Improving Mechanical Property of Hyper-Eutectic Al-Si Alloys via Regulating the Microstructure by Rheo-Die-Casting

Abstract

The microstructure plays a key role in the mechanical properties of hyper-eutectic Al-Si alloys. In this study, we investigate the microstructural evolution of rheo-die-casting (RDC) on the Al-15Si-4Cu-0.5Mg alloy using a mechanical rotational barrel system. Our findings demonstrate that higher rotational speed and pouring temperature reduce the size and roundness of primary Si particles in the semisolid slurry. Additionally, during RDC, the dendritic aluminum matrix and skeletal iron-containing inter metallics are sheared off, leading to a more uniform and dispersed Al₂Cu phase. Ultimately, our rheo-diecasting results indicate the formation of a near globular aluminum matrix, fine primary Si particles, and a homogeneous Al₂Cu phase, thus highlighting the efficacy of this processing method for improving the microstructure and properties of the Al-15Si-4Cu-0.5Mg alloy. We suggest that these results hold promise for enhancing the quality of aluminum-based alloys in various industrial applications.

1. Introduction

Hypereutectic Al-Si alloys possess a plethora of exceptional properties that make them particularly appealing for use in the automotive and aeronautical industries. These include their high strength at elevated temperatures, low coefficient of thermal expansion, and excellent wear resistance [1,2,3]. Regrettably, the industrial application of hypereutectic Al-Si alloys as structural materials has been limited by their low ductility. The mechanical performance of hypereutectic Al-Si alloys is predominantly dictated by the size and morphology of the primary silicon particle and eutectic structure. Conventionally, cast hypereutectic Al-Si alloys tend to exhibit large, irregular primary Si particles and needle-like eutectic structures. Primary Si particles tend to display facet growth patterns and may attain sizes of up to 100 μm, adopting polygonal, pentagonal, flake-like, and plate-like morphologies. The large, irregular shape of Si particles can cause severe disseverance to the matrix and stress concentration at the particle edges, ultimately undermining the mechanical properties of these alloys [4,5,6,7]. Therefore, the attainment of a fine and homogenous Si phase in hypereutectic Al-Si alloys is a mandatory prerequisite for their extended industrial application.

In hypereutectic Al-Si alloys, the morphology and size of the Si phase have traditionally been improved by the addition of inoculants such as P, Na, Sr, and RE. The introduction of P into aluminum melts leads to the formation of aluminum phosphide (AlP) particles, which provide effective nucleation sites for primary Si and lead to a significant reduction in its size [3]. Na and Sr have been widely recognized for their effectiveness in eutectic structure modification and are commonly used in commercial production [8,9]. The incorporation of Rare Earth elements can effectively inhibit the Si growth steps by adsorbing onto the growth front, thereby leading to isotropic growth of Si crystals [10]. While inoculants, such as P, Na, and Sr, have demonstrated effectiveness in either primary or eutectic Si refinement, none of them alone is capable of refining both primary and eutectic Si. Combining different inoculants for refinement has shown promise, yet the process is complicated and may result in a “poisoning immune” phenomenon [11,12,13,14].

Extensive attention has been devoted to developing advanced production processes that yield superior microstructure in hypereutectic Al-Si alloys. Among such processes, rheo-die-casting (RDC) is a notable branch of semisolid processing that has demonstrated significant advantages over traditional material processing techniques. These benefits include good net shape capability, low energy cost, reduction in entrapped air, fine grain size, and diminished solidification shrinkage, porosity, and segregation [15,16,17,18]. Notably, RDC is more than just a method of melt treatment; it represents a direct step in the path towards the final casting and stands in contrast to traditional inoculant modification methods used in Al-Si alloys. The majority of laboratory studies regarding Al-Si alloys have been focused on the hypoeutectic composition, with limited research on hypereutectic alloys using RDC. Guan [19] refined the microstructures of hypereutectic Al-Si-Fe alloys using a wavelike sloping plate method, while Wu [20] investigated the effect of direct ultrasonic vibration (DUV) on A390 and Al-20Si-2Fe-2Cu-0.4Mg-1Ni-0.5Mn alloys, showing the benefits of RDC on microstructure and mechanical strength. However, there is a lack of systematic research on hypereutectic Al-Si alloys using semisolid slurry, a key element of RDC.

