Carbon nanotubes-reinforced aluminum alloy functionally graded materials fabricated by powder extrusion process

Author’s Accepted Manuscript Carbon nanotubes-reinforced aluminum alloy functionally graded materials fabricated by powder extrusion process Dasom Kim, Kwangjae Park, Kyungju Kim, Takamichi Miyazaki, Sungwook Joo, Sanghwui Hong, Hansang Kwon www.elsevier.com/locate/msea

PII: DOI: Reference:

S0921-5093(18)31835-5 https://doi.org/10.1016/j.msea.2018.12.128 MSA37400

To appear in: Materials Science & Engineering A Received date: 17 October 2018 Revised date: 30 December 2018 Accepted date: 31 December 2018 Cite this article as: Dasom Kim, Kwangjae Park, Kyungju Kim, Takamichi Miyazaki, Sungwook Joo, Sanghwui Hong and Hansang Kwon, Carbon nanotubes-reinforced aluminum alloy functionally graded materials fabricated by powder extrusion process, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2018.12.128 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Carbon nanotubes-reinforced aluminum alloy functionally graded materials fabricated by powder extrusion process

Dasom Kima, Kwangjae Parka, Kyungju Kimb Takamichi Miyazakic Sungwook Jood Sanghwui Hongd Hansang Kwona,e,*

a

Department of Materials System Engineering, Pukyong National University, 365, Sinseon-ro, Nam-gu, Busan 48547, Korea b

The International Science Technology Research Center, Pukyong National University, 365, Sinseon-ro, Nam-gu, Busan 48547, Korea c

Department of Instrumental Analysis, Tohoku University, 6-6-11 Aramaki-aza Aoba, Aoba-ku, Sendai, 980-8579, Japan

d

Department of Converged Technology Research, Gyeongbuk Hybrid Technology Institute, Goiyean-dong, Yeongcheon, Gyeongbuk 38899, Republic of Korea

e

Department of R&D, Next Generation Materials Co., Ltd., 365, Sinseon-ro, Nam-gu, Busan 48547, Korea

Abstract Tubular shape aluminum (Al) 6063/Al-3vol.% carbon nanotubes (CNT)/Al3003 functionally graded materials (FGMs) were fabricated by hot extrusion process. FGMs is consisted by Al6063 which exhibits high strength and hardness, Al-3vol.% CNT which exhibit superior mechanical and thermal properties and Al3003 which has high ductility. The Al-3vol.%CNT powder, which is middle layer of FGMs, was used to increase contact surface area with other materials resulting in improved interface properties. The composite powder was observed using a field-emission scanning electron microscope (FE-SEM). To analyze the microstructures, FGMs were analyzed by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy(TEM). Al-3vol.%CNT was recrystallized by friction with bulk. In this zone, the Vickers hardness value was almost 120 HV, which is twice of the values of Al6063and Al3003. Although the FGMs has high strength of 142 MPa, it also realizes high elongation of 22 %. We considered some possible strengthening mechanisms, including CNT strengthening, grain refinement, precipitation hardening, and oxide dispersion strengthening.

Keywords: Aluminum alloy, Multi-walled carbon nanotubes, Functionally graded materials, High-energy ball milling, Hot extrusion process

*Corresponding author. Tel.: +82 51 629 6383; fax: +82 51 629 6373 E-mail address: [email protected]

1

1. Introduction

Recently, functionally graded materials (FGMs) have been developed to meet the demand of achieving multiple functions in one material. FGMs can solve the problem of simple joining material, where the internal residual stress is generated due to the difference in the thermal properties of different materials by gradually changing the compositions of these materials [1,2]. However, it is difficult to control the composition when manufacturing FGMs; hence, considerable internal residual stress is generated virtually [3,4]. Some researchers have attempted to solve this problem by developing new methods for FGM fabrication. Shahrjerdi et al. attempted to fabricate hydroxyapatite–Titanium FGMs through pressure-less sintering. The fabricated FGMs exhibit linear shrinkage and higher Vickers hardness [5]. However, the degree of improvement in the mechanical properties is not large compared to that obtained using the complicate procedure which includes determination of powder percentage, blending, drying, ball milling, cold pressing and sintering. Hulbert et al. fabricated boron carbide–aluminum FGMs using spark plasma sintering (SPS) [6]. Although the fabricated FGMs exhibited improved hardness (8.36–32.3 GPa) and superior fracture toughness, considerable residual stress is generated in the FGMs owing to rapid heating and quenching. Chen et al. attempted to fabricate A2014/A1050 bimetallic pipes by using the multi-billet extrusion method. They demonstrated that the extrusion method can transform FGMs with novel interface properties into a new form, compared to other methods that have limitations in terms of FGM shapes that can be manufactured [7]. A number of combination such as metals and metals, metals and ceramics could be adaptable to FGM, as well as mechanical properties could be improved by adding reinforcements. Carbon nanotubes (CNT) is widely used as a reinforcement added to FGM owing to its superior mechanical, thermal, chemical properties. 2

