Laser Sintering Technology for Co-Cr alloys

An alternative in the use of Co-Cr alloys is the laser sintering technique. The objective of this study is to assess the quality of Co-Cr prosthetic restorations, obtained with this method. Selective laser sintering technique uses a high power CO2 laser to fuse small particles of metal powders into a mass representing a desired 3-dimensional object. The laser selectively fuses metal powders by scanning cross-sections generated from a 3-D digital description by CAD file or scans data on the surface of a powder bed. After each cross-section is scanned, the powder bed is lowered by one layer thickness, a new layer of material is applied on top, and the process is repeated until the part is completed. The topography of the dental restoration is designed by numerical monitoring after having scanned devised prostheses. The laser is thus programmed in such a way that it only becomes active at the site where the element should be achieved. In our application the alloy consists of 64-67% Co, 28-30% Cr and 5-6% Mo, and has at equilibrium of a γ monophasic structure. Evaluation and characterisation was made by: Microstructure analysis; Corrosion resistance evaluation; Polarization test; Crevice corrosion test; Release of cations. Dimensional observations of Co-Cr restorations show that adjustment leads to satisfactory clinical results with a precision of 25 µm. The average hardness is 395HV, comparable to metallo-ceramic elements obtained by casting technique. The metallographic observations show a slight porosity in the horizontal plan (sintering plane). Punctual analysis shows a high regularity of the local chemical composition: 62.6-64%Co, 29.3-30.5%Cr and 4.9-6.4%Mo. Potentiodynamic polarization curves confirm the presence of the porosity in the structure of the restoration. SEM micrograph of the corroded surface after the polarization test shows: O 24.53, Na 1.19, Si 1.12, Cl 0.59, Cr 28.76, Co 34.30, Mo 9.52. The technique of manufacturing by selective laser sintering allows obtaining prosthetic elements of high dimensional precision which present mechanical properties in agreement with the clinical requirements. However, the residual porosity inherent to the sintering process may present a risk for fracture and crevice corrosion. Keywords: laser sintering, Co-Cr alloys