The study of slurry preparation and mechanical properties of hypereutectic Al-Si alloy by RDC is scientifically intriguing and essential, as it sheds light on the solidification behavior of Si and its strengthening mechanism [21]. However, there is a dearth of fundamental literature in this area. Previous research has successfully developed a mechanical rotational barrel (MRB) system for RDC [22], obtaining sound semisolid slurry with fine and spherical solid particles and near-net shape rheo-diecasting components with high integrity. Building upon this, the current study utilizes the MRB system to investigate the effect of RDC on the slurry preparation of Al-15Si-4Cu-0.5Mg alloy, with a focus on microstructural characterization and theoretical analysis. The objective is to not only provide insight into the theoretical principles but also offer practical guidance for industrial application.

2. Materials and Methods

2.1. Material Composition

The Al-15Si-4Cu-0.5Mg alloy was synthesized using a combination of raw materials including Al-20%Si and Al-10%Cu master alloys, commercial pure Al (99.99%), and pure Mg (99.99%). The Al-15Si-4Cu-0.5Mg alloy was subjected to a melting process and degassed with argon in a resistance furnace at 680 °C, without undergoing any form of modification or refinement. The actual chemical composition of the resulting samples is tabulated in

Table 1. Chemical composition of the Al-15Si-4Cu-0.5Mg alloy in weight percentage.

Alloy	Si	Cu	Mg	Mn	Fe	Al
Al-15Si-4Cu-0.5Mg	14.72	3.67	0.49	0.13	0.72	balanced

.

2.2. Material Preparation by Rheo-Die-Casting

The MRB system for rheo-die-casting (RDC) is illustrated in Figure 1a. This method is a two-step process involving the preparation of semisolid slurry, followed by rheo-diecasting. The preparation of the semisolid slurry was conducted using the MRB system and then flowed into a copper mold for microstructure observation. The barrel used in the MRB system was made of stainless steel and had a length of 500 mm and a diameter of 150 mm, with an inclination angle of 30 degrees. The MRB system operates by utilizing high shear rate and turbulence through solidification to convert the alloy melt into high-quality semisolid slurry. During the process, the solidifying melt and the barrel create a large number of interfacial areas, resulting in high efficiency in heat extraction during the slurry-making process.

The MRB system’s rotational speed and pouring temperature were identified as key parameters in the fabrication of the fine Si phase Al-15Si-4Cu-0.5Mg alloy. To scrutinize the impact of the barrel’s rotational speed on the slurry, experiments were conducted under the following conditions: (a) the barrel was preheated to 200 °C, (b) the rotational speeds were varied from 30 r/min, 60 r/min, 90 r/min, and 120 r/min, (c) the melt was poured at 660 °C. Furthermore, investigations into the effect of the pouring temperature were also conducted using the following testing conditions: (a) the barrel was preheated to 200 °C, (b) the pouring temperatures were varied from 600 °C, 620 °C, 640 °C, and 660 °C, (c) the barrel rotational speed was maintained at 60 r/min. The resulting rheo-processed slurry was rapidly quenched in an inverted trapezoid copper mold, as shown in Figure 1b, for subsequent microstructure analysis.

Upon completion of the slurry preparation process, the rheo-diecasting process was carried out using a high-pressure die-casting (HPDC) machine. The machine boasted a clamping system with a capacity of 180 t, while the shot sleeve and die were maintained at 250 and 200 °C, respectively. To obtain standard tensile samples that conform to ASTM B557-06, with a gauge diameter of 6.4 mm and a gauge length of 64 mm, the alloy melt was poured into the MRB system at a temperature of 660 °C, with a barrel rotational speed of 60 r/min.

