190 HV, and an elongation of 8-10%, and have good corrosion resistance. Furthermore, the method significantly shortens the processing time and aging time, thereby improving production efficiency.

[0010] To achieve the above object, the present invention provides the following technical solutions:

[0011] A method for preparing high-strength, corrosion-resistant rivets using 7050 aluminum alloy is characterized by comprising the following steps: A. homogenization heat treatment: subjecting a cast 7050 aluminum alloy wire to homogenization heat treatment; B. equal channel angular extrusion: drawing the 7050 aluminum alloy wire after the homogenization heat treatment into an equal channel angular extrusion die and performing continuous equal channel angular extrusion processing at room temperature to obtain a fine homogeneous wire; C. cold heading: processing the fine homogeneous 7050 aluminum alloy wire into a rivet in a cold heading machine, comprising the following steps: blanking, head preforming, head forming, and shank diameter reduction; D. aging heat treatment; E. nano-coating spraying: spraying a nano-coating on the surface of the aged rivet in a coating device, wherein the coating includes three functional zones: an adhesive layer, a transition layer, and a wear-resistant layer.

[0012] Preferably, the aging heat treatment in step D is: keeping warm at 100-120° C. for 2-3 hours.

[0013] Preferably, the number of equal channel angular extrusion passes in step B is 4 to 8.

[0014] Preferably, the homogenization heat treatment in step A is a two-stage homogenization heat treatment: that is, the 7050 aluminum alloy cast wire is heated from room temperature to T1, kept at T1 for t1, and then heated from T1 to T2, kept at T2 for t2, wherein T1 is 435°C to 465°C, t1 is 12 to 24 hours; T2 is 475°C, and t2 is 4 to 10 hours.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] 1. Because the present invention adopts the homogenization-room temperature equal channel angular extrusion-cold heading composite process to form the cast 7050 aluminum alloy wire into high-strength rivets, it not only simplifies the process flow and shortens the processing time, but also reduces the peak aging time of the material performance after forming, shortening the production cycle.

[0017] 2. Because the present invention adopts room temperature equal channel angular extrusion to refine the structure of 7050 aluminum alloy wire, on the basis of ensuring the strength and toughness of the material, the hardening effect in the rivet forming process is maintained, thereby ensuring the hardness of the rivet after forming.

[0018] 3. Because the present invention adopts the cold heading process to form the rivets, the process is simple, the cost is low and the efficiency is high.

[0019] 4. Because the present invention applies a nano coating to the rivet after forming and aging, the mechanical properties of the rivet, such as fracture resistance, wear resistance and corrosion resistance, are improved while also improving the hardness of the rivet. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a process flow chart for preparing high-strength, corrosion-resistant rivets using 7050 aluminum alloy according to an embodiment of the present invention;

[0021] Figure 2 This is a metallographic microstructure diagram of the 7050 aluminum alloy as-cast wire (465° C.-24 h) after single-stage homogenization heat treatment according to an embodiment of the present invention;

[0022] Figure 3 This is a metallographic microstructure diagram of the 7050 aluminum alloy as-cast wire after a two-stage homogenization heat treatment (465°C-24h+475°C-4h) according to an embodiment of the present invention;

[0023] Figure 4 A comparison diagram of the metallographic structures of the 7050 aluminum alloy after homogenization heat treatment and equal channel angular extrusion according to an embodiment of the present invention;

[0024] Figure 5 This is the equivalent strain distribution diagram of the 7050 aluminum alloy material after one to four passes of Bc path equal channel angular extrusion (ECAP) processing in an embodiment of the present invention. In the figure, I, II, III, and IV are the shear force action surfaces generated by the ECAP die corners. DETAILED DESCRIPTION

[0025] In order to make the technical means, creative features, objectives and effects achieved by the present invention easy to understand, the present invention is specifically described in the following examples in conjunction with the accompanying drawings. It should be noted that the description of these implementation methods is used to help understand the present invention, but does not constitute a limitation of the present invention.

