Aluminum foams are very effective porous materials for the absorption of sound, the absorption of impact energy and vibration, electromagnetic protection and, in addition, they are not flammable and remain stable at high temperature.
They are recyclable and non-polluting and offer a combination of physical, mechanical, thermal and acoustic properties characteristic of a homogeneous material. All these characteristics mean that they have important applications in different sectors such as automotive, aerospace, naval and construction.
Aluminum foam is a relatively isotropic, highly porous metallic material with a random distribution of pores within the structure. The pores, essentially spheroidal, (open or closed) occupy 50% to 90% of the total volume. The mechanical and physical properties strongly depend on the density, which varies in the range of 0.4 to 0.8 g / cm3 (floating in the water).
The manufacture of SAF aluminum is possible because this element in liquid state allows the introduction of gas bubbles that are trapped inside. Under normal conditions, gas bubbles introduced into a liquid metal tend to reach the surface very quickly due to their lower density. But an increase in the viscosity of the molten metal and an adequate modification of the pressure and temperature conditions can hinder the migration of the gas and temporarily stabilize its permanence inside the molten metal until it is solidified. This requires some means of gas generation, either by the addition of foaming agents or by the injection of gases (air, nitrogen, argon, dioxide or carbon monoxide).
The most used foaming agent for aluminium alloys is titanium hydride, more expensive than the calcium carbonate used in UPM, due to the high specific volume of hydrogen and the rapid kinetics in its decomposition reaction. However, it is a material that, in addition to being expensive, is dangerous when handled.
Features of Aluminum foam
- The precursor starts foaming at 570 ° C, obtaining the best characteristics of cellular structure at 670 ° C, in which the viscosity is sufficient to not let the gas escape and to procure a homogeneous structure.
- The shape and homogeneity of the cell structure depends directly on its density, in addition to having a great relationship with the details of the parameters of the manufacturing process (temperature, baking time, alloy and cooling process).
- The data obtained in the tests indicate that there is a direct relationship between the density and the mechanical behaviour of the alusion. The foams of lower density have a heterogeneous cellular structure, a large pore size and very thin cell walls that require lower load values for their cell structure to fail, giving rise to lower mechanical characteristics. If we consider the values of the specific mechanical properties we obtain similar results.
- The macroscopic morphology of the fracture in the impact tests is ductile, observing a great deformation of the samples tested. The samples subjected to compression suffer a collapse by parallel layers of fragile nature. The tensile tested samples show brittle fracture with little deformation and practically no nip. The values of the mechanical tests show a high dependence on the operational conditions of the tests carried out.
- The correlation obtained between the estimated values of compression elastic limit, according to the theoretical models and the real values, has been limited. The best approach has been achieved in low density foams.