CNT, which were discovered by Lijima in 1991, have nearly 100 times higher strength and superior thermal properties compared to steel. Several studies have been conducted to develop reinforced CNT composites, whose matrix generally involves metals, polymers, or ceramics [8-11]. Among them, the studies on aluminum (Al)-CNT composite have actively progressed in accordance with the demand for lightweight properties of materials, which are required for transport equipment such as automobiles, ships, and aircrafts [12,13]. In addition, in the case of Al-CNT FGM, it is suitable for expressing various functions in one material due to considerable variation of mechanical characteristics and microstructure according to the CNT content. Kwon et al. fabricated Al-CNT FGMs stacking Al-5,10,15 vol.% CNT powders on pure Al powder by SPS [14]. Each layer was well bonded without any interlayer defect, although the mechanical properties such as Vickers hardness are considerably different from 30 HV to 280 HV. Kumar et al. [15] reported that FGMs was prepared by sequentially stacking Al-0.5, 1 and 1.5% CNT plates on Al pate, and the mechanical and thermal properties of FGMs were superior to those of conventional Al plates. In this study, tubular Al6063/Al-3vol.% CNT/Al3003 FGMs was fabricated by hot extrusion process. The fabricated FGMs comprise Al3003, which has high elongation on the inner side, Al-3vol.% CNT, which exhibit high strength and harness on the middle side, and Al6063, which exhibits high strength on the outer side. To increase the interface contact area between the two materials, Al6063 and Al3003 bulk and Al-3vol.% CNT powder are used as the raw materials. The microstructure of the fabricated FGMs is analyzed by XRD, SEM with EDS, FE-SEM, and TEM. The Vickers hardness test and a tensile test are conducted to determine the mechanical properties. In addition, some strengthening mechanisms such as CNT strengthening, grain refinement, precipitation hardening, and oxide dispersion strengthening are considered by using the Hall–Petch equation and Kelly–Tyson equation.

3

2. Experimental procedure

2.1 Analysis of raw powder In this study, we used four raw materials: Al powder (ECKA Granules Japan Co. Ltd., purity 99.85%) with a particle size of 76 μm; MWCNT powder (Iljin Co. Ltd., purity 99.5%) with a length of ~10 μm and diameter of 20 nm; and Al6063 bulk, which is a hollow tube of outer diameter 100 mm and inner diameter of 60 mm and Al3003 bulk, which also has hollow tube shape. The Al and MWCNT powders were observed through SEM (TESCAN, VEGA II LSU, Czech) and TEM (Hitachi HF-2000), respectively.

2.2 Fabrication and analysis of Al-3vol.% CNT composite powder First, pure Al and MWCNT powders were mixed in 97:3 volume ratio, and then subjected to HEBM (Retsch GmbH, PM400). The 10-mm-diameter ball and raw materials (Al and MWCNT powders) were poured into a ball mill jar as 8:1 weight percent. Heptan (20 ml) was used as the process control agent(PCA), and the milling speed was set to 360 rpm and milling time was set to 6 hours. The ball milled powder was observed by FE-SEM (TESCAN, MIRA 3 LMH In-Beam Detector, Czech) to observe the dispersed aspect of MWCNT on the surface of Al powder. The phases in the composite powder were detected by XRD (Rigaku, Ultima IV, Japan) analysis with a Cu Kα radiation source (λ = 1.5148 Å, 40 kV, and 40 mA). The range of 2θ obtained using a linear detector (D/tex ultra) was 20–80°, the step size was 0.02°, and the scan rate was 0.06°/s.

2.3 Fabrication of Al6063/Al-3vol.% CNT FGMs

4

The FGMs were fabricated using the composite powder prepared by a prior process, Al6063 and Al3003 bulk by using the hot extrusion process. The process temperature was 470 °C, the ram speed was 2.6 mm/s, and the extrusion ratio was 100. 2.4 Characterization of the FGMs The density of the FGMs was measured using the Archimedes method (KERN ABJ 120-4M) to analyze if they were successfully bulked. After that, the Vickers hardness was measured using a 0.3 kg load in the cross section and the longitudinal section of the extrusion direction (HM-101, Mitutoyo Co., Japan). The Vickers hardness test was performed from the inside (Al-3vol.% CNT region) of the sample to the outward (Al6063 region) direction at four points and the average value was calculated with three measured values per point. In addition, a tensile test was conducted to investigate the mechanical properties (Universal Testing Machine, SHIMADZU), and a tensile test was carried out three times and the average value was indicated. XRD (Rigaku, Ultima IV, Japan) was conducted to detect phases presented in each material and interface between the other materials in FGMs. In the case of Al-3vol.%CNT region, the thickness on cross section is too thin to be difficult to detect with XRD beam. Thus, the outside of FGM, where is Al6063 region was polished until the Al-3vol. % CNT region could be exposed, and then measure XRD in Al-3vol.% CNT in the specimen. To observe the microstructure, the specimens were polished with silicon carbide papers to grades 330, 800 and 1200, finally with 3 µm and 1 µm diamond solution in turn. And then, the polished surface was etched by 40% NaOH solution. SEM with EDS (TESCAN, VEGA Ⅱ LSU, Czech) and FE-SEM equipped with EBSD (JEOL JSM-7800F) was used to observe and

5

Fig. 1. Digital images of fabricated Al6063/Al-3vol.% CNT/Al3003 FGMs: (a) overall view, (b) upside view, and (c) front view.

analyze the microstructures of the section of FGMs fabricated by hot extrusion. In addition, internal phases and structures were observed through TEM (Hitachi HF-2000) and SAD patterns. After the tensile test, the microstructure analysis was completed by observing the shape of the fracture surface through FE-SEM (TESCAN, MIRA 3 LMH In-Beam Detector, Czech). 3. Results and discussion

