Joining Of Alumina Ceramics By Whiskers And Joining Of Alumina Ceramics With TC4Titanium Alloy

Alumina is a kind of material widely used in space industry, chemical industry, etc.due to its low electrical conductibility, excellent mechanical strength and high thermalinsulation capability. Residual stress tends to be produced in the joint of ceramic to metaldue to the mismatch of the properties between them. The joining of ceramics withceramic or metal was realized with the help of whiskers. When joining ceramics withceramics, whiskers were synthesized both on the surface of the ceramics and in the seamto form the joint. When joining ceramic with metal, whiskers were firstly grown on thesurface of the ceramics and then brazed with metal. The morphology of the whiskers andthe microstructure revolution as well as the variation of the strength of the joints wereresearched by means of SEM, EDS, XRD and strength test.Al₁₈B₄O₃₃and Al₄B₂O₉whiskers were generated on the surface of alumina ceramicswith B2O3and Al₂O₃as raw materials and K2SO4, Fe2O3, MnO2as flux. The length of thewhiskers ranges from5-20μm, and the diameters of the whiskers were about1-2μm. Thegrowth direction of most whiskers had a small angle to the surface of the ceramics. Withthe increase of the holding time and the temperature, the length of the whiskers firstlygrew and then remained the same. When K2SO4was used as flux, the length of whiskersgrew up to15-20μm. When MnO₂was used as flux, the growth direction of mostwhiskers had a large angle to the surface of the ceramicsAlumina was joined to itself in air environment by B2O3+Al₂O₃powder mixture asthe interlayer to synthesize aluminum borate whiskers in the joint. The microstructureevolution was analyzed by means of scanning electron microscope （SEM） and X-raydiffraction （XRD）. The possible mechanism of microstructure revolution was alsodiscussed. The effects of parameters such as heating cycles and temperature on themicrostructure as well as the3-point flexural strength of the joint were investigated. Itwas found that heating the specimen for several heating cycles instead of holding at aconstant temperature for a long time can eliminate the voids among the whiskers toachieve a dense microstructure, and the phenomenon that higher temperatures canaccelerate the reaction was also observed. The highest strength of the joint reaches90.3MPa with the heating temperature of800℃after four heating cycles.The joining of TC4alloy to alumina ceramic with whiskers was realized usingAgCu interlayer. The microstructure of the joint was alumina/Ti4（Al,Cu）₂O/Ag（s,s）+Ti4（Al,Cu）₂O+TiB₂+Al₂TiO₅/Ag（s,s）/Ti₃Cu₄/TiCu/Ti₂Cu/α-βTi/TC₄titanium alloy.With the increase of the brazing temperature and the holing time, the ratio of the reactionproductions located near the interface grew up. The strength of the joint went up firstlyand then dropped with the increase of the joining temperature as well as the holding time. The highest strength of46MPa was reached when using the ceramic with in-situ grownwhiskers generated by B2O3+K2SO4as raw materials under the condition of825℃for5min.