A processing method for reducing coarse grains in high-strength 6xxx series aluminum alloy profiles

Abstract

The present invention belongs to the field of aluminum alloy metal material processing technology, and specifically relates to a processing method for reducing coarse grains in high-strength 6xxx series aluminum alloy profiles. The alloy components by mass percentage are: Mg: 0.6-1.2%, Si: 0.7-1.1%, Mn: 0.6-0.9%, Cr: 0.1-0.3%, Cu: 0.01-0.6%, Zr: 0.01-0.15%, Ti: 0.02-0.06%, Fe ≤ 0.17%, with the balance being Al and impurities totaling less than 0.01%. A processing method is also disclosed. The resulting extruded product has a tensile strength of 320-410 MPa, a yield strength of 270-360 MPa, an elongation of 12-16%, a coarse grain layer thickness of ≤ 0.5 mm, and no color difference, orange peel, or mottled defects on the anodized surface.

Description

Technical Field

[0001] The present invention belongs to the field of technical processing of aluminum alloy metal materials, and in particular relates to a processing method for reducing coarse grains in high-strength 6xxx series aluminum alloy profiles. Background Art

[0002] 6xxx series aluminum alloys, thanks to their lightweight, high-strength, and corrosion-resistant properties, have been widely used in automotive parts, aerospace structures, wings, and wheels. In motorcycle manufacturing, with the market's continued pursuit of lightweight and aesthetically pleasing design, 6xxx series aluminum alloy wheels have become a key option for achieving lightweight, high-strength, and aesthetically pleasing designs. However, existing 6xxx series aluminum alloy products based on conventional alloy compositions and production processes face significant challenges in certain complex wheel manufacturing scenarios, where cold working steps are lengthy and deformation energy storage accumulates significantly. During subsequent high-temperature heat treatment, the energy stored within the metal is released, which can easily induce varying degrees of abnormal recrystallized grain growth on the surface of the product. This phenomenon leads to 1) deterioration in surface quality: extremely poor anodizing results in noticeable color differences, severely impacting aesthetics; 2) decreased mechanical properties: coarse surface grains significantly reduce the material's mechanical properties, such as strength, hardness, and fatigue resistance; and 3) reliability risks: ultimately impacting surface quality and service life, failing to meet the requirements of high-end customers.

[0003] Currently, observed coarse grains are primarily concentrated in the surface of extruded profiles, particularly after cold working deformation, resulting in an uneven distribution of coarse grains across the entire cross-section. This poses a major obstacle for high-end products with demanding surface and overall performance requirements, such as high-performance motorcycle wheels. Currently, the only way to mitigate this uneven coarse recrystallization phenomenon is through alloy composition optimization, but this optimization often fails to meet customer needs. Patent CN 113355616 A proposes a two-stage homogenization process combined with rapid cooling to promote the precipitation of a high-density α-Al(MnCr)Si phase, thereby suppressing recrystallization and coarsening. This innovation lies in an innovative homogenization process. Patent CN 114921698 A utilizes a specific two-stage homogenization system, precisely controlling the heating rate and holding time, and maintaining a relatively low extrusion temperature of 440°C to 460°C and a speed of 0.5m / min to 0.7m / min to reduce the coarse grain ring. This core approach relies on combining a homogenization process with low-temperature and low-speed extrusion. The common limitation of these existing methods is that they have failed to break through the traditional process framework of "complex homogenization treatment is required" and "medium and low temperature extrusion". Its harm lies in that it eliminates the segregation of cast dendrites through high-temperature diffusion, depriving the Mn, Cr, and Zr elements of the basis for rapid precipitation of nano-scale dispersed phases Al6Mn, Al7Cr, Al3Zr and less than 3µm in the low temperature range of 300-400℃, resulting in grain boundary pinning failure; at the same time, medium and low temperature extrusion ≤500℃ causes insufficient dissolution of the Mg2Si strengthening phase, and the subsequent cold working deformation energy storage accumulation induces abnormal growth of recrystallized grains during solution treatment, ultimately resulting in overall coarse grains, reduced strength, tensile strength less than 300MPa, and severe color difference and mottled defects in anodizing. They failed to explore new ways to fundamentally reduce the formation of coarse grains by combining non-homogeneous treated cast billets with extrusion technology, precisely controlling the dissolution state of the Mg2Si strengthening phase and the formation, size and distribution of Al6Mn, Al7Cr and Al3Zr dispersed phase particles, especially by using low-temperature precipitated dispersed phases to pin grain boundaries and co-extrude high-temperature dissolved strengthening phases to simultaneously suppress coarse grains and achieve supersaturated solid solution.

