SpringerSeriesin materials science 124 SpringerSeriesin materials science Editors: R.Hull R.M.Osgood,Jr. J.Parisi H.Warlimont The Springer Series in Materials Science covers the complete spectrum of materials physics, includingfundamentalprinciples,physicalproperties,materialstheoryanddesign.Recognizing theincreasingimportanceofmaterialsscienceinfuturedevicetechnologies,thebooktitlesinthis seriesreflectthestate-of-the-artinunderstandingandcontrollingthestructureandproperties ofallimportantclassesofmaterials. PleaseviewavailabletitlesinSpringerSeriesinMaterialsScience onserieshomepagehttp://www.springer.com/series/856 David H. Kirkwood Michel Sue´ry Plato Kapranos Helen V. Atkinson Kenneth P. Young Semi-solid Processing of Alloys With 103 Figures 123 Dr.DavidH.Kirkwood Dr.MichelSue´ry Dr.PlatoKapranos GrenobleINP UniversityofSheffield Laboratorie SIMAP DepartmentofEngineeringMaterials Groupe GPM2 MappinStreet, Sheffield S1 3JD, UK BP46,38402St.-Martind’He`res,France E-mail:d.h.kirkwood@sheffield.ac.uk E-mail:[email protected] p.kapranos@sheffield.ac.uk Dr.KennethP.Young ProfessorHelenV.Atkinson VforgeInc. University of Leicester, 5567West6thAve,Lakewood,CO80214,USA Department of Engineering E-mail:[email protected] University Road, LE2 3TJ Leicester, UK E-mail:[email protected] SeriesEditors: ProfessorRobertHull ProfessorJürgenParisi UniversityofVirginia Universita¨tOldenburg,FachbereichPhysik Dept.ofMaterialsScienceandEngineering Abt.Energie-undHalbleiterforschung ThorntonHall Carl-von-Ossietzky-Straße9–11 Charlottesville,VA22903-2442,USA 26129Oldenburg,Germany ProfessorR.M.Osgood,Jr. ProfessorHansWarlimont MicroelectronicsScienceLaboratory DSLDresdenMaterial-InnovationGmbH DepartmentofElectricalEngineering PirnaerLandstr.176 ColumbiaUniversity 01257Dresden,Germany SeeleyW.MuddBuilding NewYork,NY10027,USA SpringerSeriesinMaterialsScience ISSN0933-033X ISBN978-3-642-00705-7 e-ISBN978-3-642-00706-4 DOI10.1007/978-3-642-00706-4 Springer Heidelberg Dordrecht London New York LibraryofCongressControlNumber: 2009929024 ©Springer-VerlagBerlinHeidelberg 2010 This work is subject to copyright. 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Coverdesign:SPi PublisherServices Printedonacid-freepaper Springer is part of Springer Science+Business Media( www.springer.com) Foreword Semisolid metallurgy (SSM) is now some 37-years-old in terms of time from its conceptionandfirstreductiontopracticeinthelaboratory.Intheinterveningyears, therehasbeenasteadilygrowingbodyofresearchonthesubjectandthebeginning ofsignificantindustrialapplications. The overall field of SSM comprises today a large number of specific process routes, almost all of which fall in the category of either “Rheocasting” or Thixo- casting.”Theformerbeginswithliquidmetalandinvolvesagitationduringpartial solidification followed by forming. The latter begins with solid metal of suitable structureandinvolvesheatingtothedesiredfractionsolidandforming. Research over the past 37 years, and particularly over the last decade, has provided a detailed picture of process fundamentals and led to a wide range of specificSSM processesandprocessinnovations.Industrialstudiesandactualpro- ductionexperienceare providinga growingpicture of the processadvantagesand limitations. Atthistime,theconditionsforeventualwideadoptionofSSMappearfavorable, bothfornonferrousandferrousalloys.Itmust,however,berecognizedthatmajor innovations,suchasSSMbecomeadoptedonlyslowlybyindustrieswherecapital costsarehigh,profitmarginsaremodest,andfailuretomeetcustomercommitments carriesahighpenalty. Asdemonstratedbythereferencesfoundinchaptersofthisbook,enthusiasmof SSM researchersthroughouttheworld ishigh,andindustrialsuccessesare begin- ningtobeevident.Thenewnessofthetechnologyandthecontinuingdevelopment of our process understandingmake it evidentthat key to increasing rate of indus- trial utilization of SSM will be (1) close collaboration between researchersin the field and production personnel, and (2) patience and persistence in the industrial developmentphase. Cambridge,MA,November2009 MertonC.Flemings v Preface SemisolidprocessingoriginateswiththeresearchatMITontheviscositybehaviour