On the nature of the armature-rail interface: liquid metal effects

Abstract

This is an experimental study of the nature of the armature-rail interface during hypervelocity launch in a railgun. It is part of a multidisciplinary modeling and experimental effort to improve the understanding of contact physics. Conditions that occurred at the interface are inferred from studies of the surfaces of recovered copper rails. Melt lubrication is observed at the armature-rail interface. Liquid aluminum metal from the contact faces of the solid aluminum armature forms at the armature-rail interface as a result of frictional and joule heating. The liquid aluminum is quenched by the relatively cool copper rail, and a quenched metal deposit is formed. Characterization of the deposit has shown a rapidly quenched microstructure. The mean grain size measured by transmission electron microscopy is 200 nm-the finest grain size reported for a melt quenched aluminum alloy film. The deposit thickness is less than 25 /spl mu/m and is rough with an oxidized surface. The thicker films crack, curl, and detach easily from the rail surface. The experiments were performed in a 25-mm square-bore railgun, with a ratio of interface current to armature contact width of 20-35 kA/mm. The liquid production rate by armature melting at the interface is found to be about 1 mg/C, We conclude that in solid armature railguns with sufficiently thick, liquid aluminum melt lubrication, the rail does not erode. Instead a deposit forms on the rail surface. Depending upon the film thickness and the quench stresses, the film detaches easily.