Mechanical, tribological and microstructural characterization of stir cast Al-6061 metal/matrix composites—a comprehensive review

Abstract

Aluminum 6061 is a heat-treated, extruded alloy used for various engineering and structural components such as wings and fuselage of aircraft, railings, window frames, driveshaft and valves, etc. Al-6061 is widely used amongst its 6000 aluminum series because of its outstanding properties like medium to high strength, excellent environmental resistance, low density, high elongation at break and superior machinability. This paper aims to review the mechanical, tribological and microstructural characterization of stir cast Al-6061 metal/matrix composites (MMCs). In this review article an attempt has been made to demonstrate the effect of different reinforcements on metallurgical behavior of Al-6061 MMC. The particulates reinforcements like SiC, Al₂O₃, Gr, B₄C, TiC, fly ash, bagasse ash and red mud were dispersed with Al-6061 alloy through stir casting route as single, dual and triple reinforcements. The experimental results showed significant improvement in tribological and mechanical properties of Al-6061 MMC as compared with Al-6061 alloy. The overall characterizations of Al-6061 MMC are summarized with key findings, which will provide a methodical approach for researchers in selecting optimized parameters for the fabrication of aluminum-based MMC.

1 Introduction

Al-6000 are heat-treatable wrought alloys also known as Mg/Si alloys and because of magnesium-silicate (Mg₂Si) the alloys retain their heat treatability [1]. The Al-6000 alloys found major applications in welding and structural components due to their excellent corrosion resistance, medium strength and good formability [2, 3]. From Al-6000 series, the commercial alloys used for the development of aluminum-based metal/matrix composite (MMC) are Al-6061, Al-6063, Al-6026 and Al-6082 and amongst them Al-6061 is reinforced maximum time with particulate reinforcements through stir casting process [4, 5].

Al-6061 is a precipitated hardening alloy containing magnesium (Mg), silicon (Si) and iron (Fe) as its chief alloying elements. It was developed in 1935, originally called “Alloy 61 S” and also known by the name “structural aluminum” [6, 7]. Al-6061 is the most versatile alloy used in the construction of marine fitting, yacht, chassis, bearing and scuba tanks, shipbuilding, transport along with aircraft structure and automobile components [8,9,10]. Al-6061 alloy possesses excellent combinations of properties like good strength at elevated temperature, high stiffness to weight ratio, superior weldability and castability [11, 12]. Al-6061 alloy is extensively used in engineering and structural applications because its strength can be enhanced through liquid heat treatment and age hardening techniques [13]. The elemental compositions of Al-6061 alloy are presented in table 1 [1]. The physical characteristics of Al-6061 are presented in table 2 [7, 14, 15]. The Al-6061 alloy is mostly available in different designations like annealed pre-tempered Al-6061(O), solution heat treatment and naturally aged Al-6061 (T4), and solution heat treatment and artificial aging Al-6061 (T6) [10]. The Al-6061 (T6) is commonly used for the construction of fly fishing reel, bicycle frames, hydraulic brake piston and bike frames [16]. The mechanical properties of different grades of Al-6061 alloy are presented in table 3 [17, 18].

Table 1 Elemental composition of un-reinforced Al-6061 alloy by weight percentage [1].

Table 2 Physical properties of un-reinforced Al-6061 (T6) alloy [9, 14,15,16].

Table 3 Mechanical properties of different grades of un-reinforced Al-6061 alloy [14, 15].

The aluminum alloy possesses good combination of physical and mechanical properties, yet these alloys failed to perform under various high-performance and high-temperature applications. However, in those specific applications, aluminum particulates metal/matrix composites (Al-PMMC) become the most valuable alternatives for researchers [19, 20]. Though the usage of Al-PMMC is not limited to structural and functional applications, these composite materials have gained wide potential in aerospace, marine, defense and automobile sectors as multi-functional materials [21, 22]. In particulates metal/matrix composites, mainly the harder reinforcement (nonmetallic material) is dispersed into softer matrix (metallic alloy) to obtain homogeneous distribution of reinforcing and matrix phase [4, 23]. The aim of PMMC is to get desired mechanical strength and hardness with improved physical and tribological properties. In recent years, the unique combination of ceramic and agro-industrial waste in the form of hybrid metal/matrix composite (HMMC) has also become a major area of research. The extensively used particulates ceramics reinforcements are SiC, B₄C, Al₂O₃, Gr and agro-industrial reinforcements such as bagasse ash (BA), bamboo leaf ash (BLA), fly ash (FA), red mud (RM) and rice husk ash (RHA) [19, 24, 25].

This review article provides a systematic case study on mechanical, tribological, microstructural and physical characterization of stir cast Al-6061 MMC. The effect of single, dual and triple reinforcements on overall properties has been reviewed and discussed. Comparative studies of Al-6061 alloy and Al-6061 MMC were also conducted and the outcomes of experimental results are presented in tabular form, bar charts, optical microscopy (OM) and scanning electron microscopy (SEM) graphs. Finally the experimental results of various characterizations are summarized in the form of key findings. The novelty of this article shows that no study has been carried out to demonstrate the overall metallurgical property of stir cast Al-6061 MMC with optimized process parameters. In future this review article will benefit academic researchers to choose the best permutation and combination of reinforcements and optimized process parameters to synthesize other aluminum-based alloys for achieving maximum metallurgical and functional properties. The basic structure of review article is illustrated in figure 1.

Figure 1

Schematic preview of review article (Al-6061 MMC).

2 Stir casting

According to literature study on fabrication routes of aluminum-based composites, the liquid metallurgical mode (stir casting) was considered as one of the best and widely accepted technique amongst all the primary production methods [26, 27]. Stir casting technique possesses many advantages over other conventional methods such as low processing cost, good homogeneity among the particulates, less moisture absorptions, applicability to mass production and suitability to a wide range of shapes, sizes and volume fractions. However, wettability between matrix and reinforcing phase, agglomeration and porosity is the key limitation of this process [28,29,30]. The greatest benefit of stir casting procedure lies in its principle to fabricate materials in conventional manners like gravity casting using a bottom pouring furnace, which makes this process much simpler than other processes; however, the saving in manufacturing cost through this process is also one-third to one-tenth [31, 32]. The systematic stir casting setup with basic components is illustrated in figure 2. The overall characteristics of stir cast MMC and distribution of reinforcing particles inside a molten matrix were depend upon various process parameters as illustrated in figure 3. The right choice of these parameters significantly affects the overall performance of composite materials in terms of enhanced mechanical, tribological and microstructural properties [23, 33, 34]. The process parameters used by various researchers on Al-6061 MMC through stir casting technique are presented in table 4.

Figure 2

Schematic description of stir casting set-up.

Figure 3

Stir cast MMC process parameters.

Table 4 Stir cast Al-6061 MMC process parameters.

3 Literature review

3.1 Mechanical characterization of stir cast Al-6061 MMC

A review on mechanical properties like ultimate tensile strength (UTS), microhardness, yield strength (YS), percentage elongation, ultimate compressive strength (UCS) and impact strength of as-cast Al-6061 MMC were discussed and compared to those of Al-6061 alloy. The results of various mechanical properties of Al-6061 MMC with sample compositions are presented in table 5. The specimens for mechanical characterization are prepared according to American Society for Testing and Materials (ASTM) standards as illustrated in figure 4 [39, 41, 49, 51, 109]. The UTS, YS, percentage elongation and UCS of fabricated composites were assessed via a Universal Testing Machine (UTS). The macrohardness of cast specimens was determined using a Rockwell and Brinell hardness tester whereas microhardness was calculated using a Vicker hardness testing machine.

Table 5 Mechanical performance of stir cast Al-6061 MMC with different reinforcements.

Figure 4

Samples used for mechanical characterization as per ASTM standards. (a) Tensile test sub-specimens. (b) Impact test specimens. (c) Compressive strength specimens. (d) Microhardness specimens [49, 109].

Hillary et al [35] studied the mechanical behavior of Al-6061 MMC reinforced with silicon carbide (SiC) and titanium diboride (TiB₂). The composite was fabricated using 5 wt% SiC and 2/8 wt% (varying step of 2) TiB₂ by conventional stir casting route. The production of Al-6061/SiC/TiB₂ HMMC was carried out at 770°C furnace temperature followed by mechanical stirring at 600 RPM for 15 min. It was observed from experimental results that 5 wt% SiC + 10 wt% TiB₂ reported 8.18%, 20.19%, 9.46% increase in microhardness, tensile strength, flexural strength, respectively, in comparison with 5 wt% SiC + 2 wt% TiB₂. The presence of harder SiC and TiB₂ enhanced the load-carrying capacity of hybrid composite, which resulted in improved mechanical performance of as-cast Al-6061 composite. The comparison of tensile strength of TiB₂/SiC/Al-6061 (HMMC) with TiB₂/Al-6061 (MMC) is illustrated in figure 5. It is finally concluded from the plot that in comparison with TiB₂/SiC (hybrid reinforced composite) the TiB₂ (single-reinforced composite) shows superior tensile strength of Al-6061MMC. In another research the impact of SiC particulates on the mechanical performance of Al-6061 MMC was observed by Sivanathan et al [36]. Particulates of size 44 µm in 0, 2 and 4 wt% of SiC were dispersed in Al-6061 matrix through stir casting technique. It was observed from results that the accumulation of SiC particles marginally improved the mechanical properties of Al-6061/SiC MMC. The micro-hardness, compressive strength and tensile strength increase from 68 to 85 VHN, 22610 to 25324 N/m² and 125 to 157 MPa, respectively. The maximum increments by 25.6%, 25% and 12% in tensile strength, microhardness and compressive strength were reported for Al-6061/4% SiC composite when compared with Al-6061 base alloy. The ductility of reinforced composite decreased from 5.87% to 1.17% with an increase in reinforcement percentage and a total decrement of 80.06% was observed in comparison with pure alloy. In another investigation Maurya et al [37] added different wt% of SiC to assess the mechanical properties of AA-6061 MMC; 1, 2, 3 and 5 wt% of SiC with 1 wt% of Mg were cast through electromagnetic stir casting technique at a stir speed of 500 RPM for 12 min. The effects of SiC reinforcement on tensile strength and hardness were investigated and it was seen from experimental results that the addition of SiC particulates significantly enhanced the hardness and UTS of AA-6061/SiC MMC. The UTS and hardness of composite improved, respectively, by 4.3% and 12.5% for optimized process parameters.

