Synthesis and Mechanical Characterization of Aluminum-Graphene Metal Matrix by Powder Metallurgy Tec

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Synthesis and Mechanical Characterization of Aluminum-Graphene Metal Matrix by Powder Metallurgy Technique A John Knoxa, Aarish Nawaza, AltafJilania,Anand Prakasha,Sudheer Reddyb a Department of Mechanical Engineering, NitteMeenakshi Institute of Technology, Bangalore, 560064,India Professor,Department of Mechanical Engineering, NitteMeenakshi Institute of Technology, Bangalore, 560064,India

b

Abstract Composite materials are the go-to materials for a huge range of applications ranging from bio-medical to aerospace, owing to their superior properties than the monolithic metals. This paper presents the synthesis of Aluminum-Graphene composite material, with Aluminum being the matrix phase and the ‘Wonder Material’ Graphene being the reinforcing phase, through powder metallurgy technique.The composite material was prepared by varying the percentage composition (by weight) of Graphene – 0.1%, 0.2%, 0.3% with the hardness and wear properties being studied. Also included is the microstructure study and the discussion on the effect of closed-die forging on these samples with conclusions being drawn on forged and unforged composites.

Keywords-

Graphene,

Aluminum,

Powder

metallurgy,

I. Introduction A Metal Matrix Composite (MMC) is a composite material in which the matrix comprises of the metal and the reinforcements are embedded in the metal matrix. Reinforcements are usually done to improve the properties such as hardness which vary before and after the addition of reinforcement and also with the percentage of reinforcement[1-2]. Thus, MMC serves as a potential substitute for the conventional metals, alloys, and the polymers in almost all the applications due to their superior properties over the unreinforced alloys.

Aluminum alloy is conventionally used for the design of medium and high strength composites for automobiles and aerospace applications. The choice of Aluminum is usually made by its easy availability and lower cost of manufacturing. Many techniques have been developed for producing particulate reinforced MMCs, such as powder metallurgy and squeeze casting. Powder metallurgy is preferred as it has flexibility to produce compositions not possible by other methods.The sintering step in the powder metallurgy densifies and strengthens the material[3].

Among all the processes powder metallurgy is the only IDL - International Digital Library

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Microstructure,

Forging,

Wear,

Hardness.

process which produces minimal or negligible scrap.

The limitation with this process is that it is not useful for low melting powder such as Zinc, Tin and Cadmium. These metals show thermal difficulties during sintering operations. Mechanical properties of all the composites are affected by the size, shape and volume fraction of the reinforcement matrix material and reaction at the interface.

Jingyue Wang et al carried out work on Aluminum as matrix material reinforced with Graphene in the form of nano sheets (GNS) through powder metallurgy process as a flake form. Through powder metallurgy technique the ultimate tensile strength of 249 MPa were observed in the Al composite reinforced with adding only 0.3 wt.% GNSs, which is 62% enhancement when compared with the unreinforced Al matrix[4].

Why Graphene?

Graphene is a perfect two-dimensional (2-D) lattice of sp2-bonded carbon atoms[5]. It is one of the strongest material ever measured with a Young’s modulus of 1TPa[6]. Not just the strength, Graphene has excellent wear and friction properties making it an ideal lubricant Copyright@IDL-2017

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International e-Journal For Technology And Research-2017 Stage 1: Powder Metallurgy

largely due to its high mechanical strength resulting in less wear [7]. It is lighter than air with superior mechanical, optical and electrical properties as well.

All these properties combined make Graphene attractive reinforcements for fabricating light weight, high strength and high performance metal matrix composites.

Stage 2: Testing

II. Experimental Plan Experimental plan is divided into two stages viz. powder metallurgy and testing.

Stage1: Involves the process of fine mixing of powders, compaction at a suitable pressure and sintering at a temperature of 0.7- 0.8 times of the melting point of the metal used.

Stage 2: Finishing operations such as grinding and lapping is done on the sintered composite samples to see the microstructure. Also, in this stage the effect of forging is carried out and hardness and wear tests are performed.

Fig. 1 Flowchart of experimental plan

III. Fabrication of Composite Graphene reinforced MMC is prepared by using dry compaction powder metallurgy technique. The quantity of pure Al taken is 20 grams in each sample and Graphene particles required to fabricate the end composite are 0.1%, 0.2% and 0.3% by weight. The process for fabrication of the MMC is same for all the different composition of Graphene. First, pure Al and the Graphene powders are weighed and then both the powders were mixed using sieve mesh of 70 and 100 grade.The ball milling method for mixing the powders was not adopted since it increases the brittle nature of the Graphene in the composite.

Then the powder mixture is filled in the die, made according to ASTM standard, with the help of a funnel.

The compaction process is done in the Universal Testing Machine at a pressure of 140MPa. Post compaction, the specimen is kept in an air oven at 100oC for degreasing and moisture removal. Sintering is carried out by keeping the composite at 500℃ in a furnace for one hour which induces strength and hardness in the specimen and then slowly cooled in air at room temperature.

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International e-Journal For Technology And Research-2017 forged

Since the density of the Al-Gr composite was around 83.6% to the theoretical density, closed die forging was performed on all compositions of Gr to increase the density of the specimens. In close die forging the specimen is kept in the furnace at 550℃ for 30 minutes and then gently hammered because of the brittle nature of the composite.

