Hypoeutectic Aluminum-silicon Alloys Research Articles

The Dispersion of Lead in Hypoeutectic Aluminium-Silicon Alloys by Spray Forming Process and their Characterization

The present paper describes the Lead dispersion in hypoeutectic Aluminium-Silicon alloys and their microstructural characteristics. The homogeneous dispersion of Lead (Pb) in Aluminium-Silicon alloys (Al-6Si+Pb (=0, 10, 15, 20, 25)) were fabricated by using of spray forming technique (also known as spray casting or spray deposition) in the form of near-net shape disc. The microstructural characterization has been done for different location of spray formed disc by using optical microscopy (OM), Scanning electron microscopy (SEM) and X-Ray Diffraction (XRD) respectively. The results of the optical image were exhibited the equiaxed and globular grain morphology of primary Aluminium with the homogeneous dispersion of extremely fine and malleable particles of Lead and spherical particles of silicon in α–Aluminium matrix. The dispersed Lead has seen clearly in the SEM images of spray formed Aluminium-Silicon alloys and result of XRD pattern confirmed the phases of Aluminium-Silicon and Lead. The grain size of Aluminium has been analyzed by intercept line method. The lattice parameter and lattice strain have been calculated with the help of the XRD pattern to determine the effect of Lead dispersion on Al-Si alloy.

Investigation on controlling the process parameters for improving the quality of investment cast parts

Investment casting is a highly flexible process which was previously perceived as an expensive process. However, when the process is compared to other optional processes which may require machining or welding, this casting can produce metallic components at highly competitive costs. There are many process variables which affect the process such as die temperature, wax temperature, injection pressure, shell firing temperature and time, cooling rate. In this study, important shell parameters such as preheat temperature, firing temperature and firing time, and melt pouring temperature have been chosen as process variables influencing the quality of the hypoeutectic aluminium–silicon alloy investment casting. The optimal input parametric condition for reduction of linear and volumetric shrinkages and increment of tensile strength of Al–Si 7%–Mg investment casting has been identified as shell preheat temperature of 200 °C, firing temperature of 900 °C, firing time of 7 h and pouring temperature of 600 °C. At this optimal setting, it was found that linear and volumetric shrinkages decreased from 0.65 and 1.89% to 0.381 and 1.546%. The tensile strength of the casting increased from 96 to 121 MPa with regard to the nine experimental runs performed. Microstructural observation revealed that higher shell preheat and pouring temperatures led to augmented porosity, increased secondary dendrite arm spacing (35.53 ± 2.4 μm), larger detrimental iron-rich intermetallics (40.49 ± 25.15 μm) followed by reduced tensile properties of the casting (96 MPa).

Studying The Effect of Nano-dispersions (Al2O3) on The Properties of A356 Hypo-eutectic Al-Si Cast Alloy

Improving the strength to weight ratio is a target aimed by many researchers. Metal Matrix Nano Composites (MMNC) have recently emerged as a new generation of composites, where the addition of nano-sized particles leads to an increase in the strength not according to conventional Metal Matrix Composites rules. Aluminium matrix composites reinforced with nano-sized Al2O3particles are widely used for high performance applications such as automotive, aerospace and electricity industries. In thiswork a numberof cast specimens made from hypoeutectic aluminium silicon alloy (A356) were cast with and without alumina (Al2O3) nanoparticles at different pouring temperatures with stirring during the addition of nano-reinforcement. The microstructure of the new castings was studied using optical microscopy and scanning electron microscopic techniques. Also, the tensile strength, hardness and ductility were evaluated. The significant factors for the tensile strength were analysed usingL4 orthogonal array and Analysis of Variance (ANOVA) techniques then building a model by Response Surface Methodology(RSM). The results obtained in this work show that adding alumina nanoparticles and pouring at different temperatures lead to an increase in the tensile strength and the hardness with less change in ductility

Effect of T6 Heat Treatment in the Microstructure and Mechanical Properties of A356 Reinforced with Nano Al2O3 Particles by Combination Effect of Stir and Squeeze Casting

