Bonding Mechanism And High Temperature Stability Of Hastelloy N/Ti/Hastelloy N Alloy Brazed Joint

Molten Salt Reactor is one of six candidate models belonging to the fourth generation nuclear reactor and a significant part of science and technology programs from Chinese Academy of Sciences called future advanced nuclear fission energy.Hastelloy N alloy has excellent resistance to corrosion in fluoride salt.As a result,Hastelloy N alloy was selected as the molten salt reactor's molten salt return piping,core vessels,heat exchangers and other structural materials.The study of the connection of Hastelloy N alloy has a very important role in the development of molten salt reactor.In this paper,we use pure Ti filler to braze the Hastelloy N alloy.The microstructure and mechanical properties of joints with the processing parameters and filler thickness was studied by means of SEM,EDS and TEM.The relationship of between microstructure and mechanical properties was discussed.The connection mechanism of Hastelloy N alloy with pure Ti filler was revealed.Through the long time high-temperature heat treatment of joints,the high-temperature-stability of joints was studied.Excellent performance of the Hastelloy N alloy joint using pure Ti filler was obtained.The shear strength of joints with 0.127 mm Ti filler at 1060 ? for 90 min is the best.The microstructure of joints can be divided into Zone 1 and Zone 2.Zone 1 was in the middle of joints and Zone 2 was near the base.The matrix of Zone 1 was NiTi,block Ni3 Ti and a large number of MoC particles distributed in NiTi matrix.In addition,Ti2C0.06 particles and NiTi2 block was distributed in the middle of joints region.Zone 2 was constituted of Ni3 Ti matrix with interspersed TiC particles and parallel Mo6Ni6 C in joints interface.Zone 1 and Zone 2 was separated by ribbon Mo2 C.The results showed that the shear strength of joints rose with the improvement of brazing temperature,in a certain range of brazing temperature.The diffusivity base metal element was enhanced,with the improvement of brazing temperature.A large number of Ni element diffused into the filler.The melting point of filler layer was increased.The ability of filler filling to the gap was decline.The response was inadequate.The best brazing temperature was 1060 ?.In order to ensure that the reaction of the filler with the base metal was sufficient,appropriate increase holding time was beneficial to improve the performance of joints in the test range.With 90 min holding time,the shear strength of joints was up to 350 MPa.In the scope of this test,the filler alloy seam was thickened with the increase of Ti foil thickness.And the strength of joints reached a maximum of 350 MPa with 0.127 mm thickness Ti foil.The connection mechanism of Hastelloy N alloy joints using Ti filler was as follows: during heating process,Ti foil first contacted with Hastelloy N parent metal alloy and reacted into liquid.Then the liquid dissolved Ni,Ti,Mo,Cr,C and other elements of the filler and base metal.There was reaction each element in the liquid phase.During cooling process,each phase separated out in turn,and the Hastelloy N/Ti/Hastelloy N joints was formation.Study on high-temperature-stability performance of joints showed that when joints after long time high temperature heat treatment,the performance of joints was first increase and then decrease.High temperature heat treatment for a long time made more base metal elements spread to the filler layer,reacted with filler,joint width increased.Heat treatment at low temperature was equivalent of annealing treatment on joints.The shear strength of joints was increased.Higher heat treatment temperature was unfavourable to the shear strength of joints.At 600 ? for 240 hours,the shear strength of joints was 396 MPa at room temperature.At 900 ?for 240 hours,the shear strength of joints remained more than 200MPa(213MPa)at room temperature.It showed that the joint had excellent and stable high temperature performance and met the requirements of actual use.In addition,SiC ceramic and Hastelloy N alloy was connected with Ag-Cu-Ti filler alloy.Shear strength of joints was 21.9MPa at 900 ? for 20 min.