钛合金超低温高速切削表面孪晶强化机理

Grain refinement or nanostructure can effectively enhance the metal material mechanical properties of strength and ductility. It has been widely implemented in metal bulk plastic deformation processes to achieve higher yield strength and ductility. The technique is equally qualified as one critical approach to significantly improve surface integrity created from high speed machining operation. This research focuses on investigating the strengthened mechanism of grain refinement and twinning generated from the high speed machining of titanium alloys under cryogenic conditions. In theory, cryogenic condition can significantly improve the machinability of titanium alloy, reduce cutting temperatures, tool wear, and prevent material phase transformation, high-hardness/brittleness, softening and grain aggrandizement tendency. Those unfavorable effects can be activated by the extremely elevated temperatures generated from high-speed machining. This research aims to achieve the mutual excellent properties of higher strength and ductility of titanium alloy in machined surface. Several interdisciplinary theories and techniques including the mechanisms of “catastrophic adiabatic shear” and “periodic crack initiation and propagation” for high-speed machining, microstructure measurement technique, and grain refinement and twinning effect under cryogenic condition are employed to explore the multi-scale strengthened mechanism of titanium alloy in the machined surface. Based on the established mechanism model, parameter sensitive analysis, and the mechanisms transition between severe plastic deformation and the twinning density of titanium alloy, service conditions are identified and coupled into a creative anti-fatigue model, which will suggest a scientific methodology to achieve an excellent performance of the machined surface by controlling the high-speed machining process of titanium alloy under cryogenic conditions.

晶粒细化或纳米化既能提高材料强度又能增强其塑性，是金属材料性能强化的有效技术途径，也是提高精密切削表面完整性及其抗疲劳性能的科学机制之一。项目针对具有重要应用价值的钛合金，拟探索超低温冷却条件下、高速切削钛合金表面晶粒细化及其孪晶效应的强化机制。理论上，超低温冷却能有效改善钛合金加工性能，降低切削温度，减缓刀具磨损，避免材料相变和晶粒粗化，实现钛合金的高效切削；综合高速切削的高应变率和超低温冷却的联合效应驱动晶粒和孪晶密度增殖，实现加工表面的“强度-塑性”复合强化。基于高速切削的剧烈绝热剪切和周期性裂纹萌生和扩展机理、微结构测试技术、材料本构方程、超低温孪晶效应的相互耦合映射关系，探明钛合金高效加工与超低温孪晶强化效应的临界条件和协调机制，建立以孪晶强化机制为核心的钛合金超低温高速切削表面的宏微观多尺度机理和实证模型，探索出有效提高精密加工表面完整性及其服役性能的科学方法和新工艺原理。

基于分子动力学，在微观尺度建立了钛金属和钛合金常温、低温和超低温条件下高速压入、正交切削表面成分演变，晶粒度细化和孪晶强化的机理模型。研究了加工参数、温度变化、初始状态等对位错密度、位错增殖、孪晶效应和成分演变的影响规律。从理论上揭示了钛金属和钛合金在低温、高速精密加工表面和次表面物理性能强化的机理方法，为宏观和介观尺度的性能控制提供了理论基础。. 基于自洽方法（SCM）提出了高速加工钛合金新的流动应力方程，提高了有限元介观尺度仿真计算模型的准确度。基于该模型采用有限元方法研究了刀具角度、磨损程度、加工参数和温度条件对加工表面和次表面的应力应变、应变速率、温度分布梯度和三轴静水压力分布，及其对材料晶粒度、位错密度和孪晶效应的影响机理。采用实验研究方法测试在大流量和干态条件下钛合金铣削力、铣屑形态；并采用微观材料表面性能测试技术对铣削表面完整性进行了较为系统的测试和分析，探究了铣削表面强化的临界加工参数、温度条件和刀具状态，并对理论和仿真模型进行实证和验证。 . 基于钛合金、钛金属加工表面的多尺度有限元和实验实证模型。将钛合金的微观特征如晶粒、孪晶密度、位错增殖等通过应力场、应变场和温度场耦合在钛合金材料本构模型中，采用晶粒度、孪晶方向、位错密度演化模型描述钛合金加工表面的微特征，进而预测超低温高速切削表面微观结构及其影响深度。基于模型参数敏感度分析、大塑性应变的孪晶-位错转换机制、钛合金的服役工况，探究超低温高速精密切削表面强化预测和控制方法。研究加工表面晶粒细化模型与材料流变应力参数、刀具刃口半径、工件-刀具摩擦状态、加工轨迹、加工系统刚度等实际工程应用中精密加工表面性能可能的形成因素和演变机制。. 基于多尺度加工表面预测模型，结合具有复杂结构零件的铣削加工接触变刚度相互耦合条件影响，探索实际工程应用中复杂零件的铣削表面完整性预测和控制的科学原理和工艺方法。.