粉坯的起泡能力和压缩性

Foamability and compressive properties of AlSi7–3vol.%

SiC –0.5wt.%TiH 2powder compact

S.Esmaeelzadeh a ,A.Simchi a,b,?

a Department of Material Science and Engineering,Sharif University of Technology,P .O.Box:11365-9466,Azadi Avenue,Tehran,Iran b

Institute for Nanoscience and Nanotechnology,Sharif University of Technology,P .O.Box:11365-9466,Azadi Avenue,Tehran,Iran

Received 16January 2007;accepted 14September 2007

Available online 11October 2007

Abstract

This paper presents experimental results on the foaming behavior and compressive properties of AlSi7-3vol.%SiC metal foam.Elemental powders of Al,Si,SiC and TiH 2were mixed,hot compacted and foamed at 750,780and 810°C.It was shown that the addition of fine SiC particles (mean diameter of 3μm)increases the melt expansion and improves the stability of AlSi7foam.A less drainage in the cell structure was also noticed.Meanwhile,large irregular cells in the macrostructure and a slight degradation of the mechanical properties under compressive load were observed.?2007Elsevier B.V .All rights reserved.

Keywords:Metal foam;AlSi7;Cell structure;Foaming behavior;Mechanical properties;Composite material

1.Introduction

Metal foams are uniform dispersion of a gaseous phase in the solid metal with low densities and novel physical and mechanical properties [1].Direct foaming of metallic melts [2]or indirect foaming of powder compacts [3]are two common and cost effective methods for fabrication of metal foams.In the both routes,the stability of the foamed melt during process is of crucial importance [4].It is known that the melt stability can be improved by adjusting the liquid metal viscosity and its surface tension by adding fine ceramic particles or introducing alloying elements into the melt [5].When the powder compact melting process is employed,the metal oxides covered the initial powder particles or the solid component of the particular alloy can also induce foam stabilization [6].By adding fine ceramic particles,further improvement in the foam stability may be obtained.For instance,Elbir et al.[7]have showed that addition of SiC particles to Al

foams increases linear expansion and compressive strength whereas drainage and cell coarsening rates are reduced.Other researchers [6,8,9]have also reported that the amount of expansion and drainage as well as the cell structure are influenced by the addition of ceramic particles.In this context,it has been found [10]that the metal matrix composition and the ceramic type have vital roles on the foaming response.For instance,while the drainage rate of aluminum foam is significantly reduced by SiC particles,it is not affected by TiB 2[9].The parameters of foaming process such as the foaming temperature and heating rate are also effective.Since the density and cell size of the foams are affected by the ceramic particles,the mechanical properties should also be influenced.Apparently,more research is still required to explore the effect of ceramic particles addition on the foaming behavior in the powder compact melting process.This paper presents experiments on foamability,foam stability and cell structure of AlSi7metal foam mixed with 3vol.%SiC particles.The effect of ceramic particle addition on the compressive mechanical prop-erties is also addressed.2.Experimental

AlSi7and AlSi7-3vol.%SiC metal foams were produced by the powder compact melting process.The detail of the production

Available online at https://www.360docs.net/doc/a115762802.html,

Materials Letters 62(2008)1561–

1564

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Corresponding author.Institute for Nanoscience and Nanotechnology,Sharif University of Technology,P.O.Box:11365-9466,Azadi Avenue,Tehran,Iran.Tel.:+982166165261;fax:+982166165262.

E-mail addresses:s_esmaeelzadeh@https://www.360docs.net/doc/a115762802.html, (S.Esmaeelzadeh),simchi@https://www.360docs.net/doc/a115762802.html, (A.Simchi).

0167-577X/$-see front matter ?2007Elsevier B.V .All rights reserved.doi:10.1016/j.matlet.2007.09.044

process was described elsewhere [11].Air atomized aluminum powder (ECKA Granules,Germany)with particle size b 160μm,elemental Si powder (Rave,Germany)with particle size b 150μm,and fine SiC powder (Saint-Gobain,Germany)with mean diameter of 3μm were used as the starting materials.TiH 2powder was supplied from Chemetall,Germany and used as the foaming agent.7wt.%Si,0.5wt.%TiH 2and 3vol.%SiC powders were blended with the base aluminum powder in a Tumbler mixer for 45min.The powder mixture was hot compacted at 450°C at pressure of 112MPa to prepare cylindrical specimens with diameter of 32mm and height of 30mm.Electro discharge machining (EDM)was used to prepare the required specimens for studying the foaming behavior and compressive mechanical properties.A mechanical expandometer [12]was employed for in-situ examination of the foaming process during heating up the specimens.The foaming process was also performed in a small laboratory furnace at a temperature in the range of 750to 810°C to prepare samples for structural analysis and evaluation of mechanical properties.Here,a closed mold with diameter of 41mm was used to produce foams with identical porosity of 87%.Five samples were performed for each foamability test to assess repeatability of the data.For analysis of the cell structure,the specimens were cut by EDM along the foaming direction and the sections were colored by black color and grind by emery papers.For the compression test,cylindrical specimens with diameter of 30mm and height of 40mm were prepared from the center of the foams.An initial strain rate of 1.1×10?3s ?1was applied and the true strain –stress curves were recorded by a Instron tensile/compression test machine.Each test was repeated three times and the average of the results are reported.

