Study On The Root Humping Suppression Mechanism Of Laser Welding Of Medium-thick Stainless Steel Plate

Laser welding has the characteristics of small heat affected zone,large aspect ratio,and small welding deformation.It has been widely used in nuclear power,ships,automobiles and other fields.However,at present,the methods and mechanisms for suppressing the root humping defects that occur during the laser welding of stainless steel plates have yet to be thoroughly studied.In order to effectively solve the problem of root humping in the process of deep penetration laser welding of medium and thick plates,this paper proposes a method of deep penetration laser welding of medium and thick plates based on prefabricated runners Experiments and theoretical studies have been conducted on the behavior and the suppression mechanism of root humping defects.The main work carried out is as follows:(1)Based on the improved "sandwich" method,the high-speed camera is used to observe the internal keyholes and molten pool of the deep penetration laser welding of the prefabricated runners in the middle and thick plates.It was found that due to the existence of the prefabricated flow channel,the molten metal on the front wall of the keyhole and the metal vapor in the keyhole flow into the flow channel,and the keyhole tends to be smooth and flat along the wall,the stability of the keyhole is good,and the flow of the bottom molten pool is stable.(2)Through high-speed camera test and numerical simulation analysis,the mechanism of root humping suppression of deep penetration laser welding of prefabricated runners was revealed.First,the metal vapor in the keyholes flows into the prefabricated runner,the recoil pressure is reduced,the impact on the bottom molten pool is reduced,and the stability of the keyholes is improved.Secondly,the prefabricated runner divides the molten metal at the high speed of the front wall of the keyhole downward,and the gravity of the bottom molten pool is reduced,so that the bottom molten pool moves smoothly.At the same time,because the molten metal under the front wall of the welding keyhole flows upward along the inclined flow path,the melting rate of the upper half of the base material is greater than that of the lower half,and the lower half of the base material can support the front wall of the keyhole The molten pool slows down the molten metal flowing down the front wall of the keyhole,thus avoiding the formation of root humping.(3)The welding parameters and flow channel parameter optimization of the deep penetration laser welding of prefabricated runners were studied.The results showed that the precast runners had a significant suppression effect on the root humping,but the upper surface had collapse.Within a certain range,the inclination angle of the flow channel has a greater influence on the weld surface formation.The larger the inclination angle of the flow channel,the smaller the upper surface collapse;as the distance between the flow channels increases,the upper surface collapse of the weld is gradually improved;When the diameter of the runner is 0.5 mm-1.0 mm,the effect on the upper surface of the weld is not obvious.When the laser power is 3.6 kW-4.0 kW,the upper surface of the weld is gradually improved,and the spatter is less.When the laser power is 4.0 kW,the weld is best formed.As the welding speed increases,the quality of the upper surface of the weld seam gradually improves.The amount of defocusing has a significant effect on the weld width of the weld,and the weld width of the weld is wider when it is defocused.(4)The microstructure and tensile properties of the deep penetration laser welding of prefabricated runners are studied.The structure of the welded joint is mainly austenite,and a little ferrite is retained.Skeleton-like,lath-like and worm-like shapes exist.The columnar crystals exist on both sides of the fusion zone on the cross section of the weld seam and are symmetrically distributed.The growth direction of the columnar crystals is toward the center of the weld seam.The tensile strength of the weld joint is 880 MPa,and the tensile strength of the test piece is close to the base material.