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Vol. 8. Issue 2.
Pages 2223-2231 (April 2019)
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Vol. 8. Issue 2.
Pages 2223-2231 (April 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.02.008
Open Access
Optimisation of mechanical stir casting parameters for fabrication of carbon nanotubes–aluminium alloy composite through Taguchi method
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Hashim Hanizama,b, Mohd Shukor Sallehc,
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shukor@utem.edu.my

Corresponding author.
, Mohd Zaidi Omarb, Abu Bakar Sulongb
a Department of Manufacturing Technology, Fakulti Teknologi Kejuruteraan Mekanikal dan Pembuatan, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, Durian Tunggal Melaka 76100, Malaysia
b Centre for Materials Engineering and Smart Manufacturing, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, UKM Bangi 43600, Selangor, Malaysia
c Department of Manufacturing Process, Fakulti Kejuruteraan Pembuatan, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, Durian Tunggal Melaka 76100, Malaysia
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Tables (4)
Table 1. Chemical composition (A356) by wt.%.
Table 2. Experimental layout of Taguchi L8 orthogonal array design.
Table 3. Modified T6 (MT6) heat treatment.
Table 4. Result of hardness and ultimate tensile strength.
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Abstract

The liquid metallurgical process route for synthesis of multiwalled carbon nanotube–A356 aluminium alloy (MWCNT-A356) composite is suitable for intricate designs and bulk production. In this work, MWCNT-CNT was fabricated through mechanical stir casting followed by thixoforming and modified by T6 (MT6) heat treatment. Hence, the optimisation and effect of variables such as amount of CNT, amount of wettability agent of Mg and mechanical stirring duration were investigated using a robust design of experiment (DOE), namely, Taguchi method, with two factorial levels. The signal-to-noise (S/N) ratio (‘larger is better’), hardness and ultimate tensile strength (UTS) were used as response variables. Results showed that the optimum hardness and UTS values of 106.4HV and 277.0MPa, respectively, were obtained from the nanoncomposite subjected to DOE run 4 and containing 0.5wt.% MWCNT, 0.5wt.% Mg and 10min of mechanical stirring. The hardness (76.3%) and UTS (108.4%) improved compared with those of the as-cast A356 alloy. The transformation of the microstructures and the porosity of the as-cast after thixoforming and MT6 modification were also discussed. This work demonstrated the optimised mechanical stir casting parameters for MWCNT-A356 fabrication and the enhanced mechanical properties achieved by thixoforming and heat treatment.

Keywords:
Thixoforming
Metal matrix composite
Multiwalled carbon nanotube
A356 alloy
Full Text
1Introduction

Metal–matrix composites (MMC) reinforced with multiwalled carbon nanotubes (MWCNTs) have been widely investigated. The strengthening efficiency of the MWCNTs is associated with several factors, such as load transfer, Orowan and thermal expansion strengthening [1]. The typical major obstacle encountered in MMC fabrication is achieving homogeneous distribution, good wetting property and interfacial phases between the reinforced particles and the matrix. Therefore, powder metallurgy (PM) processing has been preferred over other methods to resolve these challenges [2]. However, PM processing is costly and limited to simple and not intricate parts. In this regard, liquid metallurgy (LM) processing is another route that is cheaper and more suitable for complex designs and bulk production [3,4]. Nevertheless, the main problem of this route is overcoming the huge density difference between MWCNT and aluminium alloy.

Strengthening composites involves considering several variables, such as pre-processing methods for purifying and activating reinforced materials, mixing processes (temperatures, mixing techniques, amount of reinforced materials and wetting agent) and post-processing methods (sintering, extrusion, compaction, thixoforming and heat treatment). According to Wu and Chang [5] and Gurkan and Cebeci [6], a robust design technique, namely, Taguchi method, is one of the best tools to optimise parameters for composite development. Ceschini et al. [7] indicated that the amount of reinforced material, process temperatures, types of wettability agent, stirring method and stirring duration play key roles in determination of the final physical and mechanical properties of aluminium alloys fabricated by stir casting.

