JournalofMaterialsProcessingTechnology123%2002)13±17
Grindingofalumina/aluminumcomposites
ZhaoweiZhong*,NguyenPhuHung
SchoolofMechanicalandProductionEngineering,NanyangTechnologicalUniversity,NanyangAvenue,Singapore639798,Singapore
Received29October2000
Abstract
Withtheanticipatedwidespreadusageofmetalmatrixcomposites%MMCs)inthenearfuture,themachinabilityofhighperformance
MMCsneedstobeunderstood.Thispaperreportsresearchresultsobtainedfromthegrindingofaluminum-basedMMCsreinforcedwith Al2O3particlesusinggrindingwheelshavingSiCinavitri®edmatrixanddiamondinaresin-bondedmatrix.Theissuesdiscussedaresurface roughness,grindingforce,typeandsizeoftheabrasives,grindingconditions,andtheconsequentialsub-surfaceintegrity.Thestudyused grindingspeedsof1100±2200m/min,agrindingdepthof15mmforroughgrindingand1mmfor®negrinding,andcross-feedsof3and1mm forroughand®negrindingrespectively,whilemaintainingaconstanttablefeed-rateof20.8m/min.Thesurfaceintegrityoftheground
surfacesandsub-surfaceswereanalyzedusingascanningelectronmicroscope%SEM)andapro®lometer.Thesurface®nishvalues,Ra ,were scatteredintherange0.15±0.70mmfortherough-groundsamples,whilstanarrowerrangeof0.20±0.35mmwasachievedforthe®ne-ground samples.Smearingofaluminumonthegroundsurfaceswasseenforroughgrinding,butwasnegligiblefor®negrindingbecausealltheAl2O3 particlesofthegroundsurfaceswereclearlyvisiblewhenobservedwiththeSEM.Grindingusinga3000-gritdiamondwheelatdepthofcutof
1mm produced many ductile streaks on the Al2O3 particles. Both the Al2O3 particles and aluminum matrix were removed by micro machining.Therewerenocracksanddefectsfoundonthegroundsurfaces.Therewasalmostnosub-surfacedamage,exceptforarare
crackedparticlebeingfound.RoughgrindingwithaSiCwheelfollowedby®negrindingwitha®ne-gritdiamondwheelisrecommendedfor thegrindingofalumina/aluminumcomposites.#2002PublishedbyElsevierScienceB.V.
Keywords: Metalmatrixcomposites;Grinding;Ductilestreaks
1. Introduction
A study on the machining of high performance MMCs, therefore, becomes important, especially where mass pro- ductionisinvolved. Aluminum-basedmetalmatrixcomposites%MMCs)rein-
forcedwithceramicparticlesareknownasadvancedmate- Sensitive cost and fabrication challenges including rials for their high speci®c strength, high wear resistance, machining must be overcome for successful application andgooddampingcharacteristics.Methodstoproducethese ofthesecomposites.Thesurface®nishandsurfaceintegrity compositesandstudiesontheirmechanicalpropertieshave areimportantforsurface-sensitivepartssubjectedtofatigue gained popularity [1]. MMCs are primarily used in the orcreep.Machine-inducedsub-surfacedamagecouldhave astronautic,militaryandautomobileindustries.Inaddition, seriousconsequences.However,manymachiningmethods thesportinggoodsindustryhasalsobeenintheforefrontof often result in cracking, splintering and pulling-out of MMCs development, capitalizing on the materials' high reinforcementparticles.Sub-surfacedamageresultedfrom speci®cproperties.Thereisalsoagrowinginterestinother conventionalandunconventionalprocessessuchasturning, industries,suchastheshippingindustry[2]. drilling,milling,electricaldischargemachining,abrasivejet Reports on the machining of aluminum-based MMCs machining,andlasermachining[5].Aproperprocesssuch reinforced with ceramic particles [3±7] are still scarce. asgrindingtoobtainagoodsurface®nishanddamage-free Despite many advantages, full implementation of MMCs surfacesiscrucialfortheapplicationofthematerials. is cost-prohibitive, partially due to the material's poor Ductile-mode machining of brittle materials, such as machinability. Although near-net shape MMC products Al2O3andSiC,hasresultedinmanyinnovativeapplications. can be manufactured, ®nal ®nishing may still be needed Itwasreportedthatbyhavingacriticaldepthofcutandwith forthe®naldesigneddimensionsandrequiredsurface®nish. ¯attenedgrainsslightlyprotrudingfromthesurfaceofthe
grindingwheel,¯awlessmachining,freeofbrittlefracture,is possible[8].Evidenceofplastic¯owwithAl2O3,Si3N4 ,and *Correspondingauthor.
