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Comparative study of metallurgical mechanical characteristics

obtained through stir casting and centrifugal casting of Al-Si alloys

Shrishail M B1*, A Chaitanya2*, Prabhu T B3*, Gajendra S4*

1,2,3,4UG Students, Department of Mechanical Engineering ,AIET, Mangaluru ,Karnataka, India – 574227.

[email protected], [email protected] [email protected], [email protected]

Abstract---Al–Si alloys are increasingly used in many applications due to its high tensile strength in relation to the density

compared to other cast alloys, such as ductile cast iron or cast steel. As the properties of a specific Al–Si alloys

(hypoeutectic, eutectic or hypereutectic) can be attributed to the individual physical properties of its main phase

constituents and to their volume fraction and morphology, different approaches have been used to control the

microstructural features of Al–Si alloys such as adding some alloying elements in order to refine the grain. However, the

most common way to improve mechanical properties of cast Al–Si alloys is by changing the casting technology. Each

technology has particular aspects that have significant influence on microstructure and consequently on mechanical

properties. The present work discusses on the mechanical and metallurgical characteristics of Al-Si hypereutectic alloy

castings developed using stir casting and centrifugal casting. Mechanical characterization of the castings is carried out

through UTM, Impact testing and Vickers hardness testing machines. Metallurgical characterization of the casting has

been carried out through OM, SEM. Specimens produced through centrifugal casting process exhibited 20% higher tensile

strength compared to the counter parts produced by stir casting, while stir cast specimens displayed 25% higher flexural

strength than those produced by centrifugal casting. Centrifugal cast specimens showed higher hardness values and less

impact resistance compared to stir cast specimens which is attributed to coarse polyhedral shaped morphology of the

primary Silicon phase observed in the microstructure of the centrifugal cast alloy.

Keywords: centrifugal casting, stir casting, Al-24Si alloy, microstructure, hypereutectic.

I. INTRODUCTION

In recent years the application of Al-Si alloys has been extended too many machine parts. Aluminium-Silicon (Al-

Si) cast alloys are commonly used in automotive applications because of their low cost, low density, high temperature

capability, and excellent cast ability. However, these alloys often suffer from low ductility, low tensile strength, and low

fatigue resistance. Al-Si alloys are increasingly used in aerospace applications due to its high tensile strength in relation to

density compared to other cast alloys, such as ductile cast iron or cast steel [5]. Hypereutectic aluminium-silicon (Al-Si)

alloys are advanced engineering materials extensively used for electronic packaging and thermal management applications

due to their low coefficient of thermal expansion (CTE), good electrical conductivity and lightweight. The microstructure

of hypereutectic Al-Si alloy comprises of hard silicon particles dispersed in soft and ductile aluminium matrix. Al-Si

composite materials are known for having poor machine ability due to the high tool wear and poor surface finish arising

from the hard abrasive particles that act as small cutting surface thus resulting in poor surface finish [9]. These alloys are

produced by several commercial casting processes. Conventional casting techniques result in coarse and segregated

microstructures in these alloys leading to inferior mechanical and wear properties and hence they cannot be applied in

critical service conditions. The microstructural features play a vital role on the mechanical properties of these alloys. These

microstructures can be refined or modified by grain refinement and modification.

INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH EXPLORER

VOLUME 5, ISSUE 5, MAY/2018

ISSN NO: 2347-6060

http://ijire.org/118

The alloy suffers from the defects such as casting porosities, accompanied by low strength, low toughness and

poor machinability which is attributed to the presence of large primary silicon particles. Further it is also observed that, the

increasing silicon content, also causes inherent brittleness, low formability and catastrophic failure of the

components[3].As the properties of a specific Al-Si alloy (hypoeutectic, eutectic or hypereutectic) can be attributed to the

individual physical properties of its main phase constituents and to their volume fraction and morphology, different

approaches have been used to control the micro structural features of Al-Si alloys such as adding some alloying elements

in order to refine the grain size. However, the most common way to improve mechanical properties of cast Al-Si alloys is

by changing the casting technology. Each technology has particular aspects that interfere on microstructure and

consequently on mechanical properties.

