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Fly-ash composite using stir casting method
Materials Today: Proceedings xxx (xxxx) xxx
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Synthesis, microstructure and hardness of Al 7075/B4C/Fly-ash composite using stir casting method Mohit Kumar Sahu, Raj Kumar Sahu ⇑ National Institute of Technology Raipur, G.E. Road, Raipur, Chhattisgarh 492010, India
a r t i c l e
i n f o
Article history: Received 14 September 2019 Accepted 23 September 2019 Available online xxxx Keywords: Hybrid aluminum matrix composites (HAMCs) Aluminum 7075 Stir-casting Homogeneous distribution Micro-hardness
a b s t r a c t Stir-casting is the appropriate and economical technique for HAMCs synthesis as it produces uniformly reinforcement distributed composite retaining finer grains. In the present work, Al 7075/x%wt.B4C/2% wt. fly-ash HAMCs were fabricated via stir casting route, where x is weight percentage of boron carbide (x = 2, 4, 6 and 8 wt%). Incorporation and distribution of reinforcement particles in HAMCs were analysed using scanning electron microscope (SEM) and optical micrographs. Also, Micro-hardness tests were conducted at seven locals points in hardness specimen to see the variation of hardness values. The coefficient of variation was calculated to ensure uniform hardness values, which further confirm the uniform distribution of reinforcements in the prepared composite. Moreover, mathematical modelling was done to obtain the empirical relation of micro-hardness of composite with respect to fly-ash and boron carbide content, which shows the correlation between the micro-hardness and reinforcements contents in the HAMC. The synthesized HAMCs may be suitable in automobile and aerospace application where low cost, lightweight, high temperature resistant and high wear resistant composite are required. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International conference on Materials and Manufacturing Methods.
1. Introduction Hybrid aluminum matrix composites (HAMCs) are attracting attentions of automobile, aerospace and electronics industries due to their adaptive mechanical, metallurgical and tribological properties by appropriate selection of materials and synthesis technique. Homogeneous properties throughout the material are essential in most of the engineering applications, which may be achieved when the reinforcements particles suspend uniformly over the aluminum matrix. Present engineering applications necessitate materials that are robust, lightweight and less costly. Hybrid aluminum matrix composites meet these necessities and has high strength to mass ratio, good tribological characteristics, less expensive and suitable for automobile and aerospace applications [1,2]. HAMCs are usually manufactured by two routes i.e. solid-state
Abbreviations: AMCs, Aluminium matrix composites; HAMCs, Hybrid aluminium matrix composites; Al 7075, Aluminium alloy 7075; FA, Fly-Ash; SC, Stir Casting; PM, Powder Metallurgy; B4C, Boron Carbide; Al, Aluminium; K2TiF6, Potassium hexa fluoro titanate; Mg, Magnesium. ⇑ Corresponding author. Tel.:+91 771 2254200; fax:+91 771 2254600. E-mail address: [email protected] (R.K. Sahu).
and liquid state manufacturing processes. Solid-state process comprises powder metallurgy (PM) techniques whereas liquid state process includes compo-casting, squeeze casting and preferably stir-casting (SC) techniques. Stir casting method has stayed the most explored technique for producing HAMCs due to its ease of production, flexibility and less expensive. But, wettability between molten metal and ceramics particles are key challenges in stir casting, which is reported by many researchers. Zheng et al. [3] reported poor wettability of B4C in aluminum due to the reaction of boron carbide with molten Al and produce Al3BC and AlB2 phases, which results in rapid production of secondary phases in the melt. Baradeswaran et al. also reported difficulty in fabricating Al–B4C composite due to poor wettability of boron carbide and aluminum below 1100 °C, he further suggested controlling of the interface of Al–B4C. Raj et al. [4], Reddy et al. [1], and Canute et al. [5] also reported the same problem, this problem was resolved by the addition of magnesium and potassium hexafluoro titanate (K2TiF6) into the melt. Adding magnesium to the aluminum melt reduces the surface tension of the melt and enhance the wettability between fly-ash and aluminum [6–8]. Fly has SiO2 and Al2O3 as its major components, Rajan et al. [9] reported
https://doi.org/10.1016/j.matpr.2019.09.150 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International conference on Materials and Manufacturing Methods.
Please cite this article as: M. K. Sahu and R. K. Sahu, Synthesis, microstructure and hardness of Al 7075/B4C/Fly-ash composite using stir casting method, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.150
2
M.K. Sahu, R.K. Sahu / Materials Today: Proceedings xxx (xxxx) xxx
the formation of MgAl2O4 by the reaction of aluminum with SiO2 and Al2O3, which changes the composition of reinforcement in the composite, which is shown in reactions (1) and (2) [10].
2AlðlÞ þ Mg ðlÞ þ 2SiO2ðsÞ ! MgAl2 O4ðsÞ þ 2SiðsÞ
ð1Þ
2Mg ðlÞ þ 4Al2 O3 ! 3MgAl2 O4ðsÞ þ 2AlðlÞ
ð2Þ
K2TiF6 flux increases the wettability of B4C particles with aluminium melt. It also improves the stability of boron carbide particles in the aluminium during the re-melt of the composite. K2TiF6 flux removes oxide films from the aluminium melt surface, removal of the oxide layer promotes the reaction of B4C particles rapidly with aluminium as per the reaction (3).
