Morphology Of Eutectic Silicon Research Articles

Effect of Alloying Elements Mg and Cu on the Modification of Eutectic Silicon in Hypoeutectic Al–Si Alloys

The modification of eutectic silicon plays a crucial role in enhancing the mechanical properties of hypoeutectic Al–Si alloys. However, there is still limited understanding regarding the factors that affect the modification of eutectic silicon in hypoeutectic Al–Si alloys, particularly in terms of the role played by alloying elements during the modification process. In order to address this gap, this study aimed to investigate the influence of two key alloying elements, Mg and Cu, on the modification effect of Sr. To achieve this, the morphology of eutectic silicon in the as-cast and heat-treated states of Al–7Si alloy, modified with Sr, was examined using scanning electron microscopy (SEM). Different levels of magnesium and copper content were used to analyze their impact on the modification effect of Sr. The results obtained from the analysis revealed that both Mg and Cu significantly weaken the modification effect of strontium on eutectic silicon. Furthermore, it was observed that the degree of deterioration in the modification effect increases progressively with higher alloying element content. Interestingly, increasing the strontium content and adjusting the cooling rate were found to be ineffective in eliminating this weakening effect caused by Mg and Cu. These findings highlight the complex interplay between alloying elements and the modification process of eutectic silicon in hypoeutectic Al–Si alloys. A deeper understanding of these factors is essential for the development of effective strategies to optimize the mechanical properties of such alloys.

Effect of trace bismuth on the solidification structure of hypereutectic Al–22Si alloys

The effect of trace Bi on the solidification structure of the Al–22Si alloy was studied by optical microscopy, scanning electron microscopy, field emission transmission electron microscopy, X-ray diffraction, and differential scanning calorimetry. Trace amounts of Bi were found to have an inhibitory effect on the growth of primary silicon in the Al–22Si alloy. The average size and area fraction of primary silicon reached a minimum when 0.05 wt% Bi was added. As the bismuth content gradually increased, the average size and area fraction of primary silicon continued to increase. The addition of Bi improved the morphology of eutectic silicon in the Al–22Si alloy, as it gradually changed from long needle flakes to short needle flakes. Stacking faults formed in eutectic silicon may have resulted in its morphogenesis.

The Effect of Heat Treating and Deformation by Rolling and Forging on the Mechanical Properties of the 4032-Type Alloy Prepared from Recycled Materials

In the present work, the relationship between deformation, microstructure and mechanical properties of the Sr-modified 4032-type eutectic aluminum alloy was studied. The alloy was prepared from recycled materials, mainly from maritime and automotive 356 alloy scrap samples. Solubilizing heat treatment was carried out at a temperature of 450 °C with a holding time of 8 h. Finally, different samples were subjected to rolling and forging processes at a temperature of 450 °C, thus achieving a reduction of 25% of the original thickness. As expected, the microstructure and properties changed significantly due to the deformation processes, where an important factor was the change in the morphology of eutectic silicon, not only produced by the application of deformation, but also on the effect of adding strontium as a modifying agent. The samples were characterized by optical microscopy and scanning electron microscopy, where it was possible to observe not only the effect of strontium on the morphology of eutectic silicon but also the effect of the heat treatment performed. The tensile tests showed that there was indeed a notable increase in the ultimate tensile stress, yield strength, and resistance to fracture, while initial hardness also considerably increased. Finally, the fracture analysis showed that, after thermal treatment and deformation, all the samples analyzed presented a fracture within the ductile regime. It was shown that the combination of deformation and the addition of strontium led to improved globulization of the eutectic silicon.

Microstructure and Hardness of an Al–8 wt%Si–2.5 wt%Bi Alloy Subjected to Solidification Cooling Rates from 0.1 to 800 K s−1

This work explores the effect of the addition of bismuth (Bi) to Al–8 wt%Si alloys. Bi in Aluminum based alloys works as a self‐lubricating agent, improving machining and wear properties. As Bi is a soft material, it is essential to evaluate how it affects microstructural features and the resulting properties of Al–8 wt%Si alloys. Herein, this hypoeutectic alloy is modified by the addition of 2.5 wt%Bi and subjected to three solidification techniques: differential scanning calorimetry, transient directional solidification, and impulse atomization. Thus, this work investigates the effect of Bi in samples solidified under a wide range of cooling rates. Of specific interest is how Bi modifies the eutectic silicon morphology and alloy hardness compared with a hypoeutectic Al–10 wt%Si alloy from the literature. The silicon (Si) morphology of Al–8 wt%Si–2.5 wt%Bi transitions from flaky (coarse) to fibrous (fine) at a critical cooling rate of 1100 K s−1. Through the combination of Bi addition and processing through impulse atomization, the ternary Al–Si–Bi alloy achieves improvements in hardness of up to 20% compared to Al–10 wt%Si. This is despite having a coarser eutectic microstructure than the binary hypoeutectic Al–Si alloy. This is due to Bi modifying the morphology of the eutectic Si.

