Abstract
Statement of problem
The selective laser melting (SLM) process has become popular for the fabrication of
frameworks for metal-ceramic restorations, although their surface roughness is greater
than with cast or milled frameworks. Limited information is available regarding the
surface mechanical characteristics of cobalt-chromium (Co-Cr) SLM-manufactured restorations.
Purpose
The purpose of this in vitro study was to adapt the laser parameters for a remelting
strategy, scanning the outer boundary of Co-Cr specimens, to reduce surface roughness
and solidification defects, to determine microhardness, to investigate surface morphology
and microstructure, and to establish surface mechanical characteristics.
Material and methods
Co-Cr specimens were SLM manufactured by using a typical melting (TM) strategy and
an adaptive remelting (AR) strategy. The AR strategy involves rescanning 50% of the
contour, varying the laser parameters. The roughness parameters considered were Ra
and Rz. Vickers hardness was measured by microindentation with a 9.81-N force (ASTM
E384-17). The surface morphology was investigated by scanning electron microscopy, the chemical
composition by energy-dispersive X-ray spectroscopy, and the phase identification
by using X-ray diffraction. The mechanical surface properties measured were the nanohardness,
elastic modulus, and dissipation energy. One-way ANOVA with the Tukey procedure was
used to compare the groups (α=.05).
Results
The innovative AR strategy reduced the surface roughness by 45% compared with TM,
comparable with their cast counterpart. The smoothest AR surface was obtained by using
75 W and 350 mm/s for the first scanning of the contour, followed by a second remelting
with 80 W and 700 mm/s. The microstructure of AR specimens had limited solidification
defects, a chemical composition similar to that of raw powder, and a surface microhardness
over 600 HV1. A fine grain structure in a single matrix phase was detected both on
TM and AR specimens. The mechanical characteristics of the smoothest Co-Cr surface
were 218 GPa elastic modulus, 746 HVIT Vickers nanohardness, 21 243 pJ plastic energy, and 26% nanoindentation work ratio.
Significant differences were observed between the melting strategies (P<.05) both for surface roughness and microhardness.
Conclusions
The laser scanning strategy affects both the surface roughness and the hardness of
SLM-manufactured specimens. The results show that using the AR strategy and proper
laser parameters can reduce the roughness and increase the surface hardness of Co-Cr
specimens made of conventional powder feedstock.
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Article info
Publication history
Published online: February 17, 2021
Footnotes
Supported by the Project “Entrepreneurial competences and excellence research in doctoral and postdoctoral programs - ANTREDOC”, project co-funded by the European Social Fund (code POCU/380/6/13/123927).
Identification
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© 2021 by the Editorial Council for the Journal of Prosthetic Dentistry.
