Methods for studying structural features of some dental cobalt alloys in welded state.

Ghiban, Alexandru ; Bortun, Cristina Maria ; Ghiban, Brandusa 等

1. INTRODUCTION

Removable partial dentures are such prosthesis, which can be inserted and take it out voluntary, by patient, in and from oral cavity. These components are consisted from a metallic part and an acrylic one (Baba et al., 2005), (Bertrand et al., 2004), (Bortun et al., 2008), (Bridgeport et al., 1993). Many problems are met when removable partial dentures made of cobalt based alloys are reoptimized by laser welding (Ghiban, 1999), (Ghiban, 2007), (Ghiban & Bortun, 2009). The aim of present paper is to make a correlation between welding parameters and macro and micro-structural aspects form some cobalt based alloys used for removable partial dentures.

2. MATERIALS AND PROCEDURES

Two dental alloys, such as alloy "C" and WIRONIT were investigated in order to put in evidence main structural characteristics after laser welding. Chemical compositions of experimental melts, in cast state with dimensions 10x20mm and thickness of 0,4mm-1mm, are shown in table 1. The welds were made in butt joint configuration without filling material. Equipment parameters of laser welding are adjustable: impulse power, period and frequency. Different investigations were made on welded samples: macrostructural analysis made on a stereomicroscope type OLIMPUS SZx and microstructural analysis made on Reichert microscope. Stereomacrostructural analysis was made using different magnification, putting in evidence the structural discontinuities. Metallographic analyses were made on Reichert microscope, in two states, etched and non etched, using IMAGE PRO soft for image processing.

3. RESULTS AND INTERPRETATIONS

Results concerning the macrostructural features of welded samples made on stereomicroscope type Olympus are given in figure 1, for "C" alloy, and in figure 2, for Wironit alloy.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Interesting aspects may be drawn from stereomicrostructural analysis. At very low spot power, about 0,8, at both alloys the join is not realized by welding, the fracture of the component taking place (see figure 1a, and figure 2a). By increasing of the spot value, at about 1,5, the join become better, no fracture of component taking place (see figure 1b for alloy "C", and figure 2b for Wironit alloy). At higher spot power, about 2, the laser welding become better and only fine radial cracks being observed at the stereomicroscope.

Microstructural analysis results are given in figure 3 for alloy "C" and figure 4 for Wironit alloy. This analysis may reveal different behavior of these alloys after laser welding. So, laser welding of alloy "C" at low spot power (about 0,8) put in evidence the bad join of the components, with multiple cracks, missing join in the center of the sample, (figure 3a). At higher spot power for welding of the "C" alloy, in weld metal many carbides are precipitated and also only fine cracks, generated through welding, up to 40[micro]m may be met, only at one part of the sample (figure 3b). At spot were about 2 for alloy "C" a heterogeneous dendrite structure may be seen and in welded metal the existence of generated cracks by laser beam, from the top to the center of the sample with length about 40-80[micro]m (figure 3c). Similar aspects concern structural fractures may be reveal from Wironit alloy analysis. So, at low spot power parent metal consists in solid solution with dendrite segregation, but the join is not possible to be made as is shown in figure 4a. At 1,8 spot power the structure of the parent metal consists in big dendrites with different orientation, and in welded metal many cracks with transcrystalline propagation, length 200[micro]m and distance about 5-8[micro]m, figure 4b. At high spot power, about 2, the parent metal has a dendritic structure, with different orientation of the grains with closed axes and big length and in welded metal there are met on both sides cracks, about 70[micro]m, with a propagation in zigzag (figure 4c).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

One may remark that with application of different welding parameters, welding join results can be either good or bad. In general, the parameter with a great influence is the spot power, which varies from different values: 0,8; 0,9; 1; 1,5, 2 or higher. Both alloys, either "C" alloy, or Wironit alloy have a similar behavior by initially applying of a small spot power. Applying an initially small spot, at a smaller value than one leads to a failed join. The sample did not join, breaking may appear at different steps: immediately after welding, or during the preparation of sample for macro- or microstructural analyses. Summarized results concerning structural behavior of laser welding cobalt based alloys are given in table 2. One may reveal the conclusion that when a spot power is applied above two value, joins start to be good, complete, without any cracks in weld metal.