Figure 5

Comparison between ultimate tensile strength of Al-6061/SiC/TiB₂ HMMC and Al-6061/TiB₂ MMC [35, 101].

Further, Selvam et al [38] manufactured AA-6061/FA/SiC HMMC through stir casting route. The mechanical properties of the developed composite were investigated and compared to those of AA-6061. SiC and FA reinforcements were added in the mutual proportion of 15 and 17.5 wt%. It was revealed from experimental outcomes that with increase in wt% of dual reinforcements, the macrohardness and microhardness increased from 49.4 to 57.2 BHN and 69.5 to 78.8 HV, respectively. The UTS of AA-6061/FA/SiC HMMC increased by 23.12% as compared with AA-6061. In another study Umanath et al [39] inspected the effect of SiC particulates on the fracture toughness of Al-6061 MMC. The composite was developed with 15 vol% of SiC of 25 µm size through stir casting technique. The K_1c of heat-treated samples was obtained as per the ASTM E399 standard by a three-point bend test. The results of fracture toughness are illustrated in figure 6, which shows a comparison among base alloy, Al-6061/10 vol% and Al-6061/15 vol%. Observations show that the incorporation of SiC decreases the K_1c of reinforced composite marginally by 72.41% as compared with Al-6061alloy. The reduced K_1c of Al-6061/SiC MMC is attributed to the presence of residual stresses on cast samples because of rapid heat treatment. In a similar study conducted by Maurya et al [40] they developed AA-6061-SiC MMC to assess the influence of SiC particulates on the mechanical properties of manufactured composite. The composite was reinforced with 2, 4, 6 and 8 wt% of SiC of 30 µm particle size via electromagnetic stirring. Mechanical results show that with increasing weight % of SiC (2/8 wt%) the hardness and tensile strength of composite increase gradually from 40 to 51 HRB and 276 to 298 MPa, respectively. Prashant et al [41] performed a comparative study on Gr-reinforced AA-6061 MMC and SiC-reinforced AA-6061 MMC. The fabrication of composite was accomplished through stir casting route using 125 µm size and equal 6, 9 and 12 wt% of Gr and SiC. It is noticed from experimental results that the addition of SiC particulates increases the microhardness from 98 to 151 VHN, whereas addition of Gr particulates decreases the microhardness from 98 to 76 VHN. The impact of 3 wt% of SiC and 3, 6, 9 wt% of cenosphere particulates on the mechanical properties of reinforced composite was studied by Ashoka et al [42]. The mechanical mixing was carried out at 700 RPM for 2/5 min. It is seen from test results that incorporation of cenosphere with SiC reinforcements increases the young modulus and hardness of HMMC from 61 to 68 GPa and 63 to 70 VHN, respectively.

Figure 6

Fracture toughness comparison between Al-6061 alloy and Al-6061 reinforced with 10/15 wt% of SiC particles [39].

In a separate study, Ramesh et al [43] assessed the mechanical characteristics of AA-6061/SiC/ZrO₂ HMMC. The composite was fabricated using 5 wt% of SiC and 2:3 wt% of SiC:ZrO₂ composition through conventional stir casting route. The significant improvements of 43.02% in UTS and 17.03% in hardness were observed at 2 wt% SiC/3 wt% ZrO₂/AA-6061 when compared with 5 wt% SiC/AA-6061. The fatigue strength of AA-6061/SiC/ZrO₂ HMMC was reported to be more than that of the single-reinforced composite. In another research Pavithram et al [44] developed AA-6061 HMMC reinforced with 8 wt% of SiC (50 μm) with 2 and 4 wt% of Gr (60 μm) particulates via liquid metallurgical route. It is reported from the experimental outcome that with the addition of dual reinforcements the UTS and YS of Al-6061/SiC/Gr HMMC increase from 200 to 218 and 110 to 132.74 MPa, respectively. The hardness of hybrid composite initially increases with the accumulation of SiC reinforcements and later decreases with dual reinforcements (SiC and Gr). The improvement in microhardness for SiC/AA-6061 composite is attributed to enhanced wettability and homogenous bonding between matrix and reinforcements in comparison with Gr/AA-6061 composite. Moreover, with an increase in SiC and Gr reinforcements the tensile strength of composite increases gradually; however, this behavior is mainly due to the increased load sharing capacity of the Al-6061 matrix due to the incorporation of SiC and Gr content.

In a separate work, Kandpal and Singh [45] inspected the mechanical characterization of AA-6061 MMC reinforced with 5, 10, 15 and 20 vol% Al₂O₃ via stir casting method. It was reported from experimental results that with the accumulation of Al₂O₃ content from 5 to 20 vol% the hardness and tensile strength of reinforced composite had enhanced from 61.15 to 89.91 VHN and 150 to 310 MPa, respectively. The tensile strength increased by 106.6% and the hardness increased by 47.03% for Al-6061/20 vol% Al₂O₃ in comparison with Al-6061/5 vol% Al₂O₃. In a study Sivananthan et al [46] have also assessed the mechanical properties of Al-6061 MMC incorporated with aluminum oxide (Al₂O₃). The composite was developed via stir casting method using 0/4 wt% Al₂O₃ of 32 µm particle size. The maximum values of tensile strength, hardness and compressive strength were reported for Al-6061/4 wt% Al₂O₃. The hardness and UCS of reinforced composite increased, respectively, by 19.1% and 9% when compared with Al-6061. The UTS of Al-6061/Al₂O₃ increased from 125 to 164 MPa with the addition of 0/4 wt% reinforcements. However, percentage elongation of the reinforced composites decreased from 5.8% to 3.03%. Ekambaram and Murugan [47] characterized the mechanical properties of Al-6061 alloy reinforced using Al₂O₃ particulates. The Al-6061 MMCs were produced through stir casting technique with 4, 6 and 8 wt% of Al₂O₃. The mechanical mixing was carried out using a mild steel stirrer at 600 RPM. Three sets of specimens were prepared to evaluate microhardness, UTS and ductility of Al-6061 MMC. It was found from mechanical tests that on increasing wt% of Al₂O₃ the microhardness and UTS of particulate-reinforced composite increased gradually. However, the hardness and UTS for Al-6061/8 wt% Al₂O₃ increased, respectively, by 25% and 13.51% when compared with Al-6061/4 wt% Al₂O₃. The ductility of cast composite decreased from 14.22% to 7.5% with the addition of Al₂O₃.

Hima Gireesh et al [48] have prepared Al-6061/Al₂O₃-hybrid-reinforced MMC strengthened with SiC and FA reinforcements via stir route of casting. Equal 5, 7.5 and 10 wt% of Al₂O₃/SiC mixed with 5 wt% of FA were used to investigate the mechanical characterization of HMMC. The experimental outcomes demonstrated that the addition of triple reinforcements up to 10:5 wt% (Al₂O₃/SiC:FA) had increased the tensile strength from 117 to 129 MPa whereas the YS and hardness of hybrid composite increased up to 7.5 wt% SiC/Al₂O₃. It was concluded from results that the tensile strength increased by 12%, hardness increased by 113% and YS increased by 122% when compared with AA-6061. Yaspal et al [49] analyzed the effect of BA and alumina particulates on the mechanical characteristics of Al-6061 HMMC. The composite was fabricated with 5 wt% of Al₂O₃ and 8 wt% of BA through stir casting technique. The particulate size of BA was 37, 53 and 75 µm. The stir action was completed at 400/500 RPM for 10 min. It was revealed from test results that UTS, impact strength and microhardness of reinforced composite decreased with increasing particulate size from 37 to 53 μm. The maximum value of hardness, UTS and impact strength, obtained at 37 μm particulate size, was 5 VHN, 180 MPa and 6.4 J, respectively. Further, Quarder et al [50] have reported the mechanical characteristics of Al-6061 MMC reinforced with Al₂O₃ and RM. The fabrication of composite was done at a cast temperature of 750°C followed by mechanical mixing at 450 RPM for 5 min by vortex method. Fabricated composites contain an equal proportion of 2.5, 5, 7.5 and 10 wt% of Al₂O₃ and RM. It was found from experimental results of Al-6061/Al₂O₃/RM HMMC that amongst different compositions, the 10 wt% of reinforcement reported maximum tensile strength (192 MPa) and hardness (21.5 HRB). The overall improvements in UTS and hardness were observed to be 12.94% and 26.47, respectively, at 10 wt% of composition when compared with 2.5 wt% of composition.

According to a separate study conducted by Pitehayyapillai et al [51] on Al-6061/Al₂O₃/MoS₂ HMMC through stir casting process the accumulation of alumina particulates increased the hardness and UTS of HMMC whereas amalgamation of MoS₂ with Al₂O₃ reduced the tensile strength and hardness. The hybrid composite was developed using 4, 8, 12 wt% of alumina (Al₂O₃) and 2, 4, 6 wt% of molybdenum disulfide (MoS₂). The mechanical mixing for molten slurry was performed at 600 RPM for 20 min. The hardness and UTS of the composite were determined according to ASTM E10-07 and E08-8 standards using Taguchi’s design of experiment. It was revealed from test results that the Taguchi design of analysis validated the best optimum parameters that showed maximum UTS (259.3 MPa) and hardness (107.56 BHN), which were 12 wt% Al₂O₃ and 4 wt% MoS₂. It is concluded from ANOVA results that Al₂O₃ wt% is the most significant factor that enhances the mechanical performance of hybrid-reinforced MMC.

