Studies On Plasma 3D Reconstruction And Energy Transmission During High Power Laser Welding

In high-power laser welding process, there is complicated interaction and energy coupling between laser, material and laser-induced plasma. The existence of laser-induced plasma influences the dimensional track of laser propagation and the energy density arrives at the material surface, thus greatly affects the whole process of laser welding and weld shape. Accurate measurement of plasma 3D physical parameters distribution plays a very important role in the research of plasma behavior and laser energy transmission. Due to the high energy density and high speed variation of plasma during laser welding process, as well as nonuniform and asymmetric internal distribution, current studies on laser-induced plasma measurement are single-point and two-dimensional. At present, there is few researches focused on plasma measurement for internal 3D distribution. The lack of experimental measurement methods for internal physical parameters 3D distribution of laser welding plasma directly limits the development of studies on plasma-laser interaction during laser welding process. Therefore, it is necessary to conduct research of 3D reconstruction for laser-induced plasma during high-power laser welding process. The high-precision measurement of plasma 3D physical parameters distribution and the research of laser-plasma interaction based on plasma 3D physical model are helpful to further understand physical characteristic of laser-induced plasma and reveal the mechanism of high-power laser welding.In this paper, a multi-channel synchronized filming system is developed to grain multi-positions of plasma projection during laser welding process. Plasma 3D reconstruction from multiple-angle projecting images is designed to accurately measure the internal 3D physical parameters distribution. 3D distribution of temperature, absorption coefficient and refraction index of Ar-He-Al plasma during aluminium alloy CO₂ laser welding with He+Ar mixture shielding gas are computed. Furthermore, absorption and refraction effect of plasma is studied based on plasma 3D physical model. Detailed research of the effect of shielding gas parameters on plasma structure, physical characteristics and laser energy transmission is also implemented in this paper. The research results are shown as follows,In order to eliminate the disturbances of plasma image noise, welding spatters, and work-piece surface reflections, P-M diffusion equation is used to denoise plasma image while retaining the sharpness of edges. Grey-scale morphological opening algorithm is adopted to remove the isolated points caused by welding spatters. Improved snake algorithm related to priori knowledge of plasma shape is employed to separate plasma and its specular reflection image, and thus avoid data error of image reconstruction. An image adjustment method is proposed based on centroid projection principle. Plasma images are slightly shifted under the rule that the projecting lines of plasma images should intersect at one point. The registering error of images descends from approximately 1mm to below 0.1mm.Plasma central projection model is built up according to the knowledge of geometrical optics. In the central projection model, projecting light is a wedge-shaped beam consisting of series of geometrical lights passing through projecting center. In this condition, the computation of projection weight is complicated. Hence a novel method of projection weight computation is proposed. Reconstructed cubic is projected to imaging plane and then calculate the contribution of projected polygon area to pixel. Consequently the computation dimension is reduced from n to n-1 and the complexity of calculation decreases. The conventional method and the method proposed in this paper are proven to be equivalent. Finally, multi-objective optimization ART algorithm is used to reconstruct 3D plasma images. Simulation is carried out to estimate the effect of parameters on reconstruction process and a set of optimized parameters is identified for experimentsPlasma 3D temperature field during aluminium alloy CO₂ laser welding is computed by LTE equations. The results show that plasma maximum temperature is between 8000 K to 12000 K and the maximum temperature point is about 0.5mm above the work-piece surface. The temperature filed of plasma core region is nonuniform and asymmetric, composed of several isolated high temperature zones and relatively low temperature zone. Plasma temperature obtained in this paper is slightly higher than the temperature calculated based on Abel inverse transform.Plasma spherical layered model and paralleled layered model（or grid model） for laser trajectory are deeply analyzed. The reason why beam trajectory in paralleled layered model（or grid model） is demonstrated as that the interface normal is on the right side of light thus the laser beam is prone to center axis, which is not consistent with experimental reality. Based on the analysis, quadric surface layered model is set up from plasma grid model to resolve the problem that interface normal of grid model can not represent the macro-scale conditionsThe effects of He+Ar shielding gas parameters on plasma morphology characteristics, physical properties, and laser energy transmission are researched. Experimental results show that, plasma temperature decreases with flowrate of shielding gas and increases linearly with Ar ratio in the mixture gas. Nonetheless, the decreasing speed with Ar ratio is reduced by the growth of gas flowrate. Plasma temperature distributions along laser incent axis under different gas parameters are similar. The maximum temperature appears at 0.5mm above the material surface and increases with Ar ratio. Laser energy loss caused by plasma absorption is 4%-15% while the loss resulted from refraction inside plasma is 6%-22%. The loss ratio from refraction is bit higher than that of absorption under a certain shielding gas parameter. The total energy loss ratio of laser decreases with flowrate of shielding gas and increases linearly with Ar ratio in the gas. In the sense of laser energy efficiency, there is not a specified upper limit of Ar ratio in He+Ar shielding gas. He in the gas can be replaced by Ar though raising the gas flowrate, while retaining the laser energy transmission efficiency.