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Journal of Materials Processing Technology 210 (2010) 1203–1208

Contents lists available at ScienceDirect

Journal of Materials Processing Technology

journa l homepage: www.e lsev ier .com/ locate / jmatprotec

abrication of aluminium foams from powder by hot extrusion and foaming

. Shiomia,∗, S. Imagamab, K. Osakadab, R. Matsumotob

Faculty of Global Engineering, Kogakuin University, Inume, Hachioji, Tokyo 193-0802, JapanGraduate School of Engineering Science, Osaka University, Machikaneyama, Toyonaka, Osaka 560-8531, Japan

r t i c l e i n f o

rticle history:eceived 12 December 2009eceived in revised form 1 March 2010ccepted 6 March 2010

a b s t r a c t

In order to produce aluminium foams for light-weight and energy absorbing structures of automobiles,a method for fabricating aluminium foam from powder mixed with a foaming agent by using a mouldis proposed. The method consists of four sequential processes: powder compacting, extruding, foamingand moulding. In the experiment for fabricating aluminium foam from powder, the conditions of powderextrusion and foaming by the heated die are determined from the density of the aluminium foams made

eywords:ight-weight structureetal foam

orous aluminiumoulding

without a mould. The experimental results show that the relative density of the aluminium foam madeunder appropriate conditions is 0.22. In moulding of aluminium foam, a stainless steel pipe is used as amould and the cylindrical aluminium foam is produced by filling into the pipe mould. The distributionof relative density within the aluminium foam bar is in a range of 0.2–0.3 by rapid cooling of the pipe.To examine the ratio of deformation energy to weight of the pipe including the aluminium foam, a

pressthe p

nergy absorption compression test using aaluminium foam filled by

. Introduction

Metal foams have plenty of pores inside and their density isuch lower than the solid one. Their cellular structure causes to

how unique features, such as energy absorbing capacity, ther-al properties and sound absorbing properties. The manufacture,

roperties and applications of metal foams have been reviewedy Banhart (2001), Davies and Zhen (1983). Ashby et al. (2000)ave shown a design guide for metal foams. These features areery attractive in transport industries (Baumeister et al., 1997),nd prospective applications of metal foams to automobiles (Itond Kobayashi, 2005), aeroplanes and railway cars (Rausch andtöbener, 2005) have been presented.

Many fabricating processes of metal foams have been devel-ped since a forming process of porous aluminium was proposedy Sosnick (1948). Among them, the practical processes for pro-ucing metal foams are classified into two routes: melt route andowder metallurgy route. In melt route, molten metal expands and

s formed to foam by adding a blowing agent or injecting bub-les of gas directly. Aluminium foams were made from the melty adding a foaming agent such as titanium hydride and zirco-

ium hydride by Elliott (1956). Blocks of aluminium foam withregular shape are commercially manufactured in batch opera-

ions (Akiyama et al., 1988), and the mechanical properties of theroduced foams are presented by Miyoshi et al. (2000). Sheets

∗ Corresponding author. Tel.: +81 042 628 2429; fax: +81 042 628 2429.E-mail address: shiomi@cc.kogakuin.ac.jp (M. Shiomi).

924-0136/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2010.03.006

is carried out. The deformation energy of the pipe can be increased withroposed method.

© 2010 Elsevier B.V. All rights reserved.

of aluminium foam are continuously fabricated by drawing outfrom the surface of the molten metal injected bubbles of gas (Jinet al., 1990). By utilising the distinction of hydrogen solubilitybetween liquid and solid metals, porous metals with elongatedpore structure, so-called lotus meal, are fabricated by unidirectionalsolidification (Shapovalov, 1993), of which the mechanical proper-ties show anisotropic behaviour (Hyun et al., 2001). The melt routeis suitable for mass production of metal foams with a simple shape.

