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Introduction

Welding is an important process for fabricating dental prostheses. In the early 1990s, laser-welding was introduced in Germany . In recent years, laser-welding has become an essential technique to repair and add separate metal segments of the frame work for metal prostheses .

The advantages of laser-welding are that the parent metals can be welded without solder and that the parent metal can be used as a solder, if necessary. Corrosion resistance is not reduced by using the same metal for the solder as for the parent metal, and the mechanical strength of the weld joint will not suffer . Zupancic et al. reported that the corrosion resistance of laser-welded joints was better than of brazed ones, primarily due to differences in passivation ability. According to the potentiodynamic curves obtained, the curves of intact CoCr alloy and laser-welded joint CoCr were similar and indicated a nearly passive state, while the potentiodynamic curves corresponding to the brazed joint CoCr showed no distinct passive region. Since laser energy can be concentrated in a small area, there are fewer effects of heating and oxidation on the area surrounding the spot to be welded . It is easy to weld dental prostheses using this method since no additional materials, such as investment material or gas torches that are used for conventional dental soldering, are needed . Laser welding has accelerated the use of Ti and its alloys for prostheses since conventional soldering techniques cannot be used for joining Ti materials because of oxygen contamination during the soldering procedure .

The wave length, peak pulse power, pulse energy, output energy, pulse duration, pulse frequency and spot diameter of the laser affect the mechanical strength of joints . In most of the laser-welding machines used for dentistry, the output energy (current or voltage), pulse duration and spot diameter of the laser can be adjusted. The success or failure of the laser-welding procedure is affected not only by these physical parameters but also by the operator’s experience, dexterity, eyesight and knowledge of the machine . Eyesight influences the adjustment of the laser beam focusing. Currently operators have to make fine adjustments during laser welding, although manufacturers claim the laser-welding technique to be a new, rapid, economic and accurate method for joining metal.

Edge preparation, penetration welding, two-side welding and overlapped welding are common techniques used in welding dental prostheses. Iwasaki et al. reported that one-sided welding caused distortion of specimens and two-side welding decreased this. Watanabe et al. reported that double-welded specimens showed significantly greater joint strength compared to single-welded specimens. Two-side welding is a common technique that does not cause distortion of specimen . The CoCr/CoCr (normal) and Ti/Ti (normal) are commonly used by dental laboratory technicians. Unorthodox techniques, such as no edge preparation and no overlapped welding were added as welding conditions evaluated in the current study. There are some situations when a metal framework is too thin to prepare an edge. Specimens with intentional welding defects, “crack” and “spatter”, were also fabricated. These welding defects can be reproduced by controlling parameters such as current, pulse duration and spot diameter. Cracks appear in a welded area due to thermal residual stress during the welding procedure and/or changes in microstructure that affect the quality of the welded parts .

The presence of inclusions and voids in the welded area is a critical issue during the welding process . In SEM micrographs of fracture surfaces after tensile testing, many small pores were observed in welded regions . Internal defects and porosity in cast structures have been assessed by radiographic and metallographic analyses . Radiographic analysis evaluates only the presence of porosity, but the quantity of porosity cannot be determined. Metallographic analysis of the specimens cannot be performed non-destructively. These analyses are two-dimensional evaluation methods. In contrast, an X-ray micro-focus computerized tomography (micro-CT) can evaluate the porosity three-dimensionally (3D) . Micro-CT, a high-resolution variant of medical CT, allows non-invasive mapping of the microstructure in 3D with spatial resolution approaching that of optical microscopy.

The purpose of this study was to evaluate the porosity of specimens welded under various welding conditions non-destructively using micro-CT. This study was based on the following two research hypotheses. The first was that many air bubbles would not combine in the laser-welded area. The second was that porosity in the laser-welded area would not affect the joint strength of laser-welded alloys.

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Materials and methods