Cooling process of 6-series aluminum alloy ingots

Abstract

This invention discloses a cooling process for 6-series aluminum alloy ingots, comprising the following steps: heating the cast 6-series aluminum alloy ingot to 540℃-580℃ and holding it at that temperature for the first time; subjecting the ingot to a first-stage cooling process to 400℃-480℃, wherein the cooling rate of the first-stage cooling is 2℃-8℃ / min; subjecting the ingot to a second-stage cooling process; subjecting the ingot to a second-stage cooling process to 250℃-300℃, wherein the cooling rate of the second-stage cooling is 5℃-10℃ / min; and subjecting the ingot to a third-stage cooling process to room temperature. This cooling process for 6-series aluminum alloy ingots can reduce the deformation resistance of the ingots, increase the extrusion speed, and improve the surface quality and mechanical properties of the ingots.

Description

Technical Field

[0001] This invention relates to the field of aluminum alloy profile processing, and in particular to a cooling process for 6-series aluminum alloy ingots. Background Technology

[0002] 6-series aluminum alloys are heat-treatable wrought aluminum alloys with Mg and Si as the main alloying elements. They possess moderate strength, are easy to process, and have excellent weldability and corrosion resistance. Their extruded profiles are widely used in the automotive, rail transportation, construction, shipbuilding, and machinery manufacturing industries. The extrusion efficiency of profiles directly affects a company's production costs and capacity. Besides the extrusion press tonnage, extrusion efficiency is directly related to the extrudability of the aluminum ingot. The internal structure of the aluminum ingot has a decisive influence on the extrusion resistance, surface quality, and mechanical properties of the profile. The solubility of the Mg2Si phase in the aluminum matrix varies significantly at different temperatures. Its solubility reaches 1.85% at the eutectic temperature, but drops to only 0.27% at 200℃. Therefore, during homogeneous cooling, Mg2Si phase precipitation and coarsening occur. At high cooling rates, Mg and Si elements dissolve in the matrix to form a supersaturated solid solution, increasing the alloy's deformation resistance. At low cooling rates, a large amount of Mg2Si phase precipitates and coarsens. Although the alloy's deformation resistance is significantly reduced, the Mg2Si phase is difficult to dissolve during extrusion, leading to a loss of profile strength. Furthermore, at higher extrusion speeds or temperatures, the coarse Mg2Si phase easily causes numerous surface quality problems during extrusion. In existing technologies, aluminum alloy ingots are typically immediately water-cooled or water mist-cooled after homogenization. While this method can increase the homogenization cooling rate, excessively rapid cooling can cause a quenching effect, increasing the alloy's deformation resistance. It can also lead to quenching deformation of the ingot, and in severe cases, even bending deformation or cracking, resulting in ingot scrap. Multi-stage cooling methods, while avoiding quenching effects and bending deformation to some extent, increase the cooling rate. However, this method has limited cooling intensity and cannot prevent the precipitation of large-sized Mg2Si phases in the alloy. Alternatively, cooling to a certain temperature followed by heat preservation can be used to reduce deformation resistance. However, the nucleation and precipitation of Mg2Si is not uniform. Therefore, it is inevitable that some Mg2Si will coarsen during the heat preservation process, which still limits the surface quality and mechanical properties of the extruded profile.

[0003] The information disclosed in this background section is intended only to enhance the understanding of the overall background of the invention and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention

[0004] The purpose of this invention is to provide a cooling process for 6-series aluminum alloy ingots, which can reduce the deformation resistance of 6-series aluminum alloy ingots, increase the extrusion speed, and improve the surface quality and mechanical properties of 6-series aluminum alloy ingots.

[0005] To achieve the above objectives, embodiments of the present invention provide a cooling process for 6-series aluminum alloy ingots, comprising the following steps: heating the cast 6-series aluminum alloy ingot to 540℃-580℃ and holding it at that temperature for the first time; subjecting the 6-series aluminum alloy ingot after the first holding to a first-stage cooling process to cool it to 400℃-480℃, wherein the cooling rate of the first-stage cooling is 2℃-8℃ / min; subjecting the 6-series aluminum alloy ingot after the first-stage cooling to a second holding; subjecting the 6-series aluminum alloy ingot after the second holding to a second-stage cooling process to cool it to 250℃-300℃, wherein the cooling rate of the second-stage cooling is 5℃-10℃ / min; and subjecting the 6-series aluminum alloy ingot after the second-stage cooling to a third-stage cooling process to cool it to room temperature.