4. CONCLUSIONS

Our investigations were made on ordinary alloys used for partial removable dentures, such as alloy "C" and alloy "Wironit. Two structural types of investigations, macro and micro-structural analyses, put in evidence the following features:

a) The applying of a small laser power up to 1 determines the obtaining of some join with many discontinuities which led to fragmentation at different preparatory steps for the structural analyze.

b) The applying of a higher laser spot power, strength, initially as 1,5 and 2, with frequency 3 and time 1,5-1,9 s may lead to obtaining of some join welding with presence of some fine cracks, with propagation in zigzag up to 70-100[micro]m. Although join resisted to the application of some medium force, it is still considerate a partial success.

c) For achievement of a proper join by laser welding, without cracks in weld metal, either at alloys type "C alloy" or Wironit alloy, the initially small spot power should be minimum 2 values.

5. REFERENCES

Baba N. & Watanabe I. (2005). Penetration depth into dental casting alloys by Nd:YAG laser. Journal Biomedical Material Response British Applied Biomaterials, Jan. 15; 72(1): 64-8

Bertrand C. et al., (2004). Optimization of operator and physical parameters for laser welding of dental materials. British Dental Journal 196, pp 413.

Bortun C., Faur N., Cernescu A., Porojan S., Gombos O., Sandu L. & Ghiban B (2008). Finite Element Analysis for Stress Distribution in Welded Zones Used In RPD Technology, European Cells and Materials Vol. 16. Suppl. 1, ISSN 1473-2262, pg. 23

Bridgeport D.A., Brantley W.A. & Herman P.F., (1993). Cobalt-chromium and nickel-chromium alloys for removable prosthodontics, Journal Prosthodontic, Part 1: Mechanical properties. Sep;2 (3), pp 144-50

Ghiban B. (1999). Metallic Biomaterials, Editura Printech, ISBN 973-9475-76-7, Bucuresti

Ghiban B. (2007). Structural features in cobalt based alloys for dental applications, Bulletin of. Transilvania University Brasov, vol.II, pp 80-86

Ghiban B., Bortun C.M (2009). Aliaje dentare de cobalt. Structura, procesare, optimizare, Editura Printech, ISBN 978-606-521-229-9, Bucuresti

Kou Sindo: Welding metallurgy, second edition, Wiley Interscience, a John Wiley & Sons, Inc, Publication, 2002

Sandu L., Topala F., Bardeanu V., Bortun C (2007) Optimization of operator parameters for microplasma welding of dental alloys, Volume No 13--Supplement 3 2007 13th General Meeting of the Swiss Society for Biomaterials, Neuchatel, Switzerland, May 9, pp 25

Srimaneepong V, Yoneyama T, Kobayashi E, Doi H, Hanawa T (2008) Comparative study on torsional strength, ductility and fracture characteristics of laser-welded alpha+beta Ti-6Al-7Nb alloy, CP Titanium and Co-Cr alloy, Dental castings Dental Material Jun; 24(6):839-45, Dec 3

Tab. 1. Chemical composition of the experimental alloys

Alloys Chemical composition, %, with Co base

 C Cr Mo Ni Fe Si

"C" 0.29 26.5 5.35 0.60 0.64 0.97

Wironit 0.35 26.4 5.38 0.85 0.74 0.89

ISO Max 26.5 4.5-7 Max Max Max
5832/4/ 0.35 -30 1.0 1.0 1.0

Tab. 2. Structural features of different laser welded samples

 Cobalt alloy
 Spot power
 (frequency "C" alloy Wironit alloy
3, time 1,4)

 0.8 failed join, quick failed join, quick
 fragmented after fragmented
 welding

 0.9 failed join, failed join,
 fragmented at fragmented during
 metallographic metallographic
 preparation preparation

 0 cracks in weld after welding the
 metal with length sample is braking
 about 200 [micro]m during the
 metallographic
 preparation

 1.5 success join, but success join, fine
 with 120 [micro]m cracks in weld metal
 cracks

 2 success join, fine success join without
 radial cracks cracks in weld metal