Nucleation and growth of eutectic cell in hypoeutectic Al-Si alloy

The nucleation and growth of eutectic cell in hypoeutectic Al-Si alloy was investigated using optical microscopy and scanning electron microscopy equipped with electron backscattering diffraction（EBSD）.By revealing the eutectic cells and analyzing the crystallographic orientation,it was found that both the eutectic Si and Al phases in an eutectic cell were not single crystal,representing an eutectic cell consisting of small ＇grains＇.It is also suggested that the eutectic nucleation mode can not be determined based on the crystallographic orientation between eutectic Al phases and the neighboring primary dendrite Al phases.However,the evolution of primary dendrite Al phases affects remarkably the following nucleation and growth of eutectic cell.The coarse flake-fine fibrous transition of eutectic Si morphology involved in impurity elements modification may be independent of eutectic nucleation.

Functions and mechanism of modification elements in eutectic solidification of Al-Si alloys:A brief review

Being used more and more widely in engineering,AlSi alloys comprise about 80%of all kinds of aluminum alloys,which are the most widely utilized nonferrous alloys.Although most Al-Si alloys consist of multiple components,the eutectics in the structure accounts for 50%-90%of the sum volume of such alloys.Therefore,understanding the modification mechanism and function rules of the AlSi eutectic solidification is the technical key in controlling the structures and properties of such casting alloys.The present paper chiefly reviews recent investigation developments and important conclusions along the lines of the functions of modification elements and their modification mechanism in the eutectic solidification of Al-Si alloys.

Solidification structure of eutectic Al-Si alloy under a high magnetic field-aid-electromagnetic vibration

An electromagnetic vibration was generated by simultaneously imposing a strong static magnetic field （up to 10 T） and an alternative electricity current to the metal. Its effects on the solidification structure of eutectic Al-Si alloy have been investigated experimentally. It is found that the eutectic structure has been refined by solely imposing high magnetic field while it is coarsened under the electromagnetic vibration. Furthermore, polyhedral Si grains and non-dendritic α-Al appeared when the electromagnetic vibration strength was strong enough. The refining of eutectic structure is attributed to the decrease of diffusion coefficient caused by the strong magnetic field. The coarseness of eutectic structure may be attributed to the convection caused by electromagnetic vibration. Strong convection may break co-operative growth of eutectic phases to form polyhedral Si grains and non-dendritic α-Al.

Influence of titanium content on wear resistance of electrolytic low-titanium eutectic Al-Si piston alloys

The wear resistance of six kinds of the electrolytic low-titanium eutectic AI-Si piston alloys with various Ti content ranging from 0.00wt.% to 0.21wt.% has been studied. A new method of adding Ti is adopted in the electrolytic low-titanium aluminum alloy ingots. The electrolytic low-titanium eutectic AI-Si piston alloys are produced by remelting the electrolytic low-titanium aluminum alloy, crystal silicon, pure magnesium, AI-50%Cu and AI-10%Mn master alloy. The wear experiments are conducted using MM200 wear testing machine under lubricating condition. The results indicate that the better wear resistance and the less weight loss are achieved in the study for the eutectic AI-Si piston alloys with 0.08wt.%-0.12wt.% Ti content. The highest ultimate tensile strength of 135.94 MPa at 300℃ and HV141.70 hardness of the alloys are obtained at 0.12wt.% and 0.08wt.% Ti content, respectively. The wear mechanism of the eutectic AI-Si piston alloys under lubricating condition is abrasive wear.

Effects of individual and combined additions of phosphorus,boron and cerium on primary and eutectic silicon in an Al-30Si alloy

The modification of silicon in an Al-30Si alloy was studied using optical microscopy, electron probe micro-analysis, transmission electron microscopy and differential scanning calorimetry. It is found that phosphorus master alloys combined with boron master alloys have good modification effect on primary silicon but no evident modification effect on eutectic silicon, while boron combined with cerium has good modification effect on eutectic silicon. The results of differential scanning calorimetry show that phosphorus, boron or cerium addition and their combined addition have different undercooling effects on eutectic silicon. Many scholars thought that AlP particles were the nuclei of eutectic silicon when phosphorus was enough in the alloy. Our results show that β-（Al,Si,Fe） can still be the nucleus of plate-like entectic silicon while AlP is the nucleus of primary silicon when there is enough phosphorus in the melt. In addition, the mechanism about the modification was also discussed.

