Investigation of Mechanical Behaviour of CNT Reinforced Al6061 Nano-Composites

Investigation of Mechanical Behaviour of CNT Reinforced Al6061 Nano-Composites

Rushikesh B. Sonawane

Department of Mechanical Engineering, P.V.P.I.T. Budhgaon, Maharashtra, India.

Dr. Pankaj P. Awate

Department of Mechanical Engineering, P.V.P.I.T. Budhgaon, Maharashtra, India.

Abstract – This study explores the influence of Multi-Walled Carbon Nanotubes (MWCNTs) on the mechanical performance of Aluminum Alloy 6061 (A|6061). Nanocomposite specimens were fabricated using stir casting with MWCNT additions ranging from 0 wt.% to 2 wt.%. Mechanical characterization included tensile strength, hardness (BHN), and yield strength evaluation. Results indicate a consistent improvement in all mechanical properties with increasing MWCNT content. The base alloy exhibited a tensile strength of 150.412 MPa, hardness of 49.71 BHN, and yield strength of 94.874 MPa. At 2 wt.% reinforcement, tensile strength increased to 191.751 MPa, hardness to

65.60 BHN, and yield strength to 126.313 MPa. The improvements correspond to enhancements of 27.48% in tensile strength, 31.96% in hardness, and 33.14% in yield strength.The strengthening is primarily attributed to effective transfer, grain refinement, increased dislocation density, and strong interfacial bonding between MCNTs and the Al6061 matrix. The findings confirm that MWCNT reinforced Al6061 nanocomposites are promising candidates for high-performance structural and lightweight engineering applications.

Keywords: CNT, Al6061, Nanocomposites, Mechanical Properties, Stir Casting

1. INTRODUCTION

   Aluminum alloy 6061 is widely used in aerospace, automotive, and structural industries due to its good strength-to-weight ratio, corrosion resistance, and machinability [1-3]. However, further enhancement in mechanical strength and wear resistance is required for advanced engineering applications.Carbon Nanotubes (CNTs) possess extraordinary tensile strength, high elastic modulus, excellent thermal conductivity, and low density [3]. Incorporating CNTs into aluminum matrices results in aluminum metal matrix nanocomposites (AMMNCs) with superior mechanical and functional performance.Previous studies have demonstrated significant improvements in strength, hardness, fracture toughness, and wear resistance in CNT-reinforced aluminum composites [4-29]. However, uniform dispersion and strong interfacial bonding remain key challenges. This research focuses on evaluating mechanical behavior improvements in Al6061 reinforced with varying percentages of MWCNT using a controlled stir casting process.

2. MATERIALS AND METHODS

   Aluminum alloy Al6061 purchased from Plus Metals in Mumbai, which was used as the fundamental alloy. Al6061 is primarily a casting alloy consisting mainly of aluminum, with the addition of silicon, magnesium, copper, chromium, and zinc to improve its properties. Due to its excellent ability to get cast, machinability, and mechanical characteristics, it is a widely used material for die casting. The table displays the percentage of each element in the Al6061 aluminum alloy.

   Table 1. Elements Al6061 Aluminum

   Elements	Al	Mg	Si	Cu	Cr
   Wt. %	97.9	1	0.6	0.28	0.2

   1. Reinforcement – Multi-Walled Carbon Nanotubes (MWCNTs)

      MWCNTs were obtained from Ad-Nano Technologies Pvt. Ltd., India.

      Properties of MWCNTs:

      • Purity: ~99%

      • Diameter: 10-20 nm

      • Length: ~10 pm

      • High aspect ratio

      • Exceptional tensile strength

      • Excellent thermal and electrical conductivity

        Even small additions significantly improve matrix performance due to their nanoscale reinforcement effect.

3. FABRICATION OF NANOCOMPOSITES

   1. Stir Casting Process

      Nanocomposites were fabricated using a fully automatic stir casting furnace capable of reaching 1200°C. Key process parameters:

      • Melting temperature: 850°C

      • Holding temperature: 670°C

      • Stirring speed: 500 rpm

      • Stirring time: 15 minutes

      • Inert atmosphere: Argon gas

      • Bottom pouring arrangement

        Controlled stirring ensured homogeneous dispersion of MWCNT particles.

        Figure 1. Sketch of the mechanism of the stir casting furnace

   2. Specimen Preparation

      Tensile specimens were machined as per ASTM E8/E8M standards. Casting molds were designed with shrinkage and machining allowances. After solidification, samples were extracted and precision machined.

      Figure 2. Machined Test Sample model

4. RESULTS AND DISCUSSION

   To assess the suitability of MWCNT/Al6061 nanocomposites for specific applications, the effect of different MWCNT weight percentages on strength, hardness, and yield stress was evaluated for each formulation. The mechanical testing outcomes are summarized in the table. It was noted that the strength, yield stress, and hardness increased up to a certain weight percentage of reinforcement, beyond which the hardness began to decline for that weight percentage composition. Samples with varying MWCNT content (wt. %) Tensile strength (MPa) Hardness (BHN) Yield strength (MPa).

