Controlling the Formation of Iron-Bearing Intermetallics in Wrought Al
Alloys by Melt Conditioned DC (MC-DC) Casting Technology
H.-T. Lia,*, J. B. Patelb, H. R. Kotadiac, Z. Fand
The EPSRC Centre - LiME, BCAST, Brunel University London, Uxbridge, UB8 3PH, UK
a
[email protected]; [email protected]; [email protected];
d
[email protected]
Keywords: DC casting, Iron-Bearing Intermetallics, Recycling, Melt conditioned DC casting.
Abstract. With the increasing use of recycled aluminium alloys from the end-of-life products more
and more iron is accumulated into the compositions of alloys. Sometimes, recycling causes the iron
levels to increase beyond the set target levels for down-stream processing. The only way to deal
with this impurity currently in industry is to increase the primary aluminium added to the furnace to
dilute the melt and re-add all other elements or cast it for re-melting or extrude it for products that is
not surface finish critical or required higher corrosion resistance. Formation of small well dispersed
spherical α- or small β- Fe-bearing intermetallics, which can be homogenised for shorter times and
has no negative effect on downstream processing, would be promising even if the iron levels are
above the targeted compositional limits. In the present paper, fine and dispersed Fe-bearing
intermetallics have been achieved by Melt Conditioned DC (MC-DC) casting technology, instead of
coarser Fe-bearing intermetallics forming network like morphology in the DC castings with grain
refiner additions (DC-GR). This suggests feasibility of an increased tolerance of iron levels by melt
conditioned DC casting technology.
Introduction
Al-Mg-Si (6xxx) alloys are widely used heat-treatable alloys with a combination of medium
strength, ductility, and superior corrosion resistance. Fe-bearing AlFe(Mn)Si and Mg2Si are major
intermetallic phases formed in these alloys. Normally observed Fe-bearing intermetallics, include
cubic Al15(Fe,Mn)3Si2 phase, α-Al8Fe2Si phase [1], β-Al5FeSi phase [1, 2]. Of them, the hard
platelet-like β- Fe-bearing intermetallic particles have been reported to severely restrict hot
workability and are to a large extent responsible for the occurrence of surface defects during
thermal mechanical processing, leading to a poor surface finish. With the increasing use of recycled
aluminium alloys from the end-of-life components, higher iron content as one of typical impurity
elements is accumulated into the compositions of alloys. The presence of high level of Fe in 6xxx
alloys can not only deteriorate the workability of downstream thermal mechanical processing but
also result in surface defects of the final products due to the formation of coarse Fe-bearing
intermetallics [3, 4]. The only way to deal with this impurity currently in industry is to increase the
primary aluminium content in the charge. If one can control the constituent Fe-bearing intermetallic
phases in such a way that small and uniformly dispersed spherical α- or small β- Fe-bearing
intermetallics form, it would be of significant benefit. We can deal with easily–alleviated negative
effect on the subsequent thermal mechanical processing and properties of the final wrought
products, even if the iron levels are above the targeted compositional limits.
Melt conditioned direct chill (MC-DC) casting is a novel process developed for the production of
high quality billets without deliberate additions of grain refiners [5, 6]. The process combines
conventional direct chill (DC) casting with high shear melt conditioning directly in the casting sump
for in situ microstructural control. Based on the understanding of melt conditioning [6-8] and
previous MC-DC casting investigations [5, 9, 10], intensive melt shearing in the sump has the
following effects: (1) effective dispersion and uniform distribution of solid particles (such as oxides
and other inclusions), which can act as nucleating sites and enhance heterogeneous nucleation
during the solidification process; (2) intensive melt flow at the solidification front enabling a
uniform compositional field; and (3) a properly controlled thermal field in the sump. As a
consequence, fine equiaxed grain structures can be achieved in the MC-DC castings.
In the present paper, the effect of intensive melt shearing on the formation of Fe-bearing
intermetallics in MC-DC casting has been investigated. For the purpose of comparison, DC castings
with grain refiner (DC-GR) additions were made. The fine and uniformly distributed Fe-bearing
intermetallics achieved suggest the potential of an increased iron tolerance in MC-DC castings.
