Study On Microstructure And Properties Of In-situ TiB Whiskers Reinforced TNM/K417G Alloy Brazed Joints

TiAl alloys have always been regarded as the most promising lightweight high-temperature structural materials to replace Ni-based superalloys and heat resistant steel due to their high specific strength,excellent oxidation resistance,low density,and good high-temperature mechanical properties.The density of TiAl alloys is only half of Ni-based superalloys.Replacing part of Ni-based superalloys with TiAl alloys in aircraft engines can significantly reduce the weight of aircraft,thereby reducing fuel consumption.Brazing was considered to be a more effective method to realize the connection between TiAl alloys and Ni-based superalloys.This experiment innovatively proposed in-situ TiB whiskers to strengthen the brazing joint between TNM alloy and K417G alloy,and developed a new type of high melting point Ti-Ni based composite brazing filler metal.The brazing process characteristics of the new composite brazing filler metal and the effects of different brazing process parameters on the microstructure and mechanical properties of the brazed joints between TiAl alloy and Ni-based superalloy were studied in detail,and the microstructure of the brazed joints was clarified.Formation mechanism and mechanism analysis of in-situ TiB whisker-enhanced shear strength of joints.The（Ti-66Ni）_1-x（TiB₂）_x（x=7.47,11.89,16.86 wt.%）ingot brazing filler metal was melted by vacuum suspension,and the ideal volume fraction corresponding to in-situ TiB whiskers is 20 vol.%,30 vol.%and 40 vol.%.The microstructure of the brazing alloy was detected and analyzed through SEM,EDS,XRD,and it was determined that30 vol.%TiB whiskers were the filler alloy for this brazing experiment.The melting characteristics and wetting and spreading properties of the filler alloy were further analyzed,and the filler alloy should have a narrow melting temperature range and reduce the difference between the wetting and spreading area of TNM alloy and K417G alloy.Finally,this brazing experiment was confirmed.The brazing temperature range and heating route.The microstructure of TNM/30 vol.%TiB/K417G brazed joint is divided into four regions,andthephasesofeachregionare TNM/TiAl+TiAl₃+Al₂Ni Ti/TiAl₃+Al₂Ni Ti/Al₂Ni Ti+TiB₂+TiB/Al₂Ni Ti+Ti Ni₃+Ni-rich（Ni,Al,Ti）ss/K417G.The microstructure and zoning of the brazed seam did not change with the change of brazing temperature,and it was proved by the same brazing of TNM/30 vol.%TiB/K417G brazed joint of Al,Ti atoms dominate the formation.And the layer?region is formed first at the TNM alloy/30 vol.%TiB interface.The brittle compound Al₂Ni Ti phase throughout the four areas becomes the matrix of the brazed joint,but due to the significant inhomogeneity of the structure on both sides of the brazed joint area,the formation of more intermetallic compound phases and the thermal stress during the cooling process of brazing is easy to cause Al₂Ni Ti coexists with microcracks and pores.Referring to relevant literature,this experiment further proposes the optimization of TNM/30 vol.%TiB/K417G brazed joints by adding TiB₂ particles.The same brazing process parameters were carried out for the two brazing methods,and the addition of TiB₂ particles in the TNM alloy and 30 vol.%TiB brazing alloy effectively inhibited the formation of the brittle phase Al₂Ni Ti in the layer II zone.Compared with only 30 vol.%TiB brazing alloy,the ratio of brittle phase layer II to the total thickness of the brazing joint in the brazed joint obtained by adding TiB₂ particles at 1170℃for 5 min decreased by 32.7%,and the in-situ self-generated the ratio of the TiB whisker reinforcement layer III to the total thickness of the brazing seam increased by 82%,and the total thickness of the brazing seam decreased by 13.8%.It was found that the thickness of the brazing seam increased from 370μm to 535μm in the brazing temperature range from 1170℃to 1180℃,and it was concluded that the TiB₂ particle inhibition effect began to fail when the brazing temperature was above 1170℃.Finally,the typical brittle fracture of the TNM/30 vol.%TiB/K417G brazed joint is analyzed,and the brittle phases Al₂Ni Ti and TiAl₃ are the main components of the fracture surface.Because the brittle phases Al₂Ni Ti and TiAl₃ in layer II are not only different in hardness,freezing point and thermal expansion coefficient,they are prone to gaps and microcracks during subsequent solidification.And the maximum shear strength of 129 MPa was obtained at 1160℃for 5 min,and the shear strength of the joint obtained by adding TiB₂ particles increased by 51.9%compared with that.In addition,the shear strength of the joint obtained by adding TiB₂ particles reaches the maximum value of 214 MPa at 1170℃for 5 min,which is 106%higher than that obtained with only 30 vol.%TiB brazing filler metal.This is not only due to the fact that the stress load needs to be absorbed for crack propagation in the layer II region,but also the fact that the fracture path takes place in the layer III region to cause TiB whisker fracture and debonding requires more energy to be absorbed.Both brazing methods obtained the maximum shear strength when the brazing holding time was 5 min.