Experimental Investigation on Wear Behaviour of AA6082 Aluminium Alloy, Tungsten Carbide and Graphite Hybrid Composites

In this experimental study, Aluminium alloy (AA) 6082 was strengthened with Tungsten Carbide and graphite through stir casting technique. Scanning Electron Microscope (SEM) was employed to study the wear performance of the Al/WC/Gr composites. Wear tests were carried out using a pin-ondisc apparatus. The input parameters in this study are the load applied (4, 8, 12, 16, and 20 kg), speed of sliding (1, 1.5, 2, 2.5 and 3 m/s) and distance slides (1000, 1500, 2000 and 2500 m). Response Surface Methodology (RSM) has been carried the use of MINITAB 14 software program to examine the rate of wear and frictional behaviour of the hybrid composites.


Introduction
Studies conducted in aerospace and automotive industries reveal that the materials used for fabricating items need to showcase good mechanical properties with higher resistance [1]. Composite elements are a choice of materials exclusively in utilizations like dry sliding areas to elevate the Tribological performance of the material. In several applications, Metal Model Composites (MMCs) with aluminium as the model material plays a vital in enhancing these properties [2]. The properties like environmental resistance strength to weight ratio and stiffness are the potential benefits of Al composites. On the other hand, they have low resistant to wear; especially at boundary or partial lubricating situations. Stir casting technique is the most commonly used method for fabricating aluminium composites because of it is economical compared to other processes. The statistical relation between the input and output variables and its influence on parameters can be estimated by using statistical modelling techniques like Response Surface Methodology (RSM). The application of RSM in various optimization problems were investigated by different authors [15][16][17][18][19]. Allwyn Kingsly Gladstonaa et al. [21] reported that RSM is one of the primary technique that deals by multi-variables problems that influences the responses.
This technique optimizes processes and determines the best combination. This idea considerably minimizes the trials required for the model to determine the optimal parameters. Although researcher studied on the various properties of wear in numerous aluminium alloys in dry conditions and wear behaviour of hybrid aluminium composite is limited. Dry sliding wear properties of materials depend on the parameters like load, speed of sliding and distance of sliding [25].In this research, the factors influencing frictional and wear characteristics of Al/WC/Gr composites of hybrid properties under stale sliding conditions are the mechanisms were analysed by SEM.

Experimental procedure 2.1. Materials
Readily available aluminium 6082 alloys has been selected for the model material in this study and its chemical compositions are depicted in Table 1. The aluminium metal model is heated at a temperature of 850°C till it attains the molten state. The molten melt is poured into the preheated WC and graphite crucible as per the ratio. It is then blended by a stirrer for about 10 mins to have a uniform diffusion of WC and graphite particles in the alloy. The blending speed is kept as constant throughout the experiment at 500 rpm approximately. The molten mix is then poured into the die of 50 mm length and 15 mm diameter. The specimens are then machined for the required size to conduct the wear tests in fig 1.

Tungsten carbide
Addition of Tungsten Carbide in aluminium enhances the mechanical and tribological behaviour of the composites [5-9]. WC is considered as one of potential high strength materials because of its high strength, high hardness and better chemical stability. Addition of Tungsten Carbide in aluminium alloy (AA) 6082 improves the wear resistance of the aluminium composites. Addition of hard particles like WC in Al alloy reduces the rate of wear, and it is influenced by parameters like speed of sliding, load applied and distance slides [13][14][15][16][17].

Graphite
Graphite is one among the mostly used lubricant of solid type due to its positive properties of combinations such as chemical inertness, low friction, film forming ability, lack of intrinsic abrasiveness etc.

. Effect of rate of wear on load
The distinction of rate of wear in to the load applied is represented in Fig. 3. The load applied influences the rate of wear in aluminium alloy and its composites of hybrid properties considerably. The rate of wear of composites varies with changes in load applied and it significantly reduces the rate of wear of the hybrid composites. The rate of wear of Al/WC/Gr increases based on load applied and the influences in hybrid composite is less compared to base AA 6082 alloy. This phenomenon is largely due to the Mg particles of AA 6082 availability. At various load conditions the resistance of wear in the hybrid composites has been better to the AA 6082 alloy model alloy [16,17,18] in Fig 3.

Effect of rate of wear in distance slides
The variation of rate of wear with variation in distance slides is represented in Fig.3. An increase in distance slides decreases the rate of wear of Al/WC/Gr hybrid composites. At higher distance slides the temperature along the sliding surface increases thereby softening the composite materials and increases the deformation at elevated distance slides. The increase in hardness of the hybrid composite results in enhancement of seizure and wear resistance of the Al/WC/Gr hybrid composites. WC and graphite particles reinforced in hybrid composite improve the plastic formability than the base AA3082 model alloy. Presence of graphite particles in hybrid composite acts as an efficient protecting layer between the two friction surfaces that reduces the rate of wear [21].

Effect of rate of wear on speed of sliding
The influence of rate of wear with respect to the variation in speed of sliding is exposed in Fig.4. The distance slides are kept at 2000 m and a load of 16 kg. An increase in speed increases the rate of wear of the base Al alloy ( AA 6082) and its hybrid composites to a certain extend. Increasing the speed of sliding increases the temperature along the sliding surfaces and softens the pin surfaces. The rate of wear of hybrid composites are less compared to the wear of AA 6082 alloy. This is probably due to the improvement in the hardness. The increase in hardness of hybrid composite results in enhancement of wear resistance. Addition of graphite and WC particles in Al alloy ( AA 6082) will reduce the rate of wear of composites [20].

Microstructure of AA6082/WC/Graphite hybrid composites
The behaviour of composites based on the microstructure characteristics and the interface between the model and reinforcements. The scanning electron microstructure of the worn out specimen made of Al/WC/Gr hybrid composites is represented in Fig.5. Clustering along with nonhomogeneous distribution of the WC and Graphite in the Al alloy were observed in few samples. This might be probably due to the difference of contact time between the WC and Graphite particles with respect to the base alloy [6]. The failure in composite sample containing less amount of reinforcement particles were characterised by delamination and adhesive wear. This mode of failure is primarily ductile in nature. An increase in reinforcement is characterised by ploughing, protrusion, particle breakup and even tearing of particles. There is predominantly brittle kind of wear influenced due to the higher hardness of the composites.

Conclusions
The Al alloy (AA3082) reinforced with WC and Graphite composites were fabricated by stir cast technique with different weight fractions of reinforcements. The following observations are were made from the study The AA 6082 hybrid composites reinforced with WC and Graphite was effectively produced by stir casting. The wear resistance of Al/WC/Gr composites increased with the addition WC and Gr particles. The rate of wear is of composites are considerably compared to base Al alloy (AA3082) model. Rate of wear of the composites increases with load and decreases with increase in addition of WC. An increase in speed increases the rate of wear of the base Al alloy ( AA 6082) and its hybrid composites to a certain extend. An increase in distance slides decreases the rate of wear of Al/WC/Gr hybrid composites. The wear of the composite specimen is characterised by delamination, ploughing, protrusion, particle breakup and tearing of particles.