microscopic analyses of welding defects in rpd technology

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a significant dental alloys’ diversification in time, in connection with manufacturing technologies. The field of Dental Technology uses especially CoCr alloys, 3-5 which have ISO 22674(2006) guidelines: Type 4- for dentures with small sections subjected to great forces and Type 5- for dentures that need a great rigidity and mechanical resistance, also for RPD. The difference between these two types is related to the elasticity conventional limit, which is higher at type 5 (360 towards 500). Knowledge of dental alloys’ structure is necessary in order to optimize some casting and welding technologies. In order to reduce the costs and to increase the dentures lifetime, there appeared the reoptimization of metallic components with help of laser welding (the method of combining two materials or assembling of two materials in plastic or fluid phase, through their local marginal fusion, with or without material addition, with help of optic energy). _____________________________ 24 TMJ 2010, Vol. 60, No. 1 Figure 1. Materials and equipment used for our experiment: a,b. Alloys c. Welding equipment; d. Welded RPD Framework; e. Inverted Metallurgical Microscope OPK-6A BEL For about 15 years, one can talk about the advantageous and safe run of the laser welding in dental technology. 6-12 Laser, refused in the past because of high costs and lack of knowledge, has become today an aim of dental technology. The modern lasers, like those belonging to: BEGO (LaserStar T plus, LaserStar PW, LaserStar PW LYNX), GIRRBACH DENTAL SYSTEM (Neolaser L 126500), VISION
INDUSTRY (LWI 4th generation), INTERDENT Laser System (Herculea), ROFIN-SINAR Hamburg/ Germany (Macro, micro), MANFREDI, have electronic programming, microscopic visualization system and laser parameter adjustment, based on the welded alloy. The welding has maximum stability, gives a biocompatible zone with mechanic resistance and can be done very fast, with maximum accuracy, at cold. Bertrand 6,7 synthesized the advantages of laser welding: the welding is done directly on the cast, which permits a right aligning of the metallic framework fragments; it is possible to weld very close to the acrylic resin or ceramic, without any cracks or color damage; potentially, all metals can be joined but particularly titanium alloys; laser welding joints have a high strength for all metals, consistent with that of the substrate alloy; high mechanic resistance; reduced thermal influence, which involves minimum deformation; it can be used in different working phases in dentures technology. If one takes into account the right adjustment of these parameters, one can obtain breaking resistance close to or even higher than that of casted pieces. The estimation of welded joints quality for some alloys used in dental technology can be done using destructive and non-destructive methods. Among the non-destructive methods are: spectrographic, microscopic or radiological analyses; metallographic analysis and micro hardness belong to destructive methods. The aim of the study was to analyze the causes of failures for welded CoCrMo alloy prostheses, using non-invasive, rapid testing methods, like optic microscopy. The purpose is to increase the use of dentures. MAteriAls And Methods We have studied 20 removable partial denture (RPD) metallic frameworks, rehabilitated through laser welding with or without filler materials (Fig. 1.d). The welding has been done following the Kou 8 specification from Welding in Metallurgy (2003) manual. CoCrMo alloys were used, such as: “C” alloy from Vaskut Kohàszati Kft - Hungary, WIRONIT from Bego - Bremen, Germany (Fig. 1. a,b). We used a type 4 laser- Mini Laser XXS Model Orotig Italia (Fig. 1.c). The quality of welding, made in dental laboratory, was observed using an Inverted Metallurgical Microscope OPK-6A BEL PHOTONICS (Bel Engineering SRL, Italia - Fig. 1. e), which has an image capture system connected to PC. resUlts The use of metallographic microscope permitted non-invasive evaluation of the investigated metallic surfaces. Thus, one can observe different processing aspects of the metallic components (Fig. 2), which are relevant for preparing the welding procedure. After welding, the chemical composition of the alloys changes a little- by decrease of the basic components. The alloy becomes harder in the welded zone. In the heat affected zone (HAZ) cracks can appear, caused by the alloy rapid cooling after welding: Co from 65% to 64.1%; Cr from 29% to 27.4%; Mo from 5% to 4.1%. The CoCr alloys presented a good weldability. In the first phase we searched the optimal working parameters, which were correlated to alloy type, its chemical composition, fault- crack type, fracture, the fault size (length and thickness), working phase (junction, deposition, smoothing). The adequate combination of working parameters can be customized with the help of analyses, for its reproducibility and practice application. The mechanic resistance of welded joints depends on the welding spots integrity. Thus, we focused on the welding soldered joint aspect, on the faults that can appear in the welding points and compromise our reparation lifetime. Choosing of optimal parameters for our laser device is connected to the existence of a Nd:YAG laser with pulsed functioning, 25J maximum energy, 6-10A Input, 1064 nm wavelength; the clearance of laser radiated surfaces and interfaces is compulsory; the faults that are bigger than 0.2-0.5 mm need addition of a foil or CoCr wire. Working parameters for our laser device: • 8 spot types between 0.3 and 1.8 mm; • Power of the laser pulse 0.5-3 Kw; • Pulse duration is 0.5-8 msec; • The radiation diameter no smaller than 1.5 mm for non precious dental alloys; • 0.8 mm penetration depth is enough for obtaining a good braking resistance, in case of long bridges; in case of higher forces, welding is done also on the opposite part; • The superposition of successive welded surfaces has to be 60-70%; • Pulse frequency no higher than 2 Hz, especially in case of small dimension pieces (for example clasps, precision attachments, individual milled attachments); • One can give up to inert protection gas. _____________________________ Cristina Bortun et al 25
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microscopic analyses of welding defects in rpd technology

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