Intermetallic phase selection in 1XXX Al alloys
Introduction
Flat rolled aluminium products account for approximately 40% of the 24 million tonnes annual world production of aluminium. These products are commonly used for packaging and canning, in electrical applications (e.g. capacitor electrodes), architectural cladding, cable wrap, lithographic printing and automotive sheet.
About 90% of flat rolled products are produced from the melt by the following manufacturing route: the melt is degassed, filtered and grain refined, then direct chill (DC) cast into rectangular or cylindrical water cooled ring moulds with removable bases. Fig. 1 shows a schematic of the DC casting process. The removable bases are withdrawn at a controlled rate as the metal solidifies, resulting in the semicontinuous casting of rectangular ingots or cylindrical billets, typically 0.5–1 m in diameter and 5–10 m in length. The cast surface is often uneven, and the outermost ∼0.2 m of the cast in from surface is often of a coarser grain structure than the interior and can contain higher levels of segregates. The cast surface is commonly scalped off therefore, as discussed in Section 5.1, and the remainder heat treated in the temperature range 450–600°C, in the form of a pre-heat in order to effect microstructural homogenization prior to rolling. Homogenization reduces segregation, encourages the transformation of metastable secondary and ternary phases into equilibrium phases, and acts to equilibrate solid solution levels of soluble elements, resulting in certain cases in the precipitation of dispersoids. A series of both hot and cold rolls with intermediate annealing treatments are then applied to produce a foil or sheet of the required final gauge, typically in the range 6–150 μm (foil) or 150–3000 μm (sheet), which is then commonly subjected to a final anneal.
A range of different aluminium alloys are DC cast and processed by the above route. The exact compositions depend upon the final application of the casting, but Cu, Zn, Mg, Mn, Si and Fe are common alloying additions. The alloys that are the subject of this review are those designated AA1xxx by the International Alloy Designation System (IADS). Commercial 1xxx series Al alloys contain typically ≤0.5 wt% Fe and ≤0.2 wt% Si, sometimes present as deliberate alloying additions, but also as impurities. Other common impurities are Cu, Cr, Mn, Mg, V and Zn. Al–Ti–B additions are frequently used to promote primary Al grain refinement.
The identity, size and distribution of the secondary and ternary inter metallic phases are critical influences on the material properties of the alloy[74], including strength, toughness, formability, fatigue resistance, corrosion resistance and anodizing response[58]. Anodizing quality and etching response are especially important in surface critical products such as lithographic printing sheet, as well as in sheet used in architectural applications. The solid solution content is particularly important in controlling properties such as electrical conductivity and recrystallization characteristics. Thermodynamic consider ations often fail to predict correctly the phase content and solid solution content of the as-cast microstructure because of the non-equilibrium nature of solidification during DC casting. The key alloy properties are controlled by solid solution levels and secondary and ternary phase crystallography and morphology, which in turn are dependent on complex kinetic competitions for nucleation and growth.
In 2 Binary Al–Fe phases, 3 Ternary Al–Fe–Si phasesthe wide range of both equilibrium and metastable secondary Al–Fe and ternary Al–Fe–Si phases reported in 1xxx alloys are examined.
Section snippets
Binary Al–Fe phases
The maximum equilibrium solid solubility of Fe in Al is very low, at ∼0.05 wt% Fe, and Fe is usually present therefore in the form of secondary Fe aluminide phases[74]. The maximum equilibrium solid solubility of Si in Al is higher at ∼1.6 wt%, and low levels (∼0.1–0.2 wt%) of Si in the bulk are readily accommodated therefore by dissolution in the Al matrix and in the Fe aluminides. Consequently, the phase contents of DC cast Al–Fe and Al–Fe–Si alloys with ≤0.1 wt% Si are similar, although in
The equilibrium α-AlFeSi and β-AlFeSi phases
Three ternary phases form under equilibrium solidification conditions in dilute Al–Fe–Si alloys of sufficiently high bulk Si content, >0.1 wt% Si in ≤0.2 wt% Fe containing alloys, and >0.2 wt% Si in ≤0.3–0.4 wt% Fe containing alloys, at temperatures below that of the liquid→Al+Fe4Al13 eutectic reaction. Fig. 10 shows the liquidus projection and associated equilibrium solidification reactions in the Al corner of the Al–Fe–Si ternary phase diagram. The three equilibrium ternary phases produced by
Factors governing phase selection in 1xxx alloys
As noted in Section 1, due to the non-equilibrium nature of solidification during DC casting thermodynamics are usually not capable of predicting the phase and solution contents of the as cast microstructure. An understanding of the factors that govern phase selection in 1xxx Al alloys under conditions of non-equilibrium solidification is important, since varying solidification conditions can lead to variations in secondary Al–Fe and ternary Al–Fe–Si phase contents at different positions in the
Fir tree zones
As was seen in 2 Binary Al–Fe phases, 3 Ternary Al–Fe–Si phases, a wide range of both equilibrium and metastable Al–Fe and Al–Fe-Si phases are observed in model and commercial 1xxx alloys. The non-equilibrium solidification conditions that prevail during conventional casting mean that equilibrium thermodynamic considerations are usually not capable of predicting cast phase content.
As was seen in Section 4, constant solidification velocity unidirectional solidification studies can characterize
Fir tree phases in DC casts
A number of workers have characterised the majority phases that form ‘outside’ the fir-tree zone, i.e. towards the cast surface, and ‘inside', i.e. towards the centre of the billet or ingot, and the critical values of cooling rate and/or solidification velocity over which fir tree formation occurs.
Simensen and Vellasamy[94], Westengen[112], Kosuge[58], Skjerpe[96], Brusethaug et al.[17] and Maggs et al.[68] examined sections of DC cast or laboratory scale DC simulated cast billets. Using
Transformation of metastable phases
Fortunately, the fir tree structure in DC cast billets can frequently be reduced or eliminated by homogenization by heat treatment. Consequently it is desirable to have a full understanding of the transformation kinetics for the each of the metastable Al–Fe and Al–Fe–Si phases so that homogenization treatments may be optimized accordingly.
Using a range of techniques including TEM, Mössbauer spectroscopy, XRD and electrical resistivity measurements Hughes and Jones[46], Kosuge[58], Lendvai et al.
Effect of impurities on phase formation in Al–Fe and Al–Fe–Si alloys
Previous workers have attributed discrepancies between DC cast and Bridgman grown data (ref. Section 6, Table 5) to be due to the effect of impurity elements on phase selection. Ping et al.[80] attributed the effect of impurities to be responsible for the scarcity of the metastable Al–Fe eutectics in his specimens at all observed cooling rates (∼1–10 K s−1). Skjerpe's[96] observation of FeAlm forming at cooling rates down to 1 K s−1 was also proposed to be due to impurities. Brobak and Brusethaug
Effect of grain refiner additions on Al–Fe and Al–Fe–Si alloys
A full review of primary grain refinement of Al alloys is outside the scope of this review, and has previously been given by McCartney[67]. A brief summary is given however as various workers6, 32, 37, 58, 68, 104 have presented evidence that grain refiners can influence secondary/ternary phase selection.
Summary
A wide range of Al–Fe secondary and Al–Fe–Si ternary phases, both equilibrium and metastable, are reported in the literature to form during the solidification of DC cast 1xxx Al alloys. Variations in casting parameters, namely local solidification velocity and cooling rate, and alloy composition, can affect phase selection during the non-equilibrium solidification experienced in DC casting, by affecting the kinetics of nucleation and growth of each of the phases.
Variations in secondary and
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