A dual-stage cold-rolled and section-aged high-strength and high-toughness 6xxx series aluminum alloy strip and a preparation method thereof

Abstract

The application discloses a kind of high-strength and high-toughness 6xxx series aluminum alloy strip of two-stage cold rolling and sectional age strengthening and a preparation method thereof, and belongs to the technical field of aluminum alloy material and deformation heat treatment.The problem of poor matching of strength and toughness of existing 6xxx series aluminum alloy is solved.The preparation method is as follows: after casting, the blank is subjected to solid solution treatment, then subjected to first cold rolling, then subjected to short-time pre-aging treatment on the cold-rolled strip, then subjected to second cold rolling or low-temperature cryogenic rolling, and finally subjected to final aging treatment.The pre-aging step is embedded between the two cold rolling processes, so that the metastable clusters formed by pre-aging are sheared by dislocations during the second cold rolling process, forming a dislocation-precipitate composite structure, which induces a high-density and uniformly distributed nano-strengthening phase during the final aging stage, while retaining part of the substructure strengthening.The obtained aluminum alloy strip has high strength and good plasticity, breaking through the technical bottleneck that conventional process is difficult to balance strength and plasticity, and is suitable for automobile lightweight, rail transportation and aerospace fields.

Description

Technical Field

[0001] This invention belongs to the field of aluminum alloy materials and deformation heat treatment technology, specifically relating to a high-strength and high-toughness 6xxx series aluminum alloy strip suitable for automotive lightweighting, rail transportation and aerospace and its preparation method, and particularly to an aluminum alloy strip that simultaneously improves strength and plasticity by synergistically controlling the microstructure through two-stage cold rolling and segmented aging. Background Technology

[0002] 6xxx series (Al-Mg-Si) aluminum alloys possess excellent heat-treatable strengthening properties, superior corrosion resistance, weldability, and moderate strength, making them the preferred material for lightweight components such as automotive body panels, rail vehicle bodies, ship superstructures, and aerospace structural parts. Under increasingly stringent energy conservation and emission reduction requirements, these strips not only need high yield strength and tensile strength to ensure load-bearing capacity but also must maintain sufficient ductility to adapt to complex stamping processes and ensure service safety.

[0003] Currently, industrial strengthening still primarily relies on solution treatment followed by aging. The standard T6 process (solution treatment + peak aging) achieves high tensile strength, but typically comes with a significant reduction in plasticity. To further improve strength, the industry widely adopts the T8 process (solution treatment + cold working + aging), which introduces a certain amount of cold rolling deformation after solution quenching. This utilizes high-density dislocations to promote the uniform nucleation of the main strengthening phase β″ during aging, thereby achieving higher yield strength. However, conventional T8 single-pass cold rolling faces several limitations. Insufficient deformation results in limited strengthening, while excessive deformation easily leads to edge cracks and microcracks in the strip. Furthermore, the dislocation configuration introduced by a single cold rolling pass is relatively simple, and rapid dislocation recovery during subsequent aging can easily lead to over-aging softening, compromising both strength and plasticity. The matching of properties is difficult to further optimize. To overcome the above limitations, two or more cold rolling processes have been attempted. Although intermediate annealing or re-solution in these processes can restore plasticity and allow for secondary large deformation, it will eliminate a large number of high-density dislocations introduced in the previous process, thus losing the dislocation strengthening and nucleation promotion effect inherent in the T8 process. The resulting strength is often lower than that of the direct single cold rolling + aging state. On the other hand, pre-aging technology has been used to control the natural aging stability of 6xxx series alloys. Usually, pre-aging is performed immediately after solution quenching, followed by cold deformation or final aging. CN 114058885 Chinese Patent No. B discloses a 6xxx series aluminum alloy sheet and its preparation and welding methods. The alloy composition, by weight percentage, is: Mg: 0.55–0.77%; Cu: 0.50–0.65%; Mn: 0.50–0.60%; Cr: 0.20–0.25%; Si: 0.65–0.79%; Fe: 0.30–0.45%, with the balance being Al. The process route includes, sequentially: casting, homogenization treatment (holding at 550–570℃ for 6–10 h), hot rolling (540–560℃), cold rolling, solution quenching (holding at 540–580℃ for 0.5–3 min followed by water quenching), first straightening (deformation 0.4%–1.5%), pre-aging (holding at 60–80℃ for 6–10 h), and second straightening (deformation less than 0.2%). The 6xxx series aluminum alloy sheet obtained by this process has an initial yield strength lower than 170 Nm. After a 2% pre-stretch followed by baking at 185℃ for 20 min, the yield strength can reach 280 MPa. However, this technical solution involves complex alloy composition and numerous process steps, resulting in low strength of the alloy sheet. It is noteworthy that, to date, few pre-aging treatments have been placed between two cold rolling processes, failing to utilize the substructure control mechanism of pre-aging precipitates being sheared by dislocations during subsequent secondary deformation.

