Influence of the thermal history on the phase composition of laser directed energy deposited Ti-8.5 wt% Mo alloy

Ti-8.5Mo (wt%) alloy was prepared by laser directed energy deposition (L-DED) from the mixture of cp-Ti and Ti-15Mo powder in the ratio of 50:50 using constant laser power of 600 W. Microstructure of as-printed material was characterized by microhardness mapping, light and scanning electron microscopy employing EDS technique and local X-ray diffraction measurements. In particular, the influence of the baseplate preheating to 500 °C on microstructure evolution was determined. The microstructure of the as-deposited samples consists of columnar grains orientated parallel to the building direction. Volume fraction of α, β and ω phases, and consequently also microhardness, vary for different positions within the sample due to different thermal history during the printing process. Bottom part of the sample contains increased fraction of α phase due to longer exposure to temperatures favorable for α phase precipitation. Preheating of the baseplate provides additional thermodynamic stabilization of the α phase. Numerical model, predicting the temperature evolution in each layer of the sample during deposition and consequently the ultimate phase composition, was developed. The results obtained from the model are consistent with the experimentally observed phase transformations.