Morphology Of Eutectic Silicon Research Articles

Effect of cadmium on the microstructure of Al–7Si alloy

Abstract In this article we discuss the effect of Cd on the morphology of eutectic silicon in Al–7Si alloy. It has been found from the results that the Cd segregates towards the inter-dendritic regions and leads to the refinement of eutectic silicon. The possible mechanism involved in refinement of eutectic silicon in Cd treated Al–7Si alloy has been proposed here in.

Effect of NaCL Upon an AI-Si Casting Alloy Hipoeutec

This work, comprehends the study about the effect of NaCl kind Halite upon the morphology of eutectic Silicon and porosity in a cast hipoeutectic Al-Si alloy. The melting was carried out in two furnaces, the first to combustion of LP gas in graphite crucible to increase the content of Si in the alloy from 0.13 to 5% wt using metallic silicon with a purity of 99.35% wt, and the second of electric resistances in the same kind of crucible to control the temperature at 750o C. The melting material was casting into molds of sand shell. To add the salt into the metallic bath, two techniques were used; one with injection by means of Argon gas and the other one by gravity, in both cases three quantities of salt were used, 0.5, 1.5 and 2.5% wt respectively, powdered and preheated at 150o C during 60 min. The obtained samples were analyzed metallographily evidencing the modification of the eutectic Si, presenting the best results the samples with 1.5&2.5% wt of salt added by gravity. Too was obtained effect of Salt upon the degassing of bath metallic, where the simples with 1.5% wt presented the best results. Finally, by means of Scanning Electron Microscopy, the precipitates present were characterized in the samples.

Effects of electromagnetic vibration on the structure and mechanical properties of Al-6%Si alloy during directional solidification

The effects of electromagnetic vibration on the grain refinement in directional solidification were investigated. It was found that the electromagnetic vibration applied in the melt not only can refine grains remarkably but also can enhance both tensile strength and ductility values of Al-6%Si alloy. SEM graphs show that coarse dendrite structure was broken up into a somewhat globular structure, and the morphology of eutectic silicon was changed from flaky to fibrous under electromagnetic vibration treatment. The refine mechanism under electromagnetic vibration was discussed.

Preparation of Al–Ti–C–Sr master alloys and their refining efficiency on A356 alloy

Al–Ti–C–Sr master alloys with various amounts of Sr were prepared through a method of liquid solidification reactions. The as-prepared Al–Ti–C–Sr master alloys were then used as grain refiners to modify A356 alloy. The microstructures of the Al–5Ti–0.25C–2Sr, Al–5Ti–0.25C–8Sr alloys and modified A356 alloy were investigated. The results showed that the Al–5Ti–0.25C–2Sr alloy consisted of phases of α-Al, lath-shaped or tiny blocky TiAl 3, granular TiC, and blocky or rim AlTiSr, while the Al–5Ti–0.25C–8Sr alloy contained an irregular blocky Al 4Sr phase besides the above-mentioned phases. Satisfactory grain refining and modifying effects were obtained by the addition of Al–Ti–C–Sr alloys (0.5 wt.%) to the A356 alloy. Meanwhile, the sizes of the α-Al dendrites / SDAS(40 µm) decreased to 32.7 µm (or 30 µm).The morphology of eutectic silicon was changed from needle-/platelike form to fibrous/globular form. The grain refinement and modification effects of Al–Ti–C–Sr alloys on A356 alloys were mutually promoted. Compared with the Al–5Ti–0.25C–2Sr alloy, the Al–5Ti–0.25C–8Sr alloy possessed higher efficiency in grain refinement and modification of the A356 alloys.

