study of the tribological properties of fly ash cenosphere ... · aluminium and its alloys are of...

Jurnal Tribologi 18 (2018) 124-135

Received 7 February 2018; received in revised form 6 April 2018; accepted 15 August 2018.

To cite this article: Bera et al. (2018). Study of the tribological properties of fly ash cenosphere particulate reinforced

LM6 (Al-Si12) matrix alloy composites produced by stir casting method. Jurnal Tribologi 18, pp.124-135.

© 2018 Malaysian Tribology Society (MYTRIBOS). All rights reserved.

Study of the tribological properties of fly ash cenosphere particulate reinforced LM6 (Al-Si12) matrix alloy composites produced by stir casting method Tanusree Bera *, Samir K. Acharya Department of Mechanical Engineering, National Institute of Technology Rourkela, Odisha 769008, INDIA. *Corresponding author: [email protected]

KEYWORD ABSTRACT

Fly ash cenosphere LM6 (Al-Si12) matrix alloy Abrasive wear SEM

Stir casting is an economical and simplest method for the production of metal matrix composites (MMCs). In this process, the mixing of the reinforcement particles into the molten metal bath is achieved by stirring. After stirring, the molten metal poured into the desired shape mould and allowed for solidification. In this study, LM6 (Al-Si12) matrix reinforced with fly ash cenosphere particulates produced MMCs through the stir casting method. The friction and wear behaviour were studied for LM6 (Al-Si12)/cenosphere composites by using pin-on-disc wear tester under dry sliding condition. The friction and wear tests were carried out at different weight percentages of cenosphere (wt. % c/s) under various loads and sliding velocity. The results showed that the wear resistance of LM6 (Al-Si12) / cenosphere composites is better than of LM6 (Al-Si12) matrix alloy. The specific wear rate and the coefficient of friction decrease as the load increases. The worn mass loss of the composites increases rapidly with the increase of the applied load. Scanning electron microscopy (SEM) studies reveal the presence of abrasive wear mechanisms.

1.0 INTRODUCTION

Aluminium and its alloys are of high specific strength and low density. Therefore, they have a wide range of application in the automotive and aerospace industries (Vencl et al., 2004; Rohatgi et al., 2006a). However, applications of these alloys are restricted due to their undesirable low


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hardness, low wear resistance. To improve the hardness and wear resistance of aluminium and its alloys, some steps has to be taken, such as preparing surface coating and adding reinforcement. One of the important measures was to reinforce the aluminium alloy matrix with various reinforcements like SiC particles Ghosh et al. (2005), Al2O3 fibers and particles (Kang and Chan, 2004) and fly ash cenosphere (Rohatgi et al., 1997; Surappa, 2008; Ramachandra and Radhakrishna, 2007). Sometimes the selection and addition of reinforcements to the aluminium alloy matrix increase the cost of the material; hence, it is important to select a low-cost reinforcement with excellent properties. Compared to various reinforcement, cenosphere is a good choice due to its low cost and better mechanical properties. The addition of cenosphere to the aluminium alloy matrix decreases the density of the aluminium alloy, which made the composites more favourable in the automobile and aerospace industries. Fly ash cenosphere was obtained in huge quantities as a solid waste by-product during coal combustion in thermal power plants. Fly ash particles contain either solid spheres called precipitator or hollow spheres named as cenosphere. The chemical constituents of fly ash cenosphere are SiO2, Al2O3, Fe2O3, and CaO (Rohatgi et al., 2006b).

LM6 (Al-Si12) matrix alloy (http://www.cast-alloys.com) has good casting characteristics. The major alloying element in LM6 (Al-Si12) is silicon. The alloy has good fluidity and hot tearing properties that make to produce thick and thin casting sections. It has a high strength which can be used for structural components in the automobile industry. The addition of high wear-resistant ceramic particles such as cenosphere to the alloy is expected to increase the mechanical and wear resistance properties (Rohatgi and Guo, 1997; Mahendra and Radhakrishna, 2007; Rohatgi et al., 1997).