oftin/leadalloysduringsolidification,publishedbySpencer,Mehrabian,andFlem- ings in 1972.The original purpose was to examine the shear stress of semisolid structures with a view to shedding light on hot tearing mechanisms in solidify- ing alloys, but it was discovered that the process of shearing clearly altered the microstructure, changing the dendritic morphology of the solid into more spheri- calformthatbroughtabouta dramaticfallin shearstressandviscosity.Thisis of course the phenomenon of “Thixotropy,” well known to ceramicists and polymer scientists, but until then not observed in metallic systems. This discovery at MIT initiated an in-depth study of thixotropy in alloys, and the awareness that stirred semisolidalloyscouldbeinjectedasin diecastingintomoldswithcertainadvan- tages:Thecontrolledhigherviscosityflowmeantthatlessairwasentrappedinthe castingandthatthefinerequiaxedgrainsformedonsolidification,bothwouldresult inbettermechanicalproperties.Theworkcarriedoutinthelast40yearsinindustry andlaboratoriesaroundtheworldandpublishedintheBiennialInternationalCon- ference Proceedings on Semisolid Processing of Alloys and Composites, as well asin academicjournals,hasborneouttheseexpectations:Thereis nowa thriving industryintheproductionofthixoformedcomponentsinaluminumandmagnesium alloysforautomotivecomponents,andalsointheelectronicsandtelecommunica- tions industries. However, this is still a small proportion of all castings produced bymoreconventionalprocesses,andthisrestrictionismainlyduetothecostofthe preformedthixoformedbilletandtheinabilitytorecyclescrapin-house.Boththese problemscouldbeovercomebytherecentdevelopmentofthenewslurrygenerating technologiespermittingrheocasting(thatisintegratingtheformationandinjection of slurry on site, see IndustrialChap. 10), obviatingthe need to partially remelt a preformedslugandprovidingimportantproductionsavings– thusallowingdirect competitionwithconventionalhighpressurediecasting. The subject matter in this monographis discussed in three parts. The first part (A) deals with the microstructure of semisolid slurries resulting from nucleation anddendritefragmentation(Chap.1),andthechangesthatoccurduringsubsequent isothermalheattreatment(Chap.3). Thisis followedin Chap.4 by an accountof themorerecentadvancesin generatingslurriesin-house,thusallowingsignificant economies in component production. The rheology of alloy slurries is examined vii viii Preface in Chap. 6 and the modeling of semisolid flow is outlined in Chap. 7, providing a tool to optimise die design and prevent defect formation. Finally the industrial applications of semisolid processing are dealt with in Chap. 10, in which differ- ent techniquesare discussed for reheatingand partially melting preformedbillets, followed by isothermally heat treating them before thixoforming.Chapters 11and 12describethe practicalconsiderationsof processcontroland diedesign,andthe empiricalrulesthathavebeenestablishedasaguidetoproducesoundcastingsona regularbasis. I wish to acknowledge the efforts of the authors in providinga balanced, self- contained,andup-to-dateaccountofsemisolidalloyprocessingthatfollows.Itmust be admitted thatthis monographwas begunsome years ago but has been delayed in completion by the remarkable progress in the industrial technology. Hopefully with the advent of new rheocasting technologies, one phase in development has been completed – that is for light alloys. Copper based alloys for the production of intricate shapes must now be an achievable goal and this should stimulate the development of semisolid forming of high melting point alloys, such as stainless steels,nimonics,andtitaniumalloysusingspecialdiematerials. I should especially express my thanks to Dr Plato Kapranos, my colleague of manyyears,forhisgeneralhelp–notleastintheareaofthecomputermanipulation of text, and to Professor Merton C. Flemings, the father of semisolid processing, forprovidingthePrefaceandhissupportfortheproject,andalsotoProfessorHans Warlimontforsuggestingitinthefirstplaceandforhiscontinuousencouragement. Sheffield DavidKirkwood,PlatoKapranos Grenoble MichelSue´ry Leicester HelenV.Atkinson Lakewood KennethP.Young November2009 Contents PartI EvolutionandDesignofMicrostructureinSemisolidAlloys D.H.Kirkwood 1 FundamentalAspects ....................................................... 