*2.1. Experimental Setup and Procedure*

The excellent castability and ductility, combined with a reduced tendency to defect formation, have been crucial factors in increasing the application of AlSi7Mg based alloys in the production of structural castings cast by sand molding [24,25]. The mechanical properties of such castings can be improved by solubilizing and aging treatments, providing uniform distribution of Mg and Si precipitates in the aluminum matrix [26,27].

The melt charges used in this work were prepared from an AlSi7Mg0.3 alloy, with the composition presented in Table 1.


**Table 1.** Chemical composition of standard AlSi7Mg0.3 alloy.

Melting charges weighing 10 kg were previously cut from a primary melt commercial ingot and later washed and dried to eliminate the cutting lubricant. After melting and overheating at the pre-established testing temperature (720 ± 5 ◦C), the melt was kept isothermal for a 30 min period for better homogenization. After this period, the melt was degassed for 5 min using ultrasound technology. The 5-minute period and the operational parameters were chosen according to works previously published by the authors [28]. After degassing, the melt was allowed to cool and was poured at 700 ± 5 ◦C into a sand mold, as shown in Figure 1. Immediately after pouring, the pre-heated acoustic radiator was positioned over the feeder with an immersion depth of 15 mm in the melt, to ensure the maximum transfer of acoustic energy to the melt from the thick zone (feeder) to the thin areas of the casting. Acoustic energy was supplied until the metal reached the solidus temperature +10 ◦C.

**Figure 1.** (**a**) Experimental setup: (1) sand mold, (1b) pouring basin, (1c) feeder, (2) acoustic radiator, (3) waveguide, (4) booster, (5) transducer 20 kHz; (**b**) Geometric model where V#1 to V#3 correspond to the positions for sample characterization (Note: mirrored symmetry).

Samples for microstructure characterization were taken from every cast sample by sectioning them perpendicularly to their vertical and horizontal axis (respectively, V#1 to V#3 and H#1 to H#3), according to Figure 1b. They were ground using 1200 μm SiC and polished down to 1 μm. An optical microscope (OM) (LEICA DM 2500M) and the ImageJ v1.46 computer application were used to determine the average grain size, *d*, and circularity, *Rn*, using Equations (1) and (2) where *A* corresponds to the area and *P* to the perimeter of the α-grains.

$$d = 2 \times \sqrt{\frac{A}{\pi}} \tag{1}$$

$$R\_{\rm ll} = \frac{4 \times \pi \times A}{P^2} \tag{2}$$
