β-Al5FeSi Phase Research Articles

Microstructure, mechanical properties and corrosion behavior of Rheo-HPDC a novel Al-8Si-Fe alloy

Microstructure, mechanical properties and corrosion behavior of a novel Al-8Si-Fe alloy fabricated by Rheo-HPDC were investigated. Compared with the HPDC alloy, the microstructure (α1/α2-Al, Si and β-Al5FeSi phases) of the alloy was greatly refined by Rheo-HPDC. The Rheo-HPDC alloy showed improved mechanical properties and thermal conductivity to those prepared by HPDC, and the increasing rates of UTS, YS, elongation, hardness and thermal conductivity were 31%, 19%, 59%, 12% and 10%, respectively. Rheo-HPDC Al-8Si-Fe alloy also indicated a significant improvement in corrosion resistance as demonstrated by the results of immersion, electrochemical and scanning kelvin probe (SCP) tests.

The Effect of Ultrasonic Melt Treatment on Macro-Segregation and Peritectic Transformation in an Al-19Si-4Fe Alloy

It is well documented that ultrasonic melt treatment (USMT) can refine dendritic and eutectic microstructures during solidification, but much less attention has been paid to the effect of USMT on macro-segregation and intermetallic transformations. In this research, macro-segregation and primary Fe-containing intermetallic peritectic transformations in an Al-19 wt pct Si-4 wt pct Fe alloy were investigated without and with USMT. Macrostructural examination showed that in the absence of USMT the ingot revealed considerable non-uniform distribution of both the primary Fe-containing intermetallic and primary Si particles, whereas the ingot with USMT exhibited near homogeneous distribution of both primary phases, i.e., reduced macro-segregation. The beneficial effect of USMT on relieving macro-segregation was further examined using quantitative microstructural metallography and the results indicated that the area fraction, number density, and size distribution of both primary phases became essentially uniform across the ingot after USMT. USMT further exerted a significant impact on the constitution of the primary Fe-containing intermetallics, where complex particles of δ-Al3FeSi2/β-Al5FeSi were prominent without USMT, while few δ-Al3FeSi2 particles were observed after USMT and the primary Fe-containing intermetallics existed mostly as the single-phase β-Al5FeSi. The underlying reason was attributed to the reduction in the size of the primary δ-Al3FeSi2 particles which ensures the complete transformation of most primary δ-Al3FeSi2 particles to the peritectic β-Al5FeSi phase.

Effects of Solidification Rate in the Microstructure of Al-Si5Cu3 Aluminum Cast Alloy

Aluminium alloy Al-Si5Cu3 (319) is one of the most commonly used casting alloys for automobile components, due to its casting characteristics. Iron is the major impurity element that influences the detrimental β-phase formation in secondary aluminum alloys. In the present study, we investigated the influence of the cooling rate on the formation of iron compounds, α-Al15(Mn,Fe)3Si2 and β-Fe5AlSi phases, in Al-Si5Cu3 containing 1.0% Fe, 0.5% Mn and 0.2% Mg, by thermal analysis and metallography. The results show that the high cooling rates, between 10-103 K/s, are able to reduce and nullify the formation of needle-like β-Al5FeSi phase, and help the formation of the Chinese scripts α-Al15(Fe,Mn)3Si2 phase. The analysis of the curves reveals that the increase of the cooling rate increases the temperature of nucleation of the α-Al dendrite and decrease the eutectics Al-Si and Al-Cu phases.

