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LaserweldmicrostructureandcorrosionstudyofAg–Pd–Au–CualloyofthedentalapplicationM.L.Santos,H.A.Acciari,L.C.O.Vercik,A.C.GuastaldiInstitutodeQuı´micadeAraraquaraUNESP,C.P.355,14800900Araraquara,Sa˜oPaulo,BrazilReceived10June2002accepted20June2002AbstractThelaserweldingprocesswasintroducedintodentistrybytheendofthe1980s,resultingonagreatimpulsetothatareawiththedevelopmentofcheaperandsmallerequipment,usingsimplertechnique.Thisallowedgreateruseofthatprocessontheconfectionofprosthesescomparedtothebrazingprocesssincetheheatsourceforthatprocessisaconcentratedlightbeamofhighpower,whichminimizesdistortionproblemsontheprostheticpieces.Ag–Pd–Au–Cualloyusedontheconfectionofdentalimplantprostheseswasobservedbeforeandaftersubjectiontothelaserweldingprocess.Themicrostructurewasanalyzedwiththeuseofopticmicroscopyandthecorrosionresistancewasstudiedbythetraditionalelectrochemicaltechniquesandbyelectrochemicalimpedance,underenvironmentalconditionssimulatingtheaggressivenessfoundinthemouthcavity.Astructuralchangewasdetectedontheweldarea,whichpresentedarefinedmicrostructurederivingfromthehighspeedcooling.Thebasemetaloutoftheweldareapresentedafusioncoarsemicrostructure.Theelectrochemicalessaysshoweddifferencesonthepotentiodynamicpolarizationbehaviorinbothweldandmetalbaseareas,indicatingsuperiorcorrosionresistanceintheweldarea.Theimpedancespectrawerecharacterizedbycapacitivedistortedcomponents,presentinglinearimpedanceinthelowfrequenciesarea.D2002ElsevierScienceB.V.Allrightsreserved.KeywordsAg–Pd–Au–CuLaserCorrosionDentalalloys1.IntroductionInsearchforalternativemetalalloysforodontologicalpurposes,someresearchershaveappliedtheAgPdalloytosubstitutethegoldalloys,tryingtoreducecostsandtoimprovemechanicalpropertiesandcorrosionresistance1–4.Duetosomedifficultyinobtainingadaptationinprostheticpieces,mainlythelargeronessuchasmetallicstructuresmoltenintoonepiece,calledcastmonoblocks,theuseofweldingisnecessarysincethistechniqueacceptstheworkwithsegmentsoftheprosthesis,whichmakespossibleabalancedforcedistributionandthebestsuitableadaptation,occurringinanaccuratepassiveway4,5.Theprocessoflaserweldingproducesacoherent,monochromatic,concentratedlightbeamofhighpower,andithasbeenappliedtosubstitutethebrazinginodontologicalprostheseswelding.Thelaserweldingprocesswasintroducedintodentistrybytheendofthe1980s,resultingonagreatimpulsetotheareawiththedevelopmentofcheaperandsmallerequipmentduetoitsadvantagesandwideapplication,whichmadepossibletouseweldinginawidevarietyofmetalsandprostheticpieces6.Theuseofelectrochemicaltechniquesinthecorrosionstudyisimportantfortheunderstandingofitsperformance,biocompatibilityandbiofunctionality,whenclinicallyapplied,fortheseareconstantlyexposedtoaggressiveenvironments.ThisresearchobservesAg–Pd–Au–Cualloymicrostructurebehaviorandthematerialsresistancetocorrosionunderenvironmentalconditionssimulatingtheaggressivenessfoundinthemouthcavity,whenusedondentalimplantprosthesesbeforeandaftersubjectedtothelaserweldingprocess.2.ExperimentalTable1presentsthemineralcompositionofthestudiedmaterial,usingWaveDispersiveSpectroscopyWDS.Thecylindricaltestspecimens,with0.27cmdiameterand1.0cmlength,havebeensubjectedtotheweldingprocessonbuttjoints7.Theweldingmachine,DentaurumDL20002S,usedforthelaserwelding,usesacrystalNdYAGassourceoflaser,andthebeampowerwasapproximately6.08kWin14ms,originatingaweldingenergyofapproximately85.12J.Thetestspecimensweremanuallyplacedinthechamber,withshieldatmosphereofargon,andspotsoflapwelding,inapproximately2/3ofthesurfaces,wereappliedinthewholesectionofthejoint,with60ofbeampenetration.Aprecisediscmodel15HCDIAMONDwasusedtoobtainthetestspecimensofAg–Pd–Au–Cualloywithareacomprehendingonlytheweldingarea,andanISOMET1000BUEHLERmachinewasusedtoseparatethebasemetalfromtheweldingareaafterthelaserprocess.Theexposedgeometricareasoftheweldingcordandofthebasemetalwere0.057cm2.Themetallographicanalysisoftheexposedsurfaceofthebasemetalandtheweldingareawasdonewithopticmicroscopy,afterpolishwithemeryclothfrom180to1000mesh,aluminawithgranulation1and0.3Amandnitromuriaticacidapplication8.Theworkelectrodeswerepreparedfromthetestspecimensusedonthemetallographicanalysis.Measuresofopencircuitpotentialversustimewereusedintheelectrochemicalessays,aswellaspotentiodynamicpolarizationandelectrochemicalimpedance.AnelectrochemicalcellcontainingNaCl0.15moll_10.9airysolutionwiththreeelectrodeswasalsoused,withthesaturatedcalomelelectrodeSCEasreferencesystemandagraffiticylinderasauxiliaryelectrode.ElectrochemicalmeasuresofcorrosionweredonewithapotentiometerSolartronSI1287.Potentiodynamicpolarizationcurveswereobservedat0.001Vs_1immediately.Impedancemeasuresweredonewiththeanalyzeroffrequencyresponse,Solartron1255,connectedtoanelectrochemicalinterface,Solartron1287,andanamplitudeof10mVwasappliedtoafrequencychannelthatvariedfrom100kHzto6MHz,obtainingfivepointsforeachfrequencydecade,controlledbythesoftwareZplot9.ThesoftwareZveiw10wasresponsiblefortheadjustments3.ResultsanddiscussionFig.1presentsacoarsebiphasicfusionmicrostructureinthebasemetalarea.Fig.2illustratesarefineddendriticalmicrostructureinthelaserweldarea,derivingfromthehighspeedycoolingimposedbythelaserweldbecauseofalocatedfusionprocess,followedbyaquickcoolingduringthewelding,whichdoesnotallowthemicrostructuretoreturntoitsinitialbiphasicstructure.Fig.3showstheopencircuitpotentialversustimecurvesforthebasemetalandlaserweldareasoftheAg–Pd–Au–Cualloy.Thestabilizationofthepotentialwasobserved3hafterimmersionforbothareas,andthelaserweldpresentedastabilizationpotential50mVhigher.SomeAgPdalloyresearchershaveobservedthat,usually,analloyopencircuitpotential
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