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Effectiveness evaluation of molten salt processing and ultrasonic cavitation techniques during the production of aluminium based hybrid nanocomposites - An experimental investigation
Journal of Alloys and Compounds 751 (2018) 183e193
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effectiveness evaluation of molten salt processing and ultrasonic cavitation techniques during the production of aluminium based hybrid nanocomposites - An experimental investigation C. Kannan*, R. Ramanujam School of Mechanical Engineering, VIT University, Vellore, 632014, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 February 2018 Received in revised form 7 April 2018 Accepted 9 April 2018 Available online 11 April 2018
The aim of this research work is to evaluate the effectiveness of two processing methods and treatments on the mechanical properties Al 7075 based hybrid nanocomposite. In this study, Al 7075 based hybrid nanocomposites were fabricated through ultrasonic assisted cavitation and molten salt processing methods, which were further subjected to T6 treatment and deep cryogenic treatment. The microstructural observation was carried out on hybrid nanocomposites using optical and scanning electron microscopy. A hybrid nanocomposite produced by molten salt processing with ultrasonic assistance and optimized mechanical stirring was found to possess superior properties in terms of tensile strength, percentage elongation and hardness over all other samples. Out of these treatments, T6 was found to better enhance the mechanical properties of aluminium alloy and hybrid nanocomposite than deep cryogenic treatment. © 2018 Elsevier B.V. All rights reserved.
Keywords: Ultrasonic assisted cavitation Molten salt processing Nanocomposite Deep cryogenic treatment Al 7075 T6 condition
1. Introduction Aluminium (Al) based metal matrix composites (AMMC) are being used in diverse applications such as automobile, aviation, construction and mining due to their enhanced mechanical and thermal properties over pure or unreinforced aluminium alloys [1,2]. Improved properties are achieved through the incorporation of ceramic reinforcements such as aluminium oxide (Al2O3), silicon carbide (SiC), boron carbide (B4C) and titanium carbide (TiC) that are uniformly distributed in the aluminium matrix [3,4]. The better toughness and ductility of the aluminium matrix is combined with the positive properties of ceramics such as high strength and high elastic modulus. Even though the ceramic reinforcements can be added to the matrix either in the form of whiskers or particles; the manufacturing easiness and low cost endorses the selection of particles over whiskers [5]. Metal matrix nanocomposites (MMnC) are a class of composites, in which the ceramic reinforcement of nanometer size are being added to the metal matrix. The nano reinforcement increases the surface area substantially at the matrix interface. This helps in
* Corresponding author. E-mail address: [email protected] (C. Kannan). https://doi.org/10.1016/j.jallcom.2018.04.112 0925-8388/© 2018 Elsevier B.V. All rights reserved.
achieving improved strength, better fatigue and creep resistance at elevated temperature without much compromise on ductile characteristics [6]. At present, the hybrid metal matrix nanocomposites (HMMnC) are drawing the attention of researchers owing to their improved properties over single reinforced nanocomposites. Enhanced properties are obtained through the reduced intermetallic formation and meniscus penetration defect [7,8]. In hybrid nanocomposites, two and more ceramic reinforcements of the same size, shape and form (whisker, particulates, fibres and tubes) or different are blended to optimise the end properties. However, the final properties of hybrid nanocomposites are majorly dependent upon the uniform distribution, hardening mechanism and thermal stability of nano-reinforcements. The material processing industries are focusing their attention towards the development of lightweight materials for diverse applications. The researchers across the globe are also putting forth their maximum effort in developing lightweight materials in the form of composites. Despite their continual effort, there has been a limited exploration of Al 7075 based hybrid nanocomposites. Al 7075 has excellent strength to weight ratio, good fatigue strength and comparatively better than many other aluminium alloys [9]. In this investigation, hybrid nanocomposite was produced with the incorporation of nano alumina (Al2O3) and nano hexagonal boron
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nitride (h-BN) particles in Al 7075 matrix with squeezing pressure of 150 MPa. Two processing methods, namely ultrasonic assisted cavitation (UAC) and molten salt processing (MSP) were adopted for the development of this hybrid nanocomposite. The developed hybrid nanocomposite was further subjected to T6 and deep cryogenic treatment (DCT) to assess the influence of individual processing method and treatment on the final mechanical properties of the composite. 2. Materials and methods Al 7075, an aluminium alloy which has zinc as a primary alloying element, was selected as base matrix. The chemical composition of Al 7075 is listed in Table 1. The aluminium alloy was melted in an electric resistance heating furnace that had an integral stirrer. About 10 g of sodium aluminium hexafluoride (Na3AlF6) was added to the melt to prevent slag formation and for improving the efficiency of the casting process. In addition, the reinforcements were preheated to 500 C to remove surface impurities, alter the surface composition and for desorbing the gases prior to their addition in the aluminium melt [10]. In this investigation, 1 wt% Al2O3 particles (avg. size: 30e50 nm) and 0.5 wt% h-BN particles (avg. size: 80e100 nm) were appended to Al 7075 matrix using two processing methods: ultrasonic assisted cavitation and a combinational approach of molten salt processing with ultrasonic assistance and optimized mechanical stirring. The physical and mechanical properties of the nano reinforcements are presented in Table 2. In both methods, a squeezing pressure of 150 MPa was applied during the solidification molten slurry. The size and morphology of nano reinforcements were determined using scanning electron microscopy (SEM) under high magnification. The SEM images of nano Al2O3 and nano h-BN particles are shown in Fig. 1a and b respectively. The photographic view of set up used for the fabrication of squeeze cast hybrid nanocomposite by different methods is shown in Fig. 1c. 2.1. Processing methods of hybrid nanocomposites In this investigation, an unreinforced aluminium alloy was produced with conventional stir casting procedure. A hybrid nanocomposite (Al 7075 þ 1 wt% Al2O3 þ 0.5 wt% h-BN) was fabricated using ultrasonic assisted cavitation and molten salt processing with ultrasonic assistance and mechanical stirring. All material samples were subjected to squeezing pressure of 150 MPa during their solidification. 2.1.1. Ultrasonic assisted cavitation based nanocomposite processing Even with optimized mechanical stirring, the uniform distribution of nanoparticles in the matrix is really difficult with a higher weight percentage of reinforcements. This might be associated with poor wettability and larger surface/volume ratio offered by nanoparticles which induce agglomeration and clustering [15,16]. In order to overcome the problems tendered by these nanoparticles, if high intensity ultrasonic waves are used, they will produce non-linear effects such as transient cavitation and acoustic streaming in the molten metal [17]. This transient cavitation involves the formation, growth, pulsating and collapsing of tiny
Table 1 Chemical composition of Al 7075 [11]. Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Al
0.13
0.42
1.42
0.12
2.42
0.21
5.4
0.11
Bal.
