A metallographic study of porosity and fracture behavior in relation to the tensile properties in 319.2 end chill castings

A metallographic study of the porosity and fracture behavior in unidirectionally solidified end chill castings of 319.2 aluminum alloy (Al-6.2 pct Si-3.8 pct Cu-0.5 pct Fe-0.14 pct Mn-0.06 pct Mg-0.073 pct Ti) was carried out using optical microscopy and scanning electron microscopy (SEM) to determine their relationship with the tensile properties. The parameters varied in the production of these castings were the hydrogen (∼0.1 and ∼0.37 mL/100 g Al), modifier (0 and 300 ppm Sr), and grain refiner (0 and 0.02 wt pct Ti) concentrations, as well as the solidification time, which increased with increasing distance from the end chill bottom of the casting, giving dendrite arm spacings (DASs) ranging from ∼15 to ∼95 /im. Image analysis and energy dispersive X-ray (EDX) analysis were employed for quantification of porosity/microstructural constituents and fracture surface analysis (phase identification), respectively. The results showed that the local solidification time(viz. DAS) significantly influences the ductility at low hydrogen levels; at higher levels, however, hydro-gen has a more pronounced effect (porosity related) on the drop in ductility. Porosity is mainly observed in the form of elongated pores along the grain boundaries, with Sr increasing the porosity volume percent and grain refining increasing the probability for pore branching. The beneficial effect of Sr modification, however, improves the alloy ductility. Fracture of the Si, β-Al₅FeSi, α- Al₁₅(Fe,Mn)₃Si₂, and Al₂Cu phases takes place within the phase particles rather than at the particle/Al matrix interface. Sensitivity of tensile properties to DAS allows for the use of the latter as an indicator of the expected properties of the alloy.     

Effect of Mg and Sr additions on the formation of intermetallics in Al-6 Wt pct Si-3.5 Wt pct Cu-(0.45) to (0.8) Wt pct Fe 319-type alloys

Al-Si alloys are materials that have been developed over the years to meet the increasing demands of the automotive industry for smaller, lighter-weight, high-performance components. An important alloy in this respect is the 319 alloy, wherein silicon and copper are the main alloying elements, and magnesium is often added in automotive versions of the alloy for strengthening purposes. The mechanical properties are also ameliorated by modifying the eutectic silicon structure (strontium being commonly employed) and by reducing the harmful effect of the β-Al₅FeSi iron intermetallic present in the cast structure. Magnesium is also found to refine the silicon structure. The present study was undertaken to investigate the individual and combined roles of Mg and Sr on the morphologies of Si, Mg₂Si, and the iron and copper intermetallics likely to form during the solidification of 319-type alloys at very slow (close to equilibrium) cooling rates. The results show that magnesium leads to the precipitation of Al₈Mg₃FeSi₆, Mg₂Si, and Al₅Mg₈Cu₂Si₆ intermetallics. With a strontium addition, dissolution of a large proportion of the needle-like β-Al₅FeSi intermetallic in the aluminum matrix takes place; no transformation of this phase into any other intermetallics (including the Al₁₅(Fe,Mn)₃Si₂ phase) is observed. When both Mg and Sr are added, the diminution of the β-Al₅FeSi phase is enhanced, through both its dissolution in the aluminum matrix as well as its transformation into Al₈Mg₃FeSi₆. The reactions and phases obtained have been analyzed using thermal analysis, optical microscopy, image analysis, and electron microprobe analysis (EMPA) coupled with energydispersive X-ray (EDX) analysis.     

Effect of Manganese on the Formation of Fe-rich Phases in Electromagnetic Stirred Hypereutectic Al-22Si Alloy with 2 % Fe

The effect of Mn on the microstructure of electromagnetic stirred hypereutectic Al-22Si-2Fe (% w/w) alloys was studied. The results show that the alloy with a Mn/Fe ratio zero, contained plentiful α-Al₄FeSi₂ phases existing as mainly intermetallic compounds in the solidified microstructures by electromagnetic stirring (EMS) process. With EMS process, the alloy with 0.61 % Mn contained acicular β-Al₅(Fe, Mn)Si, δ-Al₄(Fe, Mn)Si₂ and blocky α-Al₁₅(Fe, Mn)₃Si₂ phases in the solidification microstructure of the stirred A1 alloy. As the Mn/Fe ratio increased to 1, intermetallic compounds were mainly in the form of blocky and fine α-Al₁₅(Fe, Mn)₃Si₂ phases in the microstructure. The intermetallic compounds were examined with an optical microscope, scanning electron microscope, and X-ray diffraction. The acicular δ-Al₄(Fe, Mn)Si₂ and blocky α-Al₁₅(Fe, Mn)₃Si₂ phases were also analyzed by transmission electron microscopy.     

