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Corresponing author: Abrar Ahamed E-mail address: abrar_gce@rediffmail.com

Doi: http://dx.doi.org/10.11127/ ijammc.2016.04.14 Copyright@GRIET Publications. All rights reserved.

74

Advanced Materials Manufacturing & Characterization Vol 6 Issue 2 (2016)

Advanced Materials Manufacturing & Characterization

journal home page: www.ijammc-griet.com

Study on Slurry Erosive Wear Behaviour of Al 6061 based Composites

Abrar Ahamed1*, Anwar Khan2

1 Department of Mechanical Engineering, Birla Institute of Technology, off shore campus, UAE 2 Department of Mechanical Engineering, Ghousia College of Engineering, Ramanagaram, Karnataka, India

Abstract In the present investigation, Titanium di oxide (TiO2) reinforced Al

6061 composites were fabricated using liquid metallurgy route. The

performance of Al 6061 based Titanium di oxide composites in slurry

erosive environment was examined. The slurry consists of equiaxed

sand particles of size approximately 600 µm in 3.5%NaCl solution. The

slurry erosive wear studies on Al 6061 alloy and Al 6061-TiO2

composites were carried out at different sand concentration, varying

rotational speed and at different time duration of exposure. It was

observed that slurry erosive weight loss decreased with increase in

TiO2 reinforcement. However, there was increase in slurry erosive

weight loss with increase in slurry concentration, rotational speed and

time duration of exposure.

Key words: Aluminium 6061 composites, TiO2, slurry erosive wear.

1. INTRODUCTION

Aluminum alloys are preferred engineering material for

automobile, aerospace and mineral processing industries for

various high performing components that are being used for

varieties of applications owing to their lower weight and

excellent thermal conductivity properties. Among several series

of aluminum alloys, heat treatable Al6061 and Al7075 are much

explored, among them Al6061 alloy are highly corrosion

resistant and are of excellent extricable in nature and exhibits

moderate strength and finds many applications in the fields of

construction (building and high way), automotive and marine

applications [1]. The composites formed out of aluminum alloys

are of wide interest owing to their high strength, fracture

toughness, wear resistance and stiffness. Further these

composites are superior in nature for elevated temperature

application when reinforced with ceramic particle [2]. In recent

years, the use of fly ash as a reinforcement material in Al alloys

has been reported to be desirable from both environmental and

economic points of view due to its availability as a low cost

waste material [3].

Slurry erosion can be defined as a type of wear or loss of

material experienced by a component, when exposed to high

velocity stream of slurry mixture of solid particles in a liquid,

usually water [1]. When the components are entrained in such

environments, the design life of the component is greatly

reduced, resulting in huge economic losses. The areas in which

components suffering from this problem include, mining

machinery components, hydraulic transport of solids in

pipelines, marine, oil gas and power generation industries [2–

5]. Erosive wear is a complex phenomenon due to presence of

too many variables such as

1. Target parameters: includes, composition, microstructure, mechanical properties [6-8].

2. Process parameters: viz particle size, shape, velocity and particle concentration [6, 8-10].

3. Environmental parameters: temperature, humidity, etc. [11].

Many peoples have studied and reported on slurry erosive

wear behavior of metal matrix composites. Caron et al. [12]

75

studied the slurry erosive wear behavior of 5083-Al2O3

composites. They have noticed that, slurry erosive wear of

composites increased with increase in Al2O3 content in the

matrix material. Ramachandra et a.l [13] have reported on

slurry erosive wear behavior of Al/SiC composites, slurry

erosive wear resistance increases with increasing if SiC content.

