Abstract
In this work, we study how ultrasonic cavitation melt treatment (UST) affects the temperature distribution, sump profile, and resulting microstructure in the direct-chill (DC) casting of an AA6008 aluminum alloy. Two 152 mm diameter billets were cast; one was treated with UST (UST-DC casting) in the hot top while the other was not (conventional DC casting). To investigate the temperature distribution, temperature was measured at multiple points in both billets. The sump profile was visualized by pouring Zn into the sump during casting. The microstructure was analyzed by measuring the grain size of as-cast billets. A numerical model of DC casting and UST-DC casting has been validated with the temperature measurements across the billets, and the experimental results agrees well with the numerical model. It is found that the sump profile quantification with thermocouple measurements is more accurate and less prone to interpretation than with Zn tracing. Numerical simulation results show that UST application in the hot top with sonotrode position at 20 mm above the graphite ring level depresses the liquidus isotherm but does not affect the solidus isotherm, resulting in a thinner transition region compared with conventional DC casting. Grain structure analysis verifies that structure refinement with UST has been achieved at the given sonotrode position. The strongest grain refinement was at the center of the billet with the average grain size 50% smaller than that without UST. The results are discussed in terms of the known mechanisms of UST, i.e. dendrite fragmentation and deagglomeration of nucleating substrates.
Highlights
Direct-chill (DC) casting is a semi-continuous casting process that is widely used in the non-ferrous metallurgical industry
It is robust in producing commercial wrought aluminum alloy billets that are suitable for further rolling, extrusion, or remelting processes [1,2]
This study shows how ultrasonic melt treatment (UST) affects the sump profile and grain structure and is a critical step towards the optimization of UST parameters in DC casting of aluminum alloys
Summary
Direct-chill (DC) casting is a semi-continuous casting process that is widely used in the non-ferrous metallurgical industry. It is robust in producing commercial wrought aluminum alloy billets that are suitable for further rolling, extrusion, or remelting processes [1,2]. To avoid casting defects and improve the thermo-mechanical properties of the as-cast billet, a finer grain structure is usually necessary. Finer grained alloys display superior mechanical characteristics such as strength, toughness, and ductility [3]; these properties are crucial for technological applications. Established techniques involve inoculants that are costly and whose production involves hazardous chemicals and processes [8]. UST has gained popularity as a more economical and environmentally friendly alternative as it enables the refining of the cast product structure with significantly less d or even without d inoculants
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