Eutectic Si Phase Research Articles

Effect and its mechanism of fixed-ratio and incremented 3Be-Sb complex modifier on microstructures and properties of hypereutectic Al-Si-Mg alloy

The effect of incremented 3Be-Sb (Be and Sb fixed-ratio at 3) complex modifier on the microstructure and mechanical properties of hypereutectic Al-Si-Mg alloy was investigated. The addition of 0.2 wt% 3Be-Sb in Al-Si-Mg alloy led the primary α-Al from dendritic to regular, the eutectic Si phase was significantly refined, and the size of the primary Mg2Si was remarkably decreased. Specifically, a further increase of 3Be-Sb addition to 0.6 wt% resulted in the microstructure of Mg2Si phase was completely modified from square to a spherical and had maximum refinement effect of primary α-Al. The refinement of the primary Mg2Si phase was attributed to the selective absorption of Be and Sb atoms on Mg2Si surfaces, which inhibited the eutectic Si phase in the preferential growth direction. During the solidification process, the Be element was the heterogeneous nucleation core, and the Mg3Sb2 phase was selected to modify the adsorption-poisoning process. Comparing to the unmodified alloy, the hypereutectic Al-Si-Mg alloy modified with 0.6 wt% 3Be-Sb exhibited significantly enhanced the yield strength, ultimate tensile strength, elongation, shear properties increased from 92.6 Mpa, 120.6 Mpa, 2.0% and 89.6–237.3 Mpa, 250.9 Mpa, 3.3% and 107.6 Mpa, respectively. The abrasive property of these alloys was also prominently improved. Meanwhile, the T6 heat treatment can further improve the mechanical properties. The refinement of Mg2Si phase after complex modification is believed to be the underlying reason for the greatly enhanced mechanical properties.

Modification of eutectic Si in hypoeutectic Al-Si alloy with novel Al-3Ti-4.35La master alloy

Grain refinement and eutectic Si modification of hypoeutectic Al-Si alloys are crucial to improving their mechanical properties. In this paper, the effects of a novel Al-3Ti-4.35La master alloy on the morphology of eutectic Si and mechanical properties of hypoeutectic Al-7Si alloy were studied, and the modification mechanism on eutectic Si was revealed. The results show that the addition of 0.2 wt% Al-3Ti-4.35La master alloy can transform the eutectic Si phase from the coarse plate and needle to fine fiber and partial granular structure. After modification, the ultimate tensile strength and elongation of the Al-7Si alloy reached 178.3 MPa and 12.0%, which are enhanced by 15.6% and 106.9%, respectively, over the unmodified alloy. A detailed analysis revealed that, on one hand, the addition of Al-3Ti-4.35La master alloy can reduce the nucleation temperature of eutectic Si in Al-7Si, shorten the eutectic reaction time, and inhibit its growth. On the other hand, the morphology of eutectic Si can also be affected by altering its growth behavior. The twinning density in the eutectic Si after the modification increases significantly while the growth direction is anisotropic. The modification mechanism of the Al-3Ti-4.35La master alloy on eutectic Si is believed to be consistent with the impurity-induced twinning mechanism (IIT). This work provides important insights into alloy modifications by rare-earth master alloys.

Triple the ductility of as-cast Al–Si alloys by phase-selective recrystallization

Very recently, a method of phase-selective recrystallization has been proposed to significantly increase the ductility of eutectic alloys. Different from the conventional methods, such as adding modifiers and severe plastic deformation, phase-selective recrystallization is a simple way to control the microstructures with selective recrystallization of the soft phase but refining the hard phase. In this paper, we reveal the microstructure evolution in the phase-selective recrystallization treatment of Al–Si alloys. The results show that through phase-selective recrystallization, the microstructure of the eutectic Si phase with high degree of spheroidization and fine grains of the α-Al matrix can be obtained. Simultaneously, the refined hard Si particles and the soft α-Al grains brought the unusual tensile elongation of 24%, triple of that in as-cast state. Meanwhile, the ultimate tensile strength increased from 208 MPa to 291 MPa. The performance of Al–Si alloy with phase-selective recrystallization is superior to all previous treatments and can be used to produce high performance Al–Si alloy sheet with large scale.

