microscopic analyses of welding defects in rpd technology

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Figure 2. Aspects of RPD metallic surfaces processing (resolution 6 MP, objective 40X / magnification 400X): a. sandblasting; b. mill processing; c. polipanto processing; d. polishing _____________________________ 26 TMJ 2010, Vol. 60, No. 1 Laser welding parameters mixing up is a very complex operation, which depends on alloys composition, welding procedure, thickness and profiles of cast samples. In fact, some of the macroscopically faultless welding contained hidden pores or cracks, which compromised the entire joint. Welding defects and cracks were discovered only at the microscopic analyses. Figure 3. Aspects of welding spots (power/ time/ frequency) at different parameters (resolution 6 MP, objective 20X, magnitude of the zoom 200X): a. little weld spot 1,6/1,4/1; b. little weld spot 1,7/1,4/1; c. medium spot: 1,7/1,6/1; d. medium spot ; e,f. welding with filler material 1,9/1,6/1; g. big spot 2/1,7/1; h. big spot 2,1/1,7/1; i,j. fracture in welding point after welding with three variants of spots These defects are mainly connected to spot size and welding power. So, the use of a smaller spot size and a lower power than the one established by the
producer can generate faultless welding points. Most of the cracks appear at a large spot size/ high welding power combination, in the process of smoothing the welding. When cracks continue from one welding point to another, it generally determines the break of the entire joint. Therefore, the highest attention is to be paid for testing a faultless welding. Figure 3 shows images captured with help of metallographic microscope video camera, which permitted the welding quality evaluation. In 72% of the cases, the cracks were found inside the welding points, but at least apparently, not always affected the weld mechanic resistance. discUssion Wulfes, Wataha, 1,4 who have vast experience, admitted the good weldability of CoCr alloys. They used welded joints for the RPD components, with clasps and also with different precision attachments, individual milled attachments. Two thirds coverage of the previous spot is ok. One considers that the complementary material has to be 45 0 headed towards the laser beam and the processed object. Also, the absorption and reflection phenomena influence the weld efficacy with chosen parameters. The blind, non retro-reflecting surfaces increase the efficiency, while those retro-reflecting (polished crowns) decrease it. In order to obtain good results, the weld parameter has to be set for each situation. Bertrand 6,7 studied 3 alloys for different types of dentures; his results showed a maximum penetration depth of 1-2 mm, which represents one of the most usual depths of the components that must be repaired. The surface structure influences the penetration rate- a shiny surface decrease laser beam penetration. Because of the laser effect, the weld zone suffered an important microstructure change- micro hardness increasing. Craig 2 opined that, „brazing is a process in which a molten filler metal wets and fills the gap between the parent metal surfaces. The filler metal has a lower melting point than the parent metal. In welding, the parent metals fuse and form the joint with or without a filler alloy”. When using filler material, half of the laser beam must fall on the wire, the rest of it- on the parent metal. Thus, we can obtain a blend of homogeneous molten material. In order to avoid cracks, pulse duration is reduced. If that is not enough, one decreases the electric tension and increases the focal diameter. Kakimoto Kazutoshi 9 evaluated the laser weldability of 7 dental CoCr alloys. Spot welds were formed on the surfaces of specimens with a pulsed YAG laser. Observation of the microstructure and the fracture surface suggested that the cracks were solidification cracks. These cracks were probably caused by the addition of elements, like silicon and carbon. Although the alloy laser weld had decreased hardness, neither cracking nor porosity was generated. The determiner factors for welding quality, respectively the operator ability of choosing the welding parameters (power, pulse duration, energy) were investigated also by Watanabe. 13-15 Regarding the inert gas atmosphere- argon, there are studies that show it isn`t absolutely necessary in case of CoCr alloy welding. The same author opine that, welding without filler material is recommended only if the gap is smaller than 0.5 mm. Cleaning of surfaces, their abrasion (2- 3mm) and filler material adjustment, is indispensable for obtaining a perfect joint. The increased thickness of the pieces to be welded has a negative influence on the weldability, because a large metallic mass cause rapid cooling of the weld and HAZ, which can generate fragile structures or even cracks. The big internal tensions (result of contraction) can lead to cracks or braking during the function in the oral cavity. Frentzen, Koort, Yamagishi, Neumann and Dobberstein 8,15-17 examined the fracture surfaces and showed that the laser welding technique was much more effective in the peripheral than in the central parts of the specimens. The tensile strength of the laser-welded joints was significantly lower than that of the brazed joints, mainly due to the smaller crosssection of the welded joints and partly due to the relatively strong brazed joints. SEM analysis revealed localized phenomena, such as pitting near the joints and pronounced corrosion in some defects on surfaces. In case of localized corrosion, and over longer periods of time, the process could become autocatalytic and more pronounced than in the study. The laser welding process could be improved by increasing the weld penetration depth. Preparation of the areas to be welded can reduce laser beam reflection and probably improve the welding efficiency. Zupancic et al. 5 opined that, large joint surfaces might ensure sufficient strength and limited thickness might enable complete joining with minimum porosity. The longevity of RPD frameworks is limited due to the mechanical or corrosive failure of the joints. The purpose of their study was to determine which joining method offers the best properties for CoCr alloy frameworks. When laser welding was used, successful joining was limited to the peripheral aspects of the weld. The welding technique did not affect significantly the joint tensile strength. Electrochemical _____________________________ Cristina Bortun et al 27
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