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Vol. 8. Issue 4.
Pages 3504-3516 (July - August 2019)
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Vol. 8. Issue 4.
Pages 3504-3516 (July - August 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.06.025
Open Access
Investigation of rare earth particulate on tribological and mechanical properties of Al-6061 alloy composites for aerospace application
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Vipin Kumar Sharma
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vipin2871985@gmail.com

Corresponding author.
, Vinod Kumar, Ravinder Singh Joshi
Thapar Institute of Engineering and Technology Patiala, India
Faculty of Meerut Institute of Engineering & Technology, Meerut, Uttar Pradesh, India
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Table 1. Comparative analysis of experimental and theoretical density analysis.
Table 2. UTS, percentage improvement, percentage elongation & impact strength values of composites.
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Abstract

Current research work emphasis on the development of rare earth particulate (REP) reinforced hybrid aluminium matrix composites processed by stir casting route. The weight percentage of CeO2 as rare-earth particulate varied from 0.5 wt% to 2.5 wt% and SiC /Al2O3 varied from 2.5 wt% to 7.5 wt%. The presence of rare earth particulate helps in grain refinement of the matrix with well defined grain boundaries is seen via Electron Backscatter Diffraction (EBSD) analysis. The Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) and Energy Dispersive Spectroscopy (EDS) examination were used to characterize the prepared samples of Al-6061 hybrid composites. Porosity analysis, Vickers microhardness, tensile behavior, flexural strength and impact strength of the hybrid composites improved significantly with the addition of rare earth particulate. The flexural strength of the Al-6061 hybrid composites is found to be increased upto 44.76% by increased weight percentage of rare earth oxide phase. The optimum quantity of CeO2 as RE that favours the better tribological and mechanical characteristics of aluminium hybrid composites was found to be 2.5 wt%. The results of wear tests showed an improvement of wear rate around 87.28% when compared to Al-6061 alloy with the addition of 2.5 wt% of CeO2. Finally, the worn out surface of the hybrid composites is examined with the help of scanning electron microscope to understand the wear mechanism of the composites for aerospace application.

Keywords:
Rare earth
Cerium oxide
Wear resistance
Abrasion
EBSD
Full Text
1Introduction

Aluminium matrix composites (AMCs) have tremendous scope in almost every field of applications including aviation sector, automobile applications, biomedical applications and advanced industrial applications because of the remarkable combination of properties including good corrosion and wear resistant, excellent sintering temperature and high specific strength [1,2]. Because of its light weight, the aluminium metal composites could be a better choice as compared to the other monothilic aluminium alloys [3]. In recent years AMCs which are made of nano-level particulate reinforcement have been explored with excellent mechanical characteristics as compared to the traditional type of AMCs with micro level particulate reinforcement [4]. Various commonly used particulate reinforcement in nano scale includes TiO2, MoS2[3], AlB2[5], Al2O3[6], and carbon nano tubes [7,8]. Further, hybrid aluminum matrix composites (HAMCs) by using two or more than two ceramic reinforcement particulates have been used in many applications due to better mechanical and metallurgical characterization properties as compared to composites having single reinforcement [9,10].

The different sizes of particulate reinforcement used in HAMCs have different role in the matrix. The purpose of nano-particles dispersed over matrix is to offer a satisfactorily level of performance to the composite structure by holding the fibers jointly and transmit the load to the fibers which finally stop the cracks formation. Nanoparticles not only help in the load carrying property of the composites but also maintain the ductility of the matrix alloy [11]. The particulate reinforcement having micro level improved the hardness, stiffness and wear characteristics of composites by sustaining the matrix skeleton. So overall performance improvement of the composite depends upon the integrated utilization of nano and micro sizes particulates with better synergy. It was further stated that if the particulate reinforcement of micro level increased by weight percent above 30% in AMCs during fabrication or in service condition leads to the creation of little voids and cracks. These defects cause brittle fracture under low tensile loads. Ultimately leads to loss in strength, low ductility and low toughness properties of ceramics particulates used in various structures. This problem can be can be further reduced by making the composites with higher content of nano particulates reinforcement as compared to micro size particulates [12]. To achieve the better properties of composites upto maximum level it is essential to properly mix the nano particulates in the matrix material so that perfect bonding takes place between them.

Silicon Cabide and aluminium oxide were found to be most common form of particulate reinforcement used to enhance the mechanical properties of various aluminium alloys in micro level. The silicon carbide in particulate form was generally responsible for enhancing the tensile strength, density and hardness whereas the aluminium oxide in particulate form was responsible for enhancing the wear resistance. It was explored that as the weight percent of mixture of (SiC + B4C) reinforcement in aluminium base alloy 6082 increases, the mechanical properties like tensile strength and microhardness improved significantly upto 15 wt% [13]. The microhardness of the composites with base alloy 6061 has been increased with the addition of mixture of fly ash and silicon carbide [14]. Madheswaran et al. [15] developed a composite with base alloy 6063 by using boron carbide as reinforcement and also see the effect of calcium carbide on mechanical properties of 6061 alloy. Mechanical properties including hardness and tensile strength were found to be increased whereas the impact strength decreases as the weight percent of reinforcement increases. Arora et al. [16] confirmed that specific wear rate of magnesium alloy based composites improved due to the microstructural refinement of grains. Singh et al. [17] developed an Al-5083 matrix composite by using silicon carbide as powder reinforcement and also see the effect of adding different weight percent of silicon carbide on wear properties of the composites. Parabhakar et al. [18] analyzed the dry sliding wear behavior of aluminum hybrid composites with traces amount of B4C and found in the improvement of specific wear rate and coefficient of friction using high value of load with minimum sliding velocity.

