ContentslistsavailableatScienceDirect
Journal of Materials Processing Technology
j ou rn a l h o m epa ge :w w w . e l s e v i e r . c o m / l o c a t e / j m a t p r o t e c
Application of a plate sonotrode to ultrasonic degassing of aluminum melt: Acoustic measurements and feasibility study
D.G.Eskina,b,∗,K.Al-Helala,I.Tzanakisa
aBrunelUniversityLondon,BrunelCentreforAdvancedSolidificationTechnology,UxbridgeUB83PH,UnitedKingdom
bTomskStateUniversity,Tomsk634050,Russia
a r t i c l e i n f o
Articlehistory:
Received20January2015
Receivedinrevisedform4March2015 Accepted5March2015
Availableonline13March2015
Keywords:
Ultrasound Cavitation Degassing Aluminum
Sonotrode,Acousticpressure
a b s t r a c t
Aflatplatesonotrodewasusedforultrasonicmeltprocessing(degassing)ofaluminummelts.Calculations showedthatthissonotrodeshouldhaveseveralantinodeswithmaximumamplitude,spacedat16.5mm.
Thedirectmeasurementsoftheamplitudeinairandindirectmeasurementsoffoilcavitationerosionin watervalidatedthesecalculations.Uniqueacousticmeasurementsofcavitationactivityinwateranda liquidaluminumalloywereperformedusingacavitometerandconfirmedthatthecavitationconditions weremetwiththisscheme.Themeltdegassingefficiencyusingtheplatesonotrodewassignificantly higher(70–80%)thanwithaconventionalcylindricalsonotrode(45–50%)inbatchoperation.Thenew schemewasalsosuitableforultrasonicmeltprocessinginthemeltflowgivingabout50%degassing efficiency,whichopensthewaytoupscalingthistechnologytotreatlargervolumesofmelt.
©2015TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).
1. Introduction
One of the objectives of good casting technology is to avoid porosity in a casting. This is achieved through melt degassing.Inliquidaluminum,theamountofhydrogencanreach 0.3–0.5cm3/100g,whiletheindustrialstandardrequiresthecon- centrationofhydrogenbeforecastingcloseto0.1cm3/100g(Waite, 1998).Currentlythemostadoptedtechnologyofdegassingisso- calledargonrotary degassing where a porousgraphite rotor is submergedintothemelt andAr ispurgedthroughtherotorto thebottomofthebulkmelt.Therotatinggraphiteshaftshearsand dispersesArbubblesthatfloattothesurface,collectinghydrogen dissolvedinthemelt.Althoughwellestablished,thistechnology hassomedrawbackssuchashighdrossformation,notrecycled expensiveAr,andpotentialdamageofthegraphiterotorwithsub- sequentmeltcontamination.
Ultrasonicdegassingisamongthefirstpotentialapplications ofacousticcavitationinliquids.Ithasbeentriedontheindustrial scaleforaluminummeltdegassinginfoundriesandcasthouses backinthe1960–1980sandprovedtobeacleanandrobusttech- nology,thoughrequiringsomespecialtyequipmentandset-upas reportedbyEskin(1965)andrecentlyreviewedbyEskinandEskin
∗Correspondingauthorat:BrunelUniversityLondon,BrunelCentreforAdvanced SolidificationTechnology,UxbridgeUB83PH,UnitedKingdom.
Tel.:+441895265317.
E-mailaddress:[email protected](D.G.Eskin).
(2014).Inrecentyearstheinteresttothistechnologyhasincreased duetoitsenvironmentfriendlinessandpotentialversatility.Ithas beenshownthattheamountofhydrogencanbedecreasedtothe levelslowerthaninthecaseofArdegassingwithmuchlessdross formation(Eskinetal.,2015).
Themechanismofultrasonicdegassingisrather wellunder- stoodandarecentoverviewcanbefoundelsewhere(Eskinand Eskin,2014).Herewewouldliketopointoutthemainfeatures.
