Eutectic Silicon Phase Research Articles

Semi-solid squeeze casting process of a ZL109 alloy

The structure evolution of the ZL109 alloy in the process of semi-solid squeeze casting and the mechanical properties of the components were investigated. The results show that (1) the eutectic silicon phase in original billets is refined in the low super-heat casting process; (2) the eutectic structure in billets starts to fuse and the crystals of the eutectic silicon phase are refined further and sphericized in the remelting process of billets; (3) in the semi-solid squeeze casting process, the sphericity of the α phase and the refining of the silicon phase occur, owing to the friction between solid and liquid; (4) in the process of heat treatment, the eutectic α phase aggregates with the primary α phase and the eutectic silicon pieces aggregate together. The elongation of the semi-solid component after heat treatment rises to 1.42%.

Relation between mechanical properties and microstructure of cast aluminum alloy AlSi9Cu3

One common material for engine applications is the AlSi9Cu3 alloy. This alloy has a good castability, excellent machinability, medium strength, and low specific weight. The study was focused on the investigation of the effect of the solution heat treatment on the microstructure and mechanical properties of the alloy (strength-Rm, hardness-HBS). The temperatures of the solution heat treatment were 505°C, 515°C, and 525°C ± 5°C and the solution time ranged from 0 to 32 h (0, 2, 4, 8, 16, and 32 h). Alloy AlSi9Cu3 contained α-matrix, eutectic silicon, and other Fe-and Cu-rich phases with different morphology (needle-like, Chinese script, skeleton-like, blocky, etc.). The results obtained revealed the relation between the mechanical properties and the morphologies of the eutectic silicon and the predominant copper-rich phase Al-Al2Cu-Si during the solution treatment.

Effect of plastic deformation on microstructure and hardness of AlSi/Al gradient composites

The effects of plastic deformation on the evolution of microstructure and micro-hardness were studied on plates made from a cylindrical ingot of AlSi/Al aluminum composite. This ingot was produced by double-stream-pouring continuous casting(DSPCC). The results show that the three layers, i.e. the external pure aluminum, internal Al-Si alloy and the transition gradient layer, are maintained after compression as well as rolling processing. With the decrease in thickness of the gradient composite plate, the fluctuation of micro-hardness in transition layer in the direction normal to the applied force is significantly reduced. A simplified lamination model was used to evaluate qualitatively the micro-hardness distribution in the direction parallel to the applied force. However, the model is invalid in the explanation of the fluctuation of the micro-hardness in the direction normal to the applied force. The micro-hardness distribution variation in this direction is mainly attributed to the deformation of α(Al) matrix incorporated the dissolution and spheroidization of eutectic silicon phase.

Determination of strontium segregation in modified hypoeutectic Al–Si alloy by micro X-ray fluorescence analysis

Analysis of intra- and inter-phase distribution of modifying elements in aluminium–silicon alloys is difficult due to the low concentrations used. This research utilises a μ-XRF (X-ray fluorescence) technique at the SPring-8 synchrotron radiation facility X-ray source and reveals that the modifying element strontium segregates exclusively to the eutectic silicon phase and the distribution of strontium within this phase is relatively homogeneous. This has important implications for the fundamental mechanisms of eutectic modification in hypoeutectic aluminium–silicon alloys.

Segregation of particles and its influence on the morphology of the eutectic silicon phase in Al–7wt.% Si alloys

A new modification phenomenon is reported for Al–Si alloys, where the Al–Si eutectic is refined by segregated TiB2 particles. The TiB2 particles are pushed to the Al–Si phase boundary during solidification of the eutectic and it is believed that at high concentrations the TiB2 particles restrict solute redistribution causing refinement of the Si.

Effects of boron on eutectic solidification in hypoeutectic Al–Si alloys

Abstract The effects of boron on the solidified microstructure and cooling curve of Al–7Si alloy were investigated. The results show that B has no significant effects on the shape factor of eutectic silicon but the mean area of eutectic silicon phase is reduced when the grain size is reduced within a certain range.