Eutectic Si Accumulation in Liquid Forged A356 Alloy Wheel and its Adverse Effect on Tensile Fracture Behavior

This investigation is aimed to study microstructural heterogeneities raised due to the inherent melt transport phenomenon and their effect on mechanical properties. Modified, Cleaned, and degassed A356 aluminum alloy melt was poured at 704°C in H13 tool steel die, while temperatures of lower die segment, side segment, and upper segment were 368°C, 375°C, and 290°C respectively. Then liquid melt was forged till its complete solidification under pressure using a hydraulic forging press. The liquid forged wheel was subjected to metallurgical investigation of microstructure, mechanical properties, and fractography of tensile test samples at different cross-sections along the protrusion of the wheel. Results conclude gradual variation in accumulation of eutectic Si phase from surface to the center of thickness and from flange to the outer rim region. The elastic properties are not affected by accumulation of the eutectic silicon however, plasticity have adversely affected with decrease in effective ductile cross-section. Also, refinement of primary α-aluminum and modification in the eutectic silicon phase in specific areas of the wheel is observed.

Investigation on heat treatment of large-sized and complex AlSi9Mg aluminum alloy components formed by squeeze casting

AlSi9Mg aluminum alloy flywheel housing components were formed via squeeze casting and T6 heat treatment was applied to the formed components. The influence of solid solution temperature, solid solution time, aging temperature and aging time on the microstructure and mechanical properties of AlSi9Mg aluminum alloy components formed by squeeze casting were investigated. The results showed that T6 heat treatment changed the morphology of the eutectic silicon in the microstructure from long fibrous to short rods and round spheres. The spheroidization of eutectic Si phase was more obvious with an increase of solution temperature, and the eutectic Si phase not dissolved in the matrix became coarse and grew with an extension of solution time. In terms of mechanical properties, the tensile strength and elongation of AlSi9Mg specimens after heat treatment showed an increasing trend with a rise of solution temperature, but the yield strength decreased, while the three mechanical properties of specimens all improved first and then decreased with an increase of solution time, aging temperature and aging time. Comprehensive consideration of three mechanical properties showed that the optimal heat treatment process parameters in this experiment were solution temperature of 540 ℃, solution time of 180 min, aging temperature of 175 ℃ and aging time of 300 min. In addition, β''−Mg2Si strengthening phases and disc-shaped strengthening phases were precipitated during T6 heat treatment, which were also one of the reasons for the improvement of the casting properties.

Corrosion of Secondary Hpdc A365 (AlSi10MnMg(Fe)) Alloys for Automotive Applications

Al alloys with suitable mechanical properties and castability are widely used for thin-wall structural HPDC components with complex shapes. Al alloy A365 (9-11.5% Si, 0.4-0.8%Mn, 0.1-0.6% Mg, 0.25% Fe max.) is typically produced using primary Al (raw material) to control/minimize the impurity Fe content. Secondary A365, made using recycled end-of-life material, with equivalent mechanical properties as the primary alloy, provides an opportunity to reduce life cycle greenhouse gas (GHG) emissions from light-duty vehicles through improved end-of-life material recovery. The primary alloy is susceptible to micro-galvanic corrosion, where the eutectic a-Al phase serves as the anode and the eutectic Si phase, and incorporated secondary intermetallic phases serves as the cathode. The purpose of this work is to determine the extent to which utilization of end-of-life materials alters the microstructure and, in turn, corrosion (anode and cathode kinetics) of secondary A365, as revealed by electrochemical measurements.

Cellular automaton simulation and experimental validation of eutectic transformation during solidification of Al-Si alloys

The morphology of eutectic silicon in solidification microstructure is critical to the performance of Al-Si-based alloys. Simulating eutectic Si phase formation has been a challenge in ICME (integrated computational materials engineering) based design and manufacturing of solidification products of Al-Si-based alloys. In this study, our previous three-dimensional (3-D) cellular automaton (CA) model for α-Al dendritic growth was extended to include eutectic (α-Al + Si) transformation in multi-dendrite domains, providing a complete solidification simulation of critically important Al-Si based alloys. The quantitative results of the Si phase in the eutectic microstructure were experimentally validated using scanning electron microscopy and deep etching techniques. The simulation results show a good agreement with the experimental observations and calculations by the Scheil model and lever rule. This 3-D CA model is useful for predicting and optimizing the solidification microstructure including eutectic transformation during solidification processing such as casting, potentially welding, and additive manufacturing.

