Modification Of Al-Si Alloys Research Articles

On the formation of Al2Si2Sr phase and its behavior in Al–Si–Sr alloys

Sr element is commonly used to modify eutectic Si in Al–Si alloys, while Al2Si2Sr phase is quite easy to precipitate when Sr content is high to tens of ppm level. In this study, the growth models of Al2Si2Sr crystal as well as its behavior in Al–xSi–2 Sr (x=5, 8, 12, 18, 25 and 40, wt%) alloys were investigated. Polyhedral, triangular pyramid and dendritic morphologies of Al2Si2Sr crystals were detected, and their growth models were discussed. Based on the crystallographic parameters, the disregistry between (0001)Al2Si2Sr and (111)Si is calculated to be 9.7%. Transmission electron microscopy analysis confirms that the Al2Si2Sr phase is not likely to be a good nucleation site for Si. Differential scanning calorimetry analysis indicates that Al2Si2Sr phase is firstly precipitated in Al–5, 8, 12, 18Si–2 Sr alloys, while its precipitating temperature is close to primary Si in Al–25Si–2 Sr alloy and it falls behind to primary Si in Al–40Si–2 Sr alloy. Interestingly, amounts of Al2Si2Sr dendrites were engulfed by primary Si in Al–40Si–2 Sr alloy. Based on the widely–known impurity–induced twinning mechanism in Sr modified Al–Si alloys, the Al2Si2Sr dendrites were deduced to be in–situ evolved from Sr–rich clusters, which are absorbed on {111}Si planes. This work may provide a new perspective for understanding the modification mechanisms of Sr element on Si crystal in Al–Si–Sr alloys.

Effects of New Modification and Refinement Techniques of Mechanical Properties and Corrosion Behaviours of Al-Si Alloys

In this paper, the effects of new techniques of phosphor-doped elements on the microstructure and mechanical properties of Al-%Si alloy to produce modifier compounds were studied. The modifiers were produced by developing a new technique and additions field (nano- doped metals) under as-cast conditions. The results show that the addition of CuPO4, Ni3(PO4)2 and Ca3(PO4)2 to the Al-Si alloys can change (refine) the primary Si phases and are less angular. In addition, the eutectic Si form has changed from a long needle-like to a fibrous shape. The modified Al-Si alloys have microstructures in the inter-dendrite region consisting of a uniformly distributed α-Al phase, eutectic Al-Si, and fine primary Si particles. The microstructure showed that the primary silicon fraction decreased 60% with increased phosphate elements content. The finest microstructure could be observed with a 0.03% addition of doped metals. Good enhancements in ultimate tensile strength and elongation with clear increases in the hardness were observed, reaching 55%, 9% and 30%, respectively. Moreover, additions give the possibility of changing corrosion behaviours from pitting corrosion to micro and general corrosion. Furthermore, the optimal holding and melting temperature is 780°C, with a holding time of 15 minutes.

Modification of Al-Si Alloys by Ce or Ce with Sr

Al-Si alloys were modified by addition of cerium (Ce) or Ce plus strontium (Sr) to study the effect on the eutectic silicon (Si) morphology. The modified alloys were characterized using scanning electron microscopy (SEM), x-ray diffraction (XRD) analysis, and thermal analysis to understand the effect of Ce and Sr on their microstructure. The results showed that addition of 1% Ce resulted in only partial modification of the Si phase, whereas addition of Ce with 0.04% Sr resulted in complete modification of the alloy. Addition of 1% Ce decreased the eutectic arrest temperature by about 10°C, compared with a 5°C drop with Sr addition only. SEM energy-dispersive spectroscopy (EDS) and XRD results revealed formation of Al2Si2Ce intermetallic in the Ce-modified Al-Si alloys. Differential scanning calorimetry (DSC) showed that the intermetallic formed just before the eutectic phase.

The effect of Fe-rich intermetallics on crack initiation in cast aluminium: An in-situ tensile study

To evaluate the role of Fe-rich intermetallics on crack initiation, two fully modified Al-Si alloys, one containing plate-like β-Fe and the second containing primary α-Fe intermetallics, were investigated by in-situ tensile testing in the scanning electron microscope. In the first alloy, large plate-like β-Fe intermetallics oriented parallel to the test direction were the first to crack at an elongation of about 1.8%. More transversely oriented intermetallics caused crack initiation in the matrix which linked up with the final fracture. In the second alloy, the cracking of α-Fe intermetallics initiated at an elongation of about 0.9%. It is concluded that large α-Fe intermetallics crack first and that clusters of α-Fe are the most potent crack initiation sites.

