Research On Brazing Parameters And Bonding Mechanism In The Graphite/hastelloy N Alloy Joints With Au Based Filler Metal

Molten salt reactor(MSR)is one of candidate models belonging to the fourth generation nuclear reactors and an important part of ?future ten-year advanced priority research program? from the government of China.The reactor core is composed of graphite,Hastelloy N alloy and molten salt.Research of brazing process and bonding mechanism in joints of graphite and Hastelloy N alloy will promote the development of molten salt reactor.In this research,Au based filler metal was used to braze graphite and Hastelloy N alloy,microstructures and mechanical properties with the change of brazing parameters and Si content were studied,formation process and bonding mechanism was obtained.When Au was used as filler metal,the microstructure of joints was as follows: graphite/Zone 1/Zone 2/Zone 3/Hastelloy N alloy.Zone 1 was mainly composed of Au(s.s)and Ni(s.s),in which some larger Mo2 C are embedded.Zone 2 was constituted by Au(s.s)and Ni(s.s).Many Mo-C carbides are precipitated in Zone 3 with lath-shaped Mo2 C in the grain interiors and continuous MoC at the grain boundaries.In Hastelloy N alloy,there are still continuous MoC at the grain boundaries but no Mo2 C can be observed.Zone 1 and Zone 2 was formed in liquid state,while Zone 3 was formed in solid state.The width of zones in liquid state was almost unchanged because the amount of Au was certain.The width of zones in solid state increased with raising brazing temperature or delaying holding time.The maximal average shear strength(34.1 MPa)was obtained at 1060? for 90 min,which is equal to graphite's.When Au+Si filler metal was used,the microstructure of joints was as follows: graphite/Zone 1/Zone 2/Hastelloy N alloy.Zone 1 was mainly composed of Au(s.s)and Ni(s.s),in which some Mo6Ni6 C are formed.The composition of Zone 2 and nearby Hastelloy N alloy was the same as Zone 2 and Zone 3 of joints using Au filler metal.Zone 1 was formed in liquid state,and Zone 2 was formed in solid state.The width of Zone 1 increased with raised Si content,and the width of Zone2 almost stayed unchanged.When 5at.% was doped into filler metal,the maximal average shear strength(32.5 MPa)was obtained at 1060? for 10 min.The bonding mechanism in graphite/Au/Hastelloy N alloy joints was as follows: Firstly,Au and Ni matrix of Hastelloy graphite N alloy turned to liquid state through mutual dissolution during brazing,and then the graphite was dissolved by molten filler metal.Excess Mo and C made the formation of Mo2 C in Zone 1.In addition,C spreading from molten filler metal promoted the generation of Mo-C carbides(Mo2C in the grain interiors,MoC at the grain boundaries)in Zone 2.Furthermore,there were some MoC at the grain boundaries in nearby Hastelloy N alloy.The bonding mechanism in graphite/ Au+Si/ Hastelloy N alloy joints was the same as ones of joints using Au filler metal except that product in Zone 1 was Mo6Ni6 C rather than Mo2 C.Owning to Mo-C carbides in filler metal and Hastelloy N alloy,a natural and hierarchical structure was formed to relieving the thermal stress and guaranteeing excellent bonding in joints.