Towards strength-ductility synergy through an optimized two-stage solution treatment in Al–7Si–3Cu-0.5Mg alloys

In order to obtain superior mechanical properties, the incipient melting and maximum dissolution of Cu-containing phases, spheroidization and coarsening of eutectic Si phase in Al–Si–Cu–Mg alloy are important factors that should be considered during solution treatment. However, the conventional single-stage solution treatment was difficult to meet this requirement. In this study, an optimized two-stage solution treatment (495 °C/8 h + 515 °C/4 h) was established for Al–7Si–3Cu-0.5 Mg casting alloys through systematically investigating the microstructure evolution at different solution temperatures (470 °C, 495 °C, 515 °C and 525 °C). The first stage (495 °C/8 h) was applied to dissolve most Cu-containing phases and avoid incipient melting, while the employ of the second one (515 °C/4 h) was to promote fast spheroidization of eutectic Si phase and prevent severe overheating. Two modified quality indexes evaluated the synergy of strength and ductility indicated that the optimized two-stage solution treatment and the subsequent aging treatment (175 °C/8 h) gave rise to an optimization (QIU = 537.1 MPa, QIT = 0.47) on the strength and ductility of Al–7Si–3Cu-0.5 Mg alloys, as compared with the alloys (QIU = 527.4 MPa, QIT = 0.46) under the conventional heat treatment including a single-stage solution treatment (515 °C/16 h). The strength-ductility synergy was arisen from the fuller dissolution of Cu-containing phases, the finer eutectic Si phase, the depletion in number density of micropores and the precipitation of nano-sized Q′ and θ′ particles. 0.3 wt% Zr addition further increased the modified quality indexes (QIU = 549.5 MPa, QIT = 0.50), which was attributed to the grain refinement and the formation of nanoscale Al–Si–Zr precipitates. Furthermore, the fracture mechanism of the obtained alloys was also discussed.

Solidification behavior and enhanced properties of semi-solid Al–8Si–0.5Fe alloys fabricated by rheo-diecasting

Compared with the high pressure diecasting, rheo-diecasting has an advantage in decreasing defects, refining microstructure and improving properties. In the present work, an Al–8Si–0.5Fe alloy with a good combination of mechanical property and thermal conductivity was obtained by rheo-diecasting. The solidification behavior, mechanical properties, and thermal conductivity of the alloy under rheo-diecasting were discussed. The results demonstrated that the chilling effect and shearing vibration not only affected the primary solidification on the vibrating contraction sloping plate, but also the secondary solidification in the remaining liquid during rheo-diecasting. The rheo-diecasting alloy possessed spherical primary α1-Al grains and secondary α2-Al grains, small-sized Al5FeSi phases, eutectic Si phases, and low porosities, and thus exhibited the high mechanical properties and thermal conductivity. With decreasing pouring temperature from 690 °C to 670 °C, the size of primary α1-Al grains, secondary α2-Al grains, Al5FeSi phases and eutectic Si phases was decreased due to high cooling rate, which further improved the mechanical properties and thermal conductivity of the alloy.

Effects of Ni content on microstructure and wear behavior of Al–13Si–3Cu–1Mg-xNi-0.6Fe-0.6Mn alloys

As one of the lightweight and wear-resistance metallic structural materials, cast Al–Si alloys have been widely studied in automobile and defense industries to meet the greater demands for elevated-temperature wear-resistance properties. In this study, the effect of Ni content on the microstructure and wear behavior of Al–13Si–3Cu–1Mg-xNi-0.6Fe-0.6Mn alloys were investigated by thermodynamic modeling, X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and ball-on-disk reciprocating sliding test. The results show that the θ-Al2Cu and γ-Al7Cu4Ni phases are disappeared, while the δ-Al3CuNi, ε-Al3Ni, and T-Fe phases are formed with the increasing of Ni contents. The 3-D network structure is composed of α-Al matrix, eutectic Si, Q, Ni-rich, and Al15(Mn, Fe)3Si2 phases, which leads to the best hardness and wear-resistance of the alloy with 2 wt% Ni. The fracture and debonding of excessive coarse ε-Al3Ni phase deteriorate the wear-resistance and ductility of the 3Ni alloy. The main wear mechanisms of the tested alloys are abrasive, delamination and oxidative wear at 25 °C and convert to adhesive wear at 350 °C.

