| Electron Beam Free Form Fabrication(EBF3) is an emerging addictive manufacturing technology in which the metal wire is deposited layer by layer by a defocused electron beam in a vacuum environment to make high performance metallic components. Currently, very little study has been made on defect formation mechanism in EBF3 process.Possible defects formation mechanism of incomplete fusion and three types of porosity observed in the EBF3 process of TC4 alloys has been presented by incorporating experiments and theoretical analysis in this paper. The main findings and innovation achievements are listed below:(1) According to the forming characteristics, four kinds of possible incomplete fusion formation patterns were proposed for the EBF3 process. Besides, a mathematical model of heat transfer and fluid flow coupling free surface evolution was established based on one of the formation patterns and the forming process of incomplete fusion was dynamically simulated. Based on the numerical simulations in EBF3 of TC4 titanium alloy, the changes of temperature fields, velocity fields and free surface evolution of the molten pool was studied.(2) The porosity in EBF3 of Ti-6-Al-4-V alloy is experimentally investigated by optical microscope(OM) and scanning electron microscopy(SEM) with energy dispersive spectrometry(EDS). Three new types of porosity are discovered, including gas-induced metallurgical porosity of type I, balls-filled porosity of type II and irregular porosity of type III. The type I porosity usually located around the fusion line with spherical contour and smooth inner surface. Porosity of type II has near-spherical contour with some unique ballfilled microstructure of radius from 1 μm to 60 μm on the inner surface. The chemical composition of type II porosity at different probing positions characterized with EDS showed that the mass fraction of aluminum reduces at both the balls and the inner surface of porosity II, from 6.00% to 3.00% and to 3.79% respectively compare to the base material Ti-6-Al-4-V. While the mass fraction of vanadium reduces from 4.00% to 2.76% at the balls but increases dramatically up to 12.06% on the inner surface. While irregular porosity of type III has obvious rough inner surface and tearing tissues.(3) Possible formation mechanism of these new types of porosities were proposed. The formation of porosity I results from nucleation and growing of entrapped gas bubbles from H2, N2, CO and CO2, etc. in the melt pool. The evaporation of Aluminum, titanium and vanadium elements in Ti-6-Al-4-V leads to the formation of metallic vapors under vacuum. If the metallic vapor meets the gas bubbles, mixed bubble formed. As the temperature of melt pool decreases, the elements of metallic vapors begin to condense and deposit on the inner surface of the mixture bubbles and eventually grow up along the solidification front, thus the porosities of type II with near-spherical contour and solid balls on the inner surface formed. The type I porosity captured by the solidification front may experience pressure drop due to Al evaporation and fail to sustain its contour. As a result, porosities of type III with obvious rough inner surface and tearing tissues formed. |
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