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Effects of mechanical stirring and short heat treatment on thixoformed of carbon nanotube aluminium alloy composite
Journal of Alloys and Compounds 788 (2019) 83e90
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Effects of mechanical stirring and short heat treatment on thixoformed of carbon nanotube aluminium alloy composite H. Hanizam a, b, M.S. Salleh c, *, M.Z. Omar b, A.B. Sulong b a Department of Manufacturing Technology, Fakulti Teknologi Kejuruteraan, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, Durian Tunggal Melaka, 76100, Malaysia b Department of Mechanical and Materials Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, Selangor, 43600, Malaysia c Department of Manufacturing Process, Fakulti Kejuruteraan Pembuatan, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, Durian Tunggal Melaka, 76100, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 October 2018 Received in revised form 9 January 2019 Accepted 18 February 2019 Available online 19 February 2019
The present work aimed to determine the effects of thixoforming and short T6 heat treatment processes on the microstructure and mechanical properties of thixoformed A356 alloy reinforced with 0.5 wt% multi-walled carbon nanotube (A356-MWCNT). The semisolid composite feedstock was produced by a mechanical stirring route followed by thixoforming, and finally, it was heat treated with a shorter solution treatment and artificial ageing hours. A premix of 0.5 wt% magnesium (Mg) as wettability agent and MWCNT was injected into molten A356 alloy at 650 C. Mixing and stirring were performed by a using three-blade impeller at 500 rpm for 10 min, and the mixture was poured into a preheated mould. Microstructure studies show the mechanical stirring effects on the transformation of dendritic arms to mostly globular and rosette structures of a-Al. The formations of more spheroidised structure of eutectic silicon (Si) were predominant after the heat treatment, thereby revealing the effectiveness of shorter T6 heat treatment. Results of field emission scanning electron microscopy images showed uniform distribution and pull-out structures of MWCNT throughout the matrix, thereby justifying the effective load transfer and wettability between reinforcement and alloy matrix. Subsequently, the mechanical properties of the composite shown significant improvements after each stage. The yield strength (YS), ultimate tensile strength (UTS) and elongation to fracture of cast A356 alloy increased from 115 MPa, 132.9 MPa and 1.8% to 135 MPa, 178.3 MPa and 3.1% respectively, in the A356-MWCNT. Consequently, these properties were further improved to 180 MPa, 255.8 MPa and 5.7% after the thixoforming process. The highest attainment of yield strength (YS), ultimate tensile strength (UTS) and elongation to fracture after short T6 of A356-MWCNT were 215 MPa, 277.0 MPa and 7.6%, respectively. The hardness of the samples was improved from 59.5 HV in as-cast alloy to 106.4 HV in thixoformed short T6 A356-MWCNT. © 2019 Elsevier B.V. All rights reserved.
Keywords: Semisolid metal processing Thixoforming T6 heat treatment Carbon nanotube Metal matrix composite
1. Introduction The utilisation of aluminium alloy metal matrix composite (MMC) has increased in recent years, especially in automotive, aerospace and military industries. Silicon carbide (SiC) and aluminium oxide (Al2O3) are frequently used as reinforcement materials in aluminium alloy matrix [1]. The composites have the
* Corresponding author. E-mail addresses: [email protected] (H. Hanizam), [email protected] (M.S. Salleh), [email protected] (M.Z. Omar), [email protected] (A.B. Sulong). https://doi.org/10.1016/j.jallcom.2019.02.217 0925-8388/© 2019 Elsevier B.V. All rights reserved.
advantages of reduction in the overall weight, fuel consumption and pollution of greenhouse gas emissions without sacrificing structural integrity [2]. Since the discovery of carbon nanotubes (CNT), more studies on various fabrications of CNT/aluminium alloy composites have been performed. Previous studies indicated that the major obstacles in the fabrication process are the agglomeration and poor distribution of CNT in the metal matrix. Therefore, to overcome these difficulties, a solid state powder metallurgy (PM) fabrication route is most commonly used [3,4]. However, many researchers have reported the occurrence of CNT structural damage due to ball milling process in PM, which affects the overall strength [5]. A liquid state (LS) route, the agglomeration due to the strong van der Waals forces between carbon atoms and large differences
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in densities are still the main issues. However, the LS route is very flexible in term of the intricate parts and bulk productions; thus, it is worth exploring. In general, the LS route involves stirring action to disperse the reinforcement throughout the matrix. The effects of stirring have been widely investigated not only for CNTs but also for other reinforcement materials [6e9]. The controlled stirring motion creates a vortex condition in the liquid matrix that helps disperse the reinforcement. Basically, the strengthening mechanisms of CNT/Al composite are load transfer, dislocations by thermal mismatch and Orowan looping system [10e13]. Therefore, besides the homogeneous distribution of the CNT, a good wettability between the reinforcement and matrix is essential in building up good interfacial bonding and effective load transfer in the regions. Wettability is the ability of the CNT to wet and break down surface tension of the matrix [14,15]. One of the methods involves adding some alloying elements (e.g. magnesium, calcium, zirconium, titanium, bismuth, lead, zinc and copper) [16] during the mixing process. For instance, magnesium (Mg) powder is commonly used to improve wettability between aluminium and CNT in LS fabrication [17,18]. In addition, the amount of reinforcement in the matrix also has a major effect on the mechanical properties of the composites. For instance, Bakr et al. [17] reported an increasing trend in hardness of liquid state fabrication of CNT/A356 composite from 0.5, 1.0 and 2.5 wt% CNT. However, compressive strength dropped considerably beyond 1.0 wt% CNT. Similarly, Bradbury et al. [19] obtained the highest hardness of 140 HV with 6.0 wt% CNT, and hardness started to deteriorate beyond this percentage. Moreover, Shayan and Niroumand [20] performed a two-stage process first by dispersing a total of 0.5 wt% of CNT powder in between the A356 alloy plates, stacking and rolling down to 50% of the original thickness and then melting it down. These processes improved the hardness and shear strength of the composite by 38% and 20%, respectively. Furthermore, the strength of the aluminium matrix composite can be further improved by additional secondary processes, such as thixoforming and heat treatment. Thixoforming is a process of reheating a thixotropic feedstock to a certain semi-solid temperature and compacting into a near-net shape mould. The thixotropic behaviour resulted from the arm breaking of the dendritic a-Al structure and formation of rosette or globular shapes microstructures due to shear forces. There are various thixotropic behaviour production routes available, but the mechanical stirring and cooling slope methods are the most common [21,22]. Moreover, during thixoforming, simultaneous coarsening of non-dendritic structure a-Al and changing of silicon phase morphology occur, thereby leading to the mechanical properties enhancement [23,24]. Likewise, heat treatment process, which also alters the microstructures, dissolves and homogenising soluble phases, thereby helping strengthen and prevent a catastrophic failure of the composites. A T6 heat treatment is a typical technique applied to A356 alloy that involves solution treatment, quenching and ageing processes. The solution treatment is performed at near eutectic temperature of the alloy ranging between 540 C and 550 C. Dissolving soluble phases of Mg2Si, Al2Cu, b-Al5FeSi, p-Al9FeMg3Si6 and Al5Cu2Mg8Si5 into a solid solution causes precipitation hardening, alloy elements homogenisation and eutectic Si spheroidisation during solidification [25]. Quenching process is carried out to cool the alloy rapidly, thereby suppressing the precipitation at room temperature. A high level supersaturation after quenching causing rapid formation of Guinier-Preston (GP) zones. These GP zones are clustered and coherent with the matrix, thereby hindering dislocation and increasing strength [26,27]. The artificial ageing between 150 C and 210 C forms clusters of precipitation that have larger sizes than GP zones.
