aluminium with an advanced melt in cavitometer Characterisation ultrasonic of acoustic the spectrum and pressureﬁeld Journal of Materials Processing Technology

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Journal of Materials Processing Technology

j o u r n a l h o m e p a g e :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

Characterisation of the ultrasonic acoustic spectrum and pressure ﬁeld in aluminium melt with an advanced cavitometer

I. Tzanakis

^a,b,∗

, G.S.B. Lebon

^c

, D.G. Eskin

^a,d

, K.A. Pericleous

^c

aBrunelUniversityLondon,BrunelCentreforAdvancedSolidiﬁcationTechnology(BCAST),Uxbridge,MiddlesexUB83PH,UK

bUniversityofOxford,DepartmentofMaterials,ParksRoad,OxfordOX13PH,UK

cUniversityofGreenwich,CentreforNumericalModellingandProcessAnalysis,LondonSE109LS,UK

dTomskStateUniversity,Tomsk634050,Russia

a r t i c l e i n f o

Articlehistory:

Received23July2015

Receivedinrevisedform7October2015 Accepted8October2015

Keywords:

Cavitationbubbles Ultrasound Liquidaluminium Cavitometer Acousticpressure Frequencyspectrum

a b s t r a c t

Currently,fundamentalexperimentalstudiesinliquidmetalsarelimitedasthereareveryfewavailable experimentaltoolsfordirectlymeasuringacousticcavitationinsuchextremeenvironments.Inthis work,acalibratedhightemperaturecavitometerwasusedformeasuringacousticemissionsandacoustic pressureinsonicatedliquidaluminiumandinwater.Theextentofthecavitationzonewasquantiﬁedin liquidaluminiumandwater.Thedifferencesbetweencavitationbehaviourofwaterandliquidaluminium wereexplainedintermsofacousticshielding,attenuation,andbubbledynamics.

1. Introduction

Ultrasonictreatmentofliquidmetalsiscloselyrelatedtocav- itationandbubbledynamicsandithasbeenprovenaseffective and promising in degassing and reﬁning thegrain structure of metallic melts asshown byEskin and Eskin (2014). Cavitation involvestheformation,growth,oscillation,collapse,andimplosion ofbubblesinliquids(Leighton,1994).Inthevicinityofcollapsing bubbles,extremetemperatures(>10000K)(FlanniganandSuslick, 2005),pressures(>400MPa)(FlanniganandSuslick,2005;Tzanakis etal.,2014)andcoolingrates(>10¹¹K/s)(Gedanken,2004)occur.

However,temperaturerequirements,opacityofmetals, andthe lackofadvancedequipmentformeasuringcavitationactivityhave imposedstrictlimitationsonthestudyofcavitationbubbledynam- icswithinliquidmetals.Onlypost-mortemanalysisisgenerally usedtocorrelatetheﬁnalstructurewithultrasonicparameters,as shownbyAtamanenkoetal.(2010)andEskinandEskin(2014).

Thisimpedestheindustrialapplicationofultrasonicprocessingto liquidmetals.

Recently,X-rayimagingtechnologyintheformofthirdgener- ationsynchrotronradiationsourceswasappliedforinsitustudies

∗Correspondingauthor.

E-mailaddress:[email protected](I.Tzanakis).

ofbubbledynamics(Xuetal.,2015)andnucleation(Huangetal., 2015)in liquidaluminium (Al) alloys. However,thesmall spa- tialandlargetemporalscalesinvolvedintheprocesshinderclear visualizationofthephysicalprocessesandconsequentlyadeeper insightintothebehaviourofcavitationbubbles.Comparedwiththe X-rayimaging,acousticemissions,i.e.cavitationnoise,couldbea morepowerfulapproachtorecordandanalysethedynamicpro- cessofcavitation.Thecavitationnoisespectracarryamultitude ofinformationintheirrespectiveultra-harmonicandbroadband componentsthathelptodistinguishdifferentregimesofacoustic cavitationandconsequentlymeasureacousticpressuresatpartic- ularfrequencies.

