Mechanical properties and corrosion behaviour of Zn-27Al...

Leonardo Electronic Journal of Practices and Technologies

ISSN 1583-1078

Issue 25, July-December, 2014

p. 58-71

Mechanical properties and corrosion behaviour of Zn-27Al based

composites reinforced with silicon carbide and bamboo leaf ash

Kenneth Kanayo ALANEME1, 2*, Sumaila Isaac ADAMA1, Samuel Ranti OKE1

1 Department of Metallurgical and Materials Engineering, Federal University of Technology,

Akure, PMB 704, Nigeria 2 Department of Mining and Metallurgical Engineering, University of Namibia, Ongwediva

Engineering Campus, Ongwediva, Namibia E-mail:* [email protected];

* Corresponding author, phone: +2348034228868

Abstract

The mechanical and corrosion behaviour of Zn27Al alloy based metal matrix

composites reinforced with silicon carbide and bamboo leaf ash (BLA) was

investigated. Double stir casting method was used to produce the composites

which contained 7 and 10wt% BLA-SiC particles consisting of 0, 20, 30 and

40% BLA, respectively. Microstructural examination, mechanical properties

and corrosion behaviour were used to characterize the composites produced.

The results show that the hardness and tensile strength of the composites

decreased with increase in the weight percent of BLA although less than 11%

for all experimental cases studied. The percent elongation (%E) improved

slightly with the addition of a maximum of 30% BLA content for both the 7

and 10 wt% composite grades while the fracture toughness increased

consistently with increase in the weight percent of BLA in both the 7 and 10

wt. % composite grades. The BLA-SiC containing Zn-27Al hybrid composites

were very stable in 3.5% NaCl solution and the corrosion resistance in 0.3M

H2SO4 solution was superior to that of the single SiC reinforced Zn-27Al

composite grade.

Keywords

Zn-27Al based composites; Mechanical properties; Bamboo leaf ash; Silicon

carbide; Corrosion

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Mechanical properties and corrosion behavior of Zn-27Al based composites reinforced with silicon carbide and bamboo leaf ash

Kenneth K. ALANEME, Sumaila I. ADAMA, Samuel R. OKE

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Introduction

Zn-Al based alloys (designated as ZA alloys) have been successfully utilized in a

variety of demanding engineering applications [1-3]. The ZA alloys possess attractive

properties such as low melting point, high strength, good cast ability, easy machinability as

well as excellent bearing capability, high wear resistance and corrosion resistance [4-5].

The Zn-27Al alloy is reported to offer a good combination of mechanical and physical

properties in comparison with other members of the ZA cast alloys [6]. It has the highest

strength and the lowest density, offer excellent bearing and wears resistance properties, and

provides the highest design stress capability at elevated temperatures in comparison with all

the commercially available ZA alloys [7]. Zn-27Al alloy has been used for pressure die

castings and gravity castings wherever very high strength is required as well as in bearings

and bushing applications as a replacement for bronze bearings because of its lower cost and

equivalent or superior bearing performances [8]. However, at elevated temperature (> 100°C)

the use of zinc-aluminium alloys are limited which has restricted their use in some

applications [9].

One promising approach to improve the elevated temperature properties as well as the

mechanical, wear and corrosion properties of the Zn-27Al alloy is by reinforcing it with

ceramic dispersoids [10]. The use of silicon carbide (SiC), alumina (Al2O3), graphite (Cg), and

glass in either fibre or particulate shapes as reinforcements in ZA alloys has been well

reported in literature [11-13]. Some of these synthetic reinforcing materials are however noted

to be quite expensive resulting in the increased cost of production of the composites.

Recently, there has been efforts aimed at developing low cost – high performance

MMCs using agro based waste materials as reinforcements [14-16]. This composite materials

design concept was given impetus by the high cost of importation of synthetic reinforcing

materials such as SiC and alumina in most developing countries. Also the potentials of

conserving foreign exchange and improving agro waste management were further incentives.

Ashes derived from controlled burning of agro wastes such as coconut shell, rice husk,

bamboo leaf, bagasse among others have been tested as reinforcements particularly with

Aluminium based composites with very promising results obtained [17-19]. The use of these

agro waste ashes as sole or hybrid reinforcements in Zn-Al based alloys has received very

little attention from researchers.


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In this research work, the mechanical and corrosion behaviour of Zn-27Al alloy based

composites reinforced with bamboo leaf ash and silicon carbide is investigated. This is aimed

at assessing the potentials of producing low cost Zn-27Al based composite grades with

comparable properties with the those reinforced with solely SiC which is a much expensive

reinforcing material.