[0004] Therefore, developing a novel processing method for 6xxx aluminum alloy profiles, particularly those that have undergone a long cold working process, without homogenization, that effectively suppresses surface grain growth while maintaining high strength, excellent appearance, and anodizing performance is of great practical significance and value. The present invention is proposed to fill this technological gap. Summary of the Invention

[0005] Addressing the limitations of existing technologies, which rely on complex homogenization processes and low-temperature extrusion, this invention provides a method for reducing coarse grains in high-strength 6xxx aluminum alloy profiles. This core innovation lies in bypassing homogenization by directly feeding the as-cast billet into the extrusion preheating furnace. This method, combined with a specific heating schedule and extrusion process, precisely controls the behavior of key secondary phases—Al6Mn, Al7Cr, and Al3Zr dispersed phases—and the Mg2Si strengthening phase, effectively suppressing the formation of surface coarse grains.

[0006] In order to achieve the above object, the specific technical solutions of the present invention are as follows:

[0007] The present invention provides a processing method for reducing coarse grains in a high-strength 6xxx series aluminum alloy profile. The method uses a 6xxx series aluminum alloy as-cast billet having the following composition by mass percentage and not subjected to homogenization treatment: Mg: 0.6-1.2%, Si: 0.7-1.1%, Mn: 0.6-0.9%, Cr: 0.1-0.3%, Cu: 0.01-0.6%, Zr: 0.01-0.15%, Ti: 0.02-0.06%, Fe≤0.17%, and the balance being Al and impurity elements with a total amount of less than 0.01%.

[0008] In the present invention, 0.02-0.06% Ti is added to form TiB2 heterogeneous nucleation particles, so that the cast grain size reaches 90-150µm, the dendrites are refined and element segregation is reduced, and the two work together to achieve low-temperature precipitation pinning + high-temperature solid solution strengthening, enhance the dragging effect on the grain boundaries, and inhibit the formation of coarse grains.

[0009] The processing method includes aluminum alloy smelting and casting processing, cast billet processing, heating, extrusion, cold processing and heat treatment, and surface treatment.

[0010] Preferably, the smelting and casting process employs the following method: Pure aluminum, Mg, Si, Mn, Cr, and Ti master alloys are selected according to the required alloy mass percentages for smelting. First, the smelting temperature is controlled between 680°C and 720°C, and the aluminum melt temperature is controlled within the range of 730°C to 770°C. The melt undergoes refining and slag removal, and utilizes online degassing, a two-stage filter plate, and a melt hydrogen content of less than 0.12mL / 100gAl. Hydraulic semi-continuous casting is used, with ingots containing internal inclusions less than 0.1mm, a grain size of 90-150µm, and a cast rod diameter controlled within the range of 178mm to 254mm.

[0011] Preferably, the as-cast billet is processed by the following method: directly using the as-cast billet of 6xxx series aluminum alloy obtained by the above-mentioned smelting and casting without homogenization treatment.

[0012] Preferably, the heat treatment adopts the following method: heating the cast billet to 300℃~400℃ in an extrusion preheating furnace and holding it for 2~6h, so as to promote the precipitation of Mn, Cr and Zr elements enriched at the interdendritic grain boundaries during casting in the form of Al6Mn, Al7Cr and Al3Zr dispersed phase particles with a size of less than 3µm within this temperature range; then heating to 530~580℃ after 0.5~2h and holding it for 2~5h, so that the Mg2Si strengthening phase particles gathered at the grain boundaries are fully dissolved in the aluminum matrix; after the billet is taken out of the furnace, the material temperature is 530~580℃.

[0013] Preferably, the extrusion adopts the following method: the billet after being heated out of the furnace is cooled to a temperature of 450-530°C at a rate of 10-50°C / min for extrusion, and the outlet temperature of the extruded material is maintained at 550-580°C. During the extrusion process, the Al3Zr dispersed phase hinders dislocation migration, reduces deformation energy storage accumulation, inhibits the coarsening of the dispersed phase, and maintains the solid solution state of the strengthening phase.

