In recent times the application of Aluminum based composites are increasing in various industries,owing to their improved mechanical properties. These materials are of much interest to theresearchers from past four decades. In this paper it is aimed to present the experimental resultsof the studies conducted regarding effect of a case hardening process like Nitriding on mechanicalproperties of Al6061-Graphite composites. The composites are prepared using the liquidmetallurgy technique, in which graphite particulates were dispersed in the base matrix in stepsof 0, 3 and 5 wt.%. The experimental results showed that, after Nitriding of the Graphite reinforcedAl6061 metal matrix composite material, the values of the mechanical properties like Brinell'shardness, Tensile strength, Young’s modulus and % elongation compared to those values withoutNitriding were found to be improved.
Keywords: Al6061, Graphite, Case hardening, Nitriding, Composites, Mechanical properties
INTRODUCTIONMetal Matrix Composites (MMC) are of wideinterest owing to their high strength, fracturetoughness and stiffness. The light metals suchas Al and its alloys form superior compositessuitable for elevated temperature applicationswhen reinforced with ceramic particulates(ASM, 2001). It was found that the matrixhardness has a strong influence on the drysliding wear behaviour of Al2O3 particulateAl6061 MMC (Martin et al., 1999). In the
ISSN 2278 – 0149 www.ijmerr.comVol. 3, No. 4, October 2014
1 Department of Mechanical Engineering, Gates Institute of Technology, Gooty, Anantapur (Dist.), India.
investigation on the tribological behavior onAl6061 reinforced with Al2O3 particles it wasconcluded that a characteristic physicalmechanism exists during the wear process(Szu Ying Yu et al., 1997). When a sufficientlyhigh load is applied on the contact, the matrixphase is plastically deformed, and the strainis partially transferred to the particulates, whichare brittle with small failure strains. It wasclearly demonstrated that the effects of appliedload and temperature on the dry sliding wear
Research Paper
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behavior of Al6061 alloy matrix compositesreinforced with SiC whiskers or SiCparticulates and concluded that, the wear ratedecreased as the applied load is increased(Basavarajappa et al., 2006). At higher normalloads (60 N), severe wear and silicon carbideparticles (SiC) cracking and seizure of thecomposite was observed in pin-on-disc testduring dry sliding wear of Al2219 alloy MMCs(Liang et al., 1995). MMCs having SiC of 3.5,10 and 20 m size with 15 vol.%, produced byP/M route displayed good wear resistance withincreasing particle size in sliding wear(Basavarajappa and Chandramohan, 2005).Sliding distance has the highest effect on thedry sliding wear of MMCs compared to loadand sliding speed (Lee et al., 1992). Additionof 20% reinforcements increases the wearresistance of the composites, but beyond thatno improvement was observed (How andBaker, 1997). In the investigation of wearbehaviour of Al6061 alloy filled with short fiber(Saffil) it was concluded that Saffilreinforcement are significant in improvingwear resistance of the composites (Jen FinLin et al., 1996). Self-lubricating graphite wasincorporated in Al6061 alloy to preparecomposites (Seah et al., 1995).
The above literature reveals that the nitridingeffects on mechanical behavior of thecomposites are not discussed, further very littleinformation is available with MMCs of Al6061reinforced with graphite particulates. Hence thepresent paper describes the mechanicalbehavior of nitrided and graphite filled Al6061metal matrix composites.
EXPERIMENTAL DETAILSAND MATERIALS USEDThe following section highlights the material,
its properties and methods of compositepreparation and testing.
The matrix material for the present study isAl6061. The reinforcing material selected wasgraphite. Table 1 gives the chemicalcomposition Al6061 and Table 2 gives thephysical and mechanical properties of Al6061and graphite.
