**1. Introduction**

Aluminum alloys are thriving in the automotive, aeronautics and aerospace industries, displacing applications that have traditionally been occupied by other alloys [1]. The performance of a mechanical component is often conditioned and limited by the characteristics of the materials themselves, as well as by limitations of the manufacturing processes and, more specifically, by their microstructure. Although, their use is widespread, the casting of aluminum alloys is not an easy process, since they are prone to nucleate and grow coarse and dendritic microstructures [2,3]. Additionally, aluminum alloys are characterized by a high absorption of hydrogen during melting and casting [4,5]. Thus, the increasing use of aluminum components with superior mechanical and fatigue properties requires suitable and high-efficiency casting processes [6]. This includes the melt treatment [7] to develop suitable microstructures, eliminate inclusions, and reduce porosities and shrinkage defects, which are the main cause of failure in aluminum components [8].

In the industrial practice of aluminum casting, three melt treatment operations are usually carried out in addition to the removal of slag, all of them chemically-based and presenting significant environmental impacts: (i) degassing, by reducing the hydrogen content of the melt, which is achieved by gas purging using mainly inert gases [9,10]; (ii) microstructure refinement, by the addition of Al-Ti-B type master alloys in proportions adjusted to the specific aluminum alloy [11–13]; (iii) eutectic silicon modification, usually carried out by the addition of master alloys containing Sr [12,14]. Although these three stages (degassing, grain refinement, modification of eutectic silicon), are vital to improving the mechanical performance of castings, the need to find more efficient, viable and clean treatment alternatives has fostered the search for new melt treatment techniques and technologies.

In the last decade, we have witnessed the development of highly efficient aluminum melt treatment techniques based on the use of acoustic energy [15–19]. The influence of ultrasound on the refinement/modification of the microstructure is based on physical phenomena due to the high acoustic intensity propagated through the liquid metal [17]. Two mechanisms have been proposed to explain the refinement of microstructures by ultrasound, dendritic fragmentation and cavitation induced heterogeneous nucleation [20,21]. However, the mechanism of heterogeneous nucleation induced by cavitation seems to be the most valid hypothesis, being supported by the majority of researchers who have worked in this field [22]. However, in order for this technique to be efficient, it is imperative that the ultrasonic system is correctly designed according to the specific needs of the casting process. This enhances the overall success of the technique and optimizes the casting component integrity.

For that purpose, the present work aimed to study the interaction between the requirements imposed by the melt conditions (i.e., the melt temperature/volume) and the constraints imposed by the manufacturing process (geometry of casting) in the optimization of an ultrasonic system and its impact on the overall microstructures [23]. Furthermore, considering their physical processing, a numerical model was used to investigate the associated acoustic pressure fields developed in the transmission medium and their role in the grain refinement.