Effect of phosphor addition on eutectic solidification and microstructure of an Al-13%Si alloy

As the refiner or modifier, the master alloys containing high concentration phosphor are widely used in preparing eutectic or hypereutectic Al-Si alloys. To study the effect of phosphor addition on the eutectic solidification and microstructure of the Al-13%Si alloy, an investigation has been undertaken by means of thermal analysis and micro/macro-structure observation. Results indicate that addition of phosphor in near eutectic Al-Si alloy promotes the nucleation of eutectic but has little refinement impact on primary Si particles as expected. Conversely, both primary Si particles and eutectic Si flakes become slightly coarser in P-rich alloys. The coarsening of eutectic Si flakes ties closely to the increased eutectic growth temperature with phosphor addition. The eutectic solidification of the alloy proceeds from the near mold zone towards the center, and it is also found that a few independent nucleation regions emerge in liquid at the solidification front due to the addition of phosphor.

Si-RICH LAYER FORMATION DURING DIRECTIONAL SOLIDIFICATION OF AI-Si EUTECTIC ALLOY IN ELECTROMAGNETIC FIELD

The formation of a special Si-rich layer on the periphery of Al-Si eutectic alloy specimen during directional solidification in rotary electromagnetic field has been investigated.This layer seems due to the migration of some Si grains onto the crucible wall,then stagnating and coarsening further.

Microstructure and properties of Mg-Al binary alloys

The effects of different amounts of added Al, ranging from 1 % to 9 %, on the microstructure and properties of Mg-Al binary alloys were investigated. The results showed that when the amount of added Al is less than 5%, the grain size of the Mg-Al binary alloys decreases dramatically from 3 097 μm to 151 μm with increasing addition of Al. Further addition of Al up to 9% makes the grain size decrease slowly to 111 μm. The α-Mg dendrite arms are also refined. Increasing the amount of added Al decreases the hot cracking susceptibility of the Mg-Al binary alloys remarkably, and enhances the micro-hardness of the α-Mg matrix.

Effect of Li content on microstructure and mechanical property of Mg-xLi-3(Al-Si) alloys

Mg-4 Li-3(Al-Si),Mg-8 Li-3(Al-Si)and Mg-12 Li-3(Al-Si)alloys based on theα-Mg,α-Mg+β-Li,β-Li phases,respectively,were produced to investigate the effect of alloying with Al-Si eutectic on the microstructure and mechanical properties of the three alloys.Alloying with the Al-Si eutectic of Mg-xLi(x=4,8 and 12 wt.%)alloys caused the formation of different types of Al-Li precipitates:Al3Li,AlLi and Li3Al2,respectively.Also,considerable quantities of Mg2Si phase particles were found in the three alloys.The results of tensile tests showed that the Mg-4Li-3(Al-Si)alloy exhibited the highest ultimate tensile strength(UTS)of 249 MPa but the lowest elongation of 6.3%.The Mg-12Li-3(Al-Si)alloy had the highest elongation of 26%but the lowest UTS of 173 MPa.The different mechanical properties were attributed to the different crystal structures of the three alloys and the precipitate with different type,morphology and distribution.

Modeling of aluminum-silicon irregular eutectic growth by cellular automaton model

Due to the extensive application of Al-Si alloys in the automotive and aerospace industries as structural components, an understanding of their microstructural formation, such as dendrite and(Al+Si) eutectic, is of great importance to control the desirable microstructure, so as to modify the performance of castings. Since previous major themes of microstructural simulation are dendrite and regular eutectic growth, few efforts have been paid to simulate the irregular eutectic growth. Therefore, a multiphase cellular automaton(CA) model is developed and applied to simulate the time-dependent Al-Si irregular eutectic growth. Prior to model establishment, related experiments were carried out to investigate the influence of cooling rate and Sr modification on the growth of eutectic Si. This CA model incorporates several aspects, including growth algorithms and nucleation criterion, to achieve the competitive and cooperative growth mechanism for nonfaceted-faceted Al-Si irregular eutectic. The growth kinetics considers thermal undercooling, constitutional undercooling, and curvature undercooling, as well as the anisotropic characteristic of eutectic Si growth. The capturing rule takes into account the effects of modification on the silicon growth behaviors.The simulated results indicate that for unmodified alloy, the higher eutectic undercooling results in the higher eutectic growth velocity, and a more refined eutectic microstructure as well as narrower eutectic lamellar spacing. For modified alloy, the eutectic silicon tends to be obvious fibrous morphology and the morphology of eutectic Si is determined by both chemical modifier and cooling rate. The predicted microstructure of Al-7Si alloy under different solidification conditions shows that this proposed model can successfully reproduce both dendrite and eutectic microstructures.

