The Surface Tension of Molten Aluminum and Al-Si-Mg Alloy under Vacuum and Hydrogen Atmospheres J.P.ANSON,R.A.L.DREW,and J.E. GRUZLESKI The surface tensions of pure molten aluminum, A356 alloy (Al-7 pct Si-0.3 pct Mg), and strontium- modified A356 alloy have been measured under vacuum and hydrogen atmospheres using the sessile drop technique. The values obtained for pure aluminum at 680 8C and for A356 alloy and modified A356 alloy at 630 8C are 1.007, 0.889, and 0.844 N/m, respectively, when measured under vacuum. The addition of hydrogen gas to the atmosphere of the liquid droplet has no significant effect on the surface tension of the unmodified A356 alloy, while itlowers the surface tension of the modified alloy to 0.801 N/m. This effect is possibly due to the formation of SrH 2 . I. INTRODUCTION surface tension of aluminum and its alloys have been carrie out under vacuum or inert gas, typically argon, and do not ACCURATE information on the surface tension of alu- necessarily represent the conditions that prevail when mic minum and its alloys is necessary for many metallurgical porosity forms in a solidifying casting. This is particularly and materials-related processes, such as casting, welding, true foraluminum alloys where the gas contained in the brazing,sintering, andthe fabricationof metal-matrix pore is hydrogen, and certain alloying elements form meta composites. [1,2] hydrides [9] (Table I). As a general rule, surface contaminants, Smallbubbles, known as microporosity, are a common such as the hydrides, reduce the surface tension of the met defect found in aluminum castings. These pores arise primar- It is possible that such hydrogen-metal interactions may be ily from the evolution of hydrogen gas during solidification responsible for increased amounts of porosity found with due to a marked decrease in the solubility of hydrogen in the addition of small amounts of certain alloying elements. liquid and solid aluminum. [3] Four forces act on a gas bubble This holds particularly true for strontium, which is added thatis present in a liquid metal. The internal gas pressure to the alloy to modify the silicon structure. Strontium form (P g ) acts to increase the bubble size, while the liquid head a stable hydride at temperatures up to 1000 K and is know or metallostatic pressure (P m ), the atmospheric pressure (P a ), to cause an increase in porosity [10,11] in cast aluminum alloys. and the surface tension forces (P st ) tend to make the bubble The present article reports on measurements of the surfa shrink. Obviously, at equilibrium, the following relationship tension of a nonmodified and strontium modified aluminum must hold: [4] casting alloy (A356) under vacuum and hydrogen gas atmo P g 5 P m 1 P a 1 P st [1] sphere. One of the main objectives of this work was to determine whether the presence of hydrogen gas has an In the majority of casting processes, the metallostatic pres- effect on the surface tension. sure is quite small in comparison to the other pressures so that P g ' P a 1 P st [2] II. MEASUREMENT OF SURFACE TENSION The surface-tension pressure is determined by the surface tension of the liquid metal ( s ) and the bubble radius. For a The surface tension is an intrinsic property of all liquids nonspherical bubble, and is due to an incomplete coordination of the atoms at P st 5 s (1/r 1 1 1/r 2 ) [3] the liquid-gas interface. Atoms in the bulk of the liquid are completely surrounded by other atoms, while those at the where r 1 and r 2 are the principle radii of curvature. If the surface are only in contact on the side and below. Interatom bubble is spherical, this expression becomes attractions cause an uneven pull on the surface atoms, draw ing them into the body of the liquid and resulting in a P st 5 2 s /r [4] curvature of the surface as the liquid tries to assume a sha There is considerable current interest in the development of that has a minimum surface area. The effect of surface ten- mathematical models to describe microporosity formation sion is commonly observed in the form of capillary action, in castparts. [4–8] Realistic models require that the surface meniscuses, and beading of droplets. tension of the liquid alloy be known. Thermodynamically, surface tension is defined as the sur The majority of the experimental measurements of the face free energy per unit area (J/m 2 ). From a dynamic point of view,the surface tension represents the work required to create one unit of additional surface area at constant J.P. ANSON,Postdoctoral Student, Department of Metallurgy Engi- temperature (N/m). The units J/m 2 are equivalent to N/m neering, R.A.L.DREW,Professor and Chair, Department of Mining and and the units dyn/cm are also frequently used (1000 dyn/ Metallurgical Engineering, and J.E.GRUZLESKI, Professor and Dean, cm 5 1 N/m). Faculty of Engineering, are with McGill University, Montreal, PQ, Canada The surface tension of a liquid can be measured by a H3A 2B2. Manuscript submitted November 18, 1998. variety of methods. [12,13,14] These are based on the change METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 30B, DECEMBER 1999—1027