CN116875863A - Novel super-strong aluminum alloy material and preparation and processing method thereof - Google Patents
Novel super-strong aluminum alloy material and preparation and processing method thereof Download PDFInfo
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- 238000003672 processing method Methods 0.000 title claims abstract description 13
- 229910045601 alloy Inorganic materials 0.000 claims abstract description 156
- 238000011282 treatment Methods 0.000 claims abstract description 44
- 238000010438 heat treatment Methods 0.000 claims abstract description 43
- 229910052782 aluminium Inorganic materials 0.000 claims abstract description 40
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- 230000032683 aging Effects 0.000 claims abstract description 37
- 229910052761 rare earth metal Inorganic materials 0.000 claims abstract description 33
- 238000003723 Smelting Methods 0.000 claims abstract description 28
- 238000005266 casting Methods 0.000 claims abstract description 16
- 239000012535 impurity Substances 0.000 claims abstract description 6
- 238000000034 method Methods 0.000 claims description 26
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- 239000007788 liquid Substances 0.000 claims description 25
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- 239000010949 copper Substances 0.000 claims description 23
- 239000011777 magnesium Substances 0.000 claims description 23
- 239000011701 zinc Substances 0.000 claims description 21
- 239000002994 raw material Substances 0.000 claims description 17
- 238000010791 quenching Methods 0.000 claims description 16
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- PEFIIJCLFMFTEP-UHFFFAOYSA-N [Nd].[Mg] Chemical compound [Nd].[Mg] PEFIIJCLFMFTEP-UHFFFAOYSA-N 0.000 claims description 15
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- 229910052802 copper Inorganic materials 0.000 claims description 11
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- FYYHWMGAXLPEAU-UHFFFAOYSA-N Magnesium Chemical compound [Mg] FYYHWMGAXLPEAU-UHFFFAOYSA-N 0.000 claims description 6
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 claims description 6
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- 238000005498 polishing Methods 0.000 claims description 4
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- PGTXKIZLOWULDJ-UHFFFAOYSA-N [Mg].[Zn] Chemical compound [Mg].[Zn] PGTXKIZLOWULDJ-UHFFFAOYSA-N 0.000 description 1
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Classifications
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C21/00—Alloys based on aluminium
- C22C21/10—Alloys based on aluminium with zinc as the next major constituent
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
- B21C23/00—Extruding metal; Impact extrusion
- B21C23/02—Making uncoated products
- B21C23/04—Making uncoated products by direct extrusion
- B21C23/06—Making sheets
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/0081—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for slabs; for billets
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C1/00—Making non-ferrous alloys
- C22C1/02—Making non-ferrous alloys by melting
- C22C1/026—Alloys based on aluminium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C1/00—Making non-ferrous alloys
- C22C1/02—Making non-ferrous alloys by melting
- C22C1/03—Making non-ferrous alloys by melting using master alloys
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
- C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
- C22F1/002—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working by rapid cooling or quenching; cooling agents used therefor
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
- C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
- C22F1/04—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon
- C22F1/053—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon of alloys with zinc as the next major constituent
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Manufacture And Refinement Of Metals (AREA)
Abstract
The invention discloses a novel super-strong aluminum alloy material and a preparation and processing method thereof, wherein in the aluminum alloy material, the content of Zn is 6.0-6.7 wt%, the content of Mg is 2.1-2.9 wt%, the content of Cu is 1.6-1.9 wt%, the content of Zr is 0.1-0.15 wt%, the content of rare earth element Nd is 0.2-0.4 wt%, the content of impurities is less than or equal to 0.1 wt%, and the balance is aluminum; the preparation and processing method comprises the steps of A, smelting and casting; step B, homogenizing; step C, thermal deformation treatment; step D, solution treatment; and E, aging treatment. The novel super-strong aluminum alloy material and the preparation and processing method thereof optimize the design of alloy components and the heat treatment mode, and improve the strength and toughness of the alloy and the high-temperature performance of the alloy.
Description
Technical Field
The invention relates to the technical field of aluminum alloy material processing. In particular to a novel super-strong aluminum alloy material and a preparation and processing method thereof.
Background
Al-Zn-Mg-Cu-Zr aluminum alloy has been widely used in aerospace because of its ultra-high strength and toughness. With the development of the aviation field, the requirements on the aspects of overall performance, service life, safety and reliability, structural quality control and the like of the aircraft are continuously improved; this places higher performance demands on the high strength aluminum alloys. As is known, microalloying is one of the most effective methods for improving the microstructure of an alloy and obtaining excellent comprehensive properties, wherein rare earth elements are the most effective microalloying elements for improving the comprehensive properties of the ultra-strong aluminum alloy at present, and the interaction of the rare earth elements with an alpha-Al matrix can form bulk compounds which are finely dispersed and precipitated phases which are coherent with the alpha-Al matrix, and the dispersed phases which are uniformly distributed in the matrix can strongly pin dislocation and subgrain boundaries and can effectively inhibit recrystallization, so that the corrosion resistance, high temperature resistance, fracture toughness and the like of the alloy are improved.
In recent years, researchers at home and abroad have begun to add a minute amount of rare earth elements (such as Sc, er, la, ce, etc.) to 7xxx series aluminum alloys in order to search for new super-strength aluminum alloys. Sc is the most effective element to improve the performance of aluminum alloys so far, because of primary and precipitated Al 3 Sc or Al 3 The (Sc, zr) phase may cause grain refinement of the aluminum alloy during solidification, heat treatment, and deformation, thereby promoting grain boundary strengthening of the aluminum alloy. The production process of the alloy mainly comprises casting, homogenizing, hot extrusion, solution treatment and aging treatment, and the process method in each production process is established to be the key for regulating and controlling the final performance of the alloy, so that the novel super-strength aluminum alloy material and the preparation and processing method are designed and are the most effective modes for developing high-strength aluminum alloy at present.