Eskin(1995)reportsthatultrasoniccavitationisrequiredtoinitiate gasbubbleformationinthealuminummelt.Althoughthereareno freegasbubblesinthemelt(unlikewater),EskinandEskin(2014) notethattherearenumerousinterfacessuchasoxideparticleswith absorbedatomichydrogenlayerorevenmolecularhydrogenin inclusion’ssurfacedefectsthatactascavitationnuclei,significantly decreasingthecavitationthreshold,i.e.,thepressurerequiredto initiatecavitationintheliquidmedium.Kapustina(1970)summa- rizedthemechanismsofultrasonicdegassingbyrectifieddiffusion intoapulsatingbubbleandEskin(1995)extendedthetheorytoliq- uidaluminum.Asabubbleoscillatesinthealternatingsoundfield, itactsasapumpextractingmoreandmorehydrogenfromthemelt witheachexpansion.Thebubblesthereforegrowandsubsequently floattothesurface,releasingthehydrogentotheatmosphere.As mentionedbyEskinandEskin(2014),meltflowscreatedbythe ultrasoundsource(acousticstreamingandsecondaryflows)facil- itatebubblesdistributionand,hence,degassing.
Aconventionalschemetointroduceultrasonicvibrationsinto themeltisthroughaconcentratedsource,i.e.,hornorsonotrode.
Inthiscasethesonotrodeissubmergedintothemeltfromthetop, http://dx.doi.org/10.1016/j.jmatprotec.2015.03.006
0924-0136/©2015TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).
transducertoexciteit.
Anotherpossibleschemeforultrasonicprocessingofliquidshas beensuggestedbyShoh(1976)forlow-temperatureliquidsandby Eskin(2002)forliquidmetalsbutnotintendedfordegassing.Aflat extendedplatesubmergedintothemeltandtransmittingflexural ratherthanlongitudinalwavesgivessomeobviousadvantages.For example,alargervibratingsurfaceincreasestheareawherethe cavitationconditionscanbemetandcavitationbubbleareformed.
Therefore,theacousticwaveandcavitationcanbetransmittedtoa largermeltvolume.Thereisalsoapossibilitytopositiontheplate inthelowerpartofthemeltvolumewithbubblenaturallydirected upwards,andthepossibilitytoperformultrasonicprocessinginthe meltflowwithoutobviouslimitationsofthedegassedvolume.
Thispaperdescribestheevaluationofthisschemeintermsof cavitationconditionsgenerated andthefeasibility studyonthe applicationofaflatplatesonotrodetotheultrasonicdegassingof liquidaluminumalloys.
2. Experimental
Theexperimentalsetupconsisted ofa 5-kW ultrasonicgen- erator,awater-cooled5-kWmagnetostrictivetransducer,asteel half-wavelengthconicalconcentrator65to40mmindiameter,a steelhalf-wavelengthextensionand atitaniumplatesonotrode 40mmwide,2mmthick,and320mmintotallength(theeffec- tivelengthworkinginthemeltdependedontheshapeoftheplate, i.e.,itsbending).Theworkingfrequencywas17.15kHz.Theplate wasattachedtothebottomoftheextensionbyasteelstudandnut.
Eskin(1965)reportsthattitaniumisnotanidealmetallicmaterial forasonotrodeasiteventuallydissolvesinliquidAl.However,its acousticcharacteristicsaresuitableforultrasonicapplicationsand thedissolutionrateinliquidmetalisrelativelysmallforshort- termexperiments,whichisfrequentlyusedinlab-scaleresearch.
Inourcasetheexperimentsinliquidaluminum didnotexceed 2minandtheamountofdissolvedTiwasmeasuredas0.05wt%.As themainpurposeoftheworkwasdegassingandnotgrainrefine- ment,thissmallamountofdissolvedTididnotaffecttheacoustic performanceofthesonotrodeandthedegassingresults.
Fourtypesofexperimentswereperformed:(i)inairwiththe aimtomeasuretheamplitudeandwavelengthofflexuralvibra- tionsinaplatesonotrode,(ii)inwaterwiththeaimtomeasure the acoustic parameters and visualize the cavitation, (iii) in a closedcompartmentwithaliquidA356-typealuminumalloyfor degassingandacousticmeasurements,and(iv)inanopenlaunder withtheflowingmeltofthesamealloyfordegassing.
