【作者】 樊玉杰；

【导师】 周建忠；

【作者基本信息】 江苏大学 ， 机械制造及其自动化， 2011， 博士

【摘要】 激光微喷丸技术(μLSP)利用μJ量级能量、ns量级脉宽和μm量级光斑的短脉冲激光束,与材料相互作用诱导冲击波压力进行表面改性,能够在微器件表层或局部产生适度残余压应力分布,增加处理区域的硬度及弹性模量,改善零件表面的耐磨损和耐腐蚀性能,提高零件的抗冲击能力,从而延长金属构件的疲劳寿命,从制造源头解决金属微结构及器件的失效问题。然而目前对微尺度激光喷丸中的尺度效应问题、强化表面力学性能及形貌的有效表征方式、激光微喷丸导致的晶粒细化机制与微观强化机理,以及实现零件表面强化处理的工艺准则尚缺乏系统的研究。本文在详细分析激光微喷丸强化机理的基础上,采用理论、数值模拟和实验相结合的方法,对纯铜试样进行激光微喷丸强化研究,主要工作如下：(1)微尺度冲击波压力传播的理论分析。在分析脉冲激光—能量转换体—冲击波压力模型的基础上,考虑高应变率条件和材料物性的非线性效应,根据微分几何、爆轰波理论、应力波基础等理论,得到了冲击波压力的二维时空分布,建立了适合微尺度下激光喷丸强化的二维压力加载模型。采用椭球波传播理论,在现有一维压力模型的基础上,根据椭球的几何特性,给出波阵面上任意一点任意时刻的径向压力与轴向压力的比值K(t,r),推导了冲击波在典型介质水中传播时的径向压力简化模型,获得了径向压力的时空分布曲线,得到了微尺度下冲击波压力的作用规律。(2)激光微喷丸强化纯铜的数值模拟。以ABAQUS软件为平台,建立了高应变率条件下激光微喷丸过程的有限元分析模型,编制专用的分析模块,考虑微尺度下材料的各向异性作用,引入了单晶体材料模型,开展了单点激光微喷丸诱导的表面动态应力和塑性变形数值模拟研究,分析了工艺参数对残余应力分布的影响规律；进行了多点搭接激光微喷丸强化纯铜的数值模拟。根据圆形光斑的搭接特性,选取过光斑中心和到光斑中心线0.5R的剖线为典型路径,表征了受喷表面及深度方向上的残余应力及塑性变形分布,探讨了不同工艺条件及喷丸方案对残余应力及表面形貌的影响。(3)工艺过程控制与参数优化。引入形状因子σSF描述了单晶体不同晶面取向下残余应力场的形状特征,更加合理地表征材料强化层的残余应力分布特性；采用Mintab软件中的Taguchi设计方法对模拟方案进行实验设计,提取各种工艺参数下的残余应力表征值数据进行处理,通过信噪比分析和方差分析确定各因素对残余应力各表征值影响的显著程度,为激光微喷丸强化过程的参数优化和残余应力控制提供指导。(4)激光微喷丸强化纯铜实验。通过测量单点激光微喷丸强化区域的表面轮廓和粗糙度,分析了工艺参数对纯铜塑性变形的影响规律；通过测量激光微喷丸区域的纳米硬度、弹性模量以及接触深度,研究了单点强化区域内力学性能的变化规律,探讨了激光微喷丸提高纯铜硬度和弹性模量的机制,并给出了影响材料表面性能变化的关键因素；在此基础上研究了多点激光微喷丸搭接条件下,能量、搭接率等主要工艺参数对表面残余应力、表面形貌及粗糙度的影响规律,获得了不同条件下合适的激光微喷丸参数和工艺准则。(5)激光微喷丸强化纯铜塑性变形层的微观组织变化及其强化机理研究。采用透射电镜(TEM)、光学显微镜(OM)等设备观察激光微喷丸后纯铜沿深度方向的晶粒大小、微观结构。在多次激光微喷丸条件下,观察微观结构演变的基础上,分析了纯铜塑性变形过程中的微观组织演化机理,系统的揭示了激光微喷丸纯铜晶粒细化机制和微观强化机理,并探讨了微观组织结构在强化处理中所起的作用。本文的研究成果,丰富了激光诱导冲击波压力的基础理论,对研究高应变率条件下,动态塑性变形过程中的微观组织演化规律有重要的理论意义,为激光微喷丸强化工艺效果的评价提供了参考和指导。更多还原

