liquid interface diffusion bonding applications for joining...

LiquidInterfaceDiffusionBondingApplicationsforJoiningPlasticInjectionDieMoldswithConformalCooling,HotRunners,andotherVentingAttributes

NormanHubele,President,Die-Bond,LLC,aDivisionofRefracSystems,ChandlerAZ

Abstract

The followingpaper discusses using liquid interfacediffusionbonding, as conductedwithin avacuumhotpressfurnace,asthepreferredmethodofjoininglayereddiemoldblanksthatmaycontainconformalcooling,venting,andother“hotrunner”passages.Discussionsaboutprocessmethods and distinctions when using a vacuum hot press furnace, particularly compared toconventionalvacuumfurnacebrazing,duringthemoldjoiningoperationsaremade.

Introduction

Approximately30yearsagoitwasacloselyguardedsecretwithintheinjectionmoldingindustrythat adding conformal cooling passages that wrap around the mold cavity made seriousimprovementstobothmoldcycletimeandreducedpartwarpage.Vacuumbrazedmoldblankscontaining cooling lines that specifically wrapped around themold cavitywere developed inGermanyandrunforthecontactlensmoldmanufacturingindustryasacloselyheldsecret.Tenyearslater,thetechnologywasre-developedbythecellphonemanufacturingindustry,andwasused to make the early “Flip” phones and other very high volume telecom components.Jumpingforwardanotherfiveyearslater,theuseofconformalcooledmoldsbecamea“whiteworld”process technology thatwaswidelypublishedand it hasbeenwidely adoptedby theworldwideplasticsmoldingindustry.Muchlessiswidelyunderstoodaboutthemethodsusedtojointhelayeredmoldelementsthatmakeupafinished“conformallycooled”moldblank.Fromafundamentalsperspectiveitshouldbeunderstoodthatthatthelaminationandjoiningof multiple 2-D layers made up of machined “wrought” bar stock materials is actually anadditive manufacturing method, but it is not one based on using powder metal as thefeedstock. A lot of material has been written over the last few years about 3-D additivemanufacturingmethodsbeingappliedtothemanufacturingofconformalcooledmolds,butasof this publication it has not found wide acceptance for manufacturing production molds,exceptforpossiblyvalidatingdesignconceptsandbuildingprototypes.Itremainstobeseenif3-D additive technologies will ever supplant the conventional 2-D layered additivemanufacturingformoldmaking,butthisauthorbelievesthatthefundamentalcostissuewithhaving tomakepowderedmetal first, and the resulting “as-cast”microstructureandpossibleresultingleakspathsmakesthisveryunlikely.

2-DLayeredMoldJoiningMethodsThemethodusedtolaminateandjoinalotoftwodimensional(2-D)machinedlayerstoforma3-D mold containing many passages running in directions that may not otherwise be easily

SPE ANTEC® Anaheim 2017 / 1903

machined,isakeyconsiderationwhendesigningmolds.Formanyyearsvariouscreativemolddesignershaveusedverycomplex“crossdrilled”holesystemsthatget“weldedover” later inthe manufacturing cycle to form interconnecting passages. Many mold shops started usingconventionalvacuumbrazingmethodstojoinmultiplelayerstogetherasawaytominimizetheweld distortion. To the present time, many mold shops are still using conventional vacuumbrazingasthemethodtoassembletheircomplexcooledmolds.Muchinformationhasalsobeenwrittenabouttheprocessknownas“DiffusionBonding”whichis also known as “DiffusionWelding” in the welding community, and “Thermo-CompressionBonding” in the semi-conductorwafer fab industries. Inmost caseseachof these specializedapplication areas have minor subtle differences, but all have similar overlapping furnacetechnology requirements. The purpose of this paper is to explore using a vacuum hot pressfurnace as needed to achieve good quality diffusion bonding, and tomake note of how thistechnology can be better utilized for joining conformally cooled injection diemolds. Variousdifferent approaches to achieving good bonds are made utilizing different kinds of furnaceequipment which will also be discussed. Firstly, let’s discuss the types of braze joints anddiffusionbonds that aremade andwhat itmeans in termsof thermal heating profile that isutilizedasshowninFigureOnebelow.

