DEVELOPMENT AND STUDY ON MICROSTRUCTURE_ HARDNESS AND WEAR PROPERTIES OF AS CAST_ HEAT TREATED AND EXTRUDED CNT- REINFORCED WITH 6061AL METAL MATRIX COMPOSITES

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DEVELOPMENT AND STUDY ON MICROSTRUCTURE_ HARDNESS AND WEAR PROPERTIES OF AS CAST_ HEAT TREATED AND EXTRUDED CNT- REINFORCED WITH 6061AL METAL MATRIX COMPOSITES Powered By Docstoc
					   International Journal Mechanical Engineering and Technology (IJMET), ISSN 0976 –
INTERNATIONALofJOURNAL OF MECHANICAL ENGINEERING
   6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME
                        AND TECHNOLOGY (IJMET)

ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)                                                   IJMET
Volume 3, Issue 3, September - December (2012), pp.583-598
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   DEVELOPMENT AND STUDY ON MICROSTRUCTURE, HARDNESS
     AND WEAR PROPERTIES OF AS CAST, HEAT TREATED AND
    EXTRUDED CNT- REINFORCED WITH 6061AL METAL MATRIX
                       COMPOSITES

                             MANJUNATHA L.H.1 P.DINESH 2
       1DEPARTMENT OF MECHANICAL ENGINEERING, REVA INSTITUTE OF TECHNOLOGY,BANGALORE,
                   2.DEPARTMENT OF MECHANICAL ENGINEERING ,MSRIT,BANGALORE.




ABSTRACT

       Mechanical properties of Al6061 alloys has necessitated the need for the development of
newer alloys and metal matrix composites of Al6061 alloy. MMC’s are quiet popular in
engineering applications because of low density and high stiffness. Carbon Nano tube to be an
excellent reinforcement material for Al6061alloy based metal matrix composites. The present
work attempts at fabricating Al6061alloy based MMC’s using MWCNT’s for reinforcement
using stir casting technique.

        The SEM images, microstructure and Tribological characteristics like wear are presented
for the various compositions of the MMC’s that have been fabricated. From the SEM images
and microstructure of the composites it is observed that there is uniform distribution of CNT and
good bonding exists between the carbon nano tubes and the matrix, establishing the stir casting
technique for production of Al6061alloy -CNT metal matrix composites. Hardness study revals
that as CNT content increases the hardnes value also increases significantly. Wear study results
for Ascast, Heat treatment and Extruded specimens have been discussed.

KEYWORDS: Microstructure, metal matrix composite, Carbon Nano Tube, Al6061 alloy, wear.




