Hypoeutectic Al-Si Alloys Research Articles

The effect of grain refinement and silicon content on grain formation in hypoeutectic Al–Si alloys

The effect of increasing the amount of added grain refiner on grain size and morphology has been investigated for a range of hypoeutectic Al-Si alloys. The results show a transition in grain size at a silicon concentration of about 3 wt% in unrefined alloys; the grain size decreasing with silicon content before the transition, and increasing beyond the transition point. A change in morphology also occurs with increased silicon content. The addition of grain refiner leads to greater refinement for silicon contents below the transition point than for those contents above the transition point, while the transition point seems to remain unchanged. The slope of the grain size versus silicon content curve after the transition seems to be unaffected by the degree of grain refinement. The results are related to the competitive processes of nucleation and constitutional effects during growth and their impact on nucleation kinetics. (C) 1999 Elsevier Science S.A. All rights reserved.

Analytical modelling of equiaxed solidification

A framework for modelling the overall microstructure evolution during equiaxed solidification is presented, based on the internal state variable approach. In its simplest form the microstructure evolution can be captured mathematically in terms of differential variation of two primary state variables with time. The model utilizes input data from thermal analysis as a means for calibrating unknown kinetic constants, and has proved useful to describe the solidification behaviour of hypoeutectic Al–Si alloys under different inoculation conditions. The results may conveniently be presented in the form of continuous cooling transformation (CCT) diagrams to show the competition between the different variables involved. Because of the analogy with solid state transformations, the proposed formalism is well suited for modelling of coupled microstructure problems in real castings or fusion welds.

Thermal expansion of isotropic Duralcan metal–matrix composites

The thermal expansion behaviour of Duralcan composites having a matrix of hypoeutectic Al–Si alloy containing SiC reinforcements ranging from 10–40 vol% was investigated. The coefficient of thermal expansion (CTE) of the MMCs was measured between 25 and 350 °C by a high-precision thermomechanical analyser, and compared to the predictions of three theoretical models. At low temperature, the experimental CTEs show substantial deviation from the predictions of the elastic analysis derived by Schapery, while the Kerner model agrees relatively well at high temperature. The overall measured CTE, in the range of 25–350 °C, as a function of the volume fraction of SiC is well predicted using Schapery's lower bound. We interpret these features as being an effect of reinforcement phase geometry and the modified microstructure derived from the Duralcan process and subsequent heat treatments. © 1998 Kluwer Academic Publishers

ＳＵＳ３０４鋼材のめっき層形成に及ぼす溶融Ａｌ‐Ｓｉめっき浴中のＳｉ濃度の影響

SUS 304 stainless steel plate was dipped in molten Al-Si alloy laving different Si concentrations, i.e., hypoeutectic, eutectic, and hypereutectic. The effect of Si concentration on coating layer formation was studied by metallographic observation and EPMA analysis. Diffusion heat treatment of hot-dipped samples was also conducted and the change in the coating layer structure examined. Metallographic examination showed that the adhered layers of samples hot-dipped in hypoeutectic and eutectic Al-Si alloy baths are the same as the solidified structures of conventionally cast Al-Si Alloys. The CrSi2-Alx phase was observed in the adhered coating layer of the sample hot-dipped in hypereutectic alloy. The amount of CrAl2-Alx increased with increasing bath temperature and immersion time. Fe-Al-Cr-Si alloy layers formed at the interface of the stainless steel and molten alloy, and grew to a maximum thickness of about 20μm. The alloy layer growth rate and maximum thickness did not depend on Si concentration in the Al-Si alloy bath.

Structure and mechanical properties of age-hardened directionally solidified AlSiCu alloys

The structure of directionally solidified Al-Si hypoeutectic alloys are generally composed of Al-matrix and Si-reinforcing phase. The growth direction of the both phases was near 〈200〉. The strength properties of an Al-Si alloy with additions of 2 wt.% and 4 wt.% copper have been investigated. These alloys were solution treated, quenched in water and aged at 200°C. Large Al2Cu precipitates present in D.S. samples dissolved partly, and after ageing, they precipitated as the Θ′ platelets, significantly increasing the mechanical properties of the alloys. Hardness, strength and elongation were measured in the course of ageing. The structure was investigated by means of: XSAS, X-ray phase analysis, lattice parameter measurements and scanning microscopy.

