【作者】 张强；

【导师】 黄卫东；

【作者基本信息】 西北工业大学 ， 材料加工工程， 2017， 博士

【摘要】 近年来,航空结构设计领域开始采用损伤容限设计准则,设计理念的改变也显著影响了钛合金的发展方向并促进了一些具有高损伤容限性能钛合金的研究和应用。激光立体成形技术能够实现较复杂结构钛合金结构件的无模具、快速、近净成形,其微观组织特征(魏氏组织和网篮组织)也契合损伤容限设计思想的选材标准。然而,激光立体成形钛合金宏观组织为外延生长的粗大柱状β晶粒,这种柱状的粗晶组织会降低结构件的力学性能并导致各向异性。如何等轴化β晶粒,减少甚至消除力学性能的各向异性是激光立体成形钛合金亟需解决的关键问题。基于此,本文以我国自主研发的高强高韧损伤容限型TC21钛合金为研究对象,重点针对激光立体成形钛合金宏微观组织演化机理及规律、组织调控方法及力学性能各向异性开展研究。本文的主要研究内容和结果如下:1.基于经验工艺参数的激光立体成形TC21钛合金非均匀组织及力学性能特征(1)采用经验工艺参数制备了TC21钛合金试样,并研究了其宏微观组织和织构特征。发现激光立体成形TC21钛合金宏观组织为典型的外延生长的柱状β晶粒,并表现出强烈的<100>纤维织构特征;α相的形貌、尺寸和体积分数等沿沉积高度方向上存在显著的不均匀性,表明合金化程度较高的TC21钛合金中α相的析出对成形过程的热历史更为敏感。此外,由于β→α相转变时存在伯氏取向关系及变体选择现象,因此,织构较强的原始柱状β晶粒中后析出的α相同样表现出较强的织构。(2)发现宏观组织为柱状β晶粒时,TC21钛合金试样的室温拉伸性能表现出明显的各向异性。外加载荷方向垂直于柱状晶(横向)时试样的抗拉强度和屈服强度均高于平行于柱状晶(纵向)试样,但总延伸率要低得多。横、纵向试样强度的差异主要由不均匀的α相引起,而总延伸率的差异主要与拉伸试样标距段内与外加载荷方向垂直的连续晶界及晶界α相的数量有关。2.激光立体成形TC21钛合金原始β晶粒演化机理及调控方法研究(1)发现同步送进粉末颗粒能够诱发熔池固液界面前沿等轴枝晶形成。采用较大的送粉量沉积TC21钛合金时,每个沉积层均由两部分组成:(i)沉积层上半部分的等轴枝晶区;(ii)沉积层下半部分的外延生长胞状晶区。底部外延生长的胞状晶被顶部的等轴枝晶区阻断。基于对较大送粉量条件下TC21钛合金显微组织的分析结果,提出了通过控制部分熔化粉末颗粒促进熔池中异质形核以获得等轴β晶粒的新思路,并进一步明确了激光能量密度和基材/已沉积层温度等影响等轴β晶粒形成的关键工艺参数。(2)研究了TC21钛合金中不同形貌柱状β晶粒的形成机理及织构特征,发现熔池后沿倾角会对柱状β晶粒的形貌产生较大的影响:单层沉积高度较小时,熔池后沿倾角较小,原始柱状β晶粒的生长方向接近平行于沉积方向;单层沉积高度增大时,熔池后沿倾角也随之增加,原始柱状β晶粒的生长方向向扫描方向倾斜较大角度并略向横向方向倾斜。此外,熔池后沿倾角改变时热流方向也随之改变,这样基材中原本处于不利取向的晶粒可能在生长过程中占据优势,从而造成倾斜柱状晶和竖直柱状晶织构的差异。3.等轴β晶粒激光立体成形TC21钛合金的拉伸性能及断裂机理(1)宏观组织为等轴β晶粒时,原始β晶粒和α相的织构强度都得到了显著的降低。此外,进行等轴β晶粒试样沉积时的多次暂停在一定程度上降低了α相的不均匀性。并发现在低于相变点温度进行热处理时,等轴β晶粒和α相的织构类型几乎不受影响,但可以显著改变α相的尺寸和体积分数。(2)研究了等轴β晶粒TC21钛合金室温拉伸性能及各向异性,发现等轴β晶粒试样的室温拉伸性能相比柱状β晶粒试样有明显改善。600~oC,2h/AC时效处理试样的室温拉伸性能各向异性较为显著;而经800~oC,1h/AC+600~oC,1h/AC和870 ~o C,1h/FC+600~oC,2h/AC两种不同的固溶时效处理后,拉伸性能各向异性都得到了明显的降低。三种热处理条件下,等轴β晶粒TC21钛合金的屈服强度与断口附近α板条宽度满足霍尔佩齐关系。等轴β晶粒试样拉伸性能各向异性与不均匀的α相有关。(3)研究了等轴β晶粒TC21钛合金室温拉伸的断裂机理,发现拉伸过程中,显微孔洞主要在板条α相与β基体的界面、α相内部及β晶界处形成。在等轴β晶界处形成的裂纹通常不会像柱状β晶粒一样导致试样的过早断裂。相比α板条较细小的显微组织,在α板条宽度较大的试样中,裂纹尖端会形成较大的塑性区以松弛应力,这样可以推迟主裂纹的形成,使材料表现出较高的塑性。4.激光立体成形TC21钛合金力学性能优化通过调整激光立体成形工艺参数及后热处理制度,可以分两步获得“等轴β晶粒+均匀α相”的组织状态,优化了激光立体成形TC21钛合金的力学性能并减小了其各向异性。更多还原

