The effect of remelting various combinations of new and used cobalt-chromium alloy on the mechanical properties and microstructure of the alloy.

ABSTRACT

Context Remelting previously cast base metal alloy can adversely affect the mechanical properties of the alloy and necessitates addition of new alloy. Aims: To study the effect of remelting different combinations of new and used cobalt-chromium (Co-Cr) alloy on its mechanical properties and microstructure. Materials and Methods: Using induction casting, 24 tensile test specimens were prepared for eight different combinations of new and used Co-Cr alloy. The test specimens were assessed for yield strength and percentage elongation. Microhardness was evaluated using Vickers's hardness tester. The tensile testing was carried out on a 50 kN servo-hydraulic universal testing machine. Microstructure analysis was done using an optical photomicroscope on the fractured samples after acid etching. Statistical Analysis: The mean values (±standard deviation) and coefficient of variation were calculated. Student's 't' test was used for statistical analysis. Statistical significance was assumed at P=.05. Results: The mean yield strength of eight different combination groups were as follows group A 849 MPa, group B 1 834 MPa, group B 2 915 MPa, group B 3 897 MPa, group C 1 874 MPa, group C 2 859 MPa, group D 1 845 MPa, and group D 2 834 MPa. The mean percentage elongation for the different groups were as follows group A 7%, group B 1 7%, group B 2 8%, group B 3 7%, group C 1 8%, group C 2 7%, group D 1 7%, and group D 2 8%. The mean hardness values were as follows group A 373 VHN, group B 1 373 VHN, group B 2 346 VHN, group B 3 346 VHN, group C 1 364 VHN, group C 2 343 VHN, group D 1 376 VHN, and group D 2 373 VHN. Conclusion: Repeated remelting of base metal alloy for dental casting without addition of new alloy can affect the mechanical properties of the alloy. Microstructure analysis shows deterioration upon remelting. However, the addition of 25% and 50% (by weight) of new alloy to the remelted alloy can bring about improvement both in mechanical properties and in microstructure.