An Experimental Study on the Role of Extrusion Ratio and the Resulting Soundness of Extrudate in Improving the Properties of Al-Cu/Sic Composites

Dhanalakshmi S, Sundaram KS and Ramesh CS

Published on: 2022-01-16


Aluminium matrix composites (AMCs) are the potential materials for many applications owing to their synergic properties. AMCs reinforced with coated ceramic particles show improved mechanical properties. In order to further improve the properties, the AMCs are subjected to secondary processing such as extrusion. In this study, aluminium matrix composites reinforced with Ni coated SiC particles, prepared by stir casting technique was subjected to plastic deformation through hot extrusion. Extrusion was carried out at three different extrusion ratios viz. 4:1, 8:1 and 10:1, at a constant billet temperature of 450 oC and a ram speed of 2 mm/s. Microstructure using OM, SEM, TEM and properties such as density, hardness, impact energy, flexural strength, tensile strength and compressive strength of the as cast and extruded composites were studied. Composite extruded at an extrusion ratio of 8:1 showed grain refinement, reduction of particle size and preferential alignment in the extrusion direction leading to increased hardness, density, impact energy and tensile ductility. While an extrusion ratio of 4:1 had resulted in the improvement of tensile strength, compressive strength and flexural strength of the composites. Composites extruded at 10:1 exhibited microstructural inhomogeneity resulting in inferior properties.


Aluminum matrix composite; Hot extrusion; Extrusion ratio; Density; Microstructure; Hardness; Mechanical properties


The demand for lightweight materials with superior properties is ever increasing [1]. Extensive research [2,3] is being carried out in the field of light weight high strength materials. Particulate reinforced aluminium matrix composites (PRAMCs) are found to have synergic properties due to the strong, tough metallic matrix combined with hard ceramic reinforcement. Owing to their attractive properties and tailor ability, PRAMCs find extensive applications in the automotive, aerospace and the defence sectors [4,5]. The ceramic particulate reinforced aluminium matrix composite exhibiting high specific strength, specific modulus and wear resistance are commonly manufactured through stir casting, powder metallurgy [6], spray deposition technique, hybrid microwave sintering [7], etc. Among the methods mentioned above, the stir casting technique is the simple, economical [8] and the most versatile technique for mass production. Inadequate care during composite preparation process and poor foundry practices will lead to poor quality cast composites. For example, improper preheating of ceramic particles, turbulence during stirring etc. causes partial de-wetting of the particulate and air bubble entrapment. In order to improve the quality of the castings and also to obtain engineering products with improved strength the Composites are often subjected to secondary processing techniques such as rolling, forging, extrusion etc. Nevertheless, the wrought aluminium alloy (Al 2XXX etc.) based composites need to be subjected to secondary processing namely extrusion, rolling, forging and so on. The particulate composites, owing to their isotropic nature, show significant improvement in the mechanical properties after secondary processing. The improvement is due to excellent formability, uniform distribution and good bonding of reinforcement particles [9], break up of agglomerates [10], reduction of porosity [11] etc. Among the various secondary processing techniques, extrusion is the most commonly used technique, which is due to the preferential axial alignment of reinforcing particles as well as the large compressive hydrostatic stress state that occurs during hot deformation [11, 12]. Apart from enhancing the strength, application of extrusion is found to improve the ductility of the composites. However, to obtain a good quality extrudate with improved mechanical properties, the extrusion process parameters need to be optimized [13]. The billet temperature, ram speed and the deformation ratio are the key variables that affect the formability and properties of hot worked metal matrix composites. Numerous researches have been carried out to study the effect of processing parameters on the microstructure, mechanical properties and the formability of composites [14] have studied the effect of microstructure and mechanical behaviour of 15 vol. % SiCnw Al2024 Composite. They found that the extrusion temperature above 520 oC was beneficial in enhancing the mechanical properties, particularly, the composites extruded at 560 oC revealed the highest strength and elastic modulus. A study on extrusion of A16061-SiC composite by CS Ramesh et al [15] reports that the hardness and tensile strength with increase in extrusion ratio [16,17] observed a more uniform particle distribution in the Al 356-10 vol% SiC composites by increasing the extrusion temperature form 450 oC to 500 oC. The reason is that higher extrusion temperature facilitates the flow of the matrix alloy under the applied stress resulting in a gradual improvement in particle distribution. However, at very high extrusion temperature of 550 oC, the particles get aligned themselves at the grain boundaries due to partial melting of matrix alloy, thus exhibiting a relatively non- uniform distribution. Contradicting to the above, Hong et al. [18] had reported that the particle alignment was not affected by the extrusion temperature [16] also reported that degree of particle breakage is higher at extrusion temperature [19] attempted to optimize the extrusion process parameters for Al2O3p reinforced Al2024 & Al 6061 aluminum alloy composites produced by squeeze casting method based on reproducible tensile properties. They optimized higher extrusion temperature of 500-560 oC based on the improved ductility of the matrix alloys and smaller size (0.15 -0.3 µm) of the reinforcement particles used. In their study, the effect of extrusion speed (5, 7.5 and 10 mm/s) on the mechanical properties was found insignificant. It was also reported that increasing the extrusion ratio above 10:1 resulted in a decreased strength and increased elongation compared to the squeeze cast condition. With this backdrop, it is understood that extensive research has been carried out on the secondary processing of Al-Si based Al 6XXX alloys and their composites [19-21], while meager information is available on the Al-Cu based Al 2XXX series alloys. Also, the effect of extrusion temperature, extrusion die shape, ram speed etc. have been extensively studied, very few research data are available on the studies of effect of extrusion ratio on the structure and properties of Al-Cu/SiCp Composites. In the light of the above, the present research deals with the study of effect of extrusion ratio on the structure and properties of the SiC particle reinforced Al2014 composite. Aluminium alloy Al2014, designated as Al-4.5%Cu, is having high strength but moderate ductility. The extrusion process parameters play an important role in controlling the microstructure and properties of the composite material. Therefore, it is essential to arrive at the appropriate extrusion process parameters, which could yield optimum properties. This paper deals with the study of effect of extrusion ratio on the microstructure, density, hardness, impact properties, compressive strength and flexural strength of hot extruded Al 2014-SiC composites.