3 1.1 GeneralandLocalEquilibriuminAlloySystems .................... 3 1.2 Nucleation............................................................... 5 1.2.1 HomogeneousNucleation ..................................... 5 1.2.2 HeterogeneousNucleation..................................... 8 1.2.3 GrainRefinement .............................................. 9 1.3 Solid–LiquidInterfaceStructure....................................... 11 1.4 MorphologicalInstability .............................................. 12 1.5 GrainMultiplication.................................................... 14 1.6 TheGrowthandSpheroidizationofEquiaxedGrains ................ 15 2 CharacterizationofMicrostructureinSemisolidSlurries.............. 17 2.1 FractionSolid........................................................... 17 2.2 TheShapeFactor ....................................................... 17 2.3 ContiguityandContinuity.............................................. 19 3 EvolutionofMicrostructureinSemisolidAlloys......................... 23 3.1 Introduction............................................................. 23 3.2 CoarseninginAl–CuAlloys........................................... 24 3.3 CoarseninginAl–SiAlloys............................................ 25 3.4 X-rayMicrotomographyofAlloySlurries............................ 29 4 RecentDevelopmentsinSlurryFormation............................... 35 ReferencesforPartIEvolutionandDesignofMicrostructure inSemisolidAlloys..................................................... 39 ix x Contents PartII RheologyandModeling M.SueryandH.Atkinson 5 IntroductionandDefinitionsforRheologyandModeling .............. 43 6 ExperimentalDeterminationofRheologicalBehavior.................. 45 6.1 PartiallySolidifiedAlloys.............................................. 45 6.1.1 ExperimentalMethods......................................... 45 6.1.2 ResultsConcerningViscosity ................................. 48 6.1.3 ResultsonFluidity............................................. 56 6.2 PartiallyRemeltedAlloys.............................................. 56 6.2.1 ExperimentalMethods......................................... 56 6.2.2 ResultsinTermsofApparentViscosity....................... 60 6.2.3 ResultsinTermsofConstitutiveBehavior.................... 65 6.3 ComparisonBetweenPartiallySolidifiedandPartially RemeltedAlloys........................................................ 66 6.4 YieldStress ............................................................. 67 6.5 ConcludingRemarks ................................................... 68 7 ModelingofSemisolidProcessing ......................................... 71 7.1 IntroductoryRemarksonModeling ................................... 71 7.2 UtilityofModeling..................................................... 74 7.3 UseoftheStructuralParameterœforModeling:Model ofBrownandCoworkers............................................... 75 7.4 UseofaYieldStressinModels........................................ 76 7.5 FiniteDifferenceModeling ............................................ 77 7.5.1 One-phaseFiniteDifferenceModeling ....................... 77 7.5.2 Two-phaseFiniteDifferenceModeling....................... 84 7.6 FiniteElementModeling............................................... 84 7.6.1 One-phaseFiniteElementModeling.......................... 84 7.6.2 Two-phaseFiniteElementModeling.......................... 92 7.7 ModelsakintoForgingModels........................................ 96 7.7.1 ViscoplasticConstitutiveModels.............................. 96 7.7.2 ModelBasedonViscoelasticityandThixotropy ............. 96 7.8 Micromodeling ......................................................... 97 7.9 ConcludingRemarksonModeling .................................... 99 8 GeneralConclusionsonRheologyandModeling........................ 101 ReferencesforPartIIRheologyandModeling ............................. 103