The formation of passive layers on the surface of the slurry

erosive specimens decreased wear loss by forming protective

layers against the impact of slurry. Ramachandra and

Radhakrishna [14] have reported on slurry erosive wear

behavior of Al-12wt%Si alloy reinforced with fly ash

composites. They have reported that use of flyash has enhanced

the slurry erosion wear resistance of the developed composites

which has been attributed to the formation of protective

passive layer on the worn surfaces. Li et al. [15] have proposed

the effect of time duration on slurry erosive wear of aluminum

alloy and have found that wear rate increases with an increase

in test time duration. Aso et al. [16] have investigated the effect

of impact velocity and sand concentration on erosive wear and

have reported that, with increase in sand concentration and

impact velocity increases the wear rate. Girish et al. [17] have

studied slurry erosion of ductile materials under normal impact

condition and reported that velocity and particle size has

strong dependence on erosion wear but solid concentration has

relatively weak dependence. Acharya et al. [18] have reported

increasing the hardness of target material decreases the wear

rate.

This paper reports on the slurry erosive wear behaviour of cast

Al6061 and Al6061-TiO2 composites. Cast Al6061 and its

composites have been produced by liquid metallurgy route

which is a very popular technique owing to its economy and

versatility coupled with large-scale production. The extent of

incorporation of TiO2 in the matrix alloy has been tried out

from 4wt% to 10wt% in steps of 2wt%. Microstructure studies,

micro hardness and slurry erosive wear tests using the

fabricated erosive test rig have been conducted on the base

matrix Al6061 and Al6061-TiO2 composites.

2. EXPERIMENTAL PROCEDURE

Aluminum 6061 was used as matrix material owing to its

excellent mechanical properties coupled with good formability

and its wide applications in industrial sector. The material was

procured from M/s Plast-Met-Chem in the form of plates. The

chemical composition of the material is given in table. 1

Table 4.1 Chemical composition of Al6061

Titanium dioxide was chosen as reinforcement owing to its

high hardness and low co-efficient of thermal expansion. TiO2 is

highly wear resistant and also has good mechanical properties,

including high temperature strength and thermal shock

resistance [19]. The properties of Titanium dioxide are listed in

Table. 2.

Table. 2 Properties of Titanium dioxide

Density 4.2g/cc

Tensile Strength 300-350 Mpa

Vickers Hardness 980 kgf/mm2

Compressive Strength 800-1000 MPa

Modulus 240 GPa

A batch of 3.5kgs of Aluminum 6061 alloy was melted using a

6KW electric furnace as shown in Fig.1. The metal was

degassed using commercially available chlorine based tablets

(Hexachloroethane). The molten metal was agitated by use of

mechanical stirrer rotating at a speed of 300 rpm to create a

fine vortex. Preheated TiO2 powders (preheated to 700C for 2

hrs) were added slowly into the vortex while continuing the

stirring process. The stirring duration was 10 min. The

composites melt maintained at a temperature of 710C was

then poured in to preheated metallic moulds. The stirrer

blades were made of stainless steel and coated with ceramic

material to minimize the iron pick up by the molten metal. The

amount of TiO2 was varied from 4 to 10 wt% in steps of 2 wt%.

76

Fig.1. Photograph of Electric Resistance Furnace.

The samples were prepared for microstructure, hardness and

slurry erosive wear tests.

The micro hardness test was performed by applying 100 grams

of load for a period of 5secs on the polished surfaces of both the

matrix alloy and composites. The hardness was noted by taking

the diagonal length of indentation produced. The test was

carried out at five different locations in order to negate the

possible effect of indenter resting on the harder particles. The

average of all the five readings was taken as hardness of

sample.

Slurry Erosion test set up was fabricated using a commercially

available domestic mixer grinder. Fig.2 shows the details of the

fabricated test set up. This machine consists of a motor with a

speed control unit having the specification of a mixer grinder

with a maximum speed of 10,500 rpm.

Fig. 2 Slurry Erosion testing set up

Slurry erosion test were carried on Al6061 alloy and Al6061-

TiO2 composites. The slurry erosive wear test samples of 8 mm

diameter and 15 mm height were machined from the castings

and prepared by polished as per standard metallographic

procedure. The samples were cleaned in acetone and weighed

using an electronic microbalance before and after the wear

tests. The polished samples were fixed on spindles with the

help of a screw. The samples were dipped into slurry pot made

of stainless steel. The slurry was prepared by mixing 3.5% of

NaCl and silica sand with distilled water. The studies were

carried out at different sand concentration, varying rotational

speed and at different time durations while keeping the

impinging particle size to be constant as an average of 300

microns. After the test, specimens were dried and cleaned

before measuring weight loss.