3K 2 TiF 6 þ 13Al ! 3Al3 Ti þ 3KAlF 4 þ K 3 AlF 6
ð3Þ
9Al þ 2B4 C ! 2Al3 BC þ 3AlB2
ð4Þ
3Al3 Ti þ B4 C ! 2TiB2 þ TiC þ 9Al
ð5Þ
Al3 Ti þ AlB2 ! TiB2 þ 4Al
ð6Þ
Moreover, compounds formed from reactions (4) and (5) provides Ti-rich layer cover and prevented the boron carbide particles from additional decomposing [11]. Table 1 shows the selection of materials and optimal stir casting parameters and temperatures for the fabrication of HAMCs. Lowcost, lightweight, high temperature resistant and high wear resistant composites are the current need of automobiles, aeronautical and electronics industries. Alumnium7075/ boron carbide/ fly-ash HAMC is the impeccable combination to fulfil demands of these industries, but the incorporation of boron carbide and fly ash particles uniformly into Al 7075 matrix itself a challenge due to low wettability between two phases and culturing of reinforcement particles. This paper may guide the research community, the synthesis of boron carbide and fly-ash reinforced HAMCs retaining uniform particle distribution.
Table 1 Selection of material, optimal stir-casting parameters and temperature.
2. Materials and methods 2.1. Material selection Aluminum 7075 were selected as base matrix due to tremendous properties such as high-temperature resistance, reasonably high tensile strength, good toughness, and is appropriate for aerospace and automobile applications. Boron carbide was selected as primary, whereas fly ash was selected as secondary reinforcement in this investigation. Boron carbide is enormously hard and is used in tribological applications, the incorporation of boron carbide into aluminum matrix enhance the properties of the developed composites [16]. Fly- ash is inexpensive industrial-waste as well it enhances mechanical properties due to the presence of SiO2, Al2O3 and TiO2 in the fly-ash [10,17]. The chemical composition of Aluminium 7075 and fly-ash are shown in Tables 2 and 3 respectively [16]. The particle size of boron carbide and fly-ash were taken in the range of 3–10 mm. 2.2. Synthesis of composite The wettability of boron carbide with aluminium is suitable at 1100 °C. Besides, such high-temperature processing leads to undesirable chemical reactions between boron carbide and aluminium and form adverse compounds like Al4C3, AlB2 and Al3BC. This problem was resolved by incorporating magnesium and potassium hexafluoro titanate (K2TiF6) flux into the melt to enhance the wettability [4,10]. The amount of K2TiF6 flux is identical to the amount of Born carbide added [12]. Al7075/x% B4C/2% Fly-ash HAMCs, where x is the weight percentage of boron carbide i.e. 2, 4, 6 and 8 wt. %, were fabricated using stir casting setup as shown in Fig. 1. Initially, 700 g Al 7075 was kept inside the furnace and the temperature was set to 850 °C in a protective atmosphere of argon gas. Simultaneously, reinforcement mixture i.e. x % wt. boron carbide and 2% wt. fly-ash with wetting agents magnesium (2% wt.) and K2TiF6 (equal to boron carbide weight) were preheated. After Preheating Al 7075, boron carbide, fly-ash, magnesium and K2TIF6 for 2 h, the graphite stirrer impeller of blade angle 30° with stainless steel rod was introduced into the aluminium melt in order to form the vortex at a constant speed at 550 rpm [2,13]. Then, reinforcements (boron carbide and fly-ash) was started feeding using the feeder and simultaneously magnesium and potassium hexafluoro titanate (K2TiF6) flux was used in the aluminium melt to increase the wettability [12]. The vortex formed by mechanical stirring dispenses reinforcement particles in the melt and it gives a homogeneous distribution of the reinforcements over the matrix. Nomenclature and composition of the casted composite are listed in Table 4. Thereafter, specimens were prepared for optical micrograph and micro-hardness test. Details of the microhardness test are discussed further.
Parameters
Value
References
Boron Carbide content Fly-ash content Melting Temperature Preheating Temperature of B4C Preheating Temperature Flyash Stirring Time Impeller Blade Angle Impeller Blade Diameter Reinforcement feed rate Stirring Speed Wetting element
2, 4, 6 and 8 wt%
[1,4]
2 wt% 850 °C 300 °C
[5] [5,10,12] [4]
400 °C
[5]
10 min 30o 50–55% of the diameter of the crucible
[13,14] [13,14] [13,14]
0.8–1.5 g/s
[13,14]
Specimens were prepared from the casted composites for microstructural characterization and micro-hardness test. Prepared specimens were undertaken for standard metallographic polishing and etched with Keller’s reagent. Further, specimens were taken for microstructural analysis using SEM and optical micrographs to confirm the incorporation and dispersion of boron carbide and fly-ash into HAMCs.
550 rpm Magnesium and potassium hexa fluoro titanate (K2TiF6) Argon environment
[1,13,14] [1,12,15]
2.4. Micro-hardness test
Protective Environment
[3]
2.3. Microstructure
Micro-hardness of all specimens were taken at various locations (C0, L1, L2, L3, R1, R2, and R3) of the specimen as shown in Fig. 2.
Please cite this article as: M. K. Sahu and R. K. Sahu, Synthesis, microstructure and hardness of Al 7075/B4C/Fly-ash composite using stir casting method, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.150
3
M.K. Sahu, R.K. Sahu / Materials Today: Proceedings xxx (xxxx) xxx Table 2 Chemical composition of Al 7075 in wt. percent. Element
Zn
Mg
Cu
Fe
Si
Cr
Ni
Al
Wt. %
5.85
2.04
1.12
0.20
0.40
0.33
O.23
Remaining
Table 3 Chemical composition of fly ash in weight percent. Element
SiO2
Al2O3
MgO
Fe2O3
TiO2
CaO
K2O
Wt. %
61.04
24.96
0.57
6.84
2.82
1.0
2.77
Fig. 1. Schematic of the stir-casting setup.