Many published studies describe the role of T6 heat treatment in strengthening the alloy. For instance, Zhu et al. [28] used the T6 heat treatment method to improve the tensile properties of A356 alloy by approximately 10%. Numerous studies have also attempted to modify the standard parameters of T6 for several reasons. Peng et al. [29] compared standard T6 versus shorter time of T6 by using solution treatment at 550 C for 2 h, quenching in hot water at 70 C and artificial ageing at 170 C for 2 h on A356 alloy. As a result, the yield strength, UTS and elongation percentage can match 90%, 95% and 80% of the standard T6. Moreover, in semisolid processing, Menargues et al. [30] have applied a new short heat treatment, as follows: solution treatment at 540 C for 15 min, soaking in water at 25 C and artificial ageing at 180 C for 3 h. The globulisation of Si particles took place in 5 min, followed by dissolution of the Mg2Si phase after 10 min, due to non-dentritic boundary conditions. Moreover, the alloy reached its maximum hardness between 15 and 20 min, as a result of a-phase hardening. Although studies on CNT/aluminium alloy matrix composites have been carried out by other researchers, their focus has been the powder route rather than the liquid route due to the difficulties of mixing two hugely different densities of CNT and aluminium materials. In addition, it is very hard to achieve homogeneous distribution of the CNT throughout the matrix in the liquid state. Scientific understanding on the behaviour of MWCNT upon thixoforming and heat treatment processes is limited. This study presents a new method for the synthesis of MWCNT into A356 matrix using mechanical stirring followed by thixoforming and short T6 heat treatment processes. Homogeneity of MWCNT throughout the matrix and evidence of effective load transfer between both materials were observed using field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectroscopy (EDX) on the fracture surface from tensile samples. Furthermore, the effects of each process on the microstructure evolution and mechanical properties were investigated. 2. Experimental procedures The alloy used as metal matrix was an as-cast A356 alloy with a chemical composition as depicted in Table 1. MWCNT produced by SigmaeAldrich with purity of more than 95%, outside diameter of 20e40 nm, inside diameter of 5e10 nm and length of 10e30 mm was utilised as reinforced material. Minimum entanglement and almost no defect were detected from the thermal electron microscopy (TEM) images of the raw MWCNT, as shown in Fig. 1. Various tube diameters with multiple wall of graphene layers were observed. As illustrated in Fig. 2, 0.5 wt% of MWCNT powder and 0.5 wt% of granule Mg with an average of 1 mm diameter as wetting agent were mixed and wrapped in aluminium foil. The mixing processes were performed as follows. First, the alloy (400 g) was heated up in an induction furnace up to 700 C until it fully melted. Next, the molten alloy was brought down to a constant temperature of 650 C to avoid the degradation of MWCNT structure following Datsyuk et al. [27]. Then, the wrapped MWCNT was placed inside a plunger and injected at the bottom of the crucible. The mix was then stirred mechanically at a medium speed of 500 rpm by using a three-blade impeller. Medium speed was used to avoid air entrapment, which could lead to high porosity of the composite [28]. The composite
Table 1 Composition of 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
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Fig. 1. TEM images of as-received MWCNT a) Magnification X100k b) Magnification X600k.
Fig. 2. Schematic diagram of the mixing process.
mixing was then poured into a preheated (150 C) mould directly to form the thixotropic feedstock billets. A cooling slope method was preferred in the initial stage, but after several attempts, large amounts of MWCNT accumulated on the top side of the billet due to the rolling effect of the cooling slope. The thixoforming process was carried out using the T30-80 KHz thixoforming machine (Fig. 3(a)). The thixotropic billet was placed on a pneumatic cylinder ram inside an induction coil and reheated up to semi-solid temperature of 580 C and yielded an equivalent to 50% liquid fraction. Differential scanning calorimetry (DCS) was
used to estimate the ratio of solid-liquid fraction as shown in Fig. 4. The reheating process was controlled by gradual increment of the heating frequency at 50 A per minute until it reached the target temperature. Then, the billet was rammed with forging load of 5 tons and speed of 1 m/s into preheated (100 C) hot work tool steel mould on top of the coil. The billet was then collected from the mould and cooled at room temperature (Fig. 3(b)). Some of the thixoformed samples underwent a short T6 heat treatment with a shorter solution treatment temperature at 540 C for 1 h followed by quenching in water at room temperature and aging artificially at 180 C for 2 h, in the Nabertherm 30 C€ller et al. [31] reported that the solution 30000 C furnace. Mo treatment time of 1 h at 540 C was sufficient to produce the highest hardness on A356 semi-solid. In addition, Menargues et al. [30] stated that short overall solution treatment was sufficient for the formation of globular microstructure of silicon. The a-phase formation reached its maximum hardness after 20 min. All samples
Fig. 3. (a) Thixoforming machine, (b) Before and after thixoformed billets (samples).
Fig. 4. Heat flow and liquid fraction profile of A356 Alloy.
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were sectioned and prepared using a standard metallographic procedure, such as grinding in different grit sizes (400, 600, 800 and 1200), polishing (6, 3 and 1 mm) with diamond paste and etching for 10 s with Kellers solution. The microstructures and distribution of A356-MWCNT composite were examined using an optical microscopy (OM) and FESEM/EDX using Hitachi SU5000 machine. The porosity volume fraction (%) was measured by using automatic iSolution DT software of the OM. The hardness and tensile strength were determined using Vickers hardness testing Matsuzawa machine (load ¼ 1 kgf and Dwell time ¼ 10 s) and tensile tests using an Autograph universal testing machine. The tensile samples were machined in accordance with ASTM E8M standard (Fig. 5) for yield strength (YS) at 0.2% plastic strain offset, ultimate tensile strength (UTS) and elongation to fracture (%). Moreover, the as-cast billet of A356 was tested and used as reference. At least three samples were tested for each step to obtain reliable results. 3. Results and discussion 3.1. Microstructures evolution Fig. 6 shows the microstructure of the as-received A356 poured at 650 C with the typical dendritic a-Al morphology with interdendritic eutectic microstructures. Fig. 7(a) shows non-uniform rosette-like and globular microstructures of a-Al after the mechanical stirring process. The dendritic arms were broken up by the external mechanical forces, and islands formed. Moreover, the nonuniform or irregular shapes of the a-Al might also have contributed to the existence of MWCNT particles in the matrix. According to Shayan and Niroumand [20], the inertia of turbulence flow minimised the formation of secondary dendritic arms and grain size microstructure as the melt solidified during the pouring process. Therefore, the inertia of the turbulent couple with the inertia of reinforced particles in the melt during stirring disturbed the formation of uniform globular microstructure, but such result was sufficient for the next thixoforming process. Furthermore, Fig. 7(b) shows the microstructures of the composite after the thixoforming process. The reheating at 50% liquid fraction and compacting of the billet resulted in globule growth and in coarsened and more rounded a-Al morphology. Zoqui et al. [32] investigated the a-Al growth of 70.2% from the as-cast A356 alloy at 580 C. In addition, eutectic Si particles were remelted, their positions were rearranged; they formed plate-like structures surrounding the a-Al globules [33]. The thixoformed and short T6 heat treatment have shown some physical changes to the microstructures (Fig. 7(c)). The eutectic Si transformed from plate-like to spheroid-like and coarsened
Fig. 5. ASTM E8M tensile test samples.
Fig. 6. OM microstructure image of as-recieved A356 at (mag. x500).