Rare earths (REs) elements have been increasingly used as reinforcement material in various aluminium matrix composites due to their high strength at room temperature, good thermal conductivity and mechanical properties. Luminescence properties as well as mechanical properties of composites reinforced with silica have been improved with different wt% of the europium as a rare earth metal [19]. Few of the authors reported that corrosion behavior of the hybrid composites based on AA-2024 alloy have been improved significantly by adding cerium sulphate [20]. The ductility of the titanium based composites have been improved by adding the optimum amount of lanthanum oxide [21]. Increase in the ductility may be the reason of very fine lanthanum particulates used in the titanium composites. A few researchers developed new hybrid composites by using Al-17 Mg as matrix material and M40 graphite powder [22]. They reported that hardness of the prepared composite improved with the addition of neodymium in traces amount. Xihua et al. [23] reported that mixed rare earth could be a better choice as mechanical property enhancement material in AlTiC hybrid composites with alumina. They have reported that bending strength and fracture toughness of the prepared composite were improved significantly with addition of optimum amount of 0.55 wt% of mixed rare earth (Nd, Ce, La and Pr). Fabrication of A-356 composite by incorporation of cerium particulate also reported [24]. They reported that an optimum concentration of 0.6 wt% of cerium favorable for enhancing the mechanical properties of A-356 alloy composite. Few of the author reported that Er and Zr could be a better choice material for enhancing the recrystallization temperature of the hybrid composites from 350 °C to 450 °C [25]. Lanthanum and cerium addition in traces amount used to enhance the mechanical properties of A356 matrix hybrid composites [26]. There are few studies focusing on preparation of advanced composites with improved ductility and strength with rare earth elements targeted for the aerospace application.

In the present work, attempts are made to fabricate a series of rare earth particulate-filled Al-6061 alloy composites via a high-temperature stir casting technique. Thereafter, the prepared alloy composites are characterized for their mechanical, tribological and microstructure studies.

2Materials and methods2.1Experimental details

Rare Earth Particulate (REP) aluminium alloy composites are fabricated using a high temperature stir-casting machine. Six different samples of Al-6061 alloy composites were fabricated by varying weight percentages of SiC + Al2O3 from 5 wt% to 15 wt% and 0.5 to 2.5 wt% of REP (CeO2). To determine the chemical composition of the Al-6061, Wavelength Dispersive X-ray Fluorescence techniques was used. Composition testing was done at Sophisticated Analytical Instrumentation Facility of Panjab University, Chandigarh India.

2.2Physical and mechanical characterization

Physical characterization is carried out to investigate the densities and void fraction of Rare Earth Particulate reinforced aluminium alloy composites. The density of Rare Earth Particulate reinforced aluminium alloy composites is calculated both theoretically and experimentally. The theoretical density is calculated by the method proposed by Bai and Xue [27] and Archimedes’ principle is used for the analysis of experimental density [28]. The mechanical properties of the composite samples were characterized by conducting hardness, tensile and percentage elongation tests. The hardness test was conducted using Rockwell hardness tester (HV) – B scale (HRB) at 100 kgf force, micro-hardness vicker tester 9 (Load range – 10 gmf – 1000 gmf, automatic loading and unloading, dwell time 5–60 s and magnifications (100X & 400X) and Standard izod impact tester with test specimen of dimension L × B×W = 75 mm × 10 mm × 10 mm [29]. The tensile test was conducted on universal testing machine with IS 1786:2008 & IS 1599:1985 test methods having capacity of 1000 kN and resolution of 0.5 kN. Cylindrical specimens (22 mm diameter and 220 mm length) were prepared according to test demand. The value of ultimate tensile strength and percentage elongation was measured with the help of results. The three point bending method is used to measure the flexural strength with a span of 20 mm and a cross head speed of 0.5 mm/min. Flexural bars are carefully ground and polished into the size of 3 mm × 4 mm × 30 mm with the surface roughness less than 0.1 µm.

2.3Sliding wear studies

Progressive wear testing was done by using Pin-on-disc tribometer which consist of a counter disc (Material of disc – EN 31, Steel Hardness – 68 HRc, Diameter – 160 mm). The specimens prepared for the wear test were of cylindrical shape with diameter 10 mm and length 12 mm. The test was performed at different sliding velocities (0.5,1.0 and 2.0 m/s) for different Sliding distance (500, 1000, 1500, 2000 m) with normal loads of 10 ,20 N and 30 sN respectively.

2.4Study of surface morphology

The surface morphology of the prepared hybrid composite samples was done with the help of Electron back scattered diffraction (EBSD), Scanning electron microscopy (SEM) (Make: JEOL JSM -6510LV, Oxford instruments) and Energy dispersive spectroscopy (EDS) techniques.