Veryfewstudieshavebeenconductedoncharacterizingcav- itationactivity in liquid metals using various means, including cavitometersasreviewed byEskinand Eskin(2014)and exem- pliﬁedbyKomarovetal.(2013).Inthelatterpaper,acavitometer wasusedforcharacterisingthecavitationintensityinamoltenAl alloy.However,theresultsweregiveninrelativetermsofelectri- caloutputofthecavitometer(mV),andnotinthephysicalunits ofpressure. No analysisoftheacousticspectrawasattempted.

As a result, thereported data cannot beapplied to, for exam- ple,validationofnumericalmodels.Thisworkisintendedtoﬁll thisgapandtodemonstratethepossibilityofusingacalibrated cavitometerfordirectlymeasuringacousticspectraandpressure in low-temperature transparent (water) and high-temperature

http://dx.doi.org/10.1016/j.jmatprotec.2015.10.009

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Fig.1. Schematicoftheexperimentaltestrig.Thecavitometermeasurestheactivity at3differentpositionsacrossthesectionofthevesselasindicatedbytheredmarks (underthesonotrode,inthemiddle(1/2R)andnearthewall(R)).Thebluedashed circleindicatesthepositionofthesonotrode(topviewofthecrucible).

opaque(moltenAl)liquids,withthebeneﬁtofgettingphysically- relevantdataaswellascharacterisingtheextentofcavitationin treatedliquids.

2. Experimentalsetup

Inthecurrentstudy,characterisationofthecavitationintensity andthecorrespondingacousticpressureﬁeldsinliquidAlandin waterwasconductedusingtheexperimentalsetupthatisschemat- icallyshowninFig1.Ultrasonicexcitationwasachievedwitha 5-kWgeneratoranda5-kWwater-cooledmagnetostrictivetrans- ducer(Reltec/Russia).A conicalNb sonotrodewitha 20mmtip diameterwasdrivenbythetransducer,whichoscillatesatanom- inalfundamentalfrequencyof17kHz.Theinputpowerfromthe generatorwasvariedintherangeof2.5–4kWinboth cases.To sonicatetheliquid,theNbsonotrodetipwasverticallyimmersed toa depthofapproximately20mmintotheliquidvolume.The sonicatedwaterliquidwascontainedinacylindrical,glass-walled vesselwithdiameterof150mm.Theliquidlevelinthevesselwas at110mm(approximately2000cm³).Topreventwaterheatingby acousticenergy,eachexperimentlastedforafewseconds.Water temperaturewasmaintainedat22±2^◦C.InthecaseofliquidAl, achargeof5.2kg(approximately2000cm³)ofcommerciallypure 99.7%Alwasmeltedinaclay–graphitecruciblecoatedinsidewith boronnitride(BN),withthesizeandgeometrybeingsimilarto thatoftheglassvessel.Themelttemperaturewasstabilisedto 710±10^◦CandwascontinuouslymonitoredbyaK-typethermo- couple.Therewasnocontrolledatmosphere.

Features of the observed spectra were captured with an advanced calibrated cavitometer ICA-3HT (BSUIR/Belorussia) equippedwitha 4-mmdiametertungstenprobe,withaspatial resolutionof50±10mmandabandwidthofupto10MHz.The cavitometerwasspeciﬁcallydesignedtomeasurecavitationactiv- ityinhightemperaturemeltsandinhighpowerultrasonicﬁelds, i.e.inmoltenmetals.Thiscavitometercanequallywellmeasure cavitationactivityin lowtemperature liquids.Afull accountof thecavitometerdesignandperformancecanbefoundinTzanakis etal.(2015a).Toinvestigatetheeffectofdistancerelativetothe sonotrodeoncavitationintensity,themeasurementsofacoustic emissionsweretakenatseveralpointsasshowninFig.1(indicated bycrosses),i.e.(i)belowthesonotrode,(ii)athalfradiusdistance (1/2R)(about38mmoffthesonotrodeaxis)and(iii)atfullradius distance(R)(about75mmoffthesonotrodeaxis)withthecavito- meterprobesubmergedat70±5mmbelowtheliquidfreesurface, whichwasmonitoredthroughtheknownlengthofthecavitometer probeoutsidethemelt.