The precipitation strengthening of directionally solidified Al Si Cu alloys

The structure of directionally solidified (DS) eutectic alloys is generally composed of matrix and reinforcing phases which reveal a sharp fibre texture. The reinforcing phase of Al-Si alloys is built of a silicon-rich solid solution, embedded in the matrix of aluminium-rich solid solution. The growth direction of the both phases is near 〈200〉. The mechanical properties of pure Al-Si alloys are low and may be improved by the addition of another element like copper. The investigations were performed in an Al-Si hypoeutectic alloy with additions of 2 and 4 wt.% copper. The alloys were solution treated at 530 °C, quenched in water followed by ageing at 200 °C. The Al 2Cu precipitates formed during ageing significantly increasing the mechanical properties of the investigated alloy. Hardness changes were measured in the course of ageing. The structure was investigated by means of scanning microscopy, X-ray phase analysis and lattice parameter measurements.

Mechanism of abrasive wear of Al-Si hypoeutectic alloycontaining 5 vol% fly ash

Fly ash, an inexpensive resource material, has been introducedinto Al-Si hypoeutectic alloy (A356) to make low-cost compositeswith decreased density and improved hardness and abrasive wearbehavior. The mechanisms of abrasive wear of stir-cast A356-5vol% fly ash composite were discussed based on the results ofwear tests of composites and the A356 base alloy. Scanningelectron microscopy was used to investigate the morphology ofthe worn surfaces, wear debris for both composite and A356 basealloy. The subsurface of the worn samples was also observed.These morphology observations provide a method to understand theabrasive wear and friction mechanism of the composites showing that the base alloy wears primarily by microcutting but thecomposite wears by microcutting and delamination caused by crackpropagation below the rubbing surface through interfaces of flyash and silicon particles with the matrix.

亜共晶Ａｌ‐Ｓｉ合金の凝固組織に及ぼすりんの影響

亜共晶Ａｌ‐Ｓｉ合金の凝固組織に及ぼすりんの影響

Grain refinement mechanisms of hypoeutectic AlSi alloys

The general concept of the grain refinement mechanism in hypoeutectic AlSi alloys has been adopted from that in wrought alloys without consideration of the influence of the silicon content. In the present investigation the commercial grain refinement practice in AlSi foundry alloys has been experimentally simulated by introducing synthetic TiB2 crystals directly into AlSi melts. To explore the possible use of a new generation of AlTiC master alloys in these alloys, the behaviour of synthetic TiC has also been studied. Experimental findings indicate that in the presence of Si, TiC crystals are highly unstable in aluminium melts. TiB2 crystals alone do not nucleate the α-phase, whereas in the presence of dissolved Ti a ternary AlTiSi interfacial layer is formed on the TiB2 which subsequently nucleates the α-Al, via a peritectic reaction. However, due to a drastic decrease in the peritectic temperature, the efficiency of grain refinement is greatly reduced. A theoretical analysis indicates that when boron is added in excess of TiB2 stoichiometry, or simply as AlB2, grain refinement occurs by a eutectic reaction.

Formation of spherical primary silicon crystals during semi-solid processing of hypereutectic Al-15.5wt%Si alloy

Semi-solid state processing is considered as an attractive processing technique because of it offers several potential advantages such as energy saving for forming, reduction of microsegregation and porosity, especially for near net shape manufacturing of components. This technique is frequently applied to make sound casting products of hypoeutectic Al-Si alloys. On the other hand, hypereutectic Al-Si alloys have limited applications because of their inherent problems associated with poor machinability and mechanical properties, although the commercial potential for their outstanding resistance to wear and corrosion, higher hardness and low coefficient of thermal expansion is well recognized. To exploit the full potential of hypereutectic Al-Si alloys, semi-solid processing is utilized in an attempt to improve the mechanical properties through the spheroidization of primary Si crystals. The purpose of the present work is to provide the experimental evidence of the fragmentation of primary Si crystals in hypereutectic Al-15.5wt%Si alloy during semi-solid processing. Microstructural evolution during the isothermal shearing above the eutectic temperature of this alloy as a function of processing time is also reported.

