Welding of Titanium using Normal Pulse Nd-YAG Laser

Abstract

Soldering has been chiefly used for jointing prosthetic materials. Titanium is very difficult to solder because it is highly susceptible to oxidation. The recent development of an infrared heater soldering system is expected to make titanium soldering easier. However, because this soldering system uses a solder different from the conventional solder, it may cause problems in the oral cavity such as low resistance to corrosion. Titanium soldering requires prior fixation with the investment material. Therefore, the authors recently attempted laser welding of titanium.
In this attempt, normal pulse Nd-YAG laser (ML-2310A, Miyachi Technos Co.) was used. Laser welding was first used to join 2 titanium cast crowns together and then 2 round tita nium rods (2mm in diameter) together under the following settings: voltage, 300V; pulse width, 6ms (for crowns) or 5ms (for round rods); output, 7J (for crowns) or 6J (for rods). The round rods were rotated during welding. The focus spot diameter was 0.6mm. Laser was applied to the materials while spraying argon gas onto them.
In both cases, laser welding resulted in glossy and strong union. Because the spot diameter was small, the heat-affected area was very narrow. Therefore, direct welding on a working model was possible, without necessitating fixation with an investment material. In case of round rods, it was possible to hold one end of both rods between fingers during laser welding. Because laser welding is a kind of sweating, it is expected to involve no risk for reduction of anti-corrosion resistance inthe welded area.
The weld strengths of titanium plate specimens obtained by two different welding techniques were compared. With a laser beam (70mm in focal distance, 0.8mm in diameter), two titanium plates (40×10×3mm), superimposed each other 2mm at the end, were welded either by lap seam welding or by spot welding. The load required to tear off the joint by pulling for the spot-welded specimen was 56kgf in average, while that for the lap-seam-welded specimen was more than 106kgf in average, where the breakage of the base metal often arose in the proximity of the weld.
A major factor hampering the utilization of titanium for prosthesis is the difficutly in processing. If laser welding is introduced to titanium, the dental application of titanium will widen.