Microstructures of DC cast light alloys under the influence of intensive
melt shearing
Y B Zuoa, B Jiangb, Z Fanc
The EPSRC Centre – LiME, BCAST, Brunel University, Uxbridge, London, UB8 3PH, UK
a
[email protected], [email protected], [email protected],
Keywords: Light alloys; Intensive melt shearing; DC casting; Microstructure; Grain refinement.
Abstract. A new direct chill (DC) casting process, melt conditioned DC (MC-DC) process has been
developed for production of high quality ingots or billets of light alloys. In the MC-DC casting
process, intensive melt shearing provided by a newly developed rotor-stator unit is used to control the
solidification process during the DC casting with a conventional DC caster. Experimental results of
DC casting of Al- and Mg-alloys with and without intensive melt shearing have demonstrated that the
MC-DC casting process can produce light alloy ingots/billets with significantly refined
microstructure with substantially reduced cast defects. The effect of intensive melt shearing on grain
refinement has been attributed to the enhanced heterogeneous nucleation on well dispersed oxides
occurring naturally in the alloy melt.
1. Introduction
Light alloys have been widely used in many industrial sectors and DC casting is still the major
technology for providing billets or slabs of light alloys for the production of wrought products.
However, the billets or slabs made by the conventional DC casting process often have coarse and
non-uniform microstructures, severe chemical segregation, porosity and hot tearing, which not only
burden the downstream thermo-mechanical processing but also have a negative influence on the
mechanical properties of the final products. Grain refinement by inoculation with chemical grain
refiners is an important and very effective for mitigating such problems [1]. However, it also causes
other problems associated with melt contamination and difficulties with downstream processing. The
search for new and effective grain refiners and methods for grain refinement still continues. Physical
method to refine microstructure such as the application of electromagnetic field [2, 3] becomes more
and more attractive recently. Fan and his co-workers [4-8] have found that intensive melt shearing
provided by a twin-screw mechanism has significant grain refining effect on both aluminium and
magnesium alloys. More recently, based on the similar principle, a new technology [9] with simpler
equipment for intensive melt shearing has been developed. In the present work, the new technology
was combined with the conventional DC casting process to form a new melt conditioned DC
(MC-DC) casting process for the production of high quality DC cast ingots/billets. This paper
presents our experimental results on microstructures of wrought Al- and Mg-alloys produced by the
new MC-DC casting process. The discussion will be focused on the mechanisms of grain refinement.
2. Experimental procedure
Commercial AA7075 Al-alloy and AZ31 and AZ91D Mg-alloys were used in the present work.
The alloys were melted under standard process and with normal cover gas for magnesium alloys and
then the ingots were made through DC casting with intensive melt shearing. Samples were cut from
the centre of DC cast ingots and then examined by optical microscopy (OM) according to the standard
metallographic procedures.
Intensive melt shearing is achieved by the application of a rotor-stator unit which comprises a rotor,
a stator and a housing for holding the stator and the rotor. A motor is set on the platform and
connected to the shaft of the rotor. During the casting
process, the motor passes the power to the rotor by
the shaft and drives the rotor to rotate and shear the
liquid metals in the gap between the rotor and the
stator and also in the openings of the stator. The melt
is conditioned by intensive shearing in the DC mould,
as shown schematically in Fig. 1.
3. Results
DC casting of AA7075 aluminium alloy. The
microstructures of AA7075 Al-alloy cast at different
temperatures with and without intensive melt
shearing are shown in Fig. 2. The conventional DC
cast ingot cast at 700oC shows a coarse and
Fig. 1 The schematic diagram of the MC-DC
non-uniform dendritic microstructure with an
process with the application of the
average grain size around 600μm (Fig. 2a). The
rotor-stator shearing unit.
microstructure cast at the same temperature but with
intensive melt shearing, (Fig. 2c), becomes much finer and more uniform. Improving, Fig. 2(b), or
decreasing, Fig. 2(d), the casting temperature, the microstructure is still much finer. Further
experiments show that the casting temperature does not have significant effect on the microstructure
when MC-DC casting. For ingots cast at 780oC (Fig. 2b), 700oC (Fig. 2c), and 650oC (Fig. 2d) by the
MC-DC casting process, the microstructures are very similar and the average grain size is in a very
close range, which is contrast to the strong dependence of DC cast microstructure on the casting
temperature in the conventional DC casting process. Fig. 3 shows the transition from a coarse
dendritic microstructure to the fine and equiaxed microstructure by simply switching on the high
shear unit.
