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A comprehensive methodology that takes into account solidification, shrinkage-driven interdendritic fluid flow, hydrogen precipitation, and porosity evolution has been developed for the prediction of the microporosity fraction and distribution in aluminum alloy castings. The approach may be used to determine the extent of gas and shrinkage porosity, i.e., the resultant microporosity which occurs due to gas precipitation and that which occurs when solidification shrinkage cannot be compensated for by the interdendritic fluid flow. A solution algorithm in which the local pressure and microporosity are coupled is presented, and details of the implementation methodology are provided. The models are implemented in a computational framework consistent with that of commonly used algorithms for fluid dynamics, allowing a straightforward incorporation into existing commercial software. The results show that the effect of microporosity on the interdendritic fluid flow cannot be neglected. The predictions of porosity profiles are validated by comparison with independent experimental measurements by other researchers on aluminum A356 alloy test castings designed to capture a variety of solidification conditions. The numerical results reproduce the characteristic microporosity profiles observed in the experimental results and also agree quantitatively with the experimentally measured porosity levels. The approach provides an enhanced capability for the design of structural castings.

A mechanism of soldering of an aluminum alloy die casting to a steel die is proposed. A soldering critical temperature is postulated, at which iron begins to react with aluminum to form an aluminumrich liquid phase and solid intermetallic compounds. The liquid joins the die with the casting upon solidification. The critical temperature is determined by the elements in both the casting alloy and the die material and is equal to the solidus temperature of the resulting alloy. The critical temperature is used to predict the onset of die soldering, and the local liquid fraction is related to the soldering tendency. Experiments have been carried out to validate the concept and to determine the critical temperature for die soldering in an iron-aluminum system. Thermodynamic calculations are used to determine the critical temperature and soldering tendency for the cases of pure aluminum and a 380 alloy in a steel mold. Factors affecting the soldering tendency are discussed, and methods for reducing die soldering are suggested.

Measurements of liquid permeability in the mushy zones of Al-15.42 pct Cu and Al-8.68 pct Cu alloy samples were performed isothermally just above the eutectic temperature, using eutectic liquid as the fluid. A modified method was developed to determine the specific permeability as a function of time (K s ) during the test from the data collected on these alloys. Factors affecting permeability measurements are discussed. The permeabilties are observed to vary throughout the experiment. This is attributed to microstructural coarsening and channeling that occur in the sample during the experiment. Coarsening rates are determined for the isothermal coarsening tests without fluid flow, and the results are observed to be less than the rates indicated from permeability tests where fluid flow is present. Careful measurement of the volume fraction of liquid (g L ) shows that g L decreases during the test. The permeability is then related to the microstructure of the sample using the Kozeny-Carman equation. The correlation between the measured K S , g L , and specific solid surface area (S V ) improves markedly when compared to previous studies, when microstructural parameters at the initial stage of the test are used.

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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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