Thevibrationamplitudewasmeasuredinairusingacontact- lessvibrometerbasedonacapacitanceprinciple(EskinandEskin, 2014).Themeasuringtipofthevibrometerwasplacedwithin1mm distancefromthevibratingsurface.The null-to-peakamplitude
wasapproximately30×1.3(timegate)=39ms.Eachexperiment wasrepeatedtoensurereproducibilityofresults.Thecavitometer probewasmadeoftungstenandwassuitablefortemperaturesup to750◦C.Duringmeasurementsthetipofthecavitometerprobe waswithin15mmfromthesurfaceofthevibratingplate.Theambi- entandinternalnoiselevelwithanidlesonotrodehasbeentaken bythecavitometerandthensubtractedfromthespectraobtained withtheworkingsonotrode.
Theexperimentswiththeliquidaluminumalloyhadthemain aimofevaluatingthedegassingefficiencythatwastakenasthe decreaseofhydrogenconcentrationrelatedtothestartinghydro- genconcentration.CutingotsfromacommercialA356alloy(7%Si, 0.3%Mg)weremeltedinatiltingfurnaceinaclay-graphitecrucible.
Themeltwasmaintainedinthefurnaceat740–750◦Cor800◦C.The samplesweretakeninthefurnacebeforepouringandduringthe experimentattheendofthelaunderfortheprocessingintheflow orattheendofdegassingfromthedegassingchamberinthebatch operation.Differentamountsofaluminum meltweretreatedas indicatedinthecorrespondingpartsofthetext.Thehydrogencon- centrationwasmeasuredusingastandardreducedpressuretest (Davies,1993).Inthistestthetwoliquidsamplesaretakensimul- taneouslyfromthemeltandaresolidified,oneinairandtheother underreducedpressure. Inthelattercase,thedissolvedhydro- genisforcedtoprecipitatefromthesolidifyingmetaland form porosity.Thedensitiesof thetwo samplesarethendetermined bytheArchimedesmethodandtheratioofthedensitydifference tothedensityofthesamplesolidifiedinairistakenasso-called densityindex.Itcanbesubsequentlyrecalculatedtothehydrogen concentrationusingapreviouslyestablishedcorrelationrelation- shipinwhichthedirectmeasurementsofhydrogenbyanAlspec-H analyzerhavebeenused(Eskinetal.,2015).Asthehydrogensol- ubilityinliquidaluminumdependsontheenvironmenthumidity, themeasurementshavebeentakenusingaweatherstation.Aprin- cipalschemeoftheexperimentinalaunderisshowninFig.1.The degassingchambermeasured150×100mmincross-sectionand 220mminlength.Inthebeginningoftheexperimenttheoutlet fromthedegassingchamberwasclosedwithaplug,assoonasthe chamberwasfilledwiththerequiredamountofmelt,thesonica- tionstartedandtheplugwastakenout,whilethemeltcontinuedto pourintothechamber.Thesizeoftheoutletassuredabout2min ofresidencetimeofthemeltduringultrasonicdegassing.Inthe caseofultrasonicprocessinginthemeltflow,themeltflowrate wascalculatedbydividingtheweightoftheliquidmetalbythe timerequiredforthismelttopassthroughthedegassingchamber.
Theflowratewascontrolledbytheoutletorificeofthedegassing chamber.Thelaunderhadthewidthandheightat100mmandthe length890mm.Thelaunderhadsomebafflestoslowdownthe meltflowandallowthebubblessufficienttimetofloattothesur- face,theresidencetimevariedbetween1and4min.Allpartsofthe melttransfersystemweremadeofarefractoryceramicboardand
Fig.1.(a)Aschemeofultrasonicdegassingexperiment:(1)ultrasonictransducer;(2)concentratorwithextension;(3)platesonotrode;(4)degassingchamber;(5)launder;
(6)baffles;and(7)pointsofacousticmeasurementsand(b)actualsetupforultrasonicdegassinginthemeltflow.
pre-heatedbeforethemelttransfer.Insomeexperimentsanelec- tricallyheatedcoverwasinstalledontopofthelaunder.Themelt temperaturewascontrolledatthepouringpointandatthelaun- deroutletbyaK-thermocouple,andwastypicallybetween750and 700◦C,respectively.