3Results and discussions3.1Effects of stir casting on microstructure evolution

For EBSD purpose hybrid composite containing 5 wt% of (SiC + Al2O3) as well as 5 wt% of (SiC + Al2O3) with 2.5 wt% of CeO2 as REP specimens processed via stir casting were prepared having size of 12 mm × 4 mm. Grinding of all the samples was done with the help of different emery papers having grades varies from 80grit to 1200grit followed by polishing with 1 μm diamond paste solution. The ground and polished hybrid composite samples were further electro polished using 20% perchloric acid and 80% ethanol solution on a very fine cloth of velvet for nearly 10 min by using a voltage of 20 V [30]. After that the samples was further processed with the help of ion-beam polishing machine (Precision etching coating system, Make: Gattan; Model: 682). Ion beam polishing was done at 65° rock angle for nearly 5 min.

Fig. 1 shows the microstructural details with pole figures and and grain size distribution as a function of area fraction of hybrid composites with 5 wt% of both SiC and Al2O3 and 5 wt% of both SiC and Al2O3 with 2.5 wt% of CeO2 as a Rare Earth Particulate. The average grain size of as received pure aluminium 6061 is very large having large area of fraction. But after incorporating of 5 wt% of both SiC and Al2O3 in aluminium matrix, it has been reduced as indicated in the graph Fig. 1(a) between grain size diameter vs area fraction. Similarly, an improvement in the grain refinement at room temperature were achieved by solidifying aluminium matrix reinforced with SiC and Al2O3 with traces amount of rare earth particulate hybrid composites as shown in Fig. 1(b). Thus, an average grain size of the hybrid composite reduced with the addition of SiC/Al2O3 and rare earth particulate as shown in Fig. 1(a,b).

Fig. 1.

Ordinary EBSD microstructural pictures (IPF and grain boundary) and grain size distribution as a function of area fraction for various composites: (a) Al6061-5 wt% (SiC + Al2O3), (b) Al6061-5 wt% (SiC + Al2O3)-2.5 wt% CeO2.

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In EBSD maps, the different grain orientations of pure aluminum alloy and hybrid composites depicted through the different colour contrast. It is largely accepted that dynamic recrystallization is the main cause of the grain refinement. In this manner, the segments affecting the nucleation and improvement of the dynamic recrystallization centered the resultant microstructural grain refinement in the Stir Zone (SZ). The factors which have significant impact on recrystallized grains include material chemistry, tool geometry, downward force in vertical direction and ambient temperature conditions [28]. For every one of the composites the normal size of the grains is fundamentally diminished after stir casting process. The [001] inverse pole figure (IPF) exhibited in Fig. 1(a,b) demonstrate the introductions created at the stir zone. The designated maximum intensities are the times random grain orientations which are 2.561 and 4.959 for the hybrid composites with 5 wt% of both SiC and Al2O3 and 5 wt% of both SiC and Al2O3 with 2.5 wt% of CeO2 as a Rare Earth Particulate.

Fig. 2 abridges the impact of stir cast processing on grain refining of Al-6061 alloy by consolidated with rare earth particulate and with SiC/Al2O3 reinforced particles through grain limit maps with image quality guide (IQM),orientation distribution function(ODF), misorientation angle histogram(MAH) and inverse pole figure(IPF). It is obvious from the IQM (Fig. 2(a,b)), diverse misorientation angle conveyance of low angle grain boundary(LAGBs)with angle (θ ≤ 15°) and high angle grain boundary (HAGBs) with angle (θ ≥ 15°) are appeared with changed hues. LAGBs additionally separated into two classifications, one from 2° to 5° spoken to by the red shading and for which the point shifts from 5° to 15° spoken to by green shading. By and large, LAGBs extending from 2° to 5° is known as subgrain limits, framed by separation improvement of disengagements into sub-grain limits happen due to high stacking deficiency vitality of aluminum dynamic recystallization (DRX) amid thermo-mechanical preparing. It is clear from the Fig. 2(b) that the fraction of LAGBs is higher for the hybrid composites reinforced with the rare earth particulates as compared to the hybrid composts with SiC and Al2O3 reinforcement. The addition in the fraction of LAGBs is observable to the detriment of HAGBs and for SiC/Al2O3/CeO2 hybrid composite significant change in LAGBs fraction occurred with modification of electromagnetic stirring. Improved estimation of LAGBs for the rare earth particulate hybrid composites is a potential indication of a sub-grain structure getting strengthened. This is likewise affirmed by the orientation distribution function (ODF) selected at  = 45° and inverse pole figure (IPF) as illustrated in Fig. 2(a,b).

Fig. 2.

Grain boundary maps with image quality map (IQM), and misorientation angle histogram for: (a) Al6061-5 wt %( SiC + Al2O3), (b) Al6061-5 wt% (SiC + Al2O3)-2.5 wt% CeO2.

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Energy dispersive spectroscopy analysis of the Al2O3/SiC/CeO2 – reinforced composites is shown in Fig. 3(a–f). It is clear from the spectroscopy analysis that oxygen and carbon peaks were observed. It is very much confirms from the EDS analysis that aluminum SiC particles and cerium oxide are present within the composites. Therefore, these results of SEM indicated that successful incorporation of ceramics Al2O3/SiC/CeO2-reinforced Al-matrix composites produced by the stir casting process.

