Structure of a 3D frame-bridge NiTi sample deposited on a low carbon steel substrate by wire arc additive manufacturing

A 3D frame-bridge sample was produced by wire arc additive manufacturing (WAAM) on a low carbon steel substrate using the Ni50.9Ti49.1 shape memory wire with a diameter of 1.2 mm. The sample consisted of a rectangular frame and three bridges. The structure and chemical composition were studied in different zones: the frame, the bridge or joint of the frame and the bridge using light and scanning electron microscopy with energy dispersive X-ray spectroscopy. It was shown that the structure of the frame and the bridge located far from the joint was close to the “walls” produced by WAAM: a columnar grain grew across the layers and the equiaxed grains appeared on the top of the layer. The structure of the joint between frame and bridge significantly differed from the “walls”: from the frame side, columnar grains were found across and alone the layers, whereas, from the bridge side the columnar gains were observed in the first layer only. The study of the chemical composition showed that the Fe and C elements diffused to the sample from the low carbon steel substrate. As a result, TiC precipitates appeared in all layers that led to the alloy hardening. Fe atoms penetrated to the NiTi phase that suppressed the martensitic transformation.