Table 4 Nomenclature and composition of casted composites. Nomenclature
Composition
Composite Composite Composite Composite
Al Al Al Al
1 2 3 4
7075/2%wt. 7075/4%wt. 7075/6%wt. 7075/8%wt.
B4C/2%wt.FA B4C/2%wt.FA B4C/2%wt.FA B4C/2%wt.FA
The figure shows, C0 is the centre of the specimen and L1, L2 and L3 are the located at the left side of the specimen R1, R2 and R3 are the located at the right of the CO, whereas the distance between each location is 2 mm. 3. Results and discussion 3.1. Microstructure The reinforcement particles were identified by Energy Dispersive Spectrum (EDS) as shown in Fig. 3. Spectrum 1 shows the peaks of boron and carbon, which indicates boron carbide in the composite. Further, Spectrum 2 shows peaks of Si, Al, K, Ti, Mg,
Fig. 2. Micro-hardness sample.
Ca which indicates SiO2, Al2O3, K2O, TiO2, MgO, and CaO. These elements are the composition of fly-ash used, which confirm that the particle is fly-ash. Fig. 4(a), (b), (c) and (d) shows the microstructure of hybrid aluminium matrix composite with fixed fly-ash content of 2 wt% and varying boron carbide content of 2, 4, 6 and 8 wt% respectively. It illustrates a good chemical bonding among the Al particles, where Al particles joined together to construct a solid structure. It was also found that particles of fly-ash and boron carbide were incorporated in the composite between the grain boundaries whereas clustering of these particles are not observed, which
Table 5 Variation coefficient of micro-hardness at various locations of composite specimen. Composites
Al/2wt%FA/2%B4C Al/2wt%FA/4%B4C Al/2wt%FA/6%B4C Al/2wt%FA/8%B4C
Locations L3
L2
L1
C0
R1
R2
R3
104.0 109.0 111.4 118.0
98.3 105.0 111.8 122.0
97.2 110.0 116.0 122.0
99.0 107.9 115.0 126.0
105.0 109.9 114.0 123.0
100.6 110.0 120.0 126.0
102.0 115.0 121.0 126.0
Mean
Standard Deviation (n-1)
Coefficient of variation
Coefficient of variation (%)
101.7 110.7 117.5 125.3
2.548 3.025 3.512 1.500
0.0251 0.0273 0.0299 0.0120
2.51 2.73 2.99 1.20
Please cite this article as: M. K. Sahu and R. K. Sahu, Synthesis, microstructure and hardness of Al 7075/B4C/Fly-ash composite using stir casting method, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.150
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Fig. 3. SEM image and EDX pattern of Composite 1 (Al 7075/2% wt. FA/2% Wt. B4C) (a) SEM image of composite 1, (b) EDX pattern of spectrum 1, (c) EDX pattern of spectrum 2.
Fig. 4. Optical micrograph of composites: (a) composite 1: Al 7075/2% Wt. FA/2% Wt. B4C; (b) Composite 2: Al 7075/2% Wt. FA/4% Wt. B4C; (c) Composite 3: Al 7075/2% Wt. FA/6% Wt. B4C composite; (d) Composite 4: Al 7075/2% Wt. FA/8% Wt. B4C.
confirms that the elements are uniformly distributed throughout the Al 7075 matrix for all 4 composites. The uniform distribution was achieved due to optimal selection of stirring parameters. 3.2. Micro-hardness Micro-hardness test results of all composites and the casted matrix material are shown in Fig. 5, which shows the variation of micro-hardness values at various location of specimen surfaces for all composites as well as for casted Al 7075 matrix. The
variation of hardness in the casted specimen is common due to the dendritic formation during solidification. Incorporation of reinforcements particles increases the hardness of the specimen. Since micro-hardness value depends on the dispersion of reinforcements in composite, reasonably low variation of the micro-hardness value indicates the uniform distribution of reinforcements. The hardness of composite is increasing with the increase of boron carbide content, which can be recognized to the fact that the boron carbide and fly-ash possess higher hardness and its presence in the aluminium matrix improves the hardness. The variation of
Please cite this article as: M. K. Sahu and R. K. Sahu, Synthesis, microstructure and hardness of Al 7075/B4C/Fly-ash composite using stir casting method, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.150
M.K. Sahu, R.K. Sahu / Materials Today: Proceedings xxx (xxxx) xxx
5
Micro hardness ¼ 89:9 þ ð2:05 fly ash wt:%Þ þ ð3:67 Boron carbide wt:%Þ
Fig. 5. Variation of micro-hardness at various locations of samples and the average hardness.
ð7Þ
3.2.3. Comparison of micro-hardness The average micro-hardness (HV) values are higher than casted Al 7075 and increasing with the increase of the content of boron carbide in the composite as shown in Figs. 5 and 6. HV value of composite 4 is increased to 123.29 HV from 89.86 HV which is of base alloy. The composite 4 specimens exhibited a maximal hardness of 123.29 HV, which was an improvement of 37.20% over that of the base alloy. This result is consistent with Dixit et al. [16] and gives better micro-hardness compared to base alloy. The results of micro-hardness of Al 7075/8 wt% boron carbide/2%wt. fly-ash gives close results to Al 7075/10 wt% boron carbide/5 wt% graphite composite [12] even after lower wt. % of boron carbide, this is due to the presence of fly-ash, which majorly consist of SiO2, Al2O3, Fe2O3, resulting enhancement of hardness. It also provides superior hardness compared to Al6061/10 wt% B4C composite [18]. Moreover, Al/2wt%FA/8%B4C composite exhibited 48% higher micro hardness than Al 7075/12 wt% Fly-ash Composite [19]. 4. Conclusion
Fig. 6. Average micro-hardness of casted composite vs. casted base alloy.
