Hypoeutectic Al-Si Research Articles

Effect of iron on wear behavior of as-cast and heat-treated hypereutectic Al–18Si–4Cu–0.5Mg alloy: A Taguchi approach

This study describes the influence of Fe (wt%) on the microstructure, hardness, and wear behavior of Al–18Si–4Cu–0.5Mg– XFe ( X = Fe, at 0.4, 2.0, and 5.0 wt%) hypereutectic Al–Si alloy. The range of alloys was selected on the basis of literature study. The literature revealed that most of the studies on the effect of iron have been focused on either hypo-eutectic Al–Si alloys or lower levels of iron in the alloy. As Fe-rich intermetallic compounds are high melting point compounds, it is expected that they should improve the high temperature strength of the Al–Si alloys. The higher iron contents were specifically selected to obtain the advantage of high temperature stability leading to high wear resistance. Microstructure has been characterized using optical microscopy, scanning electron micrography, and image analysis studies. Maximum hardness of the alloy was obtained when solutionized (at 500 ℃) for 4 h followed by artificial aging (at 190 ℃) for 6 h. Heat-treated samples with only maximum hardness were further tested for wear behavior. The wear behavior of alloy in the as-cast and heat-treated conditions has been analyzed using Taguchi approach and analysis of variance analysis to find out the factors that affect wear significantly.

Fractography as a tool to assess the occurrence of fatigue fractures in complex microstructure structural components

Fractographic assessment of fatigue fractures may be difficult if they occur in metallic components characterized by low ductility complex microstructures. In these cases reconciliation of known fatigue rupture mechanisms with fractographic appearance of fatigue fractured surfaces is challenging. Special techniques coupled with theory development may be necessary. Pearlitic steels or steels with predominant pearlitic microstructures are among the ones that are visited and their fatigue fractures interpreted. Analogously, fatigued Al foundry alloys, with hypoeutectic Al-Si compositions, are also illustrated.

Fractography as a tool to assess the occurrence of fatigue fractures in complex microstructure structural components

Fractographic assessment of fatigue fractures may be difficult if they occur in metallic components characterized by low ductility complex microstructures. In these cases reconciliation of known fatigue rupture mechanisms with fractographic appearance of fatigue fractured surfaces is challenging. Special techniques coupled with theory development may be necessary. Pearlitic steels or steels with predominant pearlitic microstructures are among the ones that are visited and their fatigue fractures interpreted. Analogously, fatigued Al foundry alloys, with hypoeutectic Al-Si compositions, are also illustrated.

Changes in Structural Characteristics of Hypoeutectic Al-Si Cast Alloy after Age Hardening

The contribution describes influence of the age-hardening consist of solution treatment at 515 °C with holding time 4 hours, water quenching at 40 °C and artificial aging at different temperature 150 °C, 170 °C and 190 °C with different holding time 2, 4, 8, 16 and 32 hours on mechanical properties (tensile strength and Brinell hardness) and changes in morphology of eutectic Si, Fe-rich and Cu-rich intermetallic phases in secondary (recycled) AlSi9Cu3 cast alloy. A combination of different analytical techniques (light microscopy upon black-white and colour etching, scanning electron microscopy (SEM) upon deep etching and energy dispersive X-ray analysis (EDX)) were therefore been used for the identification of the various phases. Quantitative study of changes in morphology of eutectic Si, Cu-rich and Fe-rich phases was carried out using Image Analyzer software NIS-Elements. Mechanical properties were measured in line with EN ISO. Age-hardening led to changes in microstructure include the spheroidization and coarsening of eutectic silicon, gradual disintegration, shortening and thinning of Fe- rich intermetallic phases, the dissolution of precipitates and the precipitation of finer hardening phase (Al 2 Cu) further increase in the hardness and tensile strength in the alloy. DOI: http://dx.doi.org/10.5755/j01.ms.18.3.2430

A probabilistic model for the high cycle fatigue behaviour of cast aluminium alloys subject to complex loads

