A thermomechanical treatment process for continuous cold rolling of ultra-high-strength Al-Zn-Mg-Cu alloy

Abstract

The invention belongs to the technical field of aluminum alloy heat treatment, and particularly relates to a thermomechanical treatment process for achieving continuous cold rolling of an ultra-high-strength Al-Zn-Mg-Cu alloy. The process comprises the following steps: performing high-temperature pre-aging on the alloy in the range of 300-420°C, with a holding time of 1-14 hours, a heating rate of 1-15°C / min in a heat treatment furnace, slowly cooling the alloy to below 200°C while holding the alloy, and then taking out the alloy; performing continuous cold rolling on the alloy at room temperature of 1-100°C, with a pass reduction controlled at 10%-30% and a total reduction of 70%-95%; and after cold rolling, performing a short-time high-temperature recrystallization annealing on the alloy at a temperature of 470-490°C and a holding time of 10-30 minutes. In the higher temperature stage, hard large-particle MgZn2 particles (1-3μm) are pre-precipitated. At this time, the alloy element content of the matrix is reduced, resulting in reduced deformation resistance, which can achieve continuous cold rolling. At the same time, the MgZn2 particles interact violently with the matrix during the cold deformation process, increasing the dislocation accumulation density of the matrix. Then, through short-term high-temperature recrystallization annealing, the recrystallization nucleation rate is greatly increased, achieving the purpose of alloy grain refinement.

Description

Technical Field

[0001] The present invention belongs to the technical field of aluminum alloy heat treatment, and in particular relates to a thermomechanical treatment process for realizing continuous cold rolling of an ultra-high-strength Al-Zn-Mg-Cu alloy. Background Art

[0002] Al-Zn-Mg-Cu alloy (7xxx series) is the strongest series of aluminum alloys. It has the advantages of high specific strength, strong stress corrosion resistance, and good hot working performance. It is recognized as a backbone material in the aerospace and weapon equipment fields. However, the forming of Al-Zn-Mg-Cu alloy is currently mainly done by hot working. The alloy has problems such as deformation / temperature unevenness and difficult to control recovery / recrystallization during hot working, which can lead to microstructural evolution or transformation such as subgrain transformation, coarse grains, and phase precipitation, thereby significantly weakening the alloy's fine grain structure and high strength advantages.

[0003] In response to these problems, in recent years, China has developed an ultra-low temperature forming process. The deformation of the solid solution Al-Zn-Mg-Cu alloy at ultra-low temperature (liquid nitrogen) can inhibit the dynamic precipitation of the strengthening phase, and only produce work hardening (dislocation accumulation), resulting in a "double increase effect" of increased deformation elongation and hardening index. The alloy does not break or fragment at a rolling amount of 70% to 90%. However, the ultra-low temperature forming process conditions are demanding and the manufacturing cost is extremely high. It is currently only used for the trial production of a small number of aerospace components. In addition, after aging treatment, high-density dislocations still exist in the Al-Zn-Mg-Cu alloy manufactured by ultra-low temperature forming, which seriously reduces the plasticity and toughness of the Al-Zn-Mg-Cu alloy and makes subsequent processing and forming difficult. Hou Longgang et al. (Hou Longgang, Liu Mingli, Wang Xindong et al., Ultra-low temperature large deformation processing and microstructure and performance control of high-strength 7050 aluminum alloy, Acta Metallurgica Sinica, 2017, 53(9):1075-1089) subjected the cast 7050 aluminum alloy to ultra-low temperature rolling with a deformation of 91% and aging at 80℃ / 72h. Compared with the hot-rolled state, the tensile strength of the alloy increased from 600MPa to 650MPa, and the elongation decreased from 16.8% to 12.0%.

[0004] At present, a large number of literatures have studied the deformation, solution treatment and aging processes of Al-Zn-Mg-Cu alloys, but there is little research on the cold deformation process of this alloy. It is generally believed that Al-Zn-Mg-Cu alloys with high element content have extremely poor room temperature forming properties and do not meet the conditions for cold processing.

