Processing Technology And Microstructure Control Of High Nb-TiAl Alloy Fabricated By Electron Beam Melting

Advanced γ-TiAl intermetallic alloys have the potential to replace the heavier Ni-base superalloys for high-temperature applications such as low-pressure turbine blades because of their low density of about 4 g/cm3,high melting point,good creep and oxidation performance up to 750℃,as well as burn resistance.High Nb-TiAl alloys have been metallurgically designed and developed to further enhance their oxidation resistance and creep properties.However,fabrication of high Nb-TiAl parts and components by conventional means including forging,rolling and welding remains as a-.challenge.This is due to γ-TiAl intrinsic room-temperature brittleness.and inadequate hot workability.Electron beam selective melting technology(EBM)is an additive manufacturing process(3D printing)that enables near net shape formation of TiAl alloys.However,due to the lack of systematic solidification phase change mechanism research,precise structural performance regulation cannot be achieved during the forming process.In this work,the microstructure and properties of high Nb-TiAl alloy prepared by EBM technology were systematically studied.Firstly,a broadened processing window was achieved with the help of parameter optimization in both preheating and melting stage.Secondly,we present an experimental investigation of the microstructural evolution in the high Nb-TiAl sample fabricated by electron beam melting(EBM).The results support that the incipient primary solid phase nucleates at the core of the melt pool,instead of the solid-liquid interface boundary.This work provides a better understanding of the nucleation and grain growth modes in y-TiAl alloy during the non-equilibrium solidification of EBM process.Finally,the effects of preheating and melting parameters on melting,solidification,phase transformation and resulting microstructure formation in as-EBM high Nb-TiAl alloy were investigated by performing a design-of-experiments.With the help of numerical simulation,fundamental principles are proposed in terms of controlling microstructure formation and fabricating fully dense high Nb-TiAl alloy in as-EBM condition.Moreover,by considering the grain morphology and texture control,a columnar lamellar colony(CLC)grain structure was obtained following a critical assessment of thermal gradient and liquid-solid interface velocity.The texture analysis and phase identification performed provided important insights in understanding the solidification and phase transformation processes during the EBM fabrication.And the nano-TiC reinforced high Nb-TiAl nanocomposites fabricated by EBM was also achieved in this study.