Reddy et al [52] studied the mechanical behavior of Al-6061 MMC reinforced using silicon carbide (SiC) and boron carbide (B₄C). The 0.5, 1, 1.5 and 2 wt% of SiC and B₄C particulates were reinforced with Al-6061 alloy by liquid metallurgical technique. It was noticed from test results that 2 wt% SiC/2 wt% B₄C/96 wt% Al-6061 showed superior flexural strength, hardness and UTS in comparison with other compositions. However, the impact energy for 0.5 wt% SiC/0.5 wt% B₄C/99 wt% Al-6061 was reported to be the maximum. Total improvement of 19.13%, 50.4% and 92.76%, respectively, in tensile strength, hardness and flexural strength of Al-6061/SiC/B₄C HMMC was reported in comparison with AA-6061. In another work Auradi and Kori [53] prepared Al-6061/B₄C MMC to assess various mechanical properties like tensile strength, compressive strength and hardness. The single-reinforced composite was fabricated by a two-stage stir casting process at a casting temperature of 750°C using 5 and 7 wt% of B₄C and K₂TiF₆ salt. It was observed from mechanical results that the UTS and UCS of Al-6061/B₄C MMC increased by 22.1%, 38.8% and 12.7%, 32.06% for 5 and 7 wt% of B₄C particulates when compared with AA-6061. Moreover, the hardness of composites increases from 70.34 VHN (AA-6061) to 120.5 VHN (7 wt% B₄C/AA-6061). According to Rajesh et al [54] the Al-6061 MMC reinforced with B₄C showed significant improvement in mechanical properties when compared with Al-6061 base alloy. The composite was fabricated by stir casting method using K₂T_iF₆ flux and pre-heated reinforcement at 9 wt%. The reinforced composite was dispersed using a zirconia coated stirrer at 250 RPM for 5/8 min. The UTS, hardness and specific strength of reinforced composite increased by 38.8%, 115.3% and 42.8%, respectively, at 9 wt% of B₄C. The percentage elongation of B₄C/Al-6061 composite decreased by 65.17% with the addition of reinforcements. In a separate work the mechanical performance of Al-6061/B₄C_p MMC was assessed by Rajesh et al [55]. The B₄C_p was reinforced inside the Al-6061 matrix via the liquid metallurgical route (two-step addition) using 7 and 9 wt% of reinforcement. Marginal improvements by 17%, 38.4% and 14.98%, 23.6%, respectively, in UTS and UCS were reported at 7 and 9 wt% of particulates. Furthermore, with the addition of reinforcements the hardness of AA-6061/B₄C_p MMC increases from 67 to 139 HV; however, ductility decreased from 13.4% to 5.0%. It is concluded from the experimental outcome [68, 81, 85, 90] that the presence of hard B₄C particulates is attributed to resistance offered by B₄C particulates to plastic deformation, which increases the strain energy of reinforced composite; however, pre-heating of B₄C in Al matrix with K₂T_iF₆ as a wetting agent also enhanced the bond strength between Al-6061 and B₄C, which resulted in better mechanical properties of Al-6061/B₄C MMC. Moreover, amongst different wt%, 9 wt% of B₄C-reinforced Al-6061 MMC showed the maximum UTS and hardness of 185.4 MPa and 151.4 HV, respectively.

Marachakkanavar et al [56] have performed the experimental study on Al-6061 alloy reinforced with iron ore to investigate the mechanical properties. Al-6061 MMC was strengthened with 2, 4, 6 wt% of iron ore of 150 μm particle size by stir route of casting method. After fabrication, cast samples were heat treated (T6) at 525°C for 6 h followed by age hardening at 175°C for 8 h. The UTS and BHN of composite increase from 173.3 to 240.5 MPa and 72.6 to 103.59 BHN, respectively, in comparison with pure Al-6061 and results are demonstrated in figure 7. The ductility of as-cast composite reduced by 20.3% when compared with base alloy. In a separate research, Phanibhusshana et al [57] examined the mechanical strength of hematite (Fe₂O₃)-reinforced Al-6061 MMC. The Fe₂O₃ particulates were incorporated within Al-6061 matrix in varying steps of 2, 4, 6 and 8 wt% by liquid metallurgical technique. The incorporation of Fe₂O₃ inside the AA-6061 was performed at 300/350 RPM for 5/10 min. It is evident from test results that with the accumulation of 2/8 wt% Fe₂O₃, the macrohardness increases from 28.5 to 37.4 BHN. The hardness and UTS were enhanced by 31.47% and 25%, respectively, in comparison with Al-6061 matrix. Moreover the mechanical strength of Al-6061/iron ore and Al-6061/Fe₂O₃ MMC was found to be better than that of the AA-6061 because of the presence of harder and brittle Fe₂O₃ micro-particles, which distributed the applied load from AA-6061 matrix to Fe₂O₃ reinforcements.

Figure 7

Effects of iron ore particles on UTS and Brinell hardness of Al-6061 metal/matrix composite [56].

In another work, the impact of surface treatment and artificial hardening on the mechanical strength of Al-6061 MMC reinforced through multi-wall carbon nanotube (MWCNT) was studied by Manjunatha and Dinesh [58]. The solution heat treatment of stir cast Al-6061/MWCNT MMC was performed at 555°C for 8 h followed by the aging process. It is evident from a result that that with an increase in aging time, the microhardness of as-cast composite increases marginally and this is due to the hardening of Al-6061 by MWCNT particulates at optimized aging temperature and time. Moreover, amongst different aging times, 175°C for 10 h yielded superior mechanical properties of heat-treated samples. Further, Ramesh et al [59] have studied the effect of frit particulates on tensile strength, hardness and UCS of Al-6061 cast composite analyzed and compared to Al-6061 alloy. The Al-6061 MMC was synthesized with 2, 4, 6, 8 and 10 wt% of frit particulates through liquid metallurgy stir casting technique. Significant enhancement in mechanical strength was reported with the accumulation of frit particles. The hardness of cast composite increases continuously with increasing frit wt%, whereas the UTS and UCS of Al-6061/frit MMC increase up to 6 wt% of reinforcement; thereafter, they get reduced. The total improvements in UTS, hardness and UCS were, respectively, reported as 46.51%, 29.17% and 55.1% at 10 wt% of frit particles in comparison with 2 wt% of frit particles. The improved mechanical strength is attributed to the close packing of pre-heated frit particles with matrix alloy and the presence of hard ceramic particles inside the soft Al-6061 matrix.

Nagaral et al [60] fabricated AA-6061/Gr MMC via two-step casting process to determine the mechanical properties. The Al-6061 alloy was mixed with 3, 6 and 9 wt% of Gr reinforcements at 250 RPM for 5/8 min. It was noticed from test results that the accumulation of Gr reinforcements marginally increases the tensile strength and ductility of reinforced AA-6061/Gr MMC. The UTS of Al-6061/Gr MMC increased by 29.94% at 6 wt% and 36.18% at 9 wt%; however, the hardness of composite decreased due to the addition of Gr content. It is concluded that the presence of graphite particulates acts as solid lubricants on the surface of as-cast composite, which deforms easily when subjected to normal load, resulting in lesser hardness than that of the base alloy.

In another investigation, Hyderali et al [61] fabricated Al-6061-reinforced MMC using 3, 6 wt% of graphite (Gr) and 6, 3 wt% of FA. The tensile strength of Gr/FA/Al-6061 HMMC was determined using a UTM and the hardness using a Brinell hardness tester. Three samples, Al-6061, Al-6061/6% Gr/13% FA and Al -6061/3% Gr/6% FA, were developed through stir casting technique at stirrer speed of 200/400 RPM for 10/20 min. The mechanical result indicated that the utmost value of hardness (30.26 BHN) was obtained at 6 wt% Gr and 13 wt% FA. The stress/strain graph showed that the maximum tensile load at specimen fracture was 2069.3 N at 10% elongation. It is concluded that the presence of graphite and FA marginally improves the mechanical strength of Al-6061 composite and it exhibits superior properties than those of the base alloy. According to Bhandakkar et al [62] the fracture toughness (K_1c) of AA-6061/FA composite decreases from 18.21 to 14.27 MPa m^1/2 when FA particles increase from 0 to 10 wt%. The total decrement of 21.63% in fracture toughness was noticed when compared with base alloy. This behavior of K_1c is due to the presence of small cracks that cause weak interfacial bonding between FA and AA-6061 matrix resulting in lower fracture toughness. Further, it was observed from mechanical results that with the accumulation of FA weight fraction, the UTS, YS and hardness of reinforced composite had increased marginally. In a separate work the mechanical behavior of Al-6061 MMC reinforced with FA and CNT was evaluated by Parswajinan et al [63]. The fabrication of hybrid composite was done using 2, 3, 4 wt% of FA and 0.2 wt% of CNT through stir route of casting. The mechanical properties like UTS, impact energy and hardness of composites were calculated and compared to those of un-reinforced alloy. It was obtained from mechanical results that the addition of 3 wt% of FA and 0.2 wt% of CNT significantly increased the UTS by 147.76 MPa; however, the hardness of composite increases to 59.2 BHN at 2 wt% of FA. The maximum improvements of 11.81% and 11.69%, respectively, in UTS and hardness of Al-6061/FA/CNT HMMC were reported in comparison with Al-6061 alloy; yet, no improvement in impact strength was noticed. Further, Varalakshmi and Kumar [64] characterized the mechanical behavior of AA-6061MMC strengthened with coconut shell ash (CSA) using optimized parameters. The composite was produced using 1, 3, 5 wt% of CSA through stir casting process. The mixing of the molten slurry was carried out at a melt temperature of 750°C and stir speed of 600 RPM for 10 min using a stainless steel mechanical stirrer. The impacts of CSA on microhardness and UTS of Al-6061/CSA MMC were determined and compared to those of AA-6061. It was found from test observation that UTS and microhardness of reinforced MMC increased remarkably with the incorporation of CSA wt%. The maximum values of hardness and UTS were observed at Al-6061/5 wt% CSA, which were 143.66 MPa and 82 VHN, respectively.

