【作者】 李永志；

【导师】 薛克敏；

【作者基本信息】 合肥工业大学 ， 材料加工工程， 2012， 博士

【摘要】 大塑性变形法(SPD)是制备块体超细晶材料的一种新型塑性加工技术,而等径角挤压(ECAP)被公认为是一种发展最为迅速、极具工程应用前景的大塑性变形法。粉末冶金材料的塑性成形致密一直是材料领域研究的难点。目前,等径角挤压(ECAP)应用于粉末冶金材料的相关研究尚处于起步阶段,特别是高熔点、低塑性、难变形的粉末材料的ECAP应用还鲜见相关报道。粉末材料的塑性成形能力差、变形机理及致密机制尤为复杂,从而限制了ECAP在该领域内的研究与应用。鉴此,本文采用理论基础研究、塑性力学解析、数值模拟分析、实验研究等相结合的方法,全面深入地研究纯钼粉末材料在ECAP过程中的变形机理、致密行为,从而为ECAP工艺在该领域的研究与应用,提供必要的理论依据及借鉴。针对钼及其粉末材料的特点,研究了不同因素对纯钼粉末烧结体材料塑性压缩变形行为的影响规律。其塑性流变应力随应变速率的增加而增加,随成形温度的升高而减小；高温条件下材料软化现象占据主导优势,其流变应力较低；初始相对密度增大,材料流变应力随之逐渐增加,材料屈服强度越高,但出现破裂时间亦越早。材料硬度增加速率对温度变化不敏感,但提高温度则有利于降低其屈服强度；压缩变形时,烧结体内孔隙和颗粒形变发生双重影响,使得材料逐渐致密,但单轴压缩并不能使材料完全致密。基于粉末材料的“可压缩连续体”假设及对现有各种屈服条件式的比较分析,建构了可压缩系数g表达的纯钼粉末体刚塑性本构关系,给出了各参数间的量化关系式。合理分析了该本构关系式的应用范围,为进行粉末体材料的塑性成形研究、成形工艺的控制、模拟建模分析等提供一定借鉴作用,同时丰富和拓展了粉末冶金材料广义塑性理论。全面研究了等径角挤压(ECAP)的塑性成形力学问题。运用滑移线法、几何推导法以及上限法,解析了ECAP的成形工艺,对应力-应变场、速度场、挤压力等进行了定量分析；考虑了摩擦的影响,在三维状态下对ECAP挤压力进行了全面解析。同时,针对“背压”对ECAP工艺的影响,运用滑移线法求解了带背压的ECAP挤压力,对等径角挤压的背压问题的理论研究进行了有益尝试。建立了用于纯钼粉末体材料塑性成形的热力耦合有限元模型。基于该模型,对不同模具结构、不同初始条件下的纯钼粉末烧结体材料成形致密进行模拟研究,获得ECAP过程中挤压力、应力-应变、相对密度、温度等场量分布规律。模拟结果表明,模具内、外角对材料的剪切变形作用是成形致密的主要动因。尽管一道次挤压变形分布呈现不均匀性,但ECAP工艺对于纯钼粉末体材料具有良好的致密效果。系统地分析了不同工艺参数对纯钼粉末体材料ECAP成形致密的影响程度。适当增大摩擦可以提高变形均匀性和致密化程度；较高的成形温度可降低材料的硬化性能,降低成形压力,提高变形均匀性和致密效果,但对组织细化不利。初始相对密度较小的试样,ECAP时易于导致孔隙被压制、致密。挤压速度大,其对坯料产生挤压效应明显,使得粉末体颗粒间压挤、形变速率加大,致密效果较好,但挤压速度与挤压力几乎成正比。建议选择相对较小的初始相对密度、挤压速度、适中的摩擦因子,以保护工装模具、延长设备使用寿命。进行了纯钼粉末烧结体材料不同挤压路径的多道次模拟研究。结果表明,路径Bc经过4道次挤压后,试件四个面均受到剪切,内部变形分布均匀,是获得变形效果和致密均匀性的最佳挤压路径。C路径ECAP时,从Y向看变形分布趋于对称和均匀。经过偶数(2n)道次挤压后,路径C的形变均匀程度要明显的优于路径A；路径A的整体致密化程度要高于路径BA。阐述了几种常用的背压方式及其作用,进行了两类背压设计的ECAP工艺的模拟研究。施加背压的大小,总体上与试样成形致密成正比。背压过大,会在小的外角模具产生“死区”,所需挤压力亦越大。第一类背压设计更符合ECAP成形特征,有利于整体变形致密、均匀,应优先选择。设计了用于纯钼粉末材料等温ECAP的实验模具及加热设备,进行了粉末烧结体-包套、粉末-包套及带背压粉末烧结体-包套等三种条件下ECAP挤压实验,并与有限元模拟结果进行对比分析。1道次粉末烧结体-包套ECAP后,试样内部的变形及密度分布不均匀,除靠近底部区域材料,其余部分的变形及孔隙焊合效果较好,且试样整体平均相对密度较高,说明ECAP具有强烈的致密效果。ECAP后,主要变形区内试样显微硬度大幅提高,且不同挤压路径、挤压温度、初始相对密度等对试样所获的硬度具有不同程度的影响。多道次ECAP显微组织的扫描电镜结果亦表明,ECAP对粉末烧结体材料具有极强的焊合作用和细化效果。经过1道次挤压后,试样主要变形区孔隙大部已经闭合,经2道次挤压后,试样接近理论压实密度,同时晶粒被明显细化。1道次ECAP对纯钼粉末-包套的致密化影响较小,主要发生颗粒重排、弹塑性变形等行为；经过2道次以上的ECAP后,试样致密化程度明显提高且主变形区域近全致密。运用自制简易背压器,进行了1道次带背压ECAP实验研究。背压能增加静水压力,使材料产生较大剪切变形,促进颗粒破碎、熔融、焊合,从而提高粉末材料的成形致密效果。当使用常规直角模具进行纯钼粉末烧结体材料的ECAP时,背压合理选择范围是20-35Mpa。最后,创新性运用模糊数学的层次分析法及灰色系统理论,进行了纯钼粉末烧结体材料的ECAP工艺多目标优化设计。仿真与实验结果证明：当模具内角不变,合理选择模具外角、成形温度、初始相对密度、成形速度和摩擦因子,能使纯钼粉末体材料的ECAP塑性变形效果好、最大损伤值降低、静水压力剧增,材料显著致密且均匀。更多还原

【Abstract】 Severe plastic deformation(SPD) is a new research techniques of preparation for bulk ultrafine-grained(UFG) materials,and as a typical method of SPD, equal channel angular pressing (ECAP) has become a hot research topics related fields in material science and engineering for most rapid development and the very prospect of industrial application.Plastic forming and densification of powder metallurgy materials has been the difficulty of materials research in the field.Currently, the related study on powder material is just beginning, and there has poor plastic workability and complication of the deformation and densification mechanism for powder materials, especially there have no related reports about ECAP application with respect to the high melting point, low plasticity and hard deformed powder materials. Therefore, the deformation mechanism and the densification behavior of pure Mo powder materials in the ECAP process are a comprehensive and in-depth studied based on the method of combining basic theoretical research, the finite element numerical simulation with experimental study in this paper. All of these will provide the essential theoretical foundation for ECAP process research and application in the field.Contrary to characteristics of Mo and its powder materials, the influence of different factors on pure Mo powder sintered materials and its plastic compression deformation behavior have been studied. The flow stress increases with increasing strain rate and decreases with increasing temperature.The yield strength of material is the higher and more prone to rupture compared with the higher initial relative