[0014] Preferably, the cold working adopts the following method: the extruded profile is subjected to necessary cold working, with the total deformation controlled at 2% to 7%, and then welding is performed.

[0015] Preferably, the heat treatment adopts the following method:

[0016] Solution treatment: The cold-worked profile is heated to 490℃~550℃ and kept warm for 0.5~2h; the residual Mg2Si strengthening phase is fully dissolved to obtain a supersaturated solid solution, while the fine Al6Mn, Al7Cr and Al3Zr dispersed phases remain stable, continuously inhibiting the growth of recrystallized grains.

[0017] Aging treatment: The profile after solution treatment is heated to 120℃~190℃ for 6~12h to promote the precipitation of nano-scale Mg2Si strengthening phase and significantly improve the strength of the alloy.

[0018] Preferably, the surface treatment adopts the following method: the product after aging treatment is subjected to CNC internal and external surface processing to remove the surface texture of the extruded profile, and anodizing process is used to make the product surface present a uniform and bright anodized layer with a film thickness of 10 to 50 μm, and the appearance color is beautiful without color difference, orange peel, spots and other defects.

[0019] Through the above-mentioned preparation method, the thickness of the coarse-grained layer on the surface of the profile is significantly reduced, fundamentally eliminating the problem of anodizing color difference caused by coarse crystals, ensuring a uniform and beautiful appearance, and obtaining excellent comprehensive performance. The material has high strength and good plasticity, with a tensile strength of 320-410MPa, a yield strength of 270-360MPa, an elongation of 12-16%, and a coarse-grained layer thickness of ≤0.5mm. All indicators meet and exceed the stringent requirements of high-end products.

[0020] Beneficial effects

[0021] This invention directly utilizes a two-stage heating process for the as-cast billet. This process not only promotes the formation of Mn / Cr / Zr dispersed phases <3μm in Al6Mn, Al7Cr, and Al3Zr during the low-temperature process (300-400°C), but also promotes the full dissolution of Mg2Si during the high-temperature process (530-580°C), providing the solid solubility of strengthening elements for subsequent solid solution strengthening. After exiting the high-temperature furnace, the billet is rapidly cooled to 450-530°C at a rate of 10-50°C / min and immediately extruded (exit temperature 550-580°C), achieving the supersaturated solid solution required for extrusion and improving mechanical properties. Conventional processes, however, require high-temperature homogenization to achieve full solid solution of strengthening elements within the grains.

[0022] The present invention provides a processing method for reducing coarse grains in high-strength 6xxx series aluminum alloy profiles. By breaking through the traditional framework and utilizing a non-homogeneous cast billet in combination with an extrusion path, the method innovatively controls the precipitation, dissolution, crushing, and pinning behaviors of the key second phase, thereby achieving efficient suppression of coarse grains on the surface of the 6xxx series aluminum alloy profile while ensuring the high strength and excellent appearance of the product. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is the observation of the macroscopic coarse-grained layer of the cross section of the product in Example 1;

[0024] Figure 2 This is the observation of the macroscopic coarse-grained layer of the cross section of the product in Example 2;

[0025] Figure 3 This is the observation of the microscopic coarse-grained layer of the cross section of the product in Example 1;

[0026] Figure 4 This is the observation of the microscopic coarse-grained layer of the cross section of the product in Example 2;

[0027] Figure 5 For comparative example 1, the macroscopic coarse-grained layer of the cross section of the product is observed;

[0028] Figure 6 For comparative example 2, the macroscopic coarse-grained layer of the cross section of the product is observed;

[0029] Figure 7 For comparative example 1, the microscopic coarse-grained layer of the cross section of the product is observed;

[0030] Figure 8 For comparative example 2, the microscopic coarse-grained layer of the cross section of the product is observed;

[0031] Figure 9 This is the SEM scanning electron microscope image of Al6Mn, Al7Cr and Al3Zr dispersed phases less than 3µm formed by the Mn, Cr and Zr elements in Example 1. DETAILED DESCRIPTION

[0032] The present invention is described in detail below based on specific implementation cases: The following implementation cases are only part of the embodiments of the present invention.

