Structural investigations of the diffusion-zone of soldered dental gold and base metal alloys

Mechanical and elemental characterization of solder joints and welds using a gold-palladium alloy

PURPOSE

This study was conducted to determine whether newer infrared or laser welding technologies created joints superior to traditional furnace or torch soldering methods of joining metals. It was designed to assess the mechanical resistance, the characteristics of the fractured surfaces, and the elemental diffusion of joints obtained by four different techniques: (1) preceramic soldering with a propane-oxygen torch, (2) postceramic soldering with a porcelain furnace, (3) preceramic and (4) postceramic soldering with an infrared heat source, and (5) laser welding.

MATERIAL AND METHODS

Mechanical resistance was determined by measuring the ultimate tensile strength of the joint and by determining their resistance to fatigue loading. Elemental diffusion to and from the joint was assessed with microprobe tracings. Scanning electron microscopy micrographs of the fractured surface were also obtained and evaluated.

RESULTS

Under monotonic tensile stress, three groups emerged: The laser welds were the strongest, the preceramic joints ranged second, and the postceramic joints were the weakest. Under fatigue stress, the order was as follows: first, the preceramic joints, and second, a group that comprised both postceramic joints and the laser welds. Inspection of the fractographs revealed several fracture modes but no consistent pattern emerged. Microprobe analyses demonstrated minor diffusion processes in the preceramic joints, whereas significant diffusion was observed in the postceramic joints.

CLINICAL IMPLICATIONS

The mechanical resistance data conflicted as to the strength that could be expected of laser welded joints. On the basis of fatigue resistance of the joints, neither infrared solder joints nor laser welds were stronger than torch or furnace soldered joints.

Fatigue strength of a Au-Pd alloy/585 solder combination

The S-N diagram (applied stress vs. cycles to failure) for the gold-palladium alloy/585 solder combination was generated. Au-Pd alloy specimens were cast, prepared for soldering, and joined with 585 solder. After milling and polishing of the joint, testing was carried out in a specially designed machine. This device loaded the cantilevered test specimen in a unidirectional mode at a rate of 70 strokes per minute. The applied tensile stress ranged between 294.2 and 686.5 MPa. Three specimens were cycled for each stress level until fracture occurred. The number of fatigue cycles to failure ranged from one cycle (stress level, 686.5 MPa) to 4.83 x 10(6) cycles (stress level, 294.2 MPa). On a logarithmic scale, the relationship between the applied stress and the number of cycles to failure was linear. Under the present (ideal) experimental conditions, the Au-Pd/585 solder combination behaved favorably on the high-cycle end of the diagram. However, no definite endurance limit could be detected within the stress range examined.