Refining Al–20Si Alloy by Appending Trace Sr–Sc–Ce Nanoparticles and Ultrasonic Vibration Processing

In this study, Al–20Si alloy is modified by appending trace Sr–Sc–Ce nanoparticles and ultrasonic vibration processing (UVP). The consequences indicate that its microstructure is significantly refined and its mechanical characteristics are improved. The effect of the favorable refinements is achieved by appending 0.2% Sr–0.15% Sc–0.3% Ce and performing UVP. The primary Si is the minimum, the grain roundness coefficient and the strength of extension are the maximum, and the spheroidizing effect of α‐Al is the best. The elongation and hardness are the highest when 0.4% Ce is appended. The research consequences afford a reasonable process scheme for manufacturing high‐quality wear‐resistant material of the automotive and aerospace industries.


Introduction
Owing to its high strength, excellent casting performance, and low density, hypereutectic Al-Si alloy is the preferred functional material to manufacture lightweight components, especially in aerospace and deep-sea equipment. [1,2]However, the platelike and blocklike primary Si, elongated needle shape, and flaky shape eutectic Si in the foundry alloy alter the continuity of the Al base, deteriorate its mechanical characteristics, and limit its application.[8] Haghayeghi et al. [9] discovered kinks were in the Sr-modified hypereutectic A390 alloy on the {111} plane.A greater strain was caused because these kinks and dislocation stacks accumulated, blocking and altering the preferred development direction of Si atoms.Therefore, the primary Si phase was refined by appending Sr.
[12][13] As Sc is both a 3D transition metal and a rare-earth metal, it has chemical properties that are identical to those of rare-earth group metals, and its physical and mechanical characteristics are similar to those of the transition group elements.[16][17] Prukkanon et al. [18] found that eutectic Si is modified into fibrous and rounded shapes by appending 0.2% Sc in the Al6Si0.25Mgalloy.Further, appending Zr and Sc simultaneously achieved an enhanced modification effect.
Cerium (Ce) is a gray active rare-earth metal that has an excellent refining effect on eutectic Si. [19] Zhao et al. [20] studied the metamorphic casting of rare-earth Ce on Mg 2 Si/Al-Si-Cu, and found that the dendritic-like structure of the primary Mg2Si was altered to a polygonal-like structure, and its size decreased from 100 to 15 μm, after which it grew to 30 μm.The eutectic Mg2Si phase transformed from a flake shape to a chrysanthemum shape, and the eutectic Si was converted into a coral shape.
The wave effect generated by propagating ultrasonic vibration processing (UVP) in the metal melt forms ultrasonic cavitation and ultrasonic sound flow effects.[23] Feng et al. [24] introduced UVP into the Al-23Si alloy and found that it had a significant grain-refining effect on primary Si achieving a more uniform distribution and smooth grain-edge transition; the α-Al was transformed from the dendritic crystal to equiaxed grains.Few studies have focused on the synergistic metamorphism of the Al-20Si by appending trace Sr-Sc-Ce nanoparticles with UVP.
In this study, five experimental configurations of Sr-Sc-Ce with UVP were used to modify Al-20Si alloy.The sample was made, and tested the mechanical characteristics, and the experimental scheme was simulated by finite-element method.The microstructure of the sample was observed on a MJ3230 metallurgical microscope and a Regulus8230 high resolution cold field-emission scanning electron microscopy (SEM and energy-dispersive X-ray spectral [EDS] analysis), and DOI: 10.1002/pssa.202300617 In this study, Al-20Si alloy is modified by appending trace Sr-Sc-Ce nanoparticles and ultrasonic vibration processing (UVP).The consequences indicate that its microstructure is significantly refined and its mechanical characteristics are improved.The effect of the favorable refinements is achieved by appending 0.2% Sr-0.15%Sc-0.3% Ce and performing UVP.The primary Si is the minimum, the grain roundness coefficient and the strength of extension are the maximum, and the spheroidizing effect of α-Al is the best.The elongation and hardness are the highest when 0.4% Ce is appended.The research consequences afford a reasonable process scheme for manufacturing high-quality wearresistant material of the automotive and aerospace industries.
the nanostructure was performed with double-spherical aberration-correction transmission electron microscope (TEM, Spectra 300S)