Figure One: Thermal Profiles of Typical Bonding Methods


• ConventionalVacuumBrazing:

Inthismethodtwoormorepartlayersarejoinedtogetherbyinsertingabrazealloy(B-material) at the bond lines and the parts are heated up to above the melt point(liquidus)oftheB-material. Theholdingtimeforthepartsabovethemeltingpointofthebrazealloymaterial,canvaryfromaveryshortholdtime(2-5minutes),toamuchmoreextendedholdtime(2-4hourstypical),asmaybeneededtoallowfordiffusionoftheparentmetalintothebrazejointforaddedstrength.

• SolidStateDiffusionBonding:

In thismethod, twoormore,mating parts are joined togetherwithout the use of aninterface aid (or a B-material such as a braze alloy foil, plating, sputtered film, ionimplantedlayer,orothermaterialfoil).Bydefinitionasolidstatebondisdefinedasthejoiningoftwomaterialswithouttheuseofanyotherinterfacialmaterialortreatment,andthattheresulting interfacemustbejoinedatorbelowthemeltingpointofeitherparentmaterialoranyresultingeutecticthatmayform(Tbond<TmpEutectic–andthisassumesthattwodifferentmaterialsthatcouldformaeutecticarepresent).

Inthisbondingmethoditisprettynormaltohave50%increaseingrainsize(ormore),and require 2-4% total part strain in order to achieve goodquality hermetic bonding,even forpartsof relativelygoodsurface finish.Toachievegoodbonding,considerableloadorunitnormalforcemustbeappliedtothebondingsurfaces.Otherwise,onemustexpecttohaveamuchlargergraingrowththanwhatmightotherwisebeemployed.

• ActivatedDiffusionBonding:

Inthisdiffusionbondingmethod,thesurfacestobebondedarecoatedwithasecond“B”material,typicallyhavingasmalleratomicdiameter,andhighervaporpressure,thantheparentmaterials.Theresultingdiffusionbondisenhancedbytheextradiffusionandmasstransportratethatwouldotherwisebeexperiencedduringasolidstatediffusionbond.Inthismethod,anditisalsobydefinition,thatthistypeofbondisalsorunatorbelow themelting point of either parentmaterial or any resulting eutectic that mayform(Tbond<TEutecticA+B).

This process is routinely done on materials that suffer strength disadvantages fromhavinganysignificantgraingrowth,orwherethestructuremaynotbestrainedasmuchduring bonding. Structures that may have a high hermeticity requirement, that havepoorsurfacefinish,orthatmaybedegradedbyanygraingrowth(suchasmanynickelandcobaltbasedsuperalloys)aretypicallybondedusingthismethod.Also,thismethodis particularly usefulwhen the structure contains very small embedded passages thatmaybepronetopluggingbyanyliquidgeneratedduringthebondingoperation.


• LiquidInterfaceDiffusion(LID)Bonding:

In this bonding method, one purposefully places a second “B” material at the bondinterface,withtheintentionthatitwillgoliquidduringthebondingoperation.Arguablythisissemanticwithconventionalvacuumbrazing,butitshouldbestressedthattherearestillsomesignificantdifferencesbetweenthetwo.Specifically,intheliquidinterfacediffusion bond, the starting interface is typically very thin (on the order of 0.00015inchesor3.81microns)whereatypicalbrazejointusuallystartsatathicknessof0.0015inches(38.1microns)ormore.Asthestartingthicknessisverythin,itisalsonormalthatduringtheLIDdiffusionbondingprocess, thesecond“B”material isnearlycompletelydiffusedoutintothelatticeoftheparentmaterialbytheendofthethermalprocess.Itis alsohelpful to thinkabout the fact that the liquidmaybeeasily squeezedoutwardintothesurfaceasperities,helpingtominimizethevoidsthatmayotherwisebecausedfrompartswithpoorsurfacefinish,orminorburrs.

Inatypicalbrazejoint,(evenanormaldiffusionbrazedjoint)themicrostructuretypicallyshowsadefinedlayerofthebrazealloychemistry,andhasfilletsattheouterinterfacesateachlayer.Theresultingstrengthofthediffusionbrazedjoint,andtheliquidinterfacediffusionbond joint,canoftenvarybysubstantialamounts,dueto relativeamountofactual parent-to-parent metal bonding that actually occurs across the much thinnerbondjointthickness.ItshouldalsobestressedthattheaddedLIDbondinglayermaybechosentoformaeutecticwiththeparentmetal,suchthatthebondingtemperaturecanbeconductedatjustabovetheeutecticliquidusoftheresultingsystem(Tbond>TEuorTliqB)