Experimental procedure
A commercial A6063 Al alloy with nominal composition: 0.41 Si, 0.45 Mg, 0.2 Fe, 0.025 Mn
(all compositions in wt.% throughout this paper), was melted at 760 °C in an electric resistance
furnace. To investigate the possibly increased tolerance to high iron level, the iron content was
increased to 0.6 wt.%, which is much higher than that in the specification of this alloy. The melt at
730 °C was then fed into a DC caster fitted with an 80 mm diameter mould via a launder and a hot
top. The high shear rotor/stator device was positioned in a predetermined height in the DC caster
sump to introduce in situ intensive melt shearing. For the purpose of comparison, DC castings with
or without grain refiner additions of Al-5Ti-1B (wt.%) master alloy were cast under the same DC
casting parameters as the MC-DC. The addition level of AlTiB was at one part per thousand by
weight (1 ppt). The samples cut from central portion of the billets were then sectioned, ground,
polished and etched following standard metallographic procedures. A Zeiss optical microscope
fitted with the Axio Vision 4.3 image analysis system was used for microstructural characterisation
and measurement of grain size and secondary dendritic arm spacing following the ASTM standard
E112-96. Anodizing with Barker’s reagent was performed to facilitate observation using polarised
light imaging. Scanning electron microscopy (SEM) was carried out with a Zeiss Supera 35
machine to reveal the real morphology of Fe-bearing intermetallics using deep-etched samples.
Results and discussions
Intensive melt shearing in the sump leads to a uniform temperature and enhanced mass transport.
Temperature measurement during steady DC casting with intensive melt shearing has confirmed
that the melt temperature in a large area within the sump is quite uniform, being a few K
below/above the alloy liquidus [5, 9, 10]. This is also reflected by the sump profile which had a
flatter bottom and a much reduced sump depth [5].
The respective roles of thermal gradient and nuclei density on the columnar to equiaxed
transition (CET) have been well established in Hunt’s pioneering work [11]. The role of a grain
refining additive/nucleating particle is, in essence, to promote the CET [12]. At higher thermal
gradients, a higher nuclei density is necessary in order to obtain the CET. For the investigated alloy,
the dominant oxide should be MgAl2O4, which has been identified to be highly potent nucleating
particles for the α-Al phase [13]. In consideration of the significantly increased number density of
oxide particles with high shearing [7, 13], the melt conditioning in the sump during DC casting
provides a unique condition for grain refinement with low temperature gradient and high growth
velocity [11]. Figs. 1a and 1b show the grain refinement achieved by DC-GR and MC-DC,
respectively. More interestingly, the two different processing conditions produce different grain
morphologies. In the case of DC casting with chemical grain refiner additions, the grain structure is
more or less similar to granular/rosette grain structure (Fig. 1c), whilst it is more like fine equiaxed
dendritic grain in the case of MC-DC casting (Fig. 1d). Table 1 shows the secondary dendritic arm
spacing of MC-DC is much finer than that of DC-GR with grain refiner addition.
For the grain structure developed in DC casting, two stages affecting the growth should be taken
into account. One is the growth under thermosolutal convection in the slurry zone, another one is
the stage in the mushy zone after the formation of a coherent skeleton (coherency point). From
Table 2, based on the different morphologies observed, it is inferred that for DC-GR, grain growth
experiences long stage coarsening in the slurry zone, where growth happens mainly under
thermosolutal convection; whilst for MC-DC, growth occurs mainly in the late stages of
solidification in the mushy zone after coherent skeleton forms. This will be discussed in detail in the
next section.
a
b
c
d
Fig. 1. Microstructures show fine grain structures in both (a) DC-GR casting; and (b) MC-DC
casting; different grain morphology in (c) granular/rosette grains in DC-GR casting; and (d) fine
equiaxed dendritic grains in MC-DC casting.
Table 1 Comparisons of grain structure parameters between DC-GR and MC-DC castings.
Samples
DC-GR
MC-DC
Grain size (µm)
170±12
185±15
SDAS (µm)
21.6±7.9
10.7±0.8
Table 2. Grain morphology dependence of the solid faction at coherency po int.
