Correlation of structural and mechanical properties for Al-Al 2 O 3 -SiC hybrid metal matrix composites

Abstract

The aim of the present paper is to examine the outcome of Al₂O₃-SiC reinforcements on structural and mechanical behavior of Al matrix based hybrid composites. Al-Al₂O₃-SiC hybrid composite has been developed through stir casting with addition of ceramics i.e. Al₂O₃-SiC (2.5 wt.%, 5.0 wt.%, 7.5 wt.% and 10.0 wt.%) in relative and symmetrical proportion. The structural characteristics, i.e. phase, microstructure, EDS; physical property i.e. density and the mechanical properties, i.e. hardness, impact strength and tensile strength of fabricated specimens have been investigated. XRD represents the transitional phase formation among Al base material and Al₂O₃-SiC ceramic phases with inter-atomic bonding between them. SEM reveals that the Al₂O₃-SiC fragments has distributed symmetrically in Al matrix. EDS spectrum of various samples are in confirmation with the XRD results. Density of hybrid composite reduces with increase in weight percentage of ceramic reinforcements i.e. Al₂O₃-SiC because ceramic particle gains low density after preheating. Hardness of hybrid composites increases upto 5 wt.% variation of ceramic reinforcements i.e. Al₂O₃-SiC after that it decreases. Impact strength of hybrid composite has been increased with an increase in weight percentage of ceramic. Al-2.5 wt.% Al₂O₃-2.5 wt.% SiC shows maximum ultimate tensile strength. It is expected that the prepared hybrid composites will be useful for fastener studs.