As the resulting diffusion rate for this type of bonding is also driven by a liquid, ittypicallyoccurs3-5ordersofmagnitude faster than thediffusion rate fora solidstatebond,andasaresult,maybedonemuchmorequickly,andatmuchlowerstrainenergy,than is required foreither thesolid stateoractivateddiffusionbondingmethods. It istypical that thismethod isused for largepartswith large internalpassagegeometries(over1/8”nominalpassages)wheretherisk fromun-wantedpluggingofthepassagesareminimized,orwherethebondingcostneedstobeaslowaspossible,orwherethepart size means that there may not be enough mechanical force available from thedifferentialexpansiontoolingorhydraulicsystem/ramstoachieveagoodbondbyanyothermethod.Byrunningthisprocesswithanextendedholdtime(3-4hourstypical)itisquiteeasytoestablishbondstrengthsof>80%oftheparentmetal tensilestrengthproperties

• TransientLiquidPhase(TLPorTLID)DiffusionBonding:

Inthisbondingmethod,thesurfacesmaybysimilarinqualityandchemistrytothethinlayer LID bondingmethod, but the process is simply sloweddownenough to preventliquid from actually forming to any significant level. It should always be rememberedthat the process heating rate can be slowed down enough that the parent metal


diffusionintothebrazealloyorplatedlayercanbesufficienttoretardtheactualmeltingofthebrazealloyorin-situeutecticthatmaybeformed.Asaresult,theriskofplugginga small passage can be substantially reduced, while otherwise still allowing thesuccessfulbondingofpartsthatmayhaveapoorsurfacefinish.

The slow ramp heating rate can be continued well above the TsolB of the secondmaterialtotemperatureswellabovetheTliqBoftheinterfacesystem,providedthattheultimate temperature never exceeds the incipient melt point of the parent material(ParentTsol).Atypicalmicrostructurefor420seriesstainlesssteelinjectionmoldingdiesbondedusingthinBNi-2/AMS4777MetglasfoilandashortcycletimeLIDbondprocessisshowninFigure2below:

Figure2.LIDBondmicrostructureof420StainlessSteelbondedwith0.0015”ThickMetglasMBF20foilat1980F-1HourHoldwiththeramprateabovethealloysolidus(1776F)setat+30F/Hour,loadedat400PSIunitnormalstressallat10-5TorrVacuum(500XMag).

Asmay be noted the LID bond joint is still quite thin, and shows very good diffusioninterfaceswith theparentmaterial. Itmustbe stressed that this typeofbond canbeuseddirectlyadjacenttopassagesthatare1/16”x1/16” insizewithvery littleriskofplugging,as theactualamountof liquid that isgeneratedusing theslowramprateof+30 F/Hour does not make much actual liquid (no substantial alloy filleting). It isnoteworthythatatthe400PSIunitnormalstressthatwasusedforthisbond,verylittleparentmetal strainwasnoted (less than0.1%strain!!)andonlyasmallpercentageofthealloy(Whichstartedat0.0015”thickness)wasactuallydisplaced.

Figure 3. Same LID bondmicrostructure shown aboveat 1,000-X Mag showing awell diffused bond line andnobondlineporosity.


EquipmentRequirementsforDiffusionBonding:

Diffusionbonding caneasilyoccurwhen it is notdesired, anytimeparts areheld at asufficienttemperaturetocauseinteratomicdiffusioninatrulyreducingatmospherethatcan remove thenascentboundoxygen layers presenton the surfaceofmostmetallicparts.Asa resultdiffusionbondingcan readilybeconducted inavacuumor reducingatmospherefurnacewherethesurfaceoxidesareallreducedbythefurnaceoperatingatmosphere (Please see more information about the Ellingham Diagram at:http://www.engr.sjsu.edu/ellingham/asHostedbySanJoseStateUniversity).

Ingeneralthevacuumreducingatmospherepressurecanbebroughtdownverycloseorjustbelowtheboilingpointofanyofthemajoralloyconstituentsundervacuum(Think:Chromium!!). Inthisway, themasstransportrateforthehighestvaporpressurealloyconstituentcanbeeasilyharnessedtoimprovethebondingrateandquality.

