A pressurized heat treatment method for rapidly obtaining an equiaxed crystal structure in aluminum alloy

Abstract

The present invention discloses a pressurized heat treatment method for rapidly obtaining an equiaxed crystal structure in aluminum alloy. The method comprises the following steps: heating a heat treatment furnace to a heat treatment temperature; placing a rolled aluminum alloy sheet in the heat treatment furnace, keeping it at the heat treatment temperature, and continuously applying a pressure perpendicular to the rolling direction to the aluminum alloy sheet by using a pressurizing device during the heat preservation process; taking out the aluminum alloy sheet and air-cooling it to room temperature. The present invention can enable the rolled 5182 aluminum alloy sheet to rapidly obtain an equiaxed crystal structure at a lower temperature and within a shorter heat preservation time, eliminate the anisotropy of the alloy structure, and effectively slow down the growth of the alloy grain size and the size of the compounds in the alloy.

Description

Technical Field

[0001] The present invention relates to the technical field of nonferrous metal materials and heat treatment thereof, and more specifically to a pressure heat treatment method for rapidly obtaining an equiaxed crystal structure of an aluminum alloy. Background Art

[0002] With the rapid development of transportation, aerospace, building decoration, electronic appliances and other fields, the application of aluminum alloy has also been rapidly developed due to its own advantages such as low density, high mechanical properties, good corrosion resistance and machining performance. Aluminum alloy rolled plates have obtained good comprehensive performance due to the elimination of a large number of internal defects during the preparation process. They are most widely used in the fields of aerospace and automobile manufacturing. Their application can greatly reduce the weight and fuel consumption of aircraft and automobiles.

[0003] However, during the rolling process of aluminum alloy rolled plates, the structure along the rolling direction will be elongated to form a strip-shaped structure with long grains, while the structure perpendicular to the rolling direction is an equiaxed crystal structure. The microstructure of aluminum alloy rolled plates shows anisotropy, which will also bring about anisotropy of alloy properties, which is very unfavorable for the application of alloys. At present, in order to solve the anisotropy of the microstructure and mechanical properties of aluminum alloy rolled plates, the aluminum alloy plates are often recrystallized by recrystallization annealing to form a large number of fine equiaxed recrystallized grains. These fine recrystallized grains will grow during the insulation process, but still maintain an equiaxed shape. In order to make the structure of aluminum alloy plates more uniform, it is often necessary to keep the alloy at a higher temperature (above the recrystallization temperature) for a period of time, which will inevitably increase the grain size of the alloy and the size of the compounds in the alloy, and ultimately cause the reduction of alloy properties. For example, after the 5052 aluminum alloy rolled plate was kept at 400°C for 1 hour, the microstructure along the rolling direction was basically transformed into a uniform and equiaxed structure. After the 5A06 aluminum alloy was kept at 390°C for 2 hours, the microstructure along the rolling direction of the alloy was basically transformed into an equiaxed crystal structure. However, as the temperature increases and the holding time increases, the microstructure of the alloy gradually coarsens.

[0004] It can be seen that in order to transform the structure of aluminum alloy rolled plate along the rolling direction into equiaxed crystal structure, it is necessary to perform recrystallization annealing at a higher temperature or for a longer time. The current recrystallization annealing treatment method increases economic and time costs, so it is urgent to develop a method that can quickly transform the structure of aluminum alloy rolled plate along the rolling direction into equiaxed crystal structure at a lower temperature to reduce costs and avoid the negative effects of reduced mechanical properties caused by grain growth and compound size growth during annealing of aluminum alloy. Summary of the invention

[0005] To this end, the technical problem to be solved by the present invention is to provide a pressure heat treatment method for rapidly obtaining an equiaxed crystal structure in aluminum alloy, which can rapidly obtain an equiaxed crystal structure finer than that obtained by conventional recrystallization annealing in aluminum alloy sheets under the conditions of reducing the heat treatment temperature and shortening the heat treatment time.

[0006] To solve the above technical problem, the present invention provides the following technical solutions:

[0007] A pressure heat treatment method for rapidly obtaining an equiaxed crystal structure in aluminum alloy, comprising the following steps:

[0008] Step A: Heat the heat treatment furnace to the heat treatment temperature;

[0009] Step B: Place the rolled aluminum alloy sheet in the heat treatment furnace, keep it at the heat treatment temperature, and continuously apply a pressure perpendicular to the rolling direction to the surface of the aluminum alloy sheet by a pressure device during the heat preservation process; if during the recrystallization annealing process, the pressure device applies pressure to the aluminum alloy sheet in other directions, it is not conducive to obtaining an equiaxed crystal structure.

[0010] Step C: Take out the aluminum alloy sheet and air-cool it to room temperature.

[0011] After the alloy is deformed such as by rolling, a large amount of strain energy will be generated inside, making the alloy in an unstable high-energy state. This strain energy will not only affect the stability of the alloy but also act as a driving force to promote the recrystallization process of the alloy. During this process, the presence of strain energy will cause the grain boundary to bow out to one side, forming the so-called "grain boundary bowing out" phenomenon. The bowed-out position provides a growth point for the recrystallization nuclei, and then recrystallized grains are formed. This recrystallization mechanism is called the strain-induced grain boundary migration mechanism, or more specifically, the grain boundary bowing-out nucleation mechanism.

[0012] During the general heat treatment (recrystallization annealing) process, the strain energy will be gradually released, and the recrystallization caused by the strain-induced grain boundary migration mechanism will gradually decrease. However, if the recrystallization annealing time is too long, the existing grains will grow excessively, resulting in poor tissue uniformity and mechanical properties of the alloy. When using a pressure device to apply a force perpendicular to the rolling direction to the surface of the aluminum alloy sheet during heat treatment, it can keep the alloy at a high strain energy during the recrystallization treatment, slow down the release of the strain energy, keep the alloy in a high-energy state all the time, thereby promoting the recrystallization process of the alloy, reducing the heat preservation time required for recrystallization annealing, preventing the nuclei from growing excessively, and enabling the banded structure along the rolling direction in the aluminum alloy to be transformed into a uniform and fine equiaxed crystal structure under lower temperature and shorter time conditions.

[0013] In the above-mentioned pressure heat treatment method for rapidly obtaining an equiaxed crystal structure in aluminum alloy, in step B: the pressure device includes a bearing component, a hydraulic component, and a pressing component; the bearing component makes surface contact with the aluminum alloy sheet and bears the aluminum alloy sheet, and then the hydraulic component drives the pressing component to make surface contact with the aluminum alloy sheet and apply pressure to the plate surface of the aluminum alloy sheet.

