| Silicon nitride(Si3N4)ceramic exhibits an excellent combination of mechanical properties,such as excellent oxidation resistance,good corrosion resistance,low density,high hardness and good wear resistance.Many of the potential engineering applications of silicon nitride ceramic require it to be functionally or structurally joined to other metallic components to take advantage of the respective superiorities of ceramics and metals.However,the difference in the physical properties of ceramics and metals,especially the large differences in the thermal expansion coefficients(CTE)will result in high residual stresses during the cooling process.In the present study,an Ag-Cu-Ti brazing alloy reinforced with Ti Np was evaluated for the purpose of obtaining high strength of Si3N4 ceramic and 42 Cr Mo steel(Invar alloy)joints.This work was financially supported by the National Natural Science Foundation of China under Grant nos.50975064,51321061 and 51372049.The stress distribution and interfacial microstructure were comprehensively investigated,and the brazing parameter and filler content were explored.The microstructure evolution mechanism and strengthening mechanism were comprehensively discussed.The wetting behavior of composite filler with different Ti Np contents on the Si3N4 ceramic indicated that the spreading of composite filler experienced three different stages: slow contact spreading,rapid change and slow change of the contact angle.Ti-N+Ti-Si were formed at the interface of filler and ceramic and extended to the front of the triple line.So the wetting of the composite filler on the Si3N4 ceramic was typical reactive wetting.The spreading ability of the composite filler was reduced with the increase of the Ti Np content.The wetting behavior of Ag-Cu,Ag-Cu-Ti and Ag-Cu-Ti+Ti Np composite filler on the 42 CrMo steel and Invar alloy was also investigated.For the nonreactive Ag-Cu/42 Cr Mo or Ag-Cu/Invar system,the solid/liquid interfacial tension was the critical factor to determine the wetting behavior.However,the addition of Ti would change the wetting mechanism to the reactive wetting or the dissolve driving wetting.With the addition of Ti and Ti Np in the Ag-Cu filler,the wettability of liquid filler on the metal substrates became worse.The typical microstructure of Si3N4/42 Cr Mo brazed with Ag-Cu-Ti+Ti Np was Si3N4/Ti N+Ti5Si3/Ag(s.s)+Cu(s.s)+ Ti Np+Cu-Ti intermetallics/Ti C/42 Cr Mo,while that of the Si3N4/Invar system altered to Si3N4/ Ti N+Ti5Si3+Fe2Ti/Ag(s.s)+Cu(s.s)+Ti Np/Fe2Ti+Ni3Ti/Ag-Cu eutectic/Invar.The formation of Fe2 Ti and Ni3 Ti compounds was greatly inhibited by using Cu as interlayer in the Si3N4/Invar brazing system.To investigate the stress distribution in the Si3N4/42 Cr Mo and Si3N4/Invar brazing system,a simplified unit cell model was designed firstly to obtain the physical properties(elastic modulus,CTE,yield stress)of the Ti Np reinforced filler.We also introduced randomly distributed reinforcements into the Si3N4/42 Cr Mo submodel to investigate the effect of the Ti Np volume fraction and particle size on the stress distribution.For the Si3N4/42 Cr Mo system,the results showed that the maximum residual stresses occurred in the vicinity of the substrates/filler alloy interfaces and the largest magnitude appeared in the vicinity of the edge at a short distance from the interface on the bottom side of ceramic.The residual stress in the Si3N4/Invar system also occurred adjacent to the filler alloy part and the largest magnitude appeared at the ceramic/filler interface.The simulated results of the Si3N4/42 Cr Mo and Si3N4/Invar systems indicated that the residual stress would be reduced and joint strength increased by increasing the Ti Np content,thickness of Cu interlayer,thickness of brazing layer,particle size and decreasing the interfacial reaction layer thickness.We varied the content of Ti Np and Ti in the Ag-Cu-Ti+Ti Np composite filler to braze the Si3N4 ceramic to 42 Cr Mo steel.The results showed that the thickness of the interfacial reaction layer was increased with the lower Ti Np content or higher Ti content,the amount of Cu-Ti intermetallics in the brazing layer was also influenced by the variation of Ti Np and Ti content.The highest four-point bending strength of 396.02 MPa was obtained when brazed at 900℃ for 5min with Ag-Cu-Ti4(wt.%)+ 8vol.%Ti Np composite filler.The joint strength of Si3N4/42 Cr Mo showed first increasing and then decreasing changing trend by increasing the particle size.When brazed the Si3N4 ceramic to Invar alloy with Ag-Cu-Ti+Ti Np composite filler,the thickness of the interfacial reaction layer at ceramic side and the number of the wavelike Fe2Ti+Ni3Ti intermetallics in the filler increased by improving the brazing temperature and extending the holding time.However,when increasing the Ti Np content,the thickness of the interfacial reaction layer at ceramic side reduced gradually and the wavelike intermetallic would be pushed to the Invar alloy side gradually.When brazed at 800℃ for 60 min,the shear strength of joint brazed with 2vol.% Ti Np composite filler was the highest,228.97 MPa.As that the chemical potential of titanium in the filler is much higher than that in the Ti N,once some Ti N particles were added into molten Ag-Cu-Ti alloy,Ti would diffuse towards and enriched in the vicinity of the Ti Np.However,the equilibrium with the titanium nitride was achieved for very low titanium concentration in the molten alloy.On the other hand,the Ti Np additions could react with Ti to form Ti2 N when the Ti content in the filler was high enough.The analysis of the strengthening mechanism indicated that the CTE reduction and the decrease of the thickness of the reaction layers at the substrates’ side when increasing the Ti Np content were beneficial to the joint.However,the plastic deformability of the brazing seam,the precipitation of Cu-Ti intermetallics and the appearance of defects such as pores would finally decrease the joint strength.The above parameters would influence final joint strength synthetically and a r eliable joints can be achieved by considering all the factors above. |
PDF Full Text Request