Casting Junction between Gold-coated Keeper of Magnetic Attachment and Ag-Pd-Au Cast Alloy

Purpose: This study examined whether coating the keeper in a magnetic attachment system with a layer of gold can ensure cast-joining to a Ag-Pd casting alloy containing 12% Au without a gap. Methods: A commercially available gold-coated keeper and experimental keepers with or without gold coating were used. The experimental keepers were specially fabricated from soft magnetic stainless steel that was identical to that used for a commercially available gold-coated keeper; they were then coated with a layer of gold (4 or 10μm thick). A commercial Ag-Pd alloy containing 12% Au was used for cast-joining. Each keeper with a wax rod was invested, and the wax pattern was burned out at 700°C. The Ag-Pd alloy containing 12% Au was then cast according to conventional laboratory procedures. The tensile cast-joint strength of the specimens was measured using a universal testing machine in air at room temperature. Also, the cross sections of the cast-joined specimens were examined by electron probe microanalysis. Results: The joint strength test showed that all the gold-coated specimens, including the commercially available gold-coated keepers, had significantly greater cast-joint strength than those without gold coating. A gradual decrease was observed in the concentration of gold from the layer of gold on the keeper to the Ag-Pd alloy containing 12% Au surface. Close contact between the cast alloy-without the presence of an oxide layer- and the surface of the keeper was seen at the cast-joined interface area in the specimens made with gold-coated keepers. Conclusions: The presence of the gold layer seems to minimize the formation of oxide films on the keeper during the casting process. This may be the main reason for an increase in the cast-joined strength of the gold-coated keepers to the cast alloy used.