Factors Affecting the Nucleation Kinetics of Microporosity Formation in Aluminum Alloy A356
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THE limited understanding of the nucleation mechanism is one challenge still to be overcome in simulating microporosity formation in aluminum alloy castings. Previous studies have shown that inclusions, especially oxide films, assist nucleation of pores and thus have a significant influence on the initial stage of porosity formation.[1–6] The conventional theory suggests that solid oxide inclusions in the melt provide heterogeneous nucleation sites, which reduce the energy barrier required for pore nucleation.[6–8] Application of the well-known Young’s equation to Figures 1(a) and (b) results in the following two expressions: cls þ cgl cos h ¼ cgs
½1a
cgs þ cgl cos h ¼ cls
½1b
where cls,cgl and cgs are the surface tensions (N/m) at the liquid–substrate, gas–liquid, and gas–substrate interfaces, respectively. In Eq. [1a], when cls is large, h is large, indicating poor wettability of the solid by the liquid, whereas in Eq. [1b], when cls is large, h is small, indicating a spreading out of the gas phase on the LU YAO, PhD Candidate, STEVE COCKCROFT, Professor, and CARL REILLY and JINDONG ZHU, Research Associates, are with the Department of Materials Engineering, University of British Columbia, Vancouver, BC V6T 1Z4, Canada. Contact e-mail: lu.ly.yao@ gmail.com Manuscript submitted April 13, 2011. Article published online December 21, 2011 1004—VOLUME 43A, MARCH 2012
substrate and a reduction in the critical free-energy for nucleation. Furthermore, the reduction in curvature acts to reduce the internal pressure associated with the liquid metal–gas surface tension, facilitating mass transfer of hydrogen to the pore. This theory has been questioned by Campbell, who pointed out that heterogeneous nucleation seems highly improbable because of the restriction on the minimum contact angle attainable between the liquid–pore interface and the solid substrate.[9] A nucleation-free mechanism for porosity formation was therefore proposed.[10,11] The theory is based on the continuous oxide film present on the surface of the metal. Because of the free-surface turbulence commonly present during pouring, these oxide films can become entrained and often folded within the melt. The dry sides of the folded oxide films cannot form bonds and thus are usually referred to as ‘‘bi-films.’’ It is also proposed that these bi-films form a gas cavity by entrapping air within the folded film. These gas cavities can be considered preexisting pores in the melt. Pore growth can occur by the ‘‘unfurling’’ of the bi-films with a pressure drop caused by insufficient liquid feeding and/or by hydrogen diffusion to the cavity. This new theory has gained considerable attention in the field of aluminum alloy casting research; however, no direct experimental evidence has been provided because of the nanometer-scale thickness and transparent nature of the bi-films. The identification of the pore/oxide film interaction mechanism is still a subject of active research. Since the 1980s, several different models have been proposed to simulate the pore nucleation process in M
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