【作者】 杨保健；

【导师】 夏琴香；

【作者基本信息】 华南理工大学 ， 机械制造及其自动化， 2013， 博士

【摘要】 纳米/超细晶材料由于特征尺寸极小、性能优异而倍受关注，但是由于受制备技术的制约，目前难以获得高纯、致密、晶界清晰的理想三维块体纳米/超细晶金属材料。随着对纳米材料研究的不断深入和纳米技术的不断发展，将强力错距旋压大变形技术与纳米技术结合起来制备纳米/超细晶筒形件引起高度关注。相关研究发现通过反复强力旋压，在金属材料表面可制备出一定厚度的纳米结构表层，即实现了筒形件表面纳米化。然而要实现筒形件整个纵截面上晶粒的纳米/超细晶化，则需要穿越宏观尺度下的表象，深入系统地研究强力错距旋压剧烈塑性变形诱导纳米/超细晶的形成机理以及初始晶粒尺寸、强力错距旋压工艺参数以及再结晶热处理对筒形件晶粒细化的影响机制。本文针对目前国内外尚未开展的采用强力错距旋压制备纳米/超细晶筒形件进行研究，研究工作围绕纳米/超细晶筒形件的强力错距旋压成形方法、成形机理以及强力错距旋压工艺参数对晶粒尺寸的影响展开。基于晶体塑性理论和ABAQUS有限元软件，建立了多晶体细观塑性本构模型和Poisson-Voronoi细观结构有限元模型（PVFEM），给出了强烈塑性变形时多晶体运动动力学基本方程，编制了相应的率无关有限元程序（通过ABAQUS用户子程序UMAT形式实现），并将此有限元程序作为用户子程序嵌入有限元软件ABAQUS中，模拟和分析了筒形件强力旋压时旋轮作用区特征晶粒在强烈塑性变形过程中形状系数比（晶粒的等效椭圆长短轴之比）随变形量的变化，详细分析了晶粒中切应力、切应变与晶粒塑性变形之间的关系，为精确预报与控制强力错距旋压制备纳米/超细晶筒形件的过程提供了可视化模型。在经对比分析确定的强力错距旋压成形方法的基础上，设计出了专用的旋压成形工艺装备，并以20钢为毛坯材料，选择合适的成形参数（如旋轮结构形状、旋压道次、道次变薄率、错距量、旋轮进给比等），拟定三种不同的实验方案分别从材料初始组织、旋压工艺参数和热处理规范等成形要素展开了系统的实验研究。实验结果表明：通过“20钢管坯——多道次强旋——静态再结晶——强旋——再结晶退火”的复合成形工艺，成功实现了组织参数（主要为铁素体晶粒尺寸和形态）在一定程度上可控的纳米/超细晶结构筒形件的制备，其晶粒平均尺寸约为600nm。在理论分析多晶体金属强力错距旋压时变形机理的基础上，借助金相分析技术、SEM及TEM技术对强力错距旋压及再结晶退火过程中20钢筒形件微观组织结构进行了表征并研究了其变化特点，获得了强力错距旋压剧烈塑性变形诱发20钢旋压件纳米/超细晶粒的形成机理，探讨了强力错距旋压过程中晶粒细化的影响因素。从选区电子衍射谱确定了晶体结构、组织结构和相互的取向关系，进而获得了变形中开动滑移系的信息，探讨了旋压过程中材料晶粒的细化机制和大角度晶界形成机理。通过对20钢筒形件金相组织、SEM/TEM微观组织表征和拉伸实验数据及断口形貌的深入分析与归纳，探索了强力错距旋压剧烈冷变形过程中珠光体片间距LSP、渗碳体沉淀对20钢旋压件的力学性能的影响，分析了强力错距旋压冷成形时20钢及淬火态20钢微裂纹产生与扩展以及破裂失效的特点与机理，研究了强力错距旋压冷变形过程中诱发的大量第二相粒子（颗粒状渗碳体）对晶界的钉扎和晶界运动的抑制作用。实验结果表明珠光体片间距LSP在强力错距旋压剧烈塑性变形过程中随筒形件壁厚减薄率的增大而显著减小，20钢旋压件抗拉强度σb随LSP的倒数增大而增大，其延伸率δ随LSP的倒数增大而减小。淬火态20钢在强力旋压起旋阶段材料流动不稳定，在拉应力σx的作用下旋轮作用区微空洞彼此连接形成微裂纹，随着强力旋压多道次成形的进一步加载，微裂纹在与裂纹所在平面垂直的拉应力σ x的作用下沿着与筒形件切向成60°的平面扩展。更多还原