[0026] The main purpose of the present invention is to simplify the manufacturing process of high-strength and corrosion-resistant aluminum alloy rivets and shorten the manufacturing cycle on the basis of improving the strength, hardness and corrosion resistance of the rivets after forming. Figure 1 As shown, a method for preparing high-strength corrosion-resistant rivets using 7050 aluminum alloy in this embodiment includes five process steps: homogenization heat treatment, equal channel angular extrusion, cold heading, aging heat treatment, and nano-coating spraying.

[0027] 1. Homogenization heat treatment

[0028] The cast aluminum alloy wires are classified and placed in a heat treatment furnace for homogenization heat treatment. The purpose is to eliminate the element segregation of the cast aluminum alloy wire structure to the greatest extent, eliminate dendritic segregation, fully dissolve and evenly distribute the alloy elements, so that the alloy has good mechanical processing properties and comprehensive mechanical properties. The main purpose is to enable the aluminum alloy wire to pass through equal channel angular extrusion smoothly and complete the material fine grain effect.

[0029] Aluminum alloy wires were analyzed using a DSC (Differential Scanning Calorimeter) instrument. The resulting DSC curves provided the target temperature basis for the homogenization heat treatment process. Below the target temperature, based on previous research, a single-stage (heating to X° + holding for Y hours), a two-stage (heating to X1° + holding for Y1 hour + heating to X2° + holding for Y2 hours), or a three-stage (heating to X1° + holding for Y1 hour + heating to X2° + holding for Y2 hours + heating to X3° + holding for Y3 hours) process was developed to complete the homogenization heat treatment process.

[0030] The preferred single-stage homogenization heat treatment process of the 7050 aluminum alloy of the present invention is as follows: the cast wire of the 7050 aluminum alloy is heated from room temperature to 435℃~465℃ and kept at this temperature for 12h~24h; the microstructure metallographic diagram of the 7050 aluminum alloy after the homogenization heat treatment at 465℃-24h is as follows: Figure 2 shown.

[0031] The preferred two-stage homogenization heat treatment process of 7050 aluminum alloy of the present invention is as follows: the cast wire of 7050 aluminum alloy is heated from room temperature to T1, and kept at T1 for t1, and then the cast wire is heated from T1 to T2, and kept at T2 for t2, wherein T1 is 435℃~465℃, t1 is 12h~24h; T2 is 475℃, and t2 is 4h~10h. The microstructure metallographic diagram of 7050 aluminum alloy after 465℃-24h+475℃-4h homogenization heat treatment is as follows: Figure 3 As shown in the figure, compared with the homogenization heat treatment at 465℃ for 24h, the lattice becomes larger, but the microstructure uniformity is better.

[0032] 2. Equal channel angular extrusion (ECAP, also known as equal channel angular extrusion)

[0033] The 7050 aluminum alloy wire after homogenization heat treatment is successively pulled into the equal channel angular extrusion die, and subjected to continuous equal channel angular Bc deformation path extrusion processing at room temperature (the Bc path is that the workpiece rotates 90° between adjacent passes, and the rotation direction does not change) to achieve wire microstructure refinement and homogenization, and obtain fine and homogeneous wire.

[0034] Experiments show that after the wire is processed by the equal channel angular extrusion die with four Bc deformation path ECAP processes, the grain refinement effect is greatly improved. Figure 4 At the same time, the multi-pass deformation Bc path ECAP deformation also ensures the uniformity of the overall deformation, reducing defects such as head warping that occur when the aluminum alloy wire undergoes equal-angle angular extrusion; Figure 5 Equivalent strain distribution diagram of ECAP processing of Bc path from one to four passes. In the figure, Ⅰ, Ⅱ, Ⅲ, and Ⅳ are the shear force action surfaces generated by the ECAP die corner. Figure 5 As can be seen, with increasing processing passes, the equivalent strain on each active surface gradually converges, and the wire material is nearly uniform after four passes of Bc deformation path ECAP processing. Multi-pass Bc deformation path ECAP also significantly improves head warping and head eccentricity during cold heading rivets, thereby improving rivet forming quality. Considering both deformation uniformity and processing efficiency, 4-8 passes of Bc deformation path ECAP processing are preferred.