[0004] Existing deformation heat treatment processes have not yet established a complete process chain for synergistic strengthening through two-stage cold rolling and segmented aging. Conventional T8 single-pass cold rolling is difficult to reconcile high dislocation density and good plasticity. Intermediate annealing or resolution treatment introduced in the two-stage cold rolling route leads to a significant degradation of dislocation strengthening effect. Furthermore, pre-aging and two-stage cold rolling remain isolated from each other, failing to achieve synergistic control of dislocation structure, subgrain structure, and nano-precipitates. Developing a method for preparing high-strength and high-toughness 6xxx series aluminum alloy strips that synergistically optimizes these three aspects through a specific sequence of two-stage cold rolling and segmented aging has become an urgent technical problem to be solved in this field. Summary of the Invention

[0005] The purpose of this invention is to provide a high-strength and high-toughness 6xxx series aluminum alloy strip with two-stage cold rolling and segmented aging strengthening, and its preparation method, to solve the problem of poor strength and toughness matching in existing 6xxx series aluminum alloys.

[0006] The present invention discloses a method for preparing high-strength and high-toughness 6xxx series aluminum alloy strip through two-stage cold rolling and segmented age hardening:

[0007] Step 1: Melt 6xxx series aluminum alloy as-cast billets;

[0008] Step 2: Perform solution treatment on the 6xxx series aluminum alloy cast billet;

[0009] Step 3: Perform the first cold rolling on the solution-treated aluminum alloy;

[0010] Step 4: Perform pre-aging treatment on the aluminum alloy after the first cold rolling;

[0011] Step 5: Perform a second cold rolling on the pre-aged strip;

[0012] Step Six: Perform final aging treatment on the strip after the second cold rolling.

[0013] Furthermore, the 6xxx series aluminum alloy cast billet, by mass percentage, consists of 0.8~1.2% Mg, 0.6~0.9% Si, 0.1~0.3% Cu, 0.1~0.3% Fe, with the balance being Al and unavoidable impurities.

[0014] Furthermore, the 6xxx series aluminum alloy as-cast billet is prepared through a melting and casting process, specifically: industrial pure aluminum ingots are heated to 720~750℃ to melt, and Al-50Mg, Al-20Si, Al-50Cu and Al-10Fe master alloys are added sequentially. After complete melting, the mixture is stirred, degassed, and slag is removed to obtain an alloy melt. Al-5Ti-1B grain refiner, accounting for 0.2% of its total mass, is added to the alloy melt and allowed to stand for 10~20 minutes. Subsequently, it is poured into a metal mold preheated to 200℃ to obtain the 6xxx series aluminum alloy as-cast billet.

[0015] Furthermore, the solution treatment in step two is carried out at a temperature of 530~540℃ for 3~6 hours, followed by water quenching.

[0016] Furthermore, in step three, the deformation amount of the first cold rolling is 50% to 60%.

[0017] Furthermore, the pre-aging treatment in step four is carried out at a temperature of 150~170℃ for 10~20 minutes, followed by water quenching.

[0018] Furthermore, in step five, the second cold rolling is performed at room temperature or at low temperature, with a deformation of 10% to 20%.

[0019] Furthermore, the cryogenic rolling process involves immersing the strip in liquid nitrogen for 10-20 minutes before rolling.

[0020] Furthermore, the final aging treatment in step six is ​​carried out at a temperature of 150~170℃ for 30~50 minutes, followed by water quenching.

[0021] The high-strength and high-toughness 6xxx series aluminum alloy strip prepared by the method of the present invention has a hardness ≥130HV, tensile strength ≥420MPa, yield strength ≥365MPa, and elongation after fracture ≥6.5%.

[0022] The alloy composition of this invention, by mass percentage, is: Mg 0.8~1.2%, Si 0.6~0.9%, Cu 0.1~0.3%, Fe 0.1~0.3%, with the balance being Al and unavoidable impurities. The addition of Cu can accelerate the age hardening response, while an appropriate amount of Fe, by forming a fine Al3Fe phase, can effectively inhibit recrystallization grain growth and improve the fracture toughness of the strip.

