**3. Results and Discussion**

Aluminum alloys are known for their high affinity to hydrogen at high temperature, promoting the appearance of numerous inclusions in the microstructure after solidification, with porosity being the most relevant and harmful defect compromising the strength of the component [4,30,31]. To assess the similarity of the porosity levels in samples, the melts were degassed before pouring at the desired temperature (700 ± 5 ◦C [31]), using the Multi-Mode Frequency, Modulated (MMM) ultrasonic technology, to ensure that the density of the bath was identical for every experiment. The average density reached in the experiments was 2.68 ± 0.1 (0.5% ± 0.07 porosity).

Figure 3 shows the variation of the α-Al grain size and circularity of AlSi7Mg0.3 alloy processed by ultrasound, with an average frequency of 19.9 ± 0.2 kHz and 40% of the system power (400 W—corresponding to approximately 60 μm), evaluated in a longitudinal section of the feeder (V#1 to V#3 samples, according to Figure 1). The respective microstructures are shown in Figure 4a–c.

**Figure 3.** Variation of the α-Al grain size and circularity with the distance to the acoustic radiator in the feeder.

**Figure 4.** Examples of microstructures of AlSi7Mg0.3 alloy processed by ultrasound, at 19.9 ± 0.2 kHz average frequency evaluated in a vertical section of the feeder: (**a**) V#1; (**b**) V#2 and (**c**) V#3 samples, according to Figure 1.

The experimental results suggest that the feeder (central zone of the feeder and directly below the acoustic radiator) presents a microstructure with a well-defined globular morphology. Overall, the variation of grain size and circularity apparent was minimal, as can confirmed by the standard deviations presented in Figure 3. Indeed, the average grain size and circularity for all characterized positions were, respectively, 120 μm and 0.8. As demonstrated by Puga et al. [32] and Eskin et al. [29] the zone below the flat acoustic radiator is able to promote extremely intense cavitation, being able to

promote nucleation and further to this, an increase in the quantity of α-Al grains, and consequently, the reduction of their diameter and a globular morphology.

Figure 5 shows the variation of the α-Al grain size and circularity of AlSi7Mg0.3 alloy processed by ultrasound, at the average frequency of 19.9 ± 0.2 kHz (resonance frequency at high temperature), with the distance to the acoustic radiator in the feeder (H#1 to H#3 samples, according to Figure 1). The respective microstructures are shown in Figure 6a–c.

**Figure 5.** Variation of the α-Al grain size and circularity with the distance to the acoustic radiator in the feeder.

**Figure 6.** Examples of microstructures of AlSi7Mg0.3 alloy processed by ultrasound, at 19.9 ± 0.2 kHz average frequency evaluated in a horizontal section of the feeder: (**a**) H#1, (**b**) H#2 and (**c**) H#3 samples, according to Figure 1.

The results presented suggest that with increasing distance to the acoustic radiator the α-Al grain size tends to increase, and the grain circularity tends to decrease. Although the grain size tends to increase, the matrix continues to present a quasi-globular morphology with some indications of rosette grains. This is an opposite tendency compared to what happens with the traditional processes of sand casting, where the matrix tends to present a dendritic morphology, even after chemical melt treatments.

Furthermore, the results presented in Figures 5 and 6 suggest that the cooling rate influences the grain morphology. Although the effect of ultrasound promotes a globular matrix, the fast cooling rates in thinner sections (e.g., position H#3 with a section solidification module 0.41) tends to overlap the effect of ultrasound. However, this effect also allows the formation of a thinner but non-dendritic microstructure, which is beneficial for the mechanical properties. However, in sections directly affected by the acoustic radiator (e.g., feeder—Figures 3 and 4), it is suggested that the morphology of the Al matrix (grain size and circularity) may be directly affected by the cavitation mechanism.

Contrary to the traditional process of refinement, where the addition of the master alloy Al-Ti-B is generally used, in the present approach this issue is not considered. Overall, the applied acoustic intensity is sufficient for: (i) converting the displacement of piezoelectric into kinetic energy in the liquid; (ii) creating acoustic cavitation and acoustic streams able to promote the nucleation and homogenization in the melt; (iii) increasing the temperature of the medium due to bubble collapse, which can promote and accelerate the α-Al grains.

A numerical simulation study was performed to validate the aforementioned hypothesis and experimental results. Figure 7 presents the numerical results of the axial displacement (evaluated in solid parts and the acoustic pressure in the melt), for the whole system, when the acoustic radiator was immersed in water at a depth of 15 mm, according to the boundary conditions represented in Figure 2.

