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Mechanical Behavior of Al6061-Al2O3 and Al6061-Graphite Composites
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ScienceDirect Materials Today: Proceedings 4 (2017) 10978–10986
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AMMMT 2016
Mechanical Behavior of Al6061-Al2O3 and Al6061-Graphite Composites Madeva Nagarala*, Shivananda B Kb, V Auradia, K I Parashivamurthyc, S A Korid, a
Department of Mechanical Engineering, Siddaganga Institute of Technology, Tumkur-572103, Karnataka, India b Department of Mechanical Engineering, UVCE, Bangalore-560001, Karnataka, India c Department of Mechanical Engineering, Government Engineering College, Chamarajnagar-571313, Karnataka, India d Department of Mechanical Engineering, Basaveshwar Engineering College, Bagalkot-587102, Karnataka, India
Abstract This work investigated the influence of Al2O3 and graphite on the microstructure and mechanical behavior of Al6061- Al2O3 and Al6061Graphite composites. The investigation reveals the effectiveness of incorporation of Al2O3 and Graphite in the Al6061 alloy for studying mechanical properties. The composites were fabricated using liquid metallurgy route. The Al6061- Al2O3 and Al6061-Graphite composites were fabricated separately by introducing 9 wt. % of Al2O3 and graphite particulates by two stage melt stirring process. In this reinforcement particulates were added in two steps to increase the wettability. The characterization was performed through Scanning Electron Microscope and Energy Dispersive Spectrum. The particle distribution was uniform in these composites. The grains were refined by addition of Al2O3 and Graphite particulates. The density, hardness, ultimate tensile strength, yield strength and percentage elongation of both Al2O3 and Graphite composites were evaluated as per ASTM standards. Further, a comparative study has been made between the Al6061- Al2O3 and Al6061Graphite composites.
© 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of Advanced Materials, Manufacturing, Management and Thermal Science (AMMMT 2016). Keywords: Al6061 Alloy; Al2O3; Graphite; Microstructure; Mechanical Properties
* Corresponding author. Tel. +91 9738304925 E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of Advanced Materials, Manufacturing, Management and Thermal Science (AMMMT 2016).
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1. Introduction Metal matrix composites (MMCs) are combinations of two or more materials (one of which is a metal) where tailored properties are achieved by systematic combinations of different constituents [1]. The commonly used metallic matrices include aluminum, magnesium, titanium, and their alloys. Metal matrix composites (MMCs) are being increasingly used in aerospace and automobile industries owing to their enhanced properties such as elastic modulus, hardness, tensile strength, wear resistance combined with significant weight savings over unreinforced alloys [2]. Aluminium and its alloys occupy third place among the commercially used engineering materials. Aluminum matrix composites have attracted enormous attention because of their high specific modulus, specific strength and excellent dimension stability. Furthermore, they have been considered as promising candidate for lightweight armors and protective materials [3, 4]. The interface between the matrix and the reinforcement plays an important role in determination of the properties of the MMCs. The advantages of particle-reinforced composites over others are their formability with cost advantage. Further, they have inherent heat and wear resistant properties [5]. A large number of fabrication techniques are currently used to manufacture the MMC materials. In the engineering materials, the MMCs can be manufactured by a unique technique such as casting as it is inexpensive and proposes many other options for materials and processing conditions. Now a day’s Aluminium alloy based composites finding wide range of applications in Aerospace domain due to light in weight compared to steel and titanium. Al alloy based metal matrix composites, are a new class of advanced materials whose properties can be tailored to satisfy the design needs by varying the reinforcements, matrices and the microstructure [6]. After about 25 years of developments for aerospace applications metal matrix composites are coming out from laboratories into commercial markets. There are several Al alloy matrices like AA2014, AA2024, AA2219, AA2618, AA6061, AA6068, AA7050, AA7025 and AA7075 are widely using aluminium alloys in an aerospace domain. The main purpose of using these alloys in aerospace is mainly due to their ageing characteristics. The required strength of the component can be achieved by heat treatment process. Further, several particulate and whisker reinforcements like SiC, Al2O3, TiC, B4C, WC and TiO2 are available as strengthening particulates for Al alloys [7]. Optimization or weight reduction plays very important role in aerospace domain. By using Al alloys alone, it is necessary to use more cross-sectional area to meet required strength, which increases the weight of the component. Since, performance of the aircraft is directly proportional to the weight of aircraft. So, it is necessary to develop lighter structures or components ready to provide required strength and stiffness in minimum cross section area. In this particular area Metal Matrix Composites play important role. So, it is necessary to develop newer materials with high strength and stiffness without increasing processing cost. Several researchers investigated the various properties of aluminium based metal matrix composites. Linlin et al. [8] investigated mechanical and tribological behavior of aluminium metal matrix composites reinforced with in situ AlB2 particles. Composites were prepared by hot rolling and solution treatment. Mechanical properties like tensile and micro hardness measurements were evaluated. The friction co-efficient, wear behavior and scratch morphology of the MMCs and pure aluminium were studied. The tensile, hardness and wear properties are higher in case of composites compared to unreinforced Al alloy. Shisheng and co-authors [9] studied the mechanical behavior of in situ carbon nano tube and silicon carbide reinforced Al6061 aluminium matrix composites. Results show that significant improvement in tensile properties with the SiC size to be 7 microns, which has ductility of 8.5% and Young’s modulus of 98 GPa, and tensile strength of 428 MPa. Harichandran and Selvakumar [10] carried out experiments on nano and micro B4C reinforced pure aluminium matrix composites. The micro and nano composites containing different weight percentages of B4C particles were fabricated by stir and ultrasonic cavitation assisted casting process. Tensile, hardness, impact and wear tests were carried out to evaluate mechanical properties of the micro and nano composites. The properties were enhanced in the case of B4C reinforced composites compared to Al alloy matrix. Very less literature is available to improve the wettability of reinforcements with base matrix. In the present investigation castings were done by using two steps particulates mixing process.