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1. INTRODUCTION

         Aerospace engineering, automobiles, electronic equipment etc., require very light
material with good mechanical properties. Al6061alloy metal matrix composites with carbon
nanotube reinforcement can be a solution for such applications. It can satisfy the requirement of
light weight with very good strength. This study focuses on preparing Al6061alloy metal matrix
composites with MWCNT reinforcements for various composition for it’s mechanical properties.
The study involves preparation of MWCNT reinforced Al6061 alloy metal matrix Composites by
stir casting method. The samples with various compositions produced will be evaluated for their
mechanical properties so that the best composition can be recommended for obtaining
composites to suit various engineering applications. Important features of MMC’s are high
strength, light weight, high stiffness, corrosion resistance, thermal stability, etc., which find
applications in aerospace, automobiles, construction industry, electronics, sports and recreations
etc.
         In the following paragraphs the state of research in the area of present study is presented.
According to[1,2,3] there are two main types of carbon nanotubes that can have high structural
perfection. Single walled nanotubes consist single graphite sheets seamless rapped in the
cylindrical tubes.CNT have electrical conductivity, thermal conductivity and mechanical strength
that conventional materials cannot match. Nanotubes can be used in wide ranges in the field
including chemical, electrical, mechanical and other applications. r young’s modulus is 800Gpa
and yield strength is 20Gpa . According to [4] the electrical properties of carbon naotubes are
influenced by small structural variations. [5] have investigated the electronic properties of carbon
nanotubes. Zhezhang et.al.,[6] have conducted scanning tunneling microscopy to investigate the
structure and electronic properties of carbon nanotubes produced from a discharge between
graphite electrodes. Toru Kuzumaki’s et.al.,[7] has conducted research on mechanical behavior
of carbon nanotubes/C60 composites . According to Eric W.Wong.,[8] the Young’s modulus,
strength and toughness of nanostructures are very important for various engineering applications,
their investigations are based on atomic force microscopy to determine the mechanical properties
of individual structural isolated silicon carbide nanorods and multi walled nano carbon tubes.
The MWCNT are two times stiffer compare to silicon carbon nanorods. R.Byron Pipes et.al.,[9]
has investigated mechanical properties of helical carbon nanotubes arrays they have proposed
two models for the prediction of effective elastic properties of helical array of CNT .Jean-paul
salvetat –Delmotte et.al.,[10] has come to conclusion based on his research that the CNT’s have
great potential has reinforcing elements for composites ,but they have identified two serious
steps has the challenges in this directions.M.M.J.Treacy et.al., [11] has found that CNT’s have
investigating mechanical properties in particular high stiffness and axial strength as a result of
their seamless cylindrical graphitic structure.Their experiments are revealed that the average
youngs modulus is 1.8Tpa.According K.T.Kashyap et.al.,[12] CNT’s are the ultimate carbon
fibers because of their high young’s modulus in the range of 1Tpa which is very useful for load
transfer in nanocomposities. L.Sridhar et.al.,[13] have investigated the characteristics of
MWCNT’s reinforced with aluminium matrix composites. The tensile yield and ultimate strength
of aluminium MWCNT’s increases to 90% with 2 wt% addition of MWCNT’s. T.Laha et.al.,[14]
has propsed that CNT’s have remarkable mechanical, electrical and thermal properties. Deepak
Srivatsava and Chenyu Wei et.al., [15] have investigated on nanomechanics of carbon nanotubes
and composites. SWCNT’s have young’s modulus slightly larger than 1TPa and tubes can

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withstand about 5 to 10% axial strength before yielding which corresponding to stress of about 50Gpa
before nanotube yield.Chunfeng Deng et.al.,[16]has found that carbon nanotube offers a kind of nanosize
reinforcement that is light weight, a hollow core, has immense aspect ratio and has remarkable
mechanical electrical and thermal properties. The investigators have used 2024Al matrix composites
reinforced with 1 wt%CNT’s which was fabricated by cold isotactic pressing followed by hot extrusion
techniques. C.Srinivasan et. al., [17] have investigated several methods available for production of CNT.
Each method has it’s strength and weakneses. Yanchen et.al.,[18] have used plasma assisted chemical
vapour deposition method for producing well aligned graphitic nano fibers.According to C.F.Deng
et.al.,[19] the demand for high performance damping materials is rapidly and continuously growing in a
variety of aerospace, mechanical and civil systems. A.M.K.Esawi et.al.,[20] has used powder metallurgy
for fabrication of CNT’s reinforced MMC’s. R.George et.al.,[21]has investigated strengthening in
CNT/Al composites. Powder metallurgy technique was used for composite fabrication, Commercial
purity Aluminum Powder was used as the matrix material. A mixture of CNT and aluminum powder(200
mesh) were ball milled at 200 rpm for 5 min. The short duration and slower milling speed ensures that
the CNT are intact and can be validated from the transmission electron microscopy(TEM) images of the
composite shown. The milled powder was compacted in a circular die with a load of 120KN,the billets
thus obtained were sintered in an inert gas environment(nitrogen) for 45min at 5800C and finally hot
extruded at 5600C.some samples were tested and the results have indicated that the mechanical properties
of CNT-Al composites including Young’s modulus have significantly improved.