There are various methods of fabrication of fly ash cenosphere reinforced MMCs, they are pressure infiltration technique (Rohatgi et al., 1998; Rohatgi et al., 2006c; Torralba et al., 2003), liquid metal stir casting technique (Rohatgi and Guo, 1997 ; Mahendra and Radhakrishna, 2007) and powder metallurgy process (Guo et al., 1997; Fogagnolo et al., 2006; Torralba et al., 2003).

The stir casting is a very economical method of manufacturing MMCs. It is the easiest liquid phase fabrication technique. In this method, the reinforcement particles are mixed with molten metal matrix alloy by stirring and then poured into the required mould of the desired shape and allowed for solidification.

Studies are available in the literature in which particulate fillers based on fly ash cenosphere particles are reported to have improved the mechanical and tribological properties of a composite. For instance, Rohatgi et al. (1997) and Rohatgi and Guo (1997) while worked with A356 Aluminum alloy observed that with fly ash content up to 10 wt. %, the abrasive wear resistance of aluminium–fly ash composite increases. Mahendra and Radhakrishna (2007) also worked on the same line for Al–4.5% Cu alloy with a different weight fraction of fly-ash as reinforcement. . The wear rate of the base alloy within 7000 s under dry sliding is about 600 µm. The wear rate of 5% fly ash is about 400 µm. The wear rate of 10% fly ash is slightly below 400 µm. The wear rate of the 15% fly ash is about 300 µm. Their results indicate maximum wear resistance of the matrix material with 15 wt. % of fly ash reinforcement. Yu et al. (2014) reported that the wear resistance of Fly ash composites FAC/AZ91D is better than that of AZ91D Mg alloy and the wear resistance of the composites increases up to the certain volume fraction of reinforcement after that it decreases.

Rohatgi (2001) worked on the centrifugally cast graphite aluminium with segregated graphite particles. The paper describes the historical evolution of Cast MMCs. It shows the effect of the reinforcement on the solidification and also discussed the microsegregation of solute,


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concentration of particles in the interdendritic regions during particle pushing phenomenon. Rohatgi et al. (2004) worked on the produced selected prototype castings of aluminium-fly ash composites using conventional foundry techniques in industrial environments. The prototype castings included intake manifolds, post caps, differential covers, brake drum, motor mounts; mounting brackets made by sand casting; squeeze casting and pressure die casting. The authors have measured the tensile strength and the coefficient of thermal expansion of the prototypes. The result shows that the tensile strength of the composite castings containing 2 to 12 vol% fly ash is higher than that of the base alloy. Kumar (2001) worked on Al-11 % Si –fly ash castings through V-process. The authors have made an attempt to visualize the effect of fly ash, on the tensile properties of the Al-11% Si alloy castings with the help of response surface methodology (RSM). These investigations emphasize that use of fly ash cenosphere as reinforcement in aluminium matrix composite yields better mechanical and tribological properties.

Many researchers used reinforcements other than fly ash. For instance, Pradhan et al. (2017) worked on the tribological properties of Al-SiC metal matrix composites under acid environment. The results show that the wear increases with increase in applied load and sliding speed but friction coefficient shows a decreasing trend with increase in load. The addition of SiC reinforcement increases the wear resistance of the metal matrix composite. Nuraliza et al. (2016) studied the tribological properties of aluminum lubricated with palm olein. The experiments had been performed under different parameters, different loads (10 N, 50 N, 100 N), and constant speeds at 3 m/s. They observed that load 100 N show high coefficient of friction compared to 10 N and 50 N. Authors found that palm olein has better performance properties in terms of friction reduction (coefficient of friction) and wear resistance (anti-wear properties) at low and high speed than mineral oil. Hirai et al. (2016) performed their experiments with the combined effects of graphite and sulfide on the tribological properties of bronze under dry conditions. They reported that the friction coefficient of the sulfide-containing bronze decreased and the seizure resistance properties significantly increased.