[0033] Example 1

[0034] A processing method for reducing coarse grains in a high-strength 6xxx series aluminum alloy profile. The aluminum alloy comprises the following weight percentages: Si 0.81%, Fe 0.12%, Cu 0.20%, Mn 0.66%, Mg 0.68%, Zr 0.07%, Ti 0.03%, and Cr 0.13%, with the balance being Al. The remaining unspecified elements are considered unavoidable impurity elements and have a content of less than 0.01%.

[0035] The preparation method comprises the following steps:

[0036] Step 1: Melting and Casting: Prepare 97.7% pure aluminum, 75% chromium agent, AlMn10 master alloy, AlSi10 master alloy, 99.95% magnesium ingot, and 99.99% copper wire. Melt the ingredients according to the mass percentage of the above ingredients. First, control the melting temperature at 700±20°C, and the aluminum melt temperature at 730±20°C. The melt undergoes refining, slagging, online degassing, and a two-stage filter plate. The hydrogen content of the melt is less than 0.12mL / 100gAl. Hydraulic semi-continuous casting is used, with inclusions less than 0.1mm, a grain size of 90-150µm, and a controlled diameter range of 178mm-254mm.

[0037] Step 2: As-cast billet treatment: The as-cast billet after casting is directly used in the subsequent heating and extrusion process without the traditional homogenization heat treatment.

[0038] Step 3: Heating and Extrusion: The as-cast billet is heated in an extrusion preheating furnace, with the first stage heating temperature at 320°C and a hold time of 5 hours. This temperature range encourages the formation of Al6Mn, Al7Cr, and Al3Zr dispersed phases <3µm in diameter, pinning grain boundaries and inhibiting recrystallization. After 1.5 hours, the temperature is raised to the second stage at 535°C and held for 2 hours. This temperature range promotes the complete dissolution of the Mg2Si strengthening phase. The billet is discharged from the furnace at a temperature of 535°C. After cooling to 480°C at a rate of 15°C / min, extrusion is immediately performed. The extrusion ratio is 35, and the extrudate exit temperature is 555±5°C. The temperature fluctuation from the head to the tail is less than 10°C, ensuring a zero tendency to overburn the product.

[0039] Step 4: Cold Working: Use the room temperature profile for the first rounding pre-forming process, with a rounding speed of 30mm / s. After the first rounding is completed, a second rounding finishing process is performed, and the total deformation of the two rounds is controlled to 4%.

[0040] Step 5: Welding: Resistance welding is used to generate high temperatures that connect the gaps to form the entire hub. After welding, the weld seam must be microscopically analyzed to ensure it is free of porosity, slag inclusions, cracks, unfused components, or undercuts. The weld must also have a tensile strength of at least 90% of the hub's strength.

[0041] Step 6: Heat Treatment: The welded wheel hub undergoes solution and aging heat treatment to strengthen it. The solution temperature is 540°C, and the temperature is increased from room temperature at 300°C / h, then held for 1.5 hours before quenching and cooling to room temperature. Aging treatment is then performed using a single-stage aging process: heating from room temperature to 190°C and holding for 10 hours. After heat treatment, the extruded product has a tensile strength of 395MPa, a yield strength of 348MPa, an elongation of 15%, and a coarse grain layer of 0.2mm.

[0042] Step 7: Surface treatment: After aging, the wheel hub is CNC machined to remove surface impurities. Anodizing is then used to create a uniform, bright anodized layer on the hub hub surface. The hub hub has a beautiful appearance and is free of color difference, orange peel, spots, and other defects.

[0043] Example 2

[0044] A processing method for reducing coarse grains in a high-strength 6xxx series aluminum alloy profile. The aluminum alloy comprises the following weight percentages: Si 1.05%, Fe 0.14%, Cu 0.52%, Mn 0.85%, Mg 0.96%, Zr 0.12%, Ti 0.05%, and Cr 0.26%, with the balance being Al. The remaining unspecified elements are considered unavoidable impurity elements and have a content of less than 0.01%.

