Size-dependent fragment shape in high-velocity anvil impact of spherical metal powder-compacts

Abstract

Room-temperature mechanical swaging of metallic powders allows the flexible synthesis of fully consolidated, composite powder compacts that can retain nearly the full strength, toughness, density, and machinability of their bulk parent elements. The swaging process offers the ability to compositionally tailor a material to exploit reactive inter-phase chemistry. Unlike some other cold powder-metallurgy fabrication methods, though, it can produce mechanically competent materials capable of serving in structurally useful applications. By means of fragment soft-catch and recovery, we evaluate the effects of initial powder size and degree of mechanical swaging on the impact fragmentation statistics of pure aluminum powder-swaged projectiles. Here, processing alone induces a wide range of impact fragmentation behavior, generally correlating with the effective toughness of the swaged product. As such, we interrogate the dynamic fragmentation of both brittle and tough metals across this range without varying composition. We observe a clear relationship between fragment size and shape, utilizing this relationship to improve high-throughput optical measurements of volume-weighted fragment size. We find surprisingly similar average fragment shape to previous hypervelocity impact experiments on geological materials, suggesting some common underlying mechanical patterns of fragmentation in the ballistic impact of otherwise disparate materials. Furthermore, post-mortem analysis suggests that the selection of appropriate starting particle size and degree of swaging in rotary-swaged metals provides some distinct and potentially valuable ability to tune dynamic fragmentation, in comparison to that of the wrought metal, while maintaining suitably high quasi-static toughness and ductility for engineering purposes.