Microstructural analysis and oxidation behavior of laser-processed Fe-Cr-Al-Y alloy coatings

Dense crack-free coatings of Fe-Cr-AI-Y quaternary alloy were produced on stainless steel 316L substrates using a continuous wave Nd-YAG solid-state laser coupled with a fiber optic beam delivery system. Experiments were performed at a laser power between 0.6 and 2.4 kW, process speed in the range 0.053 to 0.423 cm/s, powder feed rate fixed at 0.083 g/s, and focused multimode laser beam with a diameter of 0.5 cm. Various microanalysis techniques demonstrated that the coatings were metallurgically bonded to the substrate and possessed thicknesses between 0.35 and 4.64 mm, refined columnar microstructures with grain sizes of 15 to 150 mu m, increased concentration of key alloying elements, and appreciably high microhardness up to 409 kg/mm super(2). The laser-processed microstructures comprised a body-centered cubic (bcc) ferrite ( alpha phase) crystal structure with a relatively large lattice parameter compared to alpha -Fe due to the enhanced dissolution of varying amounts of Cr, Al, Ni, and Y, depending on the dilution from the substrate material. Oxidation tests conducted in air at temperatures of 1100 degree C to 1200 degree C for 95 hours revealed the formation of an approximately 5 mu m-thick dense alpha -Al sub(2)O sub(3) oxide scale of a rhombohedral (hexagonal) crystal structure. The alpha -Al sub(2)O sub(3), scale exhibited remarkable high-temperature resistance and strong adherence to the coating surface. Extensive oxidation of the uncoated stainless steel substrate produced a porous and heavily spalled alloy oxide scale about 60- mu m thick consisting of FeCr sub(2)O sub(4) and NiCr sub(2)O sub(4) with cubic and tetragonal crystal structures, respectively. The retention of the bcc alpha phase and the insignificant grain growth after oxidation are indicative of the thermal stability of the laser-processed coating microstructures. The obtained results demonstrate that Fe-Cr-Al-Y alloy coatings exhibiting fine-grained hard microstructures, high-temperature oxidation resistance, and strong adherence to stainless steel can be developed by means of laser processing.