Laser Beam Rapid Solidification of Plasma Sprayed Ni-Al Coatings on Steel

Abstract

Hard wear-resistant surfaces of mandrill tips manufactured from hot worked steels are used for extrusion to offer smooth inside surfaces of seamless pipes. In the present study, alloyed surfaces are synthesized on steel substrates using laser surface melting. Processing parameters are optimized to achieve wear-resistant surface layers of adequate thickness. Powders of nickel and Aluminium are simultaneously deposited on steel surfaces using plasma spraying technique. Plasma coated surface layers are fragile, brittle and non-adherent. These surfaces are subjected to laser beam melting of surface, followed by rapid solidification. Beam power and spot size of focused beam are 800 watts and 3 mm, respectively. Translation speed of sample with respect to beam is varied from 100 to 300 mm.s -1 to synthesize uniform surface layers over a range of thicknesses. Melt zone is hardly visible at higher velocities while consistent and uniform depth of 0.2 to 0.6 mm is achieved at velocities from 100 to 250 mm.s -1 . Microstructural study is conducted using optical and scanning electron microscopes while micro-hardness measurements are taken as a measure of mechanical behaviour and surface pin on disk tribometer for wear testing. Rapid solidification followed by laser melting has redefined the microstructure and developed strong metallurgical bond with steel substrate. Fine microstructure, along with super-saturation in alloying elements renders hardness and strength to the material enhancing wear properties, confirmed by micro hardness test results. Hardness depth profiles clearly differentiate three distant regions i.e. laser melted zone, intermediate zone and parent substrate. The typical hardness values of these regions are found to be 540±30, 470±50 and 200±15 Hv, respectively. Refinement of microstructure as well as uniformity and reduced segregation in the laser melted zone resulted in improved resistance to wear.