Process Scheme and Experiment Simulation
The experimental base material was Al-20Si with appending Al-10Ce, Al-10Sr, and Al-2Sc intermediate alloys.The Al-20Si was melted at 760 °C in the graphite crucible, and the dosage of Sr-Sc-Ce nanoparticles was adding the same Sr-Sc (0.2%-0.15%) and different Ce (0.1%, 0.2%, 0.3%, 0.4%, and 0.5%).When the melt was cooled down to 700 °C, and then was held for heat preservation for 15 min.An ultrasonic metal refining processing equipment (HC-MM2010GL) with ultrasonic transducer, amplitude converter, tool head, and ultrasonic generator were used.The melt was brought in UVP for 120 s before importing the tool head into the melt to a depth of 25 mm.A mold made of 45 steel was preheated to 300 °C and filled with the melt.After cooling, the sample size of Φ15 mm Â 180 mm was removed from the mold.
This experimental 3D model was built.The assumptions proposed to facilitate the calculation are as follows: 1) the simulation system is adiabatic and free of thermal convection; 2) the Al-20Si melt is an incompressible non-Newtonian fluid; and 3) Sr-Sc-Ce is added as nanoparticles.
Assuming linear wave propagation without considering shear forces, the sound pressure can be obtained by solving the wave equation.
where ρ, c, and p represent the density of the aluminum alloy melt (kg m À3 ), propagation speed of ultrasonic waves in the melt (m s À1 ), and sound pressure (Pa), respectively.
The cavitation effect caused by the standard k-ε method is used in Ansys Fluent 19 [25,26] to study the distribution of the nanoparticles in the molten metal after UVP.Compared with the without Ce and UVP, the HD rose from 64.2 HBV (Sr-Sc) and 64.8 HBV (Sr-Sc-UVP) to 68.5 HBV (Sr-Sc-0.4%Ce) and 69 HBV (Sr-Sc-0.4%Ce-UVP), i.e., a fall to 6.7% and 6.5%, respectively.

Mechanical Characteristics
The strength first rose with rising the Ce content and then fell.The maximum ultimate tensile strength was acquired with 0.3% Ce, as shown in Figure 1.Without Ce and UV, the consequences rose from 142.78 MPa (Sr-Sc-Ce) and 148.48 MPa (Sr-Sc-Ce-UVP) to 168.46 MPa (Sr-Sc-0.3%Ce) and 177.19 MP (Sr-Sc-0.3%Ce-UVP), i.e., a fall to 17.99% and 19.34%, respectively.

