高强7000系铝合金中析出相协助的有效晶粒细化及其成形性
发布时间：2018-08-11 10:16
【摘要】：高强7000(Al-Zn-Mg-Cu)系铝合金可作为替代汽车钢结构件的潜在材料使用,但其受成形性与耐蚀性方面的限制而无法在汽车领域获得广泛应用。目前7000系铝合金成形性方面的研究主要集中在成形工艺的改进上而忽略了材料本身组织和性能的影响。晶粒细化能提高5000(Al-Mg)、6000(Al-Mg-Si)系铝合金及BH烘烤硬化钢等汽车材料的成形性,7000系铝合金成形性的改善必然也会受益于晶粒细化。晶粒细化可通过中间形变热处理工艺实现,其主要利用大尺寸MgZn2相在再结晶过程中的粒子诱导形核作用实现组织细化。本文利用形变诱导析出和粒子诱导形核机制研发了两套短流程的晶粒细化工艺,使高强7000系铝合金的塑性和成形性获得极大提高,并为其在汽车结构件上的应用提供了组织和工艺调控原理。 研究发现,与过时效相比,温/冷变形都明显加速固溶态7075和7050铝合金中MgZn2相的时效动力学,短时内可获得大量0.5-0.6μm均匀分布的MgZn2粒子,该粒子的析出受热力学和动力学两方面因素控制。基于此提出的两套晶粒细化工艺(W-TMT和C-TMT)均能显著缩短板材制造时间(从RI-ITMT工艺所需的8h缩短至1h内),且在总变形量相同或相近时,两套工艺均可获得与RI-ITMT工艺相似的细晶组织。 分析了不同参数对最终再结晶晶粒尺寸的影响并深入研究了再结晶机理。研究发现Mg,Zn2尺寸越大、初始晶粒尺寸越小、变形储能越大、固溶再结晶升温速率越快均能明显促使形成细晶组织,而固溶保温时间对晶粒尺寸影响较小。400℃/6s和480℃C/2s退火可使W-TMT轧板完全再结晶,且再结晶速度远高于大尺寸MgZn2的回溶速度,因此Mg,Zn2能很好的发挥粒子诱导形核作用：大尺寸MgZn2粒子强烈增加再结晶形核点数量,降低再结晶温度,缩短再结晶时间。而且,大尺寸MgZn2的引入可促使再结晶困难的7050铝合金很容易实现完全再结晶和组织细化,避免了无限增大变形储能或延长固溶再结晶时间的常规处理手段。 W-TMT工艺所获细晶7075铝合金T6态的抗拉/屈服强度与RI-ITMT细晶及传统热轧(HR)粗晶样品基本一致,而延伸率明显高于HR粗晶样品,细晶样品性能各向异性也明显改善,表明组织细化在保证高强度的同时可显著提高7075铝合金的延伸率并消除强度、塑性各向异性。同样,W-TMT和C-TMT加工的细晶7050铝合金在保证高强度的同时可实现20%以上的延伸率,远高于文献报道值。晶粒尺寸对7000系铝合金的强度影响微弱,但却显著影响其延伸率,其中晶粒尺寸为～9μm时所表现出来的最高加工硬化能力确保了此时7075和7050铝合金可获得最高延伸率 W-TMT-7075细晶样品的杯突值与RI-ITMT细晶样品相当,W-TMT细晶样品200℃时杯突值(10.7mm)远高于HR粗晶样品200℃杯突值(8.5mm)。晶粒尺寸是影响成形性的关键因素,其对成形性和延伸率的影响规律一致,且晶粒尺寸为～9μm时的成形性最好。同时,W-TMT-7075铝合金经200℃温成形+烤漆处理后可获得类似于回归再时效处理的组织和性能,因此在细化7000系铝合金晶粒的基础上采用200℃温成形+烤漆的方法可获得高强、耐腐蚀的成形样品,符合汽车结构材料对综合性能的要求。
[Abstract]:High strength 7000 (Al-Zn-Mg-Cu) aluminum alloy can be used as a potential substitute for automotive steel structural parts, but it can not be widely used in automotive field because of the limitation of formability and corrosion resistance. Grain refinement can improve the formability of 5000 (Al-Mg), 6000 (Al-Mg-Si) aluminum alloy and BH bake hardening steel, and the improvement of 7000 series aluminum alloy formability will inevitably benefit from grain refinement. In this paper, two short-process grain refinement processes have been developed by using the deformation-induced precipitation and particle-induced nucleation mechanisms, which greatly improve the plasticity and formability of 7000 series aluminum alloys and provide the principle of organization and process control for their application in automotive structural parts.
