Al-Si Melt Research Articles

B Removal by Zr Addition in Electromagnetic Solidification Refinement of Si with Si-Al Melt

This study investigated a new process of enhancing B removal by adding small amounts of Zr in the electromagnetic solidification refinement of Si with Si-Al melt. B in Si was removed by as much as 97.2 pct by adding less than 1057 ppma Zr, and the added Zr was removed by as much as 99.7 pct. In addition, Zr is more effective in enhancing B removal than Ti in the same electromagnetic solidification refining process.

Effect of rotational speed on rapid separation of primary silicon from an Al–Si alloy by novel centrifugation

The optimum rotational speed (RS) of a novel centrifugation method for obtaining high-purity Si separated from Al–Si melts was evaluated numerically and experimentally. Numerical calculations showed that the separation efficiency (Si to Al ratio in the separated solid) increases and finally reaches a plateau at a high RS. The calculated relationship between RS and the separation efficiency was consistent with experimentally measured data except for the removal of primary Si crystallites, which happened at centrifugation at 800 revolutions per minute (RPM). Based on the acquired data, it was established that 700RPM is the optimal RS for our centrifugation apparatus (developed in a previous study) to maximize the separation efficiency.

Effect of Zirconium on Boron Removal in Electromagnetic Solidification Refinement of Silicon from Si-Al Melt

This work investigates the removal of B in Si by the addition of Zr in the electromagnetic solidification refinement of silicon-aluminum melts. As Zr has a strong affinity for B and can form the thermodynamically stable compound of ZrB2, the B content of lower-grade Si is expected to be effectively removed by adding a small amount of Zr to the Si-55 at% Al melt. The results show that Zr is strongly responsible for the decrease in B content of refined Si. The removal fraction of B significantly increased from 60.2% to 97.3% by adding a small amount of Zr (0 to 3500 ppmw). In addition, the removal fraction of Zr from Si was found to be as high as 98.6%; however, its residual content in the refined Si was significantly larger than its solid solubility in Si, possibly due to the non-equilibrium solidification occurring during the refining process.

Segregation Behavior and Evolution Mechanism of Iron-Rich Phases in Molten Magnesium Alloys

A new method has been proposed to prepare Mg–Al–Si master alloys by utilizing scrap Al–Si–Fe alloys with higher Fe levels, which aims to segregate Fe from Al–Si–Fe alloys by Mg melt. The segregation behaviors, microstructure morphology and evolution mechanism of iron-rich phases in Mg–Al–Si alloy melts were studied, after Al–14Si–4Fe (wt%) alloys were added and dissolved completely. In the Mg–Al–Si alloys, iron has very little solubility and tends to combine with other elements to form intermetallic phases, which grow into a deposition layer due to the higher density. During the cooling and solidifying process of Mg–Al–Si melts, the needle-like Al 5 SiFe phase in Al–14Si–4Fe alloy evolved into blocky Al 5 Fe 2 and Al 0.7 Fe 3 Si 0.3 phases. Besides, the Fe levels of the Mg–Al–Si master alloys were reduced to 0.017 wt% from nominal content of 0.164 wt%. Based on the above results, this work carried out a semi-quantitative phase-compositions analysis for the deposition layer by relative intensity ratio (RIR) method, and evolution mechanism of the iron-rich phases had also been discussed. This study has paved a new way to regenerate the scrap Al–Si–Fe alloys, which has a great significance of promoting the recycling of aluminum resources.

The Mechanism of P Removal by Solvent Refining in Al-Si-P System

To study P removal from Si by solvent refining in Al-Si-P system, series of quench experiments are carried out, which confirms a high P removal rate. An apparent segregation coefficient is introduced to characterize the segregation between primary Si and Al-Si melt, which are determined to be 0.00670, 0.0117, and 0.0201 when Si contents are 20, 30, and 40 wt pct, respectively, at the cooling rate of 0.556 mK s−1. A calculation of Gibbs free energy of AlP in the alloys is also carried out and proves AlP precipitates before the primary Si formation. The formation of the AlP particles causes the decrease of P content in the Al-Si melt and contributes to the high P removal. Models to explain the P contents in the refined Si of each sample in this work are presented. These include three different cases to describe the interaction between the AlP particles and the primary Si.

Effect of traveling magnetic field on separation and purification of Si from Al–Si melt during solidification

Separation and purification of the Si crystal during solidification process of hypereutectic Al–30Si melt under traveling magnetic field (TMF) were investigated in the present study. The results showed that under a proper condition the Si-rich layer can be formed in the periphery of the ingot while the inner microstructure is mainly the Al–Si eutectic structure. The intense melt flow carries the bulk liquid with higher Si content to promote the growth of the primary Si phase which is first precipitated close to the inner wall of the crucible with a relatively lower temperature, which resulting in the remarkable segregation of the primary Si phase. The impurity contents of the refined Si can be reduced to a very low level. The typical metallic impurities have removal fraction higher than 99.5%. In addition, there is a significant difference in the P contents between the primary and eutectic Si phases, which might be ascribed to the formation of AlP phase that acts as the heterogeneous nucleation sites. Furthermore, a considerable amount of Fe-containing particles with a size about 100–300nm is found inside the eutectic Si phase, indicating an unintended entrapment of Fe in Si.