[0005] In summary, it is difficult to simultaneously obtain ultra-high strength and good plasticity through hot working and ultra-low temperature processing of Al-Zn-Mg-Cu alloys, and there are problems such as complex process, long remelting and holding time, high energy consumption, and high processing cost. Summary of the Invention

[0006] The object of the present invention is to provide a thermomechanical treatment process for realizing continuous cold rolling of an ultra-high-strength Al-Zn-Mg-Cu alloy, which has the advantages of refining the grains while realizing cold rolling of the alloy, thereby increasing plasticity, toughness and corrosion resistance, and solves the problems that hot processing and ultra-low temperature processing of Al-Zn-Mg-Cu alloy are difficult to simultaneously obtain ultra-high strength and good plasticity, and there are also problems such as complex process, long remelting and holding time, high energy consumption and high processing cost.

[0007] In order to achieve the above technical objectives and the above technical effects, the present invention is implemented through the following technical solutions:

[0008] A thermomechanical treatment process for achieving continuous cold rolling of an ultra-high-strength Al-Zn-Mg-Cu alloy comprises the following steps:

[0009] 1) subjecting the ultra-high strength Al-Zn-Mg-Cu alloy to high temperature pre-aging at 300-420° C., with a heating rate of 1-15° C. / min in the heat treatment furnace and a holding time of 1-14 hours; cooling the alloy to below 200° C. after the holding period, and removing the ultra-high strength Al-Zn-Mg-Cu alloy;

[0010] 2) continuously cold rolling the extracted ultra-high strength Al-Zn-Mg-Cu alloy at 1-100° C., with each pass reduction controlled at 10%-30% and a total reduction of 70%-95%;

[0011] 3) After cold rolling, the ultra-high strength Al-Zn-Mg-Cu alloy is subjected to a short high-temperature recrystallization annealing at a temperature of 470-490°C for 10-30 minutes;

[0012] 4) Finally, the ultra-high strength Al-Zn-Mg-Cu alloy is subjected to a final aging treatment at a temperature of 80 to 150° C. for a holding time of 1 to 36 hours, and then cooled to obtain a finished product.

[0013] Furthermore, in step 1), the mass percentage of the components of the ultra-high strength Al-Zn-Mg-Cu alloy is:

[0014] Zn: 5% to 12%

[0015] Mg: 1.2% to 2.8%

[0016] Cu: 1.0% to 2.3%

[0017] Zr: 0.1%~0.2%

[0018] Fe: <0.12%

[0019] Si: <0.12%

[0020] The rest is Al.

[0021] Furthermore, in step 1), the cooling method is furnace cooling or air cooling with the furnace door open.

[0022] Furthermore, in step 2), the cold rolling method is synchronous rolling or asynchronous rolling.

[0023] Furthermore, when asynchronous rolling is adopted, the speed ratio is 1.05 to 1.28, and the reduction per pass is controlled at 10% to 20%.

[0024] Furthermore, in step 3), the short-time high-temperature recrystallization annealing is performed in a muffle furnace or a salt bath furnace.

[0025] Furthermore, when a salt bath furnace is used, the heating medium is potassium nitrate.

[0026] Furthermore, in step 4), after the aging treatment, the cooling method is water quenching.

[0027] The beneficial effects of the present invention are:

[0028] 1. Based on previous research, the present invention takes the commonly used Al-Zn-Mg-Cu high-strength aviation aluminum alloy as an example and adopts a deformation heat treatment method that combines high-temperature pre-aging, cold rolling deformation and short-time high-temperature solid solution. While achieving room temperature cold rolling of the alloy, it significantly refines the grains and achieves a good match between strength and plasticity, with the characteristics of energy saving, high efficiency and low cost.