【Abstract】 Micro scale laser shock peening (μLSP) has been proposed to improve the fatigue durability, corrosion and wear resistance of metals with laser beam diameter at the order ofμm, pulse duration at ns and laser energy at pJ, during which shock wave pressure generated by the interaction between laser beam and material can introduce moderate residual stress distribution and enhance surface nano-hardness and elastic modulus. It fundamentally resolves the failure problem of micro metal components. Nevertheless, up to now, problems of scale effect, response of mechanical properties, efficient characterization of surface morphology, micro strengthening mechanism and technological criterion ofμLSP are still stacking. After detailed analyzing the strengthening mechanism ofμLSP, theory, numerical simulation and experiments were adopted to research the copper specimen treated byμLSP. The main work are as follows:(1) Theoretical analysis of the shock wave propagation in micro scale. According to differential geometry, detonation wave theory and stress wave basis theory,2-D shock pressure distribution was obtained.2-D pressure model was established based on the analysis of pulse laser-energy transfer medium-shock wave pressure model with high strain rate conditions and material property of nonlinear effects. Comprehensively considering properties of shock wave ellipsoidal propagation in time and spatial distribution and geometric features of ellipse, ratio of pressure in axial direction and radial direction K(t, r) were put forward, and spatial pressure distribution in radial direction at different time was derived.(2) Numerical simulation of copper treated byμLSP. Based on ABAQUS software, the finite element models ofμLSP under high strain ration condition was established. The special analytic module forμLSP was generated. The influence of material anisotropy was considered, material model for single crystal was introduced. Surface dynamic stress and plastic deformation under single spotμLSP were analyzed, and influences of processing parameters on residual stress were researched. Meanwhile, simulation of multi-spotμLSP was also conducted. Typical paths with profile across laser spot centre and 0.5R away from laser spot centre were selected to characterize the distribution of residual stress and plastic deformation according to characteristic of overlapping circle spot. Residual stress and plastic deformation distribution at surface and along depth direction were explored, and the influence of processing conditions on residual stress and surface morphology were also discussed.(3) Technical process control and parameters optimization. Shaper factorσSF was introduced to precisely describe shape feature of residual stress under different crystal orientations. Taguchi method of Minitab software was used to carry out experimental design according to derived simulated results, the characteristics of residual stress measured under various processing parameters were extracted and treated, the influence degree were determined by the analysis of signal-to-noise ratio and variance, which provides good guidance for optimizing the processing parameters and controlling residual stress duringμLSP.(4) Experiments ofμLSP were conducted. After measuring surface profile and roughness of single spotμLSP region, influence of processing parameters on copper plastic deformation were analyzed. Mechanical properties in single shot region, enhancement mechanism ofμLSP on copper nano-hardenss and elastic modulus afterμLSP were explored by discussing the measured nano-hardenss, elastic modulus and contact depth inμLSP region. Key factor of surface performance was proposed. Multi-spotμLSP experiments were conducted to study the influence of main processing parameters (laser energy and overlapping rate) on residual stress and surface morphology. Appropriate processing parameter and processing rule were obtained under different conditions.(5) The microstructure evolution of copper and micro strengthening mechanism in plastic deformation region treated byμLSP was investigated. The grain size and microstructure of copper in depth direction were observed by transmission electro microscopy (TEM) and optical microscope (OM) technique. The microstructure evolution mechanism of copper was investigated based on the analysis of microstructure change under multipleμLSP treatment. Grain refinement mechanism and micro strengthening mechanism was proposed systematically, and the effects of microstructure in strengthening treatment were discussed.The researches enrich the basic theory of pressure induced by laser, which is theoretical significant for microstructure evolution in dynamic plastic deformation under high strain rate, and also provide the helpful reference and guidance to evaluation of enhanced result.更多还原