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In the present work an attempt is being made to process Al6061- 9 wt. % Al2O3 and Al6061-9 wt. % graphite particulate reinforced composites by two steps reinforcement mixing by using liquid stir casting technique. In this research the tensile properties of composites like hardness, ultimate tensile strength, yield strength and percentage elongations were evaluated as per ASTM standards and were compared with base alloy Al6061. 2. Experimental Details 2.1. Materials Used Al6061 alloy was used as a matrix material and Al2O3 and graphite particles with an average particle size 90-100 µm were used as reinforcement. Liquid stir casting method was used to fabricate composites. In order to enhance the wettability, the two step addition method used in the present work which helps to overcome the problem of agglomeration of reinforcing particles [11]. Table 1 represents the chemical composition of base alloy. Figure 1 shows the scanning electron microphotographs of Al2O3 and graphite particulates used to fabricate the composites. Table1. Chemical composition of Al6061 alloy in weight percentage Mg
Si
Fe
Cu
Zn
Mn
Ti
Cr
Al
0.82
0.64
0.23
0.17
0.03
0.07
0.01
0.01
Bal
(a)
(b)
Fig. 1.Shows scanning electron micrographs of (a) Al2O3 particles (b) Graphite particles
2.2. Fabrication Procedure The Al6061-9 wt. % Al2O3 and Al6061-9 wt. % graphite particulates composites were prepared using liquid stir casting technique. The equipment used for casting is shown in figure 2. Initially required amount of charge or matrix material was placed in a silicon carbide crucible, which was placed in electric resistance furnace at a temperature of around 750°C. After complete melting of Al6061 matrix, degassing was carried out by using solid hexachloro ethane degassing tablet, which helps to remove unwanted adsorbed gases from the melt [12]. Once degassing is over, the preheated ceramic reinforcement Al2O3 particles were introduced into matrix, which involves two-stage additions of reinforcement during stirring. This novel two stage additions of reinforcement into matrix Al6061 will increases wettability of the matrix and ceramic reinforcement and there will be uniform distribution of the particles. Similarly, 9 wt. % of graphite particulates reinforced Al6061 alloy composites were prepared by same procedure. Thus, the objective of this work was to investigate the effect of the Al2O3 and Graphite particles on the properties Al6061 alloy composites.
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Fig. 2.Stir casting equipment
2.3. Characterization and Testing of Composites The microstructural study was carried out on the investigating composites using scanning electron microscope. Samples around 5 mm diameter cut from the castings and were polished properly. Keller’s reagent was used to etch the samples. Indentation response of as cast Al matrix alloy and its micro composites were evaluated by micro Vickers hardness testing machine. The required specimens were prepared according to standard metallographic procedures. The experiments were carried out by applying a load of 100gms and dwell time of 15 seconds. The indentation load depth values were recorded and the Vickers hardness was determined. For each sample, the indentation test was repeated 10 times and the averaged data were reported. Tensile specimens were machined from the cast samples. The tensile specimens of circular cross section with a diameter of 9 mm and gauge length of 45mm were prepared according to the ASTM E8 standard testing procedure. The tests were conducted on a universal testing machine. All the tests were conducted in a displacement control mode at a rate of 0.1 mm/min. Multiple tests were conducted and the best results were averaged. Various tensile properties like ultimate tensile strength, yield strength and percentage elongation were evaluated for as cast Al6061 alloy, Al6061-9 wt. % Al2O3 and Al6061- 9 wt. % graphite composites. Figure 3 showing the tensile test specimen dimensions used to conduct the experiments.
Fig. 3.Showing the dimensions of tensile test specimen.
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3. Results and Discussion 3.1. Microstructure Study
(a)
(b)
(c) Fig. 4.Scanning Electron Micro-photographs (a) as cast Al6061 alloy (b) Al6061-9 wt. % Al2O3 (c) Al6061-9 wt. % Graphite composites.
