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The microstructure evolution of Cu-30Zn during the deformation and cooling stages of friction stir welding was separately investigated by employing the in-process rapid cooling, tool “stop action”, and subsequent short-time annealing. A pure copper foil was inserted into the butting surfaces of the two workpieces to show the microstructure evolution path. The microstructure along the material flow path was investigated by the EBSD technique. At the weld’s upper part, continuous material flow occurs nearly over the whole range of the shoulder. The initial coarse grains are refined by the discontinuous dynamic recrystallization (DDRX) accompanied by the annealing twinning. During the material flow, the grain structure evolution is dominated by the annealing twinning during the thermally activated grain boundary migration and the subsequent twin destruction due to further deformation, resulting in a nearly constant grain size. Finally, normal grain growth occurs at the cooling period. At the lower part, the material transfers in a very thin layer near the probe surface and rapidly forms stable band structures. Due to the lower heat generation and higher strain rate, the mechanical twinning occurs in front of the probe. These deformation twins can provide additional nucleation sites for the DDRX via the twin destruction caused by further deformation. The higher strain rate in the weld’s lower part contributes to the finer grains than that of the upper part grains. However, due to the shoulder’s coverage, the lower part undergoes a longer cooling period than the upper part, and thus more significant grain growth occurs. [ABSTRACT FROM AUTHOR]

Radiography using a synchrotron radiation X-ray source was performed to examine solidification and melting behaviors in hypereutectic cast iron specimens containing 0.002 and 0.05mass pctMg. The solidification sequence in the alloy containing 0.002mass pctMg was (1) nucleation and growth of graphite particles of which transformed to a flake-like shape, (2) growth of γ-Fe dendrites, (3) nucleation of graphite particles ahead of the interface just prior to the eutectic solidification, and (4) the eutectic solidification. In contrast, (1) and (2) occurred nearly at the same time in the specimen containing 0.05 mass pct Mg. The addition of 0.05mass pctMg significantly reduced the temperature range in which the graphite particles grew as the primary phase. Image-based analysis of melting behavior showed that even 0.05 mass pct addition was sufficient to modify the phase equilibrium of the liquid, γ-Fe, and graphite phases. Thus, the observed influence of Mg on the solidification sequence was attributed to the modification of the phase equilibrium. The influence was consistently explained by considering the shift of the eutectic composition to the carbon side in the pseudo-ternary system. It was also suggested that supersaturation of carbon in the melt increased as the temperature decreased even though the primary graphite particles existed. The supersaturation may cause the nucleation of the graphite particles just before the eutectic solidification. [ABSTRACT FROM AUTHOR]

An improved understanding of the response of solidifying microstructures to load is required to further minimize casting defects and optimize casting processes. This article overviews synchrotron radiography studies that directly measure the micromechanics of semisolid alloy deformation in a thin-sample direct-shear cell. It is shown that shear-induced dilation (also known as Reynolds' dilatancy) occurs in semisolid alloys with morphologies ranging from equiaxed-dendritic to globular, at solid fractions from the dendrite coherency point to ~90% solid, and it occurs in both Al alloys and carbon steels. Discrete-element method simulations that treat solidifying microstructures as granular materials are then used to explore the origins of dilatancy in semisolid alloys. [ABSTRACT FROM AUTHOR]

Due to a production error, the email address for co-corresponding author H. Fujii was omitted. The email address is fujii@jwri.osaka-u.ac.jp. [ABSTRACT FROM AUTHOR]