Abstract
Aluminum Hybrid Reinforcement Technology is a response to the dynamic ever-increasing service requirement of industries such as transportation, aerospace, automobile, and marine, due to its attractive properties like high ductility, highly conductivity, light weight, and high strength to weight ratio. In this evolution, an attempt has been made to investigate the wear rate of Al6061 hybrid metal matrix composite reinforced with the hard ceramic alumina (4, 8, and 12 wt.% of Al2O3) and soft solid lubricant of molybdenum disulphide (2, 4, and 6 wt.% of MoS2) is fabricated by using stir casting method. The unlubricated pins on disc wear tests were conducted to examine the wear behaviour of Al6061/12 wt.% of Al2O3/MoS2 composites. The sliding wear tests were carried out at various loads of 15, 30, and 45 N, sliding velocity (1.25, 2.50, and 3.25 m/sec), and different MoS2 wt.% (2, 4, and 6 wt.%). In addition, the CNC turning experiments were conducted on Al6061/12 wt.% Al2O3/6 wt.% MoS2 using CNMG 120408 uncoated carbide cutting tool under cutting of 100, 150, and 200 m/min, feed of 0.1, 0.2, and 0.3 mm/rev, and depth of cut of 1, 1.5, and 2 mm.
1. Introduction
Aluminum hybrid matrix composites have become better substitutes for the conventional aluminum alloys because of their characters like improved strength to weight ratio, energy saving, and better wear resistance [1]. Reinforced aluminum hybrid matrix, a composite for the use of the automotive industry and other structural applications, has been increased because of the physical and mechanical properties it possesses [2]. Aluminum based hybrid matrix composites with a variety of particulate reinforcements such as Al2O3, TiC, Ti2B, B4C, TiN, Si3N4, graphite, and industrial waste byproduct have been conceived and developed for various potential applications [3]. Researchers have developed aluminum hybrid matrix composites and they are used in many commercial and industrial applications [4]. Newer compositions have been continuously explored and the related valuable studies presented by the past researchers were discussed. Veeresh Kumar et al. [5] conducted the experiment on Al6061/SiC and Al7075/Al2O3 metal matrix composites and presented the experimental results of the studies regarding hardness, tensile strength, and wear resistance properties of Al6061/SiC and Al7075/Al2O3 composites. The SiC and Al2O3 resulted in improving the hardness and density of their respective composites.
Anilkumar et al. [6] have conducted the mechanical properties of fly ash reinforced Al6061 composites and found that the tensile strength, compressive strength, and hardness of the Al6061 composites decrease with the increase in particle size of reinforced fly ash. Increase in the weight fractions of the fly ash particles increases the ultimate ensile strength, compressive strength, and hardness and decreases the ductility of the composite. Asif et al. [7] have investigated the development of aluminum based hybrid metal matrix composites for heavy duty applications and investigated the dry sliding wear behaviour of aluminum alloy based composites, reinforced with silicon carbide particles and solid lubricants such as graphite/antimony trisulphide. The results revealed that wear rate of hybrid composites is lower than the binary composite. The wear rate decreases with the increasing load and increases with increasing speed.
Singh et al. [8] have conducted an experimental investigation on mechanical behaviour of aluminum by adding SiC and alumina. This work was focused on studying the change in behaviour of aluminum by adding a different percentage age amount of SiC and Al2O3 composite and it was concluded that the mechanical properties such as hardness, yield strength, and ultimate strength increasing with a weight percentage of reinforcement increase. But at the same time elongation is decreased and the behaviour of material is changed from ductile to brittle. Gaitonde1 et al. [9] have done an experiment on wear and corrosion properties of Al/Al2O3/graphite hybrid composites. The effects of reinforcement, time duration, and particle size on prepared samples of composites have been studied on slurry erosive wear. The static and accelerated corrosion tests have been performed and the microhardness of the developed composites was also investigated. The experimental results on Al5083/Al2O3/graphite hybrid composites revealed that the addition of reinforcement improves the hardness and reduces corrosion and wear rates.
Sharanabasappa and Motgi [10] were conducting experiments on mechanical properties of fly ash and alumina reinforced aluminum alloy composites. The mechanical properties of fly ash and alumina reinforced aluminum alloy (LM25) composite samples, processed by stir casting route, were reported. It was found that the tensile strength and hardness of the aluminum alloy composites increase with the increase in %wt. of Al2O3. Kuma and Singh [11] investigated a comparative investigation of mechanical properties of aluminum based hybrid metal matrix composites. The result indicated that there is an increase in the value of tensile strength, ultimate tensile strength, hardness value, and flexural strength of newly developed composite having SiC and B4C particulates in comparison to the SiC, graphite reinforced composite.