This article is dedicated to the high cycle fatigue behaviour of cast hypo-eutectic Al–Si alloys and in particular the AlSi7Cu05Mg03 alloy. In a vast experimental campaign undertaken to investigate the fatigue damage mechanisms operating in this alloy, subject to complex loading conditions, it was shown that two different coexisting fatigue damage mechanisms occur in this materials, depending on the presence of different microstructural heterogeneities (i.e. micro-shrinkage pores, Si particles, Fe-rich intermetallic phases, DAS of the Al-matrix, etc.). In order to take into account both of these damage mechanisms, a probabilistic approach using the weakest link concept is introduced to model the competition between the two mechanisms. This approach leads naturally to a probabilistic Kitagawa type diagram, which explains the relationship between the fatigue behaviour of the material and the different casting processes or post-treatments (e.g. gravity casting and HIP).It is shown that the sensitivity to the different loading modes (i.e. uniaxial with and without mean stress, torsion and equibiaxial tension) depends on the microstructural heterogeneities responsible for crack initiation. For a porosity-free alloy, the predictions are very good for combined tension–torsion loading modes. When pores are present and control the fatigue strength, the predictions are very satisfactory for the uniaxial loads with different R-ratios and slightly conservative for multiaxial loads (i.e. torsion and equibiaxial tension). Never-the-less, they are much better than the predictions of the Dang Van criterion [1].

High cycle fatigue damage mechanisms in cast aluminium subject to complex loads

This article is dedicated to the high cycle fatigue behaviour of cast hypo-eutectic Al–Si alloys. In particular, the AlSi7Cu05Mg03 alloy is investigated. It presents the results of a vast experimental campaign undertaken to investigate the fatigue behaviour, and more specifically the fatigue damage mechanisms observed under complex loading conditions: plane bending with different load ratios, fully reversed torsion and equibiaxial bending with a load ratio of R=0.1. A specific test set-up has been designed to create an equibiaxial stress state using disk shaped specimens. A tomographic analysis is also presented with the aim of characterising the micro-shrinkage pore population of the material.It is shown that two distinct and coexisting fatigue damage mechanisms occur in this material, depending on the presence of different microstructural heterogeneities (i.e. micro-shrinkage pores, Silicon particles in the eutectic zones, Fe-rich intermetallic phases, etc.). Furthermore, it is concluded that the effect of an equibiaxial tensile stress state is not detrimental in terms of high cycle fatigue. It is also shown that the Dang Van criterion is not able to simultaneously predict the multiaxial effect (i.e. torsion and equibiaxial tension) and the mean stress effect for this material.

Effect of main alloying elements on strength of Al–Si foundry alloys at elevated temperatures

In this study, we describe the effect of the main alloying elements Ni, Cu and Mg on the mechanical properties of near eutectic and hypoeutectic Al–Si foundry alloys at 250°C after a long term exposure to test temperature. Systematic compositional variations illustrate the significant hardening effect of secondary precipitates such as Al2Cu and Mg2Si. It is also shown that the strength is increased by the addition of Ni, albeit only to a certain level, depending on the fraction of eutectic phase in the alloy. The alloys are considered as coarse two-phase systems, where a hardening effect is caused by load transfer to the harder phase, which requires a certain contiguity of the latter. This paper discusses the individual influences of Ni, Cu and Mg on the high temperature strength and describes a potentially adverse effect of the combinations Cu/Ni and Cu/Mg.

