HeatTreatmentsofAluminumAlloys

Dr.KaustubhKulkarniDepartmentofMaterialsScienceandEngineering

IndianIns@tuteofTechnologyKanpurKanpur,U.P.2080216

Date:21stOctober2015

TEQIP Workshop on Microstructure Engineering through Heat Treatments

1

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

6

SSSSθ” θ’

θ

Q”

Q

G

Reac@onCoordinate

WhatCons@tutesAc@va@onBarrier?

•  Forma@onofnewinterfaceisassociatedwithposi@veinterfaceenergy•  Thesurfacetovolumera@oishighatthesmallsizeofnuclei•  Nucleiarenotstablebelowcertaincri@calsizeatagiventemperature•  Beyondthecri@calsize,surfacetovolumera@obecomessmall

7

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

8

Semi-coherentInterface–  Largerlalcemismatch–  Strainrelievedabitby

misfitdisloca@on

IncoherentInterface–  Verylargemismatch

–  Atomicarrangementdisturbedcausinglargeinterfaceenergy

Theagehardeningprecipitatehastobecoherentorsemi-coherentinnatureEquilibriumprecipitatesareusuallyincoherentandcannotgivemuchhardening

Region with Tensile Stresses

Compressively stressed region

FiguresborrowedfromProf.Anandh,IITKanpur

Kine@csofPrecipita@on

9

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

10

•  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

11

SomeTEMmicrographsofAgehardenedAlloys

•  Agehardeningprecipitatednotresolvedbyop@calmicroscopeorevenSEM

•  Theshapeoftheprecipitatesusuallydecidedbyitsorienta@onrela@onshipwiththematrix

12

RodshapedprecipitatesinAA6061 θ”PrecipitatedinanAl-CuAlloy

SomeAgeHardeningSystems:Al-Cu

•  PrecipitateSequence–  SSSS->G.P.Zones->θ”->θ’->θ

13

Al (Aluminum) Binary Alloy Phase Diagrams

Al-Cu (Aluminum - Copper) J.L. Murray, 1985

<Previous section in this article Next section in this article>

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 . . .

Page 1 of 2Al (Aluminum) Binary Alloy Phase Diagrams

9/23/2011http://products.asminternational.org/hbk/do/highlight/content/V03/D02/A03/S0038351.html

SomeAgeHardeningSystems:Al-Li

•  Al3Limetastableprecipitatesformthatarecoeherentwithmatrix14

Al (Aluminum) Binary Alloy Phase Diagrams

Al-Li (Aluminum - Lithium) A.J. McAlister, 1991

<Previous section in this article Next section in this article>

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

Copyright © 2002 ASM International®. All Rights Reserved.

<Previous section in this article Next section in this article>

Page 1 of 2Al (Aluminum) Binary Alloy Phase Diagrams

9/23/2011http://products.asminternational.org/hbk/do/highlight/content/V03/D02/A03/S0038362.html

IssueswithCastAlAlloys

•  CastAl-alloyscanalsobeprecipitatehardenedbuttheresponsetohardeningnotasgoodasinwroughtalloys

•  Intermetallicprecipitaesformedduringsolidificaitondonotdissolveeasilyduringsolu@onizing

•  Heterogeneousnuclea@onofprecipitates –  Forma@onofnon-hardeningequilibriumprecipitatesfavored

•  Heterogeneousmicrostructureaffectsthesolu@onizingandageingkine@cs–  Ageingresponsemaybedifferentatsurfaceandinthecoreinthicksec@ons

•  IncipientMel@ng–  Lowmel@ngnon-equilibriumphasesformedduringsolidifica@ontendtomeltifsolu@on

temperatureishigh

15

NomenclatureofAlAlloys•  1xxx=>PureAl•  2xxx=>CuHeatTreatable•  3xxx=>Mn•  4xxx=>Si•  5xxx=>Mg•  6xxx=>Mg,SiHeatTreatable•  7xxx=>ZnHeatTreatable•  8xxx=>Otherelements

16

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

17

Thankyou

AnyQues@ons????

18