density,and increasing rate of hardness is not sensitive to the change of temperature, while the increase in temperature is beneficial to the reduction of yield strength. There is double impact of the pore and particle deformation and makes the material gradually dense but does not make fully dense for pure Mo powder sintered materials based on uniaxial compression.Based on "compressible continuum" assumptions of the powder material and analysis for various existing yield, the plastic constitutive equations adapted to pure Mo powder sintered materials are researched. Rigid-plastic constitutive equation of compressibility coefficient g is deduced and the quantitative relationship between the various parameters are given. The constitutive equations and their applications are reasonable analyzed,too. All of these will give available reference for the plastic forming process control and simulation analysis and enrich and expand the powder metallurgy generalized plasticity theory.On the basis of the comprehensive analysis of the slip line method, geometric derivation method and upper bound method to solve the plastic mechanics,the ECAP forming process is analytical resolved and the stress-strain field, velocity field, the extrusion pressure are quantitative analyzed.Considering three dimensions and the friction during the ECAP processing and using the upper limit of the theory, ECAP pressure is a comprehensive carried out and mathematical expression of the extrusion pressure is given.it will provide a theoretical basis of ECAP equipment selection and tooling design.In addition, for the "back pressure" on the ECAP process, the extrusion pressure with back pressure ECAP is solved and quantitative analyzed by using slip-line method,too.Thermodynamic coupled finite element model for powder material plastic forming is deduced,and based on different die structures and different initial conditions of pure Mo powder sintered materials, forming dense simulation is studied and distribution regularity of access extrusion pressure, stress-strain, the relative density and temperature field are obtained.The simulation results show that the shear deformation caused by the die corners is the forming and dense motivation,and despite ECAP deformation distributes the inhomogeneity but the ECAP processing for pure Mo powder material has a good densification effect.Die geometry shape is the key factor affecting the densification. On the basis of higher die strength and the allow of material processed,it is essential to choose a smaller die angles with the appropriate exterior angle improving the material flow of the outer corner and being satisfied with the amount of deformation and densification effect.Systematic analysis of influence of different process parameters to densification on pure Mo powder material.Appropriate to increase the friction can increase the extent of uniform deformation and densification,and the hardening properties of the material and the forming pressure will be reduced with increasing temperature and improve deformation homogeneity and compact effect but also adverses the organization refine.The pore is easy to be suppressed and acquire high degree of densification when initial relative density is smaller. Otherwise, there have good densification mechanism for grain crushing when the initial relative density is larger.When extrusion speed is large, the compact is better because of making the powder body particles squeeze, increasing deformation rate; and due to the extrusion stroke lengthened, forming a dense process more fully, there has well dense effect if extrusion speed is small.In the actual process, it is recommended to select a smaller value of the initial relative density,extrusion speed and moderate friction factor to protect the die equipment and to extend the equipment utilization,a reasonable choice of different process parameters should be considered during ECAP of pure Mo powder sintered materials.Multiple passes