2.3. Sample Testing and Characterization

The specimens under investigation were prepared according to standard protocol and subjected to etching in a mixture composed of 2 vol.% hydrofluoric acid (HF), 3 vol.% hydrochloric acid (HCl), 5 vol.% nitric acid (HNO₃), and the remainder of water. For 3D observation of the primary and eutectic Si features, etching was performed using a 20% dilute solution of NaOH. Microstructural analysis was conducted using a ZEISS Axio Observer A1 optical microscope, complemented with a quantitative image analysis system. A Netzsch STA449F3 differential scanning calorimeter (DSC) was utilized to determine the solidification interval of the Al-15Si-4Cu-0.5Mg alloy. Disc samples with a diameter of 3 mm and a weight of 5 mg were subjected to a scanning process in an argon atmosphere between 450 °C and 630 °C at a rate of 5 k/min. X-ray diffraction (XRD) with Cu Kα radiation was utilized to conduct the structural investigations using a Rigaku D/Max-2500 V diffractometer. The phase identification was accomplished by employing Materials Data Inc. software Jade 5.0 and Powder Diffraction File (PDF release 2002). To observe the microstructure and identify the phase composition, a scanning electron microscope (SEM) equipped with an energy-dispersive X-ray (EDX) spectrometer was used. In addition, electron backscattered diffraction (EBSD) was performed to measure the twinning of primary Si, using an LEO VP SEM equipped with TSL data acquisition software. The room temperature tensile properties were tested on a Zwick/Roell testing machine, and the average of results from more than three samples was considered as each datum. The alloy porosity (η) was analyzed according to the density calculation. The density of the sample (ρ) was measured using Equation (1), where m is the weight of the sample in the air, m_a is the weight of the sample in alcohol, ρ_a is the density of alcohol. Equation (2) was used to calculate the porosity, where ρ_s is the standard density of 2.7 g/cm³.

$$\rho =\frac{m{\rho }_a}{m-m_a}$$

(1)

$$\eta =\frac{{\rho }_s-\rho }{{\rho }_s}$$

(2)

3. Results

DSC thermal analysis is used to determine the precipitation temperature of each phase of the alloy melt during solidification. Figure 2a displays the DSC thermogram obtained during the solidification of the Al-15Si-4Cu-0.5Mg alloy. The thermogram reveals three prominent peaks, which have been identified based on the liquid projection and isothermal section of the Al-Si-Cu ternary system [23]. These peaks correspond to the precipitation of primary Si, the Al-Si binary eutectic reaction, and the Al, Si, Al₂Cu ternary eutectic reaction, which predominantly occurs at three typical temperatures of 613 °C, 542 °C, and 490 °C, respectively. The XRD patterns also illustrate that both samples by the traditional permanent mold casting (PMC) and the quenched semisolid slurry mainly consist of these three phases: Al, Si, and Al₂Cu (Figure 2b). Furthermore, there is no significant shift in peak intensity, indicating that rheo-die-casting (RDC) does not affect the solid solubility of the phases.

The optimization of processing parameters is critical for the successful production of high-quality hypereutectic Al-Si alloy through mechanical rotational barrel (MRB) RDC. In this study, the effect of two key parameters, barrel rotational speed and pouring temperature, on the formation of a fine Si phase was systematically investigated. Figure 3 shows the microstructure evolution by PMC and rheo die-casting (RDC) from 30 r/min to 120 r/min. The facet features of the primary Si are observed in the optical microscope (OM) images of the PMC sample as shown in Figure 3a. Facet-columnar structures are observed for Si particles over 100 μm long. The eutectic flake Si particles display acicular morphology with an aspect ratio of 12.6, uniformly distributed in the inter dendritic regions of the Al matrix. The primary Si phase exhibits a predominantly polygonal plate-like morphology with an average size of 27.9 μm and a roundness value of 0.27 for the primary Si particles (Figure 3b). Notably, more than half of the primary Si particles exceed 30 μm in size (Figure 3c). Iron-containing phases, such as α-AlFeSi, are known to significantly influence the mechanical properties of aluminum alloys during subsequent processing or service. Here, the platelet-shaped morphology of the α-AlFeSi phase is observed, which acts as a potential site for crack initiation [24]. However, the addition of Mn to the alloy results in the transformation of the α-AlFeSi phase to α-AlFeMnSi, which assumes a modified continuous Chinese script or skeleton shape, as depicted in Figure 3a. This modified phase exhibits improved compatibility with the matrix, with a potential for enhancing the mechanical performance of the alloy. In addition, our observations, as depicted in Figure 3a, show that the Al₂Cu phase primarily appears as pockets of (Al + Al₂Cu) lamellar eutectic dispersed throughout the matrix. Normally, the formation of the copper phase Al₂Cu during the final stages of solidification can significantly impact material properties.