Pre-nucleation clusters mediated crystallization in Al–Si melts

Triggered by exogenous local ordering structures, pre-nucleation clusters appear in Al–Si melts and significantly alter the crystallization process. In absence of potent nucleating substrates (in high-purity melts), they delay silicon crystal nucleation, otherwise (in commercial-purity melts), they enhance nucleation and growth as well as change the growth mechanism of silicon. This makes the silicon grain size a competition result of nucleation and growth processes and some empirical rules of classical nucleation unreliable, e.g., easy nucleation no longer means grain-refined microstructures. These phenomena are further explained by characterizing the remains of these pre-nucleation clusters in solidified alloys.

Separation Mechanism of Primary Silicon from Hypereutectic Al-Si Melts Under Alternating Electromagnetic Fields

Solar grade silicon (SOG-Si) and hypereutectic Al-Si alloys with low silicon (silicon composition below 25 pct) can be successfully obtained by separation of hypereutectic Al-Si alloy with high silicon (silicon composition above 30 pct) under an alternating electromagnetic field after post-processing. To explore the separation mechanism in detail, experiments were conducted in this study using a high-frequency induction furnace with different pulling conditions of the crucible which is loaded with Al-45 wt pct Si melt. Results demonstrate that the separation of hypereutectic Al-Si alloy is feasible through either a pull-up or drop-down process. The height of each separation interface between the compact and sparse parts of the primary silicon decrease as the pull-up distance rose. When the pulling rate is very low, resultant morphologies of compact primary silicon are rounded and polygonal, allowing for more effective separation of the primary silicon. A novel physical model is presented here based on the experimental results and simulation. The model can be used to effectively describe the separation mechanism of primary silicon from hypereutectic Al-Si melts under alternating electromagnetic fields.

Refining of metallurgical grade Si by solidification of Al–Si melt under electromagnetic stirring

Aiming at developing a Si solidification purification process in the Al–Si alloy system, series of experiments on the effect of electromagnetic stirring on refining of metallurgical grade Si(MG–Si) were carried out. By the use of electromagnetic stirring, an increase of size and average mass of primary Si flakes with the increasing voltage of the electromagnet were obtained. Impurity elements formed intermetallic compounds with Al and Si, such as Al7Fe2Si(α phase) and Al5FeSi(β phase). An intermetallic compound type transformation with electromagnetic stirring was observed. The refined Si under the alternative electromagnetic field contains much lower B and P than those without the electromagnetic stirring. An apparent segregation coefficient was introduced to describe the segregation between solid Si and Al–Si melt. The apparent segregation coefficients of B and P are determined to be 0.37 and 0.17, respectively, when the voltage of the electromagnet is 210V and cooling rate is 0.5Kmin−1.

Effect of Si content on separation and purification of the primary Si phase from hypereutectic Al–Si alloy using rotating magnetic field

Effect of Si content on separation and purification of the primary Si phase from hypereutectic Al–Si alloy using rotating magnetic field (RMF) was investigated in the present study. The results showed that the Si content has an obvious influence on the thickness of Si-rich layer formed around the sample and impurity contents in the purified Si. The thickness of Si-rich layer increases and the Si content in the Si-rich layer decreases with increasing Si content of the hypereutectic Al–Si alloy. The impurity contents of the purified Si can be reduced to a very low level. The typical metallic impurities (Fe, Ti, and Ca) have a high removal fraction. Electron probe microanalyzer (EPMA) analysis clearly reveals that the B content is lower in the primary Si phase than that in the Al–Si eutectic, suggesting the segregation of B element between the primary Si phase and the remaining liquid during solidification. In addition, the contents of B and metallic impurities (Fe, Ti and Ca) increase with increasing Si content of the Al–Si melt due to the segregation process starting at higher temperatures with higher Si content.