[0023] The technical principle and beneficial effects of the preparation method of the high-strength and high-toughness aluminum alloy strip of the present invention are as follows:

[0024] The 6xxx series aluminum alloy strip obtained through a two-stage cold rolling and segmented aging strengthening process exhibits the following microstructure characteristics: a deformed grain matrix with high dislocation density, uniformly distributed high-density β″ strengthening phases within the grains, and dispersed Fe-containing heat-resistant phases at the grain boundaries. This strip possesses a hardness ≥130HV, tensile strength ≥420MPa, yield strength ≥365MPa, and elongation after fracture ≥6.5%. The process demonstrates strong robustness; low-temperature rolling can be achieved using conventional equipment; pre-aging eliminates sensitivity to natural aging, resulting in high batch stability and suitability for industrial production. This invention, through a two-stage cold rolling and segmented aging strengthening control strategy, organically integrates deformation strengthening, precipitation strengthening, and dislocation substructure strengthening, breaking the traditional constraint between strength and plasticity. It has broad application prospects in fields requiring high material strength and toughness, such as aerospace, rail transportation, and automotive lightweighting. Attached Figure Description

[0025] Figure 1 This is a process flow diagram for preparing a high-strength and high-toughness aluminum alloy according to the present invention;

[0026] Figure 2 The images show the microstructure of the cast aluminum alloy prepared according to the present invention; wherein, (a) is the microstructure of the cast aluminum alloy prepared in Example 1, and (b) is the microstructure of the cast aluminum alloy prepared in Example 2.

[0027] Figure 3 The images show scanning electron microscope (SEM) images of the microstructure of the aluminum alloy prepared in Example 1 of this invention after solution aging without double-stage cold rolling; (a) is a 500x SEM image; (b) is a 1000x SEM image.

[0028] Figure 4 The images show the microstructure of the high-strength and high-toughness aluminum alloy prepared in Example 2 of this invention; (a) is a black and white microstructure image, and (b) is a color microstructure image.

[0029] Figure 5 This is a transmission electron microscope (TEM) image of the precipitated phase of the high-strength and high-toughness aluminum alloy prepared in Example 3 of the present invention.

[0030] Figure 6 The images show the microstructure of the high-strength and high-toughness aluminum alloy prepared in Example 3 of this invention; (a) is a black and white microstructure image, and (b) is a color microstructure image.

[0031] Figure 7 This is a transmission electron microscope (TEM) image of the precipitated phase of the high-strength and high-toughness aluminum alloy prepared in Example 4 of the present invention.

[0032] Figure 8This is a comparison chart of the tensile properties of the aluminum alloy prepared in Example 4 of the present invention and the aluminum alloy prepared by the same composition but different processes; the blue curve in the figure is the room temperature tensile stress-strain curve of the aluminum alloy prepared by the method of Example 4; the red curve in the figure is the room temperature tensile stress-strain curve of the aluminum alloy prepared by solution treatment-cold rolling-pre-aging-final aging treatment (i.e., without double-stage cold rolling); the black curve in the figure is the room temperature tensile stress-strain curve of the cast aluminum alloy prepared by melting only. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the spirit of the contents disclosed in the present invention will be described in detail below. After understanding the embodiments of the present invention, any person skilled in the art can make changes and modifications based on the technology taught in the present invention without departing from the spirit and scope of the present invention.

[0034] The illustrative embodiments and descriptions of the present invention are used to explain the present invention, but are not intended to limit the present invention.

[0035] This invention provides a method for preparing high-strength and high-toughness 6xxx series aluminum alloy strip through two-stage cold rolling and segmented age hardening. Figure 1 This is a flow chart of the preparation process for this aluminum alloy. The main alloying elements in this aluminum alloy, by mass fraction, are: Mg 0.8%–1.2%, Si 0.6%–0.9%, Cu 0.1%–0.3%, Fe 0.1%–0.3%, with the balance being Al and unavoidable impurities. The preparation method includes the following steps:

[0036] S1, Alloy casting:

[0037] The alloy is prepared according to the target composition of 0.8%–1.2% Mg, 0.6%–0.9% Si, 0.1%–0.3% Cu, 0.1%–0.3% Fe, with the balance being Al, and the Mg / Si mass ratio is controlled at 1:1.1–1.6. Industrial pure aluminum ingots are heated to 720–750℃ to melt, followed by the sequential addition of Al-50Mg, Al-20Si, Al-50Cu, and Al-10Fe master alloys. After complete melting, the mixture is stirred, degassed, and slag removed. 0.2% of the total melt mass of Al-5Ti-1B grain refiner is added, and the mixture is allowed to stand for 10–20 minutes. The alloy melt is then poured into a metal mold preheated to 200℃ to obtain a cast slab. Preheating the mold to 200℃ helps control the cooling rate, thereby obtaining a fine and uniform initial grain structure.