[0029] 2. The present invention pre-precipitates hard and large-grained MgZn2 particles (1 to 3 μm) at a relatively high temperature stage. At this time, the alloy element content of the matrix is reduced, resulting in reduced deformation resistance, which can achieve continuous cold rolling. At the same time, the MgZn2 particles interact violently with the matrix during the cold deformation process, increasing the dislocation accumulation density of the matrix. Then, through short-term high-temperature recrystallization annealing, the recrystallization nucleation rate is greatly increased, achieving the purpose of alloy grain refinement, thereby overcoming the problem that the strength and plasticity of the Al-Zn-Mg-Cu alloy cannot be taken into account at the same time.

[0030] Of course, any product implementing the present invention does not necessarily need to achieve all of the above advantages at the same time. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0032] Figure 1 Schematic diagram of the process of the present invention;

[0033] Figure 2 This is an example of intergranular corrosion cross-sectional morphology of the present invention:

[0034] Among them, a: process of the present invention, b: control experiment of ordinary hot rolling;

[0035] Figure 3 This is an exfoliation corrosion cross-sectional morphology diagram of an example of the present invention:

[0036] Among them, a: process of the present invention, b: control experiment of ordinary hot rolling;

[0037] Figure 4 An example of microstructure analysis diagram of the present invention:

[0038] Among them, a: extruded alloy, b: high temperature pre-aging, c: coarse MgZn2 particles,

[0039] d: Metallographic image of the final alloy;

[0040] Figure 5 This is a schematic diagram of the process principle of an example of the present invention. DETAILED DESCRIPTION

[0041] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0042] The present invention provides a thermomechanical treatment process for achieving continuous cold rolling of an ultra-high-strength Al-Zn-Mg-Cu alloy, comprising the following steps:

[0043] Step 1: Pre-aging the alloy at a temperature of 300-420°C for 1-14 hours at a heating rate of 1-15°C / min. Slowly cool the alloy to below 200°C and remove it.

[0044] Step 2: The alloy is continuously cold rolled at room temperature of 1 to 100°C, with each pass reducing by 10% to 30% and a total reducing by 70% to 95%.

[0045] Step 3: After cold rolling, the alloy is subjected to a short high-temperature recrystallization annealing at 470-490°C for 10-30 minutes. Hard, large MgZn2 particles (1-3 μm) are pre-precipitated at this higher temperature. This reduces the matrix alloying element content, resulting in lower deformation resistance and enabling continuous cold rolling. Furthermore, the intense interaction of the MgZn2 particles with the matrix during cold deformation increases the matrix's dislocation accumulation density.

[0046] Step 4: A short-term high-temperature recrystallization annealing is then performed to greatly increase the recrystallization nucleation rate and achieve the purpose of alloy grain refinement.

[0047] The relevant specific embodiments of the present invention are:

[0048] Example 1

[0049] A thermomechanical treatment process for achieving continuous cold rolling of an ultra-high-strength Al-Zn-Mg-Cu alloy is disclosed. The process utilizes a 6mm thick Al-Zn-Cu-Mg alloy hot-rolled plate containing the following alloy components by mass: Zn 7.8%, Mg 2.0%, Cu 2.2%, Zr 0.12%, with the remainder being Al. First, the sample is placed in a muffle furnace and heated at a rate of 10°C / min. When the temperature reaches 400°C, it is held for 3 hours. The furnace is then powered off and the door opened to cool the sample to room temperature. The sample is then cold-rolled on a twin-roll mill with a 300mm diameter roll, with a 15% reduction per pass and a total deformation of 70%. The cold-rolled plate is then placed in a muffle furnace and held for 10 minutes at 480°C. After the holding period, the plate is rapidly water quenched. Finally, the sample is aged in a 100°C forced air drying oven for 32 hours.

