Abstract: Wire arc additive manufacturing (WAAM) is used for fabrication of NiTi parts using a dedicated Ni-rich(50.50 at.%) NiTi wire as the feedstock material. The microstructure, phase transformation characteristics and mechanical properties of the as-built parts are investigated. Experimental results show that the microstructure in each deposition layer is different as a result of the different thermal cycle conditions. Along the part height,the first deposited layer is larger equiaxed grains. As the heat input gradually accumulates, the grain growth tends to the finer equiaxed form, and there are columnar grains between two deposited layers. The as-built parts are completely austenite phase at room temperature. The Ni content of deposited layers which have higher hardness, wider transformational range and hysteresis compared to the as-received NiTi wire is 51.10 at.%. The tensile strenght of the as-built parts is of about 611.30 MPa with a corresponding fracture strain of 19.50%. The fracture surface with dimple fracture had good ductility. Additionally, it can be seen that the WAAM specimens exhibit good superelastic properties, evidenced by a low irrecoverable strain of 1.01% upon unloading in the first cycle. The plastic strain increases during the first 8 cycles, and reaches near a constant value of 2.68%.

Key words: wire arc additive manufacturing, NiTi shape memory alloy, microstructure, phase transformation, mechanical properties