Wire melting and droplet atomization in a high velocity oxy-fuel jet

摘要

Coatings produced by feeding a steel wire into a high-velocity oxy-fuel (HVOF) torch are being intensively studied by the automotive industry as a cost-effective alternative to the more expensive cast iron sleeves currently used in aluminum engine blocks. The microstructure and properties of the sprayed coatings and the overall economics of the process depend critically on the melting and atomization occurring at the wire tip. This paper presents results characterizing several aspects of wire melting and droplet breakup in an HVOF device. Fluctuations in the incandescent emission of the plume one centimeter downstream from the wire tip were recorded using a fast photodiode. A Fourier transform of the light traces provided a measure of the stripping rate of molten material from the wire tip. Simultaneous in-flight measurement of atomized particle size and velocity distributions were made using a Phase Doppler Particle Analyzer (PDPA). The recorded size distributions approximate a log-normal distribution. Small particles traveled faster than large particles, but the difference was considerably smaller than simple aerodynamic drag arguments would suggest. A set of experiments was carried out to determine the effect that variations in torch gas flow rates have on wire melt rate, average particle size, and average particle velocity. The observed variation of particle size with spray condition is qualitatively consistent with a Weber breakup of the droplets coming off the wire. The measurements also showed that it was possible to significantly alter atomized particle size and velocity without appreciably changing the wire melt rate.