Watanabe et al.

Int Chin J Dent 2005; 5: 47-51.

Distortion of cobalt-chromium frameworks induced by laserwelding
Ikuya Watanabe, DDS, PhD,a Jennifer Chang, BS,b and Yvonne Chiu, BSb
aDepartment of Biomaterials Science, Baylor College of Dentistry, Texas A&M University Systemb
Health Science Center, and Baylor College of Dentistry, Texas A&M University System Health
Science Center, Dallas, TX, USA

Purpose: This study examined the amount of distortion of prosthetic metal frameworks caused by laser welding
when the welding pattern and the method of fixing the ends of the specimens in the jig were varied during thewelding process.
Materials and Methods: Wax plate patterns (0.5x3.0x10 mm) were prepared to cast the Co-Cr plates. After thewelded surfaces (3.0x0.5 mm) were polished with No. 600 SiC paper, the two plates were matched and weldedwith a Nd:YAG laser at constant welding parameters. The specimens were welded unilaterally or bilaterallywith either three or five spots as follows: two ends fixed unilaterally (A) or bilaterally (AA), one end fixedunilaterally on one surface (B), two ends fixed on one surface with the opposite surface fixed at one end (AB), or
one end fixed bilaterally on two surfaces (BB). The bilateral specimens were made by inverting the unilateralspecimens and welding them on the other side opposite the first weld. Distortion was determined by measuring
the gap from the jig base at one end of the plate after each welding application. The results (n=3) were analyzed
using a two-way ANOVA and Tukey’s HSD (p<0.05).
Results: Compared to the two-end fixed specimens (A and AA), the specimens fixed at only one end on either
surface (B, AB, and BB) had significantly more distortion. When both ends were fixed (A and AA), more
distortion appeared to occur for three-spot welding compared with five-spot welding.
Conclusion: The method of fixing the ends of the specimens in the jig significantly affected the amount ofdistortion for both the three- and five-spot welding patterns. (Int Chin J Dent 2005; 5: 47-51.)
Key Words: cobalt-chromium, distortion, laser welding, metal framework.

Introduction
Due to its convenience, laser welding has gained popularity in the past decade for connecting metal prostheticframeworks. Since the laser energy is concentrated on a small area, the effect of heating around the weld isreduced. Thus, broken metal frameworks that include combustible acrylic can be repaired using laser welding,preserving any denture base resin already affixed to the framework. Other advantages of laser welding are: 1)
Oxidation on the area surrounding the welding spot can be minimized because of the argon shielding capabilityof the welding machine; 2) any type of dental alloys, including titanium, can be welded, and 3) there is no needfor extra materials and equipment such as investment, flux, a furnace or a gas torch, as needed for conventionalsoldering. The disadvantage of laser welding is that it is troublesome to fill the large space left after cutting theframework because of the small welding area (adjustable spot diameter: 0.2-2.0 mm).
There have been numerous studies on laser welding, most of them dealing with the mechanical strength ofwelded joints.1-9 In our previous studies using titanium7,9 and Co-Cr alloy,8 laser welding was proven to produce
joint strength equivalent to that of the non-welded parent alloys. One of the focuses of research connecting theparts of frameworks is distortion, which directly affects the dimensional integrity of prostheses. Althoughconventional soldering is known to create distortion,10-13 there is little information about distortion induced bylaser welding.14,15

Huling and Clark14 evaluated three different joint-forming procedures: laser welding,soldering, and one-piece casting.

They reported that there is significantly less post-joining distortion for47

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Int Chin J Dent 2005; 5: 47-51.

laser-welded and one-piece castings than for soldering, and significantly greater reliability (the least errorvariance) for joining with the laser than either one-piece casting or soldering. Iwasaki et al.15 investigated thedistortion of laser-welded titanium, and reported smaller distortion for two-side welding compared to one-side
welding and that pre-welding significantly reduced the distortion by stabilizing the welding assembly. In clinicalpractice, laser welding is always performed on the master cast as shown in Fig. 1.

Note that the metalframework is tightly screwed onto implant analogs on the master cast. After the top is welded, the metalframework sometimes distorts when the bottom is welded after the framework is unscrewed and removed fromthe implant analogs.
This study examined the distortion of metal frameworks caused by laser welding when the welding pattern
and the method of fixing the ends of the specimens in the jig were varied during the welding process.

Materials and Methods
The alloy used in this study was a Co-Cr alloy (60.6% Co, 31.5% Cr, 6.0% Mo; Vitallium, Austenal, Chicago,IL, USA).

AB

Side viewFig. 1.
Metal framework for
implant supported
superstructure
screwed onto implantanalogs of the mastercast before (A) andafter (B) laser welding.

End view

Screw

Specimen
Base

Jig

8.0

6.0

4.0

3.0

Topview
3.0Fig. 2. Jig apparatus setup (unit: mm).
To prepare the cast Co-Cr plates, wax plate patterns (0.5x3.0x10 mm) were invested in mold rings using aphosphate investment (V.R. Investment, Austenal).

The Co-Cr alloy ingots were then cast using aninduction-melting centrifugal casting machine (Modular 4, CMP Industries Inc., Albany, NY, USA). After the48

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Int Chin J Dent 2005; 5: 47-51.

cast surfaces were air-abraded with 50 µm Al2O3 particles, the welded surfaces (3.0x0.5 mm) were polished with
No. 600 SiC paper, and the two plates were matched and butted together.

