Influence of Solution-Annealing Parameters on the Continuous Cooling Precipitation of Aluminum ...

16 Pages·2017·18.8 MB·English

metals
Article
Influence of Solution-Annealing Parameters on the
Continuous Cooling Precipitation of Aluminum
Alloy 6082
HannesFröck1,BenjaminMilkereit1,2,* ID,PhilippWiechmann1,ArminSpringer3,
ManuelaSander4,OlafKessler1,2andMichaelReich1
1 ChairofMaterialsScience,FacultyofMechanicalEngineeringandMarineTechnology,Universityof
Rostock,AlbertEinstein-Str.2,18059Rostock,Germany;[email protected](H.F.);
[email protected](P.W.);[email protected](O.K.);
[email protected](M.R.)
2 CompetenceCentre◦CALOR,DepartmentLife,Light&Matter,FacultyofInterdisciplinaryResearch,
UniversityofRostock,Albert-Einstein-Str.25,18059Rostock,Germany
3 MedicalBiologyandElectronMicroscopyCentre,UniversityMedicalCenterRostock,Strempelstraße14,
18057Rostock,Germany;[email protected]
4 ChairofStructuralMechanics,FacultyofMechanicalEngineeringandMarineTechnology,Universityof
Rostock,AlbertEinstein-Str.2,18059Rostock,Germany;[email protected]
* Correspondence:[email protected];Tel.:[phone]
(cid:1)(cid:2)(cid:3)(cid:1)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:1)
(cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)
Received:26February2018;Accepted:3April2018;Published:13April2018
Abstract: Weuseasystematicapproachtoinvestigatetheinfluenceofthespecificsolutioncondition
onquench-inducedprecipitationofcoarsesecondaryphaseparticlesduringsubsequentcoolingfora
widerangeofcoolingrates. CommerciallyproducedplatematerialofaluminumalloyENAW-6082
was investigated and the applied solution treatment conditions were chosen based on heating
differential scanning calorimetry experiments of the initial T651 condition. The kinetics of the
quench-inducedprecipitationwereinvestigatedbyinsitucoolingdifferentialscanningcalorimetry
forawiderangeofcoolingrates. Thenatureofthosequench-inducedprecipitateswasanalyzed
by electron microscopy. The experimental data was evaluated with respect to the detrimental
effectofincompletedissolutionontheage-hardeningpotential. Weshowthatifthechosensolution
temperatureandsoakingdurationaretooloworshort,thesolutiontreatmentresultsinanincomplete
dissolution of secondary phase particles. This involves precipitation during subsequent cooling
to start concurrently with the onset of cooling, which increases the quench sensitivity. However,
ifthesolutionconditionsallowtheformationofacompletesolidsolution,precipitationwillstart
after a certain degree of undercooling, thus keeping the upper critical cooling rate at the usual
alloy-specificlevel.
Keywords: EN AW-6082; AlMgSi alloy; solution treatment; solution temperature; differential
scanningcalorimetry(DSC);scanningelectronmicroscopy(SEM);continuouscoolingprecipitation;
quenchsensitivity;completesolution;incompletedissolution
1. Introduction
Themostimportantheattreatmentwhichallowstoincreasethestrengthofseveralaluminum
alloysistheage-hardeningheattreatment[1]. Withinthisheattreatment,solutionannealing,whichis
thegenerationofasolidsolution,establishesthebasisforthepotentialdecompositionofsolidsolutions
by quench-induced precipitation and/or supersaturation of the solid solution during subsequent
cooling. Afullysupersaturatedsolidsolutionisthepreconditiontoobtainmaximumstrengthafter
Metals2018,8,265;doi:10.3390/met8040265 www.mdpi.com/journal/metals
Metals2018,8,265 2of16
the final process “aging” step [1–3]. If a full solution is obtained during the solution treatment,
a complete supersaturation can be achieved by overcritical cooling. If cooling is done “to slow”,
“quench-induced”precipitationwilloccur. Quench-inducedprecipitationreducestheage-hardening
potential by lowering the amount of solutes after cooling (prior to aging). This phenomenon is
commonlycalledquenchsensitivity. Theuppercriticalcoolingrates(uCCR)requiredtocompletely
suppressquench-inducedprecipitation,dependonseveralaspects,forinstance,alloycomposition.
