Dynamic consolidation of type 304 stainless-steel powders in gas gun experiments

Abstract

Specimens of type 304 stainless-steel powder consolidated using projectile impacts from a 10.2-cm gas gun have been recovered intact and the microstructures have been characterized. Variables for the study included two types of 304 stainless-steel powder, cavity depth, and peak stresses ranging from 11.7 to 21.1 GPa. All specimens exhibited three-dimensional contractions with approximately one-half of the densification associated with thickness. Some porosity from entrapped gas as a result of not evacuating the powder-filled cavities and microcracks due to the recovery method were observed. In the regions free of these defects, full densification was observed. The 11.7-GPa experiment produced considerable work hardening. The bonding around the particle perimeters ranged from melted regions to mechanical abutment. The 21.1-GPa experiment resulted in lower hardness, reduced metastable bcc phase, and more extensive bonding around the particle perimeters. These observations suggest that very high localized, and relatively high bulk temperatures were achieved for this peak stress. A two-dimensional numerical simulation, with the powder region considered as a two-component mixture, was used to examine the overall response of the powder-filled cavities and surrounding target to the stress wave. The calculations predicted the observed density change and the observed final shape for the compacted powder. Two-dimensional numerical simulations, in which the particles were individually modeled, were also performed to assess the behavior at the particle level. The deformation of the particles necessary to fill the interstitial regions along with the corresponding temperatures around the perimeters of the particles and their interiors were calculated. The deformation varies markedly around the particle periphery and the corresponding computed temperature rise ranges from slightly above ambient to above melting in regions of maximum particle extrusion. The nonuniform temperature rise around the particles correlates with the heterogeneous nature of the bonding observed experimentally. The model also predicts higher and broader temperature zones with increasing dynamic stress, in agreement with the experimental inferences.