COM

MUNIC

ATIO

N

DOI: 10.1002/adem.200900195

Study on Crack-Like Pores of Al Foams Made via thePowder-Metallurgy Route**

By Lei Wang*, Guangchun Yao, Yihan Liu and Guoyin Zu

Mechanisms for the formation and disappearance of the crack-like pores generated during the earlystage of Al foaming are investigated. A model for their disappearance process is proposed for the firsttime. The stress, perpendicular to the compaction direction in uniaxial cold compaction, is caused bythe interaction of the Al powder under a high compaction pressure and is the main reason for theformation of the crack-like pores. The results of the model analysis and theoretical calculations suggestthat the pressure differenceDP between the initial, round bubbles and the crack-like pores is the drivingforce for their disappearance. The rapid reduction of DP is attributed to the decomposition charac-teristics of the TiH2 powder.

The manufacturing method of Al foams made via the

powder metallurgy (PM) route is fascinating because of the

feature of making near-net-shaped parts.[1–4] The process

involves mixing Al or Al-alloy powder and Mg powder with a

powdered foaming agent, typically titanium hydride (TiH2),

and then pressing the mixture into a dense billet called a

foamable precursor. Subsequently, heating the precursor

leads to its expansion and the formation of bubbles due to

gas release from the TiH2 powder. However, the precursor’s

density being close to that of bulk Al is a prerequisite

for obtaining a good expansion and uniform cell structures

of the foamed sample. A high compaction pressure is

necessary for the high density of the precursor from the

viewpoint of pressing. It has been reported that cold

compaction for pure Al powder is enough to produce a

dense precursor.[5]

As is well known, the general characteristics of bubble

formation in the metal-foaming process are the initial bubble

nucleation, growth, rupture, coalescence and finally col-

lapse.[6] A common phenomenon, however, is also observed

in samples during the early stage of foaming; namely, the

appearance of crack-like pores.[7–10] These subsequently

[*] Dr. L. Wang, Prof. G. C. Yao, Prof. Y. H. Liu, Prof. G. Y. ZuEngineering and Researching Center of Material AdvancedPreparing Technology (Ministry ofEducation), Northeastern University 117 BoxShengyang, Liaoning, 110004, PR ChinaE-mail: chongqing_20101@163.com

[**] This work was financially supported by the National NaturalScience Foundation of China (Grant No. 50774021 and50704012).

50 � 2010 WILEY-VCH Verlag GmbH & Co.

disappear with foaming time and uniform cell structures

are obtained. It has also been reported that the presence of

crack-like pores is independent of the type of Al alloy and the

phenomenon is inherent in PM foams. The reasons for their

formation and disappearance have not been explained and

reported, although Banhart et al. described the phenomenon

in detail.[10] Further studies on the crack-like pores of Al foams

made via the PM route will be very helpful in understanding

the bubble-evolution process completely.

In this study, Al foams were prepared by the PM route, on

the basis of uniaxial cold compaction. The formation of

crack-like pores was analyzed in detail and a model for the

process of their disappearance is proposed. Theoretical

calculations were carried out based on the simplified Rayleigh

equation for a gas-liquid interface.

Experimental

The particle sizes used in this experiment were measured

using a Malvern Mastersizer 2000 laser particle-size analyzer.

Foamable precursors were prepared by mixing air-atomized

Al powder, which has a D50 value of 117.078mm (pur-

ity¼ 99.0 wt %), with 0.6 wt % TiH2 powder, with a D50 of

32.544mm, (purity¼ 99.8 wt %) in a tumbling mixer for 30 min.

To improve the foam stability, 1.0 wt % Mg powder, with a D50

of 129.325mm, was added prior to compaction.[11] The powder

mixture was then cold compacted using uniaxial compaction

in a 50 mm diameter, lubricated tool-steel die at a pressure of

600 MPa to produce dense foamable precursors.

The foamable precursors were foamed in a preheated

furnace, up to a temperature of 800 8C. The foaming times (tf)

were 40 s, 60 s, 120 s and 160 s. A stainless-steel mould with a

50 mm diameter was placed into the preheated furnace for the

foaming experiments. Each foaming experiment, with the

KGaA, Weinheim ADVANCED ENGINEERING MATERIALS 2010, 12, No. 1--2

COM

MUNIC

ATIO

N

L. Wang et al./Study on Crack-like Pores of Al Foams Made . . .

same foaming time, was repeated four times to ensure the

reliability of the results in our observations. The foamed

samples were cooled down with the help of a water quench,

and then sectioned using electro-discharge machining. A

scanner at a resolution of 300 dpi was used for examination of

the bubble macrostructure.