Although researchers have worked with the different matrix alloy composite, the work on LM6 (Al-Si12) matrix alloy with cenosphere as reinforcement is very limited. Hence in the present work, an attempt has been made to study the wear and friction behaviour of LM6 matrix alloy reinforced with various weight percentages of fly ash (5, 7.5, 10, and 12.5 wt. % c/s) through stir casting technique..

2.0 MATERIALS AND METHODS 2.1 Materials and synthesis of composites

The raw materials include fly ash cenosphere and LM6 (Al-Si12) matrix alloy ingot. The fly ash cenosphere was procured from National power Engineers Kolkata; India has a size range from 30-200 μm with a density of 0.6 g/cm3. The colour of the cenosphere is light grey. LM6 (Al-Si12) matrix alloy ingot is obtained from Kolkata, India. The physical properties and chemical composition of LM 6 (Al-Si12) is shown in Table 1 and Table 2 respectively.

Fly ash cenosphere/LM6 (Al-Si12) composites for the present investigation were prepared through stir casting method. The calculated matrix alloy was melted at 750-800°C in graphite crucible using an electric furnace for 2-3 hours; the matrix alloy has been reinforced with 5, 7.5, 10, and 12.5 wt. % c/s. The cenosphere particles were preheated at 40-50 °C in a separate muffle furnace for 2-3 hours to remove moisture and other gasses. To attain a good bonding between the


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matrix and reinforcement particle, very small amount of magnesium is added to the molten matrix alloy. The addition of magnesium increases the wettability and the fluidity of the molten matrix alloy. The preheated cenosphere particles were then added to the molten matrix alloy, and the molten matrix alloy was stirred at 100-150 rpm by a mechanical stirring system. The composite molten matrix alloy was poured into cylindrical graphite mould having 136 mm in height and 26 mm in diameter. After solidification, the prepared sample was taken out of the mould and was cut to required dimension for further experimentation. The same sample making procedure was repeated for unreinforced composites (LM6 (Al-Si12) matrix alloy). Figure 1 shows the complete procedure of sample preparation.

Table 1: The physical properties of LM6 (Al-Si12) matrix alloy.

Properties Values

Coefficient of Thermal Expansion (per °C @ 20-100°C) Thermal conductivity (cal/cm2/cm/°C @ 25°C)

0.000020 0.34

Electrical conductivity (% copper standard @ 20°C) 37 Density (g/cm3) 2.65 Freezing range (°C) approx. 575-565

Table 2: Chemical compositions of LM6 (Al-Si12) alloy.

Element percentage (%) Cu Mg Si Fe Mn Ni Zn Pb Ti Sb Al 0.1 0.5 11.7 0.6 0.5 0.1 0.1 0.1 0.2 0.05 remainder

2.2 Abrasion wear test Abrasive wear tests were carried out on a pin-on-disc wear testing machine designed as per

ASTM G-99 standard, supplied by Magnum Engineers, Bangalore, India (Figure 2). The tests were performed under a dry condition at room temperature. During the test, an abrasive paper of 150 grit size (93μm) was fixed on the rotating disc (EN 31 steel) of a diameter of 120 mm using Araldite paste. The abrasive paper has to be changed after each test because it becomes smooth due to sliding of a sample on it. The test samples were a cylindrical shape with a diameter of 10 mm, and a height of 26 mm was fixed on the tool holder and made to slide against the rotating disc. The load was placed by dead weights on the loading pan which was attached to the loading lever. The parameters like load, rotational speed, and wear track radius were fixed in the machine (Table 3). The frictional force was monitored through the electronics sensors and displayed on the machine. The sample was abraded under different loads for six intervals of 5 minutes. The samples were cleaned before and after each test with acetone to remove any wear debris. The weight loss (in grams) in the sample was measured by an electronic weighing balance. To examine the surface morphology of the worn out samples, SEM studies were carried out using a Field Emission Scanning Electron Microscope (FESEM) (Nova NANO SEM 450) make.


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Figure 1: Complete procedure of sample preparation.

Figure 2: Experimental setup of abrasive wear test on Pin-on disc wear tester.