Wang and Yan [12] investigated the feasibility of machining Al6061/Al2O3 composite materials by electro discharge machining for blind hole drilling and evaluated the material removal rate, tool wear rate, and surface roughness with various input parameters. Yan and Wang [13] observed that material removal rate is increased with peak current and was erratic with pulse duration. The material removal rate, tool wear rate, and surface roughness are increased with flushing pressure during machining of Al6061/Al2O3 composites using rotary electro discharge machining with a tube electrode. Yan et al. [14] found that, in case of rotary electro discharge machining process, the main challenge is using a disk like electrode for machining Al6061/Al2O3 composites. The influence of reinforcement of SiC and Al2O3 in different matrix of Al6061 and Al7075 on mechanical properties was investigated by Veeresh kumar et al. [15] and they found that microhardness of the composites was increased linearly with the increase of filler content. The surface roughness increases with increase in volume fraction of Al2O3 particles reinforced with aluminium metal matrix composites when machined by carbide coated tools [16]. The cutting velocity has 20.8% influences on the surface roughness during machining of A356/20/SiC/T6 composite [17]. Better surface finish was achieved while machining aluminum metal matrix composites with 5% SiCp at higher cutting speed produces [18]. Metin Kok [19] concluded that while machining Al2024/Al2O3 composites using coated and uncoated carbide cutting tools the surface roughness of the work piece was mostly affected by cutting speed. The machined surfaces of Al-Si alloy-graphite composites tend to be rougher than the surfaces machined at identical conditions on matrix material without reinforcement because of the presence of deeper holes or valleys [20]. The surface roughness value decreases with increasing cutting speed.
However many researchers have carried out the mechanical and wear properties and machining characteristics of aluminum metal matrix composites with SiC and Al2O3 as reinforcing materials. In the case of hybrid Al6061/Al2O3/MoS2 composites, limited literature is available, encompassing various aspects such as mechanical properties and wear behaviour and conducting the machining study of the composites. Aluminum based Al2O3 composite material has properties such as low weight, heat resistant, wear resistant, and low cost. These are found in various engineering applications such as cylinder block liners, vehicle drive shafts, automotive pistons, and bicycle frames. These materials are known as difficult to machine materials, because of the hardness and abrasive nature of reinforcement element like alumina particle [21]. The addition of MoS2 reinforcement in composites as a hybrid reinforcement further increases the wear resistance of the composite. This is attributed to the stable and MoS2 rich mechanically mixed layer, which prevents metal to metal contact and reduces the wear of the composite. The unstable mechanically mixed layer in the absence of the lubricant phase leads to lower wear resistance in hybrid composite [22]. So an attempt was made to study the tribological, mechanical, and machining characteristics on Al6061/Al2O3/MoS2 hybrid composite.
2. Experimental Details
The stir casting method is simple and the most economical way of fabricating particulate reinforced composites. In this technique, to accomplish the optimum properties of the hybrid composites, the distribution of the reinforced particles in the base material should be homogeneous and the wettability among the olden materials and particulates ought to be optimized. The moisture level in the cast composite must be reduced and the element reactions among the particles and the base material have to be avoided. The whirlpool method is one of the enhanced recognized tactics used to build a high quality allocation of the reinforced material in the base matrix. In this method, the base material is melted followed by a forceful stirring by automatic agitator to form a whirlpool at the face of dissolve, and the particle is subsequently introduced at the region of the vortex.
2.1. Material Fabrication
The chemical composition of Al6061 alloy is given in Table 1 and Al6061 alloy was melted in a crucible furnace at 725°C. Subsequent to melting and degassing by nitrogen, the molten metal was stirred by an alumina coated stainless steel stirrer at 600 rpm for 20 min time duration. During stirring, it is preheated at 600°C; particles alumina and MoS2 powder were added. After the addition of MoS2, the composite was poured into the preheated (250°C) permanent mould. The Al6061 alloy with different wt.% of hybrid composites is produced and test specimens were machined according to ASTM standards.
2.2. Mechanical Behaviour
The hardness tests were carried out according to ASTM E10-07 standards using Brinell hardness testing machine with a 10 mm ball indenter and 500 kg load for 30 sec. The test was carried out at atmospheric temperature (30°C) and the measurement of hardness was taken at three different locations on each sample to obtain an average value of hardness. As per the ASTM E08-8 standard, the tensile strength was evaluated on the cylindrical rod of casted composites. The 1200 grit grindings silicon carbide paper was used to polish the test specimens in order to decrease the machining scratches and the effects of surface defects on the sample. The universal testing machine was loaded with 10 KN; load cell was used to conduct the tensile test. The affecting factors and levels selected for mechanical behaviour Al6061/Al2O3/MoS2 are given in Table 2.