In-situ study of phase transformations during homogenization of 6005 and 6082 Al alloys

Intermetallic β-Al5FeSi phase and coarse Mg2Si particles have negative effects on extrudability and workability of 6xxx Al alloy billets. To achieve extruded products with a high surface quality as-cast billets are therefore heat-treated before extrusion. During heat treatment the undesired intermetallic particles, i.e., β-AlFeSi platelets, are transformed to a rounded α-Al(FeMn)Si intermetallic phase. This transformation was studied in-situ by TEM for 6005 and 6082 Al alloys. It was observed that the Mg2Si particles precipitate in the Al matrix at about 250 °C; this precipitation also occurred at the edge and faces of beta intermetallic particles, and the Mg2Si particles were the preferred sites for α-Al(FeMn)Si particle nucleation. The transformation proceeded faster and at lower temperatures, 350–450 °C, than what has been reported earlier for homogenization studies of bulk samples and industrial billets. This could be associated with the thin characteristic of used samples in TEM giving contribution from fast surface diffusion, but it was also concluded that the phase boundary layer diffusion was important for the understanding of how the transformations proceed.

Microstructural evolution during semisolid processing of Al–Si–Cu alloy with different Mg contents

A series of Al–6Si–3Cu–(0.3–2)Mg alloys were produced by a conventional casting process. Cooling slope technique was employed to produce feedstocks before they were thixoformed at 50% liquid fraction. The effect of Mg on the microstructure of Al–Si–Cu aluminium alloys under as-cast and semisolid conditions was investigated. It was found that by adding Mg to Al–Si–Cu alloy, some of the Al2Cu phase and silicon were consumed to form Al5Cu2Mg3Si5 and Mg2Si phases. The needle-like β-Al5FeSi phase transformed to Chinese-script-like π-Al8Mg3FeSi6 with the addition of Mg. In the as-cast alloys, the primary α(Al) was dendritic, but as the Mg content increased, the phase became less dendritic. Moreover, the Mg addition considerably modified the size of the α(Al) phase, but it had no significant effect on the silicon morphology. In the thixoformed alloys, the microstructure showed a fine globular primary phase surrounded by uniformly distributed silicon and fragmented intermetallic phases. The eutectic silicon was modified from a flaky and acicular shape to fine fibrous particles. The effect of Mg on eutectic silicon during semisolid processing was evident. The primary Mg2Si particles were modified from big polygonal particles to become smaller and more globular, whereas the morphology of the Chinese-script-like π-Al8Mg3FeSi6 changed to a compact shape. The results also exhibit that as the Mg content in the A319 alloy increased, the hardness, yield strength and ultimate tensile strength of the thixoformed alloys significantly improved, but the elongation to fracture dropped.

Effect of the Pressure on Microstructure of A380 Aluminum Alloy Prepared by Squeeze Casting

Effects of the solidification pressure on microstructure and mechanical properties of A380 alloy have been experimentally investigated. The obtained results show that the microstructures of the A380 aluminum alloy is fined, the porosity is decreased and the mechanical properties is improved remarkably with the increase of the solidification pressure. When the squeeze pressure increases from 0 MPa to 75MPa, the size of dendrite arm space decreases from 914 μm to 313 μm, reduced by 66%; the eutectic tissue volume fraction increases from 22.5% to 41.36%; the porosity decreases from 4.91% to 1.23%; the secondary dendrite spacing decreases from 39 μm to 18 μm; the size of needle-like β-Al5FeSi phase decreases significantly, and a few Chinese script α-Fe phases was observed in the grain boundary.

Effect of β-Al5FeSi and π-Al8Mg3FeSi6 Phases on the Impact Toughness and Fractography of Al–Si–Mg-Based Alloys

This study investigated the decomposition of π-AlMgFeSi into β-Al5FeSi phase needles during extended periods of solution heat treatment and its effects on the mechanical properties of Al–7Si–0.55Mg–0.1Fe alloys. An analysis of the results obtained from the Charpy impact test using unnotched samples shows that the highest initiation and propagation energies are obtained for the as-cast and heat-treated alloys when these alloys are solidified at a high cooling rate and modified with strontium. An increase in the solution treatment time improves the impact properties of the alloys compared to the as-cast condition. In accordance with this finding, the recommended solution treatment time at which the maximum initiation and propagation energy values can be obtained is 20 h for all alloys studied. The results also show that the impact properties are more sensitive to the changes occurring in the microstructure which result from solution heat treatment and Sr modification, namely, the eutectic Si and π-phase morphologies, rather than those related to the tensile properties, i.e., to the Mg content in the matrix. Fracture analysis was carried out using a scanning electron microscope equipped with an EDX system for element analysis. The results obtained show that the tensile and impact fracture behavior of the Al–7Si–0.55Mg–0.1Fe alloys is controlled mainly by the morphology of the eutectic silicon. The π-phase iron intermetallics act as crack initiation sites and provide an easy path for crack propagation in both non-modified and Sr-modified alloys. The fracture analysis of the 80-h solution-treated sample shows the presence of newly formed β-phase needles which provide an additional source for crack initiation, and thus all the 80-h solution-treated alloys show the lowest energy values.

