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F. M. Braz Fernandes^a, P.F. Rodrigues^a, R. Magalhães^a, E. Camacho^a, P. Inácio^b, T. Santos^b, N. Schell^c

^aCENIMAT / I3N - Departamento Ciência dos Materiais, Universidade Nova de Lisboa / FCT, 2829-516 Caparica, Portugal

^bUNIDEMI – Departamento de Engenharia Mecânica e Industrial, Universidade Nova de Lisboa / FCT, 2829-516 Caparica, Portugal

^cHigh Energy Materials Science, PETRA-III, DESY, Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max Planck-Str. 1,21502, Geesthacht, Germany

The introduction of graded functionality in commercial superleastic orthodontic archwires (Morelli SE) was analyzed. There are few studies concerning the manufacturing of the functionally graded Ni-Ti orthodontic archwires reported in the literature. The load developed by these wires depends on the geometrical characteristics, the temperature and the strain. Normally, the conventional archwires generate constant forces in a wide range of displacement during the orthodontic treatment [1]. In order to have different forces of actuation in the incisive region (lower) and in the molar region (higher), different fabrication strategies have been proposed, such as (i) laser welding different wire segments [2], or (ii) separate heat treatment of different segments in a special furnace [3]. In the present study, a functional gradient was introduced in a superelastic Ni-Ti orthodontic archwire (Morelli - SE) by localized heat treatment using Joule heat effect. DSC measurements at different positions of the wire show that a microstructural gradient is present in such treated archwires (Fig. 1-a). A localized analysis along the heat-treated archwire, using synchrotron radiation based X-ray diffraction (SR-XRD) was used to identify the microstructure of the heat treated archwire with a finer spatial resolution (Fig. 1-b).

Figure 1 - Ni-Ti orthodontic archwire heat treated at 300 ºC during 10 min:

(a) DSC curves of the three zones of the wire, where internal is the central part (16 mm) of the heat-treated zone (32 mm), transition is the intermediate zone (~8mm) between the central uniformly heated (16 mm) and non heat treated segment ( external). (b) SR-XRD along the Ni-Ti orthodontic wire.

Figure 2 – XRD analysis during full superelastic cycle: full scan of the gauge length at selected points of the stress-strain curve during the superelastic cycle.