Si Fe Cu Mn Mg Cr Zn Ti Al
0.62 0.23 0.22 0.03 0.84 0.22 0.10 0.01 Bal
Table 1: Chemical Composition of Al6061by wt%
Al6061 70-80 2.7 30 115
Graphite 8-15 2.09 1.7* 20-200**
Table 2: Physical and MechanicalProperties of Al6061 and Graphite
PREPARATION OFCOMPOSITESThe liquid metallurgy route (stir castingtechnique) has been adopted to prepare thecast composites as described below.Preheated graphite powder of laboratorygrade purity was introduced into the vortex ofthe molten alloy after effective degassing.Mechanical stirring of the molten alloy forduration of 10 min was achieved by usingceramic-coated steel impeller. A speed of400 rpm was maintained. A pouringtemperature of 730 °C was adopted and themolten composite was poured into cast ironmoulds. The extent of incorporation ofgraphite in the matrix alloy was varied from0, 3 and 5 wt%. Thus composites containingparticles 0, 3 and 5 wt% were obtained in the
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form of cylinders of diameter 22 mm andlength 210 mm.
CASE HARDENING BYNITRIDING OF COMPOSITESAmong various case hardening processes,Nitriding is selected in the present research.The nitriding process is primarily used toincrease the hardness, corrosion resistanceand wear resistance, etc., of the components/parts used in the various industrialapplications. There are various methods fornitriding. For this experimentation GasNitriding was used. Because of the absenceof a quenching requirement with attendantvolume changes, and the comparatively lowtemperatures employed in this process, gasnitriding produces less distortion anddeformation than either carburizing orconventional hardening. Some growth occursas a result of nitriding but volumetric changesare relatively small. The cast composites wereheated to a temperature of 500 °C in a gastight furnace and then ammonia gas wasintroduced into the furnace chamber. Thisprocess of nitriding was carried out, in thepresence of the nitrogen that is evolved fromthe decomposition of the ammonia gas, for aperiod of 24 hrs maintaining the temperatureof 500 °C.
TESTING OF COMPOSITESThe cast composites were machined and thespecimens for the measurement of hardness,as well as for mechanical behavior wereprepared as per ASTM standards. Brinell’shardness tester was used to measure theHardness of the composites before and afternitriding. The mechanical properties wereevaluated before and after nitriding using
Akash make computerized universal testingmachine of 40-ton capacity.
RESULTS AND DISCUSSIONThe test results of Al6061 and its compositescontaining graphite at various weightpercentages, without and with nitriding, arepresented in these sections.
Effect of Nitriding on theMechanical PropertiesThe effect of nitriding on mechanical propertiessuch as hardness, tensile strength property, %elongation, Young’s modulus, compressivestrength property test results of Al6061 andAl6061 composites containing graphite atvarious weight percentages are presented inthese sections.
HardnessThe change in the hardness of composites withvarying content of graphite reinforcement,without and with nitriding is shown in Figure 1.
Variation in Hardness without andwith Nitriding
30
35
40
45
50
55
60
0% 3% 5%
% Graphite in Al6061
Har
dnes
s B
HN
Without Nitriding With Nitriding
different wt% of graphite (Without and with
nitriding)
Figure 1: Variation in the Hardness withDifferent wt% of Graphite (Without and
with Nitriding)
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The figure represents the variation in hardnessevaluated at a load of 500 kg with increasingpercentage of graphite in Al6061 with andwithout nitriding. It is observed that thehardness of Al6061 composites decreaseswith increased content of the graphitereinforcement.
There is a good reason for thisphenomenon, though, since graphite, being asoft dispersoid, does not contribute positivelyto the hardness of the composite. Seah et al.(1995) have reported a reduction in hardnessfrom 107 BHN to 77 BHN (about 28%differences) on addition of similar weightpercentages of graphite to ZA-27 (ZincAluminium) alloy. Such a monotonic decreasein the hardness of the composite as graphitecontent is increased poses a limit to how muchgraphite may be added to enhance its othermechanical properties, since hardness isdirectly related to wear resistance.Consequently, a compromise is necessarywhen deciding how much graphite should beadded to enhance the ductility, UTS,compressive strength, and Young’s modulusof the composite without sacrificing too muchof its hardness, especially in components likeengine bearings, pistons, piston rings andcylinder liners, in which wear resistance is ofparamount importance. Further the hardnessvalues of the composites were found to beincreased after nitriding compared to thosevalues before nitriding. This is clearly depictedin the Figure 1.
