Study On Microstructural Evolution Of TiAl Alloys Fabricated By Electron Beam Freeform Fabrication Process

Titanium aluminum（Ti Al）alloy has been applied in the field of aerospace and vehicle engine manufacturing due to its excellent properties.Additive manufacturing（AM）has the advantage of a near net shape,and presents application potential in the manufacturing of Ti Al alloys.Research on the microstructural evolution behavior of Ti Al alloys during the AM process provides an important theoretical basis for their microstructure regulation and application.In this paper,a series of Ti Al binary model alloys were designed and prepared,and their composition,microstructure characteristics and grain orientation were systematically analyzed by OM,ICP-OES,XRD,SEM,EDS,TEM and EBSD.The solidification behavior,the transition of the growth mode ofγdendrites and dynamic recrystallization during the AM process were systematically studied by the combination of finite element simulation and theoretical analysis.The main results are as follows:TA_β-α,TA_β-αγ,TA_β-γand TA_α-γalloys with uniform and controllable compositions were prepared by electron beam freeform fabrication（EBF³）using titanium wire and aluminum wire as raw materials.With increasing Al content,the microstructures of the as-deposited alloys displayed an equiaxedα₂ structure,a lamellarα₂+γstructure,a lamellar and massiveγstructure and a dendriticγstructure.The microhardness and compressive strength of the as-deposited alloys were equivalent to those of Ti Al binary alloys fabricated by other AM processes,and the microhardness and yield strength decreased with increasing Al content.The solidification process of Ti Al binary alloys during EBF³ has the characteristics of high superheating and large undercooling.The primary phase of the TA_β-αγ,TA_β-γand TA_α-γalloys changed from theβphase,βphase andαphase to theαphase,αphase andγphase,respectively.Combined with the analysis of classical nucleation theory and transient nucleation theory,finite element simulation and experimental verification,the transformation of the primary phase required a large undercooling,and the calculated thermodynamic and kinetic undercooling are204～281 K and 240～247 K,respectively,which was caused by the superheating of the molten pool during the EBF³ process.Compared withγdendrites fabricated by the conventional process,the growth mode ofγdendrites fabricated by the EBF³ process changed from faceted growth to nonfaceted growth,and the solid–liquid interface changed from a smooth interface to a rough interface.The growth direction ofγdendrites changed from the<100>direction to the<110>direction,and the high superheating in the molten pool caused the melt to undergo order disorder transformation,causing the reduction of the latent heat of phase transformation,which reduced the Jackson factor and provided thermodynamic conditions for the solid–liquid interface transformation ofγdendrites.The large undercooling caused by the superheating in the molten pool was higher than the 150 K critical undercooling of the growth mode transition,which provided dynamic conditions for the nonfaceted growth ofγdendrites.Dynamic recrystallization occurred in the middle region of the as-deposited TA_β-γalloy,causing the microstructure to change from a lamellarα₂+γstructure to a lamellar and massiveγstructure,the parallel orientation relationship of grains to disappear,and obvious grain refinement to occur.The finite element simulation results showed that dynamic recrystallization of theγphase was the result of the thermal mechanical interaction during the EBF³ process.The heat accumulation made samples at a high temperature due to the low cooling rate,and thermal cycling increased the stress causing deformation and the increasing of the dislocation density in the samples.The thermal mechanical interaction during the EBF³ process provides the conditions and driving force for the dynamic recrystallization of theγphase.