Keywords

Hybrid metal matrix composites stir casting XRD SEM EDS,mechanical behavior

Introduction

From the last few decades, the role of Al metal matrix composites has increased in the field of automobile industry, material manufacturing industry and aerospace industry. With the recent advancement taking place, composites are an option for the replacement of conventional materials. Kala et al. reviewed the different properties and application of aluminium based composites with different reinforcements. It is examined that the tensile and yield strength of the composite has improved by using fly ash and coconut ash as reinforcements. Graphite has self-lubricating properties which upgrades machinability of the composites.¹ Krishna et al. studied the various characteristics of Al6061-silicon carbide composite and Al6061-Silicon carbide/Graphite hybrid composite developed using stir casting. Density of composite decreases and tensile strength increases with wear resistance. Overall mechanical property improves with an increase in ceramic particles.² Kulkarni et al. found the density, tensile strength and impact strength of Al356-fly ash composites. It has been investigated that density decreased whereas compressive strength improves with the variation of fly ash.³ Raviraj et al. again examined the microstructure, hardness and tensile strength of Al-TiC composites. Hardness and tensile strength of prepared composites upgraded upto 5 wt. % of TiC reinforcement after that hardness decreased.⁴ Kumar et al. examined the structural and tribological characteristics of Al-B₄C-MoS₂ hybrid composites. The Al2219-3% B₄C-5%MoS₂ hybrid metal matrix composites showed lowest tensile strength with respect to pure Al2219. Density and Microhardness of the composites is found to be higher.⁵ Dwivedi et al. found the hardness, tensile strength and fatigue failure of Al356-SiC composites. It is observed that hardness, tensile strength, toughness and fatigue life were improved with variation of SiC ceramic particles.⁶ Padmavathi et al. examined the various properties of wear test of Al-SiC-MWCNT hybrid composites. The hardness and specific wear resistance of Al-SiC-MWCNT is improved with variation of ceramic reinforcements.⁷ Viswanath et al. examined the creep, tensile and compressive strength of AZ91-SiC metal matrix composite. AZ91-SiC composite has good tensile strength and compressive strength respectively with the addition of ceramic particles apart from that creep resistance of the composite was also found to improve.⁸ Ghasali et al. has observed the XRD, density and hardness of Al-B₄C composite which was prepared by microwave sintering. It is found that maximum bending and compressive strength are measured at 750°C and 950°C respectively. Al-20 wt.% B₄C has a maximum Vickers hardness at 850°C.⁹ Pradhan et al. has studied that the hardness and tensile strength of Al-Al₁₃Fe composite is improved without affecting the ductility of metallic matrix.¹⁰ Dinaharan et al. examined the various properties of Cu-(SiC, Al₂O₃, TiC and B₄C) metal matrix composite developed by friction stir process. Copper-B₄C has higher hardness and wear resistance as compared to other copper matrix composite.¹¹ Mohapatra et al. examined the density, hardness and young’s modulus of Al-TiC composite developed by hot consolidation technique. Al-TiC metal matrix composite has maximum hardness, compressive strength and young’s modulus. TiC reinforcement distributed uniformly throughout the Al matrix.¹² Singh et al. reviewed the characteristics of hybrid metal matrix composites for advanced applications. Hybrid metal matrix composite has lower density as compared to Al matrix with reinforcement (fly ash, rice husk, ash, mica etc.) apart from this porosity level was found within the limit.¹³ Narayan et al. examined the mechanical characteristics of aluminium composite developed by hot forging of aluminium composite in powder form. It is found that the hardness and impact strength values were higher for quenching with respect to the heat treatment carried out by annealing process.¹⁴ Alizadeh et al. observed the characteristic i.e. hardness, tensile strength and fracture analysis of Al5083-B₄C metal matrix composites prepared by hot extrusion multi scale processing. It is observed that due to mechanical milling process and B₄C reinforcement there is an improvement in the yield strength from 130 MPa to 556 MPa but at the same time elongation of the composites was decreased.¹⁵ Bandil et al. examined the density, hardness and corrosion of Al-Si-SiC hybrid composites. In prepared composites, the 20% reinforced composite has maximum hardness whereas 20% reinforcement composite has minimum corrosion rate. The wear resistance increased with an increase in reinforcement.¹⁶ Jamwal et al. in another study examined the mechanical and tribological properties of Cu-SiC-graphite hybrid composites. It is found that the density and hardness decreased due to the variation of reinforcements. The wear rate is lowest and the corrosion resistance is highest at 5% addition of reinforcement for hybrid composites.¹⁷ Garg et al. examined the progress in the field of aluminium composites and also discussed fabrication processes as well as commercialization of aluminium composite. In recent year, the consumption of aluminium composites has enhanced by leaps and bounds.¹⁸ Jamwal et al. in another study found the hardness and tensile properties of Al-Al₂O₃-TiC composite. The hardness, tensile strength and wear resistance of the Al-Al₂O₃-TiC composite was found to increase with an increase in the concentration of ceramic reinforcements.¹⁹ Kumar et al. studied the hardness and wear properties of Al-SiC-TiC hybrid composite. In Al-SiC-TiC, the hardness and wear resistance of hybrid composite Al-SiC-TiC increased with addition of reinforcements.²⁰ Sohag et al. examined the density, hardness and wear properties of Al-Cu-SiC/TiC composite. The hybrid composite Al-Cu-SiC/TiC has low density, highest hardness and low wear rate.²¹ Hossain et al. studied the various properties of epoxy based composite and aluminium based hybrid composite. It is found that epoxy based composite have low tensile strength as well as high flexural strength and aluminium based composite have low density as well as low wear rate.^22,23 Nayim et al. examined the machining process with the help of EDM followed by optimization technique and reported that EDM is an important technique which can machine hard materials as well as composites. He also discussed about the density, hardness and wear rate of Al-CNT-TiC hybrid composites. It is observed that Al-CNT-TiC hybrid composites have highest hardness and low wear rate.^24,25 Jamwal et al. again examined the various properties of Cu-Al₂O₃/SiC/TiC/ZrO₂ composites and also discussed its various applications.²⁶ Ahamad et al. studied the structural, density and hardness properties of Al-Al₂O₃-TiO₂/Al-Al₂O₃-C hybrid composites. Al-Al₂O₃-TiO₂/Al-Al₂O₃-C hybrid composites show low density, high tensile strength and highest impact strength.^27,28 Ahamad et al. also examined the vicker’s hardness and wear of Al-Al₂O₃-C composite. Vickers hardness and wear resistance of fabricated specimen increased with variation of ceramics in Al matrix. Wear rate of Al-Al₂O₃-C composite increased with addition of applied load.²⁹

In the present study, Al-Al₂O₃-SiC hybrid composites were prepared by the stir casting technique with the addition of ceramic reinforcements in equal proportion. The aim of this research is to examine the phase, microstructure, and mechanical behavior of Al-Al₂O₃-SiC hybrid composites. It is expected that prepared hybrid composite will be useful for fasteners studs.

