Aluminum foam (porous metal based on aluminum and its alloys) has been known for more than half a century, but still has a few applications. The main reason for this was the low reproducibility of the structure and the material properties in the production of aluminum foams. The development of aluminum foam technology has made it possible to overcome some of these difficulties. Different methods for producing metal foams have been developed and investigated.
Initially, the inventors of such materials were inspired by natural porous materials such as wood, bone, coral, pumice and some other porous minerals. The common feature of these materials is the combination of high strength, low specific weight and good energy absorption. Nowadays, aluminum foam manufacturers can relatively easily produce metals with similar properties. The scope of aluminum foams applications has become much broader and has dozens of different uses.
Aluminum foams types
Depending on the manufacturing technology, porous metals have open or closed pores. In case of open pore structure such materials are called cellular metals. Structures with close pore will be called metal foam. Within each of these groups there are variations which results in a broad spectrum of aluminum foams with different properties.
Manufacturing of aluminum foam
There are different approaches for manufacturing of aluminum foams.
In one case, the metal foam will be produced from the molten metal in different ways.
Upon cooling and hardening of the aluminum, the metal foam stabilizes. Flowing gas or blowing agents are used for foam formation. Under certain conditions the blowing agent decomposes and produces gas emissions. To prevent the foam from downfall before hardening, special additives are sometimes used to stabilize the gas bubbles. By applying this manufacturing technology aluminum foam sheets, products with simple shapes and aluminum foam sandwiches can be obtained. Such products have metal foam as a core, which is covered on both sides with a solid aluminium layer.
Another manufacturing technology is casting with a filling material. As a filling material could be used e.g. salt grains, which are then washed out of the aluminum matrix, the pores occur instead of the salt particles. Polyurethane foam with a temperature-resistant coating can also be used as a filling material, which will be burned out during the heat treatment.
For the next aluminum foam manufacturing process, hollow ceramic or metal balls are used which are incorporated into the metal matrix.
By injection molding process, polymer granules can be used to create pores. Due to the very short process time, these granules do not decompose under the influence of high temperature. They either remain as a filling material in the metal matrix or are disappeared by prolonged heat treatment. In the second case, an open-pore aluminum foam is obtained.
The production of metal foams has one more concept, so called powder metallurgy. As a first step a mixture of aluminum powder with special additives is produced. When such a mixture is heated, foaming and sintering processes occur. After cooling aluminum foam is obtained.
The most modern way to obtain porous metals is additive technology. With additive manufacturing, the shape and the 3D structure of the component can be controlled very precisely. However, this process still remains very costly and is applicable only to a limited number of metals.
Properties and applications
Closed-cell aluminum foam has a volume porosity of 80 to 98%. Its strength significantly exceeds the strength of a solid metal with the same weight. The use of such materials has great potential, especially for the means of transport of the automotive industry, aerospace and shipbuilding. The weight reduction leads to fuel savings or makes it possible to increase the load weight to be transported.
The porous structure of the aluminum foam absorbs the impact energy. This feature can be used to protect against collision, armor and explosion protection during transportation. Metal foam dampens the sound. Therefore, aluminum foam panels can be used as sound absorption walls along roads and railways. The soundproofing of cinemas and concert halls can also be effective. Aluminum foam is used for the design of interiors and facades due to its light weight and beautiful surface.
Open-pore aluminum foam is permeable to gases and liquids, has low weight and high strength of structure. Permeability significantly expands the application areas of aluminum foam. Such materials are used to filter gases and liquids, to reduce the noise of pneumatic equipment, as moulds for the production of EPP / EPS products, for vacuum tables, for different heat exchangers, etc.