A considerable number of studies have been published on the effects of CNT on the mechanical properties of the metal composite [8]. Bakr et al. [9] stated that mechanical stirring mixing in the liquid state increased the hardness of A356 alloy with CNT weight fraction of 0.5–1.0wt.% and further the value up to 2.5wt.%. However, the increase in the compressive strength significantly dropped in the alloy with more than 1.0wt.% CNT. In the solid state mixing of ball milling, Yang et al. [10] added 4.5wt.% CNT into pure aluminium powder and reported that the hardness and UTS of the composite increased from 60HV to 130HV and from 123MPa to 420MPa, respectively. Bradbury et al. [11] obtained the highest hardness of 140HV when using 6.0wt.% CNT, but the value decreased as the amount of CNT further increased. Shayan et al. [12] subjected A356 alloy added with 0.5wt.% CNT to two stages of rolling and melting. Kim et al. [13] claimed that increasing CNT to more than 2vol% decreased the hardness of the composite due to agglomeration and promoted plastic flow in the metal matrix.

This study focused on the wettability issue between CNT and metal matrix to obtain effective interfacial region and load transfer. Wettability is the ability to wet and break down surface tensions between the materials involved; such tensions include CNT surface tension (100–200mN/m) compared with liquid aluminium surface tension (865mN/m) [14]. According to Hashim et al. [15], the three methods that can be applied to promote wettability are (1) addition of alloying elements (Mg, Ca, Zr, Ti, Bi, Pb, Zn and Cu), (2) coating and (3) treatment of ceramic materials [15]. Mg is one of the most widely used materials to improve the wettability of the aluminium matrix with reinforced materials [16,17]. For instance, Bakr et al. [9] added 0.75wt.% Mg into melt to improve wettability.

The effects of stirring have been widely investigated not only on CNT but also on other reinforcement materials. Wang et al. [18] used a graphite stirrer to mix SiC in the pure aluminium matrix at 500rpm for 5min. Stirring helped to effectively disperse and form the interfacial reaction and consequently improved the strength and cracking resistance. Sekar et al. [19] and Alhawari et al. [20] applied 10 and 15min of stirring, respectively, to improve the uniform distribution of Al2O3 into the A356 matrix alloy. Mansoor and Shahid [21] reported 45% and 52% improvements in hardness and UTS, respectively, when using induction stir casting for MWCNT and pure aluminium composite. Hence, fabrication of MWCNT–aluminium alloy is feasible and has received considerable attention with the field of MMC. However, the influence of mechanical stirring, wetting agent and optimised amount of MWCNT on the composite remains unclear.

This study presents an original work on the synthesis of MWCNT and A356 aluminium alloy by using mechanical stir casting followed by thixoforming and T6 heat treatment. Taguchi method (DOE) was used to optimise three variables, namely, amount of CNT, amount of wettability agent (Mg) and mechanical stirring duration, with two factorial levels while keeping the other factors as constant. The effects on mechanical properties such as hardness and ultimate tensile strength (UTS) were determined, and signal-to-noise ratio (S/N) with target values of larger is better. The evolution of the microstructures was also discussed. Finally, the optimised parameters and their effects on MWCNT-reinforced A356 alloy matrix composite were presented.

2Experimental procedures

This study investigated the optimal parameters for fabrication of MWCNT/A356 alloy composite by mechanical stir casting. The robust Taguchi method and Minitab 17 software were also used in the experiment Commercially available A356 aluminium alloy (Table 1) and industrial-grade MWCNT (Sigma–Aldrich, purity >88%, outside ϕ 20–40nm, inside ϕ 5–10nm and length 10–30nm) were used as metal matrix and reinforced particles, respectively. Pre-weighed Mg pellets (ϕ 1mm) were added to the mixture as wettability agent. Fig. 1 shows the scanning electron microscopy (SEM) image of the as-received MWCNT.

Table 1.

Chemical composition (A356) by wt.%.

Al  Si  Cu  Mg  Mn  Zn  Ni  Fe  Pb  Ti 
Balanced %  6.5  0.2  0.2  0.3  0.1  0.1  0.5  0.1  0.2 
Fig. 1.

SEM image of as-received MWCNT.

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The effects of the amounts of MWCNT and Mg in weight percentage (wt.%) and mechanical stirring duration (min) were determined. The response functions were the Vickers hardness (HV) and UTS of the composites. The L8 orthogonal array of Taguchi method that involves eight experiments with two levels of the main factors were performed randomly (Table 2). The signal-to-noise (S/N) ratio (larger is better) of the responses was analysed to quantify the variations.

Table 2.

Experimental layout of Taguchi L8 orthogonal array design.