Results and Discussion

Formation of Crack-like Pores

Figure 1 shows the cell structures of the Al foams at the

different foaming times, and also the bubble-evolution

process. As seen in Figure 1a, a crack-like pore is clearly

visible during the early stage of foaming, and other small

bubbles, which we call initial round bubbles, are dispersed

around the crack-like pore. The crack-like pore’s elongation is

perpendicular to the compaction direction, which is from top

to bottom of the longitudinal axis of the foamed samples. Most

of these small bubbles are rounded off due to a little

decomposition of the TiH2 powder. The original crack-like

pore is lost in Figure 1b, but several, which are approximately

perpendicular to compaction direction, are still visible.

However, they have disappeared in Figure 1c–d, at longer

foaming times. Polyhedral bubbles are achieved in Figure 1c,

with the expansion of the initial round bubbles and the

disappearance of the crack-like pore. The morphology of the

bubble coalescence and a dense metal layer are observed

significantly in Figure 1d. It can be concluded that the

formation and disappearance processes of the crack-like pores

of Al foams made via the PM route are not accidental events

during the whole foaming process, according to the observa-

tions of the foamed samples in these experiments and those in

the literature.[7–10]

The formation of the crack-like pores is determined by the

compaction process. It is known that the compaction pressure

acting on a powder mixture may be divided into two parts

when a uniaxial compaction is performed: one part of the

compaction pressure is used to densify the powders by

the moving, deforming and through the internal friction of the

powders; the other is unfavorable for densification of powder

Fig. 1. Cell structures of Al foams with different foaming times: a) tf¼ 40 s; b) tf¼ 60 s;c) tf¼ 120 s; d) tf¼ 160 s.

ADVANCED ENGINEERING MATERIALS 2010, 12, No. 1--2 � 2010 WILEY-VCH Ve

mixtures, and is depleted by friction losses between the

powder particles and the inner surface of the die. The main

bonding formed in powders of foamable precursors is

mechanical bonding, although metallurgical bonds are

possible through rupture of the oxide film on the surface of

the Al powder when only using uniaxial cold compaction.

Stress caused by the movement and deformation of the

particles, which is perpendicular to the compaction direction,

is inevitable in the presence of a high compaction pressure.

The stress value is not equal to the initial compaction pressure,

due to friction losses between the powder particles and

between the powder particles and the die. As mentioned

above, the stress opposite to the compaction direction is

mainly considered, and stresses for other directions are

neglected because they are much smaller than the compaction

pressure.

The formation of crack-like pores is made possible through

the mechanical bonds of the powder particles, since this kind

of bonding method is weak, and is relative to metallurgical

bonds. It has already been established that the stress will be

released along the opposite direction to itself. The direction of

the stress formed via the compaction process is opposite to

that of the compaction pressure in this study. Heating of the

foamable precursor results in the release of the stress at the

early stage of foaming, whilst the foamable precursor is still in

the solid or semi-solid state. It is certain that the release of the

stress results in the formation of cracks perpendicular to the

direction of the compaction pressure, and is strongly

demonstrated by the crack-like pore shown in Figure 1a.

Disappearance of the Crack-like Pores

Figure 2 shows a model for the disappearance of the

crack-like pores. As seen in Figure 2a, a number of initial

round bubbles are distributed around the crack-like pore.

Hydrogen gas would be released continually from the TiHx

nuclei (the black points in Figure 2a–b represent TiHx nuclei:

TiHx is the reduced stoichiometry of TiH2: a little hydrogen

gas is released after the crack-like pore is formed) during the

early stage of foaming. Incompletely decomposed TiH2

powder (TiHx) gives the initial round bubbles motive power.

