Numerical Simulation and Experimental Study on Plasma Arc-induction Hybrid Melting of Nb-Ti-Si Alloy

Author:Meng Xian Yu

Supervisor:su yan qing luo liang shun


Degree Year:2019





Nb-Ti-Si based superalloys are important candidate materials for the high-pressure turbine blades of aerospace engines with high thrust-to-weight ratio and the high-temperature structural components(>1200°C)of the new generation high-speed aircraft.They possess the characteristics of high melting point,high activity,complex composition and contain some low melting point alloying elements.In order to decrease the melting and preparation difficulties and improve macrosegregation,the plasma arc-induction hybrid melting technology was proposed and the corresponding plasma arc-water cooling copper crucible induction levitation hybrid melting equipment was established.The investigations including numerical simulation on the the transfer behavior of hybrid melting and single induction melting and composition distribution analysis are carried out.This work can helps to reveal the essential causes of the related physical transportation phenomena,which possesses a significant theoretical guidance and engineering application value for enriching CFD theoratical system and optimizing the actual technology.Firstly,based on the in-depth understanding of heating characteristic and physical transportation phenomena of single induction melting and plasma arc-induction hybrid melting process,a three-dimensional mathematical model coupling electromagnetic field,temperature field and flow field is established.On the basis of the single heat source model,"induction heat source+heat flow varying with keyhole depth"model,various kinds of heat and mass transfer phenomena during the heating and cooling stages are described in detail,by combining the VOF tracking algorithm,enthalpy-porosity method and various fluid driving forces such as electromagnetic levitation force,surface tension,Marangoni shear force,thermal buoyancy and gravity,etc.Ansys Maxwell electromagnetic field finite element software and Ansys Fluent fluid dynamics analysis software are mainly used here.Heat source and electromagnetic force source terms are firstly obtained by Ansys Maxwell,and then introduced into Ansys Fluent to realize mutual coupling.The heat source model,various heat dissipation boundary conditions and fluid driving force boundary conditions are added into Ansys Fluent software,based on the user-defined function UDF programming.The discrete of numerical program is completed using the finite volume method and PISO algorithm,and then sloved.Secondly,based on the three-dimensional numerical model of single induction melting,the heat transportation behavior as well as the formation and evolution of hump during the heating and cooling stages are systematically analyzed.The studies indicate that at the initial stage of heating,the furnace burden melt is melted from the surface to the inside along the radial direction.An annular bulge with an acute angle to the horizontal plane is formed on the upper surface of melt;then the annular bulge continuously approaches the center until it forms a complete initial hump;After the hump is formed,lifting process continues until dynamic stability is achieved;on the other hand,heat flow begins to transfer from the tip of the hump to the low temperature region below,which results in a height decrease of the low-temperature solid region.Due to the limitation of heat transfer capability in the depth direction and the attenuation of the radial power density of the induction heat source,a localized solidified shell region appears at the bottom of crucible finally.During the cooling stage,the hump falls back and undergoes multiple rebound oscillations,eventually forms Nb-Ti-Si alloy ingot.By comparing with the morphology of the Nb-Ti-Si superalloy ingot fabricated by the single induction melting method,the reliability of its three dimensional model is validated.Thirdly,based on the established three-dimensional numerical model of plasma arc-induction hybrid melting,the heat transfer behavior at the heating and cooling stage and the formation and evolution of hump were systematically analyzed.Studies have shown that,at the initial stage of heating,the furnace burden melt is melted through the radial heat transfer induced by the electromagnetic induction and the downward heat transfer induced by the plasma arc heat source and arc pressure simultaneously.The burden is completely melted under the two transportation mechanisms.Afterwards,closing the two heat sources at the same time,the levitated hump falls back and undergoes multiple rebound oscillations,and finally forms Nb-Ti-Si alloy ingot.By comparing with the morphologies of the hump and Nb-Ti-Si superalloy ingot fabricated by the plasma arc and induction hybrid melting method,the correctness of its computational results are validated.Fourthly,the thermal cycle curve and the hump growth curve of different points to the core of melt at the same penetration section under two melting parameters prepared by single induction melting and two kinds of hybrid melting methods are compared and analyzed.The obtained regularities are summarized as follow:(1)The keyhole effect of plasma arc heat source accelerates heat flow transfer to the bottom of crucible efficiently on the basis of the radial heat transfer mechanism;(2)Under the premise of keeping the other conditions fixed,the increase of frequency of induced heat source leads to the reduce of the amplitude of electromagnetic force,and thus reduce the formation time and height of quasi-stable hump,and further improve the heat transfer efficiency of plasma arc heat source and the melting efficiency.Finally,the composition distribution regularities of melt prepared by plasma arc-induction hybrid melting process is systematically detected and analyzed;Then,a kind of method evaluating the composition homogeneity by means of variance is proposed and employed to validate the feasibility.Afterward,the composition distribution regularities of Nb-Ti-Si alloy ingot and transformation regularities with technological parameters are obtained.Micromechanics of Nbss and(Nb,Ti)3Si phases in Nb-Ti-Si alloy are investigated by means of nanoindentation and microcolumn compression techniques.The obtained quantitative performances of each phase in this study are beneficial to better understand the macroscopical deformation behaviors of Nb-Ti-Si based bulk materials and can be applied to the mechanical model of numerical investigations.