mechanical and corrosion behaviour of zn-27al

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Vol.36,No.2(2014)195‐203

TribologyinIndustry

www.tribology.fink.rs

MechanicalandCorrosionBehaviourofZn‐27AlBasedCompositesReinforcedwithGroundnutShell

AshandSiliconCarbideK.K.Alaneme

a,b,B.O.Fatile

a,J.O.Borodea,

b

aDepartmentofMetallurgicalandMaterialsEngineering,FederalUniversityofTechnology,Akure,PMB704,Nigeria,bDepartmentofMiningandMetallurgicalEngineering,UniversityofNamibia,OngwedivaEngineeringCampus,Ongwediva,Namibia.

Keywords:

StircastingZn‐27AlalloyHybridcompositesGroundnutshellashMechanicalpropertiesCorrosionbehaviour

ABSTRACT

ThemechanicalandcorrosionbehaviourofZn‐27Alalloybasedcompositesreinforcedwith groundnut shell ash and silicon carbidewas investigated.Experimental test composite samples were prepared by melting Zn‐27Alalloywithpre‐determinedproportionsofgroundnutashandsiliconcarbideas reinforcements using double stir casting.Microstructural examination,mechanical properties and corrosion behaviourwere used to characterizethe composites produced. The results show that hardness and ultimatetensile strength of the hybrid composites decreasedwith increase in GSAcontent.Although the%Elongationsomewhatdecreasedwith increase theGSAcontent,thetrendwasnotasconsistentasthatofhardnessandtensilestrength. The fracture toughness of the hybrid composites however,increasedwithincreaseintheGSAcontentofthecomposites.In3.5%NaClsolution,thecompositeswereresistanttocorrosionwithsomeofthehybridcomposite grades containing GSA exhibiting relatively superior corrosionresistancetothegradeswithoutGSA.In0.3MH2SO4solution,thecompositesweregenerallyabitmoresusceptibletocorrosion(comparedto3.5%NaClsolution), but the effect of GSA content on the corrosion resistance of thecompositeswasnotconsistentfortheZn‐27Alalloybasedcomposites.

©2014PublishedbyFacultyofEngineering

Correspondingauthor:

KennethK.AlanemeDepartmentofMetallurgicalandMaterialsEngineeringFederalUniversityofTechnology,Akure,P.M.B704,NigeriaE‐mail:[email protected]

1. INTRODUCTIONIn recent years, Zn‐Al based composites aregainingwidespread importance inseveralhightechnologicalapplications[1‐2].TheZn‐Alalloybasedmatrices are designated ZA‐8, ZA‐12 andZA‐27 corresponding to the approximatealuminiumcontent inthealloy[3].Thesealloysare known to possess good combination of

physical, mechanical and technologicalproperties (low melting point, excellentcastability, high strength, good machining andtribological properties), as well as by lowmanufacturing costs [4‐5]. The Zn‐Al basedalloys are suitable for a wide range ofapplications such as for manufacture ofindustrial fittings and hardware, pressure tighthousings, sleeve bearings, thrust washers and

RESEARCH

K.K.Alenemeetal.,TribologyinIndustryVol.36,No.2(2014)195‐203

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wearplates [6‐7].Of allmembersof ZA castingalloys,ZA‐27alloyhas thehighest strengthandthe lowest density, aswell as excellent bearingand wear resistance properties [8]. This alloyalsoprovidesthehighestdesignstresscapabilityatelevatedtemperaturesofallthecommerciallyavailablezinc‐basedalloys[9].However,theZA‐27 alloy as well as other ZA alloys are notapplicableatoperatingconditionsmorethan80°C, due to thedeteriorationof theirmechanicalproperties at elevated temperatures [9].Thusstrengthening by the use of refractory ceramicparticlesorfibershasbeenexploredtoimprovetensile strength, strength to weight ratio andcreepresistanceathigherworkingtemperatures[10‐11].Hardreinforcementssuchassiliconcarbideandalumina have been successfully utilized toenhancethehardness,strength,elasticmodulusand abrasive wear resistance of ZA basedcompositeswhile some properties like fracturetoughness, impactresistance,dampingcapacity,corrosion resistance, machinability andconductivityhavebeenimprovedwiththeuseofgraphiteascomplementingreinforcementtoSiCoralumina[11‐15].Presently, there is increasing interest indeveloping low cost ‐ high performance metalmatrix composites (MMCs) using ashesprocessedfromagriculturalwastesaspartialorcomplete replacements for conventionalsynthetic reinforcements such as SiC andalumina [16]. Very encouraging results havebeen observed from Aluminium matrixcomposites reinforced with hybrid mix of theagriculturalwasteashes (suchas ricehuskash,bamboo leaf ash) and the syntheticreinforcements (such as SiC and alumina)[17].The development of Zn‐Al based hybridcomposites has been reported in literature butverylittleresourcecanbefoundforZn‐Albasedcomposites developed with the use ofagricultural waste ashes as single or hybridreinforcements. These agro waste ashes arenoted to contain reasonable amounts of silica(SiO2) and other refractory oxides such asalumina Al2O3 and hematite (Fe2O3). Theyusually are of lower densities compared withconventional reinforcements such as SiC (3.18g/cm3) and alumina (3.9 g/cm3), and can beprocessed cheaply using simplereactors/burners[16,18].