2.MATERIALS AND EXPERIMENTAL DETAILS

2.1 Matrix material
        Aluminium alloy 6061 is one of the most extensively used of the 6000 series aluminium
alloys.
                                             Table.1
                      Typical Chemical composition of Aluminum alloy 6061

Component            Al      Mg      Si    Fe   Cu    Zn   Ti   Mn   Cr    Others
Amount (Wt. %)       Bala    0.8-1.2 0.4 – Max. 0.15- Max. Max. Max. 0.04-
                     nce             0.8   0.7  0.40 0.25 0.15 0.15 0.35 0.05
                                                 Table 2
                                         Properties Al6061 alloy
             Young's             Shear modulus Bulk modulus              Poisson ratio
             modulus
             70 GPa              26 GPa              76 GPa              0.35

2.2 Reinforcement material
        Carbon nanotubes are the strongest and stiffest materials Multi-walled Carbon nano
tubes were procured from nano shell(USA) .
                                              Table.3
                          Properties : Comparison of mechanical properties
                  Young's modulus (TPa)     Tensilestrength         Elongation at break
Material                                    (GPa)
MWCNT                    0.2–0.8–0.95               11–63–150              15.4
Stainless steel          0.186–0.214                0.38–1.55              15–50

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2.3 Fabrication of MMC by stir casting method
        In the present inveatigation Al6061alloy-Carbon nanotube composite is prepared by stir
casting technique. The various proportions of Carbon Nanotubes like 0, 0.1wt%, 0.2wt%,
0.3%,0.4% & 0.5wt% volumes are tried and castings are prepared. Al6061alloy is melted in the
furnace to a temperature of 7200C & then Carbon Nanotubes(MWCNT) which is in the
powdered form(1nm) is poured slowly, simultaneously stirrer is made to rotate at an optimum
speed of 450 rpm for a period of 5-10 minutes, then the melt is degassed by passing Nitrogen
gas. Finally, the molten metal is poured into the finger metal mould. The mould is coated with
chalk powder to prevent sticking of the molten metal into the surface of the mould. The cast
samples are then subjected to heat treatment and hot extrusion. The solidified metal is removed
from the die & is subjected to heat treatment where solutionizing is done at 5900C for a period of
10 hours and then it is immersed in hot water maintained at 1000C and allowed it to cool for 12
hours. And finally ageing is done at 1750C for a period of 5 hours, then the samples for wear test
is prepared according to ASTM standards. Ascast metal is removed from die and is subjected to
hot extrusion where the specimen is heated for 5900 C for about 1 hour and then it is subjected to
hot extrusion. The extruded samples are prepared for wear test according to ASTM standards.
The results were tabulated and the conclusions will be made based on the results. In the present
work for the Ascast, Heat treated and Extruded Al6061alloy-CNT MMc’s specimen tests were
conducted to evaluate the properties.

3.0 RESULTS AND DISCUSSIONS

3.1 Wear Test
       A typical pin is cylindrical in shape with diameter equal to 8mm and length equal to
25mm. A typical disc has diameter of 180 mm and thickness of 12mm. The disc is ground to get
a surface roughness of 0.8 micrometer. The disc is made of highly polished EN-25 steel with
surface hardened to about 60 RHN.During wear testing height loss experienced by the pin
specimen is measured in microns. Measurement of wear height loss of the pin was used to
evaluate the wear loss during the wear test.

3.1.1 Effect of Sliding Time on Wear Loss on Al6061alloy- With weight% MWCT MMC’s
for AsCast Materials
                                            Table.4
                             Effect of Sliding Time on Wear Loss
       Wt% of CNT                         Wear loss in (Microns) for Various Loads
                                          10 N                 20 N                30 N
       Al6061                             204.6                241.32              261.82
       Al6061-0.1 Wt%CNT                  174.83               212.13              234.02
       Al6061-0.2 Wt%CNT                  166.79               206.88              201.91
       Al6061-0.3 Wt%CNT                  132.10               205.61              180.42
       Al6061-0.4 Wt%CNT                  107.67               166.3               162.50
       Al6061-0.5 Wt%CNT                  103.35               132.42              153.023




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Weight loss (gms)




                                                                Sliding time (min.)

