Research On Laser Welding With Activating Flux Quality Controlling Of Stainless Steel For Vehicle Body Based On The Dynamic Behavior Of Molten Pool

Activating laser welding technique is an international leading welding technique based on activating welding in recent years.Combining laser and activating welding could not only improve penetration and welding efficiency significantly,but also realize the goal of increasing laser absorptivity and lowering cost.It is the important development direction of laser welding in future.Based on the above background,under the support of the National Natural Science Foundation of China,aim at SiO₂,TiO₂,Cr₂O₃,NaF,CeO₂ five kinds of activating flux on 304 stainless steel for welding process monitoring,observing the dynamic behavior of molten pool surface,the numerical simulation analysis of temperature field,based on the molten pool behavior and temperature field analysis,study of common welding defects formation process,mechanism of the influence of surfactant on welding quality inspection,the joint mechanical properties,microstructure and composition distribution to verify the feasibility of the activity of low power laser welding thick plate.The following works are carried out:?1?Based on the single factor variable method,the influence of the defocus,the laser power and the welding speed with the activating flux was explored by the penetration depth as the evaluation indexes.The welding quality of the same group of process parameters with different activating flux was tested and analyzed,and compared with without the activating flux.The results show that the activating flux will affect the welding depth,and the effect of Cr₂O₃ is the most significant.No evident diffusion and segregation of active elements was observed in the weld joint.Due to the activating flux,although the joint microstructure is dominated by refined isometric crystals and columnar crystals and similar to that of laser welding.The hardness of the weld joint meets the hardness requirement.Besides the testpiece of TiO₂ is poor,the test piece strength of others meet the requirement.?2?To observe the behavior of the pool under the conditions with the activating flux and analyze the change of molten pool with different activating flux via high speed camera.In addition to adding SiO₂ will cause the molten pool to accelerate the shock,addition of the activating flux will make molten pool becomes more stable,of which the most significant effect of TiO₂ surfactant.?3?Ansys Mechanical APDL software was used to simulate the temperature field of active laser welding,and the change trend of temperature with the activating flux laser welding was measured by thermal imager.The results show that the activating flux have a weak influence on the temperature of molten pool,and the NaF can reduce the temperature of the molten pool by about 4%.Other activating flux can raise the temperature of the molten pool by about 7-9%.?4?Based on the non-destructive testing,macroscopic and microscopic appearance of the weld,the defects of the internal pore and external splashing,hump and inclusions of the testpiece were analyzed.Based on the dynamic behavior of molten pool,the mechanism of weld defects with different activating flux is analyzed.The results show that SiO₂,Cr₂O₃ and CeO₂ activating flux have inhibitory effects on the internal pore of the weld,TiO₂ activating flux is easy to cause pore in the weld.On the external defect,the addition of activating flux also changed the periodicity of opening and closing of holes and the eruption of metal vapor,thus affect the type of splashing.The effect of NaF on the temperature gradient is different from other activating flux,The surface tension gradient exceeds the viscous force,so that the liquid flows and the Marangoni convection can lead to the surface hump.