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The present paper deals with the behavior of porosity under an influence of convection in the weld pool during laser welding of aluminum by means of a numerical simulation developed by the authors. The simulation system based on the finite element method can calculate and represent the formation of “key-hole” and bubbles called porosities, together with the flowing behavior of porosities under convection in the weld pool during entire process from the onset of melting until the completion of solidification. Porosities appeared behind the keyhole where the flow under a compressive force dominantly affects the formation of them. The upward flow behind the keyhole was composed of two modes such as a circulating flow and a flow along the solid/liquid interface. The optimized welding condition to produce sound joints with no porosity was derived from these two distinct flows.

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This study investigated cluster formation in the early stages of natural aging in Al-1.04 mass%Si-0.55 mass%Mg alloys by soft X-ray XAFS measurements and first-principles calculation. XAFS measurements at the Mg-K and Si-K edges were carried out at the BL27SU beamline at SPring-8. It was found that the absorption edge energies changed as aging proceeded. Density functional theory (DFT) calculations were used to determine the valence electron densities near Si and Mg atoms and to simulate the Si-K and Mg-K edge spectra for some cluster models. On the basis of the results, it was demonstrated that Si and Mg atoms formed clusters in four stages (I-IV) during natural aging. In stage I, Si-vacancy pairs, Mg-vacancy pairs, and a combination of both were formed. In stage II, vacancies were released from the clusters formed in stage I. In stage III, Mg-vacancy pairs were included in the clusters. In stage IV, the clusters coarsened through the release of vacancies. These results indicate that soft X-ray XAFS, which is capable of identifying individual elements, has the ability to provide information on such clusters.

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