锻造加工效应下的钛合金汽轮机叶片环境致裂及寿命预测研究

Sustainable manufacturing is the one of the utmost importance to all socially responsible manufacturers. To be competitive on a global scale manufacturing to the large equipments needs to be aligned with strict energy consumption regulations. The forging manufacturing industry at large is striving to improve productivity and product quality while maintaining minimizing energy consumption. this can only be achieved by adopting sustainable forging techniques of manufacturing and perfect forging quality evaluation system which mainly depending on the long-life and reliable service performance of the forgings. Therefore, the aim of this project is to address the performance evaluation and life prediction of the last-stage blade in the steam turbine considering the structure integrity due to forging process. This research can be used to ensure the long-life and reliable service of last stage blade which is fabricated by titanium alloy. The following issues will be focused in the present project: (1)Clarifying the formation and evolution mechanisms of the microstructure and residual stress which are related to typical forging process used for Ti6Al4V alloy. Then, the key parameters which can be used to characterize the structure integrity can be identified. (2) Describing the relaxation of residual stress due to large plastic deformation by forging technology during the environmental fatigue process in high-temperature water containing contain NaCl. Then, the contributions of environments, residual stress and microstructure on the environmentlly assisted cracking will be reflected. (3) Describing the surface coupling chemo-mechanical behavior near fatigue crack tip and the value of residual stress along with fatigue cycles. The prediction model of multi-scale fatigue crack propagation rate will be developed through considering the crack-tip strain rate and its gradient distribution.Through the accomplishment of this project, the environmentlly assisted cracking behavior of the titanium alloy used in long last-stage blade will be identified. In this model, this project could provide some important insights on the improvement of the long-life design and manufacturing of the key components used in advanced large turbine. Also, the effect of sustainable forging machining will be considered, this will recommend ways to improve the forging technology while simultaneously ensuring forging quality.

以“可持续制造”为主题的大型装备制造领域，能源消耗大户的锻造业需要进行锻件性能评价的完善和锻造工艺的提升，这依赖于此锻件品质在服役环境中高性能、长寿命方面的表现。因此，本研究针对汽轮机钛合金长叶片展开锻造加工效应下的性能合理评价及寿命可靠预测模型的研究，主要包括：（1）通过揭示钛合金锻件工艺相关的微观结构和残余应力的形成与演变规律，实现锻造加工相关的完整性关键特征参量的提取；（2）澄清服役环境中锻造加工诱发的残余应力释放机制，建立环境－残余应力－微观组织综合效应的映射关系，合理描述三者对材料环境致裂抗力的竞争机制；（3）通过对裂尖微区表面化学－力学－加工效应的残余应力参量的合理描述，建立物理意义明确的多尺度裂纹扩展的环境致裂寿命预测模型。本研究可望获得长叶片材料环境致裂的基本规律并建立预测模型的同时，为我国大型锻件完整性评价的完善及锻造水平的提升提供科学支持。

钛合金因其较高的比强度，耐热性和耐腐蚀性被广泛用做发动机或汽轮机叶片等关键部件的制造材料，而工业领域中承载零部件大多采用锻造塑性成形。随着全球性资源和环境问题的日益凸显，现代成形制造尺度与服役环境日趋极端化，结构完整性的研究除了从本质上把握构件失效的规律，还要着眼于材料塑性成形制造的创新与拓展，即从材料/构件在极端服役条件下的结构完整性研究来促进制造技术的发展和创新。.本项目重点研究了复杂服役环境下钛合金材料的结构完整性和相对应的锻造塑性成形提升技术与方法。研究内容总体划分为两个方面，一方面研究当材料/结构长时间处于腐蚀环境-载荷交互作用的恶劣工况条件下，在多场耦合作用中钛合金材料应力应变响应随时间的演变，即从微细观层面研究钛合金微观结构的演化和微缺陷的萌生与扩展的过程；另一方面研究锻造塑性变形工艺中诱发的残余应力及微观组织结构演变对钛合金的力学性能和蠕变疲劳寿命等的影响，明确工业领域常用的基于动量定理的能量限定和基于静压原理的载荷限定两种不同工艺方式和工艺参数对锻件质量稳定性和可靠性的影响程度，进而澄清锻造工艺参数与材料服役寿命之间的复杂映射关联。.项目建立了裂尖材料力学-环境因素耦合的模型，实现了裂尖应力腐蚀力学效应的科学表征，建立了包含裂尖力学和腐蚀环境耦合的裂纹扩展行为预测模型，量化材料组织结构对应力腐蚀裂纹扩展的影响；项目通过物理实验和数值模拟方法对比了两种工艺方式下钛合金塑性成形微观组织和力学性能，发现了单一工艺方式的局限性，进一步明确了锻造温度、变形量和变形速率对钛合金锻件残余应力、疲劳寿命和蠕变特性的影响，可实现针对钛合金不同服役性能的要求制定最优工艺参数组，确保科学技术研究成果成为企业经济效益与社会效益的必要保证。本项目为后续突破传统塑性成形工艺的局限性，创新复合工艺并进一步深化钛合金塑性成形的强化机制研究奠定了坚实的基础。.