Grain-refining mechanism in hypereutectic Al-20Si alloy with minor Sr - Sc - La and ultrasonic vibration treatment

Abstract

The grain-refining mechanism with minor Sr - Sc - La and ultrasonic vibration treatment (UVT) in the hypereutectic Al-20Si alloy were studied. The results demonstrated that the microstructure of the hypereutectic Al-20Si alloy could be refined significantly, further improve its mechanical properties. The desirable refinement of the microstructure was achieved using 0.2% Sr, 0.15% Sc, and 0.3% La under UVT, achieving the highest grain circularity coefficient, hardness, elongation, and area reduction. The tensile strength was the largest with the addition of 0.2% La. The findings of this study provide theoretical and experimental guidelines for the fabrication of structural materials for application in automotive, aerospace, and deep-sea equipment.

1. Introduction

Al-Si alloys are extensively serviced in automobile, spacecraft, aviation fields, and deep-sea equipment, due to their small thermal expansion coefficient, good thermal stability, wear and corrosion resistance, and low density [1,2]. However, inordinance massive and plate-like primary Si, coarse needle and lamellar eutectic Si, and dendritic α-Al phase occur in the as-cast microstructure of the hypereutectic Al-20Si alloys, which alter the α-Al matrix's continuity and compromise the alloy's mechanical properties and workability, making it hard to broaden its application. Therefore, it is necessary to carry out the metamorphic refinement treatment [[3], [4], [5]]. Currently, Sr has been demonstrated to be a successful metamorphic agent in Al-Si alloys, with its primary Si or eutectic Si being particularly affected [[6], [7], [8]]. Recently, Sc has been found to be a good modifier for Al-Si alloys. The Al-Sc dual-phase diagram reveals that the incorporation of Sc into Al alloys during solidification leads to the production of fine Al₃Sc particles through eutectic reaction, which encourages heterogeneous nucleation and grain refinement, thus leading to fine grain strengthening and precipitation strengthening, thereby enhancing the alloy's mechanical properties [9,10]. La is a silvery-white reactive metal that can react directly with C, N, B, Se, Si, etc. Ding et al. research [11] showed that the incorporation of Al-3Ti-4.35La could decrease the nucleation temperature of eutectic Si in Al-7Si, hasten the eutectic reaction time, impede its growth, and modify its form. Jiang et al. [12] found that the eutectic Si was modified its morphology from strip to fiber or granular, and its average length is about 1.04 μm by adding 0.5% Mg-15La master alloy in the Al-11Si-2.5Cu-0.8Fe alloy. At the same time, it was observed that a large number of La containing nanoclusters in the metastable microstructure would become effective heterogeneous nucleation points for eutectic Si due to their low matching degree with the matrix eutectic Si. The mechanical properties of the alloy were significantly improved.

Similarly, Ultrasonic field treatment caused the microstructure of Al-Si alloy to transform from a coarse dendritic to a fine equiaxed crystal [13]. H. Puga et al. [14] discovered that the introduction of UVT to Al-Si-Cu alloys occurred the α-Al phase to spheroidize, the eutectic Si phase to become point-like fragments, porosity to decrease, and tensile strength and elongation to increase significantly.

However, there are few studies on the modification behavior of hypereutectic Al-20Si alloy when minor Sr-Sc-La and UVT are combined. In this paper, the effects of the same Sr-Sc with varying La mass fractions and UVT on the microstructure and mechanical properties of hypereutectic Al-20Si alloy were explored. The grain-refining mechanism of UVT-driven Sr-Sc-La nanoparticles movement in hypereutectic Al-20Si alloy was simulated and discussed. In order to find the optimal configuration process of minor Sr - Sc - La and UVT and provide theoretical and experimental reference for the manufacture of structural materials with high abrasive wear and tear in the fields of automotive industry, aerospace and deep-sea equipment.