A digital image of FGMs finally manufactured by the hot extrusion process is shown in Fig. 1. Fig. 1(a) shows a view of the whole FGM tube, (b) shows the top view, and (c) shows the front view. As seen in the photographs, the FGM tube exhibits a uniform thickness, and no faults are observed between other materials. The fabricated FGMs are tubular with a thickness of 1.0 mm. The thickness of each Al6063, Al-3vol.% CNT, and Al3003 is 0.6 mm, 6

0.08 mm, 0.32 mm. The chemical composition of Al3003 and Al6063 bulk used in this study is in range of general composition of Al3003 and Al6063 shown in Table 1. The SEM micrographs of pure Al and Al-3vol.% CNT powder and SEM and TEM micrographs of CNT, which are used as the raw materials, are shown in Fig. 2. As shown in Fig. 2(a), Al particles are irregular rather than spherical, and the particle size is various. Fig. 2(d) shows that long and thin CNT are intertwined, which is attributed to the strong cohesive force of CNT. Fig. 2(c) shows TEM images of MWCNT; here, the CNT tips are observed, and the CNT are found to be hollow with a distance of about 0.34 nm between the CNT walls. Fig. 2(e) and (f) show an FE-SEM micrograph of an Al particle with dispersed CNT on the surface after the HEBM process. As compared to the pre-ball milled Al powder shown in Fig. 2(b)., the shape of the Al particle is more irregular and the surface is rougher, which implies that the surface of aluminum particles is broken by the high energy during HEBM and fine particles or fragments are stuck on the surface because Al is a ductile material. Although there are a number of CNT on Al surface, white arrow indicates some of CNT shown in Fig. 2(f), which is a highly magnified image of Fig. 2(e).

Table 1 General chemical composition of the components Al3003 and Al6063 [16,17]. Chemical composition (%) Material

Al3003

Si

Fe

0.6

0.7

Cu

Mn

0.05

1.0

~ 0.20

~ 1.5

0.10

0.10

Zn

Mg

Cr

Ti

0.10

-

-

-

96.90 ~ 97.55

0.20 Al6063

0.45 0.35

Al

0.10

~ 0.60

~ 0.90

7

97.65 0.10

0.10 ~ 98.50

Fig. 2. SEM micrographs of (a) pure Al powder, (b) pure Al particle, and; (c) TEM micrographs of CNT; (d) SEM micrograph of CNT; (e) FE-SEM of ball-milled Al particle with CNT, (f) highly magnified ball-milled Al particle surface with dispersed CNT indicated by white arrows.

A comparison of CNT composite powder with low-energy ball milling and HEBM shows that the number of CNT observed on the matrix surface decreases as the energy increases (increasing milling time or speed) and the number of aggregated CNT decreases [18,19]. The length of the CNT is about 100–300 nm, which is 30–100 times smaller than the length (~10 μm) of raw CNT before mixing. After HEBM, CNT often embedded in ductile Al due to the high energy and destruction, such as breakage of CNT, occur for the same reason. Fig. 3 shows the XRD results of each part of the extruded FGMs (Al3003 Al-3vol.% CNT, and Al6063). In Al3003, only Al was detected because other peaks are difficult to detect. Al, Al2O3, and Al4O4C phases are detected in Al-3vol.% CNT region. Al2O3 phase is often detected in the bulked Al composites because Al can be easily oxidized in air, resulting from 8

Fig. 3. XRD graphs of Al-3003 region, Al-3vol.% CNT region, and Al6063 region of FGMs fabricated by hot extrusion process (the peak of detected phases are marked with black circle, regular square, triangle, star figures.)

the remained Al oxide from surface of Al particles. In the case of Al4O4C, it is considered to be a compound produced during hot extrusion process due to the reaction between Al particle and CNT. In general, the Mg2Si phase is present as main precipitate in the Al6063 material [20]. The XRD peak of Mg2Si is detected in Al6063 region of Al alloy FGMs fabricated in our study. Other phases which are expected to generated at interface between other materials by chemical reaction didn’t detected. Fig. 4 shows the microstructures of FGM cross-sections observed by SEM and the results of analysis of constituents analyzed by EDS. First of all, no cracks and pores except some crater by deep etching are observed at the interface between other materials shown in Fig. 4 (d, h). 9

Fig. 4. SEM micrographs of (a) Al6063 region, (b) Al-3vol.% CNT region, (c) Al3003 region, and (d) interface between Al6063 and Al-3vol.%CNT; EDS of (e) Al6063 region, (f) Al-3vol.% CNT region, and (g) Al3003 region; (h) SEM micrographs of interface between Al-3vol.%CNT and Al3003 on cross section in FGMs fabricated by hot extrusion (the cross section is shown with schematic figure in (a)).