Thefrequency spectrumwasacquiredby anexternal digital oscilloscopedevice“Picoscope”attachedtothecavitometer.This Picoscopedeviceallowedreal-timesignalmonitoringofthecav- itometersensor’sdataandultrasonicparameters.Therawvoltage

signalistransformedtothefrequencydomainviaaFastFourier Transform.Anumberof30signalaveragesoftheacquiredsignal weretakenusingaresolutionbandwidthof500Hz.Thetimeforthis signalacquisitionwasapproximately30×2ms(timegate)=60ms.

Atotalof1000wavepatternswereanalysedineachofthemea- surementpointsasshowninFig.1.

3. Resultsanddiscussion

Thelocalcavitationphenomenainthevesselwereexplained basedonthespectralcharacteristicsofacousticemissions.Acous- ticpressuresatthedrivingfrequencyof17kHzandatanacoustic frequency of1MHz (associatedwithacousticpressuresexerted fromtheactivityofthecavitationbubbles)werecalculatedusing themethodologydescribedinTzanakisetal.(2015a).Resultswere theninterpretedbasedonaveragingthevaluestakeninequiva- lentmeasurementpointsinFig.1,exceptforthepositionunder thesonotrode.Bybridgingthesetwoliquidenvironments,amore comprehensivepictureofthephenomenagoverningthecavitation processwithinliquidAlcanbeconstructed,thusadvancingthe existingknowledgeinliquidmetalprocessing.

Typical acoustic spectrafor water and liquid aluminium, as receivedfromtheinsideofthecavitationzonebythecavitome- ter,areshowninFig2.Thefundamentalfrequencycomponentat 17kHz(f₀)isapparentproducingbroadbandsignalswellintothe highfrequencydomainassociatedwiththeactivityfromcavita- tionbubbleswithfurthercontributionsfromharmonics,sub-and ultra-harmonic frequencies (numerousirregularly spaced peaks superimposedoncavitationnoisebackground).Afulldiscretization ofthespectrumatlowerfrequenciesisdescribedintheprevious workbyEskinetal.(2015)andTzanakisetal.(2015b).Addition- ally,discretizationoffrequency peaksintheregionof1MHzis shown bytheinsetsinFig.2.Abackgroundnoiseis shownfor reference.Thebackground“noise”measurementwasperformed withthetransducerswitchedoffandthecavitometersubmerged intothetestedliquid.Thesedatashowthatthelevelofnoise(note thatdBuaxisislogarithmic)isverysmallanddoesnotaffectthe acousticmeasurements.

Thegeneralshapeofthespectruminliquidaluminiumiscom- parable to that of the water, further reinforcing the accepted viewsthatwaterandaluminiumsharesimilarﬂuidanddynamic behaviouraspreviouslydemonstratedbyEskinetal.(2015)for ultrasonicprocessingandbyXuetal.(1998)forcastingprocess- ing.Inthecaseofultrasoniccavitation,themaindifferenceisthat thebroadbandspectrumgeneratedbythecollapsingbubblesofa widerangeofsizeswiththeirshockemissionsandliquidjetsis signiﬁcantlyhigherinthecaseofliquidaluminium(about10dBu onaverage)ascomparedwithwater.Insomeparticularhighfre- quencies,itcanbeashighas20dBu(seedashedarrowinFig.2b).

