Microstructural Characterization and Mechanical Behaviour of SiC and Kaoline Reinforced Aluminium Metal Matrix Composites Fabricated Through Powder Metallurgy Technique

Aluminium metal matrix composites are widely used in the automobile industry due to their superior properties like high strength to weight ratio, high ductility and better corrosion resistance. In this study, the effect of naturally available and low-cost kaoline particles on the microstructural and mechanical behaviour of Al- SiC- Kaoline Hybrid metal matrix composite was investigated. Al-10 % SiC- X% Kaoline (X = 0, 2, 4, 6, 8) composite samples were fabricated through powder metallurgy technique by applying a compaction pressure of 350 MPa. The fabricated composite samples were subjected to Density, Hardness, Tensile and impact tests to study the mechanical behaviour of fabricated hybrid composite. The presence of SiC and Kaoline reinforcements was confirmed by using SEM and X-Ray Diffraction analysis. It was observed that the maximum ultimate tensile strength ( U.T.S ) and maximum Yield Strength ( Y.S) of the hybrid composite were found to be 263 MPa and 202 MPa for Al-10 %SiC-4 %kaoline reinforcement. The formation of the intermetallic compound such as Al2Cu was observed in XRD and SEM analysis for Al-10 % SiC-6 % kaoline and Al-10 % SiC-8 % of kaoline reinforcement which leads to decrease in the U.T.S and Y.S of fabricated specimens. The impact strength of Al-10 %SiC-8 % kaoline found to be decreased by 44.4 % compared to unreinforced Aluminium due to the presence of harder SiC and Kaoline reinforcements particles. To study the fracture mechanism, Scanning Electron Microscopy study was carried on the fractured tensile specimens which reveal that ductile fracture in unreinforced Al, Al-10 % SiC, Al-10 % SiC-2 % Kaoline due to the formation of dimples and brittle fracture was observed in Al-10 % SiC-4 % Kaoline, Al-10 % SiC-6 % Kaoline and Al-10 % SiC-8 % Kaoline due to the existence of cleavages and microcracks. The best suitable combination of mechanical properties was obtained at Al-10 % SiC-4 % Kaoline hybrid composite.


Introduction
Hybrid Aluminium metal matrix composites contain two or more reinforcement elements dispersed in the Aluminium matrix phase, finds application in fabricating brake calliper, piston and rocker arms in the automobile industry due to their superior properties like high strength to weight ratio, higher hardness, higher compressive strength and higher ductility [1][2][3][4][5]. The incorporation of harder ceramic reinforcements into the softer aluminium matrix enhances the hardness and reduces the ductility of base matrix material [6][7][8][9][10]. The improved mechanical and metallurgical properties of the composite can be achieved by the appropriate selection of the fabrication process and the type of reinforcement particles dispersed in the matrix phase [11][12][13]. Out of all fabrication processes available in fabricating HMMC samples, the Powder metallurgy technique was adopted due to the lesser porosity problems and the proper wettability between the matrix and reinforcement particles [14] In addition to this, the capability to produce required densification and fabricate composites nearly net shape made powder metallurgy is a novel method to produce composites [15][16][17]. The evaluation of composite materials can be classified into different stages. In the first stage single hard reinforcement material is reinforced into the softer aluminium matrix to enhance the properties of the matrix material. The reinforcement material can be a discontinuous type (particles, whiskers and short fibers) or continuous type (fibers). Among these particle reinforced like oxides ( Al 2 O 3 , ZrO 2 , MgO), carbides ( TiC, SiC, B 4 C) and Nitrides ( BN, AlN) are widely used as a dispersed phase in aluminium matrix material [13,[18][19][20][21][22]. In the second stage, hybrid metal matrix composites were introduced to enhance the properties of composite material than with single reinforcement attributed to improved mechanical properties [23]. In the third stage, incorporation of industrial waste like fly ash and agro-waste like bagasse ash, corn cob ash, bean shell waste ash etc., are reinforced in matrix material along with harder ceramic reinforcements without deteriorating the properties of composite material [24][25][26]. Manohar et al. [27] studied the effect of kaoline clay on the mechanical behaviour of Al 7075-B 4 C hybrid composite and results concluded that the maximum tensile strength of 228 MPa was obtained up to the incorporation of 7 % kaoline. In addition to this, it was observed that the porosity of the HMMC was increased by the incorporation of kaoline more than 3 % due to the formation of the Al 3 BC intermetallic compound. Junwen Zhu et al. [28] analyzed the mechanical behaviour of AS CAST Al6082-SiC np composite and concluded that there was an 8.92 %, 12.16 %,12.60 % enhancement in the U.T.S, Y.S, % Elongation of composite material. V.S.S Venkatesh et al. [14] fabricated Al-Kaoline MMC through powder metallurgy technique. Results concluded that there was a 54.8 % enhancement in tensile strength of composite having 20 % kaoline reinforcement than the unreinforced aluminium. Prabu et al. [29] studied the effect of SiC reinforcements on the mechanical behaviour of Al 7075 composite and found that the hardness and compression strength was increased with an increase in % of SiC reinforcement. D.K.Q Mu et al. [30] analysed Al/SiC composite fabricated through powder metallurgy technique. Results revealed that optimal Y.S, U.T.S and % elongation was obtained at 24 (Table 1). Hence an attempt has been made to reinforce Kaoline as secondary phase reinforcements in the Al matrix.
In this present work, Al-SiC-Kaoline HMMC was fabricated through powder metallurgy technique by varying the Kaoline percentage from 0 to 8 % and the effect of kaoline on the mechanical properties like Hardness, Tensile strength, Impact strength are investigated. The distribution of reinforcements and formation of intermetallic compounds are analyzed with X-ray diffraction analysis (XRD) and scanning electron microscope (SEM) with energy dispersive x-ray analysis (EDS) analysis. The fractured tensile samples were subjected to Fractography analysis to know the type of fracture that occurred in the fabricated samples.