Figure 3d–k reveal notable differences in primary Si morphology, with a significantly smaller polygonal plate-like shape and the absence of facet-columnar primary Si. Further examination of primary Si features indicates that an increase in rotational speed leads to a refined primary Si structure (Figure 3d–j), with a decrease in an average size and an increase in roundness, as well as a marked increase in the portion of primary Si less than 10 μm (Figure 3h,i). We attribute these findings to more intensive heat extraction and shearing forces acting on the alloy melt at higher rotational speeds.

Experimental observations have established the production of non-dendritic structures through melt stirring during solidification as a crucial advantage of RDC [11,12]. The results of the present study demonstrate a clear modification of the dendritic aluminum matrix. While Figure 3d shows a non-dendritic morphology for most of the aluminum, some rosette-like and dendritic remnants are still present. However, higher rotational speeds yield a much finer and more homogeneous non-dendritic matrix, with only a few dendritic remnants remaining, as shown in Figure 3e–g. Additionally, Figure 3k shows that the aspect ratio of eutectic Si is much smaller under RDC than under PMC (a value of 12.6), with a value of around 6.7 at all rotational speeds. This change is attributed to the quenching modification effects, which produce a fine fibrous structure that is microscopically smooth and has a low density of incidental twins [25]. The similarity of aspect ratio results under different rotational speeds may also be due to analogous cooling rates during RDC (

Table 2. Loss of temperature of the Al-15Si-4Cu-0.5Mg alloy for Rheo-casting with different rotational speed.

Rotational Speed (r/min)	Pouring Temperature (°C)	Temperature at the End of the Barrel (°C)	Temperature Drop (°C)	Cooling Rate on Barrel (°C/S)
30	660	622	38	34.5
60	660	619	41	37.2
90	660	617	43	39.1
120	660	614	46	41.8

).

The slurry microstructures at different pouring temperatures from 600 °C to 660 °C are investigated by RDC with a rotational speed of 60 r/min (Figure 4a–d). The morphologies of primary Si are significantly affected by pouring temperatures. At 600 °C, some irregular polygonal Si, as large as over 100 μm, appears. At 620 °C, the primary Si exhibits a variety of morphologies, including coarser polygonal and plate-like shapes, with a tendency to segregate in the matrix. Further, at 640 °C, the size increases, and some irregular coarse primary Si appears. At 660 °C, the primary Si mainly exhibits a fine regular polygonal blocky shape, dispersed homogeneously in the matrix. Additionally, some rosette-like aluminum structures are observed in lower pouring temperatures (Figure 4a–c). Results indicate that the size of primary Si becomes finer with increasing pouring temperatures (Figure 4e). The average primary Si size of 660 °C is about 14 μm, which is much lower than that of 600 °C, about 30 μm (Figure 4d). The roundness analysis (Figure 4f) reflects the regular to irregular morphology transition, which decreases from 0.3 to 0.2 significantly with lowering pouring temperature. Primary Si frequency analysis shows that slurry obtained at 660 °C and 640 °C is composed mainly of particles less than 20 μm, while lower temperature slurry comprises a larger proportion of primary Si particles over 30 μm (Figure 4g). Additionally, the large irregular polygonal Si particles are not perfect polyhedrons and are embedded in the matrix with sharp edges. The relationship between undercooling of Si at the growth front and the cooling rate and temperature gradient can be expressed as follows: the undercooling is inversely proportional to the cooling rate and reverse proportional to the temperature gradient, which is described as

∆T = BV^1/2G^−1/2,

(3)

where B, V, and G are the constant, cooling rate and temperature gradient, respectively [26]. The current study delves into the interplay between the pouring temperature of Al-Si alloys and the resultant microstructure, with a specific focus on primary Si particle size. Specifically, the eutectic Si evolves from a short-clubbed shape to a long acicular shape as the pouring temperature is lowered, accompanied by an abrupt increase in aspect ratio (Figure 4h). The data presented in

Table 3. Loss of temperature of the Al-15Si-4Cu-0.5Mg alloy for Rheo-casting with different pouring temperatures.