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.

Eutectic grains in unmodified and strontium-modified hypoeutectic aluminum-silicon alloys

Additions of strontium to hypoeutectic aluminum-silicon alloys modify the morphology of the eutectic silicon phase from a coarse platelike structure to a fine fibrous structure. Thermal analysis, interrupted solidification, and microstructural examination of sand castings in this work revealed that, in addition to a change in silicon morphology, modification with strontium also causes an increase in the size of eutectic grains. The eutectic grain size increases because fewer grains nucleate, possibly due to poisoning of the phosphorus-based nucleants, that are active in the unmodified alloy. A simple growth model is developed to estimate the interface velocity during solidification of a eutectic grain. The model confirms, independent of microstructural observations, that the addition of 100 ppm strontium increases the eutectic grain size by at least an order of magnitude compared with the equivalent unmodified alloy. The model predicts that the growth velocity varies significantly during eutectic growth. At low strontium levels, these variations may be sufficient to cause transitions between flake and fibrous silicon morphologies depending on the casting conditions. The model can be used to rationally interpret the eutectic grain structure and silicon morphology of fully solidified aluminum-silicon castings and, when coupled with reliable thermal data, can be used to estimate the eutectic grain size.

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.

Structure factors of modified liquid Al–Si alloys

The structure factor and the coordination numbers of liquid Al–Si alloys with different Si content have been measured by a high temperature X-ray diffractometer. Radial distribution functions (RDFs), the nearest atomic distance and the coordination numbers of eutectic Al–Si alloys before and after being modified with Sr and Sb were studied. The RDFs of the liquid alloy were decomposed by five Gaussian peaks. The results show that a Si–Si covalent bond exists in the liquid of eutectic and hyper-eutectic alloys. Sr in the liquid Al–Si has a capability to weaken the covalent bonds of Si–Si, suppressing the nucleation of the eutectic silicon phase. On the other hand, Sb in the liquid Al–Si increases the order degree of Si atoms, decreasing the supercooling degree of the nucleation and promoting the nucleation of eutectic silicon.

The fracture behaviour of A356 alloys with different iron contents under resonant vibration

The fracture behaviour of A356 alloys with different iron contents under resonant vibration has been investigated by examining the morphology of eutectic silicon and Fe-rich intermetallic phases. The test materials were prepared by varying the iron content within the range of 0.14 to 0.97 wt%. The experimental results shows that while the iron content is below 0.57 wt%, the Fe-rich intermetallic phases are mainly π-Al8Mg3FeSi6 and α-Al15Fe3Si2 and there is no obvious difference in the fracture resistance under resonant vibration. However, when the specimen has higher iron content (0.97 wt%Fe), the fracture resistance drops due to the formation of β-Al5FeSi. Hence, the β-Al5FeSi morphology is more detrimental to the fracture resistance than that of π-Al8Mg3FeSi6 or α-Al15Fe3Si2. The results indicate that the vibration fatigue cracks initiate at the surface and the fracture paths go predominantly through the silicon particles, and occasionally through the Fe-rich intermetallic phase particles. The detrimental β-Al5FeSi becomes influential in the specimen with higher iron content. The results also show that the line intercepted density (LID) and projected crack intercepted density (PCID) values of Fe-rich intermetallic phases increased with increasing area fraction of Fe-rich intermetallic phases (or iron content). This shows that an increase in area fraction of Fe-rich intermetallic phases (or iron content) may promote the crack propagation and growth under resonant vibration.

Structures of Liquid Al-Si Alloy Modified by Sr

The structures of liquid Al-Si alloys modified by Sr were measured by using a high temperature x-ray diffractometer. The structure factor and the coordination numbers of liquid Al-Si alloys with different Si content have been obtained by Radial distribution functions, the nearest atomic distance and the coordination numbers of eutectic Al-Si alloys were studied, before and after being modified with Sr. The radial distribution functions of the liquid alloys were decomposed by 5 Gaussian peaks. These results show that Si-Si covalent bond exists in the liquid eutectic and hypereutectic alloys. Sr in the liquid Al-Si has a capability to weaken the covalent bonds of Si-Si, suppressing the nucleation of eutectic silicon phase.