The Effect of Mg Adding Order on the Liquid Structure and Solidified Microstructure of the Al-Si-Mg-P Alloy: An Experiment and ab Initio Study

In this paper, the relationship between the liquid structure and the corresponding solidified microstructure of an Al-Si-Mg-P alloy was studied. Experimental results show that Mg can reduce the phosphorous-modification effect if it was added after adding Al-P alloy. However, when it is added before adding Al-P alloy, Mg has no effect on the phosphorous-modification. It is considered that the difference in liquid structure induced by changing the adding order of Mg should be responsible for the above phenomenon, and was investigated by ab initio molecular dynamics simulation (AIMD). It was believed that the high-active Mg atoms could bond P atoms to form P-Mg clusters and then reduce the modification effect of AlP, when pure Mg was added into the prepared Al-Si-P melt. When the pure Mg was added into Al-Si melt before adding Al-P alloy, the Mg atoms would be first occupied by Si atoms to form Mg-Si clusters, and thus lose the ability to eliminate P-Al clusters which dissolve into melt later, leading to a good phosphorous-modification effect.

Experimental Study on the Segregation of Primary Silicon from the Hypereutectic Al-Si Melts in the Electromagnetic Directional Solidification

Solar grade silicon and modified Al-Si alloys are widely used in the world. These two favored materials are expected to obtain by the electromagnetic directional solidification of hypereutectic Al-Si melts. The segregation of hypereutectic Al-Si alloy is feasible with respect to the process of either pull-up or drop-down in electromagnetic directional solidification. To explore the segregation efficiency, experiments have been conducted in a high-frequency induction furnace with different pulling conditions of hypereutectic Al-Si melts. The results show that the segregation efficiency of dropping-down is higher than that of pulling-up; the slower dropping-down rate lead to higher segregation efficiency in the electromagnetic directional solidification; increasing the temperature gradient may promote the separation efficiency. A separation mechanism model responsible for the formation of the distribution of primary silicon in the cross-sections is also proposed. Wish to provide theoretical basis for the more efficiency of the segregation of hypereutectic Al-Si melts.

Grain refining potency of Nb–B inoculation on Al–12Si–0.6Fe–0.5Mn alloy

In a recent work we reported that the combined addition of Nb and B has higher grain refinement effectiveness than the addition of these individual chemical elements. In this work a commercial Al–12Si–0.6Fe–0.5Mn alloy is employed to fully characterise the refining potency of Nb–B inoculation. It is found that the addition of Nb powder and KBF4 flux to Al–Si melt introduces potent nuclei with low lattice mismatch with the primary α-Al dendrites (i.e. low undercooling) and, thus, promote noteworthy grain refinement through heterogeneous nucleation. Nb–B inoculation induces a significant increment of the volumetric number of grains, decreases the undercooling needed for solidification and makes the grain size of the Nb–B inoculated material less sensitive to the cooling rate employed to solidify the material.

Effect of electromagnetic stirring on the enrichment of primary silicon from Al–Si melt

The effect of electromagnetic stirring on the enrichment of primary silicon from Al–Si melt during the process of electromagnetic separation was investigated. It is shown that the enrichment of primary silicon in Al–Si melt strongly depends on the melt flowing and viscosity gradient. The efficient enrichment of primary silicon was achieved by implementing a high current intensity, which induced a high intense melt flowing. Also, the remaining primary Si in Al–Si alloy could be precipitated by gradually decreasing the current intensity. Additionally, the inductively coupled plasma mass spectrometry (ICP-MS) results show that Si purification is attributed with the enrichment of primary silicon. In this work, the impurity content in primary silicon is 43.3ppmw, which is much smaller than 777.6ppmw in metallurgical silicon. Therefore, a potential low-cost technology would be provided for the Si purification.

In-situ synthesis of SiC particles by the structural evolution of TiCx in Al–Si melt

A facile method has been developed to in-situ synthesize SiC particles utilizing the structural instability and evolution of TiCx in Al–Si melt. It is considered that the synthesis of SiC particles occurs via the gradual reaction between TiCx and Si atoms, whilst Si content plays the crucial role in this approach. If the Si content in the melt is above 30%, TiCx directly reacts with Si and Al to form SiC, but the needle-like TiAlxSiy phase formed simultaneously will do harm to the mechanical properties of the composites. Thus, it is proposed to add B element in the melt to transform the TiAlxSiy into TiB2 particles. Therefore, the SiC and (SiC+TiB2) hybrid-particles reinforced Al–18Si composites were successfully prepared using the method. In the composites, the SiC particles have the size range from 2.5 to 7.5μm and a block-like morphology. Furthermore, the mechanical properties of base alloy, including the wear resistance and macro-hardness, have been obviously improved by the in-situ SiC particles. Besides, the relevant underlying mechanisms are also discussed.

Si purity control and separation from Si–Al alloy melt with Zn addition

During solidification refining of Si, the eutectic alloy ultimately solidifies and becomes enriched with impurity elements according to their small segregation coefficients, which may contaminate the primary Si through the cooling process. Combined with analysis of the morphology of the primary Si, the contamination mechanism was verified for a Si–63.8wt%Al alloy using various cooling rates in the final cooling stage (873–293K). By a quenching treatment, the B content in Si was decreased from the initial content of 14.8 to 1.4ppmw, which is quite close to the value predicted by Scheil’s equation. Si cooled down at a rate of 1K/min showed an increase in impurity content (e.g., B: 4.2ppmw) owing to solid-state back-diffusion. In order to prevent such contamination, Zn was added to the Si–Al melt at 873K, which decreased the eutectic temperature and separated Si from the alloy. A final Si amount of 94.1wt% was recycled without further contamination.