Figure 4 (a-c) shows the SEM (Scanning Electron Microscope) micro photographs of as cast Al6061 alloy (Fig. 4-a), Al6061 with 9 wt. % Al2O3 (Fig. 4-b) and 9 wt. % Graphite (Fig. 4-c) composites. Figures 4(b-c) reveals the distribution of Al2O3 and Graphite particulates in different specimens and it can be observed that there is fairly uniform distribution of particles. Further, from the photographs, it is evident that composites show very low agglomeration; this is mainly due to the adoption of two stage reinforcement mixing method. In this method instead of adding entire reinforcement at once, it is added twice by dividing entire calculated amount of reinforcement into two portions. Figure 4 (b-c) also showing the good bonding between the Al6061 alloy and Al2O3 and Graphite particles. This good bonding between matrix and ceramic constituents enhances the mechanical properties of the Al6061 alloy [13].
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Figure 5a-b shows energy dispersive X-Ray spectrographs of Al6061-9 wt. % of Al2O3 and Al6061-9 wt. % of graphite composites respectively. The EDS analysis confirmed the presence of Al2O3and Graphite in the Al matrix alloy. The presence of Al2O3 in the Al alloy matrix confirmed by the O (Oxygen) content in EDS analysis. The presence of graphite shows in the form of Carbon (C), which is evident from the EDS graph 5b.
(a)
(b)
Fig. 5.Energy Dispersive Spectrographs of (a) Al6061-9 wt. % Al2O3 and (b) Al60621-9 wt. % Graphite composites.
3.2. Density Theoretical Density Experimental Density 2.78 2.76 2.74
Density (g/cm3)
2.72 2.70 2.68 2.66 2.64 2.62 2.60 2.58 Al6061
Al6061+9% Al2O3
Al6061+9% Gr
Al6061 Alloy and its composites
Fig. 6.Theoretical and experimental densities of Al6061 alloy and 9 wt. % Al2O3 and graphite composites.
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The measured densities of as cast Al6061 alloy, Al6061-9 wt. % Al2O3 and 9 wt. % Graphite composites are presented in the figure 6. It is observed that, by the addition of 9 wt. % of Al2O3 particles the density of the composite is slightly increased. This increase in density is mainly due to higher density of Al2O3 particles as compared to the base Al6061alloy. But, in the case of 9 wt. % graphite reinforced composites, it is lesser than the base alloy. This decrease in density of Al6061-graphite composite is due to lower density of graphite (2.20 g/cm3). Further, from figure 6, the experimental densities for both alloy and composites are in line with the theoretical densities but slightly lesser than the theoretical densities. 3.3. Hardness
Fig. 7.Hardness of Al6061 alloy and 9 wt. % Al2O3 and graphite composites.
In the present work, hardness values of the Al6061 alloy, Al6061- 9 wt. % Al2O3 and 9 wt. % graphite composites have been obtained by Vickers hardness tester. The variation of hardness with Al alloy and its composite is shown in figure 7. It is noticed that the hardness of Al6061-9 wt. % Al2O3 composite is more than Al6061 alloy and 9 wt. % of graphite reinforced composites. A notable rise in the hardness of the alloy matrix can be seen with the addition of Al2O3 particles. This is mainly due to the presence of Al2O3 particles in the matrix Al6061 alloy. Whenever a hard reinforcement is incorporated into a soft ductile matrix, the hardness of the matrix material is enhanced. It is evident from the figure 7 shows decrease in the hardness as the graphite reinforcement content is added. This drop in the hardness is due to softness of the graphite particles, which being soft dispersed do not contribute to the hardness of the composite as it cannot act as barriers to the movement of dislocations within the matrix. The graphite particles being softer than the matrix metal which cause reduction in the hardness by about 9%. 3.4. Ultimate Tensile and Yield Strength Figure 8 shows variation of ultimate tensile and yield strength (YS) with 9 wt. % of Al2O3 and 9 wt. % of graphite particulates reinforced Al6061 alloy composites. The ultimate tensile strength (UTS) of Al6061-9 wt. % Al2O3 and Al6061-9 wt. % of graphite composite material increases by an amount of 22.69% and 35% respectively as compared to as cast Al6061 alloy matrix. The microstructure and properties of hard oxide Al2O3 particulates control the mechanical properties of the composites. Due to the strong interface bonding load from the matrix transfers to the reinforcement exhibiting increased ultimate tensile strength [14].
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This increase in UTS and YS mainly is due to Al2O3 and graphite particles acting as barrier to dislocations in the microstructure. The improvement in strength may be due to the matrix strengthening following a reduction in Al6061-Al2O3 and Al6061-graphite grain size, and the generation of a high dislocation density in the Al6061 alloy matrix a result of the difference in the thermal expansion between the metal matrix and the Al2O3 and graphite reinforcement.