A large number of initial round bubbles having a motive

power (see pi in Fig. 2a) can generate a powerful pressure that

tries to squeeze or divide the crack-like pore into one or

several bubbles (see Fig. 2b) (Pi in Fig. 2b is defined as a variant

of pi in Fig. 2a) because there is little residual hydrogen gas to

be released in the crack-like pore. When the initial round

bubbles grow up gradually, they lose the motive power due to

the hydrogen-gas depletion of the foaming agent. Finally,

polygonal bubbles are formed (see Fig. 2c).

A simplified form of the Rayleigh equation is introduced to

supporting the model. The Rayleigh equation for a gas-liquid

interface during the formation process of a single bubble in a

metallic melt is given as follows in Equation (1):[12]

Pbubble ¼ P0 þ rghþ 2s

R(1)

rlag GmbH & Co. KGaA, Weinheim http://www.aem-journal.com 51

COM

MUNIC

ATIO

N

L. Wang et al./Study on Crack-like Pores of Al Foams Made . . .

Fig. 2. The model for the disappearance of the crack-like pores: a) the early stage of foaming; b) the growth stage offoaming; c) the matured stage of foaming.

In Equation (1), Pbubble is the pressure of a single bubble, P0 is

standard atmospheric pressure, r is the density of the metallic

melt, g is the acceleration due to gravity, h is the bubble depth

located in themelt,R is the radius of curvature of a bubble in the

metallic melt and s is the surface energy of themetallic melt (for

an Al melt, s¼ 0.2 J�m�2).[12]

Combining the model in Figure 2 with Equation (1) leads to

Equation (2):

Pcrack�like�pore ¼ P0 þ rghþ 2s

R(2)

In Equation (2), Pcrack-like-pore is the pressure of the crack-like

pore, and R is the radius of curvature of the crack-like pore.

Meanwhile, for all of the initial round bubbles around the

crack-like pore, the total pressure is expressed as Equation (3):

Pinitial�round�bubbles ¼ P0 þ rghþXni¼1

pi

¼ P0 þ rghþ 2sXni¼1

1

ri(3)

In Equation (3), Pinitial-round-bubbles is the total pressure of all of

the initial round bubbles around the crack-like pore, pi is the

pressure of single initial round bubble, and ri is the radius of

curvature of this corresponding bubble. From the analysis

mentioned above, the pressure difference DP between the

initial round bubbles and the crack-like pore may be expressed

as Equation (4) which is the difference of Equation (3) and

Equation (2):

DP ¼ 2sXni¼1

1

ri� 1

R

!(4)

Assuming that the radii of curvature of all of the initial

round bubbles are identical, the average radius of curvature

can then be defined as r and Equation (4) simplifies to:

DP � 2sn

r� 1

R

� �(5)

It is clear that the radius of curvature of a crack-like pore is

much larger than that of the initial round bubbles, from

Figure 1a and Figure 2a. In combination with Equation (5), it is

also concluded that the pressure difference, DP, has a positive

value during the whole foaming process. Employing a

52 http://www.aem-journal.com � 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

quantitative analysis of Equation (5) is very

natural to understand the process of further

change of the crack-like pore. If r¼ 0.1 mm,

R¼ 30 mm, and n¼ 100, the pressure differ-

ence DP is about equal to 4.0 MPa. Following

the foaming time, r will increase while n and

R decrease accordingly. For the next foaming

stage, where r¼ 0.5 mm, R¼ 10 mm and

n¼ 50, DP is reduced to about 0.2 MPa. It

can be seen that the pressure difference, DP,

during the early stages of foaming is quite

large and the difference of DP at the different

foaming times is also quite obvious from the

calculation results. The crack-like pore would be deformed

and split into one or several bubbles in the presence of the high

pressure of the initial round bubbles, which originates from

the ongoing hydrogen release of the foaming agent. The

results of the theoretical calculations are in good agreement

with the process of the change of the crack-like pore in

Figure 1. However, the calculation results for DP suggest that

DP decreases rapidly with foaming time, resulting from the

decomposition of the TiH2.[13–15] As a consequence, it is

concluded that disappearance of the crack-like pores during

the early stage of foaming is attributed to the pressure

difference DP between the crack-like pore and the many initial

round bubbles that have motive power.

The event of bubble coalescence in Figure 1d is also

explained by the model and Equation (5). The TiH2 foaming

agent almost decomposes completely in late stages of

foaming, called the matured stage of foaming (see Fig. 2c),

and leads to the disappearance of the crack-like pores.

