Effect of Sintering Mechanisms on the Mechanical Behaviour of SiC and Kaoline Reinforced Hybrid Aluminium Metal Matrix Composite Fabricated through Powder Metallurgy Technique

Present work focusses on the fabrication of Al-10% SiC-4% Kaoline HMMC by using conventional sintering, Microwave- assisted sintering (MAS) and Spark Plasma Sintering (SPS) techniques. Tensile, Compression and hardness tests were performed as per ASTM standards to study the effect of sintering mechanisms on the fabricated HMMC specimens. Results reveal that an enhancement of 13.3% in U.T.S and 11.7% Compression strength was observed in the Spark Plasma Sintered HMMC when compared to conventional sintered composite specimens because of lesser sintering temperature, time and the absence of intermetallic compounds in the Spark Plasma Sintering process. The formation of the Al2Cu intermetallic compound was identified in the XRD pattern of conventionally sintered Al-10% SiC-4% Kaoline HMMC sample due to the high sintering time and temperature which leads to inadequate mechanical properties. The fractured surface of tensile specimens reveals the presence of cleavages on the conventionally sintered HMMC which confirms the brittle fracture, and the existence of dimples on the Microwave sintered and Spark Plasma Sintered samples which signify that the ductile mode of failure in HMMC samples. Out of the three sintering techniques, Spark Plasma Sintering exhibits superior mechanical properties and lesser porosity levels.


Introduction
Aluminium based metal matrix composites are predominantly used in defence and automobile industries due to the superior properties such as high corrosion resistance, high strength to weight ratio, higher stiffness and comparatively lesser cost when compared to monoilithic materials [1,2]. The strength of the aluminium matrix can be improved by incorporating the harder ceramic reinforcements such as Al 2 O 3 , SiC, B 4 C, TiO 2 and ZrC etc., [3][4][5]. There are different fabrication processes available for fabricating composite materials like stir casting, compocasting, spray forming, powder metallurgy and in-situ fabrication techniques [6]. Out of all fabrication processes, the powder metallurgy technique was frequently used due to the lesser defects, lesser chance of formation of agglomerations and possibility to fabricate composites nearly net shape in powder metallurgy technique [7][8][9]. In powder metallurgy technique, sintering of cold compacts was carried by conventional sintering, SPS and MAS processes [10,11]. In the case of conventional sintering process, the availability of higher sintering temperature and time creates chemical interactions between the matrix and reinforcements that leads to the formation of agglomerations which reduces the mechanical strength of the composite [12,13]. This limitation in the conventional sintering process can be overcome by adopting advanced sintering processes like SPS and MAS. In the SPS technique, the composite specimen is undergone sintering as a result of simultaneous application of pressure and high-temperature plasma created at the interfaces of composite powder particles. The impurities present on the surface of the matrix and reinforcement particles were cleaned and the interfaces are activated to establish the strong bonding between the powder particles [14]. With the presence of lesser sintering time in the SPS process, the formation of intermetallic compounds was minimised and the composite samples with lesser porosities and better mechanical properties were fabricated by using the SPS technique [15]. In addition to this, the composites fabricated through SPS technique achieve high density and retain fine microstructure due to the reduced sintering time and temperature [16]. In MAS process, the heat generated in the powder particles is due to the absorption of microwaves and oscillation of powder particles was created at the frequency of the microwave frequencies [3,17]. The presence of higher heating rates, energy efficiency and process simplicity makes the MAS process better utilization in industrial applications to sinter ceramic powders [18]. Nouari Saheb et al., [19] fabricated Al-Mg-Zr composite through SPS process and concluded that Al-5 Mg-1 Zr composite sintered at 620°C achieved higher hardness when compared to Al-10 Mg-5 Zr composite. Nouari saheb [16] studied the mechanical behaviour of Al6061 and Al2124 synthesised by SPS and MAS processes. Results concluded that the hardness values increase with the increase in sintering temperature from 400°C to 500°C and the maximum value of hardness obtained was 70.16 VHN and 117.10 VHN for SPS Al6061 and Al2124 specimens. V.S.S Venkatesh et al., [20] fabricated Al-SiC-Kaoline HMMC and found that the maximum U.T.S of 263 MPa was obtained at Al-10%SiC-4% Kaioline HMMC. Inaddtion to this, addition of kaoline more than 4% leads to formation of agglomerations which diminishes the U.T.S of HMMC. Manohar et al., [21] synthesised Al-B 4 C-Kaoline HMMC through powder metallurgy technique. Results reveal that the maximum tensile and compression strength of 226 MPa and 228 MPa was obtained at 7% incorporation of kaoline. Based on the above literature, limited work was carried on the mechanical chaearcerization of kaoline reinforced HMMC fabricated through SPS and MAS techniques.
Kaoline clay was abundantly available, low-cost ceramic material contains Al 2 O 3 , SiO 2 , K 2 O, Fe 2 O 3 , MgO, CaO and Na 2 O which improves the mechanical properties of the composite material [22]. The presence of CaO in the kaoline reacts with Al 2 O 3 , SiO 2 and forms aluminates and silicates which enhances the bonding strength between the adjacent particles and helps to better load transfer from softer aluminium to reinforcement particles [23].
Hence, In this present work, an attempt has been made to fabricate the hybrid aluminium matrix composite by incorporating SiC as primary reinforcement and naturally available Kaoline Clay as secondary reinforcement. The effect of sintering mechanisms (Conventional sintering, SPS, MAS) on the mechanical characteristics of Al-SiC-Kaoline was investigated.

