Repairing method of fixed partial prostheses in dentistry: laser welding

Meda L.Negrutiua, Cosmin Sinescua, Dalibor Cozarovb, Laurentiu Culeac, Mihai Rominua,

Daniela M.Popa

a Department of Prostheses Technology and Dental Materials, Faculty of Dental Medicine, University of Medicine and Pharmacy "Victor Babeş" Timişoara; bDali's Dentech, private dental laboratory , Timisoara (Romania);

c Faculty of Mechanics, Politechnica University Timişoara (Romania)

ABSTRACT

Laser Welding is an advantageous method of connecting or repairing metal prosthetic frameworks because there are fewer effects of heating on the area surrounding the spot to be welded, and no further procedures, such as those used for conventional soldering, are necessary. Laser welding has been increasingly applied for fabricating the metal frameworks of prostheses and for other procedures, such as recovering the metal ridge and cusp, blocking holes on the occlusal surfaces after excess occlusal adjustment, thickening the metal framework, or adding contact points after excess grinding and adjusting of the crown margins. The objective of this study are represented by investigation and evaluation of three types of soldering compare to laser welding. The method is represented by the laser welding with a pulsed Nd-Yag Laser equipment. Other joints were produced, using three different soldering techniques. These joining areas were investigated for their quality and their corrosion properties. Corrosion attack was confirmed by electron microscopy. The investigations confirm the quality of laser welding. The investigated different laser welding methods revealed minimal corrosion and offers clear-cut advantages compared to the other soldering methods. Dental alloys are subjected to functional influences in the oral cavity and interact with the intraoral enviroment.

Keywords: laser welding, soldering methods, corrosion, argon gas shilding, alloy casting

1. INTRODUCTION One of the earliest and most widely practiced applications of laser material processing was joining of metallic parts using a continuous wave laser1,2. Laser joining is applicable to inorganic/organic and similar/dissimilar materials with an extremely high precision, versatility and productivity that can only be matched by electron beam welding.. Moreover, laser welding can be done in air, unlike the vacuum processing needed in electron beam welding 1, 2. In comparison to conventional or arc welding, laser welding scores several advantages like narrow welds with controlled bead size, faster welding with a higher productivity, less distortion, narrow heat affected zone and minimum contamination 3. From the laser joining methods with use in dental practice, we mention: laser welding, laser brazing, laser sintering, laser soldering and laser tissue welding 1, 2.

Laser welding constitutes the most important operations among the laser joining processes 2,4. The image in figure 1 shows the front view of the schematic set-up for laser welding without a filler rod. The focused laser beam is made to irradiate the work piece or joint at the given level and speed. A shroud gas protects the weld pool from undue oxidation and provides with the required oxygen flow. Laser heating fuses the work piece or plate edges and joins once the beam is withdrawn. In case of welding with filler, melting is primarily confined to the feeding wire tip while a part of the substrate being irradiated melts to insure a smooth joint. In either case, the work piece rather than the beam travels at a rate conducive for welding and maintaining a minimum heat affected zone 3,4,5.

Lasers in Dentistry XIV, edited by Peter Rechmann, Daniel Fried,Proc. of SPIE Vol. 6843, 684309, (2008)

1605-7422/08/$18 · doi: 10.1117/12.764995

Proc. of SPIE Vol. 6843 684309-12008 SPIE Digital Library -- Subscriber Archive Copy

FocusingLens

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CNC Tabe Ar shrouding

Liquid metalwe'd pool

Laser//wHeat aftect zone

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longitudinal section

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top view

Fig.1. The front view of the schematic set-up for laser welding without a filler rod.

There are two fundamental modes of laser welding depending on the beam power/configuration and its focus with respect to the work piece: conduction welding and keyhole or penetration welding 6. Conduction limited welding (fig.2 a) occurs when the beam is out of focus and power density is low/insufficient to cause boiling at the given welding speed.

Fig.2. a .Conduction limited welding; b. Deep penetration or keyhole welding.

In deep penetration or keyhole welding (fig.2 b), there is sufficient energy/unit length to cause evapouration and hence, a hole forms in the melt pool. The ‘keyhole’ behaves like an optical black body in that the radiation enters the hole and is subjected to multiple reflections before being able to escape(fig.3). The transition from conduction mode to deep penetration mode occurs with increase in laser intensity and duration of laser pulse applied to the work piece.

Fig.3. Diagram of the keyhole and the surrounding melt pool.

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Brazing is a thermal bonding process, where only the soldering metal is melted – leaving the base material unaffected. Brazing operates at lower temperatures than welding, creating lower levels of thermal stress and consequently causing less component distortion. Laser brazing can be used for various metals that are not weldable due to their composition or for joining non-ferrous metals/alloys and ceramics/glasses. Laser brazed seams fulfil high optical demands on the seams 1, 2, 6.

Lase-sintering is a layer manufacturing process, which - depending on size and geometry -can economically manufacture small to large series. In this process, the products are directly built up 100 percent dense, without any molding tools whatsoever, in a generative process where layers of powder material are fused by a laser beam positioned via precision optics. The necessary technical control data for the laser technique are generated from geometric data using wellknown 3D-CAD programs such as Solid Works, ProEngineer, Catia and AutoCAD. The laser-sintered metallic component of the prosthesis leaves the machine almost "ready to use" 1, 2.