Experimental work

Sample preparation

Aluminium (Al-6351) is used as a base material in this study, which is having composition as 96.93% Al, 0.17% Si, 0.44% Mg, 0.54% Mn and 1.93% O. In this study, Al₂O₃ and SiC are used as reinforcements. Al₂O₃ and SiC powders are having mesh size of 70-230 and 400 respectively. Percentage purity of both Al₂O₃ and SiC powders is 99%. Nomenclatures of the specimen are shown in Table 1 which is used for the present investigation. The stir casting technique is used for the fabrication of Al-Al₂O₃-SiC hybrid composite. The following are the components, i.e. stirrer rod, crucible, electrical furnace, setup controller, and die which are used in the fabrication of samples. The material of the stirrer rod is SiC, material of the crucible is graphite and the cylindrical die is made up of high chrome high carbon steel. Initially pre heating of the crucible is done for 25 min at 505°C for withdrawing the volatile content from it. After that preheating of the Al₂O₃ & SiC ceramic reinforcements is done in an electrical furnace for 40 minutes at 535°C to withdraw the water content and minimize the activity of oxidation. Power hacksaw is used for cutting the Al matrix in smaller parts and electronic weighing machine is used for weighing according to the requirement. After the weighing of the Al ingots it is placed in a crucible and then it is placed in an electrical furnace. Temperature of the furnace is made to rise very slowly up to 925°C and then it is held there for 35 mins. Al matrix was thus converted into pure liquid form after 35 min.

Table 1.

Nomenclature of pure Al and AAS (Al-6351-Al₂O₃-SiC) composite samples.

S. No.	Al-6351 (Weight %)	Al₂O₃ (Weight %)	SiC (Weight %)	Name of sample (Sample code)
1	100	0.0	0.0	AAS1
2	95	2.5	2.5	AAS2
3	90	5.0	5.0	AAS3
4	85	7.5	7.5	AAS4
5	80	10.0	10.0	AAS5

Once entire Al metal converts into liquid pool the stirring was done for 5 min at 190 rpm. Along with it preheated Al₂O₃-SiC reinforcement was mixed in molten Al and mixing is continued at 210 rpm for 35 minutes to form semi liquid slurry. The furnace temperature was further increased upto 945°C for 12 minutes and the process of stirring is continued. Thereafter, preheated cylindrical die is used for pouring of molten metal. After 15 min, casted cylindrical rod was solidified and removed from the die. The lathe machine is used for removing surface defects and prepared sample was then subjected to different types of testing. Table 1 shows total five categories of prepared samples out of which four samples belongs to the category of hybrid composites and one is of the pure Al-6351.

Characterizations

ZEISS X-ray diffractometer is used for Phase analysis at 2Ɵ (20°–80° range) angle for finding the compounds present in fabricated specimens using Ni-filter and Cu-Kα radiation. The morphological studies of the specimen at 500X magnification are carried out using Scanning Electron Microscope (SEM-ZEISS). The polishing process is used for the preparation of circular samples (diameter = 15 mm, thickness = 3 mm) which is used for various test i.e. XRD, SEM & EDS. First of all, initially the emery paper (grit size P300, P500 & P1000) has been used for polishing upto 30 minutes. The emery paper with fine grit number P1500 is then used for polishing upto 10 minutes. Now next step is carried out on polishing machine which has emery paper (fine grit number P2000) and this process was continued for 12 minutes. After that the polishing process was carried out with alumina gel for 25 min. For mirror finishing, diamond paste with hifin fluid is used upto 30 minutes. After all these polishing processes, samples are now ready for XRD, SEM and EDS studies. For the density measurement of the sample, dimensions and mass is measured thereafter density is calculated with ratio of mass to the volume of the specimen. Hardness of the sample is measured by the Rockwell Hardness Tester. Rockwell hardness tester has specifications i.e. indenter = steel ball of 1/16 inch, minor load= 10 kg, major load = 90 kg, and hardness scale is HRB.

The tensile test specimen with respective dimensions is represented in Figure 1 as per ASTM standards. Tensile strength and deflection of the sample is determined with the help of Electronic Tensometer (KUDALE INSP LTD.) - PC2000).

