【作者】 陈磊；

【导师】 王宗申；

【作者基本信息】 山东理工大学 ， 材料加工工程， 2022， 硕士

【摘要】 随着微小型化技术的快速发展,微机电系统(Micro Electro-Mechanical System,MEMS)在航空航天、微电子、智能材料、生物医学等新兴领域展现出了巨大的应用潜力,但是其金属微构件的疲劳失效问题成为制约其发展的关键因素。微尺度激光冲击强化(Micro-Scale Laser Shock Peening,μLSP)技术作为一种新兴的表面改性技术,具有强化效果显著、可控性强、适应性好等优点,为解决金属微构件的失效问题提供了新的方向。目前,关于μLSP的相关研究已经证实μLSP在改善材料耐疲劳、耐磨损和耐腐蚀等方面的优越性。然而,传统的μLSP多借助纳秒激光实现,在处理金属微构件时存在表面完整性差、难以加工复杂形状等诸多问题。飞秒激光微尺度冲击强化(Femtosecond Micro-Scale Laser Shock Peening,fs-μLSP)技术具有冲击波压力高、热影响小、能量利用率高等优点,能够解决纳秒激光微尺度冲击强化(Nanosecond Micro-Scale Laser Shock Peening,ns-μLSP)技术存在的问题。但是,目前关于fs-μLSP还缺乏系统的研究,对相关机理的解释也不清楚。因此,系统地研究飞秒激光与材料之间的作用机制以及激光工艺参数对fs-μLSP效果的影响规律与机理具有重要的理论意义与实际价值。本文将数值模拟与实验研究相结合,详细讨论T2纯铜在ns-μLSP和fs-μLSP过程中的位移、塑性应变和等效应力的动态响应情况以及残余应力分布情况,系统分析激光工艺参数对塑性变形和残余应力的影响规律与作用机制,实验研究飞秒激光工艺参数对fs-μLSP纯铜的表面完整性和力学性能的影响规律与作用机理,主要研究工作和成果如下:(1)研究了μLSP过程的有限元建模技术,建立了ns-μLSP和fs-μLSP过程的数值模型,模拟研究了μLSP过程中纯铜的位移、塑性应变和等效应力的变化规律以及残余应力的分布规律。Ns-μLSP纯铜的位移、塑性应变和等效应力随冲击波压力的加载逐渐增大。飞秒激光诱导冲击波压力的持续时间较短,fs-μLSP纯铜的位移、塑性应变和等效应力在冲击波压力加载结束后逐渐增大,水平远小于ns-μLSP。此外,ns-μLSP和fs-μLSP均出现“残余应力洞”现象。(2)通过数值模拟,分别研究了激光工艺参数对ns-μLSP和fs-μLSP纯铜的塑性变形和残余应力的影响规律与内在机理。与ns-μLSP相比,fs-μLSP的影响深度较浅,塑性变形与残余压应力水平远小于ns-μLSP。激光脉冲能量是主要的影响因素,对ns-μLSP和fs-μLSP效果的影响规律一致。随着脉冲能量的增大,塑性变形逐渐增大,残余压应力先增大后减小。光斑直径的增大导致“残余应力洞”现象更加明显。随着脉宽与冲击次数的增大,塑性变形和残余压应力水平逐渐提高。(3)进行了T2纯铜板材的ns-μLSP工艺实验,研究了K9玻璃与纯水作为约束层,胶带、油漆和铝箔作为牺牲层时的ns-μLSP效果。K9玻璃与牺牲层之间难以紧密贴合,同时受纳秒激光器脉冲能量的限制,导致严重的激光烧蚀,K9玻璃被破坏,而纯水可以解决K9玻璃的问题。激光烧蚀胶带与油漆的产物会导致污染问题,不利于ns-μLSP。使用纯水与铝箔研究了激光工艺参数对ns-μLSP效果的影响规律,验证了有限元模型的合理性和可靠性。(4)进行了T2纯铜板材的fs-μLSP工艺实验,借助SEM、EDS、表面粗糙度测量、金相组织观察、XRD测试、残余应力测试、显微硬度测试、纳米压痕测试和摩擦磨损实验等分析测试手段,研究了飞秒激光脉冲能量、脉宽、光斑搭接率和冲击次数对纯铜的表面形貌和力学性能变化过程的影响规律,着重分析了脉冲能量对纯铜的微观组织、表面形貌和力学性能以及摩擦磨损性能的影响规律和内在机理。在较低能量fs-μLSP下,纯铜表面硬度和残余压应力水平显著提高,摩擦磨损性能明显改善,同时能够保持较好的表面完整性。更多还原