[0045] The preparation method comprises the following steps:

[0046] Step 1: Melting and Casting: Prepare 97.7% pure aluminum, 75% chromium additive, AlMn10 master alloy, AlSi10 master alloy, 99.95% magnesium ingot, and 99.99% copper wire. Melt the ingredients according to the mass percentages of the above components. First, control the melting temperature at 700±20°C, and the molten aluminum temperature at 730±20°C. The molten aluminum is refined, deslagging, and degassing online through a two-stage filter. The hydrogen content of the molten aluminum is less than 0.12mL / 100gAl. Hydraulic semi-continuous casting is used, and inclusions within the cast bars are less than 0.1mm. The grain size is 90-150µm, and the cast bar diameter is controlled within the range of 178mm-254mm.

[0047] Step 2: As-cast billet treatment: The as-cast billet after casting is directly used in the subsequent heating and extrusion process without the traditional homogenization heat treatment.

[0048] Step 3: Heating and Extrusion: The as-cast billet is heated in an extrusion preheating furnace, with the first stage heating temperature at 365°C and a hold time of 3 hours. This temperature range encourages the Mn, Cr, and Zr elements to form Al6Mn, Al7Cr, and Al3Zr dispersed phases <3µm, pinning grain boundaries and inhibiting recrystallization. After 0.8 hours, the temperature is raised to the second stage at 570°C and held for 4 hours. This temperature range promotes the complete dissolution of the Mg2Si strengthening phase. The billet is discharged from the furnace at a temperature of 570°C. After cooling to 510°C at a rate of 26°C / min, extrusion is immediately performed. The extrusion ratio is 49, and the extrudate exit temperature is 565±5°C. The temperature fluctuation from the head to the tail is less than 10°C, ensuring a zero tendency to overburn the product.

[0049] Step 4: Cold Working: Use the room temperature profile for the first rounding pre-forming process, with a rounding speed of 30mm / s. After the first rounding is completed, a second rounding finishing process is performed, and the total deformation of the two rounds is controlled to 4%.

[0050] Step 5: Welding: Resistance welding is used to generate high temperatures that connect the gaps to form the entire hub. After welding, the weld seam must be microscopically analyzed to ensure it is free of porosity, slag inclusions, cracks, unfused components, or undercuts. The weld must also have a tensile strength of at least 90% of the hub's strength.

[0051] Step 6: Heat Treatment: The welded wheel hub undergoes solution and aging heat treatment to strengthen it. The solution temperature is 540°C, and the temperature is increased from room temperature at 300°C / h, then held for 1.5 hours before quenching and cooling to room temperature. Aging treatment is then performed using a single-stage aging process: from room temperature to 190°C and held for 10 hours. After heat treatment, the extruded product has a tensile strength of 402MPa, a yield strength of 365MPa, an elongation of 15%, and a coarse grain layer of 0.3mm.

[0052] Step 7: Surface treatment: After aging, the wheel hub is CNC machined to remove impurities. Anodizing is then used to create a uniform, bright anodized layer on the hub hub surface. The hub hub has a beautiful appearance and is free of color difference, orange peel, or mottling.

[0053] Comparative Example 1

[0054] A processing method for reducing coarse grains in a high-strength 6xxx series aluminum alloy profile. The aluminum alloy material comprises, by weight, 0.81% Si, 0.12% Fe, 0.20% Cu, 0.66% Mn, 0.68% Mg, 0.07% Zr, 0.03% Ti, and 0.13% Cr, with the balance being Al. The remaining unspecified elements are considered unavoidable impurity elements and have a content of less than 0.01%.

[0055] The preparation method comprises the following steps:

[0056] Step 1: Melting and Casting: Prepare 97.7% pure aluminum, 75% chromium additive, AlMn10 master alloy, AlSi10 master alloy, 99.95% magnesium ingot, and 99.99% copper wire. Melt the ingredients according to the mass percentages of the above components. First, control the melting temperature at 700±20°C, and the molten aluminum temperature at 730±20°C. The molten aluminum is refined, deslagging, and degassing online through a two-stage filter. The hydrogen content of the molten aluminum is less than 0.12mL / 100gAl. Hydraulic semi-continuous casting is used, and inclusions within the cast bars are less than 0.1mm. The grain size is 90-150µm, and the cast bar diameter is controlled within the range of 178mm-254mm.

