【作者】 张兴权；

【导师】 张永康；

【作者基本信息】 江苏大学 ， 机械制造及其自动化， 2007， 博士

【摘要】 强激光诱导的冲击波在材料加工中已得到应用,本文在介绍强激光在材料加工应用基础上,着重介绍激光喷丸成形新技术,并就激光喷丸成形机理及其相关技术进行了系统研究,对激光喷丸下板料的变形特性进行了实验,对激光喷丸成形件的表面质量进行了分析,用有限元分析方法对板料变形进行了数值模拟。主要内容有以下几个方面:在简要分析应力波形成、传播和衰减的基础上,着重分析了残余压应力形成的机理,首次建立了激光喷丸成形的数学模型,并根据纯弯曲理论,推导出激光喷丸条件下,板料变形的曲率半径,使板料变形的曲率和激光脉冲参数联系起来,为激光喷丸成形时选择参数提供了依据。根据弯矩的合成理论,得出了在喷丸轨迹为直线情况下,板料变形的基本形状为倒立的V形,复杂形状是可以看作是基本形状的复合。在此基础上,将自由状态下激光喷丸成形曲率半径和预弯状态下激光喷丸成形下曲率半径进行了比较,从理论上说明了预弯状态下激光喷丸成形能够有效改善板料在自由状态下成形时出现的球面双曲现象。实验研究了激光喷丸条件下板料的变形特性,探讨了板料变形量与激光脉冲参数、光斑覆盖率、板料几何尺寸以及喷丸路径关系。实验结果表明,在喷丸轨迹为直线的情况下,板料变形量随激光能量的加大而加大,随覆盖率的增加而增加,随喷丸次数的增加而增加,随厚度的增加而减小。当板料几何尺寸发生变化时,板料的曲率将发生变化。通过选择喷丸区域和激光喷丸强度,能够实现板料复杂三维形状的成形。在进行自由状态下板料变形实验基础上,进一步研究了预弯状态下板料的变形特性。实验研究表明,单向施加弹性预弯减小了预弯方向的曲率半径,加大了与预弯方向相垂直方向上的曲率半径,并且随着预弯量的增加,两个方向上曲率半径变化的效应更加显著。在此基础上,首次研究了激光喷丸成形精度。结果表明,激光喷丸成形的板料在半径方向上精度能达到0.4玎[1In,高于使用传统方法成形得到的成形精度,说明了激光喷丸成形能实现板料的精密成形。用XRD方法研究测量了在激波加载区残余压应力分布和板料变形后的表面残余应力。结果表明,在激光喷丸冲击下,激光诱导的残余压应力随激光能量的增加而增大,随冲击次数的增加而增加。在激光喷丸成形件的表面都存在着残余压应力,受喷丸面的冲击区的残余应力大于未冲击区的残余压应力,受喷丸面的残余应力要高于未喷丸面的残余压应力值,成形件的表面存在有益的残余压应力,这是使用传统模具弯曲件无法比拟的。研究还表明激光冲击波诱导的残余压应力层的深度是传统机械喷丸形成的残余压应力层深度的3倍多,使得激光喷丸成形板料的能力比传统的喷丸成形板料的能力强。研究了激光喷丸件表面的粗糙度、硬度、微观组织等表面质量因素及其对工件机械性能的影响。研究表明,激光喷丸后,在板料表面留有微米级的光滑的凹坑,因而导致表面粗糙度下降,表面粗糙度下降程度取决于激光喷丸件成形的曲率和材料机械性能,对于给定的零件,不同的工艺会导致不同的表面粗糙度。激光喷丸后,板料表层的硬度得到了提高,组织得到了细化,位错密度提高,意味着工件耐磨性和抗应力腐蚀的能力得到了提高,加之表层存在有益的残余压应力,使工件的抗疲劳性能得到提高,疲劳寿命得以延长,说明激光喷丸成形是集激光冲击强化和塑性成形与一体的技术。建立了激光冲击下有限元模拟的模型,并对建模过程中的一些关键问题进行了处理。采用有限元分析的方法,对激光诱导的应力波传播、表面动态应力、残余压应力和板料的变形进行了数值模拟,模拟结果和试验结果、理论分析取得结果较为一致,说明建立的模型是可靠的、合理的,采用模拟的方法是可行的。在此基础上,对板料成形的部分工艺进行了优化,对板料复杂形状的变形进行了预测,为合理选取激光加工参数和实现冲击路径的优化,实现板料的精密成形提供了指导。更多还原

