Solidification characterization and its correlation with the mechanical properties and functional response of NiTi shape memory alloy manufactured by electron beam freeform fabrication

Compared with the common additive manufacturing techniques, the vacuum operating environment and high deposition efficiency of wire-based vacuum additive manufacturing are more expective in the preparation of NiTi shape memory alloys. In the present work, the strategy of interlaminar residence was adopted to deposit the dense wall-structure NiTi shape memory alloy on the commercially pure titanium substrate using the electron beam freeform fabrication (EBF3) technique. The principle of phase constitution, microstructure evolution, martensitic transformation behavior, mechanical properties, and the functional response was studied in detail. The stable regions of as-deposited NiTi alloy were composed of B2 austenite at ambient temperature, and a small amount of B19 ' martensite also was observed. Meanwhile, the nano-scaled Ti4Ni2Ox precipitate existed in the interdendritic and grain interior of the EBF3-fabricated NiTi alloy. The stable regions of EBF3-fabricated NiTi alloy presented a single reversible martensitic transformation (B2 <-> B19 ') during the cooling and heating process. In addition, the solidification characterization, such as microstructure evolution and crystallographic orientation in the building direction and horizon direction, brought about certain differences in the mechanical properties and functional response, such as tensile strength, elongation, superelasticity, and damping capacity. The corresponding intrinsic mechanism has been discussed. Our work also provides a promising method for the flexible manufacture of dense shape memory alloys with large scale and complex geometry, which is beneficial to expanding their application fields.