Alloy Development for Intelligent Magnesium Designs文档格式.docx

Alloy Development for Intelligent Magnesium Designs文档格式.docx

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Itshouldcomeasnosurprisethatmagnesiumbecameastrategicmaterialduringtwoworldwarsandthatmostofthetechnologicaladvancesweremadeunderconditionsweretimewastheonlylimitingfactor.Fundsandresourceswerenotlimited.

Peace-timeapplicationofmagnesiumhoweverwasnotequallyspectacular-apartfrompossiblytheuseintheVWBeetle.

ThestoryofDr.Ing.h.c.F.PorschewhowascommissionedtomakeaVolkswagenin1934outofGermanrawmaterialsistrulyoutstandingbothintermsAutomotiveDevelopmentandintermsofMagnesiumDesign.

ThefamousFirstSketchoftheengine（fig1）didnotputeverythinginplacebutwasanoutlineofapowerplantthatwouldpropelacarunmatchedinthehistoryofautomotive.

Fig.1.ThefirstsketchoftheBeetleengine,madein1934.

Thatthechoiceofmagnesiumforcrankcaseandtransmissionhousingswasmadeasaresultofasystematicevaluationofcandidatematerialsorevenperformancerequirementsforthecarwouldbeanoverstatement.No,thematerialchoicewassimple.MagnesiumwastheonlyavailableGermanmaterialandenjoyedperformancestatusafterhavingbeendomesticallyproducedandusedsince1886.Atthetimeonewouldsaythattechnologywasinplace.

TodaytheBeetleisstillbeingproducedandmagnesiumcanlookbackata50yearmarriagewherehundredsofthousandsoftonsofmagnesiumgaveexcellentservicetotheseapplications.

ThefactthatVolkswagen,attimes,usedmorethan30%ofworldoutputofmagnesiummusthavebeenamajorreasonforothercarmanufacturersnottopickupmagnesiumintheirmaterialsportfolio.

Ittookanoilcrisisortwoandanincreasingawarenessofforwardthinkingpeopleonthepollutionproblemsinitiatedbytheautomotiveindustrybeforeautomakerstookasecondlookatmagnesiumanddecidedtointroducemagnesiumasaweightsaver.

Induecoursehighpurityalloys（toimprovemagnesium’scorrosionresistance）andflux-freemoltenmagnesiumhandlingtechniquesbecamestate-of-the-arttechnologiesandnewapplicationsweredevelopedintheautomotiveandelectronicindustries.

Atlastbutnotleastmustbementionedthemanyrecentstrategicalliancesbetweencarproducersandexistingaswellasnewmagnesiumprimaryproducers.

Thishasledtoanalmostinflationaryincreaseinnewmagnesiumprojectsandshowsthatmagnesiumhasagoldenopportunitytopositionitselfascommonstructuralmaterial.

Itsrawmaterialoccurrence,low（volume）productioncostasaprimarymetal,andrecyclingpotentialdoesinthisrespectcountasmuchasitslowspecificweight.

MagnesiumAlloys

TheEuropeanStandardformagnesiumalloysisnowcalledEN1753andwassubmittedinJune1977,（table1foringots）,（table2forcastings）,（table3,4,5forproperties）.BroadlythesearedividedintoalloyswithandwithoutZirconium.ForthepurposeofdiecastingonlytheAl-containingalloyswouldcomeintoconsideration.

Other,potentiallysuitedalloysfordiecastingpossessingpropertiessuperiortothosegiveninthestandard,doallhaveadevelopmentalstatus.

Itisworthnotingthatingotspecificationsaretighterthanthoseforcastings.Thisisduetothepossiblepick-upofimpuritiesduringprocessing.

Table1.TheEuropeanStandardEN1753（Mgingots）.

Table2.TheEuropeanStandardEN1753（Mgcastings）.

Table3,4,5.TheEuropeanStandardEN1753（MechanicalProperties）.

PropertyComparisonofMg,AlandZnDieCastingAlloys

Inlookingattypicalalloys,（table6）itisworthwhiletolookatdifferenceswhichhaveanimportanteffectoneitherwhichalloytouseandorwhattolookforwhenusingthealloy.

Table6.

Theratherlargedifferencesindensitiesareremarkableinthesensethat,whenusingmagnesiumtheweightpenaltyfactor（WPF）is.5whensubstitutingwithaluminumandastunning2,7whensubstitutingwithzincassumingequalvolume.Forstressedcomponents,liketransmissionhousingstheweightpenaltyfactorisreducedtoabout.3whensubstitutingwithaluminum.

Thedifferencetozinc,however,ismoredramatic.Inautomotivethishasledtoaclosetoacross-the-boardswitchtomagnesiumforsteeringcolumnlockhousings.

Theelasticmodulusdifferencesareremarkableandmustbetakenintoaccountwhendesigningforstiffness.Byincreasingthesectionmodulus-byribbing-thiscaneasilybecompensatedforbyonlyaminorweightincreasepenaltyformagnesium.Magnesium’sexcellentcastingpropertycomestoitsownrightwhenribbingisincorporated.

