Hypoeutectic Al-Si Research Articles

Effect of Ca addition on solidification microstructure of hypoeutectic Al-Si casting alloys

The effect of Ca addition on the solidification microstructure of hypoeutectic Al-Si casting alloys was investigated using scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) methods with particular attention focused on the change of morphologies of primary α-Al and eutectic silicon. Meanwhile, solute distribution in solid-liquid was simulated according to Scheil-Gulliver law. Results showed that primary α-Al and eutectic silicon were refned as Ca content increased because of the variation of the solute concentration in liquid. The addition of Ca increased the nucleation sites like Al3Ti or Al2Cu to promote the nucleation of primary α-Al. Affected by modifcation effect of Ca, the shape of eutectic silicon changed from fake to fbrous structure. In addition, coarse plate Ca-rich phases could be found along the grain boundary with high Ca content.

Thermocapillary model of formation of nanostructures on the surface irradiated by low-energy high-current electron beams

The paper examines nano and microstructures formed on the surface of hypoeutectic Al–Si alloy (silumin) irradiated by low-energy high-current electron beams. A combination of thermocapillary instability and vapor pressure is assumed to be a principal mechanism initiating formation of these structures. Taking into consideration these phenomena, a dispersion equation is stated and analyzed for low hydrodynamic disturbances on the melt surface. The authors came to a conclusion vapor pressure in low-frequency approximation results in a dual mode correlation between growth rate and wavelength. The first maximum initiated by thermocapillary forces is associated with a wavelength of ∼1 μm; whereas the second one resulted from space-time modulation of vapor pressure is in a range of ∼10 μm. The paper highlights conditions for initiation of thermocapillary instability, taking into account evaporation in a nano-dimensional range of wavelengths. In general, a dispersion equation is demonstrated to have two instable solutions irrespectively of vapor pressure modulation. Therefore, two correlations of growth rate and wavelength are identified. The first one has its maximum in the range 200 to 800 nm, being similar to dimensions of crystallization cells. Apparently, the second correlation furthers formation of periodical structures on the surface.

The effect of reinforced SiC on the mechanical properties of the fabricated hypoeutectic Al-Si alloy by centrifugal casting

Centrifugal casting is one of the advanced casting techniques widely used in modern metallurgical industries. The current work is concerned and focuses on study the effects of a selected horizontal and vertical centrifugal casting parameters including, the type of the process, the mold rotation (spinning) speeds and the adding ratio of carbide silicone (SiC) micro powder on the quality of the hypoeutectic AlSi alloy hollow cylindrical fabricated workpieces by using the response surface methodology (RSM) and the full factorial design (FFD) for the design of experiments (DOE). After examined the qualities of the producing workpieces, it is found that the finest porosities were obtained when using the vertical centrifugal casting processes, especially at mold speed of 920 RPM without adding the SiC powder and the porosity sizes increased at higher mold speeds up to 1440 RPM. At these mold speeds the inner surfaces of the produced workpieces seen with lowest raining and shrinkage cavities defects, and with the best outer surfaces qualities especially when using the adding of 0.6% SiC powder. The maximum tensile strength obtained when using the higher mold speed and the corresponding G factor values, without using the added SiC mixed powder and when using the vertical centrifugal casting, reached 167 MPa. The maximum proof stresses were obtained when using the higher mold speed values, the added SiC mixed micro powder and using the vertical centrifugal casting, reached 13.6 MPa. Finally, the maximum percentage elongation was obtained when using the mold speed of 920 RPM, which equivalent to G factor value of 56 with using of the added SiC mixed powder and vertical centrifugal casting reached 1.6%. This means that the values of maximum tensile strength, proof stresses and percentage elongation obtained for the hypoeutectic Al-Si Alloy hollow cylindrical fabricated workpieces when using the centrifugal casting processes, increased by 355, 13.3 and 87,5%, respectively when comparing with the using of the conventional gravitational casting process.