Grain refinement mechanism in an Al–Si–Mg alloy with scandium

Grain refinement of the primary aluminum (α-Al) phase in a hypoeutectic Al–Si alloy using scandium (Sc) was studied to identify the grain refinement mechanism. Optical microscopy (OM), Scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD) techniques were extensively used in this study. We found that Sc refined grains of primary aluminum. However, the grain refinement efficiency of Sc was considerably lower than that of titanium (Ti) in the Al–Si–Mg foundry alloy. It was evident that the precipitated Sc-containing phases acted as heterogeneous nucleation sites for the primary aluminum phase. The Sc-containing heterogeneous sites are irregular in shape with sizes between 3 and 5μm. At least three groups of nuclei based on their chemical composition were found, i.e., (i) Al and Sc, (ii) Al, Si, Mg, and Sc, and (iii) Al, Si, Mg, Sc, and Fe. Crystal orientation mapping showed primary aluminum dendrites with one orientation in each grain near Al3Sc particles. The grain refinement mechanism of Sc for aluminum relies on heterogeneous nucleation of Al3Sc particles, with less responsibly for grain growth restriction. Many intermetallic phases with Al, Si, Fe, Mg and Sc as their major components were found, and these phases could not effectively act as heterogeneous nuclei.

Machinability Study of Stir Cast Hypoeutectic Aluminum-Silicon Alloys During Turning

The influence of Be and Cd (iron correctors) on mechanical properties and machining behavior of hypoeutectic Al-Si alloy (Al-7Si-0.5Mg-1.2Fe) processed by conventional and semi-solid metal (SSM) processing (stir casting) techniques is investigated. The alloys under investigation were prepared by controlling melt in an induction melting furnace. The stirring of SSM was carried out at a constant stirring speed of 400 rpm under constant cooling conditions from liquidus temperature. The turning operations were carried out under dry conditions on a CNC turning center using coated-carbide insert by varying cutting speed, feed rate, depth of cut, and approaching angle. An orthogonal array, the signal-to-noise ratio, and analysis of variance were employed to study the machining performance characteristics. The results indicate that Be/Cd modification of the alloy and selected cutting parameters significantly affect the machining characteristics. The feed rate, cutting speed, and Cd as an iron corrector have more effect on the machining behavior of the alloys under study.

Effects of Low-Frequency Mechanical Vibration and Casting Temperatures on Microstructure of Semisolid AlSi8Cu3Fe Alloy

This paper is concerned with the influence of vibration on the cooling slope casting and gravity casting. Mechanical vibration during cooling slope casting is a new technique. Also isothermal holding period of hypoeutectic Al–Si alloy to prepare semisolid slurry has been studied. The convection caused by the vibration during solidification had remarkable effects on the formation of spherical α-Al particles. The main vibration effects include evolution and increase of nucleation and thus reducing as-cast grain size; globularization of particles and production of a more homogenous metal structure. In this work, mechanical mould vibration and mechanical-inclined plate vibration was applied to an AlSi8Cu3Fe alloy at fixed frequency. Metallographic examinations and grain analysis were done on specimens obtained with different pouring temperatures and casting methods. The α-Al particles were spherical in cooling slope casting under vibration, as compared with cooling slope casting without vibration and gravity casting with vibration. A grain analysis along with different casting techniques was performed in order to understand the vibration effect. A heat-transfer mechanism seems responsible for the vibration effect in grain formation.

Growth mechanism of primary silicon in cast hypoeutectic Al-Si alloys

The microstructural features of hypoeutectic Al-10%Si alloy were observed using optical microscopy and electron backscatter diffraction. The results show that primary silicon particles are frequently found in hypoeutectic alloys. Hence, the nucleation and growth mechanisms of the precipitation of primary silicon of hypoeutectic Al-10%Si alloy melts were investigated. It was discovered that Si atoms are easy to segregate and form Si-Si clusters, which results in the formation of primary silicon even in eutectic or hypoeutectic Al-Si alloys. In addition, solute redistribution caused by chemical driving force and large pile-ups or micro-segregation of the solute play an important role in the formation of the primary silicon, and the solute redistribution equations were derived from Jackson-Chalmers equations. Once Si solute concentration exceeds eutectic composition, primary silicon precipitates are formed at the front of solid/liquid interface.