In powder metallurgy route, metallic powder is mixed with ablowing agent and it is compacted to form a foamable precursor.Then the precursor is heated and formed to foam in a furnace (Allenand Sabroff, 1963). The advantage of this route is that the precursorcan expand in a heated mould and the foam with a complicatedshape can be made by mould filling (Baumgärtner et al., 2000).The precursor should be carefully produced because residual openporosities or other defects lead to poor results in the followingprocesses (Banhart, 2005). Although the precursor is made frompowder by iso-static compression, extrusion or rolling, Kitazonoet al. (2004) have proposed a method for making a precursorfrom steel sheets stacking a blowing agent by using a roll-bondingtechnique. Hangai and Utsunomiya (2009) have used friction stirprocessing (FSP) to fabricate a precursor with mixing blowing agentpowder and stabilization powder into aluminium plates. Pow-der metallurgy route requires preparation of precursor to produce

foams. It is desirable to produce metal foams from powder in ashort-time process for manufacturing of automotive parts.

In this paper, a method for fabricating aluminium foams frompowder including a foaming agent is proposed for light-weightstructures of automobiles. Aluminium powder mixed with a blow-

1204 M. Shiomi et al. / Journal of Materials Processing Technology 210 (2010) 1203–1208

Fm

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Fig. 2. Die for foaming of aluminium compact extruded from container.

Fig. 3. Mould with heating and cooling system for forming of aluminium foam.

Table 1Experimental conditions for hot powder extrusion and foaming.

Aluminium powder A6061Foaming agent TiH2

Temperature of container (◦C) 420Extrusion ratio 3.3

ing TiH2 of 1 mass% was carried out at 420 C. The punch speed wasvaried in a range of 0.2–0.6 mm/s, and the forming speed �, that wasa speed of the aluminium compact going through the die withoutfoaming, changed from 0.66 to 2 mm/s because of the extrusionratio of 3.3. The experiments of hot extrusion and foaming from

ig. 1. Experimental equipment for hot powder extrusion and moulding of alu-inium foam.

ng agent is heated and extruded into a mould through the heatedie, and the extruded aluminium expands and is formed in aequential process with an apparatus. The cylindrical aluminiumoam is produced by the proposed method, and the distribution ofhe relative density and the deformation energy of the foam are

easured.

. Hot powder extrusion and foaming

Fig. 1 shows the experimental equipment for fabricating alu-inium foams from powder. The equipment consists of two parts:

ot powder extrusion on the left side and moulding of foam on theight side. Aluminium powder (A6061) mixed with TiH2 powders a foaming agent is poured into the container of the hot powderxtrusion and is compacted in a room temperature at a pressuref 100 MPa. The container is heated to 420 ◦C for hot extrusion ofluminium, and then the aluminium powder with TiH2 powder isxtruded into the heated die for foaming of the aluminium compact.n the moulding part of aluminium foam, the die for foaming of theompact extruded from the container is heated to a temperatureigher than the liquidus point (652 ◦C) of the aluminium alloy. Thextruded aluminium compact goes into the heated die and startsoaming in the vicinity of the die exit. Then, the aluminium foam islled into the mould heated with an electric heater.

The container of hot powder extrusion is equipped with electriceaters of band type (375 W) and stick type (400, 500 W). The tem-erature of the container is measured on the surface and controlledo 420 ◦C. The inlet and outlet diameters of the container are 30 and6.5 mm, respectively, so that the extrusion ratio is 3.3. The relativeensity of the aluminium compact reaches more than 0.99.

The die for foaming of the aluminium compact including a foam-ng agent is shown in Fig. 2. The length of the die is 93 mm, andhe inner diameter of the die changes from 17 mm in the inlet to6 mm in the exit at a point of 28 mm from the exit, because theompact is heated to a temperature higher than the liquidus pointor expansion due to heat conduction from the heated die. The IHoil is located around the die and used to heat. The temperaturef the die is measured at a point of 8 mm from the exit, and it isontrolled during the process.