In a separate research, the comparative study of tungsten carbide (WC) and graphite (Gr) reinforcement on the mechanical performance of Al-6061 MMC was performed by Swamy et al [65]. The composite was developed using equal proportions of 1, 2, 3 and 4 wt% of WC and Gr reinforcements through vortex method. It was seen from experimental results that in comparison with Gr-reinforced composite the WC-reinforced composite showed higher hardness, compressive strength and tensile strength. The maximum value of mechanical properties was obtained at 3 wt% of WC; afterwards the properties get reduced. However, the UTS and UCS of Al-6061 MMC increased with addition of Gr content. The ductility of Al-6061/WC MMC decreased with the accumulation of WC particles, whereas Al-6061/Gr MMC exhibited superior ductility than Al-6061/WC MMC. Further, Kumar et al [66] fabricated Al-6061 HMMC reinforced with titanium (Ti) and E-glass fiber particulates. The experiment was performed using 1, 3 and 5 equal wt% of Ti/E glass via stir casting method. The pre-heated reinforcements were added into a furnace at 750°C temperature followed by impeller mixing at 450 RPM. The results obtained through mechanical tests showed a maximum increment in tensile strength and hardness at 1:3 wt% of Ti and E-glass reinforcements. The microhardness of composite increased by 25% when compared with base alloy; however, no such improvement was reported in UTS. It was also observed that in comparison with Ti particulates the E-glass fiber significantly improved the mechanical properties of the Al-6061/Ti/E-glass HMMC. In a study, Raviraj et al [67] observed the influence of titanium carbide (TiC) reinforcements on fracture toughness (K_1c) of Al-6061 MMC. The TiC particles varied in steps of 3, 5 and 7 wt% inside the Al-6061 matrix through liquid casting method. The particulate size of TiC reinforcement was 3/4 µm. The specimens for K_1c were prepared according to ASTM E399 standards. It is reported from the experimental study that incorporation of TiC particulates decreases the K_1c of Al-6061/TiC MMC from 19.2 to 16.4 MPa m^1/2, though composites with sample composition of 3 and 7 wt% of TiC show higher K_1c than 5 wt% of TiC particulates.

The metallurgical properties of stir cast Al-6061 (T6) MMC reinforced with 5, 10, 15 and 20 wt% of aluminum nitride particles (AINp) were evaluated by Kumar and Murugan [68]. In this study, a stir casting setup equipped with bottom pouring arrangement was used to fabricate the composites. The results of mechanical properties indicated that the maximum values of macrohardness and microhardness were obtained at 20 wt% of AINp, which were 79 BHN (107.89% from base alloy) and 91 VHN (106.81% from base alloy), respectively. The variation of the stress/strain curve shows that 20 wt% of AINp composite possesses the maximum strength with less ductility. It was also reported that YS and UTS of AA-6061/AINp MMC increased, respectively, by 95.12% and 46.95% compared with the AA-6061 base matrix. In another investigation, Pandiyarajan et al [69] inspected the mechanical behavior of Al-6061 HMMC reinforced with zirconium dioxide (ZrO₂) and graphite (Gr). The AA-6061/ZrO₂/Gr HMMCs were fabricated using 2:2, 6:2, 2:6 and 6:6 weight ratios of ZrO₂ and Gr through stir route of casting process. It was observed from experimental outcomes that for addition of reinforcements (ZrO₂:Gr) up to 6:2, the microhardness and UTS increased from 30 to 43 HRC and 128 to 175 MPa, respectively. The maximum improvements in hardness and UTS reported were, respectively, 30.23% and 26.85% at 6:6 wt% of ZrO₂/Gr when compared with AA-6061. In a separate work a similar finding was obtained by Udayshankar and Ramamurthy [70] on mechanical performance of Al-6061 MMC reinforced with 3, 6, 9 and 12 wt% of ZrO₂ (60 µm) through stir casting method. It is found from observations that above 9 wt% of reinforcement the wettability between the Al-6061 matrix and ZrO₂ decreases, which causes poor bonding between matrix and reinforcement, resulting in reduced strength and hardness. The samples were casted at melting temperature of 750°C followed by reinforcement mixing by 500/700 RPM for 5 min. Further, it was concluded that up to 9 wt% of ZrO₂ the UTS and microhardness of composite increase marginally from 110 to 145 MPa and 80 to 98 BHN, respectively, and thereafter get reduced to 115 MPa and 82 BHN at 12 wt% of ZrO₂. The maximum improvement in hardness and UTS by 31.81% and 22.5% was reported at 9 wt% of ZrO₂ when compared with 3 wt% of ZrO₂.

The comparative analyses of tensile strength (figures 8 and 9), microhardness (figure 10) and macrohardness (figure 11) with single- and hybrid-reinforced MMC are illustrated in figures 8–11. The plots revealed that the single-reinforced composite showed superior mechanical properties than hybrid-reinforced composite. Moreover the UTS, UCS and hardness of reinforced Al-6061 composites increased with increasing wt% and reducing particulate size of reinforcements. Amongst all the significant research works on Al-6061 MMC [44, 45, 51, 56, 65, 79, 87, 95], the maximum UTS (310 and 259.5 MPa) is reported with Al₂O₃ as single-reinforced composite and with Al₂O₃/MoS₂ as hybrid-reinforced composite, respectively [45, 51]. The maximum macrohardness (107.5 and 103.59 BHN) is obtained for Al₂O₃/MoS₂/Al-6061 MMC (hybrid-reinforced composite) and iron ore/Al-6061 MMC (single-reinforced composite), respectively [51, 56], whereas the maximum microhardness (194 and 139 VHN) is obtained for SiC/Al-6061 MMC (single-reinforced composite) and Fe₂O₃/B₄C/Al-6061 MMC (as hybrid-reinforced composite) [41, 95]. Moreover, the highest UCS (894.13 MPa), % elongation (25%) and impact strength (56 J) are reported with WC-, REP/Al₂O₃/SiC- and Gr/SiC-reinforced Al-6061 MMC, respectively [44, 65, 79].

Figure 8

Maximum tensile strength of single-reinforced Al-6061 MMC at different wt% of reinforcements [40, 45, 92, 101, 102].

Figure 9

Maximum tensile strength of hybrid-reinforced Al-6061 MMC at different wt% of reinforcement [44, 51, 63, 66, 69].

Figure 10

Comparison between maximum microhardness of single- and hybrid-reinforced Al-6061 MMC [35, 41, 68, 79, 87, 95, 101, 109].

Figure 11

Comparison between maximum macrohardness of single- and hybrid-reinforced Al-6061 MMC [44, 51, 52, 56, 70, 74, 84, 104].

Finally, it was concluded from mechanical results that as compared with Al-6061 alloy the Al-6061 MMC composite exhibited superior mechanical properties. The addition of primary reinforcements (SiC, Al₂O₃, TiB₂ and B₄C) marginally improves the UTS, UCS and hardness of Al-6061 MMC. This may be due the fact that these reinforcements are harder and stiffer than base alloy, which act as obstacles in the motion of dislocations [37, 38]. Another theory reported by several researchers is that during solidification thermal stresses are developed in composites due to the variation in the coefficient of thermal expansion between matrix and reinforcements that cause formation of dislocation density at the interface of reinforcement and base matrix. This results in improvement of mechanical strength of cast composite due to dislocation density [45, 46, 50]. Further, pre-heating of reinforcements, optimization of stir parameters and heat treatment of specimens also add strength to the reinforced composite [56, 57]. However, the reason for lesser mechanical strength was porosity and clustering of reinforcements also quoted by several researchers [49, 51, 59, 62].

3.2 Tribological characterization of stir cast Al-6061 MMC

In this section, the tribological aspect of Al-6061 MMC has been discussed and compared to that of the base alloy. The outcomes of tribological properties are demonstrated in table 6 with sample composition and wear parameters. The specimens of wear analysis were prepared as per ASTM G99 standards [77, 78, 84, 88, 95, 99]. The wear properties of as-cast specimens were determined using a pin on disk wear tester. The systematic arrangements of the pin on disk setup, sample configuration and counterpart disk are illustrated in figure 12(a)/(c) [59, 61, 63]. The input process parameters used for wears study were nominal applied load, rotational speed, sliding velocity and sliding distance to determine wear characteristics such as weight loss (WL), specific wear rate (SWR), wear resistance (WR), coefficient of friction (COF) and volumetric wear loss (VWL) [72, 79, 92].

Table 6 Tribological analysis of stir cast Al-6061 metal/matrix composites

Figure 12

(a) Wear testing machine (pin on disk) setup [61]. (b) Wear specimen configuration [59]. (c) Pin and counterpart arrangement [63].

A study was conducted by Mishra et al [71] on stir cast Al-6061/SiC MMC to investigate the tribological properties; 10 and 15 wt% of SiC and 150/160- µm size of as-cast composite were amalgamated with Al-6061 alloy to determine wear and COF of fabricated composite using a pin on disk experimental setup. Taguchi L9 orthogonal array was used to determine the effect of wear parameters on tribological characteristics. The results obtained at 10 wt% of SiC show that the sliding distance has a great effect on wear rate in comparison with sliding velocity and applied load. However, at 15 wt% of SiC, applied load has major influence on wear rate over sliding speed and sliding distance. The influence of applied load on COF was significantly higher than those of sliding distance and sliding velocity. In another research, Bhat and Kakandikar [72] revealed the dry sliding wear performance of Al-6061 MMC incorporated with 5 wt% SiC through surface response methodology (RMS). The wear test of Al-6061 MMC was carried out using a pin on disk tribometer. Nine experiments were performed under variable rotational speed and normal load to estimate the SWR of AA-6061 and Al-6061 composite. It is revealed from wear results as shown in figure 13 that Al-6061 composite has better WR as compared with Al-6061 alloy. The tribological results show that by increasing rotational speed (200/1200 RPM) and keeping load constant, the wear rate increases significantly; however, a similar finding was reported with increasing load (20/200 N) and keeping speed constant.

