Wire-arc spraying system: Particle production, transport, and deposition

Protective coatings are important to metal working. Thermal spray is a rapidly growing market, and wire-arc spraying is gaining a significant share of this market because of its low operating/equipment costs and high material/energy efficiency. Although wire-arc spraying is widely used, many of its underlying processes are not yet fundamentally understood. This work examines and explains different aspects of a wire-arc system.;An imaging system was developed to take pictures of the arc, and determine its length and shape. Using the information extracted from such pictures, a computational fluid dynamic model of the wire-arc torch was developed to estimate the shear stresses on the wire-tips and also sizes of primary breakups from the two electrodes.;Shortly after primary breakups, the detached particles break up into smaller particles (secondary atomization). The size and velocity of such particles were measured in-flight using a DPV-2000 system for a range of operating parameters. A technique was developed to identify and separate the size distributions of particles produced by atomization of molten metal at either the anode or cathode by assuming that both follow a log-normal distribution. (This assumption was also verified experimentally.) It was shown that particles produced by the anode are almost two times larger than those originating from the cathode. Furthermore, effect of operating parameters on size distribution of anodic and cathodic particles was investigated.;Experiments were also conducted to study the effect of impact velocity and substrate temperature on the properties of individual wire-arc splats and coatings. Aluminum was sprayed onto polished stainless-steel coupons maintained at temperatures ranging from 25°C to 450°C. At low substrate temperature, droplets splashed, forming irregular splats; at higher temperatures there was no splashing and splats formed circular disks. The temperature at which the transition occurred decreased with increasing impact velocity.;In wire-arc spraying, two consumable wires are continuously fed into the gun. An electric arc is struck between the tips of these two wires and continuously melts their material. A cross-flow gas removes the molten material from the wire-tips and accelerates them towards a substrate, over which the detached particles form a protective coating layer.