Microstructural and mechanical behaviour of zinc aluminium cast alloys

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –

6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 4, July - August (2013) © IAEME

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MICROSTRUCTURAL AND MECHANICAL BEHAVIOUR OF ZINC-

ALUMINIUM CAST ALLOYS

L.Nirmala1, C.Yuvaraj

2, K. Prahlada Rao

3, Seenappa

4

1

Research Scholar, Department of Mechanical Engineering, JNTUCE, Anantapur,

Andhra Pradesh, India 2Professor, Department of Mechanical Engineering, MITS, Madanapalle, Andhra Pradesh, India

3Professor, Department of Mechanical Engineering, JNTUCE, Anantapur, Andhra Pradesh, India

4Associate Professor, Department of Mechanical Engineering, GEC, Ramanagara, Karnataka, India

ABSTRACT

The objective of this paper is to study the micro structural and mechanical behaviour of ZA27

alloy containing nickel in the range from 1 to 3 wt. %. Elemental analysis was performed by means

of energy dispersive X-ray spectroscopy (EDS) analyzer and XRD technique was employed to

identify the phase formation of the alloy. The microstructure of the alloy was also examined by

SEM. The increase in tensile strength, yield strength and hardness has been discussed and it was

noticed that the tensile strength, yield strength and hardness of the alloys increases with increase in

nickel content. Metallographic studies showed that addition of nickel resulted in microstructural

modifications of the alloy involving the formation of complex intermetallic compounds.

Keywords: EDS, Microstructure, SEM, XRD, Zinc-Aluminum alloys

1. INTRODUCTION

The expeditious advancement in industrial activities, in the past decades, has resulted in the

need for new multifunctional materials that possess characteristics not obtainable from any

individual material. ZA alloys possess significantly higher strength and have been used increasingly

in recent years for industrial applications mainly because of their good mechanical properties,

excellent fluidity and castability [1]. These alloys have been used increasingly during the past few

years. In recent times, zinc-aluminium based alloys have found considerable industrial applications

mainly due to their good mechanical properties, excellent fluidity and castability and present

advantages in comparison with copper based and aluminium based alloys, especially high strength

with a low casting temperature [2,3]. Studies on ZA alloy with high aluminium percentage showed

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that they exhibit good mechanical properties, excellent wear resistance, superior hardness and

modulus. Cu and Mg are added to the Zn-Al based alloys to improve the mechanical properties

[4,5,11].The Mg additions used in these alloys to increase the hardness, tensile strength and

intergranular corrosion resistance of these alloys. The nickel content used in these alloys increases

the hardness, tensile strength and creep resistance of these materials [1,10].More recently, some

investigators have determined the mechanical properties and microstructural features of standard ZA

alloys [3,9], but no previous work appears to have been reported on ZA27 alloy containing higher

percentage of nickel (1-3%). Therefore this study has been carried out to quantify the effect of nickel

content on mechanical properties and microstructure of the modified ZA27 alloy.

2. EXPERIMENTAL PROCEDURE

2.1. Materials and Moulds used

Zinc based alloys were prepared by liquid metallurgy route. The raw materials used to

prepare these alloys are 99.99 % Zinc and 99.99 % aluminium and are melted up to 6300C, then

poured into a preheated die of cylindrical pattern of diameter 22 mm and 165 mm long.

2.2. Casting practice The compositions were melted in an electrical resistance furnace, using graphite crucible and

poured into a permanent metal mould, which was preheated to 1000C, the alloys were cast with a

1500C melt superheat. The aluminium and nickel master alloy was stirred carefully into the molten

Zn-27% A1 alloys before casting.

2.3. Chemical composition analysis

The composition of the alloy was determined using wet chemical (atomic absorption)

spectroscopy and also chemistry of the same alloy was quantified by EDAX analysis as a double

check.

2.4. Measurement of tensile, impact and hardness properties Tensile test was conducted as per ASTM A370 standards at ambient temperature with a strain

rate of 1.3x10-3

/s using universal testing machine. The impact test was performed in accordance with

ASTM E23 standards. Hardness of metallographically prepared samples was measured using a

Micro Vickers hardness tester as per ASTM E10 standards.

2.5. Microstructural studies Microstructural studies of the alloys were carried out using Scanning electron microscope

(SEM) and Energy dispersive X-ray spectroscopic (EDXS) facility. The specimens were polished

according to standard metallographic practice and etched. The reagent for etching was a solution

containing 5 g CrO3, 0.5g Na2SO4 and 100 ml H2O and etched for 10-15 minutes.

3. CHEMICAL COMPOSITION

Elemental quantification was performed by atomic absorption spectroscopy. The following

table gives chemistry of different developed alloys.