Figure 13

Comparison between specific wear rate of Al-6061 alloy and Al-6061 + SiC MMC [72].

Further, Mishra and Srivastava [73] also examined the dry sliding WR of stir cast Al-6061 MMC reinforced with 5/40 wt% SiC in the varying step of 5. The particulate sizes of 150 and 600 µm were used to investigate tribological properties. The wear analysis of Al-6061-SiC MMC was performed using a tribometer, normal load of 10, 20, 30 N, rubbing distance of 2 km and sliding velocity of 2 m/s. The wear results showed an increase in SWR with increasing sliding distance and normal load. However as mesh size increased from 150 to 600 µm, reduction in WR was also reported. The COF vs. load results showed reduction in COF with the addition of load; however, 600-µm size specimens have better COF than 150-µm size. Finally, it was concluded that 600-µm particles size at 35 wt% of SiC was the optimized parameter that showed higher WR and COF. In a similar research conducted by Kumar et al [74] on dry sliding wear behavior, SiC-reinforced Al-6061 MMC under all testing conditions exhibits better WR than base alloy. The composite was reinforced with 2, 4 and 6 wt% of SiC and 150 µm size particles by liquid metallurgy technique. The variation of VWL with different weight fractions under various wear parameters demonstrates that on increasing sliding distance and load, the VWL increases marginally; however, with accumulation of particulates (2/6 wt%), the VWR decreases. The wear result of SiC-reinforced Al-6061 MMC shows that SiC enhances the hardness of composite marginally, which further increases the WR of Al-6061 MMC.

In another investigation, Kumar et al [75] did comparative study on wear factor of SiC/AA-6061 MMC and Al₂O₃/AA-6061 MMC. The composites were prepared through liquid metallurgical route using equal 2, 4 and 6 wt% of SiC and Al₂O₃. In one trial the wear factors were tested under applied load of 10 N at rotating speed of 100, 300 and 500 RPM and in another trial under applied load of 20, 30 and 50 N at constant speed of 100 RPM. It was concluded from the wear study that in comparison with Al-7075 reinforced composite, the Al-6061 reinforced composite exhibited higher WR. Moreover the Al-6061/SiC composite at 6 wt% of reinforcement showed the lowest SWR in comparison with other weight percentages. In a separate study, Murthy et al [76] discovered the tribological characteristics of Al-6061/SiC particulates MMC under a dry sliding condition. The fabrication of composite was done via stir casting process using 5, 7.5 and 10 wt% of SiC. It is observed from wear results that the SWR of Al-6061/SiC MMC increases with an increase in applied load and sliding speed. Further, it is concluded from tribological results that amongst other parameters the sliding speed has a maximum influence over wear mechanism of Al-6061-SiC MMC; moreover, the composite fabricated with 10 wt% SiC showed the maximum COF.

In a separate research, Reddy et al [77] studied the tribological performance of as-cast AA 6061/SiC/Gr HMMC. The effects of SiC (2 wt%) and Gr (0.5, 1, 1.5, 2 and 3 wt%) on WR and COF were assessed using a pin on disc apparatus. It is seen from wear results as illustrated in figure 14 that wear rate of Al-6061 alloy and Al-6061/SiC/Gr hybrid composite increases gradually with increasing load, velocity and sliding distance. The wear rate of the hybrid composite showed total reduction of 20%, 40.2%, 47.3%, 57%, 73% and 64% in comparison with pure AA 6061. Moreover, with addition in applied load the COF of composite enhances and with sliding distance and velocity it reduces. In another relative study, the tribological performance of SiC and Gr particulates dispersed in Al-6061 MMC was evaluated by Manivanan et al [78] at 10/40 N applied load, 0.5 m/s sliding velocity and 1000 m sliding distance to analyze wear performance of Al-6061/SiC/Gr HMMC. The experimental outcome relates the wear rate of HMMC with the hardness of composite using archer equation (V = KWS/3H); that is more the hardness, higher the WR. Therefore enhancement in WRis attributed to the hardness of SiC particulates on the surface of hybrid composite and presence of Gr content, which acts as a thick lubricating layer on the surface of hybrid composite [48, 50]. The COF of the hybrid composite is observed to be less as compared with base alloy and this is due to the hard SiC, which reduces the plastic deformation, and adhesion at the contact surface of composite results in lower COF.

Figure 14

Wear rate of Al-6061/SiC/Gr HMMC. (a) Wear rate vs. load. (b) Wear rate vs. sliding velocity. (c) Wear rate vs. sliding distance. (d) Wear rate vs. reinforcing materials [77].

In another comparative research, Sharma et al [79] investigated the tribological characteristics of stir cast Al-6061 HMMC reinforced with silicon carbide, alumina and cerium oxide (CeO₂). The wear rate of Al-6061 hybrid-reinforced composite was determined using a pin on disk setup, 10, 20 and 30 N load, 0.5, 1.0 and 2.0 m/s sliding velocity and 0.5, 1, 1.5 and 2 km sliding distance. From test results, it was analyzed that the WR of Al-6061 with triple reinforcements (SiC/Al₂O₃/CeO₂) improved marginally by 87.28% when compared with dual reinforcements (SiC/Al₂O₃). The marginal improvements in WR of HMMC are primarily due to the presence of CeO₂ (rare earth metal), which works as a barrier in resistance of plastic deformation. Further, Umanath et al [80] have also examined the wear performance of stir cast Al-6061 (T6) MMC reinforced through SiC and Al₂O₃ under dry sliding condition. It was reported from the study that 15% SiC/Al₂O₃/Al HMMC showed higher WR than 5% SiC/Al₂O₃/Al HMMC. Amongst all the input factors, the volume fraction was the most significant parameter for improved wear performance. From surface response graph it was also noticed that the wear rate of the hybrid composite increased with increasing applied load and rotational speed from 39.24 to 58.86 N and 200 to 400 RPM, respectively. In another investigation, Umanath et al [81] also studied the wear behavior of Al-6061/SiC/Al₂O₃ HMMC under dry sliding conditions. In this study, HMMCs were manufactured by stir casting technique using 25 µm size and 5/25 vol% of SiC and Al₂O₃ in equal proportions. It was observed from wear results that height loss of specimen was directly related to the rubbing distance and maximum height loss was obtained at 1413 m. It is also revealed from the results that as load progresses the wear loss of the composite increases, whereas the height loss of Al-6061 (T6) base alloy increases more rapidly than than that of the Al-6061 (T6) cast alloy. Finally it is concluded from the research that dual reinforcements decrease the wear loss of the hybrid composite gradually. Bhandare and Sonawane [82] fabricated Al-6061 particulates MMC through stir route of casting technology using SiC, Al₂O₃ and Gr reinforcements; 1, 2.5, 4 wt% of SiC, 3, 5, 7 wt% of Al₂O₃ and 2.5, 4.6 wt% of Gr reinforcements were used to evaluate tribological behavior of Al-6061 HMMC. The tribological properties such as SWR and COF were investigated at a sliding velocity of 1.0446 and 1.832 m/s. It was noticed from wear analysis that with the addition of reinforcements (SiC/Al₂O₃/Gr) the WR of hybrid composite decreased marginally. However the value of COF increases at the smaller weight fraction of reinforcements but after this, it reduces as reinforcements content increase.

In another work, Uvaraja and Natarajan [83] reveal from tribological results that SWR of AA-6061-SiC-B₄C HMMC decreases with an increase in sliding distance and increases with increase in applied load. They concluded that the developed HMMC showed superior tribological properties than pure AA-6061 and in comparison with SiC, the B₄C content significantly enhanced the WR of the developed composite. The improved SWR and COF of Al-6061/SiC/B₄C HMMC are attributed to the addition of boron particles, which has excellent abrasion resistance. In a similar study (AA-6061/SiC/B₄C) with additional reinforcement (talc) Kumar et al [84] evaluated the tribological characteristic of Al-6061/B₄C/talc HMMC; 5, 10, 15 wt% of silicon carbide particulates (SiCp), 3 wt% of B₄C and 2 wt% of talc were dispersed into Al-6061 through stir casting process at 450 RPM stir speed for 10 min. Total improvements by 12.5%, 45% and 62.5% in WR and 4.4%, 19% and 41% in the friction coefficient were noticed for 5, 10, 15 wt% of SiCp/Al-6061 MMC when compared with AA-6061. It was reported from the research that talc showed solid lubrication property although boron carbide possessed good hardness, which resulted in better tribological performance than that of AA-6061. It is also revealed from the tribological outcome that material loss occurs due to the thermo-mechanical effect, which causes heat generation by rubbing action at the interface of the tool and specimen. In another work, Benal and Shivanand [85] used 9 wt% SiC and 1, 3, 5 wt% E-glass fiber to compare the effect of heat-treated samples and non-heat-treated samples on wear properties of Al-6061 HMMC. The cast samples were heat-treated at 530°C for 12 h followed by water quenching and artificial aging at 175°C for 3, 5 and 7 h. The results demonstrate that non-heat-treated samples possess less WR than heat-treated samples. Moreover, the specimen with 5 h duration at 175°C and 1000 m sliding distance showed superior wear properties than other samples. This specimen however possessed the maximum hardness, which also contributed to enhancement in wear properties of Al-6061/SiC/E-glass HMMC. Further, Padmavathi and Ramakrishnan [86] studied the tribological properties of Al-6061/SiC/MWCNT HMMC. The Al-6061 composite was reinforced with 0.5 and 1 wt% of MWCNT and 15 wt% of SiC through stir route of casting method. The wear tests were conducted on a pin on disk wear test rig under 0.5, 1 and 1.5 N load and 636 RPM rotational speed. It is noticed from tribological results that the SWR of hybrid composite decreases with an increase in wt% of reinforcements under all loading conditions. However, with the addition of SiC and MWCNT reinforcements, a marginal decrement in friction coefficient was also observed. It is finally concluded from outcome results that the presence of MWCNT significantly advances the tribological performance of Al-6061/SiC/MWCNT hybrid composite.