Experimentswerealsoperformedinbatchoperationwhenthe similaramountofmeltwasprocessedinthecompartmentsimilar insizetothedegassingchamber.
Forcomparisonsomeacoustic(forwater)andgasmeasurement (forliquidAl)weretakenuponultrasonicprocessingwithacon- ventionalcylindricalsonotrodeplacedinthedegassingchamber (forliquidaluminum)orinasimilarindimensionsglasscompart- ment(forwater)andsubmerged10mmfromthetopthemelt.
Theparametersoftheultrasonicprocessingwerekeptsimilarto theexperimentwiththeplatesonotrode.Inthiscase,thetipof thecavitometerprobewasplacedabout20mmfromthetipofthe sonotrode.
3. Resultsanddiscussion 3.1. Wavelengthassessment
Westarted withtheassessment ofwave characteristicsof a thinplate.Althoughan analysisofthevibrationof a thinplate fixedatonesideisa complicatedmatterthatwearenotgoing totackleinthispaper,KitaigorodskyandYakhimovich(1982)and laterHambric(2006)offerarathersimpleanalyticalassessmentof theflexuralwavelength.Theflexuralwavespeedinathinplateof thicknesshcanbecalculatedas:
Cf=Dω
h
14
, (1)
whereDistheflexuralrigiditycalculatedas D= Eh3
12(1−2), (2)
whereω=2fistheangularfrequency,fisthedrivingfrequency, istheplatematerialdensity,histheplatethickness,EisYoung’s modulusoftheplatematerial,andisPoisson’sratio.
Thewavelengthisthencalculatedas
=Cf/f. (3)
UsingthedataofourvibratorysystemandTipropertiessum- marizedisTable1,wearriveatawavelengthof33mm.
3.2. Acousticmeasurements
Themeasurementofthevibrationalamplitudeshowedthefol- lowing.Thevibrationalfrequencyofthetransducerwas17.15kHz.
Atthisfrequencyand3.5–4kWpowerofthegenerator,thenull- to-peakamplitudeatthetipofthesteelextension(wheretheplate wasconnected)was 11–12mas measuredbythe contactless vibrometer.AlongtheTiplatetheamplitudevariedaccordingto thewavelengthoftheflexuralvibrationsfrom1minthenodes to18–25mintheantinodes.Thedistancebetweenthenodesand antinodeswasabout6–8mm,whichisingoodagreementwiththe quarterwavelengthoftheflexuralwave.
ItisknownthatthecavitationthresholdinwaterandliquidAl correspondstothenull-peakamplitudesofabout2.5and5m, respectively,at20kHz(Komarovetal.,2012).Thereforethemea- suredamplitudes (18–25m) shouldbeenoughto achievethe cavitationconditioninthemelt,whichisanimportantcondition forultrasonicdegassing.
Arathersimplebutvividtestforcavitationactivitycanbedone byplacingathinfoilinthecavitationregion.Cavitatingandimplod- ingbubbleswillcausedamageofthefoil,testifyingforthepresence ofcavitation.A10-mthickaluminumfoilwasfixedonasteel wireframeandplaced5mmabovetheplatesonotrodesubmerged inwater.Theultrasonicprocessingthenstartedandcontinuedfor 30s.TheresultsareshowninFig.2.Thecavitationdamageisclearly seen.Itisalsoobviousthatthisdamagefollowsa periodicpat- terncorrespondingtothehalfwavelengthoftheflexuralwave.
Themeasurementshowsaspacingof14–15mm,whichisremark- ablyclosetotheanalyticallyobtainedvalueof16.5mm(seethe wavelengthinTable1).Notethatthewavelengthmaychangein thepresenceofaload,e.g.,waterorliquidaluminum.
Thesevaluesarealsoclosetotheexperimentallymeasuredhalf- wavelengthsinaNbplate(12mmat18kHz(Eskin,2002))andina stainlesssteelplate(15mmat20kHz(Shoh,1976)).Thedifferences areduetotheacousticpropertiesofthematerialandtheoperation frequency.
In thenextstage, we performedacousticmeasurements for cylindricalandplatesonotrodeinwaterandtheliquidaluminum alloy.