Raw Materials
To fabricate hybrid composite, Pure aluminium powder (99% purity) having particle size less than 30 μm was used as matrix material. SiC particles having a size less than 30 μm used as primary reinforcement because of their higher hardness (280 BHN), high melting point (2730°C) and compression strength (3900 MPa) [24]. Kaoline clay having a particle size less than 30 μm used as secondary reinforcement in this present study. The morphology of SiC and Kaoline is shown in Fig. 1(a,b). The presence of oxides of Silicon, Aluminium, Magnesium makes kaoline as a hard and brittle material [25]. The chemical composition of kaoline clay is shown in Table 1.

Ball Milling
A measured quantity of aluminium matrix and reinforcement powders are loaded in a chromium steel vial with powder to ball ratio of 1:10. Stearic acid was used as a process control agent to avoid the excess cold welding between the matrix and reinforcement powders and prevent the welding between the tungsten balls and powders. The chromium steel vial was sealed with Ar gas to avoid the oxidation of aluminium powder. Ball milling was performed in RESTECH 100 planetary ball mill to distribute the reinforcements uniformly throughout the matrix and to initiate the strain hardening in the powder particles [9,26,27]. The parameters adopted for an adequate ball milling process was shown in Table 2 [28].

Cold Compaction and Sintering
The milled powders were subjected to a compaction process by using a manual pallet press with an application of 600 MPa pressure [29]. H-13 steel dies with rectangular and circular crosssection were used to prepare composite samples for tensile and compression strength. The mixture of zinc stearate and acetone was applied to the contact surfaces of punch and die as a lubricant to eject the green compacted composite specimens. To study the mechanical properties of fabricated composite specimens, green compacts were subjected to three different sintering process such as (1) Conventional sintering at 620°C for 3 h [28]. (2) MAS at 500°C for 30 min [14]. (3) SPS in a graphite die at 500°C for 5 min with simultaneous application of 10 MPa and heat rate of 50°C/min [17]. Al-10% SiC-4% Kaoline HMMC specimens were fabricated by using conventional sintering, MAS and SPS process to study the effect of sintering mechanisms on the mechanical properties of the fabricated HMMC specimen.

Characterization of Composite Samples
Fabricated composite specimens were subjected to compression test as per ASTM E9 standards on micro universal testing machine M 30 model. Tensile test was performed as per ASTM E8 standards having specimen gauge length of 25 mm with crosshead movement of 0.5 mm/min. The fabricated composite specimens for compression, Hardness and tensile test is depicted in Fig. 2(a,b). The prepared composite specimens were subjected to impact test according to ASTM A370 having a notch area of 100 mm 2 [31]. Microhardness of composite samples was measured on ECONOMET VH1MD Vickers hardness tester as per ASTM E384-16 standards [32]. The average of five readings was considered for each sample for better accuracy of the result. To reveal the distribution of reinforcements in the matrix material, fabricated composite samples subjected to W-SEM equipped with an EDS analyser. Prior to W-SEM, To visualize the grain boundaries the specimens were etched by using Kellar's reagent (Mixture of Hydrofluoric Acid (2 ml), Hydrochloric Acid (3 ml), Nitric Acid (2 ml) and Distilled water (190 ml)).

