表面层微观组织结构变化决定了零件的宏观性能，准确实现零件加工表层微观组织演变预测，进而提高零件加工表层力学性能（如硬度），是改善零件服役效能以及实现零件长服役寿命可控加工的一种有效方法。切削加工是钛合金TC4零件制造工艺中最基本的加工方法之一，切削过程中材料的剧烈塑性变形导致TC4加工表层微观组织变化复杂，本文针对TC4切削过程中的晶粒细化现象，对不同切削速度（100 ~ 500 m/min）下TC4组织结构多尺度分布特征、晶粒细化演变规律及其对表面材料硬度影响进行了研究。结果表明：TC4加工表层介观尺度（10^-6 ~ 10^-5 m）晶粒细化程度随切削速度提高呈现先增大后减小趋势，切削速度为300 m/min时，加工表面晶粒细化程度达69.7%，切屑剪切带晶粒尺寸细化至2 ~ 6 μm；微观尺度（10^-8 ~ 10^-7 m）上表现为复杂位错组态和纳米孪晶，纳米孪晶类型主要为｛10\bar{\mathrm{1}}1｝压缩孪晶，且纳米孪晶在较高切削速度（> 200 m/min）下产生；基于修正的Z-H晶粒细化模型和纳米孪晶体积分数预测模型，并考虑切削过程中晶粒细化引起的硬化作用，实现了TC4切削过程中晶粒尺寸以及硬化规律预测；构建了切削过程中晶粒细化引起的硬化规律与材料显微硬度的关系，通过模拟钛合金切削过程中晶粒细化程度和纳米孪晶形成，实现了TC4加工表层硬度预测，揭示了TC4加工表层微观尺度硬化机理。

The microstructure variations of machined surface determine the performance of machined components. Accurately predicting the microstructure evolution of machined surface and thus enhancing surface hardness of machined components is an effective way to improve the service performance and realize the controllable machining of components. Machining is one the fundamental manufacturing techniques of TC4 components and the severe plastic deformation during machining process induces the complex evolutions of microstructure for TC4 machined surface. For the grain refinement phenomenon during TC4 cutting, this paper studied the multi-scale distribution characteristics of microstructure, evolution mechanisms of grain refinement and its effect on the material hardness under different cutting speeds (100 ~ 500 m/min). The results show that grain refinement degree at meso-scale (10^-6 ~ 10^-5 m) increases first and then decreases with the increasing of cutting speed. At cutting speed of 300 m/min, grain refinement degree of machined surface is 69.7% and the grains in the shear bands of chips are refined to 2 ~ 6 μm. Complex dislocation patterns and nano twining are the features of microstructure at micro-scale (10^-8 ~ 10^-7 m). The deformation twinning type is mainly characterized as {10\bar{\mathrm{1}}1} compression twinning and it is generated at higher cutting speed (> 200 m/min). Grain refinement during machining of TC4 was predicted based on the modified Z-H grain refinement model and nano twining volume fraction prediction model. The hardening effect of grain refinement was also considered in the prediction model. The evolution of grain sizes and the work hardening was predicted. The relation between grain refinement and material hardness was established, and the hardness of TC4 machined surface was predicted with directional controlling the grain refinement degree and formation of nano twinning. And the hardening mechanism in micro-scale of TC4 machined surface was revealed.