Fig. 3 shows the details of the mould where aluminium foam

s filled in. A stainless steel pipe is used as a mould in the experi-

ent, and the dimensions of the pipe are 25 mm in inner diameter,00 mm in length and 1.5 mm in thickness. The pipe mould is cov-red with a line heater of 1000 W, a copper pipe and a heat insulator

Punch speed (mm/s) 0.20, 0.30, 0.45, 0.60Forming speed � (mm/s) 0.66, 1.0, 1.5, 2.0Punch stroke (mm) 15Die temperature Tdie (◦C) 720, 760

(ceramics fibre). The copper pipe has a hole of 10 mm in diame-ter at the centre, and the pipe mould is heated with the heaterand is cooled by blowing air from the hole of the copper pipe intothe space between the heater around the mould and the copperpipe. The temperature of the mould is measured with thermo-couples at four points: 10, 40, 60 and 90 mm from the inlet of themould.

Table 1 shows the experimental conditions for hot powderextrusion and foaming. The chemical compositions of the alu-minium alloy (A6061) are shown in Table 2. Since TiH2 powder usedas a foaming agent tended to start decomposing at around 480 ◦C(Kennedy, 2002), the hot extrusion of aluminium powder includ-

◦

Table 2Chemical compositions of aluminium powder (A6061) (mass%).

Mg Si Cu Fe Cr Al

1.04 0.61 0.28 0.23 0.21 Bal.

M. Shiomi et al. / Journal of Materials Processing Technology 210 (2010) 1203–1208 1205

minium foam extruded on open mould.

pte

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edsatwotTfehabf

fiasWifwo

Fig. 4. Cross-sectional shape of alu

owder were carried out at 720 and 760 ◦C of the temperature ofhe die. MoS2 and carbon lubricants were used for the hot powderxtrusion.

. Fabrication of aluminium foam from powder

Before moulding, the aluminium foam was produced on a hemi-ylindrical pipe, and the relative density of the aluminium foam waseasured so as to obtain appropriate conditions for fabricating alu-inium foam. In the experiment, the aluminium powder including

iH2 was extruded into the die of a room temperature as an initialondition. After the die was heated to a target temperature withhe induction heater in a couple of minutes, the aluminium com-act was extruded through the heated die and the aluminium foamas made on the open pipe.

Fig. 4 shows the cross-sectional shapes of the aluminium foamsxtruded on the open pipe. When the temperature of the heatedie, Tdie, is 760 ◦C and the forming speed, �, is 1.0 mm/s, there areome large-sized cells in the foam. The size of the cell becomes smalls the forming speed increases to 1.5 mm/s or the temperature ofhe die decreases to 720 ◦C. It may be said that aluminium foamith small-sized cells is obtained under the appropriate conditions

f the die temperature and the forming speed. The front part ofhe foam made at the first stage of extrusion shows high density.he front part of the foam was in the vicinity of the die exit beforeoaming. It is considered that the compact in the die was not heatednough for expansion at the first stage of extrusion. The insufficienteating of the front part of the compact in the die, however, can bevoided in manufacturing process because aluminium foams cane sequentially produced by changing the mould and aluminiumoam already exists in the die at the next moulding.

The relationship between the relative density of the aluminiumoams and the forming speed for deferent temperatures of the dies shown in Fig. 5. In both the cases of the die temperature, theluminium foam shows high density at low speed area. The den-ity of the foam goes down and up as the forming speed increases.

hen the forming speed is low, the extruded aluminium compacts heated enough to high temperature in the heated die. After theoam of high temperature is extruded into the mould, the gas (H2)ithin the aluminium foam goes out during cooling in air. On the

ther hand, when the forming speed is high, the aluminium com-

Fig. 5. Relationship between relative density of aluminium foam and forming speedfor different die temperature.

pact is not heated enough for expansion in the die and the densitybecomes high. The aluminium foam, of which the relative density is0.22, can be obtained from powder by the proposed method whenthe forming speed is 1.5 mm/s and the temperature of the die forfoaming of aluminium compact is 760 ◦C. Those conditions are usedfor the experiments of moulding the aluminium foam.

