Structure, martensitic transformations and mechanical behaviour of NiTi shape memory alloy produced by wire arc additive manufacturing

Abstract

The gas metal arc welding (GMAW) based wire arc additive manufacturing (WAAM) process has been employed to deposit 5-layered NiTi alloy on the Titanium substrate using Ni50.9Ti49.1 wire as the feedstock. The heterogeneity of the piled up layers has been evaluated in terms of the variation in microstructure, composition and phases present. The melting of the Ti substrate under the first layer led to a substantial increase in Ti concentration in the melt during the deposition of the first layer and facilitated the formation of Ti-rich NiTi/Ti2Ni mixture during the solidification. In the 2nd – 5th layers columnar grains appeared in the inner space, whereas equiaxed grains formed on the top of the layers. The chemical composition of the 1st – 3rd layers differed from the nominal composition of the feedstock wire i.e. the layers in proximity of the substrate had lesser Ni concentration. As the result, the temperatures of the B2 ↔ B19’ martensitic transformation were different across the layers and the start temperature of the forward transformation changed from 73 °C (1st layer) to −16 °C (5th layer). Using the EDX and calorimetric data, the Ni distribution in each layer was determined and its influence on the martensitic transformation temperatures was discussed in detail. The difference in Ni concentration has made various layers to be present in different states (martensite or austenite) at room temperature. In this case, the layers (2–4) were deformed by different mechanisms during tension at room temperature. The deformation of the layers by reversible mechanisms was confirmed by the shape memory effect on heating of the pre-deformed NiTi sample produced by WAAM.