Thereby,higherconcentrationsofalloyingelementscausehigheruCCRs[4,5]. Inpracticalapplications,
thecoolingratescanbeadaptedbychoosingasuitablecoolingmedia[6].
For the solution treatment, lower temperatures and shorter soaking durations are preferable for
economicreasons,suchasthecontinuousannealinginsheetandplateproductionorquenchingfrom
extrusionheat.Anotherexampleofaveryshort“solutiontreatment”occursintheheat-affectedzoneof
weldingprocesses[7]orintheproductionoftailoredheat-treatedpre-products[8–11].Theseshort-solution
treatments,particularlyiftheyareperformedatrelativelylowtemperatures,mightendwithanincomplete
solution.However,incompletedissolutionofthesecondaryphaseparticlesmightleadtoreducedtime
spentfornucleationofthequench-inducedprecipitation.Therebyincompletedissolutionhasthepotential
toinfluencequenchsensitivity.Thesolutiontreatmentwaskeptconstantinmostquenchsensitivitystudies.
Thus,tothebestknowledgeoftheauthors,norecentstudyexiststhatsystematicallyvariesthesolution
temperatureandsoakingdurationforinvestigationsofAl-alloyquenchsensitivity.
The quench sensitivity, and thus the precipitation behavior during cooling from solution
treatmentanditskinetics, havebeeninvestigatedtoagreatextentinrecentyears(e.g., inAlMgSi
alloys[4,5,12–14]). Differentialscanningcalorimetry(DSC)hasproventobeaveryhelpfulmethodin
theseinvestigations,particularlyifcombinedwithmicro-andnanostructureanalyses[4,5,15].
Themainobjectiveofthisworkistoinvestigatethedetrimentaleffectofincompletedissolution
duringthesolutiontreatmentwithintheage-hardeningheattreatmentofAlalloyswithrespecttothe
kineticsofquench-inducedprecipitation. Therefore,thesolution-annealingtemperature,aswellasthe
soakingduration,wassystematicallyvaried.ThesolutionconditionswerechosenbasedonDSCheating
experimentsoftheinitialT651condition.TheprecipitationbehaviorduringthecoolingoftheENAW-6082
alloywasanalyzedbyDSCforawiderangeofcoolingrates.TheDSCfindingsarediscussedincomparison
withhardnessafterartificialaging,previousstudies,andtheobservedmicrostructures,whichareanalyzed
byscanningelectronmicroscopy(SEM)andtransmissionelectronmicroscopy(TEM).
2. MaterialsandMethods
2.1. InvestigatedAluminumAlloys
Thesampleswerepreparedfroma10-mm-thickplateofcommerciallysuppliedaluminumalloy,
EN AW–6082, in the initial T651 delivery condition. Aluminum wrought alloy EN AW-6082 has
been chosen as it is a very widely used age-hardening aluminum alloy. This condition comprises
solutiontreatment,quenching,stressreductionbystretchingacontrolledamount(1.5–3%oftheplate),
andfinalartificialaging. Thechemicalcompositionwasanalyzedbyopticalemissionspectroscopy
(OES).ThemassfractionsofalloyingelementsareshowninTable1.
Table1.MassfractionsofthealloyingelementsintheinvestigatedENAW–6082alloy,in%.
MassFraction,in%
ENAW-6082
Si Fe Cu Mn Mg Cr Zn Ti
OES 0.83 0.38 0.06 0.48 0.92 0.03 0.01 0.02
DINEN573-3 0.7–1.3 ≤0.5 ≤0.1 0.4–1.0 0.6–1.2 ≤0.25 ≤0.2 ≤0.1
Metals2018,8,265 3of16
2.2. DifferentialScanningCalorimetry(DSC)
Here,heatingandcoolingratesinthe0.01–5K/srangewereinvestigatedusingtwodifferent
typesofDSCdevices. Theslowerscanningrates(upto0.1K/s)wereperformedinaCALVET-type
heat-fluxDSC(SetaramDSC121,Caluire-et-Cuire,France),whereasthefasterrates(0.1–5K/s)were
examinedinpower-compensatedDSCs(PerkinElmerPyrisDiamondDSC,PerkinElmerDSC8500,
Waltham,MA,USA).ThetwoDSCdevicetypesrequireddifferentsamplegeometries. Cylindrical
samples with a diameter of 6 mm, a length of 21.65 mm, and a mass of about 1600 mg were used
forthemeasurementsintheheat-fluxDSC.Thesamplesforthepower-compensatedDSCsalsohave
a cylindrical shape, but with a diameter of 6.4 mm, a height of 1 mm, and a mass of about 80 mg.
