In near-isothermal local loading forming (NLLF) of large-size Ti-alloy parts, one of the challenging issues is to achieve accurately prediction of complex microstructural development. In this work, a thermo-mechanical-microstructural FE model considering complicated phase transformation and morphology transformation of the whole forming process is developed and experimentally verified. The simulated distribution of phase amount for primary α (α_p) and secondary α (α_s) phases agrees well with the experimental results. Based on the model, the evolution characteristics of α_p phase, lamellar α_s (α_ls) phase, and globularized α_s (α_gs) phase for large-size Ti-alloy parts at different stages were investigated. It is found that α_sphase amount in the first loading region is significantly higher than that in the second loading region, while α_gsphase amount shows a reverse trend. Moreover, processing parameters including initial heating temperature, die loading speed, and loading pass greatly affect the relative amount and distribution uniformity of constituent phases among different loading regions. Finally, the formation mechanism of quasi-trimodal microstructures under NLLF of complex Ti-alloy parts is clarified by analyzing the coupling effects of applied deformation and phase transformation on the microstructure morphology. To achieve the regulation and control of the tri-modal microstructure, an optimized processing scheme is proposed.

在大尺寸钛合金零件的近等温局部加载成形 (NLLF) 中，具有挑战性的问题之一是实现对复杂微观结构发展的准确预测。在这项工作中，开发了一种考虑整个成形过程的复杂相变和形貌转变的热-机械-显微结构有限元模型并进行了实验验证。初级α (α _p ) 和次级α (α _s )相量的模拟分布与实验结果非常吻合。基于该模型，α _p相、层状 α _s (α _ls ) 相和球状 α _s (α _gs )的演化特征) 相在不同阶段对大尺寸钛合金零件进行了研究。发现第一加载区的α _s相量明显高于第二加载区_的相量，而α _gs相量呈反向趋势。此外，初始加热温度、模具加载速度和加载道次等加工参数对不同加载区域之间组成相的相对数量和分布均匀性有很大影响。最后，通过分析外加变形和相变对微观组织形貌的耦合作用，阐明复杂钛合金零件NLLF下准三峰微观组织的形成机制。为实现三峰显微组织的调控，提出了优化的加工方案。