Aluminium grain structures in Al-Si eutectic alloys

X-ray diffraction evidence is presented to support the contention that the aluminium phase in the aluminium-silicon unmodified flake-silicon eutectic is polycrystalline as solidified. In association with published evidence of preferred oritentation relationships between the eutectic phases, it follows that the aluminium phase is repeatedly re-nucleated on the silicon flakes as substrate during growth of the unmodified eutectic.     

Eutectic nucleation and growth in hypoeutectic Al-Si alloys at different strontium levels

The effects of different levels of strontium on nucleation and growth of the eutectic in a commercial hypoeutectic Al-Si foundry alloy have been investigated by optical microscopy and electron backscattering diffraction (EBSD) mapping by scanning electron microscopy (SEM). The microstructural evolution of each specimen during solidification was studied by a quenching technique at different temperatures and Sr contents. By comparing the orientation of the aluminum in the eutectic to that of the surrounding primary aluminum dendrites by EBSD, the eutectic formation mechanism could be determined. The results of these studies show that the eutectic nucleation mode, and subsequent growth mode, is strongly dependent on Sr level. Three distinctly different eutectic growth modes were found, in isolation or sometimes together, but different for each Sr content. At very low Sr contents, the eutectic nucleated and grew from the primary phase. Increasing the Sr level to between 70 and 110 ppm resulted in nucleation of independent eutectic grains with no relation to the primary dendrites. At a Sr level of 500 ppm, the eutectic again nucleated on and grew from the primary phase while a well-modified eutectic structure was still present. A slight dependency of eutectic growth radially from the mold wall opposite the thermal gradient was observed in all specimens in the early stages of eutectic solidification.     

Shape, wear & mechanical properties of spray formed hypoeutectic Al-Si alloys

Manufacturing process through spray forming leads to give near-net-shape and fine grain microstructure. In this process Si particles, which are not distributed uniformly in conventional casting process, are distributed uniformly throughout the casting. In the present study disc shape spray form castings were made of Al-6.91Si and Al-10.1Si alloys, and then their shape, wear and mechanical properties were studied. The shape of the deposit was observed to be the most uniform at 30o inclination angle of the substrate. The hardness and tensile strength value of spray formed alloys shows the increment in the mechanical property in contrast to as cast alloys. The wearing properties of Al-10.1Si alloy were found to be better than that of the Al-6.91Si alloy.     

Growth of interdendritic eutectic in directionally solidified Al-Si alloys

Interdendritic eutectic microstructures in Al-Si (6 to 12.6 wt pct Si) alloys have been investigated as a function of growth velocity and temperature gradient. The interface morphology, as well as the behavior of the eutectic spacing and undercooling, suggest that the resultant microstructure is governed by two different growth processes. That is, at low growth rates, steady-state columnar eutectic growth is found and obeys the relationship, λ²V = constant, where λ is the eutectic spacing andV is the growth rate. At higher growth rates, the nucleation of equiaxed eutectic grains occurs in the interdendritic liquid. The experimental findings are interpreted in the light of recently developed models for the columnar to equiaxed transition and for irregular eutectic growth.     

Growth of interdendritic eutectic in directionally solidified Al-Si alloys

Metallurgical and Materials Transactions A - Interdendritic eutectic microstructures in Al-Si (6 to 12.6 wt pct Si) alloys have been investigated as a function of growth velocity and temperature...     

Growth of interdendritic eutectic in directionally solidified Al-Si alloys

Interdendritic eutectic microstructures in Al-Si (6 to 12.6 wt pct Si) alloys have been investigated as a function of growth velocity and temperature gradient. The interface morphology, as well as the behavior of the eutectic spacing and undercooling, suggest that the resultant microstructure is governed by two different growth processes. That is, at low growth rates, steady-state columnar eutectic growth is found and obeys the relationship, λ²V = constant, where λ is the eutectic spacing andV is the growth rate. At higher growth rates, the nucleation of equiaxed eutectic grains occurs in the interdendritic liquid. The experimental findings are interpreted in the light of recently developed models for the columnar to equiaxed transition and for irregular eutectic growth.     