micro-hardness values was analysed mathematically using coefficient of variation, which is calculated further. 3.2.1. Mathematical confirmation of uniform hardness The variations of micro-hardness for all four composite and base matrix were shown in Fig. 5 and the corresponding coefficient of variation is calculated and listed in Table 5. It has been observed that the coefficient of variation % of micro-hardness at various locations of hardness specimen does not exceed 3%, indicating the reasonably low variation of the micro-hardness of casted composite, which also confirms the uniform distribution of reinforcements particles in casted composites. 3.2.2. Mathematical modelling of micro-hardness All the micro-hardness test results were analysed using linear regression by Minitab 15 statistical software package to obtain an empirical relation of micro-hardness for fly ash and boron carbide content (wt.%). The mathematical expression for the micro-hardness is shown in equation (7).
In this study, Al 7075/B4C/Fly-Ash hybrid aluminium matrix composites (HAMCs) with constant weight fraction of fly-ash (2 wt%) and different weight fractions of boron carbide (2, 4, 6 and 8 wt%) have been successfully synthesized by stir casting technique. The internal structure and incorporation of fly ash and boron carbide of casted composites were observed using optical micrographs and found the homogeneous distribution of reinforcement particles. The coefficient of variations of micro-hardness values was calculated at seven local points of hardness specimen and found very low value of the coefficient of variation (less than 3%), which confirms uniform hardness values as well as uniform dispersion of reinforcements throughout the composite. The mathematical empirical expression of micro-hardness in terms of fly ash and boron carbide wt% was obtained by regression analysis. The composite 4 (Al 7075/2 wt% FA/8 wt% B4C) provides the excellent value of micro-hardness i.e. 123.29 HV, which is 37.2% higher than base matrix alloy. This study also reveals that the addition of B4C and fly-ash in the Al matrix resulted in a boost of microhardness of casted composite and it increases with the increase of boron carbide content in the matrix. Further, investigation of mechanical and tribological properties will be carried in future work. References [1] P.S. Reddy, R. Kesavan, B. Vijaya Ramnath, Silicon 10 (2018) 495–502. [2] M.K. Sahu, A. Valarmathi, S. Baskaran, V. Anandakrishnan, R.K. Pandey, Proc. Inst. Mech. Eng., Part B: J. Eng. Manuf. 228 (2014) 1501–1507. [3] J. Zheng, W. Li, Q. Liu, G. Shu, J. Composite Mater. 50 (2016) 3843–3852. [4] R. Raj, Ritesh, D.G. Thakur, Proc. Inst. Mech. Eng., Part C: J. Mech. Eng. Sci. 233 (2019) 1345–1356. [5] X. Canute, M.C. Majumder, J. Eng. Sci. Technol. 13 (2018) 755–777. [6] P.K. Rohatgi, M. Gajdardziska-Josifovska, D.P. Robertson, J.K. Kim, R.Q. Guo, Metall. Mater. Trans. A 33 (2002) 1541–1547. [7] S. Zahi, A.R. Daud, Mater. Des. 32 (2011) 1337–1346. [8] S. Sarkar, S. Sen, S.C. Mishra, M.K. Kudelwar, S. Mohan, J. Reinforced Plast. Composites 29 (2010) 144–148. [9] T.P.D. Rajan, R.M. Pillai, B.C. Pai, K.G. Satyanarayana, P.K. Rohatgi, Composites Sci. Technol. 67 (2007) 3369–3377. [10] L.J. Fan, S.H. Juang, Mater. Des. 89 (2016) 941–949. [11] Q. Hu, H. Zhao, F. Li, Mater. Manuf. Process. 31 (2016) 1292–1300. [12] A. Baradeswaran, S.C. Vettivel, A. Elaya Perumal, N. Selvakumar, R. Franklin Issac, Mater. Des. 63 (2014) 620–632. [13] M.K. Sahu, R.K. Sahu, Fabrication of aluminum matrix composites by stir casting technique and stirring process parameters optimization, advanced casting technologies, IntechOpen (2018) 111–126. [14] M.K. Sahu, R.K. Sahu, Trans. Indian Inst. Met. 70 (2017) 2563–2570.
Please cite this article as: M. K. Sahu and R. K. Sahu, Synthesis, microstructure and hardness of Al 7075/B4C/Fly-ash composite using stir casting method, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.150
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M.K. Sahu, R.K. Sahu / Materials Today: Proceedings xxx (xxxx) xxx
[15] A. Baradeswaran, A.E. Perumal, Composites Part B: Eng. 54 (2013) 146–152. [16] A.C. Dixit, B.K. Sridhara, M.V. Achutha, Evaluation of Critical Speed for Aluminum-Boron Carbide Metal Matrix Composite Shaft, Springer, Singapore, 2019, pp. 527–534.
[17] J. Selvam, D. Raja, D.S. Robinson Smart, I. Dinaharan, Mater. Des. 49 (2013) 28– 34. [18] B. Ravi, B.B. Naik, J.U. Prakash, Mater. Today: Proc. 2 (2015) 2984–2990. [19] V. Mohanavel, S.S. Kumar, R.V. Srinivasan, P. Ganeshan, K.T. Anand, J. Chem. Pharm. Sci. (2017) 217–220.
