Imagine a strong, lightweight material that could be used in structural applications such as automotive and aerospace design, biomedical devices, and sporting goods, as well as in applications such as filtration, heat exchangers and battery electrodes. Well, you don’t have to imagine. It’s here and has been for a while.
The material - or class of materials, really - is metal foam. Metal foams, more appropriately called cellular metals, have been around for decades. Despite their long history, limitations on processing have largely limited their breadth of application. New research aims to help clear that hurdle.
Theoretically, metal foams can be any metal or alloy. Practically, aluminum dominates the market. This is because aluminum has favorable characteristics for liquid-state foaming where gas is incorporated into the molten metal and then quickly cooled to maintain its shape.
Other industrially vital metals such as titanium, copper, steel, etc. are much more difficult to foam because they have higher melting points and/or an adverse reaction to oxygen during processing in the liquid state.
These “difficult” metals are often foamed through solid-state processing where the conditions can be more tightly controlled. Unfortunately, the cost tends to rise and throughput decreases with these processes. Not only that, but porosity tends to be significantly lower as well.
The new process takes a simple, powder metallurgy approach to foaming. The Additive Expansion by the Reduction of Oxides (AERO) method mixes fine, oxide particles into the metal powder through mechanical alloying. These entrained oxide particles are then reduced with hydrogen at a moderate temperature to form steam which expands and “foams” the metal.
Unlike other processes, the AERO process can create foamed particles as well as sintered components. Because each particle is “active,” the technique can also be applied to other, currently established methods of solid-state foaming to provide additional expansion. This was rudimentarily demonstrated with a small component, which was pressed and annealed to create a part with > 65% porosity.
This also allows for graded structures to be created which have a solid, non-foaming core and porous exterior, ideal for biomedical applications.
The product is unique as well. The porosity is finer than most other processes (an order of magnitude smaller) and the resulting grain size is also very fine. These factors may prove to enhance surface area and increase strength in components, respectively.
The AERO process has been demonstrated using copper, but with slight modification, it is hoped the principle can be extended to other metals as well. The simplicity and universality of the process are expected to assist in fielding new, low-density metal structures in a variety of fields.
Want to watch a video of a foamed particle being “machined?” The link below directs to a microscopic serial sectioning video which shows how individual particles are foamed throughout.
Disclaimer: The AERO process was co-developed by the author. All statements in this article are consistent with the original, scientific article, which was peer-reviewed and accepted for publication. All images are original to the authors of that work and do not appear in any other publication.