Abstract
The present study investigates the influence of ultrasonic treatment on the grain refinement of commercial purity aluminium with a range of Al3Ti1B master alloy additions. When the aluminium contains the smallest amount of added master alloy, ultrasonics caused significant additional grain refinement compared to that provided by the master alloy alone. However, the influence of ultrasonics on grain size reduces with increasing addition of the master alloy which adds additional TiB2 particles and Ti solute with each incremental addition. Applying the Interdependence model to analyse the experimentally measured grain sizes revealed that the results of this study and those from similar experiments on an Al-2Cu alloy were consistent when the alloy compositions are converted to their growth restriction factors (Q) and that increasing Q had a major effect on reducing grain size and increasing grain number density. Compared with the application of ultrasonic treatment where an order of magnitude increase in the grain number density is achieved, an increase in the Ti content over the range of master alloy additions, causes the grain number density to increase by approximately three times.
Highlights
The present study investigates the influence of ultrasonic treatment on the grain refinement of commercial purity aluminium with a range of Al3Ti1B master alloy additions
The microstructure of the sample from the central part of the ingot subjected to Ultrasonic Treatment (UST) is shown in Fig. 1c, and the average grain size is about 160 μm indicating that significant grain refinement results from the application of UST during solidification
It is proposed that in addition to the possible effects of UST on wetting, deagglomeration, nucleation and fragmentation, acoustic streaming effectively distributes nucleated grains and lowers the temperature gradient in the melt which decreases the size of the nucleation-free zone increasing the number of activated TiB2 particles and survived aluminium fragments
Summary
1 kg of commercial purity Al ingot (99.7%) was melted in a graphite-clay crucible with 90 mm top diameter, 60 mm bottom diameter and 120 mm in height by an electric resistance furnace. When UST was not applied the alloy was allowed to cool and solidify without the insertion of the sonotrode. When UST was applied to the same range of compositions as listed above, the ultrasonic sonotrode was switched-on without preheating and immersed 15 mm below the top surface of the melt in the graphite-clay crucible. Two K-type thermocouples were inserted into the melt beside the sonotrode: one adjacent to the wall of the crucible and the other being placed 12.5 mm towards the centre of the crucible. Both thermocouples were positioned 45 mm above the bottom of the crucible. At the applied amplitude of 20 μm, the ultrasonic intensity (I) level is about 975 w cm−2, estimated using Equation (4)[4]
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