[28,30], after which further coarsening of a-Al was observed. However, the presence of MWCNT particles in the regions prevented the creation of uniform a-Al. 3.2. Porosity Some porosity was detected even with slow stirring of the molten composite because of air entrapment and shrinkage during solidification. The porosity volume fraction (PVF) % was measured at 14.0% for samples after mechanical stirring, as shown in Fig. 8. The PVF value was slightly higher as compared with the as-cast A356 alloy of PVF (13.2%) [34]. Gas entrapped with the reinforcement materials during injection into the melt could be the cause of this problem. However, the PVF value decreased to only 1.2% after thixoforming and with short T6. The substantial drop in the porosity was due to the compression effect on the semi-solid structure during thixoforming and gas released by the eutectic Si particles evolution during the heat treatment process. 3.3. Homogenous distribution and wettability of MWCNT Interestingly, the MWCNTs were uniformly distributed without agglomeration on the composite tensile fracture surface, as shown in Fig. 9. This evidence suggested that by injecting the MWCNT at the bottom of the crucible followed by mechanical stirring, homogenous dispersion of the reinforcements was obtained throughout the matrix. Some MWCNTs appeared on top of the molten alloy during injection and disappeared after 5 min of stirring, thereby indicating that the vortex condition created by the impeller centrifugal action slowly distributed the MWCNT [10,35]. Then, the additional stirring time helped maintain the MWCNT in a state of suspension in the matrix. Another important finding was no sign of deterioration of the MWCNT tube structures due to thermal effect at 650 C that create bridges in between the grains for the effective mechanical load transfer [36,37]. At higher magnification image of FESEM, some of MWCNTs have bridged the gap across the crack matrix, as shown in Fig. 10(a). Moreover, there were some pull-out structures of MWCNT on the fracture surface, as shown in Fig. 10(a) and (b), thereby confirming the strong load transfer. The carbon element of MWCNT was confirmed by the EDX analysis in Fig. 11. Both of these bridging and pull-out phenomena confirmed that proper wettability and good
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Fig. 7. OM microstructure images of (a) mechanical stirred (b) after thixoforming and (c) after short T6.
Fig. 8. OM microstructure images of (a) mechanical stirred and (b) after thixoforming and short T6.
interfacial bonding between MWCNTs and the matrix had been achieved. The addition of surfactant Mg into the molten composite decreased the surface tension of the liquid matrix by producing transient layers between them, thereby improving wettability and enhancing the interfacial bonding [18,35,38]. It is generally accepted that the interface between the MWCNT and the matrix plays a major role in strengthening the composite. A strong interfacial bond provides an effective load transfer from the matrix to the reinforced particle, as stated by Rikhtegar et al. [38] and Chen et al. [39]. Further analysis on the mechanical properties of the composite is discussed in the next section to confirm the contribution of the interface. 3.4. Short heat treatment The nature of semisolid microstructures allows the application of shorter heat treatment duration to aluminium alloys. Menargues et al. [30] have recommended that the minimum solution treatment needs to be at 540 C for 15 min for dissolving, homogenising
and spheroidisation of eutectic Si. Long et al. [39] reported the negative influence of prolonged solution treatment, which causes diffusion of Mg out of the Al matrix and increased alloy porosity. Moreover, both studies agreed that most of the eutectic Si particles gradually changed to spheroid shape starting from 30 min duration onwards. Therefore, with the presence of MWCNT in the matrix, the duration of 1 h solid solution treatment is sufficient to allow integration between the soluble phases and reinforced particles. Likewise, timely homogenisation of alloy elements, spheroidisation and coarsening of eutectic Si should be permitted, as shown in Fig. 7(c). The high quenching rate at room temperature (27 C) retained the Mg2Si, Al2Cu and reinforced particles in the solid solution. High artificial ageing of 180 C allowed shorter time for the distribution of small precipitation to take place and eventually increased the strength. In addition, the fracture surface of the sample as shown in Fig. 12, showed domination of dimple fractures, thereby confirming the improvement in ductility or elongation rate with thixoforming and short T6 alloy [40]. Therefore, the short T6 heat treatment after
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Fig. 9. FESEM image of MWCNTs homogenously dispersed in the sintered A356 composite.
thixoforming further improved the microstructure, homogeneous distribution of reinforcements and ductility of the composite. 3.5. Mechanical properties In additional to the dispersion and wettability of the MWCNT in the matrix, the thixoforming and short T6 heat treatment also played significant roles in the improvement of the composite strength. As mentioned earlier, the strengthening process is the sum of load transfer, thermal mismatch dislocation and Orowan
Fig. 12. Fracture behaviour of the short T6 MWCNT-A356 alloy composite.
looping system. Therefore, the influence of the MWCNT reinforcement on the composite is obtained from the results after the mechanical stirring stage, as shown in Fig. 13. The yield strength (YS), ultimate tensile strength (UTS) and elongation to fracture were improved by 14.4% (135.0 MPa), 34.2% (178 MPa) and 72.2% (3.1%), as compared with the as-cast alloy 115.0 MPa, 132.9 MPa and 1.8%, respectively. According to Park et al. [10], load transfer is the main contributor to the yield strength improvement, as compared with thermal mismatch, as shown by using a shear lag model. This model is only valid when it is assumed that there is a perfect interfacial bonding between reinforcement and matrix. Thus, in
Fig. 10. FESEM images of MWCNTs (a) bridging across grain boundaries, (b) pull outs of MWCNT from fracture surface.
Fig. 11. FESEM e EDX at location (red box). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 13. Yield strength (YS) and ultimate tensile strength (UTS) comparison.
this study, the bridging and pull-out of MWCNT structures justified the assumption. Furthermore, the tensile strength involved a plastic deformation of the material. Therefore, the Orowan looping system was considered. The Orowan looping is the obstruction made through the reinforcement to the motion of dislocations by the thermal mismatch. As a result, back stress is produced, thereby preventing further dislocation and enhancing the strength of the composite [11]. Moreover, the strong interfacial bonding will not only contribute to the effective load transfer but also improve the elongation or ductility simultaneously [41]. The YS, UTS and elongation in Fig. 13 for the samples after thixoforming revealed further increments by 33.3% (180.0 MPa), 43.3% (255.8 MPa) and 83.9% (5.7%), respectively, as compared with the non-thixoformed composite samples. As mentioned in Section 3.2, the reduction or elimination of porosity using thixoforming is well established. Hence, with minimum porosity, the strength and ductility of the thixoformed samples are expected to be increased [29,42,43]. Besides, it is generally believed that during partial remelting of thixoforming, the homogeneous distribution of MWCNT in the matrix remains unchanged and simultaneously fills the pores, thereby increasing the compactness of the microstructure [44]. The short T6 heat treatment slightly increased the YS, UTS and elongation by 19.4% (215.0 MPa), 8.3% (277 MPa) and 33.3% (7.6%), respectively, as compared with the thixoformed composite, as depicted in Fig. 13. Based on the results, 1 h of solution treatment was sufficient to homogenise and complete the dissolution of eutectic phases involved. At the same time, further stabilisation of the reinforcement position in the matrix further improves the mechanical properties. The combination of mechanical stirring, thixoforming and short T6 of A356-MWCNT resulted in tremendous improvement in mechanical properties by up to 87.0%, 108.4% and 322.2% for YS, UTS and elongation, respectively. These results showed a good agreement with Elshalakany et al. [35], who also found that the maximum YS, UTS and elongation to fracture of 1.5 wt% of MWCNTA356 composite fabricated using rheocasting or squeeze casting were improved by 60%, 50% and 320%, respectively. The strength of the composite obtained from this studies met the recommended range of 250e300 MPa for automotive parts, according to TavitasMedrano et al. [45]. The hardness of the studied samples is shown in Fig. 14. The hardness of A356-MWCNT composite (73.9 HV) improved by 24% as compared with the as-cast A356 alloy (59.5 HV), thereby showing the direct influence of MWCNT in the matrix. Subsequently, the hardness of the composites further increased to 91.8 and 106.4 HV
Fig. 14. Hardness values of each stage.
after being subjected to thixoforming and short T6 heat treatment processes, respectively. The increase in hardness can be attributed to the homogeneous distribution of MWCNT in the matrix, porosity reduction and reduction of microstructure grain size. The effects of the distribution and porosity filling have been discussed in the previous section. Thus, grain refinement could be predicted based on Hall-Petch strengthening mechanism [46]. However, the grain size alone contributed only about 70%e80% of the reinforcement strengthening mechanism [19]. According to Hall-Petch strengthening mechanism, the hardness increases with decreasing crystallite size [19]. Therefore, in this study, it is suggested that the high hardness of the composite was due to two actions, as follows: first, by the mechanical stirring action that resulted in smaller and more uniform fragmentation of dendritic structures; and second, by the addition of MWCNT that acts as heterogeneous nucleation, thereby promoting further grain refinement to the grain sizes [17,47].