E-mailaddress:mzwzhong@ntu.edu.sg%Z.Zhong). SiCwasshownandamodelbasedonthecombinationoftwo
0924-0136/02/$±seefrontmatter#2002PublishedbyElsevierScienceB.V. PII:S0924-0136%02)00075-4
14 Z.Zhong,N.P.Hung/JournalofMaterialsProcessingTechnology123.2002) 13±17
Table2
Grindingwheelsandgrindingconditionsforroughandfinegrinding
Roughgrinding
Grindingwheel Grain Gritsize Bond
Diameter%mm) Width%mm) Dressingstick Grain Gritsize
Grindingspeed
Lowspeed%m/min) Mediumspeed%m/min) Highspeed%m/min) Depthofcut%mm) Feed-rate%m/min) Cross-feed%mm)
GreenSiC 80 Vitrified-bond 350 38 GreenSiC 60 1100 1650 2200 15 20.8 3
Finegrinding Diamond 3000
Resin-bond 350 10 WA 320 1100 1650 2200 1 20.8 1
theorieswasproposed[9].However,reportsontheductile- modemachiningofaluminum-basedMMCsreinforcedwith ceramic particles are still very scarce. Therefore, further studiesontheductile-modemachiningofthematerialsare required.
This paper reports research results obtained from the grinding of aluminum-based MMCs reinforced with Al2O3 particles using grinding wheels having SiC in a vitri®ed matrix and diamond in a resin-bonded matrix. Theissuesdiscussedaresurfaceroughness,grindingforce, typeandsizeoftheabrasives,grindingconditionsandthe consequentialsub-surfaceintegrity.
2. Experiments
The MMCs chosen for the grinding experiments were 2618/Al2O3/10p%W2F10A)and2618/Al2O320p%W2F20A) detailedinTable1.Thegrindingwheelsusedandgrinding conditionsareshowninTable2.Six2618/Al2O3/10pandsix 2618/Al2O3/20pworkpiecesweremilledtothesamesize. The surface roughness of the ground MMCs in the Three 2618/Al2O3/10p and three 2618/Al2O3/20p work- grinding %table feed) direction and cross-feed direction pieces were rough-ground with an 80-grit vitri®ed-bond was measured using a Tokyo Seimitsu roughness tester. SiCgrindingwheelatlow,mediumandhighgrindingspeed. The cut-off was 0.8mm and the evaluation length was The remaining three pieces of 2618/Al2O3/10p and 2618/ 4mm. The average value was calculated from three mea- Al2O3/20pworkpieceswere®rstrough-groundwiththeSiC suredvaluesinthesamedirectiononeachgroundsurface. grindingwheelatlowgrindingspeed,andthen®ne-ground Thesurface integrityofthegroundsurfaceswasanalyzed witha3000-gritresin-bondeddiamondwheelatlow,med- using a Cambridge±Leica scanning electron microscope iumandhighgrindingspeed. %SEM).Furthermore,thegroundsurfaceswerecleanedwith
Grinding experiments were carried out on an Okamoto alcohol, protected with epoxy, then sectioned with a dia- precision surface-grinding machine %PSG-64DX). A Fuji mondsawonaBuehlerIsometmetallographiccutter.The inverter%FVR-G75)wasattachedtothemachinesothatthe specimens were cold vacuum mounted, ground, polished, mainspindleofthemachinewascapableofbeingchanged. and then etched in a solution of 2ml HF?48%?? 3ml Adynamometer%Kistler9257A)wasmountedonthetableof concentrated concentrated
HCl? 5ml
the grinding machine to measure thegrinding forces. The
HNO3?190mlH2O. Sub-surface damage was assessed
dynamometer was connected to charge ampli®ers %Kis- byobservingtheetchedsamplesusinganSEM. tler5011)andthemeasuredgrindingforceswererecorded
usingachartrecorder%YokogawaLR8100).Thegrindingforce reportedhereistheforceperpendiculartoagroundsurface. 3. Resultsanddiscussion ASiCwheelmountedonabrake-controlledtruingdevice andasinglediamonddresserwereusedfortruingthegrinding The surface roughness values of rough and ®ne-ground wheels.SiCandWAdressingstickswereusedfordressing MMCsareshowninFigs.1and2,respectively.Thevalues the SiC and diamond grinding wheels, respectively. Dres- ofthemaximumgrindingforcemeasuredduringtherough singwascarriedoutbeforeeverygrindingexperiment. and ®ne grinding experiments are shown in Figs. 3 and
Table1
MMCworkpiecesusedforthegrindingexperiments Matrix Reinforcement Process
2618Aluminumalloy
10vol.% Al2O3particulatefor2618/Al2O3/10p%W2F10A);Al2O3 particlesize:9.3mm 20vol.% Al2O3particulatefor2618/Al2O3/20p%W2F20A);Al2O3 particlesize:21mm Directextrusionwithaflatfacedie Extrusionratio20:1 Billettemperature420±4308C Extrusionspeed3m/min
19mm?length?Â17mm?width?Â17mm?height?