Hypoeutectic aluminum silicon alloys as (A356) exhibit several specific and interesting properties that qualify them to be used in many automotive and aeronautical applications, aluminum matrix composites have good physical and mechanical properties, when reinforced with nano sized Al2O3 particle, they have high tensile, compressive and fatigue strengths along with superior wear resistance and thermal stability. In this work, the number of cast samples of A356 alloy was prepared by combination effect of stir and squeeze casting, specially designed and built furnace unit allowing for the addition of Al2O3 nano- particles (30nm). The percentage inclusions were varied from 0.5 to 1.5wt% into the molten Al-Si alloy in the liquid state with stirring at 400rpm and squeeze casting at 750̊C and pressure of 600MPa. The microstructural features and the mechanical properties of the as cast and T6 heat treated samples were investigated. The results obtained in the work showed enhance in the mechanical properties of the Al2O3 nano-dispersed alloys. Optical microscopic examination revealed uniform distribution of nano particles. Measurements of hardness, compressive strength and double shear strength showed an increase with increase in percentage addition of these nano particles.

Microstructure and Mechanical Properties of MWCNTs Reinforced A356 Aluminum Alloys Cast Nanocomposites Fabricated by Using a Combination of Rheocasting and Squeeze Casting Techniques

A356 hypoeutectic aluminum‐silicon alloys matrix composites reinforced by different contents of multiwalled carbon nanotubes (MWCNTs) were fabricated using a combination of rheocasting and squeeze casting techniques. A novel approach by adding MWCNTs into A356 aluminum alloy matrix with CNTs has been performed. This method is significant in debundling and preventing flotation of the CNTs within the molten alloy. The microstructures of nanocomposites and the interface between the aluminum alloy matrix and the MWCNTs were examined by using an optical microscopy (OM) and scanning electron microscopy (SEM) equipped with an energy dispersive X‐ray analysis (EDX). This method remarkably facilitated a uniform dispersion of nanotubes within A356 aluminum alloy matrix as well as a refinement of grain size. In addition, the effects of weight fraction (0.5, 1.0, 1.5, 2.0, and 2.5 wt%) of the CNT‐blended matrix on mechanical properties were evaluated. The results have indicated that a significant improvement in ultimate tensile strength and elongation percentage of nanocomposite occurred at the optimal amount of 1.5 wt% MWCNTs which represents an increase in their values by a ratio of about 50% and 280%, respectively, compared to their corresponding values of monolithic alloy. Hardness of the samples was also significantly increased by the addition of CNTs.

Influence of Phosphorus on the Nucleation of Eutectic Silicon in Al-Si Alloys

The role of phosphorus (P) in the heterogeneous nucleation of eutectic silicon (Si) and the evolution of eutectic grains in hypoeutectic aluminum-silicon alloys were investigated. Systematic additions of P in the range of 0.5 to 20 ppm to Al-7 wt pct Si alloys of different purities have shown that the morphology of the eutectic Si changes from a fine plate- to a coarse flake-like structure. The growth of eutectic grains was investigated by interrupting the eutectic reaction by quenching experiments. Moreover, the macroscopic growth mode of the eutectic grains was characterized by electron backscatter diffraction. An increase in P concentration from 2 to 3 ppm resulted in a transition of the macroscopic growth mode of the Al-Si eutectic in high purity alloys from growth with a planar front with a strong dependence of the thermal gradient, to nucleation in the vicinity of the primary Al dendrites and subsequent growth of distinct eutectic grains. It is suggested that AlP particles are the key impurities acting as potential nucleation sites for eutectic Si. This is further substantiated as with increasing P concentration nucleation and growth of the Al-Si occurred at higher temperatures close the equilibrium Al-Si eutectic solidification temperature at 850 K (577 °C). In addition, the recalescence undercooling ΔT R,eu was reduced from 4.5 K (0.5 ppm P) to 1.5 K (20 ppm P) in high purity alloys. This was accompanied by a drastic increase of the nucleation rate of the eutectic grains.

