1 Introduction

Aluminum matrix composites (AMCs) are engineering materials having useful characteristics such as high tensile strength, compressive strength, hardness and stiffness. These materials have better abrasive resistance compared to unreinforced alloys. These materials are being utilized in various structural implementation in several industries such as marine, aerospace, automobile etc. [1, 2]. Al 6061 is most utilized matrix material because of its low density, high strength, low electrical resistance, high corrosion resistance and higher machinable properties. However, lower wear resistance has restricted its application.

The mechanical and tribological characteristics of both particulate and fiber reinforced Al 6061composite have improved significantly in recent years. Abdullah et al. [3] investigated that the tensile strength and hardness of aluminum improved with the increase of SiC particles, but the plasticity and malleability of the composites decrease significantly. Idrisi et al. [4] fabricated AMC reinforced with a different fraction of SiC particles using traditional stir casting and ultrasonic vibration based stir casting process. They observed significant increase in density, tensile strength, hardness and compressive strength of the AMCs developed through ultrasonic stir casting route. The stir casting method allows uniform dispersion of reinforcement to the base material and scattering of the agglomeration and reduces the crack like [5,6,7,8,9,10,11,12,13,14,15] flows which affects the physical and mechanical behavior of the composites. Hassan et al. [16] reported a decrease in hardness with an increase in percentage reinforcement of graphite (Gr) in Al/Gr composites. Akhlaghi et al. [17] fabricated Al 2024 composites by adding varying fraction ( 5% to 20%) of Gr as reinforcement and observed that hardness and fracture toughness of AMCs reduced with addition of Gr content. Baradeswaran et al. [18] have recorded a reduction in tensile stress and hardness with the increase in Gr content of AA7075/Gr composites fabricated by liquid casting technique.

Idrisi et al. [19] used the stir-casting technique for the manufacturing of AMC different fraction of SiC microparticles (5% and 10%) and nanoparticles (1 and 2%). Developed AMCs were used to fabricate gears and tested using a unique wear test machine at different applied load and operating time The AMC reinforced with two percent nanoparticles has shown highest wear resistance. Baradeswaran et al. [20] developed Al 7075/Al2O3 composite through liquid metallurgy route and added Gr particles to observe its influence on the wear properties of composite. They reported that Gr reinforced composites had higher wear resistance than Al 7075/Al2O3 composites. Suresh et al. [21,22,23,24,25] investigated the impact of Gr particles on the mechanical behavior of Al6061/TiB2 composites, produced by high energy stir casting technique. The mechanical properties such as tensile strength, hardness and elongation percentage of hybrid composite improved compared to the Al6061 alloy and Al6061/TiB2 composites. Kumar et al. [26] analyzed wear properties of Al7075/(SiC + Gr) composite developed using stir casting method and observed that Gr as a primary reinforcement improved the wear-resistance of the Al7075 alloy and it further improved by adding SiC as a second reinforcement. Ravindran et al. [27] have studied the wear resistance of the hybrid composites, Al2024/(SiC + Gr), produced by powder metallurgy and have reported substantial enhancement in the wear behavior of both soft (Gr) and hard (SiC) reinforced composite as compared to the Al 2024 alloy. The Al/Gr composites and Al/(SiC + Gr) hybrid composites were fabricated through stir casting technique by Suresha et al. [28]. They observed that the hardness of Al/Gr composite was less as compared to the base metal and reduced with adding the fraction of Gr reinforcement. On the other side, the hardness of Al/(SiC + Gr) composites were more compared to Al/Gr composites and base metal.

It is evident from the literature that the main focus has been on the mechanical properties of Gr and SiC reinforced composite using different alloys but minimal research available on the Gr/SiC hybrid composites. It observed that limited literatures are available in which Al 6061 alloy has been used as a base material in the fabrication of Gr/SiC composites. This point has provided a motivation for the current work. The weld-ability and formability of Al6061 alloy make it suitable for many general-purpose applications. Its high strength and corrosion resistance lend type 6061 alloy particularly useful in architectural, structural, and automobile applications. In this work, Al 6061 Gr/SiC hybrid composites fabricated using stir casting technique. The tensile strength and hardness of the fabricated composite was determined. Furthermore, the effect of applied load and sliding velocity on wear rate of Al–SiC–Gr hybrid composites was analyzed.

