Some factors affecting porosity in directed energy deposition of AlMgScZr-alloys

Porosity is a limiting factor in the processing of aluminum-alloys with Directed Energy Deposition (DED) and other Additive Manufacturing technologies, such as Laser Powder Bed Fusion. Especially the mechanical properties of additively manufactured samples are strongly dependent from the porosity, and thus, the understanding of the parameters influencing the porosity is crucial. In this work, a detailed study of factors, which usually are not considered but can affect the porosity of AlMgScZr-alloy DED samples, is presented. To this end, the influence of powder drying, remelting of deposited layers, shielding gas conditions (argon), addition of ceramic nanoparticles, powder morphology and substrate surface conditions are studied and the emerging of shell-like pores is discussed. Experiments were carried out with two DED process variants, namely conventional DED and EHLA (a German acronym for extreme high-speed directed energy deposition), working in significantly different process regimes. DED samples ranging from high porosity (with cross-sectional porosities of more than 10%) to almost fully dense (with cross-sectional porosities of 0.01–0.5%) were manufactured. The findings of this study are considered to be applicable to other aluminum alloys.