Modification of hypoeutectic Al-Si alloys

Modification of the Al-Si eutectic microstructure has been studied by using the entrained droplet technique in a range of hypoeutectic Al-Si alloys doped with different levels of phosphorus and sodium. Differential scanning calorimetry has been used to investigate the kinetics of silicon nucleation during eutectic solidification, and transmission and scanning transmission electron microscopy has been used to investigate the eutectic microstructure and microchemistry. The Al-Si eutectic microstructure can be modified from coarse-faceted silicon particles nucleating at low undercooling to clusters of fine-scale silicon particles nucleating at high undercooling by either increasing alloy purity, adding sodium, or increasing cooling rate during solidification. The unmodified coarse silicon particles are heterogeneously nucleated on pre-existing AIP particles. Increasing alloy purity removes phosphorus and prevents the formation of AIP; adding sodium leads to the formation of Na3P in preference to AIP; and increasing cooling rate allows insufficient time for the precipitation of AIP from low levels of dissolved phosphorus. At a cooling rate of ∼ 10 K min−1, the transition from an unmodified to a modified Al-Si eutectic microstructure takes place when phosphorus levels fall below 0.25–2 p.p.m., and when sodium levels increase to 80–850 p.p.m.

Application of an ultrasound technique to control the modification of AlSi alloys

The results obtained after the application of a non-destructive technique to determine the modification rating of AlSi alloys are presented. Research focused on the effect of antimony concentration over modification rating in an AlSi hypoeutectic alloy, via ultrasonic attenuation. The ultrasonic attenuation was determined at frequencies of 1,2.25, 5 and 10 MHz. Results showed that attenuation decreases significantly as the concentration of antimony in the alloy increases. A linear correlation between attenuation and modification rating is presented, where attenuation decreases progressively as the silicon structure becomes finer. Results are compared for the alloy without alloying elements and with controlled grain size.

Quantitative Gefügeanalyse von Aluminium-Silicium- Gußlegierungen / Quantitative Microstructural Analysis of Aluminium-Silicon Cast Alloys

A method is described which, using the Quantimet 500, enables the semi-automatic quantitative characterisation of hypoeutectic Al-Si microstructures. Microstructural parameters are the volume fraction and mean chord length of the primary phase and eutectic phases, and the dendrite arm spacing. The difficulty in determining the microstructural parameters is that a portion of the eutectic Al phase crystallises on the primary Al dendrites. In this work the crystallisation is taken into consideration using image processing. The method used is also applicable to other alloy systems where crystallisation of a eutectic phase onto the primary phase occurs. The subjective measurement of the dendrite arm spacing is replaced by a measurement of the mean chord length of the primary dendrites and a proportionality constant is given for hypoeutectic Al-Si alloys wich relates the two parameters

Structure-Property Related Changes in a Hypoeutectic Aluminium-Silicon Alloy Induced by Solutionizing

An attempt has been made to optimize the solutionizing temperature and duration by observing microstructural changes along with physical and mechanical properties of a hypoeutectic Al-Si (BS LM16) alloy. The specimens were solutionized over a range of temperatures (763, 778, 793, 813 and 833 K) and duration. Solutionizing at 763, 778 and 793 K revealed significant morphological changes in the microconstituents while the specimens at 813 and 833 K suffered from partial melting. The former group of temperatures offered significant improvement in physical and mechanical properties of the alloy while the latter one caused deterioration in properties. Coarsening of phases was found to cause a reduction in hardness, density, strength and ductility of the alloy beyond certain solutionizing duration at 763, 778 and 793 K. The resolidified regions, where partial melting had occurred as a result of solutionizing at 813 and 833 K, possessed significantly higher hardness resulting from their refined microstructure as compared to the unaffected part of the specimens. Inspite of tits, the specimens suffered from deterioration in their overall hardness due to the formation of secondary porosity and microcracking. Reduced density, strength and ductility and increased electrical resistivity of the alloy also agreed well with the generation of porosity and microcracks at these temperatures. The positive influence of increased hardness and refined microstructure of the resolidified regions seemed to have been outweighed by the predominant ill-effects of the porosity and microcracking as observed by the deterioration in the properties of the alloy at 813 and 833 K. The study indicated that solutionizing the alloy in the temperature range of 778-793 K for 2-8 h offers spheroidized constituents and improved properties which can exhibit good results during its ageing