【Abstract】 With the design philosophy of aviation industry evolving from static strength design to damage tolerance design,the materials selection criteria has been changed accordingly and promoted the applications of damage tolerance titanium alloys.Laser solid forming（LSF）is an additive manufacture technique for fabricating high-performance complex metallic parts,which has been widely researched in the past decades.Especially,the LSF process has been considered as an appropriate and cost-effective technology to fabricate titanium alloy products.However,the macrostructure of LSFedα+βtitanium alloys usually consists of coarse columnar priorβgrains.The coarse columnarβgrains will deteriorate the mechanical properties and lead to anisotropy.Given the deficiency of strongly textured columnar grains and the desire of randomly textured equiaxed grains,a method to fabricate equiaxed priorβgrains is urgently needed to be developed.In the present study,microstructure evolution,microstructure controlling method and anisotropy of mechanical properties of LSFed TC21 titanium alloys will be investigated.A brief introduction to the present work and the main achievements obtained are as follows:1.The inhomogeneous microstructure and mechanical properties of LSFed TC21 titanium alloy based on the empirical processing parameters（1）The microstructure and texture characterization of LSFed TC21 titanium alloy based on the empirical processing parameters were investigated.The results showed that the as-deposited TC21 titanium alloy exhibits a macrostructure comprising coarse columnar priorβgrains and exhibits strong<100>fiber texture.TC21 titanium alloy which contains medium alloying elements shows quite inhomogeneousαlaths along the build direction as the precipitation ofαlaths is sensitive to the thermal history they experienced.Due to the Burger orientation relationship and variants selection,theαphase texture is directly affected by the original solidifiedβgrains texture.（2）The room temperature tensile properties and fracture mechanism of the LSFed TC21titanium alloy containing columnar priorβgrains were investigated.The tensile tests showed significant anisotropic mechanical properties under two different heat treatment conditions.The horizontal samples exhibit stronger strength than the vertical samples.And the ductility of the horizontal sample is much inferior to that of the vertical samples.The inferior ductility of the horizontal samples is caused by larger amounts ofα_GB layers andβgrain boundaries which perpendicular to the tensile direction.Coarsening ofα_GB layers can further reduce the ductility.2.Priorβgrain morphology evolution and control of LSFed TC21 titanium alloy（1）When the LSFed TC21 titanium alloy were deposited by deliberately increasing the powder feed rate,the microstructure of each cladding layer is composed of two regions:（i）randomly orientated cellular structure region caused by partially melted powders at the top of each cladding layer;and（ii）epitaxial cellular structure region adjacent to the fusion line.A novel design to fabricate wholly equiaxed priorβgrains structure in LSFed titanium alloy was proposed based on the experimental results.The lower laser energy density is critical for the survival of the captured powders.The underlying cladding layer temperature is also an important control parameter for the preservation of the equiaxed dendrites when a new cladding layer was deposited.In general,the equiaxed priorβgrains were obtained with lower laser energy density and a lower underlying cladding layer temperature.（2）The formation mechanism and texture characterization of the columnar priorβgrains were studied.The results showed that both vertical and inclined columnar priorβgrains were obtained,and in the samples with a thicker cladding layer the columnarβgrains show a larger inclined angle.The tilted molten pool boundary resulted in the deviation of heat flow from the build direction and will affect the finalβgrains morphologies and orientations.3.Mechanical property and fracture mechanism of LSFed TC21 titanium alloy containing equiaxed priorβgrains（1）The texture intensity of the priorβgrains andαphase was reduced in the LSFed TC21titanium alloy containing equiaxed priorβgrains.Besides,the repetitive pauses during the LSF process lead to a more homogeneous microstructure.The texture of priorβgrains andαphase almost unaffected when the heat treatment temperature is belowβtransition temperature.However,the microstructure characterization（αlaths width and volume fraction）is strongly depended on the heat treatments.（2）The room temperature tensile properties and anisotropy of the LSFed TC21 titanium alloy containing equiaxed priorβgrains were investigated.The results showed the tensile properties of the equiaxed priorβgrains are superior than the columnar priorβgrains.The aging treated sample are anisotropy while the sotion+aging treated sample are isotropy.The yield strength of the samples increased with a decreaingαlaths width,and follows the Hall-Petch relationship.The anisotropy of the LSFed TC21 titanium alloy containing equiaxed priorβgrains can be contributed to the inhomogeneous distribution of theαlaths.（3）The fracture mechanism of the LSFed TC21 titanium alloy containing equiaxed priorβgrains were investigated.The result showed that voids could nucleate at theα/βinterfaces,βgrain boundaries and theαlaths.In the samples containg equiaxed priorβgrains,the cracks can be confined in a single priorβgrain and delayed the occurrence of fracture.4.Mechanical property optimization of LSFed TC21 titanium alloyTC21 titanium parts containing equiaxed priorβgrains and homogeneous microstructure can be obtained by adjusting the LSF processing parameters and heat treatment,respectively.The mechanical properties comparable to the wrought could be achieved by appropriate microstructure control.更多还原