Primary Silicon Grains Research Articles

Effect of Different Melt Treatment Methods on Microstructure and Thermal Stability of Al-Si Alloy

The effects of modification, electromagnetic stirring and compound treatment (modification and electromagnetic stirring) on the microstructure and thermal stability of Al-13% Si alloy are experimentally investigated. The results show that the three melt treatments can change the microstructure of the alloy. The modification has obvious refining effect on primary silicon and eutectic silicon grains, electromagnetic stirring has refining effect on primary silicon grains, and eutectic silicon grains appear coarsening phenomenon. The latent heat is obviously decreased by electromagnetic stirring, while the latent heat is increased by modification. Refining eutectic silicon grains will increase the latent heat of phase transformation of the alloy. When 560 °C remains unchanged, the melting temperature of the alloy generally decreases with the increase of holding time, but fluctuates, with a decrease of 7 °C. The latent heat decreases with the increase of constant temperature time, and the decrease amplitude reaches 8.4%.

Механохимически синтезированные никельсодержащие алюмосиликатные модификаторы для триботехнических алюминиевых сплавов

Silumins are promising antifriction alloys for the manufacture of friction units. They have a low density, high wear and corrosion resistance. However, they own low hardness and strength, which leads to the need to develop modifying and strengthening additives. Aluminosilicates have a density close to silumin and can be used for its strengthening, but they are characterized by low wettability by metals. To improve wettability, it is necessary to modify the aluminosilicates with metals. The aim of this work is to study the mechanochemical synthesis of aluminosilicate/Nickel composites and modification of silumin using them. Structural changes in powder mixtures of natural aluminosilicates with Nickel during high-energy mechanical processing in a planetary ball mill, as well as in alloys of the composition silumin + potassium fluoroborate and silumin + potassium fluoroborate + aluminosilicate/Nickel obtained by hot pressing of mechanoactivated powders, were studied using Fourier-infrared spectroscopy and x-ray diffraction analysis. It is shown that mechanical activation of a mixture of silumin + potassium fluoroborate powders leads to a redistribution of silicon during sintering with the formation of large primary silicon crystals and a decrease in micro hardness by ~ 20 %. Mechanical activation of aluminosilicates with Nickel leads to the formation of composite powders with the removal of OH groups and the formation of SiO2 framework structures. The use of such a modifier during sintering of the silumin alloy leads to the formation of a homogeneous fine structure of the alloy with the grinding of eutectic and primary silicon grains, which increases the micro hardness of the alloy by 1.5 times.

The Influence of Neodymium Element on the Crater Structure Formed on Al-17.5Si Alloy Surface Processed by High-Current Pulsed Electron Beam

The effect of neodymium element on the elimination of crater structures on the surface of Al-17.5Si metallic materials processed by high-current pulsed electron beam was investigated in this study. Field emission scanning electron microscopy analysis indicated that the grain sizes of Al-17.5Si metallic materials were reduced and craters were removed from surfaces of the processed Al-17.5Si metallic material after addition of Nd. This can be attributed to the efficient transfer of heat accumulated in rich-silicon (primary silicon) areas without the eruption of a primary silicon phase if the size of primary silicon grains are small. The X-ray diffraction analysis indicates that all diffraction peaks are broadened because of the presence of structural defects, grain refinement and stress state. Electron probe micro-analyzer analysis demonstrated that Al and Nd were evenly distributed on the surface of the treated alloy, which could be attributed to the diffusion of the element. Transmission electron microscopy analysis showed that nano-Al and nano-Si cellular textures were generated during the treated process. The formation of these structures can be attributed to rapid heating and cooling effects by the treatment. Finally, electrochemical tests revealed that the corrosion current density of Al-17.5Si metallic materials (with Nd, 0.3 wt.%.) surface decreased by three orders of magnitude compared with that of the processed Al-17.5Si metallic material surfaces (without Nd). This can be attributed to the elimination of craters and grain refining.