Microstructural Analysis
Figure 2 illustrates the microstructure of the Al-20Si after appending trace Sr-Sc-Ce nanoparticles with UVP. Figure 2a indicates the eutectic Si and primary Si in the platelike or the massive-and-needle-rod form in the absence of Sr-Sc-Ce and UVP.The size of the primary Si was %70 μm, and α-Al was dendritic, coarse acicular, and rosy with many secondary phases at the grain boundaries.Figure 2b displays that, by appending 0.2% Sr-0.15%Sc, the size and form of the α-Al and Si phases were greatly altered, the number of minor dendritic grains rose obviously.Further, the α-Al phases were gradually spheroidized, and the Si phases illustrated finer and equally distribution.The average primary Si phases were %45 μm. Figure 2d,c illustrates that the appending 0.2% Sr-0.15%Sc-0.1% Ce and UVP resulted in α-Al spheroidization; however, it refined the Si phases and intermetallics to a certain extent.Figure 2f,e indicates that, with and without after appending 0.2% Sr-0.15%Sc-0.3% Ce and UVP, the α-Al were the best spheroidized; the eutectic Si, primary Si, and intermetallics displayed finer and equal distribution.The minimum primary Si was obtained and the average primary Si phases were of %28 μm.
Figure 2g,h shows that, with rising the Ce content and UVP, the α-Al began to thicken and grow; the eutectic Si, primary Si, and intermetallics illustrated to gather and bond accordingly and the size of grains grew up obviously.Comparing Figure 2d,f, h with Figure 2c,e,g reveals that the number of grains was higher, average size was smaller, and spheroidization was better because the ultrasonic cavitation and ultrasonic flow effect broke the dendrites, increased the heterogeneous nucleation, and accelerated heat transfer, mass transfer, and flow of the Sr, Sc, and Ce element.Further, this encouraged grain refinement and improved the uniform distribution of the solute elements.The result was consistent with that in Figure 1.
Figure 3a,b displays the SEM images of the sample with 0.3% Ce and UVP.The refining effect of the eutectic Si and primary Si was observed, and they appeared to be distributed more uniformly.More punctate-like and fine needlelike phases, and secondary Si particles and intermetallics (Al x Si x Sc x Ce x phase) were found around α-Al. Figure 3b-d shows the punctate-like phase in Figure 3b by EDS; Sr, Sc, and Ce were fully fused into the samples.
Image-Pro Plus 6.0 was employed to calculate, measure, and analyze the roundness coefficient and grain size of the Al-20Si.Figure 4 displays the effect of α-Al spheroidization.The refined eutectic Si, primary Si, and other phases in the roundness of the grains first rose and then fell with rising the Ce content.The maximum roundness was 0.761 (Sr-Sc-Ce) and 0.776 (Sr-Sc-Ce-UVP) by appending 0.3% Ce; this result was consistent with that shown in Figure 2e,f.Meanwhile, the minimum grain size was %5.3 μm, and α-Al spheroidizing effect was the best.

Formation Mechanism of Eutectic Si
Figure 5 displays the bright-field (BF) mode TEM eutectic Si precipitates of the sample of Al-20Si alloy by appending 0.2% Sr-0.15%Sc-0.3% Ce-UVP.The eutectic Si and α-Al in Figure 5a,b illustrate fine punctate, needlelike, fibrous, and granular particles.The platelike and coarse rodlike eutectic Si was obviously refined while its morphological structure appeared as fine, short rods.Modification with the impurity-induced twinning [3,18] caused changes in the rare-earth Ce (0.125 nm) to Si (0.118 nm) with an atomic radii ratio of 1.56, which was closer to the optimal atomic radius ratio of 1.65 for a perfect metamorphic function.Appending Sr-Sc-Ce elements easily divided the <112> birth, which indicated that the twin precipitates and nanoparticles on the eutectic Si increased significantly as illustrated in Figure 5c.The resulting directions of the Si atoms and the high number density twins and nanocluster particles are observed in Figure 5c,d.Abdelaziz et al. [13] discovered that nanocluster particles were born inside eutectic Si, and some of them were adsorbed on the twin bands.They could not only serve as the heterogeneous nucleation core of eutectic Si but also prevent the birth of eutectic Si to improve grains refinement.Figure 5d illustrates the twins and nanoclusters, indicating that they have a relatively uniform distribution.Considering the selected area electron diffraction patterns in Figure 5e, these twins and nanoparticles were of the <112> Si type.][20] Figure 5c displays the dispersed nanoprecipitates and twins, and the size was %10 nm; it was equally distributed in the Al-Si base.A good chemical affinity between the diffusion nanoparticles and Al-Si base was observed.These particles included Al precipitation and Al x Si x Sc x , Al x Si x Sr x , Al x Si x Ce x , and Al x Si x Sc x Ce x in the eutectic Si, which were related to the addition of microalloying Sr-Sc-Ce elements.Thus, metamorphic elements gathered near twin boundaries and were easily adhered on the birth interface in front of the Si dendritic crystals, thereby altering the original birth steps of the eutectic Si and increasing the number of twins.In addition, they obtained the effect of refined crystalline strengthening as observed in Figure 5b-d (Figure 6). [27]