Thisimpliesthattransientcavitationassociatedwiththelevelof thebroadbandcavitationnoiseismoreprominentinliquidalu- miniumand thushigher activityfromthecavitationbubblesis expected.Additionally,single(dashedarrow)orpopulated(solid arrows)discretepeaks(seeFig.2)athigherfrequencies,i.e.inthe rangeof200–250kHzand300–600kHz,suggestnon-linearstable ortransientcavitationactivityfromnumerouscavitationbubbles.

Thisleadsustotheconclusionthat,inthestudiedmelt,theactivity ofbubbleswithresonancesizesof5–15␮maccordingtoMinnaert (1933)canprevailinthecavitationregime,increasingtheinten- sityofthecorrespondingpeaksinthesefrequencyranges.Thisis inagreementwithaninsitustudyofcavitationinAlmelts,show- ingthatthemajorityofcavitationbubblesareindeedfallinthat particularsizerange(Xuetal.,2015).

InFig.3,acousticpressuresintherangeofthedrivingfrequency f0forbothofthestudiedliquidsareshownforthethreedifferent

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Fig.2.Typicalexamplesofacousticspectrageneratedbymagnetostrictiveultrasonictransducerat17kHzdrivingfrequency(f0)andmeasuredwiththecavitometertip positionedabout3–4cmbelowthesonotrode’stipin(a)waterand(b)liquidAl.Insetshowsthespectrumintherangeof1MHz.

Fig.3.VariationinRMSacousticpressuresandmaximumRMSacousticpressures(representingtheaverageofthemaximumRMSacousticpressuremeasuredforeach position)ofthedrivingfrequency(17kHz)in(a–b)waterand(c–d)liquidAl.TheaverageofthreedifferentpositionsasindicatedinFig.1wastaken.

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Fig.4.VariationinRMSacousticpressuresat1MHz(associatedwithbubblescavitationactivity)for(a)waterand(b)liquidaluminium.

positionsacrossthevessel.Fig.3demonstratesthatanincreasein powerinputdoesnotresultinanequivalentincreaseincavitation activity.Thisismoreobviousinthecaseofwaterwhereresults clearlyshowthatthehighestacousticpressureinwater(Fig.3a andb)wasobtainedneartothewall(R)whenthelowestpower setting(2.5kW)wasused.Athigherinputpowers,thelargebubbly cloudthatisformedinsidethecavitationshieldsandscattersthe bubbleenergyreleasedfromthecavitationbubbles,asshownby Rozenberg(1968).Asaresult,thepropagationofacousticwaves intotheliquidbulkisobstructed.Incontrast,atlowerpowerset- tings,bubblesmigratemoreeasilytowardsthesidewallsdueto primaryBjerknesforces.Withfewerbubblesandbubblyclouds, thepropagationofwavesintotheliquidbulkislessdisturbed.The bubblesformedortransportedtothevesselwallcollapsemoreeas- ily.Consequently,theoverallcavitationactivityinthebulkliquid isincreasedfurther.

ThemaximumRMSacousticpressuresrepresentingtheaver- ageofmaximumRMSacousticpressure atthethree equivalent measurementpointsareshowninFig.3banddforwaterandalu- miniumrespectively.Inthecaseofliquidaluminium,thehighest cavitationpressurewasmeasuredat3.5kW,closelyfollowedby therestofthepowersettings(Fig.3candd).Significantly(20–30%) higherpressurevaluesinwatercomparedwithliquidaluminium wereobserved.Ateachmeasuringpoint,themaximumpressure is40–50%higherthantheaveragepressurevalue(Fig.3a): this indicatesthepresenceofamorefluctuatingpressurefield,result- inginamorechaoticcavitationactivitywithinthewaterbulk.On thecontrary,inliquidaluminium,themaximumRMSpressureis onlya20–25%higherimplyingthatamorestableandcontrolled cavitatingenvironmentwithoutsignificantpressurefluctuationsis achieved.Controllingcavitationintensityisoneofthekeyfactors fortheoptimizationofmeltprocessingandsolidification.