Effects of combined additions of Sr and AlTiB grain refiners in hypoeutectic Al–Si foundry alloys

Strontium is the most widely used and a very effective element for modifying the morphology of eutectic silicon, while Ti and B are commonly present in the commercial grain refiners used for Al–Si alloys. Systematic studies on the effects of combined additions of Sr and different AlTiB grain refiners on the Al + Si eutectic and primary aluminium solidification have been performed. While slight coarsening of both eutectic Si and primary aluminium grains occurs during holding, no obvious interactions are observed between Sr and AlTiB grain refiners when the addition level of grain refiners is low. As a result, a well-modified and grain refined structure was obtained. However, strong negative interactions between Sr and Al1.5Ti1.5B were observed as the addition level of the grain refiner increases. It was found that these interactions have a much more profound impact on the eutectic solidification than the primary Al solidification. The melt treated with combined additions of Sr and Al1.5Ti1.5B still shows good grain refinement efficiency even after losing its modification completely. The mechanism responsible for such negative interactions is further discussed.

Effects of electromagnetic stirring and superheat on the microstructural characteristics of Al–Si–Fe alloy

Semisolid metal (SSM) processing is getting momentum to be the preferred fabrication route for near or net-shaped castings. For SSM processing of Al–Si alloys, there are two important microstructural features which influence the properties of finished products. These are the size and morphology of the primary α-Al phase and the eutectic matrix which consists of silicon, aluminum, and intermetallic phases. The presence of iron is beneficial to reduce the soldering effects in permanent mold castings, but it is unfavorable due to formation of a range of intermetallic phases. The casting parameters play an important role in avoiding the segregation of thin platelets of Fe-intermetallic, which is detrimental to mechanical properties. In this context, it is of particular interest to see the effects of melt stirring on the formation of eutectic silicon flakes and iron-based intermetallics during SSM processing. The effects of process parameters such as cooling rate and superheat are investigated on the morphology and size distribution of eutectic silicon and iron-based intermetallics during application of electromagnetic stirring (EMS). It is concluded that melt stirring not only alters the morphology of α-Al phase to rosette or globule shape, but also refines the eutectic silicon and iron-intermetallics.

Influence of Silicon Morphology and Mechanical Properties of Piston Alloys

In this paper, the influence of silicon particle morphology on mechanical properties of two piston alloys has been reported. Alloys having nominal composition Al-12%Si-1%Ni-0.8%Cu-0.6%Mg (LM13) and Al-17%Si-1%Ni-0.8%Cu-0.6%Mg (LM28) were prepared by melting and casting. The morphology of silicon crystals was changed using additives during the melt treatment and subsequently by heat treatment. Mechanical properties such as hardness, ductility, and tensile strength of experimental alloys were tested. Test results showed that the melt treatment and heat treatment of both the alloys increased the tensile strength, hardness, and ductility. However, the influence of melt treatment on mechanical properties was not as significant as that of heat treatment. Tensile strength and ductility of LM13 was found to be higher than the LM28 alloy under similar conditions. Hardness of LM28 was higher than the LM13 alloy. Optical and SEM studies showed a change in eutectic silicon morphology on melt treatment of LM13. However, change in morphology of primary silicon particles was not significant as for LM28. Heat treatment of both alloys caused spheroidization and better distribution of eutectic silicon crystals. Sharp-edged primary silicon particles were rounded off after the heat treatment of LM28. Scanning electron microscopy (SEM) of tensile fractured surfaces was carried out to study the influence of microstructure on fracture mode. Heat treatment of both alloys promotes dimple fracture. However, as-cast and melt-treated alloys show predominantly cleavage fracture.

Solidification microstructure and heterogeneous nucleation phenomena in cast SiCp‐reinforced hypereutectic Al–Si alloy composites

The microstructure and heterogeneous nucleation phenomena in cast SiCp‐reinforced hypereutectic Al–Si/alloy composites have been studied. Composite specimens were prepared using the compocasting technique and followed by remelting at 1023 K. The microstructure evolution of the composite has been investigated by water quenching the composite specimens at different stages of solidification. It has been observed that SiC particles can serve as substrate for the primary Si particles but not for α‐Al dendrites. Instead primary α‐Al dendrites push SiC particles into the last solidified liquid.In the presence of SiC particles, the eutectic silicon morphology changes from plate and needle shape of the unreinforced alloy to just plate shape. Globularisation of primary α‐Al dendrites occurs at the eutectic temperature while primary Si crystals tend to agglomerate as solidification proceeds. As a result both the average size and the average volume fraction of the primary Si crystals increases. A significant refinement in the size of eutectic silicon can also be observed but only in the regions where SiC particles are present.