Fig. 4(e), which shows the EDS result of (a), indicates that Fig. 4(a) is the SEM of Al6063 because Mg, which is a major alloy element contained in Al6063, is detected. Fig. 4(f) shows an EDS of Fig. 4(b), which is an SEM micrograph of Al-3vol.% CNT region, where the highest content of C is detected. This implies this region is Al-3vol.%CNT. Fig. 4(g) is the EDS result of Al3003 region, where manganese (Mn), which is contented in Al3003 from 1.0 to 1.5 wt.% generally. A SEM image of the Al6063 part (Fig. 4(a)) shows that it has a smooth surface with no serious pores or cracks, except for white fine particles, which are composed of Al, Mg, and Fe. This indicates that the white particles are Al alloys or precipitates of Al6063. Fig. 4(b), which is the SEM micrograph of Al-3vol.% CNT region show no pores exist but some particles. Fig. 5 shows the longitudinal section of the FGMs and the EDS results. Similar to the cross section, some pores due to etching exist in the order of Al3003 and Al6063. However, as seen in the form of fine grains and pores generated by etching, the material appears to be stretched in a direction parallel to the extrusion direction as a whole. It is judged that the phenomenon 10

Fig. 5. SEM micrographs of (a) Al6063 region, (b) Al-3vol.% CNT region, (c) Al3003 region, and (d) interface between Al6063 and Al-3vol.%CNT; EDS of (e) Al6063 region and (f) Al-3vol.% CNT region, and (g) Al3003 region; (h) SEM micrographs of interface between Al-3vol.%CNT and Al3003 on longitudinal section of FGMs fabricated by hot extrusion (the longitudinal section is shown with schematic figure in (a)).

occurs when the extruded material is in a direction perpendicular to that of extrusion and a tensile force is generated in a parallel direction. After etching, fine white particles, consisting of Fe, Si, and Al (about 90%) are observed. This result indicates that the fine particles are precipitates of Al6063 or Al alloy particles. The elemental mapping of the fabricated FGMs in the region that contains all materials (Al6063, Al-3vol.% CNT, and Al3003) is performed to analyze overall microstructure shown in Fig. 6. The content of Al decreases and that of O is increased in Al-3vol.% CNT, which is attributed to corrosion when etching is conducted. This region will be more active because the stress is concentrated in the boundary region where the two bulks press and because it has very wider surface area than that of Al3003 and Al6063. C element was detected in Al3003 and Al6063, which indicates that CNT could be penetrated into other materials and occur chemical reaction. However, it needs more specific analyzation for the results because the small amount of C contents could be considered in error range.

11

Fig. 6. Line mapping of Al6063/Al-3vol.% CNT/Al3003 tube: (a) SEM and overall line mapping of cross section of the fabricated Al6063/Al-3vol.% CNT/Al3003 FGMs tube, (b) line mapping of respective elements (carbon, oxygen, magnesium, aluminum) of cross section of the tube, (c) SEM and overall line mapping of longitudinal section of the FGM tube, and (d) line mapping of respective elements of longitudinal section of the tube.

EBSD analysis was conducted to analyze microstructures at each region as shown in Fig. 7(a-b). The grains in Al-3vol.%CNT region appear to be very fine, where the average grain size is approximately 1 μm, which is about 0.01 times the grain size of Al6063and Al3003.The grains of Al6063 texture are aligned with (001) orientation. In case of the Al3003 region, although the grains are more disorganized than those of the Al6063 texture, they are aligned with (001) orientation. The high-alignment properties are attributed to the effect of extrusion. Even though the grains in Al-3vol.%CNT region have rather strong (001) and (111) orientations, it has the most disorganized texture among the three regions shown in Fig. 7(d). The fine grain size would be resulted from recrystallization by extrusion because the grain 12

size is much smaller than the particle size of Al. This part will be discussed later.

Fig. 7. EBSD of (a) Al6063/Al-3vol.% CNT/Al3003 FGMs, (b) high magnification of (a); (c) grain size graph of Al-3vol.%CNT region, and (d) {111} and {001} pole figures of the microstructure of Al-3vol.%CNT region.

Fig. 8 shows the TEM images and SAD patterns analyzed to observe the internal structure of the FGMs. The crystallites with shape of an angular circle and various sizes are clearly distinguished in Fig. 8(a). Fig. 8(d) and (e) show the TEM image of a rod-shaped Al4C3, which is judged with the SAD pattern, with a length of about 40 nm shown in Fig. 8(f). It is generally known that small amounts of Al4C3 are often produced when fabricating an Al-CNT composite by methods such as spark plasma sintering and hot extrusion [21,22]. Although Al4C3 can be harmful to the composite materials because Al4C3 is brittle and hygroscopic, it

13

strengthens the interface bonding between Al and CNT.

Fig. 8. TEM micrographs of (a) Al6063 region, (b,c) Al-3vol.%CNT region where CNT covered by Al-O was present in fabricated FGMs, (d) Al-3vol.%CNT region where Al4C3 was observed (e) high magnification of (d); (f) SAD pattern of Al4C3 shown in (e).

The contact angle between Al and C is 130°–140°, while that between Al and Al4C3 is 50°– 70°, which implies that Al4C3 acts as a medium bonding Al and CNT chemically [23,24]. This also indicates that the load can be transferred more efficiently from the Al matrix to CNT reinforcement through the Al4C3 phase, resulting in attaining improved mechanical properties of the Al-CNT composite. In Al-CNT composites, the side wall of CNT is difficult to be reacted with Al liquid due to the integrity. However, for defects of CNT that contain open tips, amorphous carbon coatings can be the nucleation spot of Al4C3. In this study, the temperature of the extrusion process is 470℃, which is much lower than the melting point of Al (660℃). Al carbide is a chemically more stable phase when the degree of defect of CNT is increased, and can be generated even if the Al liquid phase is not generated [25-27]. In fact, a larger number of Al4C3 phases can be easily produced, as the milling energy is high, which 14