Laser-welding patterns
3-Spot10.0
0.5

5-Spot3.0
20.0

Unilateral

Bilateral

AAA
4.0

4.
0

4.0

4.0ABBBBFig. 3. Experimental patterns for laser welding and fixing the ends of specimens (unit: mm).
Welding was performed with a Nd:YAG laser (Neolaser L, Girrbach Dental Systems, Pforzheim, Germany) ata constant voltage of 200 V, pulse duration of 10 ms, and spot diameter of 1 mm. The specimens were weldedusing a jig apparatus (Fig. 2). Methods of laser welding and fixing the specimens in the jig are shown in Fig. 3.
Either three or five spots were applied unilaterally or bilaterally. Specimens welded with two ends fixedunilaterally (A) or bilaterally (AA) in the jig served as controls. The non-control group specimens had one end
fixed unilaterally on one surface (B), two ends fixed on one surface with the opposite surface fixed at one end(AB), or one end fixed bilaterally on two surfaces (BB). Bilateral specimens were made after inverting theunilateral specimens (AA from A, AB from A, BB from B). Distortion was determined by measuring the gap
from the jig base at one end of the plate after each welding application. Three specimens were evaluated foreach experimental condition. The results (n=3) were analyzed using a two-way ANOVA and Tukey’s HSD(p<0.05).

Results
Distortions (mm) induced by laser-welding for each experimental condition are summarized in Fig. 4.
Statistical significance was found in the fixed condition factor (F=49.965, p<0.001). Compared to the two
ends-fixed specimens (A and AA), the specimens fixed at only one end on either surface (B, AB, and BB) were49

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Int Chin J Dent 2005; 5: 47-51.

significantly more) distorted. When both ends were fixed (A and AA), there appeared to be more distortion forthree-spot weldingmcompared with five-spot welding.

Distortion (mm)m( 0.6no 0.5it 0.4
rot 0.3
si
D0.2

3-spot5-spot

0.10A
1

AA2

AB
3

B
4

BB
5

Method of fixing
Fig. 4. Distortion induced by laser welding.

Discussion
Compared to groups A and AA in which both ends were fixed (for both the three-spot and five-spot welding
methods), distortion of the Co-Cr metal specimens increased significantly for those experimental groups withonly one end fixed (AB, B, and BB) (Fig. 4) because the other end of these specimens was free to distort. Whenboth ends were fixed either unilaterally (A) or bilaterally (AA), more distortion was found using the three-spot
welding pattern compared with the five-spot welding pattern. Since the welded area covering the joint width
was smaller with three spots compared to the area covered by five spots, the weld joint strength of the Co-Cr
plates was stronger with five spots. The stronger joint produced by five spots might reduce the residual stressinduced by laser welding when the specimens are tightly fixed at both ends. The relief of residual stress wasalso compared in the specimens welded unilaterally and bilaterally. Bilaterally welded specimens (AA) shouldhave a larger welded area and higher strength compared with the unilaterally welded specimens (A). As a result,
there was less distortion of the bilaterally welded specimens compared to the unilaterally welded specimens forboth three-spot and five-spot welds. Distortion takes place in the direction of the welded side because of themelting-solidification process that causes metal shrinkage and induces residual stress on the welded side of thetwo plates.
When the specimens were fixed at one end (AB, B, and BB), the one fixed end and unilaterally welded (B)specimens had the least distortion compared with the two ends fixed on one surface with the opposite surface
fixed at one end (AB) and one end fixed bilaterally on two surfaces (BB) (Figs. 3 and 4). This finding can also
be attributed to the difference in the number of laser shots. Under these conditions, since the specimens were50

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Int Chin J Dent 2005; 5: 47-51.

fixed at only one end, the residual stress at the welded region was freed. Consequently, increasing the number oflaser shots (AB and BB > B) increased the amount of residual stress and resulted the increase of distortion.
From the results obtained in this study, it is suggested that the metal framework should be tightly fixed at acertain position to prevent the distortion during the laser welding. In the clinical case shown in Fig. 1, making
an access hole in the casting beneath the framework enabled us to weld the bottom of the framework without
removing it from the implant analog on the cast and minimizes the occurrence of distortion on the welded metalframework.

Conclusion
The method of fixing the ends of the specimens in the jig significantly affected the amount of distortion forboth the three- and five-spot welding patterns.

To minimize the occurrence of distortion on the weldedspecimens, both ends should be fixed in the jig. Once they are tightly fixed, the distortion may decrease with agreater number of bilaterally-applied laser spots.
Acknowledgment
This study was supported in part by NIH grant DE 07188, Baylor Oral Health Foundation (Y2001-Z). Editorial assistanceby Mrs. Jeanne Santa Cruz is also appreciated.

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Reprint request to:
Dr. Ikuya Watanabe
Department of Biomaterials Science, Baylor College of Dentistry
Texas A&M University System Health Science Center
3302 Gaston Ave., Dallas, TX 75246, USA
Fax: +1-214-370-7001 E-mail: iwatanabe@bcd.tamhsc.edu
Received June 14, 2005. Revised June 21, 2005. Accepted June 27, 2005.
Copyright ©2005 by the Editorial Council of the International Chinese Journal of Dentistry.

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