Fe-Bi-Mn三元合金多相相变-扩散体系中易切削相析出规律的数值研究

NUMERICAL STUDY ON FREE- CUTTING PHASE PRECIPITATION BEHAVIOR IN Fe-Bi-Mn TERNARY ALLOY MULTIPHASE TRANSFORMATION-DIFFUSION SYSTEM

以扩散支配相变动力学方法为基础,建立了多相三维流动凝固模型.模型考虑了固、液、气三相扩散相变对Fe-BiMn三元合金凝固的影响,模拟研究了合金体系中Bi和Mn S易切削相的析出过程,并分析了易切削相的多相相变过程和多相扩散路径.结果表明:易切削相的析出过程受多相相变-扩散作用影响,Mls,Mn S(Mn S的固-液质量相变速率)较大,Mn S的分配系数大而扩散系数小,当c*s,Mn S(Mn S的固相界面浓度)大于cl,Mn S(Mn S的液相浓度)时,液相Mn S在固-液界面处浓度降低,最终被固相完全"捕获",导致Mn S不再富集;Mls,Bi(Bi的固-液质量相变速率)较小且Mgl,Bi(Bi的液-气质量相变速率)为负值,Bi的分配系数小而扩散系数大,凝固过程中存在气相Bi且cl,Bi(Bi的液相浓度)始终大于c*s,Bi(Bi的固相界面浓度),故Bi持续流动富集于Mn S周围,直至凝固结束.研究工作将模拟结果与实验结果进行了对比,两者吻合较好.

The solidification process of alloys are not just liquid to solid phase transformation, in fact in some alloys liquid to gas and gas to liquid phase transformation processes happen. A method incorporating the full diffusion-governed phase transformation kinetics into a multiphase volume average solidification model is present-ed. The motivation to develop such a model is to predict the multiple effect of inclusions precipitation behavior in castings. A key feature of this model, different from most previous ones which usually assume an infinite solute mixing in liquid lead to erroneous estimation of the multiphase diffusion path, is that diffusions in solid, liquid and gas phases are considered. Here solidification of Fe-Bi-Mn ternary alloy is examined. As Mn S and Bi have large differences in the solute partition coefficient, diffusion coefficient and liquidus slope, the multiphase diffusion path shows differently from those predicted by infinite liquid mixing models. In this work, a three-dimensional mathematical model for a three-phase flow during its horizontai solidification was studied based on diffusion-governed phase transformation kinetics. Effects of Fe-Bi-Mn ternary alloy solidification on solid-liquid-gas phase transformation were considered. The free-cutting phase precipitation behavior was studied and multiphase transformation and multiphase diffusion path of free-cutting phase precipitation behavior were analyzed. Results show that the multiphase transformation-diffusion is strongly influenced by free-cutting phases precipitation behavior： Mn S has a relatively large partition coefficient and small diffusion coefficient with larger Mls,Mn S（solid-liquid mass transfer rate of Mn S）. During solidification, c＊s,Mn S（solid interface concentration of Mn S） may become even larger than cl,Mn S（liquid concentration of Mn S）, Mn S in liquid is assumed to be fully‘trapped’in solid and there is no longer any enrichment of Mn S; however Bi has a relatively small partition coefficient and large diffusion coefficient with smaller Mls,Bi（solid-liquid mass transfer rate of Bi） and negative Mgl,Bi（liquid-gas mass transfer rate of Bi）, during solidification, cl,Bi（liquid concentration of Bi） always greater than c＊s,Bi（solid interface concentration of Bi）. In addition,due to the existence of Bi-gas phase, Bi continuous to flow, enriched in the solidified around Mn S. Calculated results show good agreement with experimental data.