HeatTreatmentsofAluminumAlloys
Dr.KaustubhKulkarniDepartmentofMaterialsScienceandEngineering
IndianIns@tuteofTechnologyKanpurKanpur,U.P.2080216
Date:21stOctober2015
TEQIP Workshop on Microstructure Engineering through Heat Treatments
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HeattreatmentsofAl-Alloys:AgeHardening
• SomeAluminumalloysundergohardeningaKerexposingtoanelevatedtemperatureforalonger@me
• AgeHardeningwasaccidentallydiscoveredin1901byDr.AlfredWilm
• Thefirstage-hardenablealloybasedonAl-Cu-Mg-Mn,whichwasdevelopedandpatentedbyWilmiss@llbeingused
• DURALUMIN2
AgeHardening:HTCycle
• Solu@onizing:Hea@ngabovesolvustemperatureforafewhours• Quenching:fromthesolu@onizingtemperature• Ar@ficialAgeing:Hea@ngtoanelevatedtemperature(aboveroom
temperaturebutwellbelowsolvustemperature)3
Solu@onizing Ar@ficialAgeing
T
t
TSol
Ta
WaterQuenching
AirCooling
RT
AgeHardening:StrengthEvolu@on
• Ageing:Strengthincreaseswith@meatTa• PeakStrength:MaximumStrengthobtainableduringageing• Over-ageing:Strengthdecreaseswith@me 4
Ageing
Over-ageingYieldStrength(M
Pa)
log(t)
Assolu@onizedstrength
Peakstrength
ThermodynamicsofAgeHardening
• Solu@onizing:Singlephaseαforms• Quenching:MetastableorSupersaturatedSolidSolu@onforms• Ageing:Drivingforceforprecipita@onofθ 5
C1Ceq
TSolu@onizing
TAgeing
SinglePhaseαSolidSolu@on
SupersaturatedSolidSolu@on
SecondPhasePrecipitates
θ
α
. .... ... .. ..
.. . ... .
CanθFormDirectly?
• Ac@va@onbarrierforforma@onofθ”islowerthanthatofθ(i.e.Q”<Q)• Hence,θ”forma@onisfavoredoverθatthebeginningoftheprecipita@on
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SSSSθ” θ’
θ
Q”
Q
G
Reac@onCoordinate
WhatCons@tutesAc@va@onBarrier?
• Forma@onofnewinterfaceisassociatedwithposi@veinterfaceenergy• Thesurfacetovolumera@oishighatthesmallsizeofnuclei• Nucleiarenotstablebelowcertaincri@calsizeatagiventemperature• Beyondthecri@calsize,surfacetovolumera@obecomessmall
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α
θ
α/θInterface
hfp://www.wikiwand.com/en/Classical_nuclea@on_theory
4πr2γ
4/3πr3Gv
r*= 2γGV
Cri@calRadiusofNuclea@on
Howthebarrierisreduced?
Coherentinterface:– Smalllalcemismatch
– Lalcestrainedattheinterface
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Semi-coherentInterface– Largerlalcemismatch– Strainrelievedabitby
misfitdisloca@on
IncoherentInterface– Verylargemismatch
– Atomicarrangementdisturbedcausinglargeinterfaceenergy
Theagehardeningprecipitatehastobecoherentorsemi-coherentinnatureEquilibriumprecipitatesareusuallyincoherentandcannotgivemuchhardening
Region with Tensile Stresses
Compressively stressed region
FiguresborrowedfromProf.Anandh,IITKanpur
Kine@csofPrecipita@on
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C1 C2Ceq
∆g1
∆g2
TSolu@onizing
TAgeing
Volumefrac@ondictatedbyLeverRule
⎟⎟⎠
⎞⎜⎜⎝
⎛=Δ
eqat CC
VkTg ln
2
3*
316
gG
Δ⋅=Δγπ
⎟⎠
⎞⎜⎝
⎛ −⎟⎟⎠
⎞⎜⎜⎝
⎛ Δ−=
tkTGZNJ τ
β expexp*
*0
DrivingForce
Ac@va@onEnergyforNuclea@on
Nuclea@onRate
HigherNuclea@onratemeansfinerprecipitates
• Highernuclea@onratesobtainedatlowertemperature• Growthiscontrolledbydiffusion• Highernuclea@onrate=>Largeamountoffinerprecipitates• Highergrowthrate=>Smalleramountoflargerprecipitates
AgeingatVariousTemperatures
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• Increasingageingtemperatureenhancethekine@csbutreducesthepeakhardness
1288 S N Samaras
Figure 15. Time evolution of hardness for the peak-aged alloy 5 during reheating at varioustemperatures.
