Study On Transient Liquid Phase Bonding Of FGH97Superalloy With Different Interlayers And Its Joint Solidification Behavior

Transient liquid phase （TLP） bonding is an advanced joining process that combinesthe benefits of brazing and diffusion joining techniques together and accomplishesconnection with the operation similar to brazing and a joint without defects, which has apotential advantage for the welding of precipitation strengthening nickel-base superalloy.TLP bonding was used to weld FGH97superalloy with different interlayers at1150℃tostudy the effect of holding time on microstructure and properties of joint. Solidificationbehavior was analyzed by microstructure evolution of joint using BNi68CrWB andBNi82CrSiB interlays with increasing holding time.The results revealed that the microstructure of joint will be influenced by chemicalcompositions of interlayer evidently. Due to that Cr, Mo and W are strongboride-forming elements （SBFE） except for Co, the more SBFE in the seam, the severeron phenomena of segregation of Si due to production of steady boride that inhibited thediffusion of boron from melt metal to the parent material and restricted formation ofmicrostructure of nickel-base solid solutions.The formation process of welding seam of FGH97superalloy was the occurrenceof isothermal solidification in the liquid metals distributing continuously in the centralof weld with prolonging the holding time, and finally the compacted γ solid solutionwere completely obtained. Isothermal solidification were controlled by melting pointdepressant and redistribution of alloying elements during solidification, and sufficientdiffusion will achieved when increase the holding time. Enrichment of elementshappened in the frontier of crystallization due to the redistribution of solute when ahigher concentration of alloying elements existed, which prevented completion ofisothermal solidification. The sequence of isothermal solidification of six interlayersfrom easy to difficult is BNi92SiB, A003, A002, BNi82CrSiB, A001, BNi68CrWB, andTLP bonding of FGH97superalloy can be achieved at the parameter of1150℃/6h usingBNi82CrSiB, BNi92SiB, A002and A003interlayer.Dimensions of γ′phase in the weld influenced by diffusion of Al and Ti. Withprolonging the holding time, Al and Ti in the seam diffusing from base metal increasedgradually in amount and γ′phase grew coarser. Two diffusion paths of melting pointdepressant elements were found, i.e. intracrystalline diffusion and grain boundary diffusion. Diffusion distance of boron in grain boundary is larger than that in the grain,and the width of diffusion affected zone containing M₃B₂compound boride in grain andboundaries and enriching in Cr, W, Co and Mo with acicular, granular and vermiculatemorphology increased with extending times. Moreover, the more far from the weld, theless quantity of precipitated phase in diffusion affected zone.The smaller secondary γ′phase increased in amount and grew coarser and theprimary γ′phase tends to be spheroidized after hold at1150℃for6h compared with theas-HIPed FGH97superalloy. However, the microhardness of the parent metal was notbadly influenced by bonding thermal cycle suiting to TLP bonding of FGH97superalloy.