Primary Si Crystals Research Articles

Optimizing the microstructure and enhancing the mechanical prsoperties of hypereutectic Al–Si alloys by Mg addition

The impact of Mg on the solidification structure of hypereutectic Al–Si alloys with different Si contents (15 wt% and 18 wt%) was investigated. 3%Mg was added to Al–15Si alloy to modify the phases in the solidification structure for the improvement of mechanical properties, and T6 heat treatment was used to optimize the microstructure and precipitate strengthening phase. The solidification structure was observed by OM and TEM respectively, and the precipitation behavior of each phase was detected through DSC. The results show that the addition of Mg decreased the undercooling of the alloy melt, thus suppressing the precipitation of primary Si phase. Nanoscale β” precipitates were precipitated in the matrix after T6 heat treatment. This resulted in an increase of 54% and 165% in the UTS and EL of the alloy, reaching 252 MP and 15.1%, respectively. Primary Si crystals were present in two forms in Al–18Si–8Mg alloy: spinel twin and non-twin, while Mg2Si always maintained the faceted crystals during the growing process, where only (100) and (111) crystal planes were observed. XRD, SEM and EBSD have been used to prove that Mg2Si crystals with different morphologies could become the heterogeneous nuclei of two types of primary Si crystals. The process of nucleation and growth of primary Si with Mg2Si as the heterogeneous nuclei was experimentally analyzed and theoretically predicted.

Enhanced separation of bulk silicon from Si-Sn melt via electromagnetic directional solidification

Developing new methods for solar photovoltaic power generation is an effective way to solve future energy shortages. Solvent refining is one of the most promising methods to achieve the low-cost mass production of solar-grade Si. In this work, a novel method for separating bulk silicon from a Si-Sn melt under intensification by an electromagnetic stirring is proposed. The growth and enrichment of Si crystals were controlled by the upward and downward directional solidification of Si-50at%Sn melt at different moving rates. Compared with traditional directional solidification, electromagnetic stirring yielded a higher growth rate of Si. A lower moving rate improved the enrichment percentage of Si, and a large area of bulk Si with an enrichment percentage of up to 88.16% was obtained by performing downward directional solidification at a rate of 3 µm/s. Moreover, the downward movement improved the Si enrichment, because Si grew along the bottom and side walls of the crucible. These results provide a new scientific perspective and experimental evidence for the separation of primary Si crystals from Si-based multicomponent solvents.

Microstructural evolution in as-cast hypereutectic Al-Si alloys with different La additions

Abstract The microstructure of as-cast hypereutectic Al-(17 or 25) Si-xLa (x - 0, 0.4, 1, 3, 6 wt.%) alloys and the effect of La on the Si phase have been investigated. The microstructure was examined using optical and electron microscopy. The modification effects of La addition on the morphology and size of both the primary and the eutectic Si were observed. However, such effects are uneven and there seems no systematic change for the effectiveness of La modification with increasing La contents. The modification of La may be explained by the suppression of the growth of Si crystals through reduction of the eutectic reaction temperature or through enveloping of primary Si crystals, or additional twinning. Large particles of La-rich compounds are always observed associated with large primary Si crystals and unmodified eutectic Si crystals, while small La-rich particles are associated with small primary Si crystals and modified eutectic Si crystals. The overall concentration of La-rich compounds, formed in the Al–Si alloys containing more than 1 wt.% La, could be represented as a ternary AlSi2La2 phase, which actually contains the LaSi2 phase and an unknown ternary Al x Si y La phase.

Microstructural evolution and hardness of rapidly solidified hypereutectic Al-Si surface layers by laser remelting

ABSTRACT Selective laser melting technique is used to manufacture Al-25 wt.%Si and Al-30 wt.%Si surface layers with a thickness of 0.5–0.6 mm followed by a laser rapid solidification process to refine the silicon to a nano-size and increase the hardness. The as-produced alloyed layers were heterogeneous and composed of clustered aggregates of primary Si crystals of size 5–7 µm, decorated with α-Al cells/dendrites, and embedded in a matrix consisting of α-Al dendritic crystals and Al-Si eutectic. However, after several remelting scanning treatments at a fast speed and low power, significant refinement of all phases was achieved, and a dramatic increase in the proportion of the interwoven nano-sized Al-Si fibrous eutectic. The primary Si sizes were reduced to ≤1 µm, while the eutectic spacing decreased and approached as low as 10–15 nm at the highest cooling rate. The high cooling rate significantly reduced the size and the amount of the α-Al cells/dendrites especially those surrounding the primary Si, thereby stimulating ultrafine Al-Si fibrous eutectic to grow adjacent to the primary Si crystals and spread massively, which in turn contributed to the increase in hardness. Transmission electron microscopy revealed block-like primary Si, worm-like Si, and nanoscale fibrous eutectic Si, which was internally nano-twinned.

