A method of making a high toughness 7xxx-series aluminum alloy

Abstract

The application relates to a preparation method of a high-toughness 7XXX series aluminum alloy, the content ratio range of Mg and Zn in the 7XXX series aluminum alloy is Zn / Mg=1.4-2.8, and the content ratio range of Mg and Cu is Mg / Cu=1.5-1.9; the 7XXX series aluminum alloy is smelted in a variable-frequency induction melting furnace, and is supplemented with rotary electromagnetic stirring, the magnetic field strength is 0.01-1T; then homogenization treatment is carried out, at least two passes of hot rolling are carried out, then solid solution treatment is carried out, then at least one pass of cold rolling treatment is carried out, and then pre-strengthening treatment is carried out, so that solute atoms are enriched to form a GPII area, and the generation of a granular second phase is avoided. The intermediate aging state plate obtained by the application can shorten the pre-strengthening treatment time on the preparation process level, can promote the subsequent heat treatment or heat forming strengthening effect, and can significantly improve the production efficiency and product performance.

Description

Technical Field

[0001] This invention relates to the field of aluminum alloy sheet manufacturing technology, and more specifically, to a method for preparing high-strength and high-toughness 7XXX series aluminum alloys. Background Technology

[0002] In recent years, aluminum alloys have been increasingly used in lightweight applications in aerospace and automotive industries. To produce more complex parts using aluminum alloys, forming processes suitable for aluminum alloy sheets have been continuously improved and diversified. Currently, based on the significantly improved formability of aluminum alloys under high-temperature conditions, hot forming has become a hot topic in international research and application. High-strength 7XXX series aluminum alloys possess greater lightweight potential than 5XXX and 6XXX series alloys, but their disadvantages in formability are also more pronounced. Therefore, the processing and manufacturing of 7XXX series aluminum alloys mainly relies on hot forming.

[0003] 7XXX series aluminum alloys are heat-treatable aluminum alloys. In traditional hot forming processes, the billet undergoes solution treatment for a certain period, then the temperature is lowered to a set temperature for forming. After forming, quenching is performed, and finally, aging treatment is used to improve strength. This process suffers from uneven heating of the parts during quenching, which easily leads to thermal deformation and a decrease in forming accuracy. Based on this, a solution heat treatment, forming and cold-die quenching integrated process has been developed. This process, which integrates forming and quenching into a single step, significantly improves the forming accuracy of parts through pressure holding quenching and has become a widely used process in industrial production. However, this process still requires preliminary solution treatment and subsequent artificial aging treatment, and the production cycle has not been significantly shortened.

[0004] Patent document 202011060526.0 discloses a method for pre-strengthening hot stamping of aluminum alloys. The method involves solution treatment and quenching of the aluminum alloy to obtain a W-state sheet, followed by pre-strengthening treatment at an appropriate temperature and time to obtain a microstructure dominated by GP zones. The pre-strengthened billet is heated to approximately 200°C and held for a short time to rapidly transform the previously formed GP zones into metastable strengthening phases. It is then transferred to a forming device for hot stamping, eliminating the need for further aging treatment. Since the pre-strengthening step can be performed in advance at the sheet supplier, the hot stamping process is significantly shortened, improving production efficiency. In this forming process, the pre-strengthening treatment requires 1–24 hours to complete, and the pre-strengthening effect is closely related to the alloy composition and preparation process. Therefore, to improve the pre-strengthening effect and minimize the required time, it is necessary to develop corresponding aluminum alloy sheets and their preparation methods. Improving the pre-strengthening effect by addressing alloy composition and preparation processes will enable more efficient pre-strengthening of aluminum alloys, thereby further enhancing the product performance and production efficiency of subsequent heat treatment or hot forming.

[0005] Patent document CN108300915A discloses an Al-Zn-Mg-Cu aerospace aluminum alloy and its preparation method. By significantly increasing the Zn content in the alloy composition, the core of the material obtains higher strength. At the same time, through intense plastic deformation, a plastic deformation layer is formed on the surface of the material, forming a uniform solid solution structure. This achieves the separation of the core properties and surface properties of the aluminum alloy material, while improving the strength and resistance to localized corrosion of the material.