Table 2 Properties of reinforcements [12e14]. Property
Al2O3
h-BN
Avg. Particle size (nm) Colour Density (g/cm3) Elastic modulus (GPa) Melting point ( C)
30e50 White 3.97 375 2055
80e100 White 2.27 47 2600
bubbles. This induces hot spots in the melt where the temperature of 5000 C, the pressure of about 1000 atm and higher heat and cooling rates of order 1010 K/s could be attained. The availability of strong impact with high local temperature improves the wettability of nanoparticles and this paves the path for the successful fabrication of nanocomposites [18]. This transient cavitation along with acoustic streaming stimulates the effective dispersion of nanoparticles in the molten metal. The schematic layout of ultrasonic assisted cavitation adopted in this investigation for the production of the nanocomposite is shown in Fig. 2. 2.1.2. Molten salt based nanocomposite processing Even with ultrasonic assisted cavitation, the effective incorporation and dispersion of high loading nanoparticles into the molten slurry are still very difficult due to high specific surface energy and Vander Walls attraction force between nanoparticles [19,20]. The oxide layer formed on the surface of the molten metal is also acting as an obstacle for an effective incorporation of nanoparticles. It is established that molten fluoride salts can dissolve oxide films and aids in the effective incorporation of nanoparticles in the melt [21,22]. In this investigation, a molten fluoride salt (KAlF4) was utilized for the self-incorporation of Al2O3 and h-BN nanoparticles in the aluminium matrix to produce a hybrid nanocomposite. A uniform distribution of nanoparticles in the aluminium matrix ensures the enhancement of mechanical properties. Fig. 3 shows the schematic of molten salt based nano processing method adopted in this work. KAlF4 powders with 1 wt% Al2O3 and 0.5 wt% h-BN were ultrasonically processed in acetone for 2 h to obtain mixed powders. The acetone was evaporated and the powder mixture was dehydrated in a vacuum furnace at 180 C for 12 h. The dehydrated mixture of mixed powder was manually loaded on the surface of molten aluminium. It was further subjected to ultrasonic agitation for an appreciable time, which then followed by mechanical stirring at optimized conditions. Then the molten melt was allowed to solidify with squeezing pressure to produce the hybrid nanocomposite. 2.2. Treatment of hybrid nanocomposites Literature revealed that the mechanical properties of aluminium alloy and composites could be considerably improved by subjecting them to various aging treatments [23,24]. In this investigation, the alloy and composite material samples processed through different processes were further subjected to two treatments: solution treatment and artificial aging (T6) and deep cycle cryogenic treatment (DCT). 2.2.1. T6 treatment A tubular sintering furnace of heating rate 1e100 C/min (Delta Power Controls, Bangalore) was being used to bring the material samples to their solutionizing temperature 470 C. The materials were kept at this temperature for one hour, which is then followed by quenching in water at room temperature. The artificial aging of the samples was carried out at 120 C for 24 h [23] using an electric oven of heating rate 1e20 C/min (Delta Power Controls,
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Fig. 1. (a). SEM image of nano Al2O3 particles. Fig. 1 (b). SEM image of nano BN particles. Fig. 1 (c). A photographic view of set up used for composite fabrication.
Bangalore). The T6 procedure adopted in this work is schematically represented in Fig. 4. 2.2.2. Deep cycle cryogenic treatment Deep cryogenic treatment (DCT) is conducted at an extremely low temperature lower than 183 C by treating materials according to the predesigned processing schedule [25]. In this investigation, DCT involved the slow cooling of the material from room temperature to 196 C with the ramp down time of 6 h. Thereafter the material was soaked or held at 196 C for 72 h.
Finally, the material was brought back to room temperature with the ramp-up time of 12 h. The typical layout of cryogenic treatment plant and processing schedule adopted for DCT in the present investigation is shown in Fig. 5a and b respectively. The hybrid nanocomposites produced through ultrasonic assisted cavitation and molten salt processing were further subjected to T6 and DCT. This was carried out to investigate the influence of processing method and treatment on the mechanical properties of thus produced hybrid nanocomposite. The notation used for different material samples used in this investigation is presented in Table 3. 3. Results and discussion 3.1. Microstructural examination
Fig. 2. A schematic layout of ultrasonic assisted cavitation based nanocomposite processing.
The optical micrographs of unreinforced aluminium alloy and hybrid nanocomposite developed through ultrasonic assisted cavitation and molten salt processing methods are shown in Fig. 6. It was evident from the micrographs (Fig. 6d and g) that the grains were considerably refined in the hybrid nanocomposite when compared unreinforced aluminium alloy (Fig. 6a) even under ascast condition. This is due to the fact that in case of nanocomposites, the grain size of the matrix depends upon reinforced particle size and their volume fraction. As the particle size decreases or volume fraction increases, the grain size of the matrix decreases. This behaviour is due to higher incidence of grain
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Fig. 3. A schematic layout of molten salt-based nanocomposite processing.
Fig. 4. T6 treatment procedure adopted for unreinforced aluminium alloy and hybrid nanocomposites.
boundary pinning that prevents grain growth [8]. It was also evident from the micrograph that molten salt processing resulted in further grain refinement over ultrasonic assisted cavitation. This might be associated with improved wettability of nanoparticles, better penetration of nanoparticles into the base matrix due to the addition of molten fluoride salt [5]. It can be inferred from the literature that Al 7075 alloy, in general, has h(MgZn2) precipitate distribution at the grain boundary. But when the alloy is further subjected to artificial aging process at 120 C for 24 h (T6), it is found to form high-density fine precipitates h0 within the grain. Typically, T6 tempered alloy microstructure is characterized by a homogeneous distribution of high-density fine precipitates in the aluminium matrix. Moreover, the fine precipitates are essentially being h0 with small amounts of GuinierePreston (GP) zones and h present. This might be imputed to the better mechanical properties of Al 7075 alloy under T6 over as-cast condition. The grains were even finer for hybrid nanocomposite under T6 and DCT as compared to as-cast condition. This might have happened due to the combined effect that is contributed by (i) the presence of reinforced particles in the matrix (ii) T6 and DCT treatment procedures. The average grain size for all the investigated materials was determined from micrographs using Image J software and presented in Table 4. The SEM images of unreinforced Al 7075 alloy under as-cast, T6 and DCT conditions are shown in Fig. 7aec in sequence. The energy
dispersive spectrometry (EDS) of aluminium alloy is shown in Fig. 7d. SEM images of hybrid nanocomposites produced through ultrasonic assisted cavitation and molten salt processing are shown in Fig. 7e and f respectively. It can be inferred from these images, the reinforced nanoparticles were uniformly distributed in the aluminium matrix and the distribution is even better in molten salt processing. The elemental analysis performed on the molten salt processed hybrid nanocomposite (Fig. 7g) showed the presence of both Al2O3 and BN particles in the matrix. It is also inferred from the elemental mapping that both reinforcements are uniformly distributed in the aluminium matrix. 3.2. Mechanical properties 3.2.1. Density and porosity The theoretical and experimental density of Al 7075 alloy and its hybrid nanocomposites that are produced via ultrasonic assisted cavitation and molten salt processing is shown in Fig. 8. For the determination of theoretical density, the rule of mixtures [26] was employed, which can be represented as
rtheoretical ¼ rm 4m þ rr 4r
(1)
where 4m and 4r represent the wt. fraction of the matrix and reinforcement; rm and rr represent the density of matrix and
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Fig. 5. (a). A schematic of cryogenic treatment plant. Fig. 5(b). DCT processing cycle.