2.3. Tribological Behaviour
Dry sliding wear behaviour of Al6061/12 wt.% Al2O3/MoS2 hybrid composites is studied in pin on disc test apparatus. Pin specimens of 6 mm diameter and 15 mm height, for wear test, were prepared from the above composites and the composites were machined and polished. The test was conducted with various loads of 15 N, 30 N, and 45 N at a sliding speed of 125, 2.50, and 3.25 m/s and 2, 4, and 6 MoS2 wt.%. It was conducted at room temperature (30°C) and relative humidity of 60–65%. The affecting factors and levels selected for Tribology Al6061/12 wt.% Al2O3/MoS2 are given in Table 3.
2.4. CNC Turning Test
The work material used for the present investigation is Al6061/12 wt.% Al2O3/6 wt.% MoS2 and diameter of the material is 20 mm and machined length is 60 mm for all trials. The experiments were conducted on Fanuc CNC lathe and CNMG 120408; uncoated carbide cutting tool is used as the insert for all machining operations. The affecting factors and levels selected for turning properties Al6061/12 wt.% Al2O3/6 wt.% MoS2 are given in Table 4. SR is measured by the Mitutoyo surface roughness tester. The three cutting parameters selected for the present investigation are cutting speed, feed, and depth of cut. Minitab14 statistical software has been used for the analysis of the experimental work.
3. Result and Discussion
3.1. Optimum Setting for TS and BHN of Al6061/Al2O3/MoS2
The tensile strength and hardness of the Al6061/4, 8, and 12 wt.% Al2O3/2, 4 wt.% MoS2 hybrid composite are shown in Figures 1(a) and 1(b). It is observed that an addition of alumina particles improves the tensile strength and hardness of hybrid composites. It is evident that tensile strength and hardness clearly increase with the addition of alumina particles. The tensile strength and hardness of the Al6061/4, 8, and 12 wt.% Al2O3/6 wt.% MoS2 hybrid composites are shown in Figure 1(c). The tensile strength and hardness of hybrid composites were decreased with increasing content of molybdenum disulphide.
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The measured values of tensile strength and BHN for Al6061/Al2O3/MoS2 under different wt.% and corresponding signal to noise ratio for all experimental runs are given in Table 5. The Taguchi analyses of tensile strength and BHN for Al6061/Al2O3/MoS2 under different wt.% are given in Table 6. It clearly shows the maximum tensile strength is obtained at 12 wt.% of Al2O3 and 2 wt.% of MoS2 and the maximum BHN is obtained at 12 wt.% of Al2O3 and 4 wt.% of MoS2. Table 7 shows the results of ANOVA for tensile strength and BHN on Al6061/Al2O3/MoS2 hybrid composite. It is observed that Al2O3 is the most significant parameter affecting the tensile strength of Al6061/Al2O3/MoS2 with F : P value of 1092.26 : 0.000 and Al2O3 is significantly affecting the BHN of Al6061/Al2O3/MoS2 with F : P value of 480.69 : 0.000.
3.2. Tribological Behaviour of Al6061/Al2O3/MoS2
The variation of wear rate for Al6061/12 wt.% Al2O3/MoS2 hybrid composite with respect to different sliding velocity and applied load is shown in Figures 2(a), 2(b), and 2(c). The applied load affects the wear rate of hybrid composites significantly. The wear rate varying with applied load is an indicative of Archard’s law and significantly lower in the case of hybrid composites. The wear rate increases with increasing applied load and it is less at 6 wt.% of MoS2 hybrid composites as compared to 2 wt.% of MoS2 [23]. This is mainly due to the presence of Al2O3 in Al6061. However, at all load conditions considered, the wear resistance of the hybrid composites is superior to the matrix alloy. The wear rate is increased with increase in sliding speed Al6061/12 wt.% Al2O3/MoS2 hybrid composite. Increasing sliding speed temperature is raised to the sliding surfaces; this leads to softening of the matrix and composite pin surfaces and it is due to the enhancement in the hardness. The increase in hybrid composite hardness results in improvement of wear resistance. The presence of MoS2 and Al2O3 particles in the hybrid composite will lead to further reduction of wear rate [24].
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The coefficient of friction for Al061/12 wt.% Al2O3/MoS2 hybrid composites under varying load and sliding velocity is shown in Figures 3(a), 3(b), and 3(c). The coefficient of friction is decreased with increasing wt.% of MoS2 and increasing the due to load. The Al2O3 particles act as load bearing elements in the hybrid composites and also this results in formation of more stable lubricating film on the tribosurface of the hybrid composites. The decreased coefficient of friction of hybrid composite with MoS2 content can be credited to the collective effects of MoS2 and Al2O3 particles in formation of a more resistant tribolayer on the contact surface. The graphite tribofilm minimizes the degree of shear stress transferred to the sliding material underneath the sliding contact area which results in less plastic deformation in the subsurface region and reduces the wear rate in the hybrid composites [25].
