钛合金梯度材料激光增材制造热循环过程中非稳态固态相变行为研究

Laser additive manufacturing technique shows many technical and economic advantages on production of large and complicated titanium graded materials. However, the change of compositions in TA2/TA15 graded material and the complicated thermal-cycle history during laser additive manufacturing process lead the solid state phase transformation behavior become very complicated. Therefore, it is very difficult to well control their microstructures. In order to understand the mechanism of solid state phase transformation during laser additive manufacturing process, this project takes the TA2/TA15 graded material as the research object. And the mechanisms of solid-station phase transformation behavior and microstructure evolution of the material are investigated by analyzing: the distribution of alloying elements in material under the fast-cooling condition and to understand their impact on the formation of the non-equilibrium microstructure; the nucleation, growth behavior and microstructure of the graded material during thermal-cycles. Then a model of composition/thermos-cycle—microstructure was established to figure out the phase transformation mechanism under the change of both compositions and thermos-cycle histories. Besides, the change of the solid state phase transformation is also explained by using both the kinetic and thermodynamic method. The objective of this program is to understand the non-stable solid state phase transformation of the titanium graded material during laser additive manufacturing process and to provide a fundamental knowledge of producing unusual microstructure during subsequent heat treatment. In addition, this project can also offer a scientific guidance for controlling the microstructure of titanium alloy functional materials in manufacturing industry.

采用激光增材制造制备钛合金梯度材料大型复杂构件具有明显的技术和经济优势。然而由于材料成分渐变且经历非稳态热循环，钛合金梯度材料在增材制造过程中固态相变行为复杂，给其组织调控带来困难。本项目以航空梁框应力优化TA2/TA15梯度材料为研究对象，对其在增材制造热循环过程中非稳态固态相变行为进行基础科学研究。具体内容包括：分析快速凝固条件下梯度材料中金属元素分布规律，掌握非平衡凝固组织形成机理；分析热循环过程中各沉积层金属固态相变形核和生长行为、显微组织特征，建立成分/热循环-显微组织演变模型，掌握成分变化及热循环耦合作用对金属显微组织演变的影响规律，阐明梯度材料非稳态固态相变热力学及动力学特征变化。本项目旨在揭示TA2/TA15梯度材料增材制造热循环过程中非稳态固态相变及显微组织演变机理，为后续热处理获得超常显微组织提供科学参考，也为增材制造梯度材料的组织调控提供理论指导和科学依据。

高机动性先进飞行器的发展，使得整体构件不同部位的服役环境之间的差异越来越大，缩短了整体构件的服役寿命。采用钛合金梯度材料则可以缓和框梁结构中材料内部应力，延长其使用寿命，激光增材制造制备钛合金梯度材料大型复杂构件具有明显的技术和经济优势，然而由于材料成分渐变且经历非稳态热循环，钛合金梯度材料在增材制造过程中固态相变行为复杂，给其组织调控带来困难。本课题针对激光增材制造技术制备了TA2/TA15梯度材料，分析了激光增材制造过程中快速冷却条件下梯度过渡区各沉积层内/间成分分布及非平衡凝固组织形貌；研究了激光逐层沉积过程中热循环-合金成分耦合作用下梯度过渡区各沉积层的显微组织特征及固态相变过程；揭示了合金成分和组织形态对TA2/TA15梯度材料力学性能的影响规律；阐明了TA2/TA15梯度材料与均质材料（TA2钛合金及TA15钛合金）在弯曲过程中变形方式的区别。主要结论有：.（1）单道沉积实验中，激光增材制造TA2/TA15梯度材料快速凝固组织为马氏体，且合金成分含量与冷速均对马氏体形态产生影响。在冷速一定时，随着合金成分从TA2逐渐过渡到TA15，快速凝固组织由板条状马氏体转变成针状马氏体。.（2）多道沉积实验中，梯度区的成分和组织变化受沉积方式的影响。TA2板材上沉积TA15粉末试样梯度区组织随合金成分由TA2过渡至TA15，组织由细长α集束逐渐演化为较为粗大的网篮组织；依次沉积TA15、TA2、TA15粉末试样内，随合金成分由TA15过渡到TA2，组织由网篮组织转变为细长α板条，β相形貌由线状、段状逐渐变为点状，且含量逐渐减少。梯度区组织的硬度均随合金成分的增加而增加。.（3）弯曲实验结果表明，TA15抗弯强度2156MPa，梯度试样抗弯强度范围为：964MPa-1817MPa；随着合金成分的增加，TA2/TA15梯度材料抗弯强度逐渐增加。TA15试样及TA2/TA15梯度材料试样的断面均出现大量韧窝，属于韧性断裂。.（4）有限元应力场模拟结果表明，TA15的三点弯曲最大拉应力出现在试样底层中心位置，最大压应力出现在试样顶层中心位置。梯度材料由于内部成分及组织变化和缓，其弹性和塑性性能存在差异，梯度层交界处容易存在应力集中。