AF9628 is an ultrahigh strength steel that offers a great combination of strength, hardness and toughness properties, and affordability compared to legacy materials like Eglin steel. AF9628 forgings are now widely used in Air Force munition components. There is a need to develop AF9628 additive manufacturing technology to further improve performance of these components.
A large program is underway to develop gas metal arc direct energy deposition (GMA-DED) and powder-laser (PL) directed energy deposition (DED) build models that maximize affordability based on feature complexity and scale. The welding metallurgy of AF9628 deposits and heat affected zones are not well understood. A goal is to evaluate whether welding temperbead approaches can be used to optimize AF9628 build properties with lower cost heat treatments. A computational design of experiment (CDoE) platform for process-feature-microstructure-property optimization is being developed for AF9628 additive manufacturing (AM). A key component of this platform is quantitative process-property-microstructure relationships for a range of DED deposit sizes, heat inputs and interpass temperature build conditions. Data generated from welding and physical simulation experiments will be used to validate and refine computer simulated process-property-microstructure relationships.
This paper presents continuous cooling transformation (CCT) diagrams that are representative of AF9628 GMA- and PL-DED deposits with a focus on the coarse-grained, fine grained, and intercritical heat affected zones (HAZ). The GleebleTM thermo-mechanical simulator was used to recreate typical single- and multiple-reheat AM thermal histories on AF9628 test samples. The phase transformation temperatures were determined using dilatometry and single sensor differential thermal analysis (SS DTA). The developed CCT diagrams were complemented with data on microstructural constituents’ content, grain size, and hardness. Microstructural characterization is being performed with light optical microscopy (LOM), scanning electron microscopy (SEM), and electron beam scattering diffraction (EBSD).
The developed CCT diagrams represent the process-microstructure relationships for AF9628 DED processes. In general, AF9628 steel develops fully martensitic microstructure in a wide range of cooling rates from above the AC3 temperature, while multiple reheats below the AC1 temperature generate significant tempering effect. Ongoing research will further develop feature-microstructure-properties relationships by tensile and Charpy impact testing of samples with simulated AM microstructures and removed from AF9628 GMA- and PL-DED procedure builds over a range of additive manufactured feature deposit sizes, heat inputs and interpass temperature conditions. Transmission electron microscopy (TEM) will be used to characterize carbide formation in AM microstructures.