SURFACE COMPOSITE GEAR AND PIPE MADE BY SEPARATED EUTECTIC TECHNOLOGY

It is found that the anomalous eutectics can be separated on macroscopic scale by flow caused by electromagnetic stirring, and the separated phase is the leading faceted phase with the solution entropy over 23 J/(mol&middotK). By using this technology, a new kind of composite pipe and gear with good abrasive properties were made without adding any reinforced particles. Emphases were paid on the researches about formation mechanism of separated eutectic and abrasive property of the composite pipe or gear. The result shows that the entropy of solution controlling the eutectic microstructure is also valid and useful as a criterion of separated eutectic, and the kind and its chemical scope of the off-eutectic used to make composite can be calculated according to this theory.

Effect of electric current on the cast micro-structure of Al-Si alloy

Effect of electric current on the cast microstructure of Al-Si alloy was investigated. It was found that themicrostructure of Al-Si alloy was refined as the electric current was applied during solidification. When DC (Direct Current)was applied in solidification, the eutectic Si flakes are similar to those solidified without current, but its length was shortenedand its distribution was changed ,with most of the Si flakes arrange in the radial direction, because of the electromagneticforce that resulted from the DC. On the other hand, when AC (Alternating Current) was applied during the solidification ofAl-Si alloy, most of the minute hooks on the silicon flakes that were found under DC or without any applied current werebroken into small silicon particles. Through silicon concentration measurement by electron microprobe, it was found that thesilicon content in the α-AI matrix increased with the current application during solidification, and the effect is more obviouswhen AC was applied.

Improving wear resistance of magnesium by droplet spraying of Al-Si alloy

Abstract： In order to improve the surface hardness and wear resistance of magnesium, Al-13%Si （mass fraction） alloy coating was deposited on pure magnesium by droplet spraying process. The microstructure was studied by electron probe microanalysis and X-ray diffraction. The micro-hardness and wear resistance of coating were investigated in comparison with those of the substrate. It is found that the coating layer is composed of a-Al cellular due to rapid solidification. Formation mechanism of the coating is due to the obstruction of diffusion by in-situ formed Mg2Si in interracial layer. The coating exhibits higher hardness compared to that of the Mg substrate. As result of its high hardness, the wear resistance of the coating layer is about ten times that of the substrate. The droplet spraying process demonstrates that the magnesium surface can be strengthened by using the existing Al-Si alloys.

Mechanism of fracture toughness property of aging-hardening eutectic Al-Si alloys

The micro structures and fracture toughness properties of the aged Al-12.5Si-0.7Mg alloy plate were studied by transmission electron microscopy(TEM),optical microscopy(OM),tensile,and fracture toughness(the fracture toughness value,K_(IC))testing.The mechanism of fracture toughness properties of the alloys was discussed for different sizes of silicon particle and aging precipitates.The results show that the refined and rounded silicon particles will improve the fracture toughness property of the alloys.However,the silicon particles size is no longer the main factor to affect the fracture toughness property when the silicon particles are small and they have no sharp corners.The morphology and distribution of the precipitates play an important role in the fracture toughness property of the aged alloys.Their fracture toughness decreases gradually as the aging time increases.

