The Influence Of Different Assist Gas HE-AR Ratios On Plasma Characteristics And Behavior In CO₂ Laser Welding

Laser-induced plasma reduces the energy density by keyhole through absorbing, refracting and reflecting the CO₂ laser energy then changes the laser energy coupling with the workpiece, which will then cause welding defects such as pores and no penetration, etc. Mixture of He-Ar as side-blown gas is often used in inhibiting plasma. Based on thermal conductivity, electron negativity, dissociation energy, ionization energy and other considerations, He is the ideal shielding gas. In industrial production, it is often to inhibit plasma by increasing side-blown gas flow rate. However, He resources are scarce and expensive, it is of great importance to do some research on the influence of side-blow gas with different He-Ar ratios on plasma for improving welding quality and reducing producing cost.In this paper, based on fluid dynamics theory, a two-dimensional unsteady flow model has been built for analyzing flow field under different side-blown gas He-Ar ratios. Through the model, pure He is used as side-blown gas and nozzle height, nozzle angle, nozzle radius and gas flow rates are changed to calculate the static pressure, horizontal velocity and He volume fraction at the laser focusing point under each condition. Besides, the above three parameters are used to optimize process parameters. Calculation result shows that the optimum nozzle angle range is 40°～55°, the optimum nozzle height range is 6～12 mm, the optimum nozzle radius range is 3mm⁴mm and the best range for nozzle in X direction is -1.5mm⁰.5mm. Based on the optimum ranges, process parameters are selected. Only change He-Ar ratios among pure He, 75%He25%Ar, 50%He50%Ar and calculate the static pressure, horizontal velocity and He volume fraction at the laser focusing point. The result shows that with the increase of Ar content, static pressure and horizontal velocity are increase whereas the volume fraction of He is reduced.In this paper, based on a comprehensive consideration of plasma's absorption and refraction for laser energy and laser beam, a laser-plasma model has been built under different side-blown gas He-Ar ratio. Particle densities, absorption coefficients, thermodynamic properties and transport properties under different temperatures and within different plasma components are calculated, which have played a significant role in ensuring the accuracy of the model. The temperature field, velocity field and volume fraction field of the plasmas under different He-Ar ratios within pure He, 75%He25%Ar, 40%He60%Ar are calculated. The results show that with the increase of Ar content, the plasma height reduces and area increases. Meanwhile, with the increase of Ar fraction, the whole velocity magnitudes increase which show great velocity increase in X direction but little change in Z direction. The growth of horizontal velocity improves heat conductivity and it can help explain why the plasma height reduced along with the increase of Ar fraction. CCSB steel plates are used in the experiment. High speed camera system and Labview-based image processing platform are used in capturing plasma images and image processing. The experiment results show that plasma height reduced with the increase of Ar fraction while its area grows. The plasma height and area increase when laser power grows. Meanwhile, along with the reduction of He fraction of side-blown gas, the weld width increases while depth reduces.