Microstructures and mechanical properties of NiTi shape memory alloys fabricated by wire arc additive manufacturing

Wire arc additive manufacturing (WAAM) has been proved to be a promising method to fabricate large expensive NiTi shape memory alloys with complex geometry. In this study, five layers of NiTi alloy with dimensions of 80 x 12 x 15 mm were deposited by the cold metal transfer (CMT) welding based WAAM process. The macroscopic morphology, microstructure evolution, phase transformation and mechanical properties of each layer were investigated and compared with each other. Each layer of the as-deposited NiTi wall shows a one-step B2 -> B19' transformation during cooling and the martensitic transformation temperature (M-s) is lower than that of the as-received wire. For the first three layers, the columnar grains growing along the building direction have a larger length-to-diameter ratio and B2 is the main constituent phase. Coarse dendritic Ti2Ni/Ti4Ni2O precipitates are precipitated at the interior of the grains besides grain boundaries. However, grains in the subsequent layers become finer and the region near the top surface are characterized by equiaxed grains, and elliptical Ti2Ni/Ti4Ni2O precipitates mainly concentrate at grain boundaries. The heat-affected zone exhibits the lowest hardness of 233.34HV, followed by a gradually increase among the as-deposited layers and the highest hardness of 331.27HV was obtained at the 5th layer. The highest ultimate tensile strength of 652.46Mpa, together with the elongation of 13.66% and recoverable strain of 2.39% was obtained in the sample which is located at 11.5 mm height away from the substrate. This research provides an innovative method and insights for additive manufacturing NiTi shape memory alloys by introducing CMT based WAAM method. (C) 2021 Published by Elsevier B.V.