**1. Introduction**

A 2A14 aluminum alloy is a typical Al-Cu-Mg-Si alloy. Because of its high copper content, it has high strength and belongs to high strength duralumin [1–3]. It has good heat resistance, forging and welding properties, and it can be manufactured into free forgings, and die forgings with complex shapes, which are widely used in the field of aerospace [4–8]. Direct chill semi-continuous casting is one of the most effective technologies to produce large aluminum alloy ingots [9]. Due to the uneven distribution of temperature field and flow field in the solidification areas during casting, there are many problems in the large-scale aluminum alloy ingots, produced by traditional casting methods, which can easily cause an uneven distribution of solidification structure and solute composition, and defects, such as coarse grain, composition segregation, hot cracking, and shrinkage cavities. In particular, macro-segregation directly affects the quality of ingots and the subsequent finished products ratio of forging, and increases the material processing loss and production cost [10–13].

At present, applying appropriate ultrasonic vibration in the process of metal solidification is a good method to reducing casting defects, obtaining good structure and improving mechanical properties of materials [14–17]. Abramov V et al. [18] studied the effect of the ultrasonic treatment of the water-cooled transducer on the microstructure and properties of different industrial aluminum based alloy. The effect of ultrasonic treatment on the microstructure of as-cast alloy can be summarized as follows: Reduction of mean grain size, variation of phase distribution, and better material homogeneity and segregation control. Eskin et al. [19] introduced ultrasonics into the semi-continuous casting of aluminum alloy, produced AA2324 aluminum alloy round ingot with a diameter of 1200 mm. It was found that the grain after ultrasonic treatment was generally refined, and the solidification structure was non-dendrite. Li Xiaoqian et al. [20–23] studied the principle and mechanism of ultrasonic treatment with the straight-rod wave guide rod, analyzed the influence of ultrasonic power, frequency, and insertion depth of ultrasonic rods on the quality of large-diameter ingot, carried out a large number of casting experiments, and successfully obtained 2XXX and 7XXX ultrasonic large-diameter aluminum alloy ingots of various sizes.

However, there are some problems in the ultrasonic treatment process, which hinder its wide application in metallurgical industry. The traditional ultrasonic radiation rod, made of titanium alloy, was seriously corroded in the molten metal, thereby, reducing its service life and polluting the aluminum melt [24–26]. Moreover, the ultrasonic transducer, connected with the traditional straight-rod titanium alloy wave guide rod, is directly above the high-temperature metal melt, and the melt produces direct high-temperature heat radiation to the ultrasonic transducer. This often leads to the detuning problem of an ultrasonic vibration system or even damage of the transducer during the casting process, so it is difficult to achieve long-term stable ultrasonic treatment [27,28]. In this work, to overcome these problems, a new L-shaped ceramic ultrasonic wave guide rod was designed to introduce ultrasonic bending vibration into 2A14 aluminum alloy melt. The differences between treatment by the L-shaped ultrasonic wave guide rod and treatment by the traditional straight-rod titanium alloy wave guide rod were compared from the aspects of macrostructure, micro-grain size and morphology, secondary phase characteristics and composition segregation. The purpose of this paper is to verify the effectiveness of applying the L-shaped ultrasonic wave guide rod on the preparation of homogeneous large-scale aluminum alloy ingots from the experimental point of view, and then to find out the optimal ultrasonic introduction process, in order to provide certain theoretical guidance for industrial production.
