【作者】 姜银方；

【导师】 张永康；

【作者基本信息】 江苏大学 ， 机械制造及其自动化， 2017， 博士

【摘要】 激光冲击强化技术是利用强脉冲激光束辐照在靶材表面的涂层上,产生高能等离子体冲击波作用于工件进行表面强化的先进制造技术。激光冲击强化技术应用于工程结构细节,能够直接大幅度的提高航空材料、部件等的使用寿命,提高系统的安全性、可靠性。激光冲击强化技术应用于工程结构细节还存在许多并未揭示的理论问题,包括激光冲击波在工程结构细节中传播、作用,残余应力的形成机制、规律和调控,激光强化考虑构件的材料、厚度、几何尺寸和形状差异,外载作用下的损伤机制等,通过研究以期提出一整套的理论解决方案和工艺技术。本文在对现有激光冲击强化技术应用于工程结构细节抗疲劳技术进行了论述,综合了国内外学者在本领域中成果,探讨了研究与应用中存在的问题和发展方向。在此基础上,采用理论、仿真和试验的方法展开相关的研究工作。通过大量的仿真和试验研究,理清了工程结构细节激光冲击强化相关理论问题,探索了工程结构细节激光冲击强化的残余应力形成和作用规律、机制及调控方法;揭示了工程结构细节激光冲击强化过程中存在的特殊性和主要特征现象,微观机制;研究激光冲击强化的工程结构细节的外载特性,建立了激光冲击强化的工程结构细节的循环加载的S-N曲线。研究工作为激光冲击强化技术在工程结构细节中应用,提高工程结构关键件和疲劳部位的寿命,奠定一定的理论基础。主要结论和创新性成果如下:（1）论述了冲击波的传播及与工程结构细节的相互作用;分析了激光诱导冲击波作用下工程结构细节残余应力的形成与分布;分析比较了激光冲击强化孔角与孔壁中间应力强度因子的差异,以及激光冲击强化圆角结构细节的应力强度因子。（2）基于ABAQUS有限元分析软件和必要的疲劳试验,对小孔件、圆角件、圆棒件残余应力形成规律和影响因素进行了分析研究;为了综合考虑光斑搭接率和冲击层数的影响,提出单面冲击平均覆盖率的概念,揭示了多次激光冲击在材料表面产生的强夯效应;通过对不同功率密度（峰压）小孔构件孔壁应力峰值与表面应力研究,揭示了孔壁应力峰值随功率密度（峰压）的增加有先增后降规律;发现了不同功率密度（峰压）下孔壁应力峰值与表面应力有一定的对应关系,为通过对表面应力的测量和控制,间接而有效地对孔壁应力峰值进行预测和控制提供思路和方法。（3）通过理论分析,推导了小孔板件进行激光冲击强化需要的功率密度I₀为（?）,并通过对不同厚度的小孔试件进行激光冲击强化和疲劳试验,求得TC4DT钛合金的经验公式为（?）;制定了不同材料和不同厚度试件的工艺参数选择步骤和方法;以TC4-DT钛合金板料和7050-T7451铝合金板料为例进行了工艺参数的选择,并通过试验进行了验证。通过对不同厚度小孔试样残余应力分布规律,不同板厚试样疲劳源位置和形态进行研究,发现了激光冲击强化的不同厚度试样疲劳寿命有较明显的不同,小孔试件厚度与激光冲击强化效果间有着必然联系和规律,提出了激光冲击强化小孔试样存在厚度效应的概念。（4）通过大量的仿真和试验,对不同曲面形状和曲率进行激光冲击残余压应力的分布进行了研究,结果认为,在模型的弯曲方向上残余应力的分布规律是不同的,凹、平和凸面三种几何模型的残余压应力影响层深度不同;结果表明,激光冲击强化参数优化后有效地提高了圆角结构细节的疲劳寿命。研究结果还表明,对圆棒试样来说,激光冲击使拉应力位置内移,拉应力峰值增大,钛合金圆棒经激光冲击后不利于拉-拉疲劳性能,表现为负增益;对铝合金圆棒试样施以较小的能量冲击,表现出较好的增益;在旋转弯曲特性载荷下,钛合金圆棒经激光冲击后其疲劳寿命大幅增加,因此认为,激光冲击强化后服役环境的选择是一个重要课题。（5）对7050T7451铝合金、TC4-DT、TC21钛合金材料,不同厚度,不同工程结构细节（小孔、圆角、棒料等）试件,进行了不同应力水平下的疲劳试验和统计分析;研究了激光冲击强化对不同材料、几何尺寸和形状的影响,探讨了工艺对工程结构细节疲劳件抗疲劳的作用;参考HB/Z 112-1986《材料疲劳试样统计分析方法》,根据成对对比分析方法,以95%的置信度,对试验结果进行了统计分析,完成各个情况下的S-N曲线的制作和研究。（6）通过对不同试验参数条件下疲劳寿命及断口特性进行研究,发现了随着激光冲击能量的加大,疲劳源发生转移的机制和规律,及其与疲劳寿命的关系;研究还揭示了外载应力水平不同,疲劳源萌生,裂纹扩展的规律和机制;研究工作还就圆角、圆棒试样应力分布及其在不同循环载荷下的疲劳断口的形貌特征和规律进行了研究。更多还原

