The Research On Microstructural Evolution And Diffusing Behaviour During TLP Bonding Using Nickel-base Interlayer

The microstructural evolution and diffusing behaviour during TLP bonding using nickel-base interlayer is studied.Joint region microstructure,the structure evaluation and element diffusion behavior of amorphous filler metal was studied by optical microscope(OM), the X-ray diffraction(XRD),electron microproble analysis(EPMA),scanning electron microcopy(SEM) and energy dispersive spectrometry(EDS),and electron microproble analysis(EPMA).Microstructural studies showed that the process of joining austenitic stainless steel with a nickel-based amorphous interlayer has the characteristics of transient liquid phase diffusion bonding.In this paper,the interlayers was used in the forms of an amorphous foil and crystalline interlayer,joint analysis results show that TLP microstructural are the main by the diffusion layer,base metal dissolution layer,a remain layer of interlayer.The amorphous bonding process is characterized as a four-step process consisting of three stages:the first stage is the production of the liquid behavior;following is isothermal solidification stage;the last stage is composition homogenization stage.During the ramp-up period,the temperature range of 940℃-990℃,we can found that bonded stainless steel with nickel-base filler has the characters of transient liquid phase bonded.When the welding temperature of amorphous and crystalline interlayer below the solid temperature,has the characteristics of solid diffusion. Amorphous interlayer at the temperature of 940℃and the crystalline interlayer at the temperature of 950℃respectively hold 30min,found the diffusion of element,the process is behind for crystalline interlayer.From the formation of welded joints can be seen,the bonded forming process of amorphous interlayer are different from the crystalline interlayer.the amorphous bonding process is characterized as "diffusion brazing",and the crystalline interlayer bonding process is characterized as "capillary brazing".The amorphous bonding process with the increase of the welding temperature,resulting in generate much liquid and the diffusion rate of the atom is faster,holding time determines the element diffusion of the interlayer,Holding time determined that the proliferation of the middle layer the degree of element.To extend the holding time,elements of B in the amorphous interlayer has diffused.The element B in amorphous interlayer has diffused into original substrate.The element Fe in original substrate dissolved at the same time.The welding process accompanied with the diffusion of elements B.This reduce the concentration of B elements in original substrate.B occurred uphill proliferation,the concentration of elements Fe increased,the interaction between interlayer and matrix metal,The diffusion coefficient of B is caculated,whether it is crystalline or amorphous interlayer,The diffusion coefficient of B is increased with the temperature rising up.At the same temperature,The diffusion coefficient of B in the amorphous interlayer is greater than in the crystalline interlayer,theoretically confirmed the results mentioned above on the diffusion of high speed.