Aluminium foams have a porous structure that makes them excellent materials to absorb sound, impacts and vibrations. Their metallic nature allows their use as electromagnetic shields and makes them stable at high temperatures. Aluminium foams are recyclable and non contaminant, while offering a combination of physical, mechanical, thermal and acoustic characteristics typical of a homogeneous material. All these characteristics are ideal for diverse and important applications in different industrial sectors, from aerospace or naval to motor and construction.
For these reasons the research group "Ingeniería y Ciencia de los Materiales" that belongs to the Escuela Técnica Superior de Ingenieros Industriales at the UPM has tried and successfully manufactured(*) these foams using calcium carbonate, greatly reducing their cost and effectively increasing the possible applications.
Aluminium foam is metallic and relatively isotropic, with many randomly distributed pores inside its structure. These pores, mainly spherical, (open or closed) occupy 50% to 90% of the total volume. The mechanical and physical properties depend greatly on density that varies from 0.4 to 0.8 gr/cm3 (floats in water).
Manufacturing aluminium foam is possible because if gas bubbles are introduced in the element while in a liquid state many remain trapped in its interior as it cools down. In normal conditions, the bubbles that are introduced in a liquid metal tend to rapidly reach the surface because of their lower density, but by increasing the viscosity of the liquid metal and carefully tuning temperature and pressure conditions it be made more difficult for the gas to move inside the metal, effectively stabilising it within the material as it cools down and solidifies. In order to achieve this, a means of producing the gas are needed, either by using foaming agents or by injecting gas directly (air, nitrogen, argon, carbon dioxide or carbon monoxide). The most used foaming agent when working with aluminium is titanium hydride, because of the high specific volume of hydrogen and the fast decomposition reaction kinetics, but this is a material that is not only expensive but also dangerous to manipulate.
To avoid such inconveniences, the researchers at the UPM have manufactured(*) aluminium foams using calcium carbonate as a foaming agent. This material decomposes inside the molten aluminium alloy releasing carbon monoxide and carbon dioxide. These reactive gases, under the right agitation conditions, form bubbles along with aluminium oxide, calcium oxide and other complex metallic oxides that stabilise the liquid metal, and modify its viscosity and surface energy, to keep bubbles from merging and the liquid from draining.The obtained foam has proven stable in a wide range of compositions, allowing the production of materials with different geometry and characteristics.
With this foaming agent, the production of aluminum foam is a continuous process, and leads to significant cost reductions since this is a cheaper product than titanium hydride and other currently used foaming agents, a fact which will effectively multiply the many applications of these materials.
The main applications of aluminum foams are found in the automotive industry (impact, acoustic and vibration absorbers), the aerospace industry as structural components in turbines and spatial cones, in the naval industry as low frequency vibration absorbers, and in construction industry as sound barriers inside tunnels and as fire proof materials, structure protection systems against explosions and even as decoration.