Effect of iron on the microstructure and mechanical properties of the spray-formed and rotary-swaged 319 aluminum alloy

The influence of iron additions (0.8, 1.2, and 1.5 wt.%) on the microstructure and tensile properties of the 319 aluminum alloy processed by spray forming and rotary swaging was investigated. The spray-formed deposits were rotary-swaged at 573 K with an area reduction ratio of 5:1. Room temperature tensile tests showed a substantial increase of elongation at fracture (5.5 to 8%) when compared to the values observed for the iron-containing conventionally cast counterpart (0.6%). The high values of elongation at fracture were obtained due to the significant microstructural refinement and decrease of volumetric phase fraction, especially the iron-rich intermetallics, promoted by the combination of spray forming and rotary swaging. Therefore, this processing route significantly reduces the deleterious effect on the ductility caused by the iron content and the presence of β-AlFeSi intermetallic phase in hypoeutectic Al-Si alloy.

The improvement of electrical conductivity of hypoeutectic Al-Si alloys achieved by composite melt treatment

Hypoeutectic Al-Si alloys are one of common material for overhead transmission lines. However, electrical conductivity and mechanical property are hard to be improved simultaneously, which limits its application. Boron treatment and Sr modification can improve electrical conductivity and mechanical property by purify Al matrix and modify eutectic Si. According to our study, Ti assistant boron treatment (TBT) or Sr modification separately is good to improve electrical conductivity, but TBT with Sr modification at the same time is not effective to improve electrical conductivity. Thus Al-3B-5Sr was proposed to modify the microstructure of hypoeutectic Al-Si, improving the electrical conductivity and mechanical property at the same time. Based on this, a new composite melt treatment was proposed and high electrical conductivity with good mechanical property can be achieved simultaneously.

Microstructural influences on the fatigue crack initiation and propagation mechanisms in hypo-eutectic Al-Si cast alloys

Al-Si cast alloys are accompanied by different defects and microstructural heterogeneities like porosity, varying secondary dendrite arm spacings (SDAS) and multiple appearances of the eutectic. These issues are well known but technically difficult to prevent for the industry. To improve the light weight performance of cast aluminum alloys the present work deals with influences on crack initiation and propagation mechanisms of such heterogeneities in the high-cycle-fatigue (HCF) and very-high-cycle-fatigue (VHCF) regime for the two hypo-eutectic cast aluminum alloys AlSi8Cu3 (in-series sand castings) and AlSi7Mg0.3 (laboratory gravity die castings). Furthermore, the occurrence of facet-regions and their role in early and late state of fatigue damage evolution is discussed by showing results from modified Kitagawa-Takahashi analysis and ex-situ computed tomography investigation during intermitted HCF experiments. In the early HCF regime crack initiation starts at multiple pores. Due to coalescence of these defects, shear-stress-controlled crack propagation along slip bands leads to the occurrence of facets. In the VHCF regime, porosity provides strong scattering of the number of cycles to failure. A reduction in SDAS increases the fatigue strength significantly. However, in absence of porosity fatigue crack initiation and propagation is controlled by shear stresses on facet-like slip-planes. Additionally, the barrier role against dislocation movement of eutectic regions can be shown by means of these specimens.

Microstructure Solidification Maps for Al-10 Wt Pct Si Alloys

Hypo-eutectic Al-Si alloys are widely used in both the automotive and aerospace industries; however, they still have limited usage as structural materials, due to the inherent morphology of the Si phase that forms within the eutectic structure. This non-ideal Si morphology can be modified, via alloy additions and/or rapid solidification (RS), but the underlying mechanism(s) behind this is poorly understood. This work focused on understanding the influence of RS on the eutectic structure, for hypo-eutectic Al-10 wt pct Si alloys produced by Impulse Atomization and Differential Scanning Calorimetry. This study found that the eutectic Si forms into four distinct morphologies: (1) flaky, (2) fibrous, (3) globular + fibrous and (4) globular, depending on the solidification conditions. As a result, two solidification maps of the Si morphology are proposed, one based on local eutectic solidification conditions and another based on a solidification continuous cooling diagram (SCCT). Both maps help identify the required conditions for certain Si morphologies to form. Hardness measurements were also carried out and it was found that the Si morphology would influence the alloy hardness, with the highest value being achieved when the eutectic Si was globular. This result indicates that the Si morphology is an important factor that can alter the mechanical properties of hypo-eutectic Al-Si alloys.

