Hypereutectic Al-Si Alloys Research Articles

Comparison of Si Refinement Efficiency of Electromagnetic Stirring and Ultrasonic Treatment for a Hypereutectic Al-Si Alloy

The effects of electromagnetic stirring, ultrasonic treatment (UT), and their combination on the Si particles refinement in a hypereutectic Al-Si alloy (A390) were examined. All three physical methods yielded a considerable change in the shape and size of the particles, with UT producing the finest size. The creation of holes inside the Si particles via the explosion of cavitation bubbles was determined to occur before fragmentation of primary Si. In addition, transmission electron microscopy analysis revealed twin formation on primary Si particles by application of an external field in the liquid. UT of the melt was sufficient to produce fine primary Si particles with average size of ~12 µm. The primary Si particles refinement and eutectic Si modification improved the mechanical properties.

Effects of ZnS modification on primary Si in hypereutectic Al-Si alloy

A new modifying agent, ZnS, was used as a refiner to modify primary silicon in hypereutectic Al-Si alloy. The factors affecting the modification results, including addition level of ZnS and holding time, were investigated. The results showed that the average size of the most effectively modified primary silicon was 28.5 μm when the ZnS mixed powder addition was 0.15wt.% with a holding time of 10 min. More important, the average size of primary silicon could remain below 40 μm despite the holding time extending to 120 min, which means ZnS is a promising modifying agent of primary Si in industrial applications.

A novel restraint spraying-Conform process for manufacturing hypereutectic Al–Si alloy with enhanced properties

A novel restraint spraying-Conform (RS-C) process, which directly combines spraying with Conform to process metals in one step, has been proposed. Al-20Si alloy selected as experimental material was successfully fabricated by the RS-C process. The microstructures were dominated with fine and uniform primary silicon phases. The tensile strength and elongation to failure of the Al–20Si alloy were 204 MPa and 7.2% respectively after the RS-C process. The wear resistance of the processed Al–20Si alloy was increased significantly, about 1.7 times over the as-cast ingot. The experimental results indicate that RS-C is a promising near net shape forming technology.

Effect of phosphorus and heat treatment on microstructure of Al-25%Si alloy

It is known that phosphorus can refine the primary silicon and heat treatment can spheroidize the eutectic silicon. This paper presents an optimal combination of heat treatment processes and P refinement on hypereutectic Al-Si alloy. The optimal P addition amount, and the solution and aging temperatures for Al-25%Si alloy were obtained through the orthogonal experiment, and their modification effects were discussed. The results show that P addition has the greatest modification effect, followed by aging temperature, and the modification effect of solution temperature is the least. The optimized modification parameters are: addition of 0.6% P, solution at 540 oC and aging at 160 oC. In addition, the cooling curve, superheating and hardness of the alloy were also analyzed.

過共晶Al–Si系合金の潤滑下の摩耗に及ぼす初晶Siの粒径と面積率の影響

The wear behavior of hyper-eutectic Al–Si alloys against bearing steels was investigated under the lubricated condition. The severe-mild wear transition of hyper-eutectic Al–Si alloys occurred when Si-particle size or area ratio exceeded critical values. Below the critical value, Al–Si alloys still remained in the sever wear mode resulting in a large amount of wear due to the insufficient sparse distribution of primary Si phase at the contact area.

Tensile properties and related microstructural aspects of hypereutectic Al-Si alloys directionally solidified under different melt superheats and transient heat flow conditions

The present investigation deals with directional solidification (DS) of Al-15wt% and 18wt% Si alloys under transient heat flow conditions and further characterization of the related microstructures. Emphasis is given on the eutectic growth being affected not only by solidification kinetics but also by melt superheat, which is varied in two degrees for each alloy, i.e., 6% and 23% above the liquidus temperature. The dependences of interphase spacing (λ) on both solidification kinetics (fast, intermediate and slow cooling conditions) and on melt superheat during solidification of hypereutectic alloys are reported for the first time. Furthermore, functional experimental interrelations of tensile mechanical properties and interphase spacing between eutectic Si particles of both alloys evaluated are proposed. It is shown by these correlations that the tensile properties increase with the decrease in these spacings. More significant variations in properties are associated with a certain range of spacings (1.0µm <λ<2.3µm) in the case of the Al-15wt%Si alloy. Tensile strength and elongation-to-fracture decrease with increasing alloy silicon content. The applicability of the eutectic growth expression λ=f (V−1/2) has been verified for the present tested alloys and experimental conditions.