200 190
Ultimate Tensile Strength Yield Strength
Strength (MPa)
180 170 160 150 140 130 120 Al6061
Al6061+9% Al2O3
Al6061+9% Gr
Al6061 Alloy and its composites
Fig. 8. Tensile strength of Al6061 alloy and 9 wt. % Al2O3 and graphite composites.
3.5.
Percentage Elongation
Fig. 9. Percentage elongation of Al6061 alloy and 9 wt. % Al2O3 and graphite composites
Figure 9 is a graph showing the effect of Al2O3 and graphite content on the percentage elongation (ductility) of the composites. It can be seen from the graph that the ductility of the composites decrease significantly with the 9 wt. % Al2O3 reinforced composites. This decrease in percentage elongation in comparison with the base alloys is a most commonly occurring disadvantage in particulate reinforced metal matrix composites. The reduced ductility in Al6061-9 wt. % composites can be attributed to the presence of Al2O3 particulates which may get fractured and have sharp corners that make the composites prone to localised crack initiation and propagation. The embrittlement effect
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that occurs due to the presence of the hard ceramic particles causing increased local stress concentration sites may also be the reason. Figure 9 also showing the effect of graphite content on the ductility of the Al alloy composite. It can be seen that as the graphite content increases, the ductility of the composite material increases by significant amount. This increase in ductility is mainly due to soft nature of graphite and makes the Al6061 alloy to undergo more plastic deformation before fracture. 4. Conclusions The present work entitled, “Mechanical behaviour of Al6061-Al2O3 and Al6061-Graphite composites” has led to the following conclusions: • Al6061- Al2O3 and Al6061-Graphite composites with 9 wt. % were successfully fabricated via melt stirring method involving two step additions. • Two stage addition methods is adopted for introducing Al2O3 and Graphite particulates into Al6061 matrix during melt stirring has resulted in homogeneous distribution of Al2O3 and Graphite particulates with no clustering or agglomeration as evident from SEM microphotographs. • The Energy Dispersive (EDS) analysis revealed the presence of Al2O3 and Graphite particles in Al6061- Al2O3-Graphite composites. • The density of Al6061-Al2O3 composites has increased after the addition of 9 wt. % Al2O3 particles into Al6061 alloy matrix. • The density of Al6061-Graphite composite has decreased after the addition of Graphite particulates into aluminium matrix. • Al6061- Al2O3 composites have shown higher hardness when compared to the hardness of Al6061 alloy. • The hardness of Al6061-Graphite composites decreases with 9 wt. % of Graphite particulates in the Al6061 alloy matrix. • Improvements in ultimate tensile strength of the Al6061 matrix were obtained with the addition of Al2O3 particulates. The extent of improvement obtained in Al6061 alloy after addition 9 wt. % of Al2O3 particulates were 22.69%. • Improvements in ultimate tensile strength of the Al6061 matrix were obtained with the addition of Graphite particulates. The extent of improvement obtained in Al6061 alloy after addition 9 wt. % of Graphite particulates were 35%. • Percentage elongation of Al6061-9 wt. % of graphite composites is more compared to base alloy and Al2O3 reinforced composites. References [1] R. Ezatpour, Torabi, S. A. Sajjadi, Transactions of Nonferrous Metals Society of China, 23 (2013), 1262-1268. [2] S. A. Sajjadi, H. R. Ezatpour, M. Torabi, Materials and Design, 34 (2012), 106-111. [3] Ranjith Bauri, M. K. Surappa, Science and Technology of Advanced Materials, 8 (2007), 494-502. [4] G. B. Veeresh Kumar, C. S. P. Rao, N. Selvaraj, Composites Part B, 43 (2012), 1185-1191. [5] B. M. Girish, K. R. Prakash, B. M. Satish, Jain, Materials and Design, 32 (2011), 1050-1056. [6] J. Hashim, L. Looney, Hashmi, Journal of Materials Processing Technology, 123 (2002), 251-257. [7] K. M. Shorowordi, T. Laoui, Haseeb, J. P. Celis, L. Froyen, Journal of Materials Processing Technology, 142 (2003), 738-743. [8] Linlin Yuan, Jingtao Han, Liu, Zhengyi Jiang, Tribology International, 98 (2016), 41-47. [9] Shisheng Li, Yishi Su, Xinhai Zhu, Huiling Jin, Quibao Ouyang, Di Zhang, Materials and Design, 107 (2016), 130-138. [10] R. Harichandran , N. Selvakumar, Archives of Civil and Mechanical Engineering, 16 (2016), 147-158. [11] D. B. Miracle, Composites Science and Technology, 65 (2005), 2526-2540. [12] K. Naplocha, K. Granat, Achieves of Materials Science and Engineering, Vol. 29, 2 (2008), 81-88. [13] K. Umanath, K. Palanikumar, Selvamani, Composites Part B, 53 (2013), 159-168. [14] F. Akhlaghi, S. A. Pelaseyyed, Materials Science and Engineering A, 385 (2004), 258-266.