[0035] The equal channel angular extrusion process has high processing efficiency. In the present invention, it only takes 5 seconds to process a 45 mm long rod, and the time required for continuous equal channel angular extrusion of 100 meters of 7050 aluminum alloy wire is about 2.5 to 3 hours.

[0036] 3. Cold heading

[0037] Processing 7050 aluminum alloy fine homogeneous wire into rivets in a multi-station cold heading machine includes the following steps: blanking, head preforming, head forming, and shank diameter reduction;

[0038] Cutting: Cut the fine homogeneous wire into 7050 aluminum alloy bars; for example, cut the fine homogeneous wire into 7050 aluminum alloy bars with a length of 35 mm; then cold-forged rivets comply with GB / T109-1986, that is, the shank diameter is 10 mm, the diameter is 20 mm, and the head thickness is 3 mm.

[0039] The head forming is completed in multiple steps, avoiding processing failure caused by one-step cold heading, improving material utilization and processing efficiency;

[0040] The stem diameter reduction can be performed or not according to the size requirements of the formed rivet.

[0041] 4. Aging heat treatment

[0042] During the cold heading process, the work hardening of the rivet head and the shank is inconsistent. In order to further improve the overall mechanical properties of the rivet and reduce residual stress, the processed aluminum alloy rivets are subjected to aging heat treatment. According to the optimal aging heat treatment parameters obtained in the experiment, the aging heat treatment process is designed to obtain high-strength aluminum alloy rivets.

[0043] The aging heat treatment selected by the present invention is: keeping warm at 100℃~120℃ for 2h~3h; the 7050 aluminum alloy rivet obtained after this step can reach a yield strength of more than 550MPa, a hardness of more than 190HV, and an elongation of 8~10%.

[0044] 5. Nano coating spraying

[0045] The aged aluminum alloy rivets are arranged and conveyed to a coating device, and nanostructured coatings are applied to the surfaces of the rivets in three functional areas: an adhesive layer, a transition layer, and a wear-resistant layer. The coating thickness can be above 16 microns, thereby further improving the mechanical properties of the rivets, such as hardness, fracture resistance, wear resistance, and corrosion resistance. Ultimately, high-strength, corrosion-resistant 7050 aluminum alloy rivets with good comprehensive mechanical properties are obtained.

[0046] The yield strength, tensile strength, hardness and elongation data of the formed specimens of 7050 aluminum alloy cast wire obtained under different process conditions are shown in Table 1.

[0047] Table 1 Performance parameters of 7050 aluminum alloy forming specimens under different process conditions

[0048]

[0049]

[0050] It can be seen from Comparative Examples 1-1, 1-2, and 1-3 that both cold heading and equal channel angular extrusion can increase the strength and hardness of the material (7050 aluminum alloy cast wire), but equal channel angular extrusion has a slightly reduced effect on the toughness of the material.

[0051] It can be seen from Comparative Examples 2-1, 2-2, 2-3, and 2-4 that as the homogenization temperature increases, the homogenization of the material presents an inverted U-shaped curve, the strength and hardness first decrease and then increase (indicating that the lattice first becomes larger and then decreases), and the elongation (ie, toughness) first increases and then decreases (indicating that the material uniformity first increases and then decreases), and the homogenization degree of the material reaches a peak at around 465°C; it can be seen from Comparative Examples 2-5, 2-6, 2-3, 2-8, and 2-9 that as the homogenization time increases, the homogenization of the material also presents an inverted U-shaped curve. At a temperature of 465°C, the homogenization time is about 24 hours, and the homogenization degree of the material reaches a peak; therefore, the primary homogenization process of the material is selected at (435°C ~ 465°C) - (12h ~ 24h).

[0052] According to the primary homogenization temperature and the DSC curve, the secondary homogenization temperature of the material is selected at 475°C. It can be seen from comparative examples 3-1, 3-2, 3-3, 3-4, and 3-5 that the homogenization degree of the material reaches a peak when the secondary homogenization time is 4 to 8 hours. Therefore, the secondary homogenization process of the material is selected at (475°C)-(4h to 10h).

