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PRODUCTION AND PROPERTIES OF CAST METAL WITH A POROUS
Aleš HANUS, Petr LICHÝ, Vlasta BEDNÁŘOVÁ
VŠB –Technical university of Ostrava, 17.listopadu 15, 708 33 Ostrava, Czech Republic, [email protected]
Abstract
Porous metals and metallic foams are metallic materials containing pores in their structure that are
intentionally created. Their specific structure then gives them a very advantageous combination of physical
and mechanical properties, such as low density and high rigidity along with high capability of absorption of
impact energy. Since the discovery of porous metallic materials numerous methods of production have been
developed. According to the state, in which the metal is processed, the manufacturing processes can be
divided into four groups. Porous metallic materials can be made from liquid metal, from powdered metal,
metal vapours, or from metal ions. Porosity may achieve 30% to 93% depending on the method of
production and material used. The aim of the experiment was to verify the possibilities of production of
metallic foams by conventional foundry processes and to study the process conditions. The work is focused
on the production of cast metallic foams with open structure by use of various types of filler material.
Mastering of production of metallic foams with defined structure and properties using gravity casting into
sand or metallic foundry moulds will contribute to an expansion of the assortment produced in foundries by
completely new type of material, which has unique service properties thanks to its structure, and which fulfils
the current demanding ecological requirements. In Czech Republic cast metallic foams are not yet produced.
Key words:
Metal foam, porous metals, filling materials, casting methods.
1.
INTRODUCTION
Porous metals and metallic foams are metallic materials containing pores in their structure that are
intentionally created. Their specific structure then gives them a very advantageous combination of physical
and mechanical properties, such as low density and high rigidity along with high capability of absorption of
impact energy. The term porous metal is a general term describing a material with high porosity, while the
terms foam metal or metallic foam are used for porous metal produced by foaming of the melt, in which the
pores are not interconnected ("structure with closed pores"). In addition, we have the term metal sponge, this
is used for highly porous materials, in which the pores are connected in a complicated manner and the
structure cannot be divided into individual cavities („structure with open pores“) [1]. The term metallic foam is,
however, very often used even in professional literature as a general designation of porous material.
2.
PRODUCTION METHODS
Since the discovery of porous metallic materials numerous methods of production have been developed.
Some technologies are similar to those for polymer foaming, others are developed with regard to the
characteristic properties of metallic materials, such as their ability to sintering or the fact that they can be
deposited electrolytically.
According to the state, in which the metal is processed, the manufacturing processes can be divided into four
groups. Porous metallic materials can be made from liquid metal, from powdered metal, metal vapours, or
from metal ions [2].
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Porosity may achieve from 30% to 93%, depending on the method of production and material used. By
changing the process parameters it is possible to obtain a porous structure with various sizes and shapes of
pores and with different types of arrangement (regular or stochastic).
2.1
Production of porous metal from liquid metal
2.1.1 Direct foaming of the melt
Currently two ways of direct foaming of the liquid metal exist:
Injection of gas into the molten metal - The simplest method of liquid metal foaming consists in blowing of
gas bubbles into the melt. The main condition is the creation of fine bubbles uniformly dispersed in the melt
volume and preservation of these bubbles in the melt till its solidification. This method is used by the
Canadian company Cymat Aluminum Corp. on the basis of patents acquired from the companies Alcan
International Ltd. and Norsk Hydro [3].
Addition of foaming agent releasing gas into metal - Foaming agent is formed by powder material or by
compound, which is stable at room temperature, but which releases at thermal decomposition a gas that
forms pores in the melt [2]. This technology, registered under the trade name ALPORAS, is used by the
Japanese company Shinko Wire Company already since 1986 [4].
2.1.2 Directed solidification of gas over-saturated melt
The method was developed in 1980 in the former Soviet Union [5] and it is known as GASAR. GASAR in
Russian acronym means "stiffened by gas". It is based on the eutectic reaction of gas, which takes place
during crystallization of metals or alloys over-saturated by gas (mostly by hydrogen).
2.1.3 Powder metallurgy
Production of metallic foams from powdered metal (usually aluminium and its alloys) and from powdered
foaming agent (TiH2, ZrH2 or CaCO3) was patented as early as 1963 [6]. Powdered metal is mixed with
foaming agent and cold compressed into the blank. The next step is melting of the base material, during
which the foaming agent, evenly distributed in the matrix, decomposes. Under the influence of released gas
the material expands, and thus highly porous structure is formed. This technology is used for production of
sandwich panels or structural parts with a solid surface layer and with inner porous structure - technology
ALULIGHT.
2.1.4 Foundry methods
Investment casting - Investment casting with use of pattern made of polymeric foam is used for production
of metallic foam with open pores, which copies the shape of the polymer foam. The company ERG, Inc.,
California, produces with use of this technology metallic foam with the trade name Duocel already since
1967 [7]. Foam porosity ranges from 80 to 97%, with pore sizes of 4.3, 2, 1 and 0.5 mm. Foams Duocel are
of high quality, however, the disadvantages of the production process are the high costs, complexity of the
manufacturing process and low production volumes.
Casting on the filler material - Porous metals can be produced by the method, at which the liquid metal is
poured into a mould filled with inorganic or organic particles, which may remain in the casting, or which are
removed by leaching or annealing [8].
2.2
Production of foam from the solid phase
Metals with a porous structure can be made not only from the melt (liquid phase), but also from the
powdered metal, when the powder remains during the entire production process in the solid state.