2 Material and methods

2.1 Materials

In this study, Al 6061 alloy used and matrix material, whereas SiC (average particle size: 40 μm) and graphite (average particle size: 75 μm) particles employed as reinforcement material for the preparation of composites. Commercially available Al 6061 alloy was procured from Miser Aluminum company, Egypt, in the form of ingots. AMCs produced by adding various volume fraction of SiC (5%, 10% and 15%) whereas fixed volume fraction (10%) of the graphite used in composites. Both the reinforcements were purchased from Sigma-Aldrich. Specimens machined to the cylindrical shape of diameter 20 mm and thickness of 10 mm. The chemical composition of matrix material is as shown in Table1. The properties of matrix and reinforcing materials used in the study, as shown in Table 2.

Table 1 Chemical composition of Al 6061 alloy
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Table 2 Properties of matrix and reinforcements
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2.2 Preparation of composites

In the first step, the pressurized air infiltration (PAI) technique utilized to prepare MMC using the capsule. A mixture of pre-weighted graphite and alcohol was used to coat the inner surface of the capsule and then dried in the muffle furnace. The SiC particles were inserted in the capsule after baking in muffle furnace at 600 °C and pieces of the Al 6061 alloy was placed over it. The SiC was baked for the uniform mixing with matrix material as it improves the wettability of the mixture. The capsule was positioned and enclosed in the furnace. The furnace temperature was raised to 720 ± 5 °C and then pressurized air valve was opened for 10 s to push the melt to pass through the powder then air valve was closed. The air was maintained at the pressure of 0.7 MPa. The developed billets were used to calculate the volume fraction of SiC particle. The second phase was to melt a pre-weighed quantity of the alloy into a stainless-steel crucible with an electric resistance furnace at a temperature of 660 °C. A chromel–alumel thermocouple set was used to regulate and measure the temperature inside the crucible. Simultaneously, the temperature of AMC billet fabricated with PAI technique was raised till 660 °C in a separate electric furnace then dipped in the molten alloy and stirred at the speed of 50 rpm for 3 min. Furthermore, the speed increased to 1000 rpm and maintained to create a necessary vortex. The mixture was poured into a steel mold and cooled at room temperature [29,30,31].

3 Characterization

3.1 Tensile testing

The tensile test specimens were prepared from developed AMCs according to the ASTM E08-8 standard. The prepared specimens are shown in Fig. 1. These specimens were tested at room temperature using MTS machine. Total of 3 specimens were tested and average was taken into consideration for each composite.

Fig. 1
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The tensile test sample geometry

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3.2 Hardness testing

The Vickers hardness has been measured on the disc specimen. The Vickers hardness tester model VM 50 (Buehler micromet II micro hardness tester) was used to determine the hardness of fabricated composites. These testers strictly conform to IS 1754 and ISO 6507-2. ASTM E10 standard was used to produce hardness specimens. The Vickers hardness of the polished samples was determined using diamond cone indenter with a load of 30 N. Five readings were taken and average considered for the analysis.

3.3 Wear testing

Wear test was carried out using Pin-on-Disc device model POD-WTM shown Fig. 2. It consists of a rotary horizontal steel disc driven by variable speed motor. The test sample was placed in the sample holder that was secure against the rough face to the loading lever. The friction coefficient can be measured through two thin spring steel sheets adhere to strain gauges.

Fig. 2
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Pin-on-disc wear tester

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The wear rate depends on the sliding speed, applied load, environmental conditions and material properties. The wear test was conducted at four different applied loads (10 N, 20 N, 30 N and 40 N) and a constant rotational speed of 600 rpm. For obtaining repeatable and reliable wear information, full interaction between the specimen and abrasive disc was assured. The steel disk EN-31 was used with the hardness of 60HRC and pin specimen was designed according to the specifications of ASTM G99-95 with a measurements of 8 mm diameter and 50 mm length.

