Modification-related porosity formation in hypoeutectic aluminum-silicon alloys
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Hydrogen-Related Effects, PG
I. INTRODUCTION
MUCH research has demonstrated that additions of strontium or sodium to hypoeutectic aluminum-silicon alloys cause an increase of the volume percent of porosity,[1–11] pore size,[1,4,6,7,12] and pore number[1,5,7] in a variety of different casting configurations. Pore morphologies change from irregular and interconnected in appearance to more rounded and isolated.[2,6,12,13] The porosity becomes more evenly dispersed with strontium additions.[8,12] Some conflicting research has shown that sodium additions actually result in a decrease in porosity,[8] or alternatively, a decrease of the required riser size to avoid porosity formation.[14] It is generally accepted that the nucleation and growth of porosity in a solidifying casting occurs when the following pressure imbalance is present:[12,15,16] PG PS 7 PATM PH Pst
[1]
where PG equilibrium pressure of dissolved gases in the melt, PS pressure drop due to shrinkage-induced flow, PATM atmospheric pressure, PH pressure due to the metallostatic head, and Ps-t pressure due to the pore-liquid surface tension. The exact mechanisms underlying the effect of impurity modification on porosity formation have remained elusive. The theories that dominate the literature can be roughly categorized with reference to Eq. [1], and are discussed in Sections A through C.
It has been demonstrated that strontium additions do not normally introduce hydrogen into the melt.[1,7,17,18] Others have actually measured a decrease in hydrogen,[19] or inferred this from the density increases of vacuum-cast samples.[3] Additions of metallic sodium have been associated with increased hydrogen pickup.[20] It has been reported that melts containing strontium have a higher rate of gas absorption from their surroundings, possibly due to a more permeable oxide layer.[21,22] Alternative techniques have not shown any changes in the regassing rate with strontium or sodium additions.[17,18,20] Vacuum degassing has been shown to be an ineffective technique in strontiumcontaining aluminum-silicon melts unless accompanied by an inclusion removal treatment.[14] This was taken as an indication that the oxides in strontium-containing melts are more effective at retaining absorbed hydrogen. In the same research, sodium additions did not affect the degassing rate. The observation of subsurface porosity in strontium-modified A356 castings[2,6] has been interpreted as an indication of increased gas absorption through reactions at the mold-metal interface.[6] Current explanations related to hydrogen phenomena cannot clearly explain the trends of increased porosity with strontium and sodium additions in aluminum-silicon foundry alloys. The fact that the research to date has employed a variety of degassing techniques, and that large changes in porosity are still detected, even in well-degassed melts, is probably the best argument against a hydrogen-based theory. B. Pore Nucleation Effects, Ps-t
STUART D. McDONALD, Research Fellow, Division of Materials Engi
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