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Advances in Mechanical Engineering
Volume 2013 (2013), Article ID 658085, 10 pages
Analysis of Dry Sliding Wear Behaviour of Aluminium-Fly Ash Composites: The Taguchi Approach
1Department of Mechanical Engineering, Karpagam College of Engineering, Coimbatore 641032, India
2Department of Metallurgical Engineering, PSG College of Technology, Coimbatore 641004, India
3Department of Mechanical Engineering, Sri Shakthi Institute of Engineering & Technology, Coimbatore 641062, India
Received 31 December 2012; Revised 25 March 2013; Accepted 8 April 2013
Academic Editor: Rehan Ahmed
Copyright © 2013 Shanmughasundaram Palanisamy et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Dry sliding wear test was conducted on Al-fly ash composites employing pin-on-disc wear testing rig. The influence of parameters such as weight percentage of fly ash, load, and sliding speed on the wear rate of specimens was investigated. Specimens were fabricated by adapting a modified two-step stir casting method. The Taguchi and analysis of variance (ANOVA) techniques were employed to investigate the influence of parameters on the wear rate of composite. Multiple linear regression models were developed to predict the wear rate of the composites. It was found that the weight percentage of fly ash was the most dominant factor influencing the wear rate followed by the applied load and sliding speed. The morphology of worn surfaces of the wear pins was analyzed by scanning electron microscope (SEM) to analyze the wear behaviour. EDS analysis was carried out to investigate the mechanical mixed layer (MML), which forms on the worn surface of the wear pins.
Aluminium Matrix Composites (AMCs) are being used in aerospace and automotive industries since they possess superior strength, stiffness, thermal stability and enhanced tribological behaviour. Several research works have been carried out for the production of composites. The stir casting method can be employed for the production of large number of complex shaped components in a cost-effective manner compared to the powder metallurgy process which has its own drawbacks such as processing cost and size of the components.
Wear is material removal from one surface of the component to another during relative motion between them. Prasad et al.  reported that adhesive wear occurs when two solid surfaces slide over one another under pressure, whereas abrasive wear may involve gouging, grooving, and plastic deformation caused by the penetration of hard abrasive reinforcement particles. Askeland and Phule  have reported that materials with a high hardness, strength, and toughness are most resistant to abrasive wear.
Ibrahim et al.  reported that addition of a particulate reinforcement in metal matrix enhances the tribological behaviors and composites can be used for many engineering situations where sliding contact is expected. Several investigations have been carried out for analyzing the wear behaviour of aluminium matrix composites. The most commonly used metal matrix composite system consists of aluminium alloy reinforced with ceramic particulates [4, 5]. Recently low-cost and low-density fly ash particulate reinforcements are being analyzed as replacements for the relatively more expensive reinforcements such as SiC, Al2O3, and B4C.
Gurcan and Baker  investigated the wear resistance of Al alloy-SiC composites and reported that the maximum wear resistance was observed in the composite containing 20 wt.% of SiC particles. Mondal et al. studied the effect of applied load, size and volume fraction of reinforcement on the abrasive wear behaviour of Al alloy–Al2O3 composites. They observed that the wear rate of composite decreases linearly with increase in Al2O3 content.
Ramachandra and Radhakrishna  have analyzed the influence of fly ash concentration on sliding wear behavior of aluminium matrix composites fabricated by the stir casting method and concluded that Al alloy-15 wt.% of fly ash particulate composite showed enhanced abrasive wear resistance. They have studied different wear mechanisms under different parameters such as normal load, sliding velocity, and weight percentage of fly ash content. They reported that an increase in normal load and sliding velocity increased the magnitude of wear and frictional force.
Sudarshan and Surappa  reported that addition of fly ash particles into Al alloy showed lower wear rates compared to the Al alloy, and subsurface delamination is the main mechanism in both the Al alloy as well as in composites at higher loads. Sarkar et al.  concluded that the addition of magnesium increases the wettability which enhances wear resistance and mechanical properties such as hardness and tensile strength of Al-fly ash composites.
