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

Abstract

Due to their high specific strength and temperature resistance, γ-titanium aluminides (γ-TiAl) have a growing importance for automotive and aerospace applications. However, conventional processing is very challenging due to the inherent brittleness of the material. Therefore, new manufacturing techniques and methods have to be established. Additive manufacturing techniques such as electron powder bed fusion (PBF-EB/M) are favored, since they enable near net shape manufacturing of highly complex geometries. The high preheating temperatures, which typically occur during PBF-EB/M, can significantly improve the processability of TiAl and facilitate the fabrication of complex parts. In this study, a previously optimized material condition of the β-solidifying TNM alloy TNM-B1 (Ti-43.5Al-4Nb-1Mo-0.1B) was manufactured by PBF-EB/M. The resulting microstructure, defect distribution and morphology, and mechanical properties were characterized by means of characterization methods, e.g., CT, SEM, light microscopy, hardness measurements, and tensile tests. A special focus was on the mechanical high-temperature behavior. The pronounced sensitivity of the material to defects and internal notches, e.g., due to lack of fusion defects (misconnections) which were found in the as-built condition, was identified as a main cause for premature failure below the yield point due to the low ductility. This failure was analyzed and potential improvements were identified.

Influence of Two‐Step Heat Treatments on Microstructure and Mechanical Properties of a β‐Solidifying Titanium Aluminide Alloy Fabricated via Electron Beam Powder Bed Fusion

“…Along with the tensile test results presented in Figure 11 and 12, this suggests that the failure mechanism in the AS condition is mainly governed by the occurring lack‐of‐fusion defects. [ 29,30 ] In contrast, failure of the heat‐treated samples was governed by microstructural features rather than defects (Figure 13c,d). The fracture surfaces of HT‐1290 and HT‐1405 reveal large lamellar colonies.…”

“…Despite a thorough parameter optimization performed in a previous study, [ 29 ] both gas porosity and lack‐of‐fusion defects were observed in the AS condition, which might be critical for the mechanical properties. However, the large and elongated lack‐of‐fusion defects could successfully be eliminated through HIP (Figure 2).…”

“…[25] Wimler et al observed a similar effect after annealing of PBF-EB/Mmanufactured Ti-44.8Al-4.1Nb-0.7W-1.1Zr-0.4Si-0.5C-0.1B slightly below the γ-solvus temperature. [27] Previous studies investigated the mechanical properties of PBF-EB/M-manufactured TNM-B1 in tensile tests for vertically manufactured specimens [29] and in tensile and fatigue tests for horizontally manufactured specimens [30] for different test temperatures up to 850 °C, respectively. Due to the low defect tolerance of titanium aluminides, the material behavior was strongly dependent on the defect size and shape, which outweighed the influence of the microstructure.…”

Locally Adapted Microstructures in an Additively Manufactured Titanium Aluminide Alloy Through Process Parameter Variation and Heat Treatment

“…While large lack-of-fusion defects in the AS condition can severely compromise the mechanical properties, it was shown that by reducing the size and number of critical defects through HIP, tensile strength and fatigue life of additively manufactured TNM-B1 can be significantly improved. [33,34] Figure 4 shows scanning electron microscope (SEM) images of the AS and HIP conditions of each specimen at two different magnifications.…”

“…While large lack‐of‐fusion defects in the AS condition can severely compromise the mechanical properties, it was shown that by reducing the size and number of critical defects through HIP, tensile strength and fatigue life of additively manufactured TNM–B1 can be significantly improved. [ 33,34 ]…”