Microstructural Evolution in AlMgSi Alloys during Solidification under Electromagnetic Stirring

Equiaxed solidification of AlMgSi alloys with Fe and Mn was studied by electromagnetic stirring to understand the effect of forced flow. The specimens solidified with a low cooling rate, low temperature gradient, and forced convection. Stirring induced by a coil system around the specimens caused a transformation from equiaxed dendritic to rosette morphology with minor dendrites and, occasionally, spheroids. This evolution was quantitatively observed with specific surface Sv. The precipitation sequence of the phases was calculated using the CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) technique. Melt flow decreased secondary dendrite arm spacing λ2 in the AlSi5Fe1.0 alloy, while λ2 increased slightly in Mg-containing alloys. The length of detrimental β-Al5FeSi phases decreased only in AlSi5Fe1.0 alloy under stirring, whereas in Mg-containing alloys, changes to the β-Al5FeSi phase were negligible; however, in all specimens, the number density increased. The modification of Mn-rich phases, spacing of eutectics, and Mg2Si phases was analyzed. It was found that the occurrence of Mg2Si phase regions reduced fluid flow in the late stages of solidification and, consequentially, reduced shortening of β-Al5FeSi, diminished secondary arm-ripening caused by forced convection, and supported diffusive ripening. However, the Mg2Si phase was found to have not disturbed stirring in the early stage of solidification, and transformation from dendrites to rosettes was unaffected.

Beta Al5FeSi phase platelets-porosity formation relationship in A319.2 type alloys

The effects of several variables on the precipitation of the β-Al5FeSi phase, and porosity formation in A319.2 alloys were examined in detail. Various techniques were used for microstructural characterization and phase identification, including optical and scanning electron microscopy, electron probe microanalysis coupled with energy-dispersive X-ray and wavelength-dispersive spectroscopy facilities, and thermal analysis. An image analyzer was used in conjunction with the optical microscope for quantification purposes. The results obtained showed that the amount of iron present in the alloy affects the size of the β-Al5FeSi platelets and their distribution, particularly at low solidification rates. Addition of strontium leads to fragmentation of co-eutectic or post-eutectic β-platelets. This effect diminishes with the increase in iron concentration, and further strontium addition leads to the precipitation of Al2Si2Sr phase particles, instead. The porosity observed at any given iron level is the result of the competition between fluidity and β-platelet size and depends on the permeability of the interdendritic regions as solidification proceeds. Although branching of the β-platelets leads to porosity formation, the platelets, on the other hand, also limit pore growth. In general, percentage porosity, maximum pore area and maximum pore length increase with an increase in the average maximum β-Al5FeSi platelet length. In the Sr-modified alloys, porosity formation is frequently associated with strontium oxides (particles or films), as well as the β-Al5FeSi platelets. These oxides (with a stoichiometric composition close to Al2SrO3) are formed during melting, due to the high oxygen affinity of strontium, and are difficult to remove via degassing.