International Journal of Mechanical Engineering and Technology

6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 4, July

Table 1. Chemical composition of the

Nickel

content

Al Ni

0 24.71 0.001

1 24.53 1.09

2 23.98 1.95

3 24.10 3.10

Table 2. Mechanical properties

Nickel

content

UTS

(MPa)

Yield Strength

(MPa)

0 236.00 203.00

1 288.65 224.57

2 295.18 238.3

3 328.84 248.95

Fig. 1 Typical EDXS of ZA27 with

Fig. 3 UTS & Yield strength Vs Wt% Ni

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976

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hemical composition of the experimental ZA27 alloy (wt %)

Cu Mg Fe Ti Pb Mn

0.03 0.105 0.09 0.00069 0.0031 0.007

0.03 0.102 0.09 0.0007 0.003 0.007

0.034 0.108 0.09 0.0007 0.003 0.007

0.035 0.120 0.092 0.0007 0.003 0.007

Mechanical properties of ZA27 obtained from the experiments

Yield Strength

(MPa)

Ductility

(%)

Hardness

(MVHN)

203.00 10.80 115

224.57 8.77 121

238.30 6.03 127

248.95 3.65 135

of ZA27 with 3% Ni Fig. 2 Typical XRD of ZA27 with 3% Ni

UTS & Yield strength Vs Wt% Ni Fig. 4 Ductility Vs Wt% Ni

(IJMET), ISSN 0976 –

August (2013) © IAEME

ZA27 alloy (wt %)

Mn Zn

0.007 Rem

0.007 Rem

0.007 Rem

0.007 Rem

obtained from the experiments

Impact

Energy(J)

4

6

7

8

27 with 3% Ni

Ductility Vs Wt% Ni

International Journal of Mechanical Engineering and Technology

6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 4, July

Fig. 5 MVHN Vs Wt % Ni

a

c

Fig. 7 SEM microstructures of as cast ZA27 alloy containing: (a) 1% Ni, (b) 2% Ni,

% Ni (5KX), Single arrow: η Zinc rich phase, Double arrow:

primary α

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976

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MVHN Vs Wt % Ni Fig. 6 Impact energy Vs Wt % Ni

b

d

SEM microstructures of as cast ZA27 alloy containing: (a) 1% Ni, (b) 2% Ni,

Zinc rich phase, Double arrow: α+ η eutectoid phase, Triple arrow:

primary α phase , ε- Meta stable phase α+ η

εεεε

(IJMET), ISSN 0976 –

August (2013) © IAEME

Impact energy Vs Wt % Ni

SEM microstructures of as cast ZA27 alloy containing: (a) 1% Ni, (b) 2% Ni, (c) 3% Ni (d)3

eutectoid phase, Triple arrow:



247

4. RESULTS AND DISCUSSION

The results obtained from the tests, clearly reveal that the addition of nickel to the developed

alloy has a significant effect on mechanical properties.

EDXS analysis and XRD was carried out on the developed alloys in order to know the exact

composition as shown Fig.1 and Fig.2. Fig.3 shows that ultimate tensile strength, yield strength

increases as nickel content increases. The increase in ultimate tensile strength, yield strength and

hardness is due to solid strengthening effect and also due to formation of intermetallic phases

observed along the grain boundaries. Fig.4 shows that ductility decreases continuously with increase

in nickel percentage. This is because the intermetallics acts as barriers for the elongation [9]. Fig.5

shows that hardness increases as nickel content increased. The impact energy increases marginally

with nickel content as shown in Fig. 6.

Microstructural examination of the alloys was conventionally carried out using Scanning

electron microscope. The etching agent used was CrO3 and Na2SO4. As the nickel content increases,

the microstructure seems to get refined appreciably as seen in Fig 7. XRD analysis was carried out to

identify the phases present in the alloys. The various intermetallic compounds noticed are AlNi3, Zn,

AlNi3 and Al0.403 Zn0.597 as shown in Fig.2. The diffraction diagram shows peaks corresponding to

those of the Zn-rich and Al-rich phases of the binary Zn–Al system and the presence of Zn, AlNi3,

AlNi3, and Al0.403 Zn0.597 were seen from the XRD pattern (Fig.2). Zn, AlNi3 and AlNi3 peaks are

observed at 2θ =36.45, 43.42 and 22.92 degrees, where as Al0.403 Zn0.597 observed at 38.94 and 45

degrees.

4.1. Effect of Nickel content on microstructure

From the experimental results it is found that the tensile strength, yield strength and hardness

of the alloys increased as wt% nickel increases. It is observed from the microstructure that the

addition of nickel led to the formation of ε phase in the interdendritic regions of the zinc–aluminum–

nickel alloys which is shown in Fig. 7(a-d). Basically the intermetallic phases are very hard and

brittle which results in increase in the strength and hardness at the cost of ductility. In addition, the

nickel- rich particles may also acts as barriers for the elongation because they are harder and more

brittle. These are the reasons why the yield strength and tensile strength of the alloys increase with

increase in nickel content [9-10]. The histogram Fig.5 shows that hardness of the alloys increases as

the nickel content increased. The increase in this property may be related to the solid solution

hardening and formation of hard and brittle nickel-rich intermetallic phases along the grain

boundaries. Similar explanations have been offered in the literature [1, 4]. The microstructure of the

alloy shown in Fig.7 consists of aluminum-rich (α) phase and zinc-rich (η) phase, while the grey

structure surrounding is a mixture of zinc-rich and aluminum rich phases (α+η). The dark regions

are the zinc-rich (η) phase.

5. CONCLUSIONS

The results of current study indicate that with the addition of nickel in the range of 1–3 wt.%,

increases the tensile strength, yield strength and hardness of the developed alloys. The addition of

nickel in Zn–Al alloys reported the formation of AlNi3, Zn, AlNi3 and Al0.403 Zn0.597 intermetallics.

Which increases hardness of alloy. The decrease in ductility was due to formation of hard and brittle

intermetallics. Impact energy absorbed will increase slightly with increase in nickel content.

Microstructural analysis revealed the alloy comprised primary α dendrites surrounded by the

eutectoid α+ η and metastable phase ε.



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Microstructural and mechanical behaviour of zinc aluminium cast alloys

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