![]() 19 investigated the effect of cooling conditions on porosity formation in Al-Si alloys, and proposed a multi-zone model to explain the porosity in various regions of a casting. AUTOMATON SIMULATOR SERIESFollowing their initial work, Lee and co-authors carried out a series of studies on the porosity formation including mathematical modeling 15, in situ observation 3, 16, and cellular automaton (CA) simulation 17, 18. This study provided useful insights into the new models which govern the pore formation and its interaction with evolving grain structure. 1 reviewed both analytical and numerical modeling approaches for micro-porosity formation in aluminum alloys. This analytical method calculates the temperature range of nucleation, the fraction of solid at nucleation, and the supersaturation of hydrogen needed for nucleation. ![]() Li and Chang 14 described an analytical solution for nucleation and growth of hydrogen pores during solidification of A356 alloy. Several studies on analytical solutions and numerical simulation methods have been carried out to understand the mechanisms of nucleation and evolution of hydrogen porosity during solidification. Modeling and prediction of porosity formation has become a major goal in modeling of solidification microstructure, which provides an essential link in integrated computational materials engineering 11, 12, 13 (ICME) for solidification products. Although experiments can be conducted to measure porosity directly after the products are made, it is difficult to understand the nucleation and evolution of porosity during solidification and predict the porosity formation in the final products for product design and process optimization. These studies suggest that reducing the hydrogen source or increasing the speed of pores moving out of melt is beneficial to reducing the total porosity and the final properties of solidification products. 7 performed in situ X-ray tomography on Al-Cu alloys, and calculated an effective hydrogen diffusion coefficient as a function of the volume fraction of solid. Additionally, Kaufmann and Rooy 2 showed that hydrogen porosity could be influenced by the cooling rate of aluminum casting. 9, 10 conducted several casting experiments of A356 alloy with different pre-treatments to obtain various hydrogen levels. 8 found that hydrogen porosity can be reduced by efficient drying of the powder in selective laser melting experiment of AlSi10Mg alloy. 4 carried out laser welding experiments of A356 and AA5038 alloys, where they found that a surface preparation (especially laser cleaning) could reduce the hydrogen porosity in the weld bead. In order to improve the mechanical properties of final products, many researchers have experimented on the gas porosity and hydrogen diffusion during casting and welding processes of aluminum alloys 3, 4, 5, 6, 7, 8. Therefore, the present research focuses on the porosity developed by the presence of atomic hydrogen dissolved in molten aluminum. Hydrogen porosity, caused by the large difference in the solubility of hydrogen between liquid and solid aluminum phases, is a major source of porosity in aluminum solidification products. There are two major types of porosity observed in solidification products 1, 2: (1) shrinkage porosity due to solidification shrinkage and inadequate feeding during processing, and (2) gas porosity due to air entrapment and insoluble gases such as hydrogen. ![]() Porosity formed during solidification has been a major issue adversely affecting the performance, in particular the ultimate strength and fatigue resistance, of solidification products such as castings, welds or additively manufactured components. ![]() Aside from the advantages of high strength-to-weight ratio, good corrosion resistance, and low cost, porosity exists ubiquitously in aluminum solidification products. Aluminum alloys are widely used in automobile, aerospace, and other industrial applications. ![]()
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