Influence of heat treatment on the structure and martensitic transformation in NiTi alloy produced by wire arc additive manufacturing

Abstract

The influence of heat treatment on the structure and martensitic transformation in the 5-layered NiTi sample deposited by wire arc additive manufacturing on Ti substrate was studied. The Ti2Ni phase was found on the boundary of the Ni-rich NiTi grains after annealing that was not typical for NiTi alloys produced by conventional techniques. This was due to the as-built sample contained Ti-rich NiTi phase in the interdendritic space. During annealing, this phase decayed to the NiTi and Ti2Ni phases and led to the formation of the Ti2Ni precipitates on grain boundaries. Annealing led to homogeneous composition of the NiTi phase within the layer but kept it to be different in various layers. This was caused by the Ti2Ni and Ni4Ti3 (Ni3Ti2) precipitates formed during annealing, but the diffusion of the Ti and Ni elements between layers hardly occurred. Annealing decreased the difference in Ni concentration in the NiTi phase of the 3rd to 5th layers that decreased the temperature range of the martensitic transformation. It was shown that the properties of the NiTi sample deposited to Ti substrate after post-production annealing are close to the properteis of the NiTi sample deposited on the NiTi substrate. © 2021