Composition Design Of Nickel-based Filler Metals And Brazing Technical Experiments Of Nickel-based Superalloy

With the development of aerospace technology,the flight distance of aircraft and the thrust weight ratio of aeroengine increased,resulting in the high demands for structural materials.Nickel-based superalloy is widely used in aerospace industry owing to its remarkable high temperature strength,machining property,hot corrosion and oxidation resistance.Welding process is inevitable and extremely important in the practical application nickel-based superalloy.Brazing as a convenient and efficient welding method is widely utilized for brazing nickel-based superalloy.Nickel-based filler metals possess the advantages of high serving temperature,superior corrosion resistance,excellent mechanical properties and low cost,hence they are the most widespread filler metal for brazing nickel-based superalloy.Nickel-based filler metals could be fabricated into amorphous foils by rapid quenching method.Compared with the crystalline filler metal(powder and paste form),amorphous filler foil is featured with thinner thickness,more uniform structure,and higher diffusion efficiency,thus it is more suitable for brazing nickel-based superalloys.In this paper,four amorphous Ni-Cr-Si-B filler metals with the cluster formula of [Ni-Ni12]B2Cr + [B-Ni8Cr]BSi Cr and [B-Ni9-x Crx]BSi Ni(x=0,1,2)were designed based on based on cluster-plus-glue-atom model,and then fabricated into filler foils with the width of ~5 mm and thickness of ~50 μm by rapid quenching technology.GH4169 superalloy was brazed with [Cr-Ni12]B2Cr +[B-Ni7Cr2]BSi Ni filler foils,and K4169 superalloy was brazed with [B-Ni9-x Crx]BSi Ni(x=0,1,2)filler foils.The effect of brazing temperature and brazing time on microstructure and shear strength of GH4169 alloy joints was investigated.The effect of Cr content on the microstructure and glass forming ability of filler metals was discussed,and the microstructure,shear strength and fracture morphology of K4169 superalloy brazed joints were studied.The typical microstructure of joints included brazed seam and diffusion zone.The GH4169 superalloy brazed seam was mainly composed of γ-Ni solid solution.MC carbides(Nb C,Ti C)and(Nb,Mo,Cr)-rich borides distributed in diffusion zones.B content in substrate decreased with the increase of diffusion distance and brazing time,which led to the(Nb,Mo)-rich borides preferentially formed because Nb and Mo had a stronger borides forming ability than Cr.Coarse precipitations in the joint brazed at 1240 °C consisted of borides,Laves phase and δ phase.The shear strength of joints was principally dominated by the brittle precipitations in diffusion zone.While increasing the brazing temperature,the shear strength of the joint first decreased from 602 MPa at 1150 °C to 500 MPa at 1180 °C,extensive needle-like borides in DZ degraded the shear strength of joint.Then the shear strength inversely increased to 624 MPa at 1210 °C due to the partial reduction of brittle precipitations in DZ.The highest shear strength of 820 MPa was obtained at 1240 °C,highly homogenized structure of joint improved shear strength more efficiently.With increasing brazed time from 10 min to 40 min,the shear strength of joints significantly improved from 566 MPa to 624 MPa due to the elimination of brittle borides.Homogeneous structure exhibited in [B-Ni9]BSi Ni and [B-Ni8Cr1]BSi Ni BSi Ni filler metals,while Cr B phase formed in [B-Ni7Cr2]BSi Ni filler metal.The glass forming ability weakened with the increase of Cr content in filler metals,which was consistent with the results predicted by 24-electron rule.The K4169 superaloy brazed seam contained γ-Ni solid solution,Nb6Ni16Si7 and Cr B phases in the joint brazed with [B-Ni7Cr2]BSi Ni filler metal.More Nb6Ni16Si7 phase formed in brazed seam with the increase of Cr content in filler metal,because Cr promoted the diffusion of Nb atoms from base metal to brazed seam.Diffusion zone was composed of needle-like Cr-rich borides and blocky MC carbides(Nb C,Ti C).The amount of Cr-rich borides increased with the increase of Cr content in filler metal.The shear strength of the joint brazed with [B-Ni9]BSi Ni filler metal reached 689 MPa,then decreased to 562 MPa with [B-Ni8Cr1]BSi Ni filler metal,because more brittle Cr-rich borides formed in diffusion zone.The shear strength of the joint brazed with [B-Ni7Cr2]BSi Ni filler metal was only 450 MPa,since continuous precipitations(Nb6Ni16Si7 and Cr B)in brazed seam deteriorated the joint properties severely.All the joints fractured in the diffusion zone or brazed seam,and the fracture surfaces presented quasi-cleavage and brittle features.The existence of borides and Laves phase in the diffusion zone and continuous precipitation(Nb6Ni16Si7 and Cr B)in brazed seam exhibited low resistance to crack,which provided the potential origin for crack growth.