Effect of silicon content in grain refining hypoeutectic Al–Si foundry alloys with boron and titanium additions

The effect of Si content on the grain refinement of hypoeutectic Al–Si alloys was investigated. Alloying with Si refines the grain structure, which tends to be coarse and columnar in commercially pure aluminium. The smallest grain size occurs at ∼2 wt-%Si, where the solidification interval of hypoeutectic Al–Si alloys is the largest. Grains become increasingly coarser with increasing Si starting from this point. The grains of Al–Si alloys with 500 ppm Ti are smaller than those cast without Ti regardless of the Si content of the alloy. The fivefold reduction in grain size in commercially pure aluminium upon Ti addition is gradually reduced with increasing Si. Finally, the grain refinement provided by Ti fails to meet the expectations once Ti starts to be removed from the melt via the formation of Ti–Si compounds above 5 wt-%Si. The B addition relies on the formation of AlB2 particles to offer grain refinement. Analysis of the Al rich corner of the calculated Al–Si–B liquidus surface suggests that the primary AlB2 is formed at a Si concentration of ∼4 wt-%. While a perfect grain refiner for hypoeutectic Al–Si alloys with at least 4 wt-%Si, B fails to refine the grain structure when the Si content is less.

Microstructures and Properties of Hypoeutectic Al-Si Alloy Extruded in Semi-Solid State

The microstructures and properties of AlSi7Mg alloy extruded at 575°C in semi-solid state were studied for analyzing the extruding feasibility of the casting aluminum alloy in high solid fraction. The results show that the microstructure of AlSi7Mg alloy through low-superheat direct chill (DC) casting mainly consists of the homogeneous, fine rosette-shaped grains. The microstructure of the billet reheated at 575°C in 15min is composed of stable and near- spherical grains, which are suitable for thixoforming in high solid fraction. Extruded at 575°C in the semi-solid state, the facial-smoothed parts of AlSi7Mg alloy with homogeneous, fine microstructure across the section is obtained, and the properties of extruded part are improved obviously.

Microstructure and Texture Evolution of Fe-Si Steels after Hot Dipping and Diffusion Annealing

A homogenous intensity distribution along the cube texture fibre is important to achieve an easy magnetization in non-oriented electrical steels. Several alternatives have been discussed in literature to achieve this goal namely, tertiary recrystallization (surface energy controlled), decarburization annealing, two step cold rolling (strain induced boundary migration), twin-roll thin strip casting (directional solidification), phase transformation (surface energy anisotropy) and columnar grains formation (selective grain growth). In the present study, a hypoeutectic Al-Si alloy was deposited on the surface of cold rolled Fe-Si steels with a hot dipping simulator and subsequently annealed at 1000°C for different times. This procedure was developed previously in order to enrich the substrate with Al and/or Si and consequently improve their resistivity. Of specific interest was the formation of columnar grains in the low Fe-Si steel after annealing. These columnar grains were found to grow from the surface towards the centre of the substrate. The microstructure and texture in the columnar grains were significantly different than those in the middle of the material. Therefore, the evolution of these features during processing was studied in detail in this work.

EFFECT OF COOLING RATE ON MECHANICAL PROPERTIES OF EUTECTIC AND HYPOEUTECTIC Al-Si ALLOYS

In this research the effect of cooling rate and mold type on mechanical properties of the eutecticand hypoeutectic (Al-Si) alloys has been studied. The alloys used in this research work were (Al- 12.6%Sialloy) and (Al- 7%Si alloy).The two alloys have been melted and poured in two types of molds withdifferent cooling rates. One of them was a sand mold and the other was metal mold. Mechanical tests(hardness, tensile test and impact test) were carried out on the specimens. Also the metallographicexamination was performed.It has been found that the values of hardness for the alloys(Al-12.6%Si and Al-7%Si) which poured inmetal mold is greater than the values of hardness for the same alloy when it poured in a heated metalmold at different temperatures or in sand mold. The strength and impact resistance for the alloys (Al-12.6%Si and Al-7%Si) are greater when these were poured in a metal molds than that when it poured in asand mold.Furthermore, the higher cooling rates enhance the strength, hardness and impact resistance for thetwo alloys, while the low cooling rates reduces these mechanical properties.The percentage of elongation and the amounts of formed porosity decreased when the cooling ratesincreased