Disclosure of Invention
Therefore, the technical problem to be solved by the invention is to provide a novel super-strong aluminum alloy material and a preparation and processing method thereof, so as to solve the problem that the strength, toughness and high-temperature performance of the existing aluminum alloy material cannot meet the use requirements.
In order to solve the technical problems, the invention provides the following technical scheme:
novel super-strong aluminum alloy material with Zn content of6.0 to 6.7wt.% of Mg, 2.1 to 2.9wt.% of Cu, 1.6 to 1.9wt.% of Cu, 0.1 to 0.15wt.% of Zr, 0.2 to 0.4wt.% of rare earth element Nd, less than or equal to 0.1wt.% of impurity, and the balance of aluminum. Compared with rare earth elements Er and Sc commonly used in aluminum alloys in recent years, the diffusion coefficient of Nd at high temperature is relatively low, the lattice mismatch rate is relatively high, and the maximum solubility in an alpha-Al matrix is not greatly different, which is specifically expressed as follows: (1) FCC-Al matrix and L1 2 -Al 3 Lattice mismatch between Sc is 1.32%, alpha-Al and L1 2 -Al 3 Lattice mismatch between Er is 4.1%, lattice mismatch rate of rare earth phase formed by Nd and alpha-Al matrix is 6.13%, and larger lattice mismatch rate leads to higher strengthening effect of alloy at room temperature and high temperature; (2) the high temperature solid solubility of Er in FCC-Al matrix is lower than that of Sc, which increases the driving force and volume fraction of precipitation; er has a diffusivity in alpha-Al of 4X 10 at 300 DEG C -19 m 2 S, higher than Sc (0.9X10) -19 m 2 /s). However, at 400 ℃, the Er diffusivity of Sc in α -Al is 1×10 -18 m 2 /s, and Sc has a diffusivity of 1.98X10 -17 m 2 Diffusion coefficient of Nd at 400℃was 3.93X 10 -19 m 2 The diffusion coefficient of Nd is lower than that of Er and Sc, coarsening of volume diffusion control is restrained, and the precipitate keeps effective obstruction of dislocation movement at high temperature; (3) for a solid solubility in Al, the maximum solubility of Sc in aluminum is 0.23at.%, the maximum solid solubility of Er in aluminum is 0.17at.%, the maximum solid solubility of Nd in aluminum is 0.28at.%, the finite solid solubility maximizes the equilibrium volume fraction of the dispersed phase. Rare earth Nd is hardly applied to Al-Zn-Mg-Cu-Zr aluminum alloys, and most of the rare earth Nd is applied to the research of the thermal stability of 2xxx aluminum alloys, al-Si alloys and magnesium alloys; therefore, the rare earth Nd is used as a new microalloying element to be added, and the novel ultrahigh-strength aluminum alloy material with strength, toughness and high-temperature performance meeting the use requirements can be designed.
The mass ratio of Zn to Mg in the novel super-strong aluminum alloy material is (2.5-2.7): 1. When w (Zn)/w (Mg) =2.5-2.7,the main strengthening phase is eta' (MgZn) 2 ) And eta (MgZn) 2 ) When w (Zn)/w (Mg) is less than 2.5, the magnesium content exceeds that of MgZn formation 2 The required amount of phase will produce a complementary strong interaction, forming a T-phase; because the diffusion rate of Cu is far lower than that of Zn and Mg, the alloy has excessive T phase, hard and brittle S phase can be formed in the homogenization process, and the alloy is easy to break in the thermal deformation process, so that the performance is reduced; in order to avoid forming more T phases, the invention designs that the ratio of w (Zn)/w (Mg) is lower than the zinc-magnesium ratio of the T phases by 2.71, so that more MgZn is formed as much as possible 2 Phase, reducing the formation of T phase.
The novel super-strength aluminum alloy material comprises 6.1wt.% of Zn, 2.3wt.% of Mg, 1.7wt.% of Cu, 0.15wt.% of Zr and 0.2wt.% of rare earth element Nd.
A preparation processing method of a novel super-strong aluminum alloy material comprises the following steps:
step A, smelting and casting: taking aluminum, aluminum zinc intermediate alloy, aluminum magnesium intermediate alloy, aluminum copper intermediate alloy, aluminum zirconium intermediate alloy and magnesium neodymium intermediate alloy as raw materials, smelting and casting to obtain an aluminum alloy cast ingot;
step B, homogenizing: homogenizing the aluminum alloy cast ingot to obtain a homogenized aluminum alloy material;
and C, heat deformation treatment: carrying out thermal deformation treatment on the homogenized aluminum alloy material to obtain an aluminum alloy plate;
step D, solution treatment: carrying out solution treatment on the aluminum alloy plate to obtain a solid-solution aluminum alloy plate;
step E, aging treatment: aging the aluminum alloy plate after solid solution to obtain a novel super-strength aluminum alloy material;
and E, in the novel super-strong aluminum alloy material obtained in the step E: 6.0 to 6.7wt.% of Zn, 2.1 to 2.9wt.% of Mg, 1.6 to 1.9wt.% of Cu, 0.1 to 0.15wt.% of Zr, 0.2 to 0.4wt.% of rare earth element Nd, less than or equal to 0.1wt.% of impurity, and the balance of aluminum. The technological method and technological parameters of each step designed by the invention are beneficial to the maximization of the strength and toughness of the alloy, and meanwhile, the addition of rare earth Nd elements is beneficial to refining grains, forming a high-melting-point dispersed phase, and the mechanical property of the alloy can be obviously improved.