TestsinwaterandliquidAlwereperformedusingaTiplate sonotrodeattachedtothetipofthesteelextension.Theplatewas submergedinsidetheliquidmeltatadistanceof50mmfromits freesurfaceandequallyfromthebottomofthevesselsimilarin dimensionstothedegassingchamber.Ahightemperaturecavit- ometerdevicecalibratedtomeasureacousticpressuresinliquid
sonotrode.
Typicalacousticspectraareshownin Fig.3.Thedrivingfre- quencymeasuredbythecavitometeras17.15kHzwasinagood agreementwiththeindicatoronthemagnetostrictivetransducer controller.Notethattheshapeofthespectrumwiththehumps inspecificfrequencydomains,i.e.,from170to270kHzinFig.3, isattributabletothevariationinthesensitivityresponseofthe cavitometersensor.Alsotheacquisitionrateformeasurementin theliquid aluminumalloywaslowerthanforwater(resolution bandwidthof380Hz).
Hodnettetal.(2004)givessomefeaturesofsuchspectrathatare indicativeofthecavitationbehavior.Thereisawell-pronounced peakatthedrivingorfundamentalfrequency(17.15kHz)thatis producedbytheacousticfieldgeneratedbythesonotrodeaswell asbytheintensityfromthesuperimposedlinearbubbleoscilla- tions.Therearealsofurtherpeaksthatareintegermultiplesofthe fundamentalfrequency(harmonics),whichresultfromnon-linear bubbleoscillations.Thenon-linearoscillationsoflargerbubbles causetheirsplittingandhigh-energeticoscillationsofsmallerbub- blesathigherfrequencies,whichaccordingtoEisneretal.(2013) isdisplayedbylargeandsharppeaksinthefrequencyspectrum.
Theharmonics,i.e.,the2ndharmonicinFig.3aandb(shownby arrow),in somecasescanbehigher thanthe fundamentalfre- quencyasacousticemissionsfromlinear(non-inertial)behaviorof thecavitationbubblesaresuperimposed,increasingthestrength and sharpness of the signal. At frequencies below the driving
sharpnessofthepeakscanbeusedfortheassessmentofthemin- imumandmaximumbubblesizesandthecharacterizationofthe cavitationregime(stable/transient).
AlthoughtheshapesofthespectrainFig.3aandbaresimilar, theprominentpeaksaremuchsharperinthecasewherethecylin- dricalsonotrodewasdeployed(Fig.3b).Thismeansthatthereare morecavitationeventsinthemeasuredareainthecaseofthecylin- dricalsonotrode,butthisalsomeansthatthecavitationintensity maybedistributedoverthelargerareainthecaseoftheplate.
Anotherdifferencebetweenthesetwospectrais thatthenoise levelintherangeabove140kHz(thisfrequencyrangeisassoci- atedwithactivityfromcavitationbubbles)ishigherinthecase ofthecylindricalsonotrode(oneshouldlookattheupperedges ofthepeaksandtakeintoaccountthatthescale isnegativeso
−50dBuislargerthan−60dBu).Thisisduetotheseverityofcavi- tationcollapsesandthenumberofcavitationevents.Theriseinthe broadbandatfrequenciesabove140kHzisanindicationofinertia (transient)cavitationwhiletheriseinthepeaklevelsisrelated withstable(non-inertial)non-linear cavitationandthenumber ofcavitationevents.ThusinFig.3a,thestablecavitationpossibly prevailscomparedtoFig.3bwhereamorechaoticcavitationenvi- ronmentisdeveloped.ThisiswellexplainedbyYasuietal.(2011) whoshowedthatthedrivingamplitude(staticpressure)canbea decisiveparameterfortheperformanceofbubblesinasonicated liquid.Iftheamplitudeisatlowerlevels,bubblesarelikelytopul- sateforalongerperiodsoftimewithoutcollapsingandeventually
Fig.2. Aluminumfoilsshowingtheregularcavitationdamagefromaplatesonotrodeinwater(arulershowstheregularityofthedamagewithabout14–15mmspacing).
Insertshowstherepeatabilityoftheresults.