Experimental Procedure
The Aluminium matrix powder mixed with a pre-determined quantity of SiC particles having a particle size less than 20 μm (Fig. 1a) and Kaoline particles having a size less than 20 μm (Fig. 1b) to prepare Al-10 % SiC-x% Kaoline (x = 0,2,4,6,8) hybrid composite. Aluminium was selected as a matrix material due to the inherent properties like lower density ( 2.7 g/ cm 3 ), soft and high ductility. SiC particles was reinforced because of their higher hardness (280 BHN), high melting point (2730 0 C), compression strength (3900 MPa) and good corrosion resistance. The sequence of processes that followed during the fabrication of composite and cold compaction press was shown in Fig. 2(a, b). The as-received matrix and reinforcement powders are initially placed in the planetary high energy ball mill (Pulverisette 5 classic Frittsch GmbH) to disperse the reinforcement powders uniformly throughout the matrix material and to induce strain hardening effect in the powders [2]. Ball milling was carried out under an inert Argon gas atmosphere to prevent the oxidation of powders during the mechanical milling process. 1.5 % stearic acid was added to the powders as a process control agent (PCA) to prevent excessive cold welding between the powders and to avoid the welding between the powders and the surface of the balls [1]. After the ball milling process, these milled powders are compacted to the desired shape in H-13 steel die by applying a pressure of 350 MPa with the cross-head movement of punch was 1 mms − 1 to initiate the cold welding between the blended powders. The mixture of Acetone and zinc stearate applied on the outer surface of the punch and inner surface of the die as a lubricant for easy ejection of the compacted specimen from the punch and die assembly and to reduce the friction on the surfaces of the die [33,34]. The process parameters used in this process are shown in Table 2 [35]. The cold compacted samples were subjected to the sintering process in a muffle furnace for 3 h at 620 0 C with a temperature rise of 10 0 C/min and allowed to cool the samples inside the furnace [14,35].

Characterization of the Fabricated Composite
The developed composite was tested to determine its mechanical and morphological characteristics. The numerous tests conducted for the characterization of developed composite are discussed below.

X-Ray Diffraction (XRD) Analysis
The elemental phases present in the fabricated samples were analyzed by using XRD. XRD was conducted on a fully computerized PANalytical powder x-ray diffractometer by supplying 40 KV voltage and 20 mA current. The generated XRD spectra were taken at an angle (2θ) ranging from 10 0 to 90 0 with a step size of 0.02 0 .