The specific operation method for smelting and casting in the step A comprises the following steps:
step (A-1), preparing an alloy raw material and polishing the alloy raw material; the alloy raw materials are as follows: pure aluminum, aluminum zinc master alloy, aluminum magnesium master alloy, aluminum copper master alloy, aluminum zirconium master alloy and magnesium neodymium master alloy; the polishing treatment is to polish the surface of the alloy material by a file to remove surface oxide skin;
smelting alloy by adopting a resistance induction smelting furnace: preheating a crucible, adding pure aluminum, heating and smelting, and sequentially adding an aluminum-zinc intermediate alloy, an aluminum-copper intermediate alloy, an aluminum-zirconium intermediate alloy wrapped by aluminum foil and an aluminum-magnesium intermediate alloy wrapped by aluminum foil after the pure aluminum is completely melted; after all the added alloy raw materials are melted, refining treatment, stirring and slag skimming are carried out to obtain smelting alloy liquid;
step (A-3), adding magnesium neodymium intermediate alloy wrapped by aluminum foil into the smelting alloy liquid, standing and stirring to obtain aluminum alloy liquid; the addition sequence of the Mg-30% Nd intermediate alloy is particularly important, and the rare earth element is added after degassing and refining, so that the burning loss condition of the rare earth element is greatly reduced, and meanwhile, the intermediate alloy is added for 2 minutes and then stirred and poured, so that the segregation phenomenon of the rare earth element in the alloy can be reduced;
and (A-4) immediately pouring the stirred aluminum alloy liquid into a preheated graphite mould, and casting to obtain an aluminum alloy cast ingot.
In the step (A-1), the purity of pure aluminum is more than or equal to 99.9wt%; the mass fraction of zinc in the aluminum-zinc intermediate alloy is 30 wt%, the mass fraction of magnesium in the aluminum-magnesium intermediate alloy is 20 wt%, the mass fraction of copper in the aluminum-copper intermediate alloy is 30 wt%, and the mass fraction of zirconium in the aluminum-zirconium intermediate alloy is 10 wt%; when preparing alloy raw materials, preparing materials according to the proportion that the burning loss rate of zinc is 2%, the burning loss rate of copper is 5%, and the burning loss rate of zirconium is 30%;
in the step (A-2): the preheating temperature of the crucible is 380-420 ℃; adding pure aluminum and heating to 720-750 ℃; the refining treatment method comprises the following steps: controlling the temperature of the alloy liquid within 725-735 ℃, immersing C into the alloy liquid 2 Cl 6 Introducing argon into the graphite bell jar, and refining for 20min; c (C) 2 Cl 6 The mass of the alloy is 0.3 to 0.5 weight percent of the total weight of the alloy liquid;
in the step (A-3), adding an aluminum foil wrapped magnesium neodymium intermediate alloy when the temperature of the smelting alloy liquid is regulated to be in the range of 700-710 ℃, and standing for 2min; the mass fraction of neodymium in the magnesium-neodymium master alloy is 30wt.%;
in the step (A-4), the preheating temperature of the graphite mold is 180-230 ℃.
In the step B, during homogenization treatment, firstly, the temperature of an aluminum alloy cast ingot is raised to 300 ℃ at a temperature raising rate of 340 ℃/h, and the temperature is kept for 6 hours; then heating the aluminum alloy ingot to 472 ℃ at a heating rate of 50 ℃/h, and preserving the heat for 24h; and (3) after the heat preservation is finished, water quenching is carried out by using water with the temperature of less than or equal to 25 ℃, and the water quenching transfer time is less than or equal to 5s.
In the step C, during thermal deformation treatment, firstly, the homogenized aluminum alloy material is placed in a resistance furnace to be pretreated for 2 hours at the temperature of 420 ℃; then, at an extrusion temperature of 410.+ -. 5 ℃, extrusion deformation was performed at an extrusion speed of 1mm/s at an extrusion ratio of 25:1, to obtain an aluminum alloy sheet.
In the step D, during solution treatment, firstly heating the aluminum alloy plate to 455 ℃ at a heating rate of 340 ℃/h, and preserving heat for 1.5h; then heating the aluminum alloy plate to 474 ℃ at a heating rate of 60 ℃/h, and preserving heat for 0.5h; and (3) after the heat preservation is finished, water quenching is carried out by using water with the temperature of less than or equal to 25 ℃, and the water quenching transfer time is less than or equal to 5s.
In the step E, during aging treatment, firstly heating the aluminum alloy plate subjected to solid solution and then heating to 105 ℃ at the heating rate of 340 ℃/h, and preserving heat for 24 hours; then heating the aluminum alloy plate subjected to solid solution to 170 ℃ at a heating rate of 3 ℃/min, preserving heat for 85min, and then performing water quenching with water at a temperature of less than or equal to 25 ℃ for a water quenching transfer time of less than or equal to 5s; then naturally aging the aluminum alloy plate after solid solution for 24 hours; finally, heating the aluminum alloy plate subjected to solid solution to 80 ℃ at a heating rate of 340 ℃/h, and preserving heat for 34h; and (5) air cooling to room temperature after the heat preservation is finished. In the aging treatment RRA, natural aging is introduced for 24 hours, supersaturated vacancies are easily combined with magnesium atoms, the diffusion of the Mg atoms is accelerated, the nucleation rate of the GP zone is improved, more nucleation positions are provided for the GP zone in the re-aging process, and slow-rate heating is adopted before the regression stage, so that the volume fraction of eta' phase is increased and eta phase is less after the regression is finished.
The technical scheme of the invention has the following beneficial technical effects:
the novel super-strong aluminum alloy material and the preparation and processing method thereof optimize from two aspects of alloy component design and heat treatment modes, and finally the element composition of the obtained aluminum alloy material is as follows: al-6.1Zn-2.3Mg-1.7Cu-0.15Zr-0.2Nd; the aluminum alloy material has excellent alloy strength and toughness and good high-temperature performance. Under the preparation and processing conditions of the invention, the addition of rare earth Nd can interact with other elements in the alloy and improve the segregation degree of the elements in the alloy, so that the fused and cast alloy has uniformity, and the invention also provides guarantee for subsequent deformation and heat treatment.
