HRL Laboratories, LLC, developed technique for successfully 3D printing high-strength aluminum alloys — including Al7075 and Al6061 — which were previously not amenable to additive manufacturing. This opens the door to the additive manufacturing of engineering-relevant alloys for aircraft and automobile parts.
Additive manufacturing of metals typically begins with alloy powders that are applied in thin layers and heated with a laser or other direct heat source to melt and solidify the layers. Normally, if high-strength unweldable aluminum alloys, such as Al7075 or AL6061 are used, the resulting parts suffer severe hot cracking, which causes the part to fall apart.
HRL’s nanoparticle functionalization technique solves this problem by decorating high-strength unweldable alloy powders with specially selected nanoparticles. The nanoparticle-functionalized powder is fed into a 3D printer, which layers the powder and laser-fuses each layer to construct a three-dimensional object. During melting and solidification, the nanoparticles act as nucleation sites for the desired alloy microstructure, preventing hot cracking and allowing for retention of full alloy strength in the manufactured part.
“Our first goal was figuring out how to eliminate the hot cracking altogether. We sought to control microstructure and the solution should be something that naturally happens with the way this material solidifies,” said Hunter Martin, who co-led the team with Brennan Yahata — both engineers in HRL’s Sensors and Materials Laboratory and PhD students at University of California, Santa Barbara.
To find the correct nanoparticles, in this case zirconium-based nanoparticles, the HRL team enlisted Citrine Informatics to help them sort through the myriad possible particles to find the one with the properties they needed. “Using informatics was key,” said Yahata. “The way metallurgy used to be done was by farming the periodic table for alloying elements and testing mostly with trial and error. The point of using informatics software was to do a selective approach to the nucleation theory. We knew to find the materials with the exact properties we needed. Once we told them what to look for, their big data analysis narrowed the field of available materials from hundreds of thousands to a select few. We went from a haystack to a handful of possible needles.”
Because melting and solidification in additive manufacturing is analogous to welding, HRL’s nanoparticle functionalization can also be used to make unweldable alloys weldable. This technique is also scalable and employs low cost materials. Conventional alloy powders and nanoparticles produce printer feedstock with nanoparticles distributed uniformly on the surface of the powder grains.
The paper, “3D printing of high-strength aluminum alloys,” authored by Martin and Yahata, as well as Jacob M. Hundley, Justin A. Mayer, Tobias A. Schaedler, and Prof. Tresa M. Pollock, was published in the September 21, 2017 issue of Nature.