Tungsten Scanning Electron Microscopy (WSEM) Analysis
The morphology and elemental composition of fabricated composite specimens are investigated on Carl Zeiss EVO 50 high resolution scanning electron microscope equipped with an energy dispersive x-ray (EDS) analyzer. To check the distribution of reinforcements in the matrix, the prepared samples were grinded on the belt grinder by using abrasive papers with grid sizes 600,800,1200,1800,2400. Finally, these specimens are polished using 2 μm and then 0.5 μm diamond paste on a twin-disc polisher. The composite samples are etched by using Keller's reagent (Mixture of Distilled water (190 ml), Nitric Acid (5ml), Hydrochloric Acid (3ml) and Hydrofluoric Acid (2 ml)) for 30 s to reveal the grains and microstructure at the micron level. The composite specimens are analyzed at a magnification range of 500X-5000X.

Density and Porosity Calculations
Composite samples having dimension 30 mm dia and 10 mm length was made by using compaction die [3]. The density of fabricated samples was determined by using the Archimedes principle [4]. The mass of the samples in the air was found by using an electronic weighing machine with the least count of 10 − 3 g [4,5]. Liquid displacement technique used to measure the mass of the composite specimens in water. According to Archimedes principle, the Density of the specimen was calculated by the following Eq. (1).
ρ Measured = Measured density of the sample. w = Weight of the sample in air. The porosity of the composite sample was determined by using the law of mixtures. The mathematical representation of % porosity calculation is represented in the Eq. 2.

Hardness Measurement
The Microhardness of the fabricated composite samples was measured as per ASTM E384-16 standards by using ECONOMET VH1MD Vickers Microhardness testing by applying a load of 25 N with 10 s dwell time [36]. The facricated composite specimen for hardness test is shown in Fig. 3 (c). The average of five hardness values was considered for each sample for better accuracy of results.

Tensile Test
To correlate the % reinforcement on the ultimate tensile strength (U.T.S) and yield strength (Y.S) of composites, Micro tensile test was conducted on a universal testing machine (M-30) model with crosshead movement of 0.5 mm/ min. Tensile specimens were fabricated according to ASTM E8 model having a gauge length of 25 mm, 6 mm thickness and 6 mm gauge width as shown in Fig. 3a. The average of three readings was considered to minimize the uncertainty in the tensile test results.

Impact Test
The prepared composite samples were subjected to the Charpy impact test according to ASTM A370 having a notch area of 100 mm 2 as shown in Fig. 3b [37]. Initially, the testing apparatus was set at 10 kgm for e 1 reading. The final reading for all prepared composite specimens during the experiment was recorded as e 2 . Three trials were conducted for each % composition and the corresponding average value was taken for Charpy impact strength. The Charpy impact value was found by using the following Eq. (3).

XRD Analysis
The XRD patterns of as received Al, SiC and Kaoline powders are shown in Fig. 4. The presence of Al 2 O 3 , SiO 2 , Cao/MgO, TiO 2 peaks were identified in the XRD pattern of Kaoline as shown in Fig. 4. XRD patterns of Al-10 % SiC-6 % Kaoline and Al-10 % SiC-6 % Kaoline reveals the formation of brittle clusters (Fig. 5) and the corresponding peak Al 2 Cu was observed at an angle of 38 0 which improves the brittleness and reduces the ductility of the composite. The obtained Al 2 Cu peak corresponding to these clusters was fairly accurate with the peak obtained by Gatea et al. [38]. No oxides peaks were identified in XRD patterns of fabricated composite. The absence of oxide peaks in the composite samples signifies that the achievement of good powder metallurgical composite samples. The increase in kinetic energy of matrix and reinforcement particles during sintering of specimens and the entrapped oxygen in the material will undergo chemical reaction at higher sintering temperatures are the main causes for the formation of clusters that degrades the strength of the composite [39][40][41].