Fig. 3.

EDS analysis of Hybrid composites, (a) 2.5% Al2O3 + 2.5% SiC without rare, (b) 2.5% Al2O3 + 2.5% SiC with 0.5% CeO2 (c) 5% Al2O3 + 5% SiC without rare, d) 5% Al2O3 + 5% SiC with 1.5% CeO2, e) 7. 5% Al2O3 + 7.5% SiC without rare and f) 7. 5% Al2O3 + 7.5% SiC with 2.5% CeO2.

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X-ray diffraction is an important technique to understand the properties of hybrid composites based on various powder mix elements like CeO2, Al2O3 and SiC. Each plot of XRD pattern indicates the phases formed in samples consisting various percentage composition of reinforcing elements. In most of the cases, the highest peak was observed on 2θ scale. The various patterns for different composition of SiC and Al2O3 powder based hybrid composites are shown in Fig. 4(a). As the parent material mainly consists of aluminium in matrix form, the large peaks corresponds to the parent metal. XRD analysis also revealed that the smaller peaks corresponds to the presence of various phases like SiO2, Mg(CO3), FeAl2Si in hybrid composites. The various patterns for different composition of CeO2, Al2O3 and SiC powder based hybrid composites are shown in Fig. 4(b). Phase analysis of the hybrid sample with CeO2 modified aluminium alloy illustrated that occurrence of α-Al,Al4Ce,Al3Ce and Al8Mg5 phases. The existence of different oxides of cerium in material is a sign of successful addition of the cerium oxide as a rare earth oxides in the composite

Fig. 4.

XRD pattern of Composites, a) with 2.5, 5, 7.5 wt% of (SiC + Al2O3), b) with 0.5, 1.5, 2.5 wt % of (CeO2).

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3.2Effect of porosity content on rare earth particulate-filled Al-6061 alloy composites

The theoretical density of the alloy composites is calculated using rule-of-mixture via Eq. (1), while experimental density is evaluated using water immersion technique based on Archimedes’ principle. Thereafter, void content is computed as per Eq. (2)[31].

The accurate value of density measured by experimentally of different grade samples of hybrid composites was calculated by dividing the estimated value of measured weight of a composite sample by its measured volume using a digital weight balance machine having a ±0.0001 mg of closed tolerance [28]. To achieve the most reliable and consistent results, each composite sample having same composition tested three times. Same composition samples of hybrid composites tested three times to ensure the measured values are consistent and reliable. The values of density both theoretical and experimental, relative density and void content are presented in Table 1. In all the cases, the relative densities of composites are more than 98%. The results of relative densities are in good agreement, when Nd rare earth particulate used in the aluminum composites [22].

Table 1.

Comparative analysis of experimental and theoretical density analysis.

Nomenclature of sample  Theoretical density (gmcc−3)  Experimental density (gmcc−3)  Relative density (%)  Void content (%) 
Al-6061 alloy  2.70  2.67  98.88  11.1 
A6061/0 wt%REP/2.5 wt% Al2O3/2.5 wt%SiC  2.73  2.71  99.26  7.3 
A6061/0 wt%REP/5 wt% Al2O3/5 wt%SiC  2.77  2.76  99.63  3.5 
A6061/0 wt%REP/7.5 wt% Al2O3/7.5 wt%SiC  2.81  2.80  99.64  13.6 
A6061/0.5 wt%REP/2.5 wt% Al2O3/2.5 wt%SiC  2.76  2.75  99.63  3.62 
A6061/1.5 wt%REP/5 wt% Al2O3/5 wt%SiC  2.80  2.79  99.64  3.57 
A6061/2.5 wt%REP/7.5 wt% Al2O3/7.5 wt%SiC  2.84  2.83  99.64  3.52 

It was clearly indicated from the Table 1 that percent porosity for Al-6061/2.5 wt% SiC/2.5 wt%Al2O3 was 7.3 and that of with 0.5 wt% of REP was 3.62. Similarly, the value of porosity in percent for 1.5 wt% of REP was 3.57 and that of 2.5 wt% was found to be 3.52.Similar trends of porosity results were found in the aluminium alloy when the Ni particulate content filled in it. The void contents were found to be 6.87, 8.57, 3.75, 7.79 and 9.11.The best result of 3.75 of porosity occurred at 1 wt% of nickel particulate [28].

3.3Effect of ultimate tensile strength on rare earth powder-filled Al-6061 alloy composites

Tensile strength of hybrid composite in axial direction is calculated by dividing the maximum load bearing capacity of a composite before failure to the average value of cross-sectional area. This strength is likely depending upon capacity of transferring of externally applied load from matrix to the reinforcement. As per the fundamental of material science, total number of major strengthening mechanisms helps in the enhancement of strength are four namely Hall-Petchment mechanism, Orowan mechanism of strengthening, Work hardening and strengthening of the hybrid composites due to the difference in the coefficient of thermal expansion(CTE) [32]. The combined overall equation that favours the strength mechanism of the hybrid composites may be written below:

Overall Tensile Strength of the composite,

where, σMatrix = strength of the matrix

σLoad = effect of load transfer

ΔσHall-Petch = increment due to refinement of grains

ΔσOrawan = effect of Orawan Strengthening

Strengthening mechanism equation could be expressed as individually:

where, ky, dnc, dm represents the Hall-Petch coefficient of aluminium matrix, grain dimension of the matrix in the REP/Al2O3/SiC/Al-6061 hybrid composite and grain size of the matrix alloy Al-6061, respectively. The diameter of the nanoparticles is represented by dp, shear modulus denoted by Gm (MPa), b represents Burgers vector of the matrix in meter, vp is a volume fraction of nano particles, σm is the yield strength of the matrix, l is the dimension of the reinforcement particles parallel to the loading direction, t is the thickness of the particles and A (or l/t) denoted the aspect ratio of the particles. For Al-based material the value of ky is determined as 0.14 MPa/m1/2[33].

Table 2 represents the ultimate tensile strength (UTS), percentage elongation and percentage improvement after stir casting of aluminium without Rare earth Particulate and with Rare Earth Particulate. At first glance, the improvement in the strength of individual composites is depending upon the refinement of the grain size into small size according to the Hall-Petch mechanism of strengthening. The overall strengthening Eq. (4) stated that grain size directly affect the strength of the Al-6061 metal matrix composite [34,35].

Table 2.

UTS, percentage improvement, percentage elongation & impact strength values of composites.

Samples  UTS (MPa)  % age improvement  % age elongation  Impact strength 
A6061/0 wt%REP/2.5 wt% Al2O3/2.5 wt%SiC  30  –  2.0  22 
A6061/0 wt%REP/5 wt% Al2O3/5 wt%SiC  54  80  2.1  30 
A6061/0 wt%REP/7.5 wt% Al2O3/7.5 wt%SiC  73  35.18  6.8  34 
A6061/0.5 wt%REP/2.5 wt% Al2O3/2.5 wt%SiC  89  21.9  7.2  50 
A6061/1.5 wt%REP/5 wt% Al2O3/5 wt%SiC  102  14.6  10.0  56 
A6061/2.5 wt%REP/7.5 wt% Al2O3/7.5 wt%SiC  123  20.6  11.5  46 

The fortifying micromechanics of the matrix are affected by (SiC + Al2O3) reinforced particles. At the point when particles support is brought into a liquid molten matrix of Al-6061 alloy, there is typically a huge increment in the dislocation density all through the composite as of the significant difference in the thermal expansion coefficient of the aluminium matrix and (SiC + Al2O3) particles [36]. However, the dispersion of (SiC + Al2O3) is improved significantly with the addition of CeO2 particulates. Moreover, the composite having CeO2 additive had a smaller grain matrix as compared to the composite without CeO2 additive as indicated by the SEM micrographs. The resistance to dislocation movements, dislocation density and their interaction with the reinforcing particles may have expanded because of these impacts, bringing about expanded tensile strength and strain hardening. Increase in the tensile elongation can be ascribed due to the diminished grain size of the matrix material and the enhancement in dispersion of (SiC + Al2O3) particles.

It can be seen from the Table 2 that with increase in wt% of reinforcements the value of tensile strength as well as percentage elongation increases. It shows that both the strength and ductility of the material increases with increase in wt % of reinforcement composition. Better homogeneity of the mixture as observed from the microstructure analysis of composites may have led to the enhancement in their properties. The graphical representations of the tensile test results showing combined effects of % age reinforcement addition on the ultimate tensile strength (UTS) and percentage elongation on both types composites i.e. Non-rare earth (NRE) as well as rare earth (RE) composites are shown in Fig. 5. The optimum composition showing reasonable ultimate tensile strength and percentage elongation is observed to be Al-6061/ 2.5 wt%REP/7.5 wt% Al2O3 /7.5 wt% SiC composite. The current results are comparable with the tensile results of composites with Nb as rare earth particulates. The optimum quantity for the better ductility and strength was found to be 5 wt% [37]. With the use of 0.15 wt% of Sc in the aluminium alloy, the yield strength, UTS increased gradually. The results are in good agreements with the current results [38].

Fig. 5.

Combined effect of weight percentage of reinforcement added on UTS and % elongation of the composites.

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3.4Effect of flexural strength on rare earth powder-filled Al-6061 alloy composites

Three point bending test was realistic instead of tensile test and compression test in investigating hybrid composites reinforced with Al2O3, SiC and CeO2 particulate reinforcement. The main reason was aluminium oxide, silicon carbide and rare earth oxide particulates make the notch effect during testing. Since machining of Al2O3, SiC and CeO2 reinforced aluminium matrix hybrid composites, surface is a very fine and special process, structure may be weakened. Machining was done with high speed diamond tools. Three point bending test was performed to reveal the flexural strength of Al-6061 aluminum alloy matrix composites with 5 wt%, 10 wt% and 15 wt% of Al2O3/SiC particulate and with different percentage additions of rare earth oxide. In three-point bending test, the maximum bending load was evaluated for different samples with varying compositions of reinforcements. Then finally; the load value of different hybrid composites was converted into flexural strength (MPa) value.