Polyhedral bubbles, especially pentagonal dodecahedra,[16]

are the principal form of liquid Al foams. It can be considered

that all of the equivalent radii of curvature of the bubbles

are identical for a uniform cell structure. According to

Equation (5), the pressure difference, DP, of bubbles

adjacent to each other is quite small, or even close to zero.

At this moment, the change of the bubbles is dominated

by drainage under gravity and capillary action, present in

the whole foaming process, although these bubbles are

stabilized by the oxide network particles.[17] Liquid metal is

drained downwards along the connecting channels that are

established by the cell walls and plateau borders by gravity

action, and meanwhile, the cell walls become thinner due to

capillary forces. Finally, the bubbles rupture and merge with

each other.

Conclusions

Mechanical bonds between powder particles of foamable

precursors are the main bonding form when using only

uniaxial cold compaction. The presence of the stress is

attributed to the high compaction pressure that is used to

obtain dense foamable precursors. Heating foamable pre-

cursors results in the release of the stress along the direction

perpendicular to the compaction pressure. Crack-like pores in

ADVANCED ENGINEERING MATERIALS 2010, 12, No. 1--2

COM

MUNIC

ATIO

N

L. Wang et al./Study on Crack-like Pores of Al Foams Made . . .

Al foams made by the PM route are generated at the early

stages of foaming and are bound to occur due to the

compaction process.

The reason for the disappearance of crack-like pores is

explained through the simplified Rayleigh equation of the

gas-liquid interface. The pressure difference, DP, between the

initial round bubbles and the crack-like pores is the driving

force of the rapid disappearance of the pores. They are

squeezed or divided into one or several bubbles in the

presence of a high DP. The rapid reduction of DP in the later

stages of foaming is attributed to the decomposition

characteristics of the TiH2 powder.

Received: June 30, 2009

Final Version: October 6, 2009

Published online: November 27, 2009

[1] J. Banhart, Prog. Mater. Sci. 2001, 46, 559.

[2] A. R. Kennedy, S. Asavavisithchai, Adv. Eng. Mater.

2004, 6, 400.

[3] S. Asavavisithchai, A. R. Kennedy, Adv. Eng. Mater.

2006, 8, 810.

[4] A. Ibrahim, C. Korner, R. F. Singer, Adv. Eng. Mater.

2008, 10, 845.

ADVANCED ENGINEERING MATERIALS 2010, 12, No. 1--2 � 2010 WILEY-VCH Ve

[5] A. R. Kennedy, S. Asavavisithchai, Scr. Materialia 2004,

50, 115.

[6] L. Bonaccorsi, E. Proverbio, Adv. Eng. Mater. 2006, 8, 864.

[7] S. W. Youn, C. G. Kang, Proc. Institution Mechanical

Engineers Part B 2003, 217, 201.

[8] J. Banhart, H. Stanzick, L. Helfen, T. Baumbach,

K. Nijhof, Adv. Eng. Mater. 2001, 3, 407.

[9] L. Helfen, T. Baumbach, H. Stanzick, J. Banhart,

A. Elmoutaouakkil, P. Cloetens, Adv. Eng. Mater. 2002,

4, 808.

[10] I. Duarte, J. Banhart, Acta Materialia 2000, 48, 2349.

[11] S. Asavavisithchai, A. R. Kennedy, Scr. Materialia 2006,

54, 1331.

[12] H. P. Degischer, B. Kriszt, Handbook of Cellular Metals:

Production, Processing, Applications, Wiley-VCH, Wein-

heim 2002, Ch. 2.5.

[13] B. Matijasevic-Lux, J. Banhart, S. Fiechter, O. Gorke,

N. Wanderka, Acta Materialia 2006, 54, 1887.

[14] A. R. Kennedy, V. H. Lopez, Mater. Sci. Eng. A 2003, 357,

258.

[15] A. R. Gromov, N. N. Kouznetsova, S. L. Yudina, V. V.

Lunin, J. Alloys Compd. 1997, 261, 269.

[16] V. Gergely, T. W. Clyne, Acta Materialia 2004, 52, 3047.

[17] C. Korner, M. Arnold, R. F. Singer, Mater. Sci. Eng. A

2005, 396, 28.

rlag GmbH & Co. KGaA, Weinheim http://www.aem-journal.com 53