Drawings
FIG. 1 is a schematic diagram of an alloy melting and casting process in an embodiment of the invention;
FIG. 2a is a diagram of a structure (200 μm) of an ingot of Nd-free aluminum alloy according to an embodiment of the invention;
FIG. 2b is a diagram of the structure of an aluminum alloy ingot (200 μm) containing 0.20wt.% Nd in an example of the invention;
FIG. 2c is a diagram of the structure of an aluminum alloy ingot (200 μm) containing 0.25wt.% Nd in an example of the invention;
FIG. 2d is a diagram of the structure of an aluminum alloy ingot (200 μm) containing 0.30wt.% Nd in an example of the invention;
FIG. 2e is a diagram of the structure of an aluminum alloy ingot (200 μm) containing 0.35wt.% Nd in an example of the invention;
FIG. 2f is a texture map (200 μm) of an aluminum alloy ingot containing 0.40wt.% Nd in an example of the invention;
FIG. 3a is a diagram of the structure (50 μm) of an aluminum alloy ingot containing 0.20wt.% Nd in an example of the invention;
FIG. 3b is an SEM image of the surface of an aluminum alloy ingot containing 0.20wt.% Nd (elemental Al) in an example of the invention;
FIG. 3c is an SEM image of the surface of an aluminum alloy ingot containing 0.20wt.% Nd (Zn element);
FIG. 3d is an SEM image of the surface of an aluminum alloy ingot containing 0.20wt.% Nd (Mg element);
FIG. 3e is an SEM image of the surface of an aluminum alloy ingot containing 0.20wt.% Nd (Cu);
FIG. 3f is an SEM image of the surface of an aluminum alloy ingot containing 0.20wt.% Nd (Zr element) according to the example of the invention;
FIG. 3g is an SEM image of the surface of an aluminum alloy ingot containing 0.20wt.% Nd (Nd element) in an example of the invention;
FIG. 4a is an SEM image of an ingot structure and copper element surface of an exemplary embodiment of the invention comprising 0wt.% Nd;
FIG. 4b is an SEM image of an ingot structure and copper element surface of an exemplary embodiment of the invention comprising 0.35wt.% Nd;
FIG. 4c is an SEM image of an ingot structure and copper element surface of an exemplary embodiment of the invention comprising 0.25wt.% Nd;
FIG. 4d is an SEM image of the structure and copper element surface of an aluminum alloy ingot containing 0.40wt.% Nd in an embodiment of the invention;
FIG. 5 is a bar graph of Brinell hardness of aluminum alloy ingots having different Nd content in an example of the invention;
FIG. 6 is a schematic drawing showing the tensile properties of aluminum alloy ingots with different Nd contents in the embodiment of the invention;
FIG. 7a is a schematic diagram showing the relationship between Al-6.1Zn-2.3Mg-1.7Cu-0.15Zr alloy equilibrium transition amount and temperature in the embodiment of the invention;
FIG. 7b S-Al of FIG. 7a 2 -schematic diagram of equilibrium transition of CuMg versus temperature;
FIG. 7c is a graph showing the equilibrium transition amount of T-AlZnMgCu in FIG. 7a versus temperature;
FIG. 7d eta-MgZn in FIG. 7a 2 A schematic diagram of the relationship between equilibrium transition amount and temperature;
FIG. 7e FIG. 7a Al 3 M-D0 23 A schematic diagram of the relationship between equilibrium transition amount and temperature;
FIG. 8 is a graph showing the homogenization kinetics of Cu elements with different dendrite spacing in an embodiment of the present invention;
FIG. 9a is an optical microscope image of Al-6.1Zn-2.3Mg-1.7Cu-0.15Zr-0Nd in an embodiment of the invention;
FIG. 9b is an optical microscope image of Al-6.1Zn-2.3Mg-1.7Cu-0.15Zr-0.2Nd in the example of the present invention;
FIG. 9c is a histogram of the recrystallized volume fractions of Al-6.1Zn-2.3Mg-1.7Cu-0.15Zr-0Nd and Al-6.1Zn-2.3Mg-1.7Cu-0.15Zr-0.2Nd in the examples of the invention;
FIG. 10 is a schematic view of the process parameters of the homogenization, heat distortion, solution treatment, and aging treatments of an embodiment of the invention;
FIG. 11 shows Al-6.1Zn-2.3Mg-1.7Cu-0.15Zr-xNd (x=0.2) in the example of the present invention
Hardness diagram of alloy after four-stage aging and 120 ℃/100h heat exposure;
FIG. 12 shows Al-6.1Zn-2.3Mg-1.7Cu-0.15Zr-xNd (x=0.2) in the example of the present invention
Engineering stress strain diagram of alloy after four-stage aging and 120 ℃ hot stretching.
Detailed Description
In this embodiment, the preparation and processing method of the novel super-strength aluminum alloy material includes the following steps:
step A, smelting and casting: taking aluminum, aluminum zinc intermediate alloy, aluminum magnesium intermediate alloy, aluminum copper intermediate alloy, aluminum zirconium intermediate alloy and magnesium neodymium intermediate alloy as raw materials, smelting and casting to obtain an aluminum alloy cast ingot (shown in figure 1);
the specific operation method of smelting and casting comprises the following steps:
step (A-1), preparing alloy raw materials and carrying out pretreatment: pure aluminum, aluminum zinc master alloy, aluminum magnesium master alloy, aluminum copper master alloy, aluminum zirconium master alloy and magnesium neodymium master alloy; pure aluminum is high purity aluminum with an aluminum content of 99.9 wt.%; the mass fraction of zinc in the aluminum-zinc intermediate alloy is 30 wt%, the mass fraction of magnesium in the aluminum-magnesium intermediate alloy is 20 wt%, the mass fraction of copper in the aluminum-copper intermediate alloy is 30 wt%, and the mass fraction of zirconium in the aluminum-zirconium intermediate alloy is 10 wt%; when preparing alloy raw materials, polishing the surface of the required alloy raw materials by using a file to remove oxide skin on the surface, and proportioning according to the conditions that the burning loss rate of zinc is 2%, the burning loss rate of copper is 5% and the burning loss rate of zirconium is 30%;
smelting alloy by adopting a resistance induction smelting furnace: preheating a crucible to 400 ℃, adding pure aluminum, heating to 740 ℃ for smelting, after the pure aluminum is completely melted, sequentially adding an aluminum-zinc intermediate alloy, an aluminum-copper intermediate alloy and an aluminum-magnesium intermediate alloy wrapped by aluminum foil, then adding the aluminum-zirconium intermediate alloy, and stirring for 5min each time; after all the added alloy raw materials are melted, refining treatment, stirring and slag skimming are sequentially carried out to obtain smelting alloy liquid; the refining treatment method comprises the following steps: the temperature of the alloy liquid is reduced to 730 ℃, and C is filled in the alloy liquid in an immersed way 2 Cl 6 Introducing argon into the graphite bell jar to carry out degassing and refining for 20min; c (C) 2 Cl 6 The mass of the alloy is 0.4 weight percent of the total weight of the alloy liquid;
step (A-3), when the temperature of the smelting alloy liquid is reduced to 710 ℃, adding magnesium neodymium intermediate alloy wrapped by aluminum foil into the smelting alloy liquid, standing for 2min, and stirring to obtain aluminum alloy liquid; the mass fraction of neodymium in the magnesium-neodymium master alloy is 30wt.%;
and (A-4) immediately pouring the stirred aluminum alloy liquid into a graphite mold preheated to 200 ℃ and casting to obtain an aluminum alloy cast ingot.