Run  MWCNT (wt.%)  Mg (wt.%)  Mechanical stirring time (min) 
0.5  0.25 
0.5  0.25  10 
0.5  0.50 
0.5  0.50  10 
1.0  0.25 
1.0  0.25  10 
1.0  0.50 
1.0  0.50  10 

Composites were fabricated by mixing a certain weight percentage (wt.%) of MWCNT and Mg according to Table 2 and were wrapped with aluminium foil (Fig. 2). The alloy (400g) was fully melted in an induction furnace at temperatures up to 700°C, which was then decreased and remained at 650°C. The alloy wrapped in the foil was placed inside a plunger, injected at the bottom of the crucible and stirred mechanically at 200rpm by using three-blade propeller (schematic diagram shown in Fig. 3) for specific durations (Table 2). The mixed composite was poured immediately into a mould to form thixo feedstock billet.

Fig. 2.

Weighted of premixed MWCNT and Mg for the run 1–8.

(0.14MB).
Fig. 3.

Schematic diagram of the mixing process.

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Thixoforming was carried out using T30-80KHz machine (Fig. 4a). The billet was placed on a pneumatic cylinder ram inside the induction coil, heated up to 580°C (semi-solid temperature) and rammed with a forging load (5tonnes) and speed (1m/s) into a preheated (100°C) hot work tool steel mould on top of the coil. The billet was removed from the mould and cooled at room temperature (Fig. 4b).

Fig. 4.

(a) Thixoforming machine, (b) before and after thixoformed billets.

(0.34MB).

MT6 heat treatment was conducted with shortened time. The samples were treated at a solution treatment temperature of 540°C and quenched in water at room temperature. Then, the samples were aged at 180°C for 1h in the Nabertherm furnace at 30–3000°C (Table 3). According to Menargues et al. [22], the globular microstructure of silicon particles would be formed after 5min of treatment, and hardness similar to that of ASTM B917 can be achieved after 20min. A shorter treatment duration was used because the samples already underwent thixoforming prior to MT6.

Table 3.

Modified T6 (MT6) heat treatment.

Solution treatment  Quenching  Artificial ageing 
540°C, 127°C (room temp)  180°C, 2

The samples before and after thixoforming/heat treatment were sectioned, prepared with standard metallographic procedures of grinding (400, 600, 800, 1200grits), polishing (6μ, 3μ, 1μ with diamond solution) and etched with Keller's solution. The microstructures and distribution of MWCNT were examined through optical microscopy (OM) and FESEM/EDX analyses using Hitachi SU5000 machine. Mechanical properties were determined using VH testing Matsuzawa machine (load=1kgf and dwell time=10s), and tensile tests were performed using Autograph universal testing machine. The samples for the tensile test were machined according to ASTM E8 (Fig. 5). A confirmation experiment was performed based on the optimum results obtained from DOE. A minimum of three samples were tested for each step to obtain reliable results.

Fig. 5.

ASTM E8 tensile test samples.

(0.09MB).
3Results and discussion3.1Mechanical property responses

Table 4 summarises the average hardness and UTS of the runs. Run 4 with 0.5wt.% MWCNT, 0.5wt.% Mg and mechanical stirring duration of 10min produced the optimum hardness and UTS, with values of 106.4HV and 277.0MPa, respectively. The as-received cast ingot of A356 possessed hardness and UTS values of 59.5HV and 132.9MPa, respectively. The confirmation experiments produced almost similar results for hardness and tensile strength, with values of 104.2HV and 271.5MPa, respectively.

Table 4.

Result of hardness and ultimate tensile strength.

Run  Hardness (HV)  UTS (MPa) 
102.8  179.4 
104.9  262.5 
98.7  163.6 
106.4  277.0 
104.1  195.2 
99.5  231.3 
101.4  207.6 
103.3  243.2 

S/N response graphs were derived from the software programme (Fig. 6). Mechanical stirring was found to be the most effective factor that influenced hardness, followed by CNT amount. The amount of Mg exhibited the least effect on the hardness of the composite. Meanwhile, mechanical stirring duration exerted the highest influence on the strength of the composite over the amounts of CNT and Mg.

Fig. 6.

Main effects of S/N ratio for hardness and UTS responses.

(0.17MB).

