Comparative study on crystallographic orientation, precipitation, phase transformation and mechanical response of Ni-rich NiTi alloy fabricated by WAAM at elevated substrate heating temperatures

In this investigation, a Ni-rich NiTi alloy was in-situ deposited with different substrate heating temperatures and the evolution of crystallographic orientation, precipitation, phase transformation, and mechanical responses were evaluated. The experimental results indicated that with the increment of substrate heating temperature from 150 °C to 350 °C, the average B2 grain size and the high angle grain boundaries (HAGBs) gradually increased from 53.44 μm to 85.38 μm and 53.6%–62.4%, respectively. The crystallographic texture exhibited a dominant, strong (001) orientation with comparatively weak (111) and (101) orientations in all conditions and the intensity of {100}<001> increased slightly as the substrate heating temperature increased. Moreover, Ni4Ti3 precipitates with an inhomogeneous size distribution were identified within the B2 NiTi matrix. Increasing the substrate heating temperature coarsened the Ni4Ti3 precipitates. All the phase transformation temperatures increased when the substrate heating temperature increased, indicating that the martensitic transformation is more likely to occur. As the substrate heating temperature increased from 150 °C to 350 °C, the yield stress and ultimate tensile stress decreased from 683.9 to 513.1 MPa and 855.2 to 743.8 MPa, respectively, and the ductility decreased from 6.90% to 6.13%. In addition, a remarkable εir, poor recovery ratio and a broad stress hysteresis were obtained during the initial deformation of the cyclic loading-unloading tension. The highest recoverable strain (εre), recovery ratio and elastic energy storage efficiency (ƞ) were obtained in samples processed with the lowest substrate heating temperature. These findings provide useful references concerning process optimization in fabricating Ni-rich NiTi components by WAAM with acceptable microstructure and mechanical properties.