The samples were packed in pure aluminum crucibles and then placed in the micro-ovens of the
calorimeter. Theheatingratetoreachthesolution-annealingtemperatureis0.1K/sintheheat-flux
DSCand2K/sinthepower-compensatedDSCs.
2.3. DataProcessingofRawMeasuredHeatFlowCurves
Itisnecessarytoeliminatethedevice-specificcurvaturetoobtainhigh-qualityDSCrawdata.This
curvaturecanbedeterminedbyrecordingabaselinemeasurement.Purealuminumreferencesareplaced
inbothmicro-ovens(thetwoDSCsensors)forthebaselinemeasurement,andtheidenticaltemperature
programtotheoneperformedduringasamplemeasurementisperformed.Purealuminum(99.9995%
purity)wasusedasthereferencematerialforDSC.Thedevice-specificcurvatureofthemeasuredcurves
canchangeslightly,especiallyforthepower-compensatedcalorimeters,withinafewhours.Thefollowing
schemeoffirstacquiringasamplemeasurement,followedbyabaselinemeasurement,andthenanother
samplemeasurement,wasperformed.Theaimistohaveafreshbaselineforeachsamplemeasurement.
However,tokeepexperimentaleffortsmall,weuseonebaselinerunfortwosamplemeasurements—one
beforeandonedirectlyafterthebaselinerun.Bysubtractingthebaselinefromthesamplemeasurement,
thedevice-specificcurvaturecanberemovedfromtherawdata.
Theheatingexperimentsconsistedoffoursamplemeasurementsandtwobaselinesintheheat-flux
DSC,andeightsingle-samplemeasurementswererecordedwithfourrelatedbaselinemeasurements
in the power-compensated DSCs for each heating rate. The cooling experiments for each cooling
rate in the heat-flux DSC consisted of two-four sample measurements with one or two associated
baselinemeasurementsintheheat-fluxDSC,andsixsamplemeasurementswiththreerelatedbaseline
measurementsinthepower-compensatedDSCs.
Comparisonofthemeasurementswithdifferentheatingorcoolingrates(β)anddifferentsample
masses(m )requiresthenormalizationofthemeasuredheatflowtothevalueofexcessspecificheat
S
. .
capacity(C )accordingtoEquation(1)[16],whereQ andQ arethesampleandbaselineheat
pexcess S Bl
flows,respectively,andβisthescan(heatingorcooling)rate.
. .
Q −Q
C = S Bl (inJg–1K–1), (1)
pexcess m ·β
S
Figure1showstheevaluationoftheDSCcurvesforaheatingcurveproducedfromaheating
rateof0.5K/sthatwasrecordedwithapower-compensatedDSC.Figure1Ashowstherecorded
rawdata. Thedevice-specificcurvatureiseliminatedbysubtractingthebaselinemeasurementfrom
thesamplemeasurement,asshowninFigure1B.Figure1Cshowsthedatanormalizedbyscanning
rate and sample mass, which still contains overshoot artefacts [15] that have to be excluded from
furtherevaluations. Figure1DshowstworesultingheatingcurvesoftheENAW-6082alloyfromthe
initialstateT651,withaheatingrateof0.5K/sdependingontemperature. AsshowninFigure1D,
thedissolutionofprecipitatesisnotcompleteevenathightemperaturesupto580◦C.Thiscanbeseen
bythefact,thattheDSCsignalisnotdroppingbacktozeroathightemperatures. Thus,azero-level
correctionbysubtractionofapolynomial,assuggestedinOstenetal.[11]andMilkereitetal.[15,17],
wasnotapplicable. Experimentspecificcurvefluctuations(e.g.,causedbyslightpositiondifferencesof
Metals2018,8,265 4of16
thesensor-lidorsample[17])aredefinitelyminimizedbyaveragingthecurvesmeasuredforasingle
setofparameters. Thisdatatreatmentwasrecentlyintroducedbyourgroup,anddiscussedindetailin
Kemsiesetal.[18]. AsthereproducibilityoftheDSCmeasurementswasfoundtobehigh,withsmall
fluctuations,asshowninFigure1D,wedonotplotthecurvefluctuationrangesheretomaximizethe
readabilityoftheplots,whichalsoallowsfortheeasycomparisonofmultipleDSCcurves.