3. RESULTS AND DISCUSSIONS

3.1 Microstructure

The optical microphotographs of the cast Al6061 and Al6061-

TiO2 composites are shown in Fig. 3. The micrographs clearly

indicate the evidence of minimal porosity in both the base alloy

and the composites. The distribution of TiO2 particles in a

matrix alloy is fairly uniform. Further these microphotographs

reveal an excellent bond between the matrix alloy and the

reinforcement particles.

Al 6061 alloy Al 6061-4wt%TiO2

composite

Al 6061-6wt%TiO2

composite

Al 6061-10wt%TiO2

composite

77

Fig. 3. Microphotographs of as cast Al 6061 alloy and its

composites

3.2 Microhardness:

The variation of micro hardness with increased contents of TiO2

particles in the matrix Al6061 alloy is shown in Fig. 4. It is

observed that with increased content of TiO2 particles in the

matrix alloy, there is a significant improvement in the micro

hardness of the composites. An improvement of around 43% is

observed in Al6061-10wt%TiO2 composites when compared

with the unreinforced Al6061 matrix alloy. This trend is similar

with the result of other researchers [20].

The improvement in hardness of composites can be attributed

to the following factors.

1. TiO2 is a hard reinforcement. Hard reinforcement in a soft and ductile matrix always enhances the hardness of the matrix alloy in general.

2. Probable increased density of dislocations due to thermal mismatch between the matrix alloy and TiO2 particles due to the large differences in coefficient of thermal expansion. The increased level of dislocations thereby increases the resistance of the materials to plastic deformation leading to improved hardness.

Fig. 4 Variation of micro hardness with increased content

of TiO2

3.3 Erosive Wear results:

3.3.1. Effect of Percentage weight of reinforcement:

There is a significant reduction in the slurry erosive wear rate

of the composites with an increase in the percentage weight of

the reinforcement as shown in the Fig. 5. This can be attributed

to the higher hardness of the composite as discussed earlier.

Higher the hardness better is the erosive wear resistance of the

materials.

Fig. 5 Variation of Slurry erosive wear rate of cast Al6061

and cast Al6061-TiO2 composites

The improved performance of Al6061-TiO2 composites when

compared with Al6061 alloy can be attributed to following

reasons:

1. Aluminum alloys when exposed to slurry media / NaCl solution, it reacts with water to form a stable passive oxide layer on the surface [21-22]. Formation of such oxide layer protects the surface of alloys from erosive and corrosive action of slurry.

2. Improved hardness of Al6061-TiO2 composites can also be attributed to improvement in slurry erosive wear resistance when compared with Al6061 alloy. Higher the surface hardness of the material lowers the material removal rate by mechanical action of solid particles in the slurry.

3. Presence of TiO2 particles in the material reduces the effective metallic area of the composite exposed to slurry and reduces the formation and growth of corrosion pits in the material which may be another reason for improved wear resistance of the composites [23]. From the microphotographs it is observed that there exist minimum micro porosities in the composites. Composites with lower the porosities exhibits better wear and corrosion resistance as reported by several researchers [24-25].

4. Good bond that between matrix and reinforcement also resist the corrosion attack and slurry erosive wear [26-27].

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3.3.2. Effect of Slurry Concentration:

Fig. 6 Variation of Slurry erosive wear rate of cast Al6061

and cast Al6061-TiO2 composites for different slurry

concentration

The slurry erosive wear rates of base Al6061 alloy and Al6061-

TiO2 composites with different concentration of silica slurry at

a given slurry rotation and time duration is as shown in Fig .6. It

is observed that increased slurry concentration results in

higher slurry erosive wear rate of both the base alloy and its

composites studied. Increased slurry erosive wear rates at

higher slurry concentrations can be attributed to the fact that

increased abrasive particle concentration in the slurry will

enhance the probability of larger impingement surface in the

slurry. This in turn will result in increased deterioration of the

material from its surface. Also, as the concentration of solid

mass increases in the slurry the sand particles can interact with

target material more strongly leading to increased mass loss

[28]. This phenomenon is clearly observed on subjecting the

worn surfaces to SEM studies. Increased slurry concentration

has resulted in higher density of cracking and also certain

deposit formations over the exposed surface as shown in Fig .7.