[0050] The yield strength, tensile strength and elongation of the alloy obtained in this embodiment are shown in Table 1. In addition, the intergranular corrosion test was carried out on the sample according to GB / T7998-2005. The etching solution contained 0.97 mol NaCl + 0.3 mol H2O2 per liter. The sample was placed in a water bath at a temperature of 35°C and etched for 6 hours. The depth of intergranular corrosion was observed. The corrosion depth is listed in Table 1. The intergranular corrosion interface morphology is shown in FIG. Figure 2 As shown, all the results are compared with ordinary hot-rolled sample specimens. It can be found that the alloy prepared by the process of the present invention has more excellent mechanical properties and corrosion resistance.

[0051] Example 2

[0052] A 10mm thick Al-Zn-Cu-Mg alloy hot-rolled plate was used, with the following alloy composition by weight: Zn 9.1%, Mg 2.1%, Cu 1.7%, Zr 0.15%, and the balance Al. The sample was first placed in a muffle furnace and heated at a rate of 5°C / min. When the temperature reached 380°C, it was held for 1 hour. The furnace was then turned off, the door opened, and the sample cooled to room temperature. The sample was then cold-rolled on an asynchronous twin-roll mill with a roll diameter of 350mm, a speed ratio of 1.15, and a total deformation of 60%. The cold-rolled plate was then placed in a muffle furnace at 480°C for 5 minutes. After the holding period, it was quickly water quenched. Finally, the sample was aged in a 120°C forced air drying oven for 20 hours.

[0053] The yield strength, tensile strength and elongation of the alloy obtained in this example are shown in Table 1. The intergranular corrosion test of the sample of Example 2 was carried out according to the method in Example 1, and the results are shown in Table 1. In addition, the sample was subjected to an exfoliation corrosion test according to GB / T22639-2008. The etching solution contained 4.0 mol NaCl + 0.5 mol KNO3 + 0.1 mol HNO3 per liter. The sample was placed in a water bath at a temperature of 25°C and etched for 48 hours. After the corrosion products were removed, the surface corrosion morphology was observed. Figure 3 All the results were compared with those of ordinary hot-rolled samples. It can be found that the alloy prepared by the process of the present invention has better mechanical properties and corrosion resistance.

[0054] Example 3

[0055] An Al-Zn-Cu-Mg alloy extruded bar with a diameter of 12 mm was used. Its alloy composition by weight consisted of 5.7% Zn, 2.4% Mg, 1.8% Cu, and 0.18% Cr, with the balance being Al. The sample was first placed in a muffle furnace and heated at a rate of 10°C / min. When the temperature reached 420°C, it was held for 1 hour. The furnace was then turned off and the sample allowed to cool to below 200°C. The sample was then cold-rolled on a twin-roll mill with a roll diameter of 300 mm, with a reduction of 20% per pass and a total deformation of 85%. The cold-rolled sheet was then placed in a salt bath furnace at 480°C for 3 minutes. After the holding period, it was quickly water quenched. Finally, the sample was aged in a 130°C forced air drying oven for 8 hours.

[0056] The yield strength, tensile strength and elongation of the alloy obtained in this example are shown in Table 1. The intergranular corrosion test of the sample of Example 3 was carried out according to the method in Example 1. The results are shown in Table 1. Compared with the ordinary hot-rolled sample, it can be found that the alloy prepared by the process of the present invention has more excellent mechanical properties and corrosion resistance. In addition, the scanning electron microscope image of the original Al-Zn-Cu-Mg alloy extruded bar, the scanning electron microscope image and transmission electron microscope image of the high-temperature pre-precipitated sample, and the metallographic photograph of the final sample are shown in Table 1. Figure 4 shown.

[0057] The above Table 1 is as follows:

[0058] Table 1

[0059]

[0060] In summary, the thermomechanical treatment process for achieving continuous cold rolling of an ultra-high-strength Al-Zn-Mg-Cu alloy, through steps one, two, three, and four, solves the problems of difficulty in simultaneously obtaining ultra-high strength and good plasticity through hot processing and ultra-low temperature processing of the Al-Zn-Mg-Cu alloy, as well as the problems of complex process, long remelting and holding time, high energy consumption, and high processing cost.

