HRL Laboratories has developed a novel technique for successfully 3D printing high-strength alloys. Image: M. Durant – Copyright HRL Laboratories.
HRL Laboratories has developed a novel technique for successfully 3D printing high-strength alloys. Image: M. Durant – Copyright HRL Laboratories.

Researchers at HRL Laboratories, a corporate research-and-development laboratory owned by The Boeing Company and General Motors, have developed a technique for successfully 3D printing high-strength aluminum alloys – including types Al7075 and Al6061 – opening the door to additive manufacturing of engineering-relevant alloys.

Such alloys are very desirable for aircraft and automobile parts, but were among thousands of alloys that were not previously amenable to additive manufacturing, or 3D printing. An added benefit of the new technique is that it can be applied to additional alloy families such as high-strength steels and nickel-based superalloys that have also been difficult to process with additive manufacturing.

"We're using a 70-year-old nucleation theory to solve a 100-year-old problem with a 21st century machine," said Hunter Martin, who co-led the team with Brennan Yahata. Both are engineers in HRL's Sensors and Materials Laboratory and PhD students at the University of California, Santa Barbara studying with Tresa Pollock, a co-author on the study, which is reported in a paper in Nature.

Additive manufacturing of metals typically begins with alloy powders that are applied in thin layers and then 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 – a condition that severely weakens the metal part, allowing it to be pulled apart like a flaky biscuit.

HRL's nanoparticle functionalization technique solves this problem by decorating high-strength unweldable alloy powders with specially selected nanoparticles. This 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 the retention of full alloy strength in the manufactured part.

Because melting and solidification in additive manufacturing is analogous to welding, HRL's nanoparticle functionalization technique can be used to make unweldable alloys weldable. It is also scalable and employs low cost materials: conventional alloy powders and nanoparticles produce printer feedstock in which the nanoparticles are distributed uniformly on the surface of the powder grains.

"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," Martin said.

To find the correct nanoparticles, in this case zirconium-based nanoparticles, the HRL team enlisted a company called Citrine Informatics to help them sort through the myriad possible particles to find the one with the properties they needed.

"Using informatics was key," explained 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."

With this exciting new technique, HRL stands at the forefront of a new chapter in additive manufacturing of metals for research, industry and defense applications.

This story is adapted from material from HRL Laboratories, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.