Efficient separation of bulk Si and enhanced B removal by Si–Sn–Cu ternary solvent refining with Zr addition

• A novel Si–Sn–Cu ternary solvent with Zr-assisted refining method was proposed. • The activity coefficients of B in Si–Sn–Cu ternary solvent were firstly measured. • Large-area bulk Si was obtained from ternary solvent by precise temperature control. • B-removal fraction was further improved compared with Si-based binary solvent. • Effective separation and purification of primary Si were achieved simultaneously. Si recycling from Si powder waste is of great significance to the production profits and alleviate hazardous effects on the ecological environment. Solvent refining is an effective way to reproduce solar-grade silicon (SoG–Si) by recycling and reusing the Si powder waste. Herein, a Si–Sn–Cu ternary solvent with Zr-assisted refining method was proposed to make full use of the high activity coefficients of B and the strong B affinity of Zr. The activity coefficients of B at the different compositions of the Si–Sn–Cu ternary solvent were firstly measured, which provided guidance on the solvent optimization to improve the B removal efficiency. The precise temperature control enabled the obtainment of large-area bulk Si from Si–Sn–Cu solvent, and the enrichment percentage of the obtained bulk Si from ternary Si–Sn–Cu solvent was 99.2%. Moreover, compared to the binary solvents, the absolute B content in primary Si further decreased to 4.7 ppmw after directional solidification, showing a better B removal fraction (95.3%). It provides a new scientific insight and an experimental evidence for the effective separation and purification of primary Si crystals from Si-based multicomponent solvents.

Analysis of piston microstructure in the most critical zones

This paper presents the results of testing the microstructure in the most critical zones of pistons made of aluminum piston alloys. The paper analyzes the shape, size and distribution of primary Si crystals and other phases in three most important piston zones (the edge of the combustion chamber, the area of rings groove and piston pin boss). The phase stoichiometry was identified by comparing the results of EDS analysis with the results reported in the studied literature. The results show that the necessary size, shape and distribution may be obtained in specific zones if appropriate solidification conditions are provided that are necessary for the formation of specific phases.

Transient heat flow parameters and microstructure of the upward solidified eutectic Al–Si alloy: a theoretical and experimental study

ABSTRACT In this work, a theoretical and experimental study has been carried out with the eutectic Al12.6Si alloy (wt.%) in order to determine and evaluate the transient heat flow parameters, such as metal/mold interfacial heat transfer coefficient (hi), growth and cooling rates (VL and TR, respectively), during upward solidification. The experimental thermal data were obtained via temperature mapping using a directional solidification device as well as via mathematical model to determine the time-varying hi values given by the general expression hi=at -n . The VL and TR values were determined from the generated simulated and experimental curves. The results showed a good agreement between the simulated and experimental values. The prediction of the microstructure has revealed the formation of irregular eutectic as well as the presence of non-faceted α-Al primary phase, like halos dendrites, and massive faceted primary Si crystals. Mathematical expressions that correlate halos dendrite spacing (λHD) as a function of hi, VL and TR have been proposed. The halo microstructure has been observed when hi, VL and TR values reach 3,608.00 WmoC−1, 0.36 mms−1 and 0.13 oCs−1, respectively. A comparative analysis with other works from literature has been conducted and the results have confirmed that the n exponent assumes values less than 0.5 for non-ferrous alloys (n < 0.5). For the particular case of binary Al–Si alloys it can be less than 0.35, that is, n < 0.35.