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.

High quality plate-shaped A356 alloy casting by a combined ablation cooling and mold heating method

High quality plate-shaped A356 plate castings, with length much greater than the feeding distance, are difficult to make by conventional sand casting processes. This article describes our initial work on combining mold heating with ablation cooling in making plate-shaped castings of high internal quality. Our experimental results indicate that shrinkage porosity can be largely eliminated when the mold temperature is higher than 200 °C. Furthermore, the morphology of eutectic silicon is modified, from flake-like in sand casting to coral-like in castings subjected to ablation cooling. As a result, the mechanical properties of the plate casting, especially ductility, are significantly enhanced by the combined effect of mold heating and ablation cooling. Numerical modeling was performed in this study to quantify the cooling conditions involved.

Influence of Alloying Element Mg on Na and Sr Modifying Al-7Si Hypoeutectic Alloy.

The influence of alloying element Mg on Na and Sr modifying Al-7Si hypoeutectic alloys was investigated. The residual content of Na and the morphology of modified eutectic silicon were characterized. It was found that the alloying element Mg had an enhanced effect on the uptake of sodium in the Al-7Si hypoeutectic alloy modified by the Na-contained modifier. Moreover, the morphology of eutectic silicon of the modified Al-7Si alloys was significantly different from that of Al-7Si-0.4Mg alloys in the present research. When the addition of the modifier is enough, both modifiers could entirely modify the eutectic silicon phase of Al-7Si alloys, while incompletely modified eutectic silicon was observed in both Na-modified and Sr-modified Al-7Si-0.4Mg alloy. It was observed that there was an adhering relationship between the partially modified eutectic silicon with Mg-rich phases. According to the results, it can be proposed that the addition of Mg will affect the solidification behavior of alloys, thereby, leading to the incomplete modification of eutectic silicon phases.

Microstructure based analysis and predictive modeling of cast Al7Si1.5Cu0.4Mg alloy mechanical properties

The heterogeneity of microstructure leads to non-uniform distribution of mechanical properties, which is also related to strength and reliability degradation of components. Taking diesel engine cylinder heads as an example, this study investigated the relationship between microstructures and tensile mechanical properties of cast Al7Si1.5Cu0.4Mg alloy with different temperatures, including secondary dendrite arm spacing (SDAS), grain size, eutectic silicon morphology. The results indicate that the mechanical properties of cast Al7Si1.5Cu0.4Mg alloy have a strong correlation with the microstructures. Based on the linear regression analysis method, several predictive models were established to quantify the tensile mechanical properties of cast Al7Si1.5Cu0.4Mg alloy with different microstructures. The Hall-Petch model was used to capture the relation between microstructure parameters and yield strength, and a linear relationship model was employed between the hardness and yield strength. The prediction model of ultimate tensile strength was proposed based on the relationship of microstructure and hardness. In addition, considering the error limit of the Hall-Petch relationship, the upper threshold of grain size and SDAS is calibrated. By considering the influence of the material's plastic deformation ability and the material strength on fatigue performance, a novel Tensile-Fatigue Bridge Model was proposed. The model provides a reference for the mechanical properties prediction work of thin-walled small parts and also contributes to the strength evaluation of components with non-uniform microstructures.

Effect of Cooling Rate and Modification by Strontium on the Thermal Conductivity of Al-8Si Alloy

Cooling rate plays a critical role in determining the thermal conductivity of Al-Si alloys. Although the effect of morphology and size of Si (changed by heat treatment) on its thermal conductivity has been investigated, the effect of cooling rates on thermal conductivity has not been well studied. In this study, we investigated the microstructure of an Al-8Si (with and without modification by Strontium (Sr)) alloy with cooling rates from 46.2 °C/s to 234 °C/s. It was found that the effect of cooling rate on thermal conductivity of Sr modification and Sr-free samples are opposite from each other. As a result, while the cooling rate increased from 46.2 °C/s to 234 °C/s, the calculated thermal conductivity increased from 145.3 MS/m to 151.5 MS/m for Sr-free Al-8Si alloy, and the calculated thermal conductivity was reduced from 187.5 MS/m to 176.7 MS/m for the Sr-modified Al-8Si alloy. By discussing how thermal conductivity correlates with eutectic silicon morphology and secondary dendrite arm spacing, the relationship between cooling rate and thermal conductivity were explained. This work suggests a new design strategy for improving the thermal conductivity of Al-Si hypoeutectic alloys.