Microstructural Characterization
The fabricated composite specimen corresponding to maximum mechanical strength (Al-10% SiC-4% Kaoline) was subjected to W-SEM analysis integrated with an EDS analyser. Results reveal that the conventionally sintered HMMC shows the existence of pores as shown in Fig. 3(a). In the case of MAS and SPS composites, SEM analysis revealed that the absence of porosities and the strong interfacial bond between the matrix and reinforcement particles (refer Fig. 3(b,c)). In addition to this, the formation of agglomerations along the grain boundaries was observed in the conventionally sintered composite specimen as shown in Fig. 4. These agglomerations or clusters were formed due to the existence of a large density difference between the matrix and reinforcement particles which leads to a decrease in the mechanical strength of the material. The mechanical strength of the material depends on the interfacial bonding strength between the matrix and reinforcement particles. The agglomerations and pores which were found in the conventional sintering process occupied the interface region of matrix and reinforcement and acts as barriers for the interface bonding between the matrix and reinforcement particles. These agglomerations also act as pre-existing microcracks that reduce the load transfer mechanism leads to early failure of composite material.
In addition to this, no agglomerations of particles were observed in the MAS and SPS composites due to the faster heating rate and shorter sintering time which decelerates the chemical interactions between the matrix and reinforcement particles. Elemental mapping of MAS and SPS composite shows the presence and uniform distribution of silicon (Si),    [39]) peaks in kaoline reinforcement powder. XRD analysis of composite specimens reveals that the presence of Al 2 Cu peak in the conventionally sintered composite specimen (refer Fig. 8). The obtained peak corresponding to Al 2 Cu (JCPDS file number: 25-0012 [40]) fairly matches with the peak identified by Gatea et al., [41] and Ma H et al., [40]. This Al 2 Cu peak in conventional sintering was formed due to the higher sintering temperature and the initiation of chemical interactions between the added reinforcements and matrix particle [42,43]. However, these intermetallic peaks were not observed in SPS and MAS composite specimens. In the case of the conventional sintering process due to the higher exposure time at higher temperatures causes accelerated diffusion, which initiates the chemical reactions between the Al matrix and harder

Mechanical Characterization
The fabricated composite specimens were subjected to mechanical testing and the results reveal that the maximum U.T.S of 263 MPa, Compession strength of 282 MPa and hardness value of 147 VHN was obtained at Al-10% SiC-4% Kaoline HMMC specimen and the corresponding values are shown in Table 3. To investigate the effect of the sintering mechanisms on the mechanical properties, Al-10% SiC-4% Kaoline was sintered through three sintering techniques, such as Conventional Sintering, MAS and SPS techniques. Results concluded that the U.T.S and compression strength of the composite samples which were sintered through the conventional sintering process is 13.3% and 11.7% lesser when compared to the HMMC fabricated through MAS and SPS techniques. This was due to the higher sintering time of green compacts in conventional sintering for achieving a good interfacial bond between matrix and reinforcements. But at higher sintering temperatures, the activation energies of matrix and reinforcement particles increases, which lead to interfacial reactions and resulting in the formation of Al 2 Cu brittle intermetallic compounds as shown in Fig. 8. This Al 2 Cu compound occupies the interfacial gap between the matrix and reinforcements and weakens the bonding strength between the adjacent particles. The existence of difference in deformation capabilities of intermetallic compound and reinforcement powders leads to generate triaxial stresses during the application of load. These internal stresses initiate the crack generation, propagation and failure of the composite specimen when the magnitude of these stresses exceeds the yield strength of the composite specimen [45]. In MAS process, the heat is generated from the core part of the ceramic powder particles due to the absorption of microwaves during sintering [28]. The developed heat inside the ceramic particles was propagated to the adjacent aluminium matrix particles. This phenomenon enhances the interfacial bonding between the composite particles and decreases the temperature gradients between the core part and surface of the composite powders [15,46]. The lower sintering time and temperature in microwave sintering improve the grain growth and eliminate the chances of formation of intermetallic phases.
The high U.T.S and compression strength of SPS composite were due to the generated plasma between the powder particles which enhances the bonding strength between the adjacent powder particles by breaking the impurities present on the surface of the matrix and reinforcement particles. In addition to this, this generated spark plasma generates joul heating with interface particles and electric discharge between the surrounding particles [14]. Further, in the SPS process, the formation of brittle clusters was completely avoided due to the accelerated grain growth due to lesser sintering time that reduces the chemical interactions between the softer aluminium matrix and harder ceramic reinforcements [44]. The effect of sintering techniques on the variation of Hardness, U.T.S and Compression Strength of HMMC is depicted in Fig. 9 and Fig. 10 respectively. Results concluded that the U.T.S, compression strength and hardness of composites sintered by MAS and SPS process can be enhanced by 6.84%, 7.44%, 6.12% and 13.3%, 11.7% and 16.3% respectively.