Manyvacuumbrazingoperationsareconductedwheresimpledeadweightsareappliedinside the furnace, as a method to insure all asperities on the surface of the bondinterfacearebrought in intimate contact.Unfortunately, thisdoesnot alwaysworkasintended.Forthickerpartswhereanyresidualmachiningstressmaycausethepartstostressrelieve inthe“outof flatcondition,”thetotalrequiredforcemaybe impracticalbecausetheforceistooclosetotheyieldstrengthofthematerialwhenhot.Historically,many users of conventional vacuum brazing technologies using dead weight loadinghaveexperiencedscrapratesexceeding20%ofthequantityofpartsbeingjoined.

Over the years, the need for higher loads has resulted in the routine use of toolingmethods thatmakeuseofeitheradifferential expansionmismatch (Think:PutahighCTEpartinsidea“cagedtool”thatislowerCTEasneededtogenerateloading)orsometypeofactive loadingsystem,suchasapressurebellowsorhydraulicramsystemthatcanbeusedtoapplyalargerloadasmaybeneeded.Forthecaseofusing“cagedtools”thatareoftenbolted together, theuserneeds tounderstand that themaximumbondloadisoftengeneratedbeforethepartactuallygetstothefullbondingtemperature,astheboltsorotherstructuralmemberstypicallybegintoyieldatfairlylowtemperatures.

The need for better control of atmosphere and applied load stress has led to thedevelopmentofthevacuumhotpresssystem(VHP).AVHPisreallyjustaconventionalvacuum furnacewith added features. As a result, a very large effective loadmay beapplied using “hot rams” that come inside the furnace in similar fashion as anyconventional arbor press does, typicallyworking on flat platens to distribute the loaduniformlyonthepartbondsurfaces.Theadvantagesthatcomewithbeingabletoapplythemuchlargerunitnormal“flattening”forceonlyafterthepartssurfacesarereducedback topuremetalare substantial, includingdramatically improvedpart surface finishandflatnesspostbonding,aswellasreducedscraprates.


Recent advances in the diffusion bonding industry have led to the development of avacuumhotpress(VHP)thatalsoincludesaGasFanQuench(GFQ)coolingcapabilityaswell,suchthatpartsmaybediffusionbondedandthendirectlyquenchhardened.Mostplastics injectionmolds requirequenchhardening tooptimize thepostbondhardnessandotherrawmaterialproperties.Quenchingmayalsobeusedtosetthematerialupinthesolutionannealedcondition,orotherwisetobesetupforpostbondaging.

RefracSystemsinChandler,AZispresentlyfundingSolarManufacturing,Souderton,PAandBeckwoodPress/Triform,St.Louis,MO,tobuildwhat is thoughttobetheworld’sfirstvacuumhotpresssystemthatincludesthegasfanquenchcoolingoption(knownasaGFQVHPsystem)asshownbelowinFigure4:


Figure4.ConceptualdrawingofthenewGasFanQuenchedVacuumHotPress(GFQVHP).

Figure 5. Author (left) standing with Mr. Bill Jones, CEO, Solar Manufacturing and SolarAtmospheres,standinginfrontofthenewGFQVHPfurnacesystemwhileunderconstructioninlate2016.

Theworld’sfirstGasFanQuenchVacuumHotPress(GFQVHP)furnacesystemwasspecificallydesignedtotakelargemulticavityinjectiondiemoldssizedupto36”widex24”tallx50”longandbothdiffusionbondatupto100tonsofappliedloadwithgasfanquenchheattreatmentsavailableforthecompletedassemblyallinonerun.

ClosingRemarks:

Thereareconsiderablechallengesconfrontingmanufacturingandprocessengineersinselectingthebestbondingprocessfortheirconformallycooleddies.Ittakesagreatdealofengineeringexperiencetounderstandthetrade-off’sbetweenthethicknessofthesecond“B”material,themaximumallowablerampheatingratesandloadingprofile.Smallflowpassagesareparticularlyproblematictoavoidtheriskofpassageplugging,whileminimizingtheoverallprocesstime.Itis always recommended that anymoldmanufacturer should consult with its bonding sourcebeforecuttinganychipsforanewmold.RefracSystemsactingthruitswhollyownedsubsidiaryDie-Bond LLC, is always ready to help any prospective mold manufacturer with the properdesign and process selection to insure the highest quality bonding can be achieved. You can


reachDie-Bond, LLC, at 6141West Erie, Street, Chandler, AZ 85226, Phone: 480-940-0024orthruitsparentcompany:RefracSystems,at7201WestErieStreet,Chandler,AZ85226,Phone:800-4-REFRAC/800-473-3722.


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