Effect of P modifiers, SiC nanomodifiers and combinations them on the structure of a hypereutectic aluminum-silicon AlSi18 alloy

Introduction. Studies have been carried out with the following types of an aluminum-silicon AlSi18 alloy: unmodified, modified by a standard P modifier, modified by a SiC nanomodifier, and modified by a combination of both types if modifiers. The paper discusses the influence of the different types of modifiers and the combination of them on the shape and size of the primary Si crystals and their distribution in the structure of the alloy, as well as on the shape and size of the Si crystals in the eutectic composition.

Effects of solidification thermal parameters and Bi doping on silicon size, morphology and mechanical properties of Al-15wt.% Si-3.2wt.% Bi and Al-18wt.% Si-3.2wt.% Bi alloys

The use of bismuth (Bi) in aluminum (Al) industries is recognized as prime importance because of possible recycling as alloying element in special applications such as those enhancing machinability. The effects of Bi interaction on solidification progress as well as on the resulting microstructure of hypereutectic Al-Si alloys, i.e. the morphology and size of Si crystals are rare information. As such, the present research work investigates the characteristic parameters of eutectic and primary Si with addition of Bi for both ternary Al-15 wt.%Si- 3.2 wt.%Bi and Al-18 wt.%Si- 3.2 wt.%Bi alloys samples directionally solidified (DS) under various cooling rates. The alloy containing 15 wt.%Si showed that Bi acts in the suppression of the growth of primary Si crystals for samples solidified at cooling rates ranging from 2.0 to 31 K/s. For lower rates, a minor fraction of five-fold branched Si was recognized in transverse specimens. A mixture of globular-fibrous eutectic Si is shown to occur for eutectic cooling rates higher than 5 K/s. The Al-18 wt.%Si- 3.2 wt.%Bi alloy is characterized by a mixed structure of eutectic flaky Si and primary angular Si with intersecting spines at nearly 90º all over the entire range of experimental cooling rates (i.e., from 0.4 to 40 K/s). Despite the intricacy of the formed microstructures, it was possible to establish relationships between tensile properties and eutectic Si spacing in order to compare the properties between the tested alloys. Tensile strength and elongation decrease with increasing alloy Si content.

The effect of the modification by ultrafine silicon carbide powder on the structure and properties of the Al-Si alloy

Purpose: Determine the possibility of modifying aluminium alloys of the Al-Si system with an ultrafine SiC modifier with a particle size of 3-5 μm. Design/methodology/approach: Processing of the Al-Si alloy was carried out by introducing an ultrafine modifier in the amount of 0.1, 0.2, or 0.3 wt.%. Silicon carbide (SiC) with a particle size in the range of 3-5 μm was used as a modifier. To study the microstructure of the formed surface layers, a metallographic analysis was performed according to the standard method on a microscope MIKPOTEX® MMT-14C using TopView software. Microhardness studies of the samples were carried out on a Vickers microhardness tester NOVOTEST TC-MKV1. The microstructure of castings of the AlSi12 grade was studied at magnification from 100 to 400 times on the horizontal and vertical surfaces of the samples after etching with a 2% NaOH aqueous solution. Findings: Aluminium cast alloy of Al-Si system has been synthesized with the addition of 0.1, 0.2, and 0.3 wt.% ultrafine SiC modifier. It was found that the modification of the AlSi12 alloy by SiC particles of 3-5 μm in size led to an improvement of its microstructure due to the reduction of the volume fraction of micropores and primary Si crystals. It was shown that the AlSi12 aluminium alloy due to the modification by 0.2 wt.% SiC has the best micromechanical properties and macrostructure density. Research limitations/implications: The obtained research results are relevant for cast specimens of the indicated sizes and shapes. The studies did not take into account the influence of the scale factor of the castings. Practical implications: The developed modification technology was recommended for use in the conditions of the foundry "Dnipropetrovsk Aggregate Plant" (Dnipro, Ukraine). Originality/value: The technology of AlSi12 alloy modification of ultrafine SIC modifier with a particle size of 3-5 μm was used for the first time.