2. Experiment results

2.1. Mechanical properties

Fig. 1, Fig. 2 show the effect of the same 0.2% Sr - 0.15% Sc and different La additions on the mechanical properties of the Al-20Si alloy under UVT. The hardness (HD) and reduction of area (RDA) increase with increasing La

Fig. 1.

Effect of the minor Sr – Sc - La and UVT on the HD and RDA of the Al-20Si alloy.

Fig. 2.

Effect of the minor Sr-Sc-La and UVT on the tensile properties of the Al-20Si alloy.

content and then decrease as shown in Fig. 1. The maximum HD and RDA were obtained for the alloy containing 0.3% La. Compared with the absence of La and UVT, the HD increases from 64.2 HBV (Sr - Sc) and 64.8 HBV (Sr – Sc - UVT) to 66.6 HBV and 68.8 HBV, i.e., 3.74% and 6.17%, respectively; and the RDA increased from 0.05% (Sr - Sc) and 0.08% (Sr - Sc - UVT) to 0.17 and 0.34, i.e., an increase of 340% and 420%, respectively, compared with no addition of La and UVT. However, the HD and RDA decrease when 0.4% of La is added.

Fig. 2 illustrates that the strength augments with the augmentation of La content, then diminishes. The alloy with 0.2% La attained its peak strength. The results increased from 142.78 MPa (Sr - Sc) and 148.48 MPa (Sr - Sc - UVT) to 171.67 MPa and 173.26 MPa, i.e.,20.23% and 16.69% increase, respectively, compared with no La addition and UVT. The maximum elongation (EL) was obtained for the alloy containing 0.3% La. Compared to no addition of La and UVT, the results increased from 0.49% (Sr - Sc) and 0.76% (Sr - Sc - UVT) to 1.15% and 2.58%, i.e., an increase of 134.69% and 239.47%, respectively.

2.2. Microstructure analysis

Fig. 3 shows the microstructure of the Al-20Si alloy for the same 0.2% Sr −0.15% Sc and different La additions under UVT or not. Fig. 3 (a) displays the primary Si and eutectic Si phases without Sr - Sc - La and UVT addition. The primary Si is plate-like or massive form, and eutectic Si is coarse needle and lamellar form. The primary α-Al phase is coarse dendritic and the secondary dendritic is mostly rose-like with many unbalanced eutectic structures observed at grain boundaries. The primary α-Al phase underwent a marked transformation in morphology and size with the addition of Sr, Sc and La, with a marked rise in the number of primary dendritic grains, a decrease in the spacing of secondary dendritic grains, a gradual spheroidization of the α-Al phase, and a finer, more uniform distribution of primary Si and eutectic Si phases. This was further evidenced by the addition of 0.2% Sr - 0.15% Sc in Fig. 3 (b) marked transformation in the morphology and size of the primary Si, eutectic Si, and α-Al phases was observed; the amount of primary dendritic grains augmented significantly, the spacing of secondary dendritic grains diminished, and the growth of the dendritic grains slowed. The α-Al phase had a tendency to spheroidize gradually, and the primary Si and eutectic Si phases had a more precise and

Fig. 3.

Microstructure of the Al-20Si alloys with different minor amounts of Sr–Sc–La and UVT. (a) Sr-Sc-La-free, (b) 0.2%Sr-0.15Sc, (c) 0.2%Sr-0.15Sc-0.1%La, (d) 0.2%Sr-0.15Sc-0.1%La-UVT, (e) 0.2%Sr-0.15Sc-0.3%La, (f) 0.2%Sr-0.15Sc-0.3%La-UVT, (g) 0.2%Sr-0.15Sc-0.4%La, and (h) 0.2%Sr-0.15Sc-0.4%La-UVT.