4. Conclusions In this study, 0.5 wt% A356-MWCNT composite was successfully fabricated using the mechanical stir casting method compounded by thixoforming and short T6 heat treatment processes. Mg was premixed with MWCNT powder and utilised as wetting agent to ensure proper mixing between reinforced and matrix materials. The following conclusions are drawn: (1) The microstructure evolved from a-Al dendritic to mostly rosette-like and large eutectic Si grains. Coarsening and spheroidisation of eutectic Si was observed for all samples after short T6. The porosity volume fraction was improved from 14.0% to 1.2% from as-cast to after thixoforming and short T6. (2) The FESEM observations proved that the MWCNT were successfully distributed homogenously in the matrix. The evidence of pull-out conditions of MWCNT at the fractured surface from tensile fracture samples demonstrated effective load transfer and the occurrence of wetting. (3) The dominated dimple fractures on the tensile fracture samples were proof of higher ductility of the composite after the modified heat treatment process. The advantage of globular microstructure of the matrix allowed shorter duration from the standard T6 heat treatment process.
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(4) The overall mechanical properties were improved by 87.0% (215.0 MPa), 108.4% (277.0 MPa) and 322.2% (7.6%) for YS, UTS and elongation rate, respectively after mechanical stirring, thixoforming and short T6. Furthermore, as compared with as-cast alloy, the hardness increased by 78.8% from 59.5 HV to 106.4 HV. 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. References [1] B.V. Ramnath, C. Elanchezhian, R.M. Annamalai, S. Aravind, T.S.A. Atreya, V. Vignesh, C. Subramanian, Aluminium metal matrix composites - a review, Rev. Adv. Mater. Sci. 38 (2014) 55e60. [2] J. Singh, A. Chauhan, Characterization of hybrid aluminum matrix composites for advanced applications e a review, J. Mater. Res. Technol. 5 (2015) 1e11. [3] G. Yamamoto, K. Shirasu, T. Hashida, T. Takagi, J.W. Suk, J. An, R.D. Piner, Nanotube fracture during the failure of carbon nanotube/alumina composites, Carbon 49 (2011) 3709e3716. [4] A.D. Moghadam, E. Omrani, P.L. Menezes, P.K. Rohatgi, Mechanical and tribological properties of self-lubricating metal matrix nanocomposites reinforced by carbon nanotubes ( CNTs ) and graphene - a review, Compos. Part B. 77 (2015) 402e420. [5] M.T.Z. Hassan, A.M.K. Esawi, S. Metwalli, Effect of carbon nanotube damage on the mechanical properties of aluminiumecarbon nanotube composites, J. Alloys Compd. 607 (2014) 215e222. [6] F. Qiu, X. Gao, J. Tang, Y.Y. Gao, S.L. Shu, X. Han, Q. Li, Microstructures and tensile properties of AleCu matrix composites reinforced with nano-sized SiCp fabricated by semisolid stirring process, Metals (Basel). 7 (2017) 49. [7] K. Sekar, K. Allesu, M.A. Joseph, Effect of T6 heat treatment in the microstructure and mechanical properties of A356 reinforced with nano Al2O3 particles by combination effect of stir and squeeze casting, Procedia Mater. Sci. 5 (2014) 444e453. [8] K.S. Alhawari, M.Z. Omar, M.J. Ghazali, M.S. Salleh, M.N. Mohammed, Wear Properties of A356/Al2O3metal matrix composites produced by semisolid processing, Procedia Eng. 68 (2013) 186e192. [9] M. Mansoor, M. Shahid, Carbon nanotube-reinforced aluminum composite produced by induction melting, J. Appl. Res. Technol. 14 (2016) 215e224. [10] J.G. Park, D.H. Keum, Y.H. Lee, Strengthening mechanisms in carbon nanotubereinforced aluminum composites, Carbon 95 (2015) 690e698. [11] F. Mokdad, D.L. Chen, Z.Y. Liu, B.L. Xiao, D.R. Ni, Z.Y. Ma, Deformation and strengthening mechanisms of a carbon nanotube reinforced aluminum composite, Carbon 104 (2016) 64e77. [12] S. Dong, J. Zhou, D. Hui, Y. Wang, S. Zhang, Size dependent strengthening mechanisms in carbon nanotube reinforced metal matrix composites, Composites Part A 68 (2015) 356e364. [13] S.R. Bakshi, A. Agarwa, An analysis of the factors affecting strengthening in carbon nanotube reinforced aluminum composites, Carbon 49 (2011) 533e544. [14] K. Sita, R. Raju, V. Ramachandra, P. Rama, M. Raju, S. Rajesh, G. Partha, Enhancement of the mechanical properties of an aluminum metal matrix nanocomposite by the hybridization technique, J. Mater. Res .Techol. 5 (2017) 241e249. [15] K.S.R. Raju, V.R. Raju, P.R.M. Raju, S. Rajesh, G. Partha, Enhancement of the mechanical properties of an aluminum metal matrix nanocomposite by the hybridization technique, J. Mater. Res. Technol. 5 (2016) 241e249. [16] M.J.S. Hashim, J. Looney, L. Hashimi, The wettability of SiC particle by molten aluminium alloy, J. Mater. Process. Technol. 199 (2001) 324e328. [17] A. Bakr, A. Khattab, T.A. Osman, B. Azzam, M. Zaki, A novel technique for dispersion of MWCNTs in aluminum alloys, Minia J. Eng. Technol. (2014) 33. [18] M.T. Sijo, K.R. Jayadevan, Analysis of stir cast aluminium silicon carbide metal matrix composite: a comprehensive review, Procedia Technol. 24 (2016) 379e385. [19] C.R. Bradbury, J.-K. Gomon, L. Kollo, H. Kwon, M. Leparoux, Hardness of multi wall carbon nanotubes reinforced aluminium matrix composites, J. Alloys Compd. 585 (2014) 362e367. [20] M. Shayan, B. Niroumand, Synthesis of A356-MWCNT nanocomposites through a novel two stage casting process, Mater. Sci. Eng. A. 582 (2013) 262e269.
