Microstructure evolution and properties development of in situ processed Ni–Ti alloys by laser directed energy deposition

This investigation proposes a flexible technique for in situ fabrication of Ni–Ti structures using laser directed energy deposition, in which nickel and titanium powders are separately fed and melted together during laser processing. The proposed mechanism enables controlled flows of powders facilitating a fine-tuning of the desired chemical composition, when compared to the conventional use of pre-mixed feedstocks, in an endeavor toward the laser processing of pseudoelastic Ni–Ti alloys. The results highlighted the possibility of fabricating a wide range of tailored Ni–Ti compositions and microstructures, depending on the powder flow ratios. The developed alloys were classified as either Ni-rich or Ti–rich compositions, in which the Ni-rich alloys were composed of different fractions of B2-NiTi austenite, NiTi/Ni3Ti eutectics, and Ni3Ti intermetallic with a minor presence of NiTi2/Ni2Ti4Ox. While the Ti–rich compositions were mainly dominated by NiTi austenite with a fraction of NiTi2/Ni2Ti4Ox dendrites. Under identical laser processing parameters, the findings showed that the in situ alloyed Ni47.6Ti52.4 composition exhibited a comparable microstructure and pseudoelastic behavior similar to that obtained from a laser processed pre-alloyed powder. The output of the investigation highlights the potential use of the in situ alloying mechanism as a cost-effective and flexible approach for fabricating Ni–Ti alloys with desired compositions and properties.