Simulation Verification
Direct experimental evidence from the effect of acoustic flow and ultrasonic cavitation of UVP are required to verify the refined crystalline strengthening in the alloy melt.the simulation results of UVP Al-20Si melt and driving Sr-Sc-Ce nanoparticles. Figure 7a,d displays that nanoparticles follow the fluid flow.The flow pattern is changed by the direction of the UVP wave effect, which results in a different distribution of nanoparticles. [28]Figure 7b-d displays the flow field distribution characteristics under UVP, velocity flow, pressure flow, and turbulent kinetic energy.Research shows that UVP plays a guiding role by influencing the alloy melt solidification process through acoustic flow and its coupling effect with macroscopic flow field and temperature field.The effect of this multifield coupling effect on ingot grain refinement manifests in three aspects.
1) It strengthens the flow of the Al alloy melt and cause strong turbulence that inhibits the unrestricted growth of the dendritic arm, promotes the dissolution of the secondary dendritic arm, reduces the secondary branching of the short dendritic arm crystal, promotes its complete fusion, and merges growth.
2) It reduces the temperature gradient at the solidification front, making the temperature distribution of the liquid hole more uniform, limits the conditions for dendritic growth, and makes the crystal growth process uniform.

Refinement Mechanism by Appending Trace Sr-Sc-Ce Nanoparticles
Sr is more likely to produce Al-Si-Sr nanoclusters in the eutectic Si when modifying the hypereutectic Al-Si alloys, which can adhere to the Si/liquid interface, alternating the birth of Si phases. [8]Further, Sc can not only refine eutectic Si but also produce eutectic reaction with Al to generate Al 3 Sc nanoprecipitation, further strengthening the spheroidization of α-Al. [12,14]e is very easy to combine with Si to form Ce-rich metal compounds that segregate on the grain boundary, and a Ce-rich active film grows at the solidification interface, thereby reducing the undercooling of the alloy to refine grains.[18][19][20] When trace Sr-Sc Ce nanoparticles were added in the Al-20Si alloy, they weaken the binding force between the Si-Si and Si-Al atomic groups, thereby strengthening the binding of the Al-Al atomic groups and causing α-Al nucleation supercooling.When the alloy is solidifying, trace Sr-Sc-Ce nanoparticles easily gather at the interface among the Al base and eutectic Si phases and adhere to their surface, intercepting the birth of eutectic Si.Meanwhile, there are many irregular atomic steps or grooves on the eutectic Si, and a twinning relationship between these atomic steps and the Si wafer substrate.A few Sr-Sc-Ce atoms preferentially adsorb on these atomic steps and grooves, sealing them, slowing down the diffusion of Si atoms toward them, and expanding branching chains.Furthermore, a few Sr-Sc-Ce atoms gather at the interface steps of the eutectic Si to generate many twins and birth twin grooves, which serve as a new birth source, contributing to its diversified birth.Further, minor Sr-Sc-Ce nanoparticles and Al-Si elements produce intermetallic compounds that increase the heterogeneous nucleation point, thereby achieving refinement and spheroidizing the grains as shown in Figure 3 and 5. [15]

