【作者】 陈勇；

【导师】 郭斌； 徐文臣；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2014， 博士

【摘要】 TA15钛合金是一种典型的近α钛合金，具有比强度高、热稳定性好、抗蠕变性能优异和可焊性强等优点，被广泛地应用于航空航天工业中关键零部件的制造。强力旋压是典型的局部加载、局部成形工艺，具有省力、省料和工装简单等优点，是制造大型薄壁筒形件的最有效方法之一，已广泛应用于航空航天及兵器工业领域。本文针对钛合金热强力旋压成形过程中微观组织和性能控制的瓶颈问题，建立了TA15钛合金热强旋微观组织及性能的BP神经网络预测模型，研究了TA15钛合金热强旋微观组织和织构的演化规律，揭示了其对热强旋筒形件性能的强化机理。开展了平面应变热压缩和筒形件热强旋对比实验，分析了平面应变热压缩微观组织演化特点。采用图像分析软件对微观组织中的晶粒尺寸和再结晶分数进行定量表征，并通过显微硬度计测试了显微硬度。系统研究了TA15钛合金平面应变热压缩工艺参数、微观组织和力学性能之间的关系，建立了TA15钛合金热强旋微观组织和力学性能的BP神经网络预测模型。该模型预测误差小于13%，表明通过平面应变热压缩实验来模拟热强旋微观组织演化具有可行性，可以用于揭示TA15钛合金热强旋工艺参数对微观组织和力学性能的影响。通过TEM和EBSD等手段研究了TA15钛合金热强旋微观组织演化规律。定量分析了不同减薄率强力旋压后，晶粒尺寸、晶粒长径比和晶界密度等的变化规律。结果表明，随着减薄率的增加，TA15钛合金微观组织形状发生显著改变。原始粗大的初生α相晶粒沿轴向显著伸长，形成纤维状组织。次生α相晶粒由原始的板条状演化成细小的等轴状，晶粒得到明显细化。微观组织中不同类型晶界的数量及分布发生显著变化。当减薄率小于19.7%时，小角度晶界和中等角度晶界主要分布在原始粗大的初生α相内及次生α相板条之间。当减薄率增至81.1%时，大量小角度和中等角度晶界转变为大角度晶界，引起旋压微观组织的细化。TA15钛合金热强力旋压过程中，初生α相的变形机制主要是滑移和孪生。当减薄率小于19.7%时，孪晶的主要类型是{1012}<1011>型拉伸孪晶。次生α相板条等轴化引起合金软化，提出了TA15钛合金次生α相在热强旋过程中的球化模型。通过压缩实验研究了微观组织和织构对热强旋TA15钛合金力学性能的影响。热强旋TA15钛合金的屈服强度随晶粒尺寸减小而增加，随初生α相长径比增大而增加，随新生细小晶粒的体积分数增大而增加。随着旋压道次及减薄率的增加，TA15钛合金晶粒晶体取向择优分布明显。当减薄率达到81.1%时，TA15钛合金中c轴与ND取向差小于20°的晶粒数超过总数的45%，平均取向差从初始状态67.5°减小到28.1°。粗大的初生α相主要发生回复，晶粒被压扁拉长，形成较强的{0001}型双峰或多峰织构，略沿TD扩散。次生α相晶粒明显细化，发生明显的动态再结晶，晶粒呈现显著的{0001}型再结晶织构，并与初生α相晶体取向分布保持很大程度的一致性。TA15钛合金热强旋筒形件晶体取向有明显的择优方向，轴向{0001}<1120>基面滑移系Schmid因子明显小于环向，导致热强旋TA15钛合金筒形件有明显的各向异性，轴向屈服强度高于环向。开展了单向拉伸、双向拉伸和液压胀形实验测试TA15钛合金热强旋筒形件的力学性能。单向拉伸实验表明旋压筒形件具有明显的各向异性：轴向拉伸屈服强度高于环向，延伸率低于环向。双向拉伸实验表明筒形件受轴向应力与环向应力比为1:2时，屈服强度明显高于单向拉伸屈服强度。室温下轴向和环向双向强化效应分别为10.2%和7.0%，500℃下轴向和环向双向强化效应分别为11.7%和8.9%。旋压筒形件液压胀形实验结果与双向拉伸实验结果吻合。通过双向拉伸实验，预测TA15钛合金强旋筒形件在500℃服役时，环向应力达到1106MPa筒形件将发生屈服。更多还原