Fabrication and Sc-Refinement of SiCp/A356 Composites by Vacuum Stirring Casting Method

Aluminum matrix composites reinforced with 10 vol.% SiC particles (SiCp/Al MMCs) were prepared throughthe vacuum stirring method from SiC particles preoxidated at high temperature, followed by posttreatments of remelting and Sc addition treatment. Major problems during vacuum stirring preparation were investigated. Results show that overoxidation of SiC particles may occur during the vacuum stirring process, which causes loss of SiC particles and increases in “impurity pores” after solidification or aggregation of SiC particles, leading to scarcely improved mechanical properties of the prepared SiCp/Al composites. After Sc addition treatment, the α-Al and eutectic Si phases were obviously refined. The composite matrix was strengthened. The result indicates that the maximum tensile strength of 10 vol.% SiCp/A356 composites with Sc addition treatment was 193.5 MPa, which was increased by 43% compared with the matrix alloy. Moreover, a fracture at particles was detected after Sc addition treatment. The stress could be effectively transferred to the SiC particles when the fracture occurred.

An Al-12Si/ZnS powder inoculant for eutectic silicon in Al-12Si alloy

To obtain high-performance Al-Si-based cast alloys, refinement and modification of Si phases are required. An Al-12Si/ZnS powder inoculant was designed and fabricated using a chemical bath deposition method. The efficiency of the inoculant for modifying the eutectic Si phase in as-cast Al-12Si alloy was studied. Results show that Al-12Si/ZnS powder can significantly refine the eutectic Si in Al-Si cast alloys. The best refinement effect for eutectic Si is achieved with 17.5wt.% Al-12Si/ZnS powder. Coarse long needle-shaped eutectic Si with a length of 18 µm was modified into approximately spherical shape with a diameter of 6.53 µm, which is evenly distributed throughout the alloy. The E2EM model calculation indicates that the inter-plane misfit (Fp) and inter-atomic spacing misfit (Fr) between ZnS and Si are all less than 0.5%, which confirms that ZnS is a potential nucleation site for Si phase. The hardness, tensile strength, and elongation of Al-12Si alloys modified with 17.5% Al-12Si/ZnS powder increase 6.30%, 16.18% and 55.45%, respectively, compared to the unmodified Al-12Si alloy. The fracture behavior of the alloy with 17.5wt.% Al-12Si/ZnS powder is dominated by transgranular fracture supplemented by intergranular fracture.

Effects of Microstructure Modification by Friction Surfacing on Wear Behavior of Al Alloys with Different Si Contents.

In this work, Al alloys with 6.6%, 10.4%, and 14.6% Si were deposited as thick coatings by Friction Surfacing (FS), resulting in grain refinement and spheroidization of needle-shaped eutectic Si phase. Lubricated sliding wear tests were performed on a pin-on-disc tribometer using Al-Si alloys in as-cast and FS processed states as pins and 42CrMo4 steel discs. The chemical composition of the worn surfaces was analyzed by X-ray photoelectron spectroscopy (XPS). The wear mechanisms were studied by scanning electron microscopy (SEM) and focused ion beam (FIB), and the wear was evaluated by measuring the weight loss of the samples. For the hypoeutectic alloys, spheroidization of the Si phase particles in particular leads to a significant improvement in wear resistance. The needle-shaped Si phase in as-cast state fractures during the wear test and small fragments easily detach from the surface. The spherical Si phase particles in the FS state also break away from the surface, but to a smaller extent. No reduction in wear due to FS was observed for the hypereutectic alloy. Here, large bulky primary Si phase particles are already present in the as-cast state and do not change significantly during FS, providing high wear resistance in both material states. This study highlights the mechanisms and limitations of improved wear resistance of Si-rich Al alloys deposited as thick coatings by Friction Surfacing.