[0038] S2. Solution treatment and quenching:

[0039] The as-cast slab obtained in step S1 is heated to 530-540°C and held for 3-6 hours to allow the soluble second phase to fully dissolve. Then, it is quenched in water to obtain a supersaturated solid solution.

[0040] It should be noted that the solution treatment temperature of 530–540℃ is selected based on the typical solution treatment window of 6xxx series aluminum alloys. If the temperature is too low (<530℃) or the holding time is too short (<3 hours), the strengthening phases such as Mg2Si will not dissolve sufficiently, resulting in insufficient driving force for subsequent aging precipitation and reduced strength. If the temperature is too high (>540℃) or the holding time is too long (>6 hours), it can easily lead to grain coarsening or even overheating, significantly deteriorating mechanical properties. This invention has determined through extensive experiments that 3–6 hours is the optimal holding time window, which can ensure sufficient solution treatment while maintaining a finer grain structure.

[0041] S3, First Stage Cold Rolling:

[0042] The slab after solution quenching in step S2 is subjected to room temperature cold rolling, with the total reduction rate controlled at 50%–60%. In this stage, the high strain strongly breaks down coarse dendrites and residual phases, accumulating high-density dislocations, which provide a large number of heterogeneous nucleation sites for subsequent aging precipitation.

[0043] Extensive experiments have demonstrated that a cold rolling reduction rate of 50%–60% can introduce sufficient deformation energy storage. If the reduction rate is below 50%, the dislocation density is insufficient, resulting in reduced dispersion and volume fraction of precipitated phases during subsequent aging, thus weakening the strengthening effect. If the reduction rate is above 60%, cracks are prone to occur at the slab edges, and excessive deformation energy storage may lead to uneven recrystallization during subsequent aging, which in turn impairs the strength-toughness balance.

[0044] S4. Pre-aging treatment:

[0045] The strip that has undergone the first stage of cold rolling is pre-aged at 150–170°C for 10–20 minutes. This step induces the formation of high-density, uniformly distributed GP regions and some fine β″ phase pre-precipitation nuclei. This pre-aging design can effectively suppress the room temperature stagnation effect caused by natural aging, avoid excessive decrease in plasticity during subsequent deformation, and provide a composite strengthening basis for subsequent deformation, so that the precipitated phase and the deformed substructure can work synergistically.

[0046] If the pre-aging temperature is below 150℃ or the time is less than 10 minutes, the GP zone will not form sufficiently, making it difficult to effectively suppress natural aging and resulting in significant plastic damage. If the temperature is above 170℃ or the time exceeds 20 minutes, coarse β″ phases may form prematurely, consuming solute atoms, reducing the strengthening potential of final aging, and reducing the deformation uniformity of secondary cold rolling.

[0047] S5, Second Stage Cold Rolling:

[0048] The strip after pre-aging in step S4 undergoes a second cold rolling process with a reduction rate of 10%–20%. This stage of rolling can be performed at room temperature, or preferably in a cryogenic environment of -196℃ to -150℃. The cryogenic environment can be achieved using a liquid nitrogen evaporation device, effectively suppressing dynamic recovery, further increasing dislocation density, and promoting deformation uniformity. During cryogenic deformation, the pre-precipitated GP region / fine β″ phase interacts strongly with the high-density dislocations, constructing a stable substructure of "pre-precipitated + dislocation cell," providing an ideal precipitation substrate for final aging.

[0049] Extensive experimental verification has shown that a second-stage cold rolling reduction rate of 10%–20% can further increase dislocation density and refine substructure, while simultaneously achieving optimal synergy with the pre-precipitated phase. If the reduction rate is below 10%, dislocation multiplication is limited, and the incremental increase in final strengthening is not significant; if the reduction rate is above 20%, work hardening is severe, the strip's plasticity reserve is excessively depleted, elongation decreases significantly, and high reduction rates under cryogenic conditions easily induce microcracks. The preferred cryogenic rolling temperature is -196℃ to -150℃. Within this temperature range, the stacking fault energy of aluminum alloys decreases, dislocation cross-slip is difficult, and a uniform planar slip dislocation structure is easily formed, which is beneficial for the uniform and dispersed precipitation of strengthening phases along dislocation lines during subsequent aging.

