Abstract
This work focuses on ultrasonic melt treatment (UST) in a launder upon pilot-scale direct chill (DC) casting of 152-mm-diameter billets from an AA6XXX alloy with Zr addition. Two casting temperatures (650°C and 665°C) were used to assess their effect on the resulting microstructure (grain size, particle size, and number density).Structure refinement results show the feasibility of UST in the DC casting launder. This is quantified through the corresponding reduction of grain size by around 50% in the billet center, or more towards the billet surface, reduction of the average Al3Zr particle size, and increase in the particle number density. A higher Al3Zr particle density was obtained when the alloy was cast at 665°C. Numerical simulation results and suggestions on how to improve the treatment quality of UST in DC casting launder are also provided.
Highlights
Direct-chill (DC) casting is a typical means of producing aluminum alloy billets suitable for further downstream processing.[1]
We demonstrated that the combined effect of Ultrasonic melt treatment (UST) and Zr17,31 could be achieved upon pilot-scale DC casting (Ø = 152 mm) while applying UST in the launder
We carried out UST in the melt flow in a launder of a pilot-scale DC casting process
Summary
Direct-chill (DC) casting is a typical means of producing aluminum alloy billets suitable for further downstream processing.[1]. The structure refinement mechanism by UST has been comprehensively reviewed by Eskin and Eskin[10] and the mechanisms can be summarized as enhanced heterogeneous nucleation through activation of latent nonmetallic inclusions (i.e., oxides) and fragmentation of primary intermetallics.[9,10] These mechanisms are profoundly dependent on the acoustic cavitation (generation and implosion of cavitation bubbles) that are formed in the melt due to high-frequency (ultrasonic) vibrations with amplitude larger than a certain threshold (Blake threshold). After an acoustic cavitation bubble is formed, it pulsates and eventually collapses, subsequently generating impact pressures which may locally reach beyond 400 MPa for a short period of time.[11] Such pressures can activate dormant inclusions by their wetting and fragment the primary intermetallics in the vicinity of the collapse.[12] In the latter case, where the fragmented intermetallics represent high-potency nucleants (e.g., Al3Ti, Al3Zr, and Al3Nb13,14), these may act as nucleation points
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