【Abstract】 Nano/ultrafine grained materials (NUGM) fabricated via SPD (severe plastic deformation)has aroused a growing interest of specialists in materials science. However in recent years, itis difficult to manufacture the ideal three dimensional block NUGM with high purity, highdensity and distinct grain boundaries for lack of optimal processing technologies.Nevertheless, with the development of nano-technologies and advanced studies, more andmore researchers began to combine the power spinning technology with nano-technologiesfor fabricating the nano/ultrafine grained cylindrical parts. Related research shows thatthrough the repeated power spinning processes, cylindrical parts with nano structured surfacewere prepared, namely surface nano crystallization. However, in order to realizenano/ultrafine crystallization of the whole cross section, studies of nano/ultrafine grainforming mechanism induced by severe shear deformation in process of power staggerspinning and the effect of initial grain sizees, power stagger spinning process parameters andrecrystallization heat treatment conditions on grain refinement were carried out.Research in this dissertation was mainly focused on the mechanism of fabricatingnano/ultrafine grained cylindrical parts by power stagger spinning which has not beeninvestigated till now, and its purposes are to discuss the forming methods and mechanism ofnano/ultrafine grained cylindrical parts by power stagger spinning and the effects of processparameters on grain sizes.Based on crystal plasticity theory and the commercial finite element software ABAQUS,the polycrystalline mesoscopic plastic constitutive model and Poisson-voronoi finite elementmodel (PVFEM) were established, and several kinematical equations of polycrystalline insevere plastic deformation were also built. Further more, a rate independent program wascompiled (by ABAQUS user subroutine UMAT). By importing the program as a UMATsubroutine, study of the variable grain shape factor (the length ratio of longer axes to shortaxes of the equivalent ellipse) in forming area was carried out, and then the relationshipbetween resolved stress of slip system and macroscopic stress was also discussed in detail. Bydoing so, a visual model for accurately forecasting and controlling of forming process fornano/ultrafine polycrystalline was completed.Selecting power stagger spinning as the optimal forming method for fabricatingnano/ultrafine grained cylindrical parts, the tooling and fixture were prepared. Theexperimental study of three different programs were carried out by selecting20steel pipe as its blank and choosing different process parameters including roller profile, spinning passes,wall thickness thinning ratio, roller intervals, roller feed ratio, and so on. The results revealthat the nano/ultrafine grained cylindrical parts were successfully fabricated by a compositeforming process that started using3-pass spinning followed by580℃×0.5h staticrecrystallization,2-pass spinning and580℃×1h static recrystallization annealing insequence, and its size is600nm on average.Based on the theoretical analysis of deformation mechanism, the microstructural evolutionof20steel both in process of power stagger spinning and subsequent annealing wasinvestigated via metallographic analysis technology, SEM and TEM techniques, and then theforming mechanism of nano/ultrafine grains induced by severe shear deformation in theprocess of power stagger spinning was also revealed. Further more, the affecting factors forgrain refinement were discussed. By the investigation of selected area electron diffraction(SAD), the crystal structures, texture and the activated slip system of specimen were obtained,then forming mechanism of large angle grain boundaries (LAGB) was also discussed.Through analysis of the metallographic organizations, SEM/TEM images, tensileexperiment datas and fracture morphology of20steel spun workpieces, effect of layer spacingof pearlite (LSP) and cementite precipitation on mechanical properties during the process ofpower stagger spinning was investigated, and then both the characteristics and its mechanismof the microcrack and the plastic fracture for20steel and quenched20steel were also studied.Further more, the pinning&hinder effect of the second phase particles (granular cementite)induced by power stagger spinning on grain boundary migrations were discussed. Theexperimental results show that the layer spacing of pearlite (LSP) decreases as the thinningratio increases during power stagger spinning, and the tensile strength σbof spun tubes isproportional to the reciprocal of LSP, while the elongation δ is inversely proportional to thereciprocal of LSP. Small tough dimples connect gradually with tensile stress σxaround cracktip, then micro crack is formed later with multi-pass and multi-direction loading. By this way,crack further extends along60°to tangential direction. Finally, ductile fracture of spunworkpiece comes into being.更多还原