Effect of Current Crowding on Microstructural Evolution at Rail-Armature Contacts in Railguns

During firing of a railgun, the armature typically undergoes melting at the rail-armature contact and deposits melt on the rail, thereby reducing rail life. This melting starts at the trailing edge of the armature, and can be attributed to current crowding and localized Joule heating. This paper reports on dual laboratory-scale armature tests, one with current and one without, and by microstructural analysis of the two armatures, demonstrates that localized Joule heating due to current crowding is the principal source of melting, with frictional heating playing a much smaller role. The paper reports on microstructural changes in the armature due to Joule heating, along with associated degradation of the rail surface. It evaluates the impact of skin effect in the rail and the electrical contact conductance at the rail-armature interface on current crowding at the points of entry and exit of the current into and from the armature. In general, reduction of current crowding decreases the peak temperature at the rail-armature interface. But while reducing interfacial contact conductance reduces current crowding, it enhances the peak temperature and causes more melting.