Fabrication of Surface Layer Enriched with Zinc on AlSi17 Aluminium Cast Alloy by Hot-Dip Galvanizing

The paper deals with the fabrication of the surface layer enriched with Zn on AlSi17 aluminium alloy to modify the microstructure and surface properties of the alloy. The continuous surface layer was fabricated on the AlSi17 substrate by the hot-dip galvanizing of AlSi17 for 15min in a Zn bath heated to 450°C. The thickness of the layer was about 100 μm. The layer was characterised by a multi-component microstructure containing the regions of a solid solution of Al in Zn and dendrites of a eutectoid composed of a solid solution of Al in Zn and a solid solution of Zn in Al. In the layer, fine particles of Si with a regular shape were distributed. The results indicated that these Si particles formed by the action of Zn on the eutectic Si precipitations in the AlSi17 substrate. In the microstructure, large primary Si crystals and multi-phase precipitations, originating from the substrate, were also observed. The surface layer had much higher microhardness than the AlSi17 substrate. The results showed that hot-dip galvanizing can be used to modify the microstructure and properties of the surface layer of AlSi17. The study indicates the possibility of conducting further research on the fabrication of joints between AlSi17 and other metallic materials using a Zn interlayer fabricated by hot-dip galvanizing.

Microcharacterization and mechanical performance of an Al-50Si alloy prepared using the sub-rapid solidification technique

In this reported work, the hypereutectic Al-50Si alloys with a wide mushy zone of 465 °C were successfully fabricated using twin roll casting technique. The experimental results showed that the overall hardness of the roll-cast alloy increased, as a result of the decrease in the size of Si crystal and the increase in the relative solubility of Si in the α-Al. However, the EBSD results showed that the primary Si crystals in the twin-roll cast alloy grew according to twin plane re-entrant edge (TPRE) mechanism.

The influence of various Mg contents on the microstructures, mechanical properties and thermal expansion behaviors of P-modified Al–50Si alloys

In this work, the evolutions on the microstructure, mechanical performance as well as thermal expansivity of hypereutectic Al–50Si alloys with different Mg contents were investigated in detail. Besides that, the growth mechanism of the five-fold branch-like primary Si crystal was also explained in the view of thermodynamics. The results indicated that the size of primary Si crystal and the thermal expansivity showed upward trends with the increase of Mg content but the mechanical property presented a downward trend. Moreover, as the Mg content rose, the morphologies of primary Si crystals in the P-modified Al–50Si–xMg (x = 0, 5, 10 wt%) alloy changed from octahedron-like to plate-like and fivefold branch-like. The reason of these adverse phenomena was attributed to the formation of Mg3P2 phase caused by the reaction between Mg element and AlP compound, which will generate a poisoning effect and worsen the performances of the alloys.

Study of the influence of both a modifier P and a nanomodifier ND on the structure of a hypereutectic aluminium-silicon alloy AlSi18

The process of hardening a hypereutectic aluminium-silicon alloy AlSi18 in unmodified, modified and nano-modified state has been studied. Two types of modifiers have been used - a standard modifier P, introduced into the melt by means of the ligature CuP 0,4wt.%, and a nanomodifier (nanodiamonds - ND) 0,1wt.%. The influence of the various types of modifiers on the process of crystallization, as well as on the size of the primary Si crystals in the structure of the alloy and their distribution in it has also been discussed.

The influence on microstructure and wear behavior of hypereutectic Al-50Si alloy with various Cu-P admixture contents and cooling rates

The effect of different Cu-P admixtures and cooling rates on microstructure and wear behavior of hypereutectic Al-50Si alloys were studied systematically in this work. The growth patterns and the modified as well as over-modified mechanisms of primary Si crystals under various P contents are also illuminated in detail. It was found that the optimum content of the modifier in Al-50Si alloy is 3 wt%. At this modifier content, the mean size of Si crystallite is the smallest and the distribution is also the most homogeneous. Vickers hardness and wear test were employed to detect the mechanical performance of materials. The maximum value of hardness and the minimum value of average friction coefficient obtained under the modifier content of 3 wt% are 198 HV and 0.407, respectively.