                               Fig.1(a). Wear loss of MMCs with Sliding Time at 10 N load for AsCast MMCs
         Weight loss (gms)




                                                                Sliding time (min.)

                                 Fig.1(b). Wear loss of MMCs with Sliding time at 20 N for Ascast MMCs



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     Weight loss (gms)




                                          Sliding time (min.)
                Fig.1(c). Wear loss of MMCs with Sliding time at 30 N for Ascast MMCs
 The above results that are conducted for the as cast materials are shown in the figures
 for various loads according to the various weight% CNT.

3.1.2 Effect of Sliding Time on Wear Loss on      Al6061 alloy- With weight% CNT MMCs
for Heat Treated Materials:

                                           Table.5
                        Effect of Sliding Time on Wear Loss
            Wt% CNT                     Wear loss in (microns) for various loads
                                        10 N          20 N             30 N
            Al6061                      165.2         223.62           237.89
                                        1
            Al6061-0.1Wt%               121.2         192.88           181.39
            CNT                         7
            Al6061-0.2    Wt%           109.0         180.34           178.62
            CNT                         8
            Al6061-0.3    Wt%           96.6          172.88           168.13
            CNT
            Al6061-0.4    Wt%           95.05         144.97           160.27
            CNT
            Al6061-0.5    Wt%           77.58         138.87           148.06
            CNT
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            Weight loss (gms)




                                        Sliding time (min.)
Fig.2(a). Wear loss of MMCs with Sliding Time at 10 N load for Heat Treated MMCs
            Weight loss (gms)




                                        Sliding time (min.)
Fig.2(b). Wear loss of MMCs with Sliding Time at 20 N load for Heat Treated MMCs
    Weight loss (gms)




                                        Sliding time (min.)
Fig.2(c). Wear loss of MMCs with Sliding Time at 30 N load for Heat Treated MMCs

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                3.1.3 Effect of Sliding Time on Wear Loss on Al6061alloy- With weight% CNT MMCs for
                Extruded Materials
                                                            Table.6
                                    Effect of Sliding Time on Wear Loss
                      Wt % of CNT           Wear loss in (microns) for various loads
                                            10 N              20 N             30 N
                      Al6061                243.67            338.16           519.95
                      Al6061-0.1 Wt%        192.04            312.10           430.61
                      CNT
                      Al6061-0.2 Wt%        177.71            311.59           369.13
                      CNT
                      Al6061-0.3 Wt%        156.19            270.01           354.82
                      CNT
                      Al6061-0.4 Wt%        130.79            257.67           350.15
                      CNT
                      Al6061-0.5 Wt%        127.15            212.72           320.14
                      CNT
Weight loss (gms)




                                                        Sliding time (min.)
                Fig.3(a). Wear loss of MMCs with Sliding Time at 10 N load for Extruded MMCs



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Weight loss (gms)




                                                                     Sliding time (min.)
                         Fig.3(b). Wear loss of MMCs with Sliding Time at 20 N load for Extruded MMCs
     Weight loss (gms)




                                                                 Sliding time (min.)
                         Fig.3(c). Wear loss of MMCs with Sliding Time at 30 N load for Extruded MMCs

                                 The variation of wear loss with sliding time is as shown in fig.3(a) to (c). With increase
                         in sliding distance, there is higher wear loss for the matrix and the composites. At larger sliding
                         distance, higher rise of temperature of sliding surfaces are unavoidable. These results in
                         softening of matrix and composite pin surface leading to heavy deformation at higher sliding
                         distances. This results to higher volumetric wear loss of matrix and the composite. As shown in
                         the fig.3(a) to (c). at all the sliding distance considered . the volumetric wear loss of the
                         composites was much lower when compared with the matrix alloy and reduces with increased
                         content of CNT in the composites. Increase in hardness results in improvement of wear.