Gravitational Influence on the Microstructure of Directionally Solidified Monotectic and Eutectic Alloys

In order to study the influence of gravity on the microstructure of directionally solidified Al-In monotectic alloy, Ag-Si and Al-Si eutectic alloys, both the downward (direction of gravity) and upward (opposite to direction of gravity) unidirectional solidifications were carried out. In the case of the downward U.D.S. by the current Bridgman method, the molten alloy must be confined between the crucible and a solid. This causes a shrinkage cavity at the solid/liquid interface during solidification. In this study, to avoid this cavity formation, a zone-melting technique was used. The results are summarized as follows. (1) Regarding the unidirectional solidifications of Al-In alloy, the monotectic In(L 2 ) phase grows continuously in both growth directions. However, the morphology in the downward U.D.S. shows a tendency to form a so-called banded structure. (2) In the case of the upward U.D.S. of the Ag-Si alloy, the eutectic silicon phase grows continuously and its morphology shows the typical faceted/non-faceted eutectic structure. On the other hand, in the case of the downward U.D.S., the eutectic silicon phase grows discontinuously and disperses in the α-Ag phase with a diameter of several micrometers. This is caused by the large difference in specific density between the silver and silicon phases. (3) The eutectic silicon phase of the Al-Si alloy grows continuously regardless of the growth direction. There is no morphological difference between the upward and the downward U.D.S specimens.

The structure and bending properties of squeeze-cast composites of A356 aluminium alloy reinforced with alumina particles

In the present work, variations in the microstructure, mechanical properties and fracture character of Al 2O 3/A356 composites fabricated by squeeze casting at different solidification cooling rates were investigated. Experimental results showed that when the solidification cooling rate decreased, the crystalline grain size of the matrix alloy and the eutectic silicon phase increased gradually, but the bending strength and the elastic modulus decreased. For Al 2O 3/A356 composites fabricated by squeeze casting, solidification occurred first in the interstices between particles and the particle surface was the substrate for the heterogeneous nucleation of the eutectic silicon phase. Fracture surfaces of the Al 2O 3/A356 composites showed brittle fracture characteristics and broken and cleaved Al 2O 3 particles were observed on the fracture surfaces of the composites.

Influence of Gravity on the Microstructure of Directionally Solidified Ag–Si Eutectic Alloy

In order to study the influence of gravity on the eutectic growth of Ag-Si and Al-Si alloys, both the downward (direction of gravity) and upward (opposite to direction of gravity) unidirectional solidifications were carried out. In the case of the downward U.D.S., the molten metal is confined between the crucible and solid. This inevitably produces the shrinkage cavity at the solid/liquid interface during solidification. In this study, the zone-melting technique was used to avoid this cavity formation. The results of this study were as follows: (1) In the case of the downward U.D.S. of the Ag-Si alloy, the eutectic silicon phase grew discontinuously and dispersed in the α-Ag phase with a diameter of several micrometers. This is caused by the large difference in specific density between silver and the silicon phase. On the other hand, in the case of the upward U.D.S., the silicon phase grew continuously and its morphology showed the typical faceted/non-faceted eutectic. (2) The eutectic silicon phase of the Al-Si alloy grew continuously regardless of growth direction. There was no morphological difference between the upward and downward U.D.S. specimens.

Effect of process variables on structure and properties of aluminium–silicon piston alloy

The majority of automotive pistons are cast in one of the groups of low expansion Al–Si eutectic alloys. The structure and mechanical properties of these alloys are highly dependent upon cooling rate, composition, modification and the heat-treatment operations. The effect of these variables were investigated in this study, using locally available automotive `scrap pistons' as basic raw materials. Strontium master alloy was used as the source of the modifier and a natural-gas-fired crucible furnace was used for melting purposes. The microstructures of the alloy, both in the modified and unmodified conditions, were studied in this investigation. Properties such as the ultimate tensile strength (UTS), percentage elongation, hardness and percentage porosity for both rates of cooling have been determined, both with and without the addition of strontium. Significant changes in structures were observed to occur, especially in the primary and eutectic silicon phases. Full heat treatment has also shown a great influence on the structure and properties of the aluminium–silicon piston alloy. The residual stress in the modified alloy is fully relieved due to thermal treatment. Finally, attempts were made to correlate the observed structures and properties obtained during the experiment.