[0014] In the above-mentioned pressure heat treatment method for rapidly obtaining an equiaxed crystal structure in aluminum alloy, in step B, both the bearing component and the pressing component are arranged in the heat treatment furnace. The plate surface of the aluminum alloy sheet facing the bearing component completely fits the surface of the bearing component, and the plate surface of the aluminum alloy sheet facing the pressing component completely fits the surface of the pressing component; the positions of the bearing component and the pressing component correspond to each other.

[0015] In the heat preservation process of step B of the above-mentioned pressure heat treatment method for rapidly obtaining an equiaxed crystal structure in aluminum alloy, the change amount of the pressure applied by the pressing component to the aluminum alloy sheet is less than or equal to 0.5 MPa / 10 min. The pressure applied by the pressing component to the aluminum alloy sheet should be kept as stable as possible to control the deformation of the aluminum alloy sheet during the processing, so that the internal strain energy of the aluminum alloy remains stable during the recrystallization annealing process; the change of the strain energy will change the grain growth rate and morphology. Therefore, keeping the pressure applied by the pressing component to the aluminum alloy sheet stable during the heat preservation process is beneficial for the grains inside the aluminum alloy to grow at a relatively uniform speed and form an equiaxed crystal structure, which further helps to maintain the uniformity of the alloy microstructure and improve the mechanical properties of the alloy.

[0016] In the above-mentioned pressure heat treatment method for rapidly obtaining an equiaxed crystal structure in aluminum alloy, in step B, within 2 s after the aluminum alloy sheet contacts the bearing component, the pressing component presses the aluminum alloy sheet under the drive of the hydraulic component. After the aluminum alloy sheet is placed on the surface of the bearing component, the pressing component should press the aluminum alloy sheet as soon as possible to reduce the time of the aluminum alloy sheet without pressure heat treatment, prevent the aluminum alloy from rising to the recrystallization annealing temperature and undergoing recrystallization annealing in the non-pressure state, and then changing to the pressurized state during the recrystallization annealing process, resulting in grain growth defects and further leading to a decrease in the uniformity of the alloy structure.

[0017] In the above-mentioned pressure heat treatment method for rapidly obtaining an equiaxed crystal structure in aluminum alloy, in step A, the heat treatment temperature is 330 - 400 °C; too high heat treatment temperature will cause excessive growth of the alloy grain size and compound size, which is not conducive to the improvement of the alloy mechanical properties; too low heat treatment temperature will require a longer time to make the alloy obtain an ideal equiaxed crystal structure, and more time cost will be paid.

[0018] In step B, the heat preservation time is 20 - 100 min, and the pressing device applies a pressure greater than 0 and less than or equal to 20 MPa to the aluminum alloy sheet. If the heat preservation time is too short, it is not conducive to the formation of equiaxed crystal structure; if the time is too long, although equiaxed crystal structure can be obtained, more time cost will be paid, and the grain size and compound size of the alloy will grow, which is also not conducive to the improvement of the mechanical properties of the alloy. If the pressure applied to the aluminum alloy sheet is too low, the efficiency of obtaining equiaxed crystal structure will be reduced, or the aluminum alloy needs to be heat-treated at a higher temperature for a longer time to obtain equiaxed crystal structure; while if the applied pressure is too high, although it helps to quickly obtain equiaxed crystal structure, it may cause the alloy to deform.

[0019] In the above-mentioned pressure-assisted heat treatment method for quickly obtaining equiaxed crystal structure in aluminum alloy, in step B, the temperature fluctuation range in the heat treatment furnace is less than or equal to ±1°C. Maintaining a relatively stable temperature during the heat treatment process is conducive to controlling the size uniformity of the equiaxed crystal structure and improving the overall quality and reliability of the aluminum alloy.

[0020] In the above-mentioned pressure-assisted heat treatment method for quickly obtaining equiaxed crystal structure in aluminum alloy, in step A, the heat treatment temperature is 350°C; in step B, the heat preservation time is 60 min, and the pressing device applies a pressure of 15 MPa to the aluminum alloy sheet.

[0021] In the above-mentioned pressure-assisted heat treatment method for quickly obtaining equiaxed crystal structure in aluminum alloy, the aluminum alloy sheet is made of 5182 aluminum alloy by cold rolling, and the reduction ratio during cold rolling is 31% - 34%; the chemical composition of 5182 aluminum alloy is 4.0 wt.% - 5.0 wt.% of magnesium, 0.2 wt.% - 0.4 wt.% of manganese, 0.1 wt.% - 0.15 wt.% of silicon, 0.15 wt.% - 0.25 wt.% of iron, 0.02 wt.% - 0.06 wt.% of copper, and 0.01 wt.% - 0.03 wt.% of chromium, with the balance being aluminum. The cold rolling reduction ratio is determined according to actual requirements and the deformation ability of 5182 aluminum alloy. When the rolling reduction ratio of the alloy is relatively large, its mechanical properties will become relatively better due to the reduction or elimination of defects; however, if the cold rolling reduction ratio (deformation amount) of the alloy is too large, the alloy may crack, causing the product to fail. At the same time, a higher reduction ratio also requires higher requirements for rolling equipment. And for 5182 aluminum alloy with the above element composition, too large a cold rolling reduction ratio will exacerbate the formation of banded structure, and greater pressure, higher heat treatment temperature, and longer heat treatment time are required during heat treatment to make the alloy obtain equiaxed crystal structure.

[0022] The above-mentioned pressure heat treatment method for rapidly obtaining an equiaxed crystal structure in aluminum alloy has a rolling reduction of 33% during cold rolling. The chemical composition of 5182 aluminum alloy is 4.6 wt.% magnesium, 0.35 wt.% manganese, 0.11 wt.% silicon, 0.21 wt.% iron, 0.04 wt.% copper, and 0.01 wt.% chromium, with the balance being aluminum.

