Surface tension and Viscosity of Mg alloys Soo-han Park and Bo-young Hur K-MEM R&D Cluster-GSNU, AMRC, Division of Advanced Materials Engineering, Gyeongsang National University, Jinju, 660-701, Korea Abstract The rheological characteristics are the most important factors in casting process and metallic foam manufacturing especially. The surface tension (by the ring method) and the viscosity (by the rotation method) of molten Mg alloys (AZ91 and AM60) have been measured under pure Ar or SF 6 +CO 2 atmosphere. The surface tension and the viscosity of Mg alloys were investigated in the temperature range of 600-850 o C, and the effects of the additional elements were investigated at the 660~680. The result show that the surface tension and viscosity of these alloys decrease with increasing temperature together. The viscosity of the AZ91 is about 6.12 ± 0.5 cp and AM60 is 4.91 ± 0.4 cp. The effect of additional elements has the tendency that is the surface tension decreased and the viscosity increased. Keywords: metallic foam, surface tension, viscosity, Mg alloys. xxx(paper reference number)/1
Introduction Thermopysical properties are very important factors in casting process. Such as molten temperature, pouring temperature, density, viscosity and surface tension in molten metals. Many metallic foam produced by Directly Melt Foaming method have coarse and irregular cell structures. A primary current aim is to fabricate foams with more uniform structure and cell size. It is important to understand the mechanisms and factors controlling. For the control of the bubble in molten metal, such as birth (foaming agent, TiH 2 ), 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[1]. Thus this paper will investigate the bubble behavior of the molten metal and it s the most important two parameters: surface tension and liquid viscosity. These two factors are considered with two liquid mechanisms operating in foam. The first is gravity-driven melt flow from the top to base 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 [2,3]. Therefore, we investigated relation between driving force for a melt flow and two parameters to make metal foam with fine cell structure. In this experiment, the drop weight method and the rotational method were used because of simple measurement method. They are also directly applicable to the fabrication process. Viscosity and surface tension of Mg alloy, which is used as foaming material, were investigated. Influences of temperature to viscosity and surface tension were studied. Therefore in this paper, the rheological characteristics, namely viscosity and surface tension of Mg-Al alloy (AZ91 and AM60) were investigated in function of temperatures. Of course in the actual foaming process the appearance of the rheological characteristics are very much influenced from the existing of particles [4]. However the fundamental rheological data do not lose the importance. Experimental Surface tension was measured by the drop weight method, which applies the capillary phenomenon to measure the maximum force and wetting angle when the ring is pulled out from the melt surface [5,6]. Fig. 1 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 acting on the rotation rod by the melt [5,7]. In this experiment, the high purity argon or SF 6 +CO 2 gas sealing was used to prevent surface oxidation. The flowing rate was set to 25(l/min). The temperature range of measurement was set from 600 o C to 850 o C for Mg alloys. The temperature was measured by Pt-Rh xxx(paper reference number)/2
thermocouples, which were put them into the bottom and the side of the crucible. The maximum force (F max ) that is measured by the ring method can be recalculated to a surface tension (σ st ) by equation (1). 3 σ F max R R = f, 4πR cosθ V r (1) Where 4πR is wetted length, F max is the total maximum force, f is the Harkins Jordan factor [8] and θ is the wetted angle. Viscosity (η visco ) can be calculated from the measured torque (T) on the rotation rotor. 15T η visco = (2) 2 3 r1 h π r1 N ( + ) ab a Where T is the measured torque and N is the revolutions per min of the rotor. r 1 is the radius of rotor : 26mm, a is side gap : 4mm, b is bottom gap : 5mm and h is wetted height : 100mm. In addition, to compare the foamability among Mg alloys, foaming tests were carried out. The testing conditions were taken as table 1. Results and discussion Fig. 2 and 3 shows the measured surface tension and viscosity 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, the surface tension of AZ91 is about 473mNm -1 near the melting point, and with increasing temperature, it decreased lineally following the relationship of σ st = 473-0.545(T-T m ), and the surface tension of the AM60 follows the relationship σ st = 557-0.468(T-T m ). Compared with reference [6], 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 melting point than magnesium. The decreasing tendency of surface tension for increasing temperature is similar. Fig. 3 shows the variation of the viscosity of AZ91 and AM60 Mg alloys with the change of temperature. As shown in Fig. 3, the viscosity of AZ91 near the melting point is about 6.12 ± 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 [5]. In the case of Mg alloys, it rapidly changes at near the melting point. xxx(paper reference number)/3