Combined Effects of Ultrasonic Melt Treatment and Cu/Mg Solute on the Microstructure and Mechanical Properties of a Hypoeutectic Al-7Si Alloy

The effect of Cu/Mg solute combined with ultrasonic melt treatment (UST) on the microstructure and mechanical properties of a hypoeutectic Al-7Si alloy was investigated. Cu up to 4 wt pct and Mg to 1 wt pct were added to the alloy and UST was performed at about 100 °C above the liquidus temperature. UST at such a high temperature was ineffective in refining grain sizes but enhanced the microstructural homogeneity of the alloys with the refinement of α-Al secondary dendrite arm spacing (SDAS). It was found that both the grain size and the SDAS were solute sensitive: In the Al-7Si alloy with UST they insignificantly changed with Cu but decreased with an increase in Mg content. Along with the structural refinement, UST was likely to affect the solute redistribution and the subsequent formation of secondary phases (e.g., eutectic Si and Cu/Mg-rich intermetallic compounds (IMCs)). Quantitative analysis indicated that UST had significantly minimized microsegregation of Cu and Mg solutes, decreasing the amount of the IMCs finely and uniformly distributed. The underlying reason was attributed to the reduction in the size of the SDAS, which enabled the solid-state diffusion to occur more efficiently upon solidification. The most significant increase in the tensile properties was achieved for the Al-7Si-2Cu-1Mg alloy with UST where the SDAS was well refined and the grain size was the smallest among the alloys investigated. The important role of UST on the casting structure refinement and the solute distribution is discussed and the resulting mechanical properties are further explored.

Friction stir welding of a hypoeutectic Al–Si alloy: microstructural, mechanical, and cyclic response

In the present study, the effect of friction stir welding (FSW) parameters, tool traverse, and rotational speeds has been studied on microstructural evolution, mechanical properties, and cyclic (fatigue) response of hypoeutectic Al–10Si aluminum sheets. The main objective of the present paper is to establish the correlations between microstructure/mechanical property/fatigue response/friction stir welding parameters. Results show significant improvement in all aspects (mechanical, microstructural, cyclic) of the friction stir-welded joins thanks to elimination of casting porosities, grain refinement, and silicon fragmentation/spheroidization. The latter one is attributed to the severe plastic deformation imposed by the FSW rotating tool, which translate as-cast blade-shaped eutectic silicon particles to fragmented/spheroidized silicon particles, which are evenly distributed in the aluminum matrix.

Multistage discretization and clustering in multivariable classification of the impact of alloying elements on properties of hypoeutectic silumin

The properties of hypoeutectic Al–Si alloy (silumin) with the addition of elements such as Cr, Mo, V and W are described. Changes in silumin microstructure under the impact of these elements result in a change of the mechanical properties. The research includes presentation of procedure for the acquisition of knowledge about these changes directly from experimental results using mixed data mining techniques. The procedure for analyzing small sets of experimental data for multistage, multivariate and multivariable models has been developed. Its use can greatly simplify such research in the future. An interesting achievement is the development of a voting procedure based on the results of classification trees and cluster analysis.

Microstructural Changes in Hypoeutectic Al–Si Alloys by Low Shear and Vibration Induced Melt Conditioning Setup

In the present work, refinement of microstructure in hypoeutectic Al–Si alloys, i.e., A356 and Al–7wt%Si has been reported. One of the mechanical technique, melt conditioning, was used to refine the microstructure. The technique involved subjecting the melt to low shear force by passing it through a slope at a temperature above its liquidus temperature. In addition to the shear force, vibration was induced into the melt during its flow by a simple setup for melt conditioning. Shear force along with vibrations led to enhanced refinement in both the alloys, the most prominent being A356 alloy. It has been suggested that higher fraction of fragmented oxide particles/films into the molten A356 alloy during the flow before solidification in the mould could enhance the nucleation centres. It was concluded that the bi-oxides such as MgAlO4 acted as effective heterogeneous nucleants for α-Al.

