| Aluminum and its alloys are becoming more and more important in modern industry especiallyin the automobile industry. In which the6061aluminum alloy was widely used in the manufacture ofcar body, engine, air conditioning parts because of the good formability, excellent weldabilty andoutstanding corrosion resistance. The temperature of commercial eutectic Al-12Si brazing alloys istoo close to6061aluminum alloy so the rate of spoiled products is very high. Meanwhile, theairtightness of the joints brazed by Zn-Al alloys is very poor and can hardly used for the production ofaluminum tube brazing. In order to solve these problems, the optimum composition of the Al-Si-Znfiller metals for6061aluminum alloy medium temperature brazing was studied, moreover, theenhancement by dopping trace elements which could improve the property of the brazing seam wasalso researched, and the mechanism of the strengthen effect was then analysed.The effect of Zn contant on the properties of Al-Si-Zn filler metals and6061aluminum alloybrazing seams was studied. The addition of Zn could lower the melting point to about520℃, butexcess Zn content would worsen the wettability of the filler metal. The strength of6061aluminumalloy joint brazed by Al-6.5Si-42Zn filler metal could achieve120MPa, and is16.7%higher than thejoint brazed by Al-0.2Si-77.8Zn. It could be found that the shrinkage could be found in the fracture ofAl-0.2Si-77.8Zn brazed joints which deteriorated the strength of the joints.From the test results of theairtightness, the Al-6.5Si-42Zn brazing joints is better than Al-0.2Si-77.8Zn and Al-2Si-36Zn. α-Al,η-Zn and Si phases could be found in the microstructure of joints brazed by all the three filler metals,and the Si element was almost solid soluted in the Al-0.2Si-77.8Zn filler metals and brazed seams.With the decrease of the Zn contant and the increase of the Si contant in the filler metal, the amount ofprimary Si increased and the acicular eutectic Al-Si phase was found. Due to the difference ofcoefficient of thermal expansion between Si and base metal, the micro cracks was found in the Siphases when cooling. It was also found that the combination property of Al-6.5Si-42Zn filler metals ismuch better than the other ones from the results of nanoindentation test and the corrosion test of thebrazing jointsThe effect of trace amont Sr addition on the microstructure and properties of Al-6.5Si-42Zn fillermetals was studied and it is also found that0.09wt%Sr addition could improve the wettabilitysignificantly, and the spreading area is11.8%larger than the filler metal without Sr addition. Theappropriate amount of Sr addition could improve the strength and extensibility could also improve 6.94%and75%respectively. Sr addition could modify the Si morphology and turned the aciculareutectic Al-Si phase to fiber type, meanwhile, the β-Fe could also be ‘deactivated’by Sr addition.Because of the chemical affinity between Sr and Si is much larger than Al-Sr and Al-Si, addition of Srelement would inhibited the growth of Si phases. Meanwhile, it is also found that the suitable heattreatment could optimize the morphology of Si phase, but the ductility droped sharply.The tensile strength of the6061aluminum alloy brazed joints was also improved by Sr addition,0.09wt%Sr gives a15%raise of the Al-6.5Si-42Zn brazed joints. From the observation of themicrostructure of the brazed joints, the Mg-Al-Zn phase was also found besides of the α-Al, η-Zn andSi phases. It was also found that the micro cracks disappeared in the fracture of brazed joints after Srdoping, and the Si phase also turn from bulk type, strip type to chrysanthemum type. Thenon-equilibrium Al-Si-Zn ternary eutectic could be found in the brazed seam after water cooling, andthe ternary eutectic area diminished after ageing. Because of the evolution of the microstructure, thetensile strength of the6061aluminum alloy brazed joints improved10.2%after ageing.Ti was commonly used as the refine element in aluminum alloys, micro addition of Sr and Ti into the filler metal do little affect on the melting point of the alloy. The tensile strength of the6061aluminum alloy brazed joints is7%higher than the ones which only modified by Sr. Sr and Ti did notbring the detrimental effect to the filler metal, and increase the Ti content to0.08wt.%when the Srcontent is low, the morphology of the Al-Si eutectic phase did not change. Though the modified effectof Ti on the eutectic phases was not as good as Sr, the refinement of Ti makes the eutectic phasesmuch finer than before, and the β-Fe was also uniformly refined.RE element Ce was another grain refiner in aluminum alloys, a new Al-Ce-Ti phase could befound in the Al-Si-Zn filler metals after Ce-Ti combined strengthen. The active Ce and Ti atomsdecreased after the formation of Al-Ce-Ti phase, and then reduced the effect of the modification. Butthe new phase disappeared in the Ce-Ti modified brazed seams, and the morphology of Si was mainlybulk type, that is to say the modify effect of Ce and Ti is weaker than Sr. Following to the dissolvetheory, the solute Ce and Ti atoms was saturated in the solvent, and the brings the precipitation.However, the precipitation disappeared due to the diffusion of brazing alloy and the base metal, whenthe content of solute in the solution reduced, the saturated solution turned to be the unsaturated stateThe empirical electron theory(EET) was used and the valence electron structure of the phases inAl-6.5Si-42Zn alloy was analyzed, it was found that the covalent electron pairs in the strongest bondof α-Al increased after the Zn and Si atoms solid soluted, and thus the difference between α-Al and Sidecreased while the difference between α-Al and η-Zn increased. So the main structure difference turn from α-Al/Si interface to α-Al/Si and α-Al/η-Zn interface. Therefor, the fracture occurred near theinterface of Si/α-Al and α-Al/η-Zn. The results ansysised by ‘average atom model’ explained that Sratoms was most likely adsorbent in the face of the Si lattice. The Si-Si bond energy decreased due tothe formation of Si-Sr bond and the accumulation of the Si-Si bond was also inhibited so the Si phaseswas distributed in the fiber like sharp.With the addition of Ce, the strongest bond in the Al-Ce-Ti phase turned from Al-Ti bond toAl-Ce bond from the valence electron structure analysis result. Meawhile, Al-Ti bond is the secondstrongest bond in the Al-Ce-Ti phase, so the Al-Ce and Al-Ti bond would combined with each otherand formed the Al-Ce-Ti phase. But the new phase disappeared in the brazed seam because of thesolute was diluted in the solution and the combination opportunity between Al-Ce and Al-Ti bond wasalso reduced. |
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