【作者】 杨国平；

【导师】 单德彬； 徐文臣；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2010， 博士

【摘要】 筒形件旋压是典型的局部加载成形工艺,具有省力、省料、工装简单等优点,是制造大型薄壁无缝筒形件的有效方法,已广泛应用于航天、航空、兵器、舰船和机械等工业领域。由于旋压变形过程非常复杂,影响因素众多,到目前为止,大量的旋压技术研究工作仍定位于零件成形,旋压缺陷和几何精度控制等方面,零件力学性能控制方面考虑得较少。单晶体的力学性能通常具有各向异性,晶体学织构能对金属材料力学性能产生重大影响,织构也就成为控制材料力学性能的重要手段之一。织构演化存在于塑性变形和热处理两个阶段,本文拟对纯钛和BT20钛合金筒形件旋压变形织构及在后续热处理中的演化进行研究。基于晶体塑性理论的有限元模拟是研究变形织构的重要手段。考虑到纯钛低温塑性变形机制的多样性和旋压变形的复杂性,本文主要开展以位错滑移为变形机制的高温纯钛单道次旋压变形织构的晶体塑性有限元模拟和实验研究,采用实验分析法研究纯钛和BT20钛合金两种材料的多道次旋压变形织构以及在热处理中的演化规律。变形织构的数值模拟需要材料的初始织构、变形系统和工艺变形历史等信息。将实测宏观织构离散成晶粒取向后可作为初始织构。纯钛的变形系统限定为基面、柱面和锥面上的<a>滑移系统,以及一级锥面{1010}、二级锥面{1122}上的<a+c>滑移系统。建立织构模拟的多晶体模型,利用商业软件ABAQUS的用户材料子程序UMAT定义率相关的晶体塑性本构关系,通过拟合750℃时纯钛简单压缩应力应变曲线和织构得到晶体塑性本构关系的参数。采用各向同性的多晶纯钛本构关系进行筒形件旋压变形过程的有限元模拟,根据该有限元模拟结果计算物质点的变形梯度。将变形梯度随时间的变化转化为多晶体模型外表面节点位移随时间的变化,以此研究不同进给比、减薄率和初始织构等条件下的旋压变形织构。结果表明:不同进给比和减薄率的旋压筒形件内外表面均形成含有{0002}<1010>型织构和{0002}型纤维织构两种组分的{0002}型织构。大减薄率和小进给比的旋压变形能够明显增强外表面的{0002}<1010>型织构组分。筒形件多道次大变形旋压变形织构分析表明:纯钛旋压后容易在筒表面法向附近形成双峰{0002}型织构,双峰强度不对称,双峰连线偏离筒形件的环向方向,而BT20钛合金旋压后则在筒表面法向附近形成不稳定的单峰或多峰{0002}型织构,当变形量很大(本文84.8%)时形成稳定的单峰{0002}型织构。对纯钛和BT20钛合金多道次大变形旋压筒形件进行热处理,通过金相组织观察确定初级再结晶和晶粒正常长大的热处理温度和保温时间。采用电子背散衍射技术(EBSD)对热处理后晶体取向进行测量,计算基于晶体取向频率和晶体体积的极图以及两种极图的差异,分析再结晶晶核的定向形核和定向生长趋势。对比两种材料经过晶粒正常长大阶段后织构演化趋势。结果表明:定向形核是纯钛和BT20钛合金多道次大变形旋压筒形件再结晶织构的主要机制,BT20钛合金晶核定向生长趋势较纯钛的大;两种材料的再结晶织构在晶粒长大过程中弱化,而在附近取向上形成较强的新织构组分。更多还原

【Abstract】 Tube spinning is a typical local-loading forming process with the advantages such as material saving, low forming load and simple tooling. Considered as an effective method to manufacture large-diameter, thin-thickness and seamless tubular part the process has been applied widely in aeronautics, aerospace, weapon, shipbuilding and mechanical industries, etc. Until now most attention has been paid on how to fabricate a workpiece without defects and improve the geometrical precision because spinning deformation is very complex and influenced by many factors. However it was rarely considered to control the mechanical property of as spun part. The mechanical property of a single crystal is usually anisotropic. Therefore material mechanical property is strongly influenced and feasibly controlled by crystal texture. Texture evolution could occur in plastic deformation and heat treatment, so the topic of the paper is oriented as texture evolutions in titanium and BT20 alloy tube spinning and heat treatment of as-spun workpiece. Finite element simulation based on crystal plasticity theory, which is an important method to investigate plastic deformation of polycrystalline aggregate, was applied to study spinning texture. Considering the plastic mechanisms of pure titanium are various at low temperature and spinning deformation is very complex, texture evolution only in single-pass spinning of pure titanium at high temperature, in which only slip systems were expected to be activated, was simulated in the paper. Texture evolutions in multi-pass spinning and subsequent heat treatment were investigated by experimental method.Initial texture, deformation systems and deformation history in forming process are necessary in deformation texture simulation. Discretized macro-texture was adopted as initial texture, and deformation systems of pure titanium were <a> slips in basal, prismatic, pyramidal plane, and <a+c> slips in first-order pyramidal plane{1010}, second-order pyramidal plane{1122} . Polycrystalline model was established and crystal plasticity constitutive equations were coded in material subroutine UMAT of commercial software ABAQUS, then the parameters in the equations were obtained by fitting stress-strain curves and texture evolution of pure titanium simple compression at 750℃. Spinning deformation was represented by deformation gradient, and calculated from FEM simulation of pure titanium tube spinning using isotropic constitutive relationship. In order to realize spinning deformation of polycrystalline model the deformation gradient as a function of time was transferred to the displacements of surface nodes on the model as boundary condition. Therefore the procedure makes it possible to investigate the influences of feed rate, thickness reduction and initial texture on spinning texture. The results show that {0002}-type texture which contains {0002}<1010> -type texture and {0002}-type fiber texture components is evolved in both outer and inner surfaces of pure titanium as-spun workpiece by different feed rates and thickness reductions. In outer surface {0002}<1010> -type texture component is enhanced by spinning with large thickness reduction and small feed rate.Texture analyses of multi-pass and large deformed tube spinning show that double peaks, the intensities of which are different, tend to appear on {0002} pole figure around the surface normal of pure titanium as-spun part, and the line through the two peaks deviates the hoop direction of tube. However, just unsteady single or several peaks appear on {0002} pole figure of BT20 alloy as-spun workpiece, and steady single peak would come into being when total thickness reduction reaches a quite large level (84.8% in the paper).As-spun workpieces of pure titanium and BT20 alloy after multi passes and large deformation were heat treated, and optical microstructures were observed to determine reasonable temperatures for primary recrystallization annealing and grain growth. Crystal orientations in heat treatment samples were recognized using electron back scattered diffraction technology. Number-based and volume-based pole figures and their difference were calculated to figure out the mechanisms of texture formation, namely oriented nucleation or growth, in primary recrystallization. Texture evolution during grain growth was also studied. The results show that oriented nucleation mechanism plays an important role in the recrystallization texture formation of both as-spun pure titanium and BT20 alloys, and oriented growth tendency in BT20 alloy is larger than that in pure titanium. Textures in the primary recrystallizations of two titanium materials were randomized in grain growth, and now components would occur at near orientations.更多还原