【作者】 王飞；

【导师】 姚振强；

【作者基本信息】 上海交通大学 ， 机械制造及其自动化， 2008， 博士

【摘要】 激光冲击是一个复杂的瞬态过程,涉及到传热学、动力学、材料科学、激光技术和计算机技术等多门学科。激光辐射到金属靶材表面后,在极短的时间内,激光的被吸收、等离子体的产生、等离子膨胀爆炸形成冲击波、冲击波的约束和冲击载荷下板材的动态响应过程,以及激光冲击波对材料的微观组织的影响和材料的力学性能的变化等等诸多因素相互影响。激光冲击成形变形场的分布和动态响应的过程是研究激光冲击成形的一个关键问题,由于成形过程耦合因素多,加载时间极短,甚至短到纳秒级,以往的研究对许多影响冲击处理的因素作了大量的假设和一些定性的分析,对激光冲击成形的成形机理、变形场的理论计算和模拟、冲击成形处理的规律需要开展进一步深入的研究。单脉冲的激光冲击成形的研究可以更加简明更加直观地反映出激光冲击波的作用机理,建立板材变形量与激光参数、约束边界、材料的性能和作用时间等参数的关系,为分析各种参数对成形过程的影响提供有力的依据。单脉冲激光冲击成形的动态响应过程的分析和理论模型的建立,也为未来加工过程中各个参数的合理优化、板材变形过程的有效控制、分析和实现大面积金属板料的激光冲击成形奠定基础。本文通过对单脉冲激光冲击板材的变形过程的推导分析,提出了激光冲击板材的初速度分布假设;应用等效恒载荷简化计算,建立了单脉冲激光冲击下板材的变形场理论解析模型,并实验验证了数学模型的准确性;采用有限元仿真对激光冲击成形进行数值模拟,分析了激光冲击成形的特点;通过数学模型和有限元仿真相结合,讨论了影响激光冲击成形变形场分布的主要加工参数和激光参数,分析了激光冲击的动态响应过程,重点分析了速度,位移,应变,应变率的空间分布和随时间的历史分布;研究了激光冲击成形对材料主要性能的的影响。主要的研究工作如下:1.建立了激光冲击成形变形场的数学模型建立了以周边环形约束、中心区域受激光冲击的板材变形分析模型,基于冲量定理,在冲击加载区域,将冲击瞬间的力作用转化为板材发生塑性变形的初始动量,根据板厚与约束圆周直径的相对值,只考虑板材塑性变形的动力学弯曲响应。在激光冲击成形中,将成形过程分为两个相,分别对两个相的变形进行了分析计算,在计算的过程中,引入等效载荷以简化计算。通过黄铜和TA2板材的激光冲击成形实验验证了数学模型的准确性。2.激光冲击成形的数值模拟建立了激光冲击成形的数值仿真模型用于研究激光冲击成形动态响应过程和成形规律。模型采用显式动态分析,有效解决激光冲击成形中载荷高、结构响应变化快的问题,以提高激光冲击成形预测的精确性。分析了激光冲击成形中的瞬态响应过程,主要包括板材的速度、位移、应变以及应变率的空间和时间分布。3.激光冲击主要参数对冲击成形影响的研究研究了不同工艺参数和变形场的对应关系。结合激光冲击成形的理论与实验研究,总结了板材厚度、激光能量、板材屈服强度等主要参数对变形场的影响规律,改变了过去只依靠少量的实验结果就对某单一影响因素进行预测的方法。掌握了这些规律,可以同时预测多个工艺参数对变形场的影响,也可以通过目标变形来规划激光冲击工艺参数。4.研究了激光冲击成形处理对金属材料性能的影响研究了激光冲击成形对材料的表面形貌的变化。约束层和表面涂层是激光冲击成形中的两个重要影响因素,分析了约束层和表面涂层的选择和主要相关参数。比较了没有涂层,涂层厚度不足和合理的表面涂层对激光冲击效果的不同影响。通过实验对比研究了激光冲击对材料表面显微硬度、材料的金相的影响。更多还原

