Parts additively manufactured with AF-9628, an Air Force steel, are about 20% stronger than conventional AM alloys, in terms of ultimate tensile strength, according to research conducted by Capt. Erin Hager, an Air Force Research Laboratory employee and recent graduate of the Air Force Institute of Technology’s Aerospace Engineering Program. AF-9628 is a steel alloy developed by AFRL’s Dr Rachel Abrahams that offers high strength and toughness. The formula, nicknamed Rachel’s steel, costs less than some other high performance steel alloys including Eglin Steel and HP-9-4-20; however, it is more expensive than common grades used in conventional munitions. AF-9628 is unique since it does not contain tungsten, like Eglin Steel or cobalt, part of the formula for HP-9-4-20, which is in the Massive Ordnance Penetrator, a 30,000-pound bomb that destroys assets in well-protected facilities.
Hager’s research, sponsored by the Air Force Research Laboratory Munitions Directorate at Eglin AFB, Florida, determined that AF-9628 is an optimal material for additive manufacturing due to its high strength. While these findings are comparable to values reported in a similar U.S. Army Combat Capabilities Development Command Army Research Laboratory study, Hager yielded similar mechanical properties to conventionally forged and heat-treated AF-9628. Dr. Sean Gibbons, a research materials engineer with the Munitions directorate with expertise in steel, describes this finding as “exciting.”
In working with Rachel’s Steel, Hager employed powder bed fusion, a type of additive manufacturing in which a laser selectively melts powder in a pattern to create three-dimensional objects. As each layer is complete, the printer dispenses more powder on the build area, and the process continues until the part is complete. (Ukrainian metal)