Under the optimised condition in run 4, the composite showed 76.3% and 108.4% improvements in hardness and UTS, respectively, compared with the as-cast A356 alloy (further discussion is presented in the next sub-chapter). Boostani and Tahamtan [23] reported that the UTS of the rheocast thixoformed at 590°C followed by 50% compaction reduction increased by 73.5% compared with that of the gravity-cast A356 alloy. Salleh and Omar [24] reported high UTS of 361MPa for A356 alloy with 6% copper after thixoforming and T6 heat treatment. Cavaliere et al. [25] indicated that the as-thixo A356 alloy exhibited UTS of 241MPa; they concluded that solution treatment slightly reduced the UTS due to the decohesion of silicon particles. Zhu et al. [26] found that the maximum UTS of 385MPa was achieved after adding 0.3wt.% misch metal of Ce and La into A356 alloy and performing T6.

3.2Homogeneity and wettability of MWCNT

Analysis of the fracture surface of the tensile samples revealed that homogeneity and wettability were achieved successfully using the proposed fabrication method. Fig. 7(a) and (b) shows the scattered distribution of MWCNT in the matrix from sample run 2, indicating an effective mechanical stirring process. In addition, no sign of thermal damage was observed in the structure of MWCNT in the matrix. Similar observations were reported by Peng and Chang [27] in their study of 0.5wt.% MWCNT sintered using wet shake mixing and Esawi et al. [28] in their study of 2wt.% CNT added to pure aluminium matrix and subjected to cold compaction and hot extrusion.

Fig. 7.

FESEM images of fracture surface.

(0.14MB).

The FESEM images at high magnification displayed evidence of wetting that occurred between the MWCNT and the alloy matrix Fig. 8(a). Although no interfacial phases, such as Al4C3, was detected due to equipment limitation, good spreading and bridging conditions of the nanotubes were observed across the grains, indicating proper wetting. Furthermore, the pull-out conditions Fig. 8(a) and (b) confirmed the presence of effective load transfer from the grains to another [29]. In addition, the enhanced UTS properties between the base and composite alloys could be due to reinforced strengthening.

Fig. 8.

FESEM images of pull out MWCNT from fracture images run 4.

(0.16MB).

Evidence of MWCNT agglomeration was also observed in sample run 5 (Fig. 9). This finding revealed that high reinforcement amount weakened the mechanical properties of the composite [3].

Fig. 9.

FESEM images of agglomeration MWCNT from fracture surface run 5.

(0.09MB).
3.3Effect of mechanical stirring

During the injection of MWCNT particles into the melt, some particles instantaneously floated and agglomerated on the surface due to the density and surface tension of the matrix. However, the particles gradually started to disappear and were released into the melt when mechanical stirring was introduced. After 5min of stirring, MWCNT particles aggregated on the surface and decreased in number as time progress. This result may be explained by the fact that stirring helped distribute the particles in the molten matrix and prevented density segregation, as reported in previous studies [30,31]. The shearing action of the rotating blades caused the deagglomeration and homogenisation of the particles and helped disturb the viscoplastic behaviour of the molten alloy. Tjong [32] reported that rigorous stirring of the molten metal in LM processing creates a vortex, which assists the transfer of nanoparticles and maintains the state of suspension into the liquid metal. As the melt solidifies during pouring, the inertia of turbulence flow reduces the formation of secondary dendritic arm spacing and grain size microstructure [12].

Stirring led to negative effects, such as voids and porosities, which contributed to decreases in the hardness and UTS of the composite. Therefore, secondary processes of thixoforming and heat treatment were mandatory to minimise such effects. Figs. 10 and 11 show the microstructures and describe the influence of the secondary processes. Fig. 11(a) and (c) shows rosette-like and globular microstructures surrounded by eutectic microconstituent obtained after mechanical stirring, which permissible for thixoforming. The grain boundaries increased with increasing temperature and became continuous α-phase microstructures. Eutectic silicon (Si) regions were detected after thixoforming/MT6 Fig. 11(b) and (d), similar to previous report [33].

Fig. 10.

OM microstructure image of as-cast A356.

(0.34MB).
Fig. 11.

OM microstructure images of mechanical stirred (a) run 2 (c) run 4 and after thixoforming/MT6 (b) run 2 and (d) run 4.

(0.7MB).