Transient directional solidification of a eutectic Al–Si–Ni alloy: Macrostructure, microstructure, dendritic growth and hardness

The Al-11 wt%Si-5 wt%Ni eutectic alloy was directionally solidified (DS) under transient heat flow conditions at cooling rates in a range of about 1–25 °C/s along the length of the casting. For comparison purposes binary Al-11 wt%Si and Al-5 wt%Ni alloys were also solidified under similar experimental conditions. Initially characterized by columnar grains, the macrostructure of the ternary DS alloy casting evolved during solidification to a columnar to equiaxed transition (CET) at critical cooling rates of about 1.3–1.6 °C/s, i.e. more than 7 times higher than the CET critical value determined in the literature for Al–Ni and Al–Si alloys. The microstructure of the ternary alloy casting is shown to be constituted by primary Si crystals and α-Al dendritic branches surrounded by the eutectic phase (α-Al+Si+Al3Ni), with the Al3Ni phase having initially a plate-like morphology, which with the decrease in the solidification kinetics, evolved to a fishbone morphology. The pertinent scaling laws representing the dendritic growth of the eutectic ternary alloy, properly compared to those of the examined Al–Si and Al–Ni binary alloys are outlined. The scale of the dendritic α-Al phase is shown to affect the Vickers hardness (HV) along the length of the ternary eutectic alloy casting, and a Hall–Petch type equation is proposed relating HV to the secondary dendritic arm spacing.

Boron removal from silicon by hydrogen assistant during the electromagnetic directional solidification of Al Si alloys

Abstract Boron (B) removal from silicon (Si) by using an Ar-20% H2 gas mixture during the Al Si solvent refining was investigated. Electromagnetic directional solidification was employed to enrich the primary Si crystals in the melt and simultaneously drive the produced BxHy gas species to migrate out of the Si enrichment zone. Thermodynamic analysis shows that the formation of BxHy gas species by the reaction of dissolved boron [B] and dissolved hydrogen [H] in Al Si melt is feasible. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) analysis confirmed that the impurity carbon (C), oxygen (O), and B adhered to the wall of the gas cavity, indicating that B removal can be attributed to the migration of gas species. As the flow time of Ar-20% H2 gas mixture increased, the B removal fraction significantly increased, and a maximum value of 96.3% for B removal fraction was achieved when the flow time was 150 min. Finally, an overall process for producing solar grade silicon (SoG-Si) from metallurgical grade silicon (MG-Si) with the combination of Ar H2 refining in Al Si solvent and vacuum directional solidification was proposed.

Effect of Trace Yttrium Addition and Heat Treatment on the Microstructure and Mechanical Properties of As-Cast ADC12 Aluminum Alloy

The effects of rare earth yttrium (Y) additions and the heat treatment process on the microstructure and mechanical properties of as-cast ADC12 aluminum alloy have been investigated. The results showed that the primary Si crystals were significantly refined from long axis to fibrous or granular when the Y content was 0.2 wt%. Compared to the matrix, the mean area and aspect ratio were decreased by 92% and 75%, respectively. Moreover, the Si and Fe-rich phases were spheroidized and refined with a small average size during the solid solution. It was also noted that the copper-rich phases were dissolved into the matrix. Correspondingly, it was found that after metamorphic and heat treatment the ultimate tensile strength (UTS), elongation, and, hardness increased by 81.9%, 69.7%, and 74.8%, respectively, compared to the matrix. The improved mechanical properties can primarily be attributed to the optimization of the microstructure and the refinement of various phases.

Modification of Primary Si Crystals and Fe-rich Intermetallics in LM28 Al Alloy Fabricated Through Thixoforming

Modification of Primary Si Crystals and Fe-rich Intermetallics in LM28 Al Alloy Fabricated Through Thixoforming

Si Purification by Removal of Entrapped Al during Electromagnetic Solidification Refining of Si-Al Alloy

To investigate the effect of the removal of entrapped Al on Si purity during Al-Si electromagnetic solidification refining, the formation mechanism and removal principle of entrapped Al in primary Si were studied. The results showed that the agglomeration and irregular shape of primary Si crystals during electromagnetic solidification were the main reasons for the formation of entrapped Al; a higher initial Si content in an Al-Si alloy may result in higher amounts of entrapped Al. The entrapped Al can be effectively removed by first grinding the primary Si to powder particles of ~20 μm and then leaching the particles with acid, which can simultaneously remove other impurities concentrated in the entrapped Al such as Fe, Ca, Ti, and B. Hence, it is proposed that the combination of electromagnetic solidification, acid leaching, and vacuum directional solidification could be used to obtain solar-grade silicon.