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[32] E.J. Zoqui, M.T. Shehata, M. Paes, V. Kao, E. Es-Sadiqi, Morphological evolution of SSM A356 during partial remelting, Mater. Sci. Eng. A. 325 (2002) 38e53. [33] M.S. Salleh, M.Z. Omar, K.S. Alhawari, M.N. Mohammed, M.A. Mad Ali, E. Mohamad, Microstructural evolution and mechanical properties of thixoformed A319 alloys containing variable amounts of magnesium, Trans. Nonferrous Metals Soc. China 26 (2016) 2029e2042. [34] S. Tahamtan, A.F. Boostani, Evaluation of pitting corrosion of thixoformed A356 alloy using a simulation model, Trans. Nonferrous Metals Soc. China 20 (2010) 1702e1706. [35] A.B. Elshalakany, T.A. Osman, A. Khattab, B. Azzam, M. Zaki, Microstructure and mechanical properties of MWCNTs reinforced A356 aluminum alloys cast nanocomposites fabricated by using a combination of rheocasting and squeeze casting techniques, J. Nanomater. (2014) 1e14. [36] I. Srikanth, N. Padmavathi, P.S.R. Prasad, P. Ghosal, R.K. Jain, C. 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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effects of mechanical stirring and short heat treatment on thixoformed of carbon nanotube aluminium alloy composite H. Hanizam a, b, M.S. Salleh c, *, M.Z. Omar b, A.B. Sulong b a Department of Manufacturing Technology, Fakulti Teknologi Kejuruteraan, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, Durian Tunggal Melaka, 76100, Malaysia b Department of Mechanical and Materials Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, Selangor, 43600, Malaysia c Department of Manufacturing Process, Fakulti Kejuruteraan Pembuatan, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, Durian Tunggal Melaka, 76100, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 October 2018 Received in revised form 9 January 2019 Accepted 18 February 2019 Available online 19 February 2019
The present work aimed to determine the effects of thixoforming and short T6 heat treatment processes on the microstructure and mechanical properties of thixoformed A356 alloy reinforced with 0.5 wt% multi-walled carbon nanotube (A356-MWCNT). The semisolid composite feedstock was produced by a mechanical stirring route followed by thixoforming, and finally, it was heat treated with a shorter solution treatment and artificial ageing hours. A premix of 0.5 wt% magnesium (Mg) as wettability agent and MWCNT was injected into molten A356 alloy at 650 C. Mixing and stirring were performed by a using three-blade impeller at 500 rpm for 10 min, and the mixture was poured into a preheated mould. Microstructure studies show the mechanical stirring effects on the transformation of dendritic arms to mostly globular and rosette structures of a-Al. The formations of more spheroidised structure of eutectic silicon (Si) were predominant after the heat treatment, thereby revealing the effectiveness of shorter T6 heat treatment. Results of field emission scanning electron microscopy images showed uniform distribution and pull-out structures of MWCNT throughout the matrix, thereby justifying the effective load transfer and wettability between reinforcement and alloy matrix. Subsequently, the mechanical properties of the composite shown significant improvements after each stage. The yield strength (YS), ultimate tensile strength (UTS) and elongation to fracture of cast A356 alloy increased from 115 MPa, 132.9 MPa and 1.8% to 135 MPa, 178.3 MPa and 3.1% respectively, in the A356-MWCNT. Consequently, these properties were further improved to 180 MPa, 255.8 MPa and 5.7% after the thixoforming process. The highest attainment of yield strength (YS), ultimate tensile strength (UTS) and elongation to fracture after short T6 of A356-MWCNT were 215 MPa, 277.0 MPa and 7.6%, respectively. The hardness of the samples was improved from 59.5 HV in as-cast alloy to 106.4 HV in thixoformed short T6 A356-MWCNT. © 2019 Elsevier B.V. All rights reserved.
Keywords: Semisolid metal processing Thixoforming T6 heat treatment Carbon nanotube Metal matrix composite
1. Introduction The utilisation of aluminium alloy metal matrix composite (MMC) has increased in recent years, especially in automotive, aerospace and military industries. Silicon carbide (SiC) and aluminium oxide (Al2O3) are frequently used as reinforcement materials in aluminium alloy matrix [1]. The composites have the
* Corresponding author. E-mail addresses: [email protected] (H. Hanizam), [email protected] (M.S. Salleh), [email protected] (M.Z. Omar), [email protected] (A.B. Sulong). https://doi.org/10.1016/j.jallcom.2019.02.217 0925-8388/© 2019 Elsevier B.V. All rights reserved.
advantages of reduction in the overall weight, fuel consumption and pollution of greenhouse gas emissions without sacrificing structural integrity [2]. Since the discovery of carbon nanotubes (CNT), more studies on various fabrications of CNT/aluminium alloy composites have been performed. Previous studies indicated that the major obstacles in the fabrication process are the agglomeration and poor distribution of CNT in the metal matrix. Therefore, to overcome these difficulties, a solid state powder metallurgy (PM) fabrication route is most commonly used [3,4]. However, many researchers have reported the occurrence of CNT structural damage due to ball milling process in PM, which affects the overall strength [5]. A liquid state (LS) route, the agglomeration due to the strong van der Waals forces between carbon atoms and large differences
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in densities are still the main issues. However, the LS route is very flexible in term of the intricate parts and bulk productions; thus, it is worth exploring. In general, the LS route involves stirring action to disperse the reinforcement throughout the matrix. The effects of stirring have been widely investigated not only for CNTs but also for other reinforcement materials [6e9]. The controlled stirring motion creates a vortex condition in the liquid matrix that helps disperse the reinforcement. Basically, the strengthening mechanisms of CNT/Al composite are load transfer, dislocations by thermal mismatch and Orowan looping system [10e13]. Therefore, besides the homogeneous distribution of the CNT, a good wettability between the reinforcement and matrix is essential in building up good interfacial bonding and effective load transfer in the regions. Wettability is the ability of the CNT to wet and break down surface tension of the matrix [14,15]. One of the methods involves adding some alloying elements (e.g. magnesium, calcium, zirconium, titanium, bismuth, lead, zinc and copper) [16] during the mixing process. For instance, magnesium (Mg) powder is commonly used to improve wettability between aluminium and CNT in LS fabrication [17,18]. In addition, the amount of reinforcement in the matrix also has a major effect on the mechanical properties of the composites. For instance, Bakr et al. [17] reported an increasing trend in hardness of liquid state fabrication of CNT/A356 composite from 0.5, 1.0 and 2.5 wt% CNT. However, compressive strength dropped considerably beyond 1.0 wt% CNT. Similarly, Bradbury et al. [19] obtained the highest hardness of 140 HV with 6.0 wt% CNT, and hardness started to deteriorate beyond this percentage. Moreover, Shayan and Niroumand [20] performed a two-stage process first by dispersing a total of 0.5 wt% of CNT powder in between the A356 alloy plates, stacking and rolling down to 50% of the original thickness and then melting it down. These processes improved the hardness and shear strength of the composite by 38% and 20%, respectively. Furthermore, the strength of the aluminium matrix composite can be further improved by additional secondary processes, such as thixoforming and heat treatment. Thixoforming is a process of reheating a thixotropic feedstock to a certain semi-solid temperature and compacting into a near-net shape mould. The thixotropic behaviour resulted from the arm breaking of the dendritic a-Al structure and formation of rosette or globular shapes microstructures due to shear forces. There are various thixotropic behaviour production routes available, but the mechanical stirring and cooling slope methods are the most common [21,22]. Moreover, during thixoforming, simultaneous coarsening of non-dendritic structure a-Al and changing of silicon phase morphology occur, thereby leading to the mechanical properties enhancement [23,24]. Likewise, heat treatment process, which also alters the microstructures, dissolves and homogenising soluble phases, thereby helping strengthen and prevent a catastrophic failure of the composites. A T6 heat treatment is a typical technique applied to A356 alloy that involves solution treatment, quenching and ageing processes. The solution treatment is performed at near eutectic temperature of the alloy ranging between 540 C and 550 C. Dissolving soluble phases of Mg2Si, Al2Cu, b-Al5FeSi, p-Al9FeMg3Si6 and Al5Cu2Mg8Si5 into a solid solution causes precipitation hardening, alloy elements homogenisation and eutectic Si spheroidisation during solidification [25]. Quenching process is carried out to cool the alloy rapidly, thereby suppressing the precipitation at room temperature. A high level supersaturation after quenching causing rapid formation of Guinier-Preston (GP) zones. These GP zones are clustered and coherent with the matrix, thereby hindering dislocation and increasing strength [26,27]. The artificial ageing between 150 C and 210 C forms clusters of precipitation that have larger sizes than GP zones.