Influence of Nanodispersions on Properties and Microstructure Features of Cast and T6 Heat-Treated Al Si Hypoeutectic Alloys

Cast light metal alloys have retained their importance and unique characteristics as first candidates when cost-function relationship is considered. Hypoeutectic aluminum silicon alloys as (A356) exhibit several specific and interesting properties that qualify them to be used in many automotive and aeronautical applications. Evidence of significant enhancement in strength in the properties of Al-Si cast alloys by incorporating nano-particles have been recently presented. The present study aims at developing nano-dispersed Al-Si alloys with suitable casting methods that assure the dispersion of the nano-particles. In this work a number of cast samples of A356 were prepared by rheo-casting in a specially designed and built furnace unit allowing for the addition of the nano-particles into the molten Al-Si alloy in the semi-solid state with mechanical stirring. The microstructural features and the mechanical properties of the cast and T6 heat treated samples were investigated. The results obtained in this work showed enhancement in the mechanical strength of the nano-dispersed alloys, accompanied by significant increase in the elongation percentage, supported by evidence of refined dendrite arms length, and inter-lamellar spacing.

Influence of Al2O3 nano-dispersions on microstructure features and mechanical properties of cast and T6 heat-treated Al Si hypoeutectic Alloys

Cast light metal alloys have retained their importance and unique characteristics as first candidates when cost–function relationship is considered. Hypoeutectic aluminum silicon alloys as (A356) exhibit several specific and interesting properties that qualify them to be used in many automotive and aeronautical applications. Evidence of significant enhancement in the properties of Al–Si cast alloys by incorporating Al2O3 nano-particles have been recently presented. The present study aims at developing Al2O3 nano-dispersed Al–Si alloys with suitable casting methods that assure the dispersion of the nano-particles. In this work a number of cast samples of A356 were prepared by rheo-casting in a specially designed and built furnace unit allowing for the addition of the Al2O3 nano-particles into the molten Al–Si alloy in the semi-solid state with mechanical stirring. The microstructural features and the mechanical properties of the cast and T6 heat treated samples were investigated. The results obtained in this work showed enhancement in the mechanical strength of the Al2O3 nano-dispersed alloys, accompanied by significant increase in the elongation percentage, supported by evidence of refined dendrite arms length, and inter-lamellar spacing.

Superheat influence on mechanical properties of cast hypoeutectic aluminium–silicon alloy

This study examined the influence of hexachloroethane coated mould on the mechanical properties of hypoeutectic aluminium–silicon alloy. A hypoeutectic aluminium–silicon alloy was grain refined using 15 wt-% hexachloroethane and cast at varied degrees of melt superheats (50–150°C). The mechanical properties of the cast ingots were measured and their microstructures studied. The grain sizes of the ingots cast at various superheats were also determined. The study revealed that the grain sizes decrease as the melt superheat decreases, thereby causing an increase in the mechanical properties of the alloy. It was also noted that there exists an optimum value of superheat at which the mechanical properties were optimum. Apart from improved mechanical properties, it was inferred that the application of volatile mould coating (hexachloroethane) and temperature control facilitates the production of porosity free hypoeutectic aluminium–silicon casting.

Investigation of columnar-to-equiaxed transition in solidification processing of AlSi alloys in microgravity – The CETSOL project

Grain structures observed in most casting processes of metallic alloys are the result of a competition between the growth of several arrays of dendrites that develop under constrained and unconstrained conditions. Often this leads to a transition from columnar to equiaxed grain growth during solidification (CET). A microgravity environment results in suppression of buoyancy-driven melt flow and so enables growth of equiaxed grains free of sedimentation and buoyancy effects. This contribution presents first results obtained in experiments on-board the International Space Station (ISS), which were performed in the frame of the ESA-MAP programme CETSOL. Hypoeutectic aluminium-silicon alloys with and without grain refiners were processed successfully in a low gradient furnace (MSL-LGF). First analysis shows that in the non grain refined samples columnar dendritic growth exists, whereas CET is observed in the grain refined samples. From analysis of the thermal data and the grain structure the critical parameters for the temperature gradient and the cooling rate describing CET are determined. These data are used for initial numerical simulations to predict the position of the columnar-to-equiaxed transition and will form a unique database for calibration and further development of numerical CET-modeling.

