Corelationship Between Powder Metallurgy and Casting Method Using SiC Powder for Metal Foam

Sang-Youl Kim1,a, N. Babcsan2,b, Duck-Kyu Ahn1,c , Young-Hwan Song 1,d and Bo-Young Hur1,e

1K-MEM R&D Cluster, AMRC-ERI, Gyeongsang National University, 900 Kajoa-dong Jinju, Korea 2Hahn-Meitner-Institute, SF3, Glienicker Strae 100, 14109 Berlin, Germany

aulsfom@naver.com, bbabcsan@hmi.de, canduckkyu@hanmail.net, dyhsong4u@nate.com, ehurby@gsnu.ac.kr

Keywords: metal foam, foaming, thickening, porosity, al foam

Abstract. Metal foams are not easy to use as materials as their manufacturing process involves all three, solid, liquid and gaseous phases occurring simultaneously at varying temperatures and moreever the morphology of the solidified foam is quite complex. There has not been any report on the stabilization of metal foams. To compare the stabilisation of metal foams using a different method, powder metallurgy and casting method using SiC and Ca alloy particle are required. Our investigation results showed deeply etched valleys with high oxygen content and secondary phases attached to line-shaped pores with higher SiC content than the matrix. These line-shaped pores - most probably oxide bifilms-form networks decorated with either secondary phases or oxide particle. We suspect that these decorated bifilms play a crucial role in the stabilisation of metal foams.

Introduction Metallic foams possess good properties such as high specific stiffness, high energy absorption

capacity, high damping, etc. They have been recently highlighted as it has become possible to produce high functional metal foam at lower cost due to technological development, and the metal foam with satisfactory mechanical, thermal and energy absorption properties can be designed as the understanding of its structure and properties has depended[1]. There are two methods for produsing metal foams: casting method and powder metallurgy. Making a homogeneous structure is very difficult. For the control of the bubble in molten metal, such as birth(foaming agent, TiH2), life, death, shape and size in molten metal, the physical properties of liquid metal which have great influence on fabricating properties of metal foam must be given adequate attention. The decomposing TiH2 releases hydrogen gas into the molten compact and a cellular structure is formed[2-4].

Liquid foams are thermodynamically unstable and coarsening can lead to the reduction of their large surgace energy, hence causing the cell wasll to rupture and the foam to cllapse[5]. Many researchers have been done on the stabilization of cell wall in the melt foaming using ceramic particle. Asavavisithchai et. Al[6,7] procides some useful evidences relating to the stavilization of melt foaming using oxide particle (Al2O3). In stead of experimental observation, Landry[8] and Weirauch[9] are insist that Al2O3 particles are not readily wetted by liquid Al. To improve their weeability, the highly reactive elements, such as Mg, can be added [10,11]. Wetting is greatly influenced by the reaction between Mg and alumina at the metal-oxide interface, forming MgAl2O4 [10,12-14], and this compound can cause modifications to the oxide film or cause to rupture on the surface of the molten Al [10] exposing clean, wettable surfaces [9]. Foam stabilization mechanism is a little different between castin and pwder metallurgy method, Powder metallurgy is used to SiC particle, whereas melt foaming method is a thickening agent as Ca particle.

In this paper, the fundamental stabilization mechanism of metal foams to different fabrication method is investigated. The proposed method can be used to produce metal foams by mixing metal powders with an appropriate thickenging agent.

Materials Science Forum Vols. 534-536 (2007) pp 945-948Online available since 2007/Jan/15 at www.scientific.net© (2007) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.534-536.945

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Experiment Metallic foams can be fabricated by powder metallurgy and melt foaming process. Metallick

foams are necessary ceramic particle or thickening agent to stabilize homogeneous pore structure. In the case of power netallurgy, SiC powder is used as starbilization materials for makin metal foam.

In the melt foaming process, Al melt is necessary for thickening to assure stability of liquid foam using Ca alloy particle. In this study, Al ingot was charged to a graphite crucible and heated to melt using an electrical resistance furnace. To optain sufficient melt wiscosity the composite melt was held at a relatively low temperature of about 660℃. The thickening agent was with of 2wt%. The foaming agent was added into the melt, and then stirred quickly mechanically at an increased speed of up to 550 rpm to disperse uniformly the foaming agent throughout the melt. The stirring process was continued for about 2min

The crucible containing the mixture remained in the furnace to maintain enough temperature, allowing TiH2 to decompose and release gas and evolving bubbles. The foamed melt was removed from the furnace and cooled in air. We compared the effect of the thickening agent on powder metallurgy and melt foaming method and the effect of solid inclusions on foam stability is considered.

