Instability of the Arc Spraying Process Using Cored Wires

Depending on used wires, the melting behavior and particle formation are changed and thus the process stability. In case of using cored wires, the filling powder, strongly affects the melting behavior, particle formation, arc fluctuations, and process stability. A high speed camera was used to study the melting behavior of the used wires. The camera was triggered with an oscilloscope to correlate fluctuations with the observed images. Melting models were established for cored and solid wires. These models comprise the effect of the heat transfer inside the used wires. Due to the thermal conductivity of metals, the arc heats the used wires in three different zones. The first zone is the direct wire interaction, where the wire materials are in a fully liquefied form. The second zone is a doughy one, where the wire materials can be easily formed. The third one is a highly heat affected zone, where a permanent deformation occurs. In case of cored wires the ignited arc heats the facing surfaces of the wires and melts them, while liquidized metal is directly atomized in the form of smaller droplets. The filling powder is set free and therefore exposed to the atomizing gas flow. In their way the powder particles penetrate against the extruded metal sheets and cause metal disintegrations nearby the parent wires. The penetration consequently causes a breakaway of the extruded metal sheets. The size of these droplets is a function of the specific properties of the metal melt, and the acting aero dynamic forces. The doughy area is the origin of the extruded metal sheets. The shapes of extruded metal sheets are mainly initiated by means of Karman vortex streets behind the electrodes more than by Kelvin-Helmholtz instability. A correlation is conducted between the adjustable spraying parameters, the used wires, and the process instability.