Enhancing mechanical properties.

As aluminum foundries continue to see an increase in demand from OEMs for more complex components with improved mechanical properties, the 27 Aluminum Div. presentations focused on these goals with an emphasis on melt treatment and casting solidification.

M.B. Djurdjevic, W.T. Kierkus, G.E. Byczynski, T.J. Stockwell and J.H. Sokolowski, Univ. of Windsor, discussed the modeling of the fraction solid for 319 aluminum (014). Accurate fraction solid information is necessary to perform computer simulations of feeding and solidification, as well as to make predictions of casting structure. The most common technique used to determine fracture solid is quantitative metallography, followed by thermal analysis, differential thermal analysis, differential scanning calorimetry and indirect methods.

Twelve different test pours of 319 aluminum were performed with a two-thermocouple thermal analysis system used to analyze solidification and determine fraction solid. From these experiments, a model was developed for predicting fraction solid based on key metallurgical reaction temperatures for 319 aluminum. The model also is based on the segmentation of the cooling curve and the application of characteristic temperatures, such as liquidus and solidus, which are calculated as functions of chemical composition.

The cooling curve showed that the range between liquidus and solidus temperature could be separated into three distinct segments. For each segment, a model for calculating the fraction solid was developed as a function of temperature.

J.P. Anson and J.E. Gruzleski, McGill Univ., relayed their findings on the effect of hydrogen (H) on the amount of shrinkage and gas microporosity in an aluminum-7% silicon allot; (026). The ability to quantify the amount of shrinkage and gas porosity allows foundries to identify whether the cause of their casting microporosity is gas evolution (necessitating control of the melt's gas quantity) or from poor feeding (necessitating changes in solidification conditions).

Experiments were performed on permanent mold cast A356 aluminum. Cluster analysis was performed on the samples to measure gas and shrinkage pores, and pore size and density were used in conjunction with the aspect and area to perimeter ratios to determine that the shrinkage pores are consistently irregular as compared to gas pores.

Other conclusions from the testing included:

* shrinkage and gas pores can be separated by microstructural differences in their position;

* gas porosity and combined gas-shrinkage porosity occur in equal amounts above the threshold H level, however, there is no evidence that pure shrinkage porosity occurs if the H level is above the threshold;

* the predominant mechanism for pore formation below the threshold H level is shrinkage, as indicated by the percent porosity and average pore size;

* more gas pores exist than shrinkage pore clusters.

In a presentation on the solidification of the eutectic in hypoeutectic aluminum-silicon (Al-Si) alloys, Q.G. Wang and D. Apelian, Worcester Polytechnic Institute, and L. Arnberg, S. Gulbrandsen-Dahl and J. Hjelen, Norwegian Univ. of Science and Technology, discussed how the mechanical properties of Al-Si alloys are strongly influenced by the formation of the binary eutectic (149).

Al-Si casting alloys have gained wide acceptance due to their excellent castability and corrosion resistance, and high strength-to-weight ratio in the heat-treated condition. A lack of understanding exists, however, in regard to the solidification mechanism of Al-Si binary eutectic. To clarify and establish an operative mechanism, the authors performed experiments using conventional metallographic and thermal analysis together with crystallographic orientation mapping.

In the experiments, it was clear from metallographic observations and electron backscattering diffraction analysis that in high purity Al-Si binary alloys, the Al-Si eutectic usually nucleates from the surface aluminum dendrites. The presence of trace elements or impurities in the alloy, however, can induce an alternative nucleation of eutectic grains from heterogeneous nuclei in the melt, leading to a dual macrostructure in some areas.

In hypoeutectic Al-Si alloys, the solidification eutectic (either modified or unmodified) takes place within the remaining interdendritic melt after the primary aluminum dendrites have formed. The primary aluminum dendrites and the composition of the remaining melt thus determine the nucleation and growth of the eutectic phases.

The experiments determined two basic nucleation mechanisms of the eutectic phase - the eutectic nucleates directly on the primary aluminum dendrites and the eutectic nucleates on the existing substrate particles in the melt. The former mechanism occurs in the unmodified alloys and the latter operates in modified alloys. The latter, however, also occurs in unmodified alloys depending on the level of trace elements and impurities in the melt. The addition of an eutectic modifier agent, such as strontium, changes the growth mechanism, in addition to the nucleation of eutectic.

Also presented were papers on mold and gating system design and panels on semisolid casting and hydrogen gas control.