It is found that the aging kinetics of MgZn2 phase in 7075 and 7050 aluminum alloys can be accelerated obviously by warm/cold deformation compared with over-aging. A large number of MgZn2 particles with uniform distribution of 0.5-0.6 micron can be obtained in a short time. The precipitation of the particles is controlled by thermodynamics and kinetics. Two sets of grain refinement processes (W-TMT and C-TM) are proposed. T) can significantly shorten the manufacturing time of sheet metal (from 8 h to 1 h required by RI-ITMT process), and the fine grains similar to RI-ITMT process can be obtained by both processes at the same or similar total deformation.
The effect of different parameters on the final recrystallization grain size was analyzed and the recrystallization mechanism was studied. It was found that the larger the size of Mg and Zn2, the smaller the initial grain size, the larger the deformation energy storage, and the faster the rate of heating up of solid solution recrystallization, the more obvious fine grain structure was formed, while the effect of solid solution holding time on grain size was less. C/2s annealing at 480 C and 480 C can completely recrystallize the W-TMT rolled sheet, and the recrystallization rate is much higher than that of large size MgZn2. Therefore, Mg and Zn2 can play a very good role in particle-induced nucleation: large size MgZn2 particles increase the number of recrystallization nucleation points, reduce the recrystallization temperature and shorten the recrystallization time. The 7050 aluminum alloy with difficulty in recrystallization can easily achieve complete recrystallization and microstructure refinement, avoiding the conventional treatment of infinitely increasing deformation energy storage or prolonging solid solution recrystallization time.
The tensile/yield strength of T6 state of fine grain 7075 aluminum alloy obtained by W-TMT process is basically the same as that of RI-ITMT fine grain and traditional hot-rolled (HR) coarse grain samples. The elongation of T6 state is obviously higher than that of HR coarse grain samples. The anisotropy of fine grain samples is also obviously improved. The results show that the elongation of 7075 aluminum alloy can be significantly increased and eliminated while the high strength is ensured. Similarly, the fine grained 7050 aluminum alloy processed by W-TMT and C-TMT can achieve more than 20% elongation while maintaining high strength, which is much higher than the reported value. Grain size has a weak influence on the strength of 7000 series aluminum alloy, but has a significant effect on its elongation, and the grain size is the highest when the grain size is between 9 micron. The work hardening ability ensures that the maximum elongation of 7075 and 7050 aluminum alloy is obtained at this time.
The cupping value of W-TMT-7075 fine-grained sample is similar to that of RI-ITMT fine-grained sample. The cupping value (10.7 mm) of W-TMT fine-grained sample is much higher than that of HR coarse-grained sample at 200 C (8.5 mm). Grain size is the key factor affecting formability, and its effect on formability and elongation is the same. The formability of W-TMT fine-grained sample with grain size of 9 um is the best. The microstructure and properties of W-TMT-7075 aluminum alloy can be obtained by warm forming at 200 C and baking varnish treatment, which is similar to that of regression re-aging treatment. Therefore, the high strength and corrosion-resistant forming samples can be obtained by warm forming at 200 C and baking varnish on the basis of refining 7000 series aluminum alloy grains, which meets the requirements of automotive structural materials for comprehensive properties.
