The Effect of Annealing on the Dynamic Mechanical Properties of Aluminum Reactive Materials

CPT Lee Wooseok, Korean Army

The lethality of a reactive material is closely tied to the dynamic fracture and fragmentation processes that occur during impact or explosive launch. This research examines how these properties change with heat treatment in a prototypical aluminum reactive material. Samples were fabricated by cold isostatic pressing at 60 ksi starting from H-2 class gas-atomized Al powders. After pressing they were annealed at 200 °C in air for up to 120 minutes. Mechanical properties were evaluated by means of quasistatic testing, high-rate compaction in a Hopkinson bar, and impact fragmentation via sabot launch from a gas gun. While the compressive yield strength does not differ dramatically between samples, even short heat treatment times (30 minutes and below) had a significant effect on the tensile ductility and fragmentation of this material. The fine fragments resulting from impact on a steel anvil at 500-550 m/s were recovered, sieved, and analyzed with standard fragmentation theories. Samples that were cold-pressed but received no heat treatment showed significant fragmentation as well as the appearance of a power-law fragment distribution postulated to arise from crack microbranching. Thirty minutes at 200 °C, a temperature at which no sintering occurs, was sufficient to increase tensile ductility such that no fragmentation was observed. Electron backscatter diffraction measurements showed that this heating did not result in significant oxidation, recrystallization, or movement of grain boundaries. The recovery process within the grains appears to be the dominant change at these annealing conditions, and has an unexpectedly large effect on the fragmentation pattern including the elimination of microbranching features.

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Jan 18, 2017

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