Figure 1. Evaluation procedure of the heating experiments. (A) Raw data measurements; (B)
subtractionofbaselinemeasurementsfromthesamplemeasurements;(C)normalizationofthedata;
(D)processeddata.
A dottedstraight line isadditionally plotted on allthe DSC heatingand cooling curves, thus
indicatingtherelatedzerolevel. Adeviationofthemeasurementcurvefromthiszerolevelsignifies
eitheranendothermicdissolutionreactionoranexothermicprecipitationreaction. Boththeheating
and cooling curves are shown here. In terms of the specific heat capacity, the endothermic area is
shownupwardsintheheatingexperiments[15,19],whereastheexothermicareaisshownupwardsin
thecoolingexperiments[5]. Anyfigurespresentedintheresultsanddiscussionsectionbelowthat
consistofDSCcurvesfromdifferentscanningratesarearrangedandshiftedinorderofthescanning
rateused,startingwiththelowestrateontop.
2.4. Microscopy
Thesamplesforthemicrostructureinvestigationswereheat-treatedinaDSC,aswellasinaquenching
dilatometer(Bähr805A/D),basedontheDSCresultspresentedbelow. Afterthermalpreparation,the
samplesforSEManalysiswerecold-embeddedinepoxyresinandthenmechanicallygroundandpolished
withwater-freelubricants.Thefinalpolishingwasdonewith0.05µmoxidepolishingsuspension.The
polishedsampleswererinsedafterpolishingandwereanalyzedinthepolishedcondition.
TheSEMsampleswereanalyzedbyafieldemissionSEM(MERLIN®VPCompact,Co. Zeiss,
Oberkochen,Germany),equippedwithanenergy-dispersiveX-ray(EDX)detector(XFlash6/30)and
analysissoftware(Quantax400,Co. Bruker,Berlin,Germany). Representativeareasofthesamples
wereanalyzedandmappedtodeterminetheelementaldistributiononbasisoftheEDX-spectradata
bytheQUANTAXESPRITMicroanalysissoftware(version2.0).
TheembeddedandpolishedsamplesweremountedontheSEMcarrierwithadhesiveconductive
carbon and aluminum tape (Co. PLANO, Wetzlar, Germany). SEM-secondary electron (SEM-SE)
Metals2018,8,265 5of16
imageswereobtainedusinghighefficiencyEverhart-Thornley-typeHE-SEdetectorat5kVacceleration
voltage. TheaccelerationvoltagefortheEDXanalysiswasvariedbetween5and15kV.
Samples for transmission electron microscopy (TEM) were prepared by mechanical grinding,
polishing up to a thickness of 100–150 µm, and further treatment by twin jet electro-polishing in
methanolandnitricacid(2:1ratio). Theelectrolyticthinningoccursatatemperatureofabout−20◦C,
withavoltageof12V.TEManalysiswasdonebyaZeissLIBRA120(Oberkochen,Germany),equipped
withaLaB cathodeandanin-columnomegaspectrometer. Representativeareasofthesampleswere
6
analyzedwith120kVacceleratingvoltageanddigitalimageswereacquiredusingabottom-mount
2×2kslowscandigitalcamerasystem(Proscan,Co. TRS,Moorenweis,Germany). Imageprocessing
wasdonewiththeiTEMsoftware(OlympusSoftImagingSolutionsGmbH,Münster,Germany).
2.5. HardnessTests
The hardness was examined for different solution-annealing conditions and cooling rates to
correlatetheresultsofthethermalanalysiswithacharacteristicmechanicalvalue,afterasubsequent
artificialagingat180◦Cfor4h. Hardnesstestswerechosenasthehardnesscanbetesteddirectlyon
theDSCsamples.Theheattreatments,withcoolingupto5K/s,werecarriedoutintheDSC.However,
fastercoolingratesof10to300K/swerealsorealizedinthequenchingdilatometerBähr805A/D.
Thehardness(HV1)wasdeterminedwithamicro-hardnesstester,HMV-2,fromShimadzu,Japan. Six
hardnessindentationswererecordedperparameterset. Theresultsplottheaverage,hardnessvalues.
Additionally,theminimumandmaximumvaluesaregivenaserrorbars.
