October_2021_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 2 1 4 1 SELECTIVE LASER PROCESSING FOR FUNCTIONALLY GRADEDSHAPEMEMORY ALLOYMEDICAL DEVICES: PROCESS ANDAPPLICATIONS INDENTISTRY Novel methods for altering the mechanical properties of functionally graded orthodontic medical devices leads to new applications. Michael L Kuntz and M. Ibraheem Khan Smarter Alloys, Cambridge, Ontario, Canada F unctional grading of shape memory alloys offers design flexibility that expands the application space of these materials. Several technologies have been employed to modify the composition and/or mechanical properties over volume; however, significant defects are introduced during these processes that limit their commercial application, in- cluding cracking, porosity, oxidation, and lack of control [1-6] . In addition to the added defects, these processes are ex- pensive and difficult to scale. Recent progress in additive manufacturing technologies has shown promise in realizing functional grading of complex shapes and work continues to overcome obstacles to commercial scale processing [7] . The authors recently described the use of a laser process to selectively vaporize constituent elements from binary Nickel-Titanium (NiTi) alloys [8] . Selective vaporiza- tion enables local control of mechanical properties of NiTi-based shape memory alloys by shifting the austenite- martensite transformation temperature range. During the process, pulsed laser energy rapidly increases the local temperatures, partially vaporizing each constituent ele- ment. Owing to its relatively lower vapor pressure, nickel evaporates faster than other elements. Subsequent rapid cooling and solidification forms a columnar dendritic mi- crostructure with a decreased bulk nickel content, and corresponding higher transformation temperature. In contrast, the original base material surrounding the pro- cessed section remains mostly unchanged as shown in Fig. 1. The high spatial resolution of the laser process en- ables tight spatial control over the stiffness of a wire across very short distances. Because the transformation tempera- ture is inversely related to the pseudoelastic plateau stress, the effect of laser processing would be to locally decrease the stiffness. In shape memory actuation applications, the effect is a multi-stage actuation. The process has been de- scribed as Multiple Memory Material. The purpose of this article is to highlight recent applica- tions of selective vaporization to enable functional grading in medical devices for the dental industry. APPLICATIONS Orthodontic archwires: Since the earliest discussion of NiTi as a functional material in the 1960s, there has been significant interest in its use for biomechanical applica- tions [9] . By 1971, the first-ever application of NiTi in medi- cal devices was being investigated in an orthodontic arch- wire [10] . The first NiTi wires were fully cold worked and did not exhibit pseudoelasticity; however, the load-deflection rate was lower than stainless steel [11] . A NiTi archwire with pseudoelastic properties, ideal for providing continuous forces for long range tooth movement, was not realized until 1985 [12,13] when modern NiTi alloys with finely tuned thermo- mechanical processing enabled the close control of transfor- mation temperature. The need for functionally graded archwires was rec- ognized in 1981 [14] , before widespread use of NiTi in ortho- dontics, with the theory that varying the material properties along the archwire, rather than the wire cross-section, could be used to optimize delivery of orthodontic forces. The re- sulting archwire could deliver ideal force proportions based on equalized stress distributions in the periodontal ligament 4 Fig. 1 — Metallurgical cross-section of laser processed wire. 5 FEATURE