However, increased content of reinforcement in the matrix

alloy reduces the slurry erosive wear rate for all the slurry

concentrations studied. This can be attributed to the higher

hardness of composites with increased content of TiO2 particles

in the matrix alloy.

60 gms/ltr 90 gms/ltr

120 gms/ltr

Fig.7 SEM photographs of worn slurry erosive wear test

specimens for different slurry concentrations.

3.3.3. Effect of speed of slurry rotation:

Fig.8: Variation of Slurry erosive wear rate of cast Al6061

and cast Al6061-TiO2 composites for different speeds of

slurry rotations

The slurry erosive wear rate of Al6061 alloy and Al6061-TiO2

composites with different speed of silica slurry rotation at

constant time duration and slurry concentration is as shown in

Fig.8. It is observed that increased speed of slurry rotation

results in higher slurry erosive wear rate of both the base alloy

and its composites studied. At very high speed of 10,500 rpm

maximum slurry erosive wear rate is observed. The increased

speed of the slurry rotation will tend to increase the velocity of

impingement of the abrasive grains present in the slurry.

Increased impingement velocity will lead to higher rates of

material removal from the surfaces resulting in higher slurry

erosive wear rate. The larger extent of impingement at higher

speed of slurry rotation is demonstrated by SEM photographs

as shown in Fig.9. These SEM photographs clearly indicate the

presence of several craters on the worn surfaces. Higher the

speed of slurry rotation larger is the extent of crater formation

noticed on the worn surfaces as evident from Fig.9. However,

79

increased content of reinforcement in the matrix alloy reduces

the slurry erosive wear rate for all the speeds of slurry

rotations studied. This can be attributed to the higher hardness

of composites with increased content of TiO2 particles in the

matrix alloy.

8000rpm 9000rpm

10500rpm

Fig.9 SEM photographs of worn slurry erosive wear test

specimens for different speeds of slurry rotations.

3.3.3. Effect of time duration:

The slurry erosive wear rates of Al 6061-TiO2 composites with

different time duration at a given slurry rotation and speed is

as shown in Fig.10. It is observed that increased time duration

results in reduction of slurry erosive wear rate for both the

base alloy and the composites. This can be attributed to the fact

that the surface of the specimen gets strain hardened with as

the abrasive particles frequently impinge over its surface. This

phenomenon will lower the material loss from the surfaces.

Further decrease in weight loss can also be due to formation of

passive layer over the exposed surface of the specimens which

retards the slurry erosive wear rate by acting as a protective

layer [29]. However, increased content of TiO2 in matrix alloy

reduces the wear rate for all time duration. This can be

attributed to the higher hardness of composites with increased

content of TiO2 in matrix alloy. Probable deposits are observed

in the present study as shown in SEM photographs in Fig.11.

Fig.10 Variation of Slurry erosive wear rate of cast Al6061

and cast Al6061-TiO2 composites for different time

durations of exposure.

30 min 45 min

60 min

Fig.11 SEM photographs of worn slurry erosive wear test

specimens for different time durations.

4. CONCLUSIONS

Al 6061-TiO2 composites have been cast successfully by liquid metallurgy route.

Micro structure clearly confirms minimum porosity in composites developed.

Significant increase of 43% is noticed in hardness for composites having TiO2 as reinforcement.

Al 6061- TiO2 composites possess superior slurry erosion resistance when compared with Al 6061 alloy.

80

With increase in TiO2 reinforcement there is increase is slurry erosive wear resistance.

With increase in speed, slurry concentration and test duration there is increase in slurry erosive wear rate observed.

However, under all test conditions studied Al 6061-TiO2 composites have exhibited lower slurry erosive wear rate when compared with Al 6061 matrix alloy.

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