Experimental Procedure

Composite Fabrication

The base aluminium alloys in the form of ingots were procured from M/s Fenfee Metallurgicals, Bengaluru. The chemical composition of the base aluminium alloy used in this study is shown in Table 1. The electro-less nickel coated silicon carbide particles of average particle size 30-50 µm were used as reinforcement. The details of electro-less coating nickel on SiC particle are discussed in detail in our earlier work [22]. Metallic coating of ceramic particles improves their wettability with metallic matrix and prevents adverse chemical interaction between the particle and the matrix at elevated temperature.

Table1: Chemical composition of base alloy.











Wt %










The composites were prepared using the stir casting method. The nickel coated SiC particles (10 % by weight) were preheated to 400 oC and added to the molten aluminium alloy, which was maintained at 700-750 oC. The melt was then stirred at a speed of 300 rpm for about 15 min. The molten metal after degassing was poured in preheated metallic mould of 75 mm dia and 200 mm height. The base alloy was also cast in a similar way without addition of particles.

Hot Extrusion of Composites

The as cast base alloy and the as cast Al-10 wt.% SiC composites were machined to obtain billets of diameter 70 mm and height 100 mm. Fig.1 shows the 200 T extrusion press with extrusion die assembly. The extrusion die inserts, which is a part of extrusion die assembly, used for obtaining different extrusion ratios are shown in Fig.2. The extrusion die insert was mounted in the dieassembly, which was covered by a split type die heater with controller. The die heater is used to preheat the die and maintain the die at the desired temperature. The die assembly was maintained at a constant temperature of 350 oC throughout the extrusion process. The composite billets are homogenized at a temperature of 450 oC for a period of 6 hrs in a separate muffle furnace. After soaking the hot billet is transferred to the die cavity and pressed by the ram. All the extrusions were carried out a constant ram speed of 2 mm/s. A high temperature graphite based lubricant ‘Molygraf’ was used. Extrusion ratios of 4:1, 8:1 and 10:1 were adopted for a constant billet temperature of 450 oC. Fig. 3 shows the composite samples before and after extrusion. The extruded samples were subjected to T6 heat treatment in a muffle furnace. The samples were solutionised at 530 oC for duration of 2 hours and quenched in cold water. Quenching was followed by artificial aging. The samples were then subjected to artificial aging at 175 oC for a period of 6 hours.