Modeling Ablation Casting

Research published in open literature on ablation casting has been mainly experimental. Modeling of this innovative technology is scarce. This article describes an initial attempt in modeling ablation casting process. Material properties and boundary conditions were defined for calculating cooling curves and microstructural length scales in A356 alloy castings made using sand casting and ablation casting processes. Model predictions were compared with published data on cooling curves and solidification times in an A356 alloy casting. The validated model was then used to predict unique solidification features associated with the ablation casting process, including the eutectic solidification time, shrinkage porosity formation, eutectic front morphology, the advancing rates of the eutectic front and the associated eutectic silicon morphology. Simulation results indicate that the progressive advancing ablation cooling is capable of reducing secondary dendrite arm spacing, modifying the eutectic silicon phase owing to the water quench effect, and eliminating shrinkage porosity using much smaller risers than usual if the ablation is correctly driven towards the risers.

Influence of natural beach mineral corundum on the wear characteristics of LM30 aluminium alloy composites

The work investigates the effect of corundum (Al2O3) reinforcement on the dry sliding wear of LM30 aluminium alloy composites. Composites with the different size range of corundum particles (fine; 1–20 μm, medium; 32–50 μm and coarse; 75–106 μm) and weight percentages varying from 5 to 20 wt% were prepared. The stir casting process was used for the fabrication of composites. LM30 billets were melted at 850 and stirred with the graphite stirrer for 3–4 min. Corundum particles were added to the melt and the molten mass was stirred for another 3–4 min. Finally, the mixture was poured into a cast iron mould. Pins of 8 mm diameter were prepared for wear test and tested against the hardened die steel disc. Optical microscopy of the composites revealed a uniform distribution of corundum particles in the matrix. The microstructure of the composite also got refined. The acicular morphology of eutectic silicon in base alloy got refined to globular for the composites leading to improvement in the hardness of the composites. Dry sliding wear analysis of the composites revealed the decrement in the wear rate of the composites over the base alloy. Similar to the wear rate, the coefficient of friction of the composites also decreased with the increase in the corundum weight percentage. Finally, the wear tracks and debris analysis of the composites revealed the underlying wear mechanisms during the dry sliding wear behavior at a low contact pressure of 0.2 MPa and high contact pressure of 1.8 MPa. The wear mechanism of the composite is dependent on contact pressure. At low contact pressure of 0.2 MPa, the abrasive type wear transformed to adhesive type at a high contact pressure of 1.8 MPa.

Hot compression and industrial extrusion characteristics of an as-cast Al-10Sr master alloy

Abstract The mechanical properties and castability of Al-Si-based alloys can be remarkably improved by adding Al-Sr master alloy, which modifies the structure and morphology of eutectic silicon. The size of Al4Sr phases in Al-Sr master alloy has been found to take key effect in Sr-modification. To obtain optimized processing parameters and understand the refinement mechanisms of Al4Sr phases during plastic deformation, the hot compression and industrial extrusion characteristics of an as-cast Al-10Sr master alloy were systematically investigated in this work. The results showed that the flow behavior of the Al-10Sr master alloy can be well described by a constitutive equation with the hot deformation activation energy of 222.23 kJ/mol. The obtained optimum processing region was located at temperature of 460–530 °C and strain rate of 0.2–1 s−1 from the established processing maps. During hot compression testing, plastic deformation mainly occurred in soft Al matrix and the Al4Sr phases were slightly broken. During industrial extrusion process, the Al4Sr phases were refined effectively based on the above optimum processing conditions. Comparing to conventional extrusion process, repetitive Conform process showed better ability in refining Al4Sr phases. After three repetitive Conform passes, the length of Al4Sr phases decreased from 80 to 11 μm and the width decreased from 25.6 to 8.3 μm, and the Al4Sr phases were also distributed homogeneously. The in-depth refinement mechanisms of Al4Sr phases during plastic forming were also discussed based on microstructural examinations and finite element analysis.