The microstructure study, hardness and the wear tests are performed on the composite samples.

IV. Test Results

Fig. 3 (c) 0.2% Gr composite unforged(d) 0.2% Gr composite forged

4.1 Microstructure Result In the microstructure several dark spots were observed as can be seen in the figures below, which can be Graphene or pores. Graphene flakes can be seen around the pores. The presence can be confirmed by comparing the microstructure of Graphene powder as shown in Fig. 5 and Al-Gr composites as shown in Fig 2(a) - Fig 4(f). It can also be seen in Fig 2(b), Fig 3(d) and Fig 4(f) that the porosity of the unforged composites has been decreased after forging and the particles are much closer to each other and hence the density of the forged composites has been increased which can be seen in the figures below. But, the density could not be increased to 99.5% of theoretical density, when the ductility sets in, due to the limitation faced in closed-die forging setup.

Fig. 4 (e) 0.3% Gr composite unforged(f) 0.3% Gr composite forged

Fig. 5Graphene powder (100x)

4.2 Hardness Test Result Rockwell hardness test is conducted on all the composites at 60 kg load. Round tip Diamond indenter is used on the specimens. Result of Table 1 shows that the 0.1% Gr composite has highest hardness value than other composites because it has least amount of Graphene in it which causes less Gr-Gr bonding and doesn’t interfere with Al-Al bonding thus resulting in high hardness value.

Table 1- Rockwell Hardness Value

Fig. 2 (a) 0.1% Gr composite unforged(b) 0.1% Gr composite IDL - International Digital Library

Graphene % in composite 3|P a g e

specimen type

Trial 1

Trial 2

Trial 3

Mean hardne ss

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0.1% 0.2% 0.3% 0%

Unforged Forged Unforged Forged Unforged Forged Pure Al

42 63 43 61 42 55 58

43 67 45 59 46 62 62

44 68 44 63 45 59 57

Graph 2- Comparison of wear rate of 0.2% Gr forged and unforged composites

value 43 66 44 61 44.33 58.67 59

4.3 Wear Test Result Wear test is conducted on wear testing machine – pin on disc setup - at 2kg load at 400 rpm speed for all the composites.The wear test couldn’t be carried out 0.3%Gr forged composite as the material broke upon forging. 0.1%Gr composite has least wear rate because it has highest hardness value and thus less wear.

The wear rate of forged and unforged composites for all the compositions of Graphene have been compared by the computer-generated graphs shown below.

Graph 3- Wear rate of 0.3% Gr composite

V.Conclusion From the tests conducted to determine the hardness, wear rate and microstructure the following conclusions can be drawn:  Microstructure of forged composites shows less pores and uniform dispersion of Graphene than in unforged composites, especially in 0.1%Gr forged composite. This is due to the increase in density by Closed-die forging that reduces the pores.

Graph 1- Comparison of wear rate of 0.1% Gr forged and unforged composites

 Hardness value of 0.1% Gr forged composite is higher than other samples and even pure Aluminum sample. This is a consequence of increasing the density which results in increased bonding between the particles providing strength and hardness to the material.  Wear rate of 0.1% Gr forged composite is lesser because of the high density and less pores in it.

Pores act as stress risers and increase the wear rate which can be seen in 0.2% Gr and 0.3%Gr samples.  0.1% Gr forged composite is the optimum percentage of Gr in the composite samples prepared due to its superior hardness and wear

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International e-Journal For Technology And Research-2017 properties and because the other samples (0.2% and 0.3%Gr) developed cracks upon closed-die forging while 0.1% Gr sample did not.  Increase in Graphene percentage more than 0.1% causes the material to become less and less ductile due to more Graphene particles interfering with Al-Al bonds.  Forged composites have superior properties over unforged composites due to increased density by closed-die forging thus having higher hardness and lesser wear than unforged composites.

Mechanical Properties of GrapheneReinforced Aluminum Matrix Composites, Mater J. Environ. Sci, 7 (5), 2016, 1461-1473.

[4]

Jingyue Wang, Zhiqiang Li, Genlian Fan, Huanhuan Pan, ZhixinChenb and Di Zhanga “Reinforcement with Graphene nanosheets in Aluminum Matrix Composites”, Vol. 66, (2012) 594-597.

[5]

A.K. Geim, K.S.Novoselov, Nat. Mater. 6 (2007) 183–191.

[6]

G.H.Lee,etal.Science30(2013)1073– 1076.

DianaBerman,AliErdemir andAnirudhaV.Sumant “Graphene: A new emerging lubricant” MaterialsToday Volume17,(2014), 3

[7]

References [1]

Ebinezar, Bheemraokamble, Analysis of Hardness Test for Aluminum Carbon nanotube metal matrix and Graphene, Indian Journal of Engineering, volume10, Number 21, April 2, (2014) 33-39.

[2]

Synthesis and Characterization of Aluminum2024 and Graphene metal matrix composites by Powder Metallurgy means,SSRG International Journal of Mechanical Engineering (SSRG-IJME) – volume 2 Issue 7–July 2015, 14-18.

[3]

Pulkit Garg, Pallav Gupta, Devendra Kumar, Om Parkash, Structural and

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