Investigation On Laser Fusion Welding-brazing Aluminium Alloy To Galvanized Steel With Filler Powder

In this dissertation, contraposed to the problems of welding Aluminiumalloy/steel joint, a laser fusion welding-brazing method using rectangular spot withfiller powder is proposed. In this method, the technique of filling powder is adopted.In this method, broadband laser spot could enlarge the molten and the size of joint.The powder filled could improve the composition of weld organization, as well aslaser energy coupling efficiency and welding efficiency.The present work is concerned with the foundational research into the techniqueof laser fusion welding-brazing aluminium alloy to steel with following objectives:(1)The joints formation, weld microstructure, as well as the mechanical properties arestudied, especially the influence of intermetallic compounds on the strength of thejoint.(2) The interactions between the laser beam, the powder steam and base metalare studied by experiments. The weld pool formation mechanism of the fusionwelding-brazing with filler powder is summed up, as well as the basic reason of highutilization ratio between laser power and base metal.(3) A finite element model of thethermal cycle in welding process is established for studies in influences of thermalcycle on the width of weld interface and intermetallic compounds growing. Base onthe results of experiments and analysis, the method for predicting the mechanicalproperties of the joint is announced.A fusion welding-brazing weld of AA6061and galvanized steel without flux canbe achieved using the welding with filler powder. By optimizing the weldingparameters, the favorable weld without crack and porosity can be achieved, and themaximum intensity of152.5N/mm is obtained. The results of welding tests indicatethat the width of the weld interface increased with the laser power and defocusdistance, and decreased with the increasing of welding velocity. When the weldingparameters are same, increasing the defocus distance could wider the weld interface.The Intermetallic compounds on the interaction surface compose by layeredand dendritic structure. The layered intermetallic compound is mainly Fe2Al5phase.Fe4Al13phase was also observed in the joint close to the aluminium alloy. Thedendritic intermetallic compounds are composed of Fe4Al13, τ1and τ5phases.Al5Fe2Zn0.4phase was found on the brazing filler/galvanized steel interface.Microhardness tests indicate that the hardness of the intermetallic compounds layer ishigher than the two base metals, especially the layered intermetallic compound.Tensile tests indicate that samples fractured brittlely on the interface of brazingfiller/galvanized steel. The joint strength increase with the ratio of. Thethickness of intermetallic compounds, especially the thickness of the layered Fe2Al5phase structure, determines mechanical resistance of the joint. Welding technicalparameters, such as high laser power and high welding speed, is the crucial factor formechanical properties in the fusion welding-brazing joint.With the interaction of the laser beam and powder beam, about10-15%of the laser energy is attenuated. The temperature of powder steam raise with theincreasing of laser power output, and enlarged the high-temperature area. Themaximum temperature of the powder is only about100°C. Inelastic collisionhappened when the powder particles impact surface of base metal. The rebound angleof powder steam is influenced by airflow of the shielding gas. There are slightdifferences at different amount of filler powder and shielding gas. After the collision,the height of powder steam is proportional to amount of filler powder and shieldinggas. The formation basis of weld pool with filler powder is the melting of powderparticles on the base metal surface, which is the basic reason why the filler powdercould improve the utilization ratio of the laser power.A finite element model is created for simulating the thermal cycle of the fusionwelding-brazing process with filler powder. A certain width of the temperate zoneappeared on the filler metal/galvanized steel interface, which is benefit to enlarge thewidth of the weld interface. The thickness of intermetallic compounds layer isaffected by laser energy input and maximum temperature of braze metal/steelinterface. Combined with the influence of welding parameters on the weld formationand intermetallic compounds, this model can be used to predict the welding results.