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Advanced Aluminum Alloys

November 6, 2017

In my youth, there were many television commercials for Reynolds Wrap aluminum foil, the cooking foil introduced in 1947 by Reynolds Metals, the second-largest aluminum company in the United States after Alcoa. Just as photocopying is often referred to generically as "xeroxing," in those days, aluminum foil of any sort was often called "Reynolds Wrap." Reynolds was eventually acquired by Alcoa in the year 2000.

Aluminum is a metal with ideal properties for many applications. It's abundant in nature, lightweight, and it's corrosion-resistant; however, until about 150 years ago, aluminum was rarely used, since it was difficult to extract from its ores. While processes to reduce iron oxide to pure iron have existed since the Iron Age, about 3,000 years ago, aluminum oxide is much, much harder to reduce.

That's because aluminum oxide is a far more stable compound than iron oxide, as shown by the difference in their free energies of formation (ΔGf), as shown for two representative temperatures in the following table.[1]

Oxide ΔGf, 1000 K
ΔGf, 1500 K
Fe2O3 -134.082 -104.585
Al2O3 -325.397 -285.975

Aluminum was first isolated in 1827, and early aluminum was prepared by a reduction of gaseous anhydrous aluminum chloride by liquid sodium metal at high temperature; viz,
Al2Cl6(g) + 6Na(l) -> 2Al(s) + 6NaCl(s)
In 1884, aluminum was as valuable as silver, about a dollar an ounce ($25/ounce in today's money), and the Washington Monument was topped with an aluminum apex in that year (see figure). This apex, intended as a lightning rod, was a solid pyramid of aluminum about a foot high, weighing about 2.85 kg. The aluminum contained some alloying elements, including iron (1.70-1.90%), and silicon (0.55-0.61%), in the aluminum matrix (97.49-97.75%).[2]

Fitting the aluminum apex on the Washington Monument, 1884

Master Mechanic, P. H. McLaughlin, fitting the aluminum apex on the Washington Monument, 1884.

(Harper's Weekly illustration by S.H. Nealy, United States Library of Congress Prints and Photographs division digital ID cph.3b44599, via Wikimedia Commons.)

The "Aluminum Age" started in 1886 with the invention of an Electrochemical process for the extraction of aluminum from molten aluminum fluoride. This process, now called the Hall–Héroult process, was independently invented by American chemist, Charles Martin Hall and French inventor, Paul Héroult. Hall founded the company that would become Alcoa in Pittsburgh in 1888.

In the Hall–Héroult process, cryolite (Na3AlF6) is melted by heated to about 1,000 °C (1,830 °F). To increase energy efficiency, aluminium fluoride (AlF3) is added to form a reduced melting point composition (see figure). Application of an electric current through a carbon cathode and anode deposits aluminum as a liquid at the cathode. The molten aluminum sinks to the bottom of the electrochemical cell where it is collected.

Portion of the cryolite-aluminum fluoride phase diagram

Cryolite-rich portion of the cryolite-aluminum fluoride phase diagram.

(Illustration by the author using Inkscape.)

(Click for larger image.)

A considerable quantity of electric power is needed for this process. It takes about 25-30 kilowatt-hours of electricity to produce two kilograms of aluminum, which is about the daily electrical consumption of an average US home. That's one reason why recycling of aluminum is important.

While the lightweight quality of aluminum is important to aviation and spacecraft applications, engineers still yearn for even lower weight materials; thus, the quest for structural graphene materials. That's why chemists from Utah State University and Russia's Southern Federal University have used computational chemistry to design a new crystalline form of aluminum that is so light it will float on water.[3-4]

New allotrope of aluminum

The new allotrope of aluminum has the diamond lattice structure in which all carbon atoms are replaced by aluminum tetrahedra.

(Image: Iliya Getmanskii, Southern Federal University, Russia, via Utah State University.

