The Effects of Casting of Dental Cobalt-Chromium Alloy on Crack Initiation by Laser Welding

Abstract

The production of new dentures imposes financial and physical burdens on elderly patients ; it is preferable for them to keep wearing their old dentures after remodeling and repair. In such denture cases, metal part welding technology may be a problem. Laser welding is frequently applied to the junction of dental cobalt-chromium alloy, considering the shortness of the welding time and narrowness of the heat-affected zone (HAZ). However, welding defects, such as cracks and porosities, occasionally develop in the welding area. Therefore, this study investigated the influence of the casting of dental cobalt-chromium alloy on the occurrence of cracks. Cobalt-chromium alloy ingots for casting (Cobaltan) and castings were used in this study. Using a phosphate bonded investment material (Snow white), castings were produced using a casting machine (Argon caster AE). A dental Nd :YAG laser welding apparatus (Alpha Laser ALP50) was used for welding. The irradiation energy was set to 10.6 or 30.2 J/pulse, and spot welding areas were formed in an argon gas atmosphere. Thereafter, to evaluate the occurrence and causes of cracks, metal tissues of the base material and welded areas were observed using an optical microscope. The fracture surface was observed using a scanning electron microscope (SEM). Element concentration analysis was performed using an energy dispersive X-ray spectrometer (EDX). The metal tissue of the welded area was finer than that of the base material. When the irradiation energy was 10.6 J/pulse, cracks were found in 5 of 15 spots on the ingots, and in 14 of 14 spots on the castings. When the irradiation energy was 30.2 J/pulse, cracks were observed in 1 of 14 spots on the ingots, and in 16 of 18 spots on the castings. Cracks in the welds were more frequently observed in the castings than in the ingots. Observation of the metal tissue and fracture surface showed that those cracks were solidification cracks. It was concluded that casting was the main cause of the solidification cracks. Furthermore, phosphorus may be a cause of the solidification cracks. (J. Jpn. Soc. Laser Dent. 23:124 ~ 130, 2012 Reprint requests to Dr. KUSUNOKI) Key words= Laser welding, Cobalt-chromium alloy, Casting キーワード=レーザー溶接部,コバルトクロム合金,鋳造