The precision of laser soldering is far superior to conventional soldering. Like conventional soldering, one of the major concerns about laser soldering remains the mechanical property, particularly creep resistance at elevated temperatures 1, 2.

Laser has now opened new possibilities in joining organic substances including human and animal tissues and organs. Laser tissue welding uses laser energy to induce thermal changes in the molecular structure of the tissues. In laser tissue welding, low-strength anastomoses and thermal damage of tissue are major sources of concerns. On the other hand, laser tissue soldering is a bonding technique in which a protein-solder is applied to the tissue surfaces to be joined, and laser energy is used to bond the solder to the tissue surfaces. The addition of protein solders to augment tissue repair procedures significantly reduces the problems of low strength and thermal damage associated with laser tissue welding techniques 1, 2.

Cast titanium restorations have been clinically applied to an increasing number of patients, particularly in those with allergies to other dental alloys, because titanium and its alloys possess excellent biocompatibility. Titanium is characteristically difficult to cast and solder and it is necessary to use special equipment to cast or solder titanium frameworks 1,2. To connect cast titanium and its alloys, infrared soldering and laser welding are commonly employed methods 7.

Laser welding is suitable to weld titanium and its alloys because they have higher rates of laser beam absorption and lower thermal conductivity than do other dental casting alloys such as gold alloys; however, due to the strong reactivity of molten titanium with oxygen in ambient air, the incorporation of oxygen during laser welding may affect the joint strength 4. The mechanical strength of a welded joint is important in terms of the longevity of the prosthesis since a weak joint can cause failure of the metal framework during habitual use 5,6,8

Laser welding is an advantageous method of connecting or repairing metal prosthetic frameworks because there are fewer effects of heating on the area surrounding the spot to be welded, and no further procedures, such as those used for conventional soldering, are necessary 2. For welding of dental alloys we can use only a few type of lasers (table 1).

Laser type Wave length (µm)

Operating mode

Power (W)

Puls energy

(J)

Puls duration

(ms)

Pulse frequency

(s-1)

Radiation divergence

(mrad)

Radiation diameter

(mm)

rubin 0,694 pulsed - 400 3 5 6 16

Nd:glass 1,06 pulsed - 100 15 3 12 30

Nd:YAG 1,06 pulsed - 120 20 10000 15 10

Nd:YAG 1,06 continuous 1800 - - - 20 8

CO2 10,6 pulsed - 1000 10-3 CW 25000 10 26

CO2 10,6 continuous 2000 - - - 10 20

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1

Laser welding has been increasingly applied for fabricating the metal frameworks of prostheses and for other procedures, such as recovering the metal ridge and cusp, blocking holes on the occlusal surfaces after excess occlusal adjustment, thickening the metal framework, or adding contact points after excess grinding and adjusting of the crown margins.

The purpose of this study are represented by investigation and evaluation of three types of soldering compare to laser welding and also, the investigation of the effect of argon gas shielding on the laser-welded strengths of cast Ti and Ti-6Al-7Nb, comparing the results to those of laser-welded conventional cast dental alloys.

2. METHODOLOGY The method is represented by the laser welding with a pulsed Nd-Yag Laser equipment (Orotig) (fig.4). The following parameters were used: 1064 nm wavelength, 10 ms repetition rate and 6.58 kJ/ cm2 energy density (for Ti specimens) and 12 ms repetition rate and 7.49 kJ/cm2 energy density for Ni-Cr and Co-Cr alloys.

Fig.4. The Nd-Yag Laser equipment (Orotig) used for the weldings.

Other joints were produced, using three different soldering techniques: Hydrogen Flame Soldering, Carbon Tip Electrode Soldering and Transfer Soldering1,2.

In the hydrogen flame soldering technique, the wire elements are heated directly, so that the temperature of the wire mostly rises to levels exceeding the fusing temperature of the solder.

A spot welding unit (Master 2002; Dentaurum, Pforzheim, Germany) was used for carbon tip electrode soldering. A small soldering pad containing flux and the workpieces to be soldered was clamped between the carbon and copper electrodes. The current running through the workpieces resulted in direct heating of the wire elements and thus in melting of the solder. The soldering time did not vary significantly among the technicians; the mean values were 2.5 to 3 seconds and thus below those observed in the hydrogen flame and transfer soldering techniques.

Transfer soldering too is performed with a spot welding unit. The spot welded wire elements are held manually against the solder-bearing end of a brass wire electrode. The latter is heated by the current flow from a carbon tip electrode until the solder fuses and flows into the soldering gap. In this technique, the parts to be soldered are heated only indirectly, so that they just reach the solder melting temperature at the point of contact with the soldering globule, whereas the temperature is lower at more distant sites.

All the joints were produced as overlapping connections between a right-angled spring-hard wire of 0.6 mm diameter and a hard wire of 0.9 mm diameter (fig.5). After the soldering/ welding procedures, the joints were immersed in in three different corrosive agents : 60 minutes in 10% ferric chloride 48 hours in Kukis® cleaner and 90 days in 70 ml artificial saliva at 37 °C 9, 10, 11.

