Microstructure, densification, and mechanical properties of titanium intermetallic alloy manufactured by laser powder bed fusion additive manufacturing with high-temperature preheating using gas atomized and mechanically alloyed plasma spheroidized powders

Abstract

In this work, Laser powder bed fusion (L-PBF) Additive Manufacturing with a high-temperature platform preheating was used to fabricate Ti2AlNb-based alloy samples. The Ti-22Al-22Nb-0.1Mo-0.3Hf-0.3Ta-1.5Zr-0.8Si-0.9Fe (at. %) powder obtained by mechanical alloying followed by plasma spheroidization as well as the Ti-24Al-25Nb-1Zr-1.4V-0.6Mo-0.3Si (at. %) gas atomized (GA) powder were used as the feedstock material. Crack-free samples were fabricated using platform preheating temperatures of 600 °C and above, while lower preheating temperatures resulted in crack formation for both types of powders. The highest relative density of 99.3 % ± 0.1 % was achieved using the mechanically alloyed plasma spheroidized (MAPS) powder, while the relative densities above 99.9 % ± 0.1 % were obtained using the gas atomized powder. Platform preheating temperature significantly affected the microstructure and phase composition of the alloys as revealed by X-Ray diffraction, scanning electron microscopy, electron backscatter diffraction analysis. The phase transition temperatures were determined by differential scanning calorimetry. A fine cellular B2/β microstructure was detected in case of relatively low preheating temperatures while using preheating temperatures of 600 °C and above resulted in in situ heat treatment with the formation of intermetallic Ti2AlNb-phase. An increased preheating temperature improved the chemical homogeneity of the samples fabricated from the MAPS powder. The resulted microstructure varied from fully B2/β to B + O in the case of the MAPS powder and from B2/β, B2 + O to fully-O in the case of the GA powder. The highest microhardness values were obtained using 700 °C preheating temperature, which corresponded to the highest O-phase volume fraction. The highest tensile strength was obtained for the samples fabricated at 980 °C preheating temperature.