Understanding the melt flow behaviour of za alloys processed through centrifugal casting

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
INTERNATIONAL JOURNAL OFFebruary (2013) © IAEME ENGINEERING
ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - MECHANICAL
AND TECHNOLOGY (IJMET)
ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 4, Issue 1, January- February (2013), pp. 163-171 IJMET
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UNDERSTANDING THE MELT FLOW BEHAVIOUR OF ZA
ALLOYS PROCESSED THROUGH CENTRIFUGAL CASTING

Jyothi P.N1, A. Shailesh Rao1, M.C. Jagath2, K. Channakeshavalu3
1
K.S. School of Engineering and Management,
Department of Mechanical Engineering
Bangalore-062, Karnataka, India
2
Bangalore Institute of Technology,
Department of Industrial Engineering and Management,
Bangalore-004, Karnataka, India
3
East West Institute of Technology, Principal and Director,
Bangalore-091 Karnataka, India

ABSTRACT:

Centrifugal casting is a process of producing casting by causing molten metal to solidify in
rotating moulds. The study of melt flow in centrifugal casting is much more important as it
determines the quality and properties of the final product. Understanding the flow of liquid
metals in centrifugal casting is much more complex as there is a drop in temperature during
the flow of molten metal.

In the present work, ZA alloys (i.e. ZA 8, ZA 12, and ZA 27) with 6mm cast tube are
prepared at 400,600 and 800 rotational speed of the mould. It was found that, for ZA8 and
Z12 alloys, a uniform cast tube was observed for 600rpm, whereas for ZA 27 a uniform cast
tube was not formed for various rotational speed of the mould due to the increased
composition of aluminum. The observation made in the behaviour of molten metal during
various rotational speeds of the moulds is explained. To know about mechanical properties of
the alloys, the microstructure and hardness are discussed finally.

KEYWORDS: Centrifugal Casting, ZA alloys, Microstructure, Hardness.

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INTRODUCTION

Centrifugal casting is a material processing technique in which the flow pattern of the
molten metal during casting strongly affects the quality of the final product. Literature about
fluid flow in centrifugal casting is very sparse. Theoretically, it should be possible to produce
a true cylinder even when the mould is rotated at low speeds. But practically, the molten
metal has to be accelerated to a certain speed to form a uniform hollow cylinder. Depending
upon the conditions of the molten metal, there must be an optimum spinning speed, at which,
the molten metal will be picked up to form a true cylinder. Jaluria [1] discussed the
importance of fluid flow in material processing. He points out several aspects of fluid flow
which changes the properties in various processing techniques. Most of the studies of liquid
metal behavior are done on continuous casting, where cold modeling experiments were
compared with the final castings [2-6]. Janco [7] indicates several important parameters
involved during the centrifugal casting process. He explains the design of gating, importance
of rotational speed, mold dimensions, etc. But he has not done much to explain the
importance of molten metal behavior during the process. Ping [8] has reported that no
systematic investigation of microstructure evolution in centrifugal casting has been done,
although this information is important to know the mechanical properties of the material.
Chang [9] studied the influence of process parameters on the microstructure formation in
vertical centrifugal casting, but not the effect of liquid metal during casting. From the
experiments, Shailesh [10] explained the optimum rotational speed for centrifugal casting of
aluminum silicon alloys, for a given diameter of the mould. Below, this speed, Couette,
Taylor and Ekmann flows are seen in the final cast tube. The mechanical properties are
evaluated to substantiate optimum rotational speed of the mould.
Even though some attempts are tried to understand the nature of melt flow in
aluminum silicon alloys [11], the comparison of various composition of alloy, in
understanding the fluid behavior is not understood. To understand the nature of melt flow,
Zinc based aluminum alloys commonly referred as ZA alloys are taken for our investigation
as it is now increasing in various commercial usages. Moreover, they have better sliding,
wear resistance, machinability and excellent corrosion resistance in various environments
[12-15].
In the present work, an effort has been made to develop ZA alloys (i.e. ZA8, ZA12,
and ZA27) through centrifugal casting process, at various rotational speeds (i.e.400, 600 and
800 rpm) of the mould. The cast tube of a true cylinder had a dimension of φ 80x120mm and
6mm thick. It is explained from the literature, that addition of aluminum into molten zinc
alloy improves the fluidity and cast ability under continuous casting [13]. In the case of
centrifugal casting, it is understood that with the increase in the Aluminum content from 12%
to 27% in zinc based alloy, fluidity decreases and do not form a true uniform cylinder under
the various rotational speed of the mould. The behaviour of the liquid metal for all the cast
tube is explained in this paper. The microstructure and micro hardness of the cast tube is
found out and explained finally.