Many published studies describe the role of T6 heat treatment in strengthening the alloy. For instance, Zhu et al. [28] used the T6 heat treatment method to improve the tensile properties of A356 alloy by approximately 10%. Numerous studies have also attempted to modify the standard parameters of T6 for several reasons. Peng et al. [29] compared standard T6 versus shorter time of T6 by using solution treatment at 550 C for 2 h, quenching in hot water at 70 C and artificial ageing at 170 C for 2 h on A356 alloy. As a result, the yield strength, UTS and elongation percentage can match 90%, 95% and 80% of the standard T6. Moreover, in semisolid processing, Menargues et al. [30] have applied a new short heat treatment, as follows: solution treatment at 540 C for 15 min, soaking in water at 25 C and artificial ageing at 180 C for 3 h. The globulisation of Si particles took place in 5 min, followed by dissolution of the Mg2Si phase after 10 min, due to non-dentritic boundary conditions. Moreover, the alloy reached its maximum hardness between 15 and 20 min, as a result of a-phase hardening. Although studies on CNT/aluminium alloy matrix composites have been carried out by other researchers, their focus has been the powder route rather than the liquid route due to the difficulties of mixing two hugely different densities of CNT and aluminium materials. In addition, it is very hard to achieve homogeneous distribution of the CNT throughout the matrix in the liquid state. Scientific understanding on the behaviour of MWCNT upon thixoforming and heat treatment processes is limited. This study presents a new method for the synthesis of MWCNT into A356 matrix using mechanical stirring followed by thixoforming and short T6 heat treatment processes. Homogeneity of MWCNT throughout the matrix and evidence of effective load transfer between both materials were observed using field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectroscopy (EDX) on the fracture surface from tensile samples. Furthermore, the effects of each process on the microstructure evolution and mechanical properties were investigated. 2. Experimental procedures The alloy used as metal matrix was an as-cast A356 alloy with a chemical composition as depicted in Table 1. MWCNT produced by SigmaeAldrich with purity of more than 95%, outside diameter of 20e40 nm, inside diameter of 5e10 nm and length of 10e30 mm was utilised as reinforced material. Minimum entanglement and almost no defect were detected from the thermal electron microscopy (TEM) images of the raw MWCNT, as shown in Fig. 1. Various tube diameters with multiple wall of graphene layers were observed. As illustrated in Fig. 2, 0.5 wt% of MWCNT powder and 0.5 wt% of granule Mg with an average of 1 mm diameter as wetting agent were mixed and wrapped in aluminium foil. The mixing processes were performed as follows. First, the alloy (400 g) was heated up in an induction furnace up to 700 C until it fully melted. Next, the molten alloy was brought down to a constant temperature of 650 C to avoid the degradation of MWCNT structure following Datsyuk et al. [27]. Then, the wrapped MWCNT was placed inside a plunger and injected at the bottom of the crucible. The mix was then stirred mechanically at a medium speed of 500 rpm by using a three-blade impeller. Medium speed was used to avoid air entrapment, which could lead to high porosity of the composite [28]. The composite
Table 1 Composition of 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
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Fig. 1. TEM images of as-received MWCNT a) Magnification X100k b) Magnification X600k.
Fig. 2. Schematic diagram of the mixing process.
mixing was then poured into a preheated (150 C) mould directly to form the thixotropic feedstock billets. A cooling slope method was preferred in the initial stage, but after several attempts, large amounts of MWCNT accumulated on the top side of the billet due to the rolling effect of the cooling slope. The thixoforming process was carried out using the T30-80 KHz thixoforming machine (Fig. 3(a)). The thixotropic billet was placed on a pneumatic cylinder ram inside an induction coil and reheated up to semi-solid temperature of 580 C and yielded an equivalent to 50% liquid fraction. Differential scanning calorimetry (DCS) was
used to estimate the ratio of solid-liquid fraction as shown in Fig. 4. The reheating process was controlled by gradual increment of the heating frequency at 50 A per minute until it reached the target temperature. Then, the billet was rammed with forging load of 5 tons and speed of 1 m/s into preheated (100 C) hot work tool steel mould on top of the coil. The billet was then collected from the mould and cooled at room temperature (Fig. 3(b)). Some of the thixoformed samples underwent a short T6 heat treatment with a shorter solution treatment temperature at 540 C for 1 h followed by quenching in water at room temperature and aging artificially at 180 C for 2 h, in the Nabertherm 30 C€ller et al. [31] reported that the solution 30000 C furnace. Mo treatment time of 1 h at 540 C was sufficient to produce the highest hardness on A356 semi-solid. In addition, Menargues et al. [30] stated that short overall solution treatment was sufficient for the formation of globular microstructure of silicon. The a-phase formation reached its maximum hardness after 20 min. All samples
Fig. 3. (a) Thixoforming machine, (b) Before and after thixoformed billets (samples).
Fig. 4. Heat flow and liquid fraction profile of A356 Alloy.
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were sectioned and prepared using a standard metallographic procedure, such as grinding in different grit sizes (400, 600, 800 and 1200), polishing (6, 3 and 1 mm) with diamond paste and etching for 10 s with Kellers solution. The microstructures and distribution of A356-MWCNT composite were examined using an optical microscopy (OM) and FESEM/EDX using Hitachi SU5000 machine. The porosity volume fraction (%) was measured by using automatic iSolution DT software of the OM. The hardness and tensile strength were determined using Vickers hardness testing Matsuzawa machine (load ¼ 1 kgf and Dwell time ¼ 10 s) and tensile tests using an Autograph universal testing machine. The tensile samples were machined in accordance with ASTM E8M standard (Fig. 5) for yield strength (YS) at 0.2% plastic strain offset, ultimate tensile strength (UTS) and elongation to fracture (%). Moreover, the as-cast billet of A356 was tested and used as reference. At least three samples were tested for each step to obtain reliable results. 3. Results and discussion 3.1. Microstructures evolution Fig. 6 shows the microstructure of the as-received A356 poured at 650 C with the typical dendritic a-Al morphology with interdendritic eutectic microstructures. Fig. 7(a) shows non-uniform rosette-like and globular microstructures of a-Al after the mechanical stirring process. The dendritic arms were broken up by the external mechanical forces, and islands formed. Moreover, the nonuniform or irregular shapes of the a-Al might also have contributed to the existence of MWCNT particles in the matrix. According to Shayan and Niroumand [20], the inertia of turbulence flow minimised the formation of secondary dendritic arms and grain size microstructure as the melt solidified during the pouring process. Therefore, the inertia of the turbulent couple with the inertia of reinforced particles in the melt during stirring disturbed the formation of uniform globular microstructure, but such result was sufficient for the next thixoforming process. Furthermore, Fig. 7(b) shows the microstructures of the composite after the thixoforming process. The reheating at 50% liquid fraction and compacting of the billet resulted in globule growth and in coarsened and more rounded a-Al morphology. Zoqui et al. [32] investigated the a-Al growth of 70.2% from the as-cast A356 alloy at 580 C. In addition, eutectic Si particles were remelted, their positions were rearranged; they formed plate-like structures surrounding the a-Al globules [33]. The thixoformed and short T6 heat treatment have shown some physical changes to the microstructures (Fig. 7(c)). The eutectic Si transformed from plate-like to spheroid-like and coarsened
Fig. 5. ASTM E8M tensile test samples.
