Effect of Nb alloying on wire arc additive manufacturing NiTi-based shape memory alloys

NiTi alloy is a widely used shape memory alloy, in which Ti₄₇Ni₄₄Nb₉ alloy is a reliable material for aviation pipe coupling. In this research, Ni₅₂Ti₄₈ alloy was fabricated by double wire arc additive manufacturing (WAAM) method and Ti₄₇Ni₄₄Nb₉ alloy was obtained by in situ alloying with Nb element. The typical microstructure, compression properties, phase transformation temperature and shape memory effect of the alloy were studied. The effects of Nb element addition on the microstructure and properties of WAAM nickel-titanium alloy were analyzed. The results show that the microstructure of NiTi alloy fabricated by WAAM is characterized by coarse B2 equivalent axonal grains in the lower part, columnar B2 phase dendrite in the middle part, and relatively fine B2 equivalent axonal grains in the top part. Besides B2 phase grains, fine β-Nb phase precipitates at the grain boundary after the addition of Nb element, which makes the compress strength increased by 7.9% and 3.1% respectively in transverse and longitudinal directions, while the shape memory recovery ratio decreased by 4.0%,which is due to the fact that the plastic deformation of β-Nb particles precipitated between grain does not recover during loading, and the hysteresis of phase transformation temperature increased from −6.4 ℃ to 40.9 ℃, which makes shape memory alloy more convenient for storage and assembly when used as pipe coupling.