Rheological Characteristics of Mg-Al Alloys with Ceramic Particles for Meta Foam

Soo-han Parka，Kwang-ho Song, Yong-su Um and Bo-young Hurb K-MEM R&D Cluster-GSNU, ERI, Div.of Mat. Sci. & Eng., Gyeongsang National University,

900, Kajoa-dong, Jinju, Gyeongnam, 660-701, Korea asuhan2121@hanmmail.net, bhurby@gsnu.ac.kr

Keywords: surface tension, viscosity, magnesium alloys, foamed metal

Abstract. Foamed metal is a kind of porous material with pores in the metal matrix. One of the possible process routes is to blow gas bubbles into liquid metals. However, many metallic foams produced by this foaming method have coarse and irregular cell structures. The industrial aim is to fabricate foams with more uniform structure and cell size. It is important to understand the mechanisms and factors controlling. The rheological characteristics are the most important factors in the metal foam manufacturing. Thus this study investigated the bubble behavior of the molten metal and its two most important two parameters: surface tension and liquid viscosity. The surface tension (by the ring method) and the viscosity (by the rotation method) of Mg-Al alloy (AZ91, AM60) have been measured under pure Ar and SF6 + CO2 atmosphere. The results show that the surface tension and the viscosity of these alloys decrease with increasing temperature. The addition of Ca and SiC to Mg alloys decreases the surface tension and increases the viscosity. This anomalous behavior is related with the preferential adsorption of high activity elements on the surface.

Introduction

Many metallic foams produced by Directly Foaming method have coarse and irregular cell structures. To fabricate foams with more uniform structure and cell size, it is important to understand the mechanisms and controlling factors. 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 a great influence on fabricating properties of metal foam must be given adequate attention [1]. Thus this paper investigated the bubble behavior of the molten metal and its two most important parameters: surface tension and liquid viscosity. These two factors are considered as two liquid mechanisms operating in foam. The first is gravity-driven melt flow from the top to the bottom of foam column. The second is capillarity-driven melt flow from cell face to plateau borders. This leads to cell face thinning and often to cell face rupture [1, 2]. Therefore, we investigated the relations between the driving force for a melt flow and the two parameters to make metal foam with a fine cell structure. There are a few reports on physical property of the melt of metal. As for the surface tension and viscosity, Grosse, Wilson, Allen and Kasama have reported on these subjects [3]. To measure the surface tension, there are various methods available, such as the sessile drop, maximum bubble pressure, pendant drop, drop weight, capillary rise and oscillating drop methods [3,4]. To measure the viscosity, there exist the following methods; capillary, oscillating-vessel, rotational and oscillating-plate methods [3,5]. These methods are applicable at low temperature, but they are not accurate at a higher temperature because of oxidation on the melt.

In this experiment, the drop weight method and the rotational method were used under the high purity Ar and SF6+CO2 gas. They are also directly applicable to the fabrication process. The rheological characteristics behavior evolution of AZ91 and AM60 Mg alloy in the liquid state were investigated. Because of its good castability, good mechanical properties and light weight, the Mg alloy is widely used. The relationship between viscosity, surface tension and temperature, additional elements and time, temperature, were studied.

Materials Science Forum Vols. 510-511 (2006) pp 742-745Online available since 2006/Mar/15 at www.scientific.net© (2006) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.510-511.742

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Experimental Procedure Surface tension was measured by the modified drop weight method, which applies the capillary

phenomenon to measure the maximum force and contact angle when the ring is pulled out from the melt surface [3]. Fig. 1(a) shows a schematic illustration of the experimental apparatus for the surface tension measurement. Viscosity was measured by the rotational method, which measures the viscosity by calculating the torque that is a resistance force of the melt to the rotating rotor [5]. Fig. 1(b) shows a schematic illustration of the experimental apparatus for the viscosity measurement. To prevent the reaction between crucible and sample, graphite and alumina were used. Besides the high purity argon and SF6 + CO2 gas were used to prevent surface oxidation. The flowing rate was set to 25ℓ/min.

The alloys studied were AZ91 and AM60 Mg alloy with adding Ca and SiC. The temperature range was set from melting point to 850oC for Mg alloys. The maximum force (Fmax) measured by the ring method can be recalculated to a surface tension (σst) and viscosity (ηvisco) can be calculated from Ref. [6]. In addition, to compare the foamability of AZ9l and AM60, foaming tests were carried out. The testing conditions were taken as shown in table 1.

Fig. 1. Schematic diagram of the apparatus used for the surface tension and viscosity

measurement.

Table 1. The condition of foaming test

Agent Ca (1.5wt% addition) or and SiC (1.5wt%) Thickening Stirring 15minutes Agent TiH2 (3wt% addition) Blowing Stirring 20 seconds Thickening & blowing 650oC for AZ91 Temperature Curing 10minutes at 500 oC for AZ91

Result and Discussion

Fig. 2 shows the measured surface tension of AZ91 and AM60 Mg alloy with the change of temperature. The comparison between the measured data of AZ91 and AM60 magnesium alloys is shown as well. As shown in Fig. 2(a), the surface tension of AZ91 is about 473 mNm-1 near the melting point, and with increasing temperature, it decreased lineally following the relationship of σ = 473 - 0.545(T-Tm), and the surface tension of the AM60 follows the relationship σ = 557 - 0.468(T-Tm). Compared with reference [3], the surface tension of pure Mg corresponds to the data measured under slight surface monolayer existence, not ultra height vacuum. As the result, error range of data is comparatively wide. The surface tension of AZ91 alloy was lower than that of AM60 alloy. It may be caused by formation of impurity elements and aluminum atom cluster having higher