Assessment of circumferential cracks in hypereutectic Al-Si clutch housings

As in situ natural composites with silicon phase acting as the reinforcing phase, Al-Si alloys are among most commonly used aluminum alloys in automotive applications (i.e. engine component). Silicon contributes to the strength of Al-Si alloys through load transfer from the Al matrix to the hard (rigid) Si phase in the microstructure (load-carrying capacity). Casting parameters (i.e. solidification rate, elemental segregation, secondary dendrite spacing…) as well as the size and distribution of the microstructural constituents in Al-Si alloys (i.e. morphology of Si particles, intermetallic compounds, secondary dendrite spacing) contribute directly to the mechanical response and failure (or fracture) behavior of the alloy within the service. In hyper-eutectic Al-Si alloys (i.e. B390.0), distribution of coarse pre-eutectic Si particle mainly contribute to stress concentration, crack initiation and propagation during the actual service condition. In the present paper, the parameters contribution to the formation of the circumferential cracks in clutch housings made of die cast hyper-eutectics B390.0 Al-Si alloys are assessed through optical microscopy and scanning electron microscopy. Casting variable, cooling rate, their effect on the cracks as well some of the possible causes are also discussed in detail.

Refinement of Primary Silicon Crystals by Novel P-Doped γ-Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Particles in Solidification of Hypereutectic Al-Si Alloys

P-doped γ-Al2O3 was found to be a potent substrate to nucleate primary silicon whilst good modification of the eutectic matrix is retained during solidification of hypereutectic Al-Si alloys. On using P-doped γ-Al2O3 could be a perfect and clean source of P without additional impurities. The optical micrographs show that the morphologies of primary silicon crystals in solidification of Al-18Si alloy are changed from irregular coarse morphologies to fine regular particles. The average particle size of primary Si decreased from 52 μm to 25 μm and 22 μm in adding P-doped α-Al2O3 and P-doped γ-Al2O3 respectively. It was clear that P-doped γ-Al2O3 led to good refinement of primary Si and the modification effect on eutectic Si was retained in solidification of commercial purity Al-18Si alloy. Adding P-doped γ-Al2O3 give a good primary Si refinement to Al-18Si alloy if compared with the addition of P and using finer P doped γ-Al2O3 powder give narrower particle size range similar to that of adding P.

High-pressure phase transitions and structure of Al–20 at % Si hypereutectic alloy

Phase transformations of an Al–20 at % Si high-silicon hypereutectic alloy have been studied by differential barothermal analysis at temperatures of up to 800°C in argon compressed to 100 MPa. High pressure has been shown to raise the melting point of the alloy by 5°C during heating and to lower the eutectic solidification temperature by 5°C during cooling in comparison with the canonical phase diagram of the Al–Si system. At a temperature of 553°C, heating and cooling lead to silicon dissolution and decomposition of the aluminum-based solid solution, respectively. After high-pressure solidification, the silicon particles in the alloy have a bimodal size distribution. Quantitative porosity characteristics in the alloy after a barothermal scanning cycle are similar to those in the as-prepared alloy. The lattice parameters of the silicon and aluminum remain unchanged. The microhardness of the aluminum matrix of the alloy corresponds to that of pure aluminum.

Микроструктура и механические свойства заэвтектического сплава Al—Si после интенсивной пластической деформации

Микроструктура и механические свойства заэвтектического сплава Al—Si после интенсивной пластической деформации

Effect of Rapid Solidification and Addition of Cu3P on the Mechanical Properties of Hypereutectic Al-Si Alloys

The combined processes; rapid solidification, addition of Cu3P compound and heat treatments to improve the mechanical properties of the hypereutectic Al-13Si, Al-20Si and Al-20Si-1.5Fe-0.7Mn alloys (in wt. %) was studied. Optical microscopy and scanning electron microscopy were used to characterize the microstructures. The mechanical properties were evaluated by tensile tests. It was found that the cooling rate (20-50°C/s) used to solidify the alloys plus the addition of Cu3P compound favored the formation of fine primary Si and the transformation of the Al/Si eutectic from acicular to semi-transformed morphology. The spheroidization of the Al/Si eutectic after heat treatment caused an increase in the elongation (%) compared with the as-cast alloys. Al-13Si alloy showed the highest UTS and elongation (%) values, reaching values up to 215 MPa and 9.6%, respectively. On the other hand, the Al-20Si-1.5Fe-0.7Mn alloy showed the lowest UTS (180MPa) and elongation (3.4%) values. The formation of the Fe-intermetallic compounds caused a negative effect on the mechanical properties of the Al-20Si-1.5Fe-0.7Mn alloy.