Figure 1: Hydraulic extrusion press

Figure 2: Die inserts for different extrusion ratios.

Figure 3a: Composite samples a) before extrusion.

Figure 3b: After extrusion (extrusion ratio 4: 1).

Figure 3C: After extrusion (extrusion ratio 8: 1).


Microstructure: The microstructures were observed using optical and scanning electron microscope (SEM). The optical microstructures before and after deformation were observed using METSCOPE 1A metallurgical microscope. For the metallographic examination, the samples were prepared by the standard metallographic polishing techniques and etched using 0.5% HF solution. Carl Zeiss make, Supra 55 model FESEM was used for SEM analysis of microstructure and fracture surface of the composite samples. Specimens for fractography studies using SEM were prepared with additional care to avoid any damage to the particle. Transmission electron microscope, JEOL JEM 2100, Japan make was used to take the high resolution TEM images of the composite samples.

Density: The density of the samples was measured by water displacement technique using the set up as shown in Figure 4. In this method, the initial weight (‘m’ in g) of the sample before immersion in water was recorded. A measuring jar was filled with 140 ml of distilled water. The weighed sample was immersed into the measuring jar with water using a weightless string. The volume of displaced water is recorded to determine the volume of the sample under immersion (‘V’ in cm3). The density, ρ in g/cm3, was calculated using the formula ρ = m/V. Average values of five each of base alloy and composites were taken.

Figure 4: Density measurement set up.

Hardness: The hardness measurement from surface to core was done across the extruded samples cut in a direction perpendicular (transverse) to the extrusion direction. Micro hardness tests were performed on the samples, using Wilson, Germany make Vickers micro hardness tester by applying 100 g of load for a dwell period of 10 s. The hardness was determined by measuring the diagonal length of indentation produced. Hardness values were measured for every 0.1 mm nearer to the surface and for every 0.5 mm nearer to the core. The test was carried out at five different locations in order to counter any possible effect of indenter resting on the harder ceramic particles. The average of five readings was taken as the hardness of the sample.

Tension, Impact, Compression, Flexural Testing: Compression test at ambient temperature was carried out on the composites extruded at different ratios as per ASTM E9 standard. Room temperature tension test as per ASTM E8 flexural test and compression test were performed in the FIE make UT 40 Universal Testing Machine. Charpy V-notch impact test as per ASTM E23-98 was carried out on the as cast and extruded composite samples to find the energy absorbed before fracture. All the above tests were carried out on the as cast and extruded base alloy as well as composites.




Results and Discussion


The SEM image of the extruded base alloy and the EDS spectrum of the precipitate zone is shown in (Figure 5), which revealed the presence of CuAl2 precipitates in the Al-Cu matrix along the grain boundaries. The EDS spectrum taken in the precipitate zone is shown in (Figure 5c). Presence of isolated porosities known as voids (indicated by circles), which are not interconnected in nature, is also seen. The optical microstructure of the as cast and extruded base alloy and the composites are shown in Figure 6. It is seen from the optical micrograph of as cast base alloy (Figure 6a), that the precipitates are more or less aligned along the grain boundaries. 

Figure 5ab: SEM images of extruded base alloy showing the presence of precipitates in the matrix and the corresponding (c) EDS spectrum.

