Size distribution of particles from individual wires and the effects of nozzle geometry in twin wire arc spraying

By separating particles from the anode and the cathode wires, particle properties of individual electrodes of the arc spray process were investigated. Bimodal size distribution was observed for particles from each single wire. In addition, after atomisation and acceleration, average diameter difference between particles from the anode and cathode becomes quite small and the difference can be reduced by increasing atomising gas pressure and/or by using a closed nozzle system with a converging–diverging orifice (CD/CL). Microstructure analysis indicates that the CD/CL nozzle tends to produce coatings with finer microstructure, lower porosity and higher oxide content, while the open nozzle system leads to very coarse coating microstructure with higher porosity but less oxide content. CFD analysis of the gas dynamics was performed for different nozzles and led to numerical prediction of splat diameter distribution. Results show a smaller average splat diameter and a narrower splat diameter distribution for the CD/CL nozzle that is in close agreements with the microstructure analysis. It was found that a major disadvantage of the open nozzle system is its relatively longer distance between the wires interception point and the nozzle exit where the gas velocity attains its maximum value.