[0053] It can be seen from Comparative Examples 4-1, 4-2, 4-3, 4-4, and 4-5 that when aging heat treatment is performed after the two-stage homogenization treatment, the strength and hardness of the material gradually increase with the increase of aging time. The yield strength reaches about 550 MPa when the aging time is 36 h. However, when the aging time is 60 h, the material strength and hardness have not yet reached peak aging.

[0054] It can be seen from Comparative Examples 5-1, 5-2, 5-3, and 5-4 that after the two-stage homogenization treatment, a secondary equal channel angular extrusion and aging heat treatment are implemented. Compared with the process without equal channel angular extrusion (Comparative Example 4-*), the strength and hardness of the material are increased; and when the aging time is about 6 hours, the strength of the material reaches a peak of 539 MPa, that is, peak aging occurs. It can be seen that equal channel angular extrusion effectively reduces the peak aging time.

[0055] Comparative Examples 6-1, 6-2, 6-3, and 6-4 show that after two-stage homogenization, a secondary equal channel angular extrusion, cold heading, and aging heat treatment are performed. Compared with the process without cold heading (Comparative Example 5-*), the strength and hardness of the material are increased again; and when the aging holding time is about 3 hours, the strength of the material reaches a peak of 557MPa, that is, peak aging occurs, which shows that the cold heading process has effectively reduced the peak aging time again. Comparative Examples 7-1, 7-2, 6-3, and 7-3 show that when the aging temperature is 100-120°C, the comprehensive mechanical properties of the material are optimal (strength, hardness, and toughness are all around the peak). Therefore, the aging process after equal channel angular extrusion and cold heading is selected as: (100-120°C) holding (2-3h).

[0056] By comparing Examples 4 and 5 with Examples 1, 2 and 3, it can be seen that multi-pass equal channel angular extrusion improves the strength, hardness and toughness of the formed specimens. However, considering the processing efficiency, it is preferred to perform 4-8 passes of equal channel angular extrusion on the homogenized 7050 aluminum alloy cast wire to form rivets.

[0057] The above-mentioned embodiments are preferred cases of the present invention and are not intended to limit the scope of protection of the present invention. Various deformations or modifications that can be made by ordinary technicians in this field without creative work within the scope of the attached claims are still within the scope of protection of this patent.

Claims

1. A method for preparing high-strength corrosion-resistant rivets using 7050 aluminum alloy, characterized in that: The preparation method comprises the following steps: A. Homogenization heat treatment: The 7050 aluminum alloy cast wire is subjected to a homogenization heat treatment; the homogenization heat treatment is a two-stage homogenization heat treatment: the 7050 aluminum alloy cast wire is heated from room temperature to T1, kept at T1 for t1, then heated from T1 to T2, kept at T2 for t2, wherein T1 is 435°C to 465°C, t1 is 12 to 24 hours; T2 is 475°C, and t2 is 4 to 10 hours; B. Equal Axis Angle Extrusion: The 7050 aluminum alloy wire after homogenization heat treatment is pulled into the equal Axis Angle Extrusion die, and continuous equal Axis Angle Extrusion is performed at room temperature to achieve wire microstructure refinement and homogenization to obtain fine and homogeneous wire; C. Cold heading: 7050 aluminum alloy bar is processed into rivets in a cold heading machine, including the following steps: blanking, head preforming, head forming, and rod diameter reduction; The blanking step includes: cutting the thin homogeneous wire into 7050 aluminum alloy bars; D. Aging heat treatment: keep warm at 100-120℃ for 2-3h; E. Nano coating spraying: In the coating equipment, the nano coating is sprayed on the surface of the rivet after aging. The coating includes three functional areas: bonding layer, transition layer and wear-resistant layer.

2. The method for preparing high-strength corrosion-resistant rivets using 7050 aluminum alloy according to claim 1, characterized in that: The number of equal-channel angle extrusion passes in step B is 4 to 8.

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