【Abstract】 With the rapid development of miniaturization technology,micro electro-mechanical system(MEMS)shows great application potential in emerging fields such as aerospace,microelectronics,intelligent materials and biomedicine.However,the fatigue failure of metal micro-components in MEMS becomes a key factor restricting the development of MEMS.Micro-scale laser shock peening(μLSP),as a new surface modification technology,has the advantages of obvious strengthening effect,strong controllability and good adaptability provides a new direction for solving the failure problem of metal micro-components.At present,research on μLSP has confirmed the superiority of μLSP in improving the fatigue resistance,wear resistance and corrosion resistance of materials.However,the traditional μLSP is mainly realized by nanosecond laser,which has many problems in processing metal micro-components,such as large surface roughness and difficulty in processing complex shapes.Femtosecond micro-scale laser shock peening(fs-μLSP)can solve the problems existing in nanosecond microscale laser shock peening(ns-μLSP)because of the advantages of high shock pressure,low thermal influence and low surface roughness.However,there is still a lack of systematic research on fs-μLSP,and the related mechanism is not clear.Therefore,it is of great theoretical significance and practical value to systematically study the interaction mechanism between femtosecond laser and materials and the influence law and mechanism of laser process parameters on the fs-μLSP effect.In this paper,numerical simulations and experimental studies are combined to discuss in detail the dynamic response of displacement,plastic strain and equivalent force and residual stress distribution in ns-μLSP and fs-μLSP processes of T2 pure copper.The effect law and mechanism of laser process parameters on plastic deformation and residual stresses is systematically analyzed.The effects and mechanism of femtosecond laser parameters on the surface integrity and mechanical properties of fs-μLSP are experimentally studied.The main research work and results are as follows:(1)The finite element modeling technique of μLSP is studied,and the numerical models of ns-μLSP and fs-μLSP are established.The variation law of displacement,plastic strain and equivalent stress and residual stress distribution in μLSP are simulated.The displacement,plastic strain and equivalent stress of copper in ns-μLSP increase with the loading of shock wave pressure.However,the duration of femtosecond laser induced shock wave pressure is short.The displacement,plastic strain and equivalent stress of copper in fs-μLSP which is much lower than that in ns-μLSP gradually increase after the shock wave pressure loading.In addition,the phenomenon of "residual stress hole" happens in both ns-μLSP and fs-μLSP.(2)The effects of laser parameters on the plastic deformation and residual stress of ns-μLSP and fs-μLSP pure copper are studied by numerical simulation.The influence depth of fs-μLSP is shallow,and the plastic deformation and residual compressive stress are far less than ns-μLSP.The laser pulse energy are the main influencing factors,which have the same effect on the results of ns-μLSP and fs-μLSP.With the increase of pulse energy,the plastic deformation increases gradually,and the residual compressive stress increases first and then decreases.The "residual stress hole" becomes more obvious with the increase of spot diameter.With the increase of pulse width and impact number,the plastic deformation and residual compressive stress increase gradually.(3)The ns-μLSP results when using K9 glass and water as confinement layer,and tape,paint and aluminum foil as ablative layer are studied.It is difficult to fit closely between K9 glass and ablative layer,leading to serious laser ablation and damage of K9 glass while water can solve the problem of K9 glass.The products of laser ablation of tape and paint can cause pollution problems,which is not conducive to ns-μLSP.The influence of laser parameters on the results of μLSP is studied by using water and aluminum foil,and the rationality and reliability of the finite element model are verified.(4)The fs-μLSP experiment of T2 pure copper sheet is carried out.By SEM,EDS,surface roughness measurement,metallographic microstructure observation,XRD test,residual stress test,microhardness test,nanoindentation test and friction and wear test,the effects of femtosecond laser pulse energy,pulse width,spot overlapping rate and impact number on the surface morphology and mechanical properties of copper are investigated.The influence law and mechanism of pulse energy on the microstructure,surface morphology and mechanical properties of pure copper are mainly explored.By low energy fs-μLSP,the surface mechanical properties of pure copper are improved significantly,and the surface integrity is maintained.更多还原