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General

Material is called foam if pores in it are all together min. 50% of total volume and these pits filled with gas - forming many tiny bubbles - are dispersed evenly. Bubbles are called cells – separating walls can be made of fluids or solid material.

Cell structural materials in the nature are well known: human or animal bones, wood, bamboo, etc. Cellular materials with high porosity are light and strong - complemented by advantageous thermal insulation, sound and energy absorption properties. Ratio of strength per mass gets higher with increasing porosity while quantity of used material lessens. That’s why material science researches and developments focus on artificial solid foams. To produce ceramic or polymer foam is no more a problem – think of everyday products as Ytong construction block or Nikecell thermal insulation foam for building industry. Not so easy is to produce metal foam - such products may be current first in the next decade. Fig.1. shows innovation trend and market demand - while science publication frequency can be seen in Fig.2. Consequently we may foresee the metal foam market as significantly growing in the coming years.

Fig.1. Innovation and market trend of aluminium foam.

Credit: Aluminiumschaum, Aluminium – Merkblätter, Aluminium-Zentrale e. V. ISBN3-87017-259-2

Fig.2. Number of science publications and those of metal foam subject
Credit: Status and recent developments on porous metals and metallic foams in North America, L.P.Lefebvre, MetFoam2009 conference, Bratislava

Metal foam types

In principle there are two types of foams : one with open and other with closed cells. In latter type pores are surrounded by continuous and non-permeable cell walls to isolate them from each others. Consequently the closed cell foams are great at insulation and energy absorption and can be applied for structure items. Owing to their mass structure and consistency they float in water.

Fig.3. Closed cell metal foam with closing sheet metal on both sides. Credit: Status and recent developments on porous metals and metallic foams in North America, L.P.Lefebvre, MetFoam2009 conference, Bratislava

In open cell foams (see Fig.4.) the whole frame structure consists of several rod-like supporting parts. In this case the foam is wholly permeable and it is to admit that cells are no real cells. They can be applied very successfully for filtration and as catalyst carrier too having large relative inner surface.

Fig.4. Typical structure of open cell foam.
Credit: Status and recent developments on porous metals and metallic foams in North America, L.P.Lefebvre, MetFoam2009 conference, Bratislava

Processes for closed cells metal foams production

Fig.5. Overview of metal foam production methods. Credit: J. Banhart, D. Wearie, On the road again:metal foams find favor, Physics Today 55, 37-42 (2002)

Fig.6. Foaming of melt metal with stabilizer particles using direct gas input.

Fig.7. Metal foam sheets produced by process shown in previous figure.


Fig.8. ALPORAS foaming technology. Gas bubbles generated by Ca and TiH2

 

Of all processes shown in Fig.5. the indirect ones are in use (see Fig.8.) featured by solid metal powder with foaming additive and processing in form. These procedures are rather expensive and produced foam structure is inhomogeneous.
Processes of direct gas input (see Fig.6. and 7.) perform products showing following disadvantages:
- outer shape of product is to be simple (mainly plain interfaces)
- rough surfaces
- machining needed is very expensive.

Consequently a process is wanted to be able to make product featuring:
- uniform cells evenly distributed
- product shape design is unrestricted
- product surface quality is fine.

That’s why a Hungarian consortium lead by ADMATIS started a project in 2008 to establish and develop his own technology for metal foam production. Click here for details.

Raw materials for metal foams

  • aluminum
  • zinc
  • nickel
  • titanium
  • steel
  • magnesium