High-velocity impact fragmentation of additively-manufactured metallic tubes

In this paper, we have developed and demonstrated a novel high-velocity impact experiment to study dynamic fragmentation of additively-manufactured metals. The experiment consists of a light-gas gun that fires a conical nosed cylindrical projectile, that impacts axially on a thin-walled cylindrical tube fabricated by 3D printing. The diameter of the cylindrical part of the projectile is approximately twice greater than the inner diameter of the cylindrical target, which is expanded as the projectile moves forward, and eventually breaks into fragments. The experiments have been performed for impact velocities ranging from approximate to 180 m/s to approximate to 390 m/s, leading to strain rates in the cylindrical target that vary between approximate to 9000 s-1 and approximate to 23500 s-1. The cylindrical samples tested are printed by Selective Laser Melting out of aluminum alloy AlSi10Mg, using two printing qualities, with two different outer diameters, 12 mm and 14 mm, and two different wall thicknesses, 1 mm and 2 mm. A salient feature of this work is that we have characterized by X-ray tomography the porous microstructure of selected specimens before testing. Three-dimensional analysis of the tomograms has shown that the initial void volume fraction of the printed cylinders varies between 1.9% and 6.1%, and the maximum equivalent diameter of the 10 largest pores ranges from 143 mu m to 216 mu m, for the two different printing conditions. Two high-speed cameras have been used to film the experiments and thus to obtain time-resolved information on the mechanics of formation and propagation of fractures. Moreover, fragments ejected from the samples have been recovered, sized, weighted and analyzed using X-ray tomography, so that we have obtained indications on the effect of porous microstructure, specimen dimensions and loading velocity on the number and distribution of fragment sizes. To the authors' knowledge, this is the first paper (i) providing a systematic experimental study (34 impact tests) on the fragmentation behavior of printed specimens, and (ii) including 3D reconstructions of dynamic cracks in porous additively-manufactured materials.