Microstructures and corrosion behaviors of Al–6.5Si–0.45Mg–xSc casting alloy

The microstructures and corrosion behaviors of the Al–6.5Si–0.45Mg casting alloys with the addition of Sc were investigated by using scanning electron microscopy, X-ray diffraction, electrochemical measurement techniques and immersion corrosion tests and compared with those of Sr-modified alloy. The results show that Sc has evident refining and modifying effects on the primary α(Al) and the eutectic Si phase of the alloy, and the effects can be enhanced with the increase of Sc content. When the Sc content is increased to 0.58 wt.%, its modifying effect on the eutectic Si is almost same as that of Sr. Sc can improve the corrosion resistance of the test alloy in NaCl solution when compared with Sr, but the excessively high Sc content cannot further increase the corrosion resistance of the alloy. The corrosion of the alloys mainly occurs in the eutectic region of the alloy, and mostly the eutectic α(Al) is dissolved. This confirms that Si phase is more noble than α(Al) phase, and the galvanic couplings can be formed between the eutectic Si and α(Al) phases.

Effects of melt thermal treatment on cast Al-Si alloys: A review

The shape and size of primary phases (α-Al and primary Si in hypoeutectic and hypereutectic alloys, respectively), and that of eutectic Si phase significantly affect the mechanical properties of cast Al-Si alloys. The Al-Si alloys prepared by the conventional casting method have large-sized primary phases and needle-shaped eutectic Si phases along with their uneven distribution in the microstructure. This leads to poor mechanical properties in those alloys. This paper presents a review on a modified casting process known as melt thermal treatment, which has the potential to positively change the shape and size of primary and eutectic phases in Al-Si alloy without any foreign additives. Various researches that have been carried out to study the effect of melt thermal treatment on microstructure and different properties of Al-Si alloys are discussed here.

Effect of Mn addition on the micromechanical response and failure of Al-12.6Si alloy using actual microstructure based RVE model

Eutectic Al-12.6Si alloy has been developed with and without Mn addition through the gravity casting method. Microstructure, mechanical properties, behavior of deformation, micromechanical response and failure propagation subjected to tension have been investigated. The representative volume elements (RVEs) technique and the deformation plasticity model (Ramberg-Osgood) have been employed to understand the effects of microstructure morphology on the micromechanical response and failure initiation under tension. It has been found that the Mn addition effectively modifies the Al-12.6Si alloy’s microstructure morphology, as an effect mechanical properties improved. Additionally, the alloy has a combination of brittle and ductile fracture modes under tension. The finite element analysis reveals that the stress and strain distribution (micromechanical response) and deformation behaviour of simulated Al-12.6Si Alloy RVEs change with changing Mn concentration in the alloy. Both the primary and eutectic Si phase and Al6Mn phase shared maximum load during tensile deformation. The primary mode of failure would be initiated by breaking of eutectic silicon flakes and unmodified primary Si particles and Al6Mn intermetallic. Both the Si particle's sharp corners and Al and Si phase interface are found as the preferable crack initiation site. Additionally, void initiation takes place at the narrow Al phase region in the microstructure. Eutectic Al-12.6Si with 1 wt% Mn is found to give more uniform stress and deformation distribution, therefore the simulated results have a great concurrence with the experimented results.

Microstructure and Mechanical Property of Al6Si2Cu Alloy Subjected to Double-Solution Heat Treatment

This study aims to develop the mechanical properties of the Al6Si2Cu aluminum alloy through the double-solution treatment. In addition to the Al matrix, large amounts of coarse eutectic Si, Al2Cu intermetallic, and Fe-rich phases were generated through thermo-calc simulation in agreement with the equilibrium phases. The eutectic Si phase is fragmented and spheroidized by the solution treatment as the heat treatment temperature and time increase. The Al2Cu intermetallic phase is dissolved into the Al matrix, resulting in an increase in both strength and elongation. The second-step solution temperature at 525 °C should be an optimum condition for enhancing the mechanical properties of the Al6Si2Cu aluminum alloy.

