A high-strength Al-Mg alloy and its preparation method

Abstract

The present invention belongs to the technical field of aluminum alloy heat treatment, and particularly relates to a high-strength Al-Mg alloy and a preparation method thereof. The Al-Mg alloy provides nucleation sites for the AlMn second phase in the aluminum matrix by alloying with Mn and Zn, and reduces the activation energy of the AlMn second phase. The preparation method mainly subjects the prepared alloy to multi-stage homogenization treatment, where the low temperature in the multi-stage homogenization is for the pre-nucleation of the second phase, the intermediate temperature is for the nucleation and precipitation of the second phase, and the high temperature is for the dissolution of the coarse crystal phase in three heat treatment stages. By using this method to perform multi-stage homogenization heat treatment on the alloy ingot, the precipitation number density of the second phase in the alloy ingot can be significantly increased, and its distribution uniformity can be improved, thereby increasing the strength of the alloy and retaining a large number of sub-grain structures inside the alloy during the subsequent hot working process, effectively inhibiting the occurrence and abnormal growth of recrystallization, and providing an optimal deformed structure for obtaining high-performance aluminum alloy products.

Description

Technical Field

[0001] The present invention belongs to the technical field of aluminum alloy heat treatment, and particularly relates to a high-strength Al-Mg alloy and a preparation method thereof. Background Art

[0002] 5XXX series Al-Mg alloys are widely used in industrial fields such as automobiles, ships, buildings, and aerospace due to their high specific strength, excellent formability, toughness, weldability, and corrosion resistance. However, as a typical non-heat-treatable aluminum alloy, 5XXX series aluminum alloys lack precipitation hardening achieved through heat treatment. Their strengthening mechanism mainly relies on the solid solution strengthening effect of magnesium (Mg) elements and the strain strengthening effect experienced during the processing. Therefore, the strength improvement of the alloy is limited to a certain extent. Under specific deformation conditions, an increase in the Mg content can improve the strength of the alloy. However, too high an Mg content will significantly promote the precipitation of the β-Al3Mg2 phase along the grain boundaries, thereby causing edge cracking and stress corrosion cracking phenomena. Microalloying technology is a widely used method in alloy design, aiming to change the precipitation behavior of the second phase to improve the mechanical strength and recrystallization resistance of the alloy. This technology has currently become a research hotspot. Therefore, in order to further improve the strength of 5XXX series aluminum alloys, introducing nano-scale second phases into the Al matrix to enhance the dispersion strengthening effect has become one of the key ways in the strengthening strategy of 5XXX series aluminum alloys.

[0003] Specifically, by adding trace transition elements (such as Mn, Zr, Sc, Cr, etc.) to 5XXX series aluminum alloys, fine and dispersed second-phase particles (also known as the second phase) are formed to improve the mechanical strength and recrystallization resistance of the alloy. Currently, common commercial 5XXX alloys all contain different amounts of Mn element, and the Mn element will form fine AlMn second phase during the homogenization treatment process. Therefore, researchers optimize the precipitation characteristics of the second phase by adjusting the homogenization treatment process. Specifically, traditional single-stage homogenization treatment is usually carried out with a long-time heat preservation in a relatively high temperature range (500 - 550 °C), which results in larger size and lower number density of the formed second phase, thus reducing its strengthening effect. In contrast, existing research has shown that multi-stage homogenization treatment can effectively increase the number density of the second phase and achieve refinement of its size. However, due to the lack of effective nucleation sites inside the Al-Mg-Mn alloy, the precipitation number density of the second phase is still limited. Even with multi-stage homogenization heat treatment, the number density of the second phase is still limited and accompanied by obvious distribution inhomogeneity. Patent CN1851019A discloses an Al-Mg-Mn alloy strengthened by Er and Zr. The improvement of the aluminum alloy performance after microalloying is mainly due to the formation of coherent or semi-coherent Al3Er second phase by Er with the matrix, but the influence brought by the AlMn second phase formed by the Mn element in the alloy is not concerned. Patent CN117448636A discloses a preparation and processing method of a high heat-resistant dispersion-strengthened Al-Mg-Mn-Zr-Cr-Mo alloy. This method uses the multi-element synergistic microalloying of Zr, Cr, Mo and the processes of pre-aging and pre-deformation treatment to promote the precipitation of nano α second phase and Al3Zr second phase, but the number density of the second phase is still low. Patent CN101497975A provides a heat treatment process for an Al-Mg-Mn-Er-Zr alloy. This method mainly provides a two-stage homogenization heat treatment system, including the first-stage homogenization heat treatment at 250 - 310 °C and the second-stage homogenization heat treatment at 430 - 510 °C. Although this method promotes the Al3Er and Al6Mn second phases through the first-stage homogenization heat treatment, it does not really consider that the precipitation temperature ranges of different second phases are different, so the ideal precipitation of the second phase cannot be obtained either. Summary of the Invention