The optical micrograph of the as cast composite shows a fairly homogenous distribution of SiC particles in the aluminium alloy matrix. No zones of particle clustering and agglomeration are seen except presence of very little porosity which arises due to gas entrapment during stirring of the composite melt. The presence of multiple solidification fronts resulted due to the SiC particle addition, has led to uniform particle distribution. The microstructure of extruded base alloy and composite shows recrystallized grains. A good bonding between the matrix and the particle is seen in the case of extruded composites. From the optical micrographs (refer Figure 5 f-h), it could be noticed that among the composites extruded at three different extrusion ratios 4:1, 8:1 and 10:1, highly refined grains are found in the case of composite extruded at 8:1. This is due to the high degree of dynamic recrystallization taking place at this extrusion ratio. The dynamic recrystallization is high in the case of composites extruded at extrusion ratio of 8:1 because of the presence of high amount of reinforcing particles [23] that result due to the breakdown of bigger particles into smaller ones. These particles act nucleation sites for recrystallization. [24,25] have reported that the process of dynamic recrystallization is highly dependent on dislocation density, presence of reinforcing particles etc. It is also observed in the case of composite extruded at 8:1 that due to severe plastic deformation, the particles get aligned themselves in the direction of extrusion. The degree of particle redistribution is not same in all the extrusion ratios. In the case of composite extruded at 4:1, during extrusion, initially particle break down occurs due to plastic deformation and the particles are pushed towards the grain boundary during recrystallization. A slightly better distribution of particles throughout the matrix is observed. This leads to increased mean free path between the SiC particles when compared to the composites extruded at 8:1 and 10:1. In the case of composite extruded at extrusion ratio of 10:1, particle clustering to certain extent is noticed. The particle clustering does not occur in lower extrusion ratios. Similar observations have been reported elsewhere [26]. For instance, [23] have reported that at lower extrusion ratios for example below 3:1, no appreciable difference in the degree of clustering before and after deformation is noticed. In the present study, at higher deformation ratio (10:1), grain coarsening with relatively non-homogenous particle distribution is also noticed. This is attributed to the increased temperature during extrusion via adiabatic heat [19] resulted because of the higher amount of deformation. The effect of plastic deformation on the alignment of particles along the extrusion direction is evidenced at all the extrusion ratios. The degree of alignment being high in the case of composites extruded at an extrusion ratio 8:1. 

Figure 6: Optical Micrographs of base alloy and composites extruded at different extrusion ratios.

The SEM image of composite extruded at an extrusion ratio of 10:1 is shown in Figure 7. The precipitates are found to be aligned along the extrusion direction (indicated by arrows), causing a weak interface. In the present work, good quality extrudate were obtained with extrusion ratios 4:1 and 8:1, while the composite extruded at 10:1 showed presence voids (indicated by circles), which is probably due to de-bonding of fractured particles.

Figure 7: SEM Micrograph of composites extruded at 10:1.

The TEM images of the interfacial microstructure of the composite extruded at an extrusion ratio of 4:1 and 8:1 are shown in (Figure 8ab) respectively. The presence of dislocation near the SiC particles, which arise due to the difference in thermal expansion coefficient between the matrix and the particle, is clearly seen. Presence of dislocations aids the plastic deformation of composite. A clear particle-matrix interface is seen, which indicates that no unwanted reaction had taken place.

Figure 8: TEM images of extruded composites showing dislocations near SiC particles.


The effect of extrusion ratio on the density of as cast and extruded base alloy and composites is illustrated in (Figure 9). The density of the as cast composite is higher than that of the as cast base alloy, which is attributed to the presence of high dense SiC particles in the composite. In the case of base alloy, as the extrusion ratio is increased, there is a marginal improvement in density. From the results, it is seen that, extrusion increases the density of both base alloy and composite, which is attributed to the healing of cast defects like porosity etc. Presence of porosity, many times, is inescapable in stir cast products. The porosity is caused due to gas entrapment, inadequate wetting of SiC particles and solidification shrinkage and so on [27]. The density of the extruded base alloy and composite increases as the extrusion ratio is increased up to a certain point. This is attributed to the elimination of porosity and uniform particle distribution caused by extrusion. An increase of 2 % and 6 % in density is noticed in the case of composites extruded at 4:1 and 8:1 respectively when compared to as cast composite. The higher density in the case of composite extruded at 8:1 is due to the reduction in the size of grain as well as that of the SiC particles is due to the high compressive stresses caused during extrusion. It is reported by researchers elsewhere that plastic working causes the coarse grains to break down to finer equi-spaced grains [28].

Figure 9: Density of base alloy and composites extruded at different extrusion ratios.

It is interesting to note that for the composite extruded at an extrusion ratio of 10:1, the density is decreased compared to that of even the as cast base alloy. The decrease in density at extrusion ratio of 10:1 is due to the increased porosity content which is shown as depicted in Figure 10. This is contradicting to the study made by [29] who reported that the extent of porosity reduction increases with increase in extrusion ratio. In the present study, the porosity reduces initially as extrusion ratio increases and further it increases at a high extrusion ratio of 10:1. The reason for this may be the removal of SiC particle due to the high extrusion forces resulted in this high extrusion ratio.

