This work describes the feasibility of continuous wave laser-raster scanprocessing under controlled atmospheric conditions as employed in three distinct
surface modification processes: (a) surface roughness reduction of indirect-Selective
Laser Sintered 420 martensitic stainless steel–40 wt. % bronze infiltrated surfaces (b)
Si-Cr-Hf-C coating consolidation over 3D carbon–carbon composites cylinders (c)
dendritic solidification structures of Mar–M 247 confined powder precursor grown
from polycrystalline Alloy 718 substrates. A heat transfer model was developed to
illustrate that the aspect ratio of the laser scanned pattern and the density of scanning
lines play a significant role in determining peak surface temperature, heating and
cooling rates and melt resident times. Comprehensive characterization of the surface
of the processed specimens was performed using scanning electron microscopy
viii
(SEM), energy dispersive spectroscopy (EDS), optical metallography, X-ray
diffraction (XRD), and, in certain cases, tactile profilometry. In Process (a), it was
observed that a 24 % to 37 % roughness Ra reduction could be accomplished from the
as-received value of 2.50+/-0.10 microns for laser energy densities ranging from 350
to 500 J/cm2
. In Process (b), complete reactive wetting of carbon-carbon composite
cylinders surface was achieved by laser melting a Si-Cr-Hf-C slurry. Coatings
showed good thermal stability at 1000ºC in argon, and, when tested in air, a percent
weight reduction rate of -6.5 wt.%/hr was achieved. A soda-glass overcoat applied
over the coated specimens by conventional means revealed a percent weight
reduction rate between -1.4 to -2.2 wt.%/hr. Finally, in Process (c), microstructure of
the Mar-M 247 single layer deposits, 1 mm in height, grown on Alloy 718
polycrystalline sheets, resulted in a sound metallurgical bond, low porosity, and
uniform thickness. Polycrystalline dendrites grew preferentially along the [001]
direction from the substrate up to 400 microns. Above that height, dendrites appear to
shift towards the [100] growth direction driven by the thermal gradient and
solidification front velocity. This research demonstrated that surface modification by
high-speed raster-scanning a high power laser beam under controlled atmospheric
conditions is a feasible and versatile technique that can accomplish diverse purposes
involving metallic as well as ceramic surfaces.This work describes the feasibility of continuous wave laser-raster scanprocessing under controlled atmospheric conditions as employed in three distinct
surface modification processes: (a) surface roughness reduction of indirect-Selective
Laser Sintered 420 martensitic stainless steel–40 wt. % bronze infiltrated surfaces (b)
Si-Cr-Hf-C coating consolidation over 3D carbon–carbon composites cylinders (c)
dendritic solidification structures of Mar–M 247 confined powder precursor grown
from polycrystalline Alloy 718 substrates. A heat transfer model was developed to
illustrate that the aspect ratio of the laser scanned pattern and the density of scanning
lines play a significant role in determining peak surface temperature, heating and
cooling rates and melt resident times. Comprehensive characterization of the surface
of the processed specimens was performed using scanning electron microscopy
viii
(SEM), energy dispersive spectroscopy (EDS), optical metallography, X-ray
diffraction (XRD), and, in certain cases, tactile profilometry. In Process (a), it was
observed that a 24 % to 37 % roughness Ra reduction could be accomplished from the
as-received value of 2.50+/-0.10 microns for laser energy densities ranging from 350
to 500 J/cm2
. In Process (b), complete reactive wetting of carbon-carbon composite
cylinders surface was achieved by laser melting a Si-Cr-Hf-C slurry. Coatings
showed good thermal stability at 1000ºC in argon, and, when tested in air, a percent
weight reduction rate of -6.5 wt.%/hr was achieved. A soda-glass overcoat applied
over the coated specimens by conventional means revealed a percent weight
reduction rate between -1.4 to -2.2 wt.%/hr. Finally, in Process (c), microstructure of
the Mar-M 247 single layer deposits, 1 mm in height, grown on Alloy 718
polycrystalline sheets, resulted in a sound metallurgical bond, low porosity, and
uniform thickness. Polycrystalline dendrites grew preferentially along the [001]
direction from the substrate up to 400 microns. Above that height, dendrites appear to
shift towards the [100] growth direction driven by the thermal gradient and
solidification front velocity. This research demonstrated that surface modification by
high-speed raster-scanning a high power laser beam under controlled atmospheric
conditions is a feasible and versatile technique that can accomplish diverse purposes
involving metallic as well as ceramic surfaces.Materials Science and Engineerin