Table 3: Test parameters of abrasive wear samples

Test parameter Units Values Weight percentage of cenosphere % 5, 7.5, 10, 12.5 Load N 5, 10, 15, 20 Track radius Mm 50 Sliding velocity m/s 0.5235, 1.0472, 1.5708, 2.0944 Temperature °C Ambient


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2.3 Density and porosity Density and porosity of the samples are given in Table 4. Experimental density was measured

by water displacement and theoretical values calculated from the rule of mixture. The porosity is calculated as:

𝑃𝑜𝑟𝑜𝑠𝑖𝑡𝑦 (%) =𝜌𝑇 − 𝜌𝐸

𝜌𝑇 𝑥 100

Where ρT is the theoretical density measured from the rule of mixture, and ρE is the density measured by water displacement method. Table 4: Density and porosity of the unreinforced and reinforced LM6 (Al-Si12)/ cenosphere composites.

Sample name Theoretical density (𝝆𝑻)

Experimental density(𝝆𝑬)

Porosity (%)

LM6 (Al-Si12) matrix alloy 2.650 2.640 0.377 5 wt. % c/s 2.675 2.633 1.570 7.5 wt. % c/s 2.687 2.630 1.865 10 wt. % c/s 2.697 2.627 2.595 12.5 wt. % c/s 2.704 2.625 2.921

3.0 RESULTS AND DISCUSSION 3.1 Wear properties

The variation of specific wear rate with the load of different wt. % c/s at a velocity of 1.57 m/s is shown in Figure 3. It can be seen that the specific wear rate of the composites decreases with increased in load. In other words, there is less removal of material at higher loads. The SWR decreased with increasing load for all samples. The wear resistance of the composite reinforced with 10 wt. % c/s is better as compared with other composites. Initially, maximum wear rate was observed. With consecutive runs, it decreased and attained the steady state for all samples. The same type of results obtained by Rohatgi et al. (1997); Rohatgi and Guo (1997). There can be two reasons. First one: in the initial stage of abrasion, abrasive is in contact with the matrix, has less hardness as compared to angular silica sand (abrasive) particles. At that particular instance, the ratio Ha (hardness of the abrasive particle)/Hs (hardness of the surface) is much more than unity, resulting in severe matrix damage and the rate of material removal is very high. Thus, the specific wear rate is more. When the load increases, cenosphere particles get in contact with abrasive particles, Ha/Hs ratio is a little more than unity; as a result, cenosphere particles provide better resistance to the process of abrasion and reduce the wear rate. Second one: the continuous increase in the load results in the work hardening of the pin contact surface and hence reduces the wear rate.

The variations of mass loss with sliding time at different load with various percentage of cenosphere are shown in Figure 4. It can be observed that the worn mass loss of sample decreases with the increase in the wt. % c/s. The worn mass loss at 10 wt. % c/s is less compared to other composites. This might be due to the addition of hard cenosphere particles in the LM6 (Al-Si12) alloy. The cenosphere particulate consists of ceramic particles, having high mechanical and wear resistant properties. It is observed that when the cenosphere addition reached 12.5 wt. %, the worn mass loss of the composites increases. This might be due to porosity, the porosity content


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increases (Table 4) in the matrix which leads to increase in the wear rate (Figure 3) and worn mass loss (Figure 4) (Yu et al., 2014; Rohatgi et al., 2009; Rajan et al., 2007).

Figure 3: Variation of specific wear rate of cenosphere/LM6 (Al-Si12) alloy composites with load.

Figure 4: Change of the worn mass loss of Al-Si12/ cenosphere composites with wearing time of 30 min at velocity of 2.09 m/s and load at (a) 5 N, (b) 10 N, (c) 15 N and (d) 20 N.


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3.2 Friction properties Figure 5 shows the variation of the coefficient of friction with the load. It is observed that with

the increase in load the coefficient of friction decreases for both matrix alloy and the reinforced composites. The coefficient of friction reduces with the increase in the addition of cenosphere. The coefficient of friction decreased with increasing load for all samples. This is an agreement with the results obtained by Rohatgi et al. (1997); Rohatgi and Guo (1997). The coefficient of friction of unreinforced composite was higher than reinforced composites and the composite with 12.5 wt. % c/s is less than other composites. There are two reasons for the decrease in coefficient of friction with the increase in load-First one: due to increasing in load the temperature of the contact surface increases and soften the surface of the pin. So, friction coefficient decreases. Second one: when the load increases more wear of pin surface occurs and the wear debris stuck in between pin and counter surface and acts as a roller ball, therefore, the coefficient of friction decreases.