Iron intermetallic phases in the Al corner of the Al-Si-Fe system

The iron intermetallics observed in six dilute Al-Si-Fe alloys were studied using thermal analysis, optical microscopy, and image, scanning electron microscopy/energy dispersive X-ray, and electron probe microanalysis/wavelength dispersive spectroscopy (EPMA/WDS) analyses. The alloys were solidified in two different molds, a preheated graphite mold (600 °C) and a cylindrical metallic mold (at room temperature), to obtain slow (∼0.2 °C/s) and rapid (∼15 °C/s) cooling rates. The results show that the volume fraction of iron intermetallics obtained increases with the increase in the amount of Fe and Si added, as well as with the decrease in cooling rate. The low cooling rate produces larger-sized intermetallics, whereas the high cooling rate results in a higher density of intermetallics. Iron addition alone is more effective than either Si or Fe+Si additions in producing intermetallics. The alloy composition and cooling rate control the stability of the intermetallic phases: binary Al-Fe phases predominate at low cooling rates and a high Fe:Si ratio; the β-Al₅FeSi phase is dominant at a high Si content and low cooling rate; the α-iron intermetallics (e.g., α-Al₈Fe₂Si) exist between these two; while Si-rich ternary phases such as the δ-iron Al₄FeSi₂ intermetallic are stabilized at high cooling rates and Si contents of 0.9 wt pct and higher. Calculations of the solidification paths representing segregations of Fe and Si to the liquid using the Scheil equation did not conform to the actual solidification paths, due to the fact that solid diffusion is not taken into account in the equation. The theoretical models of Brody and Flemings^[44] and Clyne and Kurz^[45] also fail to explain the observed departure from the Scheil behavior, because these models give less weight to the effect of solid back-diffusion. An adjusted 500 °C metastable isothermal section of the Al-Si-Fe phase diagram has been proposed (in place of the equilibrium one), which correctly predicts the intermetallic phases that occur in this part of the system at low cooling rates (∼0.2 °C/s).     

The role of oxides in the formation of primary iron intermetallics in an Al-11.6Si-0.37Mg alloy

Recent research suggest that the iron-rich intermetallic phases, such as α-Fe Al₁₅(Fe,Mn)₃Si₂ and β-Fe Al₅FeSi, nucleate on oxide films entrained in aluminum casting alloys. This is evidenced by the presence of crack-like defects within these iron-rich intermetallics. In an attempt to verify the role of oxides in nucleating iron-rich intermetallics, experiments have been conducted under conditions where in-situ entrained oxide films and deliberately added oxide particles were present. Iron-rich intermetallics are observed to be associated with the oxides in the final microstructure, and crack-like defects are often observed in the β-Fe plates. The physical association of the Fe-rich intermetallic phases with these solid oxides, either formed in situ or added, is in accordance with the mechanism suggesting that iron-rich intermetallics nucleate upon the wetted sides of double oxide films. This article is based on a presentation made in the John Campbell Symposium on Shape Casting, held during the TMS Annual Meeting, February 13–17, 2005, in San Francisco, CA.     

The Al-Cu-Fe phase diagram: 0 to 25 At. pct Fe and 50

Isothermal sections of the Al-Cu-Fe equilibrium phase diagram at temperatures from 680 °C to 800 °C were determined in the region with 50 to 75 at. pct Al and 0 to 25 at. pct Fe using scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) techniques. This re- gion includes the face-centered icosahedral phase (Ψ-Al₆Cu₂Fe) which has unprecedented struc- tural perfection and no apparent phason strain. The icosahedral phase has equilibrium phase fields with four distinct phases at 700 °C and 720 °C (β-Al(Fe, Cu), λ-Al₁₃Fe₄, ω-Al₇Cu₂Fe, and liquid) and three phases at 680 °C(β, ω, and λ) and 800 °C (β, λ, and liquid). The B2 ordered β phase has considerably greater solubility for Cu than previously reported, extending from AlFe to ∼Al₅₀Fe₅Cu₄₅. The equilibrium range of composition for the icosahedral phase at these temperatures was determined, and a liquidus projection is proposed.     