Electronic Tensometer has following specifications i.e. Test speed is 3 mm/min and applied load ranges between 1 N to 20050 N. As per ASTM standards, Figure 2 represents the dimension of the charpy test sample. Impact strength of the specimen has been carried out with Impact Testing Machine (PSI LTD.).

Results

X-ray diffraction (XRD)

X-Ray diffraction of prepared samples (as per Table 1), is plotted between 2Ɵ angle and intensity as shown in Figure 3. It is observed that pure specimen shows maximum percentage of Al similarly the specimen having minimum percentage of ceramics is represented by lowest peak of compounds i.e. Si, Mg and SiC which is represented in Figure 3. In Figure 3, Al, Mg, Si and SiC has respective miller indices which is marked on their respective peaks. In sample AAS1, it is observed that lowest peak is of Si and Mg whereas highest peak is represented by Al because percentage of Al is higher with respect to Si and Mg which is shown in Figure 3. In sample AAS2, at 65° angle (2-theta) one extra peak is observed which is not present in AAS1.

After matching this peak with JCPDS file, it is observed that this extra peak corresponds to silicon carbide. Silicon carbide has lower peak in AAS2 due to the low percentage of silicon carbide reinforcement. In sample AAS3, silicon carbide has a highest peak as compared to specimen AAS2 because specimen AAS3 has a large amount of reinforcement with respect to specimen AAS2. Specimen AAS4 has a large amount of ceramic reinforcements i.e. Al₂O₃ and SiC with respect to sample AAS2 & AAS3. It is found that specimen AAS4 has more intensive peak as compared to sample AAS2 & AAS3. Specimen AAS5 has a 20% reinforcement which is maximum as compared to other samples and it is observed that more intensive peaks are developed in this specimen. However, AAS5 has a high peak intensity of silicon carbide but the numbers of phases are same as is shown in samples AAS2, AAS3, AAS4. After investigation of XRD, it is found that Al and SiC has no reaction among each other and no reactive phase patterns are developed in between Al and SiC. All the compounds show varying patterns in the various specimens. Also, Al has a high intensity peak and silicon carbide has a less intense peak in the hybrid composite.

Scanning electron microscopy (SEM)

The morphological behavior of the fabricated composites is examined by studying the microstructure. The microstructure of all the samples, i.e. AAS2, AAS3, AAS4 and AAS5 are shown in Figure 4 at 500X magnification. It is investigated that Al₂O₃ and SiC is distributed evenly in Al matrix. It is also observed that no. of reinforcement particles increased on the face of the Al matrix due to variation of ceramic reinforcements. Figure 4(a) represents the microstructure of specimen AAS2 which has 5 wt.% reinforcement and it is evenly distributed in Al matrix. It is examined that the black spots are SiC and white spots are Al₂O₃. In Figure 4, the ceramic particles are marked with an arrow on micrograph. The microstructure of sample AAS3 is represented in Figure 4(b). Figure 4(b) shows that intensity of ceramic reinforcement particles in micrograph of AAS3 are more with respect to micrograph of AAS2 because sample AAS3 has large amount of ceramic reinforcements i.e. 10 wt.% with respect to sample AAS2. Figure 4(c) represents the micrograph of specimen AAS4. In Figure 4(c) reinforcement percentage is 15% and, Al₂O₃ as well as SiC particles are shown by marking on micrograph. It is examined that Figure 4(c) has agglomeration between particles.

This agglomeration is also found in specimen AlAlOS05 because this specimen has highest concentration of reinforcements i.e. Al₂O₃ and SiC as shown by arrows in Figure 4(d). The proper mixing of reinforcement with the stir casting in base material is a complicated task. It is found that the lower percentage of ceramic particle is scattered evenly and the higher percentage of ceramic particles are scattered unevenly in Al base material. The reason behind this is large density variation between Al matrix and ceramic reinforcements.

In stir casting technique, it is examined that there is a collection of (Al₂O₃-SiC) ceramic particles during the stirring action. The Al₂O₃ and SiC reinforcement particles are insoluble in Al, as a result leads to uneven distribution of large quantity of ceramic particles in the matrix.

Energy dispersive spectroscopy (EDS)

SEM micrograph at 10,000X and Energy Dispersive Spectroscopy (EDS) spectrum of sample AAS1 are represented in Figure 5(a) and (b) respectively.

