摘要
激光清洗是一种环境友好的新型表面清洁技术，自上个世纪六十年代末诞生以来取得了迅速发展。激光清洗技术根据其工作过程中是否使用辅助材料分为干式激光清洗和湿式激光清洗，其中干式激光清洗凭借其独特的清洗作用机制以及传统清洗技术所不具备的诸多优点，受到了研究人员的高度重视，理论模型和清洗装置的研究在表面处理、材料改性和基底保护方面得到持续和快速的发展。
本论文以脉冲激光干式清洗漆层的过程作为研究对象，针对目前已经存在的干式激光清洗模型的理论中存在的不足，特别是热振动模型中存在的结构简单、一维研究过程中假设条件多且忽视了实际存在的中间界面层的作用等问题，提出了一种更贴近实际工作状态的三层吸收界面烧蚀振动模型，并利用该模型对激光清洗漆涂覆层的机制、清洗阈值等模拟与实际工作密切相关的因素进行了比较分析。用数学方法确定清洗模型中的各层吸收的激光和热转化，并使用有限元方法对其进行了数值求解，给出了清洗过程中的温度分布、应力分布和位移值的数值计算结果。与实验结果相比较，该模型模拟结果较为符合实际情况，结合中间界面层吸收特性调节激光参数能提高清洗效果并降低对于基底造成的热损伤。
通过对激光光束整形的方法获得平顶激光光束，数值计算了干式激光清洗中使用平顶激光和高斯激光对于激光除漆过程中表面温度、热应力以及基底和漆涂层位移的影响，计算和实验验证了三种不同情况的平顶激光的不同除漆效果，发现采用合适的平顶激光进行除漆能够降低对于基底的损伤。使用高重复频率的平顶脉冲激光并结合适当的扫描速度，与采用高斯激光清洗相比，可以提高清洗效率达7倍。
在激光清洗应用方面，研究了激光能量、光斑外形和入射角度对于清洗效率的影响。通过研究峰值功率和改变激光扫描搭接率确定优化激光除漆的方案，在激光功率密度达到清洗阈值情况下圆形光斑为80%以上搭接率时，可完全清除基板表面漆层而不损伤基底。
通过本课题的研究，获得了漆层清洗的三层吸收界面烧蚀振动模型，较为符合实际情况，对于平顶激光的研究有助于提高清洗速度而不形成对基底的损伤，同时，激光清除漆层的工艺研究对于激光清洗应用中的漆层清洗和再涂装具有参考意义。
Laser cleaning is a new type of surface cleaning technique. Compared with traditional cleaning techniques, the technique has excellent cleaning effect and is environmental friendly, so it attracts more attention and has achieved great improvement since the first use of last century. According to whether assistant materials are used or not during cleaning process, laser cleaning can be divided into two different types:dry laser cleaning and steam laser cleaning. Dry laser cleaning has unique cleaning mechanisms and advantages that exceed traditional cleaning techniques. More and more scientists and engineers have been attracted into this domain, and theoretical models and cleaning devices have been developed and studied successively.
In this dissertation, theoretical modeling of paint removal by dry laser cleaning was chosen as research object, to overcome the shortage of the existing cleaning models, especially the thermal expansion model with the shortcoming of simple structure, many assumed condition and the neglect of the effect of interlayer. Based on the existing removal model of large area coating a new thermal ablation vibration model is constructed. The model has three absorptive layers and is more accord with practical situations. Through this model, the mechanisms of laser paint stripping are further analyzed. Moreover other coatings removal threshold and key factors that influence the actual cleaning effect are simulated. In this model of three absorptive layers, the absorbencies of each layer are defined and the heat converting process is analyzed. With the computer aid of finite element method, the theoretical cleaning threshold, temperature, stress, and displacement distribution be calculated. A series of experiments of dry laser paint stripping also be carried out and measured. The experimental and theoretical results are compared at different working environments. The results demonstrated that this model is effective. The cleaning effect can be better and the thermal damage of substrate can be reduced if the interlayer is taken into account appropriately.
Through beam shaping, flattened laser beam can be obtained from Gaussian beam. The surface temperature, the thermal stress, and the displacement of paint and substrate layer are numerically calculated, which are resulted from pulsed laser heating of Gaussian and flattened laser beam respectively. Comparing these theoretical results, we can find out differences between these two kinds of laser energy distribution. On the basis of former numerical calculation, the cleaning effects and efficiency of different working are compared. The results demonstrated that the high repetition flattened pulsed laser can reduce the substrate damage, and the cleaning efficiency can be increased about seven times.
In this dissertation, the application of laser cleaning is also studied. The optimization method of dry laser cleaning is investigated by control the laser parameters, including laser fluence, beam shape, and angle of incidence. Through the change of intensity of laser and overlap ratio of circle laser spot, the optimized cleaning efficiency can be obtained, especially with the overlapped ratio of80%and more.
In this dissertation, the model of three layer thermal ablation vibration is helpful to enhance the laser cleaning efficiency in view of interlayer. The investigation of flattened laser can improve the stripping rate and reduce the substrate damage. Moreover the studies of laser cleaning processes have great value to paint stripping and recoating in industrial protection.
引文
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