Primary Silicon Phase Research Articles

Development of Nano-Fibrous Eutectic and Primary Silicon Phases in Al-Si Cast Alloys for High Strength Structural Applications

Among the high strength lightweight materials, hypereutectic Al-Si cast alloys with constituent phases that possess nano-scale dimensions are highly desirable for automotive and aerospace applications. Recently, we have achieved ultra-high strength Al-Si alloys via directional solidification technology. In these directionally solidified alloys, both the eutectic and primary silicon phases have nano-sized fibrous morphologies. This paper describes the microstructure evolution and manufacturing of these advanced alloys and introduces ways of implementing these findings of directionally solidified casting into near-net shaped castings.

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.

Thermo-mechanical fatigue damage and failure of modern high performance diesel pistons

This study resulted from an engineering failure investigation related to diesel engine piston failures which occurred during a bench dynamometer engine durability test programme. The test programme aimed at evaluating the effects of various fuel types on the durability of fuel system components in passenger car diesel engines. A number of unexpected cylinder head, turbocharger and piston failures were experienced during the course of the test programme. This study focused on the cause of the piston failures experienced during these tests. Analyses of the fractured pistons revealed that thermo-mechanical fatigue initiation occurred as a result of primary silicon phase cracking and subsequent micro-crack formation due to excessive thermo-mechanical loading. Progressive formations of such micro-cracks lead to flaws that were of sufficient magnitude to initiate propagation by high cycle fatigue mechanisms. The investigation also revealed that the excessive thermo-mechanical piston loading was caused by over-fuelling and a combination of elevated and poorly controlled post intercooler air temperature. There was no evidence to suggest that the failures were related to the test fuel formulations.

Effect of neodymium on primary silicon and mechanical properties of hypereutectic Al-15%Si alloy

The mechanical properties of hypereutectic Al-Si alloys are mainly determined by size and morphology of the primary silicon phase. So, optical microscopy (OM) and X-ray diffraction (XRD) were adopted to study affection of Nd on primary silicon of hypereutectic Al-15%Si alloy in this paper. The results of OM showed that pure Nd could effectively refine primary silicon of hypereutectic Al-15%Si alloy. When Nd addition was 0.3%, the average size of primary silicon reduced from 20-40 μm of initial sample to 10-20 μm of modified sample. XRD pattern showed that no new phase was formed after Nd modification. The results of mechanical properties test showed that whole properties of modified samples were significantly improved. Tensile strength increased about 32.6% from 147 MPa to 195 MPa. Elongation was increased about 160% from 1.0% to 2.6%. The improvement of mechanical properties should attribute to primary silicon refinement after modification.

Effect of Nd on microstructure and wear resistance of hypereutectic Al–20%Si alloy

In this paper, pure Nd was adopted to modify hypereutectic aluminum–silicon alloy (Al–20%Si). The morphology of eutectic and primary silicon phases was analyzed by OM and SEM. OM and SEM results show that pure Nd (0.3 wt.%) can significantly refine both eutectic and primary silicon of hypereutectic Al–20%Si alloy. Morphology of primary silicon was transformed from star-shaped and irregular morphology to fine polyhedral and grain size of primary silicon was refined from 80–120 μm to 20–50 μm. TEM results show that a new needle shape ternary phase (AlSi x Nd y ) forms in modified alloy. XRD results show that three little unknown diffraction peaks appear after Nd modification and it is induced that they should be diffraction peaks of ternary AlSi x Nd y intermetallic phase in the modified alloy by analyzing both TEM and XRD results. Friction and wear resistance tests show that friction coefficient of Al–20%Si alloy decreases after Nd modification. Wear resistance of Al–20%Si alloy after 0.3 wt.%Nd modification was significantly improved as compared to the initial sample. The improvement of wear resistance was mainly attributed to change of morphology, size and distribution of eutectic silicon and primary silicon after Nd modification. The dominant wear mechanism for 0.3 wt.%Nd modified alloy was abrasive wear, adhesive wear and oxidative wear mechanism, but wear mechanism for unmodified alloy was abrasive wear and adhesive wear mechanism.

