The Effects of Temperature and Time of Heat Treatment on Thermo-Mechanical Properties of Custom-Made NiTi Orthodontic Closed Coil Springs

Abstract

Nickel-Titanium (NiTi) springs have been increasingly used in orthodontics; however, no optimum condition of heat treatment has been reported. Therefore, this research was conducted to determine the optimum heat-treatment temperature and duration for the fabrication of NiTi-closed coil springs by investigating their effects on thermo-mechanical properties. As-drawn straight NiTi wires of 0.2 mm diameter were used to fabricate closed coil springs of 0.9 mm lumen diameter. The springs were heat-treated at three different temperatures (400, 450, and 500 °C) with three different durations (20, 40, and 60 min). Electron Probe Micro-Analysis (EPMA) and Differential Scanning Calorimetry (DSC) were used to investigate element composition and thermo-mechanical properties, respectively. Custom-made NiTi closed coil springs composed of 49.41%-Ti and 50.57%-Ni by atomic weight, where their DSC curves of 500 °C presented the obvious endothermic and exothermic peaks, and the austenite finish temperature (Af) were approximately 25 °C. With increasing temperature, deactivation curves presented decreased plateau slopes generating higher superelastic ratios (SE ratios). At 500 °C, closed coil springs showed superelastic tendency with lower stress hysteresis. The thermo-mechanical properties were significantly influenced by heat-treatment temperature rather than duration. The optimum parameter appeared to be 500 °C for 40 min to produce appropriate force delivery levels, relatively low plateau slope, and lower hysteresis for orthodontic use.

Mechanical Performance and Microstructural Evolution of Rotary Friction Welding of Acrylonitrile Butadiene Styrene and Polycarbonate Rods

“…The heat capacities for the friction-welded parts welded by rotation speeds of 330 rpm, 490 rpm, 650 rpm, 950 rpm, and 1350 rpm are −1.002 mW/mg, −0.8127 mW/mg, −0.626 4mW/mg, −1.759 mW/mg, and -2.287 mW/mg, respectively. The DSC curve for the friction-welded parts welded by a rotation speed of 1350 rpm shows an endothermic peak [31] between 400 to 440 °C and an exothermic peak [32] between 600 to 700 °C. This means that higher rotation speed contributes to higher molecular orientation in the weld joints, showing that the friction-welded parts have better mechanical properties.…”

Multi-Force Bio-Active™ Archwires and Various Contemporary NiTi Multi-Force Archwires: Properties and Characteristics—A Review

“…Because DSC enables the determination of the phase transition temperature and the amount of energy emitted or absorbed during the heating or cooling processes, it is one of the most helpful techniques for assessing the TTR of orthodontic archwires [ 42 , 43 ]. These investigations focus on the relationship between temperature variations and the thermomechanical characteristics of NiTi alloy, particularly the phase changeover temperatures [ 17 , 44 , 45 , 46 , 47 , 48 ]. In addition to influencing the mechanical properties, specific heat treatment in the designated areas of the archwire results in a change in the modulus of elasticity and the distinct transition temperature range (TTR) in the three locations (the lowest in the frontal area, increasing at the premolar, and the maximum at the molar).…”

Optimizing ageing conditions for commercial NiTi archwires: Insights from thermal phase transformation and tensile deformation analysis

“…Differential scanning calorimetry is the technique used to determine the crystalline phase of NiTi wires, as well as the transition temperature. Martensite start, 𝑀 𝑠 , martensite finish, 𝑀 𝑓 , austenite start, 𝐴 𝑠 and austenite finish, 𝐴 𝑓 are the phase transformation temperatures that can be examined using differential scanning calorimetry [7,8]. Specifically, for orthodontic wires ANSI/ADA Standard No.…”