Inthisinvestigationthemechanicalpropertiesand corrosion behaviour of Zn‐27Al alloyreinforcedwithvariedweightratiosofgroundnut shell ash and silicon carbide is reported.Groundnut is produced in large quantity inNigeriaandmanyotherpartsoftheworld[19‐20]; and to date limited use of the groundnutshells after extraction of the seeds has beenreported in literature. Conversion of theseagricultural wastes to products suitable foruseas reinforcements in compositeswill helpextend the frontiers of knowledge ofcomposite materials development andapplications, and contribute in agriculturalwastemanagement.2. MATERIALS AND METHOD

2.1MaterialsandPreparationCommercial pure Aluminium and Zinc (withchemical composition presented in Table 1)were selected for the production of the Zn‐Alalloy matrix. Chemically pure silicon carbide(SiC) particles having average particle size of30 μm and groundnut shell ash (<50 µm)derivedfromcontrolledburningandsievingofdry groundnut shell ash were used asreinforcement for the Zn‐27Al basedcomposites tobeproduced.Graphiteparticleshaving particle size of < 50 μm was alsoprocured foraddition in thecompositemix tohelpimprovemachinabilityofthecomposites.The chemical composition of the groundnutshellashispresentedinTable2.Table 1. Elemental Composition of the Zinc andAluminium used for the production of the Zn‐27AlbasedCompositematrix.

Elements Weightpercentage

Al 99.92

Fe 0.003Si 0.033Mn 0.021others 0.023Element Weightpercentage

Zn 99.96

Fe 0.02Si 0.006Pb 0.004others 0.01


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Table2.CompositionofGroundnutShellAsh.

Elements WeightpercentageSiO2 41.42Al2O3 11.75Fe2O3 12.60CaO 11.23MgO 3.51TiO2 0.63Na2O 1.02K2O 11.89P2O3 1.71MnO 0.23SO3 0.44*LOI 3.57*LOIrepresentslossonIgnitionA two step stir castingprocess as describedbyAlaneme and Aluko [20] was adopted for theproduction of the test composite samples.Charge calculation was used to determine theamountofgroundnutshellash(GSA)andsiliconcarbide(SiC)requiredtoprepare7and10wt.%of the GSA‐SiC reinforcements (in the Zn‐27Alalloy matrix) consisting of 0, 20, 30 and 40 %GSA. 0.25 wt.% graphite was added to eachcharge to improve machinability of thecompositesaftercasting.The groundnut shell ash, silicon carbide andgraphite particles were initially preheatedseparatelyatatemperatureof250oCtoremovemoisture and to help improve wettability withthemoltenZn‐Alalloy.TheAluminiumbilletwascharged into a gas fired crucible furnace (fittedwith a temperature probe), and heated to atemperature of 670 oC until the Aluminiummelted completely. The temperature of thefurnacewas lowered to500 oCbeforeZincwasintroduced. After the Zinc has meltedcompletely,themeltwasthenallowedtocoolinthe furnace to a semi solid state (atatemperature of about 450 oC) and it wasstirred at 200 rpm for 5 minutes to achievehomogenization.Thepreheatedgroundnutshellash,SiCparticlesandgraphiteparticleswerethenchargedintothemelt and stirring of the slurry was performedmanuallyfor5‐10minutes.Thecompositeslurrywas superheated to atemperature of 530 oC,stirredmechanicallyat400 rpm for10minsandthereafter sand casted. The sample designationsfor the grades of Zn‐27Al based compositesproducedarepresentedinTable3.