[0057] Step 2: Treatment of the as-cast billet: The as-cast billet is subjected to a two-stage homogenization treatment, with the first stage being kept at 330°C for 5 hours and the second stage being kept at 550°C for 6 hours. After being taken out of the furnace, it is cooled with water mist and cooled to room temperature at a cooling rate of 300°C per hour.

[0058] Step 3, heating and extrusion: The billet after homogenization is heated in an extrusion preheating furnace at 480℃±10℃. After heating for 6 hours, it is loaded and extruded. The extrusion ratio is 40, the outlet temperature of the extruded profile is 555±5℃, the temperature fluctuation range from the head end to the tail end is <10℃, and the product has no tendency to overburn.

[0059] Step 4: Cold Working: The first rounding pre-forming process is performed on the profile at room temperature, with a rounding speed of 30mm / s. After the first rounding is completed, a second rounding finishing process is performed, and the total deformation is controlled at 8%.

[0060] Step 5: Welding: Resistance welding is used to generate high temperatures that connect the gaps to form the entire hub. After welding, the weld seam must be microscopically analyzed to ensure it is free of porosity, slag inclusions, cracks, unfused components, or undercuts. The weld must also have a tensile strength of at least 90% of the hub's strength.

[0061] Step 6: Heat Treatment: The welded hub undergoes solution and aging heat treatment to strengthen it. The solution temperature is 540°C. The temperature is raised from room temperature at a rate of 300°C / hour, then held for 1.5 hours before quenching and cooling to room temperature. Aging treatment is then performed using a single-stage aging process: rising the temperature from room temperature to 190°C and holding for 10 hours. After heat treatment, the extruded product has a tensile strength of 295MPa, a yield strength of 252MPa, an elongation of 14%, and an overall coarse grain.

[0062] Step 7, Surface Treatment: After the aging process, the wheel hub is CNC machined to remove surface impurities. After the anodic oxidation process, the wheel hub surface will show defects such as dull gloss and spots.

[0063] Comparative Example 2

[0064] A processing method for reducing coarse grains in a high-strength 6xxx series aluminum alloy profile. The aluminum alloy comprises the following weight percentages: Si 1.05%, Fe 0.14%, Cu 0.52%, Mn 0.85%, Mg 0.96%, Zr 0.12%, Ti 0.05%, and Cr 0.26%, with the balance being Al. The remaining unspecified elements are considered unavoidable impurity elements and have a content of less than 0.01%.

[0065] The preparation method comprises the following steps:

[0066] Step 1: Melting and Casting: Prepare 97.7% pure aluminum, 75% chromium additive, AlMn10 master alloy, AlSi10 master alloy, 99.95% magnesium ingot, and 99.99% copper wire. Melt the ingredients according to the mass percentages of the above components. First, control the melting temperature at 700±20°C, and the molten aluminum temperature at 730±20°C. The molten aluminum is refined, deslagging, and degassing online through a two-stage filter. The hydrogen content of the molten aluminum is less than 0.12mL / 100gAl. Hydraulic semi-continuous casting is used, and inclusions within the cast bars are less than 0.1mm. The grain size is 90-150µm, and the cast bar diameter is controlled within the range of 178mm-254mm.

[0067] Step 2: Raw material processing: The as-cast billet is subjected to a two-stage homogenization treatment, with the first stage temperature at 460°C for 8 hours and the second stage temperature at 540°C for 9 hours. It is then cooled with water mist after being taken out of the furnace and cooled to room temperature at a cooling rate of 350°C per hour.

[0068] Step 3, heating and extrusion: The billet after homogenization is heated in an extrusion preheating furnace at 500℃±10℃. After heating for 7 hours, it is loaded and extruded. The extrusion ratio is 54, the outlet temperature of the extruded profile is 570℃±5℃, the temperature fluctuation range from the head end to the tail end is <10℃, and the product has no tendency to overburn.

[0069] Step 4: Cold Working: The first rounding pre-forming process is performed on the profile at room temperature, with a rounding speed of 30mm / s. After the first rounding is completed, a second rounding finishing process is performed, and the total deformation is controlled at 8%.

