高功率激光焊接等离子体三维重建及能量传输研究

发布时间：2018-05-25 09:47

本文选题：高功率激光焊接 + 光致等离子体　；参考：《上海交通大学》2015年博士论文

【摘要】：在高功率激光焊接的过程中,光致等离子体-激光-材料三者之间有着复杂的相互制约与能量耦合关系。光致等离子体将会影响激光穿越等离子体的空间路径以及最终到达工件表面的激光能量密度,从而极大地影响整个焊接过程以及焊缝成形。准确测量激光焊接等离子体内部的三维物理参数分布是研究等离子体行为特性以及激光传输过程的重要前提。由于高功率激光焊接过程中光致等离子体高能量密度、高速动态变化的特性,以及不均匀非对称分布的内部特征,现阶段针对高功率激光焊接等离子体物理参数的测量绝大部分是单点测量或二维分布测量,对等离子体内部三维物理场测量的研究非常有限。由于对激光焊接等离子体的内部结构及物理特性缺乏准确而可行的实验测量手段,直接限制了对激光焊接过程中发生的光致等离子体-激光能量交互作用的研究。因此,迫切需要研究适用于高功率激光焊接的光致等离子体三维重建方法,准确测量等离子体内部的三维物理参数场,并基于等离子体的三维模型研究等离子体-激光能量交互机制。这对于认识高功率激光焊接等离子体的物理特性,揭示激光焊接机理具有十分重要的意义。本文开发了一套多通道同步摄影系统来获得激光焊接过程中光致等离子体在空间不同角度的投影数据。设计了由多角度投影光谱图像进行激光焊接等离子体三维重建的方法,准确测量等离子体温度场等三维物理参数场。并以此为手段计算了He+Ar混合气体保护下铝合金CO2激光焊接过程中Ar-He-Al三元等离子体的三维温度场以及吸收系数与折射率的空间分布,并以三维物理模型为基础研究了等离子体对激光的吸收与折射机制。深入研究了侧吹气体参数对等离子体的三维形态、物理特性以及激光能量传输的影响。本文的主要研究成果如下:针对等离子体图像中存在的噪音污染、飞溅颗粒以及工件表面反光等严重影响重建精度的干扰因素,采用P-M扩散法对等离子体图像去噪处理,在去除噪声的同时保留等离子体的边缘;运用灰度形态学开运算滤除飞溅颗粒引起的孤立点集,减小飞溅物对等离子体投影光路的影响;采用改进的snake算法,利用等离子体形状的先验知识将等离子体本体与像的边缘剥离,避免了将高亮度的等离子体像错误地划分进重建区域。提出了一种基于质心投影准则的图像校正方法,以等离子体图像的质心投影线在空间交于一点为约束条件,对图像进行平移微调。校正后等离子体图像质心配准误差由1mm左右减少到0.1mm以内,实现了校正摄像机标定参数的优化效果。根据几何光学知识构建了等离子体中心投影模型。在中心投影模型中,投影光线是由一系列经过投影中心的几何光线所组成的楔形光线束,此时投影权重计算变得复杂。对此提出了一种新的投影权重计算方法,将重建区域的格子投影到成像面后计算投影面积对像素点的贡献。这样可将计算空间从n维变换为n-1维,降低了计算量与计算复杂度。通过分析可知本文提出的投影权重计算方法与传统的投影光线路径法的计算结果等效。采用多目标优化的ART算法进行等离子体三维重建,通过模拟计算分析了重建参数对重建过程的影响,获得了一组优化的重建参数。结合局部热力学平衡态方程计算了铝合金CO_2激光焊接等离子体三维温度场。计算结果表明:等离子体最高温度在8000K-12000K之间,最高温度出现在工件上方约0.5mm处。等离子体中心区域温度场的分布具有不连续性和不平滑性,由多个孤立高温区域和相对低温区域组成。采用本文方法计算得到的等离子体温度略高于基于Abel逆变换重建算法的计算值。深入分析了球形分层模型和平行分层模型(或网格模型)对激光传播轨迹的影响,揭示了平行分层模型(或网格模型)计算的激光轨迹与实际情况相悖的根本原因是:当激光进入分层界面时,由于分层界面的法线位于光线右侧,激光趋向于更接近中心线,因此等离子体起到了正透镜的作用,与实际情况不符。建立了可从网格模型转化的等离子体二次曲面分层模型,可以解决网格分界面的法线方向无法反映宏观尺度实际情况的问题。研究了He+Ar混合侧吹气体流量及组分对等离子体的形态特征、物理特性以及激光能量传输效率的影响。研究发现:等离子体的温度随着气体流量的增加而减小,在各个气体流量下随着Ar含量的增加呈线性增长的趋势。但是随着气体流量的增加,等离子体温度的下降速度逐步减小。各组气体参数下等离子体沿激光入射方向的温度分布趋势较为相似,最高温度均出现在距离工件上方约0.5mm处,且最高温度随Ar含量的增加而上升。等离子体的折射作用造成的激光能量损失率在6%-22%范围内,等离子体吸收造成的能量损失率在4%-15%范围内。在同样的气体参数下折射造成的能量损失略高于吸收。激光穿越等离子体后总的能量损失率随着气体流量增大而减小,随Ar含量的增加而增加。从能量损失的角度来看,Ar在He+Ar混合保护气体中的含量并没有一个特定上限。可以通过增大流量的方法将保护气体中的He替换成Ar,而并不影响激光能量的传输效率。
[Abstract]:In the process of high power laser welding, there is a complex interaction and energy coupling relationship between the three light induced plasma laser materials. The light induced plasma will affect the space path of the laser through the plasma and the laser energy density that eventually reaches the surface of the workpiece, thus greatly affecting the whole welding process and welding. The accurate measurement of the distribution of the three-dimensional physical parameters inside the plasma of the laser welded plasma is an important prerequisite for the study of the characteristics of plasma behavior and the process of laser transmission. The characteristics of high energy density, high speed dynamic change and inhomogeneous asymmetric distribution in the process of high power laser welding are present. Most of the measurements of plasma physical parameters in high power laser welding are single point measurement or two-dimensional distribution measurement. The study of three-dimensional physical field measurement inside the plasma is very limited. Because of the lack of accurate and feasible experimental measurement methods for the internal structure and physical properties of the laser welded plasma, the direct limitation is limited. The interaction between photoinduced plasma and laser energy in laser welding is studied. Therefore, it is urgent to study the three-dimensional reconstruction method of photoinduced plasma for high power laser welding, accurately measure the three-dimensional physical parameter field inside the plasma, and study the plasma excitation based on the three-dimensional model of plasma. The interaction mechanism of light energy is of great significance for understanding the physical characteristics of high power laser welding plasma and revealing the mechanism of laser welding. In this paper, a multi channel synchronous photographic system is developed to obtain the projection data of the photoplasma at different angles in the process of laser welding. The multi angle projection is designed. The three-dimensional reconstruction of laser plasma by laser welding is used to accurately measure the three-dimensional physical parameters of plasma temperature field, and the three-dimensional temperature field of Ar-He-Al three element plasma and the spatial distribution of the absorption coefficient and refractive index of the Ar-He-Al three element plasma in the CO2 laser welding of aluminum alloy under the protection of the mixed gas are calculated. The absorption and refraction mechanism of plasma to laser is studied on the basis of a three-dimensional physical model. The effects of the parameters of the side blowing gas on the three-dimensional shape, physical properties and laser energy transmission are deeply studied. The main research results of this paper are as follows: noise pollution