Pouring Temperature (°C)	Rotational Speed (r/min)	Temperature at the End of the Barrel (°C)	Temperature Drop (°C)	Cooling Rate on Barrel (°C/S)
660	60	619	41	37.2
640	60	608	32	24.6
620	60	606	14	9.3
600	60	591	9	5.6

indicate that higher pouring temperatures lead to faster cooling rates, resulting in greater undercooling and refinement of primary Si. Prior research has suggested that decahedral Si clusters, formed from pre-existing tetrahedral Si, may serve as nuclei for primary Si [27]. Moreover, the presence of Si atom clusters in the Al-Si alloy melt is well-established, and increased melt superheated temperatures have been shown to destroy some Si-Si bonds, enabling the dissolution of Si atoms into the Al bulk melt [28,29,30]. On the other hand, shearing forces generated by barrel rotation can disrupt Si clusters and foster greater homogeneity in the alloy composition. As a result, higher melt temperatures promote smaller Si-Si clusters and a more uniform composition.

SEM images in Figure 5 provide further details on the microstructural features of the alloys. The RDC Al-15Si-4Cu-0.5Mg alloy exhibits finer and more homogeneous lamellar precipitates, in contrast to the PMC sample. As mentioned above, the presence of Fe is the most harmful impurity in aluminum alloys, although the addition of Mn can mitigate its detrimental effects. However, the α-AlFeMnSi phase exhibits a 3D skeleton shape (Figure 5a). Interestingly, the α-AlFeMnSi phase is obviously fragmented and transforms from skeleton to facet shape after RDC Figure 5b).

The present study investigates the effects of RDC on the mechanical properties of HPDC and rheo die-casting (RDC) samples. The results presented in Figure 6 and

Table 4. Mechanical properties of the HPDC and RDC Al-15Si-4Cu-0.5Mg alloy.

Processing Conditions	Yield Strength (MPa)	Ultimate Tensile Strength (MPa)	Elongation (%)
HPDC, F	280	304	0.4
RDC, F	288	326	0.6
RDC, T6	391	411	0.8

demonstrate a clear enhancement in the yield strength (YS), ultimate tensile strength (UTS), and elongation (EL) of both HPDC and RDC samples after RDC. Notably, the T6 of RDC samples exhibits superior mechanical properties. It is widely recognized that the mechanical properties of diecasting samples are largely influenced by macro-defects such as gas entrapment, shrinkage porosity, large intermetallics, and microstructural non-uniformity. The present study utilizes slurry analysis to reveal that the MRB system effectively breaks dendrites and enhances the rheology of the alloy melt. The improved rheology facilitates liquidus penetration for forming and results in reduced shrinkage. Furthermore, the semisolid slurry exhibits higher viscosity due to the presence of a preliminary solidified phase, which leads to a steady filling with a “solid-front” instead of “spraying” during the subsequent die-casting process. As a result, gas entrapment is minimized, and a more compact microstructure is achieved. This finding is supported by previous research (references [31,32]).

This study investigates the microstructural characteristics of Al-15Si-4Cu-0.5Mg alloy using HPDC and RDC samples, as presented in Figure 7. The HPDC sample displays a dendritic shape for the Al phase and a facet-columnar morphology for the primary Si phase (Figure 7a). Conversely, the primary Si phase in the RDC sample is smaller, rounder, and more complex, as demonstrated in Figure 7b–d. The RDC sample’s microstructure is primarily composed of fine β₃-Si particles resulting from secondary solidification, with a few intermediate-sized β₂-Si particles. These β₂-Si particles are believed to form due to rapid temperature drops during slurry transfer and in the shot sleeve of the die-casting machine, as reported in previous studies [33]. Overall, the microstructural analysis suggests that the RDC process produces a more refined and complex microstructure, which contributes to the observed improvement in mechanical properties.