【Abstract】 Laser peen forming(LPF) is an emerging modern technique to bend metal plate. It employes the short-duration high-power laser to induce residual compressive stress distributed unevenly along the thickness of plate, and then the moment exerted by the residual compressive stress impetus the plate to develop plastic curvature. In this article, based on the analysis of the applications of the short-duration and high-power laser in material processing, a novel technique on LPF of plate was presented. A systematical study on the mechanism of LPF and the related technology were carried out from the theory and experiment, and the main contents as following:On the basis of analyzing propagation and attenuation of the stress wave in material, the mechanism of residual compressive stress was analyzed and the mathematic model on LPF was established. According to the theory of pure bending, the formulation of curvature radius after laser peening was established to correlate the curvature of deformation with laser pulse parameters. The shape of deformation of plate peened by laser along the centerline was the bulge shape of V, which is the basic shape and the intricate shape can be composed of it. The main advantage of LPF under the condition of pre-bend state was to reduce the spherical surface double phenomenon curvatures induced by LPF under the condition of free state.The charateristics of bending and the relationship among the plate deformation and the material laser pulse prameters, covering ratio of laser spot, geometrical parameters of plate as well as laser peening paths were investigated in detail from the experiments. The experimental results show that the deformation of plate increases nonlinearly with the increase of laser power density, with the increase of covering ratio of laser spot and the numbers of laser shot, while decreases with the increaseing thickness of plate. When the size of plate changed, the curvatures in the different directions changed correspondingly, The location of plate and the intensity of laser were selected purposefully to laser peening, the complex 3-D shape of plate can be gained.The experiments on LPF under the condition of pre-bend state were also carried out. The results show that the pre-bending can decrease the curvature parrallel to the direction of moment exerted and increase the curvature radius in the vertical diretion of pre-bending, that the larger the strain exerted by pre-bend is, the distincter the curvature radius in two direction is. The precision of LPF was investigated further and the precision of LPF can attain to 0.4mm in the radius direction, which goes beyond other forming methods and indicates that the LPF can fulfill the exact forming.Residual compressive stresses along the thickness and on both surfaces of workpiece were measured with X-rays diffraction after laser peening. The results show that the magnitude of residual compressive stress increases with the increasing laser power fluence and numbers of laser shot. The residual compressive stress lies on both surfaces. Comparing the magnitude of residual compressive stress induced by traditional shot peening and by laser peenig, the magnitude of residual stress induced by laser shot peening is two to three times as large as that induced by traditional shot peening, which indicates that thickness of plate formed by laser peening is greater than that by shot peening.The surface quality of workpiece after laser peening and its effect on mechanical properties were investigated. The surface quality includes the surface finish, hardness, the magnitude of residual compressive stress and so on. The experimental dada show that the surface finish decreases because of the dents in surface impacted by high pressure plasma, and the surface finish mainly depended on the curvature radius of workpiece and the material mechanical properties. The hardness after laser peening enhanced and the grain of material was refined, which reduces the rate of fatigue cracking and stress corrosion cracking in structural metals needed for aerospace and extends the fatiguelife of workiece. It indicates that the LPF combines the laser shock processing with plastic deformation.Based on the above analysises and experiments, the model of finite element method (FEM) was established and the processes of stress wave propagating, dynamic stress on surface, residual compressive stress and deformation of plate were simulated with ABAQUS software. The simulation results were agreed well with the analytic results and the experimental results. So the developed finite element method is able to predict the deformation and residual stress field in the processing of LPF. It can help to optimize the laser parameters and select the peening paths to implement the precise forming.更多还原