Fig. 6. OM microstructure image of as-recieved A356 at (mag. x500).
[28,30], after which further coarsening of a-Al was observed. However, the presence of MWCNT particles in the regions prevented the creation of uniform a-Al. 3.2. Porosity Some porosity was detected even with slow stirring of the molten composite because of air entrapment and shrinkage during solidification. The porosity volume fraction (PVF) % was measured at 14.0% for samples after mechanical stirring, as shown in Fig. 8. The PVF value was slightly higher as compared with the as-cast A356 alloy of PVF (13.2%) [34]. Gas entrapped with the reinforcement materials during injection into the melt could be the cause of this problem. However, the PVF value decreased to only 1.2% after thixoforming and with short T6. The substantial drop in the porosity was due to the compression effect on the semi-solid structure during thixoforming and gas released by the eutectic Si particles evolution during the heat treatment process. 3.3. Homogenous distribution and wettability of MWCNT Interestingly, the MWCNTs were uniformly distributed without agglomeration on the composite tensile fracture surface, as shown in Fig. 9. This evidence suggested that by injecting the MWCNT at the bottom of the crucible followed by mechanical stirring, homogenous dispersion of the reinforcements was obtained throughout the matrix. Some MWCNTs appeared on top of the molten alloy during injection and disappeared after 5 min of stirring, thereby indicating that the vortex condition created by the impeller centrifugal action slowly distributed the MWCNT [10,35]. Then, the additional stirring time helped maintain the MWCNT in a state of suspension in the matrix. Another important finding was no sign of deterioration of the MWCNT tube structures due to thermal effect at 650 C that create bridges in between the grains for the effective mechanical load transfer [36,37]. At higher magnification image of FESEM, some of MWCNTs have bridged the gap across the crack matrix, as shown in Fig. 10(a). Moreover, there were some pull-out structures of MWCNT on the fracture surface, as shown in Fig. 10(a) and (b), thereby confirming the strong load transfer. The carbon element of MWCNT was confirmed by the EDX analysis in Fig. 11. Both of these bridging and pull-out phenomena confirmed that proper wettability and good
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Fig. 7. OM microstructure images of (a) mechanical stirred (b) after thixoforming and (c) after short T6.
Fig. 8. OM microstructure images of (a) mechanical stirred and (b) after thixoforming and short T6.
interfacial bonding between MWCNTs and the matrix had been achieved. The addition of surfactant Mg into the molten composite decreased the surface tension of the liquid matrix by producing transient layers between them, thereby improving wettability and enhancing the interfacial bonding [18,35,38]. It is generally accepted that the interface between the MWCNT and the matrix plays a major role in strengthening the composite. A strong interfacial bond provides an effective load transfer from the matrix to the reinforced particle, as stated by Rikhtegar et al. [38] and Chen et al. [39]. Further analysis on the mechanical properties of the composite is discussed in the next section to confirm the contribution of the interface. 3.4. Short heat treatment The nature of semisolid microstructures allows the application of shorter heat treatment duration to aluminium alloys. Menargues et al. [30] have recommended that the minimum solution treatment needs to be at 540 C for 15 min for dissolving, homogenising
and spheroidisation of eutectic Si. Long et al. [39] reported the negative influence of prolonged solution treatment, which causes diffusion of Mg out of the Al matrix and increased alloy porosity. Moreover, both studies agreed that most of the eutectic Si particles gradually changed to spheroid shape starting from 30 min duration onwards. Therefore, with the presence of MWCNT in the matrix, the duration of 1 h solid solution treatment is sufficient to allow integration between the soluble phases and reinforced particles. Likewise, timely homogenisation of alloy elements, spheroidisation and coarsening of eutectic Si should be permitted, as shown in Fig. 7(c). The high quenching rate at room temperature (27 C) retained the Mg2Si, Al2Cu and reinforced particles in the solid solution. High artificial ageing of 180 C allowed shorter time for the distribution of small precipitation to take place and eventually increased the strength. In addition, the fracture surface of the sample as shown in Fig. 12, showed domination of dimple fractures, thereby confirming the improvement in ductility or elongation rate with thixoforming and short T6 alloy [40]. Therefore, the short T6 heat treatment after
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Fig. 9. FESEM image of MWCNTs homogenously dispersed in the sintered A356 composite.
thixoforming further improved the microstructure, homogeneous distribution of reinforcements and ductility of the composite. 3.5. Mechanical properties In additional to the dispersion and wettability of the MWCNT in the matrix, the thixoforming and short T6 heat treatment also played significant roles in the improvement of the composite strength. As mentioned earlier, the strengthening process is the sum of load transfer, thermal mismatch dislocation and Orowan
Fig. 12. Fracture behaviour of the short T6 MWCNT-A356 alloy composite.
looping system. Therefore, the influence of the MWCNT reinforcement on the composite is obtained from the results after the mechanical stirring stage, as shown in Fig. 13. The yield strength (YS), ultimate tensile strength (UTS) and elongation to fracture were improved by 14.4% (135.0 MPa), 34.2% (178 MPa) and 72.2% (3.1%), as compared with the as-cast alloy 115.0 MPa, 132.9 MPa and 1.8%, respectively. According to Park et al. [10], load transfer is the main contributor to the yield strength improvement, as compared with thermal mismatch, as shown by using a shear lag model. This model is only valid when it is assumed that there is a perfect interfacial bonding between reinforcement and matrix. Thus, in
Fig. 10. FESEM images of MWCNTs (a) bridging across grain boundaries, (b) pull outs of MWCNT from fracture surface.
Fig. 11. FESEM e EDX at location (red box). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 13. Yield strength (YS) and ultimate tensile strength (UTS) comparison.
this study, the bridging and pull-out of MWCNT structures justified the assumption. Furthermore, the tensile strength involved a plastic deformation of the material. Therefore, the Orowan looping system was considered. The Orowan looping is the obstruction made through the reinforcement to the motion of dislocations by the thermal mismatch. As a result, back stress is produced, thereby preventing further dislocation and enhancing the strength of the composite [11]. Moreover, the strong interfacial bonding will not only contribute to the effective load transfer but also improve the elongation or ductility simultaneously [41]. The YS, UTS and elongation in Fig. 13 for the samples after thixoforming revealed further increments by 33.3% (180.0 MPa), 43.3% (255.8 MPa) and 83.9% (5.7%), respectively, as compared with the non-thixoformed composite samples. As mentioned in Section 3.2, the reduction or elimination of porosity using thixoforming is well established. Hence, with minimum porosity, the strength and ductility of the thixoformed samples are expected to be increased [29,42,43]. Besides, it is generally believed that during partial remelting of thixoforming, the homogeneous distribution of MWCNT in the matrix remains unchanged and simultaneously fills the pores, thereby increasing the compactness of the microstructure [44]. The short T6 heat treatment slightly increased the YS, UTS and elongation by 19.4% (215.0 MPa), 8.3% (277 MPa) and 33.3% (7.6%), respectively, as compared with the thixoformed composite, as depicted in Fig. 13. Based on the results, 1 h of solution treatment was sufficient to homogenise and complete the dissolution of eutectic phases involved. At the same time, further stabilisation of the reinforcement position in the matrix further improves the mechanical properties. The combination of mechanical stirring, thixoforming and short T6 of A356-MWCNT resulted in tremendous improvement in mechanical properties by up to 87.0%, 108.4% and 322.2% for YS, UTS and elongation, respectively. These results showed a good agreement with Elshalakany et al. [35], who also found that the maximum YS, UTS and elongation to fracture of 1.5 wt% of MWCNTA356 composite fabricated using rheocasting or squeeze casting were improved by 60%, 50% and 320%, respectively. The strength of the composite obtained from this studies met the recommended range of 250e300 MPa for automotive parts, according to TavitasMedrano et al. [45]. The hardness of the studied samples is shown in Fig. 14. The hardness of A356-MWCNT composite (73.9 HV) improved by 24% as compared with the as-cast A356 alloy (59.5 HV), thereby showing the direct influence of MWCNT in the matrix. Subsequently, the hardness of the composites further increased to 91.8 and 106.4 HV
Fig. 14. Hardness values of each stage.
