Fabrication, Characterization And Mechanism Of Microfine Spherical TiAl Based Alloy Powders

TiAl intermetallic compound has excellent strength,creep resistance and oxidation resistance at elevated temperatures,but also has great resistance against acid or alkali corrosion and biological activity,which makes TiAl alloys not only widely used materials in aerospace and automotive applications but also potential materials for filtration,catalysis and biomaterials.Powder metallurgy is an important method to prepare TiAl alloy parts.It helps reducing fabrication costs and broadening application fields of TiAl alloys.Hence,powder preparation,especially for the powders with outstanding properties has great significance for the development of TiAl alloys.In this paper,TiAl alloyed powders with micro-fine size were successfully fabricated by three different means based on high energy ball milling and reaction synthesis techniques from Ti,Al and other elemental powder stock.Then the alloyed powders were processed by radio frequency inductively coupled plasma spheroidization system from which spherical powders were obtained.The characteristics of powder,the mechanism of powder preparation and the spheroidization process were investigated.The main contents and results are as follows:Mechanical alloying,reaction synthesis and RF plasma spheroidization were combined to fabricate high Nb containing TiAl alloy powder with micro-fine size,uniform size distribution and good sphericity from TiH₂,Al and Nb powders.TiAl-Nb alloyed powders prepared by 10h ball milling and subsequent 800? annealing had the finest particle size of 7.8?m and the best size distribution uniformity of 0.486.Besides,the powder had good composition uniformity and was dominated by ? and a small amount of ?2 phase.After processed by RF plasma spheroidization,the mean particle size and the size distribution uniformity index of the powder rose to 9.6?m and 0.622,respectively.The spherical powder was dominated by ?2 phase with a dense structure.Besides,the powder had good composition uniformity,and the oxygen content was 0.65%.High Nb containing TiAl alloy powder with micro-fine size and good sphericity was prepared from Ti,Al and Nb elemental powders by a combination of double-step ball milling,reaction synthesis and RF plasma spheroidization techniques.Composite powder with uniform composition was obtained by primary milling.In the subsequent annealing,completely alloyed coarse porous high Nb containing TiAl alloy powder was obtained at 1400? and the alloy powder was dominated by y phase and a small amount of a2 phase.The coarse powder was milled to micro fine powder with the mean size of 8.2pm by means of 30min secondary milling.Processed by RF plasma spheroidization,the sphericity of powder was closed to 100%with the oxygen content of 0.48%and the mean size went up to 8.8?m.Fabrication of TiAl based alloy powders was realized by reaction synthesis in molten salt media from Ti and Al elemental powder stocks and the mechanism as well as the influences of process parameters on alloying were investigated.It turned out that molten salt contributes to the acceleration of reaction and affects the morphology and size of TiAl alloy powders.Powder prepared with the reaction temperature of 800?,heating rate of 3°/min,holding time of 2h,molten salt to metal powder ratio of 5:1,and argon flow rate of 500ml/min,showed a phase component of y and a2.Besides,the powder had a mean diameter of 20.42?m with the oxygen content of 0.40%.The electromagnetic field,temperature field and fluid field of RF inductively coupled plasma was established based on Maxwell equations and Navier-Stokes equations utilizing Fluent and Matlab softwares.Models of particle behavior were built based on energy and momentum equations,with which the trajectories and phase transitions of particles in plasma torch were simulated.Spheroidization results for powders with different initial particle granularity features were predicted and were verified by experimental data.It was proved that plasma torch occurred inside the current coil but the maximum temperature was not along the axis of plasma system.The velocity distribution showed apparent relationship with the temperature field.Trajectories of particle inside plasma torch were not significantly influenced by particle size and injection angle,but spread outward with the increase of injection angle.The residence time of particle was increased by the increase of particle diameter which promoted the energy absorption of particle.With the given parameters of system,powder collection rate and spheroidization rate could be elevated by restricting the size range of raw powders.Moreover,spheroidization experiments of powders with different granular features were conducted and the results verified the accuracy of the numerical models.