| Electron beam selective melting(EBSM)uses the electron beam as a heat source to melt powder through the selection to form the required parts layer by layer,and can freely form a variety of metal materials.Compared with other heat sources such as laser,electron beam has the advantages of high power density,high energy utilization rate and fast scanning speed,and can obtain higher forming efficiency.It has important application prospects in many fields such as aviation,aerospace,ships,electric power and medical implants.At the same time,in order to realize the continuous high quality processing of mesoscopic scale parts,EBSM has higher requirements for high brightness electron beam spot compared with electron beam welding,electron beam melting and other technologies.Aiming at the requirement of higher quality and life of electron gun by electron beam selective melting technology,to solve the problem that it is difficult to match suitable filament materials for parts processing at the present stage,quasi-macroscopic carbon fiber(QMCFs)prepared by our team in previous experiments is used as the cathode of the filament,and the field emission mode of applying high voltage between electrodes replaces the heating electron emission mode of traditional filament.Based on EBSM forming technology and direct heat electron gun,an electron beam additive manufacturing electron gun system for EBSM forming process was developed.The main research work and achievements are as follows:(1)In this paper,under the action of iron catalyst,chemical vapor deposition(CVD)method was used to prepare QMCFs at 1000℃ with methane as carbon source,and realized the macroscopic preparation of carbon fiber.(2)Based on the finite element method,CST software was used to model the filament structure of the electron gun beam source.With the help of the electron beam imaging principle,the optimized structure of the electron gun was obtained through the simulation results of the current distribution near the electron beam source structure and the electron motion trajectory.(3)Based on the finite element method,COMSOL software was used to model the thermodynamic effect of electron beam interaction with materials,and the heat transport process of materials under the action of electron beam was analyzed.The distribution law of molten pool temperature field under different scanning power,scanning speed and electron beam spot diameter was obtained,and a method to predict the working parameters of additive electron gun of different metal powder materials was established.Through the above method,the optimization results of electron gun structure show that the QMCFs filament cathode with the diameter of emission surface of 20μm,the diameter of electron microbeam of 30μm,the acceleration voltage of 60 k V can achieve the design requirements of 1m A electron beam,and the heating power of 20 W.When the electro-optical system is used for the electron microbeam additive manufacturing of Ti-6Al-4V titanium alloy,the melting zone temperature can reach about 2500 K at the scanning speed of 0.3m/s,and the scanning speed is no higher than 1.3m/s.The electron beam scanning speed can reach 10m/s for the metal powder whose working temperature is near 606 K.This paper provides theoretical and technical guidance for forming refractory metal and alloy parts under microbeam conditions,and verifies the feasibility of forming meso-scale parts by QMCFs cathode electron gun optical system in additive manufacturing.The above work is of reference value in the design of electron gun,electron optical system and the formulation of electron beam motion control scheme for additive manufacturing of QMCFs filament. |
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