Microstructure Refinement by Channel Die Pressing

Continued interest in the enhancement of warhead liner performance has led to a number of studies focused on liner material ductility and consistency. Specifically, cold upset forging and annealing have been shown to produce significant grain refinementand texture development in liner plates of various materials. However, in tantalum, and possibly other warhead metals, variations in thermo-mechanical processing among different mills can persist as inconsistencies in the final plate, even following theextensive changes in microstructure occurring in cold forging and subsequent annealing. Equal-channel angular pressing (ECA pressing) is a material conditioning process which by intensive shear deformation is able to produce ultrafine grain sizes, withoutchanging the overall shape of the piece. Application of this process to condition bar stock segments prior to the cold forging of plates could potentially erase any material bias from previous processing and provide for complete uniformity of materialproperties in forged plates, regardless of the material supplier used. It appears that ECA pressing, followed by cold upset forging and annealing, is the optimum process sequence for the manufacture of warhead liners. This project will apply this sequenceto three warhead materials and characterize the results.Successful completion of Phases I and II of this effort will result in a manufacturing sequence for warhead liners which offers improved ductility and consistency over methods used currently.