| The brazing temperature are almost above the solidus temperature of6061aluminium alloywith the common aluminium based brazing filler metals. So the brazing process can make theerosion of6061aluminium alloy base metal and deteriorate the mechanical property of joints. Todevelop brzaing filler metals for6061alluminium alloy at middle temperature, the research onAl-Si-Zn filler metal and the exploration about the related brazing technology of6061aluminiumalloy have been done in this thesis for the purpose of lowering the melting temperature of Al-Si-Znfiller metals, improving the spreadability of Al-Si-Zn on6061and the property of joint.In this thesis, the effects of Si on the melting point and spreadability of Al-Si-Zn filler metalswere studied. The results show that the liquidus temperature of filler metals decreased, thenbecame constant at549℃when the Si was more than3%. While the solidus temperature increasedfirstly and then decreased, at the same time, the range of melting temperature became more narrowwith the addition of Si in the Al-Si-Zn filler metal between2wt.%to6wt.%. When the Si contentis more than6%, the range of melting temperature became more wider. The filler metall presentedthe best spreadability on6061aluminum alloy and the lowest liquidus temperature when Si contentwas4wt.%. The mirostructures of Al-Si-Zn filler metals were consist of α-Al solid solution、η-Znsolid solution and Si phase. The chance of the Si content in Al-Si-Zn filler metals only changed themorphology and distribution of Si phase.Among the Al-xSi-Zn/6061joints, the Al-4Si-Zn/6061joint possessed the peak value of shearstrength and the value was160.23MPa. As the content of Si in filler metal increased from2%to6%, the columnar α-Al solid solution at the interface and the dentritic α-Al solid solutiondistributed in the fillet became more finer, at the same time, the proportion of eutectic structure wasboosted gradually. The eutectic structure possessed the peak hardness value and the value increasedas the content of Si increased. When the Si content was up to6%, the eutectic structure deterioratedthe mechanical property of joint and made the fracture morphology changed from ductile fractureto brittle fracture because of its high hardness value.With the use of Al-4Si-Zn filler metal, the related brazing technology was studied. The resultsshow that the microstructures of Al-4Si-Zn/6061fillet were consist of eutectic structure and α-Alsolid solution which was perpendicular to interface between filler metal and6061base metal. Andthe eutectic structure was consist of α-Al solid solution and silicon phase uniformly distributed in the eutectic structure. At each same brazing temperature, the dentritic α-Al was the most coarse andthe proportion of the eutectic structure was the lowest in the fillet after furnace cooling, while thedentritic α-Al was the most finer and the proportion of the eutectic structure was the largest in thefillet after air cooling. Under water cooling, the α-Al solid solution in the fillet was almost dentriticand was perpendicular to interface between filler metal and6061base metal. With the cooling rateincreasing, the shear strength of the joints increased and the joint after water cooling possessed themaximum value of shear strength. Under water cooling, the fillet brazed at580℃had themaximum shear strength when the brazing temperature increased from570℃to590℃, which wasas same as under air cooling. While under furnace cooling, the brazing temperature had little effecton the shear strength of the joints.The more coarse α-Al solid solution and the more dispersible eutectic structure made the shearstrength of joint increased synchronously as the annealing time increased from5min to15min.When the annealing time was in the range from15min to60min, the shear strength decreasedgradually because the brazing technology made the erosion phenomenon of6061base metal moreseriously. Finally, the optimum brazing technology is: brazing at580℃，annealing15min, andwater cooling after brazing. |
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