Surface modification of ceramic and metallic alloy substrates by laser raster-scanning
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  direction from the substrate up to 400 microns. Above that height, dendrites appear to shift towards the  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.