A Study On The Mechanical Properties And Microstructure Of Laser-welded CoCr-NiCr Dissimilar Alloys Commonly Used In Dentistry

Objective: Applied laser welding technique to CoCr -NiCr dissimilar alloys commonly used in dentistry, in order to resolve the connection between dissimilar alloys which was the difficult process at precision attachment and telescopic prostheses. The aim of the study was to simplified the manufacture skill and improve the quality of prostheses.Methods: Welded CoCr-NiCr dissimilar alloys samples using Nd- YAG laser under different electric voltage condition, optimized the welding parameters through the tensile strength and elongation. And then observed the microstructure and chemical element distribution of the fusion zone at the condition of optimized welding parameters. Evaluated the influence of heat treatment and porcelain processing on the mechanical properties and microstructure. Applied the technique to ten telescopic prostheses manufactured with laser welding and observed the result.Results:1. The tensile strength of laser welding CoCr-NiCr dissimilar alloys increased with the electric voltage, but which of all groups were lower than the base materials. The elongation increased with the electric voltage at the electric voltage lower than 280 V, but showed contrary trend when the electric voltage higher than 280Vs. The tensile strength and elongation was 563.59MP and 6.12% respectively at the optimized welding parameters.2. The fusion zone of laser welded CoCr-NiCr dissimilar alloys was found clearly and the heat affect zone was indistinct. There was no crack and pore defect at fusion zone. The joining between the fusion zone and the two side base materials was metallurgical combination. But an intercrystal strip was found at the NiCr side beside the fusion zone. Nickel, as the advantage chemical element, was existent in solidsolution and the compound. Neonatal compounds were not found, but specimens fractured in a hard and brittle manner.3. Heat treatment and porcelain processing had slightly effect on tensile strength but obviously on elongation. Tensile fracture surface of porcelain processing group appeared obvious ductile structure in which heat treatment group was more obvious. The intercrystal corrosion strip was f aintness in porcelain processing group and disappeared in heat treatment group. The results of energy spectrum analysis and X ray diffraction analysis indicated that a certain cobalt chrome side composition diffused towards fusion zone and NiCr side after heat treatment and porcelain processing. Compared with porcelain processing group, the diffusion degree of the heat treatment group was more obviously.4. Neither prostheses, manufactured with CoCr-NiCr dissimilar alloys laser welding technique, was split and fracture nor was corrosion and decay after 6 to 12 mouths following up.Conclusions:1. The tensile strength and elongation of laser welded CoCr-NiCr dissimilar alloys under the optimized parameters (280 V, 10 ms, spot diameter 0.6 mm) can be satisfied with the clinical requirement.2. Heat treatment and porcelain processing can improve the mechanical properties, microstructure and compound distribution of laser-welded CoCr-NiCr dissimilar alloys in some extent which indicated that, when using laser welding technique on dissimilar alloys in clinic, the laser welded samples were needed heat treatment or welded before porcelain.3. Clinical application of laser-welded CoCr-NiCr dissimilar alloys was favorable.