A new technology for treating liquid metals with intensive melt shearing
Z Fana, Y B Zuob and B Jiangc
The EPSRC Centre – LiME, BCAST, Brunel University, Uxbridge, London, UB8 3PH, UK
a
[email protected], [email protected], [email protected]
Keywords: liquid metal; intensive melt shearing; degassing; grain refining; semisolid processing;
metal matrix composites.
Abstract. Melt quality is crucial for both continuous and shape casting of light alloys. Gas, oxides
and other inclusions in the melt usually deteriorate the quality of the casting products. Conventional
refining techniques, such as filtration and rotary degassing, can refine the melt by removing the
inclusions although they are costly and time-consuming. A new technology for liquid metal treatment
through intensive melt shearing was developed recently to improve the melt quality prior to metal
casting. The new technology uses a simple rotor-stator unit to provide intensive melt shearing, which
disperses effectively the harmful inclusions into fine particles to enhance nucleation during the
subsequent solidification processing. Experimental results have demonstrated that the high shear unit
can be used for general melt treatment, physical grain refinement, degassing and preparation of metal
matrix composites and semisolid slurries. In this paper we offer an overview of the high shear device
and its application in processing light alloys.
1 Introduction
Liquid metal treatment prior to solidification processing is crucial for ensuring the high quality of
the cast products regardless of which cast process is used. The existing methods for liquid metal
treatment mainly include melt filtering, mechanical stirring by an impeller, electromagnetic stirring,
and rotary degassing. However, such processes are time comsuming, energy intensive and high cost.
Recently, melt conditioning by intensive melt shearing in a twin-screw machine was studied and used
to treat both liquid and semi-solid light alloys [1-3]. Improved microstructure and mechanical
properties of both wrought and cast light alloys were achieved by the application of this technology [4,
5]. Based on the similar principle, a new technology [6] but with simpler equipment has been
developed for treating liquid metals using a high shear unit. In this paper we provide an overview on
the high shear unit and its applications in degassing, preparing semi-solid slurry and metal matrix
composite and grain refinement in DC casting.
2 The high shear unit for melt treatment
Intensive melt shearing for melt treatment can be achieved in a rotor-stator unit [6]. This unit
comprises a rotor, a stator and housing for holding the stator and the rotor. A motor is set on the
platform and connected to the shaft to drive the rotor. A schematic diagram of the apparatus for
intensive melt shearing is shown in Fig. 1. While working, 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 rotation speed can be in the order of 104 rpm. The
high shear device can provide not only macro flow in a volume of the melt for distributive mixing but
also intensive shearing of the melt near the tip of the high shear device for dispersive mixing. The
main advantages of the high shear device include significantly enhanced kinetics for any chemical
reactions or phase transformations, well dispersed and uniformly distributed solid particles or gas
bubbles, size reduction of solid particles or gas bubbles, improved homogenization of chemical
composition and temperature fields, and forced wetting of the usually difficult-to-wet solid particles
in the liquid metal. Therefore, the high shear device can be used for conditioning alloy melt prior to
solidification processing, physical grain refinement by dispersing naturally occurring oxides,
degassing, preparation of metal matrix composites, and supplying of semisolid slurries. In this work,
AA7075 Al-alloy was used to study the degassing effect of the high shear device. The reduced
pressure test (RPT) [7, 8] was used to assess the hydrogen level in the molten alloy. AZ31 and AZ91D
magnesium alloys were used for preparing DC cast ingot, semi-solid slurry and MMCs. The alloys
were melted in a resistance furnace with normal cover gas for magnesium alloys. Samples were
examined using optical microscopy (OM) according to the standard metallographic procedures.
2.80
Rotor
Liquid metal
Container
2.60
2.40
12.0%
Dv
Di
8.0%
2.20
4.0%
2.00
0.0%
Non-shearing
Shearing
Fig. 2 Comparison of density and density index
before and after degassing by the high shear unit.
Fig.1 Schematic illustration of the high shear device
for liquid metal treatment.
(a)
Da
Di
Stator
Da, Dv, g/cm3
Shaft
16.0%
(b)
(c)
(d)
Fig. 3 Sectioned samples of AA7075 alloy solidified (a), (c) under atmospheric pressure and (b), (d) under partial vacuum
(80 mbar). (a), (b) without degassing; (c), (d) with degassing of 60 seconds.