ScienceDirect Materials Today: Proceedings 4 (2017) 10978–10986
www.materialstoday.com/proceedings
AMMMT 2016
Mechanical Behavior of Al6061-Al2O3 and Al6061-Graphite Composites Madeva Nagarala*, Shivananda B Kb, V Auradia, K I Parashivamurthyc, S A Korid, a
Department of Mechanical Engineering, Siddaganga Institute of Technology, Tumkur-572103, Karnataka, India b Department of Mechanical Engineering, UVCE, Bangalore-560001, Karnataka, India c Department of Mechanical Engineering, Government Engineering College, Chamarajnagar-571313, Karnataka, India d Department of Mechanical Engineering, Basaveshwar Engineering College, Bagalkot-587102, Karnataka, India
Abstract This work investigated the influence of Al2O3 and graphite on the microstructure and mechanical behavior of Al6061- Al2O3 and Al6061Graphite composites. The investigation reveals the effectiveness of incorporation of Al2O3 and Graphite in the Al6061 alloy for studying mechanical properties. The composites were fabricated using liquid metallurgy route. The Al6061- Al2O3 and Al6061-Graphite composites were fabricated separately by introducing 9 wt. % of Al2O3 and graphite particulates by two stage melt stirring process. In this reinforcement particulates were added in two steps to increase the wettability. The characterization was performed through Scanning Electron Microscope and Energy Dispersive Spectrum. The particle distribution was uniform in these composites. The grains were refined by addition of Al2O3 and Graphite particulates. The density, hardness, ultimate tensile strength, yield strength and percentage elongation of both Al2O3 and Graphite composites were evaluated as per ASTM standards. Further, a comparative study has been made between the Al6061- Al2O3 and Al6061Graphite composites.
© 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of Advanced Materials, Manufacturing, Management and Thermal Science (AMMMT 2016). Keywords: Al6061 Alloy; Al2O3; Graphite; Microstructure; Mechanical Properties
* Corresponding author. Tel. +91 9738304925 E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of Advanced Materials, Manufacturing, Management and Thermal Science (AMMMT 2016).
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1. Introduction Metal matrix composites (MMCs) are combinations of two or more materials (one of which is a metal) where tailored properties are achieved by systematic combinations of different constituents [1]. The commonly used metallic matrices include aluminum, magnesium, titanium, and their alloys. Metal matrix composites (MMCs) are being increasingly used in aerospace and automobile industries owing to their enhanced properties such as elastic modulus, hardness, tensile strength, wear resistance combined with significant weight savings over unreinforced alloys [2]. Aluminium and its alloys occupy third place among the commercially used engineering materials. Aluminum matrix composites have attracted enormous attention because of their high specific modulus, specific strength and excellent dimension stability. Furthermore, they have been considered as promising candidate for lightweight armors and protective materials [3, 4]. The interface between the matrix and the reinforcement plays an important role in determination of the properties of the MMCs. The advantages of particle-reinforced composites over others are their formability with cost advantage. Further, they have inherent heat and wear resistant properties [5]. A large number of fabrication techniques are currently used to manufacture the MMC materials. In the engineering materials, the MMCs can be manufactured by a unique technique such as casting as it is inexpensive and proposes many other options for materials and processing conditions. Now a day’s Aluminium alloy based composites finding wide range of applications in Aerospace domain due to light in weight compared to steel and titanium. Al alloy based metal matrix composites, are a new class of advanced materials whose properties can be tailored to satisfy the design needs by varying the reinforcements, matrices and the microstructure [6]. After about 25 years of developments for aerospace applications metal matrix composites are coming out from laboratories into commercial markets. There are several Al alloy matrices like AA2014, AA2024, AA2219, AA2618, AA6061, AA6068, AA7050, AA7025 and AA7075 are widely using aluminium alloys in an aerospace domain. The main purpose of using these alloys in aerospace is mainly due to their ageing characteristics. The required strength of the component can be achieved by heat treatment process. Further, several particulate and whisker reinforcements like SiC, Al2O3, TiC, B4C, WC and TiO2 are available as strengthening particulates for Al alloys [7]. Optimization or weight reduction plays very important role in aerospace domain. By using Al alloys alone, it is necessary to use more cross-sectional area to meet required strength, which increases the weight of the component. Since, performance of the aircraft is directly proportional to the weight of aircraft. So, it is necessary to develop lighter structures or components ready to provide required strength and stiffness in minimum cross section area. In this particular area Metal Matrix Composites play important role. So, it is necessary to develop newer materials with high strength and stiffness without increasing processing cost. Several researchers investigated the various properties of aluminium based metal matrix composites. Linlin et al. [8] investigated mechanical and tribological behavior of aluminium metal matrix composites reinforced with in situ AlB2 particles. Composites were prepared by hot rolling and solution treatment. Mechanical properties like tensile and micro hardness measurements were evaluated. The friction co-efficient, wear behavior and scratch morphology of the MMCs and pure aluminium were studied. The tensile, hardness and wear properties are higher in case of composites compared to unreinforced Al alloy. Shisheng and co-authors [9] studied the mechanical behavior of in situ carbon nano tube and silicon carbide reinforced Al6061 aluminium matrix composites. Results show that significant improvement in tensile properties with the SiC size to be 7 microns, which has ductility of 8.5% and Young’s modulus of 98 GPa, and tensile strength of 428 MPa. Harichandran and Selvakumar [10] carried out experiments on nano and micro B4C reinforced pure aluminium matrix composites. The micro and nano composites containing different weight percentages of B4C particles were fabricated by stir and ultrasonic cavitation assisted casting process. Tensile, hardness, impact and wear tests were carried out to evaluate mechanical properties of the micro and nano composites. The properties were enhanced in the case of B4C reinforced composites compared to Al alloy matrix. Very less literature is available to improve the wettability of reinforcements with base matrix. In the present investigation castings were done by using two steps particulates mixing process.