The present work aims to investigate the wear behaviour of fly ash particles reinforced by pure aluminium composites fabricated by a modified two-step stir casting method by employing the Taguchi and ANOVA techniques. Scanning electronic microscope (SEM) was employed to analyze the microstructure as well as wear surface morphology to study the wear behaviour of the composites and identify the operating wear mechanism as well as other typical features of wear. Energy dispersive X-ray spectroscopy (EDS) was employed to characterize the mechanical mixed layers that are formed on the surface of the worn surfaces during sliding and its influences on the wear behaviour of composites.
In this study, 99.5% pure aluminium ingot was used as the matrix material, and fly ash particles (<75 μm) were used as the reinforcement material which consists of SiO2, Al2O3, Fe2O3, CaO, and MgO. SEM image of fly ash particles is shown in Figure 1. Composites were produced by employing modified two-step stir casting method. The detail of fabrication of composites is presented in the paper .
In the modified two-step stir casting method, Al was charged in to the graphite crucible and the furnace temperature was raised up to liquidus temperature (670°C) of Al in order to melt the Al scraps completely. During stirring, preheated fly ash particles were added into the crucible at the side of the vortex. 1.5 wt.% Mg was incorporated to the melt to promote the wetting action between Al matrix and fly ash particles. The melt temperature was brought down to 620°C to achieve the semisolid state. Stirring was done for 5 minutes in the semisolid state. The composite slurry was again reheated to the liquidus temperature of 655°C and stirred at 300 rpm for 5 minutes. Finally, the composite slurry was poured into the steel mould to solidify. Argon gas was continuously blown at the rate of 2 cc/min into the furnace during the process to minimize the high temperature oxidation problems.
During the stirring process, the impeller was continuously moved vertically within the slurry at a rate of 2 mm/s by means of stirrer position control unit. Radial impeller with 0.7 IOD/CID ratio (impeller outer dia to crucible inner dia) was employed to achieve the sufficient turbulence in the margin area and prevent deposition of the fly ash clusters on the wall surface of the crucible. Reciprocating as well as rotary movement of impeller in the composite slurry during stirring was done using a mechanism. It prevents the settling of wetted fly ash particles and maintains the particles in a state of suspension to enhance the uniform distribution.
2.1. Microstructural Analysis
Various researchers [11, 12] have emphasized that wear behaviour exhibited by composites are greatly influenced by the type, size, and weight percentage of reinforcement and distribution of reinforcing particles in the metal matrix. Microstructures were examined on the Al-15 wt % fly ash composite samples which are produced by liquid state stirring and modified two-step stir casting method to reveal the fly ash particle distribution.
Figure 2 shows the micrograph of Al-15 wt.% fly ash composite developed with the help of liquid stirring. It can be observed from the micrograph that the clustering of fly ash particles was seen in the composite. At the same time, some regions were identified without fly ash particles.
It can be concluded that the liquid state stirring could not solve the problem of poor wetting. Because when the stirring is stopped, the fly ash particles tended to return to the molten surface due to its higher surface tension. On the other hand, when the stirring is done at liquid state, wetted fly ash particles tend to accumulate at the bottom of the crucible due to higher weight density.
Figure 3 shows the micrograph of Al-15 wt.% composite developed with the help of modified two-step stirring. It can be observed from the micrograph that the clustering of fly ash particles was not seen in the composite. Fly ash particles are being distributed more uniformly in the Al matrix. Since factors such as interfacial bond between the reinforcement particles and matrix material are significant in influencing the wear resistance of the particulate metal matrix composites, the modified two-step stir casting method was adapted for production of Al-fly ash composites.
2.2. Dry Sliding Wear Test
Dry sliding wear test was conducted using pin-on-disc wear testing rig. The wear loss of the composite pin material was recorded with an accuracy of 1.0 μm by the LVDT which is provided in the wear testing apparatus. Cylindrical pins (10 mm diameter and 15 mm height) were prepared and loaded in a computer interfaced pin-on-disc wear testing rig. Prior to testing, the surface of the specimens was polished by using 1000 grit paper. The rotating disc was made of EN 32 steel and hardness of 65 HRC. Wear tests were carried out at 29°C room temperature and 65% relative humidity for 20 minutes.