スラリー・溶湯混合法による半凝固過共晶Ａｌ―Ｓｉ合金の見掛けの粘性挙動

A semi-solid slurry of a hypereutectic Al-Si alloy containing fine primary silicon crystals is producible by slurry-melt mixing (SMM) process. In the SMM process, a melt of a phosphorus-added hypereutectic Al-Si alloy (second-alloy melt) is mixed with a semi-solid slurry of a hypoeutectic Al-Si alloy (first-alloy slurry) which has been solidified with stirring in a furnace-cooled vessel. In this study, the change in apparent viscosity of the stirred slurry, whose silicon content changed from 8 to 20mass% by mixing of molten Al-32mass%Si alloy, was measured continuously during the SMM process. Immediately after the mixing of the second-alloy melt, the apparent viscosity decreases drastically from 7 to about 0 Pa•s. This abrupt change is attributed to the following facts: (1) The primary alpha-phase which has been crystallized in the first alloy remelts rapidly in the mixed alloy. (2) The fraction of the primary silicon in the mixed alloy is lower than that of the primary alpha-phase in the first alloy, though there is a strong driving force of crystallization of the primary silicon in the mixed alloy in comparison with the conventionally solidified Al-20mass%Si alloy. (3) The apparent viscosity of the hypereutectic alloy is similar in order of magnitude to that of the hypoeutectic alloy of the same fraction solid.

亜共晶Al-Si合金の半凝固スラリーとリンを添加した過共晶Al-Si合金溶湯との混合による微細初晶シリコンの生成

The principle of a new process to produce the semi-solid slurry of hypereutectic Al-Si alloy containing fine primary silicon crystals has been proposed: the process was named “slurry-melt mixing (SMM)” process. In this process, a phosphorus-added hypereutectic Al-Si alloy melt (second-alloy melt) was mixed with a semi-solid slurry of a hypoeutectic Al-Si alloy (first-alloy slurry) under a furnace-cooling atmosphere. As the two kinds of alloys were brought into contact with each other with agitation, the second-alloy melt was rapidly quenched to precipitate numerous fine primary silicon crystals, while the equiaxed alpha solid solution in the first-alloy slurry was remelted absorbing the latent heat of fusion. And the enthalpy balance of the two alloys led the mixed alloy to a stable solid-liquid coexisting state.Al-20 mass%Si alloy slurry was produced by the SMM process with the combination of an Al-8 mass%Si alloy and a phosphorus-added Al-32 mass%Si alloy. The arithmetic mean of area-equivalent diameters of the primary silicon crystals in the SMM slurry was 27 μm. The fluidity of the SMM slurry could be kept for about 1000 s because of the low cooling rate of the atmosphere. Sodium-addition into the SMM slurry makes it possible to achieve the modification of the eutectic silicon crystals without the coarsening of the primary silicon crystals.