Eutectic solidification and its role in casting porosity formation

Understanding and controlling the eutectic solidification process in Al-Si alloys permits prediction of the formation of casting porosity, eventually leading to methods for its control and elimination. In addition, it enables control of eutectic structure, silicon morphology, and eutectic grain size to further improve the alloy properties. This paper presents the current understanding of eutectic solidification in hypoeutectic Al-Si foundry alloys and the relationship between eutectic solidification and porosity formation. New concepts in engineering eutectic solidification are also explored.

Eutectic grains in unmodified and strontium-modified hypoeutectic aluminum-silicon alloys

Additions of strontium to hypoeutectic aluminum-silicon alloys modify the morphology of the eutectic silicon phase from a coarse platelike structure to a fine fibrous structure. Thermal analysis, interrupted solidification, and microstructural examination of sand castings in this work revealed that, in addition to a change in silicon morphology, modification with strontium also causes an increase in the size of eutectic grains. The eutectic grain size increases because fewer grains nucleate, possibly due to poisoning of the phosphorus-based nucleants, that are active in the unmodified alloy. A simple growth model is developed to estimate the interface velocity during solidification of a eutectic grain. The model confirms, independent of microstructural observations, that the addition of 100 ppm strontium increases the eutectic grain size by at least an order of magnitude compared with the equivalent unmodified alloy. The model predicts that the growth velocity varies significantly during eutectic growth. At low strontium levels, these variations may be sufficient to cause transitions between flake and fibrous silicon morphologies depending on the casting conditions. The model can be used to rationally interpret the eutectic grain structure and silicon morphology of fully solidified aluminum-silicon castings and, when coupled with reliable thermal data, can be used to estimate the eutectic grain size.

Effect of Al-5Ti-1B on the microstructure of near-eutectic Al-13.0%Si alloys modified with Sr

The addition of Sr in Al-Si alloys as a modifier causes a transition of the morphology of eutectic silicon from coarse plate-like or acicular to fine fibrous. However, it may also lead to the formation of long columnar dendritic α-Al phase. Al-5Ti-1B master alloy is often used as a grain refiner to achieve the fine equaixed grains in aluminum and aluminum alloys. The aim of the present study is to highlight the effect of Al-5Ti-1B master alloy additions on the microstructure of near-eutectic Al-Si alloys modified with Sr. When the addition of Al-5Ti-1B master alloy was below 0.82 mass%, the dendritic α-Al phase changed from long columnar to equiaxed, and there were no noticeable changes of the morphology and size of eutectic silicon, while the size of eutectic cells decreased slightly. However, when the addition was above 0.82 mass%, the deleterious influence of Al-5Ti-1B master alloy on the modification effect of Sr emerged and with further increases in addition level a fully unmodified microstructure was finally produced. The results indicate that the effective Sr in the melt decreases with increased addition of Al-5Ti-1B. The poisoning event of Al-5Ti-1B master alloy on the modification of Sr is supposed to be related with the interaction between Sr and Ti.