Through the Al-Si composition to compare with the two type of casting technology stir and centrifugal casting, there are

many parameters in this process which affect the final microstructure and mechanical properties of the composites. These

two type of casting comparison is done with the conducting the several mechanical tests such as tensile, bending and

impact test. And also we conclude by the SEM and XRD. Each casting technique has its own effect on the microstructure

and consequently on the mechanical properties. Efforts have been taken to improve conventional casting techniques.

Amongst them, centrifugal casting and stir such technique. The centrifugal cast alloy shows a fine to coarse microstructure

from the inner mould surface to the centre [4]. The better microstructure at the outer surface in a centrifugal cast alloy is

due to a high rate of heat extraction at mould wall and the metal interface. These castings have a high degree of

metallurgical cleanliness and homogeneous microstructures and they do not exhibit anisotropy of mechanical properties

evident in rolled / welded or forged parts [5]. Though centrifugal casting refines the microstructure to a certain extent, the

results are not much appreciable. The properties of conventionally cast alloys can be improved by secondary processing

but increases the cost of production

II. EXPERIMENTAL DETAILS

2.1 MELTING AND POURING PROCESS FOR ALLOY PREPARATION

The quantity of the alloy to be melted is calculated based on the samples and castings that are being poured. Based

on the quantity, the alloy to be added is calculated. Keep all the alloys to be added ready to add to the furnace. The

Electrical Resistance Furnace is a 36kW furnace with Kanthal heating elements all around the crucible. The crucible is a

Silicon carbide crucible of superior quality and which has high refractoriness and good conductivity. The heating coils

surround the crucible. When the furnace is switched ON, current passes in the heating elements and become red hot. This

heat is transferred to the metallic charge inside the crucible by conduction. The metal heats inside the crucible and melts

after continuous heating.

The electrical furnace is switched ON. Calculated quantity of pure aluminium is added to the furnace. Allow the pure

aluminium to melt completely. Add the required quantity of alloying elements except Magnesium. Allow it to melt.

Continue heating till the metal attains the required pouring temperature. Measure the temperature using a dip type

pyrometer. Once the temperature is attained, add the required quantity of pure Magnesium and immediately pour the

metal to the required die/ingot mold which is heated to a temperature of 200 0C. Once the casting / ingot is cooled to

around 200 0C, the casting/ingot is removed from the die. This is allowed to cool by keeping it in open air. Once it attains

the room temperature, the alloy casting/specimen is taken for further processing like cutting, machining etc to prepare the

test samples, prepared alloy elements are given in the table 2.1



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Fig 2.1 ingots prepared by die casting

TABLE 2.1: composition of Al-Si alloy

Element Aluminiu

m (Al)

Silico

n (Si)

Coppe

r (Cu)

Ferrou

s (Fe)

Manganes

e (Mn)

Nicke

l (Ni)

Zin

c

(Zn

)

Titaniu

m (Ti)

Tin

(Sn

)

Lea

d

(Pb)

Chromiu

m (Cr)

Percentage

of

Compositio

n

71.8 24 1.2 1.2 0.2 1.1 0.2 0.20 0.0

5

0.05 0.5

2.2 STIR CASTING

In this process 1.5 kg of Al-4928 B alloy was cut into small piece using hydraulic blade and filled in furnace initial

temperature of the furnace is 230C and start the furnace to increase the temperature up to 8000C and stirring of the molten

composite were accomplished for 10 minutes at 400 rpm and preheat the metal mould up to 200 0C to remove the

moisture content from the metal mould and finally poured into metallic die at temperature of 720 0C.At the end a slab of

190mm×125mm×12mm cast composite of Al-4928 were obtained along with samples required for mechanical testing

Fig 2.2 Stir casting product (190×125×12)

Fig 2.3 Metallic die

The above fig 2.3 is indicate that metallic die which is used for the preparing the stir casting specimen in the size of length

of 190 mm and thickens 12mm, width =125



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Fig 2.4 Pouring of molten metal Fig 2.5 String of the molten metal (100 rpm)

