Additive manufacturing of high strength aluminium alloys by laser powder bed fusion

High strength aluminium alloys of the 7000 series show unsurpassed strength-to-weight ratio and therefore attract much of the attention of the aerospace industry where lightweight design is paramount in material development. Despite a tendency to undergo hot cracking and elemental vaporization of Zn and Mg, the additive manufacturing of nearly fully dense parts of AA7075 by laser powder bed fusion (L-PBF) has proven successful in recent years thanks to the addition of grain refiners and a systematic process parameters optimisation. However, L-PBF parts display poorer mechanical properties than their conventional wrought counterparts and their fatigue behaviour remains unstudied so far. In this study, hydrogen stabilized zirconium nucleants (ZrH2) were added to the pre-alloyed 7075 powder feedstock by means of two different in-situ alloying methods: a mechanical blending of the two powders in a tumbling mixer and an innovative atmospheric cold plasma discharge method creating a zirconium coating directly onto the aluminium powder particles, in development in the AM 4 AM company. Initially, we compared the different powders in terms of their ability to produce fully dense parts. After selection of the best powder (a mechanical mixing of Al7075 with 2wt% ZrH2), the static mechanical properties and fatigue performance were assessed. Two different conditions were tested: as-built (state directly after L-PBF manufacture) and heat treated samples. After an optimized T6 heat treatment, the samples manufactured from the 2%Zr powder presents outstanding mechanical properties delivering a yield strength of 515 MPa and an elongation at fracture of 11,4%. Additionally, the T6-samples demonstrated excellent fatigue life, with a fatigue crack growth rate comparable to that of conventional high-strength aluminium alloys.