Dimensions
Z.Zhong,N.P.Hung/JournalofMaterialsProcessingTechnology123.2002) 13±17 15
Fig. 3. Grinding forces for rough grinding %80-grit vitrified-bond SiC
wheel;depthofcut:15mm;cross-feed:3mm;feed-rate:20.8m/min).
Fig. 1. Roughness values of ground MMC surfaces. Rough grinding %80-gritvitrified-bondSiCwheel;depthofcut:15mm;cross-feed:3mm; feed-rate:20.8m/min).
4, respectively. Figs. 5 and 6 show the top and the cross- sectionoftherough-groundsurfaces,whileFigs.7,8and9 showthoseofthe®ne-groundsurfaces.
Theroughnessvaluesoftherough-groundMMCsurfaces
in the cross-feed direction were higher than those in the grinding %table feed) direction. However, the effect of the measurementdirection onthesurfaceroughnessvaluesof
the ®ne-ground MMC surfaces was not signi®cant. The surface ®nish values, Ra, were scattered in the range of 0.15±0.70mm fortherough-groundsamples, whilst anar- rowerrangeof0.20±0.35mmwasachievedfor®ne-ground Fig. 4. Grinding forces for fine grinding %3000-grit resin-bond diamond
wheel;depthofcut:1mm;cross-feed:1mm;feed-rate:20.8m/min). samples.The surfaces ground bythe80-gritSiCwheelat
speedof1100and1650m/minatadepthofcutof15mm
hadroughnessvaluesclosetothoseofsurfacesproducedby the3000-gritdiamondgrindingwheelatadepthofcutof 1mm:thiswasduetothesmearingofthealuminummatrix. Smearingofaluminumonthegroundsurfaceswasseenfor
Fig. 2. Roughness values of ground MMC surfaces. Fine grinding %3000-grit resin-bond diamond wheel; depth of cut: 1mm; cross-feed: 1mm;feed-rate:20.8m/min). Fig.5. AnSEMmicrographofagroundMMCsurface%2618/Al2O3/20p). Roughgrinding%80-gritvitrified-bondSiCwheel;grindingspeed:2200m/ min;depthofcut:15mm,cross-feed:3mm;feed-rate:20.8m/min).
16 Z.Zhong,N.P.Hung/JournalofMaterialsProcessingTechnology123.2002) 13±17
Fig. 8. Sub-surface of ground MMC %2618/Al2O3/20p). Fine grinding
%3000-gritresin-bonddiamondwheel;grindingspeed:1100m/min;depth ofcut:1mm;cross-feed:1mm;feed-rate:20.8m/min).
Fig.6. Sub-surfaceofgroundMMC%2618/Al2O3/20p).Thearrowsshow commonlyfoundcracksAl2O3particles.Roughgrinding%80-gritvitrified- bondSiCwheel;grindingspeed:2200m/min;depthofcut:15mm;cross- feed:3mm;feed-rate:20.8m/min).
speedandreducedthegrindingforcecomponentperpendi-
cular to the ground surface. In the case of ®ne grinding, becausethedepthofcutwas1mm,thethermaleffectmight be negligible. Further investigation is needed to better understand the micro machining mechanism of both the soft matrix and hard, brittle particles at the same time. roughgrinding,butwasnegligiblefor®negrindingbecause
alltheAl2O3particlesofthegroundsurfaceswereclearly However, from Figs. 3 and 4 it can be seen clearly that
the grinding force required for grinding the MMCs with visiblewhenobservedwiththeSEM.