Table 3 Processing and treatments for unreinforced aluminium alloy and hybrid nanocomposite. Sample notation
Processing method
Treatment
1A 1C 1D 2A 2C 2D 3A 3C 3D
Mechanical stirring þ squeeze cast Mechanical stirring þ squeeze cast Mechanical stirring þ squeeze cast Ultrasonic assisted cavitation þ mechanical stirring þ squeeze cast Ultrasonic assisted cavitation þ mechanical stirring þ squeeze cast Ultrasonic assisted cavitation þ mechanical stirring þ squeeze cast Molten salt processing þ ultrasonic assistance þ mechanical stirring þ squeeze cast Molten salt processing þ ultrasonic assistance þ mechanical stirring þ squeeze cast Molten salt processing þ ultrasonic assistance þ mechanical stirring þ squeeze cast
As cast T6 DCT As cast T6 DCT As cast T6 DCT
reinforcement; rtheoretical represents the theoretical density of the material under investigation. The experimental density was determined using the water immersion displacement method according to ASTM C135 e 2003 standards. Using a physical weighing balance, the weight of the samples were measured initially in the air and then in water to calculate the experimental density using
the following equation
rexperimental ¼
Mass in air Mass in air Mass in water
(2)
For the fabrication of hybrid nanocomposite, 1 wt% Al2O3 nanoparticles were mixed with 0.5 wt% of h-BN nanoparticles and
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Fig. 6. Optical micrographs of (aec) unreinforced aluminium alloy (def) ultrasonic cavitation processed hybrid nanocomposite (gei) molten salt processed hybrid nanocomposite under as cast, T6 and DCT condition in sequence.
incorporated in the molten metal of aluminium alloy. Al2O3 and hBN particles used in this work are having density values of 3.97 g/ cm3 and 2.27 g/cm3 respectively. The higher wt. fraction of alumina resulted in an increase in the theoretical density of hybrid nanocomposite over the unreinforced Al 7075 alloy. The porosity associated with unreinforced and hybrid nano-reinforced aluminium alloy was determined using the relation
% porosity ¼
rtheoretical rexperimental rtheoretical
(3)
The percentage porosity calculated for the unreinforced aluminium alloy and hybrid nanocomposite is represented in Table 5. The porosity of a metal matrix composite is normally associated with poor wettability characteristics, particle agglomeration, clustering, gas entrapment during mixing, shrinkage during solidification and presence of air bubbles in the melt [27]. In this experiment, the wettability issues imposed by nanoparticles were considerably reduced by preheating them to 500 C prior to their incorporation in the melt [28]. In addition, ultrasonic assisted cavitation technique was adopted to avoid agglomeration and
Table 4 Average grain size of unreinforced aluminium alloy and hybrid nanocomposites. Material
Processing Method
Treatment
Grain size (mm)
Unreinforced Al 7075 alloy
Mechanical stirring þ squeeze cast
Al 7075 þ 1 wt% Al2O3 þ 0.5 wt% BN hybrid nanocomposite
Ultrasonic assisted cavitation þ mechanical stirring þ squeeze cast
Al 7075 þ 1 wt% Al2O3 þ 0.5 wt% BN hybrid nanocomposite
Molten salt processing þ ultrasonic assistance þ mechanical stirring þ squeeze cast
as cast T6 DCT as cast T6 DCT as cast T6 DCT
53 37 49 42 32 37 40 30 35
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Fig. 7. SEM images of (a) Al 7075 e as cast (b) Al 7075 e T6 (c) Al 7075-DCT (d) EDS of unreinforced Al 7075 alloy (e) ultrasonic cavitation processed hybrid nanocomposite (f) molten salt processed hybrid nanocomposite (g) elemental mapping of molten salt processed hybrid nanocomposite.
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Fig. 9. Dimensions of a tensile test specimen.
Fig. 8. Theoretical and experimental density of Al 7075 alloy, ultrasonic cavitation and molten salt processed hybrid nanocomposites.
clustering of nanoparticles. The free energy at the interface, convection properties and temperature gradient that exists between particles and the melt during solidification may also institute high porosity in metal matrix composites [29]. This is also manifested from high porosity associated with ultrasonic cavitation processed hybrid nanocomposite over unreinforced aluminium alloy. With the adoption of molten salt processing with ultrasonic assistance and optimized mechanical stirring, the porosity was reduced from 3.5% to 3.1% during the nanocomposite fabrication. This might be associated with the presence of molten fluoride salt, which has the capability of dissolving oxide layer that usually forms on the upper surface of the melt under unprotected environment [21]. This could have resulted in the better incorporation of nanoparticles in the melt and improved wettability. The effectiveness of these two manufacturing processes can be evaluated in terms of porosity reduction associated with the final fabricated composite. Molten salt processing was found to result in 12% higher effectiveness over ultrasonic cavitation processing. 3.2.2. Ultimate tensile strength and % elongation According to ASTM E08-8standards, the tensile tests were conducted on the specimens. The universal testing machine (UTMINSTRON 4000) loaded with 10 kN load cell was used to conduct the tensile test. The ultimate tensile strength (UTS) was evaluated at the crosshead speed of 0.5 mm/min. The dimension of the tensile test sample is shown in Fig. 9. The ultimate tensile strength and % elongation for different material samples under investigation is shown in Fig. 10. The unreinforced aluminium alloy has highest UTS under T6 over as-cast and DCT condition, which might be associated with grain refinement. In spite of grain refinement, the grain boundaries around the proeutectic globules have lost their definition during the solution treatment process (Fig. 6b). Generally, a coarse intermetallic phase such as MgZn2 is formed below solidus line during the gradual cooling of Al 7075 castings, which may impair the mechanical properties [30]. Higher solution
Fig. 10. Ultimate tensile strength and % Elongation of unreinforced aluminium alloy and hybrid nanocomposite.
temperature and longer soaking time were adopted in this investigation to dissolve most of this intermetallic phase to form a homogeneous phase and thus achieving enhanced tensile strength [31]. This intermetallic can be dissolved to form a homogeneous phase under the influence of T6. This can be manifested from the enhanced strength of Al 7075 alloy under T6 condition. An improvement in UTS of about 112% was achieved under T6 condition over as-cast condition. A marginal improvement of about 5.8% was observed in tensile strength of aluminium alloy under DCT. A similar trend was reported by an earlier researcher [32]. Higher tensile strength was observed in hybrid nanocomposite over unreinforced aluminium alloy under all conditions. Previous researchers [33,34] proposed that the higher strength of the hybrid nanocomposite is associated with the different strengthening mechanisms such as grain refinement, load sharing, particle strengthening, and thermal mismatch strengthening imposed by nanoparticles.
Ds ¼ Dsload þ DsHallPetch þ ðDsOrowan Þ2 þ ðDsCTE Þ2
1=2 (4)
Out of these mechanisms, the influence of load sharing is minimal [34] and the enhancement in tensile strength is mainly due to
Table 5 Calculated porosity of unreinforced aluminium alloy and hybrid nanocomposites. Material
Description of processing method
% Porosity
Unreinforced Al 7075 alloy Al 7075 þ 1 wt% Al2O3 þ 0.5 wt% BN hybrid nanocomposite Al 7075 þ 1 wt% Al2O3 þ 0.5 wt% BN hybrid nanocomposite
Mechanical stirring þ squeeze cast Ultrasonic assisted cavitation þ mechanical stirring þ squeeze cast Molten salt processing þ ultrasonic assistance þ mechanical stirring þ squeeze cast
1.8 3.5 3.1
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Fig. 11. Tensile stress-strain for (a) unreinforced aluminium alloy (b) ultrasonic cavitation processed hybrid nanocomposite (c) molten salt processed hybrid nanocomposite.