The research team, whose work was supported by the US National Science Foundation and the Russian Ministry of Science and Education, used density functional calculations to design this metastable allotrope of aluminum that has yet to be synthesized.[3-4] The crystal structure is essentially a diamond lattice in which the carbon atom positions are now tetrahedra of Al4. This structure has a calculated density of just 0.61 g/cm3, so it would float on water (density, 1.00 g/cm3).[3] The density of conventional aluminum is 2.7 g/cm3. Calculations indicate that this new form of aluminum is a semimetal, and it should be a highly plastic material.[3]

Says Alexander Boldyrev, a professor of Chemistry and Biochemistry at Utah State University,

"An amazing aspect of this research is the approach: using a known structure to design a new material... My colleagues' approach to this challenge was very innovative... They started with a known crystal lattice, in this case, a diamond, and substituted every carbon atom with an aluminum tetrahedron... Of course, it's very early to speculate about how this material could be used. There are many unknowns. For one thing, we don’t know anything about its strength."[4]

While this new allotrope of aluminum has yet to be synthesized, researchers at HRL Laboratories, Malibu, California, have developed a 3D method for printing high-strength aluminum alloys, including types Al7075 and Al6061 that are useful for aircraft and automobile parts.[5-6] The HRL Laboratories method is also amenable to the additive manufacture of many other useful alloys, such as high-strength steels, and the nickel-based superalloys used in turbine engines.[6]

Printing of metals typically proceeds by laser heating of applied thin alloy powder layers so they melt and solidify, but this isn't possible for unweldable high-strength aluminum alloys such as Al7075 or Al6061. These alloys exhibit hot cracking in which the metal part can be pulled apart in layers.[6] Of the more than 5,000 common alloys, just a few like AlSi10Mg, TiAl6V4, CoCr and Inconel 718, can be laser printed.[5] Melting and solidification of the others changes the alloy microstructure to produce large columnar grains and periodic cracks.[5]

The HRL Laboratory process overcomes these problems by addition of specially selected nanoparticles to the unweldable alloy powders. As the laser melts the powder and solidification proceeds, these nanoparticles act as nucleation sites to produce the desired alloy microstructure, thus eliminating hot cracking.[6] As a result, the printed alloy retains its full alloy strength.[6] The nanoparticles are uniformly distributed on the surface of the conventional powder grains to act as nucleation sites that control solidification.[5-6] For the aluminum alloys, zirconium-based nanoparticles were used.[6]

A 3D-printed aluminum alloy component

A 3D-printed aluminum alloy component.

(Still image from an HRL Laboratory YouTube Video.)[7]

The resultant printed parts were crack-free, with an equiaxed fine-grained microstructure; that is, the alloy grains were roughly equal in length, width and height. This produced material strengths that were comparable to that of wrought material.[5] This technique can be applied, also, to non-weldable nickel superalloys and intermetallics, as well as in joining, casting and injection molding operations in which solidification cracking is a common issue.[5] Says Hunter Martin, an engineer at HRL's Sensors and Materials Laboratory, co-principal investigator of the research project, and a Ph.D. candidate at the University of California, Santa Barbara,

"We're using a 70-year-old nucleation theory to solve a 100-year-old problem with a 21st century machine... 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."[6]


  1. L. B. Pankratz, "Thermodynamic Properties of Elements and Oxides," U. S. Bureau of Mines Bulletin 672, U. S. Government Printing Office (1982).
  2. George J. Binczewski, "The Point of a Monument: A History of the Aluminum Cap of the Washington Monument," JOM, vol. 47, no. 11 (1995), pp. 20-25.
  3. Iliya V. Getmanskii, Vitaliy V. Koval, Ruslan M. Minyaev, Alexander I. Boldyrev, and Vladimir Isaak Minkin, "Supertetrahedral Aluminum - a New Allotropic Ultra-Light Crystalline Form of Aluminum," J. Phys. Chem. C, Just Accepted Manuscript (September 18, 2017), DOI: 10.1021/acs.jpcc.7b07565.
  4. Ultra-Light Aluminum: USU Chemist Reports Material Design Breakthrough, Utah State Today, Friday, September 22, 2017.
  5. John H. Martin, Brennan D. Yahata, Jacob M. Hundley, Justin A. Mayer, Tobias A. Schaedler, and Tresa M. Pollock, "3D printing of high-strength aluminium alloys," Nature, vol. 549, no. 7672 (September 21, 2017), pp. 365-369, doi:10.1038/nature23894.
  6. Metallurgy Breakthrough: HRL Engineers 3D Print High-Strength Aluminum, Solve Ages-Old Welding Problem Using Nanoparticles, HRL Laboratories Press Release, September 20, 2017.
  7. Metallurgy Breakthrough: 3D Printing High-Strength Aluminum, HRL Laboratories YouTube Video, September 20, 2017.

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