Study On Process And Mechanism Of Transient Liquid Phase Bonding And Post-weld Heat Treatment Of Inconel 718 Superalloy

Inconel 718 superalloy has excellent combination of corrosion resistance,oxidation resistance,and good tensile and creep properties.It is an important material for aerospace engines.In order to meet the demand of actual project subject of bonding engine case,transient liquid phase(TLP)bonding of Inconel 718 superalloy that formed at 1150°C for different bonding time in this paper.Respectively using BNi-2 interlayer with 50 ?m thickness and BNi-1a interlayer with 50 ?m thickness.The microstructure of joints was analyzed by means of optical microscope(OM),scanning electron microscope(SEM),energy dispersive x-ray detector(EDS),and the mechanical properties of joints was tested in Vickers hardness tester and tensile test machine.The effect of interlayer composition and TLP bonding parameters on microstructure and mechanical properties was investigated.Post-weld heat treatment was carried out on the joints which was using BNi-2 interlayer,and the effect of post-weld heat treatment parameters on microstructure and mechanical properties was investigated.When TLP bonding Inconel 718 with the BNi-1a interlayer,the bonding temperatures were chosen as 1150°C.The holding times were chosen as 1 min,10 min,20 min,30 min,60 min,120 min and 240 min.When the holding time was less than 240 min,the joints had not completed isothermal solidification.The joint interface structure was divided into four distinct zone,which were athermally solidified zone(ASZ),isothermally solidified zone(ISZ),diffusion affected zone(DAZ)and base metal(BM).The ASZ consists of a large number of intermetallic compounds,which was composed of Cr-rich boride,Ni-rich silicide and Ni-Nb-rich boride.The ISZ was composed of nickel-based solid solution.DAZ was composed of different borides.When the holding time was 240 min,the joints had completed isothermal solidification.All intermetallic compounds dissolved,and the ASZ disappeared.The joint interface structure was composed of ISZ,DAZ and BM.The mechanical properties of joints showed that the shear strength of joints increased with the prolongation of holding time.The biggest shear strength of 367.5 MPa was obtained at 240 min.When TLP bonding Inconel 718 with the BNi-2 interlayer,the bonding temperatures were chosen as 1150°C.The holding times were chosen as 1 min,10 min,20 min,30 min and 60 min.When the holding time was 240 min,the joints had completed isothermal solidification.The mechanical properties of joints showed that the shear strength of joints increased with the extension of holding time before that and declined after that.The biggest shear strength of 389.4 MPa was obtained at 20 min.BNi-1a solder contains more Cr elements than BNi-2 solder.The higher content of Cr in the filler metal can increase the melting point of the filler metal and inhibit the diffusion of Si and B from the bonding zone into the base metals.So,under the same parameters of TLP,it would took a longer time to complete isothermal solidification when TLP bonding Inconel 718 with the BNi-1a interlayer.When the interlayer and holding time were defined.With increasing the bonding time,the fewer compounds precipitated were found on DAZ,and the size of compounds precipitated in DAZ decreased,and the distribution of compounds precipitated in DAZ more uniform.Weld width,DAZ size,ISA size and average grain size increased with the extension of bonding time.With increasing the bonding time,the average hardness of ISZ and BM decreased,and the maximum hardness of DAZ decreased.Post-weld heat treatment was carried out on the joints which was using BNi-2 as the interlayer in order to improve the joining quality.After solution treatment,the ASZ was disappeared,and Ni-Si-Nb ternary precipitates were observed at ISZ,and with increasing bonding time,the amount of compounds increased.After solution treatment and aging treatment,the Ni-Si-Nb ternary precipitates were observed at ISZ,and the shape and distribution was similar to solution treatment.But the content of Nb in the compound was reduced.A fine ?'' phase was observed at base material and the weld zone after aging treatment.The effect of solution treatment on the shear strength were related to the original microstructure of the joints.The joints with short bonding time had ASZ.After solution treatment,the ASZ would disappeared,and the less Ni-Si-Nb ternary precipitates was observed in ISZ,so the shear strength would improve.The joints with long bonding time,the more Ni-Si-Nb ternary precipitates was observed in ISZ,and the average grain of ISZ increased,so the shear strength decreased.The shear strength has improved obviously after solution treatment and aging treatment.After solution treatment and aging treatment of the TLP joints for 1 min,shear strength increased from 202.7 MPa to 693.3 MPa.The hardness of base metal and weld zone decreased by solution treatment,while the hardness of base metal and weld zone increased by solution treatment and aging treatment.