grain refinement according to Hall-Petch theory, restricted movement of dislocations in the matrix due to nanoparticles according to Orowan mechanism [33] and difference in co-efficient of thermal expansion (CTE) of matrix and nanoparticles when it is cooled to room temperature [36]. Under as-cast condition, the improved tensile strength of hybrid nanocomposite might be associated with the combined influence of Hall-Petch, Orowan and CTE mismatch mechanisms. This was evident from the refined grain structure of hybrid nanocomposite processed through two different routes (Fig. 6d and g) as compared to unreinforced aluminium alloy (Fig. 6a). Moreover, the reinforced nanoparticles in the matrix restrict the dislocation movement and huge difference in CTE of matrix and reinforcement also contributes to this enhancement. It was observed that the tensile strength of ultrasonic cavitation processed hybrid nanocomposite was higher by 21%, 19.8% and 22% under as cast, T6 and DCT conditions than their unreinforced aluminium alloy counterparts. While molten salt processed hybrid nanocomposite was producing 24.5%, 21.5% and 25.6% higher tensile strength than their unreinforced counterparts under as-cast, T6 and DCT respectively. In case of the hybrid nanocomposite, utmost 3.2% improvement was achieved with molten salt processing method over ultrasonic assisted cavitation. This might be associated with the effective incorporation of reinforcements in the matrix due to the presence of molten fluoride salts. Lower percentage elongation was observed for hybrid nanocomposites over unreinforced aluminium alloy which might due to the presence of hard ceramic particles (Al2O3 and h-BN) introduced into the matrix. These ceramic reinforcements might have introduced the
brittleness and lowered the percentage elongation. However, due to their nanosize, the variation in percentage elongation of the hybrid nanocomposite in comparison to pure alloy was almost negligible. The results are consistent with trends reported by other investigators [8,35]. Highest percentage elongation was achieved under T6 condition in both unreinforced alloy and hybrid nanocomposite due to grain refinement. The test results revealed that the cryogenic treatment could improve the percentage elongation by a maximum of 8% over casted samples. The tensile stress-strain obtained for different materials under investigation is shown in Fig. 11 (aec). The tensile modulus and ultimate tensile strength of those materials are also presented in Table 6. 3.2.3. Vicker hardness The hardness tests were performed on the polished surface of all material samples under investigation. A Vickers microhardness testing machine [MMT-X series, Matsuzawa, Japan] with a diamond indenter was used for this purpose. The hardness was measured for the applied load of 200 g with a dwell time of 10 s. The tests were carried out according to IS 1501: 2002 ASTM standards. All the tests were carried out at room temperature of 28 C and measurements were taken at seven random places and the average value was determined. This was done to eliminate any possibility of error that might be occurred during the measurement procedure. The hardness values of unreinforced aluminium alloy, ultrasonic cavitation processed and molten salt processed hybrid nanocomposites are shown in Fig. 12. For an unreinforced aluminium alloy, the Vicker hardness was
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strength, % elongation are evaluated and presented. The significant findings of this investigation are as follows:
Table 6 Tensile modulus and UTS for different samples. Material
Young's Modulus (GPa)
UTS (MPa)
1A 1C 1D 2A 2C 2D 3A 3C 3D
65.2 70.8 66.5 68.3 72.1 69.4 68.7 72.7 70.6
192 408 203 232 489 247 239 496 255
Fig. 12. Vicker hardness of unreinforced aluminium alloy and hybrid nanocomposite.
increased by 44.1% and 5.1% under T6 and DCT respectively. The improved hardness of unreinforced Al 7075 under T6 may be imputed to their refined structure. It is evident from Fig. 12 that the hardness of hybrid nanocomposite is higher than pure aluminium alloy under all conditions. When cast composites are cooled to room temperature, the ceramic reinforcements viz. nano Al2O3 and BN particles used in this investigation tend to strengthen the matrix due to their CTE mismatch with the matrix. This in turn induced mismatch strains at the interfaces of reinforced nanoparticles and matrix which hinder the dislocation movement and resulting in an improved hardness of hybrid nanocomposite. The ultrasonic cavitation processed hybrid nanocomposite was found to produce 25%, 18.8% and 26.6% higher hardness than unreinforced aluminium alloy under as-cast, T6 and DCT conditions, while in molten salt processed hybrid nanocomposite, an enhancement of 30.5%, 26.4% and 34.6% were observed under identical treatment conditions in the same sequence. In general, in case of the hybrid nanocomposite, the presence of nanoparticles leads to ultra-grain refinement, particle strengthening and CTE mismatch strengthening [37] which resulted in higher hardness over unreinforced alloy. A similar kind of result trend was observed by an earlier investigator [18].
4. Conclusions This research work investigated the influence of two processing methods (ultrasonic assisted cavitation and molten salt processing) and different treatment procedures (T6 and DCT) on the mechanical and microstructural characteristics of 1 wt% Al2O3 and 0.5 wt% BN reinforced Al 7075 based hybrid nanocomposite. The mechanical properties such density, porosity, Vickers hardness, ultimate tensile
The molten salt processing aids in the effective incorporation of nanoparticles and their uniform distribution in the aluminium matrix; as molten salt is capable of addressing the wettability issues imposed by the nanoparticles to a greater extent. Higher porosity levels associated with conventional stir cast aluminium nanocomposites can be considerably scaled down with the adoption of molten salt processing with ultrasonic assistance and optimized mechanical stirring. The molten salt processed hybrid nanocomposites are found to be resulting in higher tensile strength, % elongation and Vickers hardness over the ultrasonic cavitation processed composites. T6 treatment is found to enhance mechanical properties to a greater extent. Even though the mechanical properties can be improved with DCT, the improvement is only marginal. Under T6 condition, the Vicker hardness can be improved to the maximum of 44.1% in unreinforced aluminium alloy and 39.6% in case of the hybrid nanocomposite; while the corresponding improvement of 5.1% and 4.5% is observed under DCT. The microstructure reveals that the grain refinement achieved through the incorporation of nanoparticles positively influences the mechanical properties. Whilst the uniform distribution of nanoparticles in the matrix can be achieved either through ultrasonic cavitation or molten salt processing, the latter method is found to result in much better distribution. From this experimental investigation, it is concluded that hybrid nanocomposite processed through molten salt processing and undergone T6 treatment exhibit superior mechanical properties over all samples that are under investigation. Hence, this method is recommended to fabricate hybrid nanocomposites that are going to be employed in automotive and aerospace sectors. Conflict of interests The authors declare that there is no conflict of interests regarding the publication of this paper. References [1] M.K. Surappa, Aluminium matrix composites: challenges and opportunities, Sadhana Acad. P Eng. S 28 (2003) 319e334. [2] M. Rosso, Ceramic and metal matrix composites: routes and properties, J. Mater. Process. Technol. 175 (2006) 364e375. [3] B.V. Ramnath, C. Elanchezhian, M. Jaivignesh, S. Rajesh, C. Parswajinan, A.S.A. Ghias, Evaluation of mechanical properties of aluminium alloyealuminaeboron carbide metal matrix composites, Mater. Des. 58 (2014) 332e338. [4] A.J. Knowles, X. Jiang, M. Galano, F. Audebert, Microstructure and mechanical properties of 6061 Al alloy based composites with SiC nanoparticles, J. Alloys Compd. 615 (2014) 401e405. [5] D.K. Das, P.C. Mishra, S. Singh, S. Pattanaik, Fabrication and heat treatment of ceramic-reinforced aluminium matrix composites-a review, Int. J. Mech. Mater. Eng. 9 (2014), 6 pages. [6] S.A. Sajjadi, M. Torabi Parizi, H.R. Ezatpour, A. Sedghi, Fabrication of A356 composite reinforced with micro and nano Al2O3 particles by a developed compocasting method and study of its properties, J. Alloys Compd. 511 (2012) 226e231. [7] T. Rajmohan, K. Palanikumar, S. Ranganathan, Evaluation of mechanical and wear properties of hybrid aluminium matrix composites, T. Nonferr. Metal. Soc. 23 (2013) 2509e2517. [8] A. Alizadeh, A. Abdollahi, H. Biukani, Creep behavior and wear resistance of Al 5083 based hybrid composites reinforced with carbon nanotubes (CNTs) and boron carbide (B4C), J. Alloys Compd. 650 (2015) 783e793. [9] R. Cobden, A. Banbury, Aluminium: physical properties, characteristics and alloys, Talat Lecture. 1501 (1994) 144e155. [10] S.H. Juang, L.J. Fan, H.P. Yang, Influence of preheating temperatures and adding rates on distributions of fly ash in aluminum matrix composites prepared by stir casting, Int. J. Precis. Eng. Manuf. 16 (2015) 1321e1327. [11] A. Baradeswaran, A.E. Perumal, Influence of B4C on the tribological and
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effectiveness evaluation of molten salt processing and ultrasonic cavitation techniques during the production of aluminium based hybrid nanocomposites - An experimental investigation C. Kannan*, R. Ramanujam School of Mechanical Engineering, VIT University, Vellore, 632014, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 February 2018 Received in revised form 7 April 2018 Accepted 9 April 2018 Available online 11 April 2018
The aim of this research work is to evaluate the effectiveness of two processing methods and treatments on the mechanical properties Al 7075 based hybrid nanocomposite. In this study, Al 7075 based hybrid nanocomposites were fabricated through ultrasonic assisted cavitation and molten salt processing methods, which were further subjected to T6 treatment and deep cryogenic treatment. The microstructural observation was carried out on hybrid nanocomposites using optical and scanning electron microscopy. A hybrid nanocomposite produced by molten salt processing with ultrasonic assistance and optimized mechanical stirring was found to possess superior properties in terms of tensile strength, percentage elongation and hardness over all other samples. Out of these treatments, T6 was found to better enhance the mechanical properties of aluminium alloy and hybrid nanocomposite than deep cryogenic treatment. © 2018 Elsevier B.V. All rights reserved.
