Microstructure formation during laser powder bed fusion of Ti-22Al-25Nb

Kurzfassung

Titanium alloys based on the orthorhombic Ti2AlNb phase have been considered as potential structural lightweight alloys since the early 1990s due to their favourable mechanical performance, including high strength and ductility at room and elevated temperatures as well as high oxidation and creep resistance. With the emergence of additive manufacturing these alloys become interesting again as they can be reliably processed, and the microstructures and properties differ considerably from conventionally processed materials offering new possibilities for materials design. In our study, the complete process chain for LPBF of Ti-22Al-25Nb including powder production is examined. We investigated the microstructure formation of the orthorhombic alloy Ti-22Al-25Nb at low as well as high pre-heating temperatures. In particular the effects of in situ and intrinsic heating during laser powder bed fusion with differing energy densities is considered and the phase transformations during high temperature LPBF are revealed by means of state-of-the-art characterization techniques such as in situ high energy synchrotron X-ray diffraction and advanced electron microscopy. The fast cooling rates during low-temperature LPBF lead to metastable weakly ordered β phase, whereas for high-temperature LPBF a Widmanstätten microstructure was observed with lenticular O phase precipitates within the β matrix.