in the plasma images, spatter particles and The interference factors such as the surface reflection of the workpiece surface seriously affect the accuracy of the reconstruction, the P-M diffusion method is used to denoise plasma images, and the edges of the plasma are retained while the noise is removed; the outlier set caused by the spatter particles is filtered by the use of grayscale morphology, and the effect of the splash on the plasma projection optical path is reduced. The snake algorithm uses the prior knowledge of plasma shape to peel off the plasma and the edges of the image, avoiding the wrong partition of the high brightness plasma image into the reconstruction area. A image correction method based on the centroid projection criterion is proposed, which is constrained by the plasma image centroid projection line at a point. The image centroid registration error is reduced from about 1mm to less than 0.1mm, and the optimization effect of calibrating the camera calibration parameters is realized. The projection model of the plasma center is constructed according to the geometric optics knowledge. In the center projection model, the projection light is a series of projection centers. The calculation of the weight of the projection becomes complex at this time. A new method of calculating the weight of projection is proposed. The projection area of the reconstructed area is projected to the imaging surface to calculate the contribution of the projection area to the pixel. This can change the computing space from the n-dimension to the N-1 dimension, which reduces the complexity of calculation and calculation. The calculation method of projection weight proposed in this paper is equivalent to the result of the traditional projection ray path method. The ART algorithm of multi-objective optimization is used to reconstruct the plasma. The effect of the reconstruction parameters on the reconstruction process is analyzed by the simulation calculation, and a set of optimized reconstruction parameters is obtained. The equilibrium state equation is used to calculate the three-dimensional temperature field of the CO_2 laser welding of aluminum alloy. The calculation results show that the highest temperature of the plasma is between 8000K-12000K, the highest temperature is about 0.5mm above the workpiece. The distribution of the temperature field in the center of the plasma is discontinuous and unsmooth, and the temperature field of the plasma center is from a number of isolated high temperature regions and phases. The plasma temperature degree calculated by this method is slightly higher than the calculated values based on the Abel inversion reconstruction algorithm. The influence of the spherical stratification model and the parallel layered model (or mesh model) on the laser propagation trajectory is deeply analyzed, and the laser trajectories and the reality of the parallel stratified model (or grid model) are revealed. The fundamental reason for the contrary is that when the laser enters the layered interface, the laser tends to be closer to the center line because the normal line of the layered interface is located on the right side of the light, so the plasma plays the role of the positive lens and does not correspond with the actual situation. The two layer surface layer model of the plasma which can be transformed from the grid model can be solved. The normal direction of the grid interface can not reflect the actual situation in the macro scale. The influence of the flow and composition of He+Ar mixed side blowing gas on the morphological characteristics, physical properties and laser energy transmission efficiency of the plasma is studied. The study shows that the temperature of the plasma decreases with the increase of the volume flow of the gas. With the increase of Ar content, the decreasing speed of plasma temperature decreases gradually with the increase of gas flow. The temperature distribution trend of plasma body along the laser incident direction is similar, the highest temperature appears at about 0.5mm above the workpiece, and the highest temperature is with the Ar content. The energy loss rate of the laser energy caused by the plasma refraction is within the range of 6%-22%. The energy loss rate caused by the plasma absorption is within the range of 4%-15%. The energy loss caused by the refraction of the same gas parameters is slightly higher than that of absorption. The total energy loss rate of the laser through the plasma increases with the gas flow rate. The decrease is increased with the increase of Ar content. From the point of view of energy loss, there is no specific upper limit for the content of Ar in the He+Ar mixed protection gas. By increasing the flow rate, the He in the protective gas is replaced with Ar without affecting the transmission efficiency of the laser energy.
【学位授予单位】：上海交通大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG456.7

【相似文献】