Mechanical Properties Of Laser-welded Joints Of Different Dental Alloy And Properties Of Laser-welded Keepers

Magnetic attachment is gaining more popularity and being used extensively in clinical work of prosthodontics, for its advantages such as reliable and constant retention, little lateral stress to the abutment tooth, ease of insertion for both dentist and patient and so on. Magnetic attachment is made up of two parts. One part is close-field magnet, the other part is keeper. Keeper is an important part of magnetic attachment. It's properties directly affect the retentive effect of the attachment. Usually, the keeper is cast in coping alloy. But the high temperature inevitably affects the keeper's property. In order to explore a better means of dealing with keeper, advanced laser-welding technique is introduced into this experiment. The research is carried out from two aspects to explore the feasibility of laser-welded keeper.To explore the mechanical properties of laser-welded joints of Fe-Cr-Mo magnetizable alloy and Co-Cr alloy, Ni-Cr alloy respectively, standard tensile testing rods of the above material were fabricated. The rods of Fe-Cr-Mo alloy were machined and cut in half. Wax pattern was made in accordance with the impression of this rod. Those rods of Co-Cr alloy and Ni-Cr alloy were cast as usual. The intact ones were picked out for test specimens. Home-made programmed laser welding apparatus was adopted to weld items of Fe-Cr-Mo alloy and Co-Cr alloy, Fe-Cr-Mo alloy and Ni-Cr alloy respectively. The laser welds of Co-Cr alloy were chosen as control group. The tensile strength and 0.2% yield strength were measured on Instron universal testing machine. The fractured surface was observed under SEM. Statistical analysis was applied to the results and no significant difference was showed. As to toughness, the fractured surface of laser-welded Fe-Cr-Mo alloy and Co-Cr alloy was quasi uncoiling, and that ofFe-Cr-Mo alloy and Ni-Cr alloy had tenacious fractured surface, while that of laser welded Co-Cr alloy took on a tenacity- uncoiling look. It meant that laser welds of Fe-Cr-Mo alloy and Ni-Cr alloy were the best. And those of Fe-Cr-Mo alloy and Co-Cr alloy were inferior to it but superior to the welds of Co-Cr alloy.Based on the above results, more research was carried out to compare the breakaway retention between magnetic attachments with cast coping keepers, laser-welded keepers and ready-made keepers. The roughness of the surface of three kinds of keepers were also measured by instrument. In this test, wax patterns were fabricated on standard canine root. Z-3 type keepers were embedded in the wax coping in cast group. The replica of keeper was taken out after wax coping was constructed so as to form a place in which keeper would be welded. Nd:YAG laser welding apparatus, Instron universal testing machine and roughness instrument were adopted respectively to weld keeper to root coping, test retentive force of magnetic attachments and analyze keepers' surface. The laser-welded keepers were superior to cast ones in terms of keepers' surface. For their smoothness is grade 0.1, the same as ready-made keepers. While the cast keepers fall into grade 0.4 after filing out pit defects, with which the smoothness can't be shown by roughness instrument. But the retentive force of the magnetic attachments with laser-welded keepers showed no difference from those with cast keepers, both were a little smaller than that of the attachments with ready-made keepers.In general, conclusion can be drawn as: the mechanical properties of laser-welded joints for Fe-Cr-Mo alloy and Co-Cr alloy or Ni-Cr alloy meet clinical demands; laser-welded keepers exhibit better surface property than cast coping keepers; laser-welded coping keepers are fit for clinical application and can be adopted in clinical work when using magnetic attachments .