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The throat region of solid armatures is subjected to significant stress and temperature during a railgun launch. Recovered aluminum alloy armatures show evidence of plastic deformation, melting, and even cracking in the throat region. Plastic deformation can result from magnetic pinch or thermal expansion forces and melting from ohmic heating. Although cracking is historically associated with grain boundary melting, the cracks observed in recently recovered armatures are likely a result of thermal stresses. Finite-element codes EMAP3D and DYNA3D were linked to assess stresses generated by magnetic forces, inertial loading, wear, and thermal expansions. For low-speed armatures, thermal stresses were found to be as important as magnetic stresses. The observed cracking is likely due to rapid cooling, similar to quench cracking, and may or may not occur during launch. Index Terms--Cracking, solid armatures, throat region.

Previous work investigated macroscopic features of recovered aluminum alloy armatures launched at five different currents. Here, microstructural features of those armatures are explored. Several distinct heat-treatment regimes present near the armature throat surface include grain deformation, melting, and material ejection. Two distinct cracking mechanisms are present. The first--presumably due to liquation cracking---occurs at higher currents, is intergranular, and has cracking directions independent of armature geometry directions. The second--likely due to thermal stresses--is intragranular and has strong dependence on orientation. Detailed photomicrographs of the recovered armatures are presented, along with calculated dimensions of the heat-treatment zones and observed cracks. Index Terms--Aluminum-alloy armature, microstructure.