evenly distributed microstructure than the unaltered alloys. Fig. 3 (d) and (c) demonstrate that the addition of 0.2%Sr-0.15Sc-0.1%La and UVT or not yields a marked spheroidization and roundedness of α-Al phase grains, as well as a notable refinement of the primary Si, eutectic Si phase, and intermediate compound phase grains. The α-Al phase grains in the samples, after 0.3% La and UVT or not, are fully spheroidized, and the primary Si, eutectic Si phase, intermediate compound phase grains are finer and more uniformly distributed as shown in Fig. 3 (f) and (e). There are visible second phases and intra crystal second phases between the grains, indicating the most suitable grain refinement and microstructure. Furthermore, Fig. 3 (h) and (g) displays the effects of 0.4% La and UVT or not that the α-Al phase starts to polarize, the grain boundaries become wider, and the second phase can be seen, while the eutectic silicon phase and the intermediate compound phase appear to lap, swell and agglomerate, and the grain size gradually increases.

Fig. 4 (a) and (b) reveals a significant refinement and spheroidization of primary Si and eutectic Si by SEM the Al-20Si alloy with addition 0.3%La and UVT, and the distribution is more evenly distributed in the α-Al phase, as well as the discovery of secondary Si particles and intermetallic compounds. Fig. 4 (c) and (d) show that the EDS of the point in Fig. 4 (b), and results that Sr, Sc and La have been deeply integrated into Al-20Si alloy.

Fig. 4.

SEM of the microstructure of the Al-20Si alloys with 0.2%Sr-0.15%Sc-0.3%La and UVT. (a) SEM of the sample morphology. (b) SEM of the EDS point used for analysis. (c) and (d) EDS results.

Fig. 5 reveals the results of Image-Pro Plus 6.0 analysis, which was conducted to evaluate the influence of grain refinement and spheroidization of the primary α-Al, primary Si, eutectic Si, and other phases of Al-20Si alloys, the grain size and circularity of the microstructure of Al-20Si alloys with the same Sr - Sc and different La additions were quantified under UVT or not. The circularity of the grains first increased and then decreased with increasing La. When 0.3% La was added, the maximum circularity was 0.751 and 0.769, respectively, as shown in Fig. 3 (e) and (f). Image-Pro Plus 6.0 was used to analyze and detect Fig. 3 (e) and (f). The remarkable grain refinement effect was evident, with the minimum grain size being approximately 11.45 μm and the average grain size being the least, around 14 μm.

Fig. 5.

Effect of minor amounts of Sr–Sc–La and UVT on the circularity coefficient of the Al-20Si alloy grains.

2.3. Micro-alloyed eutectic Si

Fig. 6 shows TEM and SEM analysis of the eutectic Si phase in Al-20Si with the adding 0.2% Sr - 0.15% Sc - 0.3% La - UVT. The α-Al and eutectic Si phases are observed in Fig. 6 (a); Fig. 6 (b) shows small massive, acicular, granular and fibrous particles (Al (Si Sc La), Al3(Si, Sc), Al (Si Sr)); which are smaller and more spherical. The coarse and lamellar eutectic Si phases are greatly refined while its morphology become granular, short rods and fibrous [[15], [16], [17]]. Ding et al. [11] investigated TEM analysis twins on the eutectic Si in Al-7Si and discovered that the distribution of metamorphic elements

Fig. 6.

TEM and SEM images, and the corresponding SAED patterns, of the internal structure in the eutectic Si phase in the as-cast Al-20Si alloy with the addition of 0.2%Sr-0.15%Sc-0.3%La-UVT: (a) and (b) The bright-field (BF)-TEM image of the eutectic Si phase with a higher magnification; (c) and (d) BF-TEM image of the eutectic Si phase, indicating a high density of nanoprecipitates and low density of twins in the eutectic Si phase; (e) the corresponding SAED pattern of the samples in (c) and (f).

