Effect of He–Ar ratio of side assisting gas on plasma 3D formation during CO2 laser welding

• A 3D reconstruction method based on CT technology for laser-induced plasma is proposed.
• The effect of He–Ar ratio in the side assisting gas on the plasma properties and laser energy transmission is studied through plasma spatial information.
• The change regulation of plasma 3D properties differs from that of plasma 2D properties.
• Plasma absorption is not very sensitive to Ar ratio. While laser refraction increases apparently with the growth of Ar ratio.

Side assisting gas plays a very important role in the laser-induced plasma suppression and the gas mixture ratio directly influences the formation and behavior of the laser-induced plasma during the laser welding process. In this paper, a photography system was set up with three synchronous CCD cameras to record the plasma plume during CO2 laser welding under different He–Ar ratios for helium–argon mixed side assisting gas. Three-dimensional reconstruction of the laser-induced plasma based on the computed tomography (CT) technology was achieved from the images shot by the cameras. Four characteristics, including the volume, uniformity, parameter PA associated with plasma absorption and parameter PR associated with laser refraction, were extracted from the 3D plasma and analyzed to investigate the effect on the plasma plume morphology as well as the laser energy attenuation. The results indicated that the He–Ar ratio of the side assisting gas has a considerable influence on some characteristics while some other characteristics are not sensitive to the mixture ratio. In addition, the effect of He–Ar ratio on the laser-induced plasma varies a lot with the flow rate of the side assisting gas.