2.3. The Taguchi Method
Taguchi’s parameter design provides a systematic and efficient methodology for determining optimum design parameters which have an effect on the process and performance. The Taguchi method utilizes orthogonal arrays to study a large number of variables with a minimum number of configurations. In this study, “smaller is better” S/N ratio is used to predict the optimum parameters because a smaller wear rate was desirable. Mathematical equation of the S/N ratio for “smaller is better” is represented as where is the observed data and is the number of observations.
In the present investigation, wear tests were conducted in the composite material as per the L27 orthogonal array (Table 1). Accordingly, 27 experiments were carried out and each experiment was repeated twice in order to minimize the experimental errors. The factors and the corresponding levels which have been used are presented in Table 2. Experimental results were analyzed using analysis of variance (ANOVA) to study the influence of the factors on wear rate.
3. Results and Discussion
3.1. Results of S/N Ratio
The S/N ratio for each parameter level is determined by averaging the S/N ratios at the corresponding level. Process parameters with the highest S/N ratio would give the optimum quality with minimum variance. The influence of parameters such as weight percentage of fly ash content, applied load, and sliding speed on wear rate have been analyzed. Measured values and S/N ratios for wear rate of composites are given in Table 3. Ranking of parameters is presented in Table 4 using signal-to-noise ratios obtained for different parameter levels. It can be observed from the Table 4 that the weight percentage of fly ash content is a dominant parameter on the wear rate followed by applied load and sliding speed.
From the response diagram of S/N ratio (Figure 4), it was found that the optimum parameters were wt.% fly ash (20 wt.% fly ash), load (5 N), and sliding speed (1 m/s) for the composites.
3.2. Results of ANOVA
ANOVA determines the optimum combination of process parameters more accurately by investigating the relative importance among the parameters. ANOVA was performed with the help of the software package MINITAB15 for a level of significance of 5% to study the contribution of the factors. In the ANOVA analysis Table 5, there is a value for each independent parameter in the model. The value is used to test the significance of each parameter and interaction between parameters. When the value is less than 0.05, then the parameter can be considered as statistically highly significant. It was observed that wt.% fly ash, applied load, and sliding speed have less than 0.05, which means that they are highly significant at 95% confidence level.
The interaction effect of applied load with sliding speed (B*C) is a significant model term influencing the wear rate of Al-fly ash composites, since it has value < 0.05. Since the value for the interaction terms (A*B) and (A*C) is greater than 0.05, the model terms may be considered as statistically insignificant and neglected. The last column of Table 5 shows the percentage contribution (Pc%) of each variable in the total variation indicating their degree of influence on the wear rate of the composites. It can be observed that the wt.% fly ash (52.22%) was the major contributing factor followed by load (40.86%) and finally sliding speed (2.64%) influencing the wear rate of the Al-fly ash composites.
3.3. Multiple Linear Regression Model
A multiple linear regression equation was developed to establish the correlation among the significant factors on the response. The value of regression coefficient (0.9904) is in good agreement with the adjusted (0.9688) for Al-fly ash composite. It can be noted that since the value of regression coefficient for the model is 0.9688, the wear data were not scattered. Since both the values are reasonably close to unity, models provide a reasonably good explanation of the relationship between the independent factors and the response (wear rate).
The regression equation developed for wear rate of the Al-fly ash composite is where : dry sliding wear loss, : fly ash content, wt.%, : Applied load, N, : Sliding speed, m/s.
It can be observed from (2) that the coefficients associated with fly ash content and sliding velocity are negative. It indicates that the wear rate of the composite decreases with increasing fly ash content and sliding velocity. Conversely the wear rate of the composite increases with increasing the applied load since the coefficient associated with the applied load is positive.