Specimen AAS1 has different weight percentage of elements as shown in Table 2. It is observed that AAS1 has Al, O, Mg, Mn and Si elements as seen in Figure 5(b). The weight percentage of Mo is zero in Al matrix as shown in Table 2. It is observed that the quantity of oxygen in sample AAS1 is very low after casting because the setup is well protected from the environment.

Table 2.

Elements weight percentage in AAS1.

Element	Al	O	Mg	Mn	Si	Mo
Wt. %	96.93	1.93	0.44	0.54	0.17	0.00

SEM micrograph at 500X and Energy Dispersive Spectroscopy (EDS) spectrum of sample AAS2 is shown in Figure 6(a) and (b) respectively. The sample AAS2 has different elements and Table 3 represents the weight percentages of respective elements in AAS2.

Table 3.

Elements weight percentage in AAS2.

Element	Al	O	Si	C
Wt. %	92.97	4.02	0.70	2.31

It is observed that Figure 6(b) have various peaks i.e. Al, O, Si and C after that this result is compared with Figure 5(b). It is found that the sample AAS2 has an extra peak of carbon with respect to sample AAS1 due to the addition of silicon carbide. Specimen AAS2 has higher peak of Al and lower peak of carbon and silicon. In both Figures 5(b) and 6(b) oxygen peak is very low with very less intensity in EDS analysis due to perfect shielding with the surrounding during casting. Figure 6(a) and (b) shows that the reinforcement is distributed uniformly and no internal reaction is developed between Al matrix and reinforcements. The specimen AAS2 has Al-2.5% Al₂O₃-2.5% SiC reinforcements which is also shown in Figure 6(b) and Table 3.

SEM micrograph at 500X and EDS spectrum of sample AAS3 is presented in Figure 7(a) and (b) respectively. The sample AAS3 has various elements which are shown in Table 4 with weight percentage and respective peak height of all elements i.e. aluminium, oxygen, silicon and carbon are shown in Figure 7(b). It is observed that sample AAS3 has a highest peak of carbon with respect to sample AAS2 because the percentage of reinforcements is higher as compared to specimen AAS3.

Table 4.

Elements weight percentage in AAS3.

Element	Al	O	Si	C
Wt. %	91.65	3.78	0.70	3.87

It is found that ceramic reinforcements (Al₂O₃, SiC) and base material Al has no transitional phase generation between each other and Al has maximum intensity of peak with reference to carbon and silicon peak in specimen AAS3 as shown in Figure 7(b).

SEM micrograph at 500X and EDS spectrum of sample AAS4 is displayed in Figure 8(a) and (b) respectively. The weight percentage of elements composition in AAS4 is shown in Table 5. The specimen AAS4 has higher peak of Al and lower peak of silicon & carbon respectively. It is observed that the intensity of peak is high in specimen AAS4 as compared to specimen AAS3 & AAS2 because specimen AAS4 has 15% reinforcement.

Table 5.

Elements weight percentage in AAS4.

Element	Al	O	Si	C
Wt. %	89.94	3.62	0.87	5.57

It is found that the maximum weight percentage of elements is Al and carbon compared to other composition as shown in Table 5.

SEM micrograph at 500X and EDS spectrum of sample AAS5 is shown in Figure 9(a) and (b) respectively. The weight percentage of elements composition in specimen AAS5 is shown in Table 6. The specimen AAS5 has a 20 weight percentage of reinforcement which has the highest weight percentage with respect to other samples. The elements i.e., Al and carbon has highest weight percentage as displayed in Table 6. It is observed that Al and carbon have high intensity peak as represented in Figure 9(b). It is found that the ceramic reinforcement (Al₂O₃-SiC) has no transitional phase formation as displayed in Figure 9(b) between each other. Figure 9(b) shows that all the element of composition has separate peak and no reaction is developed between the elements.

Table 6.

Elements weight percentage in AAS5.

Element	Al	O	Si	C
Wt. %	88.54	2.54	0.87	8.05

Density and hardness

Density and Rockwell hardness of the fabricated specimens are shown in Figure 10. In Figure 10, it is found that AAS1 has higher density with respect to other casted hybrid composite specimens. Hybrid composite specimens have lower density due to variation of reinforcements percentage i.e. Al₂O₃-SiC. It is investigated that the density of hybrid composite specimen is reduced with the addition of weight percentage of Al₂O₃-SiC reinforcements. It is found that the specimen AAS1 has highest density whereas the specimen AAS5 has lowest density as represented by bar chart in Figure 10. The density of the composite is reduced due to shape, size and the fraction of ceramic reinforcement as well as the nature of the Al matrix material.