[0023] The technical solution of the present invention has achieved the following beneficial technical effects:

[0024] 1. A pressure heat treatment method proposed by the present invention can rapidly transform the banded structure of 5182 aluminum alloy along the rolling direction into an equiaxed crystal structure and eliminate the texture anisotropy of the rolled aluminum alloy sheet by maintaining the temperature at 330 - 400 °C for 20 - 100 min during heat treatment and applying a pressure greater than 0 and less than or equal to 20 MPa to the 5182 aluminum alloy during the holding process. Compared with conventional recrystallization annealing, the pressure heat treatment method proposed by the present invention can shorten the time required to transform the banded structure into an equiaxed crystal structure to a maximum of 40 min, and the required temperature can be as low as 330 °C at the lowest, preventing the coarsening of the aluminum alloy microstructure caused by too high a temperature or too long a holding time during recrystallization annealing, and enabling fine equiaxed crystal structures to be formed along the rolling direction in the 5182 aluminum alloy rolled sheet.

[0025] 2. When using the pressure heat treatment method provided in the present invention, an aluminum alloy sheet with a cold rolling reduction of 30% - 40% is used as the raw material. Based on the suitable morphology and size of the banded structure in this kind of aluminum alloy sheet, a pressure perpendicular to the rolling direction is continuously applied to the surface of the aluminum alloy sheet. This pressure can slow down the release of strain energy, enabling the aluminum alloy to always maintain a high strain energy during the recrystallization annealing (heat treatment) process, promoting the rapid formation of new grain boundaries (i.e., the formation of new grains) inside the aluminum alloy, and reducing the time required for heat treatment. At the same time, during the heat treatment process, the alloy temperature and internal strain energy remain stable, enabling the newly formed grains to grow at a relatively uniform rate during the heat treatment (recrystallization annealing) process and form equiaxed crystal structures with uniform sizes, reducing grain distortion. Under the heat treatment conditions where the temperature of the alloy is 330 - 400 °C, the holding time is 20 - 100 min, and the pressure applying device applies a pressure greater than 0 and less than or equal to 20 MPa to the aluminum alloy sheet, the banded structure in the alloy can be fully transformed into fine and uniform equiaxed crystal structures, avoiding the problem of excessive grain growth in the alloy caused by too high a heat treatment temperature or too long a time, eliminating the anisotropy of the alloy structure, and thus making the alloy structure more uniform, which will be beneficial to the improvement of the mechanical properties of the alloy.

[0026] 3. The pressure heat treatment method proposed by the present invention is simple, has good process stability, and high process controllability. Description of the Drawings

[0027] Figure 1 Microstructure photograph of the 5182 aluminum alloy prepared in Example 5 of the present invention along the rolling direction;

[0028] Figure 2 Microstructure photograph of the 5182 aluminum alloy prepared in Example 8 of the present invention along the rolling direction;

[0029] Figure 3 Microstructure photograph of the 5182 aluminum alloy prepared in Comparative Example 10 of the present invention along the rolling direction. Detailed implementation manners

[0030] Example 1

[0031] The composition of the 5182 aluminum alloy used in this example is 4.6 wt.% Mg, 0.35 wt.% Mn, 0.11 wt.% Si, 0.21 wt.% Fe, 0.04 wt.% Cu, 0.01 wt.% Cr, and the balance is Al. The 5182 aluminum alloy is cold-rolled into an aluminum alloy sheet. During cold rolling, the 5182 aluminum alloy with a thickness of about 6 mm is rolled into an aluminum alloy sheet with a thickness of about 4 mm on a cold rolling mill (after actual measurement, the reduction ratio during cold rolling in this example is 31.6%). After cold rolling, the aluminum alloy sheet is wire-cut into blocks with dimensions of 10 mm × 10 mm × 4 mm, thus obtaining aluminum alloy specimens.

[0032] In the heat treatment furnace used in this example, a pressurizing device is provided. The pressurizing device includes a hydraulic component, a first heat-resistant stainless steel rod as a bearing component, and a second heat-resistant stainless steel rod as a pressing component. Among them, the lower surface of the first heat-resistant stainless steel rod contacts the experimental table, and the upper surface is flush with the lower surface of the furnace chamber of the heat treatment furnace; at the position corresponding to the first heat-resistant stainless steel rod on the upper surface of the furnace chamber, a second heat-resistant stainless steel rod is installed, and the hydraulic device is drivingly connected to the second heat-resistant stainless steel rod. During the heat treatment process, one plate surface of the aluminum alloy specimen faces the first heat-resistant stainless steel rod and is placed on the upper surface of the first heat-resistant stainless steel rod (that is, the aluminum alloy sheet is supported by the first heat-resistant stainless steel rod, and a surface contact is formed between the first heat-resistant stainless steel rod and the aluminum alloy sheet), the hydraulic device drives the second heat-resistant stainless steel rod to contact the aluminum alloy specimen (that is, a surface contact is formed between the second heat-resistant stainless steel rod and the aluminum alloy specimen), and a pressure perpendicular to the rolling direction is applied to the aluminum alloy specimen according to the set pressure value, and during the entire heat treatment process, the change amount of the pressure applied by the second heat-resistant stainless steel rod to the aluminum alloy specimen is less than or equal to 0.5 MPa / 10 min.

[0033] In this embodiment, the diameters of the first heat-resistant stainless steel rod serving as the bearing component and the second heat-resistant stainless steel rod serving as the pressing component are both 20 mm, and the areas of the circular surfaces of both in contact with the aluminum alloy are larger than the area of the aluminum alloy test sample surface (10 mm × 10 mm). In some other embodiments, the bearing component and the pressing component can also be of other shapes, as long as the dimensions of both can "cover" the aluminum alloy sheet in contact (that is, the surface of the aluminum alloy sheet facing the bearing component is completely attached to the surface of the bearing component, and the surface of the aluminum alloy sheet facing the pressing component is completely attached to the surface of the pressing component).

[0034] The process of rapidly obtaining equiaxed crystal structure for 5182 aluminum alloy by the pressure-assisted heat treatment method in this embodiment includes the following steps:

[0035] (1) Preheating of the heat treatment furnace: Power on the heat treatment furnace, and use the PID program to control the temperature rise to 330 °C.

[0036] (2) Heat treatment process: Open the furnace door of the heat treatment furnace, quickly place the 5182 aluminum alloy sample on the upper surface of the first heat-resistant stainless steel rod arranged at the middle position at the bottom of the heat treatment furnace, quickly (within 2 seconds) bring the second heat-resistant stainless steel rod into contact with the upper surface of the aluminum alloy sample, apply a pressure of 5 MPa, and quickly close the furnace door of the heat treatment furnace. The pressing direction is perpendicular to the rolling direction of the aluminum alloy. During the heat treatment process, the temperature in the furnace chamber of the heat treatment furnace remains stable, with a maximum fluctuation of no more than ±1 °C.