The porosity volume fraction % (PVF) levels increased before and after thixoforming/MT6 processes (Fig. 12). This finding indicated the effects of these processes on the matrix alloy. All samples showed similar significant reduction in porosity, and two (run 4/2) samples with the highest responses were examined. This study assumes that low porosity produces higher hardness, despite the slightly higher value for sample run 2 than for run 4.

Fig. 12.

OM microstructure images of mechanical stirred (a) run 2 (c) run 4 and after thixoforming/T6 (b) run 2 and (d) run 4.

(0.73MB).
3.4Effect of CNT and magnesium contents

According to the DOE results, CNT content exerted more significant effect on hardness than on tensile properties. A lower CNT content of 0.5wt.% led to higher hardness and tensile values than 1.0wt.%. These results could be due to the higher overall metal matrix density of 0.5wt.% than that of 1.0wt.%, leading to low porosity and high-density dislocation. The same observations and conclusions were reported by a previous study [34] on semi-solid aluminium composites. Furthermore, stirring contributed to the grain refinement of the microstructures. In the present study, CNT content led to less significant changes in UTS than in hardness possibly because of the presence of a reinforcement material with some agglomeration as per previous discussion.

The higher amount (0.50wt.%) of Mg in the matrix contributed to higher and consistent values for both properties of hardness and UTS compared with 0.25wt.%. Pure Mg acts as a wettability agent, which reduces the surface tension of the molten matrix during mixing and allows the CNT to penetrate the grain boundaries. Given that 0.2wt.% Mg was added to the A356 alloy, the additional 0.5wt.% might alter the composition but was still within the maximum range of Mg in the alloy. However, the precise amounts of MWCNT and Mg inside the metal matrix were not identified accurately. Despite previous experimental analyses on various percentages of reinforced particles to be added into the matrix, determining the real amount of these particles remains challenging.

4Conclusion

This study determined the effects of CNT, Mg and mechanical stirring on hardness and UTS of MWCNT–A356 composite. The robust L8 orthogonal array of Taguchi method with two levels of main factors was used to optimise the fabrication parameters. Analysis of signal-to-noise (S/N) ratio of the responses was also conducted to quantify the variations in hardness and UTS.

MWCNT–A356 composite was successfully fabricated using LM processing considering the homogeneous distribution and pull-outs of MWCNT across the grain boundaries observed in the FESEM images. These observations were critical in assuming the effective load transfer and wettability between the two materials. However, some MWCNT particles agglomerated in the sample with high (1.0wt%) amount of MWCNT. The composite was then subjected to thixoforming and heat treat treated using short-solution treatment of modified T6 (MT6).

The highest hardness and UTS values of 106.4HV and 277.0MPa, respectively, were obtained from the DOE run 4 sample containing 0.5wt.% MWCNT and 0.5wt.% Mg and subjected to 10min of mechanical stirring. Thus, the hardness (76.3%) and UTS (108.4%) of the composite improved compared with those of the as-cast A356 alloy. Based on the Taguchi analysis, the duration of mechanical stirring was the major factor that influenced the improvement in the mechanical properties of the composite. The amount of MWCNT played a more important role in improving the hardness of the material than in increasing the UTS. The amount of Mg exhibited the least effect on both properties of the composite. Comparison of the present and previous findings confirmed that vortex action during stirring helps break the surface tension and supplies and suspends nanoparticles throughout the matrix. Furthermore, a high amount of CNT may surpass the threshold limit of nanoparticles in the matrix, as evident in MWCNT agglomeration in the FESEM image. Mg content also contributed to the enhancement of the wettability, but the effect was quite minimal.

The microstructures evolved from dendritic arms into rosette-like or near-globular forms after mechanical stirring. The size of α-Al was continuous and expanded after thixoforming and MT6. Moreover, the porosity decreased by 11% after MT6, resulting in increased hardness and UTS.

This work demonstrated the potential of a simple and cheap mechanical stir casting process for fabrication of MWCNT–A356 composite. The mechanical properties of the composite can be further enhanced through thixoforming and heat treatments. Further research using controlled trials must be conducted to confirm the findings and determine the association between thixoforming/heat treatment and metal composites.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgments

The authors would like to thank Universiti Teknikal Malaysia Melaka (UTeM), Universiti Kebangsaan Malaysia (UKM) and the Ministry of Education Malaysia for financial support received under research grant FRGS/2018/FKP-AMC/F00379

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Journal of Materials Research and Technology

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