causes CNT defects. In addition, CNT and Al-O compounds are observed through TEM in Al-3vol.% CNT shown in Fig. 8 (b, c). Both materials coexist in a form where the Al-O compound surrounds CNT. This Al-O compound is predicted to be Al2O3 detected in XRD of Al-3vol.% CNT powder. The surface of the Al powder is easily oxidized when it is exposed to air even at room temperature. Although the oxide film, which is mainly Al2O3, breaks during HEBM, the remaining oxide on the Al surface also prevents CNT dispersion on the Al surface from transforming to Al carbide. These CNT play the role of a pinning effect in preventing the grain growth of Al. Based on these analysis results, the mechanical strength of FGMs is expected to be high. The three other materials (Al6063, Al-3vol.% CNT and Al3003) are successfully fabricated into a bulk by the extrusion process, with the results of relative density reaching 100% (Table 3). The Vickers hardness was then measured in the part of Al3003, Al-3vol.%CNT, and Al6063 respectively (Fig. 9). The value of Al-3vol.% CNT is 110-130 HV, which is due to the very fine grain size shown in Fig. 7. The Vickers hardness values of Al3003 region and Al6063 are higher than the generally known conventional Vickers hardness of Al3003 and Al6063 (Table 2). It indicates that the reinforcement by hot extrusion process was occur.

Table 2 General mechanical properties of Al6063, pure Al, and CNT [16,17,28] Materials

Density (g/𝒄𝒎𝟑 )

Vickers hardness (HV)

Elongation (%)

0.2% Offset yield strength (MPa)

3003-T0

2.730

29

40

41.4

110

68.9

6063-T0

2.700

26

18

50

90

68.9

Pure Al

2.700

16

39

17

73

68

MWCNT

1.8-2.6

-

20

-

63,000

1800

15

Ultimate tensile strength (MPa)

Elastic modulus (GPa)

Fig. 9. (a) Digital and schematic image of cross section and (b) micrographs where Vickers hardness was measured in cross section of FGMs; (c) digital and schematic image of longitudinal section and, (d) micrographs where Vickers hardness was measured in longitudinal section of FGMs.

To investigate the overall strength of FGMs, the tensile test is performed as shown in Fig. 10. The tensile graph shows the generally known Al6063, Al3003 fabricated Al-3vol.% CNT with same materials, and the FGMs. The average maximum tensile strength and elongation are 142.77 MPa and 22.03%, respectively, which are higher than the theoretical values calculated using the rule of mixture equation (113.71 MPa, 25.46%). The elastic modulus of FGMs, which is shown by the inner graph in Fig. 10 (263.78 GPa), is almost 4 times of theoretical value (69.43 GPa), even higher than Al-3vol.% CNT bulk (152.76 GPa) fabricated

16

Fig. 10. Tensile strength–strain graphs of Al6063/Al-3vol.% CNT/Al3003 FGMs, general Al6063, Al-3vol.% CNT and Al3003; the inset tensile strength–strain graph indicates a very initial stage; the dot line indicates the theoretical strength calculated with Kelly-Tyson equation.

with same materials. This initial tangent modulus was measured with initial stress-strain line, which is under proportional limit, in tensile graph. Gowda et al. insisted that initial linear portion curves show the elastic behavior of jute laminate polyster resin composite [29]. It can be predicted that Young’s modulus of FGMs was mainly improved by reinforcement of CNTs which of Young’s modulus is 1800 GPa shown in Table 2. Generally, Young’s modulus is affected from lattice distance and bonding force of atoms, and these factors are changed by not temperature but impurities or second phases. Thus, it could 17

be considered that Al4C3 formed in Al-CNT region indirectly increase the Young’s modulus acting as transforming the lattice. As well as, the young’s modulus of Al4C3 is 231.93 GPa to 345.11 GPa, which can improve the young’s modulus of FGMs [30]. The FGMs with superior young’s modulus so strong to the strain that it has advantage when it is used to structural frame application such as solar installation system frame, electric vehicle body and so on. More specific strengthening mechanisms considered in this study are discussed. The Hall–Petch equation is used to compare the theoretical strengths when Al-3vol.%CNT region was recrystallized [31]. According to study of Kwon et al [32], the average grain size of the bulk when 14.82 μm size aluminum powder was mixed with CNT at 95: 5 vol.%. The grain size of the extruded bulk which was sintered with composite powder first was 13.3 µm. As a result, it can be considered that a considerable decrease in grain size does not occur when the bulk was sintered and extruded. The average particle size of the aluminum powder used in this study is 76 μm, and the grain size of Al-3vol.%CNT region was about 1 μm. It indicates that when bulk and powder were extruded contacting each other, recrystallization could be occurred [33]. If the recrystallization does not occur when the powder is bulked, it is expected that a grain size of about 50 μm in Al-3vol.%CNT area will be formed.

Table 3 Experimental mechanical properties of Al6063/Al-3vol.% CNT FGMs Materials

Al3003/ Al-3vol.%CNT/ Al6063 FGMs

Bulk density (g/𝒄𝒎𝟑 )±0.01

2.70

Relative density (%)±0.74

100

Vickers hardness (HV)±5.15

0.2% offset yield strength (MPa)±1.27

Al6063

52.54

Al-3vol.%CNT

120.37

Al3003

39.30

18

55.25

Ultimate tensile strength (MPa)±2.76

142.77

Elastic Modulus (GPa)±60.53

263.78

Elongation (%)±0.74

22.03

Fig. 11. FE-SEM micrographs of (a) Al6063 region, (b) overall region which contains Al6063, Al-3vol.%CNT and Al3003, (c) Al3003 region, (d) interface between Al6063 and Al-3vol.%CNT, (e) Al-3vol.%CNT region and (f) interface between Al3003 and Al-3vol.%CNT of broken section of FGMs after tensile test.