Please cite this article as: M. K. Sahu and R. K. Sahu, Synthesis, microstructure and hardness of Al 7075/B4C/Fly-ash composite using stir casting method, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.150
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Synthesis, microstructure and hardness of Al 7075/B4C/Fly-ash composite using stir casting method Mohit Kumar Sahu, Raj Kumar Sahu ⇑ National Institute of Technology Raipur, G.E. Road, Raipur, Chhattisgarh 492010, India
a r t i c l e
i n f o
Article history: Received 14 September 2019 Accepted 23 September 2019 Available online xxxx Keywords: Hybrid aluminum matrix composites (HAMCs) Aluminum 7075 Stir-casting Homogeneous distribution Micro-hardness
a b s t r a c t Stir-casting is the appropriate and economical technique for HAMCs synthesis as it produces uniformly reinforcement distributed composite retaining finer grains. In the present work, Al 7075/x%wt.B4C/2% wt. fly-ash HAMCs were fabricated via stir casting route, where x is weight percentage of boron carbide (x = 2, 4, 6 and 8 wt%). Incorporation and distribution of reinforcement particles in HAMCs were analysed using scanning electron microscope (SEM) and optical micrographs. Also, Micro-hardness tests were conducted at seven locals points in hardness specimen to see the variation of hardness values. The coefficient of variation was calculated to ensure uniform hardness values, which further confirm the uniform distribution of reinforcements in the prepared composite. Moreover, mathematical modelling was done to obtain the empirical relation of micro-hardness of composite with respect to fly-ash and boron carbide content, which shows the correlation between the micro-hardness and reinforcements contents in the HAMC. The synthesized HAMCs may be suitable in automobile and aerospace application where low cost, lightweight, high temperature resistant and high wear resistant composite are required. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International conference on Materials and Manufacturing Methods.
1. Introduction Hybrid aluminum matrix composites (HAMCs) are attracting attentions of automobile, aerospace and electronics industries due to their adaptive mechanical, metallurgical and tribological properties by appropriate selection of materials and synthesis technique. Homogeneous properties throughout the material are essential in most of the engineering applications, which may be achieved when the reinforcements particles suspend uniformly over the aluminum matrix. Present engineering applications necessitate materials that are robust, lightweight and less costly. Hybrid aluminum matrix composites meet these necessities and has high strength to mass ratio, good tribological characteristics, less expensive and suitable for automobile and aerospace applications [1,2]. HAMCs are usually manufactured by two routes i.e. solid-state
Abbreviations: AMCs, Aluminium matrix composites; HAMCs, Hybrid aluminium matrix composites; Al 7075, Aluminium alloy 7075; FA, Fly-Ash; SC, Stir Casting; PM, Powder Metallurgy; B4C, Boron Carbide; Al, Aluminium; K2TiF6, Potassium hexa fluoro titanate; Mg, Magnesium. ⇑ Corresponding author. Tel.:+91 771 2254200; fax:+91 771 2254600. E-mail address: [email protected] (R.K. Sahu).
and liquid state manufacturing processes. Solid-state process comprises powder metallurgy (PM) techniques whereas liquid state process includes compo-casting, squeeze casting and preferably stir-casting (SC) techniques. Stir casting method has stayed the most explored technique for producing HAMCs due to its ease of production, flexibility and less expensive. But, wettability between molten metal and ceramics particles are key challenges in stir casting, which is reported by many researchers. Zheng et al. [3] reported poor wettability of B4C in aluminum due to the reaction of boron carbide with molten Al and produce Al3BC and AlB2 phases, which results in rapid production of secondary phases in the melt. Baradeswaran et al. also reported difficulty in fabricating Al–B4C composite due to poor wettability of boron carbide and aluminum below 1100 °C, he further suggested controlling of the interface of Al–B4C. Raj et al. [4], Reddy et al. [1], and Canute et al. [5] also reported the same problem, this problem was resolved by the addition of magnesium and potassium hexafluoro titanate (K2TiF6) into the melt. Adding magnesium to the aluminum melt reduces the surface tension of the melt and enhance the wettability between fly-ash and aluminum [6–8]. Fly has SiO2 and Al2O3 as its major components, Rajan et al. [9] reported
https://doi.org/10.1016/j.matpr.2019.09.150 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International conference on Materials and Manufacturing Methods.
Please cite this article as: M. K. Sahu and R. K. Sahu, Synthesis, microstructure and hardness of Al 7075/B4C/Fly-ash composite using stir casting method, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.150
2
M.K. Sahu, R.K. Sahu / Materials Today: Proceedings xxx (xxxx) xxx
the formation of MgAl2O4 by the reaction of aluminum with SiO2 and Al2O3, which changes the composition of reinforcement in the composite, which is shown in reactions (1) and (2) [10].
2AlðlÞ þ Mg ðlÞ þ 2SiO2ðsÞ ! MgAl2 O4ðsÞ þ 2SiðsÞ
ð1Þ
2Mg ðlÞ þ 4Al2 O3 ! 3MgAl2 O4ðsÞ þ 2AlðlÞ
ð2Þ
K2TiF6 flux increases the wettability of B4C particles with aluminium melt. It also improves the stability of boron carbide particles in the aluminium during the re-melt of the composite. K2TiF6 flux removes oxide films from the aluminium melt surface, removal of the oxide layer promotes the reaction of B4C particles rapidly with aluminium as per the reaction (3).