[0037] (3) When the holding time reaches 80 min, open the furnace door, use crucible tongs to take out the sample and place it on the pre-prepared refractory brick to air-cool to room temperature.

[0038] The surface of the 5182 aluminum alloy sample along the rolling direction is polished successively through 400#, 800#, 1200#, 1500#, and 2000#, and after polishing, it is polished to a mirror surface with almost no scratches on the metallographic polishing machine, then etched with Keller's reagent (190 ml of distilled water, 5 ml of nitric acid, 3 ml of hydrochloric acid, 2 ml of hydrofluoric acid), and then the microstructure of the alloy along the rolling direction is observed through an optical metallographic microscope to judge whether an equiaxed crystal structure is obtained.

[0039] The grain morphology of the aluminum alloy prepared in Example 1 along the rolling direction is shown in Table 1.

[0040] Example 2

[0041] The composition of the 5182 aluminum alloy, the heat treatment furnace, and the pressing device used in this embodiment are the same as those in Example 1. The difference is that the cold rolling reduction rate of the 5182 aluminum alloy used in this embodiment is 32.1%.

[0042] The process of rapidly obtaining an equiaxed grain structure for 5182 aluminum alloy by the pressure heat treatment method in this embodiment includes the following steps:

[0043] (1) Preheating the heat treatment furnace: Power on the heat treatment furnace, and use the PID program to control the temperature rise to 330 °C.

[0044] (2) Heat treatment process: Open the furnace door of the heat treatment furnace, quickly place the 5182 aluminum alloy sample on the upper surface of the first heat-resistant stainless steel rod arranged at the middle position at the bottom of the heat treatment furnace, quickly (within 2 seconds) contact the second heat-resistant stainless steel rod with the upper surface of the aluminum alloy sample, apply a pressure of 10 MPa, and quickly close the furnace door of the heat treatment furnace. The pressure application direction is perpendicular to the rolling direction of the aluminum alloy. During the heat treatment process, the temperature in the furnace cavity of the heat treatment furnace remains stable, with a maximum fluctuation not exceeding ±1 °C.

[0045] (3) When the holding time reaches 60 min, open the furnace door, use crucible tongs to take out the sample and place it on the pre-prepared refractory brick to air-cool to room temperature.

[0046] Observe the microstructure of the alloy along the rolling direction through an optical metallographic microscope to judge whether an equiaxed grain structure is obtained. The observation method is the same as that in Example 1.

[0047] The grain morphology of the aluminum alloy prepared in Example 2 along the rolling direction is shown in Table 1.

[0048] Example 3

[0049] The composition of the 5182 aluminum alloy, the heat treatment furnace, and the pressure application device used in this embodiment are the same as those in Example 1. The difference is that the cold rolling reduction rate of the 5182 aluminum alloy used in this embodiment is 32.5%.

[0050] The process of rapidly obtaining an equiaxed grain structure for 5182 aluminum alloy by the pressure heat treatment method in this embodiment includes the following steps:

[0051] (1) Preheating the heat treatment furnace: Power on the heat treatment furnace, and use the PID program to control the temperature rise to 330 °C.

[0052] (2) Heat treatment process: Open the furnace door of the heat treatment furnace, quickly place the 5182 aluminum alloy sample on the upper surface of the first heat-resistant stainless steel rod arranged at the middle position at the bottom of the heat treatment furnace, quickly (within 2 seconds) contact the second heat-resistant stainless steel rod with the upper surface of the aluminum alloy sample, apply a pressure of 10 MPa, and quickly close the furnace door of the heat treatment furnace. The pressure application direction is perpendicular to the rolling direction of the aluminum alloy. During the heat treatment process, the temperature in the furnace cavity of the heat treatment furnace remains stable, with a maximum fluctuation not exceeding ±1 °C.

[0053] (3) When the holding time reaches 80 min, open the furnace door, use crucible tongs to take out the sample and place it on the pre-prepared refractory brick to air-cool to room temperature.

[0054] The microstructure of the alloy along the rolling direction was observed by an optical metallographic microscope to determine whether an equiaxed grain structure was obtained. The observation method was the same as that in Example 1.

[0055] The grain morphology of the aluminum alloy prepared in Example 3 along the rolling direction is shown in Table 1.

[0056] Example 4

[0057] The composition of the 5182 aluminum alloy, the heat treatment furnace and the pressurizing device used in this example were the same as those in Example 1, except that the cold rolling reduction rate of the 5182 aluminum alloy used in this example was 33.1%.

[0058] The process of rapidly obtaining an equiaxed grain structure for the 5182 aluminum alloy by the pressure heat treatment method in this example included the following steps:

[0059] (1) Heat treatment furnace preheating: The heat treatment furnace was powered on, and the temperature was raised to 330 °C by PID program control.

[0060] (2) Heat treatment process: Open the furnace door of the heat treatment furnace, quickly place the 5182 aluminum alloy sample on the upper surface of the first heat-resistant stainless steel rod arranged at the middle position at the bottom of the heat treatment furnace, quickly (within 2 seconds) contact the second heat-resistant stainless steel rod with the upper surface of the aluminum alloy sample, apply a pressure of 20 MPa, and quickly close the furnace door of the heat treatment furnace. The pressurizing direction was perpendicular to the rolling direction of the aluminum alloy. During the heat treatment process, the temperature in the furnace cavity of the heat treatment furnace remained stable, with a maximum fluctuation of no more than ±1 °C.

[0061] (3) When the holding time reached 60 min, open the furnace door, use crucible tongs to take out the sample and place it on the pre-prepared refractory brick to air-cool to room temperature.

[0062] The microstructure of the alloy along the rolling direction was observed by an optical metallographic microscope to determine whether an equiaxed grain structure was obtained. The observation method was the same as that in Example 1.

[0063] The grain morphology of the aluminum alloy prepared in Example 4 along the rolling direction is shown in Table 1.

[0064] Example 5

[0065] The composition of the 5182 aluminum alloy, the heat treatment furnace and the pressurizing device used in this example were the same as those in Example 1, except that the cold rolling reduction rate of the 5182 aluminum alloy used in this example was 32.8%.