In the novel super-strong aluminum alloy material aluminum alloy cast ingot prepared by the embodiment: 6.1wt.% Zn, 2.3wt.% Mg, 1.7wt.% Cu, 0.15wt.% Zr, 0.2wt.% rare earth Nd, less than or equal to 0.1wt.% impurities, the balance being aluminum;
by adopting the same preparation method in the step A, the aluminum alloy materials with the rare earth element Nd content of 0, 0.25wt.%, 0.3wt.%, 0.35wt.% and 0.4wt.% are respectively prepared by controlling the addition amount of Mg-30% Nd of the magnesium-neodymium intermediate alloy.
As a result of microstructure analysis of the above-prepared aluminum alloy ingots with different Nd contents, as shown in fig. 2a to 2f, it can be seen from fig. 2a to 2f that when rare earth element Nd is added to the alloy ingots, the addition of Nd clearly observes that the as-cast structure of the alloy is significantly refined, the eutectic structure of the alloy is thinned and discontinuous, and the dendrite spacing and the unbalanced eutectic phase at the grain boundary are also relatively reduced, wherein the refining effect is most obvious when 0.2wt.% Nd is added. The rare earth element Nd has higher chemical activity, in the process of alloy casting solidification, the Nd element is limited by solidification diffusion dynamics conditions, aggregation can occur at the front edge of a solid/liquid interface, solute redistribution can occur in the processes of crystallization nucleation and growth, so that a component supercooling region is changed, the growth of dendrites in the alloy solidification process is inhibited, and the grain size is thinned. As can be seen from fig. 3a to 4d, the addition of 0.2wt.% Nd of the rare earth element also improves the segregation effect of Cu element to some extent, reduces the segregation effect at the grain boundaries, and increases the distribution density of Cu element in the crystal. In terms of mechanical properties, with the continuous addition of Nd content, the Brinell hardness is kept in a relatively stable state as a whole, which indicates that the Nd-containing alloy has good structural uniformity. When the Nd content is 0.2wt.%, the refining effect is best, the comprehensive performance is optimal, and the room temperature hardness is increased by 44.9 percent (105.2 HBW, see figure 5); the room temperature tensile strength and elongation increased by 34.3% (163 MPa, see FIG. 6) and 44% (2.5%, see FIG. 6), respectively.
Step B, homogenizing: homogenizing the aluminum alloy cast ingot to obtain a homogenized aluminum alloy material; during homogenization treatment, firstly heating an aluminum alloy ingot to 300 ℃ at a heating rate of 50 ℃/h, and preserving heat for 6 hours; then heating the aluminum alloy ingot to 472 ℃ at a heating rate of 50 ℃/h, and preserving the heat for 24h; and (3) after the heat preservation is finished, water quenching is carried out by using water at 15 ℃, and the water quenching transfer time is less than or equal to 5s.
The homogenization process of this example was determined by thermodynamic simulation calculations of Al-6.1Zn-2.3Mg-1.7Cu-0.15Zr alloy using thermodynamic software to determine the two-stage homogeneityThe first stage temperature in homogenization; the temperature and time of the second stage were determined by DSC testing and establishing a homogenization kinetic equation. Specifically, the thermodynamic software is utilized to cast aluminum alloy ingots with different Nd contents prepared in the step A: performing thermodynamic simulation calculation of homogenization treatment on the Al-6.1Zn-2.3Mg-1.7Cu-0.15Zr alloy, and further selecting the first stage temperature of the homogenization treatment, wherein the liquidus temperature and the solidus temperature of the alloy are 634 ℃ and 532 ℃ respectively, the precipitation temperature of each low-melting eutectic phase can be determined from a vector diagram of the temperature and the content of elements in the alloy, the S phase starting precipitation temperature 465 ℃ can be determined respectively in fig. 7b to 7e, and the content of the S phase is at most 1.71% when the temperature is 414 ℃; the precipitation temperature of the phase T is 227 ℃ until the room temperature; the precipitation temperature of eta phase is 414 ℃, and the content of eta phase is 5.39% at 227 ℃; al (Al) 3 M-D0 23 The phase begins to precipitate at 700 ℃ and Al at 634 DEG C 3 M-D0 23 The phase had a short downward trend and continued to rise after 610℃and the amount of precipitation was relatively stable and reached a maximum of 0.18% when the temperature reached 300 ℃. To obtain more dispersed Al 3 The Zr phase is selected to have a homogenization degree of 300 ℃/6h in the first stage (the temperature in the first stage is close to the dissolution temperature of eta phase).
