A quantification and certification paradigm for additively manufactured materials

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Initial spallation testing indicates significant dynamic ductility

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Additive manufacture (AM) is the process of joining materials to make objects from 3-D model data, usually layer upon layer, as opposed to current subtractive manufacturing methodologies. Additive manufacturing is an agile model for designing, producing, and implementing the process-aware materials of the future. However, no “ASTM-type” additive manufacturing certified process or AM-material produced specifications exist. Therefore, the additive manufacturing certification and qualification paradigm needs to evolve. For example, even for small changes in such variables as starting feed material (powder or wire), component geometry, build process, and post-build thermo-mechanical processing, the qualification cycle can be complicated–leading to long implementation times.

In large part, this is because researchers cannot predict and control the processing-structure-property-performance relationships in additively manufactured materials at present. Metallic-component certification requirements have been documented elsewhere for specific materials. The requirements generally involve meeting engineering and physics requirements tied to the functional performance requirements of the engineering component, and process and product qualification. Key microstructural parameters and defects need to be quantified in order to establish minimum performance requirements.

As part of an NNSA complex-wide effort researching techniques toward a certification and qualification process for additively manufactured materials, Los Alamos researchers compared and quantified the constitutive behavior of an additively manufactured stainless steel in the as-built condition with that of the same that had undergone recrystallization and yet again to a conventionally manufactured annealed and wrought stainless steel.

In this initial study, researchers produced cylinders of 316L stainless steel (SS) using a LENS MR-7 laser additive manufacturing system from Optomec (Albuquerque, NM) equipped with a 1kW Yb-fiber laser. The team characterized the microstructure of the additively manufactured-316L SS in both the as-built condition and following heat-treatments to obtain full recrystallization. The scientists measured constitutive behavior as a function of strain rate and temperature and compared it with that of nominal annealed wrought 316L SS plate. They probed the dynamic damage evolution and failure response of all three materials using flyer-plate impact driven spallation experiments at two peak stress levels, 4.3 and 6.2 GPa, to examine incipient and full spallation response.

The spall strength of wrought 316L SS did not vary for the two-peak shock stresses studied. The AM-316L SS spall strengths, in the as-built and following recrystallization, decreased with increasing peak shock stress. Researchers are characterizing the damage evolution as a function of microstructure and peak shock stress through optical metallography, electron-back-scatter diffraction, and scanning-electron microscopy methods. The experimental results demonstrate that the macroscopic spall strength of the additively manufactured 316L SS is quite similar to that of wrought 316L SS. However, the details of the damage evolution, as controlled by the complex microstructure in the additively manufactured material, are substantially different.

The results of their mechanical behavior and spallation testing reinforce the concept that additive manufacturing will force a shift from “material” qualification (ASTM) to science-based qualification and certification. Researchers presented the initial results of the spallation investigation on 316L-SS during the complex-wide JOWOG (Joint Working Group) on additive manufacturing held at Lawrence Livermore National Laboratory in October.

The work is part of the qualification and certification research thrust of a DOE additive manufacturing initiative that includes Los Alamos National Laboratory, Sandia National Laboratories, Lawrence Livermore National Laboratory, Kansas City Plant, and Savannah River Site. Using materials additively manufactured at these sites, Los Alamos is conducting further fundamental dynamic spall tests and characterizing the resulting structure/property relations. Los Alamos participants included G.T. (Rusty) Gray III (Materials Science in Radiation and Dynamics Extremes, MST-8) who serves as principal investigator for the complex-wide program and the LANL activities, John Carpenter and Thomas Lienert (Metallurgy, MST-6), and Veronica Livescu, Carl Trujillo, Shuh-Rong Chen, Carl Cady, Saryu Fensin, and Danial Martinez (MST-8).

Science Campaign 2 (LANL Program Manager Russell Olson, acting) and the Joint Munitions Program (LANL Program Manager Tom Mason) funded the research. The work supports the Laboratory’s Nuclear Deterrence mission area and Materials for the Future science pillar through the development of additive manufacturing methods to produce designed materials with desired structure/property relations. Future facilities, such as MaRIE (Matter-Radiation Interactions in Extremes), could build upon this research linking process-aware materials behavior to performanceby enabling in-situ quantification of deformation and damage evolution during dynamic loading.This information could be used in the qualification and certification paradigm for additively manufactured materials.

Technical contact: Rusty Gray

 

 

 

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The results of flyer-­‐plate impact-­‐driven spallation on (from top) conventionally manufactured annealed and wrought stainless steel, additively manufactured stainless steel that has been recrystallized, and the as-­‐built additively manufactured 316L SS. The optical microscopy results show void formation and damage along solidification boundaries in the as-­‐built additively manufactured steel, classic spherical void nucleation, growth, and shear coalescence in the wrought steel; and reduced void formation and shear in the recrystallized additively manufactured steel.