Keywords: Ultrasonic assisted cavitation Molten salt processing Nanocomposite Deep cryogenic treatment Al 7075 T6 condition
1. Introduction Aluminium (Al) based metal matrix composites (AMMC) are being used in diverse applications such as automobile, aviation, construction and mining due to their enhanced mechanical and thermal properties over pure or unreinforced aluminium alloys [1,2]. Improved properties are achieved through the incorporation of ceramic reinforcements such as aluminium oxide (Al2O3), silicon carbide (SiC), boron carbide (B4C) and titanium carbide (TiC) that are uniformly distributed in the aluminium matrix [3,4]. The better toughness and ductility of the aluminium matrix is combined with the positive properties of ceramics such as high strength and high elastic modulus. Even though the ceramic reinforcements can be added to the matrix either in the form of whiskers or particles; the manufacturing easiness and low cost endorses the selection of particles over whiskers [5]. Metal matrix nanocomposites (MMnC) are a class of composites, in which the ceramic reinforcement of nanometer size are being added to the metal matrix. The nano reinforcement increases the surface area substantially at the matrix interface. This helps in
* Corresponding author. E-mail address: [email protected] (C. Kannan). https://doi.org/10.1016/j.jallcom.2018.04.112 0925-8388/© 2018 Elsevier B.V. All rights reserved.
achieving improved strength, better fatigue and creep resistance at elevated temperature without much compromise on ductile characteristics [6]. At present, the hybrid metal matrix nanocomposites (HMMnC) are drawing the attention of researchers owing to their improved properties over single reinforced nanocomposites. Enhanced properties are obtained through the reduced intermetallic formation and meniscus penetration defect [7,8]. In hybrid nanocomposites, two and more ceramic reinforcements of the same size, shape and form (whisker, particulates, fibres and tubes) or different are blended to optimise the end properties. However, the final properties of hybrid nanocomposites are majorly dependent upon the uniform distribution, hardening mechanism and thermal stability of nano-reinforcements. The material processing industries are focusing their attention towards the development of lightweight materials for diverse applications. The researchers across the globe are also putting forth their maximum effort in developing lightweight materials in the form of composites. Despite their continual effort, there has been a limited exploration of Al 7075 based hybrid nanocomposites. Al 7075 has excellent strength to weight ratio, good fatigue strength and comparatively better than many other aluminium alloys [9]. In this investigation, hybrid nanocomposite was produced with the incorporation of nano alumina (Al2O3) and nano hexagonal boron
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nitride (h-BN) particles in Al 7075 matrix with squeezing pressure of 150 MPa. Two processing methods, namely ultrasonic assisted cavitation (UAC) and molten salt processing (MSP) were adopted for the development of this hybrid nanocomposite. The developed hybrid nanocomposite was further subjected to T6 and deep cryogenic treatment (DCT) to assess the influence of individual processing method and treatment on the final mechanical properties of the composite. 2. Materials and methods Al 7075, an aluminium alloy which has zinc as a primary alloying element, was selected as base matrix. The chemical composition of Al 7075 is listed in Table 1. The aluminium alloy was melted in an electric resistance heating furnace that had an integral stirrer. About 10 g of sodium aluminium hexafluoride (Na3AlF6) was added to the melt to prevent slag formation and for improving the efficiency of the casting process. In addition, the reinforcements were preheated to 500 C to remove surface impurities, alter the surface composition and for desorbing the gases prior to their addition in the aluminium melt [10]. In this investigation, 1 wt% Al2O3 particles (avg. size: 30e50 nm) and 0.5 wt% h-BN particles (avg. size: 80e100 nm) were appended to Al 7075 matrix using two processing methods: ultrasonic assisted cavitation and a combinational approach of molten salt processing with ultrasonic assistance and optimized mechanical stirring. The physical and mechanical properties of the nano reinforcements are presented in Table 2. In both methods, a squeezing pressure of 150 MPa was applied during the solidification molten slurry. The size and morphology of nano reinforcements were determined using scanning electron microscopy (SEM) under high magnification. The SEM images of nano Al2O3 and nano h-BN particles are shown in Fig. 1a and b respectively. The photographic view of set up used for the fabrication of squeeze cast hybrid nanocomposite by different methods is shown in Fig. 1c. 2.1. Processing methods of hybrid nanocomposites In this investigation, an unreinforced aluminium alloy was produced with conventional stir casting procedure. A hybrid nanocomposite (Al 7075 þ 1 wt% Al2O3 þ 0.5 wt% h-BN) was fabricated using ultrasonic assisted cavitation and molten salt processing with ultrasonic assistance and mechanical stirring. All material samples were subjected to squeezing pressure of 150 MPa during their solidification. 2.1.1. Ultrasonic assisted cavitation based nanocomposite processing Even with optimized mechanical stirring, the uniform distribution of nanoparticles in the matrix is really difficult with a higher weight percentage of reinforcements. This might be associated with poor wettability and larger surface/volume ratio offered by nanoparticles which induce agglomeration and clustering [15,16]. In order to overcome the problems tendered by these nanoparticles, if high intensity ultrasonic waves are used, they will produce non-linear effects such as transient cavitation and acoustic streaming in the molten metal [17]. This transient cavitation involves the formation, growth, pulsating and collapsing of tiny
Table 1 Chemical composition of Al 7075 [11]. Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Al
0.13
0.42
1.42
0.12
2.42
0.21
5.4
0.11
Bal.