Effect of high-intensity ultrasonic treatment on microstructure, hardness and wear behaviour of the hypereutectic Mg-5Si alloy

The effect of high-intensity ultrasonic treatment (HIUST) on microstructure, hardness and wear behavior in Mg-5wt.%Si hypereutectic alloy has been investigated. The results showed clearly that without HIUST, most of primary Mg2Si appeared as coarse dendritic morphology with average size of about 200 µm. With HIUST, the average size of primary Mg2Si decreased significantly to about 33 µm and their morphologies changed to polyhedral shape. The modification mechanism is mainly attributed conjugation of two mechanisms: cavitation-enhanced heterogeneous nucleation and cavitation-induced dendrite fragmentation. The alloy treated with HIUST has higher hardness and wear resistance than that untreated with HIUST. The wear mechanism of investigated alloys at low applied load (10 N) and low sliding speed (0.3 m/s) is a mild abrasive oxidative wear with little adhesion. However, the wear mechanism due to the applied high loads (30, 50 N) at low sliding speed (0.3 m/s) and/or to the applied high sliding speeds (0.6, 0.9 m/s) under low load (10 N), could be described as delamination mechanism. The microstructures of the specimens were analyzed by optical microscope (OM) (model OPTIKA M-790, Italy). Energy dispersion spectrum (EDS) affiliated to field emission scanning electron microscopy (FESEM) (model Quanta FEG, The Netherlands) were performed to reveal the concentration of alloying elements in selected areas of the microstructure.

The microstructure of the surface layer of magnesium laser alloyed with aluminum and silicon

The surface layer under analysis was formed as a result of diffusion bonding of a thin AlSi20 plate to a magnesium substrate followed by laser melting. Depending on the process parameters, the laser beam melted the AlSi20 plate only or the AlSi20 plate and a layer of the magnesium surface adjacent to it. Two types of microstructure of the remelted layer were thus analyzed. If the melting zone was limited to the AlSi20 plate, the microstructure of the surface layer was typical of a rapidly solidified hypereutectic Al–Si alloy. Since, however, the liquid AlSi20 reacted with the magnesium substrate, the following intermetallic phases formed: Al3Mg2, Mg17Al12 and Mg2Si. The microstructure of the modified surface layer of magnesium was examined using optical, scanning electron and transmission electron microscopy. The analysis of the surface properties of the laser modified magnesium revealed that the thin layer has a microstructure of a rapidly solidified Al–Si alloy offering good protection against corrosion. By contrast, the surface layer containing particles of intermetallic phases was more resistant to abrasion but had lower corrosion resistance than the silumin type layer.

Enhancing the machinability of hypereutectic Al-30Si alloy by friction stir processing

Chip formation ability and chip characteristics during machining in as-cast as well as friction stir processed hypereutectic Al-Si alloy are investigated. The effect of microstructural refinement by friction stir processing on chip size, shape and morphology are characterized using optical microscopy and scanning electron microscopy techniques. It was observed that with the increase in number of FSP passes morphology of chips transformed from an irregular brittle powder to regular curly ductile chips with the corresponding decrease in the surface roughness values, thus enhancing the machinability. The change of chip behavior in cast and that after FSP was primarily due to the reduction in diameter and uniform redistribution of primary silicon.

Hypereutectic AlSi Alloy: Gathering of 3D Microstructure Data

Hypereutectic and eutectic AlSi-base alloys find frequent application in casting automotive components. The properties of this type of alloy depend significantly on their solidification microstructure, especially the size, shape, and distribution of primary and eutectic silicon. The serial sectioning technique was applied for determining the three-dimensional (3D) microstructure of an Al-18wt.%Si alloy. For clear identification of both the larger primary Si particles grown in the melt and the fine lamellar eutectic Si, a series of two-dimensional equidistant cross sections were metallographically prepared. The microstructure in these cross sections was detected and observed at high resolution using a light microscope. The images were stored in a digital library. The 3D reconstruction of primary Si particles and AlSi eutectic was achieved through the application of various software tools. This provided data about the faceted growth behavior of octahedral Si particles and feathery eutectic Si. The image stack was also imported to hierarchical data format (version 5) (HDF5) open source format, thus, enabling availability of the 3D image data to the wider community. In this way, 3D reconstructions of this kind can contribute to a greater understanding of processing/microstructure property relationships in hypereutectic AlSi alloys.