Effect of Modification with Different Contents of Sb and Sr on the Thermal Conductivity of Hypoeutectic Al-Si Alloy

In this paper, the effects of size, morphology and distribution of eutectic silicon on the thermal conductivity of Al-8Si alloy modified by Sr (0.04, 0.08, 0.12 wt.%) and Sb (0.1, 0.3, 0.5 wt.%) elements with T6 heat treatment were investigated. The results show that the modified fibrous eutectic silicon has a significant capability of improvement of thermal conductivity, while the amount of the modifier has a relatively weak effect on thermal conductivity. After T6 treatment, the fracture or spheroidization of the flake eutectic silicon and the disappearance of clustering phenomenon could raise thermal conductivity, but the coarsening of fibrous eutectic silicon is inconducive to thermal conductivity. Finally, the effect of eutectic silicon on electron transport is analyzed in detail, which could provide a reference for enhancing the thermal (or electrical) conductivity of hypoeutectic Al-Si alloy through effective microstructure control.

Simultaneously improving strength and ductility of hybrid Al–Si matrix composite with polyphasic and multi-scale ceramic particles

Hybrid aluminum matrix composites have received widespread attention because of their superior performance to single-phase reinforced aluminum matrix composites. In situ VB2–AlN–Al4C3–VSi2/Al ribbons with polyphasic and multi-scale reinforcement particles were prepared by powder metallurgy method and controlled by melt-spinning and then verified the reinforcement efficacy in Al–12Si alloy. It was found that the addition of VB2–AlN–Al4C3–VSi2/Al ribbons not only refined the grain of α-Al, but also optimized the morphology of eutectic silicon for Al–12Si matrix composite. Thereafter, the mechanical properties of the VB2–AlN–Al4C3–VSi2/Al–12Si composite, including the tensile strength, ductility and hardness were increased by 36.2%, 82.2% and 33.4%, respectively, with the addition of VB2–AlN–Al4C3–VSi2/Al ribbons. The excellent mechanical properties can be attributed to the micron-sized VSi2 particles, nano-sized AlN, VB2, and Al4C3 as well as the well-bonded particle/matrix interface. In addition, multiple strengthening mechanisms have also made outstanding contributions to the improvement of the mechanical properties, including (1) Orowan strengthening; (2) Coefficient of thermal expansion (CTE) mismatch strengthening; (3) the Hall-Petch effect caused by grain refinement. The present experimental results provide an insight into the understanding of polyphasic and multi-scale reinforcement particles, and extend the effort for the development of Al–Si composite.

Corrosion of Cast Aluminum Alloys: A Review

Research on corrosion resistance of cast aluminum alloys is reviewed in this article. The effect of the main microstructural features of cast aluminum alloys such as secondary dendrite arm spacing (SDAS), eutectic silicon morphology, grain size, macrosegregation, microsegregation, and intermetallic compounds is discussed. Moreover, the corrosion resistance of cast aluminum alloys obtained by modern manufacturing processes such as semi-solid and additive manufacturing are analyzed. Finally, the protective effects provided by different coatings on the aluminum cast alloys—such as anodized, plasma electrolytic oxidation (PEO), and laser—is reviewed. Some conclusions and future guidelines for future works are proposed.

Optimizing the Microstructure and Mechanical Properties of Vacuum Counter-Pressure Casting ZL114A Aluminum Alloy via Ultrasonic Treatment

The effect of ultrasonic temperature on density, microstructure and mechanical properties of vacuum counter-pressure casting ZL114A alloy during solidification was investigated by optical microscopy (OM), scanning electron microscope (SEM) and a tensile test. The results show that compared with the traditional vacuum counter-pressure casting aluminum alloy, the primary phase and eutectic silicon of the alloy with ultrasonic treatment has been greatly refined due to the dendrites broken by ultrasonic vibration. However, the refining effect of ultrasonic treatment on vacuum counter-pressure casting aluminum alloy will be significantly affected by ultrasonic temperature. When the ultrasonic temperature increases from 680 °C to 720 °C, the primary phase is gradually refined, and the morphology of eutectic silicon also changes from coarse needle-like flakes to fine short rods. With a further increase in the ultrasonic temperature, the microstructure will coarse again. The tensile strength and elongation of vacuum counter-pressure casting ZL114A alloy increases first and then decreases with the increase of ultrasonic temperature. The optimal mechanical properties were achieved with tensile strength of 327 MPa and the elongation of 5.57% at ultrasonic temperature of 720 °C, which is 6.3% and 8.2%, respectively, higher than that of alloy without ultrasonic treatment.