【Abstract】 As a typical near-α titanium alloy, TA15titanium alloy exhibits excellentproperties, such as high specific strength, good thermal stability, considerable creepresistance and great weldability. Therefore, it has been widely used to produce the keycomponents in the aviation and aerospace industry. Power spinning is a typicallocal-loading forming technology with many advantages, such as low forming load,high material utilization ratio and simple tooling. As an effective method to manufacturetubes with large diameter and thin thickness, power spinning has been widely applied inmany industries, such as aviation, aerospace, armament, marine and machinery. Thisthesis focused on the control bottlenecks in microstructure and properties of titaniumalloy during hot power spinning process. The BP neural network prediction model ofTA15titanium during hot power spinning was established, and the evolution law of themicrostructure and texture and the strengthening mechanism of TA15titanium alloyduring hot power spinning were systematically investigated.The microstructure evolution features of the plane strain thermal compressionsamples were compared with those of hot power spun tubes to identify their correlations.The grain size and recrystallization ratio of microstructure were quantitativelycharacterized using image processing software, and the microhardness was measured bythe microhardness tester. The relation between the processing parameters of the planestrain thermal compression and the microstructure and mechanical properties of TA15titanium alloy was systematically studied, and then a BP neural network predictionmodel for microstructure and mechanical properties of TA15titanium alloy during hotpower spinning was established. The prediction error were within13%compared toexperimental results. It indicates that it is feasable to simulate the microstructureevolution during hot power spinning using the plane strain thermal compressionexperiment, which can be further used to reveal the effect of the processing parametersof hot power spinning on the microstructure and mechanical properties of TA15titanium alloy.The microstructure evolution (i.e. grain size, grain aspect ratio, grain boundarydensity, etc) during hot power spinning in different passes was deeply investigated byTEM and EBSD. The results show significant changes in the microstructure shape ofTA15titanium alloy with the increasing of spinning pass. The coarse grains of theoriginal primary α phase were significantly elongated along the axial direction, resultingin the formation of a fibrous structure. The lamellar grains of the original secondary αphase evolved into a large number of small equaxial grains. Additionally, there werealso significant changes in the number and distribution of the grain boundaries with different angles in microstructure. When the thickness reduction ratio was below19.7%,most small-and medium-angle grain boundaries presented inside the original coarseprimary α phase and between the lamellas of the secondary α phase. When the thicknessreduction ratio increased to81.1%, a large number of small-and medium-angle grainboundaries evolved into large-angle ones, leading to the refinement of as-spunmicrostructure. During the hot power spinning process of the TA15titanium alloy, slipand twinning were the main deformation mechanisms of the primary α phase. When thethickness reduction ratio was below19.7%,{1012}<1011> tensile twinningpredominated the deformation processing. The globularization of the secondary α phaselamellas lead to the softenning of alloy. The spheroidization model of the secondary αphase during hot power spinning process was also proposed.The effects of the microstructure and texture on the mechanical properties of TA15titanium alloy during hot power spinning were analysed by compression test. The yieldstrength of TA15titanium alloy during hot power spinning increased with the decreaseof the grain size, the increase of the aspect ratio of the primary α phase and the increaseof the volume fraction of fresh small grains. With the increase of spinning pass andthickness reduction, the grains of TA15titanium alloy showed significantly preferredcrystal orientation distribution. When the thickness reduction ratio reached81.1%, the caxis of more than45%TA15titanium alloy grains rotated to the direction with less than20°difference relative to the ND, and the average misorientation of grains reduced fromoriginal67.5°to28.1°. Recovery happened in the coarse primary α phase, grainselongated and flattened to form the obvious {0001} bimodal or multimodal textureswith slightly diffused distribution along TD. Dynamic recrystallization happened insecondary α-phase finer grains and formed {0001} texture. The crystal orientations ofthe primary α phase grains and small secondary α-phase finer grain size were wellconsistent in some locations. The orientation consistency weakened with the increase ofthe thickness reduction ratio. The TA15titanium alloy tube have prefered orientation,the Schmid factors of the {0001}<1120> basal slip of grains in RD was obviouslysmaller than that in TD, which resulted in anisotropy of the tube, and yield stress in RDwas higher than yield stress in TD.Uniaxial, biaxial tensile tests and hydraulic bulging test were carried out to test themechanical properties of TA15titanium alloy tubes after hot power spinning. The axialtensile test showed that as-spun tubes exhibited obvious anisotropic properties: thetensile yield strength along the axial direction was higher than that along thecircumferential direction, while the elongation along the axial direction was lower thanthat along the circumferential direction. Biaxial tensile test showed that the yieldstrength of tubes under biaxial tensile test were significantly higher than that under uniaxial tensile test when the ratio between the axial stress and the circumferential stresswas1:2, and the values of bixial strengthening effect in axial and circumfentialdierection were10.2%and7.0%at room temperature,11.7%and8.9%at500℃,respectively. The result of hydraulic bulging test of as-spun tubes coincided with thebiaxial tensile test result. Therefore, the service mechanical property of TA15titaniumalloy hot power spun tubes at500℃was predicteded using the biaxial tensile test,which showed that the tube would yield with a circumferential stress of1106MPa.更多还原