On development of hypoeutectic aluminium–silicon powder metallurgy alloy

Powder metallurgy allows for the rapid, automated and efficient production of many different types of automotive components. However, a drawback is the limited selection of readily available light alloy blends. Owing to the wide spread use of aluminium–silicon casting alloys for existing components it is logical to develop aluminium–silicon PM options. Therefore, an experimental hypoeutectic aluminium–silicon alloy was chosen for study and an optimum processing route developed. Tests were performed to determine the green strength and density as a function of compaction pressure. Sintering conditions were optimised based on sintered density, hardness and dimensional changes. Metallography, differential scanning calorimetry and energy dispersive X-ray spectroscopy analysis provided insight into post-sinter furnace cooling and heat treatment parameters. An appropriate T6 heat treatment was developed and samples were tested in tension. The alloy was able to achieve a high sintered density approaching 98% and a yield strength of 232 MPa under the T6 condition.

Grain size and porosity of cast hypoeutectic aluminium-silicon alloys in hexachloroethane coated mould

This paper examined the effect of concentration of hexachloroethane (C2Cl6) in alumina mould wash on the grain size and apparent porosity of cast hypoeutectic aluminium silicon alloy. A hypoeutectic aluminium-silicon alloy was prepared and cast in metal mould coated with alumina slurry to which varied concentrations (0–40 wt %) of volatile organic compound, C2Cl6, have been added. The effects of the amount of the hexachloroethane in the mould coating on the grain size and apparent porosity of cast hypoeutectic aluminium-silicon alloy were assessed. There was an observed initial decrease and subsequent increase in the grain size as the concentration of C2Cl6 in the mould wash increased while there was no significant change in the apparent porosity of the cast alloy. It was concluded that coating metal mould with alumina mould wash containing approximately 15 wt % C2Cl6 produces optima grain size. The study also revealed that the use of such volatile mould coating does not introduce any noticeable gas porosity in the cast hypoeutectic aluminium-silicon ingots but rather aids the expulsion of possible trapped gases.

The role of trace element segregation in the eutectic modification of hypoeutectic Al–Si alloys

Modification of the eutectic silicon in hypoeutectic Al–Si alloys causes a structural transformation of the silicon phase from a needle-like to a fine fibrous morphology and is carried out extensively in industry to improve mechanical properties. It has been documented that the fibrous silicon phase in chemically modified alloys is heavily twinned. It has been proposed that this increased density of twinning results from the adsorption of impurity-elements at the solid–liquid interface perpetuating further growth according to a twin plane re-entrant edge (TPRE) model. In this paper, we discuss this mechanism of eutectic modification in light of experimental data obtained by synchrotron radiation and predictions based on theoretical calculations. This research utilizes a μ-XRF (X-ray fluorescence) technique at the SPring-8 synchrotron radiation facility X-ray source and reveals that different elements which theoretically satisfy the geometric requirements of the impurity-induced TPRE model do not have the same effect on the growth of silicon during solidification. Europium for instance is distributed relatively homogeneously in the silicon phase while ytterbium was not found in the silicon phase at all. This has important implications for the fundamental mechanisms of eutectic modification in hypoeutectic aluminium–silicon alloys.

The effects of a eutectic modifier on microstructure and surface corrosion behavior of Al-Si hypoeutectic alloys

Hypoeutectic aluminum–silicon alloys can have significant improvements in mechanical properties by inducing structural modification in the normally occurring eutectic. The eutectic modification may affect not only the mechanical properties but also the corrosion resistance of such alloys. It is well known that structural parameters such as grain size and interdendritic spacing can significantly affect corrosion resistance of alloys. However, to date, few researches have been performed to experimentally evaluate the effects of an effective modification of eutectic morphology on surface corrosion behavior of Al–Si alloys. In the present study, modified and unmodified samples of an Al 9 wt.% Si alloy were solidified under similar solidification conditions, and after metallographic procedures, the corrosion resistance was analyzed by both the electrochemical impedance spectroscopy technique and the Tafel extrapolation method carried out in a 0.5 M NaCl test solution at 25 °C. The impedance parameters and corrosion rate were obtained from an equivalent circuit analysis. It was found that the Al-9 wt.% Si alloy casting in the modified condition tends to have its corrosion resistance decreased when compared to the unmodified alloy.