2. EXPERIMENTAL DETAILS

The experimental alloys were prepared as per ASTM B86-11 Standard for Zinc-
Aluminum (ZA) Alloy Foundry, by liquid metallurgy route. The alloy was melted and 200OC
as super heat was maintained as teeming temperature for all the cast tubes. Horizontal type

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centrifugal casting machine as shown in Fig [1] is employed to cast ZA alloys samples in this
investigation. It is driven by a 2 HP DC motor for varying speed from 20 to 2000rpm.

Fig. 1 Centrifugal Casting Machine Set up

For determining the microstructure, the samples were cut from the casting and were
characterized using optical microscope. The samples were polished metallographically and
etched suitably prior to their micro structural examination. Microstructures at the three
regions (inner, middle and outer surface of the specimen) were examined to know changes in
the mechanical properties along the radial direction of the cast tube. Finally, the Micro
hardness for the samples at three regions were determined and explained.

3. RESULTS AND DISCUSSIONS

3.1 APPEARANCE OF THE CAST TUBE
The ZA8 alloy heated in a furnace to form 6 mm thick cast tube is poured into rotating
mould. An irregular pattern of the casting is formed at 400 rpm as shown in Fig. [2, a]. The
molten metal, which is poured, leads to its stickiness at particular locations on the inner
surface of the mould due to the lifting of the melt. Some quantity of molten metal is also
found to move in the axial direction and a lump of mass settles in on one side at low
rotational speed of the mould. Similar observations are also found in ZA12 alloy as seen from
figure. With ZA27 alloy, an irregular pattern is seen in the final casting as observed from the
figure. This is possibly due to the quick lifting of the liquid metal and hence limiting its axial
movements.
Further increase in the rotational speed of the mould to 600 rpm, a uniformly thick full
cylinder is observed at 600 rpm with ZA8 and ZA12 alloy. This is possibly due to the molten
metal getting along the circumference of the inner mould and thus avoiding other types of
flows (Couette, Taylor Ekmann flows). The driving force which is acting on the molten metal
should be sufficient, so that it is carried along the inner surface of the mould before it gets
solidified. In case of ZA 27, the mould enables the rapid movement of the liquid metal in
circumferential direction resulting in the formation of non-uniform thickness of the casting
Fig [2, b].
Further increase in rotational speed to 800 rpm, enables the molten metal of all the
alloys to move along the inner surface of the mould due to larger driving force. Only small
amount of metal succeeds in moving along the axis getting solidified during its motion.
Finally the casting which is formed has a varying thickness on its side Fig [2, c].

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From the above, it is understood that a uniform cast tube is formed at a rotational
speed of 600rpm for ZA8 and ZA12 alloy for a given dimension of mould. Moreover,
uniform cylinder is not formed in ZA27 alloy for different rotational speeds of the mould.
Viscosity of the melt finds a major role in the formation of uniformly thick cylinder
castings. A full cylinder is formed with irregular patterns inside the cast tube when rotated at
speed of 400rpm. Since the viscosity and the driving force of the melt is low, it moves axially
and obtains a lift from the inner walls of the mould during teeming process. The melt
consumes more time for completing solidification which leads to formation of lumps in the
final casting. Further with gradual increase in the magnitude of viscosity, the melt moves
easily along the circumference of the mould and impression of the bands is formed in the
final casting. The solidification rate and driving force of the melt finds more dependent on the
viscosity and rotational speed of the mould. The melt must rotate at larger rotational speed
after pouring, since it has low viscosity during teeming process. A uniform full cylinder is
observed, when the mould is rotated at a speed of 600 rpm. Further increase in the rotational
speed to 800rpm, the driving force plays a predominant role; it guides the molten metal to
move along the circumference rather than moving along the axis. Finally, an irregular cast
tube is formed.