船载微型Ga-In共晶固定点的现场复现方法研究

将温标固定点技术应用于船载现场是实现温度量值扁平化,提升海洋温度测量水平的重要手段。为了弥补ITS90温标在海洋温区(-5~35℃)范围内固定点数目不足,不能对NTC热敏电阻进行有效校准的缺陷,研制了微型Ga-In共晶固定点,开展了船载控温箱微型Ga-In固定点温坪复现研究,同时探索了环境温度条件下微型Ga-In共晶固定点的复现。采用切线交点法和三次多项式拟合法对微型Ga-In固定点进行了评估和分析。结果表明:微型Ga-In共晶固定点重复性优于1 mK;微型Ga-In共晶固定点应该在固态并低于熔点的环境下保存。采用切线交点法赋值得到微型Ga-In共晶固定点相变温度为15.6549℃,扩展不确定度为1.54 mK(k=2)。

固溶时间对3种典型Al-Si二元合金组织与力学性能的影响

考察了固溶时间对3种典型Al-Si合金变质前后组织与力学性能的影响。结果表明:随着固溶时间的延长,变质前后的Al-Si合金抗拉强度逐渐降低,而伸长率有大幅度地提高;Al-12Si共晶合金抗拉强度及伸长率变化幅度最大;535℃固溶6h可获得综合性能较优的Al-Si合金;Sr的变质有助于固溶过程中共晶硅的粒化。

Eutectic Solidification in Near-eutectic Al-Si Casting Alloys

Eutectic solidification in near-eutectic Al-13 wt pct Si casting alloys and the effect of trace addition of boron or strontium on it have been investigated using thermal analysis and microstructural characterization. In unmodified alloy, dual eutectic structure has been observed. The coarse eutectic （dendrite-like Al＋ coarse Si flakes） is formed above the equilibrium temperature of eutectic （Al＋Si） reaction （577℃）. The coarse eutectic （CE） grains nucleate from the primary silicon particles formed earlier due to local enrichment of silicon solute and grow in a divorced mode between the dendritic Al phase and large silicon flakes. The fine eutectic （FE） grains nucleate later on other potential sites activated by melt undercooling and grow in coupled-growing mode with the silicon crystals as fine flakes. The formation of the FE grains is favored in the alloys containing boron because of a great number of potential nucleation sites being added from boron-containing particles. Addition of strontium to the alloys restrains completely the formation of primary silicon particles and hence limits the nucleation of the CE. This is because the eutectic point has moved far enough to make the alloy, at this composition （Al-13 wt pct Si）, hypo-eutectic. Local cooling rate during solidification has an important influence on competition formation of these two eutectics.

Effect of erbium on properties and microstructure of Al-Si eutectic alloy

Eutectic Al-12.6 wt.%Si alloys with various contents of the rare earth element Er were prepared by the conventional casting technique. The effect of Er on the microstructure and properties of the eutectic Al-Si alloys was investigated using optical microscopy, scanning electron microscopy as well as the friction and wear tests. It was found that the addition of Er obviously improved the anti-wear properties, and reduced the friction coefficient of the alloys. The appropriate addition of Er would change the size and shape of the eutectic silicon, and thereby refine the microstructure of the Al-Si alloys. The refinement mechanism was also discussed.

Effect of bismuth on microstructure of unmodified and Sr-modified Al-7Si-0.4Mg alloys

The effects of bismuth and the combination of bismuth and strontium on the eutectic silicon structure in Al-7Si-0.4Mg alloys were investigated under different solidification conditions.The results show that bismuth has a refining effect on the eutectic silicon and its refinement behavior increases with increasing Bi content up to 0.5% （mass fraction）.When bismuth is added into the molten alloy modified with strontium,a higher Sr/Bi mass ratio of at least 0.45 is required to attain full modification of the eutectic silicon.

Effect of addition of Al-Si eutectic alloy on microstructure and mechanical properties of Mg-12wt%Li alloy

Mg-12 Li, Mg-12 Li-3（Al-Si）, Mg-12 Li-7（Al-Si） and Mg-12 Li-9（Al-Si） alloys（all in wt%） were fabricated by high frequency vacuum induction melting in a water cooled copper crucible. After subsequently hotrolling and annealing, their microstructure and mechanical properties were investigated. Experimental results show that mechanical properties of Mg-12 Li alloy were significantly improved by the addition of Al-Si eutectic alloy. Mg-12 Li-7（Al-Si） alloy shows the highest strength of 196 MPa of the investigated alloys, which is about 1.8 times of the strength of Mg-12 Li alloy, and maintains high elongation of 27%.The improved mechanical property with addition of Al and Si in the eutectic proportion into Mg-12 Li alloy was attributed to the solution strengthening effect of A1 and precipitation hardening effect from AlLi and Mg_2 Si precipitates.