Cast iron-platinum magnetic alloy laser-welded to Au- and Ag- alloys

 

Ikuya Watanabe¹, Yasuhiro Tanaka², Kunihiro Hisatsune², Mitsuru Atsuta²,

Khoi Nguyen¹, P. Andrew Benson¹, Jennifer Chang¹, Yvonne Chiu¹.

 

¹Department of Biomaterials Science, Baylor College of Dentistry

Texas A&M University System Health Science Center, Dallas, Texas USA

 

²Department of Developmental and Reconstructive Medicine

Nagasaki University Graduate School of Biomedical Sciences, Nagasaki, Japan

 

INTRODUCTION

            Magnetic attachments have increasingly been applied for retention of removable partial dentures and maxillofacial prostheses. Although most dental magnetic attachments are available as prefabricated products in several shapes and sizes, highly corrosion-resistant castable Fe-Pt magnetic attachments were developed that can be made in any size and shape needed to connect to each individual patient$B!G(Bs cast metal framework. These cast magnetic attachments are usually connected to the metal frameworks by means of soldering or welding. However, the heat generated during soldering and welding reduces the magnetic properties of the Fe-Pt magnetic alloys. Therefore, laser welding is the most suitable method for the Fe-Pt alloys because there are fewer effects of heating on the area surrounding the weld due to the concentrated laser energy. The joint strengths of cast Fe-Pt magnetic attachments laser-welded to dental casting alloys are important in terms of longevity after the habitual use of the prostheses. However, there is little information regarding the joint properties of different types of noble dental casting gold alloys laser-welded to a Fe-Pt magnetic alloy using Nd:YAG laser.

 

OBJECTIVE

            To investigate the joint properties of a cast Fe-Pt magnetic alloy laser-welded to dental casting Au- and Ag- alloys.

 

MATERIAL AND METHODS

Preparation of cast plates

            Custom-made Fe-Pt alloy ingots (Fe-36at%Pt) were prepared for casting of specimens. The three gold casting alloys used were: ADA Type II gold alloy (Ney-Oro A-1, Au: 77.5%, Ag: 12.5%, Cu: 7.3%, Pd: 2%, Zn: <1%, Degussa-Ney Inc.), Type IV gold alloy (Ney-Oro 60, Au: 56%, Ag: 19.9%, Cu: 17%, Pd: 4%, Zn: 3%, Degussa-Ney Inc.), and Ag-Au alloy (New Gold Para, Au: 12%, Ag: 51%, Pd: 20%, Cu: 14.5%, Ishifuku Corp.). Two types of wax plate patterns were prepared for the laser-welded (0.5 x 3.0 x 10 mm) and non-welded control (0.5 x 3.0 x 20 mm) specimens.  The wax patterns were invested in the molds and then cast with each metal. The gold alloys were cast conventionally using a broken-arm centrifugal casting unit (Kerr Centrifico, Kerr Corp.) and a cristobalite investment (Cristobalite, Whip-Mix Corp.). Each casting procedure followed the manufacturer$B!G(Bs instructions. The Fe-Pt alloy was cast using an induction melting centrifugal casting machine (Eagle, Jelenko) and a magnesia-based investment (Titavest CB, Morita). After casting, the molds were allowed to bench-cool to room temperature.  The cast plates were then divested, air-abraded with 50 $B&L(Bm Al₂O₃ particles and ultrasonically cleaned with acetone for 10 minutes.

Heat treatment of cast Fe-Pt plates

            The cast Fe-Pt plates were heat-treated to obtain the magnetic properties appropriate for keepers.  They were sealed in an evacuated silica tube and solution-treated at 1325$B!k(BC for 45 min, and then quenched in ice water before laser welding.

Preparation of laser-welded specimens

After the 3.0 x 0.5 mm surfaces of two plates (0.5 x 3.0 x 10 mm) were polished with No. 600 SiC paper, they were butted against one another using a jig. Fe-Pt alloy plates were butted to each of the three gold alloys. Homogeneously welded specimens were also prepared for each alloy. The assembled cast plates were then welded with a Nd:YAG laser (Neolaser L, Girrbach Dental System) at a constant voltage of 200 V, pulse duration of 10 ms and spot diameter of 1 mm. The laser was perpendicularly applied to the interface of each pair of plates. Five specimens were prepared for each experimental condition. Each one was bilaterally welded with five laser spots per side (Fig. 1).

Tensile Testing

Tensile testing of the specimens was conducted with a universal testing machine (Model 1125, Instron Corp.) 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 (Ff: N) and elongation (El: %) were recorded, and the means (n=5) and standard deviations were calculated.  The data were statistically analyzed using an ANOVA and Tukey$B!G(Bs HSD test at a significance level of $B&A(B=0.05.

 

RESULTS

The results of failure load (Ff: N) and elongation (El: %) are presented in Figs. 2 and 3, respectively. The highest Ff value was obtained for the control group, followed by the group of alloys welded homogeneously, and the group welded to the Fe-Pt alloy. In the control group, the highest Ff values were obtained for the Ag-Au alloy, and the other gold (Types II and IV) alloys had similar Ff values (no significant difference at p>0.05). The Ff values of the group of alloys welded homogeneously were ranked as follows:  Ag-Au alloy > Type IV alloy > Type II alloy > Fe-Pt alloy (There was no difference between the Ag-Au and Type IV alloys).  The Type IV alloy welded to the Fe-Pt alloy had the highest Ff value among the three alloys tested. The El results tended to be similar to the Ff results, namely, the highest El value was obtained for the control group followed by the group of alloys welded to the same alloys, and the group welded to the Fe-Pt alloy. The El value of the Type II control was the greatest among all of the conditions examined.

 

CONCLUSIONS

1.    The highest value of failure load was obtained for the control group followed by the group of alloys welded to the same alloys, and the group welded to the Fe-Pt alloy. The elongation results tended to be similar to the results of failure load.

2.    Of the homogeneously-welded alloys, the Ag-Au alloy had the greatest value of failure load, followed by Type IV alloy, Type II alloy, and Fe-Pt alloy.

3.    Since the Type IV alloy welded to the Fe-Pt alloy had the highest value of failure load among the three alloys tested, the results suggested that the Type IV alloy is suitable for metal frameworks to which cast Fe-Pt magnetic alloy is attached by laser welding.

 

Acknowledgement

            Supported in part by NIH/NIDCR grant DE07188 and grant Y2001-Z from the BCDOHF.

 

Figure 1.           Laser-welding configurations and tensile test specimen (0.5 x 3.0 x 20.0 mm) used in this study.

                        Arrows indicate load applied for tensile testing.

 

Figure 2.           Failure load (N) of the specimens.  

 

Figure 3.           Elongation (%) of the specimens.