Sintering of metallic powders and fibres
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2.3

Sintering of hollow metallic balls

Reaction sintering [2,9].

Capture of gas - Low Density Core Process (LDC).

Foaming of suspension

Use of filler material
Production of foam from ionized metal
This method uses electric plating of polymer foam with open pores, which are afterwards removed. In order
to perform electroplating, the polymer foam must be electrically conductive [2].
2.4
Production of foams from the gaseous phase of metal alloys
Metallic foams can be made from metal vapour or from gas metallic compound. Metal vapours are formed in
a vacuum chamber and they condensate on the cold precursor. Solid porous structure, such as polyurethane
foam or polymeric material with lattice structure, serves as a precursor [10].
3.
Properties and applications of metallic foams
The material offers particularly the following properties and possibilities of application:
•
Reduction of weight: porous metals themselves are lightweight and when connected with thin ribs, they
can achieve the same values of mechanical strength as it is required for conventional heavy structures
•
Absorption (damping) of energy, the most promising feature: it uses the ability of this type of material
to deform under a constant and relatively low stress, and thus to absorb in the relatively small volume
large amounts of energy. This property can be used in transport, especially in personal vehicles for
protection of the interior in the case of car crash (e.g. aluminium foam materials, produced in small series
for the Audi Q7, and dampers for railway production [14]).
•
Absorption of sound and vibrations: substitute of organic foam materials in an environment with
extreme thermal and mechanical stress;
•
Thermal insulation: metallic porous materials retain high mechanical properties even at high
temperatures and even when exposed to flames they do not release any organic vapours;
•
Protection against explosions and impacts: both for non-military and military purposes
•
Exchange of heat or electricity: metallic porous materials with open structure have very large specific
surface area, which gives them better exchange capabilities. Nickel foam material, used as electrodes in
rechargeable batteries for portable devices (mobile phones, laptops), are manufactured in series
production.
•
Medicine: which uses porous metals for bone and dental implants - (titanium metallic foams), their
structure is closer to the structure of human bone, and titanium is well tolerated by the organism.
Despite the uniqueness of properties and wide range of possibilities of use, the examples of practical
permanent applications are only few. The reasons are primarily economic. That’s why the current interest (as
reflected by a growing number of new published studies and possibilities of industrial applications [11, 12,
13, 14] is focused mainly on the development of technologies enabling production of foam materials at the
costs, which would allow their widespread use.
Automotive industry seems to be one of the most promising fields of application in the Czech Republic. Since
the effort in the recent years was aimed at reduction of the weight of automobile parts in order to reduce fuel
consumption and emissions, the porous metallic material fully complies with these requirements and it
provides also other unique properties (energy absorption).
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6.
Experimental part
The aim of the experiment was to verify the possibilities of production of metallic foams by conventional
foundry processes and to study the process conditions. The work was focused on the production of cast
metallic foams with open structure by use of various types of filler material.
6.1
Use of ceramics particles
Precursors made of ceramic material with fractions of 8 16 mm (Fig. 1) and 3-4 mm, were tested, which were
inserted into the cavity of the mould made from bentonite bonded mixture. Apart from different granulometry
