基于扫描电子束的新型抛光技术机理和数学建模研究

Some difficult problems need to be solved in the surface modification technology by electron beam for the die steels. Some defects such as craters，depth-to-width ratio too much and phase structure cracks exist in the modified layer. These problems cannot be solved in the existing modification technologies by electron beam. A new polishing technology by scanning electron beam is studied in this project. The new technology is an important breakthrough and can promote greatly the development and application of surface modification technology by electron beam.. The mechanisms and related scientific problems of surface micro-melting and polishing technology by scanning electron beam were studied by the theory and experiment. Based on the thermodynamics, fluid dynamics and metal solidification theory, the quasi-static, dynamic equations and GBE model were established during the process of surface micro-melting and polishing. According to the theoretical and microcosmic viewpoints, the relationship between grains growth, melting depth and melting time were obtained during the process of rapid melting and solidification. The three-dimensional transient coupling model of temperature field, structure field and stress field was established based on microstructure calculation theory and experimental testing. The coupling and distribution laws of three-dimensions were analyzed during the phase transformation hardening process of surface micro-melting and polishing. The effects of the original roughness and surface morphology on microstructures, properties and roughness were studied. The processing conditions and basic mechanisms of composite strengthening by surface polishing of scanning electron beam were obtained.

在模具钢电子束表面改性技术研究工作中，改性层存在“深宽比过大”、“火山坑”和“相结构裂纹”等缺陷是一些亟待解决的问题，目前现有电子束表面改性技术未能很好的解决这些难题，因此项目研究扫描电子束的新型抛光技术,极有可能成为一个重要的突破点，对电子束表面改性技术研究工作的发展和应用起到极大的推动作用。. 本项目从理论和实验两方面研究扫描电子束热源加热时，表面浅熔抛光技术机理和表面复合强化相关科学问题，基于热力学、流体动力学等理论，建立表面浅熔抛光过程的GBE模型、准静态和动态方程，从理论和微观角度揭示表面抛光快速熔凝过程中晶粒生长及熔深和熔融时间的变化规律；基于组织计算理论和实验测试，建立表面浅熔抛光相变硬化过程中温度场、组织场、应力场三维瞬态耦合模型,采用SEM和TEM等手段从表面和截面分析被处理材料，并结合温度场、组织场与应力场计算从本质上揭示扫描电子束表面抛光复合机理。

电子束表面处理是一种新型材料表面处理技术。本项目提出了扫描电子束表面处理的新型技术方法。基于热力学、流体动力学、相变理论等，建立了温度场、应力场、组织场和流场的三维有限元模型，探讨了扫描电子束加热及冷却凝固过程的温度分布及三场之间的相互作用规律；采用元胞自动机法和蒙特卡罗法，分别建立微观组织形核与生长模型；研究了相变硬化过程中多场耦合的分布规律；讨论了电子束工艺参数对抛光组织和性能的影响规律。研究了扫描电子束表面合金化和多道扫描对组织和性能的影响。. 结果表明：扫描电子束处理后组织形貌分为强化层、热影响区和基体三部分，熔池分界线为抛物线；凝固区底部最先发生非均质形核，形成细小晶粒层，中部呈柱状晶生长，上部为等轴晶区，机械性能明显提高。表面温度分布均匀，横截面温度呈梯度分布；电子束扫描带附近为拉应力，其它位置为压应力。表面最大应力为115.4MPa；束斑中心处液态金属流速最大0.28 m/s，熔池底部流速最小为0.11 m/s；加热速度和降温速度分别高达3.2×〖10〗^3K/s和1.6×〖10〗^3K/s；扫描电子束表面微熔抛光其表面粗糙度最低为0.39μm。有时表面会出现微小的“熔坑”，且含碳量越高“熔坑”产生的概率越大；多道扫描处理搭接区组织依次为回火的索氏体、屈氏体和马氏体；45钢扫描电子束合金化区组织主要由针状马氏体和碳化物颗粒组成，表面硬度是基体的5倍，磨损失重是基体的五分之一；当扫描电子束功率增加到280W时，合金化区将出现裂纹；随着束流的增加，表面应力、硬度和耐磨性提高，熔池深度、宽度增加，表面粗糙度呈现先下降后上升的规律，扫描速度的增加则呈相反的变化规律。. 项目研究获得的工艺条件与相关机理，可为扫描电子束材料表面抛光、强化与合金化处理技术的实际应用提供理论技术支持。研究的方法和结论可为电子束调控金属材料表面性能、实验设备的研发与设计提供参考依据。