Research On Microstructure And Performance Of Vacuum Brazed Joint Of W-cu Composite And Stainless Steel

The application of tungsten-copper composite was constrained under special conditionbecause of the weak oxidation resistance in high-temperature and poor toughness inlow-temperature. For joining W-Cu and stainless steel to form compound structure, it canobtain the performance complementary of both and then widen the application prospect.While the great difference in characteristics between W-Cu and stainless steel becomesthe primary difficulty, including fusion point, coefficient of linear expansion, thermalconductivity and gas sensitivity, together with the hard metallurgical compatibility.Thereby, an excellent joint via conventional fusion welding could not be received easily.But brazing characterized by kind of solid-liquid connection make the weld and substratesnearby be suffered less from complicated thermal cycle and stress field, and thenstrengthen the bonding strength at interface.In this study, vacuum brazing of W-Cu composite to stainless steel with the filermetals of BNi2、CuMnCo and AgCu would be conducted. Microstructure, elementsdiffusion, fracture feature together with flexure strength were analyzed for the bondingmechanism. Finally, effect of brazing clearance on the microstructure and property of Nibased joint was conducted.Microstructure of Ni based joint was composed of γ-Ni(Cu) solid solution and someboride and silicide. The bend strength was as high as188MPa and fractured in the brazedlayer. It was identified as brittle fracture owing to few dimple but many cleavage planesappearing at fracture surface. However microhardness in the brazing layer was clearlyhigher then that of the parent metals, which testified the existence of certain intermetallicformed in the brazed seam.Microstructure of Cu based joint maily contained of matirx solid solution (Cu-Mn)and precipitated solid solution(Fe-Co) with good plasticity and microhardness of the brazedseam was lower than that of the substrates. The fracture happened at W-Cu side with ahighest bending strength of780MPa. On condition of lots of dimples existed and severaltearing cracks, it illustrated as a mixed fracture feature containing brittle fracture of W-Winterface and plasticity facture of solid solution in the brazed region.Microstructure of Ag based joint mainly contained eutectic phase near W-Cucomposite side and Cu riched phase near stainless steel side. And the microhardness was lower than that of substrates, showing a special pattern of “hard clip soft”. Thefracture occurred at the bonding interface near1Cr18Ni9steel side with576MPa bendingstrength, together with such as shaft dense dimples and several tear ridges along the sheardirection. The fracture behavior was identified as traditional shear fracture in45angle ofductility so that the joint.Bending strength of Ni based joint of20μm clearance obtained the highest flexurestrength. There was almost little hard intermetallic compounds formed in the brazed jointrelying on homogeneous diffusion of the active elements and balanced transition of thematrix composition; While the brazed seam region of100μm clearance came into beingobvious brittle reaction layers. The big brazing clearance was hardly filled by the meltingfiller alloy, which resulted in several micro-holes, brittle phase and ununited area formed.Thus the joint was evaluated badly in the aspects of mechanical property. Whilecombination property of the brazed joint with50μm clearance was between that of thejoints with20μm and100μm clearance, whose strength was obviously limited.