The flexural testing specimens of fabricated hybrid composites were machined according to ASTM standards. The composite were tested for flexural strength by using FIE make UTE100 model universal testing machine at room temperature. The Flexural strength results of the samples are shown in Fig. 6. These results reveal that, effect of Al2O3, SiC and CeO2 particulate reinforcement content lead to decrease the interspatial distance between various reinforcements and matrix grain, which reduces the number of dislocations at the grain boundaries in the microstructure. Hence, the enhancement in the Flexural strength was obtained than that of the Al-6061 matrix alloy as presented in the Fig. 6. The enhancement of the flexural strength was possible due the strong bonding strength between the reinforcement and matrix.

Fig. 6.

Effect of weight percentage of reinforcement added on Flexural Strength of the hybrid composites.

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The structure and properties of the reinforcements controls the mechanical properties of the composites that are reasoned to strong interface that transfers and distributes the load from the matrix to the reinforcements exhibiting increased elastic modulus and strength [39]. The brittleness of the Al2O3/SiC particles together with the interface between rare earth oxides and matrix leads to the increase in the flexural strength of the composites and it was higher than that of the Al-6061 base alloy. The optimum characteristic is gained by the description given in the fewer cases about the optimum rare earth oxides quantity [40,41]. In order to strengthen the tribological and mechanical characteristics the hard reinforcing particles into the matrix alloy is incorporated. Nevertheless this may result with rapid counter face along with deteriorated machinability [42,43]. The hard and solid lubricant materials can be obtained from the hybrid composites which has strengthened tribological and mechanical properties been developed [44–48]. The flexural strength of hybrid composites reinforced with 15 wt% of Al2O3 + SiC was increased 44.76% with the addition of 2.5 wt% of CeO2 than that of Al-6061. It was found that with the addition of 15 wt% of Al2O3 + SiC without rare earth oxides, flexural strength was increased only 24.44%. Addition of Al2O3 particle increases the flexural strength of the hybrid composite of Al-7075 and it was higher than that of base alloy. Addition of graphite to aluminium alloy is known to decrease the flexural strength and it was overcome by the addition of Al2O3 particulates in the hybrid composites. The flexural strength was increased 23% than that of Al-7075 [28]. These results are good agreement with the current results. Similar kind of flexural strength was found in the AlTiC master alloy with traces of mixed rare earth elements (Nd, Ce, La and Pr). The optimum flexural strength was found to be 617.6 MPa with 0.5% of rare earth elements [23].

3.5Effect of hardness on rare earth powder-filled Al-6061 alloy composites

The hardness test was conducted with the help of Vicker hardness tester (Micro-hardness). The test shows an increase in hardness of the composites with increase in weight percentage of the reinforcements. Micro-hardness value of the base alloy Al-6061 was increased from 79.3 HV to 92.8 HV on addition of 2.5 wt% rare earth particulate along with (Al2 O3 + SiC) mixture and the percentage increase was nearly 17.02% and as compared to non-rare earth element (Alloy + 7.5 wt%(Al2O3 + SiC)) mixture the hardness increased from 79.3 HV to 90.17 HV with a percentage increase of 13.70%. Whereas the value of Rockwell hardness of the base alloy increased from 61.73HRB to 82.6HRB with a percentage increase of 33.80% by addition of rare earth metals and as compared to non-rare the hardness increased the value increased from 61.73HRB to 71.8HRB with a percentage increase of 16.32%. This can be due to strong bonding and better refinement of the composites which can be seen from the SEM images. It has been clearly seen that the hybrid composites structure refined with the addition of rare earth element. It further helps in the strengthens the bonds between the particles and hence provide more hardness to the surface of the composites. Also, increase in the reinforcement contents helps in increasing the density of the material and further helps in increasing the hardness. The graphs showing relation between hardness value and percentage reinforcement are shown in Figs. 7 and 8. Similar trends of increase in hardness were observed in Gr/Al composites reinforced with neodymium contents vary from 0.2 wt%, 0.5 wt% and 2.0 wt% [22]. The hardness of master alloy made of aluminium was found to be in an increased manner with increased in wt% of rare earth elements. The optimum value of 20.70 GPa hardness was achieved at 0.5 wt% of rare earth element [23]. These results are very much in-line with the current research.

Fig. 7.

Bar chart showing maximum hardness of composite with 2.5 wt% rare earth particulate.

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Fig. 8.

Rockwell hardness distribution for Non-rare earth and rare earth composites.

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3.6Effect of impact strength on rare earth powder-filled Al- 6061 alloy composites

Fig. 9 shows the effect of rare earth powder content on the impact strength of Al-6061 alloy rare earth particulate composites. The graph indicates the impact energy increases with rare earth particulate content from 50 J at 0.5 wt% of CeO2 to 56 J at 1.5 wt% of CeO2 in the unfilled matrix. The impact strength of the base metal alloy Al-6061 and samples of hybrid composites are represented in Table 2. It can be seen in Table 2 that the value of impact strength continue to increase with increase in the percentage of SiC and rare earth elements Ce O2 but on increasing the amount of rare earth elements to 2.5 wt% impact strength value of the composite decreases. On adding 2.5 wt% rare earth particulate the value of impact strength decreases because with increase in the amount of rare earth material properties tends to change from ductile to brittle which cause reduction in hardness and because of which ability of a material to absorb shock decreases and it break with less force as compared to other prepared composites.