Effect of Solidification Processing Parameters and Silicon Content on the Dendritic Spacing and Hardness in Hypoeutectic Al-Si Alloys

Aluminium with silicon as alloying element, form a class of material providing the most significant part of all material casting manufactured materials. These alloys have a wide range of applications in the automotive and aerospace industries. It is widely recognized that moderate addition of silicon to aluminum, significantly improves the resulting mechanical properties. Hypoeutectic Al-Si alloys were used in the present experimental study to investigate the solidification parameters effect and Si 3-5 wt% addition on the microstructural features and resulting hardness in a vertical directional solidification system. The hypoeutectic alloys were directionally solidified under transient heat flow conditions in a range of cooling rates from 0.3 to 4 oC/s. Characterization analyses by optical microscopy indicate clearly that secondary dendritic arm spacing (λ2) increases significantly with the decrease in solidification speed (SP) and cooling rate (Ṫ). On the other hand, experimental growth laws relating the secondary dendritic arm spacing (λ2) to the solidification speed (SP) and cooling rate (Ṫ) indicate that Si addition from 3 wt% to 5 wt% has induced a thickening effect leading to increase in λ2 . While, experimental results have shown that the resulting hardness increase as solidification processing parameters (SP and Ṫ ) increase. Results of Vickers hardness test for Al-5 wt% Si alloy has hardness increase of 18 % from that of Al-3 wt% Si alloy with mean values of 26 and 22 HV, respectively.

Effect of Structural Modification on Corrosion Behavior of Hypo Eutectic Al-Si Alloy

Al-Si eutectic cast alloys are widely used in aeronautical and automobile industries where significantly high strength, toughness and wear resistance are required. This class of cast alloys exhibit relatively low corrosion resistance in brine environments. The mechanical properties of the alloy system mainly depend upon the shape of Si rich eutectic phase, which mainly has acicular geometry. In present research, the effect of modified microstructure of 12 wt. % Si-Al alloy on corrosion behavior was studied. The needle like Si rich eutectic phase was modified to disperse spherical structure using rare earth metal halides. The corrosion rate and pitting behavior of modified and unmodified alloy were evaluated in 3.5% NaCl solution by general corrosion for calculated time. It was observed that the corrosion rate and pitting tendency of modified alloy had been appreciably reduced as compare to unmodified alloy. The improvement of corrosion properties were the attributes of changed morphology and distribution of Si rich eutectic phase.

Effect of Sr addition on coarse Si particle formation in hypo-eutectic Al–Si filler alloy

Effect of Sr addition on coarse Si particle formation in hypo-eutectic Al–Si filler alloy

Microstructures, mechanical properties, and corrosion behavior of novel high-thermal-conductivity hypoeutectic Al-Si alloys prepared by rheological high pressure die-casting and high pressure die-casting

The microstructures, mechanical properties, thermal conductivities, and corrosion behavior of novel high-thermal-conductivity hypoeutectic Al-Si alloys, prepared by rheological high-pressure die-casting (Rheo-HPDC) and HPDC, were investigated and compared. The microstructure of the Rheo-HPDC alloy, including the primary α-Al (α1), secondary α-Al (α2-Al), β-Al5FeSi, and Si phases, was significantly more refined than that of the HPDC alloy. Compared to the HPDC alloy, the Rheo-HPDC alloy exhibited superior mechanical properties and thermal conductivity, and its ultimate tensile strength (UTS), yield strength (YS), elongation, hardness, and thermal conductivity were improved by 27%, 21%, 72%, 10% and 8%, respectively. Pitting corrosion of the Rheo-HPDC alloy was observed to originate at the interfaces between the Fe-rich intermetallics and the α2-Al particles. Corrosion was observed to propagate in the eutectic areas upon prolonged immersion in 3.5 wt% NaCl solution. Electrochemical testing, scanning Kelvin probe (SKP) measurements, and corrosion-morphology analyses reveal that the Rheo-HPDC alloy has better corrosion resistance, primarily due to the decrease in the potential difference between the Fe-rich intermetallics and the matrix, the refinement of the eutectic Si and intermetallics, and the decrease in the areal ratio of eutectic Si to α2-Al.

Fluidity and Hot Cracking Susceptibility of A356 Alloys with Sc Additions

A356 with scandium (Sc) addition provides interesting results beyond costs. For the practical use of Sc, the effects of Sc on castability must be considered. Fluidity and hot cracking are important factors defining the castability of aluminum casting alloys. In the present work, the influence of Sc addition on the castability of A356 hypoeutectic Al–Si alloy was investigated, which was evaluated through fluidity and hot cracking susceptibility. The fluidity of the alloys was studied by measuring the total volume of solidified aluminum in a multi-channel mold. The hot cracking susceptibility of the alloys was evaluated by using a constrained-rod casting mold test. The results of the fluidity and hot cracking susceptibility test were supported by microstructural analysis. The results indicate that 0.2 wt% Sc addition significantly increases the fluidity of A356 alloy, due to the grain refinement and eutectic Si modification by changing the solidification mode. However, the fluidity slightly decreases when the Sc content increases to 0.4 wt% due to the formation of primary Al2Si2Sc intermetallic phase. The hot cracking of A356 alloy was completely diminished when Sc was added to the alloy.