coincided with the location of the twins. Fig. 6 (d) displays the magnified, high-resolution crystal lattice fringes of the twins in eutectic Si, which were previously seen in Fig. 6 (c). The results indicate that the twins and nano-particles in eutectic Si become significantly more pronounced with the inclusion of Sr-Sc-La, and their distribution is relatively uniform. Combined with the SAED patterns in Fig. 6 (e) displays a <112> Si type of twins and nano-particles. The growth of eutectic Si along the <112> directions on the {111} Si planes is anisotropic, thus decreasing interfacial energy [18,19]. Fig. 6 (c) displays the dispersed grains, with a horseshoe shape and a size of around 20 nm, which were evenly distributed in the Almatrix. The diffraction pattern in Fig. 6 (e) further demonstrates the good coherence between the diffusion particles and the Al matrix. A powerful strengthening effect was noticed in the particle composition, which was composed of Al (Si Sr), Al (Si Sc La) and Al3(Si, Sc). This indicates that the modified elements gather near the twin boundaries and are easily adsorbed on the front edge of the growth interface of Si crystals, intercepting the previous growth direction of eutectic Si,

and accelerating the growth of the number of twins [11]. Nano-cluster particles of high density emerge within the eutectic Si, some of which are spread across the twinned strip. These particles can also act as heterogeneous nucleation sites of eutectic Si during solidification, and even more can impede the growth of eutectic Si to achieve a refinement strengthening effect, as shown in Fig. 6 (b) and (d). To clarify the overall distribution of the Al-10Sr and Al-2Sc and Al-10La intermediate alloy in the hypoeutectic Al-20Si alloy, the SEM EDS of line scanning pattern analysis of Line was used to observe element distributions, as shown in Figs. 6 (f) and Fig. 7(a–e). From the maps, the Sr-Sc-La are uniformly dissolved in the Al matrix, evidencing that the Al-10Sr and Al-2Sc and Al-10La intermediate alloy are dissolved. At the eutectic Si phase surface, Sr-Sc-La elements are adsorbed, and the eutectic Si production orientation is altered to refine grains. The new phases contain Al, Si and Sr-Sc-La elements, confirming that the Al3(Si, Sc), Al (Si Sr), Al2Si2La, and Al (Si Sc La) phase formed. When the content of La is more than 0.4%, a large number of intermediate compounds based on Al-Si-La are generated during the alloy's solidification. These compounds exist in primary Si, eutectic Si, and α-Al or between their dendrites can easily become a source of cracks, reducing alloy's plasticity. Consequently, the alloy's mechanical properties are detrimentally affected by an overabundance of La [19]. The results are consistent with that in Fig. 1, Fig. 2, Fig. 3, and 5.

Fig. 7.

SEM of the cast Al-20Si alloy with the addition of 0.2%Sr-0.15%Sc-0.3%La-UVT: SEM EDS of the sample and mapping of Al, Si, Sr, Sc, and La (a–e).

3. Simulation verification

Verifying the refinement and strengthening of the microstructure of Al alloy melt, due to the high temperature and opacity of the alloy melt during solidification, requires direct experimental evidence to be obtained from ultrasonic cavitation and acoustic flow effects of UVT. In this paper, ANSYS FLUENT19 was used to simulate the direct UVT of Al-20Si melt and the crystallization process of adding minor nano Sr, Sc and La particles, and to explore the grain-refining mechanism of adding UVT and minor Sr-Sc-La on the microstructure of Al-20Si alloy.

ANSYS FLUENT19 was used to build the 3D model for the conventional calculations. In order to reduce the computational load, the geometric model of the crucible in this paper is reduced according to the actual geometric model. The crucible is conical: the bottom diameter is 100 mm, the opening diameter is 160 mm, the height is 200 mm, the ultrasonic tool rod diameter is 30 mm, and the direct depth into the melt is 20 mm.

