高塑性材料的高应变率脆化效应及纯铁脆性切削机理的研究

Pure iron has extensive and important applications in weapon, express, and nuclear power industries. However, it is very difficult to be fabricated into high-quality components by machining due its high ductility and toughness. Theoretically, material become brittle and be removed brittlely in machining if the cutting speed is increased near the sound speed in the material. The chip generated in the machining then will change from continuous to segmented as well. In this project, embrittlement of pure iron and its influences in the machining proess are stuied to find a way to machine pure iron with high quality. Firstly, dynamic material ductility is represented by material max fracture strain, and the material dynamic elasticity is represented by the quotient of material yielding stress dividing by elastic modulous. Then the dynamic ductility evolution of pure iron is obtained by a series of high-velocity impact tests. Secondly, the coupling between stress, temperature, strain rate, and fracture strain in the primary shear zone is studied. The evolution of fracture strain of the primary shear zone with the increasing of cutting speed is assessed. Through the analysis of stain-stress evolution under different material ductility states due to different cutting speeds, the mechanism of chip transition from serrated to segmented is explained. Finally, the chip segmentation is predicted by a model based on energy balance in cutting. The results of the this project are useful for the high-quality machining of pure iron and other high-ductility materials.

纯铁在兵器、高铁、核电等方面有广泛且重要的应用前景，但因塑性和韧性高切削加工时断屑难、易粘刀和产生表面鳞刺，加工质量难控制。理论上，塑性材料以其声速当量切削速度切削时，塑性变形表现为脆性破坏，变形区材料由弹粘塑性转变为弹脆性，切屑形态从带状转变成单元状。本项目研究高塑高韧性材料——纯铁在高速变形时的脆化行为，探索脆性状态下纯铁精密切削的机理。首先基于损伤力学损伤度高应变率敏感性建立断裂失效应变模型，定量表征纯铁高应变率快速脆化行为；以率相关屈服强度与弹性模量之比表征纯铁动态弹性，揭示高应变率阶段纯铁动态弹塑性演化规律。其次，建立第一变形区材料热-力-弹塑性耦合模型，探索高速切削中纯铁动态弹塑性变化对切削热、力和变形的影响机制。最后明确纯铁弹塑性变化对切削变形能的影响规律，基于能量平衡提出单元切屑形成临界切削条件判据和纯铁脆性加工方法。研究成果对其他软粘性材料精密切削加工亦具有理论指导意义。

纯铁在兵器、高铁、核电等方面有广泛且重要的应用，但因塑性和韧性高切削加工时断屑难、易粘刀和产生表面鳞刺，加工质量难控制。.本项目基于材料的高应变率脆化效应，通过提高切削速度提高材料变形速度和相应材料物理机械性能，探索解决软黏材料切削加工中的粘刀、已加工表面质量差和难以断屑问题的可能性，开展了如下研究：首先，研究纯铁在大应变率范围的变形和断裂行为，探寻断裂失效应变和应力、温度、应变率之间的关系，建立纯铁断裂失效应变模型，揭示高应变率阶段纯铁弹塑性演化规律。其次，研究切削层材料流经第一变形区时变形区温度、应力、弹塑性的耦合演化过程，分析随切削速度提高变形区材料热、力、弹塑性的变化规律，探索这些变化引起的切削变形失稳机制的转变机制和规律。最后，研究纯铁弹塑变化对切削变形能的影响规律，分析不同切削参数下切削变形区能量状态和变化规律，基于能量法提出切削脆化条件形成临界切削条件判据和纯铁脆性加工方法。.本项目通过大量的高速冲击和切削实验，获得了纯铁材料由等温塑性状态到绝热剪切状态，再到高速变形脆化状态的全应变率范围断裂形貌和金相图谱，揭示了纯铁材料在应变率约为10^5/s时开始断裂应变快速降低，至约10^6/s时成为完全脆性材料。基于J-C断裂应变模型建立了能够同时描述材料高/超高应变率脆化效应的断裂应变模型，探明了纯铁动态弹塑性演化（脆化）规律，揭示了不同变形速度下纯铁材料的等温断裂失效、绝热剪切失效和高速变形脆性断裂失效的发生发展和演化规律。通过二次开发将纯铁断裂失效应变模型嵌入第三方有限元分析软件，基于此程序（软件）对纯铁进行了高速切削仿真，揭示了在不同切削速度下切削纯铁的切削力、热、温度分布和耗能的变化规律，进行了基于热软化和高应变率脆化的纯铁高精度切削加工工艺探索。以上结果可为纯铁切削加工实践中制定合理高效、高精高质的加工工艺提供理论和数据支撑。