Table 2 Properties of reinforcements [12e14]. Property
Al2O3
h-BN
Avg. Particle size (nm) Colour Density (g/cm3) Elastic modulus (GPa) Melting point ( C)
30e50 White 3.97 375 2055
80e100 White 2.27 47 2600
bubbles. This induces hot spots in the melt where the temperature of 5000 C, the pressure of about 1000 atm and higher heat and cooling rates of order 1010 K/s could be attained. The availability of strong impact with high local temperature improves the wettability of nanoparticles and this paves the path for the successful fabrication of nanocomposites [18]. This transient cavitation along with acoustic streaming stimulates the effective dispersion of nanoparticles in the molten metal. The schematic layout of ultrasonic assisted cavitation adopted in this investigation for the production of the nanocomposite is shown in Fig. 2. 2.1.2. Molten salt based nanocomposite processing Even with ultrasonic assisted cavitation, the effective incorporation and dispersion of high loading nanoparticles into the molten slurry are still very difficult due to high specific surface energy and Vander Walls attraction force between nanoparticles [19,20]. The oxide layer formed on the surface of the molten metal is also acting as an obstacle for an effective incorporation of nanoparticles. It is established that molten fluoride salts can dissolve oxide films and aids in the effective incorporation of nanoparticles in the melt [21,22]. In this investigation, a molten fluoride salt (KAlF4) was utilized for the self-incorporation of Al2O3 and h-BN nanoparticles in the aluminium matrix to produce a hybrid nanocomposite. A uniform distribution of nanoparticles in the aluminium matrix ensures the enhancement of mechanical properties. Fig. 3 shows the schematic of molten salt based nano processing method adopted in this work. KAlF4 powders with 1 wt% Al2O3 and 0.5 wt% h-BN were ultrasonically processed in acetone for 2 h to obtain mixed powders. The acetone was evaporated and the powder mixture was dehydrated in a vacuum furnace at 180 C for 12 h. The dehydrated mixture of mixed powder was manually loaded on the surface of molten aluminium. It was further subjected to ultrasonic agitation for an appreciable time, which then followed by mechanical stirring at optimized conditions. Then the molten melt was allowed to solidify with squeezing pressure to produce the hybrid nanocomposite. 2.2. Treatment of hybrid nanocomposites Literature revealed that the mechanical properties of aluminium alloy and composites could be considerably improved by subjecting them to various aging treatments [23,24]. In this investigation, the alloy and composite material samples processed through different processes were further subjected to two treatments: solution treatment and artificial aging (T6) and deep cycle cryogenic treatment (DCT). 2.2.1. T6 treatment A tubular sintering furnace of heating rate 1e100 C/min (Delta Power Controls, Bangalore) was being used to bring the material samples to their solutionizing temperature 470 C. The materials were kept at this temperature for one hour, which is then followed by quenching in water at room temperature. The artificial aging of the samples was carried out at 120 C for 24 h [23] using an electric oven of heating rate 1e20 C/min (Delta Power Controls,
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Fig. 1. (a). SEM image of nano Al2O3 particles. Fig. 1 (b). SEM image of nano BN particles. Fig. 1 (c). A photographic view of set up used for composite fabrication.
Bangalore). The T6 procedure adopted in this work is schematically represented in Fig. 4. 2.2.2. Deep cycle cryogenic treatment Deep cryogenic treatment (DCT) is conducted at an extremely low temperature lower than 183 C by treating materials according to the predesigned processing schedule [25]. In this investigation, DCT involved the slow cooling of the material from room temperature to 196 C with the ramp down time of 6 h. Thereafter the material was soaked or held at 196 C for 72 h.
Finally, the material was brought back to room temperature with the ramp-up time of 12 h. The typical layout of cryogenic treatment plant and processing schedule adopted for DCT in the present investigation is shown in Fig. 5a and b respectively. The hybrid nanocomposites produced through ultrasonic assisted cavitation and molten salt processing were further subjected to T6 and DCT. This was carried out to investigate the influence of processing method and treatment on the mechanical properties of thus produced hybrid nanocomposite. The notation used for different material samples used in this investigation is presented in Table 3. 3. Results and discussion 3.1. Microstructural examination
Fig. 2. A schematic layout of ultrasonic assisted cavitation based nanocomposite processing.
The optical micrographs of unreinforced aluminium alloy and hybrid nanocomposite developed through ultrasonic assisted cavitation and molten salt processing methods are shown in Fig. 6. It was evident from the micrographs (Fig. 6d and g) that the grains were considerably refined in the hybrid nanocomposite when compared unreinforced aluminium alloy (Fig. 6a) even under ascast condition. This is due to the fact that in case of nanocomposites, the grain size of the matrix depends upon reinforced particle size and their volume fraction. As the particle size decreases or volume fraction increases, the grain size of the matrix decreases. This behaviour is due to higher incidence of grain
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Fig. 3. A schematic layout of molten salt-based nanocomposite processing.
Fig. 4. T6 treatment procedure adopted for unreinforced aluminium alloy and hybrid nanocomposites.
boundary pinning that prevents grain growth [8]. It was also evident from the micrograph that molten salt processing resulted in further grain refinement over ultrasonic assisted cavitation. This might be associated with improved wettability of nanoparticles, better penetration of nanoparticles into the base matrix due to the addition of molten fluoride salt [5]. It can be inferred from the literature that Al 7075 alloy, in general, has h(MgZn2) precipitate distribution at the grain boundary. But when the alloy is further subjected to artificial aging process at 120 C for 24 h (T6), it is found to form high-density fine precipitates h0 within the grain. Typically, T6 tempered alloy microstructure is characterized by a homogeneous distribution of high-density fine precipitates in the aluminium matrix. Moreover, the fine precipitates are essentially being h0 with small amounts of GuinierePreston (GP) zones and h present. This might be imputed to the better mechanical properties of Al 7075 alloy under T6 over as-cast condition. The grains were even finer for hybrid nanocomposite under T6 and DCT as compared to as-cast condition. This might have happened due to the combined effect that is contributed by (i) the presence of reinforced particles in the matrix (ii) T6 and DCT treatment procedures. The average grain size for all the investigated materials was determined from micrographs using Image J software and presented in Table 4. The SEM images of unreinforced Al 7075 alloy under as-cast, T6 and DCT conditions are shown in Fig. 7aec in sequence. The energy
dispersive spectrometry (EDS) of aluminium alloy is shown in Fig. 7d. SEM images of hybrid nanocomposites produced through ultrasonic assisted cavitation and molten salt processing are shown in Fig. 7e and f respectively. It can be inferred from these images, the reinforced nanoparticles were uniformly distributed in the aluminium matrix and the distribution is even better in molten salt processing. The elemental analysis performed on the molten salt processed hybrid nanocomposite (Fig. 7g) showed the presence of both Al2O3 and BN particles in the matrix. It is also inferred from the elemental mapping that both reinforcements are uniformly distributed in the aluminium matrix. 3.2. Mechanical properties 3.2.1. Density and porosity The theoretical and experimental density of Al 7075 alloy and its hybrid nanocomposites that are produced via ultrasonic assisted cavitation and molten salt processing is shown in Fig. 8. For the determination of theoretical density, the rule of mixtures [26] was employed, which can be represented as
rtheoretical ¼ rm 4m þ rr 4r
(1)
where 4m and 4r represent the wt. fraction of the matrix and reinforcement; rm and rr represent the density of matrix and
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Fig. 5. (a). A schematic of cryogenic treatment plant. Fig. 5(b). DCT processing cycle.
Table 3 Processing and treatments for unreinforced aluminium alloy and hybrid nanocomposite. Sample notation
Processing method
Treatment
1A 1C 1D 2A 2C 2D 3A 3C 3D
Mechanical stirring þ squeeze cast Mechanical stirring þ squeeze cast Mechanical stirring þ squeeze cast Ultrasonic assisted cavitation þ mechanical stirring þ squeeze cast Ultrasonic assisted cavitation þ mechanical stirring þ squeeze cast Ultrasonic assisted cavitation þ mechanical stirring þ squeeze cast Molten salt processing þ ultrasonic assistance þ mechanical stirring þ squeeze cast Molten salt processing þ ultrasonic assistance þ mechanical stirring þ squeeze cast Molten salt processing þ ultrasonic assistance þ mechanical stirring þ squeeze cast
As cast T6 DCT As cast T6 DCT As cast T6 DCT
reinforcement; rtheoretical represents the theoretical density of the material under investigation. The experimental density was determined using the water immersion displacement method according to ASTM C135 e 2003 standards. Using a physical weighing balance, the weight of the samples were measured initially in the air and then in water to calculate the experimental density using
the following equation
rexperimental ¼
Mass in air Mass in air Mass in water
(2)
For the fabrication of hybrid nanocomposite, 1 wt% Al2O3 nanoparticles were mixed with 0.5 wt% of h-BN nanoparticles and
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Fig. 6. Optical micrographs of (aec) unreinforced aluminium alloy (def) ultrasonic cavitation processed hybrid nanocomposite (gei) molten salt processed hybrid nanocomposite under as cast, T6 and DCT condition in sequence.
