Microstructure Evolution And Properties Of TA15 Alloy Fabricated By Electron Beam Freeform Fabrication

As a typical nearly?-type titanium alloy,TA15 titanium alloy is widely used in aerospace and other fields with good thermal strength,weldability and process plasticity.The traditional manufacturing process of TA15 alloy often faces the disadvantages of low forming efficiency,poor matching of strength and plasticity,high die cost and low material utilization rate.The electron beam free form fabrication(EBF~3)technology has the advantages of high energy density,fast scanning frequency and no pollution in vacuum,which is suitable for forming large structural parts of active metals such as titanium alloy.In this study,the microstructure of TA15 alloy fabricated using EBF~3 have been investigated by using the techniques of optical microscope,scanning electron microscopy(SEM),transmission electron microscopy(TEM)and X-ray diffraction,combining with the ANSYS finite element analysis of temperature field,Study systematically and deeply to microstructure evolution,evaluate mechanical properties of samples through the vickers hardness tester and room temperature tensile experiment.Based on ANSYS APDL programming language and combined with experimental results to establish thermal-structure model of TA15 titanium alloy deposited by EBF~3,through pretreatment,loading stage and post-processing stage,research on temperature field distribution.Study the temperature field distribution and molten pool morphology under different process parameters,then optimize the process parameters.In the meantime,it provides a basis for analyzing the influence of temperature field on the structure in the subsequent experiment.The results show that with the increase of deposition height,the peak temperature of each layer gradually increases,when the beam density is 25m A,the peak temperature increases from 2176.64?to 2824.5?.And the whole process is a process of cooling after rapid heating.With the increase of beam density or the decrease of printing speed,the length and depth of molten pool increase,and the effect on the length of molten pool is greater.By adjusting the beam density,printing speed and wire feeding speed,the macromorphology,?-grain morphology and?-phase microstructure of TA15 deposition formed by EBF~3 under different process parameters were studied.The results show that the overall morphology of?-grains from equiaxed grains to columnar grains in the direction of deposition height.With the increase of printing speed,columnar grains are formed,and even some of them tend to be equiaxed grains.The?phase is mainly composed of the lamellar structure,basketweave structure and Widmanst?tten structure growing inwards from the original?grain boundary.The thickness of the lamellar structure increases first and then decreases.The?lath in the middle and bottom of the specimen is coarsened obviously due to the thermal cycling.By increasing the printing speed or decreasing the wire feeding speed,the burning problem of aluminum element is improved.Through microhardness test and room temperature tensile test,the effects of different process parameters on the mechanical properties of TA15 formed by EBF~3 were explored,and combining with microstructure to analyse the relationship between microstructure and properties.The results show that the hardness of TA15 alloy is between 335-345HV,which is higher than that formed by forging process.With the increase of beam density,the tensile strength increases slightly,and the maximum value reaches 1037.67MPa.The tensile strength,yield strength and elongation all meet the requirements of forgings;With the increase of printing speed and wire feeding speed,the tensile strength can meet the requirements of forgings,but the plasticity of the top of the sample is relatively poor.