Effect of Sintering Mechanisms on Porosity of the HMMC
The percentage of voids present in the given specimen is referred as porosity. Porosity present in the material degrades the mechanical properties of the material. Pores generate the weak interfaces between the matrix and reinforcement particles that act as obstacles for the propagation of heat during the sintering. The existence of thermal mismatch between the matrix and reinforcements causes the differences in the deformation capabilities of softer aluminium matrix and harder reinforcements that create the pores at the interface regions. In the case of SPS due to the simultaneous application of pressure and heat over a short time period and the presence of vacuum during the SPS process, the possibility of pore formation was minimized and causes an efficient diffusion process between the particles. During the compaction process, it was observed that the compressibility of hybrid composite powders decreases with an increase in harder ceramic reinforcement particles. At the low compaction pressure rates (i.e., Quasi-static condition) the powder particles are having sufficient time to rearrangement and fill the interfacial gaps and produce the densified green compacted composite specimen. But at dynamic compaction pressure conditions, the powder particles don't have time to rearrangement such that interlocking occurs about their mean positions and undergone plastic deformation during compaction. This causes very few chances to fill the interfacial gaps that cause high porosity in the composite specimens that degrade the mechanical strength of the fabricated samples [45]. In this experiment higher porosity levels of 4.5% was obtained for conventionally sintered composite. The SPS and microwave sintered composite specimens showed porosity levels of 1.9% and 3.1% respectively as shown in Fig. 11. In the case of the SPS technique, porosity negatively affects the sintering process. Porosities present in the composite acts as electrical resistance and causes acute diversion of electric currents at the junctions of Al-SiC-Al particles, which prone to decreasing joul heating regions and reduces the densification of material [14]. To eradicate this defect, the reinforcement particles must be dispersed uniformly throughout the matrix which can be achieved by an appropriate selection of process parameters during the ball milling process.

Impact Energy
The energy absorbed by the material before fracture is termed impact energy. Figure 12 Indicates that there was an enhancement of 59.3% and 75.58% impact energy for MAS and SPS HMMC when compared to Conventional sintered HMMC.
The lower impact energy of conventional sintered composite was due to the presence of brittle Al 2 Cu intermetallic compound which was formed as a result of chemical interactions between matrix and reinforcement particles [41]. The brittle intermetallics reduces the energy absorption capability of the material [47,48]. In addition to this, the presence of higher porosity levels and microcracks in conventional sintered composites is the key factor for the lesser impact energy of HMMC [49]. In the case of MAS and SPS HMMC's, due to the presence of shorter sintering time and temperature makes the specimens free from agglomerations and causes a strong interfacial bond which makes the large plastic deformation at high-stress concentration areas before the fracture [50]. The high impact energy of MAS and SPS HMMC leads to ductile fracture under the application of load.

Fractography
Fracture in the tensile specimen is classified into the brittle fracture and ductile fracture. The distribution of reinforcement particles in the matrix material, interfacial bonding between the particles and the agglomerations which are formed due to the chemical interactions between the adjacent particles are  the essential factors that affect the type of fracture in tensile specimens. The non-uniform dispersion of reinforcements leads to variation in a strain carrying potentiality between the softer aluminium matrix and harder ceramic reinforcements (SiC, Kaoline). The presence of clusters weakens the interfacial bond which promotes the early stage failure of the composite under the application of load. Fractured tensile specimens are subjected to SEM analysis to know the fractured patterns like dimples and cleavage facts. The formation of dimples was generally formed on the fracture surface of the ductile specimen with large plastic deformation and localized stresses [51,52]. Transgranular cleavages occur on the brittle fracture which enhances the propagation of crack through the intergranular grain boundaries [47,48,53,54]. Figure 13(a-c) represents the SEM micrographs for the tensile fractured surfaces. The presence of harder reinforcements and the formation of Al 2 Cu intermetallic compound due to the large sintering time of conventional sintering promotes cleavage facts on the fractured surface as shown in Fig. 13(a). In the case of MAS and SPS, as the composite specimens are sintered in less time brittle agglomerations was not found in XRD patterns of SPS and MAS. Figure 13(b,c) reveals the presence of dimples due to the absence of Al 2 Cu intermetallic compound which causes ductile failure and enhances the elastic nature of composite specimens [55].

Economical Analysis
From an economical study, it was clear that the productivity is high for the conventional sintering process when compared to MAS and SPS. The conventional sintering process was preferred for the fabrication of components that are less important in engineering applications. In the case of MAS, the sintering cost is high compared to conventional sintering as the availability of equipment is less and high power consumption during SPS process (refer Table 4.) In the SPS process, the high cost of the initial set-up and cost of graphite die makes the SPS process costlier compared to the microwave sintering process. To fabricate the high precision components with lesser defects, these modern sintering techniques (MAS and SPS) are adopted as the lesser sintering time and temperature that can reduce the formation of intermetallic compounds. SPS technique was generally employed for the materials which are not

Conclusions
The present investigation explored the fabrication of Al-10% SiC-4% Kaoline HMMC through Conventional Sintering, MAS and SPS processes. The conclusions drawn from this present study were summarized below.