Metal-based additive manufacturing, or three-dimensional (3D) printing, is a potentially disruptive technology across multiple industries, including the aerospace, biomedical and automotive industries. Building up metal components layer by layer increases design freedom and manufacturing flexibility, thereby enabling complex geometries, increased product customization and shorter time to market, while eliminating traditional economy-of-scale constraints. However, currently only a few alloys, the most relevant being AlSi₁₀Mg, TiAl₆V₄, CoCr and Inconel 718, can be reliably printed; the vast majority of the more than 5,500 alloys in use today cannot be additively manufactured because the melting and solidification dynamics during the printing process lead to intolerable microstructures with large columnar grains and periodic cracks. Here we demonstrate that these issues can be resolved by introducing nanoparticles of nucleants that control solidification during additive manufacturing.

—Martin et al.

Additive manufacturing of metal alloys via selective laser melting. The central schematic represents an overview of the additive manufacturing process, whereby a direct energy source (laser or electron beam) melts a layer of metal powder (yellow), which solidifies (red to blue), fusing it to the previous (underlying) layer of metal (grey). a, Conventional Al7075 powder feedstock. b, Al7075 powder functionalized with nanoparticles. c, Many alloys including Al7075 tend to solidify by columnar growth of dendrites, resulting in cracks due to solidification shrinkage. d, Suitable nanoparticles can induce heterogeneous nucleation and facilitate equiaxed grain growth, thereby reducing the effect of solidification strain. e, Many alloys exhibit intolerable microstructure with large grains and periodic cracks when 3D-printed using conventional approaches, as illustrated by the inverse pole figure. f, Functionalizing the powder feedstock with nanoparticles produces fine equiaxed grain growth and eliminates hot cracking. g, A 3D-printed, topologically optimized Al6061 piston on the build plate. h, 3D-printed Al7075 HRL logo. Martin et al. Click to enlarge.

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—a condition that renders a metal part able to be pulled apart like a flaky biscuit.

HRL solves this problem by decorating high-strength unweldable alloy powders with specially selected nanoparticles— nanoparticle functionalization. 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.

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.

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.

—Hunter Martin, who co-led the team with Brennan Yahata

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. 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.

—Brennan Yahata

Other authors on the paper were Jacob Hundley, Justin Mayer, and Tobias A. Schaedler all of HRL.

HRL Laboratories is a corporate research-and-development laboratory owned by The Boeing Company and General Motors specializing in research into sensors and materials, information and systems sciences, applied electromagnetics, and microelectronics. HRL provides custom research and development and performs additional R&D contract services for its LLC member companies, the U.S. government, and other commercial companies.

Resources

• John H. Martin, Brennan D. Yahata, Jacob M. Hundley, Justin A. Mayer, Tobias A. Schaedler & Tresa M. Pollock (2017) “3D printing of high-strength aluminium alloys” Nature 549, 365–369 doi: 10.1038/nature23894