Inspired by Nature and Tough as Iron: Metal Foams

Inspired by nature, metal foams combine much of the strength of metal with the lighter, porous structure of foam. Metal foams have many interesting applications in various industries. They can be made from a variety of metals and tend to be incredibly light and strong. 

Here, we explore what metal foams are and how they are made, and take a quick look at where they are used. 

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What is metal foam?

Metal foams are usually made up of a solid metal, with gas-filled pores inside. The foam has a dense outer layer, and the interior is in the form of a matrix of pores. The structure of these foams allows gases and even liquids to pass through them. They are based on materials with a similar structure, such as natural bone, pumice stone, and natural sponges. 

The pores can be sealed (closed-cell foam) or interconnected (open-cell foam). Metal foams are incredibly lightweight, yet very strong. 

On the whole, metal foams tend to be made from aluminum but they can also be formed from a variety of other metals, including titanium, tantalum, copper, zinc, or lead. 

 

Metal foams have some very important key features that make them particularly useful for material scientists, amongst others. 

The key properties of metal foams are as follows: 

• They are ultralight materials (75–95% of the volume consists of empty or gas-filled space)
• They have a very high porosity
• They have high compressive strengths combined with good energy absorption characteristics
• Their thermal conductivity is low
• They are very strong

What are metal foams used for?

Metal foams have a variety of applications. These range from construction materials to biomedical solutions. 

Some notable metal foam applications include, but are not limited to: 

• Closed-cell varieties are used for structural applications requiring load-bearing features, and for weight-saving and impact-absorbing structures in vehicles.
• Open-cell varieties are ideal for vibration and sound absorption, filtration, and catalysis at high temperatures, for heat exchange, and in medical devices.
• Open-cell varieties are also useful in functional applications such as filtration and damping.
• More recently, metal foams are being used as an experimental prosthetic in animals. Some researchers are even exploring using them to replace defective bones too. 
• Metal foams with high strengths can act as high-capacity impact-energy absorbers.
• In the automotive industry, metal foams can be used to reduce the weight of parts in the car frame, and facilitate assembly, thereby reducing costs and improving performance.

How is a metal foam made?

Metallic foams are formed through a series of steps. First, metallic melts are foamed by creating gas bubbles in the molten metal. 

Because the gas bubbles are less dense and have higher buoyancy than the metal melt, they rise to the surface. To stop this, the molten metal’s density is increased to trap the air bubbles in the melt. 

Fine ceramic powders are often employed to achieve this, or alloying elements are used to stabilize particles in the molten metal.

The introduction of gas bubbles is usually done through one of three methods. The most often used are as follows — though other methods also exist, like the aforementioned one devised by BASF: 

• Injecting gas into the liquid metal from an external source.
• Causing the precipitation of gas that had just been dissolved in the liquid.
• Causing an in-situ gas formation in the liquid by admixing gas-releasing blowing agents to the melt.

Metal powder or metal chips can also be mixed with titanium hydride, or other explosive agents, and heated above the melting point of the metal. In doing this, gaseous hydrogen is released which foams the mixture.

What is titanium foam used for?

Titanium metal foam is a very low density, permeable material that has many applications. Its defining characteristics are its very high porosity (between 75 and 95% by volume) so that it is mostly comprised of void spaces.

This incredibly strong metal foam is often used for:

• Thermal insulation
• Acoustic insulation
• Adsorption of environmen
  tal pollutants
• Filtration of molten metal alloys
• As a substrate for catalysts requiring a large internal surface area

Titanium foam also has some other limited medical applications. Researchers at Fraunhofer-Gesellschaft, a leading applied research organization, have attempted to use it as an ultra-strong implant to replace lost or damaged bone. 

“In the TiFoam Project, the research partners concentrated on demonstrating the viability of titanium foam for [the] replacement of defective vertebral bodies. The foam is equally suitable for repairing other severely stressed bones,” notes Fraunhofer. 

Is aluminum a porous metal?

Some forms of aluminum are porous. They have some similarities to metal foam, i.e. they are both permeable to gases and certain liquids, but there are also some major differences.

The main difference is in the manufacturing process. Porous aluminum is formed by casting the metal together with crystal salt into the desired shape. 

After the molding is complete, the salt is washed out. This process dissolves and removes the salt. Areas that contained salt grains are left as pores.

This porous aluminum has some advantages over metal foams. For example, one of the main difficulties in the production of metal foams is finding the right combination of metal and explosive agents. Another difficulty is finding the correct temperatures and times to heating them to produce the required texture of the foam.

Because of this, it is difficult to predict how the porosity, pore size, and distribution of pores will turn out.

“For these reasons, porous aluminum, with its simple manufacturing technology, is a valuable alternative. The broad range of precisely defined filter grades from 5µm to 250µm can differentiate the application accurately. The extra strong cast interface between porous and non-porous parts opens up a wide range of different applications,” explains a site concerned about the topic.

Some of the main differences between porous aluminum and metal foam are as follows: 

• A broad range of precisely defined filter grades from 5µm to 250µm
• Extra strong cast interface between porous and non-porous parts
• Extrem strength thanks to cast structure
• Simple post-processing (e.g. threading)
• Cost-effective manufacturing technology

Is stainless steel porous?

Stainless steel isn’t inherently porous, but porous stainless steel can be created. For example, a team of researchers at the Universidade Federal de São Paulo were able to produce porous stainless steel for medical purposes.

“Austenitic stainless steels are frequently used in biomedical applications, such as orthopedic implants, due to properties like high corrosion and fatigue resistance as well as high fracture toughness. In addition to biocompatibility, these properties are important in the selection and adaptation of material for biomedical applications,” notes an article on researchgate.net. 

Metal foams are an interesting material not only for their physical properties and production process but also for the various applications for them. Inspired by nature, these materials may find some other incredible uses in the not too distant future.