4. Moulding of aluminium foam

To confirm the moulding of aluminium foam, the aluminiumfoam is filled into a pipe mould for simple shape forming. Table 3shows the experimental conditions of extrusion and moulding. Theconditions for extruding the aluminium foams from the die werethe same as those where the lightest foams were obtained on theopen pipe. The punch stroke of 18 mm was determined from thevolume calculation, in which the solid aluminium extruded fromthe die would expand and fill into the mould, and the relative den-

sity of aluminium foam would be 0.25. The pipe mould was heatedfor 1200 s to be a constant temperature before moulding. In theexperimental processes, the aluminium powder with a foamingagent was poured into the container and extruded into the die. Thedie was heated to a temperature higher than the liquidus point of

1206 M. Shiomi et al. / Journal of Materials Processing Technology 210 (2010) 1203–1208

Table 3Experimental conditions of extrusion and moulding ofaluminium foam.

Aluminium powder A6061Foaming agent TiH2

Container temperature (◦C) 420Punch speed (mm/s) 0.45Forming speed (mm/s) 1.5Extrusion ratio 3.3

thwti

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tteafonfThdpaa

F(

Punch stroke (mm) 18Heating time of mould (s) 1200

he aluminium alloy, and the overflow of aluminium foam from theeated die due to expansion was removed. Then, the pipe mouldas attached to the cooled die and heated again for moulding. After

he preparation, the experiment for moulding the aluminium foamnto the pipe mould was carried out.

Fig. 6 shows the temperature history of the mould from theeginning of moulding. The temperatures on the mould surface at0, 40 and 60 mm from the mould inlet increase during forming,ecause the aluminium foam touches the mould step by step. Theluminium foam reaches the mould surface at 90 mm from the inletfter moulding because the surface temperature at 90 mm indi-ated by the line (d) increases after moulding. Although the punchn hot powder extrusion was stopped at the end of forming, thextruding of the foam from the die was not stopped immediatelyue to foaming. The mould is cooled after forming and the foam isonverted to its solid state. The solidification time is defined by theeriod from the end of forming to the time when the temperaturef the mould surface reaches the solidus line of the aluminium alloy572 ◦C).

The cross-sectional shapes of the aluminium foams moulded inhe pipe for different solidification time are shown in Fig. 7. Whenhe solidification time is 40 s that is the minimum time for thexperimental equipment, the aluminium foam is filled in the pipend overflowed from the pipe end. The relative density of the wholeoam is 0.24, and the range of the cell size is not large. In the casef solidification time of 125 s when the aluminium foam is cooledaturally without blowing air, the cell size at the centre part of the

oam becomes larger than that at the other parts due to coalescence.he centre part of the mould is not cooled quickly because of theeat insulator around the mould. The end part of the foam is con-

ensed at the mould inlet. Although the punch stops pushing theowder in the container of hot powder extrusion, foaming of theluminium compact in the die does not stop at the same time. Theluminium compact in the die still expands after forming and the

ig. 6. History of mould temperature from moulding of aluminium foam into pipeTdie = 760 ◦C and � = 1.5 mm/s, solidification time: 70 s).

Fig. 7. Cross-sectional shape of aluminium foam moulded into pipe for differentsolidification time.

foam around the mould inlet is condensed. The density of the foamsis in a wide range, though the relative density of the whole foam is0.26. It is important to control the temperature of the mould andthe die for the density distribution of the moulded foam. To obtainmore uniform cell size of the foam, the aluminium foam should becooled rapidly after forming.

To measure the density distribution within the aluminiumfoams, the foams are cut at every 20 mm from the end in the lon-gitudinal direction and the densities are obtained by Archimedesmethod. Fig. 8 shows the density distribution of the foam produceddirectly from powder for different solidification time. The mea-sured densities are plotted at the centre position of the cut portion.When the solidification time is 125 s, the density at the centre ofthe foam is very low. On the other hand the densities at the endparts are very high because of the gas leak and the condensationof the overflowed foam. In the case of the shortest solidificationtime: 40 s, the density of the foam is in a range of 0.2–0.3, andthe average relative density is 0.24. The density distribution withinthe foam fabricated by rapid cooling becomes more uniform thanthat by natural cooling. It is considered that the high cooling speed

after forming is important to mould the aluminium foam with asmall range of density distribution within the foam. It is necessaryto control the temperature of the mould to successful moulding ofaluminium foams.