[0006] In one or more embodiments of the present invention, the heat preservation time of the first heat preservation is between 5h and 10h.

[0007] In one or more embodiments of the present invention, the cooling method of the first stage cooling is selected from one or more of air cooling and water mist cooling.

[0008] In one or more embodiments of the present invention, the heat preservation time of the second heat preservation is between 0.5h and 1h.

[0009] In one or more embodiments of the present invention, the first stage of cooling is performed in a cooling chamber.

[0010] In one or more embodiments of the present invention, the second heat preservation is performed in the cooling chamber, and during the second heat preservation process, the cooling system in the cooling chamber is turned off.

[0011] In one or more embodiments of the present invention, the cooling method of the second stage cooling is selected from one or more of air cooling and water cooling.

[0012] In one or more embodiments of the present invention, the third-stage cooling method is air cooling.

[0013] Compared with the prior art, the cooling process for 6-series aluminum alloy ingots according to an embodiment of the present invention adopts a multi-stage cooling method, using different cooling methods and controlling the cooling rate at each stage, and reasonably controlling the size of the Mg2Si phase in the 6-series aluminum alloy crystals, thereby reducing the deformation resistance of the 6-series aluminum alloy ingot, increasing the extrusion speed, and improving the surface quality and mechanical properties of the 6-series aluminum alloy ingot. Attached Figure Description

[0014] Figure 1 This is a flowchart of the cooling process for 6-series aluminum alloy ingots according to an embodiment of the present invention.

[0015] Figure 2 These are SEM images of the grain boundaries and intragranular Mg2Si phase in Example 1 of an embodiment of the present invention.

[0016] Figure 3 These are SEM images of the grain boundaries and intragranular Mg2Si phase in Example 2 of an embodiment of the present invention.

[0017] Figure 4 These are SEM images of the grain boundaries and intragranular Mg2Si phase in Example 3 of an embodiment of the present invention.

[0018] Figure 5 These are SEM images of the grain boundaries and intragranular Mg2Si phase in Example 4 of an embodiment of the present invention.

[0019] Figure 6 These are SEM images of the grain boundaries and intragranular Mg2Si phase in Example 5 of an embodiment of the present invention.

[0020] Figure 7 These are SEM images of the grain boundaries and intragranular Mg2Si phase in Comparative Example 1 according to an embodiment of the present invention.

[0021] Figure 8 These are SEM images of the grain boundaries and intragranular Mg2Si phase in Comparative Example 2 according to an embodiment of the present invention.

[0022] Figure 9 These are SEM images of the grain boundaries and intragranular Mg2Si phase in Comparative Example 3 according to an embodiment of the present invention.

[0023] Figure 10 These are SEM images of the grain boundaries and intragranular Mg2Si phase in Comparative Example 4 according to an embodiment of the present invention. Detailed Implementation

[0024] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0025] Unless otherwise expressly stated, throughout the specification and claims, the term "comprising" or its variations such as "including" or "comprises" shall be understood to include the stated elements or components without excluding other elements or other components.

[0026] This invention provides a cooling process for 6-series aluminum alloy ingots, employing a multi-stage cooling method. Different cooling methods are used in each stage, and the cooling rate is controlled to regulate the size of the Mg2Si phase within the 6-series aluminum alloy grains. During the cooling process, the precipitation of β or β′ phases is inevitable in 6-series aluminum alloys, reducing their deformation resistance. At high temperatures, Mg2Si preferentially nucleates and precipitates near grain boundaries or within the grains of the β-Al(Mn,Fe)Si or α-Al(Mn,Fe)Si phases, and continues to grow and coarsen in subsequent cooling stages. The nucleation, precipitation, and coarsening of the Mg2Si phase within the 6-series aluminum alloy grains affect the deformation resistance, surface quality, and mechanical properties of the 6-series aluminum alloy ingot.

[0027] like Figure 1 As shown, a cooling process for 6-series aluminum alloy ingots according to a preferred embodiment of the present invention includes steps S1 to S5.

