Microstructural Refinement of As-Cast Al-Mg Alloy by Ultrasonic Melt Treatment Using a Titanium Sonotrode Under Fully Liquid Condition

The effects of ultrasonic treatment (UST) using a titanium (Ti) sonotrode in a fully liquid stage on the grain refinement and mechanical properties of as-cast Al–Mg alloy billets were investigated. To clarify the grain refinement mechanism, the grain size (GS) and dendrite arm spacing (DAS) were examined as functions of the growth restriction factor (Q) dependent on the Ti dissolution from the sonotrode, and compared to those in the as-cast and re-melted states of the samples inoculated using an Al–10 mass%Ti master alloy. In addition, the formation and dissolution behavior of Al3Ti intermetallic particles acting as a heterogeneous nuclei was indirectly observed by measuring the electrical resistivity during isochronal annealing. In comparison with the chemical refiner inoculation, the UST effectively refined not only the GS but also the DAS, both of which showed similar slightly concave upward curves with an increasing slope against 1/Q. Electrical resistivity measurement results provided indirect evidence that the dissolved Ti was present as a solute during the solidification stage. The GS, DAS, and electrical resistivity results all suggest that the dissolved Ti refined the GS primarily by solute-induced growth restriction effect rather than by providing heterogeneous nucleation sites. The UST effect on the microstructure refinement was efficient when the Ti-dissolution content was as low as less than 0.05 mass%. The refinement of grains, Al3Fe particles, dendrites, and pores by the UST significantly improved mechanical properties, especially the elongation at break

Heat Dissipation of Open-Cell-Type Aluminum Foams Manufactured by Replication-Casting Process

Open-cell-type aluminum foam demonstrates excellent heat dissipation owing to interconnected pores. In this study, open-cell-type aluminum foams with various pore sizes and porosities were fabricated using the replication-casting process, which is a relatively simple process. The porosity of the manufactured foams ranged from approximately 55% to 62%. To assess the heat dissipation of the manufactured foams, an air-cooling system was designed. The device could pass a controlled amount of air through the connected pores, simultaneously measuring pressure drop ∆P and temperature changes. It was confirmed that the open-cell-type aluminum foams exhibited a very high cooling rate in the initial cooling phase, and the thermal behavior is influenced by structural characteristics. At a porosity of 62%, the initial maximum cooling rate was measured to be 1.41 ℃/s for a pore size of 0.7~1.0 mm, and it was observed to significantly increase to 3.82 ℃/s for a pore size of 2.8~3.4 mm. Furthermore, for the same pore size, an increase in porosity resulted in an increase in the initial cooling rate. Lager pore sizes and higher porosities led to lower pressure drop ∆P and improved airflow, enhancing the cooling efficiency of open-cell-type aluminum foams