Microstructural Formation During Wire Arc Spraying of Alloy 625 on Wrought Alloy 625 Substrate
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INTRODUCTION
THERMAL spraying is a general term for a group of coating techniques such as air plasma spraying (APS), vacuum plasma spraying (VPS), high velocity oxygen fuel (HVOF), and wire arc spraying. These techniques are used to deposit various materials available in powder or wire forms as molten or semimolten particles onto the surface of a substrate. There has been a growing worldwide interest in the use of thermal spray processes to deposit Ni-based superalloys as protective coatings onto the surfaces of engineering parts and components to improve their performances under severe service condition.[1,2] Wire arc spray (electric arc spray) is a well-developed technology to deposit coating layers up to a few millimeters in thickness. Wire arc spraying is a flexible, rapid, and relatively cheap method compared to the other deposition techniques. In this process, an electric arc forms between two wires as consumable electrodes made from the desired coating material. Thermal energy resulting from the arc melts both wires, which continuously are fed into the system. Molten metal in the form of droplets is broken up (atomized) into finer particles F. AZARMI, Assistant Professor, and C.P. LEITHER, Graduate Student, are with the Department of Mechanical Engineering, North Dakota State University, Fargo, ND 58108. Contact e-mail: fardad. [email protected] Manuscript submitted January 19, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
and accelerated toward the substrate by a high-velocity air stream.[3] The thermal efficiency of the process is higher than other thermal spraying techniques due to the direct melting of coating materials by the arc. Since there is no flame associated with the melting process, the heat transfer into the substrate is kept to a minimum level. Wire arc spraying is a suitable method for deposition on any type of substrate with minimum built up thermal stress. Coatings are built up from the impact, flattening, and solidification of fine molten particles, splats, connected to each other by mechanical and metallurgical bonding, forming a layered structure with anisotropic behaviors. The coatings exhibit different thermomechanical properties in longitudinal and transverse directions.[4,5] Zhao et al. considered the anisotropy of the microstructure to develop a model to estimate elastic moduli of the coatings in both the parallel (transverse) and perpendicular (longitudinal) directions relative to the coating plane.[6] The structural integrity of coatings depends highly on microstructural properties of the deposition. Therefore, the microstructural characteristics are more critical in thermal-sprayed depositions and require thorough investigation. Porosity, oxidation, microcracks, and voids are the most important microstructural characteristic features of arc-sprayed coatings. Porosity is known to decrease the elastic modulus and strength of thermal-sprayed coatings.[7] During spraying of reactive metals in air, rapid oxidation reactions may occur. This oxidation can be considered
a crucial problem in wi
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