Interactions between iron, manganese, and the Al-Si eutectic in hypoeutectic Al-Si alloys

Sand-cast plates were used to determine the effect of iron and manganese concentrations on porosity levels in Al-9 pct Si-0.5 pct Mg alloys. Iron increased porosity levels. Manganese additions increased porosity levels in alloys with 0.1 pct Fe, but reduced porosity in alloys with 0.6 and 1 pct Fe. Thermal analysis and quenching were undertaken to determine the effect of iron and managanese on the solidification of the Al-Si eutectic. At high iron levels, the presence of large β-Al₅FeSi was found to reduce the number of eutectic nucleation events and increase the eutectic grain size. The preferential formation of α-Al₁₅Mn₃Si₂ upon addition of manganese reversed these effects. It is proposed that this interaction is due to β-Al₅FeSi and the Al-Si eutectic having common nuclei. Porosity levels are proposed to be controlled by the eutectic grain size and the size of the iron-bearing intermetallic particles rather than the specific intermetallic phase that forms.     

The effect of strontium on the Mg2Si precipitation process in 6201 aluminum alloy

A transmission electron microscopy (TEM) study of a 6201 aluminum alloy to which controlled strontium additions were made has revealed important differences compared to the same alloy free of strontium. In the as-cast state, strontium favors the formation of α-AlFeSi (Al₈Fe₂Si) rather than β-AlFeSi (Al₅FeSi) phase, resulting in a greater quantity of excess silicon present in the strontium-treated alloy. During heat treatment, the excess silicon allows a greater density of finer β″-Mg₂Si precipitates to form, leading to increased tensile strength values and increased electrical resistivity. Strontium also retards the growth of the precipitates formed during heat treatment and inhibits formation of the equilibrium β-Mg₂Si phase. As a result, the strontium-treated alloy exhibits a resistance to overaging.     

Primary crystallization in the Al-Fe-Mn-Ni-Si system as applied to casting alloys based on aluminum-nickel eutectic

Liquidus projections in characteristic sections of the Al-Fe-Mn-Ni-Si system in a range of up to 9% Ni, to 3% Fe, to 2% Mn, and 3% Si (wt %) are computed. The element concentrations at which the primary crystallization of the Al₃Ni, Al₉FeNi, Al₃Fe, Al₆(Fe, Mn), and Al₁₅(Fe,Mn)₃Si₂ intermetallide phases are determined. A good agreement between experimental and computed data is shown. When casting into metallic molds in the region of nickalin compositions, the formation of primary crystals of two Al₉FeNi and Al₆(Fe, Mn) phases is most probable. The primary crystallization of Al₃Ni and Al₁₅(Fe, Mn)₃Si₂ is possible only upon increased concentrations of nickel and silicon, and it takes slow solidification during casting in temporary molds to form the Al₃Fe phase.     

Determination of the isothermal sections of the Al-Ni-Si ternary system at 750 °C and 850 °C

The phase equilibria of the Al-Ni-Si ternary system at 850 °C and 750 °C have been investigated using scanning electron microscopy (SEM) and electron-probe microanalysis (EPMA). Isothermal sections at 850 °C and 750 °C were constructed based on experimental data from 53 alloys heat treated at 850 °C for 1200 hours and at 750 °C for 1440 hours, respectively. The phase equilibria among the following intermetallics and solid-solution phases are described: Ll₂-Ni₃(Al,Si), B2-NiAl, Ni₅Si₂, δ-Ni₂Si, ϑ-Ni₂Si(τ ₄), Ni₃Si₂, NiSi, NiSi₂, Ni₂Al₃, NiAl₃, Ni₂AlSi(τ ₂), Ni₃Al₆Si(τ ₃), Ni₁₆AlSi₉(τ ₅), the fcc solid solution, and the diamond (Si) phase. In addition, a phase, temporarily designated as Ni₅(Al,Si)₃(τ ₆), was observed for the first time at both 750 °C and 850 °C. This phase is probably the stabilization of Ni₅Al₃ by Si to higher temperatures than the binary Ni₅Al₃, which is only stable at <∼700 °C.     