The starting melting point of the precipitated phase was 475.4 ℃to 505.3 ℃according to the DSC result, and in order to prevent the occurrence of the overburning phenomenon, the temperature was selected to be slightly lower than the starting melting point of the unbalanced eutectic phase, and finally 472℃was selected. In the two-stage homogenizing heating stage, slope heating is adopted, the heating rate is 50 degrees/h, in order to determine the time used in the homogenizing process, a homogenizing kinetic equation is adopted, as shown in fig. 8, the distance between secondary dendrite arms of 0.2wt.% Nd alloy is 53 μm, the total homogenizing time is determined to be 33h, and the homogenizing time and the heating rate in the first stage are combined, so that the second stage process is finally determined to be 472 ℃/24h.
Therefore, the homogenization system was selected to be 300 ℃/6h (temperature rise rate 340 °/h) to 472 ℃/24h (temperature rise rate 50 °/h, water quenching).
And C, heat deformation treatment: carrying out thermal deformation treatment on the homogenized aluminum alloy material to obtain an aluminum alloy plate; when in thermal deformation treatment, firstly, the homogenized aluminum alloy material is placed in a resistance furnace to be pretreated for 2 hours at the temperature of 420 ℃; then, extrusion deformation was performed at an extrusion temperature of 400℃at an extrusion speed of 1mm/s at an extrusion ratio of 25:1 to obtain an aluminum alloy sheet.
Step D, solution treatment: carrying out solution treatment on the aluminum alloy plate to obtain a solid-solution aluminum alloy plate;
the evolution of the second phase particles during solution treatment comprises three phases: (1) In the low temperature stage, i.e. in the range of room temperature to 350 deg.C, mgZn is precipitated 2 Separating out; (2) MgZn at 350-450 deg.C 2 Is not less than a certain proportion of Al 2 Coarsening CuMg; (3) At a high temperature of 450 ℃ or higher, al 2 Dissolution of the CuMg phase. To avoid MgZn 2 The phase transition to S phase is made by designing the temperature at the low temperature stage to be 450 ℃ or above, and the initial melting temperature of the low melting point unbalanced eutectic phase is 489 ℃ and the phase initial transition temperature is 477 ℃ according to DSC result, in order to prevent the phenomena of overburning during solution treatment, the second phase is prevented from melting, defects are prevented from occurring on the bonding interface between the phase and the matrix, and the temperature is reduced by 3-5 ℃. The final solution treatment process is 455 ℃/1.5h (the heating rate of 340 ℃/h), and the heating rate of 60 ℃/h is heated to 474 ℃/0.5h; and (3) after the heat preservation is finished, water quenching is carried out by using water at 25 ℃, and the water quenching transfer time is less than or equal to 5s.
As can be seen from fig. 9a to 9c, the alloy to which rare earth Nd was not added had a remarkable recrystallization phenomenon in which the recrystallization ratio was 60.3%, whereas the alloy to which Nd was added in an amount of 0.2wt% maintained substantially a non-recrystallized fibrous structure in which the recrystallization ratio was 44.5%; the inhibition of recrystallization by the addition of 0.2wt% Nd is enhanced by 26.2% compared to the absence of rare earth addition; compared with the rare earth not added, the compound addition of Nd has better recrystallization inhibition effect, can better retain deformation recovery structure, stabilizes the substructure of the deformation structure, prevents the process of developing the subgrain boundary into a large-angle grain boundary, effectively inhibits the recrystallization nucleation and growth process, retains more subgrain of small-angle grain boundaries, and improves the final comprehensive mechanical property; this shows that the addition of Nd has a significant improvement in the overall properties of the alloy.
Step E, aging treatment: aging the aluminum alloy plate after solid solution to obtain a novel super-strength aluminum alloy material; during aging treatment, firstly heating the aluminum alloy plate subjected to solid solution to 105 ℃ at a heating rate of 340 ℃/h, and preserving heat for 24 hours; then heating the aluminum alloy plate subjected to solid solution to 170 ℃ at a heating rate of 3 ℃/min, preserving heat for 85min, and performing water quenching with water at 25 ℃, wherein the water quenching transfer time is less than or equal to 5s; then naturally aging the aluminum alloy plate after solid solution for 24 hours; finally, heating the aluminum alloy plate subjected to solid solution to 80 ℃ at a heating rate of 340 ℃/h, and preserving heat for 34h; and (5) air cooling to room temperature after the heat preservation is finished.
In the aging treatment RRA, natural aging is added for 24 hours, so that a more dispersed GP zone can be obtained after aging, and oversaturated vacancies are easily combined with magnesium atoms instead of copper atoms in the natural aging process. This accelerates the diffusion of Mg atoms and increases the nucleation rate of the GP zone, providing more nucleation sites for the GP zone during the reaging process, consuming a large amount of supersaturated vacancies and stored energy in the matrix, and can reduce the diffusion rate and strength loss of solute atoms at 120 ℃. Natural aging for 24 hours prior to re-aging may promote the formation of GP zones, thereby effectively improving the strength properties and thermal stability of the alloys under investigation.
The regression and reaging RRA process comprises a preaging stage, a regression stage and a reaging stage, wherein the novel four-stage ageing is that natural ageing is added for 24 hours before reaging. The low temperature 105 ℃ is selected for pre-ageing, because compared with the conventional 120 ℃, the thermal stability of the precipitated phase at 105 ℃ is low, the size of the precipitated phase is small, the re-dissolution degree of the precipitated phase after the pre-ageing stage is slightly large, the coarsening degree is reduced, the volume fraction is the lowest, and a foundation is provided for the re-dissolution of the precipitated phase in the regression stage. Finally, the pre-ageing schedule of the first stage is selected to be 105 ℃/24 hours. The temperature rising rate of 3 degrees/min is selected in the temperature rising stage of the regression stage, the regression temperature is approximately 165-180 ℃, the selection of the time of the regression stage is determined according to an LSW coarsening dynamics model researched by Deng Yun, and the relationship between the average size Rt of a grain boundary precipitated phase, the regression aging time T and the aging temperature T is expressed by a formula:
selecting the temperature of 170 ℃ and the size of a grain boundary precipitated phase of 40nm, and carrying out formula (1) to obtain 96min of time t; in combination with slow rate of temperature rise, the regression phase schedule is 170 ℃/85min. The diffusion rate of the Mg element is far lower than that of Zn and Mg because of the combination of the Mg element and the supersaturated vacancies, and the diffusion rate of the Mg element is higher in the natural aging process after the regression stage, so that the combination can be gathered and formed into GP zones at the supersaturated vacancies to enhance the nucleation effect of the GP zones. Finally, in the re-ageing stage (80 ℃ C./34 h), the GP zones precipitate again, so that eventually more GP zones and eta' phases are present.
