Effect Of Narrow Side Gas Jet On The Process And Bead Formation Using Fiber Laser Welding Method

SUS 304 stainless steel is a common austenitic stainless steel and it is widely applied in ships,aerospace,nuclear and other industries.Laser welding has a wide range of applications because of its fast welding speed,small heat affected zone,high depth-width ratio and accurate control ability.Due to the complex energy transfer and mass exchange process in thick section welding,it is common to achieve poor quality bead.In order to increase the penetration and improve welding bead quality,a 2mm-diameter gas jet nozzle was employed to straightly blow the laser keyhole during the fiber laser welding of 8mm-thickness SUS 304 stainless steel and the effect of narrow side gas jet on the process and bead formation using fiber laser welding method was studied.The behaviors of the keyhole,molten pool,metallic vapor and the state of keyhole inside were observed by high-speed video camera to study the mechanism of narrow side gas effects on welding process and bead formation.The spectrum of metallic vapor during SUS 304 stainless steel fiber laser welding was collected and analyzed.It was found that the wavelength of high spectral line was in the range of 500~700nm and the intensity of spectral line with wavelength above 800 nm was very weak with argon or nitrogen or without side gas.As a result,the high-speed camera can capture the image of the molten pool,the keyhole and the inside of the keyhole clearly with 810nm-wavelength background laser source and filter regardless of the influence from metallic vapor.Comparison experiment between argon and nitrogen was conducted.It was found that the frequency of keyhole opening was lower,the spatters were less,the bead was sounder and the penetration was deeper with argon than nitrogen.So for the purpose of this paper,it is much better to choose argon as the side gas.Using argon as the side gas,the weld penetration tends to rise with the increasing of gas flow at the constant laser power,welding speed,defocused distance,the gas jet position and other welding parameters.The max penetration sound bead was achieved when the side gas flow was 10L/min.It was also found that the depth of the keyhole tends to rise with the increasing of gas flow though the state of keyhole inside observation.When the metallic vapor height was higher,the corresponding keyhole depth was shallow,and when the metallic vapor height was suppressed,the keyhole depth was deeper.Suppressing the metal vapor height makes for increasing the weld penetration.It was found that the area and length of the molten pool and the area and width of the keyhole tends to rise with the increasing of gas flow through the observation of molten pool and keyhole by fast-speed camera.Adding narrow side gas can reduce the frequency of the keyhole opening,lessen the weld spatters and improve the welding quality.But when the side gas flow was too much high,the gas would push the melt metal out of the molten pool,resulting into the poor-quality bead.Welding of GG17 glass and SUS 304 stainless steel was overserved by fast-speed camera and background laser source.It was found that the depth of the keyhole tends to rise with the increasing of gas.It was also found that the keyhole front wall was inclined and the metal at the keyhole front tend to flow to the bottom of the molten pool after turning into liquid.The keyhole back wall tends to produce different degree of hump and deformation state along with the increasing of the side gas flow.It was found that the area and height of metallic vapor tends to reduce with the increasing of gas flow through the observation of metallic vapor by fast-speed camera.The area and height of metallic vapor changed little,when the side gas flow increased from 0 to 5 L/min.But the area and height of metallic vapor changed much when the side gas flow increased from 5 to 10 L/min.In addition,the area and height of metallic vapor had little difference when the side gas flow increasing from 10 to 15 or 20L/min.It is the best choice to use 10L/min as the side gas rate considering the suppressing of area and height of the metallic vapor.