2.6 Schematic diagram of the stir casting

2.3 CENTRIFUGAL CASTING

In this process, 1.5 kg of Al4928 B alloy was re-melted to a temperature of 800ºC in oil fired furnace and the

molten metal was poured from the crucible in to a cast iron mould rotating at 1000 rpm. The mold has been pre-heated to

100 degree Celsius to remove the moisture .The molten metal was centrifugally thrown to the inside mold wall and then

allowed to cool to get the required casting of length 106mm& 16mm(wall thickness).

Fig.2.7 pouring of the molten metal into centrifugal die (1000RPM)

Fig 2.8 Centrifugal casting product (l=106,t=16)



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Fig 2.9 Schematic diagram of the centrifugal casting

2.4 Metallographic

Samples were extracted from central regions of both the Al-24Si alloys for microstructural examination. The samples were

prepared by polishing using standard metallographic techniques of grinding on water proof emery paper with grit size in

the order of 100,500,1000,1500,2000, in specifications. Final polishing was done on a valuate cloth using diamond paste

and kerosene. The polished samples were etched with Keller’s reagent (1% vol. HF, 1.5% vol. HCl, 2.5%vol. HNO3 and

rest water). We prepared for 200ml in which HF= 2ml, HCL=3ml, HNO3=5ml distilled water=190ml.The

microstructures of the sampleswere examined under ZEISS optical microscope.

Fig 2.10 Stir cast alloy prepared for microstructure purpose

Fig 2.11Centrifugal cast alloy prepared for microstructure purpose

2.5 Tensile testing

Tensile samples of 25 mm gauge length and 100 mm gauge length were prepared from both Al–24Si alloys.

Samples were extracted from the central regions of the castings. Tensile tests were conducted at room temperature on an

INSTRON Universal Testing Machine at a cross speed of 0.05cm/min and a chart speed of 2 cm/min. Three samples have

been considered for tensile testing. The average values of tensile strength for both the alloys have been reported in below

table



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TABLE 2.2

Types of casting Percentage of elongation Ultimate stress in

MPa

Ultimate force

in N

Stir casting 6.28 98.06 3680

Centrifugal casting 7.56 123.2 4613.3

Fig 2.12 Tensile test specimen (ASTM E10) Fig 2.13 Tensile specimen after conducting test

2.6 Hardness testing

Hardness is the resistance of the martial to the indentation, penetration are called as the hardness. Hardness testing of Al–

24Si alloys was carried out on Vickers hardness tester. All the samples were polished before conducting the test. A weight

of 5 kg was considered for this study. At least five measurements were taken for each sample and the average values of

hardness have been reported in Table 2.3

TABLE 2.3

Type of casting Load in kg HV in Kgf/mm2

Stir casting 5 48 .90

Centrifugal casting 5 58.60

2.7 Three point bending testing

3 point bending test of 32 mm gauge length and 100 mm length and thickness is 4mm,width is 15 mm specimen

were prepared from both Al–24Si alloys. Samples were extracted from the central in the stir casting and inner periphery

from the centrifugal castings. Bending tests were conducted at room temperature on an INSTRON Universal Testing

Machine at a cross speed of 0.05cm/min and a chart speed of 2 cm/min. Three samples have been considered for bending

testing. The average values of bending strength for both the alloys have been reported in table 2.4

TABLE 2.4

Types of

casting

Elastic modulus in

MPa

Ultimate stress in

MPa

Ultimate strain

in%

Ultimate force in

N

Stir casting 25300 173.15 0.779 868.5

Centrifugal 27700 129 0.646 647.5

Fig 2.14 Bending specimen with ASTM E190-92Fig 2.15 Bending specimen after conducting test



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2.8 Impact testing (Charpy Impact Test)

Impact test is undoubtedly the most commonly used test that is done to characterize the ductile to brittle

transition behaviour in materials. The impact test is done by placing a square shaped V-notched specimen in the machine.