3K 2 TiF 6 þ 13Al ! 3Al3 Ti þ 3KAlF 4 þ K 3 AlF 6
ð3Þ
9Al þ 2B4 C ! 2Al3 BC þ 3AlB2
ð4Þ
3Al3 Ti þ B4 C ! 2TiB2 þ TiC þ 9Al
ð5Þ
Al3 Ti þ AlB2 ! TiB2 þ 4Al
ð6Þ
Moreover, compounds formed from reactions (4) and (5) provides Ti-rich layer cover and prevented the boron carbide particles from additional decomposing [11]. Table 1 shows the selection of materials and optimal stir casting parameters and temperatures for the fabrication of HAMCs. Lowcost, lightweight, high temperature resistant and high wear resistant composites are the current need of automobiles, aeronautical and electronics industries. Alumnium7075/ boron carbide/ fly-ash HAMC is the impeccable combination to fulfil demands of these industries, but the incorporation of boron carbide and fly ash particles uniformly into Al 7075 matrix itself a challenge due to low wettability between two phases and culturing of reinforcement particles. This paper may guide the research community, the synthesis of boron carbide and fly-ash reinforced HAMCs retaining uniform particle distribution.
Table 1 Selection of material, optimal stir-casting parameters and temperature.
2. Materials and methods 2.1. Material selection Aluminum 7075 were selected as base matrix due to tremendous properties such as high-temperature resistance, reasonably high tensile strength, good toughness, and is appropriate for aerospace and automobile applications. Boron carbide was selected as primary, whereas fly ash was selected as secondary reinforcement in this investigation. Boron carbide is enormously hard and is used in tribological applications, the incorporation of boron carbide into aluminum matrix enhance the properties of the developed composites [16]. Fly- ash is inexpensive industrial-waste as well it enhances mechanical properties due to the presence of SiO2, Al2O3 and TiO2 in the fly-ash [10,17]. The chemical composition of Aluminium 7075 and fly-ash are shown in Tables 2 and 3 respectively [16]. The particle size of boron carbide and fly-ash were taken in the range of 3–10 mm. 2.2. Synthesis of composite The wettability of boron carbide with aluminium is suitable at 1100 °C. Besides, such high-temperature processing leads to undesirable chemical reactions between boron carbide and aluminium and form adverse compounds like Al4C3, AlB2 and Al3BC. This problem was resolved by incorporating magnesium and potassium hexafluoro titanate (K2TiF6) flux into the melt to enhance the wettability [4,10]. The amount of K2TiF6 flux is identical to the amount of Born carbide added [12]. Al7075/x% B4C/2% Fly-ash HAMCs, where x is the weight percentage of boron carbide i.e. 2, 4, 6 and 8 wt. %, were fabricated using stir casting setup as shown in Fig. 1. Initially, 700 g Al 7075 was kept inside the furnace and the temperature was set to 850 °C in a protective atmosphere of argon gas. Simultaneously, reinforcement mixture i.e. x % wt. boron carbide and 2% wt. fly-ash with wetting agents magnesium (2% wt.) and K2TiF6 (equal to boron carbide weight) were preheated. After Preheating Al 7075, boron carbide, fly-ash, magnesium and K2TIF6 for 2 h, the graphite stirrer impeller of blade angle 30° with stainless steel rod was introduced into the aluminium melt in order to form the vortex at a constant speed at 550 rpm [2,13]. Then, reinforcements (boron carbide and fly-ash) was started feeding using the feeder and simultaneously magnesium and potassium hexafluoro titanate (K2TiF6) flux was used in the aluminium melt to increase the wettability [12]. The vortex formed by mechanical stirring dispenses reinforcement particles in the melt and it gives a homogeneous distribution of the reinforcements over the matrix. Nomenclature and composition of the casted composite are listed in Table 4. Thereafter, specimens were prepared for optical micrograph and micro-hardness test. Details of the microhardness test are discussed further.
Parameters
Value
References
Boron Carbide content Fly-ash content Melting Temperature Preheating Temperature of B4C Preheating Temperature Flyash Stirring Time Impeller Blade Angle Impeller Blade Diameter Reinforcement feed rate Stirring Speed Wetting element
2, 4, 6 and 8 wt%
[1,4]
2 wt% 850 °C 300 °C
[5] [5,10,12] [4]
400 °C
[5]
10 min 30o 50–55% of the diameter of the crucible
[13,14] [13,14] [13,14]
0.8–1.5 g/s
[13,14]
Specimens were prepared from the casted composites for microstructural characterization and micro-hardness test. Prepared specimens were undertaken for standard metallographic polishing and etched with Keller’s reagent. Further, specimens were taken for microstructural analysis using SEM and optical micrographs to confirm the incorporation and dispersion of boron carbide and fly-ash into HAMCs.
550 rpm Magnesium and potassium hexa fluoro titanate (K2TiF6) Argon environment
[1,13,14] [1,12,15]
2.4. Micro-hardness test
Protective Environment
[3]
2.3. Microstructure
Micro-hardness of all specimens were taken at various locations (C0, L1, L2, L3, R1, R2, and R3) of the specimen as shown in Fig. 2.
Please cite this article as: M. K. Sahu and R. K. Sahu, Synthesis, microstructure and hardness of Al 7075/B4C/Fly-ash composite using stir casting method, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.150
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M.K. Sahu, R.K. Sahu / Materials Today: Proceedings xxx (xxxx) xxx Table 2 Chemical composition of Al 7075 in wt. percent. Element
Zn
Mg
Cu
Fe
Si
Cr
Ni
Al
Wt. %
5.85
2.04
1.12
0.20
0.40
0.33
O.23
Remaining
Table 3 Chemical composition of fly ash in weight percent. Element
SiO2
Al2O3
MgO
Fe2O3
TiO2
CaO
K2O
Wt. %
61.04
24.96
0.57
6.84
2.82
1.0
2.77
Fig. 1. Schematic of the stir-casting setup.