(a)
(b)
(c)
(d)
Fig.2 Microstructure of AA7075 Al-alloy at the centre of the DC ingot. (a) cast at 700oC without melt shearing;
(b), (c) and (d) cast at 780oC, 700oC and 650oC all with melt shearing, respectively.
DC casting of AZ31 magnesium alloy. Fig. 4
shows the microstructure of AZ31 Mg-alloy ingot
without (Fig. 4a) and with (Fig. 4b) melt shearing.
Without melt shearing, the microstructure is coarse
and dendritic, but with melt shearing the
microstructure becomes fine and equiaxed. This
clearly demonstrated the power of intensive melt shear
for grain refinement of Al-containing Mg-alloys.
DC casting of AZ91D magnesium alloy. Fig. 5
shows the microstructure of AZ91D Mg-alloy ingot
with and without shearing and the transition zone from
non-shearing to shearing. The MC-DC casting process
also exhibits significant grain refinement of Mg-alloy
even with much higher Al contents.
Fig. 3 The transition zone from
non-shearing to shearing (left to right) of
AA7075 alloy ingot cast at 650oC.
(b)
(a)
Fig.4 Microstructure of DC cast AZ31 Mg-alloy. (a) without and (b) with intensive melt shearing.
(a)
Without shearing
(b)
(c)
With shearing
Fig.5 Microstructure of DC cast AZ91D Mg-alloy. (a) without and (c) with melt shearing; (b) transition zone.
4. Discussions
Alloy melt inevitably contains oxides in forms of particle clusters or films with a non-uniform
distribution. Such oxides may act as substrates for nucleation but not effective for grain refinement
due to their poor wetability and low number density. However, such oxide clusters and films can be
dispersed effectively by intensive melt shearing. It has been confirmed that the well dispersed oxide
particles can act as effective nucleation sites[4]. The high shear unit can not only disperse oxides but
also provides macro flow in the melt, which can distribute the individual particles uniformly in the
entire melt. Intensive melt shearing is also beleived to be helpful for improving wetability between the
liquid alloy and the dispersed oxide particles. It is therefore concluded that the intensive melt shearing
can enhance nucleation and makes significant contribution to the formation of fine and unifrom
microstructure.
To further understand the mechanism of grain
refinement by intensive melt shearing temperature
Start shearing from here
measurement during DC casting was carried out.
The temperature curve during DC casting of AZ91D
magnesium alloy is shown in Fig. 6, which was
obtained through the thermocouple set 5 mm below
the head of the high shear unit. With the application
With shearing
of intensive shearing there is a sharp decrease of
melt temperature around the head of the rotor-stator
unit, which is caused by the macro flow in the melt.
Lower temperature of the melt is helpful for the
reduction of the remelting rate of the nuclei and
consequently increasing the effective nucleation. In
Fig. 6 The measured temperature curve
addition, both macro flow and intensive shearing
during DC casting AZ91D alloy.
can also result in the improved homogenization of
the composition and temperature fields. All these factors contribute to the grain refinement.
5. Summaries
A new DC casting process, melt conditioned DC casting (MC-DC) process, has been developed by
application of a high shear unit inside the DC mould. The MC-DC process has been applied to
produce Al- and Mg-alloy ingots. The experimental results have demonstrated clearly that intensive
melt shearing can result in significant grain refinement of the DC cast ingot. The mechanism of grain
refinement is believed to be that under intensive melt shearing the well dispersed and uniformly
distributed oxide particles can act as nucleation sites for enhancing heterogeneous nucleation. In
addition, the homogenized temperature and composition fields by intensive melt shearing can
improve the nucleation efficiency.
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