Formation of intermetallic δ phase in Al-10Si-0.3Fe alloy investigated by in-situ 4D X-ray synchrotron tomography

During solidification of Fe-containing Al-Si casting alloys, various different intermetallic phases are formed. As this can have an impact on mechanical properties we investigated phase formation by applying in-situ time resolved synchrotron X-ray tomography. For a commercially pure Al-10Si-0.3 wt.% Fe alloy, phase morphology and spatial arrangement during solidification was revealed with 1 °C temperature resolution. We find that many of the intermetallic phases that have been identified as β-AlFeSi phases in previous studies are in fact δ phases. It was found that δ plates mainly nucleate on the eutectic Al and Si. δ phase formation terminates almost simultaneously with the final solidification of eutectic phases. Features of δ phase growth such as impingement, branching and deformation of plates are discussed. The phase separation sequence during solidification is described and explained by the existence of “cells” of residual liquid in which δ phase formation progresses rapidly due to the supersaturation of solute atoms.

Influence of Sr, Fe and Mn content and casting process on the microstructures and mechanical properties of AlSi7Cu3 alloy

The effects of Strontium (Sr), Iron (Fe) and Manganese (Mn) additions, casting process (i.e., cooling rate) on the microstructures and mechanical properties of AlSi7Cu3 alloy were investigated. 2D and 3D metallographic and image analysis have been performed to measure the microstructural changes occurring at different Sr, Fe and Mn levels and casting process. The evolution of mechanical properties of the alloys has been monitored by Brinell and Vickers hardness measurement and tensile tests. Addition of Sr slightly refines the eutectic silicon particles but it also introduces more pores. The combined addition of Fe and Mn induces an increase of Fe-rich intermetallic compounds which include both α-Al15(Fe,Mn)3Si2 and β-Al5FeSi phase, while the volume fraction of porosity decreases with the Fe and Mn content increase. The secondary dendrite arm spacing slightly decreases with the addition of Sr, Fe and Mn alloying elements.

Phase precipitation in transition metal-containing 354-type alloys

Abstract The present study was carried out to investigate the effects of Ni, Mn, Zr, and Sc additions, individually or in combination, on the microstructure of 354 casting alloy (Al-9 wt.% Si-1.8 wt.% Cu-0.5 wt.% Mg). Microstructural examination and thermal analysis data showed that the main reactions detected during the solidification of the six 354 alloys (G1, G6–G10) investigated are: formation of the α-Al dendritic network; precipitation of Al-Si eutectic and post-eutectic β-Al5FeSi; Mg2Si phase; transformation of the β-phase into π-Al8Mg3FeSi6 phase; and precipitation of Al2Cu and Q-Al5Mg8Cu2Si6 phases. With 2 wt.% Ni addition, the formation of Al9FeNi and Al3CuNi phases is observed. In the base 354 alloy the main phases are restricted to Cu-, Mg-, and Fe-rich intermetallic phases. The Si particle characteristics and volume fraction of intermetallics are influenced by the solidification rate and Mg level, whereas addition of Fe and/or Mn has no significant influence. In alloy G9, Fe, Mn and Ni interact to form new intermetallic phases. An increased Fe content leads to formation of polyhedral/star-like sludge particles in addition to α-Fe and β-Al5FeSi phases; the presence of the hard sludge particles within the soft α-Al dendrites improves the alloy properties.

Morphological evolution and strengthening behavior of α-Al(Fe,Mn)Si in Al–6Si–2Fe–xMn alloys

β-Al5FeSi is preferred to form in Al–Si–Fe alloys, normally exhibiting needlelike, which is harmful for the mechanical properties. In this paper, with the addition of 1%, 1.5% and 3% Mn into an Al–6Si–2Fe alloy, β-Al5FeSi phase was found to transform to skeleton, flower-like and coarse dendritic α-Al(Fe,Mn)Si, respectively. The novel flower-like α-Al(Fe,Mn)Si crystals contain developed branches with the average diameter of ∼200nm, performing strengthening effect on the tensile property. Detailed morphologies of α-Al(Fe,Mn)Si phase and the formation mechanism were discussed.

