Microstructure of cobalt based alloy coatings produced by laser deposit welding

Abstract

Coatings of cobalt based alloy HMSP 2528, deposited on a nickel based superalloy using a transverse flow CO₂ laser, are investigated. Sections of the coatings have been examined to reveal their microstructure and phase composition using an Olympus PME–3 type optical microscope, scanning electron microscope, transmission electron microscope, and X-ray diffractometer. The results showed that primary phase (γ-Co) and eutectic (γ-Co + Cr₂₃C₆) were present in the coatings. Rapid directional solidification was found at the interface, where the growth direction was perpendicular to the interface. Multidirectional growth and growth parallel to the scanning direction were also found in the central region and near the top surface, respectively. At the remelted boundary between two tracks, a novel microstructure was found in which the crystallisation direction of the first track was transmitted to the second track. As more tracks were deposited, coarsening dendrites could be found at the interfaces.