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orIgInal arTIClES

Microscopic AnAlyses of Welding defects in rpd technology

Bortun Cristina1, Ghiban Brandusa2, Barlea Romeo3, Gombos Otilia4

reZUMAtIntroducere: Aliajele pe bază de cobalt sunt frecvent folosite la realizarea scheletelor metalice ale protezelor parţiale mobilizabile scheletate (PPMS). Stresul mecanic, la care acestea sunt expuse, poate genera apariţia unor fisuri. Autorii au studiat o metodă de reparaţie, şi anume sudura cu laser într-un sistem pulsatoriu. Obiective: creşterea duratei de viaţă a PPMS, reparaţia şi analiza cauzelor eşecului la protezele sudate, realizate din aliaje de cobalt. Material şi metodă: 20 suprafeţe ale unor schelete metalice sudate au fost analizate la microscop. Procedeul de sudură s-a realizat cu sau fără material adiţional. S-au utilizat aliaje de CoCrMo: aliaj “C” (Vaskut Kohàszati Kft – Ungaria) şi WIRONIT (Bego - Bremen, Germania). Calitatea sudurii (realizată în laboratorul de tehnică dentară cu Mini Laser XXS Model.Orotig Italia) a fost observată cu un microscop metalografic (OPK-6A BEL PHOTONICS Bel Engineering SRL, Italia).Rezultate: La analiza microscopică au fost descoperite defecte ale sudurii şi fisuri, care sunt legate în special de mărimea spotului şi puterea laserului de sudură. Majoritatea fisurilor apar în cazul combinaţiei spot mare/putere înaltă, în cursul procesului de netezire/finisare a sudurii. Folosind un spot şi o putere mai mică decât cea stabilită de producător, putem obţine puncte de sudură fără defecte.Concluzii: Alegerea unei combinaţii adecvate- energia impulsului, durata acestuia şi putere mai slabă pentru fiecare etapă a sudurii, este decisivă pentru succesul acesteia. Este important să cunoaştem calitatea şi defectele structurale ale reparaţiilor, pentru a obţine proteze cu o rezistenţă superioară. Cuvinte cheie: eşec, sudura aliajelor de CoCrMo, proteze parţiale mobilizabile scheletate.

ABstrActIntroduction: Cobalt based alloys are frequently used in manufacturing metallic framework of removable partial dentures. Because of mechanical stress to which they are exposed, cracks in dental prostheses can develop. The authors studied a repairing method, namely welding through a laser beam in a pulse operating system. Objectives: to increase the rpd lifetime, repair and analysis of the causes of failure for welded cocrmo alloy prostheses. Material and methods: Surfaces of 20 welded metallic frameworks were microscopic analyzed. The welding procedures were: butt joint with or without filler materials. There were used CoCrMo alloys: “C” alloy (vaskut kohàszati kft – hungary), wironit (bego - bremen, germany). The quality of welding (made in dental laboratory with mini laser xxs model, orotig italia), was observed using an inverted metallurgical microscope (opk-6a bel photonics bel engineering srl, italy).Results: at microscopic analysis, we discovered welding defects and cracks, which are mainly connected to spot size and welding power. Most of the cracks appear at large spot size/high welding power combination, in the process of smoothing the welding. Using of smaller spot size and lower power than the one established by the producer can generate faultless welding points.Conclusions: Selecting the adequate combination of pulse energy, pulse duration and peak power for each welding step is decisive for the success of welding procedure. It is important to know the quality and structural defects of our repairs, in order to obtain high resistance prostheses.Key words: failure, welding CoCrMo alloys, removable partial dentures

Received for publication: Oct. 03, 2009. Revised: Nov. 14, 2009.

1Department of Removable Partial Dentistry Technology, Faculty of Dental Medicine, “Victor Babeş” University of Medicine and Pharmacy Timişoara, Dental Technology Specialization2Politehnica University Bucharest

Correspondence to:Borţun CristinaDepartment of Removable Partial Dentures Technology, Dental Technology Specialization, Blv. Revoluţiei 1989, no. 9Phone: 0745378254E-mail: cristinabortun@yahoo.com; cristinabortun@gmail.com

introdUction

CoCrMo alloys are frequently used for manufacturing metallic framework of removable partial dentures and for their rehabilitation.1 These dentures are used for treatment of patients with medium incomes, who are between 50 and 65 years old.

Cobalt based alloys are used in dentistry with great interest, due to their simultaneous properties, such as: high mechanical characteristics (yielding strength, ultimate tensile strength and hardness), biocompatibility or wear resistance.2 One can notice

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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).

