Metal foams have a low density due to pores and cavities , but have a high specific rigidity and strength . In addition, metal foams enable a priori increased absorption of kinetic energy, which means that they offer improved solutions in particular for some classic main construction problems in vehicle, plant and mechanical engineering, such as vibrations ( damping ) and safety ( crash test ).
Aluminum or aluminum alloys are preferred because, due to their very low density, they are ideal for the advantages of lightweight construction . Using suitable blowing agents, it is also possible to produce foams from copper , zinc , lead or steel / iron .
Metal foams are often made using metal powder and a metal hydride , e.g. B. the titanium dihydride produced. Both powders are mixed with one another and compacted into a primary material by sintering or extrusion . The starting material is then heated to a temperature above the melting point of the metal. The titanium dihydride releases gaseous hydrogen and foams the mixture.
There are other ways to produce metal foams. For example, gas can be blown into a metal melt which has previously been made foamable by adding solid constituents. For aluminum alloys, 10-20% by volume of silicon carbide or aluminum oxide are added for stabilization . An alloy is formed by adding calcium . This increases the viscosity and stabilizes the melt.
As a further variant of the method is S chlicker r eaktions s chaum s inter-method (SRSS) method available, can be produced with the mainly iron, steel and nickel foams. In this process, a slip is foamed by means of hydrogen, which is formed by the reaction of acid with the respective metal powder . The foam structure is set by further reaction products and dried in a mold. The resulting green compact is then sintered in a reducing atmosphere or in a vacuum.
For low-melting metals (aluminum, zinc, lead) there is a technology for the production of open-pore metal foams by pouring them with salt. The liquid aluminum (or zinc, lead) is poured together with crystal salt (NaCl) and after mechanical treatment the salt is washed out. The grains of salt and their contact points are replaced by pores. With this manufacturing technology, all pores are interconnected and therefore open. Such a metal foam has a porosity of 50 to 65% and adjustable pore sizes of 0.2 mm to 4 mm.
Metallic foams show good mechanical energy absorption capacity, which is why aluminum foams are beginning to be used as crash protection elements in automobile construction . In the Audi Q7 , crash absorbers made from this material are used in the luggage net holder to ensure occupant safety even if the body is deformed.
In machine tool construction , metal foams are of interest for use in machines with high dynamic loads. In particular, improved vibration damping usually means a significant increase in manufacturing quality in machine tools, which in turn usually lowers operating costs or increases productivity . One example is the universal slide of the Mikron HPM 1850 U machine tool, which was developed in cooperation between the Chemnitz-based Niles-Simmons industrial plants and the Fraunhofer Institute for Machine Tools and Forming Technology . The machine was presented for the first time at the METAV 2004 and is manufactured in series. Compared to the pure steel welded construction, the mass could be significantly reduced. The dynamic stiffness and damping properties have been improved analogously.
Other fields of application for metallic foams are heat shields , heat encapsulation , filters , catalyst carriers , sound-absorbing cladding or the production of very light, foam- filled rollers for the printing or paper industry.
Composite metal foam (CMF)
Composite metal foam (CMF) is formed from hollow metal spheres in a solid matrix of another metal, such as steel in aluminum. CMF has a 5 to 6 times higher strength-density ratio and more than 7 times higher energy absorption than the previous metal foams. In 2016, North Carolina State University demonstrated its usefulness as a kinetic absorption layer in multi-layer armor. A one inch thick Kevlar -Metallschaum- ceramic - composite was able armor-piercing projectiles to intercept 7.62 × 63 mm M2 AP caliber.
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