Abstract
Most of the models for predicting porosity formation in aluminum alloy castings use a simple mass balance, such as the lever rule, to track hydrogen enrichment in the interdendritic liquid. However, the hydrogen concentration predicted by the lever rule is typically too low to satisfy the threshold concentration for pore nucleation based on classical nucleation and growth theory. As a result, important features of microporosity such as the size and spacing of pores cannot be treated properly. In this article, the hydrogen concentration during the directional solidification of an Al-4.5 pct Cu alloy is calculated, assuming hydrogen rejection during solidification and diffusion in the mushy zone. The calculation shows that the use of the lever rule greatly underestimates the hydrogen concentration at the eutectic front. This is due to the fact that the eutectic front also rejects hydrogen and that this is not considered in the use of the lever rule. Results of numerical simulations that consider hydrogen rejection and diffusion are compared with results obtained using the lever rule. The comparison indicates that actual hydrogen concentrations may be orders of magnitude higher than that predicted by the lever rule. It is suggested that the lever rule should not be used in predicting porosity nucleation. The model outlined in this article is used to propose and explain the formation of a wavelike distribution of pores during directional solidification.
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Han, Q., Viswanathan, S. Hydrogen evolution during directional solidification and its effect on porosity formation in aluminum alloys. Metall Mater Trans A 33, 2067–2072 (2002). https://doi.org/10.1007/s11661-002-0038-0
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DOI: https://doi.org/10.1007/s11661-002-0038-0