Laser Powder Bed Fusion of Titanium Aluminides: An Investigation on Site-Specific Microstructure Evolution Mechanism

Metal additive manufacturing (AM) improves the design flexibility of titanium aluminides (TiAl-based alloys) as a new class of high-temperature alloys towards widespread applications. In this work, the underlying mechanisms responsible for the site-specific thermal history and grain evolution during laser powder bed fusion (LPBF) of TiAl-based alloys are investigated through an integrated computational and experimental effort. In specific, a multi-physics modeling framework integrating a finite element thermal model with a highly efficient phase-field method is developed to simulate the solidification microstructure at different locations within the melt pool during LPBF processing. The investigation of process-microstructure relationship is accomplished using a Ti-45Al (at.%) alloy for a binary approximation, with a focus on site-specific primary dendrite arm spacing (PDAS) and non-equilibrium micro-segregation. The microstructural sensitivity to spatial variations, individual processing parameters, and mis-orientation angle between the preferred crystalline orientation and the temperature gradient direction are studied to thoroughly understand the rapid solidification during LPBF. LPBF experiments are carried out to validate the modeling results in terms of melt pool dimensions and site-specific PDAS across the melt pool. The knowledge gained in this work will benefit the development of AM processing routine for fabrication of high-performance TiAl-based alloys towards extensive applications.