[0066] The process of rapidly obtaining an equiaxed grain structure for the 5182 aluminum alloy by the pressure heat treatment method in this example included the following steps:

[0067] (1)Preheating of the heat treatment furnace: Power on the heat treatment furnace, and use the PID program to control the temperature rise to 350 °C.

[0068] (2)Heat treatment process: Open the furnace door of the heat treatment furnace, quickly place the 5182 aluminum alloy sample on the upper surface of the first heat-resistant stainless steel rod arranged at the middle position of the bottom of the heat treatment furnace, quickly (within 2 seconds) bring the second heat-resistant stainless steel rod into contact with the upper surface of the aluminum alloy sample, apply a pressure of 5 MPa, and quickly close the furnace door of the heat treatment furnace. The pressing direction is perpendicular to the rolling direction of the aluminum alloy. During the heat treatment process, the temperature in the furnace cavity of the heat treatment furnace remains stable, with a maximum fluctuation of no more than ±1 °C.

[0069] (3)When the holding time reaches 60 min, open the furnace door, use crucible tongs to take out the sample and place it on the pre-prepared refractory brick to air-cool to room temperature.

[0070] Observe the microstructure of the alloy along the rolling direction through an optical metallographic microscope to determine whether an equiaxed crystal structure is obtained. The observation method is the same as that in Example 1.

[0071] The grain morphology of the aluminum alloy prepared in Example 5 along the rolling direction is shown in Table 1.

[0072] Example 6

[0073] The composition of the 5182 aluminum alloy, the heat treatment furnace and the pressing device used in this example are the same as those in Example 1. The difference is that the cold rolling reduction rate of the 5182 aluminum alloy used in this example is 33.4%.

[0074] The process of quickly obtaining an equiaxed crystal structure for the 5182 aluminum alloy by the pressure heat treatment method in this example includes the following steps:

[0075] (1)Preheating of the heat treatment furnace: Power on the heat treatment furnace, and use the PID program to control the temperature rise to 350 °C.

[0076] (2)Heat treatment process: Open the furnace door of the heat treatment furnace, quickly place the 5182 aluminum alloy sample on the upper surface of the first heat-resistant stainless steel rod arranged at the middle position of the bottom of the heat treatment furnace, quickly (within 2 seconds) bring the second heat-resistant stainless steel rod into contact with the upper surface of the aluminum alloy sample, apply a pressure of 10 MPa, and quickly close the furnace door of the heat treatment furnace. The pressing direction is perpendicular to the rolling direction of the aluminum alloy. During the heat treatment process, the temperature in the furnace cavity of the heat treatment furnace remains stable, with a maximum fluctuation of no more than ±1 °C.

[0077] (3)When the holding time reaches 60 min, open the furnace door, use crucible tongs to take out the sample and place it on the pre-prepared refractory brick to air-cool to room temperature.

[0078] The microstructure of the alloy along the rolling direction was observed by an optical metallographic microscope to determine whether an equiaxed crystal structure was obtained. The observation method was the same as that in Example 1.

[0079] The grain morphology of the aluminum alloy prepared in Example 6 along the rolling direction is shown in Table 1.

[0080] Example 7

[0081] The composition of the 5182 aluminum alloy, the heat treatment furnace, and the pressurizing device used in this example were the same as those in Example 1, except that the cold rolling reduction rate of the 5182 aluminum alloy used in this example was 32.5%.

[0082] The process of rapidly obtaining an equiaxed crystal structure of the 5182 aluminum alloy by the pressure heat treatment method in this example included the following steps:

[0083] (1) Heat treatment furnace preheating: The heat treatment furnace was powered on, and the temperature was raised to 350 °C by PID program control.

[0084] (2) Heat treatment process: Open the furnace door of the heat treatment furnace, quickly place the 5182 aluminum alloy specimen on the upper surface of the first heat-resistant stainless steel rod arranged at the middle position at the bottom of the heat treatment furnace, quickly (within 2 seconds) contact the second heat-resistant stainless steel rod with the upper surface of the aluminum alloy specimen, apply a pressure of 10 MPa, and quickly close the furnace door of the heat treatment furnace. The pressurizing direction was perpendicular to the rolling direction of the aluminum alloy. During the heat treatment process, the temperature in the furnace cavity of the heat treatment furnace remained stable, with a maximum fluctuation of no more than ±1 °C.

[0085] (3) When the holding time reached 80 min, open the furnace door, use crucible tongs to take out the specimen and place it on the pre-prepared refractory brick to air-cool to room temperature.

[0086] The microstructure of the alloy along the rolling direction was observed by an optical metallographic microscope to determine whether an equiaxed crystal structure was obtained. The observation method was the same as that in Example 1.

[0087] The grain morphology of the aluminum alloy prepared in Example 7 along the rolling direction is shown in Table 1.

[0088] Example 8

[0089] The composition of the 5182 aluminum alloy, the heat treatment furnace, and the pressurizing device used in this example were the same as those in Example 1, except that the cold rolling reduction rate of the 5182 aluminum alloy used in this example was 33.8%.

[0090] The process of rapidly obtaining an equiaxed crystal structure of the 5182 aluminum alloy by the pressure heat treatment method in this example included the following steps:

[0091] (1) Heat treatment furnace preheating: The heat treatment furnace was powered on, and the temperature was raised to 350 °C by PID program control.

[0092] (2) Heat treatment process: Open the furnace door of the heat treatment furnace, quickly place the 5182 aluminum alloy sample on the upper surface of the first heat-resistant stainless steel rod arranged at the middle position at the bottom of the heat treatment furnace, quickly (within 2 seconds) bring the second heat-resistant stainless steel rod into contact with the upper surface of the aluminum alloy sample, apply a pressure of 15 MPa, and quickly close the furnace door of the heat treatment furnace. The pressing direction is perpendicular to the rolling direction of the aluminum alloy. During the heat treatment process, the temperature in the furnace cavity of the heat treatment furnace remains stable, with a maximum fluctuation of no more than ±1°C.

[0093] (3) When the holding time reaches 60 min, open the furnace door, use crucible tongs to take out the sample and place it on the pre-prepared refractory brick to air-cool to room temperature.

[0094] Observe the microstructure of the alloy along the rolling direction through an optical metallographic microscope to determine whether an equiaxed crystal structure is obtained. The observation method is the same as that in Example 1.

[0095] The grain morphology of the aluminum alloy prepared in Example 8 along the rolling direction is shown in Table 1.