[0050] S6. Final Timeliness Processing:

[0051] The strip after the secondary rolling in step S5 is held at 150–170°C for 30–50 minutes to complete the final aging. This stage promotes the further transformation of the GP region formed by pre-aging into the β″ phase, and with the help of high-density dislocations as a fast diffusion channel, a large number of dispersed and fine β″ strengthening phases preferentially precipitate on the dislocation lines, giving full play to the synergistic effect of precipitation strengthening and dislocation strengthening, so as to obtain high strength while retaining good plasticity.

[0052] The embodiments of this invention employ a final aging regime of 150–170°C for 30–50 minutes, based on the matching of the peak aging characteristics of 6xxx series aluminum alloys with the pre-deformed microstructure. If the aging temperature is too low or the time is too short, insufficient precipitation of the β″ phase will result in incomplete strength potential; if the aging temperature is too high or the time is too long, the β″ phase is prone to coarsening and transformation into β′ or β phase, leading to a decrease in strength and no significant improvement in elongation. A 30–50 minute window achieves the optimal balance between strength and plasticity.

[0053] More preferably, the second stage of cold rolling in step S5 can be carried out by multi-pass rolling, with a reduction rate of 5% to 10% per pass. If deep cold rolling is carried out between passes, the strip needs to be re-immersed in liquid nitrogen for 10 to 20 minutes to ensure uniform core temperature and avoid dynamic recovery and uneven local deformation caused by adiabatic temperature rise.

[0054] It should be noted that the smelting, casting, rolling, solution treatment, quenching, and aging operations described in the embodiments of the present invention can all be performed using conventional equipment in the field of metal processing. For example, smelting is carried out in a pit-type resistance furnace or a medium-frequency induction furnace, rolling is carried out on a two-roll or four-roll experimental rolling mill, solution treatment and aging are carried out in a heat treatment furnace with forced air circulation, and the cryogenic environment is achieved through a liquid nitrogen container and a temperature control device.

[0055] This invention utilizes precise control of the Mg / Si ratio, the synergistic effect of trace amounts of Cu and Fe, and a coupled two-stage cold rolling and segmented aging process—solution treatment, first-stage high-reduction cold rolling, pre-aging, second-stage cold rolling (room temperature or cryogenic), and final aging—to fully activate the synergistic effects of precipitation strengthening, dislocation strengthening, and fine-grain / substructure strengthening. This results in 6xxx series aluminum alloy strips possessing high yield strength, high tensile strength, and excellent plasticity. The room temperature mechanical properties of the obtained aluminum alloy strips are: hardness ≥130HV, ≥420MPa, yield strength ≥365MPa, and elongation after fracture ≥6.5%. This makes them suitable for applications such as automotive body panels where lightweighting and a high-formability-high-strength balance are critical.

[0056] The features and performance of the present invention will be further described in detail below with reference to specific embodiments and comparative examples.

[0057] The present invention will be further described in detail below through specific embodiments, but the implementation of the present invention is not limited thereto.

[0058] Example 1:

[0059] This embodiment provides a high-strength and high-toughness 6xxx series aluminum alloy strip, whose alloy composition (mass fraction) is: Mg 1.0%, Si 0.75%, Cu 0.2%, Fe 0.2%, with the balance being Al and unavoidable impurities, and the Mg / Si mass ratio is 1.33.

[0060] This embodiment also provides a method for preparing the high-strength and high-toughness aluminum alloy strip: According to the above-mentioned composition, industrial pure aluminum ingots are heated and melted, and Al-50Mg, Al-20Si, Al-50Cu, and Al-10Fe master alloys are added sequentially. After complete melting, the mixture is stirred, degassed, and slag removed. When the melt temperature reaches 730℃, 0.2% of the total melt mass of Al-5Ti-1B grain refiner is added. The mixture is allowed to stand for 15 minutes, and then cast into a preheated metal mold at 200℃ to obtain a cast slab. The cast slab is heated to 535℃ and held for 4 hours for solution treatment, followed by water quenching. The solution-quenched slab undergoes a first-stage cold rolling with a total reduction of 55%, completed at room temperature. The cold-rolled strip is pre-aged at 160℃ for 15 minutes. A second-stage cold rolling is then performed at room temperature with a reduction of 15%. Finally, the second-stage cold-rolled strip is aged at 160℃ for 40 minutes and then water-quenched to obtain the target strip.