Refinement Effect under UVP
When UVP is introduced in the Al-20Si melt, the effect of ultrasonic flow and ultrasonic cavitation are generated, as shown in Figure 7 and 8. Ultrasonic cavitation is a physical phenomenon caused by ultrasound, and it is the reason that ultrasound can refine metal melts.Ultrasonic waves propagating in a solution medium generate a large number of small vacuum bubbles.At the moment of its collapse, small bubbles generate a pressure of up to tens of thousands of atmospheres that results in a violent impact on nearby liquids, thereby crushing α-Al and eutectic Si dendrites and producing a large number of small grains that become new nucleation centers, refining the grains of the melt, [26,27] as displayed in Figure 8b,c.The effect of ultrasonic flow is a phenomenon of continuous attenuation because the propagation distance increases when ultrasonic waves propagate in the medium.The attenuation gradually weakens the pressure on the liquid medium from the sound source, forming a certain pressure difference and forcing the liquid to flow at a high speed along the pressure gradient direction.When the acoustic wave reaches a certain intensity, the jet flow caused by the acoustic flow effect circulates through the entire metal melt, forming a circular flow.Acoustic flow has generated the significant sound pressure gradient in the melt (Figure 7d) and excited melt flow at high speed (Figure 7b,c) further formed particle flow (Figure 7a).This alters the direction of grain growth, hinders the sliding of dislocations, and enhances grain refinement and spheroidization. [28,29]

Conclusion
We tested the effect of the combination of trace Sr-Sc-Ce modifier and UVP on the grain refinement and spheroidization of Al-20Si.We found that using 0.3% Ce and 0.3% Ce-UVP achieves the favorable refinement of its microstructure.We obtained the minimum primary Si and the average primary Si phases are %28 μm, and the maximum roundness and tensile strength, when compared to those obtained at the other Ce concentrations.Further, we found the maximum EL of 2.28% and1.32% and the highest HD of 69 and 68.5 HBV at 0.4% Ce with and without UVP, respectively.Finally, the simulation results of UVP Al-20Si melt and driving Sr-Sc-Ce nanoparticles had verified the achievements of experimental test and theoretical analysis.Therefore, this article renders to afford a reasonable process scheme for manufacturing high-quality wear-resistant material of the automotive and aerospace industries.

Figure 1
Figure1displays the mechanical characteristics of the Al-20Si alloy after appending trace Sr-Sc-Ce nanoparticles and the UVP.The hardness (HD) rose and then fell with rising the Ce content.The maximum HD was acquired at 0.4% Ce.Compared with the without Ce and UVP, the HD rose from 64.2 HBV (Sr-Sc) and 64.8 HBV (Sr-Sc-UVP) to 68.5 HBV (Sr-Sc-0.4%Ce) and 69 HBV (Sr-Sc-0.4%Ce-UVP), i.e., a fall to 6.7% and 6.5%, respectively.The strength first rose with rising the Ce content and then fell.The maximum ultimate tensile strength was acquired with 0.3% Ce, as shown in Figure1.Without Ce and UV, the consequences rose from 142.78 MPa (Sr-Sc-Ce) and 148.48 MPa (Sr-Sc-Ce-UVP) to 168.46 MPa (Sr-Sc-0.3%Ce) and 177.19 MP (Sr-Sc-0.3%Ce-UVP), i.e., a fall to 17.99% and 19.34%, respectively.The maximum elongation (EL) was acquired at 0.4% Ce, as shown in Figure1.Compared to those when not appending Ce and UVP, the consequences rose from 0.49% (Sr-Sc) and 0.76% (Sr-Sc-UVP) to 1.32% (Sr-Sc-0.4%Ce) and 2.28% (Sr-Sc-0.4%Ce-UVP), i.e., a rising to 169.39% and 200%, respectively.

Figure 1 .
Figure 1.The mechanical characteristics of Al-20Si alloy by appending the trace Sr-Sc-Ce and ultrasonic vibration processing (UVP).

Figure 4 .
Figure 4.The roundness coefficient of grains of Al-20Si.

Figure 5 .
Figure 5.The nanostructure of the Al-20Si by appending 0.2% Sr-0.15%Sc-0.3% Ce-UVP.a,b) The bright-field transmission electron microscope (BF-TEM) image of Si precipitates.c,d) The BF-TEM image of nanoprecipitates and twins.e) The corresponding selected area electron diffraction pattern of (c), showing the signal of twins and the high density of nanoprecipitates.

Figure 7 .Figure 8 .
Figure 7.The simulation results of UVP Al-20Si melt: a) particle path line, b,c) velocity flow, and d) press flow.