3.Results and discussion 3.1.Melt expansion and drainage

Fig.1shows the expansion of AlSi7and AlSi7-SiC precursors versus time at different foaming temperatures obtained by the expandometer

instrument.Each graph is reproduced by averaging five foaming tests at any given condition.The maximum error of measurement is b 8%.It is seen that the addition of 3vol.%SiC particles significantly improved the maximum expansion of the AlSi7foam,and the effect became less pronounced as the foaming temperature increases.This observation is linked to the melt viscosity.It is known that the presence of solid particles increases the bulk viscosity of liquids [13].Therefore,the higher viscosity of the melt containing ceramic particles improves capturing of the released gas from the foaming agent.Since the viscosity of aluminum melt decreases with increasing temperature,the positive influence of SiC particles should be lower at higher foaming temperatures.

Fig.1also shows that in the all examined foaming experiments,a drop in the expansion curve occurred after passing the maximum expansion time.This drop is an indicator of melt drainage and cell structure collapse during processing.Evidently,metal foams are more stable when the amount of the reduced expansion decreases.Therefore,it becomes apparent that the addition of SiC particles not only enhances the amount of maximum expansion,but also improves the foam stability.To characterize the stability of different foams quantitatively,the amount of reduced expansion was determined.This could be done by calculating the difference between the integral of expansion –time curve of the examined foams and fully stable foams (without any collapse)[14].If the stability parameter (S )is defined as the difference between these two integrals,the higher S-value means worse stability because it indicates a higher melt drainage occurring upon processing.Based on the data presented in Fig.1,the stability parameter (S )of different examined foams was determined.The results are reported in Table 1.First at all,it appears that the stability parameter of AlSi7-SiC composite foams is lower than that of AlSi7(ΔS N 0);the stability of AlSi7foams is improved by introducing SiC particles.On the other hand,it is seen that with increasing temperature,the amount of ΔS increases,i.e.the difference between the stability parameter of the AlSi7foam with and without SiC becomes higher.This indicates that the effect of solid particle addition at higher temperatures is more pronounced.The other point that should be noticed is the dependency of S -value to temperature.While the stability of AlSi7-SiC foam is improved as the temperature increases (lower S value at higher temperature),the stability of AlSi7foam is decreased (higher S value at higher temperature).3.2.Cell structure

The effect of SiC addition on the cell structure of AlSi7foam is shown in Fig.2.The structure of AlSi7illustrates relatively uniform cells.A dense layer at the bottom of the specimen,which shows the melt drainage,is also visible.When 3vol.%SiC particles were introduced,larger irregular cells were formed and the amount of drainage decreased.It was shown elsewhere [14]that in AlSi7-SiC foams,SiC particles are gathered within the cell walls during foaming.This observation indicates that the aluminum melt wetted the ceramic particles;otherwise,the particles should be located at the gas-metal interface.It is known that the wettability of SiC particles by pure aluminum melt is low,i.e.the contact angle is 150°at 900°C [15].A better wettability of AlSi7is related to the dissolved Si,which reduces the surface tension of aluminum [15]

and

Fig.1.AlSi7and AlSi7-SiC expansion at different foaming temperatures.

Table 1

Effect of SiC addition on the stability parameter (S )of AlSi7metal foam Temperature (°C)AlSi7AlSi7-SiC ΔS Improvement (%)750135±11123±7129780199±25104±189548810

225±17

72±20

153

68

1562S.Esmaeelzadeh,A.Simchi /Materials Letters 62(2008)1561–1564

prevent formation of Al 4C 3at the interface [16].If Al melt wets the ceramic particles,the presence of the solid particles increases the viscosity of the system,thereby contributing in foaming by decreasing the melt drainage.On the other hand,the particles increase the cell wall thinning rate [17],and as a consequence,larger cells are formed.

3.3.Mechanical properties

Fig.3shows the compressive stress –strain curves of AlSi7and AlSi7-SiC foams produced,respectively,at 750and 810°C.These tests were repeated three times to check the reproducibility and reliability of the results.As it is seen,the addition of SiC particles increased the yield strength of AlSi7for about 18%.The slope of the semi-plateau region was also increased whereas the densification strain was decreased.Although higher strength was achieved,the maximum energy absorption efficiency [18]was slightly reduced (～5%).The stress –strain curve of the AlSi7-SiC foam exhibits more fluctuations in the semi-plateau region,indicating non-uniformity in the plastic deformation during loading [19].

This can be due to non-uniform cell structure of the composite foam compared with that of AlSi7(Fig.2).

4.Conclusion

Effects of SiC addition on the cell structure,foaming behavior and mechanical properties of AlSi7metal foams were studied.It was found that SiC particles are gathered within the cell walls.Under this condition,the particles increase the bulk viscosity of the metal melt and improve the maximum expansion and the foam stability.It was also shown that SiC particles influence the cell structure of AlSi7foam.While the drainage is reduced,the cells became larger and non-uniform.The composite foam exhibits more brittle and non-uniform deformation with less energy absorption efficiency as compared with AlSi7foam.Acknowledgment

The authors sincerely acknowledge with thank Dr. D.Lehmhus (Fraunhofer Institute IFAM,Bremen,Germany)for his assistance in the experimental work and advice during valu-able discussion.References

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