[0004] Although the existing technical conditions have realized that introducing nano second phase into the aluminum matrix through microalloying to enhance the mechanical strength and recrystallization resistance of the alloy, due to the segregation of Mn element in the Al-Mg-Mn alloy and the lack of nucleation sites for AlMn second phase in the alloy, the precipitation number density of the second phase is limited and unevenly distributed, thus resulting in limited improvement of the alloy strength.

[0005] Based on the above problems, the present invention proposes a new method for strengthening aluminum alloys, particularly a multi-stage homogenization heat treatment method and its application for 5XXX series aluminum alloys. That is, zinc (Zn) is introduced into the Al-Mg-Mn based alloy to form a low-melting-point MgZn second phase as an effective nucleation site for the high-melting-point AlMn second phase, and combined with the heat treatment process, the precipitation behavior of these two second phases is precisely controlled to enhance the number density of the second phase and its distribution uniformity in the alloy matrix, and the refinement of the second phase size is achieved. This strategy aims to improve the comprehensive performance of the alloy by optimizing the microstructure.

[0006] To solve the above existing technical problems, the present application provides the following technical solutions:

[0007] The present invention provides a method for preparing a high-strength Al-Mg based alloy, comprising the following steps:

[0008] S11: Heating the alloy ingot at a rate of 1-5 °C / min to 150-200 °C for the first-stage homogenization heat treatment for 1-4 h to obtain a first-stage ingot; the alloy ingot is obtained by semi-continuous casting of master alloys; calculated by mass fraction, the alloy ingot comprises the following elements: 2-6% Mg, 0.4-0.9% Mn, 1-5% Zn, Si ≤0.15%, Fe≤0.15%, Ti≤0.1%, and the balance is Al; the master alloys are selected from high-purity Al (99.9%), pure Mg (99.9%), pure Zn (99.9%) and Al-10Mn;

[0009] S12: Heating the first-stage ingot at a rate of 1-5 °C / min to 250-350 °C for the second-stage homogenization heat treatment for 1-4 h to obtain a second-stage ingot;

[0010] S13: Heating the second-stage ingot at a rate of 1-5 °C / min to 460-520 °C for the third-stage homogenization heat treatment for 1-8 h to obtain a third-stage ingot;

[0011] S14: Cooling the third-stage ingot to room temperature and then performing hot rolling deformation and annealing treatment to obtain the high-strength Al-Mg based alloy.

[0012] Preferably, in the step S14, in the third-stage ingot after three-stage homogenization heat treatment, nano-scale AlMn second phases are precipitated.

[0013] Furthermore, the number density of the AlMn second phase > 30 μm -3 .

[0014] Furthermore, the average equivalent diameter of the nano-scale AlMn second phase is less than 80 nm.

[0015] Furthermore, the dispersion strengthening value brought by the nanoscale AlMn second phase is greater than 50 MPa.

[0016] Preferably, in the step S14, the cooling is carried out within 2 minutes after the three-stage ingot is taken out of the furnace.

[0017] Preferably, in the step S14, the cooling method is air cooling or water mist cooling.

[0018] Preferably, in the step S14, the recrystallization fraction after hot rolling deformation is less than 10%.

[0019] Preferably, in the step S14, after cooling, it is held at 400 °C for 1 h, and then hot rolling deformation is carried out at 400 °C, and the total reduction is 70%; the temperature of the annealing heat treatment is 400 °C and the time is 1 h.

[0020] The present invention also provides a high-strength Al-Mg alloy system prepared by the above preparation method.

