Investigation on Formation Mechanism of Irregular Shape Porosity in Hypoeutectic Aluminum Alloy by X-Ray Real Time Obser

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ity in cast aluminum alloys has long been recognized as one of the most detrimental factors affecting mechanical properties,[1,2] especially fatigue resistance of the material.[3–8] In general, porosity in casting can be classified into gas porosity and shrinkage porosity. The large difference in hydrogen solubility between the solid and liquid phases is the main cause for hydrogen gas porosity in aluminum castings.[9] As solidification proceeds, the excess atomic hydrogen is rejected from the newly formed solid into the surrounding liquid. When the partitioned hydrogen in the solidification front reaches a critical solubility level, molecular hydrogen pores (gas porosity) form that may grow or shrink depending on the local hydrogen concentration levels and the rate of hydrogen diffusion. Theoretically, the Scheil model may be used to simply determine the hydrogen content in the remaining liquid and, subsequently, the fraction of solid at which hydrogen saturation and associated pore nucleation would occur. However, the Scheil equation should be HENGCHENG LIAO and YE PAN, Professors, LEI ZHAO and YUNA WU, Doctoral Students, and RAN FAN, Master Student, are with the Jiangsu Key Laboratory for Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, District of Jiangning, Campus of Southeast University, Nanjing 211189, P.R. China. Contact e-mail: [email protected] QIGUI WANG, Materials Specialist, is with the Materials Technology Department, GM Global Powertrain Engineering, Pontiac, MI 48340. Manuscript submitted October 12, 2011. Article published online May 16, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

modified due to the high diffusion coefficient of hydrogen in solid aluminum and the competition of pore nucleation and growth for hydrogen solute. The gas porosity usually has a relatively regular spherical configuration. The shrinkage porosity refers to the density difference between solid and liquid. When the volumetric shrinkage cannot be fed by liquid metal, voids (shrinkage-porosity) form in the final solidifying liquid metal pools, spreading along the configuration of the solidified solid. The shrinkage porosity thus has a fairly irregular morphology. In hypoeutectic aluminum alloys, shrinkage porosity is relatively easy to form due to the large mushy zone. The detrimental influence from the irregular shape porosity on mechanical properties is much more severe than the spherical shape gas porosity, due to the high stress concentration and thus cracking around the sharp corners of the irregular pores.[4,10,11] Porosity formation can be divided into two stages: nucleation and growth. In cast aluminum alloys, the nucleation of porosity is in no way homogeneous. The heterogeneous nucleation occurs at sites such as oxide inclusions with gas gaps, grooves of mold wall, undissolved fine gas bubbles in liquid, etc.[12–14] During melting or pouring of aluminum alloys, oxide inclusions can readily form, especially with turbulent flow. In oxide inclusions, there are a great number of nan