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 azhongyun.fan@brunel.ac.uk,

byubo.zuo@brunel.ac.uk,

cbo.jiang@brunel.ac.uk

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.

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

Degassing. Fig. 2 shows the density and

density index of AA7075 Al-alloy with and

without degassing by the high shear device.

After 1 minute degassing, the density index

was significantly reduced from 13% to less

than 0.2%, which indicates that hydrogen

in AA7075 alloy can be effectively reduced

with very short period of time by this new

intensive melt shearing technology. Fig. 3

provides another evidence of the degassing

effect of this technology. The sectioned

samples show that both the number and

size of the porosity were remarkably

Liquid metal

Stator

Shaft

Rotor

Container

Fig.1 Schematic illustration of the high shear device

for liquid metal treatment.

Fig. 2 Comparison of density and density index

before and after degassing by the high shear unit.

2.00

2.20

2.40

2.60

2.80

Non-shearing Shearing

Da,

Dv,

g/c

m3

0.0%

4.0%

8.0%

12.0%

16.0%

Di

Da

Dv

Di

2.60

2.65

2.70

2.75

2.80

0 20 40 60 80 100

Holding time, minutes

Da,

Dv,

g/c

m3

0.0%

1.0%

2.0%

3.0%

4.0%

Di

Da

Dv

Di

Fig. 4 Variation of density and density index of AA7075 alloy as a

function of isothermal holding time after 60s degassing at 700oC.

(a) (b) (c) (d)

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.

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.5 Microstructure of magnesium-SiC composite prepared by intensive melt shearing with the new high shear

device.

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.

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.

References

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Scamans: Int. J. Cast Met. Res., 2009, 22, 103-107.

[3] Z. Fan, et al: Int. J. Cast Met. Res., 2009, 22, 318-322.

[4] Y. B. Zuo, et al, Mater. Sci. and Technol., 2011, 27, 101-107.

[5] Y. Zuo, et al, Scripta Mater., 2011, 64, 209–212

[6] Z. Fan, Y. Zuo, B. Jiang, UK Patent, Application No.1015498.7, filed on 16 September 2010.

[7] D. Dispinar, J. Campbell, Int. J. Cast Met. Res., 2004, 17 (5), 280–286.

[8] D. Dispinar, J. Campbell, Int. J. Cast Met. Res., 2004, 17 (5), 287–294.

Fig.7 The transition zone of DC cast AZ31 magnesium

alloy from non-shearing to shearing (right to left).