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                    Effect of Material loss on Wt% of MMC due to Wear for Ascast Heat Treated and
                    Extruded Materials
                                                        Table.7
                    Effect of Material lost on Wt% of MMC due to Wear for As Cast Materials.

                          Wt %
                          of CNT             0N                10 N               20 N               30N
                          0 wt%              5.836             5.812              5.784              5.71
                          0.1
                          wt%                5.562             5.538              5.509              5.464
                          0.2
                          wt%                5.427             5.406              5.378              5.304
                          0.3
                          wt%                5.232             5.215              5.186              5.112
                          0.4
                          wt%                5.716             5.682              5.647              5.589
                          0.5
                          wt%                5.966             5.954              5.92               5.886
Weight loss (gms)




                                                              Wt.% of CNT
                      Fig.4(a). Weight loss of MMCs with various Wt% CNT at different load for As cast MMCs


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                                                              Table.8
                        Effect of Material lost on Wt% of MMC due to Wear for Heat Treated
                                                      Materials.

                           Wt
                           % of
                           CNT               0N                10N              20N              30N
                           0
                           wt%               5.353             5.344            5.306            5.276
                           0.1
                           wt%               5.215             5.198            5.17             5.141
                           0.2
                           wt%               6.138             6.121            6.09             5.989
                           0.3
                           wt%               5.815             5.802            5.773            5.457
                           0.4
                           wt%               5.166             5.144            5.113            5.084
                           0.5
                           wt%               5.676             5.664            5.627            5.607
Weight loss (gms)




                                                              Wt.% of CNT
                    Fig.4(b). Weight loss of MMCs with various Wt% CNT at different load for Heat treated
                                                           MMCs
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                                                              Table.9
                    Effect of Material lost on Wt% of MMC due to Wear for Extruded Materials.
                           Wt
                           %
                           CNT             0N           10 N          20 N           30 N
                           0
                           wt%             4.25         4.228         4.203          4.189
                           0.1
                           wt%             3.865        3.847         3.828          3.778
                           0.2
                           wt%             4.327        4.299         4.287          4.227
                           0.3
                           wt%             4.412        4.4           4.379          4.349
                           0.4
                           wt%             4.174        4.155         4.123          4.069
                           0.5
                           wt%             2.858        2.834         2.803          2.779
Weight loss (gms)




                                                                   Wt.% of CNT
                      Fig.4(c). Weight loss of MMCs with various Wt% CNT at different load for Extruded MMCs
                    It is reported that, by the addition of the CNT its wear property will be decreased by the
                    reinforcement of CNT with Al6061.

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3.2 Hardness Test:
        The Brinell hardness test method consists of indenting the test material with a 4 mm
diameter hardened steel or carbide ball subjected to a load of 500 kg. The full load is normally
applied for 10 to 15 seconds for at least 30 seconds. The diameter of the indentation left in the
test material is measured with a low powered microscope. The Brinell harness number is
calculated by dividing the load applied by the surface area of the indentation. The Ascast , Heat
treated and Extruded specimens were subjected to Brinell Hardness Test and the BHN are
tabulated in below Table .4

                                             Table.10
         Effect hardness on Ascast , Heat treated and Extruded specimens
       Wt % of CNT                            Hardness (BHN)
       Reinforcement                 Ascast        Heat Treated Extruded
             Al6061                  30.12            32.08        32.08
       Al6061-0.1 Wt%CNT             32.08            39.95        35.26
       Al6061-0.2 Wt%CNT             35.26            41.21        38.56
       Al6061-0.3 Wt%CNT             39.42            52.81        42.95
       Al6061-0.4 Wt%CNT             41.57            58.79        39.02
       Al6061-0.5 Wt%CNT             43.37            69.54        45.75




                        Fig.5.BHN v/s wt.% of MWCNT
Fig.5 shows the graphical representation of the hardness(BHN) v/s Wt % of CNT in Ascast, Heat
Treated and Extruded samples.
In this observation of graph it is noticed that, there is an increase in the hardness in heat treated
when compared to Ascast and Extruded
.