[0096] Example 9

[0097] The composition of the 5182 aluminum alloy, the heat treatment furnace, and the pressing device used in this example are the same as those in Example 1. The difference is that the cold rolling reduction rate of the 5182 aluminum alloy used in this example is 32.6%.

[0098] The process of quickly obtaining an equiaxed crystal structure for the 5182 aluminum alloy by the pressure-assisted heat treatment method in this example includes the following steps:

[0099] (1) Preheating of the heat treatment furnace: Electrify the heat treatment furnace, and use the PID program to control the temperature rise to 370°C.

[0100] (2) Heat treatment process: Open the furnace door of the heat treatment furnace, quickly place the 5182 aluminum alloy sample on the upper surface of the first heat-resistant stainless steel rod arranged at the middle position at the bottom of the heat treatment furnace, quickly (within 2 seconds) bring the second heat-resistant stainless steel rod into contact with the upper surface of the aluminum alloy sample, apply a pressure of 5 MPa, and quickly close the furnace door of the heat treatment furnace. The pressing direction is perpendicular to the rolling direction of the aluminum alloy. During the heat treatment process, the temperature in the furnace cavity of the heat treatment furnace remains stable, with a maximum fluctuation of no more than ±1°C.

[0101] (3) When the holding time reaches 60 min, open the furnace door, use crucible tongs to take out the sample and place it on the pre-prepared refractory brick to air-cool to room temperature.

[0102] Observe the microstructure of the alloy along the rolling direction through an optical metallographic microscope to determine whether an equiaxed crystal structure is obtained. The observation method is the same as that in Example 1.

[0103] The grain morphology of the aluminum alloy obtained in Example 9 along the rolling direction is shown in Table 1.

[0104] Example 10

[0105] The composition of the 5182 aluminum alloy, the heat treatment furnace, and the pressurizing device used in this example are the same as those in Example 1. The difference is that the cold rolling reduction rate of the 5182 aluminum alloy used in this example is 32.7%.

[0106] The process of rapidly obtaining an equiaxed grain structure for the 5182 aluminum alloy by the pressure heat treatment method in this example includes the following steps:

[0107] (1) Heat treatment furnace preheating: Power on the heat treatment furnace, and use the PID program to control the temperature rise to 370 °C.

[0108] (2) Heat treatment process: Open the furnace door of the heat treatment furnace, quickly place the 5182 aluminum alloy specimen on the upper surface of the first heat-resistant stainless steel rod arranged in the middle of the bottom of the heat treatment furnace, quickly (within 2 seconds) contact the second heat-resistant stainless steel rod with the upper surface of the aluminum alloy specimen, apply a pressure of 10 MPa, and quickly close the furnace door of the heat treatment furnace. The pressurizing direction is perpendicular to the rolling direction of the aluminum alloy. During the heat treatment process, the temperature in the furnace cavity of the heat treatment furnace remains stable, with a maximum fluctuation of no more than ±1 °C.

[0109] (3) When the holding time reaches 60 min, open the furnace door, use crucible tongs to take out the specimen and place it on the pre-prepared refractory brick to air-cool to room temperature.

[0110] Observe the microstructure of the alloy along the rolling direction through an optical metallographic microscope to determine whether an equiaxed grain structure is obtained. The observation method is the same as that in Example 1.

[0111] The grain morphology of the aluminum alloy obtained in Example 10 along the rolling direction is shown in Table 1.

[0112] Example 11

[0113] The composition of the 5182 aluminum alloy, the heat treatment furnace, and the pressurizing device used in this example are the same as those in Example 1. The difference is that the cold rolling reduction rate of the 5182 aluminum alloy used in this example is 33.5%.

[0114] The process of rapidly obtaining an equiaxed grain structure for the 5182 aluminum alloy by the pressure heat treatment method in this example includes the following steps:

[0115] (1) Heat treatment furnace preheating: Power on the heat treatment furnace, and use the PID program to control the temperature rise to 370 °C.

[0116] (2) Heat treatment process: Open the furnace door of the heat treatment furnace, quickly place the 5182 aluminum alloy sample on the upper surface of the first heat-resistant stainless steel rod configured at the middle position of the bottom of the heat treatment furnace, quickly (within 2 seconds) bring the second heat-resistant stainless steel rod into contact with the upper surface of the aluminum alloy sample, apply a pressure of 15 MPa, and quickly close the furnace door of the heat treatment furnace. The pressing direction is perpendicular to the rolling direction of the aluminum alloy. During the heat treatment process, the temperature in the furnace cavity of the heat treatment furnace remains stable, with a maximum fluctuation of no more than ±1°C.

[0117] (3) When the holding time reaches 40 min, open the furnace door, use crucible tongs to take out the sample and place it on the pre-prepared refractory brick to air-cool to room temperature.

[0118] Observe the microstructure of the alloy along the rolling direction through an optical metallographic microscope to determine whether an equiaxed crystal structure is obtained. The observation method is the same as that in Example 1.

[0119] The grain morphology of the aluminum alloy prepared in Example 11 along the rolling direction is shown in Table 1.

[0120] Example 12

[0121] The composition of the 5182 aluminum alloy, the heat treatment furnace, and the pressing device used in this example are the same as those in Example 1. The difference is that the cold rolling reduction rate of the 5182 aluminum alloy used in this example is 33.6%.

[0122] The process of quickly obtaining an equiaxed crystal structure for the 5182 aluminum alloy by the pressure heat treatment method in this example includes the following steps:

[0123] (1) Heat treatment furnace preheating: Electrify the heat treatment furnace, and use the PID program to control the temperature rise to 370°C.

[0124] (2) Heat treatment process: Open the furnace door of the heat treatment furnace, quickly place the 5182 aluminum alloy sample on the upper surface of the first heat-resistant stainless steel rod configured at the middle position of the bottom of the heat treatment furnace, quickly (within 2 seconds) bring the second heat-resistant stainless steel rod into contact with the upper surface of the aluminum alloy sample, apply a pressure of 20 MPa, and quickly close the furnace door of the heat treatment furnace. The pressing direction is perpendicular to the rolling direction of the aluminum alloy. During the heat treatment process, the temperature in the furnace cavity of the heat treatment furnace remains stable, with a maximum fluctuation of no more than ±1°C.

[0125] (3) When the holding time reaches 40 min, open the furnace door, use crucible tongs to take out the sample and place it on the pre-prepared refractory brick to air-cool to room temperature.