Influence of Sc on microstructure and mechanical properties of Al–Si–Mg–Cu–Zr alloy

In the present study, the effects of Mg, Cu, Sc and Zr combined additions on the microstructure and mechanical properties of hypoeutectic Al–Si cast alloy were systematically investigated. Characterization techniques such as optical microscopy (OM), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), electron back-scatter diffraction (EBSD), atomic force microscopy (AFM), transmission electron microscope (TEM), Brinell hardness tester and universal testing machine were employed to analyze the microstructure and mechanical properties. The results showed that Sc served as modifier on the microstructure of Al–3Si–0.45Mg–0.45Cu–0.2Zr alloys, including modification of eutectic Si and grains. Extraordinarily, grain refinement was found to be related to the primary particles, which exhibited a close orientation to matrix. After T6 heat treatment, the grain structures were composed of nano-scaled secondary Al3(Sc, Zr) precipitates and spherical eutectic Si. Combined with T6 heat treatment, the highest hardness, yield strength, ultimate tensile strength and elongation were achieved in 0.56 wt.% Sc-modified alloy. Interestingly, the strength and ductility had a similar tendency. This paper demonstrated that combined additions of Mg, Cu, Sc and Zr could significantly improve the microstructure and performance of the hypoeutectic Al–Si cast alloy.

Fabrication and characterization of hypoeutectic open-cell Al-Si foams using gravity die casting and squeeze casting

Gravity die casting and squeeze casting are the techniques used for the fabrication of hypoeutectic open-cell Al-Si foams which are characterized and studied for their energy absorbing quality in compression tests. The effect of different amounts of sodium chloride (NaCl) (up to 56 vol.%) as a space holder in the casting of aluminum foam on the morphology, density, porosity, compressive and energy absorption properties of aluminum foam was studied. The hypoeutectic Al-Si alloy with NaCl particles as a space holder was used to fabricate the aluminum foam using gravity die casting and squeeze casting. The hypoeutectic open-cell Al-Si foams produced by squeeze casting showed smaller pore size, better pore distribution, higher porosity, good compressive strength and greater energy absorption energy compared to that of gravity die casting. The hypoeutectic open-cell Al-Si foams with 44 vol.% NaCl using squeeze casting showed the best properties among all foams due to its moderate and well-distributed porosity.

Impression creep properties of hypoeutectic Al-Si alloys with scandium additions

The present work pertains to an improvement of high temperature stability of age hardenable hypoeutectic Al-Si alloy. Addition of scandium (Sc) was attempted to modify the microstructure and to improve the high temperature stability of the hypoeutectic Al-Si alloy. The high temperature stability was determined by impression creep techniques. The tests were performed at temperature 250°C and applied stresses in the range of 160 to 200 MPa for dwell times up to 3000 seconds. The results showed that the creep strength of the hypoeutectic Al-Si alloys was improved. For all loads, hypoeutectic Al-Si alloy with 0.4 wt% Sc had the lowest creep rates and thus the highest creep resistance among all materials tested. This may be attributed to the possible microstructural changes, solute enrichment of the matrix and pinning of the grain boundaries by the fine precipitates.

Metallurgical Processing of Al-Si Alloys with Increased Iron Content Using Sodium, Strontium, and Tellurium

This paper presents the results of research focused on the processing of the hypoeutectic Al-Si alloy with an increased iron content of 1.75 wt.%. The physical-metallurgical method was used to eliminate the undesirable effect of the iron rich b-phase on the mechanical properties of the casting material using liquid metal modification by strontium, sodium, and tellurium. The chemical composition of an experimental material didn’t contain any elements for the solid-solution strengthening of a matrix; therefore, the presented results can document the capabilities of modifying the b-phase under a minimum influence on the properties of the basic Al-Si system.