**1. Introduction**

Aluminum alloy is the most widely used non-ferrous metal structure material in the metallurgy, chemical, construction, transportation, aerospace, and weapons industries [1,2]. With the application of aluminum alloys in the high-tech field, stricter requirements are imposed on the structure and properties of aluminum alloys [3,4]. At present, grain refinement by adding an Al-Ti-B refiner is an effective way to improve the performance of aluminum alloy strips in casting and rolling production [5–7]. A large number of researchers have studied and improved the Al-Ti-B refiner to improve its refining efficiency [8–11]. However, the addition of the Al-Ti-B refiner will generate several undesirable

by-products, including the formation of particle aggregates, local defects, and impurities; in addition, the refiner when used as a consumable also increases the cost of the casting and rolling production.

In recent years, the use of additional physical fields such as ultrasonic vibration or pulsed magneto-oscillation (PMO) to replace the refiner used to refine the grains has aroused interest among researchers [12–16]. Xia et al. [17] used ultrasonic vibration to treat a 3003 aluminum alloy in continuous casting and rolling and found that the effect of ultrasonic treatment was better than that of adding Al-Ti-B refiner and that the refiner could be completely replaced, reducing the production cost and improving the material properties of the strip. Shi et al. [18] applied ultrasonic vibration to the process of 8011 aluminum alloy twin roll casting and rolling, and the research results showed that through the effect of ultrasonic vibration, the grains of the strip were made smaller and the mechanical properties were improved to a certain extent. Xu et al. [19] reduced the amount of Al-Ti-B by applying a electromagnetic field to the cast rolling of the 1100 aluminum alloy. It was found that the refining effect of adding 0.1 wt% Al-Ti-B refiner under the electromagnetic field roll-casting conditions was better than that of adding 0.4 wt% Al-Ti-B refiner under the conventional roll-casting conditions.

The key to the realization of ultrasonic-assisted metal solidification forming is the introduction of the ultrasonic wave. High-energy ultrasound can cause a cavitation effect and acoustic flow effect in the melt. With the increase in ultrasonic power, the effects will be significantly enhanced [20]. Due to these effects, the homogeneity of solute elements and fluidity of the melt can be enhanced, as well as improving the grain refinement and melt degassing efficiency [21–24]. At present, most researchers use a method of directly introducing ultrasonic waves in the upper part of the melt to treat the metal melt. The ultrasonic transducer is subjected to direct thermal radiation and a thermal shock from the molten metal, which often causes the transducer to be detuned or even damaged during the casting process. In addition, the titanium alloy ultrasonic radiator in contact with the melt is easily eroded [25–27]. These have made it difficult to achieve long-term ultrasonic effects in metal melts.

In recent years, our research group has developed a new type of L-shaped ultrasonic rod which uses a nano ceramic radiator to avoid the ultrasonic transducer being directly affected by high-temperature heat radiation and resist the erosion of metal melt, which can meet the requirements of long-term continuous work and is conducive to industrial application. Shi et al. [28] applied the L-shaped ultrasonic device to the solidification process of a large 2A14 aluminum alloy ingot (ϕ830 mm × 6000 mm). The study showed that it can significantly refine the grains of the large ingot, effectively decrease the degree of solute segregation, and improve its mechanical properties. The mechanical vibration generated by the ultrasonic transducer was conducted by an L-shaped ultrasonic rod, which formed an ultrasonic bending vibration at the head of the ceramic tool and led to metal melt. Based on the industrial test, this paper studies the influence of ultrasonic bending vibrations on the microstructure and properties of a 1060 aluminum alloy strip in the casting and rolling process and explores the ultrasonic-assisted refining effect of Al-Ti-B refiner in different usage conditions.