[0028] In step S1, the 6-series aluminum alloy ingot is heated to 540℃-580℃ and held at that temperature for the first time.

[0029] Specifically, the cast aluminum alloy ingot is heated to 540-580℃ and held for the first time for 5-10 hours. In this step S1, the holding time of 5-10 hours not only allows the low-melting-point eutectic in the as-cast microstructure to dissolve and eliminate dendritic segregation, but also allows for the precipitation of dispersed α-AlFe(Mn,Cr)Si phase within the grains. The α-AlFe(Mn,Cr)Si phase can act as a heterogeneous nucleation site, promoting the nucleation, precipitation, and coarsening of the Mg2Si phase during cooling.

[0030] In step S2: the 6-series aluminum alloy ingot after the first heat preservation is subjected to the first stage of cooling to cool it to 400℃-480℃, wherein the cooling rate of the first stage of cooling is 2℃-8℃ / min.

[0031] Specifically, the 6-series aluminum alloy ingot, after the first heat preservation, is placed in a cooling chamber, and the 6-series aluminum alloy ingot is cooled to 400℃-480℃ by air cooling, water mist cooling, or air cooling plus water mist cooling. The cooling rate of the first stage of cooling is 2℃-8℃ / min.

[0032] In step S2, the cooling method can be either air cooling or water mist cooling alone, or a combination of both. These cooling methods provide uniform cooling and easier control of the cooling rate. Furthermore, controlling the cooling rate at 2-8℃ / min and the temperature between 400-480℃ avoids prolonged exposure of the aluminum alloy ingot to high temperatures, thereby suppressing severe coarsening of the Mg2Si phase precipitated at grain boundaries and within grains, and thus preventing surface quality problems and strength loss in the aluminum alloy ingot due to coarse Mg2Si phases. On the other hand, controlling the cooling rate at 2-8℃ / min also suppresses Mg2Si phase coarsening in the high-temperature range while preventing quenching stress caused by rapid cooling, which could lead to bending deformation of the aluminum alloy ingot.

[0033] In step S3: The 6-series aluminum alloy ingot, after the first stage of cooling, is subjected to a second heat preservation.

[0034] Specifically, the 6-series aluminum alloy ingot, after the first stage of cooling, undergoes a second holding period in a cooling chamber, with the holding time ranging from 0.5 to 1 hour. During this second holding period, the cooling system in the cooling chamber is shut off. This holding time of 0.5 to 1 hour improves the uniformity of the internal microstructure of the aluminum alloy ingot and promotes the extensive dispersed nucleation and precipitation of the Mg2Si phase within the grains without excessive coarsening, thus reducing deformation resistance.

[0035] In step S4: the 6-series aluminum alloy ingot after the second heat preservation is subjected to a second stage of cooling to cool it to 250℃-300℃, wherein the cooling rate of the second stage of cooling is 5℃-10℃ / min.

[0036] Specifically, after the second heat treatment, the 6-series aluminum alloy ingot is cooled to 250℃-300℃ using air cooling or water cooling, with a cooling rate of 5℃-10℃ / min. In step S4, as the temperature gradually decreases, in addition to the rapid precipitation and coarsening of the Mg2Si phase that was pre-precipitated at the grain boundaries and within the grains during the high-temperature stage, a large amount of uniformly dispersed Mg2Si phase also begins to precipitate within the grains and rapidly coarsens. Therefore, a relatively fast cooling rate should be adopted at this stage to ensure that the Mg2Si phase in the alloy does not coarsen excessively. At this point, since the ingot temperature has dropped to a relatively low temperature, even if the cooling rate is increased, the ingot will not bend. Controlling the cooling rate at 5-10℃ / min can suppress the coarsening of the Mg2Si pre-precipitated at the high-temperature stage during the rapid precipitation stage; it can also induce the uniform dispersion of nano-sized Mg2Si phase within the grains, reducing the deformation resistance of the aluminum alloy ingot.

[0037] In step S5: the 6-series aluminum alloy ingot after the second stage of cooling is subjected to a third stage of cooling and cooled to room temperature.