[0061] Although the alloys used in the specific embodiments of the present invention are the three components of the age-hardened Al-Zn-Mg-Cu alloy of the present invention, the composition changes within the range of the Al-Zn-Mg-Cu alloy of the present invention are essentially age-hardened Al-Zn-Mg-Cu alloys, and the types and precipitation patterns of the precipitated phases are basically the same. Therefore, the present invention is applicable to all alloy compositions. In addition, the process parameters of the present invention are not limited to the specific processes selected in the embodiments, and the same effect can be achieved within the process parameter range.

[0062] The preferred embodiments of the present invention disclosed above are intended only to help illustrate the present invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the present invention to specific embodiments. Obviously, many modifications and variations are possible based on the contents of this specification. These embodiments are selected and described in detail in this specification to better explain the principles and practical applications of the present invention, thereby enabling those skilled in the art to better understand and utilize the present invention. The present invention is limited only by the claims and their full scope and equivalents.

Claims

1. A thermomechanical treatment process for achieving continuous cold rolling of high-strength Al-Zn-Mg-Cu alloy, characterized in that: The following steps are involved: 1) Pre-aging the high-strength Al-Zn-Mg-Cu alloy at 300-420°C, with a heating rate of 1-15°C / min and a holding time of 1-14 hours; cooling the alloy to below 200°C after the holding period, and removing the high-strength Al-Zn-Mg-Cu alloy; wherein the mass percentages of the components of the high-strength Al-Zn-Mg-Cu alloy are as follows: Zn: 5%~12% Mg: 1.2%~2.8% Cu: 1.0%~2.3% Zr：0.1%~0.2% Fe: <0.12% Si: <0.12% The rest is Al; 2) The extracted high-strength Al-Zn-Mg-Cu alloy is continuously cold rolled at 1-100°C, with each pass reduction controlled at 10%-30% and a total reduction of 70%-95%; 3) After cold rolling, the high-strength Al-Zn-Mg-Cu alloy is subjected to a short-time high-temperature recrystallization annealing at a temperature of 470-490°C for 10-30 minutes; 4) Finally, the high-strength Al-Zn-Mg-Cu alloy is subjected to a final aging treatment at a temperature of 80-150°C for a holding time of 1-36 hours, and then cooled to obtain a finished product.

2. The thermomechanical treatment process for achieving continuous cold rolling of a high-strength Al-Zn-Mg-Cu alloy according to claim 1, characterized in that: In step 1), the cooling method is furnace cooling or air cooling with the furnace door open.

3. The thermomechanical treatment process for achieving continuous cold rolling of a high-strength Al-Zn-Mg-Cu alloy according to claim 1, characterized in that: In step 2), the cold rolling method is synchronous rolling or asynchronous rolling.

4. The thermomechanical treatment process for achieving continuous cold rolling of a high-strength Al-Zn-Mg-Cu alloy according to claim 3, characterized in that: When asynchronous rolling is adopted, the speed ratio is 1.05~1.28, and the pass reduction is controlled at 10%~20%.

5. The thermomechanical treatment process for achieving continuous cold rolling of a high-strength Al-Zn-Mg-Cu alloy according to claim 1, characterized in that: In step 3), short-time high-temperature recrystallization annealing is performed in a muffle furnace or a salt bath furnace.

6. The thermomechanical treatment process for achieving continuous cold rolling of a high-strength Al-Zn-Mg-Cu alloy according to claim 5, characterized in that: When a salt bath furnace is used, the heating medium is potassium nitrate.

7. The thermomechanical treatment process for achieving continuous cold rolling of a high-strength Al-Zn-Mg-Cu alloy according to claim 1, characterized in that: In step 4), after aging treatment, water quenching is used for cooling.

Images