The fracture behaviour of A356 alloys with different iron contents under resonant vibration

The fracture behaviour of A356 alloys with different iron contents under resonant vibration has been investigated by examining the morphology of eutectic silicon and Fe-rich intermetallic phases. The test materials were prepared by varying the iron content within the range of 0.14 to 0.97 wt%. The experimental results shows that while the iron content is below 0.57 wt%, the Fe-rich intermetallic phases are mainly π-Al8Mg3FeSi6 and α-Al15Fe3Si2 and there is no obvious difference in the fracture resistance under resonant vibration. However, when the specimen has higher iron content (0.97 wt%Fe), the fracture resistance drops due to the formation of β-Al5FeSi. Hence, the β-Al5FeSi morphology is more detrimental to the fracture resistance than that of π-Al8Mg3FeSi6 or α-Al15Fe3Si2. The results indicate that the vibration fatigue cracks initiate at the surface and the fracture paths go predominantly through the silicon particles, and occasionally through the Fe-rich intermetallic phase particles. The detrimental β-Al5FeSi becomes influential in the specimen with higher iron content. The results also show that the line intercepted density (LID) and projected crack intercepted density (PCID) values of Fe-rich intermetallic phases increased with increasing area fraction of Fe-rich intermetallic phases (or iron content). This shows that an increase in area fraction of Fe-rich intermetallic phases (or iron content) may promote the crack propagation and growth under resonant vibration.

The effect of SiC particles on the size and morphology of eutectic silicon in cast A356/SiC p composites

Properties of cast aluminium matrix composites are greatly influenced by the nature of distribution of reinforcing phase in the matrix and matrix microstructural length scales, such as grain size, dendrite arm spacing, size and morphology of secondary matrix phases, etc. Earlier workers have shown that SiC reinforcements can act as heterogeneous nucleation sites for Si during solidification of Al–Si–SiC composites. The present study aims at a quantitative understanding of the effect of SiC reinforcements on secondary matrix phases, namely eutectic Si, during solidification of A356 Al–SiC composites. Effect of volume fraction of SiC particulate on size and shape of eutectic Si has been studied at different cooling rates. Results indicate that an increase in SiC volume fraction leads to a reduction in the size of eutectic Si and also changes its morphology from needle-like to equiaxed. This is attributed to the heterogeneous nucleation of eutectic Si on SiC particles. However, SiC particles are found to have negligible influence on DAS. Under all the solidification conditions studied in the present investigation, SiC particles are found to be rejected by the growing dendrites.

Refinement of Cast Microstructure of Hypereutectic Al-Si Alloys Through the Addition of Rare Earth Metals

Microstructural observation and thermal analysis of Al-21 wt % Si alloys with different rare earth metals were performed to examine the effect of rare earth metal on the refinement of primary silicon phase. Simultaneous refinement of both primary and eutectic silicon morphology is achieved with the addition of rare earth and its effect increases with the amount of rare earth addition and cooling rate. Depression of 12–17 °C in primary reaction temperature and 2–7 °C in eutectic temperature is measured with the addition of rare earth. Rare earth bearing compounds were not believed to act as a nucleation agent of primary silicon phase. Some rare earth bearing compounds determined to AlCe were around primary silicon in the matrix. The twin density of eutectic silicon remains same regardless of the addition of rare earth. The refinement of silicon in rare earth treated hypereutectic Al-Si alloys is supposed to be due to the suppression of the nucleation temperature of silicon phase.

Effect of calcium on mechanical properties of recycled aluminiumcasting alloys

The effect of the addition of calcium on tensile properties, fracture toughness, and fatigue characteristics was investigated in JIS AC4C-T6, JIS AC2B-T6, and JIS AC4D–T6 aluminium casting alloys with various iron contents. The number of coarse platelike Al-Fe based intermetallic compounds increased with increasing iron content. The modification of the eutectic silicon morphology is successfully achieved by the addition of 40 ppm calcium. The addition of calcium makes the shape of the intermetallic compound, as well as the eutectic silicon, finer and round, and this seems to contribute to the improvement of mechanical properties, i.e. elongation, Charpy absorbed energy, both impact and conventional fatigue strengths, dynamic fracture toughness, and impact fatigue fracture toughness. Impact fatigue strength, especially in the high cycle region, increases with the addition of calcium. In the lower stress high cycle fatigue region, a higher fatigue strength with increasing iron content is observed owing to the deflected crack path along Al-Fe based intermetallic compounds.

