【作者】 秦渊；

【导师】 倪晓武；

【作者基本信息】 南京理工大学 ， 光学工程， 2012， 博士

【摘要】 毫秒激光与金属材料相互作用中将出现一系列的热学和力学效应。本文针对塑性屈服、熔融及小孔成形过程进行了理论计算、有限元模拟和实验研究。从实验和数值模拟两个方面研究了毫秒激光致金属材料产生塑性屈服的现象。在测量了薄铝板上反鼓包的成形过程的基础上,依据热传导理论及热弹塑性力学理论,用有限元法模拟了相关变形过程,通过比较实验和数值结果验证了数值算法的合理性。继而建立了毫秒激光与厚航空铝合金板相互作用的物理模型,用有限元法模拟了温度场和应力场的分布,得到了塑性屈服时间、范围以及残余应力的大小。并且将这几方面作为评判标准,比较了高斯型与顶帽型激光使厚铝合金板产生塑性屈服的过程。依据毫秒激光致金属厚板产生熔融相变情况,建立了半无限大轴对称模型。基于热传导理论及温度分布形式,得到了温度和熔融深度的解析解,并利用实验研究获得的熔池形貌验证了解析计算结果。进而讨论了熔融深度与作用激光脉宽及能量的关系,对深度先随脉宽增加然后又减小的现象进行了解释。提出了毫秒激光逆重力方向对厚铝板打孔的实验方法,并根据实验和温度解得到了小孔深度解析式。进而比较了激光顺打孔(顺重力方向)和逆打孔实验中熔融物的迁移质量、挂渣现象和小孔体积,讨论了重力对于物质迁移的作用。证明了逆打孔时激光能量更多的用于将材料加热至熔融,在重力辅助作用下,熔融物质更易迁移出孔,因而打孔效率更高。继而改变激光能量进行实验研究,发现大能量毫秒激光打孔较深,且算法需考虑离焦效应的影响。然后利用改进后的算法研究了毫秒激光对不同金属材料的打孔速率。针对激光打孔时光在小孔中传输的实际情况建立物理模型,分别采用物理光学法及光线追迹法研究了光束在小孔内多次反射过程,计算了小孔对入射光的总吸收率和吸收光强在小孔内的分布情况。通过比较计算结果得到结论：当孔口衍射作用明显或孔直径小于入射光波长时,光线追迹法将不再适用于计算小孔吸收,其中衍射的作用取决于偏振态、入射角度、光斑大小及小孔边缘处的相对光强。本文的研究结果可为进一步研究毫秒激光与金属相互作用中出现的温升、热应力、塑性屈服、熔融及小孔成形提供理论和实验研究依据,亦有助于毫秒激光在加工和军事中得到进一步的应用。更多还原

【Abstract】 Series of thermal and mechanical effect will occur during millisecond laser heating of metals. In this dissertation, the plastic damage, melting and hole formation processes are investigated by theoretical calculation, finite element simulation and experiment.By means of experimental and numerical simulation, the plastic yielding process of metals induced by millisecond laser is studied. The reverse bulging in a thin aluminum plate is detected. According to which, this deformation is simulated by the finite element method (FEM) based on the heat conduction equation and thermal elasto-plastic constitutive relation. And the numerical algorithm is verified by comparing the experimental and the calculated results. Furthermore, a physical model of the millisecond laser interaction with an aluminum alloy slab is established. By using of FEM, the distributions of the temperature field and the stress field are studied numerically. In particular, the yielding time, the range of plastic damage region and the magnitude of residual stress are obtained. And the plastic damage for Gaussian and Top hat laser are compared according to these three results.A semi-infinite axisymmetric model is established for millisecond laser melting of a metal slab. The analytical solutions of the entire temperature filed and the melting depth are obtained based on the heat conduction theory and the distribution of temperature field. The molten pool obtained in calculation agrees with the experiment results. Moreover, the melting depth versus the laser pulse width is investigated. It is found that with the increasing of the pulsed width, the depth increases at first and reaches the maximum value, then turn to decrease. This phenomenon is discussed.An upward drilling method is proposed. In the experiment, the laser beam is designed to transmit along the opposite direction of the gravity and drill hole at the bottom of an aluminum slab. Based on the experiment and the temperature solution, the analytical solution of hole depth is obtained. For further verifying the gravity action, the downward (along the gravity direction) and the upward drilling cases are carried out in experiment. Meanwhile, the removed mass of molten material, the adherent dross and the hole volume are compared. The results show that more molten material is expelled with the assistance of the gravity, and more laser energy is used to melt the aluminum slab in the upward drilling. In a word, it is more efficient to drill hole upwardly. Thereafter, the hole depth versus laser energy is studied. It is found that the depth with high energy is deep and the defocusing effect should be considered in the calculation. In addition, the modified solution is used to calculate the drilling speed of millisecond laser for different metals.The model of light propagation in a hole during laser drilling is established. The total absorptance and absorbed intensity distribution inside the hole are calculated with the ray-tracing and physical optics methods respectively. The comparison of the results show that the ray-tracing method is not applicable if the diffraction at the hole entrance is important, or the hole diameter is smaller than the wavelength of the incident beam. Particularly, the influence of diffraction depends on the polarization state, the incident angle, the beam waist and the relative intensity on the hole entrance.The research consequences may offer theoretical and experimental references to the further study of the temperature rising, the thermal stress, the plastic damage, the melting and the formation of hole, and accelerate the application of millisecond laser in manufacturing and military.更多还原