Material and method

Commercial pure Aluminium and Zinc (with chemical composition presented in Table

1) were selected for the production of the Zn-Al alloy matrix. Chemically pure silicon carbide

(SiC) particles having average particle size of 30 μm and bamboo leaf ash (ash) (<50 µm)

derived from controlled burning and sieving of dry bamboo leaves were selected as

reinforcement for the Zn-27Al based composites to be produced. Graphite particles having

particle size of < 50 μm was also procured for addition in the composite mix to help improve

machinability of the composites [20]. The chemical composition of the bamboo leaf ash

utilized in this research is presented in Table 2.

Table 1. Elemental composition of the zinc and aluminium used for the production of the Zn-27Al based composite matrix

Elements Weight percentageAl 99.92 Fe 0.003 Si 0.033 Mn 0.021 others 0.023

Element Weight percentage Zn 99.96 Fe 0.02 Si 0.006 Pb 0.004 others 0.01

Table 2: Composition of bamboo leaf ash taken into study [19] SiO2 Al2O3 Fe2O3 CaO MgO K2O TiO275.9 4.13 1.22 7.47 1.85 5.62 0.20

Production of the Zn-27 Al based composites

Double stir casting process performed in accordance with Alaneme and Aluko [21]

was adopted for the production of the composites. The amounts of bamboo leaf ash (BLA and

silicon carbide (SiC) required to prepare 7 and 10 wt. % of the BLA-SiC reinforcements (in

the Zn-27Al alloy matrix) was first determined using charge calculations. Four different

BLA-SiC reinforcement weight mix ratios were prepared consisting of 0, 20, 30 and 40%



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BLA. Graphite (0.25 wt. %) was added to each charge for improved machinability of the

composites after casting. In order to remove moisture, and improve wettability between the

reinforcements and the molten Zn-Al alloy; preheating of the bamboo leaf ash, silicon carbide

and graphite particles were performed separately at a temperature of 250°C. The melting

operation was carried out using a gas fired crucible furnace. The Aluminium billets was

charged into the furnace and heated to a temperature of 670°C until it has melted completely.

(The temperature of the furnace was monitored during melting using a temperature probe

fitted to the furnace). The temperature of the furnace was lowered to 500°C before Zinc was

added in to the crucible and allowed to melt completely. The liquid alloy was then allowed to

cool in the furnace to a semi solid state (temperature of 450°C); after which it was stirred at

200rpm for 5minutes to achieve homogenization. The preheated bamboo leaf ash, SiC and

graphite particles were then charged into the melt and the ensuing slurry stirred for 5-10

minutes. The composite slurry was subsequently superheated to 530°C and second stirring

performed using a mechanical stirrer. The stirring operation was performed at a speed of 400

rpm for 10 minutes before casting into prepared sand moulds. The sample designations for the

different compositions of the Zn-27Al based composites produced are presented in Table 3.

Table 3: Sample designations for the composites produced Sample designation Weight percent BLA:SiC7wt% Reinforcement A0 0: 100 A20 20:80 A30 30:70 A40 40:60 10wt% Reinforcement B0 0: 100 B20 20:80 B30 30:70 B40 40:60

Mechanical testing

Hardness tests (HRC) were performed on the composites produced using Indentec

Hardness Testing Machine. The composite specimen surfaces were well polished following

standard metallographic procedures to ensure a smooth surface is produced to allow for

reliable measurement of the hardness values. Multiple hardness tests were performed on each

sample and the average from values within the range of ± 2% was taken as the hardness of the

specimen.


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Tensile tests were also performed on the composites produced following the

specifications of ASTM 8M-91 standards [22]. The samples for the test were machined to

round specimen configuration with 6 mm diameter and 30 mm gauge length. The test was

carried out at room temperature using an Instron universal testing machine operated at a strain

rate of 10-3/s. The tensile properties evaluated from the tension tests are the ultimate tensile

strength and the strain to fracture. Three repeat tests were performed for each grade of the Zn-

27Al based composite produced to ensure repeatability and reliability of the data generated.

Circumferential notch tensile (CNT) specimens were used for the determination of the

fracture toughness of the composites following specimen preparation and test procedures in

accordance with Alaneme [23]. The CNT testing was also performed at room temperature

using an Instron universal testing machine. The samples for the test were machined having

gauge length, specimen diameter (D), and notch diameter (d) of 30, 6, and 4.5 in mm

respectively; and notch angle of 60°. The specimens were subjected to tensile loading to

fracture and the fracture load (Pf) obtained from the load – extension plots were used to

evaluate the fracture toughness using the equation [24]:

K1C = Pf/(D)3/2[1.72(D/d)–1.27] (1)

where, D and d are respectively the specimen diameter and the diameter of the notched

section. Plane strain conditions and by extension, the validity of the fracture toughness values

obtained was determined using the relations in accordance with Nath and Das [25]:

D ≥ (K1C/σy)2 (1)

Three repeat tests were performed for each grade of the Zn-27Al based composite

produced to ensure repeatability and reliability of the data generated.