[0021] The technical solution of the present invention has the following advantages compared with the prior art:

[0022] The present invention forms the MgZn second phase as an effective nucleation site by introducing Zn into the Al-Mg-Mn alloy, and optimizes the heat treatment process in combination with the precipitation kinetics characteristics of the second phase, significantly improving the number density of the AlMn second phase in the alloy, refining the second phase size, and improving the distribution uniformity of the second phase in the alloy matrix. Thus, these improvements in the microstructure provide an additional dispersion strengthening effect for the alloy, thereby enhancing the mechanical strength and recrystallization resistance of the alloy.

[0023] Compared with the existing 5XXX series homogenization heat treatment method, in the single-stage homogenization treatment, the precipitation number density of the AlMn second phase is low and the size is large. In the multi-stage homogenization treatment, the precipitation number density of the AlMn second phase has increased, but it is still limited, resulting in a limited additional dispersion strengthening effect obtained by the alloy. At the same time, in the traditional single-stage and multi-stage homogenization treatment processes, the holding time is long, resulting in high energy consumption and low production efficiency. The method of the present invention has a simple process, wide applicability, and significant industrial application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a scanning electron microscope (SEM) characterization diagram of Al-Mg-Mn-xZn alloys with different Zn contents in Example 1 under the same homogenization heat treatment conditions.

[0025] Figure 2 It is an electron backscatter diffraction (EBSD) characterization diagram of Al-Mg-Mn-xZn alloys with different Zn contents in Example 1 under the same homogenization heat treatment conditions.

[0026] Figure 3 TEM characterization diagrams of Al-Mg-Mn-Zn alloys with the same Zn content as in Example 2 under different homogenization heat treatment conditions.

[0027] Figure 4 EBSD characterization diagrams of Al-Mg-Mn-Zn alloys with the same Zn content as in Example 2 under different homogenization heat treatment conditions.

[0028] Figure 5 TEM characterization diagrams of Al-Mg-Mn-xZn alloys with different Zn contents as in Example 3 under different homogenization heat treatment conditions.

[0029] Figure 6 EBSD characterization diagrams of Al-Mg-Mn-xZn alloys with different Zn contents as in Example 3 under different homogenization heat treatment conditions. Detailed implementation manners

[0030] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, so that those skilled in the art can better understand the present invention and be able to implement it, but the specific embodiments cited do not limit the present invention.

[0031] Example 1:

[0032] In this example, Al-Mg-Mn-xZn alloys with different Zn contents (x = 0, 3, 5 wt.%) were used as the research objects. The alloys were prepared according to the following mass percentages: Mg: 4.5%; Mn: 0.7%; Zn: 0 - 5%; the balance was aluminum and unavoidable impurities, and the content of its unavoidable impurities was: Fe: 0.02%; Si: 0.02%. Intermediate alloys high-purity Al (99.9%), pure Mg (99.9%), pure Zn (99.9%) and Al-10Mn were selected. After batching, the raw materials were heated to 750 °C, held for 30 min after complete melting and stirred multiple times, then cooled to 740 °C, degassed with argon for 15 min, the surface slag was removed and then left standing for 20 min, and cast into a cold water copper mold at 700 - 710 °C to obtain ingots. Samples were taken from different ingots and the same homogenization heat treatment tests were carried out. Al-Mg-Mn alloys with different Zn contents are shown in Table 1. After the homogenization treatment of different alloys was completed, they were immediately water-cooled to 25 °C.

[0033] The alloy after homogenization treatment is first heat-insulated at 400 °C for 1 h, then hot-rolled at 400 °C with a total reduction of 70%. The cooling method is water spray cooling. Subsequently, the alloy is annealed at 400 °C for 1 h to eliminate the internal stress in the alloy.

[0034] The analysis and testing are carried out in accordance with relevant standards, and the results are shown in Table 2. By comparing Figure 1 and Figure 2 the scanning electron microscopy and deformed microstructure results of the second-phase precipitation characteristics inside the grains shown, it can be seen that there are significant differences in the second-phase characteristics of Al-Mg-Mn alloys with different Zn contents. Adding Zn to the Al-Mg-Mn alloy proposed in the present invention can significantly increase the number density of second-phase precipitation, thereby increasing the contribution value of the second phase to the dispersion strengthening of the alloy. At the same time, good recovery microstructure is retained during the hot deformation process.