[0006] Patent document CN103757507A discloses a high paint hardening aluminum alloy material for automobile body outer panels and its preparation method. Through composition design and optimization, the content of Zn element is increased on the basis of Al-Mg-Si (6XXX series) aluminum alloy. The Mg element can interact with Si and Zn to form multiple reinforcing phases such as Mg2Si and MgZn2, thereby increasing the paint hardening increment of Al-Mg-Si aluminum alloy. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a method for preparing high-strength and high-toughness 7XXX series aluminum alloys. On the one hand, it shortens the time of pre-strengthening treatment at the preparation process level, and on the other hand, it enables the parts after subsequent heat treatment or hot forming to obtain better strengthening effect, thereby further improving production efficiency and product performance.

[0008] The technical solution adopted by the present invention to solve its technical problem is: designing a method for preparing a high-strength and high-toughness 7XXX series aluminum alloy, wherein the content ratio of Mg to Zn in the 7XXX series aluminum alloy is Zn / Mg = 1.4–2.8, and the content ratio of Mg to Cu is Mg / Cu = 1.5–1.9;

[0009] The preparation method of the high-strength and high-toughness 7XXX series aluminum alloy includes the following steps:

[0010] (1) Melting is carried out in a variable frequency induction melting furnace, with the assistance of rotating electromagnetic stirring, and the magnetic field strength is 0.01–1T;

[0011] (2) Homogenization treatment;

[0012] (3) Perform at least two passes of hot rolling;

[0013] (4) Perform solution treatment;

[0014] (5) Perform at least one cold rolling process and control the cold rolling deformation to be 2%–20%;

[0015] (6) Perform pre-strengthening treatment to promote the enrichment of solute atoms in the matrix to form a circular GPII region with a thickness of 1–2 nm and a diameter of 3–10 nm; at the same time, control the pre-strengthening treatment temperature to 65–95 °C to avoid the formation of particulate second phase;

[0016] (7) Obtain intermediate aged sheet material.

[0017] In the above scheme, the mass percentage content of each element in the 7XXX series aluminum alloy is as follows: Zn: 4.9%–8.9%, Mg: 1.7%–3.1%, Cu: 1.1%–2.9%, Mn≤0.2%, Ti≤0.12%, Si≤0.15%, Fe≤0.29%, and the remainder is Al.

[0018] In the above scheme, the second phase of the particles is the η′ phase and the η phase.

[0019] In the above scheme, the obtained 7XXX series aluminum alloy sheet undergoes subsequent forming or heat treatment processes within 6 months.

[0020] The mechanism of this invention is as follows:

[0021] For 7XXX series (Al-Zn-Mg-Cu) aluminum alloys, the precipitation sequence during aging is: SSS (supersaturated solid solution) → GP (Guinier-Preston) region → η′ phase → η phase (MgZn2). The GP region is mainly a solute-rich region formed by the segregation of Mg and Zn atoms in the matrix. The η′ phase mainly evolves from the GP region during aging at higher temperatures. It has a semi-coherent relationship with the matrix and is the main strengthening phase in the peak-aged (T6) state of 7XXX series aluminum alloys. The GP region can be divided into GPⅠ region and GPⅡ region. GPⅠ region is the main precipitate that appears early after solution treatment and can be formed at temperatures from room temperature to 150℃. GPⅡ region is generally formed under conditions of quenching above 450℃ and aging above 70℃. In this invention, the main purpose of the pre-strengthening treatment is to promote the rapid formation of the GPII region in the alloy microstructure, so as to serve as the nucleation core for the η′ phase in the subsequent heat treatment (hot forming) process. This allows the transformation of the GP region to the η′ phase to proceed more efficiently and completely, resulting in the formation of a large amount of the η′ phase (or the coexistence of the GP region and the η′ phase) in the alloy microstructure after heat treatment (or hot forming). This eliminates the need for additional long-term artificial aging to achieve higher mechanical properties. Therefore, for the pre-strengthening process, the more efficiently solute atoms are enriched, the more favorable it is for the formation of the GPII region, which in turn facilitates the transformation of the GP region to the η′ phase in the subsequent heat treatment or hot forming process, resulting in higher performance of the parts.