Observance of the soldering instructions by the dental technicians was checked. Corrosion attack was confirmed by electron microscopy 12.

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5mm

0 0.6 DIDI

0 0.9 DIDI

Fig.5. The Welder/Solder joint geometry (schematic drawing).

For the investigation of the effect of argon gas shielding on the laser-welded strengths of cast Ti, Ti-6Al-7Nb, and conventional dental alloys, were used the commercially pure Ti, Ti-6Al-7Nb and three dental casting alloys (Ni-Cr, gold and Co-Cr). Two types of wax plate patterns were prepared for the laser-welded (0.5×3.0× 10 mm) and nonwelded (control) (0.5 × 3.0 × 20 mm) specimens. The plate patterns were invested in the molds and then cast with each metal. Each casting procedure followed the manufacturer’s instructions. After casting, the molds were allowed to bench-cool to room temperature. The cast plates were then divested, airabraded with 50-µmAl2O3 particles, and ultrasonically cleaned with acetone for 10 minutes.

Laser welding configurations and tensile specimen used: three or five laser spots were bilaterally applied perpendicular to the surface at the interface.

The assembled cast plates were then welded with an Nd:YAG laser (Orotig) at a constant voltage of 200 V, pulse duration of 10 ms, and spot diameter of 1 mm.

Tensile testing was conducted with a universal testing machine (Model 1125, Instron Corp., Canton, MA) at a crosshead speed of 1 mm/min and a gauge length of 10 mm (grips were attached 5 mm from both ends). Failure load (N) and elongation (%) were recorded, and the means and standard deviations were calculated.

3. RESULTS The investigations confirm the quality of laser welding 12, 13, 14. In figure 6 is represented the surface appearance of the laser weld at the transition from conduction mode to keyhole mode welding (fig.6 c) and cross-sections of both, a conduction mode weld just before the transition (fig.6 a) and a keyhole mode weld directly after the transition (fig.6 b)6.

Incomplete filling of the soldering gaps, porosities resulting from the production process, poor corrosion properties and in particular a high variability of the measured values point out the insufficient reliability of classical soldering technique. Figure 7 shows Scanning electron microscopic images of various solder joints in 17x (a) and 50x (b and c) magnification after immersion in ferric chloride. Massive defects in the steel wire occurred primarily at the uncovered snipped-off spring ends (fig.7 a, b), uncovered soldering gap areas (fig.7 c), and impressions of the carbon tip electrode (fig.7 a).

Variations in quality were also detected related to technicians’ modes of operation.

Proc. of SPIE Vol. 6843 684309-5

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1mm,

t- ,t- 4

Fig.6. The surface appearance of the laser weld at the transition from conduction mode to keyhole mode welding (c) and

cross-sections of both, a conduction mode weld just before the transition (a) and a keyhole mode weld directly after the transition (b).

Fig.7. Scanning electron microscopic images of various solder joints in 17x (a) and 50x (b and c) magnification after

immersion in ferric chloride.

The results obtained by the investigation of the effect of argon gas shielding on the laser-welded strengths indicate that if cast pure titanium and Ti-6Al-Nb metal frameworks for prostheses are joined by laser welding under appropriate conditions in conjunction with argon gas shielding, these metal frameworks will have mechanical strength equivalent to that of the nonwelded one piece cast metal frameworks (fig.8, 9).

a b c Fig.8. SEM micrographs of the fracture surfaces of Ti specimens. (A) laser welded with argon shielding; (B) laser welded

without argon shielding; (C) control.

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t.

a b c Fig.9. SEM micrographs of the fracture surfaces of Ti-6Al-7Nb (a) laser welded with argon shielding; (b) laser welded

without argon shielding; (c) control.

However, the laser welded gold alloy metal framework may not be reliable for long-term usage, regardless of argon shielding, due to the pores created in the alloy (fig.10). As for the Co-Cr alloy, the use of argon shielding may disturb the effective welding of Co-Cr frameworks under conditions similar to those used in this study (fig.10).

Fig.10. Representative SEM photographs of entire fracture surface (cross-section) for A- Co-Cr control, B- gold martor, C-

welded Co-Cr with Ar, D- Co-Cr without Ar, E – gold with Ar, F – gold without Ar

4. CONCLUSIONS Under the limitations of this study, the following conclusions can be drawn:

1. The investigated laser welding methods revealed minimal corrosion and offers clear-cut advantages compared to the other soldering methods.

2. Dental alloys are subjected to functional influences in the oral cavity and interact with the intraoral environment. These interactions are of a complex nature and are influenced by a wide range of environmental factors (quantity and quality of saliva, dental plaque, pH value, foods and drugs).

3. The results indicated that argon shielding is definitely necessary when laser-welding CP Ti and Ti-6Al-7Nb.

4. Regardless of the argon gas shielding, the failure load for the laser-welded gold alloy was half that of the control specimens because of the pores created during the welding process.

5. Argon shielding was found to be detrimental to effective welding of Co-Cr alloy under the conditions used in this study.

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