Microstructure formation during laser powder bed fusion of Ti-22Al-25Nb with low and high pre-heating temperatures

We compare microstructure formation during laser powder bed fusion (LPBF) of a Ti-22Al-25Nb alloy applying low and high pre-heating build plate temperatures. Fast cooling rates during low-temperature LPBF lead to metastable weakly ordered & beta; phase, i.e., bcc-(Ti,Al,Nb) with pronounced 100 texture in build direction and nanosized segregations within grains elongated in the build direction. For high-temperature LPBF a Wid-mansta & BULL;tten microstructure was observed with lenticular O phase precipitates within the & beta; matrix. Microscopical and in situ high-energy synchrotron diffraction investigations demonstrate that the microstructure formation can be widely explained by the precipitation of O phase from supersaturated & beta; rather than by a sequence of solid-state phase transformations, as it would be expected under thermodynamic equilibrium conditions. A detailed analysis of the microstructural gradient in the subsurface region, however, demonstrates that precipitation from metastable & beta; cannot fully explain the observed microstructures, and that the process energy density, i.e., the intensity of the intrinsic heat treatment of LPBF, plays a relevant role. The investigation of the graded area near the surface of the LPBF materials produced with high pre-heating temperature is particularly interesting as it reveals preferred nucleation of the O phase at subgrain boundaries and dislocations.

Automated summary

We studied the microstructure formation of a Ti-22Al-25Nb alloy during laser powder bed fusion (LPBF) at low and high pre-heating temperatures. Low-temperature LPBF resulted in a metastable weakly ordered β phase with a pronounced 100 texture in the build direction and nanosized segregations within elongated grains. High-temperature LPBF showed a Widmanstätten microstructure with lenticular O phase precipitates within the β matrix. The microstructure formation can be explained by the precipitation of O phase from supersaturated β rather than solid-state phase transformations expected under thermodynamic equilibrium conditions. However, the process energy density plays a relevant role, and precipitation from metastable β alone cannot explain the observed microstructures.