20vol.% Al2O3 particles is always higher than that for Themaximumgrindingforcedecreasedwithincreasing
grinding speed for the rough grinding, but increased with grindingtheMMCswith10vol.% Al2O3particles. increasinggrindingspeedforthe®negrinding.Thiscouldbe Figs.5and6showthatcracksoftheAl2O3particlesoccur
due to the different abrasives,grit sizes and depths of cut on and under the rough-ground surfaces. As mentioned
above, the surfaces rough-ground at speed of 1100 and used for the rough- and ®ne-grinding experiments, the
1650m/min have roughness values close to those of ®ne- thermallyinducedsoftenedmatrixathighspeedforrough
grinding, etc. For example, because the depth of cut was ground surfaces. However, SEM pictures show that the 15mm for the rough grinding of Al2O3 particles %particle surfacetopographiesoftherough-and®ne-groundsurfaces size:9.3or21mm),moreheatwasgeneratedinthedefor- withclosesurfaceroughnessvaluesaresigni®cantlydiffer-
ent. No Al2O3 particles were seen on the rough-ground mationzone.Thissoftenedthematrixatthehighergrinding
surfaces, except for some small holes showing fractured Al2O3 particles, as seen in Fig. 5. Almost the whole of thesurfacesweresmearedwiththesoftaluminummatrix.
Fig.7. SEMmicrographofgroundMMCsurface%2618/Al2O3/20p).Fine
grinding %3000-grit resin-bond diamond wheel; grinding speed: 1100m/ min;depthofcut:1mm;cross-feed:1mm;feed-rate:20.8m/min).
Fig. 9. The sub-surface of ground MMC %2618/Al2O3/20p). The arrow shows theveryrare crack of an Al2O3 particle. Fine grinding %3000-grit resin-bond diamond wheel; grinding speed: 2200m/min; depth of cut: 1mm;cross-feed:1mm;feed-rate:20.8m/min).
Z.Zhong,N.P.Hung/JournalofMaterialsProcessingTechnology123.2002) 13±17 17
Someofthealuminumchipswereback-transferredontothe ground surfaces were clearly visible when observed with
topofthesurfaces. anSEM. SiCwheelsaremuchcheaperthandiamondwheels,the Grindingusingthe3,000-gritdiamondwheelatdepthof costratiobeingroughly1:10±20.Becausethedepthofcut cut of 1mm produced many ductile streaks on the Al2O3 and cross-feed used were 15 and 3 times those for ®ne particles. Both the Al2O3 particles and aluminum matrix grindingrespectively,thestockremovalwassubstantialand wereremovedbymicro machining.Therewerenocracks thegrindingtimewasmuchshorterascomparedtothatfor anddefectsfoundonthegroundsurfaces.Therewasalmost ®negrinding.Hence, the potential of using SiCwheelsat no sub-surface damage except for a rare cracked particle leastforroughgrindingishigh.Roughgrindingparameters beingfound. anddressingfrequencyshouldbeoptimizedtomakerough ThepotentialofusingSiCwheelsatleastfortherough grindingusingSiCwheelsmoreattractive. grindingofalumina/aluminumcompositesishigh,because As shown in Fig. 7, grinding of the alumina/aluminum SiCgrainsareharderthanAl2O3 reinforcingparticlesand composite2618/Al2O3/20p%20vol.% Al2O3)usingthe®ne- muchlessexpensivethandiamondgrains.Roughgrinding grit diamond wheel at 1mm in-feed %depth of grinding) withaSiCwheelfollowedby®negrindingwitha®ne-grit producedvisibleductilestreaksontheAl2O3particles.Both diamondwheelisrecommendedforthegrindingofalumina/ thematrixandtheAl2O3particleswereremovedbymicro aluminumcomposites. machiningbecausetheductilegrindingmarkswereclearly
seen on the Al2O3 particles. There were no cracks and
defects found on the ground surfaces. There was almost References nosub-surfacedamageasshowninFig.8,exceptforavery
[1] M.J.Tan,L.H.Koh,K.A.Khor,F.Y.C.Boey,Y.Murakoshi,T.Sano,J. rarecrackedparticle,asshowninFig.9.
4. Summaryofresults
Grindingexperimentsusingboth80-gritSiCinavitri®ed
matrixand3000-gritdiamondinaresin-bondedmatrixwere performed on aluminum-based MMCs reinforced with Al2O3particles.
Thesurface®nishvalues,Ra,werescatteredintherange of 0.15±0.70mm for the rough-ground samples, whilst a narrower range of 0.20±0.35mm was achieved for ®ne- ground samples. Smearing of aluminum on the ground surfaces was seen in rough grinding, but was negligible for ®ne grinding because all the Al2O3 particles of the
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