Once the laser heat source that is converged by the laser beam is irradiated to the surface in the pre-determined axis direction, only a local layer on the surface is heated by the thermal conductivity that occurs inside, and the heat on the surface is rapidly cooled down as soon as the laser beam is removed. The absorbed part is changed into heat energy from the metal surface, thereby producing an important heat source for laser welding.

The amount of laser beam absorbed into the surface of the metal can be different depending upon the wavelength of the laser beam surface roughness and the thermal conductivity of the metal. Only those laser beams that are absorbed are used for material processing. When laser welding is performed, some of the laser beams are reflected from the surface of the metal, while the rest of the beams are absorbed into the metal. As such, laser welding has been proven to be a more effective technology than existing soldering methods. It is also performed under an Argon atmosphere so that it has the advantage that oxidation is minimized in the area surrounding the diameter where the laser is irradiated. Since laser welding concentrates energy on a small area, it creates a small HAZ. These shortcomings in the existing soldering methods can be overcome using Nd:YAG laser welding. It can also cause increased corrosion due to the direct contact between the deposited metals and the other different types of soldering materials, resulting in the reduction in strength of the joint parts.

The soldering methods generally used in existing dental fields inevitably introduce gas into the soldering connecting part, thereby leaving bubbles and degrading the strength of the connecting parts using gas also results in the formation of a wide heat-affected zone (HAZ), which can change the particle size of the joint metals and decrease their tensile strength. Welding is required when metals need to be joined for a variety of reasons, such as re-attachment of the severed prosthesis or fractures in order to fix the deformation or errors after casting during the manufacturing of the metal structures of the fixed prosthesis. The importance of joining the connecting parts of casts for prosthesis manufacturing in the dental field has increased significantly. Through this study, the laser fusing zone had better hardenability due to the inter-metallic compound and grain refinement effect. These results suggest that each Mo and Si combined inter-metallic compounds were formed on the interdendritic area. The observation result of the diffusion of the constituent elements and microstructure using field emission scanning electron microscopy, energy dispersive spectroscopy (EDS), and electron probe micro-analyzer showed that the fusing zone revealed a much finer dendritic form than the base metal due to the self-quenching effect after laser melting, while no change in constituent elements was found although some evaporation of the main elements was observed.

After the laser irradiation, a cross-section of the fusing zone in the specimens was observed in terms of laser melted depth, hardness depth profile, and the microstructure of each alloy. The laser was then irradiated as the spot diameter condition varied between 0.5 mm and 1.4 mm in accordance with defocusing distance from 0.0 mm to 2.0 mm. This was followed by setting the Nd:YAG laser output, pulse duration, and frequency to 60 W, 10 ms, and 5 Hz, respectively. The specimens were cut to have a plate shape with a size of 0.5 × 3.0 × 2.5 mm. The alloys used in this study were Verabond 2 V, Noritake Super, and Bellabond Plus, which are commercially used non-precious dental alloys. In addition, the hardness values in places beyond the fusing zone and the HAZ were measured as being quantitatively lower. In all groups, the hardness depth profiles in the laser fusing zone and heat-affecteded zone (HAZ) had larger values than those of the base metal. This study aimed to determine the effect of hardness change according to penetration depth in the laser fusing zone and observed the correlation of the microstructure as an Nd:YAG laser was irradiated to Ni-Cr alloy for dental use by setting the spot diameter size with respect to defocusing distances.

Arloy nonprecious high fusing metal