Effect of Fe content and microstructural features on the tensile and fatigue properties of the Al–Si10–Cu2 alloy

As the automotive industry has to meet the requirements of fuel efficiency and environmental concerns, the use of aluminium alloys is steadily increasing. A number of papers have been published about the correlation between microstructure and mechanical properties of the widely used A356/A357 aluminium alloys, while relatively few data are available on others hypoeutectic Al–Si alloys, such as Al–Si–Cu alloys with higher Si content. In this work the effect of different amounts of Fe and Mn on the tensile and fatigue behaviour of the Al–Si10–Cu2 casting alloy was studied. The reason of this study comes from the fact that cast components are mostly made by secondary Al alloys that inevitably contain Fe, which in turn forms intermetallic compounds, negatively affecting the mechanical behaviour of the alloy. Fatigue specimens were subjected to hot isostatic pressing (HIP) before tests, in order to eliminate the internal pores (gas pores and interdendritic shrinkages) and therefore to solely investigate the effect of microstructural features, rather than solidification defects, on the fatigue propagation stage. The microstructural characterisation of the alloy was carried out by optical and scanning electron microscopy. Proof and ultimate tensile strength, as well as fatigue life of the investigated alloy were greatly enhanced by high Fe and Mn content, which reduced the micro-crack propagation rate; on the contrary Fe, without Mn, negatively affected the elongation to failure.

Solute Redistribution Resulting in Growth of Primary Silicon in Cast Hypoeutectic Al-Si Alloys

The experimental data present that primary silicon is precipitated from eutectic and hypoeutectic Al–Si alloy melts. It attributes to solute redistribution on the chemical driving force. Si atoms are easy to segregate to form Si-Si clusters, resulting in primary silicon precipitated from hypereutectic Al–Si alloy. Another reason is that primary silicon is precipitated from solid-liquid interface front once solute concentration in the solidification front exceeds eutectic composition. Solute redistribution equations are derivate from Jackson-Chalmers equation. The third reason is that precious little impurity turn Heterogeneous Nucleation of Si atoms.

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.

Multiscale characterization of nanostructured Al-Si-Zr alloys obtained by rapid solidification method.

Properties of engineering metallic alloys (e.g., fracture toughness, corrosion resistance) are often limited by the presence of primary intermetallic particles which form during conventional solidification. Rapid solidification brings about much more homogenous amorphous and/or nanocrystalline structure with reduced density of primary particles. Rapidly solidified thin ribbons obtained by melt spinning are usually considered as intrinsically homogenous. However, due to different cooling conditions at the wheel surface and on the side exposed to the ambient environment, structure of such ribbons may vary significantly across its thickness. The materials studied in this study were 30–40 μm thickness ribbons of nanocrystalline hyper- and hypo-eutectic Al–Si–Zr alloys produced by melt-spinning method. Transmission electron microscopy and high resolution scanning transmission electron microscopy were used to characterize the structure homogeneity across the ribbons. Thin foils for transmission observations were prepared by focused ion beam system. Microstructural observations confirmed nanocrystalline character of Al–Si–Zr alloys. However, these observations revealed inhomogeneity of the structure across the ribbon width.

Grain refining mechanism of Al-3B master alloy on hypoeutectic Al-Si alloys

Al-3B master alloy is a kind of efficient grain refiner for hypoeutectic Al-Si alloys. Experiments were carried out to evaluate the effect of undissolved AlB 2 particles in Al-3B master alloy on the grain refinement of Al-7Si. It is found that the number and the settlement of AlB 2 particles in the melt all have effect on the grain refining efficiency. On the basis of experiments and theoretical analysis, a new grain refinement mechanism was proposed to explain the grain refinement action of Al-3B on hypoeutectic Al-Si alloys. The formation of ‘Al-AlB 2’ shell structure is the direct reason for grain refinement and the undissolved AlB 2 particles is the indirect nucleating base for subsequent α(Al) phase.