[0061] Figure 2 (a) shows the as-cast microstructure of the aluminum alloy, which exhibits a columnar crystal morphology with a well-developed dendritic network. Figure 3 The images show scanning electron microscope (SEM) images of the microstructure of aluminum alloys with the same composition, obtained by solution aging without double-stage cold rolling in this embodiment, at different magnifications (left and right figures). Because the samples did not undergo double-stage cold rolling pretreatment, the as-cast structure was not completely eliminated after solution aging. Room temperature mechanical property testing showed that the high-strength and tough aluminum alloy strip prepared in this embodiment had a hardness of 134 HV, a tensile strength of 435 MPa, a yield strength of 365 MPa, and an elongation after fracture of 7.1%.

[0062] Example 2:

[0063] This embodiment provides a high-strength and high-toughness 6xxx series aluminum alloy strip, whose alloy composition (mass fraction) is: Mg 1.1%, Si 0.8%, Cu 0.25%, Fe 0.15%, with the balance being Al and unavoidable impurities, and the Mg / Si mass ratio is 1.375.

[0064] The preparation method in this embodiment is as follows: The raw materials are mixed and smelted according to the same process as in Example 1. The melt is cast into a metal mold at 200°C to obtain a cast slab. The slab is heated to 540°C and held for 3 hours for solution treatment, followed by water quenching. A first-stage cold rolling process is performed with a total reduction of 60%. This is followed by pre-aging at 150°C for 20 minutes. A second-stage cold rolling process is then performed using deep cryogenic rolling in a liquid nitrogen environment, with a total reduction of 10%, rolled in two passes with a reduction of 5% per pass. Between passes, the strip is re-immersed in liquid nitrogen for 10 minutes to ensure temperature uniformity. Finally, it is aged at 170°C for 30 minutes and then water quenched.

[0065] Figure 2 (b) shows the microstructure of the as-cast alloy in this embodiment, which is mainly composed of a dendritic α-Al matrix and a second phase. Figure 4 The images shown are photographs of the final microstructure of the as-cast alloy in this embodiment (black and white and color images, respectively). After two-stage cold rolling and segmented aging strengthening treatment, the microstructure exhibits a significantly elongated fibrous deformed structure along the rolling direction. The room temperature mechanical properties of the high-strength and high-toughness aluminum alloy strip obtained in this embodiment are as follows: hardness 131HV, tensile strength 427MPa, yield strength 368MPa, and elongation after fracture 7.6%.

[0066] Example 3:

[0067] This embodiment provides a high-strength and high-toughness 6xxx series aluminum alloy strip, whose alloy composition (mass fraction) is: Mg 0.9%, Si 0.7%, Cu 0.15%, Fe 0.25%, with the balance being Al and unavoidable impurities, and the Mg / Si mass ratio is 1.286.

[0068] The preparation method in this embodiment is as follows: According to the above-mentioned ingredient ratio, industrial pure aluminum ingots are heated and melted. Al-50Mg, Al-20Si, Al-50Cu, and Al-10Fe master alloys are added sequentially. After complete melting, the mixture is stirred, degassed, and slag removed. When the melt temperature reaches 740℃, 0.2% of the total melt mass of Al-5Ti-1B grain refiner is added. The mixture is allowed to stand for 20 minutes, then cast into a preheated metal mold at 200℃. The mold temperature is 200℃, resulting in a cast slab. Solution treatment is performed at 530℃ for 6 hours, followed by water quenching. The first stage cold rolling reduction is 50%. Pre-aging is carried out at 170℃ for 10 minutes. Then, the second stage room temperature cold rolling is performed with a reduction of 20%. Finally, aging is carried out at 150℃ for 50 minutes, followed by water quenching.

[0069] Figure 5 This is a transmission electron microscope (TEM) image of the high-strength and tough aluminum alloy prepared in Example 3 of the present invention. As can be seen from the image, nanoscale rod-shaped precipitates of strengthening phase are uniformly dispersed in the alloy matrix. Figure 6 The images show the microstructure of the aluminum alloy (black and white and color images, respectively). The grains in the microstructure are significantly elongated along the rolling direction, exhibiting a fibrous deformed structure. The room temperature mechanical properties of the high-strength and high-toughness aluminum alloy strip prepared in this embodiment are as follows: hardness 135 HV, tensile strength 442 MPa, yield strength 370 MPa, and elongation after fracture 6.8%. Compared with other treatment methods, the strength is significantly improved.