**Figure 7.** Numerical results of the solid displacement and acoustic pressure obtained for the ultrasonic system apparatus.

According to the eigenfrequency results of the acoustic radiator, the first longitudinal compression mode was located at 20.23 kHz. This matches well with the operation frequency of the transducer, as can be proved by the experimental electrical impedance results measured by the authors. Thus, in order to numerically quantify the acoustic pressure in the medium, a domain frequency study was performed at *f* <sup>0</sup> = 20.20 kHz. Figure 8a,b presents the numerical values of acoustic pressure along the horizontal section with the distance to the acoustic radiator in the feeder (H#1 to H#2) and the vertical component beneath the feeder (V#1 to V#3).

**Figure 8.** Numerical results of the acoustic pressure obtained in the (**a**) vertical and (**b**) horizontal directions.

As can be observed in Figure 8a, a sinusoidal curve with a positive and negative acoustic pressure of at least 5 MPa in the vertical component is reached in the melt. According to Eskin's work [29] the threshold of acoustic pressure to verify cavitation is around 2 MPa. Indeed, according to experimental results for parameters numerically evaluated, the level of cavitation and acoustic streams at 40% system power (using the same acoustic radiator that numerically simulated) is totally defined, as can be observed in Figure 9. It may be clearly observed that in corresponding conditions (i.e., similar fluid and boundary conditions) that are initially resting (Figure 9a), it is possible to promote streaming and cavitation effects by the use of the detailed ultrasonic system.

**Figure 9.** Photograph of resonance cavitation field in the experimental container (400 W): (**a**) No-US, (**b**) with US activated.

Contrary to high values of acoustic pressure measured along the longitudinal section of the feeder (V#1 to V#3), in the horizontal cylindrical section (H#1 to H#2) the maximum acoustic pressure registered was 1 MPa (Figure 8b). Considering the high level of cavitation and acoustic streams created below the flat surface of the acoustic radiator, as well as the long interval time of solidification, it is suggested that a mixing and distribution of nuclei can travel along the cylindrical shape and contribute to the refining of Al grain reported in results of Figures 5 and 6. Furthermore, according to Figure 10, there is a correlation between the acoustic pressure and the grain size. It is shown that as the pressure increases, the grain size tends to decrease by an exponential decay function.

**Figure 10.** Effect of acoustic pressure versus grain size.

It is suggested that there is a threshold for the acoustic pressure grain refinement effect. In these particular conditions, it is apparent that for acoustic pressures higher than 2 MPa there are no significant benefits in terms of grain refinement.

## **4. Conclusions**

This study explored the influence of acoustic pressure in the overall refinement of sand cast aluminum alloys, using experimental and numerical approaches to detail the overall pressure distribution and distance to acoustic excitation. According to the results, the following conclusions may be drawn:

(1) The mechanism of refinement/modification of the α-Al matrix is a consequence of the acoustic activation caused in the liquid metal directly below the face of the acoustic radiator and is able to be distributed by different mold cavity area branches.

(2) In areas near the feeder there is a clear homogeneity in the morphology of the α-Al with respect to grain size and grain circularity. That is, the influence of the acoustic radiator in the liquid medium immediately below its top flat face is evident.

(3) In areas that are more distant to the feeder, the acoustic pressure directly caused by the acoustic radiator tends not to induce significant alteration in the grain size due to the lower pressure; however, this is compensated for by the higher cooling rates.

(4) Knowledge of the acoustic pressure profile, together with the analysis of the positioning of the acoustic radiator for refinement/modification the α-Al matrix validated through the use of numerical models, will allow high integrity castings to be obtained, with a tendency towards increased mechanical properties when compared to traditional treatment methods.

**Author Contributions:** Conceptualization, V.H.C. and H.P.; Methodology, V.H.P. and H.P.; Validation, H.P. and J.B.; Formal analysis, V.H.C. and H.P.; Resources, H.P.; Data curation, V.H.P. and J.B.; Writing—original draft preparation, V.H.P. and H.P.

**Funding:** This research received no external funding.

**Acknowledgments:** This work was supported by FCT funding through the project PTDC/EMEEME/30967/2017 (NORTE-0145-FEDER-030967) and by FEDER funding through COMPETE 2020, NORTE2020, PORTUGAL2020 – Programa Operacional Competitividade e Internacionalização (POCI). Additionally, this work was supported by FCT with the reference project UID/EEA/04436/2019.

**Conflicts of Interest:** The authors declare no conflicts of interest.