Effect of Pouring Temperature on Mechanical Properties and Microstructures of Aluminium Matrix Composite Strengthened by CNT with Stir Casting Method

Aluminium matrix composite reinforced Carbon nanoTubes are widely developed because it can increase mechanical strength without reducing its ductility. One of the AMC / CNT manufacturing processes is through the stir casting method. The challenge of the Al / CNT manufacturing process is the occurrence of agglomeration and CNT not homogeneous so that the right casting parameters are needed to obtain optimal results. The purpose of this study was to analyse the effect of pouring temperature on the mechanical strength and microstructure of AA6061 by adding Carbon Nanotube (CNT) through the stir casting method. The CNT is added by 0.1% wt and pouring temperature at 700 °C, 730 °C and 760 °C. Mechanical tests carried out were tensile test, hardness test, and impact test. The highest value of hardness and tensile strength was obtained at the pouring temperature of 700 °C are78 HV and 80.97 MPa. Lower pouring temperature causes smaller grain size so that it has higher strength. The distribution of hardness values ​​at the top, middle and bottom of the specimen is not evenly distributed, but does not differ greatly for all pouring temperatures. The highest value of impact strength is obtained at the pouring temperature of 760 °C which is 0.128 J/mm2. Microstructure was shown the addition of CNTs caused the size of primary silicon and aluminium grains to be small which would increase the mechanical properties.

Preparation of Semi-Solid A390 Aluminum Alloy Slurry through a Serpentine Pouring Channel

In order to study the possibility of refining the primary silicon grains in the microstructure of hypereutectic Al-Si alloy through a serpentine pouring channel, the semi-solid slurry of A390 aluminum alloy was prepared through a water-cooled copper serpentine pouring channel, which is a new method proposed recently for semi-solid forming process, and the effect of pouring temperature on the slurry microstructure was investigated. The results show that the slurry of A390 aluminum alloy with refined primary silicon grains can be prepared under given conditions and especially when the pouring temperatures is 690°C, the primary silicon grains can be refined obviously, the equivalent silicon grain size is 19.7mm and the average shape factor is about 0.7. The analysis shows that the chilling effect of the inner channel wall precipitates a large number of primary silicon nuclei, and so the primary silicon grains are refined greatly. Meanwhile, the subsequent alloy melt washing also promotes the separation of primary silicon grains from the inner wall and the primary silicon grains are further refined. The work undertaken demonstrates eventually that the serpentine pouring channel process is a good method for refining the primary silicon grains in hypereutectic A390 alloy rather than using chemical fining agent phosphorus as in traditional process, and provides an alternative process choice.

Refinement of Primary Silicon Grains in Semi-Solid Al-25%Si Hypereutectic Aluminum Alloy Slurry

s: The semi-solid slurry of Al-25%Si hypereutectic aluminum alloy was prepared through a copper serpentine pouring channel, the effect of pouring temperature and numbers of channel bend on the slurry microstructure was investigated. The results show that the primary silicon grains in Al-25%Si hypereutectic alloy solidified at a traditional condition are very large and the average silicon grain size is about 65.3μm, however, when the liquid Al-25%Si alloy is poured through a copper serpentine pouring channel, the primary silicon grains are fined obviously. If the channel has three bends and the pouring temperature is 747°C,the average silicon grain size in the slurry is about 33.7μm. If the channel has four bends and the pouring temperature is 747°C, the average silicon grain size in the slurry is about 30.9μm. If the channel has seven bends and the pouring temperature is 747°C, the average silicon grain size in the slurry is about 28.6μm. The analysis shows that the chilling effect of the inner channel wall precipitates primary silicon nuclei, and so the primary silicon grains of Al-25%Si alloy are fined greatly. Meanwhile, the subsequent washing of the alloy melt also promotes the separation of primary silicon grains from the inner wall and the primary silicon grains are further fined. The above research work demonstrates eventually that the copper serpentine pouring channel process is a good method for fining the primary silicon grains in hypereutectic Al-25%Si alloy rather than using chemical fining agent phosphorus as in traditional process.