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In the present work an attempt is being made to process Al6061- 9 wt. % Al2O3 and Al6061-9 wt. % graphite particulate reinforced composites by two steps reinforcement mixing by using liquid stir casting technique. In this research the tensile properties of composites like hardness, ultimate tensile strength, yield strength and percentage elongations were evaluated as per ASTM standards and were compared with base alloy Al6061. 2. Experimental Details 2.1. Materials Used Al6061 alloy was used as a matrix material and Al2O3 and graphite particles with an average particle size 90-100 µm were used as reinforcement. Liquid stir casting method was used to fabricate composites. In order to enhance the wettability, the two step addition method used in the present work which helps to overcome the problem of agglomeration of reinforcing particles [11]. Table 1 represents the chemical composition of base alloy. Figure 1 shows the scanning electron microphotographs of Al2O3 and graphite particulates used to fabricate the composites. Table1. Chemical composition of Al6061 alloy in weight percentage Mg
Si
Fe
Cu
Zn
Mn
Ti
Cr
Al
0.82
0.64
0.23
0.17
0.03
0.07
0.01
0.01
Bal
(a)
(b)
Fig. 1.Shows scanning electron micrographs of (a) Al2O3 particles (b) Graphite particles
2.2. Fabrication Procedure The Al6061-9 wt. % Al2O3 and Al6061-9 wt. % graphite particulates composites were prepared using liquid stir casting technique. The equipment used for casting is shown in figure 2. Initially required amount of charge or matrix material was placed in a silicon carbide crucible, which was placed in electric resistance furnace at a temperature of around 750°C. After complete melting of Al6061 matrix, degassing was carried out by using solid hexachloro ethane degassing tablet, which helps to remove unwanted adsorbed gases from the melt [12]. Once degassing is over, the preheated ceramic reinforcement Al2O3 particles were introduced into matrix, which involves two-stage additions of reinforcement during stirring. This novel two stage additions of reinforcement into matrix Al6061 will increases wettability of the matrix and ceramic reinforcement and there will be uniform distribution of the particles. Similarly, 9 wt. % of graphite particulates reinforced Al6061 alloy composites were prepared by same procedure. Thus, the objective of this work was to investigate the effect of the Al2O3 and Graphite particles on the properties Al6061 alloy composites.
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Fig. 2.Stir casting equipment
2.3. Characterization and Testing of Composites The microstructural study was carried out on the investigating composites using scanning electron microscope. Samples around 5 mm diameter cut from the castings and were polished properly. Keller’s reagent was used to etch the samples. Indentation response of as cast Al matrix alloy and its micro composites were evaluated by micro Vickers hardness testing machine. The required specimens were prepared according to standard metallographic procedures. The experiments were carried out by applying a load of 100gms and dwell time of 15 seconds. The indentation load depth values were recorded and the Vickers hardness was determined. For each sample, the indentation test was repeated 10 times and the averaged data were reported. Tensile specimens were machined from the cast samples. The tensile specimens of circular cross section with a diameter of 9 mm and gauge length of 45mm were prepared according to the ASTM E8 standard testing procedure. The tests were conducted on a universal testing machine. All the tests were conducted in a displacement control mode at a rate of 0.1 mm/min. Multiple tests were conducted and the best results were averaged. Various tensile properties like ultimate tensile strength, yield strength and percentage elongation were evaluated for as cast Al6061 alloy, Al6061-9 wt. % Al2O3 and Al6061- 9 wt. % graphite composites. Figure 3 showing the tensile test specimen dimensions used to conduct the experiments.
Fig. 3.Showing the dimensions of tensile test specimen.
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3. Results and Discussion 3.1. Microstructure Study
(a)
(b)
(c) Fig. 4.Scanning Electron Micro-photographs (a) as cast Al6061 alloy (b) Al6061-9 wt. % Al2O3 (c) Al6061-9 wt. % Graphite composites.