Influence of Oxides on Porosity Formation in Sr-Treated Alloys

The aim of the present study has been to investigate the metallurgical parameters controlling the microstructural evolution and porosity formation in Al–Si–Mg and Al–Si–Cu alloys, through a study of the microstructural characteristics of directionally solidified A356 and 319 type alloys as a function of iron content, Sr addition (~250 ppm), and solidification rate. In the Sr-modified alloys, porosity formation is frequently associated with strontium oxides (particles or films), as well as β-Al5FeSi phase platelets. These oxides (with a stoichiometric composition close to Al2SrO3) are formed during melting, due to the high oxygen affinity of strontium, and are difficult to remove via degassing. The pore morphology (round or irregular) is determined by the form of the oxide, i.e. fine dispersed particles or thick films. Round pores are also observed surrounded by Al–Si eutectic regions. Aluminum oxide films trapped in the molten metal lead to the formation of coarser and deeper pores than those formed due to the strontium oxides. These pores can also link with each other and are characterized by the presence of solidified metal trapped within the aluminum oxide films, close to the periphery. The form of these pores is controlled by the amount of gases trapped within the pores during solidification.

Influence of Al grain structure on Fe bearing intermetallics during DC casting of an Al-Mg-Si alloy

207mm diameter direct chill (DC) cast billets of 6063 aluminium-magnesium-silicon (Al-Mg-Si) alloy were produced with various different primary aluminium (α-Al) grain structures including feathery-dendrites, equiaxed-dendrites and equiaxed-globular morphologies. To control the α-Al grain structure (grain morphology and grain size) an intensive shearing melt conditioning technique and Al-5Ti-1B grain refiner were used. For the first time, due to the variety of controlled microstructures produced in the DC billets, it was possible to study and determine the role of the α-Al grain structure on the iron (Fe) bearing intermetallics (Fe-IMCs). The size, shape and three dimensional (3D) inter-connectivity of the Fe-IMCs were observed to be affected by the modification of the primary α-Al grain morphology and grain size. Although both “αc-AlFeSi” and “β-AlFeSi” phases are present in all billets, β-AlFeSi phase is dominant in the billets with grain refiner addition while αc-AlFeSi is dominant in the billets without grain refiner. This suggests that the addition of Al-5Ti-1B grain refiner plays a more significant role in the intermetallic phase selection than the primary α-Al grain morphology or grain size.

Microstructure and mechanical properties of AlSi10Cu3 alloy with (La+Yb) addition processed by heat treatment

The optical microscopy, scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS) were used to assess the influence of micro-addition of (La+Yb) on the microstructure and mechanical performance of the AlSi10Cu3 alloy in heat treatment conditions. It was shown that the appropriate (La+Yb) addition (0.3 wt.% or 0.6 wt.%) transformed the needle-like β-Al5FeSi phase into Chinese script or spherical α-Al8Fe2Si phase. Eutectic silicon refined the long needle-like particles into granular or round particles at 0.6 wt.% (La+Yb) content. Moreover, the La3Al11 and YbAl3 phases acted as strengthening phases during the heat treatment processing in the alloy with the addition of (La+Yb). Consequently, the alloy with 0.6 wt.% (La+Yb) exhibited an enhanced mechanical properties response with ultimate tensile strength, elongation, and hardness at 69.35%, 113.26% and 23.61% higher than those of the unmodified alloy, respectively. Further addition (0.9 wt.%) of (La+Yb) resulted in the increasing of the black acicular RE-rich intermetallics during heat treatment, which could aggravate the situation of stress concentration leading to deterioration of the mechanical properties of alloy.