extrusion on various paths are simulated.The results show that the path BC after four passes extrusion completes a cycle and four surfaces are subjected to shear in the specimen, internal deformation is evenly distributed,so it is best extrusion path.That C path makes the deformation distribution tends to be symmetrical and uniform because of exchanging of the upper and lower contact surfaces. In view of uniformity of densification, path C is significantly better than path A after even (2n)passes extrusion, and the overall degree of densification of path A is higher than that of the path BA,too.Several common backpressure way and its role are described and simulation of two types BP-ECAP process are studied. The results show that forming dense is general proportional to the applied back pressure,However, that back pressure is too large will form a "dead zone" in die with a small exterior angle; Simultaneously there has too large extrusion pressure required, this will have serious implications for the ECAP forming die structure.It is conclusion of comprehensive comparison that the first class of the BP-ECAP design should be given firstly because of being more in line with the ECAP forming characteristics and more overall uniform for material deformation.On the basis of the theoretical foundation, numerical simulation studies, die and heating equipment are designed for pure Mo powder materials and experimental program is prepared. Extrusion experiments of the tube-powder sintered, tube-powder for ECAP and tube-powder sintered for BP-ECAP are carried out with comparative analysis of the finite element simulation results. After a single pass of tube-powder sintered ECAP.there is uneven distribution of deformation and density of the sample which is low relative density within near the bottom and the rest of the deformation and pore welding is better, while the sample overall has average higher relative density, it is confirmed that the ECAP has a strong densification effect. Microhardness of the specimen obtained has greatly improved depends on the extrusion path, extrusion temperature, the initial relative density.The ovservation and analysis of microstructure, scanning electron microscopy after multi-passes ECAP show that, ECAP has a powerful effect of the welding and refining effect of sintered powder materials. After a single pass, most of the pore of the sample deformation zone has been closed, specimen is close to the theoretical compaction density at the same time the grain has been significantly refined after2passes ECAP. Experiment results of2passes ECAP for tube-pure Mo powder show that there has smaller effect to deformation and densification besides occuring mainly particle rearrangement, elastic and plastic deformation behavior; the degree of densification of the specimen is significantly improved, the main deformation zone is near full density after2passes ECAP processing. A single pass BP-ECAP experiments using homemade simple back pressure equipment combined with the simulation results show,proper back pressure can increase the hydrostatic pressure and a greater shear deformation, promote particle crushing, melting, welding,and contribute effectively to the powder material densification. Reasonable range of back pressure for ECAP of pure Mo powder sintered materials is20-35Mpa when using the conventional right angle die.Finally, based on the orthogonal experimental design and numerical simulation,and innovative use of the Analytic Hierarchy of fuzzy mathematics and gray system theory, multi-objective optimization design of pure Mo powder sintered material during the ECAP process is studied. Results have proved that a reasonable choice of the die outer corner, forming temperature, the initial relative density, forming speed and the friction factor can significantly reduce the maximum damage value, obtain high hydrostatic pressure and make material significantly dense on the premis of the die inner angles to determine.更多还原