Quantitative analysis of the primary crystallization of iron-containing phases as applied to aluminum alloys of various doping systems

Aluminum-based multicomponent systems are analyzed using the Thermo-Calc program in order to determine the concentration boundaries in which the first primary crystals of Fe-containing phases appear. Projections of the liquidus surfaces are calculated as applied relative to the industrial cast alloys of three main groups, namely, Al-Si (silumins), Al-Cu (the AM5 type), and Al-Mg (magnaliums). It is shown that the primary crystallization of the following Fe-containing phases is most probable: in silumins these are the Al₅FeSi and Al₁₅(Fe, Mn)₃Si₂ phases and in magnaliums and the AM5-type alloys these are Al₃Fe and Al₆(Fe, Mn) phases. Based on the calculation of parameters of the primary crystallization of the Fe-containing phases, the possibility of evaluating the efficiency of iron removal from aluminum alloys is shown.     

Microstructure and properties of duplex δ-Al3(Ti, V)/β-(Ti, V) alloys

The microstructures of multiphase intermetallic alloys with compositions Al₇₀Ti₁₀V₂₀ and Al₆₂Ti₁₀V₂₈ based on the trialuminide Al₃Ti have been characterized, following chill casting and postsolidification heat treatment, using a combination of scanning electron microscopy and transmission electron microscopy (TEM). Evidence of a eutectic reaction of the form L → δ-Al₃(Ti, V)+ζ-Al₈V₅, not previously reported in the Al-Ti-V system, has been observed in both alloys solidified at sufficient levels of undercooling. The ζ phase is replaced by metallic β-(Ti, V) phase during subsequent heat treatment in the range 1073 to 1273 K, and differential thermal analysis (DTA) of samples prean-nealed at 1173 K revealed an endothermic peak at ∼1560 K, consistent with equilibrium eutectic melting of the form (δ+β) → L. Although the chill-cast alloys retained metastable intermediate high-temperature phases, duplex metallic-intermetallic microstructures, containing uniform fine-scale distributions of metallic β-(Ti, V) solid solution in a δ-Al₃(Ti, V) intermetallic matrix, have been produced in both alloys during isothermal heat treatments at temperatures in the range 1073 to 1273 K. For both alloys, the bulk Vickers hardness of such microstructures remained in excess of that of binary Al₃Ti, while in the Al₆₂Ti₁₀V₂₈ alloy, where the increased volume fraction of β phase took the form of a near-continuous network within δ matrix, there was evidence arising from indentation tests of a substantial improvement in the cracking resistance compared to both chill-cast ternary alloy and binary Al₃Ti.     

Microstructural evolution and mechanical properties of AC8A/Al2O3 short-fiber composites after thermal exposure at 150 °C to 350 °C

The microstructural evolution and mechanical properties of an AC8A/12 vol Pct A1₂O₃ (sf) composite fabricated by squeeze casting were characterized. Thermal treatments included the normal T6 temper and thermal exposure at 150 °C, 250 °C, 300 °C, and 350 °C for 400 hours. The predominant strengthening phase in the matrix appeared to be β′ (Mg₂Si) needles. Bulk pure Si particles and dendrites were commonly seen. Large particles, termed asB-type phase, might include hexagonal Al₃(Ni, Cu, Fe, Si, Mg)₂ and orthorhombic Al₃(Ni, Cu, Fe, Si, Mg) phases. Both the Si andB dispersoids were not obviously affected by artificial aging at 150 °C to 350 °C. In certain cases, large cubic β (Mg₂Si) particles, hexagonalQ′ orQ (Al₄Cu₂Mg₈Si₇) precipitates, and numerous small Al particles inside Si dispersoids were also seen. No interfacial reaction product was observed along the fiber/ matrix interface even after long exposure at 350 °C. Amorphous SiO₂ gels, which were used as a binder during fabrication, were occasionally observed. The tensile and fatigue behavior of the AC8A alloys and composites after the preceding thermal exposures were evaluated over the temperature range of 25 °C to 350 °C. The composites showed similar strength as the matrix alloy at room temperature but exhibited higher strength at temperatures above 250 °C, with the sacrifice of the lower ductility. The strength levels of both the alloys and composites were significantly reduced after long thermal exposure, especially for temperatures higher than 250 °C. The loss of strength after long-term exposure at elevated temperatures may be attributed to age-softening of the matrix.     

