Synthesis Of CNTs/AlSiCu Powder And The Strengthening Mechanism Of MBO_w/Al Brazed Joint

Magnesium borate wisker reinforced Al martix composite (MBOW/Al) is akind of discontinuous reinforced aerospace material which is widely concerned forthe excellent bearing capacity, wear resistance and dimensional stability. Due to thepoor workablity, however, it’s hard to be machined into complex components andthat greatly limits the practical use. Therefore, achieving a high strength brazedjoint should be meaningful.There are two common difficulties in brazing Al martix composite(AMC).Firstly, the residual stress caused by CTE mismatch between filler and base metalcould weaken the brazed joint. So we prefer Al-base filler for its CTE close to thatof AMC. Secondly, a limited brazing temperature has been difined by the lowmelting point of AMC. Generally, it is hard for low temperature filler to achieve ahigh strength joint for its poor mechanical proformance. Severely, a new type offiller owning both low CTE and superior mechanical properties is in need.In this paper, we proposed an approach that in-site growing carbon nanotubes(CNTs) on AlSiCu powder at low temperature by PECVD so as to synthesize theCNTs/AlSiCu powder. After that, we analyzed the thermal and mechanicalproperties of it. And also the influence of different parameters upon CNTs growthwas studied. The content, dispersibility, morphology, purity, and microstructure ofCNTs in CNTs/AlSiCu powder were characterized. Under the optimized parameters,CNTs content is4.85wt%and spead uniformly with high purity and structuralintegrity. The diameter is about11nm and length reaches1μm.An integrated high strength brazd joint was achieved by CNTs/AlSiCu with atypical interface of MBOW/Al composite/Mg2B2O5+Al2Cu+Al(s,s)+Al4C/MBOW/Alcomposite. Results showed that the mutual diffusion was positively correlated withbrazing temperature and holding time. The shear strength reached up to94MPawhen brazing at590℃/10min by0.6wt.%CNTs/AlSiCu, which had been improvedby130%than that of the joint without CNTs.Results showed that CTE regulation and load transfer contributed to theinforcement of CNTs/AlSiCu on joint strength. The addition of CNTs could lowerthe CTE of brazed seam, decrease residual stress and effectively block the formulation and propagation of cracks. The interfacial behavior between CNTs andAlSiCu during brazing was simulated by evaporating filler metal onto CNTs.Results showed the amorphous carbon and defects on the surface of CNTs couldreact with Al and form Al4C3thin film, which could improve the interfacialinteraction between Al and CNTs. Continuously and evenly coated by filler metal,the CNTs after reaction still kept perfect structure and contributed to theenhancement of mechanical properties of the joint.