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BLOWING AGENT FOR METAL
FOAM
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CONTENTS
Introduction.
Foaming Process.
Hydrides As Blowing Agents.
Carbonates As Blowing Agent.
Decomposition Temperature.
Reactivity With Base Metal.
Performance Enhancement.
References.
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INTRODUCTION
A blowing agent can be defined as a material that decomposes under
the influence of heat and releases gas which then propels the foaming
process .
The quality of final foam is influenced by several parameters such as
composition, size and volume fraction of the blowing agent used.
Different chemicals such as carbonates and metal hydrides fit into the
blowing agent description.
Blowing agents are typically applied when the blown material is in a
liquid stage. Generally the foaming agents must undergo thermaldecomposition and generate gas pressure in the matrix sufficient to
overcome atmospheric pressure and cause foaming.
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FOAMING PROCESS
During the production of metallic foam as soon as the metal softens, the
blowing agent expands giving rise to small spherical bubbles. During the
growth of these bubbles interaction between cells results in change of
shape of bubbles from spherical to polygon and thus final shape of
bubble formed is polygon surrounded by liquid layer.
Figure 1.Steps of formation of the metallic foam
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HYDRIDES AS BLOWING AGENTS
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CARBONATES AS BLOWING
AGENT
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SELECTION CRITERIA
The selection of blowing agents mainly depends upon the following
parameters:
Particle size of blowing agents: The particle size of blowing agent
must be selected according to the intended cell size of the foam also
the particles of base metal must be smaller than particle size of
blowing agent (for powder compaction method).
Decomposition Temperature.
Reactivity of blowing agent with base metal.
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DECOMPOSITION TEMPERATURE
The melting temperature of the metal and the gas decomposition
temperature range of the blowing agent have to be match to ensure
satisfactory foaming result. If the blowing agent releases gas below the
foam matrix melting point, the metal would be expanded in the solid
state, which would lead to cracks. On the other hand, if the blowingagent decomposes above the melting point, the viscosity of the melt will
be too low for the formation of stable foams.
Table 1:Decomposition temperature of blowing agents
S.N. `Blowing Agent Starting melting temp.(K) End melting temp.(K)1 TiH2 703 933
2 ZrH2 660 740
3 Mg(OH)2 633 753
4 4MgCO3.Mg(OH)2.5H2O Ambient temperature 613
5 MgCO3 793 9936 CaCO3 993 1203
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REACTIVITY WITH BASE METAL
The blowing agent must not react with the base metal and form
intermetallic brittle compounds as they temper the mechanical
properties of the foam produced.
In case of ZAM alloy when TiH2
was used as blowing agent
intermetallic Al3Ti was formed which lowered the ductility of cell
walls and reduced toughness of foam whereas when CaCo3was used
Al+Al4Ca was formed which is less brittle and thus does not impair
toughness of foam.
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PERFORMANCE ENHANCEMENT
Performance of metal foam can be significantly improved by applying
suitable coating agent on blowing agents.
Takashi Nakamura et al. showed that applying CaF2coating on CaCo3
(blowing agent) low density foams were produced due to wettability
enhancement.
Guo-qiang LU et al. found that coating of SiO2 on CaCo3 enhanced
thermal stability and reduced foaming speed and thus Mg foam
expanding gradually with homogeneous structure without shielding
gas.
Guo-qiang LU et al. showed that more enhanced compressive strength
and homogeneous cell structure was obtained when Ni coated TiH2
blowing agent was used in Al foam manufacturing.
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REFERENCES
Banhart, J., 2001, "Manufacture, Characterization and Application of
Cellular Metals and Metal Foams
Bart-Smith, H., Bastawros, A. F., Mumm, D. R., Evans, A. G, Sypeck,
D. J., and Wadley, H. N. G., 1998, "Compressive Deformation and
Yielding Mechanisms in Cellular Al Alloys.
Banhart, J., 2000, "Manufacturing Routes for Metallic Foams.
Banhart, J., 2001, "Manufacture, Characterisation and Application of
Cellular Metals and Metal Foams.
LI Ai-bin, XU Hong-yu, GENG Lin, LI Bin-ling, TAN Zheng-bin,
REN Wei,2012 Preparation and characterization of SiCp/2024Al
composite foams by powder metallurgy.
Baumgrtner, F., Duarte, and Banhart, J., 2000, "Industrialization of
Powder Compact Foaming Process
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Bhosle, V, Baburaj, E. G., Miranova, M., and Salama, K., 2003,
"Dehydrogenation of TiH2
Kennedy, A. R., 2002, "The Effect of TiH2 Heat Treatment on Gas
Release and Foaming in Al-TiH2 Preform
Guo-qiang LU, Hai HAO, Fu-yun WANG, Xing-guo ZHANG 2013
Prepration of closed cell Mg foam using SiO2coated CaCo3as
blowing agent in atmosphere
Takashi Nakamura,Ryiochi Ishikawa,Kazuhiro Sakamoto 2002
Development of New Foaming Agent for Metal Foam
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