after being subjected to thixoforming and short T6 heat treatment processes, respectively. The increase in hardness can be attributed to the homogeneous distribution of MWCNT in the matrix, porosity reduction and reduction of microstructure grain size. The effects of the distribution and porosity filling have been discussed in the previous section. Thus, grain refinement could be predicted based on Hall-Petch strengthening mechanism [46]. However, the grain size alone contributed only about 70%e80% of the reinforcement strengthening mechanism [19]. According to Hall-Petch strengthening mechanism, the hardness increases with decreasing crystallite size [19]. Therefore, in this study, it is suggested that the high hardness of the composite was due to two actions, as follows: first, by the mechanical stirring action that resulted in smaller and more uniform fragmentation of dendritic structures; and second, by the addition of MWCNT that acts as heterogeneous nucleation, thereby promoting further grain refinement to the grain sizes [17,47].
4. Conclusions In this study, 0.5 wt% A356-MWCNT composite was successfully fabricated using the mechanical stir casting method compounded by thixoforming and short T6 heat treatment processes. Mg was premixed with MWCNT powder and utilised as wetting agent to ensure proper mixing between reinforced and matrix materials. The following conclusions are drawn: (1) The microstructure evolved from a-Al dendritic to mostly rosette-like and large eutectic Si grains. Coarsening and spheroidisation of eutectic Si was observed for all samples after short T6. The porosity volume fraction was improved from 14.0% to 1.2% from as-cast to after thixoforming and short T6. (2) The FESEM observations proved that the MWCNT were successfully distributed homogenously in the matrix. The evidence of pull-out conditions of MWCNT at the fractured surface from tensile fracture samples demonstrated effective load transfer and the occurrence of wetting. (3) The dominated dimple fractures on the tensile fracture samples were proof of higher ductility of the composite after the modified heat treatment process. The advantage of globular microstructure of the matrix allowed shorter duration from the standard T6 heat treatment process.
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(4) The overall mechanical properties were improved by 87.0% (215.0 MPa), 108.4% (277.0 MPa) and 322.2% (7.6%) for YS, UTS and elongation rate, respectively after mechanical stirring, thixoforming and short T6. Furthermore, as compared with as-cast alloy, the hardness increased by 78.8% from 59.5 HV to 106.4 HV. 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. References [1] B.V. Ramnath, C. Elanchezhian, R.M. Annamalai, S. Aravind, T.S.A. Atreya, V. Vignesh, C. Subramanian, Aluminium metal matrix composites - a review, Rev. Adv. Mater. Sci. 38 (2014) 55e60. [2] J. Singh, A. Chauhan, Characterization of hybrid aluminum matrix composites for advanced applications e a review, J. Mater. Res. Technol. 5 (2015) 1e11. [3] G. Yamamoto, K. Shirasu, T. Hashida, T. Takagi, J.W. Suk, J. An, R.D. Piner, Nanotube fracture during the failure of carbon nanotube/alumina composites, Carbon 49 (2011) 3709e3716. [4] A.D. Moghadam, E. Omrani, P.L. Menezes, P.K. Rohatgi, Mechanical and tribological properties of self-lubricating metal matrix nanocomposites reinforced by carbon nanotubes ( CNTs ) and graphene - a review, Compos. Part B. 77 (2015) 402e420. [5] M.T.Z. Hassan, A.M.K. Esawi, S. Metwalli, Effect of carbon nanotube damage on the mechanical properties of aluminiumecarbon nanotube composites, J. Alloys Compd. 607 (2014) 215e222. [6] F. Qiu, X. Gao, J. Tang, Y.Y. Gao, S.L. Shu, X. Han, Q. Li, Microstructures and tensile properties of AleCu matrix composites reinforced with nano-sized SiCp fabricated by semisolid stirring process, Metals (Basel). 7 (2017) 49. [7] K. Sekar, K. Allesu, M.A. Joseph, Effect of T6 heat treatment in the microstructure and mechanical properties of A356 reinforced with nano Al2O3 particles by combination effect of stir and squeeze casting, Procedia Mater. Sci. 5 (2014) 444e453. [8] K.S. Alhawari, M.Z. Omar, M.J. Ghazali, M.S. Salleh, M.N. Mohammed, Wear Properties of A356/Al2O3metal matrix composites produced by semisolid processing, Procedia Eng. 68 (2013) 186e192. [9] M. Mansoor, M. Shahid, Carbon nanotube-reinforced aluminum composite produced by induction melting, J. Appl. Res. Technol. 14 (2016) 215e224. [10] J.G. Park, D.H. Keum, Y.H. Lee, Strengthening mechanisms in carbon nanotubereinforced aluminum composites, Carbon 95 (2015) 690e698. [11] F. Mokdad, D.L. Chen, Z.Y. Liu, B.L. Xiao, D.R. Ni, Z.Y. Ma, Deformation and strengthening mechanisms of a carbon nanotube reinforced aluminum composite, Carbon 104 (2016) 64e77. [12] S. Dong, J. Zhou, D. Hui, Y. Wang, S. Zhang, Size dependent strengthening mechanisms in carbon nanotube reinforced metal matrix composites, Composites Part A 68 (2015) 356e364. [13] S.R. Bakshi, A. Agarwa, An analysis of the factors affecting strengthening in carbon nanotube reinforced aluminum composites, Carbon 49 (2011) 533e544. [14] K. Sita, R. Raju, V. Ramachandra, P. Rama, M. Raju, S. Rajesh, G. Partha, Enhancement of the mechanical properties of an aluminum metal matrix nanocomposite by the hybridization technique, J. Mater. Res .Techol. 5 (2017) 241e249. [15] K.S.R. Raju, V.R. Raju, P.R.M. Raju, S. Rajesh, G. Partha, Enhancement of the mechanical properties of an aluminum metal matrix nanocomposite by the hybridization technique, J. Mater. Res. Technol. 5 (2016) 241e249. [16] M.J.S. Hashim, J. Looney, L. Hashimi, The wettability of SiC particle by molten aluminium alloy, J. Mater. Process. Technol. 199 (2001) 324e328. [17] A. Bakr, A. Khattab, T.A. Osman, B. Azzam, M. Zaki, A novel technique for dispersion of MWCNTs in aluminum alloys, Minia J. Eng. Technol. (2014) 33. [18] M.T. Sijo, K.R. Jayadevan, Analysis of stir cast aluminium silicon carbide metal matrix composite: a comprehensive review, Procedia Technol. 24 (2016) 379e385. [19] C.R. Bradbury, J.-K. Gomon, L. Kollo, H. Kwon, M. Leparoux, Hardness of multi wall carbon nanotubes reinforced aluminium matrix composites, J. Alloys Compd. 585 (2014) 362e367. [20] M. Shayan, B. Niroumand, Synthesis of A356-MWCNT nanocomposites through a novel two stage casting process, Mater. Sci. Eng. A. 582 (2013) 262e269.
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