Therefore, the intensity increases by 0.86 Ky when a band region with a grain size of 1 µm is formed due to recrystallization phenomenon compared to when no recrystallization occurs. The static recrystallization, dynamic recrystallization, and grain growth occur during the hot extrusion process, which is a deformation process [34]. Generally, the microstructure of a hot extruded billet is divided into three zones, including the dynamic recrystallization zone near the billet surface billet, the shear intensive zone, and the metal flow zone. The dynamic recrystallization zone has fine grains, resulting from the generation of severe shear stress by the friction between the billet and die. During dynamic recrystallization, nucleation and growth of the grain occur through the deformation of the materials [35,36]. In this study, Al-3vol.% CNT powder was extruded between Al3003 and Al6063 bulk contacting each other at one surface, where friction occurs similar to the case of the billet and die. However, the friction between bulk and powder is different from that between bulk and

19

bulk, resulting in different microstructures that have very fine grains. The Al-3vol.% CNT that has equal thickness across materials is a dynamically recrystallized zone receiving uniform loads at all sites. Fig. 11 is an FE-SEM micrograph of the fracture surface of the FGMs obtained after conducting the tensile test. The several dimples observed on the fracture surface of the regions (Al6063, Al-3vol.% CNT, Al3003) shown in Fig. 11 (a, c, e) imply the occurrence of ductile fracture. Among them, the fracture surface of the Al-3vol.% CNT is filled with fine dimples. The grain size in this region is about 1 μm, as shown in Fig. 7, which will strengthen the FGMs. This implies that Al-3vol.% CNT exhibits proper elongation and superior strength, which positively affects the mechanical properties of FGMs. In the Al3003 region, smaller amounts of dimples exist than in Al6063 region shown in Fig. 11(c). However, it can be observed that ripple, which is shown in ductile fracture surface with dimples. Thus, it implies that Al3003 has high elongation. The FE-SEM micrographs in Fig. 12 indicate that CNT are extruded from the Al matrix and Al carbide embedded in the Al matrix. Fig. 12 (a–c) shows CNT surrounded with the Al. In case of the composite composed of the matrix and fiber, load transfer from the matrix to the fiber often occurs.In this study, CNT act as a fiber in the Al-3vol.% CNT region and the Al matrix and CNT are chemically bonded through Al4C3. This implies that Al and CNT are strongly bonded, and that the load applied in the Al matrix can be transferred to CNT efficiently. In addition, Fig. 12(d) shows that three types of stretched portion of Al matrix, which indicate that stress was concentrated on this area. According to the study of Al-CNT composite by Kwon et al, the types can be distinguished by the shape of matrix, containing triangular type, pointed type and square dull-stick type, where Al carbides and CNT are embedded in the Al matrix with TEM micrographs [37]. Thus, it can be predicted that Al

20

Fig. 12. FE-SEM micrographs (a-c) Al-3vol.%CNT region where CNT, which is shown in which circle was observed on the broken surface of FGM after tensile test, (d) Al-3vol.%CNT region where protruded Al matrix were observed (white arrow indicates the protruded Al), which is shown with contained schematic image.

carbide, which was observed in Fig. 8, is embedded in Al matrix like the schematic picture in Fig. 12(d). The Al4C3 that is transformed from CNT also contributes to strengthening through load transfer from an Al matrix similar to CNT. The theoretical strength of Al-3vol.% CNT is calculated using the Kelly–Tyson equation to confirm the strengthening effect of CNT referring the study of Kwon et al., in which the theoretical strength of Al-5vol.% CNT composite was calculated using the Kelly–Tyson equation [38,39]. The tensile strength calculated similarly is 188.8 MPa, which is relatively higher than the experimental strength of Al-3vol.% CNT indicated in Fig. 10. However, the difference is less than 5%, implying that the reinforcement effect induced by the addition of CNT is enough to occur in FGMs.

21

Besides these strengthening effects, other effects are also predicted. Al6063 is an Al alloy containing alloying elements such as Mg and Si, and it is known that precipitates such as Mg2Si exist in the interior. The pressure will be applied by extrusion and the effect of reducing the grain size will occur. The precipitates are much harder than aluminum, and therefore, will shrink significantly less than aluminum. As a result, the proportion occupied by the precipitates in one grain after the extrusion is increased. These results further strengthen the material by disturbing the movement of dislocations. The results of improved Vickers hardness of the Al6063 region than that of general Al6063 implies that this prediction is valuable. In addition, oxide dispersion strengthening, which is referred to strengthening obtained by an oxide-dispersed reinforced alloy using a stable oxide at a high temperature, is prepared and used [40,41]. To uniformly disperse the fine oxide particles in the material, a strengthening effect is generated. High-energy milling can also lead to finer and more dispersed oxides. In this study, high-energy milling is used to fabricate Al-3vol.% CNT. It can be predicted that Al oxide, which is present on the surface of Al powder, is cracked and broken during the milling process. It is likely that the oxides, along with the CNT, would be dispersed between the Al matrixes, resulting in reinforcement.