【Abstract】 Laser Shock Forming (LSF) of metal sheet is a new and competitive laser based manufacturing technology. The dynamic response of the metal sheet and the deformation field are two key problems in the research of LSF rescently. LSF is a very complicated dynamic process. Lots of science and technology such as the diathermaneity,dynamics, material science, laser technology and computer science are all involved in LSF.. The laser irradiates on the metal sheet. And in a very short time, the absorption of the laser energy, the production and the combustion of the plasma, the expansion and the production of the laser shock waves, the confinement of the waves, the procession of the dynamical response of the sheet to the laser shock loads, the effect of the laser shock waves on the material microstructure, the change of the mechanical capability of the material and etc, all these factors have effect on each other. Although some theories, experiments and simulation of the LSF have been performed by some researchers and some important accomplishments are achieved, lots of supposion of the influencing factors and some simplified analysis are made because of too many interactional factors and the extremely short time about nanoseconds. The deforming mechanism of the LSF treatment, the mathematical computation and simulation of the displacement field and the discipline of the LSF are all need further research. The research of LSP with single laser pulse can simplify the model and display the mechanism directly. The relationship among the displacement of the sheet, the laser parameters, the confined conditions, the material properties and the loading time can be established and offers strong proof for the analysis of process of LSP. In this paper, through the analysis of the deforming procession of the metal sheet, reasonable supposition of velocity field is made. Applying equivalent constant load to simplify the computation, the mathematical model with single laser pulse of the displacement field is established and its precision is validated by experiments; applying finite elements, the numerical simulation is used to analyze the procession and results of the LSF. Combining the mathematical model and the numerical simulation, the main parameters of process are discussed and the dynamic process of the LSF treatment is analyzed. In the analysis, the deforming time, the displacement history, the strain and the strain rate are intensively discussed. In the end of the paper, the effect of the LSF on the material is studied. The main research work is listed as the following:1. The mathematical model of the LSF displacement field was established. Selecting the LSF with single laser pulse as the research object, the dynamic response of the metal sheet to the LSF load and the mechanism of the plastic deforming with high strain rate are studied. When the pressure generated by the laser shock waves is bigger than the dynamic yield strength of the structure, the plastic deformation is occurred. In the LSF treatment, the load time is extremely short and the reasonable muzzy velocity model is established. The process of the LSF can be divided into two phases. And then the displacement of the two phases is discussed and computed. In the process of the computation, the equivalent constant load is used to simpfy the work. In the end of this part, select the metal sheet of Brass and TA2 as the object, the precision of the computation is validated by experiments.2. The numerical simulation of the LSF treatment. According to the features of the LSF, the numerical simulation research is done to study the LSF. How to choose the parameter of the FEM model is analyzed. Comparing the simulated results with experiments, the precision and reliability are validated. The displacement history and the energy history of the structure changed with time are analyzed, which will be beneficial to the research of dynamic response of LSF. 3. The effect of the main LSF parameters is studied. In the LSF treatment, given the main parameters such as the material properties and the size of metal sheet, the displacement field is mainly determined by the laser energy and the size of the laser pulse. In order to study the effect of different parameters, the dynamic deforming process is analyzed with the numerical simulation and mathematical model. And then the displacement field can be predicted with the LSF treatment parameters. The parameters can also be specified with the objective displacement field. The relation of the parameters and the displacement field is studied qualitatively.4. The effect on the mechanic properties is studied. The surface quality of the LSF is examined and analyzed. The confining layer and the coating, which are two important factors in the treatment, are discussed. Three conditions, without coating, insufficient and reasonable coating are compared according to the experimental results. The investigation shows that the surface quality and the microstructure have no remarkable change; LSF is a mechanical process, not a thermal process and no visible ablation is observed. The micro-hardness of brass specimen increases by about 10~20% after the LSP treatment and the TA2 specimen has no obvious change.更多还原