Experimental Research On Laser Welding-Brazing Of Dissimilar Metals Aluminum/steel And Optimization Of Process Parameters

The situation of environment protection and energy supply has been deteriorating in recent years, and a stricter requirement for energy conservation and emission reduction calls for the lightweight as an effective solution. The composite structure which consists of aluminum alloys and steels can reduce the structure weight remarkably for energy saving and the reduction of gas emission. Therefore, lightweight displays its unique advantages and has broader prospects in the field of aviation, shipbuilding and automobile and other industries.Through the melting-brazing experiment for aluminum/steel dissimilar metal butt joint with laser as heat sources, during whose process the filler wires with different silicon content are added, the influence of different silicon contents and process parameters on the butt joint microstructure and mechanical property is studied in this dissertation. In addition, the process parameters of welding process have been optimized by the response surface method(RSM). The following key aspects are included in this dissertation.(1) Theoretical aspects: The interaction between laser and mental, and the calefaction of laser on material surface are studied. The difficulties of welding aluminum/steel dissimilar metal are analyzed from the perspective of dissimilar metal weldability afterwards. Then the factors influencing the dampening and spreading process of liquid brazing filler on base metal, and the interaction between liquid filler and solid base metal during the welding-brazing process are also discussed.(2) Experimental aspects:The laser welding-brazing experiment of aluminum alloys and stainless steels is conducted with a specially designed welding fixture at first. Then influencees of the content of Si element and process parameters on the formation of weld is studied, and the reasons on the formation of pores and cracks during welding, as well as the states of grains at different locations of weld joints are illustrated. The cause of the differences of intermetallic compounds’(IMC) shapes and thickness at the interface are analyzed after taking into the consideration of the thermal distribution of molten pool, the generation of compounds and its mechanism of generation during welding. The phase compositions of IMC in the section and the fracture of welding joints with different contents of Si are identified. Afterwards, the mechanical property of joints, and the fracture mechanism are also comprehensively analyzed by considering the relationship between process, structure and property. The experiment results show that the IMC’s types and the diffusion between aluminum and iron elements have great impacts on the tensile strength. The weld beam with ER4047 filler wire can generate τ5 and τ6 phases, and the diffusion between aluminum and iron can be suppressed, which leads to the damage of joint property; joint fracture displays a mixed fracture mode. Based on the thermodynamic and kinetic researches of compounds reactions, ultimately, this dissertation also put forward an influence model of different Si contents on the compound generation at the interface.(3) The optimization of process parameters: A mathematical model between tensile strength and process parameters are built based on the data of the experiment with central composite experiment design. Significance testing and variance analysis of the appropriateness of this model are made afterwards, to find out the main effects and the interactive relationship between process parameters as well. By setting the maximum tensile strength as the optimization objective, the parameters during the welding process are optimized by the response surface method. Finally, using the optimized parameters to conduct experiments, the mechanical property of the weld joint under the condition of optimized parameters is detected and analyzed. Experiment results show that the defocus and the laser power have the great impact on the tensile strength, the predicted value of the regress model shows a good consistency with the experiment value. The joint tensile strength under the optimized parameters can reach 185 MPa.