Modification of Eutectic Si in Al-Si-(Ba) Alloy by Inducing a Novel 9R Structure in Twins.

The change of twinning morphology plays an important role in the modification of Al-Si alloys, which are widely used in industrial applications. However, the interpretation of this change is still insufficient. In this work, the microstructure of twins was investigated in two kinds of Al-Si alloys with different additions of Ba using high-resolution transmission electron microscopy (HRTEM). Unlike the normal {111} twin that exists in Ba-free alloy, discontinuous twins and multiple twins were observed in the Ba-containing alloy. In addition, the 9R structure formed by the dissociation of twins was firstly observed at the turning of discontinuous twins and the intersection of multiple twins in Al-Si alloys.

3D Atom Probe Tomography Study on Segregation of Yttrium in Modified Al-Si Alloys

Yttrium segregation behavior in Al-Si alloys has been studied using the three-dimensional atom probe tomography technique. Al-Si alloys were prepared by casting method, and yttrium was added to modify the eutectic silicon morphology in these alloys. The results indicated that yttrium is preferentially located within the Si phase, with the highest concentration at the interface between eutectic Al and eutectic Si.

3D imaging and quantification of porosity and intermetallic particles in strontium modified Al-Si alloys

Al-Si alloys are widely used in the automotive and aerospace industries. Porosity is considered to be a major defect in these cast alloys. In this study, the effect of strontium (Sr) modification on porosity formation and intermetallic formation in Al-Si alloys is quantified using X-ray tomography. Quantitative information such as average pore size, shape, pore distribution and intermetallic particle size distribution were evaluated for Sr modified and unmodified Al-Si alloys. Results show that the addition of 400 ppm of Sr increases the porosity by 10% as compared to unmodified Al-Si alloy. Further, Sr addition increases the average equivalent diameter of pores from 2 μm to 18 μm compared to unmodified alloys. A greater number of larger pores were found in the Sr modified alloy, whereas the number of small sized pores was significantly higher in unmodified alloy. The addition of 400 ppm of Sr results in formation of Al-Si-Sr intermetallics with an average equivalent diameter of 4.5 μm. It can be hypothesised that these intermetallic particles act as heterogeneous sites for pore nucleation and significant further growth of porosity in Sr modified alloys.

Cluster formation at the Si/liquid interface in Sr and Na modified Al–Si alloys

Atom probe tomography was used to compare Na and Sr modified Al–Si hypoeutectic alloys. Both Na and Sr promote the formation of nanometre-sized clusters in the Si eutectic phase. Compositional analyses of the clusters show an Al:Sr ratio of 2.92±0.46 and an Al:Na ratio of 1.07±0.23. It is proposed that SrAl2Si2 and NaAlSi clusters are formed at the Si/liquid interface and take part in the modification process by altering the eutectic Si growth.

Experimental Study on the Segregation of Primary Silicon from the Hypereutectic Al-Si Melts in the Electromagnetic Directional Solidification

Solar grade silicon and modified Al-Si alloys are widely used in the world. These two favored materials are expected to obtain by the electromagnetic directional solidification of hypereutectic Al-Si melts. The segregation of hypereutectic Al-Si alloy is feasible with respect to the process of either pull-up or drop-down in electromagnetic directional solidification. To explore the segregation efficiency, experiments have been conducted in a high-frequency induction furnace with different pulling conditions of hypereutectic Al-Si melts. The results show that the segregation efficiency of dropping-down is higher than that of pulling-up; the slower dropping-down rate lead to higher segregation efficiency in the electromagnetic directional solidification; increasing the temperature gradient may promote the separation efficiency. A separation mechanism model responsible for the formation of the distribution of primary silicon in the cross-sections is also proposed. Wish to provide theoretical basis for the more efficiency of the segregation of hypereutectic Al-Si melts.