Effects of Mg Content on Microstructure and Mechanical Properties of Al-14.5Si-4.5Cu Alloy

This study elucidates how Mg content affects the microstructure and mechanical properties of Al-14.5Si-4.5Cu alloy by adding 0.45 and 0.90 wt pct Mg. Primary silicon, eutectic silicon, acicular β-Al5FeSi, Al2Cu, and Al5Cu2Mg8Si6 phases were observed under the as-cast condition in low-Mg alloy. In high-Mg alloy, a large proportion of the acicular β-Al5FeSi phase was converted to Chinese script Al8Mg3FeSi6 phase. Neither the acicular β-Al5FeSi phase nor the Chinese script Al8Mg3FeSi6 phase dissolved in the Al matrix during solution heat treatment. Tensile testing indicates that T6-treated high-Mg alloy containing the Chinese script Al8Mg3FeSi6 phase is better than the T6-treated low-Mg alloy that contains the acicular β-Al5FeSi phase. The morphologies of Chinese script Al8Mg3FeSi6 and acicular β-Al5FeSi phases affect the tensile properties of Mg-containing Al-14.5Si-4.5Cu alloys. Additionally, high-Mg alloy contains a higher concentration of Mg solute atoms in the Al matrix than low-Mg alloy, and so the former contains more λ′ precipitates (Al5Cu2Mg8Si6) after T6-tempering treatment. The large amount of λ′ that precipitates in T6-treated high-Mg alloy may also enhance its tensile strength. The ultimate tensile strength (UTS) and elongation of the high-Mg alloy are superior to those of the low-Mg alloy.

Effect of (Mn + Cr) addition on the microstructure and thermal stability of spray-formed hypereutectic Al–Si alloys

Microstructures and thermal stability of hypereutectic Al–Si alloys with or without (Mn + Cr) addition, prepared via Spray Forming technique, are studied and compared with traditional cast alloys with same composition, using scanning electron microscopy with energy diffraction spectrum, X-ray diffraction, transmission electron microscopy and differential scanning calorimeter. The results show that the Fe-bearing and primary silicon phases in SF-3C alloy can be refined to less than 10 μm, especially in SF-MC21 alloy the Fe-bearing phase is refined into uniformly distributed α-Al(Fe,Mn,Cr)Si phase particles with sizes smaller than 5–6 μm, contributing to the decrease/elimination of the deleterious effect of needle-like Fe-bearing phases. The results of different heat treatments show SF-MC21 alloy possesses excellent thermal stability than SF-3C alloy which is unstable below 750 K for the coarsening of β-Al 5FeSi phase and formation of Al 7Cu 2Fe phase. The study indicates that both the existence of thermodynamically stable α-Al(Fe,Mn,Cr)Si particles and the increase of solidus temperature of SF-3C alloy induced by adding (2Mn + 1Cr) elements contribute to the high thermal stability of SF-MC21 alloy. Contemporarily, combined the phase reactions or transformation occurred during the melting and solidification processes of both spray-formed hypereutectic Al–Si alloys, the microstructure formation of spray-formed alloys is discussed.

Hypereutectic Al-Si Alloys: Practical Casting Considerations

Hypereutectic Al-Si systems such as 390 alloy became popular in the early 1970s in premium German engines and compressors, and grew in popularity in North America for use in pumps, compressors, transmission components, pistons and similar wear applications. Hypereutectic alloys are, in reality, in-situ metal matrix composites (MMC) of hard silicon crystals disbursed throughout a eutectic matrix; thus, like other MMCs, they combine light weight and excellent wear properties with high elastic moduli, low thermal expansion and exceptional resistance to elevated temperature service environments. Traditionally, the downsides to casting such alloys are certain difficulties associated with controlling the size and distribution of the primary silicon phase and dealing with the high heat of fusion of silicon. Techniques used to minimize those difficulties are described.

EET research on the Al-22%Si alloy under the action of electric pulses

Based on the empirical electron theory of solids and molecules (EET), the phase valence electron structure parameters of Al-22%Si alloy are calculated, and the sensitivity of the bond network of Al-Si alloy melt to temperature (energy) and the effect of the sensitivity on the morphology are studied. The results show that the Si-Si clusters with larger n A in the slightly superheated Al-Si alloy melt supply the nucleation core to the primary silicon phase in the hypereutectic Al-22%Si alloy, and strongly generate the drag-like effect for the Al-Si clusters around them; that the variation of temperature significantly affects the stability of bonds of the core so that the solidified structure is changed; that the electric pulse applied to the alloy melt can irrecoverably alter the stability of Si-Si clusters, then the modification of the solidified structure morphology of alloys is generated; that the higher the energy of the electric pulse, the less stable the Si-Si clusters, and the more significant the electric pulse modification.

Effect of P/M value on the preforms and microstructures of spray formed 70Si30Al alloy

A novel 70Si30Al alloy was prepared by the spray forming process for electronic packaging materials. The effect of the ratio of atomization pressure to metal melt mass flux rate ( PIM) on the preforms and microstructures of the spray-deposited 70Si30Al alloy was studied. The results indicate that the PIM value has a considerable influence on the formation of the preforms and the optimal value is in the range of 0.209–0.231 MPa/(kg-min -1). The microstructure of the spray formed 70Si30Al alloy is fine and homogenous. The primary silicon phases distributing in the aluminum matrix evenly are fine and irregular. The aluminum matrix is divided into wo groups: supersaturated α-Al phase or α-Al phase and Al-Si pseudoeutectic phase or Al-Si eutectic phase.

