Computational Methods in Engineering

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The formation of microporosity in modified Al-Si alloys has been reviewed in the present study. A major concern in modification is the increased tendency to form microporosity in the macro-shrinkage free Al-Si alloy castings. It has also been demonstrated that at low hydrogen contents (0.1cc/ 100g, Al), where only shrinkage porosity should occur, the effect of Sr-modification on porosity content is not considerable, indicating that the increase in porosity is due to an increase in gas porosity. Modification treatment, however, does not add hydrogen to the melt, nor does it increase the rate of regassing of the liquid, revealing that it can not enhance pore formation by increasing the melt hydrogen content. Modification treatment raises the freezing range (4-10 oC), but this increased freezing range exerts only a very small effect on microporosity formation, which cannot, by itself, explain the increased tendency to microporosity formation observed in modified alloys. The presence of modifiers slightly decreases the surface tension of the melt (5%), although this decrease in surface tension is not sufficiently high to considerably enhance pore formation in modified alloys. Many researchers have reported that modification treatment might favour the formation of porosity due to its effect on oxide use in the heterogeneous pore formation although the systematic observation of pores has shown that SrO does not take part in pore fomation in Sr-modified alloys. Strontium and other modifiers which increase pore formation (Na and Ca) in Al-Si alloys have a high chemical affinity to form complex intermetallic compounds with Si and Al. Systematic observation of pores have shown that Sr-rich intermetallics take part in pore formation. Thus, Sr-modification may increase the porosity content through the formation of Sr-rich compounds during solidification.

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Fabrication and characterization of aluminum matrix composites containing different volume fractions of Ni3Al powder (5-40 Vol%) were investigated. Ni3Al powder was produced by mechanical alloying of elemental nickel and aluminum powder mixture. Al-Ni3Al composite parts were prepared using a powder metallurgy route involving two stages Al and Ni3Al powder mixtures were first compacted under 500MPa and then hot-pressed under 250MPa at 420 oC for 10min. The microstructure and hardness of consolidated parts were investigated by x-ray diffractometery, optical and scanning electron microscopy and hardness measurements. Results showed that consolidated Al-Ni3Al samples included no significant porosity with a nearly uniform distribution of Ni3Al particles. Additionally, structural examinations showed that no significant reaction between Ni3Al and aluminum matrix occurred during sintering process. Al-Ni3Al composites exhibited a higher hardness value compared with pure aluminum sample prepared under identical conditions. The hardness value of Al-Ni3Al composites increased linearly as Ni3Al content increased.