[0035] Table 1 Al-Mg-Mn alloys with different Zn contents

[0036]

[0037] Table 2 Analysis results of the second phase and dispersion strengthening contribution values in Al-Mg-Mn alloys with different Zn contents

[0038]

[0039] Example 2:

[0040] In this example, the alloy composition and ingot preparation method are different from those in Example 1. This example takes the Al-Mg-Mn-Zn alloy as the research object, and its alloy composition is: Mg: 4.5%, Mn: 0.7%, Zn: 5%, and the balance is aluminum and inevitable impurities, and the content of its inevitable impurities is: Fe: 0.02%, Si: 0.02%. Select intermediate alloys high-purity Al (99.9%), pure Mg (99.9%), pure Zn (99.9%) and Al-10Mn. After batching, heat the raw materials to 750 °C, keep them molten for 30 min and stir them several times, then cool down to 740 °C, degas with argon for 15 min, remove the surface slag and then stand for 20 min, and cast into a cold water copper mold at 700 - 710 °C to obtain an ingot. Carry out homogenization heat treatment tests with different schemes on this alloy ingot, and the homogenization treatment regime is shown in Table 3. Among them, Process 1# - 3# are all the methods of the present invention. Immediately after homogenization treatment, water-cool to 25 °C.

[0041] The alloy after homogenization treatment is first heat-insulated at 400 °C for 1 h, then hot-rolled at 400 °C with a total reduction of 70%. The cooling method is water spray cooling. Subsequently, the alloy is annealed at 400 °C for 1 h to eliminate the internal stress in the alloy.

[0042] The analysis and testing are carried out in accordance with relevant standards. The analysis results of the second phase and the contribution values of dispersion strengthening after different homogenization heat treatments are shown in Table 4. It can be found by comparison that, compared with the homogenization heat treatment processes 1# and 2#, the stepwise homogenization heat treatment process 3# proposed in the present invention significantly reduces the average equivalent diameter of the second phase, increases the number density of the second phase, and thus significantly enhances the dispersion strength. By comparing Figure 3 and Figure 4 the scanning electron microscope and deformed microstructure results showing the precipitation characteristics of the second phase inside the grains as shown, it can be seen that in the stepwise homogenization heat treatment process 3# proposed in the present invention, the average equivalent diameter of the second phase is smaller, the number density is significantly increased, and the corresponding contribution value of dispersion strengthening is higher. Therefore, it can be seen by comparing the homogenization heat treatment processes 1# to 3# that after heat treatment using the stepwise homogenization heat treatment process 3#, under the same hot working conditions, the obtained second phase has the smallest size, the largest number density, the largest contribution value of dispersion strengthening, and has a good recovery microstructure.

[0043] Table 3 Different homogenization heat treatment process plans

[0044]

[0045] Table 4 Analysis results of the second phase and contribution values of dispersion strengthening after different homogenization heat treatments

[0046]

[0047] Example 3:

[0048] In this example, the alloy composition and the ingot preparation method are different from those in Example 2. In this example, the Al-Mg-Mn-xZn alloy (x = 0, 5 wt.%) is taken as the research object, and the alloy composition is: Mg: 4.5%, Mn: 0.7%, Zn: 0 - 5%, and the balance is aluminum and inevitable impurities, and the content of its inevitable impurities is: Fe: 0.02%, Si: 0.02%. Intermediate alloys of high-purity Al (99.9%), pure Mg (99.9%), pure Zn (99.9%) and Al-10Mn are selected. After batching, the raw materials are heated to 750 °C, completely melted, held for 30 min and stirred multiple times, then cooled to 740 °C, degassed with argon for 15 min, the surface slag is removed, and then left standing for 20 min, and cast into a cold water copper mold at 700 - 710 °C to obtain an ingot. Homogenization heat treatment tests with the same plan are carried out on different alloy ingots. The Al-Mg-Mn alloys with different Zn contents are shown in Table 5. After different alloys are homogenized, they are immediately water-cooled to 25 °C.