Grain morphology
Secondary dendritic arm
Solid faction at coherency
Granular/rosettes
Equiaxed dendrites
Ref.
Less developed and coarse Well developed and finer This work
25-53
20-30
[14]
In DC casting process, to achieve high quality (less casting defects, such as hot tearing, cold
cracking, etc.) and high productivity, chemical grain refiner additions are the standard practice in
industry. However, very little has been reported in the literature on the refining of secondary
dendritic arm spacings (SDAS), which is, in effect, more important for the distribution of
intermetallic particles, which form in the late stages of solidification [14, 15].
a
b
Fig. 2. OM micrographs showing the distributions of Fe-bearing intermetallics formed in A6063
alloy in (a) DC-GR with 1ppt AlTiB grain refiners added and (b) MC-DC casting.
Fig. 3. SEM micrographs of the deep etched samples showing the morphology of Fe-bearing
intermetallics formed in DC castings. (a) Plate like Fe-bearing intermetallic particles on the grain
boundaries of DC-GR casting; and (b) Chinese script like Fe-bearing intermetallic particles in
interdendritic regions of MC-DC casting, and sometimes also on grain boundaries (arrowed).
In the present study, due to the different grain refining mechanisms compared to the usual
chemical grain refiner scheme, the grain structure development in MC-DC is quite different
(Figs.1c and 1d, Table 2). Considering the different grain morphology (Figs. 1c and 1d), as
indicated in Table 2, the solid fraction at coherency point of the DC-GR, could be double that of the
MC-DC. This means for DC-GR, grain structures mainly developed in the slurry zone, which is
characterised by thermosolutal convection, higher melt temperature and lower cooling rate, whilst
for MC-DC, dendritic structures mainly developed in the mushy zone (after coherent skeleton
formed), which is characterised by less melt convection, lower melt temperature and higher cooling
rate. As a result, one can see the significant refinement of SDAS by melt conditioning (Table 1).
The morphology, size and distribution of the intermetallics all have a critical effect on the
homogenisation and workability during subsequent thermal mechanical processing and the
development of surface defects of the final wrought products. Figs. 2 and 3 show the distribution
and typical morphology of Fe-bearing intermetallics formed in DC castings. Under conventional
DC casting with grain refiner additions, with the formation of primary α-Al phase with a coarse
SDAS, solute elements would be rejected and built-up on the grain boundaries of the α-Al phase
rather than being well distributed on either interdendritic regions or grain boundaries, which is the
case of MC-DC with a fine SDAS. The latter enables more even distribution of the solute elements.
Theoretically, the fine dendritic grain structure provides more sites for intermetallic particles to
form compared to the granular/rosette grains, where the sites provided is much less and more or less
connected. According to the solidification sequence, Fe-bearing intermetallics form in the late
stages of solidification after the formation of the α-Al phase. As a result, in MC-DC casting, a more
even distribution of Fe-bearing intermetallics formed either along grain boundaries or interdendritic
regions inside bulk of grains. As shown in Fig. 3, intermetallics display a plate-like morphology
larger than 30-50 µm in DC-GR (Fig. 3a); whilst in the MC-DC casting (Fig. 3b), the refined,
uniformly distributed Fe-bearing intermetallics show a typical size of 10-20 µm, indicating
improved morphology, distribution and smaller size of Fe-bearing intermetallics.
Summary
Fine equiaxed dendritic grain structures with finer secondary dendritic arm spacing can be
achieved by melt conditioned DC (MC-DC) casting compared with the similarly fine grain size but
much coarser secondary dendritic arm spacing in the DC castings with grain refiner additions (DCGR). As a result, finer and uniformly distributed Fe-bearing intermetallics formed in MC-DC
casting rather than the coarser and network like Fe-bearing intermetallics formed in the DC-GR
casting. This suggests a potential to increase tolerance of iron levels in DC castings by MC-DC
casting technology. Further investigations are ongoing to characterise the intermetallics formed in
MC-DC and to evaluate its effects on downstream thermal mechanical processing.
Acknowledgements EPSRC-UK is gratefully acknowledged for supporting the EPSRC Centre –
LiME under grant EP/H026177/1.
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