Microstructure Evolution and Solidification Behavior of a Novel Semi-Solid Alloy Slurry Prepared by Vibrating Contraction Inclined Plate

In this work, based on the A356 alloy, a novel Al–Si–Mg–Cu–Fe–Sr alloy with good mechanical property and high thermal conductivity was developed. The semi-solid slurry of the alloy was prepared via the vibrating contraction inclined plate. The microstructure evolution and solidification behavior of the alloy were investigated. The results demonstrated that, compared with the A356 alloy, the enhanced property of the Al–Si–Mg–Cu–Fe–Sr alloy was associated with the size of primary α-Al grains and morphology of eutectic Si phases. In addition, the preparation process parameters of semi-solid slurries, including the pouring temperature, inclination angle, and vibration frequency, had a crucial effect on the size and morphology of primary α-Al grains. The optimized pouring temperature, inclination angle, and vibration frequency were 670 °C, 45°, and 60 Hz, respectively. In this condition, for the primary α-Al grains, a minimum grain diameter of 64.31 µm and a maximum shape factor of 0.80 were obtained. This work provides a reference for the application of the alloy with high performance in the field of automobile and communication.

The Applicability of Die Cast A356 Alloy to Additive Friction Stir Deposition at Various Feeding Speeds.

In the current investigation, additive friction stir-deposition (AFS-D) of as-cast hypoeutectic A356 Al alloy was conducted. The effect of feeding speeds of 3, 4, and 5 mm/min at a constant rotational speed of 1200 rpm on the macrostructure, microstructure, and hardness of the additive manufacturing parts (AMPs) was investigated. Various techniques (OM, SEM, and XRD) were used to evaluate grain microstructure, presence phases, and intermetallics for the as-cast material and the AMPs. The results showed that the friction stir deposition technique successfully produced sound additive manufactured parts at all the applied feeding speeds. The friction stir deposition process significantly improved the microstructure of the as-cast alloy by eliminating porosity and refining the dendritic α-Al grains, eutectic Si phase, and the primary Si plates in addition to intermetallic fragmentation. The mean values of the grain size of the produced AMPs at the feeding speeds of 3, 4, and 5 mm/min were 0.62 ± 0.1, 1.54 ± 0.2, and 2.40 ± 0.15 µm, respectively, compared to the grain size value of 30.85 ± 2 for the as-cast alloy. The AMPs exhibited higher hardness values than the as-cast A356 alloy. The as-cast A356 alloy showed highly scattered hardness values between 55 and 75.8 VHN. The AMP fabricated at a 3 mm/min feeding speed exhibited the maximum hardness values between 88 and 98.1 VHN.

Microstructures and corrosion resistances of hypoeutectic Al-6.5Si-0.45 Mg casting alloy with addition of Sc and Zr

The microstructures and corrosion resistances of hypoeutectic Al-6.5Si-0.45 Mg casting alloys with addition of both Sc and Zr were investigated by using scanning electron microscope, X-ray diffraction, electrochemical measurement techniques and immersion corrosion tests. The results show that the addition of lower content of Sc has some modifying effect on the eutectic Si phases, but doesn't exhibit obvious refining action on the primary α (Al) phases of the alloy. The combined addition of both Sc and Zr can enhance the refining and modifying effects of Sc, and transform the primary α(Al) phases to equiaxed crystal, and the eutectic Si phases to the fine granular shapes. The corrosion resistance of the test alloys is firstly weakened, and then recovered, to some extent, with the increase of the exposure time in the 3.5 wt%NaCl solution. The test alloys with fine eutectic Si have poorer corrosion resistance than the test alloys with coarse eutectic Si. The formation of more galvanic couplings in the test alloys with fine eutectic Si is responsible for the poor corrosion resistance. The immersion corrosion rate of the test alloys with fine eutectic Si is higher than that of the test alloys with coarse eutectic Si. This is consistent with the results of the electrochemical corrosion studies. The corrosion of the alloys mainly occurs in the eutectic region, and the eutectic α(Al) phases are dissolved. It indicates that the Si phase is more noble than the α(Al) phase, and the galvanic couplings can be formed between the eutectic Si and α(Al) phases.