3. ResultsandDiscussion
3.1. ContinuousHeatingoftheT651InitialCondition
Thesolution-annealingtemperaturewaschosenas540◦Cinseveralpreviousinvestigationsofthe
quenchsensitivityofAlMgSialloys[4,5,12]. However,theDSCheatingexperimentsontheparticular
alloy batch and the initial T651 condition used here (shown in Figure 2), even for slow heating at
0.01K/s,indicatethatdissolutionofthesecondaryprecipitatesofinterestisjustcompletedatabout
560◦C.Furthermore,Figure2showsthatthetemperatureatwhichthedissolutionofthesecondary
phasesiscompletedstronglydependsontheheatingrate. Anincreaseintheheatingrateshiftsthis
dissolutioncompletiontohighertemperatures. AsderivedfromfindingsinSchumacheretal.[20],it
canbeexpectedthatthespecificsolvustemperaturewillbereducedbyuptoafewtensofKelvinat
evenlowerheatingratesorunderisothermalsolution-annealingconditions. Onesolution-annealing
temperature was thus chosen as 560 ◦C, which, according to the heating experiments, should be
above the equilibrium solvus temperature in any case. As mentioned above, 540 ◦C is a typical
solution-annealingtemperaturethathasbeenusedinpreviousworks,and550◦Cwaschosenasthe
temperatureinbetweenthesevalues. Twodifferentsolution-annealingsoakingdurationsof1and
20minwerealsoexamined. SincetheslowerCALVET-typeDSCrequiredatleast5mintoachieve
thermalequilibration[13],asoakingdurationof1minisonlyinvestigatedinthefasterDSCs(0.3–5
K/s). Therefore,asystematicinvestigationofthequenchsensitivityorquench-inducedprecipitation
behaviorafterdifferentsolution-annealingconditionswasconducted,andtheDSCexperimentswere
performedwiththreesignificantparametersvaried,asfollows:
• Solution-annealingtemperatures: 540,550,and560◦C;
• Solution-annealingsoakingdurations: 1and20min;
• coolingrates: 0.01–5K/s.
Thiscorrespondsto33setsofparametersandmorethan200singleDSCexperiments.
Metals2018,8,265 6of16
Figure2. Differentialscanningcalorimetry(DSC)heatingcurvesofENAW–6082initiallyT651asa
functionoftemperature.Thesolutiontemperatureschosenforlaterexperimentsareindicated.
3.2. ContinuousCoolingExperiments,StartingfromDifferentSolutionTreatmentConditions
Figure3highlightsoneparametersetofthecoolingexperiments,plottedaccordingtothe540,
550,and560◦Csolution-annealingtemperatures. TheDSCcoolingcurvesofthetwodifferentsoaking
durationsareplottedintwocolors. TheareabetweenaDSCcurvepeakanditscorrespondingzero
levelisproportionaltotheprecipitationenthalpy,andthusthevolumefraction,oftheprecipitates
grownduringacertainreaction/peak[4].
Figure 3A shows that the DSC curves/reactions are insensitive to the soaking time at 540 ◦C.
Furthermore,abroadhigh-temperaturereactions(HTRs)peakdominates,evenathighcoolingrates.
Asimilarpatternisobservedifcoolingstartedfrom560◦Cregardingtheinsensitivitytosoakingtime
(Figure3C).However,theshapeofthecoolingcurvesvariessignificantlywhencomparingsoakingat
thesetwotemperatures. Twomainexothermicprecipitationreactionpeaksareobservedforthesoaking
at560◦C.Thelowtemperaturereactions(LTRs)peakbeginstodominatetheprecipitationbehavior
duringfastercoolingfrom560◦C.Themediumsolution-annealingtemperatureat550◦Chighlights
thatthesoakingdurationsignificantlyimpactsthecurvatureoftheobservedDSCcurves,asshownin
Figure3B.Here,theshortersoakingtimes(1min)yieldDSCcurvesthataresimilartothoseat540◦C,
whereasthelongersoakingtimes(20min)yieldDSCcurvesthataresimilartothoseat560◦C.