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3.3. SEM of worn wear surface :
       Fig.6. show the SEM photographs of worn surfaces of ascast, heat treated and extruded
Al6061 alloy and its composites at an applied load of 30 N and sliding distance of 3 km.


 (a)                            (b)                           (c)




    Fig.6(a).As cast                      Heat
                                Fig.6.(b).Heat treated        Fig.6.(c).Extruded

                   (a)
The above fig.6.(a) to(c) shown micrograph is of ascast, heat treated and extruded SEM
microstructure. we can analyze that the presence of CNT in the material wh       where the black
indicates the CNT. The SEM microstructure also indicates the wear surface happens due to wear
,which indicates the surface after wear and also the particles direction after wear caused by the
heat. There is evidence of adhesion and ploughing on the Al worn surface, which shows a
distinct characteristic of abrasive and adhesive wear. The counterpart pin is seized twice in the
wear tests, which may be the result of the weld contact surface metal at some protruding points.
6061Al wear particles on the worn surfaces are laminated by the pin on the contact area, forming
plough. SEM images of worn surfaces of the composite and the delamination theory of wear, it
can be deduced that the delamination wear could be the main wear mechanism.

3.4 Microstructure test :
     The optical microphotography of Al6061alloy and Carbon Nano Tubes composites are as
shown in the Fig. 1. (a) to (d)


 (a)                   (b)                   (c)                    (d)




Fig.7.(a)Al 6061        Fig.7.(b)Al6061        Fig.7.(c)Al6061       Fig.7.(d)Al6061
Alloy                      Alloy                -0.1wt% CNT          0.1
                                                                    -0.1 wt% CNT

       (a)
Fig.7.(a) to (d) shows the Scanning Electron Micrographs and it is clearly indicates the
homogeneity in the distribution of Carbon Nano Tubes particulates through the matrix alloy. The
microstructure of the carbon nano tubes reinforced composite showed a reason  reasonably uniform
distribution of particles and good interracial bonding of dispersed particles with al6061 matrix
alloy.


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4. CONCLUSIONS

         Al6061 metal matrix composites reinforced with carbon nano tubes was successfully
fabricated by stir casting Technique .
Wear behaviour of 6061Al alloy and it’s composites was carried out successfully by friction and
wear monitor.The carbon nano tubes content in Al6061 alloy plays a significant role in
increasing the wear resistance of the material.Al6061 with 0.5%CNT have high resistance as
indicated by lowest wear. The wear loss tends to decrease with increasing particles volume,
which confirms that addition of CNT is beneficial in reducing the wear loss of the composite.In
adhesive wear,the material loss for Al6061-CNT composites is lower when compared to the
Al6061 matrix alloy.
Wear loss increases with increasing sliding distance due to the work hardening of the surface
leading to abrasion wear.Heat treatment has a important role in adhesive wear mechanism. The
results indicate that quenching of heat treated specimen worm water gives better wear resistance.
Hot extruded specimen gives better wear resistance compared heat treated and without heat
treated specimen.
Co-efficient of friction of Al6061-CNT composites is higher than the Al6061 Matrix alloy. This
is due to the hard reinforcement particles in the soft Al6061 matrix leads to the higher coefficient
of friction. There is an increase in the hardness in heat treated when compared to Ascast and
Extruded.
The microstructure of the carbon nano tubes reinforced composite showed a reasonably uniform
distribution of particles and good interracial bonding of dispersed particles with al6061 matrix
alloy.There is a good interfacial bonding between Al6061alloy and carbon nanotube metal
matrix which improves the hardness of the composites and also the wear behavior of the
composites.
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