Variable Strain Rate Properties of Low Porosity Additively Manufactured AF 9628 Steel

Additively manufactured munitions steel AF 9628 powder was developed by the United States Air Force as a high strength, low-alloy, printable material. Powder-bed-printed AF 9628 was tested under variable strain rate conditions. The porosity of the samples varied from 0.2% to 4.8% porosity. Tests were limited to stress below 13 GPa. Initial testing at quasistatic strain rates showed loose anisotropic yield characteristics in tensile and compression testing. Gas gun driven symmetric flyer plate impacts show a "crush up" region and spall strength that are impacted by the porosity level of the samples. The Hugoniot relationship resulting from this test was consistent with accepted literature for iron-based materials of similar porosity. Split Hopkinson Bar tests, in concert with aforementioned testing, indicated material strain rate hardening behavior. Electron backscatter diffraction (EBSD) revealed the presence of retained austenite that likely impacted the mechanical behavior at variable strain rates. These results were later benchmarked with Taylor impact tests, comparing fit response to actual response.