we prepared , the Charpy specimen has a square cross-section of dimensions L=56mm ,t=5mm and contains a 450 V

notch of 2 mm deep with root radius of 0.25 mm. Impact testing machine used for this experiment contains a heavy swing

pendulum. This pendulum has the maximum capability of impacting energy of 264 ft pound force= 264 × 0.3048m

×9.8ms-2 × 0.45362 Kg = 343.977 J. A heavy pendulum released from a known height strikes the sample on its downward

swing and fractures it. After the test bar is broken, the pendulum rebounds to a height that decreases as the energy

absorbed in fracture increases. by using this machine conduct experiment and take average of that the result is shown in

the table 2.5

Fig 2.16 impact specimen with ASTM E23 Fig 2.17 Impact specimen after the test

TABLE 2.5

Type of

casting

Energy

absorbed by

pendulum for

frictional

resistance in

joule

Energy

absorbed by

pendulum for

fracture of the

of the

specimen in

joule

Net energy

(b-a)

Impact

strength

=impact

energy/area

of cross

section

Mean

value in

j/mm2

Centrifugal 00

00

02

02

02

02

0.066

0.066

0.066

Stir 00

00

03

03

03

03

0.1

0.1

0.1

III. RESULT AND DISCUSSION

3.1Micro structural features

Scanning electron microscopy (SEM) and optical microscope (OM) was used to study the microstructures of Al–24Si

alloy for stir cast and centrifugal cast alloys are shown in Fig.3.1 and 3.3 respectively. In microstructure the white region

represent Al matrix and the globular shaped particles represent Si. Coarse acicular Si particle are distributed along the

primary aluminium boundaries which indicated the non-uniform distribution of the Si particle throughout the aluminium

matrix are shown in fig 3.1(a) and primary Si ,aluminium dendrites are shown in fig 3.1 (b) .

In centrifugal cast alloy at the outer periphery we observed that refinement of the microstructure and homogeneous

distribution of second Si phase particle and less porosity at the outer periphery shown fig 3.3 (a) , and the centrifugal cast

specimen has less α (Al) matrix and a less number of eutectic silicon and primary silicon toward the inner periphery

shown fig 3.3 (b)



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(a) (b)

Fig 3.1: Optical microscopy micrograph of (a) coarse microstructure (b) primary silicon and aluminium dendrites

Fig 3.2:SEM micrograph of stir cast alloy

(a) (b)

Fig 3.3 SEM microstructure (a) inner periphery (b) outer periphery

3.2 Mechanical properties

3.2 .1 Tensile characteristics

The results of tensile testing of Al-24Si alloys were reported in Table 2.2. The results show that the centrifugal cast

alloy gave considerable improvement in its tensile strength and % elongation compared to the stir cast alloy. The high

strength and ductility observed in centrifugal cast alloy mainly due to the less porosities at the outer periphery, refinement

of the microstructure and homogeneous distribution of primary Si phase particle [5] .due to which centrifugal cast alloy

specimen shows better tensile strength .Dendrite structure more present in stir cast alloy specimen compare to centrifugal

cast alloy, these dendrite structure reduce the tensile strength and percentage of elongation [4].



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Fig 3.4: Tensile graph of stress v/s stain

From the above graph concluded that, Specimens produced through centrifugal casting process exhibited 20% higher

tensile strength compared to the counter parts produced by stir casting

3.2.2 Hardness characteristics

The result of hardness testing were reported in the table 2.3.In this study ,bulk hardness values have been

considered because in centrifugal cast alloy it becomes difficult to indent a particular phase .it is seen form the table 2.3

that hardness value of the centrifugal casting is higher than that of the stir casting alloy .the higher the values of hardness

in the centrifugal Cast alloy is due to the micro structural refinement owing to the rapid solidification effect [11] and lower

values in the stir cast alloy

3, 2.3 Bending characteristics

The results of three points bending testing of Al-24Si alloys were reported in Table 2.4 .The bending test

behaviour of the stir and centrifugal cast alloy specimen was tested as per ASTM E190-92 standards. The 3 point bending