SEM Analysis of Fabricated HMMC
Scanning Electron Micrograph images for fabricated composites was shown in Fig. 6 (a-d). Figure 6 (a, b) depicts the uniform distribution of reinforcements and no clusters were identified for the samples having Al-10 % SiC-2 % Kaoline and Al-10 % SiC-4 % Kaoline. The incorporation of kaoline reinforcements of more than 4 % causes the formation of agglomerations and clusters of Al 2 Cu which was identified in Fig. 6 (c,d) and the corresponding peaks shown in XRD pattern of Fig. (5). The obtained peaks for Al 2 Cu cluster was similar to Shakie et al. [38]. The existence of thermal mismatch between the agglomerations and reinforcements leads to decreases the bonding strength and the load-bearing capability of the composite. This, in turn, leads to the degradation of the strength of the composite [11,42]. The SEM micrographs of all composite samples ( Fig. 6 (a-d)) infers that the samples are free from voids and pores, which signifies that the samples were not undergone any oxidation during sintering and are free from defects. Figure 7

Influence of Reinforcement Particles on Density and Porosity of Hybrid AMMC
The effect of SiC and Kaoline on the density of hybrid Aluminium MMC is shown in Fig. 9. The theoretical density of the composite decreases from 2.71 g/cc for unreinforced Aluminium to 2.7552 g/cc for Al-10 %SiC-8 % Kaoline. The experimental densities decline from 2.6706 g/cc for unreinforced Aluminium to 2.7151 g/cc for Al-10 %SiC-8 % Kaoline. This decrement in density is due to the lower density of Kaoline (2.65 g/cc) against the density of SiC (3.21 g/cc) and density of Aluminium (2.71 g/cc). As the melting temperature of SiC reinforcement (2730 0 C) much higher than that of kaoline (740 0 C), These SiC reinforcements were not properly sintered at the sintering temperature (620 0 C) and acts as diffusion barriers could be the reason for the decrement in relative density of Hybrid composite [43]. The obtained experimental density values are close to the theoretical density which shows the porosity defects are less in the fabricating sample and the maximum porosity of 1.453 % was obtained for unreinforced Aluminium (shown in Fig. 10). This was attributed due to the formation of oxides and gas entrapment during the sintering of specimen [44]. In addition to this, the presence of intermetallic compounds at 6 % kaoline and 8 % kaoline reinforcements occupies the interfacial gap present in between the softer aluminium matrix and harder ceramic particles. This phenomenon further reduces the porosity levels of the composite to 1.287 % and 1.279 % for Al-10 % SiC-6 % Kaoline HMMC and Al-10 % SiC-8 % Kaoline respectively.    [47]. In addition to this, uniform distribution of reinforcements facilitates strong interfacial bonding between matrix-reinforcements offers resistance to dislocation movement which improves the strength of the composite up to 4 % Kaoline reinforcement [48]. However, The Addition of kaoline reinforcement beyond 4 % leads to decreases in the U.T.S from 263 MPa to 198.3 MPa and yield strength from 202 MPa to 131 MPa for 4 % kaoline to 8 % kaoline reinforcement. This was attributed to the fact that the incorporation of a higher volume fraction of harder reinforcements tends to improve the slip planes. During application of tensile load, the atoms present along the slip planes can easily find the path for movement along these slip planes and at the lower applied loads, the plastic deformation of material takes place. Similar results are reported by Sudarshan and surappa for different percentage of fly ash reinforcements [49]. The existence of deformation capabilities of intermetallic compounds and ceramic reinforcements leads to generate the triaxial stresses between the interfacial regions, thereby crack initiation and propagation starts at these zones and the early stage failure occur in the HMMC samples [44,[50][51][52].
The ratio of Yield Strength to U.T.S was found to be 0.76. It can be clear that the early stage yielding and strain hardening occurs in the composite material. Hence the fabricated composite is suitable for fabricating Metal forming related operations.