4. Conclusions

In this study, Al6063/ Al-3vol.% CNT/ Al3003 FGMs were successfully fabricated using the hot extrusion process. The FGMs showed the surface with no cracks or pores except some pores generated with intentional deep etching. The Al-3vol.% CNT region which has microstructure of mean grain size 1 µm was observed

22

between Al3003 and Al6063. It was predicted that ultra-fine grains might be generated by unique hot extrusion state, which powder and bulk are contacted and extruded with high friction energy. The ultra-fine grains Al-CNT region has high Vickers hardness (120 HV), which is over twice as high as those of Al6063 and Al3003. As well as the fracture surface of FGMs with several dimples proved the occurrence of ductile fracture, which indicates that the recrystallization zone exhibits excellent hardness and strength and good ductility, which will positively affect the mechanical properties of FGMs. The FGMs including such recrystallized zone also showed improved strength and elastic modulus than expected values calculated with rule of mixture. Some of strengthening mechanisms such as load transfer effect, grain refinement and other factors was introduced in this study to explain the higher mechanical properties than theoretical values. The CNTs which was used as a fiber mainly improved the strength and young’s modulus through load transfer effect. The small amount of nano-size Al4C3 generated in the Al-3vol.% CNT region also offer chemical connection of matrix and fiber, which make interfacial bonds strong and strengthened the FGMs with load transfer coordinating with CNT. The theoretical strength of Al-3vol.% CNT bulk and fabricated FGMs in this study was calculated respectively based on the Kelly-Tyson equation and Hall-Petch equation to compare with experimental values. In the results, it was shown theoretical and experimental values significantly were matched. Thus, those equations might be used as one of the theoretical analysis tool for prediction of Al-CNT composites. These Al3003/Al-3vol.% CNT/Al6063 FGMs can be easily used as the material of the tube, which requires properties such as high strength and light weight. The new type of CNT reinforced Al FGMs can be applicable to the transportation equipment parts and air-hydro cylinder tube body.

23

References [1] R. Andrew, D. Sun, Functionally Craded Materials (FGM) and their Production Methods. http://www.azom.com/details.asp?ArticleID=1592, 2012 (accessed 07 June 2012). [2] P. Gu, R. J. Asaro, Cracks in functionally graded materials, International J. Solids Structures 34, January (1997), 1-17. [3] X. F. Yao, W. Xu, S. L. Bai, H. Y. Yeh, Caustics analysis of the crack initiation and propagation of graded materials, Compos. Sci. Technol. 68 (2008), 953-962. [4] K. S. Ravichandran, Thermal residual stresses in a functionally graded material system, Mater. Sci. Eng.; A 201 (1995), 269-276. [5] A. Shahrjerdi1, F. Mustapha, M. Bayat, S. M. Sapuan, D. L. A. Majid, Fabrication of functionally graded Hydroxyapatite-Titanium by applying optimal sintering procedure and powder metallurgy, International J. Phys. Sci. 6 (2011), 2258-2267. [6] D. M. Hulbert, D. Jiang, U. Anselmi-Tamburini, C. Unuvar, A. K. Mukherjee, Continuous functionally graded boron carbide-aluminum nanocomposites by spark plasma sintering, Mater. Sci. Eng.; A 493 (2008), 251-255. [7] Z. Chena, K. Ikedab, T. Murakamib, T. Takedaa, Jian-Xin Xiec, Fabrication of composite pipes by multi-billet extrusion technique, J. Mater. Process. Technol. 137 (2003) 10–16. [8] M. F. Yu, Fundamental mechanical properties of carbon nanotubes: current understanding and the related experimental studies, J. Eng. Mater. Technol. 126 (2004), 271-278. [9] M. H. Al-saleh, U. Sundararaj, Electromagnetic interference shielding mechanisms of CNT/polymer composites, Carbon 47 (2009), 1739-1746. [10] G. Yamamoto, M. Omori, T. Hashida, H. Kimura, A novel structure for carbon nanotube 24

reinforced alumina composites with improved mechanical properties, Nanotechnology 19 (2008), 315708. [11] S. I. Cha, K. T. Kim, S. N. Arshad, C. B. Mo, S. H. Hong, Extraordinary strengthening effect of carbon nanotubes in metal-matrix nanocomposites processed by molecular-level mixing, Adv. Mater. 17 (2005), 1377-1381. [12] A. M. K. Esawi, K. Morsi, A. Sayed, A. A. Gawad, P. Borah, Fabrication and properties of dispersed carbon nanotube-aluminum composites, Mater. Sci. Eng.; A 508 (2009), 167-173. [13] R. Pres-Bustamante, C. D. Gomez-Esparza, I. Estrada-Guel, M. Miki-Yoshida, L. Licea-Jimenez, S. A. Perez-Garcia, R. Martinez-Sanchez, Microstructural and mechanical characterization of Al-MWCNT composites produced by mechanical milling, Mater. Sci. Eng.; A 502 (2009), 159-163. [14] H. Kwon, C. R. Bradbury, M. Leparoux, Fabrication of functionally graded carbon nanotube-reinforced aluminum matrix composite, Adv. Mater. 13 (2011), 325-329. [15] L. Kumar, S. P. Harsha, Effect of carbon nanotubes in CNT reinforced FGM nano plate under thermos mechanical loading, Procedia Technol. 23 (2016), 130-137. [16] https://www.makeitfrom.com/material-properties/3003-O-Aluminum, 2018 (accessed 01 April 2018). [17]