It is found that sample AAS5 has a minimum density, which has maximum reinforcement i.e. 10 wt. % of Al₂O₃ and 10 wt. % SiC in Al matrix. The reinforcement has higher density at the time of production due to its moisture content. The reinforcement is heated in a furnace for removing the moisture content, followed by that it is used for fabrication since preheating the reinforcement leads to lower density as compared to the as received reinforcement. The hardness of composite specimen is measured with Rockwell hardness testing machine. For Al composite, the total applied load is 100 kg where 10 kg is minor load and 90 kg is a major load on the Rockwell hardness tester. As per standards for Al composite, the indenter is steel ball and hardness is measured on the B scale. Figure 10 shows the hardness of various composite samples. The hardness of the composite samples changes with an increase in weight percentage of ceramic reinforcements i.e. Al₂O₃-SiC. It is found that the hardness of the composite improved upto 5 wt.% of ceramic reinforcements thereafter hardness decreased with an increase in reinforcements in Al matrix. After 15 wt.% reinforcements, hardness of the composite is increased. There is a decrease in hardness because SiC has low density and is hard in nature. It is investigated that the maximum hardness is observed in specimen AAS2 as shown in Figure 10. It is found that sample AAS4 has minimum hardness value of 50.5 HRB and sample AAS2 has 70.8 HRB maximum hardness value.

Impact Strength

Charpy test is used for the investigation of toughness due to sudden applied load on composite sample and toughness is measured in the form of strain energy which is known as impact strength of the samples. The impact strength of various samples is showed in Figure 11. From Figure 11; it is found that the impact strength is affected with variation of ceramic reinforcements i.e. Al₂O₃-SiC. The sample AAS1 has lowest impact strength i.e. 29 joule and sample AAS5 has highest impact strength i.e. 37.5 Joule.

The surface of the sample becomes soft due to the increased weight percentage of reinforcements, within this region specimen resists maximum force before fracture. Hence, the impact strength of the hybrid composite specimens improved with variation in weight of ceramic reinforcement in hybrid composites.

Tensile strength

The tensile test graph is plotted between load v/s deflection for the specimen AAS2 which is represented in Figure 12. Table 7 shows the peak and break load, peak and break displacement as well as tensile strength for AAS2. It is measured that sample AAS2 has peak load of 200.6 N & peak displacement of 0.89 mm respectively as shown in Table 7. The peak load of represented the maximum strength of the sample. The graph between load and displacement vary linearly up to peak point after that applied load decreases whereas displacement increases up to break point as shown in Figure 12.

Table 7.

Tensile test values for AAS2.

Peak load in(N)	Peak disp. in (mm)	Peakdisp. (%)	Break load in (N)	Break disp. in (mm)	Breakdisp. (%)	Engg. UTS (MPa)	True UTS (MPa)
2000.6	0.89	2.99	402.1	1.43	4.79	70.7	72.8

It is also measured that peak load of the sample is maximum as compared to break load as shown in Figure 12. Engineering ultimate tensile strength & true ultimate tensile strength of AAS2 are 70.7 MPa and 72.8 MPa respectively as presented in Table 7.

Figure 13 represents load and displacement graph for sample AAS3 which shows that the peak load and peak displacement is 1529.89 N and 1.04 mm respectively. It is examined that sample AAS3 has low peak load and tensile strength as compared to sample AAS2.

Figure 13 represented that the applied load is directly proportional to deflection for AAS3 upto a load of 1529.89 N hence load decreased whereas deflection increased upto the fracture point. Table 8 represents the numerical value of peak & break load as well as displacement & UTS for sample AAS3. Engineering UTS is 54.08 MPa whereas true UTS are 55.7 MPa which is lower as compared to AAS2 composite specimen, as represented in Table 8.

Table 8.

Tensile test values for AAS3.