Table 4 Nomenclature and composition of casted composites. Nomenclature
Composition
Composite Composite Composite Composite
Al Al Al Al
1 2 3 4
7075/2%wt. 7075/4%wt. 7075/6%wt. 7075/8%wt.
B4C/2%wt.FA B4C/2%wt.FA B4C/2%wt.FA B4C/2%wt.FA
The figure shows, C0 is the centre of the specimen and L1, L2 and L3 are the located at the left side of the specimen R1, R2 and R3 are the located at the right of the CO, whereas the distance between each location is 2 mm. 3. Results and discussion 3.1. Microstructure The reinforcement particles were identified by Energy Dispersive Spectrum (EDS) as shown in Fig. 3. Spectrum 1 shows the peaks of boron and carbon, which indicates boron carbide in the composite. Further, Spectrum 2 shows peaks of Si, Al, K, Ti, Mg,
Fig. 2. Micro-hardness sample.
Ca which indicates SiO2, Al2O3, K2O, TiO2, MgO, and CaO. These elements are the composition of fly-ash used, which confirm that the particle is fly-ash. Fig. 4(a), (b), (c) and (d) shows the microstructure of hybrid aluminium matrix composite with fixed fly-ash content of 2 wt% and varying boron carbide content of 2, 4, 6 and 8 wt% respectively. It illustrates a good chemical bonding among the Al particles, where Al particles joined together to construct a solid structure. It was also found that particles of fly-ash and boron carbide were incorporated in the composite between the grain boundaries whereas clustering of these particles are not observed, which
Table 5 Variation coefficient of micro-hardness at various locations of composite specimen. Composites
Al/2wt%FA/2%B4C Al/2wt%FA/4%B4C Al/2wt%FA/6%B4C Al/2wt%FA/8%B4C
Locations L3
L2
L1
C0
R1
R2
R3
104.0 109.0 111.4 118.0
98.3 105.0 111.8 122.0
97.2 110.0 116.0 122.0
99.0 107.9 115.0 126.0
105.0 109.9 114.0 123.0
100.6 110.0 120.0 126.0
102.0 115.0 121.0 126.0
Mean
Standard Deviation (n-1)
Coefficient of variation
Coefficient of variation (%)
101.7 110.7 117.5 125.3
2.548 3.025 3.512 1.500
0.0251 0.0273 0.0299 0.0120
2.51 2.73 2.99 1.20
Please cite this article as: M. K. Sahu and R. K. Sahu, Synthesis, microstructure and hardness of Al 7075/B4C/Fly-ash composite using stir casting method, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.150
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M.K. Sahu, R.K. Sahu / Materials Today: Proceedings xxx (xxxx) xxx
Fig. 3. SEM image and EDX pattern of Composite 1 (Al 7075/2% wt. FA/2% Wt. B4C) (a) SEM image of composite 1, (b) EDX pattern of spectrum 1, (c) EDX pattern of spectrum 2.
Fig. 4. Optical micrograph of composites: (a) composite 1: Al 7075/2% Wt. FA/2% Wt. B4C; (b) Composite 2: Al 7075/2% Wt. FA/4% Wt. B4C; (c) Composite 3: Al 7075/2% Wt. FA/6% Wt. B4C composite; (d) Composite 4: Al 7075/2% Wt. FA/8% Wt. B4C.
confirms that the elements are uniformly distributed throughout the Al 7075 matrix for all 4 composites. The uniform distribution was achieved due to optimal selection of stirring parameters. 3.2. Micro-hardness Micro-hardness test results of all composites and the casted matrix material are shown in Fig. 5, which shows the variation of micro-hardness values at various location of specimen surfaces for all composites as well as for casted Al 7075 matrix. The
variation of hardness in the casted specimen is common due to the dendritic formation during solidification. Incorporation of reinforcements particles increases the hardness of the specimen. Since micro-hardness value depends on the dispersion of reinforcements in composite, reasonably low variation of the micro-hardness value indicates the uniform distribution of reinforcements. The hardness of composite is increasing with the increase of boron carbide content, which can be recognized to the fact that the boron carbide and fly-ash possess higher hardness and its presence in the aluminium matrix improves the hardness. The variation of
Please cite this article as: M. K. Sahu and R. K. Sahu, Synthesis, microstructure and hardness of Al 7075/B4C/Fly-ash composite using stir casting method, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.150
M.K. Sahu, R.K. Sahu / Materials Today: Proceedings xxx (xxxx) xxx
5
Micro hardness ¼ 89:9 þ ð2:05 fly ash wt:%Þ þ ð3:67 Boron carbide wt:%Þ
Fig. 5. Variation of micro-hardness at various locations of samples and the average hardness.
ð7Þ
3.2.3. Comparison of micro-hardness The average micro-hardness (HV) values are higher than casted Al 7075 and increasing with the increase of the content of boron carbide in the composite as shown in Figs. 5 and 6. HV value of composite 4 is increased to 123.29 HV from 89.86 HV which is of base alloy. The composite 4 specimens exhibited a maximal hardness of 123.29 HV, which was an improvement of 37.20% over that of the base alloy. This result is consistent with Dixit et al. [16] and gives better micro-hardness compared to base alloy. The results of micro-hardness of Al 7075/8 wt% boron carbide/2%wt. fly-ash gives close results to Al 7075/10 wt% boron carbide/5 wt% graphite composite [12] even after lower wt. % of boron carbide, this is due to the presence of fly-ash, which majorly consist of SiO2, Al2O3, Fe2O3, resulting enhancement of hardness. It also provides superior hardness compared to Al6061/10 wt% B4C composite [18]. Moreover, Al/2wt%FA/8%B4C composite exhibited 48% higher micro hardness than Al 7075/12 wt% Fly-ash Composite [19]. 4. Conclusion
Fig. 6. Average micro-hardness of casted composite vs. casted base alloy.
