The Research On Elements Volatilization In Micro-bath Of TC4Titanium During High-energy Beam Melting Process

The characteristic of the high-energy beam melting is melting metal in use ofthe high density of energy to make a bath on surface of the alloy. Although the highdensity of energy can melt the metals with high melting point, partially elements inthe melt with higher saturated vapor pressure will appear the phenomenon ofevaporation due to the high temperature in the center of the bath,thereby thecomponent of alloy producted will deviated from the design value which is notallowed in aerospace science that demanding for composition and performance. Andno matter under high vacuum or atmospheric pressure, evaporation of elements inmelting alloy is a common phenomenon, in this paper, the author is trying to build aunified model with broad description of the scope to describe the phenomenon ofevaporation mechanism in both thermodynamic and kinetic.In the Thermodynamics, we use the Miedema heat generated binary solutionmodel and Kolher ternary solution model to calculate the activity coefficients andthe saturation vapor pressure of each component in TC4alloy, proved the mainlyevaporate element is Al when melting and we deduced the component concentrationof the evaporation element near melt surface at the the beginning of melting; in thedynamics we establish a physical model of the spread of evaporation component inthe vacuum chamber based on kinetic theory of gases thought and molecular freepath, and we establish a evaporation component diffusion differential equations byuse of the finite difference method and calculate the evaporation componentconcentration distribution in vacuum chamber indoor,vaporation rate,the amount ofevaporation loss, mass transfer coefficient, and mass fraction curve and the spreadreached the time required for equilibrium under different temperature, pressure andvacuum melting time.Results show that: the diffusion of evaporation component in the vacuumchamber satisfy the Fick’s second law, over the time, the non-steady state diffusionwill channge to the steady-state diffusion, the concentration gradient will alwaysconstant when the system reaches steady-state; when the pressure is constant, thehigher temperature of the melt, the more lively of the metal melt, the evaporationrate will increase;when the temperature is constant, the gas diffusion resistancebecomes larger and the diffusion rate slows down with increasing pressure, when thevacuum chamber pressure is lower than the critical pressure, the evaporation rate isalmost free change from vacuum chamber pressure changes, when vacuum chamberpressure is higher than the critical pressure,the evaporation rate decreases when thevacuum chamber pressure is increased; the result of calculations show the process that evaporation rate increases until the evaporation rate constant; the calculations ofmass transfer coefficient confirmed the shift of volatilization control mechanisms,when the vacuum chamber pressure is lower than the critical pressure, theevaporation is controlled by the interface reaction, when vacuum chamber pressureis higher than the critical pressure, evaporation is under control of gas diffusionprocess; the calculation results of diffusion time required to reach equilibrium willbe the proof to the the evaporation rate and the concentration distribution.Through TC4alloy electron beam melting under vacuum and laser meltingexperiments under atmospheric pressure, and we obtained mass fraction curves of Alat different temperatures and under different time in use the analysis of spectroscopy,with the results of relative ratio with theoretical calculations results,we can say thatcalculation model can reflect the actual change in the Al content in the meltingprocess, and calculation model is proved not only can be used to describe theelements evaporation under high vacuum, but also can be used to calculate theelements evaporation under atmospheric pressure, thus,we prove the reliability ofthe theoretical model.