Hypereutectic silumin modification by ion-electron-plasma method

In this paper, hypereutectic silumin (22-24 wt/%) was used. The structure was studied before modification. It is represented by primary silicon grains with a size of about 100 μm. Intermetallic compounds are also non-uniform distributed. After modification by an electron beam, the crystallite size was 0.4-0.5 μm. There are several layers. The modification depth was 130 μm. Hardness increased in 1.7 times, wear resistance in 1.2 times. Also, in this paper, data was given on the variation of the elementary composition with respect to the depth of the sample. The film was melt into the surface of the sample by an electron beam after depositing. The hardness increased by 1.7 times, the wear resistance increased in 2.6 times in comparison with the untreated samples. The coefficient of friction was 0.39.

Effect of acid leaching conditions on impurity removal from silicon doped by magnesium

The effect of magnesium addition into a commercial silicon and its leaching refining behavior is studied for producing solar grade silicon feedstock. Two different levels of Mg is added into a commercial silicon and the leaching of the produced alloys by 10% HCl solution at 60 ℃ for different durations is performed. It is shown that the microstructure of the alloy and in particular the distribution of eutectic phases is dependent on the amount of the added Mg. Moreover, the metallic impurities in silicon such as Fe, Al, Ca and Ti are mainly forming silicide particles with different compositions. These silicides are physically more detached from the primary silicon grains and their removal through chemical and physical separation in leaching is better for higher Mg additions. It is observed that the leaching is more effective for the purification of smaller silicon particles produced from each Mg-doped silicon alloy. It is shown that acid leaching by the applied method is effective to reach more than 70% of phosphorous removal. It is also shown that the purity of silicon is dependent on the total Mg removal and effectiveness of leaching on removing the Mg2Si phase.

On the Modification of Hypereutectic Al-15Si Alloy Using Rare Earth Ce

Hypereutectic Al-15Si (wt pct) alloys with various content of rare earth Ce were prepared by conventional casting technique. The influence of the primary silicon phase and eutectic silicon on the solidification process of hypereutectic Al-15Si alloys with the addition of Ce is presented. The microstructures were examined with conventional light microscope and scanning electron microscope. The simultaneous refinement of both the primary and the eutectic silicon particles wasntachieved with Ce additions. The results showed that the addition of cerium doesnt cause to refinement of the primary silicon grains, while cerium is caused to stress on concentration regions.The results showed that the addingthe cerium had a negative influence on the alloys tensile strength.

Effects of cerium and phosphorus on microstructures and properties of hypereutectic Al-21%Si alloy

The influences of P and rare earth (RE) complex modifier on the microstructure and mechanical properties of hypereutectic Al-21%Si alloy were studied. The ingots were made by metal mold casting and the proportion of Ce+P ingredient was different. The result showed that the size of grains could be refined obviously by the Ce+P modifier and the effect of phosphorus was more intensive. The primary silicon crystal was refined, while the needle-like eutectic silicon was turned fibrous or short. The alloy mechanical properties had the best performance when 0.08% P and 0.6% Ce were added. The modification of primary silicon grains mainly depended on the heterogeneous nucleation mechanism, and the metamorphic mechanism of eutectic silicon was explained by adsorbing-twinning theory. The strengthening mechanism of experimental alloy was also discussed. The σb, 20 °C increases from 236.2 to 287.6 MPa and σb, 300 °C increases from 142.5 to 210 MPa.

Characterization of acoustic cavitation in water and molten aluminum alloy

High-intensive ultrasonic vibrations have been recognized as an attractive tool for refining the grain structure of metals in casting technology. However, the practical application of ultrasonics in this area remains rather limited. One of the reasons is a lack of data needed to optimize the ultrasonic treatment conditions, particularly those concerning characteristics of cavitation zone in molten aluminum.The main aim of the present study was to investigate the intensity and spectral characteristics of cavitation noise generated during radiation of ultrasonic waves into water and molten aluminum alloys, and to establish a measure for evaluating the cavitation intensity. The measurements were performed by using a high temperature cavitometer capable of measuring the level of cavitation noise within five frequency bands from 0.01 to 10MHz. The effect of cavitation treatment was verified by applying high-intense ultrasonic vibrations to a DC caster to refine the primary silicon grains of a model Al–17Si alloy. It was found that the level of high frequency noise components is the most adequate parameter for evaluating the cavitation intensity. Based on this finding, it was concluded that implosions of cavitation bubbles play a decisive role in refinement of the alloy structure.