The effect of heat treatment on die casting aluminum to apply self-pierce riveting

Vehicle weight reduction has been discussed by automotive industry for decades. The current solution for weight reduction purpose is the use of aluminum stamping and casting parts to replace the steel parts. However, joining aluminum with aluminum or aluminum with steel parts would be difficult to select a proper joining technology to ensure a qualified joint. Self-pierce riveting (SPR) as a mechanical joining technique has been used by automotive industry owing to its simplicity, versatility, and environment friendly nature. Although SPR can avoid the joining quality problems caused by welding to join aluminum with steel parts, the other joining problems from SPR still hinder the industry to widely use this joining technology. This article analyzed the cause of cracks on joint button from the understanding of joined material. The modified heat treatments were used to investigate the relationship between eutectic silicon morphology and cracking condition on joint button. In addition, the lap-shear strength according to the samples with various heat treatments was also tested and analyzed. Eventually, the results indicated that silicon content and its morphology take main responsibility on crack initiation and propagation on SPR joint button. But the influence of cracks on joining strength still needs to be investigated and studied in the future work.

The Role of Silicon Morphology in the Electrical Conductivity and Mechanical Properties of As-Cast B319 Aluminum Alloy

The enhanced performance of automotive B319 aluminum alloys can be realized via the improvement of both strength and conductivity. Yet, vastly dissimilar mechanisms are responsible for each property, and the incomplete understanding of their respective dominant microstructural features impedes effective alloy design. In this study, permanent mold cast B319 alloy was systematically produced with total solidification rates between 0.14 and 5.89 °C s−1 and strontium contents up to 300 ppm to isolate their respective effects on material properties. The as-cast samples were characterized by their dendritic structure, eutectic silicon morphology, porosity content, hardness, tensile strength, ductility, and electrical conductivity. With increasing solidification rate, the refinement of microstructure considerably improved all mechanical properties analyzed. Nonetheless, these properties were found to be independent of strontium content, attributed to the role of the coarse and brittle intermetallic phases in fracture initiation. In contrast, conductivity was minimally affected by solidification rate in the unmodified condition. However, the synergistic silicon modification promoted by increasing both solidification rate and strontium enhanced conductivity by up to 3 pct IACS. The correlations developed with the quantified silicon characteristics establish this phase as dominant in the conductivity of B319 alloy, and they elucidate opportunities for the further enhancement of automotive materials.

Interactions Between Strontium and Sodium Fluorosilicate in the Combined Treatment of A319 Al Alloy

The mechanical properties of cast aluminum–silicon alloys are typically improved by eutectic silicon modification and melt degassing. First, trace strontium additions can modify the silicon phase and relieve stress concentrations at the naturally coarse, acicular particles. Second, chemical degasser additions, such as sodium fluorosilicate, can remove dissolved hydrogen gas from the aluminum melt to prevent the formation of deleterious gas porosity. However, combining both treatments is difficult, due to the tendency of the two chemicals to react and reduce their effectiveness. Nonetheless, there is limited research outlining specific procedures to enable their maximum potency, with both additions. In this work, the effects of strontium and sodium fluorosilicate on the eutectic silicon morphology and casting porosity in A319 aluminum alloy were investigated. This included a comprehensive study of the fading of their individual additions as well as the influence of separating the two chemical additions by various time intervals. The results of the melt treatments were characterized using optical microscopy, density measurements, and analysis of strontium contents using optical emission spectrometry. It was found that the maximum potency of both treatments was achieved by degassing prior to strontium additions, featuring both satisfactory silicon modification and porosity reductions.

Processing and characterisation of modified strain-induced melt activation processed Al–Si alloys

In the present study, the microstructural features and its correlation with mechanical properties of modified strain-induced melt activation (modified SIMA) processed Al- (7, 12, 14 wt-%) Si alloys are investigated. The microstructure of modified SIMA process Al–Si alloys consists of globular α-Al grains, primary Si particles and Al–Si eutectic phase. The casting defects and surface roughness are minimised and the morphology of eutectic silicon is modified after modified SIMA treatment. The mechanical properties of modified SIMA processed samples are increased significantly as compare to as-cast alloy due to reduction of porosity, formation of fine globular microstructu+re and accumulation of high dislocation tangle near primary Si particles.