Effect of laser welding on mechanical properties of round-rectangular nickel-titanium (NiTi) orthodontic wires

Objectives. To investigate the welding parameters on preparation of nickel-titanium (NiTi) orthodontic arch wires that can simultaneously level and retract anterior teeth by combining round wire to provide high flexibility with rectangular wire to provide high rigidity. The effects of voltage and number of welding points on tensile strength, transition temperature range (TTR), and penetration of laser beam were investigated. Materials and Methods. 15 mm segments of 0.018 in. round wire and 15 mm of 0.019 × 0.025 in. rectangular wire were laser welded under argon shielding. The laser voltage was varied between 170 and 210 V for one-point and two-point welding. Results. Incomplete welding was found only in one-point welding at low voltage while all others were joined completely. Mean maximum tensile force at fracture was increased with increasing voltage. One-point welding at 210 V gave rise to the highest force at fracture. Differential scanning calorimetry (DSC) of the welded joints showed a lack of the original TTR curve, indicating the loss of superelastic properties. Significance. Laser welding of NiTi orthodontic arch wires with different geometries provided acceptable strength for use in orthodontic treatment.