In another investigation Bharath et al [87] developed AA-6061 MMC with 6, 9, 12 wt% of alumina through liquid casting route to assess the wear characteristics at rotational speed of 300 RPM and applied load of 19.62 N. It was analyzed from the experimental outcome that WL of developed composite decreased with the addition of alumina particulates. This behavior was attributed to the presence of hard Al₂O₃ particles inside the matrix alloy, which resisted the plowing action and resulted in improved WR of the developed composite. According to Hariharasakthisudhan et al [88] the triple-reinforced composite (Al₂O₃/Gr/Si₃N₄) showed lesser SWR and COF than dual-reinforced composite (Al₂O₃/Gr). The wear properties of Al-6061/Al₂O₃/Gr/Si₃N₄ HMMC were evaluated under dry sliding condition. The SWR of composite with Si₃N₄ particles increased with an increase in normal load (10/30 N) and sliding speed (1/4 m/s). Finally, it was concluded that Si₃N₄ developed a stable bond between AA-6061 and Gr reinforcement that marginally improved the WR of reinforced composite. The material removal mechanism was the thermal softening of the composite due to abrasive wear and plastic deformation of materials. In another study Premnath et al [89] analyzed the tribological characteristics of Al-6061/Al₂O₃/Gr HMMC. It was seen from 3D surface plots that SWR of composite improved with addition in load; however, SWR reduced with an increase in sliding velocity. It was also reported from the research that the presence of self-lubricated graphite content and hard Al₂O₃ particulates improved the WR of HMMC significantly. It was concluded from the experimental design that load was the most significant factor followed by sliding velocity and wt% of particle reinforcements on SWR of HMMC. Moreover the friction coefficient of dual-reinforced composite decreased with an increase in normal load. In a separate work Kumar et al [90] correlated the wear characteristics of FA/Mg/Al-6061HMMC and graphite/Mg/Al-6061HMMC. Seven experiments were performed with the sample composition of Al-6061/4 wt% Mg/10, 15, 20 wt% FA and Al-6061/4% Mg/10, 15, 20 wt% FA/4 wt% Gr. The wear plots indicate that on increasing speed from 500 to 2500 RPM, the SWR of composite increases for Gr reinforcement. However, the value of SWR decreased with the addition of FA content up to 15 wt%; thereafter, SWR increased. It is finally concluded from the comparison between Gr and FA reinforcements (shown in figure 15) that the presence of Gr slightly improves the WR of reinforced composite due to its lubrication effect. Further, Ponugati et al [91] have optimized the tribological performance of stir cast Al-6061 HMMC. The composite was fabricated using 9 wt% of Gr and 1, 2, 3 wt% of WC at 300 RPM stir speed for 15 min to reinforce particles inside the matrix. A total of 30 experiments were performed as per the statistical technique (design of experiment) to investigate the wear loss and COF. The wear loss and COF were also optimized using Fuzzy Gray Relation Analysis (FGRA). It was concluded that amongst different weights % and wear parameters the 9:3 wt% of Gr and WC at 30 N applied load, 3 m/s velocity and 0.5 km sliding distance reported minimum SWR and COF of Al-6061/Gr/WC HMMC.

Figure 15

Comparison between specific wear rates of (a) 10, (b) 15 and (c) 20 wt% fly ash and, respectively, 10, 15, 20 wt% fly ash + 4 wt% graphite [90].

The AA-6061/TiC MMC was fabricated by Gopalkrishan and Murgun [92] through stir route of casting technique to assess the wear behavior of single-reinforced composite. The effects of TiC particulates (3, 5, 7 wt% of TiC) on tribological properties of reinforced composite were studied at different loads and sliding velocities. It was seen from wear plots that WR of composite increased gradually with increasing normal load and sliding velocity. However, the SWR of TiC/AA-6061 MMC increased steeply with the accumulation of TiC particulates for different wear parameters. This behavior is mainly due to the thermal softening of material, which causes sharp plastic deformation on the surface of AA-6061/TiC resulting in high wear rate. In a separate research Ramesh et al [93] developed Al-6061/TiO₂ MMC by stir casting route to evaluate the wear coefficient (WC) of the reinforced composite; 2/10 wt% of TiO₂ and 1 wt% of Mg with 20 µm particle size were cast to analyze the VWL and WC. The WC was validated using predicted value (Archer and Yang model) and experimental value under various sliding distance and loading conditions. The results indicated that the wear loss of reinforced composite was much lower than that of the un-reinforced alloy. The VWL of composite is attributed to heavy deformation and materials loss due to the thermal softening of composite at higher load. From the Archard model of WC, it was observed that the WC of reinforced Al-6061 MMC decreased with incorporation of 2/10 wt% of TiO₂ at 30 N load and 0.36 km rubbing distance. This behavior is mainly because of the harder TiO₂ content, which increases the hardness of the composite. The values obtained from experimental and predicted models for WC are approximately similar, which confirms that the results attained by the Archard model for WC are accurately predicted. Further, tribological properties of AA-6061/titanium debride (TiB₂)-reinforced MMC were investigated by Suresh and Moorthi [94]. Varied weight fractions of TiB₂ v were 0, 4, 8 and 12%. The wear tests of AA-6061/TiB₂ MMC were studied under different applied loads and rotational speeds. From wear results it was observed that on increasing wt% from 0 to 12, the WR of reinforced composite increased marginally. However, the value of COF was maximum at a low wt%; thereafter it declined gradually with increasing wt% of TiB₂.

Mummourthni et al [95] studied the Al–Mg–Si (Al-6061) MMC using 5 wt% of Fe₂O₃ and 2, 4, 6 wt% of B₄C. In this research the wear characteristics like WL, SWR and COF were evaluated. The wear specimen was dispersed at stir speed of 250 RPM for 10 min. The result obtained from wear study indicated that the SWR of hybrid composite decreased by 77% at 20 N load and 85% at 40 N load when compared with AA-6061. The marginal improvement in WR was due to work hardening effect of harder Fe₂O₃ and B₄C particles on the surface of matrix alloy (Al-6061). The COF of Al-6061/Fe₂O₃/B₄C increased up to 5:2 wt% of Fe₂O₃:B₄C. In other relative research by Monikadan et al [96], the tribological performance (WL and COF) of AA-6061 MMC with single-reinforced B₄C particles (10 wt%) was investigated. The wear test on fabricated AA-6061/B₄C composite was performed according to G99-05 ASTM standards using a pin on disk wear tribometer. The wear parameters like load, sliding velocity and rubbing distance were used to evaluate tribological characteristics. It is noticed from results that with the addition of normal load and sliding distance, the WL and COF of reinforced composite increase gradually; however the WL and COF reduced at higher sliding speed. This performance is attributed to the formation of a mechanical mixed layer (MML) on the contact surface of specimen, which reduces the WR and COF of AA-6061/B₄C MMC.

Further, Lakshmikanthan and Prabu [97] observed the effect of input parameters on tribological properties of stir cast Al-6061 MMC reinforced with 3, 6, 9, 12 and 15 wt% of CSA; 10 N applied load, 15 m/s sliding velocity and 1 km sliding distance were selected in this research to investigate the wear behavior of fabricated composite. It is noticed from wear graphs that up to 6 wt% of CSA, the wear rate decreases at a slower rate; thereafter it advances steeply with the addition of CSA. However, on increasing sliding distance, the wear rate gets reduced under all loading conditions. In a separate work, Prakash et al [98] evaluated the effect of rock dust (RD), cenosphere (CENO) and E-waste glass reinforcements on dry sliding wear properties of Al-6061 HMMC. Nine experiments were performed using an equivalent proportion of 5, 10 and 15 wt% with 40 µm particle size. The results obtained for Al-6061/RD/E-waste HMMC shown in Fig 16(a) show that the accumulation of particulate reinforcements significantly reduces the wear rate of composite. However, from figure 16(b) it is analyzed that the wear rate exhibited by MMC is higher than that by HMMC. Finally, it was concluded from research that cenosphere and E glass reinforcements showed better wear results than RD. According to Sharma [99] input parameters have significantly affected the SWR and COF of Al-6061/garnet MMC. The SWR increases with an increase in applied load and sliding velocity from 10 to 50 N and 1.25 to 3.05 m/s, respectively. The average COF of developed composite reduced with increasing reinforcement particulates, sliding velocity and sliding distance. It is finally concluded from the results that formation of the hard layer at the surface of composite acts as a solid lubricant, which reduces wear rate and frictional coefficient. Further, Chethan et al [100] studied the tribological performance of stir cast Al-6061 hybrid-reinforced MMC using 1, 2 and 3 equal wt% of bamboo char (BC) and B₄C. It was analyzed through results that the WR of the HMMC increased with the incorporation of BC and B₄C reinforcements whereas the SWR of composite decreased with increasing applied load and sliding speed. Furthermore, it is revealed from the study that the average COF of hybrid composite gets stabilized at lower rotational speed and longer sliding distance. The enhanced wear behavior of HMMC is attributed to the dispersion of B₄C and BC contents in which BC particulates act as solid lubricants and B₄C particulates prevent surface deformation, resulting in improved WR of Al-6061/BC/B₄C HMMC.

Figure 16

(a) Wear rate of Al-6061+RD/CENO/CRT HMMC. (b) Wear plot of Al-6061 MMC and Al-6061 HMMC [98].

Amongst several considerable studies on tribological performance of Al-6061 MMC, the presence of Gr significantly improves the WR and COF of composite [77, 78, 82]. This is due to the fact that graphite itself acts as a solid lubricant, which forms a protective layer between wear sample and counterpart that possibly reduces friction between rubbing areas [88, 89]. In several research works on Al-6061 MMC the hardness of SiC, Al₂O₃, TiC and B₄C particulates resisted the plowing action due to establishment of MML on the contact surfaces, resulting in improved WR [71, 72, 77, 85, 88, 89, 96]. Further, agro-industrial waste (CSA, FA, BC and RM) also contributed to reduce the WR and COF of AA-6061 MMC [90, 97, 100].