Microstructural examination

The microstructure of the composites was examined using a Zeiss Metallurgical

Microscope with accessories for image analysis. The specimens for the test were

metallographically polished and etched in dilute aqua regal solution before they were viewed

under the microscope.

Corrosion test

The corrosion behaviour of the Zn-27Al alloy based composites produced was studied

using weight loss method. The mass loss (g/cm2) and corrosion rate (mmy) measurements

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determined from the weight loss data (in accordance with ASTM G31 standard recommended

practice [26]) were used to access the corrosion behaviour of the composites. The corrosion

test was performed by at room temperature (25°C) by immersion of the test specimens in still

solutions of 0.3M H2SO4 and 3.5% NaCl which were prepared following standard procedures.

The specimens for the test were cut to size 15×15×10 mm and then mechanically polished

with progressively finer grit size emery papers to produce a smooth surface. The samples

were de-greased with acetone, rinsed in distilled water, and then dried in air before immersion

in the solutions. The solution-to-specimen surface area ratio was about 150 ml cm-2, and the

corrosion setups were exposed to atmospheric air for the duration of the immersion test. The

weight loss readings were monitored on two day intervals for a period of 42 days. Three

repeat corrosion tests were carried out for each grade of the Zn-27Al based composites

produced to guarantee reliability of results generated.

Results and discussion

Microstructure

Representative microstructures of the Zn-27Al based composites reinforced with BLA

and SiC are presented in Figure 1. For the two microstructural images, the dendritic

solidification structure of the Zn-27Al alloy which serves as the composite matrix are visible

alongside the dispersed reinforcements.

a. b. Figure 1: Representative microstructures of (a) the Zn-27Al based composite containing 7

wt% BLA-SiC (with 20% BLA: 80% SiC in wt %) and (b) the Zn-27Al based composite containing 10 wt% BLA-SiC (with 30% BLA: 70% SiC in wt. %)


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Mechanical behaviour

The results of the mechanical properties of the Zn-27Al alloy based composites

produced are summarized in Table 3.

Table 3. Mechanical properties of Zn27Al based composites produced Sample designation

BLA (%)

HRC UTS (MPa)

E (%)

KIC

(MPam1/2) 7wt% A0 0 26.2 273.2 6.55 7.3 A20 20 25.6 269.7 6.7 8.8 A30 30 24.7 266 6.9 10.2 A40 40 23.4 258 6.2 9.6 10wt% B0 0 28.3 316.8 6.3 7 B20 20 27.4 310.2 6.4 7.8 B30 30 26.7 292.4 6.5 9.6 B40 40 25.8 283.7 6.1 8.9

It is observed from Table 3 that the hardness and tensile strength of the composites

decreased with increase in the weight percent of BLA. For hardness, reductions of 2.3, 5.7,

and 10.6% was observed for the 7 wt% BLA-SiC reinforced Zn-27Al composite grades

containing 20, 30, and 40% BLA (A20, A30, and A40) respectively. In the case of the 10

wt.% BLA-SiC reinforced Zn-27Al composite grades, 3.2, 5.6 and 8.8% reduction in hardness

was observed for the composites containing 20, 30, and 40% BLA (B20, B30, and B40)

respectively. For tensile strength, reductions of 1.3, 2.6 and 5.6% were obtained for the 7 wt%

BLA-SiC reinforced Zn-27Al composite grades containing 20, 30, and 40% BLA (A20, A30,

and A40) respectively. For the 10 wt% BLA-SiC reinforced Zn-27Al composite grades 2.1,

7.7 and 10.5% were observed for the composites containing 20, 30, and 40% BLA (B20, B30,

and B40) respectively. For both cases of reduction in hardness and tensile strength with

increase in BLA content, the predominance of silica in BLA (Table 2) which is known to be

softer compared to SiC is largely responsible [19]. The percent elongation (%E) is observed

to improve slightly with the addition of a maximum of 30% BLA content for both the 7 and

10 wt% composite grades. Further increase to 40% BLA content is noted to result in decrease

in the % Elongation of the composites. The fracture toughness also increased with increase in

the weight percent of BLA in both the 7 and 10 wt. % composite grades which is in contrast

with the trend observed for the hardness and tensile strength. For the 7 wt.% BLA-SiC

reinforced Zn-27Al composite grades, significant increases of 17.1 , 28.4 and 24.0 % were

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obtained for the composites containing 20, 30, and 40% BLA (A20, A30, and A40)

respectively. In the case of the 10 wt.% BLA-SiC reinforced Zn-27Al composite grades 10.3,

27.1 and 21.4 % increment in fracture toughness was observed for the composites containing

20, 30, and 40% BLA (B20, B30, and B40) respectively. The improvement in the fracture

toughness with increase in BLA content of the composites is due to the reduced amount of

relatively harder and brittle SiC particles in the Zn-27Al based composites produced. The SiC

particles like most hard and brittle ceramic particles have a higher tendency to undergo rapid

crack propagation [19]. Thus it is noted that the addition of BLA in the Zn-27Al based

composites results in improved resistance to crack propagation and fracture.

