Study On Microstructure And Technology Of Lanthanum Oxide Dispersion Strengthened Molybdenum Alloy Electron Beam Welding

As a refractory metal,molybdenum alloy has a melting point of up to 2620?,high temperature strength and good creep resistance.It is widely used in aerospace industry,nuclear power industry,smelting industry,military industry and other fields.With its increasingly wide application,there is more and more demand for its welding.In this paper,La 2 O 3 dispersion strengthened molybdenum alloy is taken as the research object,and the forming,microstructure,welding defects and mechanical properties of its joints are analyzed in detail.Firstly,the weldability of molybdenum alloy is analyzed by direct welding,and the reason of low strength of molybdenum alloy welding joint is explained.Two improved processes are put forward,one is pre Hf intermediate layer welding process and the other is TC4 alloy filling process.The reliable connection of La2 O3 dispersion strengthened molybdenum alloy is realized.First of all,it is necessary to clearly understand all kinds of welding problems and causes of La 2 O3 dispersion strengthened molybdenum alloy electron beam welding,so as to provide guidance for the development of subsequent solutions and direct welding.The weld grain is very coarse,and there are a lot of porosity defects in the weld.The diameter of the big porosity can reach more than 100 ?m.at the same time,there are a lot of crystal crack defects in the upper area of the weld.These welding problems have greatly deteriorated the performance of the joint,the joint has almost no strength,and the fracture form is intergranular fracture.Because of the recrystallization of the HAZ and the coarse grain of the weld,the microhardness of the welded joint is reduced by about 20%.In order to restrain the welding defects,refine the grain and improve the structure,the welding process of pre Hf interlayer was adopted in this paper.Because of the stronger affinity between Hf and impurity o,impurity o will preferentially combine with HF to form Hf O 2 particles with high melting point,thus realizing the purification of grain boundary,eliminating the defects of crystal cracks,and preferentially precipitated Hf O 2 particles can be used as effective heterogeneous nucleation particles.The liquid molybdenum atom can be attached to the particle for nucleation,which greatly increases the number of nucleation.Therefore,the grain in the weld area is refined.The strength of the joint has been greatly improved,reaching 333.44 MPa.The fracture mode has changed from direct welding intergranular fracture to intergranular and cleavage mixed fracture,but the hardness of the weld has greatly increased,which makes the weld embrittlement.Although the introduction of Hf element improves the strength of the joint to a large extent,but the improvement is limited,there are still a certain amount of porosity defects in the weld,which has a negative impact on the strength of the joint.In order to avoid the problem of Mo alloy welding,another improved process is put forward,which is to fill the weld with TC4 alloy with good weldability and replace the weld with Mo alloy with poor weldability.The microstructure in the middle part of the weld is mainly storage martensite,and the microstructure on both sides is mainly ? phase.The porosity defects of the joint are significantly reduced,only a small amount of small porosity defects exist near the fusion line,and the tensile strength of the joint is further improved,reaching 385.30 mpa.The tensile fracture occurs in the fusion line,which is intergranular and cleavage mixed fracture.