The analysis of primary Si size frequency and morphology is a crucial step towards understanding the microstructural properties of cast materials. In this study, we investigated two samples produced using distinct casting techniques: rheo die-casting (RDC) and high-pressure die casting (HPDC). Our results indicate that RDC produces a higher proportion of primary Si particles smaller than 10 μm, suggesting that this method facilitates the production of fine primary Si particles. Notably, the RDC sample displayed a near globular morphology for the primary Al phase, with minimal dendritic structures observed. This is likely due to the effective shearing of the dendritic primary Al during the slurry preparation stage in the MRB system. Subsequently, rapid solidification during high-pressure die casting results in the observed microstructure presented in Figure 7b–d. These findings provide valuable insight into the influence of processing techniques on the microstructure of cast materials, emphasizing the potential of RDC in achieving finer and more uniform microstructures.

Figure 7f illustrates the porosity of the alloy at various processing conditions, and the results indicate that significantly less gas entrapment is achieved after RDC. The lower porosity observed in the RDC sample allows for post-treatment aging to improve its mechanical properties. The combination of finer and rounder primary Si particles, refined and homogeneous Al₂Cu eutectics, fragmented ferrous-intermetallics, and reduced porosity collectively contribute to the superior mechanical properties of the RDC sample. These findings highlight the potential of RDC as a method for producing high-quality cast materials with improved mechanical performance.

4. Discussion

4.1. Nucleation of the Primary Si under RDC

The compositional heterogeneity of Al-Si alloy is well-documented in the literature, with Si clusters taking on the form of both tetrahedrons and decahedrons present in the melt [28,29,30]. This liquid structure has a notable impact on the formation of primary Si, which has been explored by Kobayashi and Hogan, who propose that star-like primary Si arises from a decahedral nucleus resulting from the agglomeration of five tetrahedral Si clusters [27]. From a thermodynamic perspective, Gui has demonstrated that the spontaneous agglomeration of five tetrahedral or decahedral Si clusters into star-like primary Si through fivefold twinning is a plausible process [34]. This implies that the primary Si nucleus may take shape through the combination of Si atoms in the form of tetrahedrons or decahedrons. The twinning structure of the primary Si was characterized via EBSD and presented in Figure 8a,b.

The microstructure of the Al-15Si-4Cu-0.5Mg alloy is known to vary markedly with processing conditions. A sudden surge in primary Si particle size was observed, in the twinning structure of the PMC sample (Figure 8a). In the course of liquid casting, primary Si particles displayed one-fold or two/three-fold twinning structures, with the absence of star-like five-fold primary Si particles. However, RDC at 660 °C and 60 r/min led to a reduction in primary Si particle size and the emergence of some particles with five-fold twinning structures (Figure 8b). Given the profound influence of microstructure on the mechanical properties of alloys, especially their strength and ductility, a thorough understanding of the correlation between processing conditions and microstructure is essential for optimal performance in specific applications. The formation of star-like primary Si particles in Al-Si alloys has been conventionally associated with a Si composition exceeding 18 wt%. However, the emergence of these particles in the Al-15Si-4Cu-0.5Mg alloy during RDC at 660 °C and 60 r/min (Figure 8b) is a striking deviation from this conventional understanding. The observation suggests that Si decahedra were formed during the processing of the alloy, leading to the development of star-like primary Si particles. These results provide valuable insights into the underlying mechanisms driving microstructural evolution in this alloy system, shedding light on the role of processing conditions in shaping the properties of Al-Si alloys and contributing to the advancement of materials science.

4.2. Effect of RDC on Crystal Growth of the Primary Si

The use of a rotational barrel in the MRB process has been shown to provide significant benefits, including localized rapid heat extraction and compositional homogeneity, as previously reported by Hu and Guo [35,36]. However, this process also results in strong heat and compositional exchange, which can lead to increased impurity diffusion near the corners and edges of the octahedral Si crystals, resulting in morphological instability and the growth of imperfect octahedral shapes with hollows inside. Octahedral Si is the most fundamental polycrystal of hypereutectic Al-Si alloys, but it can also grow as other morphological polycrystals, such as spinel twins and star-like crystals, through layer growth, which involves the generation of {111} planes on {111} surface facets in a manner of spiral dislocation [37].

