Electric Current Induced Liquid Al Deposition, Reaction and Flow on Cu Rails at Rail-Armature Contacts in Railguns

When a railgun is fired, the aluminum armature typically undergoes melting at the rail-armature contacts and deposits liquid metal on the rails. Rail-life is affected by these deposits. Localized aluminum armature melting has been attributed to current crowding and localized Joule heating. This paper reports on (1) characterization of debris on fired rails and their role in rail damage, and (2) experiments to elucidate the role of electric current in the transport of liquefied armature material along the rail surfaces. A miniature static railgun model was constructed by wrapping an Al foil around a quartz core, and placing the Al foil in contact with two thin film Cu stripes deposited on Si substrates. In this arrangement, the Cu thin films simulated the surface skins on the two rails, whereas the Al foil represented the skin of the armature. The circuit was subjected to a constant current under high vacuum and various ambient temperatures. The part of the Al foil in contact with Cu melts quickly due to Joule heating, and the liquefied Al starts flowing down the negative rail. In all instances, the direction of liquid flow was from the positive to the negative terminal. The kinetics of Al migration was measured, and it was found that the activation energy for Al migration is close to that for self-diffusion in liquid Al. The observed liquid transport is attributable to liquid electromigration under the influence of the applied current.