Study On Laser Brazing With Filler Wire For Galvanized Steel Sheets And Numerical Simulation Of The Thermal Process

The joining of galvanized steel sheets is difficult due to the existence of Zn coat. The evaporation and oxidation of zinc can lead to an unstable arc, low corrosion resistance and other welding defects such as porosity, inadequate fusion and cracks and so on. These problems can be avoided with the development of laser brazing process, which provides the possibility to improve the structure design and reduce the cost in production. With the laser heating feature and real-time wire feeding, the process is complex and the influencing factors are various, and process stability and joint quality are required to improve further. So, comprehensive investigation and quantitative evaluation of the influence of process parameters are significant in theory and practice. Aiming at the laser brazing of flange butt joints of galvanized steel sheets, the influence regularity of process parameters on brazing seam forming and interface behaviors was studied in this paper. Based on the analysis, the filler metal flow and temperature distribution in brazing process with different heating condition were calculated. These results provide technique support and fundamental theory in comprehending mechanics of laser brazing process and developing new process. The melting and spreading process of filler metal with static laser heating were observed and analyzed using high speed photographing system in this paper.The influence of process parameters such as laser power, spot diameter and heating time on spreading ability was evaluated. In further investigation, the melting and spreading process of feeding wire in real brazing process was analyzed. The experimental results indicate that the surface tension is the most important influencing factor in the spreading flow and forming of filler metal, the temperature of filler metal and base metal determine the spreading ability. Furthermore, the matching of parameters such as laser power, brazing speed, wire feeding speed and laser heating position should be considered for a good appearance brazing joint.Based on the investigation of individual laser beam brazing, a new brazing process, dual-spot laser beam brazing was presented. Compared with individual beam brazing, new beam parameters include beam power ratio, focus distance, defocusing mode and spot arrange method, which make the brazing process more flexible. Combining the heating parameters and beam parameters, energy intensity distribution of brazing heat source can be changed to realize the different thermal distribution. Experimental results show that the better process adaptability and higher laser power availability ratio can be achieved in dual-spot beam brazing than in individual beam brazing.In order to expand the interaction time between filler metal and base metal and increase the interface combination quality at the bottom joint area, rectangular spot laser beam was used as brazing heat source. Integrator mirror was used to obtain rectangular laser spot with uniform laser power distribution in spot length direction. The rectangular spot brazing results indicate that narrower brazing seam surface and more joint interface area with suitable brazing parameters can be achieved, and the process can reduce the burning loss of Zn coat. Whereas, the adaptability and laser power availability ratio are lower.Further investigation was focus on the laser brazing joint structure and interface behaviors, in which metallographic microscope, electron microprobe scanning, SEM and EDS analysis were used. In experiments, cellular dendritic crystal ofαsosoloid was found in brazing seam with filler material of CuSi3 and the adjacent interface area of base metal is coarse grain zone. The joint SEM analysis with different heating condition was carried out to understand the mechanics of intermetallic compounds formed at the interface, and the results indicate that the thickness and shape of intermetallic compounds layer relate to brazing thermal cycle and interface temperature gradient.A FEM model of thermal-fluid coupling of laser brazing was developed. With the model, fixed beam heating, individual beam, dual-spot beam and rectangular spot brazing process were simulated, and the temperature distribution, flow field and filler metal shape were calculated. The simulated results show that beam spot size is the determining factor of brazing droplet shape, the temperature gradient on free surface and droplet shape determine the flow of filler metal for fixed beam heating. In individual beam brazing, the temperature of filler metal increase rapidly, and a great temperature gradient can be obtained. For dual-spot and rectangular spot brazing, the temperature filed is more uniform. The simulated results of flow field show that, the downward flow filling the joint gap is the main flow situation of the filler metal at the early stage of individual beam heating, and Marangoni flow is more important after brazing seam shape forming. For dual-spot laser brazing, the downflow is dominant in the whole process and the time of filling joint gap process is longer. The average velocity of filler metal flow is small in rectangular spot brazing and the filler metal adjacent seam center line flow downwards and Marangoni flow is dominant for the filler metal at the upper part of brazing seam. Simulated and experimental brazing shapes were compared, and a good agreement is obtained.