4 Result and discussion

4.1 Tensile strength

In Fig. 3 shows the stress–strain curve variation of the developed composites with different volume fraction of reinforcement. It represents that the tensile strength of the SiC/Gr reinforced composite is improved as compared to the Al 6061 alloy. The tensile strength of Al 6061 with 10 vol% SiC + 10 vol% Gr hybrid reinforced composite is higher as compared to Al 6061 with 5 vol% SiC + 10 vol% Gr reinforcement. AMC with 15 vol% SiCp particles and 10 vol% Gr found to be the strongest among all fabricated composites. The tensile strength of the composites improved with the increase of reinforcement fraction. An improvement of 50% in the tensile strength was achieved in AMC with 15 vol% SiC + 10 vol% Gr reinforcement compared to the base material. It is due to the fact that Gr and SiC reinforcement exhibits a decent bonding with Al 6061 alloy which helps in enduring more load as compared to Al 6061 alloy. The surge in tensile strength in AMCs can be ascribed to grain refining, increased dislocation density close to the matrix reinforcement interface, and transfer of tensile force to the strongly bonded SiC and Gr reinforced particles in the aluminum matrix [32, 33]. The tensile strength improvement may be caused by the tight packing of the reinforcement within the matrix phase. The performance of AMCs considerably subjected to the interface properties of the base material and the reinforcements, the interfacial properties further depend on the wettability between the matrix and reinforcements in the fabrication process [34, 35]. Strong bonding between the base material and reinforcements supports an enhancement in the tensile strength of the composites [36,37,38,39,40,41,42,43,44,45].

Fig. 3
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Stress–strain curve of Al 6061 alloy and composite with varying reinforcement fraction

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4.2 Macro hardness

The surface hardness of the AMC is measured as one of the most significant factors which affects the wear rate of the composites. Figure 4 shows the effect of reinforcement on the hardness of AMC and it increased with the addition of SiC and Gr particles. The SiC/Gr reinforcement composites found to be harder as compared to the Al 6061 alloy. The hardness of Al 6061 with 10 vol% SiC + 10 vol% Gr hybrid reinforced composite was higher as compared to Al 6061 with 5 vol% SiC + 10 vol% Gr reinforcement. AMC with 15% SiC and 10 vol% Gr particles have the highest hardness among all fabricated composites. It is known that the yield stress is the minimum stress required to make dislocations move. If the material is brittle with no yielding due to the incorporation of brittle silicon carbide and graphite (which are very brittle) in the aluminum, this will act as a barrier for dislocation formation which will take place under the flow stress after yielding [46,47,48,49,50]. These particles protect matrix material from slides cutting, deformation and penetration on the surface of AMCs.

Fig. 4
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Effect of reinforcement on the hardness of AMC

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4.3 Wear properties

The wear test was performed on cylindrical pin specimens against a rotating steel disc. The electronic sensors were installed in wear test machine for monitoring the tangential friction force and wear rate. The wear rate was determined based on two variables: applied load and sliding velocity.

The wear test was conducted at 10 N, 20 N, 30 N and 40 N nominal load and rotational speed of 600 rpm as shown in Fig. 5. It was observed that the wear rate increased with increase in the applied load. Idrisi et al. [51] and Miyajima et al. [52] also detected the similar linear trend. In addition, the wear resistance of the unreinforced alloy was determined to be lower than the reinforced AMC with the increase of applied load. The wear resistance of Al 6061 with 10% SiC + 10% Gr hybrid reinforced composite was higher as compared to Al 6061 with 5 vol% SiC + 10 vol% Gr reinforcement. AMC with 15 vol% SiC and 10 vol% Gr particles found to have the largest wear resistance among all fabricated composites. The wear rate reduced by 62.5% at 10 N and 55% at 40 N of applied load for the AMC with 15 vol% SiC + 10 vol% Gr compared to the unreinforced alloy.