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3.3. Effect of Alumina and Molybdenum Disulfide Content in Al6061
The wear surface of the Al6061/12 wt.% Al2O3/6 wt.% MoS2 under sliding velocity of 2.50 m/sec is given in Figures 4(a) and 4(b). It clearly reveals the formation of tribolayer on the contact surfaces of aluminum composites tested at 15 N and 45 N conditions. Figure 4(a) reveals that the tribolayer is formed only to a limited extent in Al6061/12 wt.% Al2O3/4 wt.% MoS2 hybrid composite at 15 N load, 2.50 m/s sliding speed. At a higher load of 45 N and 2.50 m/s sliding speed the tribolayer that is formed increases, due to squeezing out of solid lubricant from the hybrid composites. The extent of tribolayer formation also increases with increasing percentage of molybdenum disulphide located immediately below the contact surface.
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The experimental values of wear rate and coefficient of friction for Al6061/12 wt.% Al2O3/MoS2 under different parameters and corresponding signal to noise ratio for all experimental runs are given in Table 8. The Taguchi analysis for Al6061/12 wt.% Al2O3/MoS2 under different load, sliding velocity, and wt.% of MoS2 is given in Table 9. It clearly shows the minimum wear rate and coefficient of friction is obtained at 15 N of load, 3.25 m/sec of sliding velocity, and 6 wt.% of MoS2. Table 10 shows the results of ANOVA for wear rate and coefficient of friction on Al6061/12 wt.% Al2O3/MoS2 hybrid composite. It is observed that load is the most significant parameter affecting the wear rate and coefficient of friction of Al6061/12 wt.% Al2O3/MoS2 with F : P value of 29.06 : 0.033 and 756.27 : 0.001.
3.4. Optimum Setting for CNC Tuning Al6061/12 wt.% Al2O3/6 wt.% MoS2
The experimental results of arithmetic mean value for turning Al6061/12 wt.% Al2O3/6 wt.% MoS2 under different cutting conditions and corresponding signal to noise ratio for all experimental runs are given in Table 11. The Taguchi analysis for Al6061/12 wt.% Al2O3/6 wt.% MoS2 using uncoated carbide cutting tool is given in Table 12. It clearly shows the minimum surface roughness is obtained at 100 m/min of cutting speed, 0.1 mm/rev of feed, and 1.5 mm of depth of cut for tuning Al6061/12 wt.% Al2O3/6 wt.% MoS2. Table 13 shows the results of ANOVA for surface roughness on Al6061/12 wt.% Al2O3/6 wt.% MoS2 hybrid composite. It is observed that feed is the most significant parameter affecting the surface roughness of Al6061/12 wt.% Al2O3/6 wt.% MoS2 with F : P value of 128.40 : 008.
4. Conclusion
In the present investigation, the Al6061/Al2O3/MoS2 hybrid composite is successfully fabricated using stir casting process. The mechanical behaviour, tribological behaviour, and machinability behaviour are evaluated. The obtained results can be summarized as follows:(i)Mechanical properties of hybrid composites increase with an increase in weight fraction of alumina particles. An increase in weight fraction of molybdenum disulphide reinforcement decreases the mechanical properties like tensile strength and BHN. The optimum parameter for maximization of tensile strength is obtained at 12 wt.% of Al2O3 and 2 wt.% of MoS2 and the maximum BHN is obtained at 12 wt.% of Al2O3 and 4 wt.% of MoS2.(ii)The incorporation of Al2O3 reinforcement to Al6061 increases the wear resistance of the composites. The addition of MoS2 reinforcement in Al6061/Al2O3 composites as a hybrid reinforcement further increases the wear and friction resistance of the composite. This is attributed to the stable and MoS2 rich mechanically mixed layer, which prevents metal to metal contact and reduces the wear of the composite. The unstable mechanically mixed layer in the absence of the lubricant phase leads to lower wear resistance in Al6061/Al2O3/MoS2 hybrid composite. The optimum parameter for minimization wear rate and coefficient of friction are obtained at 15 N of load, 3.25 m/sec of sliding velocity, and 6 wt.% of MoS2.(iii)The optimum parameter for minimization surface roughness is obtained at 100 m/min of cutting speed, 0.1 mm/rev of feed, and 1.5 mm of depth of cut for CNC turning Al6061/12 wt.% Al2O3/6 wt.% MoS2 hybrid composite.
Competing Interests
The authors declare that they have no competing interests.