Influence of Reinforcement Particles on the Impact Strength of HMMC
The influence of kaoline and SiC reinforcements on the impact strength of HAMMC is shown in Fig. 13. The impact strength of hybrid composite found to be decreased with the increase in Kaoline reinforcement particles. The impact strength is higher for unreinforced Aluminium compared to composite with 10 % SiC and 8 % Kaoline reinforcement. The impact strength was decreased from 18 J for unreinforced Aluminium to 10 J for Al-10 % SiC-8 % Kaoline, i.e. 44 % decrement in impact strength observed from the base Aluminium alloy to Al-10 %SiC-8 %Kaoline composite. The impact energy of the specimen is due to the energy absorbed by the material before fracture. The higher impact of Aluminium was attributed due to the presence of higher ductility which tends to heavier plastic deformation before the fracturing at higher stress concentration areas. The presence of harder SiC and Kaoline reinforcements acts as barriers for dislocation movement which leads to improving the hardness and thereby decreases the plastic deformation energy of composite [53,54]. The incorporation of ceramic reinforcements (SiC, Kaoline) enhances the brittleness of the composite thereby, the energy absorption capability of the HMMC samples found to be decreased with an increase in percentage reinforcements. In addition to this, The existence of Al 2 Cu peaks for Al-10 % SiC-6 % Kaoline and Al-10 % SiC-8 % Kaoline composite weakens the interfacial bonding between the matrix and reinforcement particles which deteriorates the energy absorbing capability of composite [2].

Fractography
Fractography analysis was conducted on fractured tensile specimens to know the type of fracture that occurred during the tensile test. In general, the mode of fracture in tensile specimens was categorized into the ductile and brittle fracture. The formation of clusters during the sintering and non-uniform distribution of reinforcements are the main factors that govern and influence the type of fracture in the specimens. Non-uniform distribution of reinforcements leads to the difference in strain carrying capacity between the Aluminium matrix and reinforcements which attribute to the initiation of the crack in the material [20,55]. Microvoid coalescence and Dimples was generally observed in ductile fractured samples due to necking that occurred in samples before the fracture occurs [56]. But in the case of brittle materials, transgranular boundary movements and cleavages are formed due to lesser deformation energy. These movements enhance the crack through grain boundaries when the applied load exceeds the tensile strength of the material. Figure 14 depicts the SEM images for the fractured tensile surface. Figure 14 (a) shows the fractured surface of unreinforced aluminium which shows the presence of dimples that indicates the ductile fracture with large plastic deformation that leads to slow propagation of microcracks in the composite material [57]. Figure 14 (b, c) shows the presence of shallower dimples and minor cleavage facts which signifies that the composite failure occurs in ductile mode. Incorporation of kaoline reinforcements more than 2 % improves the brittleness of the specimen due to the presence of more wt% of oxides of Al, Si and Ti in kaoline reinforcements leads to cleavage patterns on the fracture surface causing the brittle fracture in the composite specimens as shown in Fig. 14 (d-f). The existence of ceramic reinforcements opposes plastic deformation due to the presence of differences in thermal expansions of softer aluminium and harder SiC and Kaoline reinforcements which causes cleavage facts in composite specimens. Transgranular cleavages lead to the initiation of crack through the grains and the intensity of crack increases with applied load and causes brittle failure when the applied load reaches beyond the yield strength of the material as shown in Fig. 14 (d) [58,59]. The presence of harder SiC and Kaoline reinforcements in the matrix reduces the elastic deformation in the matrix which attributes to debonding and clustering of reinforcement particles from softer ductile matrix causes microcracks during application of tensile load.

Conclusions
Following conclusions are drawn on the Al-SiC-Kaoline hybrid metal matrix composite fabricated through powder metallurgy technique. & XRD peaks reveal the presence of SiC and Kaoline reinforcements in the fabricated samples. In addition to this, the existence of intermetallic compound such as Al 2 Cu peak was observed in Al-10 % SiC-6 % Kaoline and Al-10 % SiC-8 % Kaoline composite samples. & The density of fabricated samples found to be decreased with increasing with kaoline reinforcement. The maximum porosity value of 1.455 was found for Al-10 % SiC-8 % Kaoline, which signifies that the fabricated samples are the absence of pores and defects. & The hardness of the composite found to be increased with increasing the kaoline reinforcement percentage. There was a 74.14 % enhancement in Vickers hardness value for Al-10 %SiC-8 %Kaoline than the unreinforced aluminium specimen. & Impact strength of fabricated composites found to be decreased with increasing the kaoline reinforcement. & Fractography analysis revealed that the ductile fracture in Al, Al-10 % SiC, Al-10 % SiC-2 % Kaoline due to the formation of dimples. The incorporation of kaoline reinforcement of more than 4 % improves the brittleness of the composite which in turn causes brittle fracture in remaining composite samples.