Mg alloy is satisfied with required value for metal foam fabrication. Viscosity value of optimal conditions for the metal foam manufacturing is about 10~14[cp]. In the case of Al and alloys, it is possible that the optimal conditions of the viscosity can be obtained through controlling the amount of adding elements (Ca or Mg) [9-12]. But the interrelation of the surface tension and viscosity shows that Mg alloy needs proper selection of addition elements, as a result of present research The foaming test results are shown Fig. 4. AZ91 foam is produced by adding Ca to increase the melt viscosity and blowing agent (TiH 2 ) to generate gas at 650 o C. Because hydrogen can react with Mg at high temperature, more amount of hydrogen generated by TiH 2 is needed, so 3wt.% TiH 2, which is twice as much as that used for Al foaming process, is added. The foaming test results are shown Fig. 4. AZ91 and AM60 foam is produced by adding Ca to increase the melt viscosity and blowing agent (TiH 2 ) to generate gas at 650 o C. Because hydrogen can react with Mg at high temperature, more amount of hydrogen generated by TiH 2 is needed, so 3wt.% TiH 2, which is twice as much as that used for Al foaming process, is added. Fig. 4(c),(d) shows the photographs of foamed AZ91 and AM60 of a horizontal section. Compared with foamed Al, Mg alloys foamed by optimal condition have very bad pore structures. Fig. 4(a),(b) shows photographs of the foamed pure Al and alloy column. According to the previous study, optimal conditions of pure Al and alloys have viscosity of 10~14 cp and surface tension of 550~650mNm -1 [9-12]. The specimen heights increased to more than eight times for Al and alloys and three times for AZ91 with Ca and two times for AM60 that of the original column height. The porosity including skins of the foamed columns was evaluated by Archimedes principle to be (a) 0.92, (b) 0.90, (c) 0.51 and (d) 0.63, respectively. A few larger pores of about 4mm exist at the center of Al and alloy though the remaining small pores of less than about 3mm are dispersed uniformly around them. The AZ91 and AM60 alloy foams have coarse and shows big pore and more irregular cell structures than foamed Al and alloys. Foamed Mg alloys satisfied viscosity level, but surface tension is too low. We can see that foamed Mg alloys has ruptured and coalescence pores Conclusion The purpose of this study was to measure the physical properties of molten Mg alloys. The results are: The surface tension of AZ91 and AM60 followed the relationship σ=473-0.545(t-t m ) and σ=557-0.468 (T-T m ). xxx(paper reference number)/4
The viscosity of the AZ91 and AM60 is about 6.12±0.5cp and 4.91±0.4 cp respectively. The porosity including skins of the foamed columns was evaluated by Archimedes principle to 0.51 and 0.63, respectively. A few larger pores of about 4mm exist at the center of Al and alloy though the remaining small pores of less than about 3mm are dispersed uniformly around them. References 1 J. Banhart, M. Ashby and N. Fleck: Cellular Metals and Metal Foaming Technology (Verlag MIT Publishing 2001). 2 H. Fusheng and Z. Zhengang, J. Mater. Sci. 34 (1999) 291. 3 M. Meier, D. Hille and G. Wallot, Cellular Metals: Manufacture, Properties, Applications, MIT-Verlag Publication, 2003, p. 65. 4 J. Banhart, Journal of Metals 52 (2000) 22-27. 5 T. Iida and P. I. L. Guthrie: The Physical Properties of Liquid Metals (Clarendon press. Oxford 1988). 6 J. P. Anson, R. A. L. Drew and J. E. Gruzleski: Met. & Mater., Trans. B, 30 (1998), pp. 1999 7 Y. Shiraishi: J.of Kor. Inst. Met & Mater., Vol. 25 (1987), pp. 11 8 W. D. Harkins and H. F. Jordan: J. of Am. Chem. Soc., Vol. 52 (1930), pp. 1751 9 B. Y. Hur, H. J. Ahn, D. C. Choi and S. Y. Kim: Limat (2001), pp. 671 10 B. Y. Hur, S. H. Cho and K. B. Kim: Proceedings of Fall Conference. Vol. 1 (2001), pp. 246 11 B. Y. Hur, H. J. Ahn, D. C. Choi, S. H. Cho, K. D. Park, Y. J. Kim and S. H. Jun: Proceedings of the Symposium on Solidification Process of Metals (2000), pp. 87 12 S. Y. Kim, B. Y. Hur, C. K. Kwon, D. K. Ahn, S, H. Park and A. Hiroshi: Proceedings of the 65th World Foundry Congress (2002), pp. 499 13 J. W. Gibbs: Thermodynamics (The Collected Works of J. W. Gibbs, vol. I, Yale Univer. Press, New Haven, CT 1948). 14 P. Kozakevitch: J. of Met. (1969), pp. 57 15 D. Skupien and D. R. Gaskell: Met & Mater., Trans. B, Vol. 31 (2000), pp. 921 16 H. C. Birkman: J. Chem. Phys., Vol. 20 (1992), pp. 571 17 Y. J. Lee and S. H. Yi: J. of Kor. Inst. Met & Mater., Vol. 35 (1997), pp. 8 Acknowledgements 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) xxx(paper reference number)/5
Tables Table 1. The conditions of foaming test. Agent Thickening Stirring Agent Blowing Stirring Ca (1.5wt% add.) 15minutes TiH 2 (3wt% add. to Mg alloy) 20 Seconds Temperature Figures Thickening & Blowing Curing 650 o C for Mg alloys 10min. at 500 o C for Mg alloys Fig. 1. Schematic diagram of the apparatus used for the surface tension and viscosity measurement. Surface Tension, mnm -1 600 500 400 300 σ = 557-0.468(T-T m ) σ = 473-0.545(T-T m ) AM60 AZ91 600 700 800 Temperature, o C Fig. 2. Temperature dependencies of surface tension of Mg alloy. xxx(paper reference number)/6
Viscosity, mpa. s 7 6 5 4 AM 60 AZ 91 3 600 700 800 900 Temperature, o C Fig. 3. Temperature dependencies of viscosity of Mg alloy. (a) foamed Al (b) foamed Al alloy (c) foamed AZ 91 (d) foamed AM 60 Fig. 4. Photographs of foamed Al and Mg alloys column. xxx(paper reference number)/7