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melting point than magnesium. The decreasing tendency of surface tension for increasing temperature is similar. Fig. 2(b) shows the variation of the viscosity of AZ91 and AM60 Mg alloys with the change of temperature. As shown in Fig. 2(b), the viscosity of AZ91 near the melting point is about 6.12 cp± 0.5 cp and the viscosity of the AM60 Mg alloy is 4.91± 0.4 cp. The viscosity value of AZ91 was higher than that of the AM60, which may be caused by adsorption behavior of solute (aluminum and zinc) in liquid AZ91 [7]. In the case of Mg alloys, it rapidly changes at near the melting point.

600 700 800

300

400

500

600

Surf

ace

Tens

ion

, mN

m-1

Temperature , oC

AM60 AZ91

600 700 800 9003

4

5

6

7

Vis

cosi

ty ,

cp

Temperature , oC

AM 60 AZ 91

(a) Surface tension (b) Viscosity

Fig. 2. Temperature dependencies of surface tension and viscosity of Mg alloy.

In Fig. 3, the effects of Ca, and SiC on the surface tension and viscosity were plotted. In the case of adding 2wt%SiC, There is no obvious change of surface tension. When 3wt% SiC was added, the surface tension begins to decrease. When Ca was added, there is a rapid decrease. The results of Ca addition showed more decreases than that of SiC addition, which indicates that Ca has higher surface activity effect than SiC. This reason can be explained well by Gibbs's adsorption equation [8], which relates the excess surface concentration with the dependence of the surface tension on the thermodynamic activity of the solute in melt.

0 2 4 60.3

0.4

0.5

0.6

Surf

ace

Tens

ion

, Ncm

-1

Particle , wt%

AM60 with Ca AM60 with SiC AZ91 with Ca AZ91 with SiC

0 5 10 15 20

0

4

8

12

16

20

Rotation speed 700rpmtemperature 600oC

Stirring time , min

Vis

csity

, c

p

AM60 with 1.5wt% Ca AM60 with 1.5wt% SiC AZ91 with 1.5wt% Ca AZ91 with 1.5 wt% SiC

Fig. 3. Effect of added elements on the surface tension and viscosity of Mg alloys at 600 ~ 610oC

Fig. 3(b) shows the measured viscosity of Mg alloy with addition of Ca and SiC. In the case of Mg alloys, we can see that the viscosity increase remarkably with Ca addition and stirring time.

However, viscosity increases relatively slower in the case of adding SiC particles. Adding Ca into the magnesium melt can increase the viscosity of the melt better. Spot line is optimal condition for foamed metal manufacturing.

AZ91 foam is produced by adding Ca to increase the melt viscosity and blowing agent (TiH2) to generate gas at 650oC. Because hydrogen can react with Mg at high temperature, more amount of

σ = 557-0.468(T-Tm)

σ = 473-0.545(T-Tm)

744 Eco-Materials Processing & Design VII

hydrogen generated by TiH2 is needed, so 3wt.% TiH2, which is twice as much as that used for Al foaming process, is added.

Fig. 4(b) shows the photographs of foamed AZ91 of a horizontal section. Compared with foamed Al, AZ91 (with Ca and SiC) foamed by optimal condition have very bad pore structures. Fig. 5(a) shows photographs of the foamed Al column. According to the previous study, optimal conditions of pure Al have viscosity of 10~14 cp and surface tension of 550~650mNm-1 [6, 9]. The specimen heights increased to more than eight times for Al and three times for AZ91 with Ca and two times for AZ91 with SiC alloy that of the original column height. The AZ91 alloy foams have coarse and shows big pore and more irregular cell structures than foamed Al. Foamed AZ91 Mg alloy satisfied viscosity level, but surface tension is too low. We can see that foamed Mg alloy has ruptured and coalescence pores.

(a) Foamed Al (b) foamed AZ91 with Ca (c) foamed AZ91 with SiC

Fig. 4. Photographs of foamed Al and Mg alloy column.

Summary The surface tension of AZ91 and AM60 are about 473mNm-1 and 557mNm-1 near the melting

point, respectively. When Ca was added, there was a rapid decrease. The results showed that the addition of Ca decreases the surface tension more than that of SiC does. The viscosity of AZ91 near the melting point is about 6.12 cp ± 0.5 cp and the viscosity of the AM60 Mg alloy is 4.91 ± 0.4 cp. We can see that the viscosity increases remarkably with Ca addition and stirring time. Foamed AZ91 Mg alloy satisfied viscosity values, but the surface tension was too low. That is why we can see that foamed Mg alloy has ruptured and coalescent pores.

Acknowledgement

This work was supported by grant No. RTI04-01-03 from the Regional Technology Innovation Program of the Ministry of Commerce, Industry and Energy (MOCIE)

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doi:10.1007/s11663-999-0108-4 [6] S.H. Park and B.Y. Hur: Materials Science Forum, 486-487 (2005), p. 464-467

doi:10.4028/www.scientific.net/MSF.486-487.464 [7] D. Skupien and D. R. Gaskell: Met & Mater., Trans. B, Vol. 31 (2000), p. 921.

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