test is conducted for three specimens extracted from the toward inner periphery of the centrifugal cast alloy and for stir

casting extracted from centre. To conducting test the variations in the bending properties for both casting. The graph is

plotted with respect to stress v/s

Strain is given below

Fig 3.5: Graph of stress v/s stain

From the above graph we concluded that stir cast specimens displayed 25% higher flexural strength than those

produced by centrifugal casting this mainly due to the more porosities and less density at the inner periphery of the

centrifugal cast specimens is shown in the figure 3.3 (b) and refinement of the microstructure and homogeneous

distribution of primary Si phase is good in stir casting compare to inner periphery of the centrifugal cast alloy specimens.

This leads to the stir casting gave better ultimate stress



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3.2.4 Impact characteristics

The results of impact testing of Al-24Si alloys were reported in Table 2.5 .The impact test behaviour of the stir

and centrifugal cast alloy specimen was tested as per ASTM E23 standards. The impact test is conducted for three

specimens extracted from the toward inner periphery of the centrifugal cast alloy the and for stir casting specimen

extracted from centre. By conducting impact test we concluded that stir casting specimen gave better impact resistance

compare to centrifugal casting, this mainly due to the centrifugal cast specimen has less α (Al) matrix and a less number of

eutectic silicon and primary silicon toward the centre at the speed range of 1000 to 1300 rpm [12], this cause reduce the

impact resistance and hardness

3.2.5 XRD RESULT

Figure 3.6 shows the analytic results of crystal structure of the sample from the stir and centrifugal cast alloy, the

material comprises aluminium, silicon, and silicon dioxide, AlCu,Al203 , the analytic results confirmed that formation of the

, Al-Cu , this phase help to increase the hardness in both cast alloy and Al203 , SiO2 reinforcement was also formed ,these

two reinforcement are help to increase the wear and creep resistance [6]

Fig 3.6 XRD pattern of the sample from centrifugal and stir alloy

IV CONCLUSIONS

Outer periphery of centrifugal casting specimens has refined microstructure of Al–24Si alloy. The Si distribution is

uniform in Al matrix. The centrifugal cast alloy showed higher strength, ductility and hardness compare to stir cast alloy.

This clearly justified the effectiveness of centrifugal casting process. But when considered the inner periphery of

centrifugal cast specimen result in less ultimate strength and impact resistance compare to stir casting specimen, we can

clearly justify the effectiveness of stir casting process.so we can conclude that best structural components are produced by

using the outer periphery of the centrifugal cast alloy.

REFERENCES

[1] Xiaoyu Huang∗, Changming Liu, XunjiaLv, Guanghui Liu, Fuqiang Li,Aluminum alloy pistons reinforced with SiC fabricated by

centrifugal casting. Accepted 12 April 2011

[2] G. Chirita a, I. Stefanescu b,1, D. Cruz a, D. Soares a, F.S. Silva a,*, Sensitivity of different Al–Si alloys to centrifugal casting effect.

Accepted 26 December 2009

[3] P. Shailesh1*, S.Sundarrajan2 , M.Komaraiah3. Optimization of process parameters of Al-Si alloy by centrifugal casting technique using

Taguchi design of experiments.



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[4] G. Chirita, D. Soares, F.S. Silva *.Advantages of the centrifugal casting technique for the production of structural components with Al–Si

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[5] P.R. Gurua, F. Khan MDa, S.K. Panigrahia,∗, G.D. JanakiRamb. Enhancing strength, ductility and machinability of a Al–Si cast

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[8] Journal of Composite Materials 2012 46: 1021 originally published online 15 August 2011, K Wang, JF Cheng, WJ Sun and HS

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[10]K. Raju1, A.P. Harsha1 and S.N. Ojha2 Effect of processing techniques on the mechanical and wearproperties of Al-20Si alloy

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[12] Zhao CHEN1,a, Effect of Centrifugal Casting on Microstructures and Properties of Hypereutectic Al-18wt.%Si Alloy



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