Thecreepstrengthofmagnesium,fortheAZ91alloy,israpidlydroppingasthetemperatureincreases.Muchmoresothanisthecaseforaluminum.Thisisofaparticularconcerninengineandtransmissionassemblies.

Thecoefficientoflinearthermalexpansionisofrealconcerninboltassemblieswhicharethermallystressed.Theuseofneckedtensionboltshasbeenfoundtobeanacceptablesolutionprovidedonisabletoincreasetheseatengagingareaanddimensiontheboltcorrectlyinordertoavoidplasticdeformationineithermaterial.

Magnesiumscorrosionpropertieshavebeenthesubjectofintensiveresearchanddevelopmentduringrecentyearsandarenot,contrarytotheopinionofmany,areasoninitselftodisqualifymagnesiumfrommanyapplications.

ManymagnesiumalloyshavebetteratmosphericcorrosionpropertiesthanmildsteelandcommonlyusedAl-alloysandmagnesiumsgalvaniccorrosionproblem（causedbythelow（est）electronegativepotentialamongstthecommonlyusedmetals）canbesolvedbyusingwellknowndesigntechniques.

SelectingtheRightMagnesiumAlloy

Table7givesanoverviewoversevendifferentmagnesiumalloysofwhichthefirst4areincommercialuseandthelast3inadevelopmentalmode.

Thechoiceofanalloyisoftendictatedbytherequirementofthepartinquestion.Thisisparticularlytruewhenpartsarestressed,operatingatelevatedtemperatures,subjectedtoshockloading,requiringtoughnessandductilityandsometimesevenrequiringacosmeticsurfaceappearance.

Fromthetable,examiningtheroomtemperatureproperties,onewouldtemptedtoratethealloysaccordingtotheirductility.

Infactthestressstraincurve（fig2）（atroomtemperature）isagoodtoolforassessingthealloysinquestion.Bearinmind,however,thatprocessingconditionshaveamajorinfluenceonthemechanicalpropertiesofanyalloy.

Table7.CompositionandpropertiesfordifferentMagnesiumalloys.

Thecreepbehavior,atelevatedtemperaturesandcommonlyusedstresslevelsinfunctionaldesigns,isalargedrawbackforthepenetrationofmagnesiumalloysintoso-calledhotapplications.Theseincludeengineblockswithhang-onpartslikebrackets,intakemanifoldsandtransmissions.

Fig3showscreepcurvesat50MPaat1500Cfordifferentalloysandfig4creepcurvesat85Mpaand1350Cforsomeexperimentalnewalloys.Itisobviousthatanimprovedalloywillgreatlycontributetoanincreasinguseofmagnesiuminhotapplications.

Fig.2.Stress-straindiagramforMagnesiumalloys.

Fig.3.Creepat1500Cand50MPa.

Fig.4.Creepat1350Cand85MPa.

DesignGuidelines

Asageneralruleoneshouldbeawareofthefactthatasuccessfulpartorcomponentisafunctionof6equallyimportantelements.Insequentialordertheyare:

1.evaluationofthefunctionofthepart-selectionofoptions

2.alloyselection

3.partdesign

4.diedesign

5.conversionprocessselection（especiallycastingmachine）

6.foundry--technologyandhumanresources--

Whendesigninginanewmaterialitisalwayshelpfultostudysuccessfulapplications.

Todaywefindanumberofthoseascanbeseeninfig5whereautomotive,electronic,tool,sports,communicationandmoreindustriesarepresented.

Allpartshave,orshouldhave,acommondenominatorinthattheyaredesignedaccordingtoacceptedmagnesiumdesignprinciples.

Thesearetextbookdesignrulesandwill,whenproperlyused,helpmakepartsmorecastable.

Fig.5.Magnesiumcastings.

Ingeneraltermswallthicknesscanbecastdownto2mmanddraftangelsdownto1.50,andcriticaltolerancesdownto+/-0.001mm/mm.Althoughthesedesignfeaturescanbeapplied,thepartbypartconsiderationintermsofrequirementsmakeoftenthesmartdesignerrelaxononeormoreoffeaturesinordertomaketheparteasiertocast.

Uniformwallthickness’andsmoothtransitionsasshowninfig6aswellastheincorporationoftexturedsurfaces,mini-ribslikeinfig.7and8.

Thegoodreasonsfortexturingthesurfaceandribbingcornersaretobraketheflowfrontduringfillingthusavoidingpreferredpathwaysofflow.This,inturn,eliminatesswirlsandhotcracks.

Fig.6.DesignRecommendationsonWallTransitions.

Fig.7.TexturedSurfacesandRibsinCorners.

Fig.8and9.RibsandTexturedSurfaces.

GoodMagnesiumDesigns

Theseatframeandcushionsupportonfig11showhowanumberoffeaturescanbeadaptedintothisdesign.Mountingorassemblylugsandsmallbracketswillmakeseatassemblyeasierandclosetolerancescanfacilitateautomationinassembly.Thepartisslowlybecomingmultifunctionalincomparisonwiththeconventionalsteelstamped/weldedassemblies.

Thecrosscarbeaminfig11isan