Table 3. Sample Designations for the Compositesproduced.

Sample Designation WeightpercentGSA:SiC7 wt%Reinforcement A0 0:100A20 20:80A30 30:70A40 40:6010 wt%Reinforcement B0 0:100B20 20:80B30 30:70B40 40:602.2MechanicalTestingHardness tests (HRC) were conducted onpolished specimens, representative of eachgrade of composite produced using IndentecHardness Testing Machine. Multiple hardnesstests were performed on each sample and themean of valueswithin the range of ± 2%wastakenasthehardnessofthespecimen.Testspecimensobtainedfrommastercompositealloys were subjected to tensile test usingInstron Universal test machine in accordancewith theASTM8M‐91standards [21].Readingswere taken to determine the ultimate tensilestrength(UTS)andstraintofailure(reportedas% Elongation). Similarly, circumferentialnotched tensile tests were carried out onstandard machined test specimens using theInstron Universal Test Machine in accordancewith the test proceduresdescribedbyAlaneme[22]. The load‐extension plots obtained wereused to determine the fracture toughness asdescribedbyDieter[23].Inall testcases,multiple testswerecarriedoutforeachgradeof theZn‐27Albasedcompositesand average of the measurements taken toensure repeatability and reliability of thegenerateddata.2.3MicrostructuralExaminationA Zeiss Metallurgical Microscope withaccessories for image analysis was used foroptical microscopic investigation of thecompositesproduced.Thespecimensforthetestwere metallographically polished and etchedbeforemicroscopicexaminationwasperformed.Thepolishedsampleswereetchedindiluteaqua


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regal solution (3HCl:1HNO3) before they wereviewedunderthemicroscope.2.4CorrosionTestThecorrosionbehaviourof thecompositeswasstudied byweight lossmethod usingmass lossand corrosion rate measurements as basis forevaluating the results generated. The corrosiontest was carried out by immersion of the testspecimens in 0.3M H2SO4 and 3.5 % NaClsolutions which were prepared followingstandardprocedures[24].Thespecimensforthetest were cut to size 15×15×10 mm and thenmechanically polishedwith emery papers from220 down to 600 grades to produce a smoothsurface. The samples were de‐greased withacetone,rinsedindistilledwater,andthendriedinairbeforeimmersioninstillsolutionsof0.3MH2SO4and3.5%NaClatroomtemperature(250C).Thesolution‐to‐specimensurfacearearatiowasabout150mlcm‐2,andthecorrosionsetupswere exposed to atmospheric air for thedurationof the immersion test.Theweight lossreadings were monitored on two day intervalsfor a period of 42 days. Themass loss (g/cm2)and the corrosion rate (mmy) for each samplewas evaluated in accordance with ASTM G31standard recommended practice [25]. Threerepeat testswere carried out for each grade oftheZn‐27Albasedcompositesproduced,andthereproducibility and repeatability of the resultsfrom the triplicates were found to have nosignificantdifferences.3.RESULTSANDDISCUSSION3.1MicrostructureFigure 1 shows the microstructural features ofthe Zn‐27Al alloy and a representative Zn‐27Al/GSA‐SiC composite grade. It is observedfromFig.1(a)thatasomewhat featheryfeaturewhichisactuallythedendriticpatternoftheZn‐27Al alloy indicating the solidification patternandgrainmorphology.Figure1(b)ontheotherhandshowsapartiallyconcealeddendriticgrainstructurearisingfromthepresenceofthe fairlyhomogeneous distribution of the GSA‐SiCparticles in the Zn‐27Al alloy. A few pockets ofparticle clusters which are very common withstir casting processedMMCs are also observedinthemicrostructure.