Iron-rich intermetallic phases and their role in casting defect formation in hypoeutectic Al-Si alloys

Iron is the most common and detrimental impurity in aluminum casting alloys and has long been associated with an increase in casting defects. While the negative effects of iron are clear, the mechanism involved is not fully understood. It is generally believed to be associated with the formation of Fe-rich intermetallic phases. Many factors, including alloy composition, melt superheating, Sr modification, cooling rate, and oxide bifilms, could play a role. In the present investigation, the interactions between iron and each individual element commonly present in aluminum casting alloys, were investigated using a combination of thermal analysis and interrupted quenching tests. The Fe-rich intermetallic phases were characterized using optical microscope, scanning electron microscope, and electron probe microanalysis (EPMA), and the results were compared with the predictions by Thermocalc. It was found that increasing the iron content changes the precipitation sequence of the β phase, leading to the precipitation of coarse binary β platelets at a higher temperature. In contrast, manganese, silicon, and strontium appear to suppress the coarse binary β platelets, and Mn further promotes the formation of a more compact and less harmful α phase. They are therefore expected to reduce the negative effects of the β phase. While reported in the literature, no effect of P on the amount of β platelets was observed. Finally, attempts are made to correlate the Fe-rich intermetallic phases to the formation of casting defects. The role of the β phase as a nucleation site for eutectic Si and the role of the oxide bifilms and AlP as a heterogeneous substrate of Fe intermetallics are also discussed.     

Modeling of irregular eutectic microstructures in solidification of Al-Si alloys

A modified cellular automaton (MCA) model was developed and applied to simulate the evolution of solidification microstructures of both eutectic and hypoeutectic Al-Si alloys. The present MCA model considers the equilibrium and metastable equilibrium solidification processes in a multiphase system. It accounts for the aspects including the nucleation of a new phase, the growth of primary α dendrites and two eutectic solid phases from a single liquid phase, as well as the coupling between the phase transformation and solute redistribution in liquid. The effects of alloy composition and eutectic undercooling on eutectic morphology and eutectic nucleation mode were investigated. The simulated results were compared with those obtained experimentally.     

Experimental and modeling studies of the thermal conditions and magnesium,iron, and copper content on the morphology of the aluminum silicon eutectic in hypoeutectic aluminum silicon alloys

Hypoeutectic aluminum silicon alloys without and with additions of magnesium, copper, iron, and strontium have been cast in a mold, giving directional solidification from a chill. Detailed temperature measurements have been carried out. Solidification modeling based on front tracking of the microstructure growth fronts allowed identification of the time and temperature at which the dendrite tips and the first eutectic pass the thermocouples. The undercooling, growth rate, and thermal gradients at the dendritic and eutectic growth fronts were derived. The effect of varying thermal parameters and alloy compositions on the microstructure was investigated. Compared to the binary alloy, a coarser eutectic was observed in the alloys with magnesium, iron, and/or copper. The coarsening is explained as a result of the transition from a eutectic forming at one specific temperature, to a eutectic forming over a temperature range. The former is likely to grow as a plane front, whereas the latter is likely to form an interdendritic eutectic mushy zone.     

铸造共晶铝硅合金中析出相对断裂行为的影响

通过对铸造共晶Al-Si合金断口及析出相的分析研究了拉伸过程中基体内析出相的断裂特点.结果发现:拉伸过程中基体内初、共晶硅以穿晶断裂为主,这与颗粒内部存在的结构缺陷有关;直径小于2.0μm的初晶硅能够抑制二次裂纹的扩展.块状富铁相表现出穿晶断裂特点,鱼骨状富铁相中的微裂纹沿一次枝晶轴向生长.富铜相对裂纹扩展具有强烈地偏折作用;以富铁相为核心析出的富铜相能抑制富铁相中微裂纹的形成.提高合金中Cu含量可以降低富铁相的有害作用,改善合金的力学性能.     