The Micro-Fabrication of Multidimensional Structure of Silicon by Selection Corrosion

Multibranched silicon microstructure, such as branched root-like microwires and branched coralline-like micorwires, have been rationally synthesized via a simple one-step, inexpensive, and catalyst-free fabrication technique. The multiple-branched root-like silicon microwires which obtained at 0.07% Sr addition of Al-Si alloys have lengths of several tens of micrometers and its transverse sizes range from 300 to 500 nm, while the transverse sizes of multiple-branched coralline-like silicon microwires are much smaller. The recommend of patterning process of multiple-branched silicon microwires show that selective corrosion of Sr modified Al-Si alloys is that the aluminum is wiped out while the silicon micorwires as a hidden microstructure revealed by etching.

NUMERICAL SIMULATION OF AL-SI ALLOYS WITH AND WITHOUT A DIRECTIONAL SOLIDIFICATION

Numerical simulations are presented to analyze the influence of the casting process on the resulting strength of Strontium modified Al–Si alloys. A relationship is identified between the mechanical behavior and the different 3D morphologies of the eutectic silicon of the samples obtained by the die cast procedure and the directional solidification. It is shown that the mechanical behavior of the die cast alloy is isotropic in all three directions. In contrary, for the directional solidified alloy, the mechanical strength in the direction of the temperature gradient is higher than in the transverse direction. This fact has to be taken into account when analyzing structures issued from different casting processes. The volume meshes for the simulations are generated from experimental 3D FIB/SEM data sets. The influence of several levels of coarsening of the meshes as well as the order of the Lagrange element in the finite element setup are also analyzed.

Effect of Rare Earth Addition on Microstructure and Mechanical Properties of Al-Si Alloys: An Overview

This paper reviews the effect of rare earth addition on aluminium-silicon (Al-Si) alloys of hypoeutectic, eutectic, and hypereutectic types. The effects of rare earth on metallurgy, tensile strength, tensile fracture surface and wear behaviour of Al-Si alloys are highlighted and discussed in this paper. It was concluded that adding rare earth element to Al-Si alloys reduces the grain size of primary Si, increases the tensile strength and decreases the friction coefficient decreases. These indicate enhanced mechanical properties for rare earth modified Al-Si alloys are likely.

Effect of a grain refiner cum modifier on mechanical properties of Al-7Si and Al-11Si alloys

This study evaluates the influence of grain refiners/modifiers on the mechanical properties of the Al-7Si and Al-11Si alloys with an experiment of quantitative and qualitative correlations with the microstructure. Modification of Al-Si alloys with strontium additions and grain refinement with Al-Ti, Al-B and Al-T-B master alloy additions are demonstrated to be efficient on Al-Si alloys. A single master alloy with combined additions of Sr and Ti and/or B was prepared and the microstructure and mechanical properties were studied. The results show that boron rich (Al-3B-Sr and Al-1Ti-3B-Sr) master alloys are more efficient than Ti rich (Al-3Ti-Sr and Al-5Ti-1B-Sr) master alloys considering their combined grain refinement and modification effect on Al-7Si and Al-11Si alloys. However, the presence of Sr does not influence the grain refinement. Similarly, presence of grain refiner does not influence the modification of eutectic Si.

Effect of Chemical Modification of Al-Si Alloys on Thermal Diffusivity and Contact Heat Transfer at the Casting–Chill Interface

Abstract The heat flow during the unidirectional downward solidification of Al-7Si and Al-12Si alloys was analyzed using thermal analysis technique and inverse modeling. Chills instrumented with thermocouples were brought into contact with a small pool of liquid metal so as to minimize the effect of convection caused by pouring and temperature gradients. Modification melt treatment resulted in an increase in the cooling rate of the solidifying casting near the casting–chill interfacial region. The corresponding interfacial heat flux transients were also found to be higher. The thermal diffusivities of alloys were measured using a laser pulse technique and were found to be higher for modified alloys. However, the increase in the heat flux transients was attributed mainly to the improvement in the casting–chill interfacial thermal contact condition brought about by the decrease in the surface tension of the liquid metal upon the addition of sodium.