From the tribological outcome with numerous reinforcements it was concluded that Al-6061 MMC exhibited superior tribological characteristics than Al-6061 base alloy. The wear rate of composite increases with increasing sliding speed, applied load and sliding distance; however, with the accumulation of reinforcement wt%, the wear rate of cast composite decreases drastically. It is also revealed from studies that amongst various wear parameters the applied load is the most significant factor that highly influences the tribological properties of Al-6061 MMC. The thermo-mechanical effect is quoted as the reason for the material loss due to the thermal softening of materials.

3.3 Microstructural characterization of stir cast Al-6061 MMC.

The surface morphology and fractography analysis of stir cast Al-6061 MMC with different reinforcements has been reviewed and discussed here with OM and SEM photomicrographs.

Hillary et al [35] assessed the microstructural behavior of Al-6061/SiC/TiB₂ HMMC with hexafluorotitanate (K₂TiF₆) salt. The pre-heating of reinforcements was done at 800°C (SiC) and 250°C (TiB₂) to remove moisture content and other impurities. The mechanical stirring was performed at 600 RPM for 15 min. It is observed from the experimental study that as the wt% of TiB₂ increases the nucleation sites increase and the grain size decreases. The microstructural images illustrated in figure 17 show the presence of SiC and TiB₂ particulates inside Al-6061 alloy. The addition of K₂TiF₆ salt improves the wettability between melt and particles, which results in homogeneous distribution of reinforcements particles inside the matrix. It was concluded from microstructural analysis that the optimized stir parameters were responsible for achieving good bonding between matrix and reinforcements. In another study, Pazhouhanfar and Eghbali [101] investigated the morphology behavior of TiB₂-reinforced Al-6061 MMC. In this study the uniform distribution of TiB₂ particles was seen through OM and SEM micrographs around the intergranular and transgranualar regions of the Al-6061 matrix without any agglomerations. The microstructure of reinforced composite illustrated in figure 18 shows the presence of Mg₂Si (thin black layer), and ternary eutectic phase is also seen in some regions of AA-6061. The preheating of TiB₂ content at 250°C for 2 h and addition of k₂TiF₆ salt were reported as the reason for strong bonding between AA-6061 and TiB₂ particles.

Figure 17

Optical microscopic images of Al-6061/SiC/TiB₂. (a) Al-6061/5 wt% SiC/2 wt%TiB₂. (b) Al-6061/5 wt% SiC/4 wt% TiB₂, adapted from [35].

Figure 18

Optical photomicrographs of aluminum 6061 reinforced with TiB₂. (a) Al-6061 (O) pure alloy. (b) Al-6061 (T3) + 3 wt% TiB₂, adapted from [101].

In a separate work, Moses et al [102] have studied the effect of various process parameters (stir speed, stir time, stir blade angle and casting temperature) on morphology analysis of stir cast AA-6061/15% TiC MMC. The microstructural results revealed that low stirring speed (100 RPM) and minimum stirring time (5 min) are responsible for poor distribution of particles inside the melt. However, as the stirring speed and stirring time increased, respectively, from 100 to 500 RPM and 5 to 25 min, the dispersion of TiC inside AA-6061 improved significantly and fair distribution of particulates was also seen. Further, it was reported that 30° blade angle gave uniform distribution of TiC inside AA-6061 with no particle clustering whereas at 0° and 60° blade angles, heterogeneous distributions with more agglomeration were also observed. Further, it was revealed from microstructure that at low casting temperature (650°C) the particles distributed heterogeneously inside the melt due to low viscosity and high frictional resistance; however, high casting temperature (1030°C) resulted in large amount of porosity content. The optimum value of casting temperature obtained is 850°C, which reduces the viscosity and porosity of as-cast composite and results in homogenous distribution of particles inside Al-6061 MMC. In another research, Guan et al [103] have also investigated the impact of stirring parameters on microstructural characteristics of Al-6061 MMC. The reinforced composite amalgamated with 5 wt% of aluminum borate whisker (ABO_w) and 5 and 15 wt% of SiCp. The sizes of ABO_w and SiCp reinforcements were 1 and 8/14 µm, respectively. The dispersion of stir cast Al-6061/SiCp/ABO_w MMC was done using a mechanical stirrer at 300 RPM for 10 min at different cast temperatures of 680, 650, 640 and 630°C. The photomicrograph reveals that the homogeneity of as-cast composite increases with decreasing stir temperature and increasing stir time. Experimental results demonstrated that amongst different casting temperatures, 640°C showed homogeneous microstructure without any particle clustering and agglomeration.

Further, Auradi et al [104] evaluated the microstructural behavior of Al-6061 MMC reinforced with B₄C. The composites were fabricated through a two-step mixing technique using 5/20 wt% B₄C of 88 µm particulates size with K₂T_iF₆ salt. The SEM micrograph of Al-6061/B₄C MMC with single-step mixing showed non-homogeneous dispersion of reinforcements inside the Al matrix with large particulates clustering. However, two-step mixing process resulted in fair distribution of B₄C particles within the Al-6061 matrix without any voids and clustering of particles. The presence of halide salt (K₂T_iF₆) also improved the wettability between Al-6061 and B₄C. In another investigation, Kalaiselvan et al [105] studied the microstructural analysis of stir cast AA-6061/B₄C MMC at 4, 6, 8 and 10 wt%. OM images showed uniform distribution of reinforcement inside the matrix and it was uniformly distributed in all regions of the matrix; however, small traces of Ti layer were also noticed. It was found from EDAX analysis that 1.35% of Ti compound was present due to the incorporation of k₂TiF₆ flux that was added for removing oxides from base matrix and to ensure uniform wettability between AA-6061 and B₄C content. The uniform distribution of reinforcement inside the matrix could be due to similar density of B₄C (2.52 g/cm³) and Al-6061 (2.7 g/cm³).

In a research, the microstructure of AA-6061/aluminum nitride (AIN) MMC was investigated by Kumar and Murugan [68]. The composites were prepared through stir casting route at an agitator speed of 450 RPM for 20 min. The SEM photomicrograph illustrated in figure 19 shows clear interfacial bonding between AIN_p and AA-6061 and the AIN_p particles are distributed uniformly within the matrix without any cracks and porosity. However some traces of Mg₂Si are also present in several regions of the matrix, which according to the study was the element present in matrix alloy and additional Mg as a wetting agent. Further, Ezatpour et al [106] investigated the microstructure characteristics of stir cast Al-6061 MMC using nano-alumina (Al₂O₃) particulates. The photomicrographs shown in figure 20 indicate the fair distribution of Al₂O₃ particles inside the Al-6061 matrix with small voids and agglomerations. It is concluded from the experimental outcome that these voids increase with increasing wt% of alumina particles and this is primarily due to the entrapment of gases during melting. SEM results of fabricated composite demonstrated the iron-rich phase (Fe₃SiAl₁₂) with Al₂O₃ agglomerated particles and also occurrences of some nucleation pores around inter-grain regions. The presence of Fe₃SiAl₁₂ was mainly due to the milling of alumina particles with steel balls. In another research Srivastava and Chaudhari [107] studied the fracture analysis of tensile specimens having 1, 2 and 3 wt% of alumina nanoparticles (70 nm) through microstructural characterization. The SEM fractography of Al-6061/Al₂O₃ specimens revealed small dimples and cleavages that showed mixed-mode fracture for 1NC and 2NC specimens as there was no uniform bonding between matrix and reinforcement; however, for 3NC specimen the brittle fractured mode was reported due to uneven cracks and grain size. In the same way Ramesh et al [108] have also investigated the tensile fractured behavior of AA-6061 alloy and AA-6061/Si₃N₄ MMC through SEM analysis; 4, 6, 8 and 10 wt% of Si₃N₄ particles of 2/10 µm particle size were reinforced inside the Al-6061 matrix via stir casting method. It was revealed from microstructure images that the base alloy (AA-6061) showed large and uniform voids, which indicated ductile fracture, whereas AA-6061/Si₃N₄ composite showed brittle fracture due to the presence of non-uniform smaller grains. The SEM results in figure 21 show that the possible fracture mechanism of tensile specimens is due to non-uniformity of voids and nucleation of microcracks that cause de-bonding between Si₃N₄ and Al-6061. Further, Mahadevan et al [110] performed fatigue fracture examination on heat-treated Al-6061/SiCp composite using SEM fractography. The samples of fatigue fracture were solutionized at 530°C for 1 and 3 h followed by aging at 170°C for 4 h. The SEM photomicrograph revealed that fractured samples failed partially due to ductile fracture as confirmed through dull marks in a perpendicular direction of crack initiation. Moreover there was no sign of dimples over the crack surface but strong evidence of striation marks within the crack propagation region, indicating ductile mode of fracture. Finally, it was concluded that the presence of ductile dimples inside the fracture surface confirmed the uniformity of SiCp inside the Al-6061matrix.

Figure 19

SEM photomicrographs of AA-6061/AIN_p composite. (a) AA-6061/10 wt% AIN_p. (b) AA-6061/15 wt% AIN_p, adapted from [68].

Figure 20

Optical microstructural images of Al-6061/nano-Al₂O₃. (a) Al-6061 matrix alloy. (b) Al-6061/0.5 wt% nano-Al₂O₃, adapted from [106]

Figure 21

SEM fractographs of Si₃N₄-reinfoced Al-6061 composite. (a) Tensile fracture of reinforcing phase and matrix. (b) De-cohesion and void between Al-6061 and Si₃N₄ reinforcement, adapted from [108].

From microstructure analysis, it is observed that the particulate reinforcements disperse randomly inside the matrix alloy without any specific orientation. The presence of particles inside the matrix phase is clearly visible with fair distribution; however, some cracks and particle clustering were also observed at some regions. The SEM fractography of tensile specimens revealed de-bonding and particles fracture, which are mainly responsible for the brittle mode of fracture. Finally, it is concluded from these photomicrographs that above certain level of reinforcements the particle clustering and agglomeration start developing, which results in non-homogeneous dispersion of particulates inside MMC.

