【作者】 陈永禄；

【导师】 陈文哲； 傅高升；

【作者基本信息】 福州大学 ， 材料学， 2006， 博士

【摘要】 针对目前我国铝材生产与研究中对内在冶金缺陷的影响作用重视不足、液态(熔体)处理与各种固态处理均是按各自系统独立进行研究的现状,作者结合本课题组的前期研究基础,以具有实用价值且量大面广的易拉罐用铝材(Al-Mn-Mg合金)作为研究对象,提出从改善铝材冶金质量这一关键技术入手,将铝材的处理状态(熔体处理和均匀化)与热变形加工有机地结合起来进行研究的新思路。借助光学显微镜(OM)、扫描电镜(SEM)、透射电镜(TEM)、能谱仪(EDAX)、X射线衍射(X-Ray)、电子拉伸机等先进的分析测试手段,系统深入地研究熔体处理与均匀化退火工艺对易拉罐用铝材微观组织与力学性能的影响规律,在此基础上采用动态热/力模拟试验技术,全面深入地探讨了易拉罐用铝材的熔体处理、热处理工艺、热塑性变形行为三者之间的相互关系,深化了对铝熔体处理的基本理论、冶金缺陷的作用机理以及铝材高温塑性变形理论的认识,对进一步挖掘易拉罐用铝材的性能潜力具有重要意义,可为实际热轧工艺的确定和优化提供试验依据与理论指导。主要研究结果如下: 1.以自行开发的高效排杂熔剂净化为主的铝熔体综合处理工艺显著改善了易拉罐用铝材的冶金质量。该工艺在提高铝基体纯净度、细化结晶组织以及改善析出相形态等方面均起到关键的作用。合适的铝熔体综合处理可有效减少材料中裂纹萌生源的数量,改变易拉罐用铝材在铸态下的断裂方式,从而显著提高该铝材的铸态力学性能(尤其是塑性)。2.揭示了均匀化退火对易拉罐用铝材析出相形态的影响规律,同时发现,均匀化退火难以消除铸锭原始组织中杂气等冶金缺陷,对经常规熔体处理的易拉罐用铝材的力学性能与断裂方式影响不大。然而,试验条件下的均匀化优化工艺表明:铝熔体综合处理有利于降低均匀化处理过程中的能耗,与该类材料常用的均匀化工艺相比,在温度降低100~120K、保温时间减少4~12h的条件下即可获得较为理想的组织与性能。3.热模拟试验研究表明易拉罐用铝材属负温度敏感性和正应变速率敏感性材料。在高温热变形条件下,易表现出明显的动态软化并最终进入稳态变形,且均匀化态铝材相对铸态铝材具有更明显的动态软化曲线特征。当T = 573~773K、ε&≤5. 0s?1时,σ~ε曲线以单峰值的连续动态再结晶曲线特征为主; 而当T = 673K、ε& = 10. 0s?1时,则表现为多峰值的非连续动态再结晶曲线特征。热变形条件对流变应力的影响大于处理状态(熔体处理和均匀化)的影响,低Z值(升高T或降低ε& )的热变形条件削弱了处理状态对铝材热变形性能的影响。回归分析表明,易拉罐用铝材的高温流变应力与应变速率、变形温度及热变形激活能之间满足用双曲正弦函数形式修正的Arrhenius关系,所建立的关系式进一步证明易拉罐用铝材的热变形是受热激活控制的塑性变形过程。更多还原