Fig. 5
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Effect of load on the wear rate of Al 6061 alloy and composites

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Figure 6 shows variation in the wear rate of Al 6061 alloy and Al 6061 alloy composites at different sliding velocity (0.4, 0.6, 0.8, 1, 1.2 and 1.6 m/s) at constant load condition of 40 N. The wear rate was higher in Al 60,061 alloy as compared to developed AMCs at all sliding velocities. It was observed that the wear rate of Al 6061 alloy increased linearly from 7 × 10–3 mm3/m to 8 × 10–3 mm3/s at the sliding velocity of 0.4 m/s to 1.2 m/s respectively then wear rate increased steeply to 13 × 10–3 mm3/m at the sliding velocity of 1.6 m/s. An opposite trend was observed for the AMCs. The wear rate increased linearly up to the sliding velocity of 1.2 m/s and then it decreased steeply at higher sliding velocity. The wear rate increase by 63% from 0.4 to 1.6 m/s for AMC with 5 vo l%SiC + 10 vol% Gr whereas it was only 10% for AMC with 15 vo l%SiC + 10 vol% Gr reinforced particles. The decrease in wear rate with increasing sliding speed could be due to the transformation of thermodynamically meta-stable structure of the carbon bonds and graphite-like structure. Such decrease might also occur at the real contact area where flash temperatures are high. This transformation can facilitate the development of a lubricating tribofilm and reduce wear of the surface [53]. The wear rate decreased with increase in volume fraction of the reinforcement could be due to the presence of graphite in the hybrid composites which is a solid lubricant will lead to reduction in wear rates at higher sliding velocity. Furthermore, SiC particles act as obstacles during the relative motion of the surfaces and oppose the wear of matrix phase.

Fig. 6
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Effect of sliding velocity on the wear rate of Al 6061 alloy and composites

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Figure 7 shows effect of SiC vol% on the friction coefficient of developed AMCs. It indicates the reduction in the coefficient of friction with increase of SiC vol%. The friction coefficient reduced from 0.6 to 0.52 for Al 6061 alloy to the AMC with 15 vol% of SiC. The friction coefficient for AMC with 5 vol% and 10 vol% was observed to be 0.58 and 0.55 respectively.

Fig. 7
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Effect of SiC vol% on the friction coefficient of Al 6061 alloy and composites

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Figure 8 shows variation in the wear rate of Al 6061 alloy and Al 6061 alloy composites at different sliding distance (250, 500, 750 and 1000 m). The wear rate was higher in Al 60061 alloy as compared to developed AMCs at all sliding distances. It was observed that the wear rate of Al 6061 alloy increased linearly from 3.1 × 10–3 mm3/m to 7.9 × 10–3 mm3/s at the sliding distance of 250 m to 1000 m respectively. The wear rate decreased by 45% and 83% at 250 m sliding distance for AMC with 5 vol%SiC + 10 vol% Gr and AMC with 15 vol%SiC + 10 vol% Gr respectively as compared to Al 6061 alloy whereas it was 49% and 69% at 1000 m sliding distance respectively. These results are in line with the trends reported by other investigators [54, 55].

Fig. 8
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Effect of sliding distance on the wear rate of Al 6061 alloy and composites

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5 Conclusion

AMCs reinforced with silicon carbide and graphite particles have been successfully developed via stir casting process. Following significant conclusions were drawn:

  1. 1.

    The tensile strength has improved by 50% for AMC with 15 vol%SiC + 10 vol%Gr particles as compared to the Al 6061 alloy.

  2. 2.

    The hardness of Al 6061 alloy increased from 65 to 85 VHN by the addition of 15 vol% SiC + 10 vol% Gr particles.

  3. 3.

    Wear resistance of developed AMCs improved with the addition of SiC and Gr particle.

  4. 4.

    It was observed that the wear rate for Al 6061 alloy increased linearly upto the sliding velocity 1.2 m/s then increased steeply up to 1.6 m/s whereas the opposite trend was observed for AMCs in which the wear rate increased linearly up to the sliding velocity of 1.2 m/s and then it decreased steeply at a higher sliding velocity