Research of the Morphology and Distribution of the Primary Silicon in Al-18Si Alloy under a Gradient High Magnetic Field

A gradient high magnetic field effected significantly the morphology and distribution of the primary silicon grains in Al-18Si alloy. Experimental studies shew that in the gradient high magnetic field the primary silicon phase grains, which are large plate-like or five-star-like in the case of solidification without magnetic field, are accumulated on the top of the specimen and refined remarkably with the morphology of polygonal or nodular shapes when the alloy solidifies from the semisolid state. In the segregated layer of the silicon, the distribution of the silicon grains is homogeneous. The size of the primary silicon grains decreases and the grain number density rises with the increase of the magnetic strength maintaining the magnetization force unchangeable. It seems that the high magnetic field influences the diffusion of silicon. Theoretical models have been proposed to explain the refining and the distribution of the silicon grains.

Effects of a high-gradient magnetic field on the migratory behavior of primary crystal silicon in hypereutectic Al–Si alloy

The migration of primary Si grains during the solidification of Al–18 wt%Si alloy under a high-gradient magnetic field has been investigated experimentally. It was found that under a gradient magnetic field, the primary Si grains migrated toward one end of the specimen, forming a Si-rich layer, and the thickness of the Si-rich layer increased with increasing magnetic flux density. No movement of Si grains was apparent under a magnetic field below 2.3 T. For magnetic fields above 6.6 T, however, the thickness of the Si-rich layer was almost constant. It was shown that the static field also played a role in impeding the movement of the grains. The primary Si grains were refined in the Si layer, even though the primary silicon grains were very dense. The effect of the magnetic flux density on the migratory behavior is discussed.

Aluminium precipitates in the primary silicon of as-sprayformed hypereutectic aluminium-silicon alloys

Precipitates have been observed in the primary silicon of as-spray-formed hypereutectic aluminium-silicon alloys with and without various alloy additions. The precipitates have been investigated using transmission electron microscopy and high-resolution transmission electron microscopy. They are aluminium based and generally adopt a spheroidal morphology, with diameters in the range of approximately 5-10nm, unless located on a stacking fault or grain boundary. Spray forming is a rapid solidification process and precipitation occurs because of the low solid solubility of aluminium in silicon (maximum of 0.016 +/- 0.003at.% at 1190 C). The morphology of the precipitates suggests that they form from aluminium or aluminium-rich liquid. The orientation relationships of several precipitates in one of the primary silicon grains have been determined.

Ａｌ‐Ｓｉ‐Ｃｕ‐Ｎｉ‐Ｍｇ系合金鋳物の中高温における疲労特性

Rotating bending fatigue tests of modified Al-Si-Cu-Ni-Mg eutectic casting alloys (AC8A) and hypereutectic casting alloys (AC9A and AC9B) were carried out in a temperature range between room temperature and 400°C. The fatigue strength of sodium modified AC8A is superior to that of phosphorus modified AC8A. The fatigue strength of AC9B with lower content of silicon is greater than that of AC9A. The decrease in fatigue strength is due to a coarse primary silicon. It is concluded that the fatigue properties of Al-Si-Cu-Ni-Mg casting alloys are influenced by the primary silicon grains.

Effect of phosphides on the structure and properties of hypereutectic silumins

1. All the phosphides investigated except gallium and indium phosphides have a considerable modifying effect on the structure and properties of hypereutectic Silumins. 2. Aluminum phosphide has the largest effect on these alloys. 3. Modification with AIIIBV phosphides increases the microhardness of the primary silicon grains. 4. If the operating characteristics of parts do not change with the addition of copper and zinc to hypereutectic Silumins at levels exceeding GOST or TU specifications then it can be recommended that these alloys be modified with copper phosphide (the most suitable technologically) or zinc phosphide (the cheapest).