To facilitate the calculation, the added elemental particles Sr, Sc and La are assumed to be nanospheres, and the forces on the particles in the crucible are mainly the impact of acoustic flow caused by UVT, centrifugal forces, gravity and resistance, and the blasting force caused by cavitation, etc. Equations of motion in Lagrangian coordinates are as follows:

$${\mathrm{m}}_{\mathrm{p}}\frac{\mathrm{d}{\mathrm{u}}_{\mathrm{p}}}{\mathrm{d}\mathrm{t}}={\mathrm{m}}_{\mathrm{g}}\frac{1}{{\tau }_{\mathrm{p}}}{\mathrm{C}}_{\mathrm{D}}\frac{\mathrm{R}{\mathrm{e}}_{\mathrm{p}}}{24}\left(\mathrm{u}-{\mathrm{u}}_{\mathrm{p}}\right)+\frac{{\rho }_{\mathrm{p}}\pi {\mathrm{d}}^3{\mathrm{u}}_{\mathrm{p}}^2}{6\mathrm{r}}$$ (1)

$${\mathrm{m}}_{\mathrm{p}}\frac{\mathrm{d}{\mathrm{v}}_{\mathrm{p}}}{\mathrm{d}\mathrm{t}}={\mathrm{m}}_{\mathrm{g}}\frac{1}{{\tau }_{\mathrm{p}}}{\mathrm{C}}_{\mathrm{D}}\frac{\mathrm{R}{\mathrm{e}}_{\mathrm{p}}}{24}\left(\mathrm{v}-{\mathrm{v}}_{\mathrm{p}}\right)+\frac{{\rho }_{\mathrm{p}}\pi {\mathrm{d}}^3{\mathrm{v}}_{\mathrm{p}}^2}{6\mathrm{r}}$$ (2)

$${\mathrm{m}}_{\mathrm{p}}\frac{\mathrm{d}{\mathrm{w}}_{\mathrm{p}}}{\mathrm{d}\mathrm{t}}={\mathrm{m}}_{\mathrm{g}}\frac{1}{{\tau }_{\mathrm{p}}}{\mathrm{C}}_{\mathrm{D}}\frac{\mathrm{R}{\mathrm{e}}_{\mathrm{p}}}{24}\left(\mathrm{w}-{\mathrm{w}}_{\mathrm{p}}\right)+\frac{{\rho }_{\mathrm{p}}\pi {\mathrm{d}}^3{\mathrm{w}}_{\mathrm{p}}^2}{6\mathrm{r}}$$ (3)

Among $\frac{\mathrm{d}\mathrm{x}}{\mathrm{d}\mathrm{t}}={\mathrm{u}}_{\mathrm{p}}$, $\frac{\mathrm{d}\mathrm{y}}{\mathrm{d}\mathrm{t}}={\mathrm{v}}_{\mathrm{p}}$, $\frac{\mathrm{d}\mathrm{z}}{\mathrm{d}\mathrm{t}}={\mathrm{w}}_{\mathrm{p}}\text{;}$ the relaxation time of particles ${\tau }_{\mathrm{p}}=\frac{{\rho }_{\mathrm{p}}{\mathrm{d}}_{\mathrm{p}}^2}{18\mu }$ , the Particle Reynolds number $\mathrm{R}{\mathrm{e}}_{\mathrm{p}}=\frac{{\rho }_{\mathrm{p}}{\mathrm{d}}_{\mathrm{p}}\left|\mathrm{u}-{\mathrm{u}}_{\mathrm{p}}\right|}{\mu }$ , m_p the mass of particle，d_p the diameter of particle，ρp the density of particles，up the velocity of particles，u the gas velocity，C_D the resistance coefficient，and μ fluid viscosity coefficient [20,21]。

Fig. 8 shows the results of the finite element simulation of the ultrasonic field-fluid and solid coupling. Fig. 8 (a) and (b) display the trajectory of the metal particles under UVT. The Al-Si alloy melt near the bottom of the tool head has a good stirring effect and a short residence time, indicating that the UVT has a good effect on the dispersion of Sr, Sc and La in the Al-Si alloy melt. Fig. 8 (c) and (d) show the flow field distribution characteristics of the Al alloy melt under UVT, namely velocity flow and pressure flow, respectively. The Al alloy melt is subjected to an intense impact flow and turbulent kinetic energy when the ultrasonic field is applied to its end face, reaching a maximum velocity of 0.54 m/s [22]. This velocity is highly beneficial in facilitating heat and mass transfer, as well as in improving the segregation of melt elements, accelerating the melting of secondary dendrite roots, and refining the grains. According to literature studies, UVT produces cavitation effects in the melt. The rupture of the cavitation bubble produced a peak pressure of 2.8 GPa in the current trials [23]. Under such strong cavitation pressure, the nucleation rate of the melt can be improved and the dendrites can be broken rapidly to achieve the effect of grain refinement [1].