【Abstract】 Aiming at the current existing problems of study and fabrication of national aluminum sheet used for easy-open can without being pay more attention to the effects of intrinsic metallurgical defects, a new idea to link organically with hot deformation process in the study of the aluminum sheet used for easy-open can has been put forward by author. Influence of melt-treatment and homogenization on microstructure and mechanical properties has been studied systematically and deeply with the aids of some modern analytical and testing techniques, such as OM, SEM, TEM, EDAX, XRD and INSTRON material testing machine, etc. Moreover, hot deformation behavior has been fully studied by adopting dynamic thermal/mechanical simulation technique, and their relationships with melt-treatments and heat treatment process have been well discussed. The results can deepen the understanding of basic theories of aluminum melt-treatment, influence of metallurgical defects and theory of hot deformation. It is far reaching importance of excavating the potential properties of the sheet, which provides the experimental and the theory base for determination and optimization of actual hot rolling. The main conclusions are as follows: 1. The synthesis technology of aluminum melt-treatment which gives priority to the purification of using the high-efficient removing-inclusion flux self-developed can effectively improved the metallurgy quality of the aluminum sheet used for easy-open can. The key role is on enhancing the purification of aluminum matrix, grain refinement and the modification of precipitating phases Also, it can reduce effectively the quantity of crack initiation source in the material and change the fracture form of the sheet and then improve the mechanical properties (especially the plasticity). 2. The influence rule of homogenization on the precipitating phase’s shapes of the aluminum sheet used for easy-open can has been revealed in this paper. The results show that the metallurgical defects, such as inclusions and gas pores, cannot be eliminated by homogenization, so there is little influence on the mechanical properties and fracture form of the aluminum sheet prepared by routine melt-treatment technology. However, the synthesis technology of aluminum melt-treatment is in favor of reducing the energy consumption during homogenization. The ideal microstructure and the properties can be obtained under the condition of temperature reduction 100~120K and holding time decrease 4~12 hours comparing with the common technology used for this material. 3. The results of thermal simulation tests indicate that the aluminum sheet used for easy-open can is a kind of material with the negative temperature sensitivity and the positive strain rate sensitivity. Under the condition of hot deformation at elevated temperature, it is easy to occur dynamic softening in the sheet and reaches the steady-state deformation stage finally. The softening effect of the homogenized materials’ is more obvious than that of the as-cast material. Under the hot deformation condition of T=573~773K and ε& ≤ 5. 0s?1, the feature of σ ~ ε curves is primarily the continuous dynamic recrystallization of single peak; while the curves’ feature of multi-peak is discontinuous dynamic recrystallization under the condition of T=673K and ε& = 10. 0s?1. Effect of hot deformation condition on the flow stress is greater than that of the processing states such as melt-treatment and homogenization. The influence of processing states on the hot deformation properties of aluminum sheet can be weakened under the condition of low Z value (increasing T or reducing ε& ). The regression results show that the relationship of flow stress of the sheet with deformation temperature and strain rate can be expressed more suitably with the Arrhenius equation modified by hyperbolic sine function, which means that the hot deformation of the sheet is the plastic deformation controlled by thermal activation. 4. Multivariate regression method has been successfully adopted to solve the hot deformation material constants (Q ,m ,n ,α and A ). The results indicate that this method is much more simple and accurate compared with the conventional graphical construction or iteration. The values of the material constants have been disclosed by incorporating the hot deformation condition (including the strain) and processing states of the sheet, and the more explicit and reasonable explanation in the physics significance of the constants has been given. The results show that the synthesis technology of high-efficient aluminum melt-treatment (particularly the purification) plays the key role on reducing the hot deformation activation energy of the sheet. The tiny circle-entire and uniformly distributing inclusions and precipitating phases as well as the even diffusion solute atoms can weaken the hindrance effect on the dislocation’s movement, which is in favor of the hot deformation. 5. The flow stress equations of the aluminum sheets deformed at elevated temperature have been constructed on the basis of Laasraoui-Jonas peak-front stress model and JMA recrystallization kinetic model by considering fully the softening mechanism of hot deformation before and after the peak stress, incorporating the activation energies of each deformation stage can determine the corresponding Z parameter values (Z0.2, Zp and Zs). The fitting curves agree with the actual curves very well, which can be used to describe accurately the relationship between flow stress of the aluminum sheet at elevated temperature and the hotdeformation condition. 6. Effects of melt-treatment and homogenization on the hot deformation microstructure of the sheets have been disclosed. Firstly, it is pointed out that both melt-treatment and homogenization have the promoting effect to improve the morphology of hot deformation grains, but the effect of latter process depends on the former. Secondly, the dynamic softening mechanism of the sheet has been reasonably explained. It is indicated that the nucleating mechanism of dynamic recrystallization of the sheet is primarily the subgrain congregation, and the subgrains’ growth also plays a role to the extent. Additionally, the other recrystallization nucleating modes are also observed, that is, bowing by the primitive grain boundary and geometry dynamic recrystallization that occurs at the high strain rate. Thirdly, the influence of metallurgical defects on dynamic softening of the sheets has been fully analyzed. It is pointed out by observing the microstructure feature that the original grain boundary (the edge of three grains), dislocation pile section and surface of massive inclusions or precipitated phases all can become the preferential nucleating position of dynamic recrystallization, the grain size before hot deformation influences relatively greater on the nucleating ratio (N) of dynamic recrystallization, and the quantity and existing morphology of the precipitating phase particles and solute atoms influence much more on the rate of the growth of grain nucleus (G). Finally, the prediction models have been created by regression, which can clearly reflect the relationship of the average size of recrystallization grains and hot deformation condition (Z parameter) or the flow stress at steady-state stage. Moreover, the critical condition at which the ideal dynamic recrystallization structure of the sheet is obtained has been established by incorporating the feature of σ ~ ε curves and microstructure observation. The results are useful for counting load, establishing and optimizing the hot working process parameter during the hot working production. 7. It is indicated by hot rolling simulation that the enhancement effect of prolonging the interval time on softening is far weaker than increasing temperature. When the strain (50%) remains constant, the stress level of the last pass is dropping gradually with increment of the pass number. Increasing the strain of the former passes properly is in favor of enhancing the strain cumulative effect of the following passes, which can obtain the fine recrystallization structure during the last interval time and reduce the inhomogeneity of hot deformation. The coexistent structure of dynamic recrystallization and the static recrystallization grains can be obtained after the multi-pass hot deformation, and the subgrains’ congregation and growth are also the primary nucleating mechanism. When the hot rolling process parameters of the sheet are achieved, that is, the temperature is 673K, the strain rate is 0.5s-1, the interval time is about1 to 3 minutes and the strain assignment is “→10%→30%→50%→water cooling”, the relatively ideal microstructure and properties can be obtained.更多还原