半溶融凝固法により製造したＡｌ‐７％Ｓｉ‐０．５％Ｍｇ合金の微細組織と機械的性質

Microstructure and mechanical properties of an as-cast and heat-treated Al–7%Si–0.5%Mg– alloy (an alloy A357) produced by both the conventional mold casting and semi-solid process have been investigated. The effect of microstructural changes during heating to semi-solid temperatures on the mechanical properties are also discussed for the alloy produced by the semi-solid process. The microstructure of the produced alloy basically consists of the spherical α phase and fine eutectic silicon phase without any segregates and defects. The average size and volume fraction of the α phase are about 52 μm and 65 ±3%, respectively. The eutectic silicon phase becomes spheroidized by solution heat treatment. The ultimate tensile strength and elongation of the peak-aged alloy are 350 MPa and 16%, respectively. As the heating and holding times increase, the mean particle size of the α phase slightly increases. The increment, however, is negligibly small in the examined condition. Therefore, the mechanical properties of the alloy produced by the semi-solid process remain almost unchanged with increasing heating and holding times, compared with those of the non-heated alloy. Coalescence between α phase particles occurs with increasing heating and holding times, resulting in the shape change of the particles.

Structure control of in-situ silicon particle reinforced Zn [sbnd]27Al composite materials

Al Si alloy is one of the most widely applied commercial alloys. Its properties can be improved by alloying with the element silicon, the morphology of the silicon phases having a direct effect on its properties. The modification of the needle-like eutectic silicon and plate primary silicon phase is importance for the mechanical properties of Al Si alloy. Zinc based aluminum alloy is also an important commercial alloy, one of its main applications being that of a wear-resistant material. It is composed of multiphases, but the microhardness differences of each phase is not great. If the number of dispersion hard phases (or particles) is well-distributed in the zinc based aluminum alloy, it is of no doubt that the abraiibility of the material is improved. The problem of how to prepare particle reinforced zinc based aluminum alloy composite is of concern to researchers. The purpose of this present study is to introduce a new process which can be used to prepare silicon particle reinforced zinc based aluminum alloy composite. The process is that the molten zinc based aluminum alloy mixes with rheologic (or semi-solid) hypercutectic Al Si alloy, the primary and eutectic silicon phases then acting as a particle reinforced phase in the zinc based aluminum alloy. The morphology of silicon phase can be controlled by stirring and modifying. The morphology of the silicon phase is regularly polyhedral, its size is about 10≈50 μm and it is well-distributed in the zinc based aluminum alloy. The results of wear-resistant test show that the silicon particle reinforced zinc based aluminum alloy composite is a good wear-resistant material.

Growth of eutectic silicon from primary silicon crystals in aluminium-silicon alloys

Technological interest in aluminium-silicon alloys derives, fundamentally, from their excellent moulding properties provided by the presence of silicon. In addition, they possess high thermal and electrical conductivities and good corrosion resistance. The possibility of modification of their cast structure by adding small quantities of alkaline and alkaline-earth elements makes their mechanical characteristics very competitive. Metallographic observations of the structures of primary and eutectic silicon crystals, and their possible synergistic influence, provide useful data on the critical stages of formation and growth of eutectic silicon phase. The nucleation and growth of eutectic silicon, removed from the cooperative precipitation zone, induce refinement in crystal size and, therefore, an improvement of the mechanical properties of the moulded structure. The aim of this investigation was to observe the influence of primary silicon crystals on the nucleation and growth of eutectic silicon.