In-situ synthesis of high-quality pseudoelastic NiTi alloys with intrinsic Ni₄Ti₃ phase precipitation using laser DED

Laser directed energy deposition (DED) has been used for fabrication of Nickel-Titanium (NiTi) alloys. The raw material can be pre-alloyed NiTi powders or premixed Ni and Ti powders. Using premixed Ni and Ti powders to in-situ synthesize NiTi alloys shows more potential with advantages of cost-effectiveness and flexibility in tailoring the mechanical and phase transformation properties. However, a common problem for in-situ synthesis of NiTi lies in the difficulty in obtaining sufficiently dense parts due to the alloying effects, liquid capillary effects, and the difference in diffusivity between Ni and Ti. The porosity could be greatly affected by the powder morphology. For the first time, this paper reports the in-situ synthesis of high-quality pseudoelastic NiTi alloys using spherical powders in the laser DED process. The comparisons of the NiTi alloys in-situ synthesized using powders with different morphologies are made. The manufacturing defect formation mechanisms with different powder morphologies (spherical and irregular) are firstly discussed. It is found that the NiTi parts in-situ synthesized using spherical powders have comparable density with those fabricated using pre-alloyed powders. Moreover, Ni₄Ti₃ precipitates are only observed in the NiTi parts in-situ synthesized using spherical powders. The mechanical properties (pseudoelasticity, Young's modulus, and microhardness) are also affected by the powder morphology. The experimental results indicate that high-quality NiTi alloys can be in-situ synthesized with reduced manufacturing defects, the formation of Ni₄Ti₃ precipitates, and improved pseudoelasticity and microhardness.