Effects of the intensity and frequency of electromagnetic vibrations on the microstructural refinement of hypoeutectic Al-Si alloys

An experimental apparatus that uses a superconducting magnet and enables the simultaneous application of an alternating electric field with a frequency of up to 50 kHz and a magnetic field of up to 10 T was designed and assembled. Electromagnetic vibrations were induced in Al-7 wt pct Si alloy during solidification by simultaneous application of the two fields. The thorough investigation, which was carried out over wide ranges of intensity (an electromagnetic pressure range of 0 to 2.25×10⁵ Pa) and frequency (0 to 50 kHz), clarified the effects of the two main parameters on the microstructural refinement brought about by electromagnetic vibrations. Low-intensity vibrations changed the highly columnar dendritic structure into one composed of large, equiaxed dendrites. As the intensity, and consequently, the magnetic pressure were increased, at about 0.93×10⁵ Pa, fine isolated grains started to appear and dominated the structure as the pressure was increased further. At low frequencies, the structure was one with large, equiaxed dendrites, which disintegrated to form a fine and homogeneous structure as the frequency was increased. At about 1.5 kHz, the trend reversed and the structure gradually became a completely columnar dendritic one at frequencies higher than 10 kHz. Metallographic observations showed that the cavitation phenomenon has been a main factor behind the observed microstructural refinement. The effects of mechanical vibrations of the experimental apparatus were also investigated and found to have no contribution to the observed effects.     

Al—3B中间合金的加工量对亚共晶Al—Si合金晶粒细化的影响

研究了Al-3B中间合金不同的加入量对亚共晶Al-Si合金晶粒尺寸的影响。该中间合金对所研究全部范围内的Al-Si合金都具有一定的细化效果,但在不同的含硅量下其细化能力并不相同,同时证实对未经细化处理的亚共晶Al-Si合金而言,在含硅量约3%（质量分数,下同）时合金具有最小的晶粒尺寸。随着Al-3B中间合金加入量的提高,出现最小晶粒尺寸的合成成分向高硅方向移动,当加入量达到1%时,在含硅量为6%的Al-Si合金中出现最小的晶粒尺寸。     

Structure formation of powders and coatings of eutectic iron base alloys

Translated from Poroshkovaya Metallurgiya, No. 7(331), pp. 29–34, July, 1990.     

Interparticle spacings and undercoolings in Al-Si eutectic microstructures

Modified methods for the measurement of interparticle spacing and undercooling of the Al-Si eutectic have been applied in an attempt to test the applicability to this system of the accepted equation for diffusion-controlled normal eutectic growth. Good agreement with some published measurements has been obtained. For the unmodified and chill-modified eutectic microstructures it seems probable that the basic equation is applicable but that the extremum condition is not. For the impurity modified eutectic there is no agreement with the theoretical prediction, although the possibility is advanced of adding an undercooling term based on twin boundary energy.     

Modification-related porosity formation in hypoeutectic aluminum-silicon alloys

A series of aluminum-10 wt pct silicon castings were produced in sand molds to investigate the effect of modification on porosity formation. Modification with individual additions of either strontium or sodium resulted in a statistically significant increase in the level of porosity compared to unmodified castings. The increase in porosity with modification is due to the presence of numerous dispersed pores, which were absent in the unmodified casting. It is proposed that these pores form as a result of differences in size of the aluminum-silicon eutectic grains between unmodified and modified alloys. A geometric model is developed to show how the size of eutectic grains can influence the amount and distribution of porosity. Unlike traditional feeding-based models, which incorporate the effect of microstructure on permeability, this model considers what happens when liquid is isolated from the riser and can no longer flow. This simple “isolation” model complements rather than contradicts existing theories on modification-related porosity formation and should be considered in the development of future comprehensive models.