Peak load in (N)	Peak disp. in (mm)	PeakDisp. (%)	Break load in (N)	Break disp. in (mm)	Breakdisp. (%)	Engg. UTS (MPa)	True UTS (MPa)
1529.89	1.04	1.32	313.82	1.23	2.12	54.08	55.7

Figure 14 represents a graph between applied load and displacement for specimen AAS4. Peak displacement and Peak load of specimen AAS4 is 2.72 mm and 1255.8 N respectively which is less than specimen AAS2 and AAS3. Initially this specimen follows hook’s law when the load for the sample is upto 1225.8 N which indicates the proportional point. On further variation in load beyond 1225.8 N, it is observed that deformation was found to increase in the sample, which further leads to vary in displacement upto the fracture point.

The sample AAS4 has following data such as peak and break load, displacement and ultimate tensile strength as represented in Table 9. Engineering UTS and true UTS are 43.33 N/mm² and 44.05 N/mm² respectively.

Table 9.

Tensile test values for AAS4.

Peak load in (N)	Peak disp. in (mm)	Peakdisp. (%)	Break load in (N)	Break disp. in (mm)	Breakdisp. (%)	Engg. UTS (MPa)	True UTS (MPa)
1225.8	0.54	2.72	205.95	0.87	3.43	43.33	44.05

The tensile graph is plotted between load and displacement for the specimen AAS5 as shown in Figure 15. The peak load & displacement as well as break load & displacement respectively and tensile strength of AAS5 is presented in Table 10.

Table 10.

Tensile test values for AAS5.

Peak load in (N)	Peak disp. in (mm)	PeakDisp. (%)	Break load in (N)	Break disp. in (mm)	Breakdisp. (%)	Engg. UTS (MPa)	True UTS (MPa)
1049.3	0.87	3.04	215.8	1.69	5.87	37.1	38.2

It is found that specimen AAS5 has peak loads of 1049.3 N, peak displacement is 0.87 mm; break load is 215.8 N and 1.69 mm is break displacement respectively as shown in Table 10 and Figure 15. The load and displacement increases linearly up to peak point thereafter load decreases and displacement increases. It is found that specimen AAS5 has 37.1 N/mm² & 38.2 N/mm² as engineering and true ultimate tensile strength respectively.

Figure 16 represents the ultimate tensile strength of all prepared samples i.e. AAS1, AAS2, AAS3, AAS4 and AAS5.

It is found that sample AAS2 has highest peak load & maximum ultimate tensile strength. The specimen AAS2 has a lower ceramic reinforcement with respect to the other fabricated samples as shown in Figure 16. It is observed that a lower percentage reinforced composite samples has more tensile strength with reference to a higher percentage reinforced specimens. It is examined that the tensile strength of the specimen decreased with variation of weight percentage of ceramic reinforcement in hybrid composite because reinforcement decreased the ductile properties of the composite.

Discussion

From the above results, X-ray diffraction studies shows that base material i.e. Al and ceramic reinforcements i.e. Al₂O₃-SiC has no transitional phase pattern between each other. It is also found that aluminium oxide and silicon carbide has no reaction between each other because both the materials have different phase patterns. Al has a high peak intensity as compared to silicon carbide and peak intensity of silicon carbide increased with an increase in percentage of reinforcements.

In SEM micrographs, it is investigated that Al₂O₃-SiC ceramic particles are scattered evenly in Al base material upto 10 wt.% reinforcement. It is observed that above 10 wt. % Al₂O₃-SiC reinforcements in Al matrix, agglomeration develops between reinforced particles. The agglomeration develops between the reinforcements after a certain concentration. So complete mixing and distribution of Al₂O₃-SiC reinforcements is a challenging task in stir casting. The ceramic reinforcement is scattered evenly in Al matrix at lower concentration, but the distribution is non-uniform in Al matrix at higher concentration of reinforcements. The ceramic reinforced particles are collected during stirring process because both Al and ceramic materials has more density variation. The reinforcements i.e. Al₂O₃-SiC particles are incapable of being dissolved in the aluminium matrix which leads to an uneven distribution of ceramic reinforcement particles in base material. In EDS analysis, it is observed that oxygen peak is shown in the EDS spectrum because at the time of pouring of molten metal, the molten metal comes in contact with surrounding oxygen. Oxygen content can be minimized by using a furnace which has protection with environment during pouring and casting process.