[0070] Example 4:

[0071] This embodiment provides a high-strength and high-toughness 6xxx series aluminum alloy strip, whose alloy composition (mass fraction) is: Mg 1.2%, Si 0.9%, Cu 0.3%, Fe 0.3%, with the balance being Al and unavoidable impurities, and the Mg / Si mass ratio is 1.333.

[0072] The preparation method in this embodiment is as follows: The components are mixed and smelted according to the same process as in Example 1. The difference between this embodiment and Example 1 is that the resulting casting undergoes solution treatment at 535℃ for 5 hours and is then water-quenched. The first stage cold rolling reduction is 58%; pre-aging at 165℃ for 15 minutes; followed by a second stage cold rolling at -196℃ with a reduction of 15%, multi-pass rolling, with a reduction of 5% per pass, and liquid nitrogen immersion for 15 minutes between passes. Finally, aging at 165℃ for 45 minutes is followed by water quenching.

[0073] The transmission electron microscopy morphology of the aluminum alloy precipitates prepared in this embodiment is as follows: Figure 7 As shown, diffusely distributed rod-shaped precipitates and tortuous dislocation lines are visible, and the dislocations are effectively pinned by the precipitates. Figure 8The tensile properties of aluminum alloys obtained by Example 4 of this invention and those obtained by different processes were compared. The results showed that the tensile properties of the aluminum alloy obtained by the two-stage cold rolling combined with segmented aging process were significantly better than those obtained by other processes. The room temperature mechanical properties of this high-strength and tough aluminum alloy strip are as follows: hardness reaches 138HV, tensile strength is 450MPa, yield strength reaches 380MPa, and elongation after fracture is 6.5%.

[0074] To better verify the beneficial effects of the present invention, the following comparative examples are provided:

[0075] Scale settings instructions:

[0076] Comparative Example 1:

[0077] This comparative example provides a 6xxx series aluminum alloy with the same alloy composition as Example 1 (Mg 1.0%, Si 0.75%, Cu 0.2%, Fe 0.2%).

[0078] The difference between this comparative example and Example 1 is that after melting and casting according to the composition of Example 1, no solution treatment, first-stage cold rolling, pre-aging, second-stage cold rolling and final aging treatment are performed.

[0079] The room temperature mechanical properties of the obtained aluminum alloy are as follows: hardness 69HV, tensile strength only 208 MPa, yield strength 135 MPa, and elongation after fracture 7.6%. It can be seen that due to the lack of cold rolling deformation and graded aging, the strength is much lower than that of the example.

[0080] Comparative Example 2:

[0081] This comparative example provides a 6xxx series aluminum alloy with the same alloy composition as Example 2 (Mg 1.1%, Si 0.8%, Cu 0.25%, Fe 0.15%).

[0082] The difference between this comparative example and Example 2 is that the second stage of cold rolling is not performed. The specific steps are as follows: after melting and casting, solution quenching is performed at 540℃ for 3 hours, followed by a first stage of cold rolling with a reduction rate of 60%, then pre-aging at 150℃ for 20 minutes, and then direct final aging at 170℃ for 30 minutes, without a second stage of cold rolling.

[0083] The room temperature mechanical properties test results of the obtained aluminum alloy showed that its hardness reached 123HV, tensile strength was 374 MPa, yield strength was 276 MPa, and elongation after fracture was 6.2%. Compared with Example 2, the strength decreased significantly.

[0084] Comparative Example 3:

[0085] This comparative example provides a 6xxx series aluminum alloy strip with the following alloy composition (mass fraction): Mg 1.4%, Si 0.8%, Cu 0.2%, Fe 0.2%, with the balance being Al and unavoidable impurities, and the Mg / Si mass ratio being 1.75.

[0086] The preparation steps of this comparative casting are exactly the same as those of Example 1, except that: after melting and casting, the casting is solution-quenched at 535℃ for 4 hours, followed by a first-stage cold rolling with a reduction rate of 55%, then pre-aged at 160℃ for 15 minutes, followed by a second-stage room-temperature cold rolling with a reduction rate of 15%, and finally aging at 160℃ for 40 minutes.