The same DSC data are rearranged in Figure 4 to highlight the role of the solution-annealing
temperature.Twomajorargumentscanbederivedfromthesedata:(1)onlyminordifferencesintheDSC
curvecurvaturesareobservedattheslowercoolingrates(upto0.1K/s,resultingincoolingdurationsof
1.4to14h)and20minofsoaking.However,(2)forfastercoolingratesof0.3to5K/sthosedifferencesare
moresignificantwhilethosecoolingratesadditionallyareconsideredtobemuchmoretechnologically
relevant,coveringcoolingdurationsrangingfrom30stoafewminutes.ConsiderablydifferentDSCcurves
areproducedatthesefastcoolingrates,thetwodifferentsoakingtimes,andtherangeofsolution-annealing
temperatures,whichexhibitthelargestdifferencesforthemediumcoolingratesof0.3and1K/s.
ThefindingsfromFigures3and4thussuggestthattherearetwomajortypesofDSCcurvaturesfor
agivencoolingrate.TheresultantDSCcurvatureobviouslydependsonthespecificsolutioncondition
appliedpriortocooling.Therefore,theDSCdataarearrangedintothesetwotypesofDSCcurves,as
showninFigure5. TheDSCcurvesinFigure5AaredominatedbyHTRs, whiletheDSCcurvesin
Figure5BaredominatedbyLTRs.ThisleadsonetoaskwhatthecauseofthesedifferentDSCcurves
andtheirquench-inducedprecipitationbehavioris. Wethuspostulatethatthetwodifferenttypesof
precipitationbehaviordependonwhetherthecoolingstartedfromanincompletesolution(asseenin
Figure5A)orifa(largely,ifnotfully)completesolutionprecedesthecooling(asseeninFigure5B).
Metals2018,8,265 7of16
Figure3.DSCcoolingcurvesofENAW-6082(initiallyT651)withcoolingratesfrom0.01upto5K/s
aftersolution-annealingat(A)540◦C,(B)550◦Cand(C)560◦Cfor1min(red)and20min(black).
Figure4.DSCcoolingcurvesofENAW-6082(initiallyT651),withcoolingratesrangingfrom0.01to
5K/sandaftersolution-annealingatvariousconditions.(A)Soakingdurationof1minand(B)20min.
Thethreedifferentappliedtemperaturesarecolor-codedtohighlightthedegreeofvariabilitywith
soakingtimeandcoolingrate.
Metals2018,8,265 8of16
Figure 5. DSC cooling curves of EN AW-6082, with cooling rates from 0.3 to 5 K/s after
solution-annealing,resultingin:(A)anincompletesolutionand(B)acompletesolution.
Thispostulationisderivedfromthreearguments:
i. AccordingtoFigure2(andtherelateddiscussion),thesolvustemperatureoftheinitialT651
conditionofthisparticularbatchofENAW-6082forslowheatingisbelow560◦C.
ii. Ifcoolingisstartingfromamore-or-lesscompletesolution(e.g.,560◦Csolution-annealing
temperatureand20minsoakingtime),themainprecipitationbeginsafteracertaindegree
ofundercooling.
iii. The quasi-binary phase diagram Al-Mg Si (Figure 6) indicates that the Mg Si equilibrium
2 2
solvustemperatureforthisalloyisnearorslightlyabove540◦C.
Ourreasoningforthesethreeargumentsisasfollows:
→ i.: Theheatingrate-specificsolvustemperaturecandropbelow560◦Cduringveryslowheating,
asdemonstratedinFigure2. ThecoolingexperimentsinFigure5wereconductedafterrelatively
fastheating(2K/s). Therefore,accordingtoFigure2,thedissolutionremainedincompleteatthe
endoftheheatingprocess.Acompletesolutioncanstillbeobtainedwiththeswitchtoisothermal
soaking,asseeninFigure5B.Here20minofsoakingat550and560◦Cresultsinprecipitation
beginningafteracertaindegreeofundercooling,whileashortsoakingof1minat560◦Cstill
causesthedirectonsetofprecipitation.
→ ii.: Taking the cooling rate of 0.3 K/s as an example, cooling from 560 ◦C and 20 min of
soakingcausestheprecipitationtostartatabout500◦C,afteracertaindegreeofundercooling.
However, starting from a condition that we claim to be an incomplete solution (e.g., 540 ◦C
solution-annealingtemperature),thesamecoolingrateof0.3K/scausestheimmediateonsetof
precipitationwhenthetemperaturedropsbelow540◦C,withprecipitationthusstarting40K
higherthanthatinthecompletesolution. Twodifferenttypesofprecipitationonset(1:immediate
precipitationonsetwiththeonsetofcooling;and2: onsetofprecipitationafteracertaindegreeof
undercooling)wereattributedtotheobservationsfromthequasi-binaryAl-Mg Siphasediagram,
2
whichdependedonwhetherthecoolingstartsfromtheα-Alsolid-solutionsingle-phaseregion
orfromtheα-Al+β-Mg Sitwo-phaseregion[21].
