Electron Beam Powder Bed Fusion of γ-Titanium Aluminide: Effect of Processing Parameters on Part Density, Surface Characteristics, and Aluminum Content

Abstract

Gamma titanium aluminides are very interesting for their use in high-performance applications such as aircraft engines due to their low density, high stiffness and favorable high-temperature properties. However, the pronounced brittleness of these intermetallic alloys is a major challenge for their processing through conventional fabrication methods. Additive manufacturing by means of electron beam powder bed fusion (EB-PBF) significantly improves the processability of titanium aluminides due to the high preheating temperatures and facilitates complex components. The objective of this study was to determine a suitable processing window for EB-PBF of the TNM-B1 alloy (Ti-43.5Al-4Nb-1Mo-0.1B), using an increased aluminum content in the powder raw material to compensate for evaporation losses during the process. Design of experiments was used to evaluate the effect of beam current, scan speed, focus offset, line offset and layer thickness on porosity. Top surface roughness was assessed through laser scanning confocal microscopy. Scanning electron microscopy, electron backscatter diffraction (EBSD) and energy-dispersive X-ray spectroscopy (EDX) were used for microstructural investigation and to analyze aluminum loss depending on the volumetric energy density used in EB-PBF. An optimized process parameter set for achieving part densities of 99.9% and smooth top surfaces was derived. The results regarding microstructures and aluminum evaporation suggest a solidification via the β-phase.

Characterization of the high-temperature behavior of PBF-EB/M manufactured γ titanium aluminides

“…A total volume of all three specimens of 261 mm 3 was analyzed and the relative density was calculated to be 99.88%. In comparison to the previously conducted study regarding process parameter optimization, where relative densities of 99.9% could be achieved using the exact same parameter, this value is considerably lower [33]. This indicates that scaling the specimen volume without adjusting the parameters may result in slightly increased density.…”

“…They occur between melting layers due to a lack of fusion (LOF). Again, these findings are in contrast to [33], where LOF defects could fully be avoided using the optimized parameter set. A possible explanation might be that the parameter set was optimized using cubic specimens with an edge length of 10 mm.…”

“…All specimens were manufactured on the Arcam A2X machine (ARCAM AB, Mölndal, Sweden) and EBM Control 3.2 software with the manufacturing parameters given in Table 2. The parameters were identified as optimal parameters with respect to achievable relative density and top surface quality in the previous studies [33]. A central composite design was used to determine the influence of beam current, scan speed, line offset, focus offset, and layer thickness on the resulting part density.…”

“…In the previous investigations, the initial condition was already characterized using EBSD scans. The exact method can be found in [33].…”

“…This paper is therefore intended to contribute to the complete characterization of the alloy TNM-B1 manufactured by PBF-EB/M. For this purpose, an as-built condition of the alloy TNM-B1 was investigated, which was identified in the previous investigations as optimal concerning porosity, surface roughness, and evaporation of aluminum [33]. Therefore, specimens were produced with this parameter set, and the as-built state was characterized by various destructive and non-destructive testing methods.…”