Fig.3canddshowanalmostlineardropoftheacousticpres- sureﬁeldwithdistanceinliquidaluminium.Asaruleofthumb, cavitationacousticpressureinliquidaluminiumdropsabout50%

ateveryhalfradiusdistancefromthesource.Thisacousticfield attenuationmaybeduetotheabsorptionofacousticenergybythe viscousenvironmentandaccumulationoflong-livedaluminium cavitationbubbles.Unlikeinwater,thebordersofthecavitation zoneinliquidaluminiumcanbewelldefined,indicatingtheregion ofactivecavitationregime.Thus,mostoftheprocessesresponsible forgrainstructurerefinement,suchasde-agglomerationandsono- fragmentation,shouldtakeplaceinthatparticularregion,unlikein waterwherecavitationtreatmentmaybeactiveeverywhereinthe vessel.

TheacousticpressureresultsinFig.3demonstratehigherval- uesforwaterthanforliquidaluminium,whiletheoverallacoustic spectruminFig.2suggeststheopposite.Thiscouldberelatedto thereleaseofbubbleenergyathighfrequencies,assupportedby theresultsinFig.4.Inwater,RMSacousticpressuresat1MHzare 2–3timeslowerthaninliquidaluminium,meaningthatenergyis indeedstoredduringrelativelylonglife-timeofAlcavitationbub- blesandthenreleasedbackintothebulkinaformofcavitating andcollapsingbubbles.ResultsareinagoodagreementwithX-ray imagingresultswherenumerouscavitationbubbleswereseensus- tainedforlongperiodsoftimeinthebulkbeforetheydisappear(Xu etal.,2015).Therefore,thesigniﬁcantshieldingandscatteringof acousticwavesatthelowerfrequenciesintherangeof17kHzdoes notexcludethepossibilityofpowerfulcollapsesofcavitationbub- bles,asattestedbystrongpressuresurgesdetectedatthehigher frequencies(1MHz).

TheresultsinFig.4conﬁrmthatcavitationtreatmentinliquid aluminiumshouldtakeplaceinsidethecavitationzoneunlikein waterwherepressureexertedfrombubblesislessdependenton thedistancefromtheultrasoundsource.RMSacousticpressures inliquidaluminiumcouldbeashighas800kPacertainlyenough tobreakclusterofparticlesfacilitatingthede-agglomarationpro- cessand wettingthroughsono-capillaryeffect(EskinandEskin, 2014).Resultsareinagoodagreementwitharecentstudywhere theﬁllingprocessdependingthegeometryandsizeofcrackscan beachievedatsimilarpressurescalestiedupwiththecollapseof cavitationbubbles(Tzanakisetal.,2015c).

4. Conclusions

1.Thecavitationacousticpressureandthebordersofthecavitation zonewerequantiﬁedusingacalibratedcavitometerinliquid aluminiumandtheresultswerecomparedwiththoseobtained inwater.

2.Inthelowfrequencydomainreﬂectingtheoverallacousticﬁeld, shieldingandacousticdampingismorepronouncedinliquid aluminium,obstructingthewavepropagationintothebulk.In contrast,amoreconsistentpressureregimeisestablishedacross thetreatedvolumeinwater.Consequently,themeasuredRMS acousticpressuresinliquidaluminiumarelowerthaninwater.

3.Athigherfrequencies,associatedwithcavitationbubbleemis- sions, the acoustic pressures are much higher in liquid aluminium than in water implying that bubbles accumulate largeamountsofenergypriortoreleasethemuponcollapse.

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4.Thisworkdemonstrated thefeasibilityandpractical valueof directacousticmeasurementsinliquidmetals.Thesemeasure- mentscanbeusedforvalidatingthenumericalmodelsthatare requiredforoptimisationofultrasonicmeltprocessing.

Acknowledgments

ThisworkisperformedwithintheUltrameltProjectﬁnancially supportedbytheUKEngineeringandPhysicalSciencesResearch Council(EPSRC)underthecontractnumbers:EP/K00588X/1and EP/K005804/1.

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