Wire Arc Spraying

Abstract

Wire arc spraying (WAS) is one of the oldest of thermal spray processes. Compared to alternate surface modification technologies, WAS is most competitive because of its high deposition rates up to 25 kg/h, favorable economics and potential use for a wide range of applications mostly for corrosion protection of infrastructure such as bridges and metallic constructions, as well as in the marine and automotive industry. The technology is based on the concept of sticking an arc between two wires, or a single wire and a non-consumable electrode. As the tip of the wire(s) melts, the formed molten metal is atomized by a high-velocity gas stream, forming a spray of molten metal droplets that is projected toward the substrate on which it is deposited forming the coating. The atomizing gas being essentially at room temperature, the substrate is not subjected to heated during the spray process. The main limitation of the technology is that it works best for the spraying of ductile metallic wires, such as aluminum, zinc, or steels. The development of the “cord–wire” approach in which a wire is formed through the use of a ductile metallic foil as envelop filled with a ceramic or composite fine powder allowed the expansion of the technology to new industrial-scale wear resistance applications involving the spraying of carbide-based cermets such as Cr₃C₂ with Fe and FeC, WC/W₂C + Fe, WC/TiC + Fe, Cr, Ni. The technology can be combined with epoxy or silicon polymer coating for the sealing open porosity in the coating provided that the service temperature is below (< 200 °C). In this chapter, the basic concepts behind the technology are discussed highlighting the fundamental phenomena involved. This is followed by a review of industrial torch designs with emphasis on their relative advantages, limitation, and arc-droplet dynamics and their impact on coating quality. Process technology is reviewed next with examples of some of the leading WAS applications.

Nomenclature

Units are indicated in parentheses; when no units are indicated, the parameter is dimensionless.

1.1 Latin Alphabet

d :

Droplet/particle diameter (m)

d _ℓ :

Liquid jet diameter (m)

I :

Arc current (A)

L _ℓ :

Characteristic dimension for the liquid (m)

L _s :

Characteristic length (m)

$\dot{Q}$ :

Gas flow rate (m³/s)

R _0.5 :

Radius at which the droplet flux reached 50% of its maximum centerline value (m)

Re :

Reynolds number (Re = ρ_ℓu_ℓd_ℓ/μ_ℓ)

T :

Droplet temperature (K)

u _ℓ :

Liquid velocity (m/s)

u _r :

Relative velocity of the liquid to gas stream (m/s)

v :

Droplet velocity (m/s)

v_s:

Substrate traverse velocity (m/s) or (m/min)

x, y, z :

Lateral, perpendicular, and axial coordinates (m)

We :

Weber number $\left( We={u}_r^2{L}_{\ell }\ {\rho}_g/{\sigma}_{\ell}\right)$

1.2 Greek Alphabet

ρ_g:

Density of gas (kg/m³)

κ :

Thermal conductivity (W/mK)

ρ _ℓ :

Liquid density (kg/m³)

σ _ℓ :

Surface tension (N/m) (kg/s²)

μ _ℓ :

Dynamic viscosity of the liquid (kg/m.s)