Study On Laser Welding Process And Joint Microstructure And Properties Of UNS N10003 Hastelloy

As the fourth generation advanced nuclear energy system,thorium based molten salt reactor nuclear energy system has the characteristics of high inherent safety,less nuclear waste,better non-proliferation performance,and economy.The key components of the molten salt reactor must work stably and effectively under multiple extreme environments such as high temperature,molten salt corrosion,and neutron irradiation.UNS N10003 Hastelloy has good corrosion resistance,high temperature mechanical properties,and radiation resistance in a 700℃fluoride environment,so it is mainly used in molten salt reactor nuclear power systems.When using the traditional fusion welding method to weld Hastelloy,due to the high heat input and the liquefaction of base metal carbides,it is easy to form a liquefaction film on the grain boundary and appear liquefaction cracks.In addition,the liquid viscosity of Hastelloy is high,and defects such as pores are easy to appear during welding.Laser welding is especially suitable for welding Hastelloy because of its concentrated energy and low heat input.In this paper,UNS N10003 Hastelloy was welded by two methods:laser self fusion welding and laser filler wire welding,and well-formed welded joints were obtained.The weld is screw-shaped when the welding heat input is low,and dumbbell-shaped when the heat input is high.Compared with filler wire welding,self fusion welding is easier to generate pores at the weld,and the occurrence of pores can be reduced by appropriately increasing the power.There are more Cr and Mo elements in the weld bead grain boundary than in the crystal,while the contents of Ni and Fe elements are lower.Under appropriate process parameters,the tensile strength of self fusion welding is 787.4MPa,that of wire filler welding is 801.2MPa,and that of base metal is 816.4MPa.The three strengths are close and the welding effect is good.The tensile fracture of wire filler welding and self fusion welding occurs at the junction of heat affected zone and base metal,and the fracture mode is a plastic brittle mixed fracture.The microhardness of both self fusion welding and filler wire welding weld bead are similar and higher than that of base metal.The grain size at the fusion line is fine,and the hardness is higher than that of the weld bead and base metal.The polarization curve test shows that the electrochemical corrosion resistance of filler wire welding weld bead,self fusion welding weld bead,and base metal are similar.After molten salt corrosion,many dense holes appear on the surface of the sample,and the corrosion mode is uniform corrosion.After the same corrosion time,the corrosion depth of self fusion welding,filler welding and base metal is similar.With the increase of corrosion time,the corrosion depth of weld bead and base metal also increases,but the growth rate became slower.After corrosion,the content of Cr and Mo on the sample surface decreases,while the content of Ni and Fe increases.The corrosion mechanism is mainly the loss of Cr and Mo.Ni and Fe elements exist on the sample surface in the form of Ni₃Fe and pure Ni phase,which can slow down the corrosion rate.After holding at 700℃for a period of time,there is no new phase in the base metal,and have good thermal stability.The double ellipsoid and elliptic cylinder compound heat source model is used to simulate the screw-shaped weld bead,and the positive cone and inverted cone compound heat source model is used to simulate the dumbbell-shaped weld bead.The simulated temperature fields are similar to the actual welding results.Because the energy distribution of the conical heat source model is more concentrated and the laser power of the dumbbell-shaped weld bead is higher,the maximum temperature of the dumbbell-shaped weld bead after the simulation is higher than that of the screw-shaped weld bead.However,the heating area of the double ellipsoid and elliptic cylinder compound heat source is larger,so the cooling speed is slower after reaching the highest temperature.During the structural analysis,because the two edges of the sample in the X direction are restrained and the temperature gradient in the X direction is high,the stress value in the X direction is high,but the deformation is low;The Y direction can release the stress through deformation,so the stress value is low and the deformation is high.The X direction of the sample produces inward deformation from both sides,while the Y direction deforms from the inside to the outside of the sample.The total deformation of the sample is similar to the deformation in Y direction,which is mainly caused by the deformation in Y direction.