Heterogeneous nucleation of solidification of Si by solid AI in hypoeutectic Al-Si alloy

Melt-spun Al-3 wt pct Si with and without ternary additions of Na and Sr has been heat-treated above the Al-Si eutectic temperature in a differential scanning calorimeter to form a microstructure of Al-Si eutectic liquid droplets embedded in the α-Al matrix. During subsequent cooling in the calorimeter, the heterogeneous nucleation temperature for solidification of Si in contact with the surrounding Al matrix depends sensitively on the alloy purity, with a nucleation undercooling which increases with increasing alloy purity from 9 to 63 K below the Al-Si eutectic temperature. These results are consistent with Southin’s hypothesis that low levels of trace P impurities are effective in catalyzing Si nucleation in contact with the surrounding Al matrix. With a low Al purity alloy, 0.1 wt pct Na addition increases the Si nucleation undercooling from 9 to 50 K, 0.15 wt pct Sr addition does not affect the Si nucleation temperature, and 0.3 wt pct Sr addition decreases the Si nucleation undercooling from 9 to 3 to 4 K. The solidified microstructure of the liquid Al-Si eutectic droplets embedded in the Al matrix depends on the Si nucleation undercooling. With low Si nucleation undercooling, each Al-Si eutectic liquid droplet solidifies to form one faceted Si particle; however, with high Si nucleation undercooling, each Al-Si eutectic droplet solidifies to form a large number of nonfaceted Si particles embedded in Al.

Friction behavior of ceramic fiber-reinforced aluminum metal-matrix composites against a 440C steel counterface

This paper describes the friction behavior of kaowool (Al 2O 3 · SiO 2) and saffil (Al 2O 3) fiber-reinforced aluminum metal-matrix composites (MMCs) prepared by the squeeze infiltration technique. A hypoeutectic Al-Si alloy was used as the matrix. Friction measurements were made using a ball-on-disc configuration, in which a 440C stainless steel ball was held against a rotating MMC disc. Wear tracks on MMC discs and material transfer to the steel counterface were examined using a scanning electron microscope equipped with an energy dispersive X-ray spectroscope. There were no significant differences in the friction behavior of the three MMCs and the unreinforced alloy when metallographically polished samples were used as test specimens. In all four cases, the friction trace was rough with large fluctuations in friction force; the coefficients of friction varied between 0.4 and 0.6. When the surface of the MMC was etched, the friction coefficient dropped to a low value of 0.18 and the stick-slip type behavior disappeared. With increase in normal load (and sliding distance), the stick-slip type behavior, characteristic of polished surfaces, reappeared. The transition from smooth to stick-slip type behavior is correlated to the microstructure and surface topography of the MMC. Factors influencing the transfer of aluminum (from the MMC disc) to steel are discussed.

Evaluation of primary dendrite trunk diameters in directionally solidified Al-Si alloys

Hypoeutectic Al-Si alloys were directionally solidified under conditions of steady-state growth. Cross-sectional views of the microstructure, i.e., perpendicular to the growth direction, were examined and measurements of the primary dendrite trunk diameters made. The results are presented as functions of growth velocity, composition, and temperature gradient, and evaluated in terms of relevant theory. It is suggested that the primary dendrite trunk diameter is a reliable, or better, a measure of the local solidification conditions during constrained growth than either the conventionally accepted eutectic ( λ E) or primary dendrite arm (λ 1) spacing.

Aluminum metal matrix composites tailored for antifriction applications

This paper describes the friction behavior of aluminum metal matrix composites (MMCs) reinforced with kaowool (Al 2O 3·SiO 2) fiber. Composites were produced by squeeze infiltration of fiber per-forms. A hypoeutectic Al-Si alloy was used as the matrix. Friction measurements were made using a ball-on-disc configuration in which a type 440C stainless steel ball was held against a rotating MMC disc. A polished surface produced by standard metallographic polishing and an etched surface (with protrusions of fibers and eutectic silicon) were used as test surfaces. Wear scars and steel balls (after the wear test) were examined in a scanning electron microscope. When metallographically polished surfaces were used as test specimens, the friction coefficient varied between 0.4 and 0.5; the friction trace was rough with large fluctuations in frictional force. Smearing of aluminum was detected on the steel ball as well as on the wear scar. When etched surfaces were used as test specimens, the friction coefficient was reduced to 0.15 and aluminum transfer to the counterface was completely eliminated. The results of this study indicate that microstructural control coupled with etching can be used to tailor the aluminum MMCs for antifriction applications.