Fig. 8. Density distribution within aluminium foam made by pipe moulding.

M. Shiomi et al. / Journal of Materials Process

5

piFimwooawiowTwc

siattalpafiws

Fig. 9. Test piece for compression test.

. Compression test

Aluminium foam inserted pipes are fabricated by the pro-osed method, and the energy absorbing capacity of the pipe

ncluding aluminium foam is examined by a compression test.ig. 9 shows the test pieces for the compression test. The pipencluding aluminium foam was fabricated by moulding of the alu-

inium foam into the pipe under the moulding conditions inhich the solidification time was 40 s and the relative density

f the foam was 0.24. To compare with the absorbing capacityf the pipe including aluminium foam, the pipe without foamnd the aluminium foam billet made by the proposed methodere also compressed. A stainless steel (SUS304) pipe of 30 mm

n outer diameter and 1.5 mm in thickness was used. The heightf the test pieces was 40 mm. The test pieces were compressedith flat tools, and a mechanical press was used for the test.

he initial strain-rate was 4.75 1/s, and the reduction in heightas 62.5%. The compression test was carried out with no lubri-

ant.The relationship between the stress and the strain in compres-

ion test is shown in Fig. 10. The pipe including the aluminium foamndicated by the line (a) shows high compression stress at first,nd then the stress goes down steeply. After the strain exceeds 0.5,he stress increases during compressing. The pipe without foam,he line (b), also shows the same tendency of compression stresss that of the pipe with the aluminium foam, though the stress isower than that of the pipe with the aluminium foam. The com-ression stress of the aluminium foam, the line (c), is the smallest

mong them and it increases gradually during compressing. To con-rm the difference of the compression stresses between the pipesith and without the aluminium foam, the sum of the compressive

tress values of the pipe, the line (b), and the aluminium foam, the

Fig. 10. Relationship between stress and strain in compression test.

ing Technology 210 (2010) 1203–1208 1207

line (c), at every strain is calculated and indicated by the doted line(d). The compression stress of the pipe with the aluminium foamis larger than that of the pipe and the aluminium foam billet as it isknown (Seitzberger et al., 1997).

To evaluate the energy absorbing capacity of the pipe includingthe aluminium foam, the ratio of deformation energy to weight incompression test is calculated from the weight and the stress multi-plied by the corresponding strain till a strain of 0.5. The deformationenergy of the pipe including the aluminium foam is 17.6 J/kg, whilethe deformation energy of the pipe only is 15.4 J/kg. It may be saidthat the deformation energy of pipe can be increased with alu-minium foam filled by the proposed method, in which a bondingprocess between the aluminium foam and the pipe is not carriedout in particular.

6. Conclusions

In order to produce aluminium foams directly from powder forautomotive parts, the combined process of hot powder extrusionand foaming was proposed. The A6061 aluminium powder mixedwith TiH2 powder as a foaming agent was extruded into the dieheated to a temperature higher than the melting point and thealuminium foam was made. The relative density of the aluminiumfoam was 0.22. The moulding of the aluminium foam into the pipewas carried out and the cylindrical aluminium foam was obtained.The distribution of relative density within the aluminium foammoulded into the pipe was in a range of 0.2–0.3. The energy absorb-ing capacity of the pipe increased with the aluminium foam filledby the proposed method.

Acknowledgments

A part of this research work was supported by the SUZUKI Foun-dation for the development of forming process of aluminium foams.M. Shiomi would like to express his thanks to Dr. S. Ishizuka andDr. H. Kaneda, Suzuki Motor Corp., for their advices to the researchwork.

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