Study On Effct Of Small Diameter Side Blowing Gas On Plasma And Weld Pool And Keyhole

Laser welding is a favorite fabrication method in modern industry like automobile,shipbuilding, etc. Laser-induced plasma and keyhole are the two most importantcharacteristics in laser welding. Although laser welding has many advantages, it also hasits drawbacks. In this paper, study was focused on two of its disadvantages. One is thatlaser-induced plasma decreased laser efficiency by refraction and absorbing, this couldmake laser welding course unstable and deteriorate welding quality. The other is that inlaser overlap welding of zinc coated steel, the zinc coating is easy to vaporize and the zincvapor between plates is hard to escape, which leads to strong splash and bad weldingquality.On account of the two problems, in this paper, experiments were designed based onliterature investigation, a side nozzle with the inner diameter of2mm was applied, usingside assisting gas to solve these2problems. Meanwhile, the connection framework wasdesigned to match side nozzle system with CO₂laser welding system in laboratory.First, the effect of laser power, welding speed and line energy on plasma control wasstudied. Linlog high speed photography system was applied to capture the dynamicbehavior of plasma, plasma images were processed by LabView software. It’s found outthat too strong a plasma would make welding unstable and deteriorate welding quality.The area and height of plasma increases with the rise of laser power, and with the deceaseof welding speed. When line energy kept instant, the area and height of plasma differ notmuch under different laser power and welding speed. While when line energy was set at ahigh level, the weld depth was low, for the plasma was strong in this condition.Side assisting gas could control the plasma effectively. In this paper, the gas flow rate,gas composition and side nozzle positioning were investigated and optimized. With gasflow rate increasing, the average area and height of plasma decreases, and weld depth increases. Helium is the preferred side gas for its high ionization energy, better plasmacontrol effect could be obtained when mixed with argon, the density of which is higherthan that of helium. The study shows that a gas composition of75%He mixed with25%Ar has the best plasma control effect. Positioning of side nozzle is also important, whenthe gas flow is at the center of laser welding point with an error of0⁰.5mm, plasma couldbe controlled effectively. The height of side nozzle affect plasma control effect followingthe same mechanism of flow rate, which could be defined by the pressure on the weldingpoint produced by side gas. The height range of5⁸mm is appropriate to get big welddepth.Side gas also has good effect on laser welding of zinc coated steel. In this paper, anoptical system was designed according to the spectrum of DP590GA galvanized steel, toobserve the behavior of welding pool and keyhole. The effect of gas flow rate and weldingspeed on zinc coated steel laser welding was studied in details. Appropriate gas flow rate（10L/min in this paper） would reduce splash when welding, and good weld bead could beobtained. When welding speed is too slow, more zinc vapor is generated and the effect ofside gas on zinc vapor escaping is faint in this condition, raising gas flow rate to a highlevel would lead to instability of welding pool. The effect of gas flow on the weldingquality improvement of zinc coated steel overlap welding could be explained by the‘keyhole opening’ mechanism. At an appropriate gas flow rate, the keyhole could beblown to a big opening, which makes a passage for the escape of zinc vapor. While toohigh a flow rate would make the welding pool unstable.