A phase-field model for the solidification of multicomponent and multiphase alloys

A phase-field model for the simulation of solidification of a multicomponent and multiphase systems has been developed, which is based on an earlier developed multiphase field model for binary alloys and a phase-field multicomponent model for single-solid-phase systems. After incorporation with alloy thermodynamics and commercial software for the calculation of phase equilibria, the model has been implemented to study the microstructural evolution of an Al–11.5 mol% Si–0.9 mol% Cu–0.4 mol% Fe alloy. Numerical results for the morphological evolution of primary aluminium, silicon and AlFeSi intermetallic phases agree with experimental observations very well.

Creep Behavior of Al-Si Die-Cast Alloys

Commercial, aluminum die-cast alloys are subject to long-term stresses leading to viscoelastic material responses resulting in inefficient engine operation and failure. Constant load creep tests were conducted on aluminum die-casting alloys: B-390, eutectic Al-Si and a 17% Si-Al alloys. Rupture occurred in the primary creep regime, with the eutectic alloy having the longest times to failure. Primary creep was modeled by Jt=A+Btn with A, B, and n dependent on stress. Poor creep performance is linked to the brittle fracture of the primary silicon phase as well as other casting defects.

Solidification of Al–50at.%Si alloy in a drop tube

Abstract Solidification of 0.1–1.0 mm diameter droplets of Al–50 at.%Si hypereutectic alloy was achieved in a 3 m drop tube. Phase morphologies of deeply etched sections of the solidified samples with different diameters are examined. With decreasing of the diameter of the droplets, refinement of the primary silicon phase is observed. A tendency for the primary silicon crystals to form granular morphology is found in the smaller samples. Stratified deposits of aluminum in the primary silicon along the silicon plate are evident in bigger samples, but less evident with decreasing of the sample size. Morphologies of Al–Si eutectics change from needle-like to short, blunt and worm-like forms with decreasing of the sample size. Through calculation of the nucleation rates at different sizes of the solidified sample and analyses of the crystal morphologies, grain refinement of the primary silicon can find its origins of copious nucleation during rapid cooling and solute-restriction to crystal growth.

Solidification mechanisms of unmodified and strontium-modified hypereutectic aluminium–silicon alloys

The effects of strontium on the solidification mode of hypereutectic aluminium–silicon alloys have been studied. Samples were prepared from an aluminium–17 wt% silicon-based alloy and strontium was added at several different concentrations. The development of the microstructure was investigated by cooling curve analysis, interrupted solidification experiments and optical and scanning electron microscopy. It was found that nucleation of primary silicon is suppressed by additions of strontium. The suppressed nucleation results in supersaturation of the liquid prior to nucleation, and an increased growth rate after nucleation. As a result, the silicon crystals become less faceted and more dendritic with increasing strontium additions. Increasing the strontium concentration slightly refined the eutectic spacing and introduced a small amount of fibrous silicon. Electron back-scatter diffraction measurements were performed to determine the crystallographic relation between the primary and eutectic silicon phases. The eutectic silicon in the unmodified alloy does not have any crystallographic relationship with the primary silicon crystals. In contrast, the eutectic silicon crystals in the strontium-modified alloys often share an identical or twin relationship with nearby primary silicon crystals. The incidence of twinning within primary silicon crystals was relatively low and did not appear to increase with strontium additions.

Corrosion resistance and properties of pump pistons coated with hard materials

Low expansion aluminium–silicon eutectic alloys are cast to produce most of the automotive pistons. The structure and properties of these alloys are very much dependent on the cooling rate, composition, modification and heat treatment operations. In this study, locally available automotive ‘scrap pistons’ were used as basic raw materials and a natural gas fired crucible furnace was used for melting purpose. The wear behaviour of both as-cast and heat treated specimens were studied under dry sliding conditions at room temperature using a pin-on-disc type wear testing apparatus. The extent of wear damage and the type of wear were investigated by means of weight loss measurement and optical microscopy techniques. The full heat treatment showed a great influence on the wear properties of the aluminium–silicon piston alloy as it reduced the wear rate of the specimens. The exceptional high tensile strength and hardness were attributed to the heat treatment condition with decrease in ductility. Significant changes in structure were also observed to occur specially in the primary and eutectic silicon phases. Some of the results of this study can be recommended for tribological use of this alloy in manufacturing automobile spare parts.