Corrosion behaviour

The results of the variation of mass loss and corrosion rate with exposure time for

composites immersed in 3.5wt% NaCl solution are presented in Figures 2 and 3.

(a)

(b) Figure 2. Variation of (a) mass loss, and (b) corrosion rate of Zn-27Al based composites

containing 7 wt% BLA-SiC in 3.5% NaCl solution


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(a)

(b)

Figure 3. Variation of (a) mass loss, and (b) corrosion rate of Zn-27Al based composites containing 10 wt % BLA-SiC in 3.5% NaCl solution

For the 7 wt% reinforced Zn-27Al based composite grades (Figure 2), it is observed

from both the plots of mass loss (Figure 2a) and corrosion rate (Figure 2b) that there are

consistent fluctuations in the mass loss and corrosion rate of the composites in NaCl solution

over the exposure period for this study. This trend is linked to repeated film formation and

breakdown on the surface of the composites. It is noted however that the composite grade

containing 20% BLA had the highest resistance to corrosion of all the composites produced in

this series. The fluctuating mass loss notwithstanding, the 20% BLA containing composite

grade is observed to exhibit weight gain for most of the period of immersion. This suggests

fairly steady film formation.

The composite with 40% BLA is observed to be the most susceptible to corrosion

compared to the other composites in this series. This is a clear indication that proper selection

of BLA/SiC mix ratio can result in improved corrosion resistance of the Zn-27 Al based

composites in NaCl environment in comparison to the grades reinforced with only SiC.

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For the 10 wt% reinforced composite grades (Figure 3), it is observed that the

composites were relatively more stable in NaCl solution compared with the 7 wt % composite

grades (Figure 2). The mass loss of the composites (Figure 3a) reduced significantly with the

addition of 30 to 40% BLA compared with the other composite compositions in this series.

The progressive weight gain observed in the composite grades containing 30 and 40%

BLA, is indicative of the consolidating nature of film deposition on the surface of the

composites with continuous exposure in the NaCl solution.

Thus the oxide films formed are very stable in NaCl solution. Similar trend in

improvement of corrosion resistance with the addition of BLA has been reported by Alaneme

et al [19]. The trends observed from the mass loss results (Figure 3a) are supported by the

corrosion rate plots (Figure 3b).

The variation of mass loss and corrosion rate with exposure time of the composites

produced in 0.3M H2SO4 solution is presented in Figures 4 and 5.

(a)


containing 7 wt% BLA-SiC in 0.3M H2SO4 solution


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For the 7 wt% reinforced composite grades (Figure 4), it is observed from the mass

loss plots (Figure 4a) that the corrosion resistance of all the hybrid composites was superior to

that of the single reinforced composite grade.

This observation is supported by the corrosion rate plot (Figure 4b) where it is noted

that peak corrosion rate was obtained at the third day of immersion. It is apparent that the

corrosion resistance of the composites roughly improved with increase in the BLA content.

This suggests that the addition of BLA (up to 40%) helps in stabilizing the passive film

formed on the surface of the composites and hence improves its protective capacity.

For the 10 wt% reinforced composite grades (Fig. 5), it is observed from the mass loss

(Fig. 5a) and the corrosion rate plots (Fig. 5b) that with the exception of the composite grade

containing 30% BLA, the corrosion resistance of the hybrid composite grades containing 20

and 40% BLA was better than that of the single SiC reinforced composite grade. This

corrosion trend is quite similar to that of the 7 wt% reinforced composite grades.

(a)


containing 10 wt% BLA-SiC in 0.3M H2SO4 solution

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Thus it can be stated the use of hybrid compositions of BLA and SiC in reinforcing

Zn-27Al based composites results in improved corrosion resistance in H2SO4 environments in

comparison with the use of single SiC reinforced Zn-27Al composite grade.

Conclusions

The mechanical and corrosion behavior of Zn27Al alloy based composites reinforced

with 7 and 10 wt % BLA and SiC in varied weight ratios was investigated. The results show

that:

• The hardness and tensile strength of the composites decreased with increase in the weight

percent of BLA although less than 11% for all experimental cases studied.

• The percent elongation (%E) improved slightly with the addition of a maximum of 30%

BLA content for both the 7 and 10 wt% composite grades.

• The fracture toughness increased consistently with increase in the weight percent of BLA

in both the 7 and 10 wt. % composite grades.

• The hybrid composites were very stable in 3.5 % NaCl solution and the corrosion

resistance in 0.3M H2SO4 solution was superior to that of the single SiC reinforced Zn-

27Al composite grade.

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