[0038] Specifically, the 6-series aluminum alloy ingot, after the second stage of cooling, undergoes a third stage of cooling using air cooling to bring it to room temperature. In this step S5, air cooling helps to further precipitate dispersed fine β′ phases inside the aluminum alloy ingot, which gradually grow and further reduce the deformation resistance of the aluminum alloy ingot. The uniform dispersion of nano-sized Mg2Si phases and β′ phases at the grain boundaries and within the grains helps to reduce the deformation resistance of the ingot, thereby increasing the extrusion speed of the aluminum alloy ingot.

[0039] Therefore, this invention employs a multi-stage cooling method, using matching cooling methods at different cooling stages and controlling the cooling rate and the critical temperature range of the transformation. Ultimately, while micron-sized Mg2Si and β′ phases are precipitated within the grains of the 6-series aluminum alloy, the size of the Mg2Si phase precipitated through heterogeneous nucleation at grain boundaries or within the grains is rationally controlled. During extrusion, the Mg2Si phase dissolves back into the 6-series aluminum alloy, thereby contributing to the improvement of the final mechanical properties, extrusion speed, and surface quality of the profile.

[0040] The present invention will be further illustrated below through specific embodiments and comparative examples.

[0041] Example 1

[0042] The 6-series aluminum alloy ingot was first held at 580℃ for 8 hours. After the first holding, the ingot was transferred to a cooling chamber for the first stage of cooling using water mist at a rate of 4℃ / min to cool it to 450℃. The cooling system was then shut off, and the ingot underwent a second holding for 0.5 hours. Following this, the ingot underwent a second stage of cooling using water at a rate of 8℃ / min to cool it to 300℃. Finally, the ingot underwent a third stage of cooling, air-cooling it to room temperature.

[0043] like Figure 2 As shown, under this process condition, the average size of the Mg2Si phase formed by heterogeneous nucleation at grain boundaries and within grains is 4.3 μm, and the average size of the Mg2Si phase dispersed within grains is 130 nm.

[0044] Example 2

[0045] The 6-series aluminum alloy ingot was first held at 580℃ for 8 hours. After the first holding, the ingot was transferred to a cooling chamber for the first stage of cooling using water mist at a rate of 2.5℃ / min to 450℃. The cooling system was then shut off, and the ingot underwent a second holding for 0.5 hours. Following this, the ingot underwent a second stage of cooling using water at a rate of 8℃ / min to 300℃. Finally, the ingot underwent a third stage of cooling, air-cooled to room temperature.

[0046] like Figure 3 As shown, under this process condition, the average size of the Mg2Si phase formed by heterogeneous nucleation at grain boundaries and within the grain is 4.2 μm, and the average size of the Mg2Si phase dispersed within the grain is 138 nm.

[0047] Example 3

[0048] The 6-series aluminum alloy ingot was first held at 580℃ for 8 hours. After the first holding, the ingot was transferred to a cooling chamber for the first stage of cooling using water mist at a rate of 4℃ / min to cool it to 400℃. The cooling system was then shut off, and the ingot underwent a second holding for 0.5 hours. Following this, the ingot underwent a second stage of cooling using water at a rate of 8℃ / min to cool it to 300℃. Finally, the ingot underwent a third stage of cooling, air-cooled to room temperature.

[0049] like Figure 4 As shown, under this process condition, the average size of the Mg2Si phase formed by heterogeneous nucleation at grain boundaries and within grains is 4.5 μm, and the average size of the Mg2Si phase dispersed within grains is 139 nm.

[0050] Example 4

[0051] The 6-series aluminum alloy ingot was first held at 580℃ for 8 hours. After the first holding, the ingot was transferred to a cooling chamber for the first stage of cooling using water mist at a rate of 4℃ / min to cool it to 450℃. The cooling system was then shut off, and the ingot underwent a second holding for 0.5 hours. Following this, the ingot underwent a second stage of cooling using water at a rate of 5℃ / min to cool it to 300℃. Finally, the ingot underwent a third stage of cooling, air-cooled to room temperature.

[0052] like Figure 5As shown, under this process condition, the average size of the Mg2Si phase formed by heterogeneous nucleation at grain boundaries and within grains is 4.7 μm, and the average size of the Mg2Si phase dispersed within grains is 152 nm.