[0049] The alloy after homogenization treatment is first heat-insulated at 400 °C for 1 h, then hot-rolled at 400 °C with a total reduction of 70%, and the cooling method is water spray cooling. Subsequently, the alloy is annealed at 400 °C for 1 h to eliminate the internal stress in the alloy.

[0050] The analysis and testing were carried out in accordance with relevant standards, and the results are shown in Table 6. It can be found by comparison that for the Al-Mg-Mn alloy with and without Zn addition, under different homogenization heat treatment processes, the Al-Mg-Mn alloy with Zn addition has a lower average equivalent diameter of the second phase and an increased number density, resulting in a higher contribution value of dispersion strengthening and a lower recrystallization fraction. By comparing Figure 5 and Figure 6 the scanning electron microscope results and the deformed microstructure results of the second phase precipitation characteristics inside the grains shown, it can be seen that the multi-stage homogenization heat treatment method of the Zn-containing Al-Mg-Mn alloy of the present invention significantly increases the precipitation number density of the second phase and reduces the size of the second phase, thereby bringing a large contribution value of dispersion strengthening to the alloy and retaining a good recovery microstructure during the hot deformation process. At the same time, it can also greatly shorten the time of homogenization heat treatment and reduce energy consumption.

[0051] Table 5 Al-Mg-Mn alloys with different Zn contents

[0052]

[0053] Table 6 Analysis results of the second phase and dispersion strengthening contribution values in Al-Mg-Mn alloys with different Zn contents

[0054]

[0055] Obviously, the above embodiments are merely examples given for clear illustration and are not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or variations can be made based on the above description. It is not necessary and impossible to enumerate all the implementation manners here. And the obvious changes or variations derived therefrom are still within the protection scope of the present invention.

Claims

1. A preparation method of a high-strength Al-Mg alloy system, characterized in that It includes the following steps: S11: Heating the alloy ingot at a rate of 1 - 5 °C / min to 150 - 200 °C for the first-stage homogenization heat treatment for 1 - 4 h to obtain a first-stage ingot; the alloy ingot is obtained by semi-continuous casting of master alloy; Calculated by mass fraction, the alloy ingot includes the following elements: 2 - 6% Mg, 0.4 - 0.9% Mn, 3 - 5% Zn, Si ≤ 0.15%, Fe ≤ 0.15%, Ti ≤ 0.1%, and the balance is Al; the master alloy is selected from high-purity Al, pure Mg, pure Zn, and Al-10Mn; S12: Heating the first-stage ingot at a rate of 1 - 5 °C / min to 250 - 350 °C for the second-stage homogenization heat treatment for 1 - 4 h to obtain a second-stage ingot; S13: Heating the second-stage ingot at a rate of 1 - 5 °C / min to 460 - 520 °C for the third-stage homogenization heat treatment for 1 - 8 h to obtain a third-stage ingot; S14: After cooling the three-stage ingot to room temperature, hot rolling deformation and annealing heat treatment are carried out to obtain the high-strength Al-Mg alloy system; in the step S14, in the three-stage ingot after three times of homogenization heat treatment, nano-scale AlMn second-phase precipitates; the number density of the AlMn second-phase > 30 μm -3 ; the average equivalent diameter of the nano-scale AlMn second-phase is less than 80 nm; the dispersion strengthening value brought by the nano-scale AlMn second-phase is greater than 50 MPa.

2. The preparation method of the high-strength Al-Mg alloy according to claim 1, characterized in that, In step S14, the cooling is carried out within 2 min after the third-stage ingot is taken out of the furnace.

3. The preparation method of the high-strength Al-Mg alloy according to claim 1, characterized in that, In step S14, the cooling method is air cooling or water mist cooling.

4. The preparation method of the high-strength Al-Mg alloy according to claim 1, characterized in that In step S14, after holding at 400 °C for 1 h, hot rolling deformation is carried out at 400 °C, and the total reduction is 70%; the temperature of the annealing heat treatment is 400 °C and the time is 1 h.

5. The preparation method of the high-strength Al-Mg alloy according to claim 1, characterized in that, In step S14, the recrystallization fraction after hot rolling deformation is less than 10%.

6. A high-strength Al-Mg alloy prepared by the preparation method according to any one of claims 1 - 5.

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