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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