6063

aluminum

alloy

from

Wikipedia,

the

free

encyclopedia,

https://en.wikipedia.org/wiki/6063_aluminium_alloy#6063-O, 2017 (accessed 20 September 2017). [18] J. Liao, M. J. Tan, Mixing of carbon nanotubes(CNT) and aluminum powder for powder

25

metallurgy use, Powder Technol. 208 (2011), 42-48. [19] B. Krause, T. Villmow, R. Boldt, M. Mende, G. Petzold, P. Potschke, Influence of dry grinding in a ball mill on the length of multiwalled carbon nanotubes, Compos. Sci. Technol. 71 (2011), 1145-1153. [20] S. K. Panigrhi, R. Jayganthan, V. Pancholi, Effect of plastic deformation conditions on microstructural characteristics and mechanical properties of Al 6063 alloy, Mater. Des. 30 (2009), 1894-1901. [21] J. Wu, H. Zhang, Y. Zhang, X. Wang, Mechanical and thermal properties of carbon nanotube/aluminum composites consolidated by spark plasma sintering, Mater. Des. 41 (2012), 344-348. [22] M. Majid, G. H. Majzoobi, G. A. Noozad, A. Reihani, S. Z. Mortazavi, M. S. Gorji, Fabrication and mechanical properties of MWCNT-reinforced aluminum composites by hot extrusion, Rare Metals 31 (2012), 372-378. [23] K. Landary, S. Kalogeropoulou, N. Eustathopoulos, Wettability of carbon by aluminum and aluminum alloys, Mater. Sci. Eng.; A 254 (1998), 99-111. [24] L. Ci, Z. Ryu, N. Y. Jin-Phillipp, M. Ruhle, Investigation of the interfacial reaction between multi-walled carbon nanotubes and aluminum, Acta. Mater. 54 (2006), 5367-5375. [25] X. Zhu, Y. G. Zhao, M. Wu, H. Y. Wang, Q. C. Jiang, Effect of initial aluminum alloy particle size on the damage of carbon nanotubes during ball milling, Materials 9 (2016), 173. [26] M. F. Yu, O. Lourie, M. J. Dyer, K. Moloni, T. F. Kelly, R. S. Ruoff, Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load, science 287, 2000, 637-640.

26

[27] W. F. Smith, Foundations of Materials Science and Engineering McGraw-Hill, 2003. [28] B. Peng, M. Locascio, P. Zapol, S. Li, S. L. Mielke, G. C. Schatz, H. D. Espinosa, Measurements

of

near-ultimate

strength

for

multiwalled

carbon

nanotubes

and

irradiation-induced crosslinking improvements". Nat. Nanotechnol. 3 (2008): 626–631. [29] T. M. Gowada, A. C. B. Naidu, P. Chhaya, Some mechanical properites of untreated jute fabric-reinforced polyster composites, composite: part A 30 (1999), 277-284. [30] ELATE, http://progs.coudert.name/elate/mp?query=mp-1591, 2018 (accessed 18 July 2018). [31] H. Fujita T. Tabata, The effect of grain size and deformation sub-structure on mechanical properties of polycrystalline aluminum, Acta. Metall. 21 (1973), 355-365. [32] H. Kwon, M. Estili, K. Takagi, T. Miyazaki, A. Kawasaki, Combination of hot extrusion and spark plasma sintering for producing carbon nanotube reinforced aluminum matrix composites, Carbon 17 (2009), 570-577. [33] R. W. Armstrong, Hall-Petch Realationship: use in characterizing properties of Aluminum and Aluminum alloys (Department of Mechanical Engineering University of Maryland College Park, 2016), pp. 1-30. [34] A. Guzel, A. Jager, F. Parvizian, H. -G. Lambers, A. E. Tekkaya, B. Svendsen, H. J. Maier, A new method for determining dynamic grain structure evolution during hot aluminum extrusion, J. Meter. Process. Technol. 212 (2012), 323-330. [35] L. Guo, F. Fujita, Influence of rolling parameters on dynamically recrystallized microstructures in AZ31 magnesium alloy sheets, J. Magn. Alloy 3 (2015), 95-105. [36] R. C. Bell, D. W. Wisander, Surface recrystallization theory of the wear of copper in

27

liquid methane, NASA Tech. Note TD D-7840 (1974). [37] H. Kwon, M. Leparoux, Hot extruded carbon nanotube reinforced aluminum matrix composite materials, Nanotechnology 23 (2012), 415701. [38] A. Kelly, W. R. Tyson. Tensile properties of fibre-reinforced metals: copper/tungsten and copper/molybdenum, J. Mech. Phys. Solids 13 (1965), 329-350. [39] H. Kwon, A. Kawasaki, Extrusion of spark plasma sintered aluminum-carbon nanotube composites at various sintering temperatures, J. Nanosci. Nanotechnol. 9 (2009), 6542-6548. [40] J. S. Benjamin, Dispersion strengthened superalloys by mechanical alloying, Metall. Trans. 1 (1970), 2943-2951. [41] J. Eckert, M. Seidel, A. Kübler, U. Klement, L.Schultz, Oxide dispersion strengthened mechanically alloyed amorphous Zr-Al-Cu-Ni composites, Scripta Mater. 38 (1998), 595-602.

28