[0022] In view of this, the present invention provides a high-strength and high-toughness 7XXX series aluminum alloy and its preparation method. The principle is to limit the content of Mg and Zn elements in the chemical composition of the material, with the Mg / Zn content ratio ranging from Zn / Mg = 1.4–2.8 and the Mg / Cu content ratio ranging from Mg / Cu = 1.5–1.9, making it easier for Mg and Zn atoms to accumulate and form GPII zones. Simultaneously, the presence of Cu element is beneficial in improving the intergranular corrosion resistance of the alloy and accelerating the precipitation process of precipitated phases during high-temperature aging. Rotary electromagnetic stirring during the melting process and the introduction of cold rolling deformation before pre-strengthening treatment can promote the precipitation of GP zones, thereby improving pre-strengthening efficiency and shortening the heat treatment time. However, the purpose of pre-strengthening is to form a large number of GPII zones as nucleation sites for the η′ phase, rather than directly forming particulate second phases (η′ and η phases). Therefore, it is necessary to control the pre-strengthening treatment parameters, keeping the pre-strengthening temperature below 95℃.

[0023] The method for preparing high-strength and high-toughness 7XXX series aluminum alloys described in this invention has the following beneficial effects:

[0024] (1) Limiting the element content and composition ratio of 7XXX series aluminum alloy plates makes Mg atoms and Zn atoms that form the strengthening phase of 7XXX series aluminum alloy more efficiently enriched during the pre-strengthening process, forming GPII zone. On the one hand, it improves the efficiency of pre-strengthening process and shortens the pre-strengthening process time, thereby shortening the production cycle of aluminum alloy billet. On the other hand, it is conducive to the more sufficient enrichment of solute atoms and their transformation into the particle strengthening phase, promoting the precipitation strengthening effect of the alloy and improving the performance of parts after subsequent heat treatment (hot forming).

[0025] (2) Improve preparation efficiency from the process level. During the preparation of aluminum alloy, rotary electromagnetic stirring is carried out during the melting process, and multiple rolling processes are performed. The amount of deformation is flexibly controlled as needed. Under the premise of ensuring the plasticity of the billet, dislocations are formed through cold rolling deformation, which provides more nucleation sites for the enrichment of solute and the precipitation of strengthening phase, thus promoting the pre-strengthening process.

[0026] (3) Optimize the production efficiency and part performance of subsequent heat treatment (hot forming). Since the subsequent heat treatment (or hot forming) stage requires heating the sheet to around 200°C, the small GP region will dissolve at this temperature. If the granular strengthening phase (η′ phase) already exists, it is easy to coarsen and transform. Therefore, in the aluminum alloy preparation process, magnetic field intervention and dislocation introduction provide a phase transformation driving force to accelerate the enrichment of solute elements, which is beneficial to shorten the time required for pre-strengthening treatment. On the other hand, the pre-strengthening treatment temperature is controlled below 95°C to avoid the generation of granular second phases (η′ phase and η phase) during the pre-strengthening process, which would affect the performance of subsequent parts. Attached Figure Description

[0027] The present invention will be further described below with reference to the accompanying drawings and embodiments. In the accompanying drawings:

[0028] Figure 1 This is a process route diagram of the method for preparing high-strength and high-toughness 7XXX series aluminum alloys according to the present invention;

[0029] Figure 2 This is a schematic diagram of the microstructure of the second phase particles described in this invention. Detailed Implementation

[0030] To provide a clearer understanding of the technical features, objectives, and effects of the present invention, specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0031] A method for preparing a high-strength and high-toughness 7XXX series aluminum alloy, characterized in that the content ratio of Mg to Zn in the 7XXX series aluminum alloy is in the range of Zn / Mg = 1.4–2.8, and the content ratio of Mg to Cu is in the range of Mg / Cu = 1.5–1.9;

[0032] The preparation method of the high-strength and high-toughness 7XXX series aluminum alloy includes the following steps:

[0033] (1) Melting is carried out in a variable frequency induction melting furnace, with the assistance of rotating electromagnetic stirring, and the magnetic field strength is 0.01–1T;

[0034] (2) Homogenization treatment;

[0035] (3) Perform at least two passes of hot rolling;

[0036] (4) Perform solution treatment;

[0037] (5) Perform at least one cold rolling process and control the cold rolling deformation to be 2%–20%;

[0038] (6) Perform pre-strengthening treatment to promote the enrichment of solute atoms in the matrix to form a circular GPII region with a thickness of 1–2 nm and a diameter of 3–10 nm; at the same time, control the pre-strengthening treatment temperature to 65–95 °C to avoid the formation of particulate second phase;

[0039] (7) Obtain intermediate aged sheet material.

[0040] Furthermore, the mass percentage content of each element in the 7XXX series aluminum alloy is as follows: Zn: 4.9%–8.9%, Mg: 1.7%–3.1%, Cu: 1.1%–2.9%, Mn≤0.2%, Ti≤0.12%, Si≤0.15%, Fe≤0.29%, and the remainder is Al.

[0041] Furthermore, the second phase of the particles is an η′ phase and an η phase.

[0042] Furthermore, the obtained 7XXX series aluminum alloy sheets undergo subsequent forming (heat treatment) processes within 6 months.

[0043] Example 1:

[0044] This invention provides a high-strength and high-toughness 7XXX series aluminum alloy:

[0045] The mass percentage content of the elements in the chemical composition of this aluminum alloy is as follows: Zn: 5.33%, Mg: 2.87%, Cu: 1.61%, Mn: 0.087%, Ti: 0.031%, Si: 0.09%, Fe: 0.18%, with the remainder being Al.

[0046] In this aluminum alloy, the ratio of Mg to Zn is Zn / Mg = 1.86, and the ratio of Mg to Cu is Mg / Cu = 1.78.

[0047] This invention provides a method for preparing high-strength and high-toughness 7XXX series aluminum alloy sheets, which mainly includes the following steps:

[0048] (1) Melting is carried out in a variable frequency induction melting furnace, with the assistance of a rotating electromagnetic stirring with a magnetic field strength of 0.1T;

[0049] (2) Homogenization treatment;

[0050] (3) First hot rolling;

[0051] (4) The second hot rolling pass can be increased in the number of hot rolling passes and deformation amount as needed;

[0052] (5) Solution treatment;

[0053] (6) Cold rolling, with the cold rolling deformation controlled at 5%;

[0054] (7) Pre-strengthening treatment, the treatment temperature is 65–95℃, and the pre-strengthening treatment time is set at 3 hours.

[0055] The high-strength and high-toughness 7XXX series aluminum alloy and its preparation method described in this invention can be used to form aluminum alloy parts through the following process steps:

[0056] (8) Heat the billet to 180–220°C and hold it for 5 minutes;

[0057] (9) Transfer the aluminum alloy sheet to the forming mold for thermoforming;

[0058] (10) Perform necessary subsequent processes such as trimming and shaping.

[0059] Example 2:

[0060] This invention provides a high-strength and high-toughness 7XXX series aluminum alloy:

[0061] The mass percentage content of the elements in the chemical composition of this aluminum alloy is as follows: Zn: 6.35%, Mg: 2.73%, Cu: 1.48%, Mn: 0.087%, Ti: 0.031%, Si: 0.09%, Fe: 0.18%, with the remainder being Al.

[0062] In this aluminum alloy, the ratio of Mg to Zn is Zn / Mg = 2.33, and the ratio of Mg to Cu is Mg / Cu = 1.84.

[0063] This invention provides a method for preparing high-strength and high-toughness 7xxx series aluminum alloy sheets, which mainly includes the following steps:

[0064] (1) Melting is carried out in a variable frequency induction melting furnace, with the assistance of a rotating electromagnetic stirring with a magnetic field strength of 0.5T;

[0065] (2) Homogenization treatment;

[0066] (3) First hot rolling;

[0067] (4) The second hot rolling pass can be increased in the number of hot rolling passes and deformation amount as needed;

[0068] (5) Cold rolling, with the cold rolling deformation controlled at 10%;

[0069] (6) Solution treatment;

[0070] (7) Pre-strengthening treatment, the treatment temperature is 65–95℃, and the pre-strengthening treatment time is set at 3 hours.