Determination of strontium segregation in modified hypoeutectic Al–Si alloy by micro X-ray fluorescence analysis

Analysis of intra- and inter-phase distribution of modifying elements in aluminium–silicon alloys is difficult due to the low concentrations used. This research utilises a μ-XRF (X-ray fluorescence) technique at the SPring-8 synchrotron radiation facility X-ray source and reveals that the modifying element strontium segregates exclusively to the eutectic silicon phase and the distribution of strontium within this phase is relatively homogeneous. This has important implications for the fundamental mechanisms of eutectic modification in hypoeutectic aluminium–silicon alloys.

Modification-related porosity formation in hypoeutectic aluminum-silicon alloys

A series of aluminum-10 wt pct silicon castings were produced in sand molds to investigate the effect of modification on porosity formation. Modification with individual additions of either strontium or sodium resulted in a statistically significant increase in the level of porosity compared to unmodified castings. The increase in porosity with modification is due to the presence of numerous dispersed pores, which were absent in the unmodified casting. It is proposed that these pores form as a result of differences in size of the aluminum-silicon eutectic grains between unmodified and modified alloys. A geometric model is developed to show how the size of eutectic grains can influence the amount and distribution of porosity. Unlike traditional feeding-based models, which incorporate the effect of microstructure on permeability, this model considers what happens when liquid is isolated from the riser and can no longer flow. This simple “isolation” model complements rather than contradicts existing theories on modification-related porosity formation and should be considered in the development of future comprehensive models.

Experimental and modeling studies of the thermal conditions and magnesium, iron, and copper content on the morphology of the aluminum silicon eutectic in hypoeutectic aluminum silicon alloys

Hypoeutectic aluminum silicon alloys without and with additions of magnesium, copper, iron, and strontium have been cast in a mold, giving directional solidification from a chill. Detailed temperature measurements have been carried out. Solidification modeling based on front tracking of the microstructure growth fronts allowed identification of the time and temperature at which the dendrite tips and the first eutectic pass the thermocouples. The undercooling, growth rate, and thermal gradients at the dendritic and eutectic growth fronts were derived. The effect of varying thermal parameters and alloy compositions on the microstructure was investigated. Compared to the binary alloy, a coarser eutectic was observed in the alloys with magnesium, iron, and/or copper. The coarsening is explained as a result of the transition from a eutectic forming at one specific temperature, to a eutectic forming over a temperature range. The former is likely to grow as a plane front, whereas the latter is likely to form an interdendritic eutectic mushy zone.

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.

The use of thermodynamic simulation for the selection of hypoeutectic aluminum–silicon alloys for semi-solid metal processing

Currently, established hypoeutectic aluminum–silicon foundry alloys such as aluminum A356 and 357 alloys are also used for semi-solid metal (SSM) processing. Process inconsistencies arising from the use of these alloys are partly due to variations in the fraction solid due to temperature variations. Consequently, the process would be more robust if the alloys were tailored such that the fraction solid variation in the temperature range of operation was minimized. For this purpose, thermodynamic simulations have been carried out to determine the fraction solid and fraction solid variation of hypoeutectic aluminum–silicon alloys as a function of processing temperature, process temperature variation, and silicon and magnesium composition. The fraction solid variation is calculated over a composition range that includes A356 and 357 alloys, and alternate and more desirable fraction solids and alloy compositions that maximize the processing temperature window are suggested for SSM processing.

Density and solidification shrinkage of hypoeutectic aluminum-silicon alloys

The density of liquid and solid hypoeutectic aluminum-silicon alloys has been measured with high accuracy in the temperature range 400 °C to 800 °C by using the indirect Archimedian method. A eutectic mixture of KCl and LiCl salts was used as reference liquid. This method allowed measurements of density for both liquid and solid in the same experiment and thus reduced the systematic error in estimating the solidification shrinkage. The results show that the density of liquid aluminum-silicon alloy increases with increasing silicon content, while silicon reduces the density in the solid state. Silicon content reduces the solidification shrinkage from 6.6 pct for pure aluminum to 4.4 pct for Al-11.6 pct Si.