3.2 MICROSTRUCTURES OF THE CAST TUBE
The results of metallographic investigations of ZA alloys during centrifugal casting
process are presented in Figure [3-5]. The microstructure of ZA8 alloy for various rotational
speed of the mould is shown in Fig. [3].The structure is typically dendritic at the inner,
middle and outer surface of the cast tube. Dendrites formed here are of complex shape and the
growth of dendrite is moved from outer to inner surface of the mould. Here the liquid metal
finds difficult to move along the circumference of the mould during teeming. It probably
oscillates along the axis and when it becomes viscous, it moves along the circumference of
the mould. This could be imagined and observed from the final cast tube. The process here
has a lower solidification rate of the liquid metal in cast tube and finally improper structures
are seen in the cast tube. Transformation of the dendritic structure into a fine structure is seen,
when the rotational speed of the mould is increased to 600rpm.The solidification rate here is
comparatively more since the liquid metal moved along the axis and simultaneously rises
along the circumference of the mould forming a good cast tube. Increase in rotational speed to
800rpm, the liquid metal moves along the circumference of the mould after teeming into it.
The driving force is too high, so that the melt has limited axial movement. Since the lump of
melt is accumulated in one portion of mould, the solidification is low and hence dendrite
structure is formed at the middle and inner surface of the mould. Due to sudden quenching of
melt into mould during pouring, fine structures are seen at the outer surface of the mould.
Similar observations are also seen for ZA12 alloy Fig [4]. Here the solidification of melt
begins with the formation of aluminum rich dendrites. Further increase in aluminum for
ZA27 alloy, a poor microstructure is observed in all the cast tube. For 400rpm, the liquid
metal moves along the axial hitting the other side of the mould. It reverses back and tries to
oscillate along the axis of the mould. Meanwhile the melt becomes viscous and then moves
along the circumference of the mould. Increase in rotational speed to 600 and 800rpm, the
melt moves along the circumference of the mould. This is due to high centrifugal force and
viscosity. The microstructure shown in the Fig [5] revealed the above explanation. A rich
primary aluminum is seen for the cast tube rotated at 400rpm. Since the aluminum restricts

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fluidity, a coarse grain structure is seen in the metal rotated at higher rotational speed of the
mould.

L R

(A) 400 RPM (B) 600 RPM (C) 800 RPM

Fig 2: ZA8, ZA12 and ZA27 alloys (L R) for Various Rotational Speeds (6mm Thick)

Outer Middle Inner

Fig 3: Microstructure of ZA8 alloy for 400rpm, 600rpm and 800 rpm (Top to bottom) with
Magnification of 400µ

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Outer Middle Inner

Fig 4: Microstructure of ZA12 alloy for 400rpm,
600rpm and 800rpm
(Top to bottom) with Magnification of 400µm

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Outer Middle Inner

Fig 5: Microstructure of ZA27 alloy for 400rpm, 600rpm and 800rpm (Top to bottom) with
Magnification of 400µm.
MICRO HARDNESS OF THE CAST TUBE
Micro Hardness measurements of the test sample are made on Vickers Hardness tester
of Model MMT-X7A with applied load of 1 kg, according to the standard testing protocols.
The hardness is carried out on a piece cut radially about 10mm square. Since the sample is
thin, the curvature is marginal and it is easily pressed flat. Averages of three readings are
taken as the hardness value for a given specimen.
The hardness values on the outer, middle and inner surface of the samples as a
function of rotational speed is shown in Fig. [6]. Hardness of centrifugally cast specimen
depends on flowability and melt filling Behaviour, which can be known by flow length and
wall thickness. In the present work hardness of ZA8, ZA12, and ZA27 centrifugally cast tube
at different rotational speeds is found out. It is found that at 400 rpm and 800 rpm, hardness is
not uniform across the cross section of the cast tube. But at 600rpm uniform hardness at all
the three layers is recorded, as the flow of melt in the rotating mould is uniform. Moreover, at
lower rotational speed of the mould, there will be no uniformity of melt over the inner surface
of the mould. Due to this, the solidification rate of the cast tube is less and hence lower
hardness values are observed in the final cast tube. In the case of higher rotational speed, the
mould itself lifts the liquid metal over its inner surface making limited movement in axial

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direction. Again, here, lower hardness values are found out due to lower solidification rate of
the cast tube. For ZA27 alloy for three rotational speeds of the mould, hardness is varying
across the section of the cast tube, as the flow of melt in to the mould is not uniform. This is
due to increase of Al content in the melt; which reduces fluidity of the molten metal forming
irregular cast tube.

(a)

(b)

(c)
Fig 6: Vickers Hardness Value versus Rotational speed of the mould for
(a) ZA 8 alloy (b) ZA 12 alloy and (c) ZA 27 alloy

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CONCLUSION
ZA alloys are cast centrifugally with varying rotational speed of the mould. The
following are the conclusions remarks from the experiments.

1) A uniform cast tube was formed for ZA8, ZA12 alloy due to lower content of
aluminum in the metal. When the aluminum content is increased, say ZA27 alloy a
uniform cast tube was not formed.
2) All the cast tube formed must be substantial with mechanical properties. A fine
microstructure was formed for uniform cast tube (i.e. both ZA8, ZA12 alloy). With
aluminum rich ZA27 alloy, a coarse grain structure was formed.
3) Hardness test for all the cast tube showed an increased value for uniform cylinder.
The hardness value is varied across the section for ZA27 alloy.

ACKNOWLEDGEMENT

The authors acknowledge Management, Principal, Staff and Non-teaching Staff of K.S.School
of Engineering and Management, and NITTE Meenakshi Institute of technology, Bangalore
for their kind support for this project.

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