Selection of a re-ageing schedule: the low temperature re-ageing (80 ℃ C./34 h) is chosen, the GP zone being the main precipitate with a re-ageing temperature of 80 ℃ and the eta' phase being the main precipitate with a re-ageing temperature of 120 ℃. After final aging treatment, more GP zones exist, and the alloy has a certain promotion effect on the high-temperature performance of the alloy.
The aging treatment process comprises the following steps: 105 ℃/24h, 170 ℃/85min, natural aging for 24h, 80 ℃/34h (heating rate is 3 DEG/min, water quenching transfer time is less than or equal to 5s, and water temperature is less than or equal to 25 ℃). For the four-stage ageing regime, the first stage ageing was 105 ℃/24h to obtain GP zones and a fine η' phase. The major change in microstructure during regression (170 ℃/85 min) is dissolution of the unstable precipitate (GP zone and fine η' phase) formed by the first aging; meanwhile, with the extension of regression time, eta' and eta phases will grow; the sample is then quenched from 170 ℃ to room temperature to preserve vacancies, and during natural aging the magnesium atoms will combine with vacancies to enhance GP zone nucleation. Finally, in the re-ageing stage (80 ℃ C./34 h), the GP zones precipitate again.
Fig. 11 shows that the addition of rare earth Nd has a very pronounced effect on the softening resistance of the alloy after thermal exposure. Room temperature hardness of 0.2wt.% Nd alloy increased by 4.9% (192.4 HV) compared to Nd-free alloy; the hardness of the alloy without added rare earth was reduced by 17.7% after 120 ℃/100h heat exposure, while the hardness was reduced by 6.6% after 0.2wt.% Nd addition. The result shows that the addition of rare earth Nd can keep a higher hardness value, and the alloy has good high-temperature stability.
FIG. 12 shows the engineering stress strain diagram of an Al-6.1Zn-2.3Mg-1.7Cu-0.15Zr-xNd (x=0; 0.2) alloy after four-stage aging and 120℃hot stretching. Table 1 shows the mechanical properties of the alloy after four-stage aging and 120℃hot stretching.
Table 1 mechanical properties of the alloys after four-stage aging and 120 ℃ hot stretching
As is clear from fig. 12 and table 1, the temperature has a remarkable effect on the mechanical properties of al—zn—mg—cu—zr alloy, and the room temperature tensile strength of the alloy added with 0.2wt.% Nd reaches 670.4MPa, which is 37.5% higher than that of the alloy without rare earth (487.5 MPa), because fine Al3Nd dispersed phase is precipitated in the crystal after aging treatment, which can strongly inhibit migration of dislocation and subgrain boundary. The addition of Nd can also inhibit recrystallization and grain growth, retain small-angle grain boundaries, reduce the ratio of fracture along the grain, and thus improve the strength of the alloy. When the alloy is stretched at 120 ℃, the tensile strength of the alloy added with 0.2wt.% of Nd reaches 396.2MPa, and the tensile strength is increased by 50.6% compared with that of the alloy without rare earth (263.0 MPa), which shows that the rare earth element Nd can obviously improve the high-temperature mechanical property of the alloy.
In the embodiment, the homogenization, solution treatment and aging treatment processes are optimized through microalloying adding rare earth Nd and combining thermodynamics, so that the alloy can improve the performance of the alloy in the aspects of composition design and process. And the natural aging at four stages still improves the subsequent thermal stability to a certain extent. The addition of rare earth Nd improves the segregation degree of elements in the alloy, so that the melted alloy has uniformity, and the subsequent deformation and heat treatment are guaranteed.
It is apparent that the above examples are given by way of illustration only and are not limiting of the embodiments. Other variations or modifications of the above teachings will be apparent to those of ordinary skill in the art. It is not necessary here nor is it exhaustive of all embodiments. While the obvious variations or modifications which are extended therefrom remain within the scope of the claims of this patent application.
Claims (10)
1. The novel super-strong aluminum alloy material is characterized in that the content of Zn is 6.0-6.7 wt%, the content of Mg is 2.1-2.9 wt%, the content of Cu is 1.6-1.9 wt%, the content of Zr is 0.1-0.15 wt%, the content of rare earth element Nd is 0.2-0.4 wt%, the content of impurities is less than or equal to 0.1 wt%, and the balance is aluminum.
2. The novel ultra-strong aluminum alloy material according to claim 1, wherein the mass ratio of Zn to Mg is (2.5-2.7): 1.
3. The novel super-strong aluminum alloy material as claimed in claim 2, wherein the Zn content is 6.1wt.%, the Mg content is 2.3wt.%, the Cu content is 1.7wt.%, the Zr content is 0.15wt.%, and the rare earth element Nd content is 0.2wt.%.