[0126] Observe the microstructure of the alloy along the rolling direction through an optical metallographic microscope to determine whether an equiaxed crystal structure is obtained. The observation method is the same as that in Example 1.

[0127] The grain morphology of the aluminum alloy prepared in Example 12 along the rolling direction is shown in Table 1.

[0128] Comparative Example 1

[0129] The composition of the 5182 aluminum alloy used in this comparative example is the same as that in Example 1, except that the cold rolling reduction rate of the 5182 aluminum alloy used in this comparative example is 33.4%.

[0130] The process of rapidly obtaining equiaxed crystal structure of 5182 aluminum alloy by pressure heat treatment in this comparative example includes the following steps:

[0131] (1) Heat treatment furnace preheating: Power on the heat treatment furnace, and use the PID program to control the temperature rise to 350 °C.

[0132] (2) Heat treatment process: Open the furnace door of the heat treatment furnace, quickly place the 5182 aluminum alloy specimen on the upper surface of the first heat-resistant stainless steel rod arranged at the middle position of the bottom of the heat treatment furnace, and quickly close the furnace door of the heat treatment furnace.

[0133] (3) When the holding time reaches 60 min, open the furnace door, take out the specimen with crucible tongs and place it on the pre-prepared refractory brick to air-cool to room temperature.

[0134] The microstructure of the alloy along the rolling direction was observed by an optical metallographic microscope to judge whether an equiaxed crystal structure was obtained. The observation method is the same as that in Example 1.

[0135] The grain morphology of the aluminum alloy prepared in Comparative Example 1 along the rolling direction is shown in Table 1.

[0136] Comparative Example 2

[0137] The composition of the 5182 aluminum alloy used in this comparative example is the same as that in Example 1, except that the cold rolling reduction rate of the 5182 aluminum alloy used in this comparative example is 33.6%.

[0138] The process of rapidly obtaining equiaxed crystal structure of 5182 aluminum alloy by pressure heat treatment in this comparative example includes the following steps:

[0139] (1) Heat treatment furnace preheating: Power on the heat treatment furnace, and use the PID program to control the temperature rise to 370 °C.

[0140] (2) Heat treatment process: Open the furnace door of the heat treatment furnace, quickly place the 5182 aluminum alloy specimen on the upper surface of the first heat-resistant stainless steel rod arranged at the middle position of the bottom of the heat treatment furnace, and quickly close the furnace door of the heat treatment furnace.

[0141] (3) When the holding time reaches 80 min, open the furnace door, take out the specimen with crucible tongs and place it on the pre-prepared refractory brick to air-cool to room temperature.

[0142] The microstructure of the alloy along the rolling direction was observed by optical metallographic microscope to determine whether an equiaxed grain structure was obtained. The observation method was the same as that in Example 1.

[0143] The grain morphology of the aluminum alloy prepared in Comparative Example 2 along the rolling direction is shown in Table 1.

[0144] Comparative Example 3

[0145] The composition of the 5182 aluminum alloy used in this comparative example was the same as that in Example 1, except that the cold rolling reduction rate of the 5182 aluminum alloy used in this comparative example was 33.2%.

[0146] The process of rapidly obtaining an equiaxed grain structure of the 5182 aluminum alloy by the pressure heat treatment method in this comparative example includes the following steps:

[0147] (1) Heat treatment furnace preheating: The heat treatment furnace was powered on, and the temperature was raised to 400 °C by PID program control.

[0148] (2) Heat treatment process: Open the furnace door of the heat treatment furnace, quickly place the 5182 aluminum alloy specimen on the upper surface of the first heat-resistant stainless steel rod arranged at the middle position at the bottom of the heat treatment furnace, and quickly close the furnace door of the heat treatment furnace.

[0149] (3) When the holding time reached 100 min, open the furnace door, take out the specimen with crucible tongs and place it on the pre-prepared refractory brick to air-cool to room temperature.

[0150] The microstructure of the alloy along the rolling direction was observed by optical metallographic microscope to determine whether an equiaxed grain structure was obtained. The observation method was the same as that in Example 1.

[0151] The grain morphology of the aluminum alloy prepared in Comparative Example 3 along the rolling direction is shown in Table 1.

[0152] Table 1 Grain morphology of 5182 aluminum alloy along the rolling direction after different pressure heat treatments

[0153]

[0154] During the processing of rolled aluminum alloy sheets, due to the action of rolling force, the grains will be elongated along the rolling direction, forming obvious banded structures. The morphologies and properties of these banded structures are quite different from those of the structures perpendicular to the rolling direction, resulting in anisotropy in the microstructure and properties of the alloy, which to a certain extent limits the application range of the alloy. However, although the traditional recrystallization annealing method can improve the microstructure of the alloy to a certain extent, it usually requires a relatively high treatment temperature and a long annealing time. This will not only increase energy consumption, but also may cause excessive growth of the alloy grain size and compound size, and it is difficult to form a uniform and fine equiaxed grain structure in the alloy, thereby reducing the mechanical properties of the alloy.

[0155] However, for the aluminum alloys prepared by the methods provided in Examples 1 to 20, most of them contain equiaxed crystal structures. As can be seen from Table 1, with the increase of the pressure applied during the heat treatment (comparing Example 1 and Example 3), or the extension of the holding time (comparing Example 2 and Example 3), or the increase of the holding temperature (comparing Example 5 and Example 9), it is beneficial to transform the structure of 5182 aluminum alloy along the rolling direction into an equiaxed crystal structure. Comparing Example 6 with a pressure of 10 Mpa and Comparative Example 1 without pressure, under the same heat treatment temperature and time conditions, applying pressure can promote the transformation of the elongated grains in the alloy into equiaxed crystals; moreover, from the comparison between Example 4 and Comparative Example 2, it can be seen that applying pressure can enable the alloy to rapidly obtain an equiaxed crystal structure at a lower temperature and a shorter holding time, effectively avoiding the growth of the alloy grain size and the size of the compounds in the alloy at a higher temperature and a longer holding time.