Figure 5: Variation of coefficient of friction with load at V=1.04 m/s.

3.3 Density and porosity

The theoretical and experimental densities of the composite along with the corresponding porosity are presented in Table 4. It may be noted that there is the difference between experimental and theoretical densities of the composites as observed from the table. This difference is a measure of voids and pores present in the composite. It is clearly seen that with the increase in cenosphere content, there is an increase in the porosity in all the composites. These results have been observed in previous investigations (Hanumanth and Irons, 1993; Kok, 2000; Ghosh and Ray, 1988). The porosity formation has the following causes:

(a) Gas entrapment during stirring. (b) Air bubbles entering the melt matrix material. (c) Hydrogen evolution. (d) The difference in the size and shape of reinforcement and matrix.


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3.4 Worn surface morphology Figure 6 (a-j) shows the worn surface morphologies of LM6 (Al-Si12) matrix alloy and

reinforced composites at 5 N and 20 N loads. An arrow has been shown in all SEM images to indicate the sliding direction. As we can see in Figure 6 (a, c, e, g, i) at low load narrow furrows, are observed and when the load increases to 20 N (Figure 6 (b, d, f, h, j)) the worn surface morphology changes from narrow to deep and wide furrows with a large amount of wear debris scattered on the surface. The furrows are parallel to the direction of sliding. The wear debris from the reinforced composites is less and shorter than that from the LM6 (Al-Si12) matrix alloy. The higher load increases the stress at the contact point and the temperature at the sliding interface. This leads to chipping of hard and soft particles i.e. cenosphere particulates and aluminium particles from the sample. The presence of furrows and wear debris on the samples indicates the ploughing action and microcutting. This reveals the presence of abrasive wear mechanism.


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Figure 6: SEM Micrographs of the worn surfaces after wear test at the wearing time of 30 min under velocity of 1.57 m/s with (a) LM6 (Al-Si12) matrix alloy at 5 N, (b) LM6 (Al-Si12) matrix alloy at 20 N, (c) 5 wt. % reinforced composite at 5 N, (d) 5 wt. % reinforced composite at 20 N, (e) 7.5 wt. % reinforced composite at 5 N, (f) 7.5 wt. % reinforced composite at 20 N, (g) 10 wt. % reinforced composite at 5 N, (h) 10 wt. % reinforced composite at 20 N, (i) 12.5 wt. % reinforced composite at 5 N and (j) 12.5 wt. % reinforced composite at 20 N.


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CONCLUSION Al-cenosphere MMCs were successfully fabricated in the laboratory by stir casting technique.

The specific wear rate decreases as load increases for all samples. The wear resistance of the composite reinforced with 10 wt. % c/s is better as compared with other composites. Incorporation of cenosphere particles into LM6 (Al-Si12) alloy significantly reduces the abrasive wear loss. The optimum wear resistance properties were obtained at the cenosphere content of 10 wt. %. But the excessive addition of cenosphere particles (12.5 wt. %) results in higher porosity, which in turn increase in wear loss. The coefficient of friction of the composites decreases with the addition of cenosphere particles. The porosity of the composites increases with the addition of reinforcement. The 12.5 wt. % c/s has highest porosity compare to other composites. SEM analysis indicates the presence of narrow furrows at low load, as the load increases, the furrows become deep and wide and more wear debris are scattered on the interface surface. Hence, the SEM morphology analysis reveals the presence of abrasive wear mechanism. REFERENCES Fogagnolo, J. B., Robert, M. H., & Torralba, J. M. (2006). Mechanically alloyed AlN particle-

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study of the tribological properties of fly ash cenosphere ... · aluminium and its alloys are of...

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