Figure 4 (a-c) shows the SEM (Scanning Electron Microscope) micro photographs of as cast Al6061 alloy (Fig. 4-a), Al6061 with 9 wt. % Al2O3 (Fig. 4-b) and 9 wt. % Graphite (Fig. 4-c) composites. Figures 4(b-c) reveals the distribution of Al2O3 and Graphite particulates in different specimens and it can be observed that there is fairly uniform distribution of particles. Further, from the photographs, it is evident that composites show very low agglomeration; this is mainly due to the adoption of two stage reinforcement mixing method. In this method instead of adding entire reinforcement at once, it is added twice by dividing entire calculated amount of reinforcement into two portions. Figure 4 (b-c) also showing the good bonding between the Al6061 alloy and Al2O3 and Graphite particles. This good bonding between matrix and ceramic constituents enhances the mechanical properties of the Al6061 alloy [13].
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Figure 5a-b shows energy dispersive X-Ray spectrographs of Al6061-9 wt. % of Al2O3 and Al6061-9 wt. % of graphite composites respectively. The EDS analysis confirmed the presence of Al2O3and Graphite in the Al matrix alloy. The presence of Al2O3 in the Al alloy matrix confirmed by the O (Oxygen) content in EDS analysis. The presence of graphite shows in the form of Carbon (C), which is evident from the EDS graph 5b.
(a)
(b)
Fig. 5.Energy Dispersive Spectrographs of (a) Al6061-9 wt. % Al2O3 and (b) Al60621-9 wt. % Graphite composites.
3.2. Density Theoretical Density Experimental Density 2.78 2.76 2.74
Density (g/cm3)
2.72 2.70 2.68 2.66 2.64 2.62 2.60 2.58 Al6061
Al6061+9% Al2O3
Al6061+9% Gr
Al6061 Alloy and its composites
Fig. 6.Theoretical and experimental densities of Al6061 alloy and 9 wt. % Al2O3 and graphite composites.
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The measured densities of as cast Al6061 alloy, Al6061-9 wt. % Al2O3 and 9 wt. % Graphite composites are presented in the figure 6. It is observed that, by the addition of 9 wt. % of Al2O3 particles the density of the composite is slightly increased. This increase in density is mainly due to higher density of Al2O3 particles as compared to the base Al6061alloy. But, in the case of 9 wt. % graphite reinforced composites, it is lesser than the base alloy. This decrease in density of Al6061-graphite composite is due to lower density of graphite (2.20 g/cm3). Further, from figure 6, the experimental densities for both alloy and composites are in line with the theoretical densities but slightly lesser than the theoretical densities. 3.3. Hardness
Fig. 7.Hardness of Al6061 alloy and 9 wt. % Al2O3 and graphite composites.
In the present work, hardness values of the Al6061 alloy, Al6061- 9 wt. % Al2O3 and 9 wt. % graphite composites have been obtained by Vickers hardness tester. The variation of hardness with Al alloy and its composite is shown in figure 7. It is noticed that the hardness of Al6061-9 wt. % Al2O3 composite is more than Al6061 alloy and 9 wt. % of graphite reinforced composites. A notable rise in the hardness of the alloy matrix can be seen with the addition of Al2O3 particles. This is mainly due to the presence of Al2O3 particles in the matrix Al6061 alloy. Whenever a hard reinforcement is incorporated into a soft ductile matrix, the hardness of the matrix material is enhanced. It is evident from the figure 7 shows decrease in the hardness as the graphite reinforcement content is added. This drop in the hardness is due to softness of the graphite particles, which being soft dispersed do not contribute to the hardness of the composite as it cannot act as barriers to the movement of dislocations within the matrix. The graphite particles being softer than the matrix metal which cause reduction in the hardness by about 9%. 3.4. Ultimate Tensile and Yield Strength Figure 8 shows variation of ultimate tensile and yield strength (YS) with 9 wt. % of Al2O3 and 9 wt. % of graphite particulates reinforced Al6061 alloy composites. The ultimate tensile strength (UTS) of Al6061-9 wt. % Al2O3 and Al6061-9 wt. % of graphite composite material increases by an amount of 22.69% and 35% respectively as compared to as cast Al6061 alloy matrix. The microstructure and properties of hard oxide Al2O3 particulates control the mechanical properties of the composites. Due to the strong interface bonding load from the matrix transfers to the reinforcement exhibiting increased ultimate tensile strength [14].
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This increase in UTS and YS mainly is due to Al2O3 and graphite particles acting as barrier to dislocations in the microstructure. The improvement in strength may be due to the matrix strengthening following a reduction in Al6061-Al2O3 and Al6061-graphite grain size, and the generation of a high dislocation density in the Al6061 alloy matrix a result of the difference in the thermal expansion between the metal matrix and the Al2O3 and graphite reinforcement.
