Engineers in California have achieved 3-D printing of high-strength aluminium and solved an ages-old welding problem using nanoparticles.
Credit: B. Ferguson - HRL Laboratories
HRL Laboratories, a research centre owned by General Motors Corporation and Boeing, has achieved a major breakthrough in metallurgy. This week, researchers at the facility announced a new technique for 3-D printing high-strength aluminium alloys – including types Al7075 and Al6061 – opening the door to additive manufacturing of engineering-relevant alloys.
These alloys are very desirable for aircraft and automobile parts. They have been among thousands that, until now, were not amenable to 3-D printing (additive manufacturing). An added benefit of the researcher's method is that it can be applied to additional alloy families, such as high-strength steels and nickel-based superalloys difficult to process with current technology.
"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 the HRL's Sensors and Materials Laboratory and PhD students at University of California, Santa Barbara studying with Professor Tresa Pollock, a co-author on the study. Their paper, 3D printing of high-strength aluminum alloys, appears this week in Nature.
Credit: M. Durant HRL Laboratories
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. If high-strength unweldable aluminium alloys (such as Al7075 or AL6061) are used, they normally suffer severe hot cracking – a defect that renders a metal part able to be pulled apart like a flaky biscuit.
HRL's new technique solves this problem by decorating high-strength unweldable alloy powders with specially selected nanoparticles. The nanoparticle-functionalised powder is fed into a 3-D printer, then layered and laser-fused to construct a three-dimensional object. During melting and solidification these 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 functionalisation 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," said Martin.
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 exact 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."