【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG146.21
本文编号：2176703
[Abstract]:High strength 7000 (Al-Zn-Mg-Cu) aluminum alloy can be used as a potential substitute for automotive steel structural parts, but it can not be widely used in automotive field because of the limitation of formability and corrosion resistance. Grain refinement can improve the formability of 5000 (Al-Mg), 6000 (Al-Mg-Si) aluminum alloy and BH bake hardening steel, and the improvement of 7000 series aluminum alloy formability will inevitably benefit from grain refinement. In this paper, two short-process grain refinement processes have been developed by using the deformation-induced precipitation and particle-induced nucleation mechanisms, which greatly improve the plasticity and formability of 7000 series aluminum alloys and provide the principle of organization and process control for their application in automotive structural parts.
It is found that the aging kinetics of MgZn2 phase in 7075 and 7050 aluminum alloys can be accelerated obviously by warm/cold deformation compared with over-aging. A large number of MgZn2 particles with uniform distribution of 0.5-0.6 micron can be obtained in a short time. The precipitation of the particles is controlled by thermodynamics and kinetics. Two sets of grain refinement processes (W-TMT and C-TM) are proposed. T) can significantly shorten the manufacturing time of sheet metal (from 8 h to 1 h required by RI-ITMT process), and the fine grains similar to RI-ITMT process can be obtained by both processes at the same or similar total deformation.
The effect of different parameters on the final recrystallization grain size was analyzed and the recrystallization mechanism was studied. It was found that the larger the size of Mg and Zn2, the smaller the initial grain size, the larger the deformation energy storage, and the faster the rate of heating up of solid solution recrystallization, the more obvious fine grain structure was formed, while the effect of solid solution holding time on grain size was less. C/2s annealing at 480 C and 480 C can completely recrystallize the W-TMT rolled sheet, and the recrystallization rate is much higher than that of large size MgZn2. Therefore, Mg and Zn2 can play a very good role in particle-induced nucleation: large size MgZn2 particles increase the number of recrystallization nucleation points, reduce the recrystallization temperature and shorten the recrystallization time. The 7050 aluminum alloy with difficulty in recrystallization can easily achieve complete recrystallization and microstructure refinement, avoiding the conventional treatment of infinitely increasing deformation energy storage or prolonging solid solution recrystallization time.
The tensile/yield strength of T6 state of fine grain 7075 aluminum alloy obtained by W-TMT process is basically the same as that of RI-ITMT fine grain and traditional hot-rolled (HR) coarse grain samples. The elongation of T6 state is obviously higher than that of HR coarse grain samples. The anisotropy of fine grain samples is also obviously improved. The results show that the elongation of 7075 aluminum alloy can be significantly increased and eliminated while the high strength is ensured. Similarly, the fine grained 7050 aluminum alloy processed by W-TMT and C-TMT can achieve more than 20% elongation while maintaining high strength, which is much higher than the reported value. Grain size has a weak influence on the strength of 7000 series aluminum alloy, but has a significant effect on its elongation, and the grain size is the highest when the grain size is between 9 micron. The work hardening ability ensures that the maximum elongation of 7075 and 7050 aluminum alloy is obtained at this time.
The cupping value of W-TMT-7075 fine-grained sample is similar to that of RI-ITMT fine-grained sample. The cupping value (10.7 mm) of W-TMT fine-grained sample is much higher than that of HR coarse-grained sample at 200 C (8.5 mm). Grain size is the key factor affecting formability, and its effect on formability and elongation is the same. The formability of W-TMT fine-grained sample with grain size of 9 um is the best. The microstructure and properties of W-TMT-7075 aluminum alloy can be obtained by warm forming at 200 C and baking varnish treatment, which is similar to that of regression re-aging treatment. Therefore, the high strength and corrosion-resistant forming samples can be obtained by warm forming at 200 C and baking varnish on the basis of refining 7000 series aluminum alloy grains, which meets the requirements of automotive structural materials for comprehensive properties.
【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG146.21
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