4. The preparation and processing method of the novel super-strong aluminum alloy material is characterized by comprising the following steps of:
step A, smelting and casting: taking aluminum, aluminum zinc intermediate alloy, aluminum magnesium intermediate alloy, aluminum copper intermediate alloy, aluminum zirconium intermediate alloy and magnesium neodymium intermediate alloy as raw materials, smelting and casting to obtain an aluminum alloy cast ingot;
step B, homogenizing: homogenizing the aluminum alloy cast ingot to obtain a homogenized aluminum alloy material;
and C, heat deformation treatment: carrying out thermal deformation treatment on the homogenized aluminum alloy material to obtain an aluminum alloy plate;
step D, solution treatment: carrying out solution treatment on the aluminum alloy plate to obtain a solid-solution aluminum alloy plate;
step E, aging treatment: aging the aluminum alloy plate after solid solution to obtain a novel super-strength aluminum alloy material;
and E, in the novel super-strong aluminum alloy material obtained in the step E: 6.0 to 6.7wt.% of Zn, 2.1 to 2.9wt.% of Mg, 1.6 to 1.9wt.% of Cu, 0.1 to 0.15wt.% of Zr, 0.2 to 0.4wt.% of rare earth element Nd, less than or equal to 0.1wt.% of impurity, and the balance of aluminum.
5. The preparation and processing method of the novel ultra-strong aluminum alloy material according to claim 4, wherein the specific operation method of smelting and casting in the step A comprises the following steps:
step (A-1), preparing an alloy raw material and polishing the alloy raw material; the alloy raw materials are as follows: pure aluminum, aluminum zinc master alloy, aluminum magnesium master alloy, aluminum copper master alloy, aluminum zirconium master alloy and magnesium neodymium master alloy;
smelting alloy by adopting a resistance induction smelting furnace: preheating a crucible, adding pure aluminum, heating and smelting, and sequentially adding an aluminum-zinc intermediate alloy, an aluminum-copper intermediate alloy, an aluminum-zirconium intermediate alloy wrapped by aluminum foil and an aluminum-magnesium intermediate alloy wrapped by aluminum foil after the pure aluminum is completely melted; after all the added alloy raw materials are melted, refining treatment, stirring and slag skimming are carried out to obtain smelting alloy liquid;
step (A-3), adding magnesium neodymium intermediate alloy wrapped by aluminum foil into the smelting alloy liquid, standing and stirring to obtain aluminum alloy liquid;
and (A-4) immediately pouring the stirred aluminum alloy liquid into a preheated graphite mould, and casting to obtain an aluminum alloy cast ingot.
6. The method for producing and processing a novel ultra-strong aluminum alloy material according to claim 5, wherein in the step (a-1), the purity of pure aluminum is 99.9wt% or more; the mass fraction of zinc in the aluminum-zinc intermediate alloy is 30 wt%, the mass fraction of magnesium in the aluminum-magnesium intermediate alloy is 20 wt%, the mass fraction of copper in the aluminum-copper intermediate alloy is 30 wt%, and the mass fraction of zirconium in the aluminum-zirconium intermediate alloy is 10 wt%; when preparing alloy raw materials, preparing materials according to the proportion that the burning loss rate of zinc is 2%, the burning loss rate of copper is 5%, and the burning loss rate of zirconium is 30%;
in the step (A-2): the preheating temperature of the crucible is 380-420 ℃; adding pure aluminum and heating to 720-750 ℃; the refining treatment method comprises the following steps: controlling the temperature of the alloy liquid within 725-735 ℃, immersing C into the alloy liquid 2 Cl 6 Introducing argon into the graphite bell jar, and refining for 20min; c (C) 2 Cl 6 The mass of the alloy is 0.3 to 0.5 weight percent of the total weight of the alloy liquid;
in the step (A-3), adding an aluminum foil wrapped magnesium neodymium intermediate alloy when the temperature of the smelting alloy liquid is regulated to be in the range of 700-710 ℃, and standing for 2min; the mass fraction of neodymium in the magnesium-neodymium master alloy is 30wt.%;
in the step (A-4), the preheating temperature of the graphite mold is 180-230 ℃.
7. The method for preparing and processing a novel ultra-strong aluminum alloy material according to claim 4, wherein in the step B, during homogenization treatment, the temperature of the aluminum alloy ingot is raised to 300 ℃ at a heating rate of 340 ℃/h, and the temperature is kept for 6 hours; then heating the aluminum alloy ingot to 472 ℃ at a heating rate of 50 ℃/h, and preserving the heat for 24h; and (3) after the heat preservation is finished, water quenching is carried out by using water with the temperature of less than or equal to 25 ℃, and the water quenching transfer time is less than or equal to 5s.
8. The method for preparing and processing a novel ultra-strong aluminum alloy material according to claim 4, wherein in the step C, during the heat deformation treatment, the homogenized aluminum alloy material is firstly placed in a resistance furnace and is pretreated for 2 hours at the temperature of 420 ℃; then, at an extrusion temperature of 405.+ -. 5 ℃ and an extrusion speed of 1mm/s, an extrusion ratio of 25:1 was subjected to extrusion deformation to obtain an aluminum alloy sheet.
9. The method for producing and processing a novel ultra-strong aluminum alloy material according to claim 4, wherein in the step D, during the solution treatment, the aluminum alloy plate is heated to 455 ℃ at a heating rate of 340 ℃/h, and is kept for 1.5h; then heating the aluminum alloy plate to 474 ℃ at a heating rate of 60 ℃/h, and preserving heat for 0.5h; and (3) after the heat preservation is finished, water quenching is carried out by using water with the temperature of less than or equal to 25 ℃, and the water quenching transfer time is less than or equal to 5s.
10. The method for producing and processing a novel super-strength aluminum alloy material according to claim 4, wherein in the step E, the temperature of the aluminum alloy plate after solid solution is raised to 105 ℃ at a temperature raising rate of 340 ℃/h and is kept for 24 hours during aging treatment; then heating the aluminum alloy plate subjected to solid solution to 170 ℃ at a heating rate of 3 ℃/min, preserving heat for 85min, and then performing water quenching with water at a temperature of less than or equal to 25 ℃ for a water quenching transfer time of less than or equal to 5s; then naturally aging the aluminum alloy plate after solid solution for 24 hours; finally, heating the aluminum alloy plate subjected to solid solution to 80 ℃ at a heating rate of 340 ℃/h, and preserving heat for 34h; and (5) air cooling to room temperature after the heat preservation is finished.
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