Semi-solid Processing of Alloys

149 Pages·2010·5.41 MB·English

by  David H. Kirkwood

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materials science
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Editors: R.Hull R.M.Osgood,Jr. J.Parisi H.Warlimont
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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]
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UniversityofVirginia Universita¨tOldenburg,FachbereichPhysik
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MicroelectronicsScienceLaboratory DSLDresdenMaterial-InnovationGmbH
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SpringerSeriesinMaterialsScience ISSN0933-033X
ISBN978-3-642-00705-7 e-ISBN978-3-642-00706-4
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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