2
Metals2018,8,265 9of16
→ iii.: Thethreeappliedsolutiontemperaturesareindicatedinthequasi-binaryphasediagram
Al-Mg SiandshowninFigure6,aswellastheestimatedmaximumamountofMg Siforthe
2 2
particular batch of EN AW-6082 that was used in the experiments. While the comparison of
ENAW-6082withthisphasediagramisasimplification,theprecipitationoftheβ-Mg Siphase
2
fromtheα-AlsolidsolutionisdominatingtheHTRsduringcoolingfromthesolutiontreatment
ofAlMgSialloys[5],includingtwobatchesofENAW-6082. Itisthusinferredthatthemostlikely
phasetobeconsideredintermsofcompletesolutionisβ-Mg Si. Theequilibriumsolvusofthe
2
estimatedβ-Mg Simassfractioniscloseto540◦C,whichisadditionallysupportedbytheDSC
2
experimentsshowninFigure7. Thelatterusedsoakingdurationsrangingfrom1to120minat
540◦Cthatprecededacoolingrateof1K/s. Acompletesolutionwasstillnotattainedaftertwo
hoursofsoakingat540◦C,withtheMg Siprecipitationstartingconcurrentlywiththeonsetof
2
cooling. Therefore,itisreasonabletoassumethatthealloybatch-specificequilibriumβ-Mg Si
2
solvustemperatureiscloseto540◦C(verylikelyafewKelvinabove540◦C).
It can thus be concluded that a relatively short soaking at a high enough temperature (some
10 K above the alloy specific solvus) can result in the intended complete solution. The formation
of a complete solution requires both an appropriate solution temperature and soaking duration
to be chosen. As seen from the above results, a complete solution as the initial condition for
subsequent cooling results in the initiation of precipitation temperatures that are some 10 Kelvin
lowerthattheinitialconditionofanincompletesolution. Thisaspectcanbequiteimportantinan
industrial/productioncontext. Forinstance,thetransportationofthesolution-treatedpartfromthe
furnacetothequenchingmediaisunavoidablygoingtoexperiencesomecoolingbeforequenching.
Ensuringacompletesolutionduringthetechnologicalapplicationofthesolutiontreatmentcanthus
helptoavoidquench-inducedprecipitation. Thisaspectmightbeofevenmorerelevance,becausethe
generalquenchsensitivityisalsoincreasedifthedissolutionremainsincompleteduringthesolution
treatment(seethediscussiononhardnessbelow).
Figure6. Sectionfromthequasi-binaryphasediagramAl-Mg Si(adoptedfromPolmear[1]). The
2
theoreticalmaximumamountofβ-Mg SiinthisbatchofENAW-6082(verticalredline),aswellasthe
2
appliedsolutiontemperatures(horizontallines),ishighlighted.
Metals2018,8,265 10of16
Figure7.DSCcoolingexperimentsonENAW-6082aftersolutiontreatmentat540◦C,soakingtimesof
1,20,and120min,andacoolingrateof1K/s.
3.3. AnalysisoftheQuench-InducedMicrostructure
Theschematictemperature-timeprofileappliedfortheheattreatmentofsamplesusedforthe
microstructureanalysisisshowninFigure8. TheparameterswerechosenbasedonthecoolingDSC
results. Forclarity,thecorrespondingDSCcurvesareplottednexttothemicrostructureimagesin
Figures9Aand10A.Figures9and10showtheDSCcurvesandrepresentativeSEM-SEimagesfrom
540◦Cand560◦C,respectively,with20minofsoakingandacoolingrateof0.3K/s. Thecoolingwas
continuedatthislowratedownroomtemperatureinFigure9B,DandFigure10B,D.Alternatively,
thelowcoolingrateof0.3K/swasapplieduntil425◦C,andsubsequentlyovercriticallygas-quenched
(~400K/s)toroomtemperature(seeFigure8)inFigure9C,EandFigure10C,E.
Figure8.Schematictime–temperatureprofileoftheheattreatmentsforthemicrostructureanalysis.