Results and discussion The stabilization mechanism for closed cell metal foams made by powder metallurgy and melt

foaming method has not yet been discussed in the literature. To be able to compare the stabilization of melt foaming and powder metallurgy method, closed cell metal foams were produced. Fig. 1 shows the microstructure of Al foam with different method, melt foaming and powder metrallurgy.

Fig. 1 microstructure of Al foam with different method (a) melt foaming method (b) powder metallurgy[15].

Fig. 2. SEM image of Al foam with SiC particle by powder metallurgy[16].

Fig 2 shows the microstructure of Al foam cell wall by powder metallurgy using SiC particles. It can be seen that the cell wall is stabilized and drainage is disturbed by Al melt being suppressed by

5 mm

a) b)

946 Progress in Powder Metallurgy

SiC particles. SiC-particule addition reduced the drainage and cell coarsening rates in Al compacts[15].

Microstructure of the cell wall cross section is shown in Fig. 3. The sample was prepared using 1 wt% Ca particle, hydrogenated Ti and was stirred for 20 min in air prior to foaming. Our SEM and EDX examination showed deeply etched valleys with high oxygen content and secondary phases attached to line-shaped pores with higher Ca and/or Ti content than the matrix. These line-shaped pores - most probably oxide bifilms - form networks decorated with secondary phases. We suspect that these decorated bifilms play a crucial role in the stabilization of closed cell type metal foams.

Fig. 3. Surface of the cell wall cross section, (a) OM image and (b) etched surface of the same

material, SEM image.

Table 1 shows chemical composition of foam cell wall by melt foaming method. The oxygen shows a higher concentration on the cell wall surface compared to the bulk concentration. Oxides are almost distributed on the surface of the cell wall, which means that oxide particles does play a role in the stabilization of metal foam by melt foaming. Drainage of molten Al is suppressed by oxide particles in foaming process. Melting and stirring a molten metal causes the oxide films to agglomerate. Viscosity increases with more CaO, CaAl2O4 and Al2O3 crystallization in molten Al alloy. Oxide particle is shown in Fig. 3. Generrally, when the second phase (solid) is added, the wiscosity of the melt increases. However, if the second phase exists with the spherical shape, the effect on wiscosity becomes induced by Einstein’s equation [17]

Table 1. Chemical composition of Al foam.

Conc. Wt% Al O Ca Ti Bulk 92.37 2.61 1.87 3.15

Surface 88.88 9.06 1.71 0.35

(a) SEM (b) TEM

Fig. 4. Microstructure of surface of the Al foam cell wall.

Fig. 4 shows microstructure of surface of Al foam cell wall. Original surface of the cell wall with Ti particles, open oxide bifilm lamellae and nano-sized particles, SEM image, particles on the

Ti

oxide bifilm

T

oxide

nanoparicle

A

particles on the surface

1.08 μm

Materials Science Forum Vols. 534-536 947

surface of a cell wall, cross section, TEM image. Ceramic particle located to surface of cell wall as surfactant that is stabilized to cell wall surface. It was increased wettability of the surface of cell wall and viscosity of Al melt. Al foam by melt foaming method is homogeneous cell structure because oxide particle plays a important roe to drainage. Generally, Ca is used to melt foaming method as thickening agent that is easily form oxide particle in thickening process. Oxide particles are increasing viscosity of Al melt which has an effect on cell structrure such as cell size, cell shape and distribution.

Conclusion The foaming behavior of the Al foams, powder metallurgy and melt foaming method was

investigated. Based on our findings, we could conclude that solid inclusions influence foam stability through their wetting behavior, their shape and their distribution in the melt (network formation, clustering or segregation). Besides the particle concentration and size, recent investigations showed that the compositions of both the metallic melts and the stabilizing particles influence stability. It is believed that the temperature and the composition of the melt act through the formation of additional surface layers on the particles.

Metal foams produced by the melt foaming method contain decorated oxide bifilms which could greatly influence the statilistion of closed cell-type metal foams. These bifilms form a network inside the foam but there was no significant surface segregation Nano- and submicron-sized particles are present in the foam but their distribution and composition remains to be further investigated.

Acknowlodgement This research was supported by a grant from the Eco-technopia 21 project by the Ministy of Environment.

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