3.4 Density and porosity analysis of stir cast Al-6061 MMC.

The aluminum alloys are conventionally reinforced with particulates reinforcements like B₄C (2.52 g/cm³), SiC (3.2 g/cm³) and Al₂O₃ (3.9 g/cm³), which enhance the density of as-cast composite [111, 112]. However, in several studies, addition of light-weight agro-industrial waste such as FA (2.1 g/cm³), BA (1.95 g/cm³) and RHA (2 g/cm³) reduces the density of MMC [113, 114]. The experimental density of cast composite is calculated using the Archimedes theory, whereas theoretical density is discovered by rule of mixture [115,116,117]. Moreover, the porosity percentage is evaluated by comparing experimental density and theoretical density [109]. In this section, the effects of various particulate reinforcements on density and porosity of Al-6061 MMC are discussed.

Ezatpour et al [106] examine the effect of nano-alumina particles on the porosity of cast Al-6061 MMC and it is observed from figure 22 that with the addition of nanoparticles from 0 to 1.5 wt%, the porosity percentage of as-cast composites increases from 0.5% to 4%. Similar findings of increased porosity % with increasing reinforcement % were also reported by several researchers [36, 54]. In another work, Reddy et al [77] studied the theoretical density, experimental density and porosity % of Al-6061 HMMC reinforced with SiC and Gr and it was analyzed from experimental results that porosity % of Al-6061/SiC/Gr HMMC increased from 0.56% to 3.2%. Moreover, as compared with single-reinforced composite (Al-6061/SiC), the hybrid-reinforced composite (Al-6061/SiC/Gr) possesses lower density and this can be due to the presence of light-weight graphite (2.26 g/cm³) particulates. A separate study was conducted by Sharma et al [79] on porosity %, theoretical density and experimental density of Al-6061 HMMC with triple reinforcements (Al₂O₃/SiC/CeO₂) and it was discovered from experimental results that Al-6061/Al₂O₃/SiC/CeO₂ composite exhibited higher density than base alloy and it increased linearly with the accumulation of Al₂O₃/SiC/CeO₂ reinforcements. In this study another finding reveals that the porosity of Al-6061 alloy is reported to be 11.1%, which significantly reduces to 3.52% with the addition of 2.5 wt% REP (CeO₂) and 7.5 wt% of SiC and Al₂O₃. Further, Kumar et al [75] perform comparative analysis on experimental density of Al-6061/SiC MMC and Al-7075/Al₂O₃ MMC; it is observed from the experimental outcome that Al-6061 reinforced composite exhibits lower density than Al-7075 reinforced composite and the comparisons of results are illustrated in figure 23. In research work [47, 52, 75], on SiC/Al-6061 MMC, it has been investigated that the incorporation of SiC inside the base matrix increases the density of Al-6061 alloy linearly. Similar results of increased density with addition of different reinforcements (Al₂O₃ and Fe₂O₃) were observed by various authors [91, 95]. The variation of theoretical and experimental density of Al-6061/Fe₂O₃/B₄C MMC is demonstrated in figure 24 [63, 95]. However, in a separate study on AA-6061-SiC/Al₂O₃/FA HMMC, Hima Gireesh et al [48] reported lesser density of reinforced HMMC than un-reinforced Al-6061 alloy. In this study the reverse trend of decreasing density (2.67/2.44 g/cm³) with incorporation of particulate reinforcements (SiC, Al₂O₃, FA) was also observed. However in separate works, a similar finding is obtained by other researchers [65, 109] in which the incorporation of agricultural waste such as CSA and BA reduces the density of as-cast Al-6061 MMC. In another research on Al-6061/B₄C MMC, Rajesh et al [55] obtained a similar behavior. In this study the fabricated composite possesses lower density than Al-6061 alloy, and according to the authors this is due to the presence of light-weight boron carbide (2.52 g/cm³) particulates inside the Al matrix. Finally, it is observed that as compared with conventional reinforcements the agro-industrial reinforcements exhibit lesser experimental density. The reported works on theoretical and experimental density of stir cast Al-6061 MMCs with porosity % are illustrated in table 7.

Figure 22

Effect of nano-Al₂O₃ wt% on porosity % of Al-6061 MMC [106].

Figure 23

Comparison of experimental density between SiC/Al-6061 and Al₂O₃/Al-7075 MMC [75].

Figure 24

Theoretical and experimental density of Al-6061/Fe₂O₃/B₄C HMMC vs. wt% of reinforcements [95].

Table 7 Density and porosity results of stir cast Al-6061 MMC.

4 Summary

In this research, the mechanical, tribological, microstructural and physical properties of Al-6061 MMC are discussed briefly and on that basis the major key findings are listed in table 8. The results obtained through considered research consolidate that the addition of particulate reinforcement in various combinations significantly improves the mechanical and tribological performance of Al-6061 MMC. However, amongst numerous reinforcements, Al₂O₃, SiC and TiB₂ marginally contribute to mechanical strength whereas Gr and B₄C enhance the tribological characteristics. The incorporation of industrial and agro-waste (FA, RM, BA, CSA, BCA) as secondary reinforcements also shows high potential with enhanced mechanical and tribological properties of

Table 8 Key findings of Al-6061 MMC on overall characterization.

Al-6061 MMC. Based on the literature survey, it is discovered that the dispersion of SiC, Al₂O₃, B₄C and Gr with Al-6061 alloy finds several engineering applications in automobile, aerospace and defense sectors. The key applications of Al-6061 MMC with specific components are illustrated in table 9. The concluding remarks of literature work on stir cast Al-6061 MMC through single, dual and triple reinforcements with various process parameters are summarized in figure 25(a)/(c). It is observed from figure 25 that in comparison with other reinforcements, SiC is reinforced maximum time (35%) with Al-6061 MMC. However, Al-6061 alloy mostly disperses with single reinforcements when compared with dual and triple reinforcements. Amongst overall properties the tribological and mechanical were investigated maximum time by researchers on Al-6061 MMC.

Table 9 Applications of Al-6061 MMC with various particulates reinforcements.

Figure 25

Analysis of stir cast Al-6061 MMC. (a) Al-6061 MMC with different reinforcing materials. (b) Al-6061 MMC with single, dual and triple reinforcements. (c) Al-6061 MMC with different properties.

5 Conclusions

The review paper attempted to present a comprehensive research review on the characterization of stir cast Al-6061 MMCs with multiple particulates reinforcements. Based on the literature, following broad conclusions were drawn and discussed as follows.

• 1. The carbide, oxides, nitrides and agro-industrial reinforcements such as SiC, B₄C, TiC, WC, TiB₂, Al₂O₃, TiO₂, ZrO₂, MoS₂, Fe₂O₃, Gr, FA, RM, CNT and MWCNT in single, dual and multiple reinforcements were successfully incorporated in Al-6061 MMC through stir route of casting process. The accumulation of these reinforcements in particulates form considerably improved the mechanical, tribological and physical performance of Al-6061 composites.

• 2. The mechanical properties of Al-6061 MMC increased marginally with the addition of particulate reinforcements. Amongst various reinforcements Al₂O₃ SiC, WC and Fe₂O₃ significantly improved the UTS, UCS and hardness of cast Al-6061 MMC. Other reinforcements like FA, RM and glass fiber also strengthened the composites. However, the ductility and impact strength of reinforced composites decreased with an increase in reinforcement wt%.

• 3. Tribological properties such as SWR, VWR, WR, WL and COF of Al-6061 MMC significantly improved with the addition of solid reinforcements. The Gr, SiC, B₄C and Al₂O₃ particulates significantly improved the SWR and COF of Al-6061 MMC. However, incorporation of Gr was considered to be the best reinforcement for tribological properties due to its self-lubrication effect; it formed a scratch-resistant layer on the surface of as-cast composites, which enhanced the WR and COF of Al-6061 MMC.

• 4. Microstructural behavior of Al-6061 MMC through photomicrographs revealed that up to some level of reinforcement, uniform interfacial bonding with homogeneous distribution inside the matrix was observed; however, at higher weight fraction some voids, particles clustering, agglomerations and cracks were also reported by many authors.

• 5. However, addition of Mg and K₂TiF₆ enhanced the bonding and wettability between solid matrix and particulate reinforcements.

• 6. It was found from physical characterization that theoretical and experimental density of Al-6061 alloy increased with an increase in SiC, Al₂O₃ and Fe₂O₃ and decreased with BA, B₄C and CSA. In the most studied feature, porosity % showed a direct relationship with wt% of reinforcements. However, the presence of light-weight B₄C, Gr, FA, BA and CSA reinforcements was considered to be the most beneficial reinforcement to reduce the density of Al-6061 composites with less porosity %.

6 Future scope

In the current scenario, the need of high-performance, light-weight, low-cost material is increasing day by day amongst academic researchers. From many alternatives, the choice of agro-industrial wastes like groundnut shell ash (GSA), RHA, BLA, maize stalk ash (MSA) and bauxite residue (BR) is gaining more attention for metallurgists in the development of Al-based MMC. The right processing and synthesis of these materials may convert waste into green reinforcements. In the last many years, these waste materials were used in the form of ashes as complementary reinforcement with conventional ceramics reinforcements; however, very less work has been reported with these wastes (agro-industrial) as primary reinforcement. Moreover, the high cost and less availability of ceramic reinforcement pose many challenges for academic researchers. On the other side, the remarkable properties of agro-industrial ashes like availability, light weight, low processing cost, good adhesion, easy to handle and cost effectiveness could maximize the potential of composite materials. The additions of these reinforcements not only reduce the cost of fabricating composites but will improve the various physical, mechanical and tribological properties of developed MMC. Economic utilization of these ashes in powder form might provide alternative engineering materials for the metallurgists in the marine, automotive, defense and aerospace sectors. Furthermore, the limitation of conventional stir casting process could be overcome with the advanced ultrasonic-assisted stir casting processes in combination with bottom pouring vacuum-assisted technique for the production of Al-6061 MMC.