Figure 10: Porosity mapping of composite extruded at 10:1.


The results of hardness survey from extreme surface (case) to the inner surface (core) of the extruded composites cut in a direction transverse to the extrusion direction are plotted in (Figure 11). It can be noted that in all the extrusion ratios the composites at the extreme surface (at 0.1 mm) shows lower hardness. The lower hardness in the extreme surface of the extruded composite is due to the shearing of the billet surface with the die wall. Further, in the near surface region (at 0.2 mm), the hardness is found to increase. A relatively higher hardness is observed in the surface beneath the extreme surface (at 0.3 mm), which is due to the presence of recrystallized grains. Towards the core, the hardness decreases and reaches almost a constant value, which is attributed to the presence of relatively coarser equiaxed grains. In all the composites, the peak hardness is observed in the core region. Hardness values of 146, 154, and 124 HV are recorded in the core region of composites extruded at 4:1, 8:1 and 10:1 respectively. The hardness of the composites extruded at an extrusion ratio of 8:1, is found to be the highest compared to the composites extruded at 4:1 and 10:1, which is attributed to the reduction in the grain size and SiC particle size, caused by the fragmentation during extrusion. Presence of large amount of dislocation as a result of difference in coefficient of thermal expansion, which is the favorite site for precipitates, has also led to the increased hardness. The above results are in good agreement with the studies made by other researchers [30-32]. The lower hardness of the composite extruded at a higher extrusion ratio of 10:1 is due to the heterogeneous particle distribution and increased porosity content as discussed in the previous section. This is in agreement with the study of [26], who studied the effect of deformation amount microstructure and mechanical properties of SiCp/AZ91 magnesium matrix composites. They reported that the larger deformation amount is unfavorable for the particle distribution.

Figure 11: Variation of hardness from case to core of the composite samples.

The optical micrographs of the case and core regions of the composite extruded at 10:1 are illustrated in (Figure12). Presence of fine cracks in the recrystallized grains at the extreme surface were observed due to die wall – billet shearing of the composite during extrusion while the core microstructure indicates crack free region with relatively coarse grains. Though producing a recrystallized grain structure and avoiding the coarse grains at the surface is beyond the scope of the present work, the hardness – distance survey and the corresponding microstructure fetch useful information in understanding the effect of extrusion ratio on the formation of defect free extrusions.

Figure 12: The a) case and b) core micrographs of composite extruded at 10:1.

Extrusion ratio is an important parameter which can influence the recrystallization behaviour which affects the mechanical properties. The dynamic recrystallization is much pronounced in the case of composite rather than the base alloy. This is due to the fact that though aluminum alloy do not undergo dynamic recrystallization, the presence of hard ceramic act as a driving force for nucleation and growth of new grains [33]. Sweet et al.[34] studied the effect of extrusion parameters on the nature of surface and sub-surface grains on the 6XXX aluminium alloys. They reported that all extrusions had a narrow band of recrystallized grains on the surface of extrusion and a fibrous grain towards the center. They also reported in their study that the thickness of the recrystallized layer decreases with decreases in the extrusion ratio from 32 to 16. Though the grain refinement increases linearly with extrusion ratio, the hardness does not improve in all the cases, because of the non-uniform particle distribution in the matrix. The presence of partially recrystallized or coarse grain crystallized structure generally depends upon alloy type homogenization treatment, die shape and extrusion ratio. This will affect the surface integrity and surface finish of the extrusion. It is reported in literature [34] that coarse grain recrystallization in aluminium extrusion is almost always at the surface and near surface of the extrusion.

Impact Energy

The effect of extrusion ratio on the impact energy of the as cast composite and composites extruded at extrusion ratios 4:1, 8:1 and 10:1 is shown in (Figure 13). It is understood that extrusion improves the impact behavior of the composites compared to the unextruded counterpart. With increase in extrusion ratio the impact energy value increases up to 8:1 (a maximum increase of around 50 % is noticed), which is due to the micro structural homogeneity and removal of defects. However, when the extrusion ratio is further increased to 10:1, the impact value is found to decrease. The SEM fractographs of impact tested composite samples are shown in (Figure 14).