[0071] The high-strength and high-toughness 7XXX series aluminum alloy sheet and its preparation method described in this invention can be used to form aluminum alloy parts through the following process steps:

[0072] (8) Heat the billet to 180–220°C and hold it for 5 minutes;

[0073] (9) Transfer the aluminum alloy sheet to the forming mold for thermoforming;

[0074] (10) Perform necessary subsequent processes such as trimming and shaping.

[0075] Example 3:

[0076] This invention provides a high-strength and high-toughness 7XXX series aluminum alloy:

[0077] The chemical composition of this aluminum alloy sheet contains the following elements by mass percentage: Zn: 5.67%, Mg: 2.27%, Cu: 1.47%, Mn: 0.087%, Ti: 0.031%, Si: 0.09%, Fe: 0.18%, with the remainder being Al.

[0078] In this aluminum alloy, the ratio of Mg to Zn is Zn / Mg = 2.5, and the ratio of Mg to Cu is Mg / Cu = 1.54.

[0079] This invention provides a method for preparing high-strength and high-toughness 7XXX series aluminum alloy sheets, which mainly includes the following steps:

[0080] (1) Melting is carried out in a variable frequency induction melting furnace, with the assistance of a rotating electromagnetic stirring with a magnetic field strength of 0.5T;

[0081] (2) Homogenization treatment;

[0082] (3) First hot rolling;

[0083] (4) The second hot rolling pass can be increased in the number of hot rolling passes and deformation amount as needed;

[0084] (5) Cold rolling, with the cold rolling deformation controlled at 5%;

[0085] (6) Solution treatment;

[0086] (7) Pre-strengthening treatment, the treatment temperature is 65–95℃, and the pre-strengthening treatment time is set at 3 hours.

[0087] The high-strength and high-toughness 7XXX series aluminum alloy sheet and its preparation method described in this invention can be used to form aluminum alloy parts through the following process steps:

[0088] (8) Heat the billet to 180–220°C and hold it for 5 minutes;

[0089] (9) Transfer the aluminum alloy sheet to the forming mold for thermoforming;

[0090] (10) Perform necessary subsequent processes such as trimming and shaping.

[0091] Comparative Example 1:

[0092] A commercially available 7075 aluminum alloy sheet is used for hot forming:

[0093] The chemical composition of this aluminum alloy sheet contains the following elements by mass percentage: Zn: 5.23%, Mg: 2.67%, Cu: 1.31%, Mn: 0.094%, Ti: 0.027%, Si: 0.07%, Fe: 0.35%, with the remainder being Al.

[0094] The commercial 7075 aluminum alloy sheet undergoes the following steps:

[0095] (1) Solution treatment;

[0096] (2) Pre-strengthening treatment, the treatment temperature is 65–95℃, and the pre-strengthening treatment time is set at 3 hours.

[0097] After pre-strengthening treatment, aluminum alloy sheets are formed into aluminum alloy parts through the following process steps:

[0098] (3) Heat the board to 180–220℃ and keep it warm for 5 minutes;

[0099] (4) Transfer the aluminum alloy sheet to the forming mold for thermoforming;

[0100] (5) Perform necessary subsequent processes such as trimming and shaping.

[0101] Comparative Example 2:

[0102] A commercially available 7075 aluminum alloy sheet is used for hot forming:

[0103] The chemical composition of this aluminum alloy sheet contains the following elements by mass percentage: Zn: 5.23%, Mg: 2.67%, Cu: 1.31%, Mn: 0.094%, Ti: 0.027%, Si: 0.07%, Fe: 0.35%, with the remainder being Al.