[0087] The room temperature mechanical property test results of the obtained aluminum alloy showed that its hardness reached 134HV, tensile strength was 430MPa, and yield strength was 363MPa, but the elongation after fracture was only 3.2%, which was lower than the plasticity requirement of 6.5%. Although an excessively high Mg / Si ratio can maintain high strength, it seriously impairs plasticity and processing stability, and cannot meet the requirement of high strength and toughness matching.

[0088] Table 1 Summary of room temperature mechanical properties comparison between the examples and comparative examples

[0089]

[0090] The results above show that:

[0091] The hardness of Examples 1-4 of this invention is ≥130HV, tensile strength is ≥420MPa, yield strength is ≥365MPa, and elongation is ≥6.5%. While maintaining good plasticity, it achieves a strength level significantly higher than conventional processes. Combining the results of Examples 1-4 and Comparative Examples 1-3, it can be seen that the two-stage cold rolling and segmented aging strengthening process of this invention, through alternating deformation and aging, precisely controls the synergistic evolution of dislocation configuration and precipitate distribution, achieving a significant increase in strength while maintaining good plasticity of 6xxx series aluminum alloys. Specifically, the first cold rolling provides initial deformation energy storage; pre-aging pre-precipitates fine, dispersed metastable phases between two rolling passes to pin dislocations and guide subsequent precipitation; the second cold rolling further multiplies dislocations and breaks up coarse phases; and the final aging fully utilizes the coupling effect of precipitation strengthening and dislocation strengthening, ultimately obtaining aluminum alloy strips with both high strength and good plasticity, demonstrating outstanding technological advancement and industrial application value.

Claims

1. A method for preparing high-strength and high-toughness 6xxx series aluminum alloy strip through two-stage cold rolling and segmented age hardening, characterized in that, The preparation method is carried out according to the following steps: Step 1: Melting 6xxx series aluminum alloy as-cast billets; the 6xxx series aluminum alloy as-cast billets, by mass percentage, consist of 0.8~1.2% Mg, 0.6~0.9% Si, 0.1~0.3% Cu, 0.1~0.3% Fe, with the balance being Al and unavoidable impurities; wherein the Mg / Si mass ratio is 1.33, 1.375, 1.286, or 1.333; Step 2: Perform solution treatment on the 6xxx series aluminum alloy cast billet; Step 3: Perform the first cold rolling on the solution-treated aluminum alloy. The first cold rolling is at room temperature, and the deformation is 50%~60%. Step 4: Perform pre-aging treatment on the aluminum alloy after the first cold rolling. The temperature of the pre-aging treatment is 150~170℃ and the holding time is 10~20 minutes, followed by water quenching. Step 5: Perform a second cold rolling on the pre-aged strip. The second cold rolling is either room temperature cold rolling or low temperature deep cold rolling, with a deformation of 10%~20%. Step 6: Perform final aging treatment on the strip after the second cold rolling. The temperature of the final aging treatment is 150~170℃, the holding time is 30~50 minutes, and then water quenching is performed.

2. The method for preparing high-strength and high-toughness 6xxx series aluminum alloy strip according to claim 1, characterized in that, The 6xxx series aluminum alloy as-cast billet mentioned in step one is prepared through a melting and casting process. Specifically, industrial pure aluminum ingots are heated to 720~750℃ to melt, and Al-50Mg, Al-20Si, Al-50Cu and Al-10Fe master alloys are added sequentially. After complete melting, the mixture is stirred, degassed and slag removed to obtain an alloy melt. Al-5Ti-1B grain refiner, accounting for 0.2% of its total mass, is added to the alloy melt and allowed to stand for 10~20 minutes. Then, it is poured into a metal mold preheated to 200℃ to obtain the 6xxx series aluminum alloy as-cast billet.

3. The method for preparing high-strength and high-toughness 6xxx series aluminum alloy strip according to claim 1, characterized in that, The solution treatment in step two is performed at a temperature of 530~540℃ for 3~6 hours, followed by water quenching.

4. The method for preparing high-strength and high-toughness 6xxx series aluminum alloy strip according to claim 1, characterized in that, The cryogenic rolling process involves immersing the strip in liquid nitrogen for 10-20 minutes before rolling.

5. The high-strength and high-toughness 6xxx series aluminum alloy strip prepared by the preparation method according to any one of claims 1 to 4, characterized in that, The high-strength and tough 6xxx series aluminum alloy strip has a hardness ≥130HV, tensile strength ≥420MPa, yield strength ≥365MPa, and elongation after fracture ≥6.5%.

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