Conclusions
In a wire-arc spray system (such as Sulzer-Metco’s ValuArc 200, which was used in this
study), particles are formed by atomization of molten metal from the tips of two consumable
wires between which an electric arc is struck. A cross-flow atomizing gas accelerates the
detached particles towards a substrate on which a protective coating is formed by deposition of
these particles.
In this study, arc voltage and arc current were experimentally measured at different
operating conditions. The measured data were analyzed to find the energy delivered to unit mass
of the fed material, and to estimate aluminum evaporation. Arc voltage fluctuations were also
looked at, and analyzed to obtain an estimate of the size of primary atomizations from the wires.
It is well known that the arc attaches to the anode over a much larger area that the
cathode and, consequentially, particles separating from the anode are larger than those from the
cathode. The sizes of these primary detachments were estimated using computational fluid
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dynamics and simplified models of arc and particle-breakup. These simplified models were
based on the information obtained from pictures of the arc (taken with Nikon E3 digital camera)
and pictures of metal detachment (taken with a custom-made UV camera with Nitrogen laser
illumination).
Shortly after primary atomization, the detached particles break up into smaller particles
(undergo secondary atomizations). These in-flight particles are a mixture of cathodic and anodic
particles. Although mixed, an algorithm was presented to identify the size distributions of the
two sets of particles. The presented algorithm assumes that both anodic and cathodic particles
follow a log-normal distribution. This assumption was also validated by spraying magnetic and
non-magnetic materials (as anode and cathode), and separating the resultant particles.
The presented separation-algorithm provides a tool to study effects of operating
parameters on each of the anodic or cathodic set of particles. Experiments showed that increasing
the atomizing gas pressure decreased the size of both anodic and cathodic particles, while
changing wire-feed-rate and operating voltage did not change particle size significantly.
Axial variations (along the spray plume) of particle velocity and temperature were also
investigated. While aluminum particles decelerate as they move towards the substrate (as
expected), their temperature remains almost constant. This was explained by analyzing the
exothermic oxidation of the surface of aluminum particles. The presented explanation was also
verified by spraying aluminum with nitrogen as atomizing gas (to prevent oxidation).
Also, effects of substrate temperature and spray velocity on the properties of aluminum
splats and coatings deposited on mirror-polished steel surfaces were studied. As substrate
temperature was increased droplets no longer splashed, but formed disk shaped splats.
Aluminum particles sprayed with an average velocity of 109 m/s had a transition temperature of
230°C; particles sprayed with an average velocity of 131 m/s had a transition temperature of
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200°C; particles sprayed with an average velocity of 143 m/s had a transition temperature of
140°C.
In addition, coating porosity levels were measured (less than 5% for all coatings
produced). Raising substrate temperature reduced the size and density of voids at the
coating/substrate interface, and also increased deposition efficiency and adhesion strength of
coatings.
By conducting the studies on a mirror-polished surface, the effect of surface roughness on
splashing was eliminated, allowing focusing on other phenomena that promote splashing.