Separation mechanism of the primary Si phase from the hypereutectic Al–Si alloy using a rotating magnetic field during solidification

Understanding solidification behavior under an intense flow field is important in controlling the microstructure and macrostructure of alloys in industry. In the present study, we show that using a rotating magnetic field (RMF) during solidification of hypereutectic Al–Si alloy can efficiently congregate the primary Si phase to the inner wall of the crucible and form a Si-rich layer with 65–698wt.% Si content. The Al–Si melt flow under an RMF and the temperature field of the liquid metal are the two dominant conditions for the segregation of the primary Si phase. The intense melt flow, i.e., secondary flow and Taylor–Görtler vortices, carries the bulk liquid with higher Si content to promote the growth of the primary Si phase formed close to the inner wall of the crucible where the temperature is low, finally resulting in the remarkable segregation of the primary Si phase. This work has demonstrated that a forced intense melt flow combined with proper cooling conditions can greatly change the solidification structure of alloys, which is beneficial to microstructure control.

Separation and purification of Si from solidification of hypereutectic Al–Si melt under rotating magnetic field

A low-cost and high-efficiency method to purify Si directly from cheap MG–Si at low temperature was proposed and demonstrated in this paper, which used power frequency rotating magnetic field (RMF) to separate the primary Si from a hypereutectic Al–Si alloy and was followed by the acid peeling. The separation mechanism was based on the flow characteristic of melt under RMF and the cooling condition of the liquid metal. A Si-rich layer with Si content of 65–59wt% was formed in the periphery of alloy, while the inner microstructure of the alloy was mainly the Al–Si eutectic structure. The refined silicon was collected after aqua regia leaching, and had much fewer typical impurities (Fe, Ti, Ca, B, P) than those in MG–Si, and the metallic impurities besides Al had removal fraction higher than 98%, which is mainly ascribed to the segregation effect of Al-30Si alloy during solidification under RMF.

Effect of Heat Treatment and Silicon Concentration on Microstructure and Formation of Intermetallic Phases on Hot Dip Aluminized Coating on Indian RAFMS

Hot dip aluminizing was tried on Indian reduced activation ferritic martensitic steel. This experiment was performed with aluminum (Al) melt, with three different silicon (Si) concentrations (3%, 5%, and 7%). Samples were dipped into the Al-Si melt, at 750°C for 30 s, which produced a hard and brittle Fe2Al5 intermetallic layer on the samples. These samples were subjected to two types of heat treatments: (I) 760°C for 30 h and (II) 980°C for 0.5 h, followed by 760°C for 1.5 h to convert the intermetallic layer into more ductile phases. The width of the Fe2Al5 layer was <10 μm for all the samples with different Si concentrations, and for the pure Al melt, it was ∼35 μm. For both the heat treatments, FeAl and α-Fe(Al) layers were observed. Out of the two heat treatments, heat treatment I gives thinner FeAl and α-Fe(Al) layers compared to heat treatment II. X-ray diffraction measurements confirmed the formation of an α-Al2O3 layer on the surface, for 3% and 5% Si concentrations for heat treatment I and for all Si concentrations for heat treatment II. The hardnesses of the Fe2Al5, FeAl, and α-Fe(Al) layers were found to be 972 to 1089 HV (hardness value)/0.01, 324 to 384 HV/0.01, and ∼200 HV/0.01, respectively.

Low-Cost Process for Silicon Purification with Bubble Adsorption in Al-Si Melt

The primary silicon and Al-Si alloy have been separated in hypereutectic Al-Si melt by the electromagnetic stirring and directional solidification processes. During the electromagnetic separation process, the behavior of a hydrogen bubble in Al-Si melt has been discussed. Furthermore, the bubble adsorption effect for the Si purification has been revealed. The results show that the bubble cavity formed in the lower part of the sample by pulling it up. The scanning electron microscope along with energy dispersive spectrometer (SEM-EDS) analysis indicated that a lot of impurities were adsorbed onto the surface of the bubble cavity that may be beneficial for the Si purification. By decreasing the pulling-up rates, the size of the bubble cavity in Al-Si alloy increased, which results in the decreasing of the impurity contents in primary silicon. In this work, the impurity content in primary silicon is 10.8 ppmw, which is obviously improved compared with the 777.57 ppmw in metallurgical silicon. It is a low-cost technology that will be a potential route for the Si purification.