【Abstract】 Laser shock processing(LSP)is an advanced surface manufacturing technology which uses the high-pulse laser beam irradiating on the surface of the metal coating to gasify and ionize the coating,producing high-energy plasma shock wave to strengthen the metal surface.The application of LSP technology to the details of engineering structures can greatly improve the service life of aviation materials and components and greatly improve the safety and reliability of the system.Many theoretical problems in the application of LSP to the engineering structure details remain to be disclosed,including the propagation and action of the laser shock wave in the engineering structure details;forming mechanism,rule and regulation of residual stress;damage mechanism of components with different material,thickness,geometrical dimension,shape.It is expected to propose a set of theoretical solutions and processing technology through research.In this paper,the existing anti-fatigue technology of LSP applied to the engineering structure details were discussed.The results of domestic and foreign scholars in the field were summarized.The problems and the development of the research and application were discussed,through which related research can be done based on theory,simulation and test methods.Through a large number of simulation and experimental studies,we can clarify the theoretical issues related to the LSP to the engineering structure details;explore the residual stress formation and the action rule,mechanism and regulating method;disclose the characteristics,main characteristic phenomena and the microscopic mechanism existing in the process of LSP;study the external load characteristics of the engineering structure details after LSP to draw the S-N curve of the cyclic loading on the engineering structure details after LSP,which lays a theoretical foundation for the application of LSP technology in engineering structure details and the improvement of the life of key parts and fatigue parts in engineering structures.Main conclusions and innovative achievements are as follows:(1)The propagation of shock wave and its interaction with the engineering structure details were discussed;the formation and distribution of residual stress of engineering structure details under laser-induced shock wave were analyzed;the stress intensity factor differences between hole corner,middle of the hole wall and the fillet structure details after LSP were analyzed.(2)The formation rules and influencing factors of residual stress in hole,fillet and round bar parts were analyzed based on the ABAQUS finite element analysis software and necessary fatigue tests;in order to comprehensively consider the effect of spot overlap rate and the number of impact layer,the concept of single-side impact average rate of coverage was proposed to reveal the dynamic compaction effect on material surface after multiple laser shock;through the study on the peak stress in wall hole and surface stress caused by LSP under different power density(peak pressure).it was revealed that the peak stress increases first and then decreases with the increase of the power density(peak pressure);it was found that the peak stress of hole wall has a certain correspondence with the surface stress which provides an idea and method for indirectly and effectively predicting and controlling the peak stress of hole wall by measuring and controlling the surface stress.(3)Through the theoretical analysis,the power density required for LSP on the hole plate are deduced as (?),and the empirical formula of the power density required for LSP on TC4DT titanium alloy is obtained as (?) by LSP and fatigue test on the hole specimens with different thickness;process parameters selection steps and methods of different materials with different thickness are formulated;TC4-DT titanium alloy sheet and7050-T7451 aluminum alloy sheet as an example were selected and verified by the test.Through the study on the distribution of residual stress and the location and shape of the fatigue source of different thickness specimens,it was found that the fatigue life of laser shock processed specimens with different thickness is obviously different.There is an inevitable connection between the effect of LSP and thickness of specimens,so the concept of the thickness effect of hole specimens after LSP is proposed.(4)Through a large number of simulations and experiments,the distribution of residual stress in models with different surface shapes and curvatures after shock has been studied.The results show that the distribution of residual stress is different in the bending direction of the model and concave,flat and convex model have different depth of influence layer;it has been proved by experiment that the LSP with optimized parameters can effectively improve the fatigue life of fillet structure.The results also showed that for the round bar sample,the laser shock caused the inward shift of stress position and the increase of peak tensile stress.The titanium alloy round bar is not conducive to the pull-pull fatigue performance after laser shock,which showed a negative gain;the aluminum alloy round bar sample which was exerted less energy shock showed a better gain;under the bending load,the fatigue life of the titanium alloy rod increased greatly after the laser shock,so it is considered that the choice of service environment after LSP is an important issue.(5)The fatigue test and statistical analysis of 7050T7451 aluminum alloy,TC4-DT,TC21titanium alloy material,and different engineering structure details(holes,fillets,bars,etc.)with different thickness were carried out under different stress levels;effect of LSP on different materials,geometric size and shape and the effects of process on the fatigue resistance of the engineering structure detail fatigue structure were studied and discussed;referring to the HB/Z 112-1986?method of statistical analysis of the material fatigue specimen’,according to the method of paired comparison analysis,under the 95%confidence level,the S-N curve in each situation were drawn and studied through the statistical analysis of the test results.(6)Through the study on fatigue life and fracture characteristics under different experimental parameters,the mechanism and rule of fatigue source transferring and its relationship with fatigue with the increase of laser shock energy life have been found;the rule and mechanism of fatigue source initiation and propagation of crack under different external stress level was also revealed;the stress distribution in fillet and round bar specimen and the morphological characteristics and rule of fatigue fracture under different cyclic loads were investigated.更多还原