Fig. 8.

Contours of the particle traces, pressure field, and velocity field in the Al-20Si melts by UVT. (a) and (b) Particle traces, (c) velocity streamline, and (d) press volume rendering.

4. Discussion

4.1. Grain-refining mechanism and effect in hypereutectic Al-20Si alloy

When Al-Si alloy is modified by Sr, the formation of Al (Si Sr) nanoclusters at the Si/liquid interface is more likely to alter the growth of Si, thereby refine the eutectic Si [15]. Rare earth Sc is more likely to cooperate with Al to produce nano-Al₃Sc particles as the heterogeneous nuclei, effectively refine α-Al [16]. With the addition of La, La is more likely to act as a surfactant to refine α-Al grains, and eutectic Si is modified by the formation of multiple Si twins [24]. In Al-Si alloys without any modifiers, Si crystals always grow faster in the <211> direction and slower in the <111> direction, resulting in anisotropic growth of eutectic Si. However, the growth mode of the Si phase changes after the addition of Sr – Sc - La elements to the alloy, resulting in a typical modified structure. Firstly, the eutectic Si in hypereutectic Al-20Si alloys is often grown from polyhedral primary Si without the need for separate nucleation, that is, then eutectic Si is grown dependent on the primary Si. The eutectic Si is grown from the primary Si, extends into the eutectic liquid, and is then mixed with α-Al phase symbiotic growth. The growth twins are easily formed due to the dislocation of atoms during crystal growth under non equilibrium conditions. The driving force for the formation of twins is the supercooling of the liquid at the front of the interface and the presence of Sr - Sc - La impurities, which lead to a decrease in the degree of crystal supercooling. However, Sr - Sc - La elements are very easily adsorbed on the Si crystal growth interface, causing it to generate multiple twins, inhibiting the preferred growth orientation of eutectic Si. In addition, Sr - Sc - La elements are also easily distributed at the ends and edges of the eutectic Si and primary Si, especially at the intersection of Si phase branching and turning points. When the concentration of Sr – Sc - La elements increases or the phase segregation of Sr – Sc - La elements promotes the necking of the Si phase here, pelletization of Si phase is achieved [[25], [26], [27], [28]], as shown in Fig. 3 (e) and (f), and Fig. 4, Fig. 6. Therefore, the synergistic modification mechanism of Sr - Sc - La elements plays a refinement and modification role by altering the growth mode of the eutectic Si during solidification.

4.2. The refinement mechanism by adding UVT

Ultrasonic cavitation effect and ultrasonic sound flow effect will be generated when UVT is introduced into Al-20Si alloy melt, as shown in Fig. 8. The ultrasonic cavitation wave effect, which activates inert solute particles into active crystal nuclei, boosts the nucleation core and accelerates the rate of nucleation; additionally, the collapse of the cavitation bubble produces a high-pressure micro-shock wave, resulting in a great number of nucleation cores, both of which are already refined α-Al phase and eutectic Si. The expansion of the cavitation bubble, with its micro-subcooling, boosts the supercooling of the solution, intensifies the solidification and crystallization force, and fortifies the nucleation of the solution; therefore, the melt grains are refined [29], as shown in Fig. 3 (e) and (f), and Fig. 4, Fig. 6. Ultrasonic acoustic flow effect: Ultrasonic acoustic flow is a combination of circulation and turbulence, which can cause severe vibration and stirring effects on metal melts, encourages the dispersion of Sr, Sc, and La particles, which are taken in by the roots or biconcave valleys of phases like Si and Al-Si, thus altering the grain orientation, hindering the sliding of dislocations, and further enhancing grain refinement [30].