[0070] Step 5: Welding: Resistance welding is used to generate high temperatures that connect the gaps to form the entire hub. After welding, the weld seam must be microscopically analyzed to ensure it is free of porosity, slag inclusions, cracks, unfused components, or undercuts. The weld must also have a tensile strength of at least 90% of the hub's strength.

[0071] Step 6: Heat Treatment: The welded hub undergoes solution and aging heat treatment to strengthen it. The solution temperature is 540°C. The temperature is raised from room temperature at a rate of 300°C / hour, then held for 1.5 hours before quenching and cooling to room temperature. A single-stage aging treatment is then performed: the temperature is raised to 190°C from room temperature and held for 10 hours. After heat treatment, the extruded product has a tensile strength of 291 MPa, a yield strength of 252 MPa, an elongation of 15%, and overall coarse grains.

[0072] Step 7: Surface treatment: After aging, the wheel hub is CNC machined to remove surface impurities. After anodizing, the wheel hub surface will show defects such as dull luster, overall coarse grains, and spots.

[0073] Observe the coarse crystal layer of Example 1-2 and Comparative Example 1-2, see Figures 1-8 . Figure 9 SEM scanning electron microscopy shows that the Mn, Cr, and Zr elements form Al6Mn, Al7Cr, and Al3Zr dispersed phases less than 3µm. The figure shows that the coarse grain layer thickness of Examples 1 and 2 is less than 0.5mm, while both Comparative Examples 1 and 2 have overall coarse grains, and the coarse grain layer thickness is much greater than 0.5mm.

[0074] The tensile strength, yield strength, elongation, and coarse-grained layer thickness of Examples 1-2 and Comparative Examples 1-2 were tested. The tensile strength test method was in accordance with GB / T 228.1-2021; the yield strength test method was in accordance with GB / T 228.1-2021; the elongation test method was in accordance with GB / T 228.1-2021; and the coarse-grained layer thickness test method was in accordance with GB / T 3246.2-2012. Specific data are shown in Table 1.

[0075] Table 1 Mechanical test table of wheel hub products

[0076]

Claims

1. A method for reducing coarse grains in high-strength 6xxx series aluminum alloy profiles, characterized in that: A 6xxx series aluminum alloy as-cast billet without homogenization treatment having the following composition by mass percentage is used: Mg: 0.6-1.2%, Si: 0.7-1.1%, Mn: 0.6-0.9%, Cr: 0.1-0.3%, Cu: 0.01-0.6%, Zr: 0.01-0.15%, Ti: 0.02-0.06%, Fe≤0.17%, and the balance is Al and impurity elements with a total amount of less than 0.01%; The process steps include the following: (a) Heating treatment: The as-cast billet is heated to 300°C–400°C in an extrusion preheating furnace for 2–6 h to promote the formation of Al6Mn, Al7Cr, and Al3Zr dispersed phases with a size of less than 3 µm. The temperature is then rapidly raised to 530°C–580°C and held for 2–5 h to fully dissolve the Mg2Si strengthening phase. (b) Extrusion: The heated cast billet is taken out of the heating furnace at a temperature of 530°C to 580°C, then rapidly cooled to 450°C to 530°C at a rate of 10 to 50°C / min, and immediately extruded at 450°C to 530°C, maintaining the outlet temperature of the extruded material at 550°C to 580°C. (c) Cold working: the total deformation is controlled within 2% to 7%; (d) The cold-worked profiles are subjected to offline solution treatment, aging, CNC machining, and surface treatment.

2. The method for reducing coarse grains in high-strength 6xxx series aluminum alloy profiles according to claim 1, wherein: The specific operation of solid solution is: heat the cold-processed profile to 490℃~550℃ and keep it warm for 0.5~2h.

3. The method for reducing coarse grains in high-strength 6xxx series aluminum alloy profiles according to claim 2, wherein: The specific operation of aging is: heat the solution treated profile to 120℃~190℃ and keep it warm for 6~12h.

4. A 6xxx series aluminum alloy profile prepared by the processing method according to any one of claims 1 to 3, characterized in that: The tensile strength is 320-410MPa, the yield strength is 270-360MPa, the elongation is 12-16%, and the surface coarse grain layer is ≤0.5mm. After anodizing treatment, the surface has no color difference, orange peel or mottled defects.

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