Multi-material wire arc additive manufacturing of a bio-inspired heterogeneous layered NiTi/Nb/Ti6Al4V structure: Microstructural evolutions and mechanical properties

Abstract

Obtaining a reliable NiTi–Ti6Al4V(TC4) heterogeneous alloy component exhibiting excellent strength and elasticity via integrating distinct material benefits can provide design flexibility in multifunctional device fabrication. However, the fabrication of this materials couple is challenging due to the development of excessive Ti2Ni intermetallic compound. Inspired by the bioheterogeneous layered structure, Nb interlayer was applied for the wire arc additive manufacturing (WAAM) of NiTi and TC4 to restrict the mixing of alloying elements on both sides. However, the strength of NiTi–Nb-TC4 alloy component was limited by pure Nb characteristics when the NiTi and TC4 was completely separated by the unmolten Nb interlayer. In this work, the tensile strength perpendicular to the interface of NiTi–Nb-TC4 alloy component was enhanced to introduce appropriate amount Ni and Ti in Nb interlayer by adjusting the composite WAAM process at the expense of Ti2Ni. Microstructure evolution, composition change and mechanical properties of composite WAAM NiTi–Nb-TC4 alloy component were investigated. It was shown that the compression strength and fracture strain perpendicular to the interface of WAAM NiTi–Nb-TC4 sample was 1549.0 ± 17 MPa and 22.3 ± 8 %, and the compressive strength can reach 63.6 % and 94.5 % of WAAM NiTi alloy and TC4 alloy, respectively. The composite WAAM NiTi–Nb-TC4 sample maintained 65.3 % recovery rate after ten compression cycles, which confirmed that partial superelasticity was maintained in the WAAM NiTi–Nb-TC4 multilayer structure. This fact further proved that WAAM can be successfully applied to deposit shape memory/superelastic alloys. The use of the Nb interlayer enhanced the tensile strength perpendicular to the interface to 460.7 ± 16 MPa. The ultimate tensile strength perpendicular to the interface of composite WAAM NiTi–Nb-TC4 sample can reach 72.0 % and 60.0 % of WAAM NiTi alloy and TC4 alloy, respectively. The formation of NiTi, TiNb and β-Nb phases in Nb interlayer and the network distribution of Ti2Ni and TiNb phases explained the improved mechanical performance of the composite WAAM NiTi–Nb-TC4 alloy component.