[0053] Example 5

[0054] The 6-series aluminum alloy ingot was first held at 580℃ for 8 hours. After the first holding, the ingot was transferred to a cooling chamber for the first stage of cooling using water mist at a rate of 2.5℃ / min to 450℃. The cooling system was then shut off, and the ingot underwent a second holding for 0.5 hours. Following this, the ingot underwent a second stage of cooling using water at a rate of 10℃ / min to 300℃. Finally, the ingot underwent a third stage of cooling, air-cooled to room temperature.

[0055] like Figure 6 As shown, under this process condition, the average size of the Mg2Si phase formed by heterogeneous nucleation at grain boundaries and within the grain is 4.4 μm, and the average size of the Mg2Si phase dispersed within the grain is 149 nm.

[0056] Comparative Example 1

[0057] The 6-series aluminum alloy ingot was first held at 580℃ for 8 hours. After the first holding, the ingot was transferred to a cooling chamber for the first stage of cooling using water mist at a rate of 1.5℃ / min to 450℃. The cooling system was then shut off, and the ingot underwent a second holding for 0.5 hours. Following this, the ingot underwent a second stage of cooling using water at a rate of 8℃ / min to 300℃. Finally, the ingot underwent a third stage of cooling, air-cooled to room temperature.

[0058] like Figure 7 As shown, under this process condition, the average size of the Mg2Si phase formed by heterogeneous nucleation at grain boundaries and within grains is 7.5 μm, while the average size of the Mg2Si phase dispersed within the grains is 601 nm, with some areas coarsening to around 2 μm. At this point, the Mg2Si phase at the grain boundaries coarsens, with the size range of 6–11.2 μm.

[0059] Comparative Example 2

[0060] The 6-series aluminum alloy ingot was first held at 580℃ for 8 hours. After the first holding, the ingot was transferred to a cooling chamber for the first stage of cooling using water mist at a rate of 4℃ / min to reduce the ingot to 350℃. The cooling system was then shut off, and the ingot underwent a second holding for 0.5 hours. Following this, the ingot underwent a second stage of cooling using water at a rate of 8℃ / min to reduce the ingot to 300℃. Finally, the ingot underwent a third stage of cooling, air-cooled to room temperature.

[0061] like Figure 8 As shown, under this process condition, the average size of the Mg2Si phase formed by heterogeneous nucleation at grain boundaries and within grains is 6.8 μm, while the average size of the Mg2Si phase dispersed within the grains is 532 nm, with the maximum size reaching approximately 1.8 μm. At this point, the Mg2Si phase at grain boundaries coarsens, and the size range of the Mg2Si phase at the grain boundaries is 6–12.1 μm.

[0062] Comparative Example 3

[0063] The 6-series aluminum alloy ingot was held at 580℃ for 10 hours, then cooled to 450℃ at a cooling rate of 195℃ / h, then cooled to 150℃ at a cooling rate of 260℃ / h, and finally cooled to room temperature at a cooling rate of 100℃ / h.

[0064] like Figure 9 As shown, the Mg2Si phase at the grain boundaries and within the grains is severely coarsened. The average size of the Mg2Si phase at the grain boundaries is 11 μm, while the Mg2Si phase within the grains is coarsened to the micrometer level, approximately 6 μm.

[0065] Comparative Example 4

[0066] The 6-series aluminum alloy ingot is held at 580℃ for 10 hours, then cooled to below 350℃ in a cooling chamber at a cooling rate of 5℃. The ingot is then transferred back to a homogenizing furnace and held for 1.5 to 2 hours.

[0067] like Figure 10 As shown, a large number of coarse Mg2Si phases are precipitated at the grain boundaries and within the grains. The average size of the Mg2Si phase at the grain boundaries is 12 μm, and the average size of the Mg2Si phase within the grains is 6.2 μm.

[0068] The 6-series aluminum alloy ingots obtained by different cooling methods in Examples 1-5 and Comparative Examples 1-4 were tested, and the specific test results are shown in Table 1.

[0069] Table 1. Test data of extrusion tests and mechanical properties of Examples 1-5 and Comparative Examples 1-4.