3.4. Confirmation Test
A confirmation test is the final step in the design of the experiment process. It was found that the optimum parameters were wt.% fly ash (20 wt.% fly ash), load (5 N), and sliding speed (1 m/s) in minimizing the wear rate of the composites. The confirmation experiments were conducted, and results are presented in Table 6. Computed values from the regression equation and the experimental values for the wear rate of the composites are nearly the same with the least error (±5%). The resulting equations seem to be capable of predicting the wear rate to the acceptable level of accuracy.
3.5. Construction of Wear Maps
The contour and response surface plots which are the graphical representation of the regression equation employed to establish and visualize the relationship between the response and experimental levels of each factor. In a contour plot, the values for two factors are represented on the - and -axes, while the values for a third factor are represented by shaded regions. A contour surface provides a two-dimensional view, while surface plot provides a three-dimensional view.
The plots (Figures 5(a) and 5(b)) show that the wear rate is increased at high load and high speed conditions, whereas wear rate is decreased at low load and high speed conditions. The higher wear rate is obtained at a load of 15 N and a sliding speed of 1 m/s. Low wear rate is obtained at a load of 5 N and a sliding speed of 1 m/s. Low load with higher sliding speed enhances the resistance to wear through the formation of stable mechanical mixed layer (MML) on the surface of the composite pin during wear process. It can be concluded that there is a significant interaction between the applied load and sliding speed on the wear rate of composites. The contour Figure 6(a) and surface plot (Figure 6(b)) illustrate that the trend of decrease of wear rate is in accordance with the gradual increasing of fly ash content and decreasing of applied load. It was found that the minimum wear rate occurred at a load of 5 N and fly ash content of 20 wt.%. It can be explained by the fact that the fly ash particles possess higher hardness than the aluminum and their incorporation increases the hardness of the resulting composites. This increase in wear resistance can also be attributed to a better interfacial bonding between Al and fly ash particles which helps in preventing the damages caused due to sliding action. Wear rate of composites tend to increase when applied load is increases. Both the contour plot (Figure 7(a)) and surface plot (Figure 7(b)) demonstrate that the wear rate tends to decrease when the fly ash content and sliding speed gradually increases. Low wear rate was observed at high fly ash content (20 wt.%) and high sliding speed (1 m/s) conditions.
3.6. Surface Morphology
Scanning electronic microscope (SEM) was employed to analyze the wear surface profile to study the wear mechanism of composites.
It can be seen from the worn out surface of Al-20 wt.% fly ash composite (Figure 8) that the wear grooves and scratches along the sliding direction were smaller at 5 N with 1 m/s sliding speed. The dark surface was found to be covered on the wearing surface of the composite pin when observing under a microscope. When the sliding speed is increased at low load, the plugged surface of the counter face (Fe) reacts and forms an oxide-like transferred layer (Fe3O4) at the sliding interface due to the frictional heating. It reduces the direct metallic contacts resulting in a lower wear rate of composite. The fly ash particles also get crushed and form very minute particles due to abrasive action of the fly ash particles against the steel counter face. So, these particles form a mechanical mixed Layer (MML) which are composed of a mixture derived from the sliding surfaces enhance the wear resistance. Antoniou and Borland  have reported that the worn surface was characterized by the formation of an iron-rich compacted debris layer and increases the wear resistance at lower loads.
Figure 9 shows the wear track morphology of Al-20 wt.% fly ash at normal load of 15 N with 1 m/s sliding speed. The worn surface is characterized by deep grooves and the cracks on the surface propagate in the subsurface zone which brings material in the form of flakes. Since the higher speed increases the interface temperature, the surface of the material becomes soft which promotes local yielding and wear mechanism changes to delamination wear. This behaviour is termed as severe wear behaviour, in which material removal occurs at an accelerated rate. Transition from mild to severe wear is associated with the existence of delamination and adhesion which are the primary wear mechanisms at higher applied load and sliding speed conditions.
EDS test was performed on the worn surface of the Al-fly ash composite pins to confirm the formation of MML. High magnification view of the compact surface layer of Al-20 wt.% fly ash at normal load of 5 N with 1 m/s sliding speed is presented in Figure 10.
