Microstructure And Mechanical Properties Evolution And Corrosion Behavior Of Laser Welds In GH3535 Alloy

GH3535 nickel base alloy is the main structural material of thorium molten salt reactor which is one of the fourth generation advanced reactor.This alloy possesses excellent high temperature properties and corrosion resistance in high temperature and high corrosion environment.Unlike the homogeneous microstructure of the base metal,the welded joints have the characteristics of microsegregation,second phase precipitation,and extremely heterogeneous grains,which make the welded joints usually become the weakest position of the components.The welding method of welding nickel base alloy with high heat input often encounter the problems of solidification crack at fusion zone,liquation crack or reheat crack at heat affected zone.Therefore,laser welding technology with small heat input is a preferred option for welding GH3535 alloy.However,the studies on laser welding GH3535 alloy are scanty.In this paper,the laser welding is used to weld GH3535 alloy.The effect of the key process parameters on the porosity of the fusion zone is studied.The porosities defects in the welding process of the nickel base alloy are solved.Based on the service characteristics of the high temperature and high corrosion for the molten salt reactor,the evolution of microstructure and properties in GH3535 alloy joints were investigated.The corrosion behavior of the joint was studied and the corrosion characteristics of the laser welded joint were revealed.The dynamic change of keyhole of laser welding process and the flow characteristics of molten pool are analyzed by means of high-speed camera technology and characterization methods.The porosity of the fusion zone is affected by the stability of the keyhole and the oscillation of the molten pool.The pulse laser mode can reduce the porosity of the CO because of the agitation of the molten pool;the continuous laser mode makes the porosity rate higher because of the instability of the laser keyhole.The welding joints with minimal porosity were obtained through the optimization of the process,and the characteristics of the welded joints were analyzed.Based on solidification theory and experimental research,There are elements Mo,Si and C segregation of k<1 in the interdendritic region of fusion zone,which makes the eutectic reaction in this region:L?L+??L+?+M₆C??+M₆C.The spheroidal M₆C carbide in the heat affected zone near fusion boundary is affected by high temperature thermal cycle and liquefied to M₆C-?eutectic carbide.The tensile fracture locations of the welded joints are not located in fusion zone and heat affeated zone,which indicate that the formation of eutectic carbides do not damage the short time tensile properties.Based on the characteristics of high temperature operation for molten salt reactor,the microstructure and properties of welded joints under different thermal aging time were analyzed.The nanoscale M₆C carbides were precipitated in the element segregation area in the fusion zone and the heat affected zone.The original M₆C-?eutectic carbides gradually evolved from the bar form to the spherical form due to free energy driving.The tensile strength of the welded joint is higher after aging 100h and the tensile strength tends to be stable after 3000h.The tensile strength of the joint is higher than that of the welded joint after aging 10000h.M₆C of the large size in heat affeated zone becomes the main crack source of the tensile process.The evolution and precipitation of carbides in the fusion zone and the heat affected zone do not damage the tensile properties of the joint.The life of stress rupture for welded joint increases about 56%after thermal aging,and the fracture position is located at the longitudinal grain boundary at the center of the fusion zone.The precipitation of M₆C and local strain homogenization increase the property of stress rupture for the joint after thermal aging.The fracture of the joint is dominated by the large angle longitudinal grain boundary at the center of fusion zone.The influence of carbide on the evolution of fusion zone was analyzed by two kinds of weldments contained different carbon content.After solution treatment,the dendritic microstructure without carbides changes to coarse equiaxed grains.Carbides can impede the migration of grain boundaries and increase the activation energy of grain growth:the activation energy of grain growth of low carbon without M₆C is 106.5kJ mol^-1,and the activation energy of grain growth of high carbon with M₆C is 934.7kJ mol^-1.The existence of carbides is very important for the microstructure stability of fusion zone.Static corrosion test is used to study the influence of the heterogeneity of welded joint on the corrosion behavior.The heterogeneity of grain structure in welded joint does not show significant difference corrosion behavior compare with the base metal under the static corrosion condition.the corrosion of the welded joint is a uniform corrosion mechanism dominated by the diffusion dissolution of Cr.It can be seen from the above conclusion that the laser welded joint of GH3535alloy has excellent weldability,high temperature thermal stability and corrosion resistance of molten salt.This study lays a foundation for laser welding application in the molten salt reactor.