超薄钛板介观尺度温热成形损伤的尺度效应及成形极限预测

Warm forming of ultra-thin titanium bipolar plates with meso-scale high-aspect-ratio channels is a critical technique for solving the bottlenecking problems of power density and service life of the proton exchange membrane fuel cell system. However, ductile brittle transition occurs during the meso-scale warm forming process of titanium sheet metals as a result of the complicated variation of grain orientations, martensite structures, twinning/dislocation slipping combined behaviors, etc. The current forming limit theories and models could not predict the fracture behavior accurately. Hence unpredicted fracture may occur during the forming process leading to the high risk of defective product..To characterize the ductile-brittle transition and failure mechanism of TA2 titanium ultra-thin sheet metals under the coupling influences of phase/microstructure evolution and size effect during the meso-scale warm forming process, the fracture behavior, mechanics and physical modeling are investigated thoroughly as follows: The damage and fracture behavior during the warm forming process are first studied by using the in-situ synchrotron X-ray computed topography method. The evolution of micro voids and cracks under different temperature and grain size conditions are characterized in detail. After that the coupling influences of phase/microstructure evolution and size effect are analyzed. Those influences on the ductile-brittle transition and the evolution of micro voids and cracks are further revealed to characterize the failure mechanism. At last, Gurson void growth model and Horii-Nemat-Nasser crack expansion model are modified by introducing the coupling influences of temperature and size effect. A novel ductile-brittle combined damage mechanical model is established to predict the forming limit of titanium sheet metals during the meso-scale forming process. Verifications are further conducted by comparing the analytical prediction and the experimental results of limit height onset of fracture during the warm forming of meso-scale channels. The present research is expected to provide technical support for the design and optimization of warm forming process of ultra-thin titanium sheet metals.

高深宽比细密流道超薄钛极板温热成形是突破燃料电池功率密度和寿命瓶颈的关键。但超薄钛板成形中晶粒取向、马氏体形态与含量、位错滑移与孪晶等随工艺温度和特征尺度而复杂变化，导致成形失效模式出现韧脆转变。现有成形极限理论难以准确预测开裂风险，成形废品率高。.本项目围绕TA2超薄钛板介观尺度温热成形中组织演化和尺度效应耦合作用下韧脆转变的失效机制，从断裂失效现象、机理分析、物理建模三方面开展研究：首先利用同步光源成像进行失效过程原位观测，揭示微孔洞及微裂纹的演化规律；再进一步揭示组织演化、尺度效应及其耦合作用对损伤失效过程韧脆转变、微孔洞、微裂纹演化行为的影响机制；最后，将温度作用下的尺度效应引入Gurson孔洞长大模型和Horii-Nemat-Nasser裂纹扩展模型，建立介观尺度温热成形极限预测的韧脆转变损伤失效模型，并通过微流道热成形进行实验验证，为超薄钛板成形工艺设计提供理论基础。

随着国家燃料电池汽车示范城市建设的深入，高功率密度、长寿命车用燃料电池成为产业发展的关键。高耐蚀、长寿命的钛双极板是车用大功率燃料电池的重要部件，为满足大功率车用电堆功率密度要求，需要实现钛极板高深宽比亚毫米级细密流道的高效高质量成形制造。但超薄钛板成形这类亚毫米级微细特征时，晶粒取向、位错滑移与孪晶等随特征尺度而复杂变化，成形失效模式出现韧脆转变，现有成形极限理论难以准确预测开裂风险，成形微流道深宽比小、成形废品率高。.为了深入揭示超薄钛板的断裂尺度效应机理、建立准确预测模型以指导成形工艺，本研究从钛薄板成形断裂尺度效应机理探究、损伤建模、成形验证三方面开展了深入研究：.（1）搭建了超薄钛板原位拉伸观测台，利用上海同步光源高能X射线成像技术，完成了钛板成形断裂过程中损伤结构演化的实时观测；结合EBSD、SEM表征等方法，对不同晶粒条件、不同温度下钛薄板试样变形损伤过程及其周围组织结构的演变开展了观测；通过实验发现了钛薄板损伤形核、长大、断裂尺度效应与孪晶诱导再结晶的相关性，揭示了小晶粒下再结晶晶界阻碍裂纹扩展、形成韧性断面，而在大晶粒下再结晶发生降低、裂纹快速扩展形成脆性断面的机制。.（2）构建了以孪晶体积分数和位错密度耦合效应为判据的诱导再结晶模型，引入晶体塑性仿真模型准确描述尺度效应对诱导再结晶体积分数的影响，以位错滑移切变能作为损伤判据，建立了结合考虑损伤的晶体塑性力学仿真方法，实现了钛薄板断裂的准确预测；在此基础上，将再结晶晶粒导致位错屏蔽效应增强、抑制裂纹扩展的机制引入损伤模型，建立了考虑尺度效应的细观损伤模型，实现了不同晶粒条件下断裂应变的准确预测。.（3）在准确预测微流道断裂极限高度基础上，提出了多步成形方法，通过分散圆角处的应变集中趋势；搭建了超薄钛板微流道多步成形试验台，开展了微流道多步成形实验，通过三步成形实现了微流道极限成形深度和深宽比的显著提升。