5. Material and methods

Al-20Si, Al-10Sr, Al-2Sc, and Al-10La intermediate alloys were used as test materials, with the mass fractions of La equal to Sr–Sc, Sr (0.2%), and Sc (0.15%), respectively, using different La mass fractions of 0.1%, 0.2%, 0.3%, 0.4%, and 0.5%. The Al-20Si alloy was melted at 750 °C, and Sr - Sc - La were added according to the weight ratio after slag removal before a heat preservation 10 min. An ultrasonic metal melt apparatus, boasting a peak power of 1000 W and a frequency of 20 kHz, was employed. Its primary components were an ultrasonic generator, a transducer, and a guide rod (horn and tool rod). Its operation was based on a piezoelectric transducer that converted electrical signals of a given power and frequency into mechanical vibrations of the same frequency. A horn amplifies the magnitude of this vibration, and the ultrasonic vibration is then injected into the metal melt via a tool rod for ultrasonic processing. The melt was subjected to ultrasonic vibration for 100 s after the tool rod was inserted 20 mm into the melt. After the UVT, the melt was injected into a steel mold and preheated to 350 °C. After cooling to room temperature, the standard casting samples, measuring ø15 mm × 180 mm, were shifted out of the mold. The samples were then subjected to the mechanical property testing standards, and tested for tensile strength, EL, RDA, and HD. The tensile strength, EL, RDA were carried out through a microcomputer control electronic universal testing machine（WDW-100C）using a 0.5 mm/min elongation rate. HD was measured by digital micrograph Vickers hardness tester (HVS-1000), in which the test force was 0.1Kgf (0.98 N) and the test force holding time was 10s, indenter material was diamond. Using a Leica DMIL LED metallurgical microscope, EVO MAI10 ZEISS scanning electron microscopy, and EDS energy dispersive spectroscopy, the sample's microstructure was observed. The nanostructure was then obtained via a Tecnai G2FA20 Transmission Electron Microscope. Precision Ion Polishing System was used to prepare the TEM specimens.

6. Conclusions

• (1)The synergistic effect of adding minor Sr, Sc, and La with UVT enabled Al-20Si alloy microstructures to be refined. When 0.3%La and 0.3%La - UVT were added, the best grain refinement and spheroidization of microstructure was achieved. The average grain size was the smallest (about 14 μm), and the circularity coefficient was the largest 0.751 and 0.769, respectively.
• (2)Studying the effect of the La content on mechanical properties of Al-20Si alloy, we found that the maximum EL of 1.15% and 2.58%, the maximum RDA of 0.17% and 0.34%, and the highest HD of 66.6 HBV and 68.8 HBV, are reached at 0.3%La and 0.3%La - UVT, respectively. The largest tensile strengths of 171.67 MPa and 173.26 MPa are obtained at 0.2%La and 0.2%La - UVT, respectively.
• (3)Simulation results show that ultrasonic cavitation and flow induced by UVT accelerate the nucleation activity of Sr, Sc, and La, and promote the adhesion of the new-formed particles to the root of the α-Al and eutectic Si phases and in the twin concave valley of dendrites. The resulting modifications of the alloy microstructure promote the refinement of grains, thus, improving the mechanical properties of the Al–20Si alloy.

Thus, this study presents the grain-refining mechanism of UVT-driven Sr–Sc–La nanoparticles in the fabrication of Al-20Si alloys and the optimal process conditions. Moreover, the findings of this study provide theoretical and experimental guidelines for the fabrication of structural materials with high abrasive wear and tear for application in automotive, aerospace, and deep-sea equipment.

Author contribution statement

Jiping Lei: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Bowen Lei: Contributed reagents, materials, analysis tools or data; Wrote the paper.

Kai Zhang: Conceived and designed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data.

Guanlin Liu: Yuanyuan Liu: Performed the experiments.

Data availability statement

Data will be made available on request.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.