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Hot Corrosion and Oxidation Behaviour of a Laser Processed Nickel-Based Superalloy
B.K. Narayanan, M. Duraiselvam, S. Natarajan and M.A. Khan

The significance of grain refinement in improving the resistance to high temperature oxidation and corrosion was studied. An as-cast nickel-based superalloy used for manufacturing blisks and vanes in advanced gas turbines was used in the present work. A Yb-doped fibre laser having 1080 μm wavelength operated in continuous wave mode was used to laser treat the surface of the test material to modify the microstructure across the treated depth. The laser beam scan parameters were optimized through pilot experiments and the surface is treated in parallel tracks having 30% overlap. The cross-section of the treated samples was studied to understand the volume fraction of grain refinement. A finely refined microstructure was observed across the treated depth and their morphology varies with the different combination of laser power and scan speed. The combination of higher laser power with reduced interaction time imparts finely refined equiaxed grain structure near the top surface and columnar grains having reduced primary arm spacing near the bottom of the treated depth. This imparts higher surface hardness near the surface which reduces linearly towards the treated depth. The effect of laser surface treatment in improving the oxidation and corrosion resistance was analysed. The samples were tested at 1100 and 800°C in a muffle furnace to study the high temperature oxidation and corrosion behaviour of the nickel-based superalloy base and treated surfaces. For high temperature corrosion testing the samples were coated with a thin film of 40%Na2SO4+ 50%NaCl+10%V2SO4 salt to accelerate the corrosion. The weight of the samples was monitored at a regular interval. The oxidized and corroded samples were examined using scanning electron microscope (SEM). The results revealed that the laser treated surface favour the formation of stable oxide layers at high temperature and resists the surface from further oxidation and the refinement of grains resist the corrosion compared with the coarse microstructure of the nickel-based superalloy base substrate.

Keywords: Fibre laser, nickel superalloy, gas turbine, hot corrosion, oxidation, microstructure, volume fraction, grain refinement, failure analysis

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