incorporated in the molten metal of aluminium alloy. Al2O3 and hBN particles used in this work are having density values of 3.97 g/ cm3 and 2.27 g/cm3 respectively. The higher wt. fraction of alumina resulted in an increase in the theoretical density of hybrid nanocomposite over the unreinforced Al 7075 alloy. The porosity associated with unreinforced and hybrid nano-reinforced aluminium alloy was determined using the relation
% porosity ¼
rtheoretical rexperimental rtheoretical
(3)
The percentage porosity calculated for the unreinforced aluminium alloy and hybrid nanocomposite is represented in Table 5. The porosity of a metal matrix composite is normally associated with poor wettability characteristics, particle agglomeration, clustering, gas entrapment during mixing, shrinkage during solidification and presence of air bubbles in the melt [27]. In this experiment, the wettability issues imposed by nanoparticles were considerably reduced by preheating them to 500 C prior to their incorporation in the melt [28]. In addition, ultrasonic assisted cavitation technique was adopted to avoid agglomeration and
Table 4 Average grain size of unreinforced aluminium alloy and hybrid nanocomposites. Material
Processing Method
Treatment
Grain size (mm)
Unreinforced Al 7075 alloy
Mechanical stirring þ squeeze cast
Al 7075 þ 1 wt% Al2O3 þ 0.5 wt% BN hybrid nanocomposite
Ultrasonic assisted cavitation þ mechanical stirring þ squeeze cast
Al 7075 þ 1 wt% Al2O3 þ 0.5 wt% BN hybrid nanocomposite
Molten salt processing þ ultrasonic assistance þ mechanical stirring þ squeeze cast
as cast T6 DCT as cast T6 DCT as cast T6 DCT
53 37 49 42 32 37 40 30 35
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189
Fig. 7. SEM images of (a) Al 7075 e as cast (b) Al 7075 e T6 (c) Al 7075-DCT (d) EDS of unreinforced Al 7075 alloy (e) ultrasonic cavitation processed hybrid nanocomposite (f) molten salt processed hybrid nanocomposite (g) elemental mapping of molten salt processed hybrid nanocomposite.
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Fig. 9. Dimensions of a tensile test specimen.
Fig. 8. Theoretical and experimental density of Al 7075 alloy, ultrasonic cavitation and molten salt processed hybrid nanocomposites.
clustering of nanoparticles. The free energy at the interface, convection properties and temperature gradient that exists between particles and the melt during solidification may also institute high porosity in metal matrix composites [29]. This is also manifested from high porosity associated with ultrasonic cavitation processed hybrid nanocomposite over unreinforced aluminium alloy. With the adoption of molten salt processing with ultrasonic assistance and optimized mechanical stirring, the porosity was reduced from 3.5% to 3.1% during the nanocomposite fabrication. This might be associated with the presence of molten fluoride salt, which has the capability of dissolving oxide layer that usually forms on the upper surface of the melt under unprotected environment [21]. This could have resulted in the better incorporation of nanoparticles in the melt and improved wettability. The effectiveness of these two manufacturing processes can be evaluated in terms of porosity reduction associated with the final fabricated composite. Molten salt processing was found to result in 12% higher effectiveness over ultrasonic cavitation processing. 3.2.2. Ultimate tensile strength and % elongation According to ASTM E08-8standards, the tensile tests were conducted on the specimens. The universal testing machine (UTMINSTRON 4000) loaded with 10 kN load cell was used to conduct the tensile test. The ultimate tensile strength (UTS) was evaluated at the crosshead speed of 0.5 mm/min. The dimension of the tensile test sample is shown in Fig. 9. The ultimate tensile strength and % elongation for different material samples under investigation is shown in Fig. 10. The unreinforced aluminium alloy has highest UTS under T6 over as-cast and DCT condition, which might be associated with grain refinement. In spite of grain refinement, the grain boundaries around the proeutectic globules have lost their definition during the solution treatment process (Fig. 6b). Generally, a coarse intermetallic phase such as MgZn2 is formed below solidus line during the gradual cooling of Al 7075 castings, which may impair the mechanical properties [30]. Higher solution
Fig. 10. Ultimate tensile strength and % Elongation of unreinforced aluminium alloy and hybrid nanocomposite.
temperature and longer soaking time were adopted in this investigation to dissolve most of this intermetallic phase to form a homogeneous phase and thus achieving enhanced tensile strength [31]. This intermetallic can be dissolved to form a homogeneous phase under the influence of T6. This can be manifested from the enhanced strength of Al 7075 alloy under T6 condition. An improvement in UTS of about 112% was achieved under T6 condition over as-cast condition. A marginal improvement of about 5.8% was observed in tensile strength of aluminium alloy under DCT. A similar trend was reported by an earlier researcher [32]. Higher tensile strength was observed in hybrid nanocomposite over unreinforced aluminium alloy under all conditions. Previous researchers [33,34] proposed that the higher strength of the hybrid nanocomposite is associated with the different strengthening mechanisms such as grain refinement, load sharing, particle strengthening, and thermal mismatch strengthening imposed by nanoparticles.
Ds ¼ Dsload þ DsHallPetch þ ðDsOrowan Þ2 þ ðDsCTE Þ2
1=2 (4)
Out of these mechanisms, the influence of load sharing is minimal [34] and the enhancement in tensile strength is mainly due to
Table 5 Calculated porosity of unreinforced aluminium alloy and hybrid nanocomposites. Material
Description of processing method
% Porosity
Unreinforced Al 7075 alloy Al 7075 þ 1 wt% Al2O3 þ 0.5 wt% BN hybrid nanocomposite Al 7075 þ 1 wt% Al2O3 þ 0.5 wt% BN hybrid nanocomposite
Mechanical stirring þ squeeze cast Ultrasonic assisted cavitation þ mechanical stirring þ squeeze cast Molten salt processing þ ultrasonic assistance þ mechanical stirring þ squeeze cast
1.8 3.5 3.1
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Fig. 11. Tensile stress-strain for (a) unreinforced aluminium alloy (b) ultrasonic cavitation processed hybrid nanocomposite (c) molten salt processed hybrid nanocomposite.
