About the problematic to deduce valid strength and durability data from LPBF of high strength Aluminum alloys

Palm, Frank

Since several decades the established way to design and manufacture strength driven aircraft structures like brackets, beams, shafts etc. based on a differential approach where the deployed high strength aluminum material is provided by a material supplier. Such semi-products like sheet or plate are quality assured and its strength properties are proven as well trackable. A material certificate enables safe computer based designs and reliable stress analysis to estimate or predict static and fatigue behavior. A close correlation between design stress assumptions and material properties like UTS or YS is possible and a strong confidence about validity on the finally manufactured (machined) 3D part is given. All this based on the trust that material test samples and machined part will have more or less the same bulk material-surface conditions leading to reproduceable 3D stress evolution and distribution.

Unfortunately, this is not the case for LPBF generated parts or test bodies. Owing its inherent surface roughness and a variety of bulk material imperfections a stress driven description and analysis of the test samples is notoriously invalid as the localized stress evolution after loading the part appears extremely undefined. Clear differentiation between machined test bodies and "as built" rough test elements is necessary. Part durability as well load bearing capabilities are affected extraordinary by the LPBF build conditions, the build platform that was used as well where it was located in the build chamber. A new more formalized approach is necessary to extract the desired/required performance data so that stress and quality engineers are satisfied and able to release and approve 3D-printed parts. Especially for high strength Al-alloy applications where LPBF has to compete against fully (easy) machined parts (from "safe" plates materials) such a "remedy" is inevitable. The presentation will show some ideas to cope with this challenge.