Microstructure and Mechanical Properties of NiTi Alloy Prepared by Double-Wire + Arc Additive Manufacturing Plus In Situ Heat Treatment.

In this study, NiTi shape memory alloy was prepared by double-wire + arc additive manufacturing plus in situ heat treatment using TA1 and ER-Ni welding wires as the raw materials. The results show that the microstructural evolution from the bottom to top is NiTi2 + NiTi → NiTi + Ni3Ti + Ni4Ti3 → NiTi + Ni4Ti3 + Ni3Ti2 + Ni3Ti + α-Ti. Complex thermal cycles led to the precipitation of Ni3Ti, which improves the hardness of the matrix (B2), and the average hardness value of the top region reaches 550.7 HV0.2. The fracture stress is 2075 ± 138.4 MPa and the fracture strain is 11.2 ± 1.27%. The sample shows 7.02% residual strain and 5.87% reversible strain after 15 cycles, and the stress hysteresis decreases with an increase in cyclic strain.