[0104] The commercial 7075 aluminum alloy sheet undergoes the following steps:

[0105] (1) Solution treatment;

[0106] (2) Pre-strengthening treatment, the treatment temperature is room temperature – 65℃, the pre-strengthening treatment time is set to 3 hours. Due to the low temperature, only GPI region or GPII region with a diameter of less than 3nm can be formed, and its thermal stability is poor.

[0107] After pre-strengthening treatment, aluminum alloy sheets are formed into aluminum alloy parts through the following process steps:

[0108] (3) Heat the board to 180–220℃ and keep it warm for 5 minutes;

[0109] (4) Transfer the aluminum alloy sheet to the forming mold for thermoforming;

[0110] (5) Perform necessary subsequent processes such as trimming and shaping.

[0111] In the above embodiments and comparative examples, Embodiments 1 to 3 were prepared using the preparation method of the high-strength and tough 7XXX series aluminum alloy sheet described in this invention, while Comparative Examples 1 and 2 used commercially available 7075 aluminum alloy sheets, and the pre-strengthening heat treatment process used differed from that of this invention. The mechanical properties of the hot-formed components obtained in Embodiments 1 to 3 and Comparative Examples 1 and 2 were tested, and their tensile strength, yield strength, and elongation are shown in the table below:

[0112]

[0113]

[0114] It is evident that by using the high-strength and tough 7XXX series aluminum alloy sheet preparation method provided by this invention for sheet preparation and forming processes, more ideal part performance can be obtained.

[0115] This invention starts with alloy composition and preparation process. By rationally controlling the chemical composition of the alloy, especially the content and ratio of Mg and Zn elements that form the strengthening phase of 7XXX series aluminum alloys, and by supplementing the melting process with rotary electromagnetic stirring, the solute atoms in the material are enriched more quickly and fully during the pre-strengthening process, forming a solute-rich zone, namely the thermally stable GPⅡ zone. This makes the pre-strengthening treatment of aluminum alloys more efficient. These GPⅡ zones can serve as nucleation sites for the η′ phase during subsequent heat treatment (or hot forming). This invention can shorten the pre-strengthening treatment time as much as possible at the process level, and enable the hot-formed parts to achieve better strengthening effects, thereby further improving the production efficiency and product performance of the hot forming process.

[0116] The embodiments of the present invention have been described above with reference to the accompanying drawings. However, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of the present invention without departing from the spirit and scope of the claims. All of these forms are within the protection scope of the present invention.

Claims

1. A method for preparing a high-strength and high-toughness 7XXX series aluminum alloy, characterized in that, The Mg / Zn content ratio in 7XXX series aluminum alloys ranges from Zn / Mg = 1.4–2.8, and the Mg / Cu content ratio ranges from Mg / Cu = 1.5–1.

9. The mass percentage content of each element in 7XXX series aluminum alloys is as follows: Zn: 4.9%–8.9%, Mg: 1.7%–3.1%, Cu: 1.1%–2.9%, Mn≤0.2%, Ti≤0.12%, Si≤0.15%, Fe≤0.29%, with the remainder being Al. The preparation method of the high-strength and high-toughness 7XXX series aluminum alloy includes the following steps: (1) Melting is carried out in a variable frequency induction melting furnace, with the assistance of a rotating electromagnetic stirring, and the magnetic field strength is 0.01–1T; (2) Homogenization treatment; (3) Perform at least two passes of hot rolling; (4) Perform solution treatment; (5) Perform at least one cold rolling process and control the cold rolling deformation to be 2%–20%; (6) Perform pre-strengthening treatment to promote the enrichment of solute atoms in the matrix to form a circular GPII region with a thickness of 1–2 nm and a diameter of 3–10 nm; at the same time, control the pre-strengthening treatment temperature to 65–95 °C to avoid the formation of particulate second phase; (7) Obtain intermediate-aged sheet material.

2. The method for preparing high-strength and high-toughness 7XXX series aluminum alloy according to claim 1, characterized in that, The second phase of the particles is Harmony Mutually.

3. The method for preparing high-strength and high-toughness 7XXX series aluminum alloy according to claim 1, characterized in that, The obtained 7XXX series aluminum alloy sheets undergo subsequent forming or heat treatment processes within 6 months.

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