   Table 2- Results of mechanical testing

   Samples	Content of MWCNT (wt. %)	Tensile strength (MPa)	Hardness (BHN)	Yield strength (MPa)
   1	0	150.412	49.71	94.874
   2	0.5	171.734	56.90	100.518
   3	1	177.788	61.95	110.145
   4	1.5	188.161	62.10	114.928
   5	2	191.751	65.60	126.313

   Table 2: Mechanical Test Results

   Table No. 2 illustrates the influence of MWCNT on the mechanical properties of Al6061. When comparing the MWCNT/Al6061 nanocomposites to the unreinforced Al6061, an improvement in the mechanical testing outcomes was observed. The increase in tensile strength, yield strength, and hardness can be attributed primarily to the utilization of a well-equipped automatic stir casting furnace, the uniform dispersion of Al6061 nanoparticles, preheating, the gradual introduction of nanoparticles, along with slow, controlled, and continuous stirring, and reduced porosity.

   Tensile Strength

   Figure 3. Tensile strength variation with MWCNT

   To investigate the effect of MWCNT on the Al6061 matrix, tensile tests were conducted based on ASTM standards using a computerized uni-axial universal testing machine. The stress-strain curves for different samples were analyzed, and mechanical properties such as strength and yield stress were assssed. The ultimate tensile strength demonstrated an increase in strength with the addition of MWCNT at 2 wt. %. A notable high tensile strength was observed for all reinforcements compared to cast Al6061, which can be attributed to grain refinement and the toughening effects of MWCNT nanoparticles. The level of MWCNT reinforcement is directly related to strength and hardness. The nano-size of the reinforced particles significantly contributed to the strengthening and hardening of the Al6061 matrix by enhancing the interface between the matrix and the nanoparticles. This interphase offers resistance to dislocation movement. An improvement in strength resulted from the thermal mismatch phenomenon occurring between the Al6061 matrix and MWCNT, which is primarily responsible for the increased dislocation density in the Al6061 matrix, thereby enhancing nanocomposite strength. The addition of 2 wt. % of MWCNT reinforcement yielded the highest tensile strength of 191.751 MPa, which is 27.48% greater than the strength of cast Al6061 alloys, measured at 150.412 MPa.

   Hardness

   Figure 4. Hardness variation with MWCNT

   Figure 4 illustrates the hardness characteristics of Al6061 nanocomposites with varying weight percentages of MWCNT. A significant enhancement in the hardness values (BHN) of the Al6061 matrix was observed across all compositions tested. This increase in hardness clearly indicates that the addition of MWCNT improves the overall hardness of the inherently soft Al6061 due to a homogeneous dispersion of nanoparticles, effective bonding between the matrix and MWCNT, and reduced porosity. The highest hardness recorded is 65.60 BHN at 2 wt. % of MWCNT reinforcement, reflecting a 31.96% increase compared to the hardness (49.71 BHN) of the cast Al6061 alloys. In the nanocomposite, particle strengthening takes place because MWCNT nanoparticles impede dislocation movement, contributing to the increase in hardness. Additional factors for the rise in hardness include grain refinement and enhanced microstructural densification. The minor decline in hardness noted after reaching the peak value can be attributed to the uneven dispersion of nanoparticles and the presence of nano-porosity. For all MWCNT nanoparticle weight percentages of 0%, 0.5%, 1%, 1.5%, and 2%, the recorded hardness values were 49.71, 56.90, 61.95, 62.10, and 65.60, respectively, measured in BHN.

   Yield Strength

   Figure 5-Yield strength variation with MWCNT

   Yield strength plays a crucial role in materials such as aluminum and magnesium, where defining yield points can be challenging. The behavior of yield strength is similar to that of tensile strength. The factors influencing the achieved proof stress values are the same as those affecting tensile strength. The maximum proof stress observed with 2 wt. % reinforcement yields a peak yield stress of 126.313 MPa, representing an increase of 33.14% over the yield stress of the cast Al6061 alloy, which is 94.874 MPa. At this level, the material exhibits its greatest capacity for plastic deformation or permanent set, along with high ductility.

5. MICROSTRUCTURAL STRENGTHENING DISCUSSION

   The uniform distribution of MWCNTs within the aluminum matrix plays a vital role in strengthening. The nanoscale reinforcement increases the effective surface area for stress transfer. Furthermore:

   • CNTs act as crack arresters.

   • Grain boundary pinning restricts grain growth.

   • Enhanced interfacial bonding improves stress transfer efficiency.

   • Reduced clustering improves mechanical consistency.

     Beyond 2 wt.%, agglomeration may occur, potentially reducing effectiveness. Therefore, 2 wt.% appears optimal under current processing conditions.

6. APPLICATIONS

   The improved nanocomposite material can be used in:

   • Aerospace structural brackets

   • Automotive lightweight engine components

   • Defense structural systems

   • Heat sinks and electronic enclosures

   • High-strength lightweight frames

7. CONCLUSION

The experimental investigation confirms that adding MWCNT to Al6061 significantly enhances mechanical performance.

Key findings:

• Tensile strength improved by 27.48%

• Hardness improved by 31.96%

• Yield strength improved by 33.14%

• Best performance observed at 2 wt.% MWCNT

The improvements are attributed to grain refinement, dislocation strengthening, thermal mismatch effects, and improved interfacial bonding. MWCNT reinforced Al6061 nanocomposites demonstrate strong potential for advanced lightweight structural applications.

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