Influence of focus offset on the microstructure of an intermetallic gamma-TiAl based alloy produced by electron beam powder bed fusion

It is well established in literature that, when processing intermetallic gamma-TiAl components by electron beam powder bed fusion, a banded microstructure is frequently formed because of an inhomogeneous Al distribution since more pronounced evaporation of Al occurs at the top of the melt pool. This feature is particularly promoted when highly energetic process parameters (high beam currents, slow beam speeds, narrow line offsets) are used. Therefore, an approach already suggested in the literature to reduce the Al loss is to minimize the energy level of the process parameter during production. However, there is a limit to such kind of approach: minimizing the beam current or increasing the beam speed, or increasing the line offset will, at a certain point, results in not being able to achieve a completely dense material and thus some process -induced porosity, the so-called lack-of-fusion defects, starts to occur in the produced parts.In this study, the effect of an additional parameter of the electron beam powder bed fusion process is taken under consideration: the focus offset (FO), i.e. the distance between the focusing plane of the electron beam with respect to the powder bed. The effect of the FO on the residual porosity, microstructure, phase composition, hardness as well as chemical composition is investigated, thus having the possibility to demonstrate that also the FO can affect the Al loss and play a fundamental role in the generation of a homogenous microstructure, contributing to mitigate the appearance of a banded microstructure

Microstructural and Phase Analysis of an Additively Manufactured Intermetallic TiAl Alloy using Metallographic Techniques and High-Energy X-Rays

Due to the unique combination of low density and their excellent properties-profile at elevated temperatures, intermetallic γ-titanium aluminide based alloys are a promising structural material for applications in aviation and the automotive industry. Additive manufacturing of a TiAl alloy of nominal composition Ti-48Al-2Cr-2Nb (in at. %), using electron beam melting, resulted in a banded and anisotropic microstructure. In this work, the present microstructure was examined by means of visible light and scanning electron microscopy with regard to morphology and phase distribution. Furthermore, a three-dimensional representation of the microstructure was generated based on differently oriented metallographic specimens. Phase analysis was performed using high-energy X-ray diffraction in order to quantitatively determine present phase fractions and to relate them to findings from microstructural analysis