Si particulate-reinforced Zn[sbnd]Al based composites joints of hypereutectic Al[sbnd]50Si alloys by ultrasonic-assisted soldering

Hypereutectic AlSi alloy plates were joined by ultrasonic-assisted flux-less method using Zn5Al filler metal at 420°C in air. The influence of ultrasonic time on the microstructure and mechanical property of joints was investigated. The introducing of ultrasonic into soldering processes had effects on accelerating Si particles distribution in a Zn-based filler metal, and also affected the solidification microstructures of ZnAl alloys. The shear strength of joints had correlation with the distribution of Si particles and types of ZnAl alloys, and increased with the ultrasonic time. The highest average shear strength of joints reached to ~108MPa, and the fracture mainly occurred at the base metal of Al50Si rather than the Si particulate-reinforced ZnAl based composites joints, indicating the strength of joints was stronger than that of Al50Si based metal.

A method for improving the mechanical properties of a hypereutectic Al–Si alloy by introducing the α-Al phase

Abstract The low ductility of hypereutectic Al–Si alloys, which is attributed to the coarse primary silicon particles in the microstructure, limits their application potential. The mechanical properties of hypereutectic Al–Si alloys strongly depend on the size and morphology of the primary Si. This study focused not only on the effect of the primary Si content, but also on the ductile α-Al phase, which does not occur in the microstructure of hypereutectic Al–Si alloys when using traditional alloying methods. We propose a novel method which allows introducing the α-Al phase in order to improve the mechanical properties of hypereutectic Al–Si alloys, e.g., the hardness, tensile strength, and ductility. The formation of the α-Al phase and a refinement of the primary Si were simultaneously achieved via an unconventional method, which is based on mixing a high-temperature molten Al–Si alloy with low Si content into a low-temperature semi-solid Al–Si alloy with high Si content. The resulting alloy showed an improved ductility, hardness and tensile strength compared to a traditionally-cast Al–Si alloy with the same nominal Si content.

Revealing heterogeneous nucleation of primary Si and eutectic Si by AlP in hypereutectic Al-Si alloys.

The heterogeneous nucleation of primary Si and eutectic Si can be attributed to the presence of AlP. Although P, in the form of AlP particles, is usually observed in the centre of primary Si, there is still a lack of detailed investigations on the distribution of P within primary Si and eutectic Si in hypereutectic Al-Si alloys at the atomic scale. Here, we report an atomic-scale experimental investigation on the distribution of P in hypereutectic Al-Si alloys. P, in the form of AlP particles, was observed in the centre of primary Si. However, no significant amount of P was detected within primary Si, eutectic Si and the Al matrix. Instead, P was observed at the interface between the Al matrix and eutectic Si, strongly indicating that P, in the form of AlP particles (or AlP ‘patch’ dependent on the P concentration), may have nucleated on the surface of the Al matrix and thereby enhanced the heterogeneous nucleation of eutectic Si. The present investigation reveals some novel insights into heterogeneous nucleation of primary Si and eutectic Si by AlP in hypereutectic Al-Si alloys and can be used to further develop heterogeneous nucleation mechanisms based on adsorption.

Wear behavior and microstructure of hypereutectic Al-Si alloys prepared by selective laser melting

This work investigates the microstructure and wear behavior of hypereutectic Al-Si alloys, in-situ fabricated using selective laser melting of a mixture of eutectic Al-12Si (wt.%) and pure Si powders. The first observation was that the size and morphology of the Si phase are strongly influenced by the laser power. In addition, it was also observed that a high laser power causes serious evaporation of aluminum during the remelting process. Dry sliding wear test and Vickers microhardness measurements were employed to characterize the mechanical properties of the material. The lowest wear rate of about 7.0×10−4mm3N−1m−1 was observed for samples having the highest value of relative density (96%) and microhardness (105Hv0.3).

Microstructure and wear behavior of in-situ hypereutectic Al–high Si alloys produced by selective laser melting

In this study, almost dense hypereutectic Al-50 Si alloys were fabricated in situ by selective laser melting (SLM) from the mixture of Al and Si powders under argon atmosphere. The effects of laser power on the microstructure and mechanical properties were determined regarding the size and morphology of primary Si phase (proeutectic Si). The phase analysis shows that the main phase of SLM-processed Al-Si alloys consists of Al and Si for all samples. Additionally, due to the vaporization behavior of Al, the Si content increases after melting process. The primary Si phase presents a smaller size (about 5μm) than that in the conventional produced hypereutectic Al-Si alloys. Moreover, while the laser power increases, the spherical morphology of primary silicon turns into irregular shape. Besides, the maximum value of microhardness (188 Hv) was detected for the samples obtained at laser power of 320W. The sample obtained at laser power of 350W shows the lowest wear rate (5.5×10−4mm3N−1m−1).