| Thermo-mechanical and microstructural study has been conducted to examine the effects of annealing, deformation and laser heat treatment on commercially available NiTi alloy dental arch wires in order to improve their shape memory property, surface and mechanical properties.; Changes in mechanical and thermal properties are investigated after various annealing (200-600{dollar}spcirc{dollar}C for 10-120 min) and deformation conditions (uniaxial prestrains of 0-16% and prebending with radius of 2 and 3 mm). Also laser heat treatment on the surface of the NiTi wire were made with Nd-Yag (max 400 W) and CO{dollar}sb2{dollar} laser (max 150 W). Microstructures were observed with optical and scanning electron microscope (SEM). Chemical composition was measured with energy dispersive x-ray microanalyzer (EDX). Hardness was measured with a microhardness tester. Thermal properties such as transition temperature A{dollar}sb{lcub}rm s{rcub}{dollar}, A{dollar}sb{lcub}rm f{rcub}{dollar}, M{dollar}sb{lcub}rm s{rcub}{dollar}, M{dollar}sb{lcub}rm f{rcub}{dollar}, and {dollar}Delta{dollar}H (shape recovery starting and finishing temperatures, martensite transformation starting and finishing temperatures, and enthalpy change respectively) were measured with a differential scanning calorimeter (DSC).; The results indicate that (1) elastic modulus shows testing temperature dependence as E = aT + b, (2) transition temperature depends on annealing time and laser duration time (transition temperature = k t {dollar}sp{lcub}rm n{rcub}{dollar}) and on annealing temperature (transition temperature = a T + b), (3) samples with larger annealed grains show the higher transition temperatures and the wider transition temperature range (TTR), (4) uniaxial deformation of more than 8% strain shows increased shape recovery temperature and broadens the TTR.; As results of laser surface treatment on NiTi samples, almost pure titanium surface film, laser melted zone (LMZ) and heat affected zone (HAZ) are produced. Their dimensions can be controlled by changing laser treatment conditions. Ti surface film formation mechanism is interpreted as rapid solidification of nucleated Ti. The back scattered electron microscope image of Nd-Yag laser treated samples show that LMZ has better chemical resistance than original matrix. Hardness of LMZ is decreased from 400 to 230 DPH by Nd-Yag laser treatment and increased higher than 400 DPH by CO{dollar}sb2{dollar} laser treatment because of the formation of fine grains. |