Influence of Grain Refiner Addition on the Precipitation of Fe-Rich Phases in Secondary AlSi7Cu3Mg Alloys

The effect of grain refiner addition on the precipitation of Fe-rich compounds in secondary AlSi7Cu3Mg has been studied. Thermal and metallographic analysis techniques have been used to examine the formation of Fe-bearing intermetallics. The results show that grain refinement with AlTi5B1 can alter the precipitation sequence of Fe-rich phases. In non-grain refined alloy and grain-refined alloy with AlTi10, two different Fe-rich phases, α-Al15(Fe,Mn)3Si2 and π-Al8Mg3FeSi6, form, whereas, with AlTi5B1 addition, the β-Al5FeSi phase also tends to crystallize, nucleating on TiB2 particles. It has also been demonstrated how at high cooling rates, the β-Al5FeSi-involved reactions occur more preferentially, when compared to α-Al15(Fe,Mn)3Si2 phase. This crystallization behaviour is explained in terms of phase diagram relationship and the nucleation kinetics of competing α-Al15(Fe,Mn)3Si2 and β-Al5FeSi phases.

Effect of Sc addition and T6 aging treatment on the microstructure modification and mechanical properties of A356 alloy

Effect of Sc addition and T6 aging treatment on the secondary dendritic arm spacing (SDAS), modification of eutectic Si morphology, β-Al5FeSiand π-Al8Mg3Si6Fe1 phases and its effect on mechanical properties in A356 alloy has been investigated. Addition of 0.4wt%Sc in A356 alloy resulted in a 50%reduction in the secondary dendritic arm spacing (SDAS). Sc addition changed the morphology of eutectic Si from plate like to fibrous and globular. The needle like morphology of β-Al5FeSi phase in A356 alloy changed to Al5Fe(Si,Sc) phase having smaller size and irregular morphology. Transmission electron microscopy (TEM) diffraction pattern and Energy dispersive spectroscopy (EDS) analysis revealed the presence of β-Al5FeSiand π-Al8Mg3Si6Fe1 phases in A356 alloy which changed to β-Al5Fe(Si,Sc), π-Al8Mg3(Si,Sc)6Fe1 and additional V-AlSi2Sc2phase was observed in Sc containing alloys. Addition of 0.4wt%Sc to A356 alloy improved its Vickers hardness, Ultimate tensile strength (UTS), Yield strength (YS) and ductility by 20%, 25%, 20% and 30% respectively. Artificial aging treatment resulted in significant improvement in the tensile properties for both A356 and Sc added A356 alloys.

The Influence of Sr Addition on the Microstructure of A380 Alloy

Whereas A380 series alloys are some of the most widely used aluminum alloys, iron can severely degrade the ductility of an Al–Si die-casting alloy because its presence leads to the precipitation of different AlFeSi intermetallic phases resulting in development of stress concentrations. Thus, controlling the fraction and morphology of AlFeSi phase, especially the β-AlFeSi phase, is an important way to refine the ductility of A380 die-casting alloys. This article describes how Sr element was added to A380 alloys to study the effect of Sr on the morphology change of the AlFeSi phase. The results show that high cooling rates and Sr addition can both change the morphology of β-AlFeSi and transfer β-AlFeSi to α-AlFeSi. In addition, both high cooling rates and Sr addition can modify the eutectic Al–Si phase. The study shows that the optimum Sr addition and cooling rate for the A380 series alloy are 0.1 wt% and 10 °C/s, respectively.

Effect of Heat Treatment on Controlling the Morphology of AlFeSi Phase in A380 Alloy

In most aluminum–silicon-based die casting alloys, iron is considered one of the most harmful elements. The presence of iron in Al–Si-based alloys leads to the precipitation of a series of iron-containing intermetallic phases. Of these, the β-AlFeSi phase is the most detrimental to the toughness of Al–Si-based alloys. Reducing the formation of the β-AlFeSi phase has the potential of significantly improving the ductility of die casting alloys. The purpose of this work is to show how combined use of high-temperature and short-time heat treatment can be an effective way of controlling the morphology and fraction of β-AlFeSi phase in A380 alloy. Experimental results indicate that if heat treatment temperature goes up to 500 °C (932 °F), the higher temperature can reduce formation of β-AlFeSi with shorter heat treatment time. Evidence of this reduction is proved in optical and scanning electron microscopy images showing both the decomposition of β-AlFeSi and the transformation of β to α-AlFeSi.