A study on hot-working as alternative post-processing method for titanium aluminides built by laser powder bed fusion and electron beam melting

Abstract

Intermetallic Titanium Aluminides (TiAl) have been designed for high-temperature lightweight applications. Powder-bed additive manufacturing (AM) processes such as laser powder bed fusion (LPBF) or electron beam melting (EBM) allow near-net-shape production of TiAl components. However, TiAl parts produced by LPBF or EBM do not reach the structural integrity and mechanical properties of forged parts. The main post-processing step of TiAl parts made by AM is hot-isostatic pressing (HIP), which has several disadvantages such as a long process time and an undesired coarsening of the microstructure. High-performance TiAl components such as turbine blades are thus still produced by isothermal forging of preforms made by investment casting and hot isostatic pressing. Due to the low workability of TiAl alloys, often more than one deformation step is required, and the tooling costs and the low material yield make this process chain very expensive. This paper explores the feasibility of combining AM with isothermal forming in order to replace HIP by thermomechanical post-processing. The hot forming behavior of the Ti-43.5Al-4Nb-1Mo-0.1B (at. %) TNM-B1 alloy, manufactured by EBM and LPBF, is analyzed concerning the evolution of the voids and grain sizes and compared to hot isostatic pressing. It is shown that the fine microstructure produced by AM yields a much lower flow stress and faster globularization kinetics in comparison to conventional cast and HIPed material. While HIP experiments are shown to significantly coarsen the microstructure, isothermal hot working is shown to convert the AM microstructure to a dense and refined state. Moderate hot working with total strains of ∼1 of AM pre-forms may thus serve as an alternative process chain to conventional large strains forging of cast pre-forms and to AM + HIP in the series production of high-performance TiAl components.