Tensile PropertiesFrom the study of Figure 2 it can be seen thatthe tensile strength increases with increasingpercentage of graphite. From the figure, it canbe observed that the tensile strength of the
composites is higher than that of the matrixalloy. Further, from the graph, the trends of thetensile strength can be found to be increasedwith increase in graphite content in thecomposites. This increase in tensile strengthmay be due to the graphite particulates actingas barriers to dislocations in themicrostructure. One great advantage of thisdispersion-strengthening effect is that it isretained even at elevated temperatures andfor extended time periods because theparticles are unreactive with the matrix phase.Also the tensile strength of the composites wasfound to be increased after nitriding comparedto that before nitriding. This is clearly seen inthe Figure 2.
Figure 2: Variation in Tensile Strength ofAl6061 with Increasing wt% of Graphite
(Without and with Nitriding)
Variation in Tensile Strengthwithout and with Nitriding
95105115125135145155
0% 3% 5%
% Graphite in Al6061
Ten
sile
Str
engt
h(M
Pa
)
Without Nitriding With Nitriding
Figure 2. Variation in tensile strengthof Al6061 with increasing wt% ofGraphite (Without and with nitriding)
% ElongationFrom the study of Figure 3 it can be seen thatthe % elongation increases with increasingpercentage of graphite. From the figure, it canbe observed that the % elongation of the
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composites is higher than that of the matrixalloy. Further, from the graph, the trends of the% elongation can be found to be increasedwith increase in graphite content in thecomposites. This considerable increase inductility is due to the graphite additions, beingan effective solid lubricant, eases themovement of grains along the slip planes. Theeffect of graphite is expected to be mechanicalin nature since the particles are un-reactivewith the matrix phase. It was also observedthat the % elongation of the composites isincreased after nitriding.
Young’s ModulusFrom the study of Figure 4 it can be seen thatthe Young's modulus increases with increasingpercentage of graphite. From the figure, it canbe observed that the Young’s modulus of thecomposites is higher than that of the matrixalloy. Further, from the graph, the trends of theYoung's modulus can be found to be increased
with increase in graphite content in thecomposites. Similar results have beenobtained in aluminium matrix compositeswhere the Young's modulus has been reportedto increase with increase in the content of thereinforcing material, regardless of the type ofreinforcement used. The values of the Young'smodulus of the composite were found to beenhanced after nitriding. This is clearly seenin the Figure 4.
Compressive PropertiesFrom the study of Figure 5 it can be seen thatthe compressive strength increases withincreasing percentage of graphite. From thefigure, it can be observed that thecompressive strength of the composites ishigher than that of the matrix alloy. Further,from the graph, the trends of the compressivestrength can be found to be increased withincrease in graphite content in thecomposites. Also the compressive strength
Figure 3: Variation in % Elongation ofAl6061 with Increasing wt% of Graphite
(Without and with Nitriding)
Variation in % Elongationwithout and with Nitriding
3
4
5
6
7
8
9
0% 3% 5%% Graphite in Al6061
% E
long
atio
n
Without Nitriding With Nitriding
Figure 3. Variation in % Elongationof Al6061 with increasing wt% ofGraphite (Without and with nitriding)
Figure 4: Variation in Young’s Modulus ofAl6061 with Increasing wt% of Graphite
(Without and with Nitriding)
Variation in Young's Moduluswithout and with Nitriding
55
65
75
85
95
0% 3% 5%
% Graphite in Al6061Y
oung's
Modula
s(G
Pa)
Without Nitriding With Nitriding
Figure 4. Variation in Young’s modulus of Al6061 with increasing wt% of Graphite (Without and with nitriding)
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values of the composites were found to beimproved after nitriding compared to thosevalues before nitriding. This is clearlydepicted in the Figure 5.
CONCLUSIONThe significant conclusions of the studiescarried out on Al6061-Graphite compositesare as follows.
Cast Al6061-Graphite composites wereprepared successfully using liquid metallurgytechniques.
The properties of the cast Al6061-graphitecomposites are significantly changed byvarying the amount of graphite therein. It wasfound that increasing the graphite contentwithin the matrix material resulted in significantimprovement in ductility, tensile strength,compressive strength and Young’s modulus,but a decrease in the hardness.
The mechanical properties like hardness,tensile strength, % elongation, Young’smodulus and compressive strength of thecomposites were found enhanced with thenitriding heat treatment process.
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