Effect of calcium on mechanical properties of recycled aluminiumcasting alloys

The effect of the addition of calcium on tensile properties, fracture toughness, and fatigue characteristics was investigated in JIS AC4C-T6, JIS AC2B-T6, and JIS AC4D–T6 aluminium casting alloys with various iron contents. The number of coarse platelike Al-Fe based intermetallic compounds increased with increasing iron content. The modification of the eutectic silicon morphology is successfully achieved by the addition of 40 ppm calcium. The addition of calcium makes the shape of the intermetallic compound, as well as the eutectic silicon, finer and round, and this seems to contribute to the improvement of mechanical properties, i.e. elongation, Charpy absorbed energy, both impact and conventional fatigue strengths, dynamic fracture toughness, and impact fatigue fracture toughness. Impact fatigue strength, especially in the high cycle region, increases with the addition of calcium. In the lower stress high cycle fatigue region, a higher fatigue strength with increasing iron content is observed owing to the deflected crack path along Al-Fe based intermetallic compounds.

Crack propagation behavior of A356 aluminum alloy under resonant vibration

During the application of a structural component, failure by resonant vibration may occur under some abnormal situations. The study of this failure is necessary in order to ensure the reliability of the component. In the current investigation, A356 aluminum alloy (Al-7wt% Si-0.3wt% Mg) was chosen to explore the crack propagation behavior under resonant vibration. This alloy, which contains eutectic silicon particles, has been used as many of the structural components in automobiles and aircrafts. Since the shape and distribution of second phase particles can affect the fatigue crack propagation of a two-phase material, they should also affect the crack propagation under resonant vibration. To study the effect of eutectic silicon morphology and distribution, the A356 alloy was tested under the as-cast condition and two solution-treated conditions.

Modification of Al-Si Microstructure - The Al-Si-Sr Phase Diagram from 0–20 WT. % Si and 0–5.0 WT. % Sr

ABSTRACTIt is well known that the morphology of primary and eutectic silicon resulting from solidification is sensitive to minor impurity additions: “modification” by alkali and alkaline earth metals is common foundry practice and the use of strontium is of recent and current interest because its effect is retained in liquid metal for relatively long times. The mechanism for modification remains obscure and is not necessarily the same from one elemental addition to the next, but it is essential to separate the equilibrium and kinetic influences if the phenomenon is to be better understood. Accordingly, the liquidus surface and eutectic reactions have been located in the relevant part of the Al-Si-Sr system by careful thermal analysis and the results correlated with the microstructure. Comparison is made with the parallel behavior of alkali metal additions.

Morphology of eutetic silicon and mechanical properties of Al-Si alloys

Studies were made on the relation between the three dimensional morphology of eutectic silicon in Al-13%Si alloy and the mechanical properties of the alloy.The examination under a scanning electron microscope or by the successive polishing method revealed that the eutectic silicon crystals, which were apparently acircularon polished surfaces (by observation under on optical microscope), were flaky or stick-like and interconnected with one another over a wide range. The specimens aged at 400550°C for 45hrs. or less were sounded and then showed the neckings of silicon flakes in several places, which would be led to division of crystals. The silicon crystals of aged specimens were larger in thickness, narrower in width, and advanced in granulation as compared with the as cast ones. The shape of the eutectic silicon crystals in sodium treated specimens was fibrous and they were interconnected with one another over a wide range.The change in interconnection of crystals and the change of shape toward granulation by aging at high temperatures resulted in slight decrease in yield and tensile strengths (at 0.2% in proof stress and ultimate tensile strength), except for the drop of strengths at the initial stage of aging. However, the duitility (total elongation and reduction in crosssectional area) was remarkably improved in proportion to the aging time.