Abstract
BackgroundThis 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. 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. In addition, the hardness values in places beyond the fusing zone and the HAZ were measured as being quantitatively lower.MethodsThe alloys used in this study were Verabond 2 V, Noritake Super, and Bellabond Plus, which are commercially used non-precious dental alloys. The specimens were cut to have a plate shape with a size of 0.5 × 3.0 × 2.5 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 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. 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.ResultsThe 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.ConclusionsThese results suggest that each Mo and Si combined inter-metallic compounds were formed on the interdendritic area. Through this study, the laser fusing zone had better hardenability due to the inter-metallic compound and grain refinement effect.
Highlights
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
It has been known that the fusing zone and the surrounding heat-affecteded zone (HAZ) experience changes in both their microstructure and hardness at the penetrated fusing zone due to the rapid cooling [9]. While this type of change in the metallurgical characteristics due to laser irradiation has been reported, this study aims to discuss the metallurgical characteristics that are changed by Nd:YAG laser welding used in the casting of a dental alloy by setting a laser spot diameter size with respect to defocusing distances, which can be selected generally in the laser welding
This study reviewed hardness change according to penetration depth in the laser fusing zone and
Summary
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. 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 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. It can 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. These shortcomings in the existing soldering methods can be overcome using Nd:YAG laser welding [2, 3].
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