MCrAlY coatings are widely used to protect superalloy components against oxidation and corrosion in hot sections of gas turbines. In this research, the effect of laser surface melting treatment on the microstructure, morphology and hot oxidation behavior of HVOF-sprayed NiCoCrAlY coating has been investigated. For this purpose, after spraying the NiCoCrAlY coating on the CMSX-4 nickel-based superalloy substrate, the surface melting treatment on the coatings was performed by pulsed Nd:YAG laser in both single-pass and multiple-pass ways. Scanning electron microscope, energy dispersive spectroscopy, optical microscope, roughness test and microhardness test were used to eva‎luate the morphology, microstructure and hardness of the coatings. The results showed that the laser surface melting process is significantly effective in improving adhesion, reducing porosity, and reducing surface roughness of the coating. At a power of 70 W (scanning speed 2, 3 and 4 mm/s), about half of the depth of the coating is remelted and an almost dense structure in the upper half of the coating is achieved. At a power of 250 W (scanning speed 8, 10 and 12 mm/s), in addition to the relatively complete removal of structural porosity, surface inhomogeneities and oxide strings in the coating structure, the entire depth of the coating is remelted and the mechanical bond in the coating-substrate interface , turned into a metallurgical bond and a better adhesion between the coating and the substrate was created. At a constant scanning speed (2 mm/s), with a 40% increase in laser power from 50 to 70 W, the depth of the remelted zone increased by about 50% and the width of the remelted zone increased by about 100%. On the other hand, in constant laser power (70 W), with a 50% increase in the scanning speed from 4 to 6 mm/s, the depth of the remelted zone decreased by about 14% and the width of the remelted zone decreased by about 0.5%. Therefore, the effect of increasing the laser power in increasing the depth of the remelted zone, reducing the porosity in the coating and increasing the width of the remelted zone is more than reducing the laser scanning speed. The cross-sectional area of the single-pass remelting zone increased linearly with increasing laser power at constant scanning speed. On the other hand, by reducing the laser scanning speed at constant power, the cross-sectional area of the single-pass remelting area increased linearly. However, the effect of increasing the laser power in increasing the cross-sectional area of the remelting zone was greater than decreasing the laser scanning speed. In the laser surface melting process, the remelting zones of multiple passes were deeper than single pass. The average roughness of Ra for NiCoCrAlY coating after HVOF deposition was about 3.45 micrometers, which was reduced by about 80% after the laser surface melting process. Also, due to the high solidification rate, it led to a decrease in grain size and an increase in hardness from the interface to the coating surface by about 15%. On the other hand, the comparison of the variables of the laser surface melting process showed that, in general, at a constant laser power, the microhardness value of the coating surface increases with the increase of the laser scanning speed due to the finer dendritic structure. In order to investigate the hot oxidation behavior of the coated samples before and after laser surface melting, the isothermal hot oxidation test was performed at 1000°C for 170 hours. The results showed that the laser surface melting process significantly increases the hot oxidation resistance of NiCoCrAlY coating by improving adhesion, reducing porosity and reducing surface roughness, and reduces the growth rate of the oxide layer and the formation of a stable α-Al2O3 layer.

احمد كرمانپور، فخرالدين اشرفي زاده

مهران نحوي، مسعود عطاپور