Influence Of Side Assisting Gas On Structure And Shielding Effect Of Laser Induced Plasma

As important physical phenomena, laser-induced plasma can seriouslyaffect welding process and weld quality in the high-power laser weldingprocess. The presence of plasma will greatly influence the energy transferbetween the material and laser beam, thus cause instable welding process,low energy efficiency, low weld quality. The influence of laser-inducedplasma on energy transfer embodied in the absorption, refraction, scattering,recoil effects, heat transfer effect, and so on[1]. Among them, absorption andrefraction are especially important. In order to suppress the adverse effectsof laser-induced plasma, a simple, effective and widely used method is toadd side assisting gas to suppress the plasma[2].Temperature is an important parameter to describe laser-inducedplasma. Ways to measure the plasma temperature, such as spectroscopy,laser probes, can only get temperature at few points of plasma. Howeverthere are few methods to measure entire three-dimensional temperature field[3,4]. And more, among these methods for3D temperature field, most areusing Inverted Abel Transform method based on the assumption that theplasma is cylindrically symmetric. The actual plasma has varying degrees ofdeviation from cylindrically symmetry, sometimes severe deviation fromcylindrically symmetry[5].Currently, researches on shielding effect of plasma mainly focus onabsorption effect[6~9]and refraction using spectroscopy method, high speedCCD system and so on.This article provides an innovative approach to use method ofthree-dimensional reconstruction to get three-dimensional temperaturedistribution in the plasma. In the experiment, blowing angle and height ofshielding gas are fixed, while gas flow rate is set to vary from10L/min to 50L/min with pure He gas. Six CCD cameras are located in3directions ofthe welding plasma, with2cameras in front,2cameras on side, while thelast2cameras obliquely above. The two cameras in each direction areequipped with670nm and587nm filter film, respectively. During weldingprocess, strict synchronization is ensured between six cameras.2-dimensional projections of2specific lines from three directions could begot from the camera system. After the welding process three-dimensionaldistribution of line intensity of the plasma could be restored by centralprojection model based on the algebraic iterative method. Then spectralintensity method is employed to calculate three-dimensional distribution ofelectron temperature. At the same time, three-dimensional distribution ofelectron density could also be acquired according to thermo dynamicalprinciples of hot plasma. And then, we can calculate absorption rate of laserplasma.Finally, after calculation of temperature field and absorption rate,analysis is launched to compare some key parameters of the temperaturefield and absorption rates under different He flow rate.Experiment and calculations show that, temperature distribution of theplasma is found to be of such characteristics: high temperature near thecanter while low temperature near the edge. In addition, the plasmatemperature distribution also changes continually over time. However,volume ratio of different temperature range would change when flow ratevaries. With higher flow rate, the plasma is smaller, plasma temperature andabsorption rate also decrease.