Fig. 9.

Impact Strength distribution for Non-rare earth and rare earth composites.

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3.7Steady-state specific wear rate analysis of aluminum alloy and its hybrid composites

Wear test were performed on the different samples of the composites reinforced with Al2O3, SiC and CeO2. The different composites having sample size of diameter 10 mm and length 12 mm were prepared. The experiments were conducted on the universal tribometer using universal pin-on-disc test configurations. The apparatus consists of pin held stationary by using the holder arrangement and kept in a contact with counter face material made up of Stainless steel disc. All the composite specimens reinforced Al2O3 and SiC with traces amount of cerium oxide were tested under applied loads of 10 N, 20 N and 30 N at different sliding velocities i.e. 0.5 m/s, 1 m/s, 2 m/.The sliding distance considered for the experiments were 500 m, 1000, 1500 and 2000 m.

The wear tests were carried out for finding the wear rate on the composites with rare earth and without rare earth having maximum hardness. Fig. 10 shows the comparative analysis of wear rate plots for Al-6063 alloy, Al-6063/7.5 wt%Al2O3/7.5 wt%SiC and Al-6063/7.5 wt%Al2O3/7.5 wt%SiC/2.5 wt%CeO2 hybrid composites at different sliding velocities and at various load conditions. The result indicated that the composites has shown better wear properties throughout the range of parameters investigated after incorporation of rare earth elements as depicted from curves 10 (a)–(c). The wear behavior of aluminium composites shows comparable results with the current results after incorporation of double synthetic ceramics [49]. It is depicted from the curve 10(a) that the least wear rate in hybrid composites shows at 0.5 m/s sliding velocity and at 10 N of normal load value after incorporation of rare earth elements. The maximum mass loss would be achieved at low value of sliding velocity and as the value sliding velocity further increased the mass loss decreased in both the composites. The wear rate of composites with 7.5 wt% of (SiC + Al2O3) mixed was improved significantly up to 87.28% after incorporation of 2.5 wt% of CeO2 when compared with A-6061 base alloy at sliding velocity of 0.5 m/s and normal load of 10 N. At similar condition of investigated parameters, the wear rate was achieved only 38% in composites with 7.5 wt% of (SiC + Al2O3) mixed reinforcement as compared to base alloy Al-6061 alloy. The wear rate of composites with rare earth elements decreased upto 77.5% at velocity of 0.5 m/s and normal load of 20 N when compared to the Al-6061 base alloy, whereas the hybrid composites with 7.5 wt% of (SiC + Al2O3) showed only 32% improvement. At sliding velocity 0.5 m/s and 30 N of normal load value, the wear rate was improved by 68.83%of composite with rare earth whereas with addition of 7.5 wt% of (SiC + Al2O3) only 22% improvement in wear rate was observed. From curve 10(b), it can be seen that the maximum mass loss per sliding distance observed at maximum load of 30 N and it decreased linearly with decrease in the value of normal load. Wear curve 10(c) depicts that the wear rate improved by 44.93% and 34.95% in composites with of 2.5 wt% of CeO2 and 7.5 wt% of (SiC + Al2O3) compared to Al-6061 base alloy at velocity of 2 m/s and normal load of 20 N and 30 N respectively. Moreover, decrease in the wear rates with change in the sliding speed from 1 m/s to 2 m/s were found to be higher in comparison to change in sliding velocity from 0.5 m/s to 1 m/s. This may be due to the various wear mechanism phenomenon exists at different sliding velocity and at different loads. It has been seen that the phenomenon of abrasion generally occurs at low value of normal load and sliding velocity. The extent of abrasion wear found to decrease with increase in normal load and sliding velocity. Similar kinds of wear mechanism were observed in the 2218 aluminium composites after incorporation of Al2O3 and TiO2[50]. After incorporation of rare earth in the Mg-Al-Zn alloy composite, the most dominating mechanism of wear like abrasion, delamination and plastic deformation were observed by SEM micrograph [51]. Results are in very much comparable with the current research results.

Fig. 10.

Progressive Wear Rate plots for Al-6061 aluminium alloy, Al-6061 aluminium alloy with SiC/Al2O3/CeO2 and Al-6061 aluminium alloy with SiC/Al2O3 at (a) 0.5 m/s sliding velocity, (b) 1 m/s sliding velocity and, (c) at 2.0 m/s sliding velocity.

(0.54MB).
3.8Surface morphology studies of rare earth particulate Al-6061 alloy hybrid composites

Fig. 11a–f shows the worn out surface of the base alloy Al-6061 alloy and hybrid composites subjected to 10 N load respectively. The various types of wear mechanism were observed due to the different nature and types of particulate reinforcements incorporated into the aluminium alloys. Abrasive wear, ploughing and plastic deformation were found to be the most dominating wear mechanism in composite sample comprises of (SiC + Al2O3) mixed reinforcment at load of 10 N and sliding velocity of 0.5 m/s as indicated by SEM micrograph 11(a). The cracked surface of hybrid composites reinforced with (SiC + Al2O3) looks like as broken particles. From EDS analysis of the hybrid composites reinforced with (SiC + Al2O3) demonstrate that a significant amount of oxygen present on its surface. Due to the presence of oxygen, an oxidation wear was resulted on the surface of composites.