It can be observed from Figure 10(a) that the brighter compacted layer was seen on the worn surface of the Al-20 wt.% fly ash composite pin. Peaks of O and Fe, which are the main constituents of MML, are clearly detected on the worn surface. The presence of oxidized material indicates that the oxidative wear is predominant, in which frictional heating causes oxidation of the surface. Though the fly ash particles contain various elements such as oxides of Al, Si, C, Ca, Mg, O and Fe, more Fe element was found on the surface. Being rich in Fe and lean in aluminium content indicates that iron has been transferred from the counter steel disc during sliding. It can be concluded that increasing speed at low load has a tendency to form MML on the worn surface of the composite, and MML was more uniform in the entire wear path of the composite pin. Hence, MML acts as a protective layer and reduces the direct contact between composite pin surface and counter face thereby enhances the wear resistance. The formation of such layers has been reported by Suh et al. .
High magnification view of the worn surface layer of Al-20 wt.% fly ash with a normal load of 15 N with 1 m/s sliding speed is presented in Figure 11(a).
It was observed from Figure 11(a) that the region marked “A” on the worn surface contains about 29.19% Al, while the region marked “B” contains about 50.46% Al. Similarly, other elements such as iron and oxides were not uniform in the entire wear track of the composite pin. Small white particles are visible, which originated from the rupture of mechanical mixed layer. Moreover, pullout of fly ash particles was seen on the compact layer. Since the MML is separated into some patches, the layer failed to withstand the higher load and sliding speed during wear process. Rosenberger et al.  reported that both MML formation rate and MML fracture rate should be necessarily equal to attain the constant MML thickness which ensures the steady state wear condition.
Wear surface of Al-20 wt.% fly ash composite contains larger amount of oxides when increasing the sliding speed from 0.5 m/s to 1 m/s at 5 N as demonstrated in Figure 12. It indicates that the oxidation wear mechanism is predominant under low load and high speed condition. EDS analysis shows that the MML contains iron of about 27 wt.%, which must come from the counter face steel disc.
On the other hand, when the load is increased from 5 N to 15 N, the amount of oxides present on the worn surface tends to decrease due to the increase of removal rate of the oxide film. It can also be noted that the substantial amount of iron has been transferred from the counter steel disc to the composite pin during wear process. However, broken and uneven oxide segments reduce the wear resistance of the composite. Delamination was found to be more extensive under the higher load of 15 N and 1 m/s since the amount of Al is increased compared to the 5 N and 1 m/s sliding conditions on the worn surface of composites. It can be concluded that the formation of MML depends on factors such as applied load and sliding speed.
This paper has presented an application of L27 orthogonal array of the Taguchi method and analysis of variance for investigating the influences of wt.% of fly ash particles, applied load, and sliding speed on the wear rate of composites. Based on this study, the following conclusions have been summarized.
The results revealed that wt.% of fly ash particles and load were the most significant parameters followed by sliding speed on the wear rate. It was found that the optimum parameters for minimum wear rate were wt.% of fly ash (20 wt.% fly ash), load (5 N), and sliding speed (1 m/s). The best combination values of optimum parameters were confirmed with the verification experiments. A multiple linear regression model was developed to predict the wear rate of the composites. The closeness of the results of predictions based on calculated S/N ratios and experimental values show that the Taguchi experimental design technique can be used successfully for both optimization and prediction.
Analysis of worn surfaces revealed that at lower load, oxidation was the dominant wear mechanism, whereas at higher load, delamination and adhesion were found to be dominant for the Al-fly ash composites. It was found that mild wear occurs at high speed and lower applied load, whereas severe wear occurs at high speed and higher applied load. It was observed that the presence of an MML certainly increases the wear resistance. However, at higher load, MML is easily broken and detached from the surface, resulting in higher wear rate.
The authors are grateful to the Faculty of Engineering, Karpagam University, Coimbatore, for providing the facilities to carry out this project successfully. The authors would like to thank Mr. S. Poovarasan and Mr. K. Prathap Singh who helped in preparing the composites.
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