Study on wear properties of aluminium–silicon piston alloy

Low expansion aluminium–silicon eutectic alloys are cast to produce most of the automotive pistons. The structure and properties of these alloys are very much dependent on the cooling rate, composition, modification and heat treatment operations. In this study, locally available automotive ‘scrap pistons’ were used as basic raw materials and a natural gas fired crucible furnace was used for melting purpose. The wear behaviour of both as-cast and heat treated specimens were studied under dry sliding conditions at room temperature using a pin-on-disc type wear testing apparatus. The extent of wear damage and the type of wear were investigated by means of weight loss measurement and optical microscopy techniques. The full heat treatment showed a great influence on the wear properties of the aluminium–silicon piston alloy as it reduced the wear rate of the specimens. The exceptional high tensile strength and hardness were attributed to the heat treatment condition with decrease in ductility. Significant changes in structure were also observed to occur specially in the primary and eutectic silicon phases. Some of the results of this study can be recommended for tribological use of this alloy in manufacturing automobile spare parts.

Interface investigations of alumina and aluminosilicate short-fiber-reinforced aluminum-alloy composites

The interfacial microstructures of alumina (Al 2O 3) and aluminosilicate (Al 2O 3·SiO 2) short-fiber-reinforced aluminum-alloy composites were studied by means of transmission electron microscopy (TEM). The experimental results showed that the SiO 2 content in the fiber has a marked effect on the interface microstructures of the composites even though a silica binder is not used. The reaction product MgAl 2O 4 spinel oxide was formed at the interface of the aluminosilicate-fiber-reinforce aluminum-alloy composite, however, no spinel oxide was observed at the interface of alumina-fiber-reinforced composite. The alumina fiber could act as a heterogeneous nucleation substrate for the primary silicon phase as a result of the low disregistry (5.62%) between the (0001) of δ-Al 2O 3 and the (110) of silicon.

The atomic-structure changes in Al-16 pct Si alloy above the liquidus

The atomic-structure changes in an Al-16 pct Si alloy above the liquidus have been studied by a θ-θ high-temperature X-ray diffractometer, rapid solidification, a vertical centrifugal casting apparatus, and differential scanning calorimetry (DSC) measurements. It was found that the diffraction intensity and structure-factor (S(Q)) curves for an Al-16 pct Si alloy have small prepeaks at small Q values when the temperature is high enough. Rapid-solidification and centrifugal-casting experimental results show that the primary silicon phase can easily coarsen and segregate under additive force after an overheat at high temperatures. The DSC measurements show that the temperature and latent heat of primary solidification rise with the temperature of overheating. These experimental results suggest that a strong interaction occurs between Si-Si atoms in a liquid Al-16 pct Si alloy at high temperatures, resulting in the microsegregation of Si atoms in the melt.

The morphology of cast zinc-based alloy reinforced by spheroidal silicon phase and its wear resistance

This paper deals with the influence of silicon phase on the wear resistance of zinc-aluminum alloy. In order to improve the wear resistance performance of ZA27 alloy under the conditions of dry friction, Si was added. It was discovered that the primary silicon phase changes from a block shape into a spheroidal one by the modification of the alloy with a compound agent containing sodium. Scanning electron microscope images (SEM) showed that the spheroidal silicon phase is a polyhedron from which needle-like eutectic silicon forms. The result of dry-friction testing indicated that this may be firmly embedded in the Zn-Al matrix and thus cannot be exfoliated during friction. The zinc-aluminum alloy reinforced by spheroidal silicon phase was found to have an increased wear resistance performance.

Rare earth concentration in the primary Si crystal in rare earth added Al-21wt.%Si alloy

Al-Si alloys containing more than about 12 wt. % Si exhibit a hypereutectic microstructure, normally consisting of a primary silicon phase in an eutectic matrix. The primary silicon in normal hypereutectic alloys is usually very coarse and thus leads to poor properties to these alloys. Therefore, alloys with a predominantly coarse primary silicon crystal must be modified to ensure adequate mechanical strength and ductility. Further improvement of mechanical properties of these alloys can be achieved by the modification of eutectic microstructure. Therefore, development of a modifier or refiner that can produce both fine primary and eutectic Si is a major factor which can lead to significant enhancement of mechanical properties in hypereutectic Al-Si alloys. Refinement of primary silicon is usually achieved by the addition of phosphor to the melt. On the other hand, it is reported that the rare earth (RE) elements are capable of modifying the eutectic structure of cast Al-Si alloys. According to the literature, Phosphor acts as a heterogeneous nucleation site of Si crystal by forming AlP intermetallic particles at high temperature, i.e., above liquidus temperature of Al-Si alloy. Unlike phosphor, RE was not known to form a stable compound with Al that can act as amore » nucleation site at high temperature. Therefore, the role of RE as a refiner should be considered by examining the behavior of RE as a solute in the melt. The distribution of RE within the primary Si and in the matrix of the alloy will provide a clue to the role of RE on the modification of primary Si during solidification.« less