1. What is Foamed Metal and Foamed Aluminium
In the course of history, the discovery of foamed metals constitutes an important milestone in the revolution of the entire field of materials engineering. A metal foam is a material that consists of a metallic structure with a large number of pores that are filled with gases along the structure. The pores of these metal foams can be in the form of interconnections or hermetic seals, known as open-cell and closed-cell foams, respectively. Among them, aluminium foam is currently the foam metal with the greatest potential for application, and is known as the “metal star” due to a series of excellent properties, such as high porosity, low density, light weight, high strength, good compression, good impact absorption, good sound absorption, and electromagnetic shielding. “metal star”, widely used in aerospace engineering, manufacturing and other processing methods in the field of physical sciences.

Structural characteristics of aluminium foam:   (a) Open-cell aluminium foam;    (b) Closed-cell aluminium foam

2. Properties and Applications of Aluminium Foam
Aluminium foam is a metal with a high porosity and porous structure, in which the sealed pores account for about three quarters of the total volume of the metal material, making the material very light. In addition to these properties, aluminium foam has impressive energy absorption properties and high compressibility. With regard to density, the researchers found that most aluminium foam samples have a density of around 0.4-1.2 g-cm-3. In addition, aluminium foam is sound-absorbing, recyclable and has low thermal conductivity.

Improved impact-resistant bilge partial structure: 1 – upper skin; 2 – aluminium foam truss; 3 – lower skin

As far as applications are concerned, the electromagnetic shielding, lightweight and absorptive properties of aluminium foam make it an ideal candidate for components used in controlled fusion devices, automotive, aerospace and construction industries due to its ability to resist extreme physical environments. In controlled fusion devices, closed cell aluminium foam is an excellent electromagnetic shielding material. Due to the interconnection of the aluminium foam skeleton, which has a huge specific surface area and a good ability to absorb and reflect electromagnetic waves, various kinds of losses, such as absorption losses and reflection losses, occur in the aluminium foam, which gives the foam aluminium excellent electromagnetic shielding properties. And it has also been found that the electromagnetic shielding effectiveness of aluminium foam is decreasing and then increasing with the increase of electromagnetic wave frequency. In the aerospace industry, aluminium foam is used as a component in the manufacture of space cones or turbines, which is particularly useful for reducing the total weight of aircraft components due to the low mass of aluminium foam. Moreover, as an excellent energy-absorbing material, aluminium foam is used in the manufacture of spacecraft buffers, vibration absorbers and the bottom of spacecraft return capsules, which enables safe landings and protects astronauts and space equipment. By effectively absorbing energy and damping vibrations, aluminium foam can ensure the safety of astronauts in the event of a spacecraft landing impact.
In the automotive industry, aluminium foam helps to reduce the total number of parts used in the frame, making the assembly process more efficient, which ultimately helps to reduce costs and improve performance. Currently, aluminium foam is used in crash bars, cross members and door beams in car body structures on the market.

(a) Diagram of localised energy-absorbing parts of the bodywork.  (b) Schematic diagram of a aluminium foam crash bar.

In the construction industry, the main use of aluminium foam is as a protective, fire-resistant barrier against severe noise. Typical examples of architectural applications of closed-cell aluminium foam are the London Underground station and the SOHO Revival Square in Shanghai, where the designers used large quantities of aluminium foam for sound absorption and insulation.

(a) (b) Photographs of aluminium foam used in the interior of SOHO Plaza, Shanghai; (c) Aluminium foam + metal cavity composite sound absorbing wall at Bermondsey Station, London Underground.

3. Conclusion
The discovery of aluminium foam is considered to be a remarkable achievement. Aluminium foam material combines excellent electromagnetic shielding properties, light weight, low density, vibration damping and energy absorption properties, which makes it very widely applicable in various industries today. Interestingly, aluminium foam is also an environmentally friendly product that can be recycled, meaning it is a sustainable material that will hopefully be used in a large number of industrial applications in the future.