Figure 16. Ageing data for alloy 6061 for 5 ageing temperatures.
Figure 17. Ageing data for alloy 6061 for 5 ageing temperatures.
(c) The asymptotic behaviour of the solution coincides with the LSW theory at the long-timeand infinite dilution regime.
(d) The problem derivation is general and different growth (e.g. multiparticle analysis) ornucleation laws (e.g. distribution of nuclei) can be applied.
CriteriaforAgeHardenability
• Phasediagramwithslopingsolvus• Existenceofametastableorstableprecipitatethatiscoherentorsemi-coherentwiththealuminummatrix
• Solu@ontemperatureselectedbaseduponsolvustemperature
• Ageingtemperatureop@mizedbaseduponpeakhardnessrequiredandageing@mefeasible
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SomeTEMmicrographsofAgehardenedAlloys
• Agehardeningprecipitatednotresolvedbyop@calmicroscopeorevenSEM
• Theshapeoftheprecipitatesusuallydecidedbyitsorienta@onrela@onshipwiththematrix
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RodshapedprecipitatesinAA6061 θ”PrecipitatedinanAl-CuAlloy
SomeAgeHardeningSystems:Al-Cu
• PrecipitateSequence– SSSS->G.P.Zones->θ”->θ’->θ
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Al (Aluminum) Binary Alloy Phase Diagrams
Al-Cu (Aluminum - Copper) J.L. Murray, 1985
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Al-Cu phase diagram
Al-Cu crystallographic data
Phase Composition,
wt% Cu Pearson symbol
Space group
(Al) 0 to 5.65 cF4 Fm m
52.5 to 53.7 tI12 I4/mcm
1 70.0 to 72.2 oP16 or oC16 Pban or Cmmm
2 70.0 to 72.1 mC20 C2/m
1 74.4 to 77.8 hP42 P6/mmm
2 74.4 to 75.2 (a) . . .
1 77.5 to 79.4 (b) . . .
2 72.2 to 78.7 hP4 P63/mmc
77.4 to 78.3 (c) R m
0 77.8 to 84 (d) . . .
1 79.7 to 84 cP52 P 3m
83.1 to 84.7 . . .
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SomeAgeHardeningSystems:Al-Li
• Al3Limetastableprecipitatesformthatarecoeherentwithmatrix14
Al (Aluminum) Binary Alloy Phase Diagrams
Al-Li (Aluminum - Lithium) A.J. McAlister, 1991
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Al-Li phase diagram
Al-Li crystallographic data
Phase Composition,
wt% Li Pearson symbol
Space group
(Al) 0 to 4 cF4 Fm m
17 to 24 cF16 Fd m
Al2Li3 28 to 29 hR15 R m
Al4Li9 36.6 mC26 C2/m
Al4Li9' 36.6 . . . . . .
( Li) 100 cI2 Im m
( Li) 100 hP2 P63/mmc
Metastable phases
Al3Li . . . cP4 Pm m
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IssueswithCastAlAlloys
• CastAl-alloyscanalsobeprecipitatehardenedbuttheresponsetohardeningnotasgoodasinwroughtalloys
• Intermetallicprecipitaesformedduringsolidificaitondonotdissolveeasilyduringsolu@onizing
• Heterogeneousnuclea@onofprecipitates – Forma@onofnon-hardeningequilibriumprecipitatesfavored
• Heterogeneousmicrostructureaffectsthesolu@onizingandageingkine@cs– Ageingresponsemaybedifferentatsurfaceandinthecoreinthicksec@ons
• IncipientMel@ng– Lowmel@ngnon-equilibriumphasesformedduringsolidifica@ontendtomeltifsolu@on
temperatureishigh
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NomenclatureofAlAlloys• 1xxx=>PureAl• 2xxx=>CuHeatTreatable• 3xxx=>Mn• 4xxx=>Si• 5xxx=>Mg• 6xxx=>Mg,SiHeatTreatable• 7xxx=>ZnHeatTreatable• 8xxx=>Otherelements
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TemperDesigna@onsofAlAlloys• F=>Asfabricated• O=>Annealed• H=>StrainHardened• W=>Solu@ontreated(usuallystableformonthsoryears)• T=>Aged
– T1=>Cooledandnaturallyaged– T2=>Cooled,coldworkedandnaturallyaged– T3=>Solu@onized,coledworkedandnaturallyaged– T4=>Solu@onizedandnaturallyaged– T5=>Cooledfromprocessandar@ficiallyaged– T6=>Solu@onizedandar@ficiallyaged– T7=>Solu@onizedandoveraged– T8=>Solu@onized,coldworkedandar@ficiallyaged
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Thankyou
AnyQues@ons????
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