Figure13: Effect of extrusion ratio on the impact property of the extruded composites.

Figure14: SEM Fractographs of impact tested composite samples a) as cast b) extruded at 4:1 c) extruded at 8:1 d) extruded at 10:1.

The fracture surface of impact tested as cast composite shows the presence of cracks as indicated by arrows. This contribute to the lower impact value of the as cast composites. The fracture surfaces of the composites extruded at 4:1 and 8:1 show presence several equiaxed dimples indicating good interfacial bonding between particle and the matrix. The fractography of extruded composites confirms the presence of both brittle and plastic deformation zone in the case of extruded composites. The fractograph of composite extruded at 10:1 [Figure 14d] reveals the particle pull out from matrix subsequent to the decohesion of particle from the matrix, as shown by circles leading to the lower impact energy of the composite. During extrusion, the SiC agglomerations have sheared and oriented towards the extrusion direction, while in the case of composite undergoing high deformation rate, the particles has pulled out creating voids. Similar observation is reported by [35] in the tensile testing of extruded SiC-Al6061 composite. They stated that large stress concentration has led to fracture and debonding of SiC particles from the matrix. Previous researchers [36, 37] have found that hot extrusion improves the particle distribution, which further enhances the mechanical properties of the magnesium metal matrix composites. However, an increase in extrusion ratio over an optimum value, may deteriorate the local interfacial cohesion between the particle and the matrix, and may degrade the properties [38]. In the present work, increasing the extrusion ratio above 8:1 does not result in improving the mechanical properties. Hence, further studies such as tensile, compressive and flexural strengths were evaluated for base alloy and composite samples extruded at 4:1 and 8:1 and compared with that of the as cast ones. 

Tensile Strength and Elongation

The yield strength (YS) and the tensile strength (TS) of the as- cast and extruded composite areshown in (Figure 15). From the figure, it is understood that extrusion improves both the yield strength and tensile strength of the composite. The YS of composite extruded at extrusion ratios 4:1 and 8:1 increases by 43% and 45% respectively compared to that of as-cast composite. The TS increases by 27% and 18% when compared to the as cast composite, for the extrusion ratios 4:1 and 8:1 respectively. The improvement in strength as compared to the unextruded composite is attributed to the microstructural homogeneity, grain refinement and reduction in porosity.

Figure 15: Effect of extrusion on the tensile properties of composite.

In the present work, the defects due to plastic deformation were not much pronounced in the composites extruded at extrusion ratios 4:1 and 8:1 [39-41] have reported that at lower extrusion ratios (< 3:1), the chances of composite degradation during plastic working in terms of particle fracture, debonding, void growth and so on reduces. Also, as reported by [42,24,43] the degree of composite damage is very high at higher extrusion ratio of above 10:1. The micron sized Silicon Carbide (SiC) particles get themselves engulfed into the relatively softer matrix thus leading to the increased strength of the composites. These particles act as second phase in the matrix and resist the movement of dislocation. When the composite is subjected to plastic deformation during hot working, the particles hinder the plastic flow of the matrix material and also lock the grain boundaries, which lead to the increased strength and ductility of the composite up to a maximum level. The increased strength of composites extruded at 4: 1, in the present study, is due to the relatively uniform distribution of the particles throughout the matrix when compared to that extruded at 8:1, wherein the particles more or less align towards the extrusion direction [20] has mentioned that the more uniform distribution of eutectic silicon particles in the matrix to be the cause for the improved tensile strength of the extruded composites compared to the as cast composites. The ductility in terms of percentage elongation of the composites is shown in Figure 16. In the extruded condition, the ductility of the composite is higher by 7% for 4:1 and 30% for 8:1 than the as- cast composite. The improvement of ductility of the extruded composite is due to the elimination of porosity, microstructural homogeneity in terms of particle distribution and grain refinement. The ductility of composite extruded at 8:1 is higher than that extruded at 4:1, which may be due to the smaller particles resulted due to fragmentation of particles due to the high shear stress during extrusion. The reduction of porosity to a greater extent and grain refinement are also considered to be the reasons for the increased tensile ductility of the composite extruded at an extrusion ratio of 8: 1.