The density of Al₂O₃-SiC reinforcement is reduced by eliminating moisture content due to preheating before being used in stir casting. Preheated SiC reinforcement has coarse nature along with porous structure, so the density of reinforcement is lower than Al matrix due to elimination of moisture content. Al matrix has soft metallic particles and they easily deform due to ductile properties and hence act as viscous plastic. Developed composite material has a low density with reference to Al matrix and it varies according to the percentage of reinforcements. It is also observed that there are some factors on which the density depends, i.e. the fraction of the ceramic particles, size & shape of the ceramic particles as well as nature of the base materials.

It is investigated that the hardness of the prepared sample depends on the variation of ceramics. It is examined that hybrid composite has highest hardness with reference to Al matrix as per the above results. Al matrix has plasticity properties due to which its density becomes low. The reinforcement has hard particles in nature due to existence of covalent and ionic bonding in reinforcements. Due to ionic bond, the reinforcement becomes hard and maximum energy is required to penetrate into the matrix. So, silicon carbide reinforcement has higher hardness with respect to Al matrix.

From the above study, it is examined that hybrid composite has higher impact strength with reference to Al base material. The impact strength of the prepared sample is increased with the variation of silicon carbide reinforcement. It is found that impact strength depends upon size, shape, nature and uniform spreading of reinforcement in hybrid composites. The SiC reinforcement ceases of dislocation movement in Al matrix, so the local stress concentration has decreased.

From the above study, it is investigated that specimen AAS2 has highest tensile strength with reference to specimen AAS5. The tensile strength of Al-Al₂O₃-SiC composite decreased with an increase in SiC reinforcement above 2.5 wt.% in Al matrix. These are the factors at which tensile strength depends such as uniform distribution of ceramic reinforcement, shape, size, density variation and the nature of the ceramic as well as base material. The tensile strength is a function of tensile load carrying capacity of the composite which depends upon the thermal expansion coefficient. The Al base material has higher value of coefficient of thermal expansion as compared to SiC reinforcement. Due to larger variation of the thermal expansion coefficient among Al and SiC, development of a large amount of dislocation in hybrid composites takes place. These dislocation works as a barrier in the tensile strength. So, the tensile strength has improved with small variations of SiC reinforcement. The tensile strength is found to decrease at higher percentage of reinforcements because the agglomeration was developed at higher percentages along with some minute amount of porosity. The agglomeration decreases the bonding strength between Al matrix and reinforcements. So, it is observed that Al-2.5% Al₂O₃-2.5% SiC hybrid metal matrix composite has higher strength as compared to Al-10% Al₂O₃-10% SiC hybrid metal matrix composite.

Conclusion

Pure Al and Al-Al₂O₃-SiC hybrid composites were efficiently developed with stir casting technique. The various properties such as phase, microstructure, density, hardness, impact and tensile strength of developed specimens were studied. The concluding comments of the above work are as follows:

XRD result of fabricated composites showed that Al matrix and Al₂O₃ as well as SiC has no reaction between them. It is found that peak intensity of silicon carbide reinforcement increases when SiC percentage increases in compositions. However, oxygen has low intensity peak.

Scanning Electron Microscopy investigated that Al₂O₃ and SiC reinforcements are scattered evenly in Al base material. It is found that Al matrix and Al₂O₃-SiC has excellent interfacial bonding. EDS spectrums are in confirmation with the XRD results.

Density of hybrid composite decreases with an increase in SiC powder. The specimen AAS5 has lowest density i.e. 2.5473 gm/cm³ which is having 10 wt.% Al₂O₃ and 10 wt.% SiC reinforcement content. It is investigated that shape, size and the fraction of reinforcement resolves the density of fabricated composites.

Al-2.5 wt.% Al₂O₃-2.5 wt.% SiC hybrid composite has highest hardness value of 70.8 HRB and after that hardness decreases due to variation of reinforcements.

The hybrid composite has higher impact strength with reference to pure Al-6351. Impact strength increases with addition of SiC reinforcement content in Al-6351 and highest impact strength is 37.5 Joule in sample AAS5 which has 10 wt.% Al₂O₃ and 10 wt.% SiC.

Al-2.5 wt.% Al₂O₃-2.5 wt.% SiC hybrid composite showed highest tensile strength such as, engineering UTS is 70.7 N/mm² and true UTS is 72.8 N/mm².

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Pallav Gupta

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