Fig. 11.

SEM images of the wear specimens at load 10 N (a), (b) composite with Al2 O3 +SiC mixture (c) RE Hybrid composite with Al2 O3 +SiC + Ce O2 mixture (d) Al-6061 base alloy (e) RE Hybrid composite with Al2 O3 +SiC + Ce O2 mixture (f) RE Hybrid composite with Al2 O3 +SiC + Ce O2 mixture.

(1.19MB).

Rare earth based hybrid composites exhibited abrasion, ploughing and delamination as the most predominating wear mechanism as depicted from Fig. 11(b). Out of these mechanism, abrasion was mostly responsible for wear to be occur on the surface. Weight loss and friction coefficient gets reduced after incorporation of CeO2 additive to the (SiC + Al2O3) mixed hybrid composites as indicated by Fig. 11(c). It was predicted that the wear rate were found to be diminished continuously. Abrasion wear in the composite reinforced with (SiC + Al2O3) was low after incorporation of cerium oxide as rare earth elements whereas delamination was found to be most severe mechanism responsible for wear to be occur on it. It has been found that as the content of cerium oxides exceeds from 2.5 wt%, the wear mechanism dominated by delamination which ultimately leads to brittle fracture as indicated by Fig. 11(c). Fig. 11(d) indicates the worn-out surface of base alloy Al-6061. It is obviously seen that the worn surface of the base alloy consists of large number of pits and microcracks. These microcracks extended continuosly over the surface of worn-out base alloy. It has been observed from micrograph Fig. 11(e) and Fig. 11(f) of hybrid composites that after incorporation of cerium oxide the strong interfacial adhesion makes SiC and Al2O3 not easily detach from the Al-6061alloy surface and prevents the rubbing-off of base alloy Al-6061. Due to the strong interfacial adhesion, wear properties aluminium alloy composites reinforced with (SiC + Al2O3) improved significantly after incorporation of cerium oxide. Further, a new intermetallic phase Al11 Ce3 having rod shape has been found in the microstructure of composites with cerium oxide. The existence of the intermettalic phase into the microstructure could be the reason of better mechanical properties at higher temperature which include the load bearing capacity as well of the base alloy Al-6061reinforced with (SiC + Al2O3).

4Conclusions

In the current work, hybrid AMCs with the successful incorporation of rare earth particulate are fabricated via stir casting technique followed by the mechanical and metallurgical characterization. Following given below findings were made:

  • 1)

    Hybrid AMCs mixed with (SiC + Al2O3) is successfully produced after incorporation of 0.5, 1.5 and 2.5 wt% of cerium oxide.

  • 2)

    SEM images of the microstructures of hybrid AMCs mixed with (SiC + Al2O3) prove the fact that the compact interfacial bonding between the cerium oxides particles and aluminium matrix.

  • 3)

    EBSD images of the microstructures of hybrid AMCs mixed with (SiC + Al2O3) prove the fact that the grains sizes are considerably finer after incorporation of cerium oxide. Further, homogenous and ultrafine equiaxed grains are obtained with well-defined grain boundaries by the effect of REP.

  • 4)

    From EDS analysis it was clear that the oxygen and carbon peaks were observed. It is very much confirms from the EDS analysis that aluminum SiC particles and cerium oxide are present within the composites. Therefore, these results of SEM indicated that successful incorporation of Hybrid ceramics Al2O3/SiC/CeO2-reinforced Al-matrix composites produced by the stir casting process.

  • 5)

    XRD analysis shows the presence of various phases like SiO2, Mg(CO3), FeAl2Si in hybrid composites The XRD study of CeO2 modified aluminium alloy showed that presence of α-Al, SiO2, Mg5Si6, MgO, Al4Ce and Al3Ce phases.The presence of Al4Ce and Al3Ce indicated successful addition of the cerium oxide as a rare earth oxides in the aluminium alloy Al-6061.

  • 6)

    Hybrid AMCs mixed with (SiC + Al2O3) shows better mechanical properties with reduced porosity after incorporation of cerium oxide. The tensile strength rises from 30 MPa to 123 MPa after successfully incorporation of 2.5 wt% of cerium oxide.

  • 7)

    Micro hardness of the Al-6061 base alloy was increased by 17.02% after incorporation of 2.5 wt% of cerium oxide. Similarly, the value of Rockwell hardness of the Al-6061 base alloy increased by 33.80% by addition 2.5 wt% of cerium oxide and increased only by 16.31% when no cerium oxide was incorporated.

  • 8)

    With the addition of 2.5 wt% of cerium oxide in hybrid AMCs mixed with 7.5 wt% of (SiC + Al2O3) yields better wear resistance than hybrid AMCs mixed with only 7.5 wt% of (SiC + Al2O3). Wear rate improved significantly by 87.28% with the addition of 2.5 wt% of cerium oxide corresponding to velocity 0.5 m/s and 10 N normal load. Wear rate of aluminium alloy improved only by 37.69% with the reinforcement 7.5 wt% of (SiC + Al2O3).

Conflicts of interest

The authors declare no conflicts of interest.

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