3 Results and discussions
2.80
4.0%
Di
Da, Dv, g/cm3
Degassing. Fig. 2 shows the density and
density index of AA7075 Al-alloy with and
2.75
3.0%
without degassing by the high shear device.
After 1 minute degassing, the density index
2.70
2.0%
Da
was significantly reduced from 13% to less
Dv
than 0.2%, which indicates that hydrogen
2.65
1.0%
Di
in AA7075 alloy can be effectively reduced
with very short period of time by this new
2.60
0.0%
intensive melt shearing technology. Fig. 3
0
20
40
60
80
100
provides another evidence of the degassing
Holding time, minutes
effect of this technology. The sectioned
a
samples show that both the number and Fig. 4 Variation of density and density index of AA7075 alloy as
function of isothermal holding time after 60s degassing at 700oC.
size of the porosity were remarkably
reduced after intensive melt shearing. Compared with the conventional rotary degassing process, this
new shearing technology shows significantly improved degassing efficiency. This is mainly due to the
improvement of the uniformity of the inert gas bubbles and the reduction of the bubble size and in
consequence the total area of gas-liquid interface. This is one of the important advantages over the
conventional degassing process. Another point is degassing by this technology, the liquid flow near
the liquid surface and the vortex of the liquid are very weak, which can avoid severe turbulence near
the liquid surface and, consequently, prevent the entrapment of contaminants.
According to Fig. 4, after degassing, during isothermal holding, the density index Di decreases first,
which is mainly due to the floating of small inert gas bubbles, and then increases, which is believed to
be caused by the absorption of hydrogen or water vapor. Although the degassing effect can be
partially kept after holding 90 minutes, the degassed melt should be used within 60 minutes after
degassing.
Preparing MMCs. 9 µm SiC particles were added to AZ91D alloy by the intensive shearing
technology and the tensile samples were cast by a 280 ton high pressure die casting machine. Fig. 5
shows the microstructure and uniformly distributed SiC particles. The intensive melt shearing can
improve the uniformity and the wettability of the particles, and is therefore very suitable to prepare
MMCs.
Fig.5 Microstructure of magnesium-SiC composite prepared by intensive melt shearing with the new high shear
device.
Preparing semi-solid slurry. Semi-solid slurry of AZ91D alloy was prepared by the new shearing
technology and the tensile samples were also cast with a 280 ton high pressure die casting machine.
Fig.6 shows the fine and uniform semi-solid microstructure of the tensile sample. The α-Mg grains are
uniformly distributed with a very narrow size range.
Fig.6 Microstructure of semi-solid AZ91D magnesium alloy prepared by intensive melt shearing with
the new high shear device.
Direct Chill casting. The effect of intensive
melt shearing in the DC casting mould on the
microstructure of AZ31 magnesium alloy was also
studied by the application of the new shearing
technology in a DC casting simulator. The results
are shown in Fig. 7. For the conventionally cast
ingot, the microstructure is large dendrites. After
the application of intensive melt shearing the
microstructure becomes much finer and more
uniform. The morphology of the grains also
changes from dendrite to equaxed. The application
of the intensive melt shearing in the DC casting
process can achieve significant grain refinement. Fig.7 The transition zone of DC cast AZ31 magnesium
alloy from non-shearing to shearing (right to left).
4 Summaries
A new technology for liquid metals treatment using a rotor-stator unit has been developed. The
high shear device can provide not only macro flow in a volume of melt for distributive mixing, but
also, intensive melt shearing of the melt near the tip of the high shear device for dispersive mixing.
The main advantages of the high shear device include, significantly enhanced kinetics for any
chemical reactions or phase transformations, well dispersed and uniformly distributed solid particles
or pas bubbles, size reduction of solid particles or pas bubbles, improved homogenization of chemical
composition and temperature fields, forced wetting of the usually difficult-to-wet solid particles in the
liquid metal. It has been demonstrated that the high shear device can be used for conditioning alloy
melt prior to solidification processing, physical grain refinement by dispersing naturally occurring
oxides, degassing, preparation of metal matrix composites, and supplying of semisolid slurries.
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