Electron Beam Selective Melting Of Nb521 Alloy And Its Microstructure And Mechanical Properties

As a new generation of high temperature refractory Nb alloy,Nb521 alloy has excellent high temperature mechanical properties,high melting point,low density and good plasticity,which occupy an important position in the aerospace field.However,with the increasing demand for complex components of high-temperature refractory materials,and the high melting point of Nb521 alloy and high temperature oxidation,its preparation has become a major problem restricting development.The electron beam selective melting additive manufacturing technology(EBSM)solves the above problems by possessing the characteristics of powdered materials and high-energy electron beam and vacuum-free environment.At present,there are few related studies on EBSM of Nb521 alloy.There are still many scientific and technical problems to be studied.Therefore,the Nb521 alloy was prepared by EBSM method and further research was carried out.In this thesis,the microscopic morphology,microstructure and hardness of Nb521 alloy powder required for EBSM process were systematically studied.The temperature field of different size powder preparation process was simulated by COMSOL Multiphysics finite element simulation software to clarify the cooling rate and solidification structure change law.The ABAQUS CAE finite element simulation software was used to simulate the thermal history of Nb521 alloy EBSM process.The morphology,microstructure and mechanical properties of Nb521 printed parts were studied.The simulation results were used to illustrate the forming mechanism of the printing process.Nb521 alloy powder was prepared by plasma atomization milling equipment.The morphology,microstructure and hardness of the powder were investigated by X-ray diffraction(XRD),metallographic microscope(OM),scanning electron microscopy(SEM)and nanoindentation.The results show that the Nb521 alloy powder has a high sphericity,its particle size distribution conforms to the standard normal distribution,the median diameter is 75?m,and the overall scale is on the micrometer scale.From the XRD results,only the Nb diffraction peak was present in the Nb521 alloy powder.By comparing the cooling rate calculation with the simulation calculation results,the average cooling rate of the aerosolized Nb521 alloy powder is 105~106K/s.The results of SEM and nanoindentation show that the smaller the particle size of Nb521 alloy powder,the faster the cooling process in the formation process,the finer the structure,the higher the hardness,and the solidification structure will become a completely planar crystal when it reaches a certain level.The ABAQUS CAE finite element simulation method was used to study the thermal history of different layers of the printing process.It was found that the temperature of the first layer was the highest and the molten pool was the largest due to the preheating of the substrate.Each print layer except the outermost layer is affected by thermal cycling,revealing the heat transfer principle of the printing process.Regardless of the thermal cycling effect,the first layer has the fastest cooling rate and the fastest surface cooling rate in the overall printing process.The Nb521 alloy molded parts were prepared by EBSM process,and combined with the simulation results,the microstructure,precipitation phase,morphology and mechanical properties(including tensile strength and nanohardness)of each layer(S1~S6)were systematically studied.In the Nb521 alloy print structure,strips and punctiform Nb2 C precipitates and punctiform(Nb,Zr)C precipitates exist,and the(Nb,Zr)C phase evolves from the Nb2 C phase during cooling.The grain size of the S1~S6 layer is firstly increased and then decreased by the heat of the printing process.The results of nanoindentation and tensile test showed a trend of decreasing first and then increasing along the printing direction S1~S6 layer,which were located at 4-4.3GPa and 500-550 MPa,respectively,which were higher than the general as-cast level.