搅拌摩擦加工碳钢表面纳米化

Surface nanocrystallization of carbon steel can be successfully obtained by friction stir processing (FSP) as a new nanotechnology. Nanocrystallization is integrated controlled by thermal and distortion activation, thus the characteristic of grain boundary has significant effect on nanocrystallization mechanism during FSP. Taking carbon steel as an object, the surface nanocrystalline layer with different grain size and proprotional high-angle grain boundary is fabricated by the modification of FSP tool, processing parameter and forced cooling condition base on the design and fabrication of FSP equipment for nanotechnology. Microstructure evolution of FSP surface nanocrystalline layer is investigated by analyzing the grain size, characteristic of grain boundary and the second phase, and dislocation configuration using TEM, SEM, EBSD and AFM technology. Therefore, Microstructure formation mechanism of FSP surface nanocrystalline layer is obtained by comprehensive analyzing the above microstructure reasearch results and mechanical properties of tensile and fatigue. On the base of above studies, the effect of the characteristic of grain boundary and the second phase and dislocation movement on the formation mechanism of surface nanocrystalline layer is revealed. Microstructure stability and the effect of high ratio high-angle grain boundary on the fatigue property of surface nanocrystalline layer are also interpreted. These results will provide theoretical basis for FSP surface nanotechnology.

搅拌摩擦加工（FSP）作为一项新型纳米化技术，可成功实现碳钢材料表面纳米化。FSP纳米化受热激活和变形激活协同作用，晶界特征对纳米化形成机制产生显著影响。本项目以碳钢为研究对象，在设计开发FSP表面纳米层制备系统的基础上，通过FSP工具设计、加工参数、强制冷却条件调整等方法制备出不同晶粒尺寸、高角度晶界分数的表面纳米层，采用TEM、SEM、EBSD、AFM等技术对表面纳米层的晶粒尺寸、晶界特征、第二相特征、位错组态等进行分析，掌握表面纳米层的组织演化规律，结合表面纳米结构材料拉伸、疲劳等力学性能参量，获得FSP表面纳米层的微观形成机制，揭示晶界特征、第二相特征和位错运动对纳米层形成机制的影响，阐明表面纳米层的组织稳定性，探讨高比率高角度晶界对材料疲劳性能的作用机理，从而为FSP表层纳米化技术提供基础理论依据。

本课题对DC04 IF钢及Q235低碳钢进行搅拌摩擦加工（Friction stir processing，FSP）和水下搅拌摩擦加工（Submerged friction stir processing, SFSP），利用OM、SEM、TEM等对改性层进行了微观组织表征，通过显微硬度测试、室温拉伸、疲劳、电化学腐蚀、中性盐雾腐蚀实验等，建立了微观组织与力学性能及腐蚀行为的关联性。结果表明：IF钢FSP后，最优参数为旋转速度950 rpm，前进速度60 mm/min，成功将晶粒尺寸从45 µm细化到5~10 μm。加工区平均硬度较母材提高了140.8%，抗拉强度提高了150.9%；通过建立IF钢母材和FSP加工区样品在空气中和3.5%NaCl溶液S-N曲线的拟合方程，疲劳裂纹扩展速率方程，结果表明，IF钢FSP加工区的疲劳寿命在腐蚀条件下比空气中降低，疲劳裂纹扩展速率比空气中快，同时FSP加工区疲劳强度远高于母材，疲劳裂纹扩展速率低于母材；IF钢在NaCl和H2SO4电化学行为试验中，FSP材料腐蚀电流密度较母材分别降低了47.0%和32.8%；在中性盐雾环境中，母材的平均腐蚀速率是细晶材料的1.3倍。低碳钢进行了FSP和SFSP试验，在相同加工参数条件下，SFSP比FSP获得的组织细化效果更加显著。在750 rpm转速SFSP时，成功将铁素体晶粒尺寸从11.7 µm细化到5.4 µm，板条状铁素体宽约650 nm，材料的硬度、抗拉强度和延伸率分别达到母材的125.4%，135.3%和87.9%，细晶强化、弥散强化、位错强化是力学性能提高的主要原因。电化学腐蚀中，750-SFSP试样的腐蚀速率仅为母材的38.4%；中性盐雾环境下，750-SFSP试样的耐蚀性明显提升，主要归结于晶粒细化和板条状铁素体的生成。