200 190
Ultimate Tensile Strength Yield Strength
Strength (MPa)
180 170 160 150 140 130 120 Al6061
Al6061+9% Al2O3
Al6061+9% Gr
Al6061 Alloy and its composites
Fig. 8. Tensile strength of Al6061 alloy and 9 wt. % Al2O3 and graphite composites.
3.5.
Percentage Elongation
Fig. 9. Percentage elongation of Al6061 alloy and 9 wt. % Al2O3 and graphite composites
Figure 9 is a graph showing the effect of Al2O3 and graphite content on the percentage elongation (ductility) of the composites. It can be seen from the graph that the ductility of the composites decrease significantly with the 9 wt. % Al2O3 reinforced composites. This decrease in percentage elongation in comparison with the base alloys is a most commonly occurring disadvantage in particulate reinforced metal matrix composites. The reduced ductility in Al6061-9 wt. % composites can be attributed to the presence of Al2O3 particulates which may get fractured and have sharp corners that make the composites prone to localised crack initiation and propagation. The embrittlement effect
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that occurs due to the presence of the hard ceramic particles causing increased local stress concentration sites may also be the reason. Figure 9 also showing the effect of graphite content on the ductility of the Al alloy composite. It can be seen that as the graphite content increases, the ductility of the composite material increases by significant amount. This increase in ductility is mainly due to soft nature of graphite and makes the Al6061 alloy to undergo more plastic deformation before fracture. 4. Conclusions The present work entitled, “Mechanical behaviour of Al6061-Al2O3 and Al6061-Graphite composites” has led to the following conclusions: • Al6061- Al2O3 and Al6061-Graphite composites with 9 wt. % were successfully fabricated via melt stirring method involving two step additions. • Two stage addition methods is adopted for introducing Al2O3 and Graphite particulates into Al6061 matrix during melt stirring has resulted in homogeneous distribution of Al2O3 and Graphite particulates with no clustering or agglomeration as evident from SEM microphotographs. • The Energy Dispersive (EDS) analysis revealed the presence of Al2O3 and Graphite particles in Al6061- Al2O3-Graphite composites. • The density of Al6061-Al2O3 composites has increased after the addition of 9 wt. % Al2O3 particles into Al6061 alloy matrix. • The density of Al6061-Graphite composite has decreased after the addition of Graphite particulates into aluminium matrix. • Al6061- Al2O3 composites have shown higher hardness when compared to the hardness of Al6061 alloy. • The hardness of Al6061-Graphite composites decreases with 9 wt. % of Graphite particulates in the Al6061 alloy matrix. • Improvements in ultimate tensile strength of the Al6061 matrix were obtained with the addition of Al2O3 particulates. The extent of improvement obtained in Al6061 alloy after addition 9 wt. % of Al2O3 particulates were 22.69%. • Improvements in ultimate tensile strength of the Al6061 matrix were obtained with the addition of Graphite particulates. The extent of improvement obtained in Al6061 alloy after addition 9 wt. % of Graphite particulates were 35%. • Percentage elongation of Al6061-9 wt. % of graphite composites is more compared to base alloy and Al2O3 reinforced composites. References [1] R. Ezatpour, Torabi, S. A. Sajjadi, Transactions of Nonferrous Metals Society of China, 23 (2013), 1262-1268. [2] S. A. Sajjadi, H. R. Ezatpour, M. Torabi, Materials and Design, 34 (2012), 106-111. [3] Ranjith Bauri, M. K. Surappa, Science and Technology of Advanced Materials, 8 (2007), 494-502. [4] G. B. Veeresh Kumar, C. S. P. Rao, N. Selvaraj, Composites Part B, 43 (2012), 1185-1191. [5] B. M. Girish, K. R. Prakash, B. M. Satish, Jain, Materials and Design, 32 (2011), 1050-1056. [6] J. Hashim, L. Looney, Hashmi, Journal of Materials Processing Technology, 123 (2002), 251-257. [7] K. M. Shorowordi, T. Laoui, Haseeb, J. P. Celis, L. Froyen, Journal of Materials Processing Technology, 142 (2003), 738-743. [8] Linlin Yuan, Jingtao Han, Liu, Zhengyi Jiang, Tribology International, 98 (2016), 41-47. [9] Shisheng Li, Yishi Su, Xinhai Zhu, Huiling Jin, Quibao Ouyang, Di Zhang, Materials and Design, 107 (2016), 130-138. [10] R. Harichandran , N. Selvakumar, Archives of Civil and Mechanical Engineering, 16 (2016), 147-158. [11] D. B. Miracle, Composites Science and Technology, 65 (2005), 2526-2540. [12] K. Naplocha, K. Granat, Achieves of Materials Science and Engineering, Vol. 29, 2 (2008), 81-88. [13] K. Umanath, K. Palanikumar, Selvamani, Composites Part B, 53 (2013), 159-168. [14] F. Akhlaghi, S. A. Pelaseyyed, Materials Science and Engineering A, 385 (2004), 258-266.