grain refinement according to Hall-Petch theory, restricted movement of dislocations in the matrix due to nanoparticles according to Orowan mechanism [33] and difference in co-efficient of thermal expansion (CTE) of matrix and nanoparticles when it is cooled to room temperature [36]. Under as-cast condition, the improved tensile strength of hybrid nanocomposite might be associated with the combined influence of Hall-Petch, Orowan and CTE mismatch mechanisms. This was evident from the refined grain structure of hybrid nanocomposite processed through two different routes (Fig. 6d and g) as compared to unreinforced aluminium alloy (Fig. 6a). Moreover, the reinforced nanoparticles in the matrix restrict the dislocation movement and huge difference in CTE of matrix and reinforcement also contributes to this enhancement. It was observed that the tensile strength of ultrasonic cavitation processed hybrid nanocomposite was higher by 21%, 19.8% and 22% under as cast, T6 and DCT conditions than their unreinforced aluminium alloy counterparts. While molten salt processed hybrid nanocomposite was producing 24.5%, 21.5% and 25.6% higher tensile strength than their unreinforced counterparts under as-cast, T6 and DCT respectively. In case of the hybrid nanocomposite, utmost 3.2% improvement was achieved with molten salt processing method over ultrasonic assisted cavitation. This might be associated with the effective incorporation of reinforcements in the matrix due to the presence of molten fluoride salts. Lower percentage elongation was observed for hybrid nanocomposites over unreinforced aluminium alloy which might due to the presence of hard ceramic particles (Al2O3 and h-BN) introduced into the matrix. These ceramic reinforcements might have introduced the
brittleness and lowered the percentage elongation. However, due to their nanosize, the variation in percentage elongation of the hybrid nanocomposite in comparison to pure alloy was almost negligible. The results are consistent with trends reported by other investigators [8,35]. Highest percentage elongation was achieved under T6 condition in both unreinforced alloy and hybrid nanocomposite due to grain refinement. The test results revealed that the cryogenic treatment could improve the percentage elongation by a maximum of 8% over casted samples. The tensile stress-strain obtained for different materials under investigation is shown in Fig. 11 (aec). The tensile modulus and ultimate tensile strength of those materials are also presented in Table 6. 3.2.3. Vicker hardness The hardness tests were performed on the polished surface of all material samples under investigation. A Vickers microhardness testing machine [MMT-X series, Matsuzawa, Japan] with a diamond indenter was used for this purpose. The hardness was measured for the applied load of 200 g with a dwell time of 10 s. The tests were carried out according to IS 1501: 2002 ASTM standards. All the tests were carried out at room temperature of 28 C and measurements were taken at seven random places and the average value was determined. This was done to eliminate any possibility of error that might be occurred during the measurement procedure. The hardness values of unreinforced aluminium alloy, ultrasonic cavitation processed and molten salt processed hybrid nanocomposites are shown in Fig. 12. For an unreinforced aluminium alloy, the Vicker hardness was
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strength, % elongation are evaluated and presented. The significant findings of this investigation are as follows:
Table 6 Tensile modulus and UTS for different samples. Material
Young's Modulus (GPa)
UTS (MPa)
1A 1C 1D 2A 2C 2D 3A 3C 3D
65.2 70.8 66.5 68.3 72.1 69.4 68.7 72.7 70.6
192 408 203 232 489 247 239 496 255
Fig. 12. Vicker hardness of unreinforced aluminium alloy and hybrid nanocomposite.
increased by 44.1% and 5.1% under T6 and DCT respectively. The improved hardness of unreinforced Al 7075 under T6 may be imputed to their refined structure. It is evident from Fig. 12 that the hardness of hybrid nanocomposite is higher than pure aluminium alloy under all conditions. When cast composites are cooled to room temperature, the ceramic reinforcements viz. nano Al2O3 and BN particles used in this investigation tend to strengthen the matrix due to their CTE mismatch with the matrix. This in turn induced mismatch strains at the interfaces of reinforced nanoparticles and matrix which hinder the dislocation movement and resulting in an improved hardness of hybrid nanocomposite. The ultrasonic cavitation processed hybrid nanocomposite was found to produce 25%, 18.8% and 26.6% higher hardness than unreinforced aluminium alloy under as-cast, T6 and DCT conditions, while in molten salt processed hybrid nanocomposite, an enhancement of 30.5%, 26.4% and 34.6% were observed under identical treatment conditions in the same sequence. In general, in case of the hybrid nanocomposite, the presence of nanoparticles leads to ultra-grain refinement, particle strengthening and CTE mismatch strengthening [37] which resulted in higher hardness over unreinforced alloy. A similar kind of result trend was observed by an earlier investigator [18].
4. Conclusions This research work investigated the influence of two processing methods (ultrasonic assisted cavitation and molten salt processing) and different treatment procedures (T6 and DCT) on the mechanical and microstructural characteristics of 1 wt% Al2O3 and 0.5 wt% BN reinforced Al 7075 based hybrid nanocomposite. The mechanical properties such density, porosity, Vickers hardness, ultimate tensile
The molten salt processing aids in the effective incorporation of nanoparticles and their uniform distribution in the aluminium matrix; as molten salt is capable of addressing the wettability issues imposed by the nanoparticles to a greater extent. Higher porosity levels associated with conventional stir cast aluminium nanocomposites can be considerably scaled down with the adoption of molten salt processing with ultrasonic assistance and optimized mechanical stirring. The molten salt processed hybrid nanocomposites are found to be resulting in higher tensile strength, % elongation and Vickers hardness over the ultrasonic cavitation processed composites. T6 treatment is found to enhance mechanical properties to a greater extent. Even though the mechanical properties can be improved with DCT, the improvement is only marginal. Under T6 condition, the Vicker hardness can be improved to the maximum of 44.1% in unreinforced aluminium alloy and 39.6% in case of the hybrid nanocomposite; while the corresponding improvement of 5.1% and 4.5% is observed under DCT. The microstructure reveals that the grain refinement achieved through the incorporation of nanoparticles positively influences the mechanical properties. Whilst the uniform distribution of nanoparticles in the matrix can be achieved either through ultrasonic cavitation or molten salt processing, the latter method is found to result in much better distribution. From this experimental investigation, it is concluded that hybrid nanocomposite processed through molten salt processing and undergone T6 treatment exhibit superior mechanical properties over all samples that are under investigation. Hence, this method is recommended to fabricate hybrid nanocomposites that are going to be employed in automotive and aerospace sectors. Conflict of interests The authors declare that there is no conflict of interests regarding the publication of this paper. References [1] M.K. Surappa, Aluminium matrix composites: challenges and opportunities, Sadhana Acad. P Eng. S 28 (2003) 319e334. [2] M. Rosso, Ceramic and metal matrix composites: routes and properties, J. Mater. Process. Technol. 175 (2006) 364e375. [3] B.V. Ramnath, C. Elanchezhian, M. Jaivignesh, S. Rajesh, C. Parswajinan, A.S.A. Ghias, Evaluation of mechanical properties of aluminium alloyealuminaeboron carbide metal matrix composites, Mater. Des. 58 (2014) 332e338. [4] A.J. Knowles, X. Jiang, M. Galano, F. Audebert, Microstructure and mechanical properties of 6061 Al alloy based composites with SiC nanoparticles, J. Alloys Compd. 615 (2014) 401e405. [5] D.K. Das, P.C. Mishra, S. Singh, S. Pattanaik, Fabrication and heat treatment of ceramic-reinforced aluminium matrix composites-a review, Int. J. Mech. Mater. Eng. 9 (2014), 6 pages. [6] S.A. Sajjadi, M. Torabi Parizi, H.R. Ezatpour, A. Sedghi, Fabrication of A356 composite reinforced with micro and nano Al2O3 particles by a developed compocasting method and study of its properties, J. Alloys Compd. 511 (2012) 226e231. [7] T. Rajmohan, K. Palanikumar, S. Ranganathan, Evaluation of mechanical and wear properties of hybrid aluminium matrix composites, T. Nonferr. Metal. Soc. 23 (2013) 2509e2517. [8] A. Alizadeh, A. Abdollahi, H. Biukani, Creep behavior and wear resistance of Al 5083 based hybrid composites reinforced with carbon nanotubes (CNTs) and boron carbide (B4C), J. Alloys Compd. 650 (2015) 783e793. [9] R. Cobden, A. Banbury, Aluminium: physical properties, characteristics and alloys, Talat Lecture. 1501 (1994) 144e155. [10] S.H. Juang, L.J. Fan, H.P. Yang, Influence of preheating temperatures and adding rates on distributions of fly ash in aluminum matrix composites prepared by stir casting, Int. J. Precis. Eng. Manuf. 16 (2015) 1321e1327. [11] A. Baradeswaran, A.E. Perumal, Influence of B4C on the tribological and
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