POSTTEST CHARACTERIZATION OF THE HARDNESS AND MICROSTRUCTURE OF COPPER RAILS FROM A 9-MJ ELECTROMAGNETIC LAUNCHER

Following a test series consisting of sixty-seven experiments, the rai ls from a 90 mm diameter x 10 m long launcher were removed for metallu rgical characterization. The rails were machined from OFHC copper extr usions. The measured Brinell hardness values of the rails ranged from HB 60 to HB 86. The starting material had a hardness of HB 86. Observe d values increased from the breech up to a distance of approximately 4 m and tended to decrease and level off beyond 4 m. These hardness val ues are normal for worked OFHC copper, falling between those for fully annealed (HB 34) and cold rolled (HB 91) conditions, The compressive flow strength of the as-received material was determined to be sigma(f ) 331 epsilon(0.12) MPa. Extensive cratering and hypervelocity gouging damage was observed except in the regions 1 m from the breech end and between 7.5 and 10 m. The microstructure of the as-received rail mate rial was fully dense and equiaxed and the average grain size was 100 m u m. Sections were removed from the 2 m, 4 m, and 8 m locations for me tallographic analysis. Average grain sizes at the 2, 4, and 8 m locati ons were 45 mu m, 100 mu m, and 65 mu m, respectively. The larger grai n size at the 4 m location was correlated with grain growth resulting from the high heat flux associated with armature transitioning. Twinni ng, a structural feature in shock-loaded copper, was observed at the 4 m and 8 m locations. X-ray dot mapping and energy dispersive spectros copy analyses revealed that the colored patches close to the 4 m locat ion were composed of a reaction product of copper (rail material) and aluminum and zinc (armature-7075 aluminum alloy). An x-ray line profil e analysis showed that aluminum had migrated to a depth of about 150 m u m from the surface of the rail. One weight percent of aluminum disso lved in copper halves its room-temperature con ductivity.