ALMA FORUM 2016 (2016.08.09@KYOTO)

Materials Processing for Light Metals
with Meso-scale Structures;
Porous Metals
Makoto Kobashi
Dept. Mater. Sci. and Eng.
Nagoya University1

Outline• Introduction:– Porous structures in nature– Man-made porous metals• Conventional processing for making porousaluminum– Precursor method, Spacer (Space holder) method• Processing for high-melting- temperature
porous aluminides– Reactive precursor method– Variety of porous structure• Summary2

When a modern man builds large load-bearing structures, he uses
dense solids; steel, concrete, glass. When nature does the same,she generally uses porous materials; wood, bone, coral. There
must be good reasons for it.
Prof. M.F.Ashby, University of Cambridge

Aligned along
the loading
direction

The thigh bone

The skull

Designed for bothmaximizing bending
and buckling strengthand minimizing weight.

Designed forabsorbing impact
energy to protect
your brain.

Sandwichstructure.

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When a modern man builds large load-bearing structures, he uses
dense solids; steel, concrete, glass. When nature does the same,she generally uses porous materials; wood, bone, coral. There
must be good reasons for it.
Prof. M.F.Ashby, University of Cambridge
The moisture in the air condenses on snail
shell surface, and repels oily dirt.
Patterned roughsurface

Waterfilm

The hollow fur of the polar bear provides
excellent thermal insulation from the cold.
Hollow
structure

Oily dirt

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Closed cell structure

Here3030

Static compression(5x10-4 m/s)
Density : 406 ~ 417 kg/m 32525
2020

圧縮応力(ＭＰa)

Drop weight test(High-speed camera)

Compressive stress (MPa)

Here

Ideal plateaustress forbumper
408.8
409.5

15158~10 MPa (forPorous passenger safety)
metal 2~3 MPa (forpedestrian safety)
Plateau stress

10105500

406.5

Dense
metal

002020406080
406080圧縮ひずみ(％)
Compressive strain (%)

405.6
417.4

100100Large deformation at a constant stress level is an
ideal property for a impact energy absorber.

Functions

Cross-section Category

The porous metal is a kind of nature inspired materials, and can be classified
according to shape and size of the pores.
Closed cell(Equi-axed)

Lotus cell(uni-directional)

Flat cell(Lamellar)

Open cell(Sponge)HighliquidHigh damping Fluidabsorbency,
capacity,Permeability,Fluid
Impact energy High specific Low thermal permeability,
absorption,modulus,
conductivity Capillary
High specific High damping
force,
moduluscapacity
Acoustic
absorption

Open cell(Nano)

Catalytic
property,Highliquidabsorbency,
Large surfaceareaFabrication route of porous aluminumby precursor method (Closed cell structure)
Precursor

TiH2

Al

1. Blending

2. Consolidation

1. Powder blending
Al(45 mm) + 0.5mass%TiH2
2. Consolidation
Hot compaction or Extrusion
3. Heating and foaming
Melting point of Al: 660℃
Decomposition of TiH２: 500℃
TiH2  Ti + H2↑

Molten Al

H2 gas

3. Heating

4. foaming

7

Fabrication route of porous aluminumby precursor method (Closed cell structure)
Closed cell structure

20 mm

Blowing behavior of plate-shape precursor heated by an induction coil8

Foamed in hollow components

Porous Al filled in a hollow tube
Filled to the edge

Foamed in a star-shapemoldStar moldPore distribution

10mm

Fabrication route of porous aluminumby spaceholder method (open cell structure)

PoreLimitations of conventional process
Material• Applicable to only low melting point Al alloys (nochance for high-melting-point aluminide intermetallics)
TiAl, Al3Ti– Sample should be melted– Appropriate blowing agents or spacer powder are notavailable.

Processing• Limited sample size
– limited by furnace size

• Difficult to fill Al hollow components with porous Al– Processing temp. is the melting point of aluminum➫Reactive Precursor Process
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Combustion reaction: a strong exothermic reaction between elementalpowders that synthesizes ceramics or intermetallics.

Schematic illustration of combustion reaction between Al and Ti

ABCombustion foaming behavior

ABA+BABex.) Ti+AlTiAlTi+3AlAl3Ti

Ignition start

200ms

700ms

1100ms

400ms

Transmission images of a precursor by X-ray during thereaction between Al and Ti

Exothermic agent addition

Exothermic agent (B4C)
3Ti+B4C  2TiB2+TiC+254 kJ/mol Ti(3Al + Ti  Al3Ti +146 kJ/mol Ti)

Temperature profile of precursors
with and without B4C additions during heating

One of the advantages of the reactive process is that thefoaming behavior self-propagates throughout theprecursor.
Self-propagation
foaming• Energy saving• Long or large scale
Precursor
products• Direct synthesis inhollow component
Heated Side

10mm

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Laser

Low melting
point
hollow tube

Selective heating ofprecursor in ahollow component

Precursor

Hollow/porous hybrid
Pore

Pore

Summary• Porous metal is a new class of materials withvarious unique properties. The most fascinating
feature of porous metal is that the properties canbe controlled by the shape, size and fraction ofthe pores.
• Reactive precursor process has been studied andvarious cell morphologies has been achieved sofar. The porous metals obtained by this method
can be applied to various engineering fields .
Thank you for your kind attention!

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