Evaluation of Mechanical Properties of MWCNT / Nanoclay Reinforced Aluminium alloy Metal Matrix Composite

Aluminium alloy 5083 (AA5083) is a widely used material in aerospace, marine, defence and structural applications were mechanical and corrosion resistance property plays a vital role. For the present work, MWCNT / Nanoclay (montmorillonite (MMT) K10) mixed with AA5083 for different composition in weight percentage to enhance the mechanical property. Semi-solid state casting method (Compo-casting) was used to fabricate the composite materials. By using Field-emission scanning electron microscope (FESEM) the uniform dispersion of the reinforcement and microstructure were studied. Finally, the addition of Nanoclay shows decrease in tensile strength compared to the AA5083 / MWCNT composites and hardness value of the composites (AA5083 / MWCNT and AA5083 / Nanoclay) was found to increase significantly.


Introduction
A carbon nanotube (CNT) is one of the widely used materials in various applications due to its high surface to volume ratio and excellent properties like thermal, mechanical and electrical [1,2]. A multi-wall carbon nanotube (MWCNT) material is used in structural based industries for various applications to improve the mechanical and dynamic property. Interfacial bonding between the matrix and reinforcement material plays a major role in improving the mechanical property of the composite material [3]. Aluminium alloy 5083 (AA5083) was used due to its low density and strength to weight ratio in aerospace, missile, LNG tanks, off-shore structure and marine applications. Abou Bakr et.al. fabricated a MWCNT based aluminium alloy composites using rheological and squeeze casting, for homogenous dispersion of MWCNT magnesium was added into the aluminium alloy [4][5][6][7][8]. In other case, Nanoclay montmorillonite (MMT) K10 powder is one of the effective reinforcement materials which has a good mechanical property, thermal stability, flame retardance and it also act as a water barrier material [9][10][11][12]. Montmorillonite (MMT) with a particle thickness of 1 nm and 70 to 100 nm crosswise silica platelets. The excellent properties lie in the unique structure of MMT, where the platelet thickness is only 1 nm, while its dimension in length and width can be around hundreds of nanometers [14][15][16]. Hence, MWCNT and Nanoclay were selected as reinforcement materials and added to the AA5083 alloy for varying compositions. In the present work, semi-solid state casting method (compo-casting) was used to fabricate the AA5083 -MWCNT / Nanoclay composites. For the fabricated composite material, the mechanical properties and characterization were studied.

Experimental Procedure
MWCNT purity of >98%, diameter (D) in 5-20 nm and length (L) 1-10 μm was selected. By varying MWCNT compositions like 1, 1.25, 1.5 and 1.75 in weight percentage and Nanoclay montmorillonite (MMT) K10 powder in 3, 5, 7 and 9 weight percentage as the reinforcement. AA5083 is selected as a matrix material (chemical composition shown in Table 1.) to fabricate the composite using semi-solid state casting method (Compo-casting). To improve the wettability of the reinforcement no further addition of material were mixed into the matrix material. The experimental setup as shown in Figure 1 is to fabricate the aluminium metal matrix composite. MWCNT and Nanoclay reinforcement materials were preheated at 773 K and AA5083 was melted in the graphite crucible at 1173 K. Then the preheated MWCNT / Nanoclay were added into matrix material and stirred for 2 -3 minutes at 350 -400 rpm. After the mixing of nanoparticle reinforcement, AA5083/MWCNT & AA5083/Nanoclay was poured into the mould and got the required shape. Thus, the composite material was fabricated for various compositions of reinforcement materials.

Micro structural observations of Composite material
The microstructure of the fabricated composite material was analyzed using Field Emission Scanning Electron Microscope (FESEM). The FESEM image and EDX patterns shown in Figure 3 of the AA5083 reinforced with 1 wt% of MWCNT, also in Figure 4 it shows the distribution of 3 wt% of Nanoclay in AA5083 matrix material. The uniform dispersion of MWCNT/Nanoclay was observed in the FESEM images [17,18]. An EDX pattern shows the presence of MWCNT and Nanoclay in the developed composite material. The interference between the nanoparticle and matrix material was shown in Figure 5

Mechanical Properties
There is no proper method to check the uniform distribution of nanoparticles (MWCNT/Nanoclay) inside the matrix material [15]. To known the uniform dispersion of nanoparticle in matrix material hardness test is one of the indirect method to analyze the dispersion. Figure 6   The density value of the developed composite materials (AA5083/MWCNT & AA5083/Nanoclay) and AA5083 was shown in Figure 7 (a) & (b). The density value was calculated using the ASTM standard -B311-08 Archimedean density method (or) water displacement method [19]. No significant changes occurred in the density value of developed AA5083/MWCNT, due to the fine particle size of the nanoparticles. Nanoclay reinforced composite shows increased density value within the limit of 2.80 g/cm3, which is in the acceptable limit for aluminium alloy applications. Therefore, the developed composite materials can be used in structural applications.  The matrix material and MWCNT reinforced composite was fabricated with varying compositions like 1, 1.25, 1.5 & 1.75wt% in semi-solid state casting method (Compo-casting) at room temperature. In Figure 8 the ultimate tensile value (UTS) of composite material is increased due to the presence of MWCNT. Comparatively, the matrix material shows less strength than MWCNT reinforced composites. The strength value of the matrix and composite materials increased from 168.2 MPa to 234.12 MPa for the composition 1.75 wt% of MWCNT. Based on the tensile strength of the developed composite material the dispersion of the MWCNT in the matrix material is uniform. From the observed experimental value, that load transfer mechanism plays a major role in increasing of Ultimate tensile strength of the AA5083 / MWCNT composite. To get a better ductility and strength at low temperature the MWCNT concentration value should lies between 1 to 1.75 wt% [18]. In the other case, AA5083 & AA5083 / Nanoclay composite with composition of 3, 5, 7 and 9wt% shows increase in mechanical strength. When compare to the AA5083 / MWCNT reinforced composite it has less ultimate tensile strength which is suitable for a varying application. Figure 9 shows the ultimate tensile strength (UTS) of matrix and Nanoclay reinforced composites.

Conclusions
From the above experimental results and analysis of this work it is concluded that, 1. Adding MWCNT reinforcement in the matrix material increases the mechanical properties to a considerable level by 36%, compare to the Nanoclay reinforced AA5083 composite and AA5083 alloy. 2. Brinell Hardness value of the AA5083 / MWCNT composite material was increased from 74 BHN to 84 BHN for and AA5083 / Nanoclay value increased from 74 BHN to 82 BHN. 3. By reducing grain size of the MWCNT and increase the weight fraction of MWCNT up to 1.75wt%, the Ultimate Tensile Strength is found to increase considerably, due to the presence of carbon percentage. 4. MWCNT can be added up to 1.75wt% with the matrix material, with increase in mechanical properties of the composite. 5. From the above study it shows, the AA5083 / MWCNT shows the better influence in the mechanical property than AA5083 / Nanoclay composite.