Heat transfer during solidification of chemically modified Al–Si alloys around a copper chill

The solidifying metal/chill contour will significantly affect the boundary heat transfer coefficients, and solidification modellers should be aware of the casting conditions for which the heat transfer coefficients are determined. The previous work carried out on solidification of Al–Si alloys in a metallic mould and solidification against bottom/top chills has shown that modification and chilling have synergetic effect resulting in a significant increase in the heat flux transients at the casting/chill interface. In the present work, the heat transfer during solidification of unmodified and chemically modified Al–Si alloys around a cylindrical copper chill was investigated. Heat flux transients were estimated using lumped heat capacitance method. Lower peak heat flux was obtained with chemically modified alloy. This is in contrast to the results reported for alloys solidifying against chills and in metallic moulds. The chill thermal behaviour and heat transfer to the chill material when surrounded by modified and unmodified alloys were explained on the basis of the decrease in the degree of undercooling in the case of modified alloy as compared to unmodified alloy and the change in contact condition and shrinkage characteristics of the alloy due to the addition of chemical modifiers.

Cerium extraction by metallothermic reduction using cerium oxide powder injection

This work presented the feasibility of cerium recovery by Al-Mg alloy through the metallothermic reduction of CeO 2 to obtain a master alloy Al-4%Ce. The master alloy obtained in this investigation was for the grain refinement and modification of Al-Si alloys. The reagent was incorporated into a molten alloy using the submerged powder injection technique, and metallic samples were obtained during injection. Chemical and microstructural analyses (by inductively coupled plasma (ICP) and scanning electron microscopy (SEM), respectively) confirmed the possibility of Ce uptake in the bath (0 to 4 wt.%), as CeO 2 was reduced through metallothermic reactions in the molten alloys. Based on the characterization of reaction products, the sequence of the reaction was proposed.

Characterization of the Morphology of Si in the Modified Al-Si Alloys

One of the methods of melt-treatment of Al-Si alloys is the modification that plays a very significant role in the development of (mechanical) properties of finished product. There are a lot of well-known processes for the investigation of effectiveness of modification but an objective, exact result cannot be obtained in each case by using these methods. The method suitable for the classifi-cation of modification worked out by us was developed by integrating two methods namely the comparison by means of etalon-images and the integration of measurement of morphological pa-rameters. This method is relatively simple, does not demand lots of measurements and gives reliable results.

Oxide Layer Formation by Micro-Arc Oxidation on Structurally Modified Al-Si Alloys and Applications for Large-Sized Articles Manufacturing

The effect of the shape, size and kind of distribution of the silicon inclusions in Al-Si alloys of hypoeutectic, eutectic, and hypereutectic compositions (6–22 % Si) on the quality of oxide layer formed by micro-arc oxidation (MAO) is studied. The hitherto unknown phenomenon of oxide layer growth inhibition by the silicon particles under MAO is discovered and researched. The filling of near-surface pores by oxidation products in MAO process is observed also. The discovered ability to increase the properties of oxide layers obtained by the MAO due to prior directed modifica¬tion of the initial structure provides the good quality of oxide layers for aluminium alloys with high level of silicon content up to hypereutectic compositions of Al-Si alloys. The physical opportunity to get the high quality oxidized layers by MAO on structurally modified Al-Si alloys is confirmed for large-sized Al-Si alloys castings (including cylinder block for engine).

The rapid prediction of the modification level of Al-Si alloys by surface tension

Both the theoretical analysis and experiments on Al-Si alloys have shown that a relationship between the surface tension and the modification of Al-Si alloys exists. A new method for the rapid prediction of the the modification level of Al-Si alloys by surface tension has been introduced. The quantity of the modifier is found to have the greatest effect, followed by the time of the modif cation and then the temperature. In Al-Si alloys, when the surface tension is above 530 mN/m, the modification level is partial. When the surface tension is between 400 mN/m and 530 mN/m, the modification level is moderate. When the surface tension is below 400 mN/m, the modification level is perfect.

Discussion of “Nucleation mechanism of eutectic phases in aluminum-silicon hypoeutectic alloys”

A previous study tried to prove that each plate of Si in the ordinary Al-Si eutectic is nucleated individually by the action of a small iron-containing particle. The study justified the need for a more complete understanding of the Al-Si eutectic reaction in the absence of chemical modifiers in order to shed light on the complicated mechanisms operating in chemically modified Al-Si alloys. Reacting to this claim, the present authors state that the mechanisms of the ordinary eutectic reaction of Al-Si alloys have been well understood for several decades, thus a very strong evidence is required to justify a revision of that understanding. For their part, the authors of the first study justified their claim for a revision by considering results of thermal analyses, optical microscopy, scanning and transmission electron microscopy, and selected area electron diffraction (SAED) analyses and elemental X-ray mapping performed on samples of precisely controlled chemistry.