Research On The Key Issues In Selective Electron Beam Melting Additive Manufacturing Of Titanium Alloys

Selective electron beam melting technology is one kind of common metal additive manufacturing technologies,which can form parts with complex shape and structure,especially suitable for the additive manufacturing of high melting point metals such as titanium alloys.However,there are still many issues in the selective electron beam melting process of titanium alloys,such as many forming defects and the gap between forming performance and technical indexes.The main factors are powder preparation process,powder properties and forming process parameters.In view of the above problems,this paper carries out the research on the key issues of titanium alloys formed by selective electron beam melting technology,so as to provide theoretical support and technical guarantee for the engineering application of titanium alloy parts formed by selective electron beam melting additive manufacturing technology.Taking the selective electron beam melting additive manufacturing process of TC4 and TA15 titanium alloy as the research object,this paper focuses on the preparation process of titanium alloy powders by gas atomization method,the powder spreading mechanism during the building process,forming defects,forming microstructure and mechanical properties.The specific research work is as follows:Firstly,the nozzle structure is designed and the jet apex angle is introduced to improve the classical prediction model of powder median particle size.On this basis,the effects of melt superheat,delivery tube diameter,atomization pressure and jet apex angle on the median particle size and fine powder yield are analyzed.The physical and chemical properties of these powders are characterized,so as to obtain high-performance titanium alloy powders.The results show that the median particle size of titanium alloy powders is 74.1 μm and the fine powder yield is 36.5% and that the chemical composition,bulk density,tap density,fluidity,hollow powder rate of these powders meet the requirements of relevant standards when the optimum atomization process parameters are melt superheat 140 ℃,delivery tube diameter 4.5 mm,atomization pressure 4.8 MPa and jet apex angle 11 °.Secondly,based on the electron beam deflection effect,the force model of titanium alloy powders and the calculation model of powder spreading current are established,which are verified by experiments,and the effects of powder preheating and specimen layout on powder spreading are analyzed.The results show that the deflection angle θ is related to the length and width of the base plate and the distance between the starting point of electron beam deflection and the center of the base plate;When increasing the deflection angle θ,the powder spreading current increases and the possibility of powder spreading increases;Increasing the number of sintering necks formed in the powder preheating process and increasing the preheating area of the samples in the building chamber can effectively prevent powder spreading.Thirdly,based on single factor experiment and orthogonal experiment,the effects of hatch spacing,speed factor and focus offset on defects and the relative density of samples during the selective electron beam melting process of TC4 and TA15 titanium alloys are studied respectively,as well as the evolution law of defects with the building height.The results show that the typical defects include spheroidization,thermal crack and pores,and the pore defect is the most obvious;Within the range of test parameters,when the focus offset and speed factor are moderate and the hatch spacing is large,the number of defects decreases and the relative density increases;For TC4 titanium alloy,there is a phenomenon of defect growth and defect healing in the forming process,as for TA15 titanium alloy,the defect morphology is irregular parallel to the build direction and spherical perpendicular to the build direction.Finally,based on the above experimental design,the effects of speed factor,focus offset and hatch spacing on the forming microstructure and mechanical properties of TC4 and TA15 titanium alloys are studied.The results show that the microstructure of TC4 and TA15 titanium alloys formed by selective electron beam melting are basket-weave,lamellar and bimodal microstructure,and these parts with bimodal microstructure have better mechanical properties;The sampling location affects the defect distribution,microstructure and Vickers hardness of the samples.Process parameters mainly affect the thickness of α lath to affect its mechanical properties;Through the optimization of process parameters,the tensile strength of TC4 and TA15 titanium alloy at room temperature is 1000 MPa and 1057 MPa,the yield strength is 870 MPa and 977 MPa,and the elongation is 14% and 13.3%,respectively;Through the above researches,the mapping correlation between powder properties,process parameters,microstructure and properties is finally established,and the performance index of titanium alloys formed by selective electron beam melting is also improved.