Figure 16: Effect of extrusion on the ductility of the composite.

Compressive Strength

The compressive strength and the force vs. stroke of the as cast and extruded (4:1 and 8:1) base alloy & composites are shown in (Figures 17 & 18) respectively. It can be seen that extrusion increases the compressive strength of both the base alloy and the composites. However, the compressive strength decreases with increase in extrusion ratio both in the case of base alloy as well as the composite. This is in agreement and contradictory to the researches elsewhere [10, 44]. Have reported that the compressive strength is found to increase with increasing extrusion ratio due to strain hardening effect. It is studied by researchers elsewhere that the increased particle size has contributed much in enhancing the compressive strength of the composites [45]. In their study on the compressive characteristics of Al6061 reinforced with different size of B4C particles, have reported that the compressive strength of the composite with bigger particles (58 µm) was higher compared to that with smaller sized particles. In the present study, the reduction of compressive strength of composites at higher extrusion ratios i.e. at an extrusion ratio of 8:1 is probably due to the reduction in particle size caused by fragmentation during extrusion. This may probably be the reason for the lower compressive strength of composites extruded at 8:1 than the composites extruded at an extrusion ratio of 4:1 [46]. Have reported that when composites subjected to large plastic strains, particle de-bonding from the particle-matrix interface occurs.

Figure 17: Compressive strength of extruded base alloy and composite.

Figure18: Force vs. Stroke in compressive testing of extruded base alloy and composites.

Though the quality of composites extruded at 8:1 is found to be better than that of composites extruded at 4:1, in terms of particle distribution, grain size reduction, porosity reduction etc the compressive strength decrease is reported in this study due to the particle size reduction. In general, the defects in the extruded composite such as porosity have no disadvantageous effect on the compressive strength of the composites. In the present study, the composite extruded at and extrusion ratio of 4:1 shows better compressive strength compared to that extruded at 8:1.

Flexural Strength

The flexural strength of the as cast and extruded base alloy and composites is plotted in Fig. 19. It is observed that the flexural strength of composite is lower compared to the base alloy both in the as cast condition and extruded condition. The value of decrease is high (around 21 %) in the as cast condition compared to the extruded (about 1 % for 4:1 and 10% for 8:1) condition. This decrease in flexural strength in the case of composite compared to the base alloy is attributed due to the presence of hard SiC ceramic particles. However, it is found that extrusion improves the flexural properties of both the as cast base alloy and composite. There is an increase of flexural strength by 65% and 34% in the case of composites extruded at 4:1 and 8:1 respectively when compared to as cast composite, which is due to the removal of defects and grain size reduction. A contradictory observation was reported by [47] in their study on the forged Saffil fibre reinforced Aluminium composites. They found that forging at room temperature reduced the flexural modulus and forging at high temperature tended towards that of as cast composite but not enhanced the flexural modulus. They reported that the reason for the less or nil improvement in the flexural property after room temperature forging is the deleterious porosity. Whereas in the present investigation, extrusion has enhanced the flexural strength of the as cast composite, which is due to the improved soundness of the casting.

Figure 19: Effect of extrusion ratio on the flexural strength of as cast and extruded base alloy and composites.



Al-10 wt% SiCp composites prepared by stir casting technique, further extruded at various extrusion ratios of 4:1, 8:1, 10:1 were studied for its microstructural homogenity, density and mechanical properties.

  • The microstructure of the extruded composites was free from particle clustering, and there is a markable decrease in particle size and grain size when compared to the as cast composite as a result of
  • The density increases with increase in extrusion ratio while at higher ratio of 10:1, the composite showed lower density than of as cast This is due to the higher porosity and non-uniform microstructure of the composite extruded at 10:1.
  • The hardness and impact energy value increases with extrusion ratio, while at higher extrusion ratio of 10:1 it decreases because of decohesion and particle pull out from the
  • The grain refinement, breakdown of particle clusters, particle size reduction due to fragmentation, preferential alignment of particle along the extrusion direction, has of the composites extruded at 8:1 has resulted in porosity 

    reduction, improvement of hardness, tensile ductility and impact properties.

  • Grain size reduction with uniform distribution of particle, with less or no particle fragmentation of the composite extruded at 4:1 has resulted in better flexural and compressive strength.


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