Oxidation behavior of in-flight molten aluminum droplets in the twin-wire electric arc thermal spray process

This work examines the in-flight oxidation of molten aluminum sprayed in air using the twin-wire electric arc (TWEA) thermal spray process. Measurements of droplet size, velocity, and temperature within the spray plume during flight were obtained using in-flight particle pyrometry and laser Doppler velocimetry and used as the basis for the estimating fluid and thermal effects. Pitot tube measurements provided information on the velocity of the freestream air. Aerodynamic shear at the droplet surface enhances the amount of in-flight oxidation by: (1) promoting entrainment of the surface oxides within the droplet and continually exposing fresh fluid available for oxidation, and (2) causing a continuous heat generation effect that increases droplet temperature over that of a rigid sphere (i.e., without internal circulation). The oxidation reaction of aluminum in air is highly exothermic and is represented by a heat generation term in the energy balance. This continual source of heat input keeps the droplets in a liquid state during flight. A linear rate law based on the Mott-Cabrera theory was used to estimate the growth of the surface oxide layer formed during droplet flight. The calculated oxide volume fraction of a "typical" droplet with internal circulation is 8.7%. This compares favorably to the experimentally determined oxide content for a typical TWEA-sprayed aluminum coating sprayed onto a room temperature substrate, which ranges from 3.3 to 12.7%. The oxide volume fraction calculated for a rigid sphere is nearly two orders of magnitude smaller than that of a droplet with internal circulation. The experimental measurements show an elevated, nearly constant droplet surface temperature (∼2000°C) during flight to the substrate. The solution of the governing differential equation for droplet temperature confirms that the continual heat generation produced by the oxidation reaction is necessary to maintain the droplet superheat. The major contribution of this research is the identification of internal circulation in molten droplets as a key mechanism for the formation of oxides in TWEA-sprayed aluminum coatings. Process variables that can be used to adjust the droplet oxide content include changing the oxygen partial pressure and the standoff distance.