Study On Powder Delivery And Process In Laser Welding Of High Strength Galvanized Steel With Powder Filling

With the development of laser technology and the lightweight of car body, thelaser butt welding will take the place of conventional lap welding because the laserbutt welding can reduce car body weight effectively. Yet laser butt welding requireshigh assembly accuracy of workpiece. To ensure welding performance and reduce theassembly accuracy, some derivative laser technology has been invented. Laserwelding with powder filling is a high flexible welding technology due to its effectivereduction of the assembly accuracy of workpiece. Hence the effectiveness of thepowder delivery and welding process with a self-developed lateral device for powderfeeding were analyzed in the thesis.Firstly, the effectiveness of the powder delivery and the utilization of powderhave been studied on the basis of the correlation between powder distribution densityand the gradation of image. The powder distribution density on weldin g plane hasbeen obtained by measuring the distribution of gradation in powder-beam section. Atthe same time, the effect of the effective delivery area on powder delivery positionhas been researched. The functional relationship of the effectiveness of the powderdelivery and powder delivery position has been set up on the basis of the dependencybetween the effective delivery area and the effectiveness of the powder delivery.When the angle of powder filling was45°, the best powder delivery position was-1.75mm as the maximum effective delivery rate was obtained. Secondly, the powerwould be fed into the welding pool ineffectively if the powder position was too closeto or far away from the laser beam. The powder would be burned as the powderposition is too close to the laser beam and if the powder position was too far awayfrom the laser beam, the effective delivery rate would be reduced. The maximumpowder utilization rate could be realized with the powder delivering position was-4mm. The copper content at the weld center section decreased linearly as the powderfeeding speed decreased. When the feeding speed was less than1.15g/min, thepowder would not feed into the weld seam effectively.Simultaneously, the process parameters in laser welding of high strengthgalvanized steel with powder filling were also optimized. Low welding speed wouldbring the accumulation of the molten mass and the welding holes. To increase thedefocusing distance would be conducive to the addition of the powder into molten pool. The optimization process parameters for laser welding of high-strengthgalvanized steel with powder filling were1500W for laser power,30mm/s for weldingspeed and12mm for the defocusing distance. Finally, the effect of powder filling onthe performance of weld joint has been researched. The result shows that: powderfilling can significantly improve the performance of the weld surface and refine thegrains in the heat-affected zone, as well as improve the weld properties. The laserwelding with powder filling can also increase the gap margin and lower therequirements of assembly accuracy of the workpieces. The broken occurred on thebase material in tensile fracture tests with powder filling as the assembly accuracywas0.25mm.The research results of the effectiveness of powder delivery provide thetheoretical basis to improve the powder utilization rate in the process of laser weldingwith powder filling. And the optimized process parameters provide reference for theoptimization of the parameters for practical production. The minimum powder feedingspeed can be the design basis of the accuracy of the powder delivery in powderfeeding device.