[0156] Such as Figure 1 is a microstructural photograph of the 5182 aluminum alloy prepared in Example 5 of the present invention along the rolling direction. Figure 2 is a microstructural photograph of the 5182 aluminum alloy prepared in Example 8 of the present invention along the rolling direction. Figure 3 is a microstructural photograph of the 5182 aluminum alloy prepared in Comparative Example 10 of the present invention along the rolling direction. The vertical direction in the figure is the rolling direction, and the dark dots in the figure are the compounds in the alloy. From Figure 1 and Figure 2 comparison, it can be seen that under the same temperature (350 °C) and heat treatment time (60 min) conditions, when the applied pressure is relatively low, it cannot transform the banded structure of the alloy along the rolling direction into an equiaxed crystal structure. As Figure 1 shows, the arrangement pattern of the compounds in the aluminum alloy obviously shows a tendency of strip-like distribution along the rolling direction, indicating that the banded structure in the alloy has not been fully transformed into an equiaxed crystal structure; while when the applied pressure increases, it can enable the alloy to obtain an equiaxed crystal structure along the rolling direction. As Figure 2 shows, the distribution state of the compounds in the alloy is more uniform than that in Figure 1 , indicating that the process of recrystallization of the banded structure in the alloy to form an equiaxed crystal structure progresses more fully. Increasing the heat treatment temperature (370 °C) can also enable the alloy to achieve the transformation of the banded structure into an equiaxed crystal structure within 60 min under the condition of relatively small applied pressure, but a higher temperature will cause the growth of the alloy grain size and the size of the compounds, which is not conducive to the improvement of the mechanical properties of the alloy.

[0157] In the above embodiments, the 5182 aluminum alloy rolled sheet is used as the material for pressure heat treatment. This method of pressure heat treatment is also applicable to other grades of rolled aluminum alloy sheets, other metal rolled sheets, extrusion profiles, etc. Those skilled in the art can also adjust parameters such as the pressure, holding temperature, and holding time according to actual needs.

[0158] In the above embodiments, the 5182 aluminum alloy is only cold-rolled once. In some other embodiments, the 5182 aluminum alloy can also be cold-rolled twice, as long as the final cold rolling reduction rate is within the range of 31% to 34%.

[0159] Obviously, the above embodiments are only 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 modifications can be made based on the above description. It is not necessary and impossible to list all the implementation manners here. And the obvious changes or modifications derived therefrom are still within the protection scope of the claims of this patent application.

Claims

1. A pressurized heat treatment method for rapidly obtaining an equiaxed grain structure in an aluminum alloy, characterized in that: The steps include: Step A: heating the heat treatment furnace to a heat treatment temperature of 330-400°C; Step B: placing the rolled aluminum alloy plate in a heat treatment furnace, keeping it warm at the heat treatment temperature, and using a pressure device to continuously apply a pressure perpendicular to the rolling direction to the plate surface of the aluminum alloy plate during the heat treatment process; the heat preservation time is 20 to 100 minutes, and during the heat preservation process, the pressure device applies a pressure greater than or equal to 10MPa and less than or equal to 20MPa to the aluminum alloy plate; so that the strip-shaped structure along the rolling direction of the aluminum alloy is transformed into a uniform and fine equiaxed crystal structure under lower temperature and shorter time conditions; the aluminum alloy plate is made of 5182 aluminum alloy by cold rolling, and the reduction rate during cold rolling is 31% to 34%; Step C: Take out the aluminum alloy plate and allow it to air cool to room temperature.

2. The pressurized heat treatment method for rapidly obtaining an equiaxed grain structure of an aluminum alloy according to claim 1, characterized in that: In step B: the pressurizing device includes a bearing component, a hydraulic component and a clamping component. The bearing component makes surface contact with the aluminum alloy plate and bears the aluminum alloy plate, and then the hydraulic component drives the clamping component to make surface contact with the aluminum alloy plate and apply pressure to the plate surface of the aluminum alloy plate.

3. The pressurized heat treatment method for rapidly obtaining an equiaxed grain structure of an aluminum alloy according to claim 2, characterized in that: In step B, the bearing component and the clamping component are both arranged in a heat treatment furnace, the plate surface of the aluminum alloy plate facing the bearing component is completely in contact with the surface of the bearing component, and the plate surface of the aluminum alloy plate facing the clamping component is completely in contact with the surface of the clamping component; the positions of the bearing component and the clamping component correspond.

4. The pressurized heat treatment method for rapidly obtaining an equiaxed grain structure of an aluminum alloy according to claim 3, characterized in that: During the heat preservation process of step B, the change in the pressure applied by the pressing component to the aluminum alloy plate is less than or equal to 0.5 MPa / 10min.

5. The pressurized heat treatment method for rapidly obtaining an equiaxed grain structure of an aluminum alloy according to claim 3, characterized in that: In step B, within 2 seconds after the aluminum alloy plate contacts the bearing component, the pressing component presses the aluminum alloy plate under the drive of the hydraulic component.

6. The pressurized heat treatment method for rapidly obtaining an equiaxed grain structure of an aluminum alloy according to claim 1, characterized in that: In step A, the heat treatment temperature is 330-370° C.; in step B, the heat preservation time is 40-60 min, and the pressure device applies a pressure of 10-20 MPa to the aluminum alloy plate.

7. The pressurized heat treatment method for rapidly obtaining an equiaxed grain structure of an aluminum alloy according to claim 6, characterized in that: In step B, the temperature fluctuation range in the heat treatment furnace is less than or equal to ±1°C.

8. The pressurized heat treatment method for rapidly obtaining an equiaxed grain structure of an aluminum alloy according to claim 6, characterized in that: In step A, the heat treatment temperature is 350° C.; in step B, the holding time is 60 min, and the pressurizing device applies a pressure of 15 MPa to the aluminum alloy plate.

9. The pressurized heat treatment method for rapidly obtaining an equiaxed grain structure of an aluminum alloy according to any one of claims 1 to 8, characterized in that: The chemical composition of 5182 aluminum alloy is 4.0wt.% to 5.0wt.% magnesium, 0.2wt.% to 0.4wt.% manganese, 0.1wt.% to 0.15wt.% silicon, 0.15wt.% to 0.25wt.% iron, 0.02wt.% to 0.06wt.% copper and 0.01wt.% to 0.03wt.% chromium, with the balance being aluminum.

10. The pressurized heat treatment method for rapidly obtaining an equiaxed grain structure of an aluminum alloy according to claim 9, characterized in that: The reduction rate during cold rolling is 33%. The chemical composition of the 5182 aluminum alloy is 4.6wt.% magnesium, 0.35wt.% manganese, 0.11wt.% silicon, 0.21wt.% iron, 0.04wt.% copper and 0.01wt.% chromium, with the balance being aluminum.

Images