micro-hardness values was analysed mathematically using coefficient of variation, which is calculated further. 3.2.1. Mathematical confirmation of uniform hardness The variations of micro-hardness for all four composite and base matrix were shown in Fig. 5 and the corresponding coefficient of variation is calculated and listed in Table 5. It has been observed that the coefficient of variation % of micro-hardness at various locations of hardness specimen does not exceed 3%, indicating the reasonably low variation of the micro-hardness of casted composite, which also confirms the uniform distribution of reinforcements particles in casted composites. 3.2.2. Mathematical modelling of micro-hardness All the micro-hardness test results were analysed using linear regression by Minitab 15 statistical software package to obtain an empirical relation of micro-hardness for fly ash and boron carbide content (wt.%). The mathematical expression for the micro-hardness is shown in equation (7).
In this study, Al 7075/B4C/Fly-Ash hybrid aluminium matrix composites (HAMCs) with constant weight fraction of fly-ash (2 wt%) and different weight fractions of boron carbide (2, 4, 6 and 8 wt%) have been successfully synthesized by stir casting technique. The internal structure and incorporation of fly ash and boron carbide of casted composites were observed using optical micrographs and found the homogeneous distribution of reinforcement particles. The coefficient of variations of micro-hardness values was calculated at seven local points of hardness specimen and found very low value of the coefficient of variation (less than 3%), which confirms uniform hardness values as well as uniform dispersion of reinforcements throughout the composite. The mathematical empirical expression of micro-hardness in terms of fly ash and boron carbide wt% was obtained by regression analysis. The composite 4 (Al 7075/2 wt% FA/8 wt% B4C) provides the excellent value of micro-hardness i.e. 123.29 HV, which is 37.2% higher than base matrix alloy. This study also reveals that the addition of B4C and fly-ash in the Al matrix resulted in a boost of microhardness of casted composite and it increases with the increase of boron carbide content in the matrix. Further, investigation of mechanical and tribological properties will be carried in future work. References [1] P.S. Reddy, R. Kesavan, B. Vijaya Ramnath, Silicon 10 (2018) 495–502. [2] M.K. Sahu, A. Valarmathi, S. Baskaran, V. Anandakrishnan, R.K. Pandey, Proc. Inst. Mech. Eng., Part B: J. Eng. Manuf. 228 (2014) 1501–1507. [3] J. Zheng, W. Li, Q. Liu, G. Shu, J. Composite Mater. 50 (2016) 3843–3852. [4] R. Raj, Ritesh, D.G. Thakur, Proc. Inst. Mech. Eng., Part C: J. Mech. Eng. Sci. 233 (2019) 1345–1356. [5] X. Canute, M.C. Majumder, J. Eng. Sci. Technol. 13 (2018) 755–777. [6] P.K. Rohatgi, M. Gajdardziska-Josifovska, D.P. Robertson, J.K. Kim, R.Q. Guo, Metall. Mater. Trans. A 33 (2002) 1541–1547. [7] S. Zahi, A.R. Daud, Mater. Des. 32 (2011) 1337–1346. [8] S. Sarkar, S. Sen, S.C. Mishra, M.K. Kudelwar, S. Mohan, J. Reinforced Plast. Composites 29 (2010) 144–148. [9] T.P.D. Rajan, R.M. Pillai, B.C. Pai, K.G. Satyanarayana, P.K. Rohatgi, Composites Sci. Technol. 67 (2007) 3369–3377. [10] L.J. Fan, S.H. Juang, Mater. Des. 89 (2016) 941–949. [11] Q. Hu, H. Zhao, F. Li, Mater. Manuf. Process. 31 (2016) 1292–1300. [12] A. Baradeswaran, S.C. Vettivel, A. Elaya Perumal, N. Selvakumar, R. Franklin Issac, Mater. Des. 63 (2014) 620–632. [13] M.K. Sahu, R.K. Sahu, Fabrication of aluminum matrix composites by stir casting technique and stirring process parameters optimization, advanced casting technologies, IntechOpen (2018) 111–126. [14] M.K. Sahu, R.K. Sahu, Trans. Indian Inst. Met. 70 (2017) 2563–2570.
Please cite this article as: M. K. Sahu and R. K. Sahu, Synthesis, microstructure and hardness of Al 7075/B4C/Fly-ash composite using stir casting method, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.150
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[15] A. Baradeswaran, A.E. Perumal, Composites Part B: Eng. 54 (2013) 146–152. [16] A.C. Dixit, B.K. Sridhara, M.V. Achutha, Evaluation of Critical Speed for Aluminum-Boron Carbide Metal Matrix Composite Shaft, Springer, Singapore, 2019, pp. 527–534.
[17] J. Selvam, D. Raja, D.S. Robinson Smart, I. Dinaharan, Mater. Des. 49 (2013) 28– 34. [18] B. Ravi, B.B. Naik, J.U. Prakash, Mater. Today: Proc. 2 (2015) 2984–2990. [19] V. Mohanavel, S.S. Kumar, R.V. Srinivasan, P. Ganeshan, K.T. Anand, J. Chem. Pharm. Sci. (2017) 217–220.
Please cite this article as: M. K. Sahu and R. K. Sahu, Synthesis, microstructure and hardness of Al 7075/B4C/Fly-ash composite using stir casting method, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.150