【作者】 王轩；

【导师】 徐达鸣；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2014， 硕士

【摘要】 高能束熔炼的特点便是利用高的能量密度在合金表面形成熔池，进而熔化金属。高的能量密度虽可以熔炼难熔金属，但由于熔池中心温度过高，合金组元会出现挥发现象，从而使生产出合金的成分偏离设计值，这在成分和性能要求极高的航空航天领域是不允许的。而且无论在高真空还是常压下，元素挥发是熔炼合金时普遍存在的现象，本文通过热力学和动力学两方面阐述了元素挥发的机制，试图建立一个适用范围较宽的描述挥发现象的统一模型。热力学方面，利用Miedema二元溶液生成热模型和Kolher三元溶液模型计算出了TC4合金中各组元的活度系数和饱和蒸汽压，证明了熔炼时主要是Al的挥发，并推导出了挥发刚开始时靠近合金熔体表面处挥发组元的浓度值；动力学方面根据气体分子运动理论和分子自由程的思想建立了挥发组元在真空室内扩散的物理模型，并利用有限差分法建立了挥发组元扩散的差分方程，通过Matlab软件计算出了不同温度、真空室压力以及熔炼时间下真空室内挥发组元的浓度分布、挥发速率、挥发损失量、传质系数、质量分数曲线以及扩散达到平衡状态所需的时间。计算结果表明：挥发组元在真空室内的扩散满足菲克第二定律，随着时间推移，扩散由非稳态转变为稳态，达到稳定状态时浓度梯度保持恒定；压力恒定时，温度越高，金属熔体越活泼，挥发速率越大；温度恒定时，随着压力的升高，气体扩散的阻力变大，扩散速率变慢，当外压低于临界压力，挥发速率几乎不受外压的变化而改变，当外压高于临界压力，挥发速率随外压升高而降低；挥发速率的计算结果显示了挥发速率逐渐增大直至恒定的过程；传质系数的计算结果证实了挥发控制机制的转变，外压低于临界压力，挥发过程受界面反应控制，外压高于临界压力，挥发反应受到气体扩散过程的控制；扩散达到平衡所需时间的与浓度分布和挥发速率的计算结果相互印证。通过对TC4合金真空条件下的电子束熔炼和常压下的激光熔炼实验，并利用能谱分析得到了不同温度以及不同作用时间下Al的质量分数曲线，与理论计算的结果相对比后，发现计算模型可以较好的反映熔炼过程中Al含量的实际变化，证明了理论计算模型不仅可以适用于高真空环境下元素挥发的描述，也同样适用于常压下元素挥发的计算，进而证明了理论模型的可靠性。更多还原

【Abstract】 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.更多还原