Modeling structure parameters of irregular eutectic growth: Modification of Magnin-Kurz theory

A new approach to irregular eutectic growth (faceted/nonfaceted crystallization) in Fe-C and Al-Si alloys has been presented in this article. The results of unidirectional crystallization of the irregular eutectic in the Fe-C, Al-Si, and Al-Fe systems were used for the experimental verification of the resulting model. For the oriented graphite, α(Al)-Si and α(Al)-Al₃Fe eutectics, a decrease of the interlamellar spacing λ and in protrusion δ_β of the nonfaceted phase (austenite, α(Al)) by the leading faceted phase (graphite, silicon, and Al₃Fe), the increase of growth rate v was observed. The Magnin-Kurz theory of irregular eutectic growth has been modified in order to better understand the physical mechanisms driving the crystallization process. A comparison of the measured and calculated average λ values has revealed good agreement for the (γ)Fe-graphite, α(Al)-Si, and α(Al)-Al₃Fe eutectics. The developed model also considered the influence of the material constants of the examined alloys on the interlamellar spacing and protrusion of the leading phase—graphite, silicon, and Al₃Fe. It has been found that material constants such as the wetting angle, diffusion coefficient, and Gibbs-Thomson coefficient are of great importance in this eutectic growth.     

The role of iron in the formation of porosity in Al-Si-Cu-based casting alloys: Part II. A phase-diagram approach

The mechanism by which iron causes casting defects in the AA309 (Al-5 pct Si-1.2 pct Cu-0.5 pct Mg) may be related to the solidification sequence of the alloy. Superimposing calculated segregation lines on the liquidus projection of the ternary Al-Si-Fe phase diagram suggests that porosity is minimized at a critical iron content when solidification proceeds directly from the primary field to the ternary Al-Si-βAl₅FeSi eutectic point. Solidification via the binary Al-βAl₅FeSi eutectic is detrimental to casting integrity. This hypothesis was tested by comparing the critical iron content observed in the standard AA309 alloy to that of a high-silicon (10 pct Si) variant of this alloy.     

Iron intermetallic phases in the Al corner of the Al-Si-Fe system

The iron intermetallics observed in six dilute Al-Si-Fe alloys were studied using thermal analysis, optical microscopy, and image, scanning electron microscopy/energy dispersive X-ray, and electron probe microanalysis/wavelength dispersive spectroscopy (EPMA/WDS) analyses. The alloys were solidified in two different molds, a preheated graphite mold (600°C) and a cylindrical metallic mold (at room temperature), to obtain slow (}0.2 °C/s) and rapid (}15 °C/s) cooling rates. The results show that the volume fraction of iron intermetallics obtained increases with the increase in the amount of Fe and Si added, as well as with the decrease in cooling rate. The low cooling rate produces larger-sized intermetallics, whereas the high cooling rate results in a higher density of intermetallics. Iron addition alone is more effective than either Si or Fe+Si additions in producing intermetallics. The alloy composition and cooling rate control the stability of the intermetallic phases: binary Al-Fe phases predominate at low cooling rates and a high Fe:Si ratio; the β-Al₅FeSi phase is dominant at a high Si content and low cooling rate; the α-iron intermetallics (e.g., α-Al₈Fe₂Si) exist between these two; while Si-rich ternary phases such as the δ-iron Al₄FeSi₂ intermetallic are stabilized at high cooling rates and Si contents of 0.9 wt pct and higher. Calculations of the solidification paths representing segregations of Fe and Si to the liquid using the Scheil equation did not conform to the actual solidification paths, due to the fact that solid diffusion is not taken into account in the equation. The theoretical models of Brody and Flemings^[44] and Clyne and Kurz^[45] also fail to explain the observed departure from the Scheil behavior, because these models give less weight to the effect of solid back-diffusion. An adjusted 500°C metastable isothermal section of the Al-Si-Fe phase diagram has been proposed (in place of the equilibrium one), which correctly predicts the intermetallic phases that occur in this part of the system at low cooling rates (}0.2 °C/s).