also different design of the gating system was used, and the temperature of the filler material was also
changed (in order to affect positively filling capability of the molten metal). Materials used for casting was
AlSi10MgMn alloy (Fig. 2 and Fig.3) and cast iron with lamellar graphite – according to EN GJL-200.
The experiment did not prove an influence of the inlet design of the gating system, or of the temperature of
the filler material on fluidity of the given alloy. Size of particles of the filler material had substantial influence
on filling capability of the molten metal, when in case of the fraction 3 4 mm in dependence on the amount of
the filler the produced castings were not fully cast by some 20-60% (rated by weight). Although annealing of
precursors to the temperature of 150°C resulted in reduction of material moisture from 1.2 to 0.28%,
however, compared to expectations, it did not lead to significant improvements of filling capability, and
temperatures of 80 - 100°C, at which the material was inserted into the mould cavity, complicated the
handling. Due to the relatively low pouring temperatures of the alloy AlSi10MgMn (690-705°C) the filler
material remained in the casting (syntactic foam [2]) and its removal would require an additional annealing of
castings, which would increase the energy consumption of such production. These problems did not occur at
pouring of castings from cast iron, in which it was possible to easily remove the used filler material from the
castings and to obtain thus porous castings with open pores in chastic arrangement.
Fig. 1 Ceramic particles (fractions 8 – 16 mm)
6.2
Fig. 2 Casting made of the alloy AlSi10MgMn with
ceramic particles (fractions 8 – 16 mm)
Use of core particles
Core particles were then manufactured from moulding mixture (3% Novanol resin, curing time 90s) using
conventional core making techniques (ie. molding mixture compaction in the prepared core box and CO2
curing). The special attachment to the supply of allowed the more uniform curing of the whole core volume
and thus also better mechanical properties of the core. Final globular shape of core precursors was achieved
by spliting in to small pieces and followed by tumbling. The precursors so produced was inserted in a mould
cavity. Mold was made from commonly used green sand (ie. bentonite bonded moulding mixture) – Fig. 4.
Material used for casting was cast iron with lamellar graphite – according to EN GJL-200 – Fig.5 and Fig. 6.
23. - 25. 5. 2012, Brno, Czech Republic, EU
6.3
Use organic filler material
For the alloy AlSi10MgMn use of different types of organic fillers, allowing easier removal from the casting,
was verified (Fig. 7). During pouring, however, some substances produced big evolution of gas (depending
on the type of material, temperature of its thermal destruction and gas evolution properties) and in this case
either the metal did not fill fully the mould cavity in case of materials with particle size below 3 mm.
The pores in the castings were distributed unevenly throughout the volume of material.
Fig. 3 Casting made of the alloy AlSi10MgMn with
ceramic particles (fractions 3 – 4 mm)
Fig. 4 Core particles in mould
Fig. 5 Casting made of the cast iron (EN GJL-200)
with core particles
Fig. 6 Casting made of the cast iron (EN GJL-200)
with core particles
6.3 Discussion of results
The main drawback of the method using a filler material is irregular structure and random distribution of
pores throughout the volume of the casting, and therefore impossibility to achieve reproducible results [15].
That’s why next experiments were focused on the preparation of porous materials with regular structure.
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Fig. 7 Cross section of the casting (AlSi10MgMn) after removal of organic filler
ACKNOWLEDGEMENTS
This work was elaborated within the frame of the research project TA02011333 (Technology Agency
of the CR) and the specific research project VŠB-TU Ostrava SP 2012/23.
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