Structure, martensitic transformations and mechanical behaviour of NiTi shape memory alloy produced by wire arc additive manufacturing N. Resnina a, * , I.A. Palani b , S. Belyaev a , S.S. Mani Prabu b , P. Liulchak a , U. Karaseva a , M. Manikandan b , S. Jayachandran b , V. Bryukhanova a , Anshu Sahu b , R. Bikbaev a a Saint-Petersburg State University, 7/9 Universitetskaya nab., Saint-Petersburg, 199034, Russia b Discipline of Mechanical Engineering, Indian Institute of Technology Indore, Indore, 453552, India article info Article history: Received 4 July 2020 Received in revised form 19 August 2020 Accepted 22 August 2020 Available online 24 August 2020 Keywords: Additive manufacturing Wire arc additive manufacturing Shape memory alloys NiTi Martensitic transformation 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 Ni 50.9 Ti 49.1 wire as the feed- stock. The heterogeneity of the piled up layers has been evaluated in terms of the variation in micro- structure, 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/Ti 2 Ni mixture during the solidification. In the 2nd e 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 e 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 temper- atures of the B2 4 B19’ martensitic transformation were different across the layers and the start tem- perature 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 (2e4) 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. © 2020 Elsevier B.V. All rights reserved. 1. Introduction Shape memory alloys are widely employed in various engi- neering fields including aerospace, automobile, robotics, micro and nanoscale industry and medicine. These alloys have unique smart capabilities for the strain recovery/stress generation on heating (the shape memory effect) or unloading (superelasticity) or under magnetic field (magnetic shape memory effect) [1]. Such functional behaviour permits the shape memory alloys to be used as sensors, micromanipulators, actuators, coupling, implants and medical de- vices [2e4]. The shape memory alloys include different alloy sys- tems such as Cu-based, Fe-based, Heusler alloys and Ni-free alloys undergoing the thermoelastic martensitic transformation. How- ever, the best combination of mechanical properties (a yield limit for dislocation slip of 400e600 MPa, a strength of 800e1000 MPa, a plasticity of about 30e50% depending on the stress state and chemical composition) and functional behaviour (a shape memory strain of about 10% and a recovery stress of about 800 MPa) is observed in NiTi-based alloys [1]. At the same time, it is very difficult and expensive to subject the NiTi-based alloys to mechanical processing such as forming, mill- ing, turning, drilling etc. for the production of complex shaped components. In this regard, the fabrication of the NiTi shape memory alloy components using additive manufacturing (AM) * Corresponding author. E-mail address: resnat@mail.ru (N. Resnina). Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom https://doi.org/10.1016/j.jallcom.2020.156851 0925-8388/© 2020 Elsevier B.V. All rights reserved. Journal of Alloys and Compounds 851 (2021) 156851