The crystal growth of primary Si subsequent to nucleation is anisotropic and diffusion-limited. During the initial solidification phase, the Si takes on an octahedral skeleton structure that includes corners, <110> edges, and {111} facets. As skeletal Si grows, a significant quantity of impurities—both incidental and alloying elements, such as Fe, Cu, and Mg—are rejected by the primary Si and accumulate on nearby surfaces. Concurrently, the {111} surfaces that grow from the corners and edges effectively sweep impurities towards the center of the {111} facets. The impurities that reach the center of the {111} facets become trapped, while those located at the edges and corners tend to diffuse back into the melt. As a result, Si concentration is higher at the corners and edges but lower in the center of the {111} facets. Consequently, a stronger driving force for crystal growth exists at the corners and edges relative to the {111} facets in the center [38]. The interplay of different forces promotes instability and triggers the outgrowth of the corners and edges in primary Si. As the concentration of impurities at the {111} facets center reaches a critical threshold, the {111} facets come to a halt in growth and may even re-dissolve, resulting in hollows within the Si structure. Theoretical analyses suggest that a perfect octahedral Si may be attained only when the ratio of the crystal growth velocities along [001] and [111] directions is equal to √3. Consequently, if V₀₀₁/V₁₁₁ > √3, the primary Si will grow as an imperfect octahedron. Conversely, if V₀₀₁/V₁₁₁ < √3, other morphologies of primary Si will arise. Ryningen and Takahashi [39,40] have reported the nucleation of large dislocation clusters at the primary Si grain boundaries close to the solid–liquid interface due to the large shear stress during solidification. These clusters grow with slip on the {110} <110> system. Notably, the dislocation growth system may intersect with the layer growth system on the twelve <110> edges and six corners of the octahedral Si that emerge on the solid-liquid front during solidification. Therefore, the neighbouring areas of the primary Si growing interface do not possess a perfect crystal structure and exhibit several defects. Some researchers have reported that the primary Si can be refined by RDC treatment, which may be attributed to the break-off of primary Si that contains many defects [19].

Figure 8c,d depict the morphological differences between the liquid casting and RDC Al-15Si-4Cu-0.5Mg alloy samples through deep etching. As evidenced by the figure, the liquid casting sample exhibits a solid inner structure, while the RDC sample assumes a typical imperfect octahedral shape with a hollow interior and cracks on the edges and corners. In a study by Lee [41], shearing forces were applied to the Al-15.5 wt% Si alloy for several hours at the isothermal semisolid state, leading to the fragmentation of the primary Si along {100} planes and cleavage along {110} planes. While Lee attributed the origin of the cracks to the mutual collision between the primary Si, no discussion was offered on crystal nucleation and growth. In Figure 8d, the crack trace exhibits a similar pattern, originating around the octahedral corner on the (100) plane and propagating along the (110) and planes. Strong shearing forces are exerted on the octahedral Si by the turbulent liquid flow resulting from the rotational barrel. Given the existence of numerous dislocation clusters on the edges and corners, these regions are more susceptible to crack initiation under shearing forces. Once the cracks have formed, they tend to propagate rapidly along the dislocation cluster {110} planes and may even pulverize the Si upon encountering the inner hollows, resulting in Si refinement.

5. Conclusions

The present study employs the MRB system to investigate the impact of RDC on the slurry preparation of Al-15Si-4Cu-0.5Mg alloy. The research focuses on the effect of microstructure on mechanical properties and aims to not only provide insight into the underlying strengthening and toughening mechanisms but also offer practical guidance for industrial application. RDC represents a novel means to achieve enhanced strength and toughness in the alloy, with crucial parameters such as pouring temperature and rotation speed being closely scrutinized.

1.

The semisolid slurry analysis revealed that increasing the rotational speed and pouring temperature resulted in the production of relatively finer and rounder primary Si. The aspect ratio of eutectic Si decreased with increasing pouring temperature but remained relatively constant with increasing rotational speed. After RDC, the dendritic aluminum matrix and iron-containing phase were sheared off.

2.

The RDC alloy exhibited smaller and rounder primary Si compared to HPDC, with the eutectic Si and Al₂Cu being finer and more dispersive. The RDC alloy demonstrated superior mechanical properties, with less porosity, making it heat treatable and resulting in an ultimate tensile property improvement of more than 20%.

3.

RDC was found to break the Si-Si clusters, increasing the nucleation of primary Si. However, the strong shearing stress led to imperfect octahedral primary Si growth and the formation of cracks originating from {100} planes and propagating along {110} planes.