(a)

(b)

Fig. 1. Representative micrograph showing (a) theZn‐27Al alloy microstructure, and (b) themicrostructure of the Zn‐27Al based compositecontaining 7 wt% GSA‐SiC in weight percent 30 %GSA:70%SiC.3.3MechanicalBehaviourThe hardness values of the composites arepresented in Fig. 2(a). There was little doubtthat the hardness values of the compositeswoulddecreasewithincreaseintheGSAcontentinthecompositesjudgingfromthepredominantsilica, alumina, and haematite content of GSA(Table2)whichare lessabrasive (harder) thanSiC [18]. The interest was to ascertain if thepercentage decrease in hardness was quitesignificant. For the 7 wt% GSA‐SiC reinforcedZn‐27Al composite grades, an averagedecreaseinhardnessof7.1,9.8,and12.6%wasobservedfor the composites containing 20, 30, and 40%GSA (A20, A30, and A40) respectively. In thecaseof the10wt%GSA‐SiC reinforcedZn‐27Alcomposite grades, 4.56, 10.2, and 12.7 %reduction in hardness was observed for the


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composites containing 20, 30, and 40 % GSA(B20,B30,andB40)respectively.

(a)

(b)

(c)

(d)

Fig.2. Variation of (a) Hardness, (b) Ultimate TensileStrength,(c)%Elongation,and(d)FractureToughnessoftheZn‐27AlbasedCompositesProduced.

A similar trend was observed for the tensilestrength values (Fig. 2b) in which the UTSdecreasedwithincreaseintheweightpercentofGSA. Reductions of 1.3, 8.1, and 12.3% whereobtained for the 7wt%GSA‐SiC reinforced Zn‐27Alcompositegradescontaining20,30,and40%GSA(A20,A30,andA40)respectively.Forthe10 wt% GSA‐SiC reinforced Zn‐27Al compositegrades 1.7, 9, 17.1 % where observed for thecomposites containing 20, 30, and 40 % GSA(B20,B30,andB40)respectively.Thevariationof % Elongation (Fig. 2c) with increase in theGSA content in the Zn‐27Al based compositesproducedwasnotasconsistentaswasthecaseof the hardness (Fig. 2a) and tensile strength(Fig. 2b) discussed earlier. It is however notedthat for both the 7 and 10 wt% reinforcedcompositeseries,thegradeswithlittleornoGSA(0and20%GSA)appeared tobemoreductilethan thegradeswithhighercontentofGSA(30and 40%GSA). The fracture toughness results(Fig. 2d) are however noted to increase withincreaseintheweightpercentofGSAinboththe7 and 10 wt% composite grades which is incontrast with the trend observed for thehardness and tensile strength. Significantincreasesof25,43,and54%wereobtainedforthe 7 wt% GSA‐SiC reinforced Zn‐27Alcomposite grades containing 20, 30, and 40%GSA(A20,A30,andA40)respectively.Forthe10wt% GSA‐SiC reinforced Zn‐27Al compositegrades increase of 15, 36, and 47 % whereobserved for the composites containing 20, 30,and40%GSA(B20,B30,andB40)respectively.Theimprovementinthefracturetoughnesswithincrease in GSA content of the compositesmaybe due to the reduced amount of relativelyharder and brittle SiC particles in the Zn‐27Albased composites produced [18]. The SiCparticles like most hard and brittle ceramicparticles have a higher tendency to undergorapid crack propagation [17]. Thus it is notedthat the addition of GSA in the Zn‐27Al basedcomposites results in improved resistance tocrackpropagationandfracture.3.3CorrosionBehaviourFigures3and4showthevariationofmasslossandcorrosionratewithexposuretimeforZn‐27Al composite grades containing 7 and 10wt.%GSA‐SiC reinforcement respectively in 3.5 %NaClsolution.FromFig.3(a),itisobservedthatthe mass loss values for the composite grades


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with 7 wt.% GSA‐SiC reinforcement weregenerallyverylowindicatingthattheZn‐27Al/7wt.% GSA‐SiC composites are very stable andcorrosionresistantin3.5%NaClsolutionwhichis representative of a typical marineenvironment. The mass loss values for agoodproportionofimmersionperiodwerenegative–an indication of weight gain and affirmation oftheprotectivenatureofthepassivefilmformedon the composites. Closer examination revealsthat the hybrid composite grades A20 andA40(which both contain 20 and 40 % GSArespectively),havesuperiorcorrosionresistancein comparison with the composite grade A0(which does not contain GSA). The corrosionrate data (Fig. 3b) is consistent with the masslosstrendobservedinFig.3(a).