[0070]

[0071] according to Figure 2-10 As shown in Table 1, the different cooling processes during the cooling of 6-series aluminum alloys lead to different evolution patterns of the Mg2Si phase. These differences in microstructure directly affect the extrusion speed, deformation resistance, surface quality, and final mechanical properties of the alloy. Examples 1-5, through a staged cooling process, achieved precise control of the size of the Mg2Si phase at grain boundaries and within grain boundaries during the cooling stage. This resulted in ingots with reduced deformation resistance, enabling high-speed extrusion, good surface quality, and high mechanical properties after online quenching. Table 1 shows that compared to Comparative Examples 1-2, Examples 1-5 showed no significant difference in extrusion breakthrough force at an extrusion speed of 2 mm / s. However, at a higher extrusion speed of 3 mm / s, Examples 1-5 maintained good surface quality, while Comparative Examples 1-2 all experienced surface cracking. The extrusion speed of Examples 1-5 was approximately 35% higher than that of Comparative Examples 1-2. Furthermore, the T5 mechanical properties of Examples 1-5 were approximately 10-20 MPa higher than those of Comparative Examples 1-2.

[0072] Furthermore, as shown in Comparative Examples 1-4, reducing the cooling rate leads to the precipitation of coarse Mg2Si phases in the alloy, with a maximum size of 12 μm. Although Comparative Example 3 also employed a staged cooling method, the homogeneous cooling intensity was insufficient, especially in the first stage of cooling. During homogeneous cooling, coarse Mg2Si phases precipitated in the alloy, and these coarse second phases limited the increase in extrusion speed and resulted in a decrease in peak strength after extrusion. Comparative Example 4, on the other hand, used a cooling + isothermal method. During the isothermal holding at 360℃, the Mg2Si phase in the alloy rapidly coarsened. Although a slow cooling rate allowed for sufficient precipitation of the Mg2Si phase to reduce deformation resistance, the excessively coarsened Mg2Si phase was difficult to fully dissolve during extrusion, consuming a large amount of the precipitation strengthening element Mg / Si, ultimately resulting in low mechanical properties in the T5 state.

[0073] The foregoing description of specific exemplary embodiments of the invention is for illustrative and explanatory purposes. These descriptions are not intended to limit the invention to the precise forms disclosed, and it will be apparent that many changes and variations can be made in accordance with the foregoing teachings. The exemplary embodiments were chosen and described in order to explain the specific principles of the invention and its practical application, thereby enabling those skilled in the art to implement and utilize various different exemplary embodiments of the invention, as well as various different choices and variations. The scope of the invention is intended to be defined by the claims and their equivalents.

Claims

1. A cooling process for 6-series aluminum alloy ingots, characterized in that, Includes the following steps: The 6-series aluminum alloy ingots, which are formed by melting and casting, are heated to 540℃-580℃ and subjected to a first heat preservation; the heat preservation time of the first heat preservation is between 5h and 10h. The 6-series aluminum alloy ingots after the first heat preservation are subjected to the first stage of cooling to cool to 400℃-480℃, wherein the cooling rate of the first stage of cooling is 2℃-8℃ / min; The 6-series aluminum alloy ingots after the first stage of cooling are subjected to a second heat preservation; the heat preservation time for the second heat preservation is between 0.5h and 1h. The 6-series aluminum alloy ingot, after the second heat treatment, undergoes a second stage of cooling to 250°C-300°C, wherein the cooling rate of the second stage is 5°C-10°C / min; and The 6-series aluminum alloy ingots, after the second stage of cooling, are subjected to a third stage of cooling to bring them to room temperature.

2. The cooling process for 6-series aluminum alloy ingots as described in claim 1, characterized in that, The cooling method for the first stage of cooling is selected from one or more of air cooling and water mist cooling.

3. The cooling process for 6-series aluminum alloy ingots as described in claim 1, characterized in that, The first stage of cooling takes place in the cooling chamber.

4. The cooling process for 6-series aluminum alloy ingots as described in claim 3, characterized in that, The second heat preservation is carried out in the cooling chamber, and during the second heat preservation process, the cooling system in the cooling chamber is turned off.

5. The cooling process for 6-series aluminum alloy ingots as described in claim 1, characterized in that, The second-stage cooling method is selected from one or more of air cooling and water cooling.

6. The cooling process for 6-series aluminum alloy ingots as described in claim 1, characterized in that, The third stage of cooling uses air cooling.

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