Study On The Root Humping Formation Mechanism With Fiber Laser Welding Of Medium-thick Plate

In recent years, with the emergence and development of high power and high quality fiber lasers, making deep penetration laser welding technology get further research in the field of welding manufacturing medium-thick plate. In the process of high power fiber laser welding of medium-thick plate, the power density of fiber laser can be up to 106~108 W/cm2. The dynamic pool is unbalanced force and very unstable. That is easy to cause welding spatter?surface's undercut and root humping, which affects the properties of the weld.In order to achieve high quality and efficiency of laser welding medium-thick plate, this paper is an attempt to probe the problem of root humping in the process of high power fiber laser welding thickness of 5 mm SUS 304 stainless steel plate, study the keyhole's dynamic changes and the internal molten pool's flow state, analyse the relationship between the vapor outside the keyhole and root humping, and study the mechanism of root humping.(1) The paper describes the current research situation of high power laser welding and root humping. Based on high-speed camera, the experiment platform of high power fiber laser welding is built.(2) Based on a modified “sandwich” novel method, the paper used high-speed camera to observe that the dropped down pool led to surface undercut. The results show that the vapor pressure and the downward momentum of the shelf on the front keyhole wall overcome the surface tension of the bottom pool of keyhole. And leads to the molten metal rolls out the inside pool, then flows into root humping.(3) Used high-speed camera to take pictures of the orifice keyhole, the flow of the molten pool and the vapor outside the keyhole from different angles. The results show that surface tension is an important cause of the generation and growth of the first root humping. The molten metal flow is not continuous in the keyhole, the moving downward shelfon the front keyhole wall and the pressure of metallic vapor inside the keyhole lead to a plurality of droplet in the bottom pool. The surface tension and the flow of melt pool cause the backward flow of molten metal. With the accumulation and solidification of molten metal, root humping forms.(4) In certain Ar surface shielding gas and top shielding gas with certain flow rate and blowing angle, the paper makes use of changing the single variable to discuss the influence of the laser power, welding speed, defocus, flow rate of bottom shielding gas and the welding position on root humping. The results show that adopting some reasonable welding parameters can efficiently suppresse root humping, improve the welding quality.