(a)

(b)

Fig. 3.Variationof (a)Mass loss,and(b)Corrosionrate of Zn‐27Al based composites containing 7wt%GSA‐SiCin3.5%NaClSolution.The Zn‐27Al alloy composite grades of the 10wt.%GSA‐SiC reinforced series alsoexhibited averyhigh resistance to corrosion in3.5%NaClsolution (Fig. 4). The negativemass loss valuesof the composites (Fig. 4a) are reflective of thestableandprotectivenatureofthepassivefilmsformed on the surfaces of the composites. It isalso noted that the corrosion resistance of the

composites improves with increase in the GSAcontent inthecomposites.ThecompositegradewithoutGSA(B0)isobservedtohavethehighestmass loss (least corrosion resistance) in 3.5%NaCl solution. The improvement in corrosionresistance observed with the addition of GSAmay not be isolated to the presence of silicawhich is themajorconstituentofGSA(Table2)asisthecasewithAluminiumbasedcompositeswithsimilarhybridreinforcementcompositions[26].ThenatureandprotectivecharacteristicsofpassivefilmsformedonZn‐Albasedcompositesimmersed in NaCl environments has beenreportedtodependonacomplexrelationshipofthealloysandcompositecomposition,structureand the surface area exposed to the corrosiveenvironment[27].

(a)

(b)

Fig. 4.Variationof (a)Mass loss,and(b)CorrosionrateofZn‐27Albasedcompositescontaining10wt%GSA‐SiCin3.5%NaClSolution.Figures5and6showthevariationofmasslossand corrosion rate with exposure time forcomposite samples with 7 and 10 wt.% ofreinforcements respectively immersed in 0.3MH2SO4solution.From Fig. 5(a), it is observed that all thecompositegradeswith7wt.%reinforcementare


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susceptible to corrosion in0.3MH2SO4withA0(the sample with 0% GSA) exhibiting the leastmass loss in comparisonwithother compositesin this series. In addition, the mass loss andcorrosion rate (Fig. 5b) increased gradually forall the composite grades after the initial sharprise observed during exposure in the acidicsolution.Theprogressioninthecorrosionofthecompositesisanindicationthatthepassivefilmformed on the composites was unable to giveadequateprotectiontothesubstrates.

(a)

(b)

Fig. 5.Variationof (a)Mass loss,and(b)Corrosionrate of Zn‐27Al based composites containing 7wt%GSA‐SiCin0.3MH2SO4Solution.InthecaseoftheZn‐27Alcompositegradeswith10 wt.% GSA‐SiC reinforcement, it is observedfrom Figure 6(a) that themass loss formost ofthe composites are comparable with theexceptionofthecompositegradeB30(containing30%GSA)whichwasrelativelymoreresistanttocorrosion in the 0.3M H2SO4 solution. Just as inthe case of the composite grades with 7 wt.%reinforcement, It is noted that the mass lossincreased steadily throughout the immersionperiodafteran initial sharp increaseat thestartoftheimmersiontest.Thecorrosionrateresults(Fig. 6b) are consistent with the mass lossobservationpresentedinFig.6(a).Thecorrosion

trends observed for the Zn‐27Al basedcomposites in 0.3MH2SO4 solution is supportedbytheinvestigationofSharmaetal.[28].

(a)

(b)

Fig.6. Variation of (a)Mass loss, and (b) CorrosionrateofZn‐27Albasedcompositescontaining10wt%GSA‐SiCin0.3MH2SO4Solution.4.CONCLUSIONSThemechanical and corrosionbehaviourof Zn‐Alhybridcompositescontaining7and10wt.%of varied weight ratios of GSA and SiC wasinvestigated.Theresultsshowthat:

Hardness andultimate tensile strength ofthe hybrid composites decreased withincreaseinGSAcontent.

Although the % Elongation somewhatdecreasedwith increase the GSA content,the trendwasnotasconsistentas thatofhardnessandtensilestrength.

The fracture toughness of the hybridcomposites however, increased withincrease in the GSA content of thecomposites.

In 3.5 % NaCl solution, the compositeswere resistant to corrosionwith some ofthe hybrid composite grades containing


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GSA exhibiting relatively superiorcorrosionresistancetothegradeswithoutGSA.

In 0.3M H2SO4 solution, the compositeswere generally abit more susceptible tocorrosion (compared to 3.5 % NaClsolution),buttheeffectofGSAcontentonthecorrosionresistanceofthecompositeswas not consistent for the Zn‐27Al alloybased composites belonging to the 7 and10wt.%series.

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mechanical and corrosion behaviour of zn-27al

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