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

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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.

Bertrand6,7 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 Kou8 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.

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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

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

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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 450 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.

Bertrand6,7 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.

Craig2 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 Kazutoshi9 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 Dobberstein8,15-17 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 cross-section 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

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measurements indicated that, the corrosion resistance of the laser-welded joints was better than that of the brazed one.

Reclaru3 shows that, precious metal based dental alloys generally have a superior corrosion resistance, in particular enhanced resistance to pitting and crevice corrosion, compared to non-precious metal based alloys such as CoCr alloys. A new generation of Co-Cr alloys enriched with precious metals (Au, Pt, Ru) have now appeared on the market.

Burkhardt, Reichert11,18 compared laser and microimpulse welding. They concluded that, laser welding is superior, but more expensive due to the working equipment.

conclUsions

Because of mechanical stress to which they are exposed, cracks can appear in dental prosthesis. The repairing method studied by the authors is welding through a laser beam in a pulse operating system. Since the cracks that tend to appear do not have a regular shape, it is highly recommended the use of welding by hand.

Laser welding is a performing method for metallic prostheses repairs. Selecting the adequate combination of pulse energy, pulse duration and peak power for each welding step is decisive in the success of welding procedure.

It is important to know the quality and structural defects of our repairs, both base metal and welded metal, in order to obtain high resistance prosthesis. Microscopically non-invasive analyses and tests show good or poor quality of the welded joints.

AcKnoWledgeMents

This study was supported by the CNCSIS Grant Ideas, 1878/2009, from the Ministry of Education and Research of Romania

references

1. Wulfes H. Precision milling and partial denture constructions. Bremen: Academia Dental 2003.

2. Craig RG. Dental materials: properties and manipulation. 7th Ed. St. Louis. Mosby 2000.

3. Reclaru L, Lüthy H, Eschler PY, et al. Corrosion behaviour of cobalt-chromium dental alloys doped with precious metals. Biomaterials 2005, 26(21):4358-65.

4. Wataha JC. Alloys for prosthodontic restorations. J Prosthet Dent 2002;87:351–63.

5. Zupancic R, Legat A, Funduk N. Electrochemical and mechanical properties of cobalt-chromium. Materials and technology 2007;6:295–300.

6. Bertrand C, Le Petitcorps Y, Albingre L, Dupuis V. Optimization of operator and physical parameters for laser welding of dental materials, Br Dent J 2004;196(7):413-8.

7. Bertrand C, Le Petitcorps Y, Albingre L, Dupuis V. The laser welding technique applied to non precious dental alloys procedure and results, Br Dent J 2001;190(5):255-7.

8. Frentzen M, Koort HJ. Lasertechnit in der zahnheilkunde. DST. Zahnarzth 1991;46:443-452

9. Kakimoto K, Moriguchi A, Uenishi K, et al. Laser weldability of dental cobalt -chromium alloy. J Osaka Odontol Soc 2001;64:35-47.

10. Kou S. Welding metallurgy, 2nd edition, Wiley Interscience, New Jersey, 2003.

11. Reichert A. Untersuchung der Schweisnahtqualitaten von Laser und Phaser Quintessenz Zahntech 2005;31:264-79.

12. Tambasco J, Anthony T, Sandven O. Laser welding in the dental laboratory: an alternative to soldering. J Dent Technol 1996;3:23-31.

13. Watanabe I, Topham S. Laser welding of cast titanium and dental alloys using argon shielding. J Prosthodont 2006;15:102-7.

14. Watanabe I, Baba N, Chang J, et al. Nd:YAG penetration into cast titanium and gold alloy with different surface preparations. J Oral Rehabil 2006;33:443-6.

15. Yamagishi T, Ito M, Fugimura Y. Mechanical properties of laser welds of titanium in dentistry by pulsed Nd:YAG laser apparatus. J Prost Dent 1993;70:264-73.

16. Neumann MO, Lindigkeit J. Mechanische Festigheit von geschweisten EMF-Legierungen, Quintessenz Zahntech 2005;31:966-73.

17. Dobberstein H. SchweiBen von kobalt-chrom-, nickel-chrom-and silber-palladium-legierungen mittles festkörperlaser. Zahn Mund Kwferheilkd 1990;78:259-61.

18. Burkhardt HJ. Ein wirtschaftlicher Weg mit Erfolgschancen - Schweißen mit dem CeHa Phaser MX1. Quintessenz Zahntech 2005;31(2):136-42.