Materials Science and Engineering A 528 (2011) 2718–2724
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Materials Science and Engineering
A
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /m s e
a
Effects of heat treatments on the microstructure and mechanical properties of a 6061aluminium alloy
D.Maisonnette a ,M.Suery b ,D.Nelias a ,∗,P.Chaudet a ,T.Epicier c
a
Universitéde Lyon,CNRS,INSA-Lyon,LaMCoS UMR5259,F-69621,France
b
Universitéde Grenoble,SIMaP,UMR CNRS 5266,BP46,Domaine Universitaire,38402Saint Martin d’Hères Cedex,France c
Universitéde Lyon,CNRS,INSA-Lyon,Mateis UMR5510,F-69621,France
a r t i c l e i n f o Article history:
Received 23August 2010
Received in revised form 3December 2010Accepted 3December 2010
Available online 9 December 2010
Keywords:
6061Aluminium alloy
Thermomechanical properties
Electron beam welding stress–strain curves Yield stress
Hardening precipitates
a b s t r a c t
This paper describes the mechanical behavior of the 6061-T6aluminium alloy at room temperature for various previous thermal histories representative of an electron beam welding.A fast-heating device has been designed to control and apply thermal loadings on tensile specimens.Tensile tests show that
the yield stress at ambient temperature decreases if the maximum temperature reached increases or if the heating rate decreases.This variation of the mechanical properties is the result of microstructural changes which have been observed by Transmission Electron Microscopy (TEM).
© 2010 Elsevier B.V. All rights reserved.
1.Introduction
The study presented in this paper is concerned with the widely used 6061-T6aluminium alloy.It is an age hardenable alloy,the mechanical properties of which being mainly controlled by the hardening precipitates contained in the material.When the material is subjected to a solution heat treatment followed by a quenching and a tempering treatment,its mechanical properties reach their highest level and become very good compared to other aluminium alloys.The as-obtained microstructure of the material is called T6temper (tempering around 175◦C).Another interest-ing characteristic of the AA6061is its good weldability.Because of these favorable properties,the AA6061alloy is used in the trans-port and the public works domains (framework,pylon,handling equipment ...)and also for complex structures assembled by weld-ing [1–3].
The present work is part of the early qualifying study of a pres-sure vessel to be used in an experiment
al nuclear reactor.The approximate size of the vessel is five meters height with a diameter of about one meter.Several ferrules in AA6061-T6should be assem-bled together by electron beam (EB)welding.The aim of the work presented in this paper is to evaluate the influence of the weld-ing process on the mechanical properties of the material at room
∗Corresponding author.
E-mail lias@insa-lyon.fr (D.Nelias).
temperature.The change of mechanical properties is due to met-allurgical phenomena such as dissolution,growth or coarsening of precipitates,which have been also observed.
It is commonly assumed that the generic precipitation sequence in Al–Mg–Si alloys is [4,5]:
SSSS →GP →␤
→␤
→␤-Mg 2Si
(1)
Here SSSS represents the super-saturated solid solution and GP stands for Guinier–Preston zones.The sequence (1)will be consid-ered in this work.However,some authors give more details about this sequence [5–12]particularly Ravi and Wolverton [5]who gave a detailed inventory of the compositions of the phases contained in an Al–Mg–Si alloy.The compositions generally accepted for the most common precipitates are listed in Table 1.
According to the literature [6–9,13,14],the T6temper of the 6XXX alloys involves very thin precipitates.They are ␤  needle-shaped precipitates oriented along the three  100 directions of the matrix.Their size is nanometric and they are partially coherent.
The study presented in this paper includes High Resolution Transmission Electron Microscopy (HRTEM)observations of the investigated 6061-T6alloy in order to characterize the precipita-tion state of the T6temper.These observations will allow defining a precipitate distribution of reference for the initial alloy.From this initial state,thermal loadings are applied on specimens which are thereafter observed by TEM.The investigated thermal loadings will also be applied on tensile specimens in order to evaluate the variation of the resulting mechanical properties.
0921-5093/$–see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.msea.2010.12.011
D.Maisonnette et al./Materials Science and Engineering A528 (2011) 2718–27242719
Table1
Compositions of the precipitates contained in Al–Mg–Si alloys.
Phase Composition
GP zone Mg1Si1
␤  Mg5Si6
␤ Mg9Si5
␤Mg2Si
For experimental convenience,the study will be limited to the solid state of the alloy.This means that the maximum temperature to be used is below582◦C(solidus temperature for the AA6061)and the phenomena occurring in the melting pool of the weld will not be taken into account here.Furthermore,the mechanical characteriza-tions and microstructural observations will be carried o
ut at room temperature after the thermal loading.This will allow the char-acterization of the material at various points of the Heat Affected Zone(HAZ)after welding(and not during the welding process). For that purpose,the required thermal loadings should reproduce the temperature evolution in the HAZ with high heating rates up to200K/s.An experimental device has been specifically developed to meet these requirements.Atfirst,the design of the device will be briefly presented.Then,the results of the mechanical charac-terizations and microstructural observations will be presented and discussed.
2.Experimental procedure
2.1.Experimental heating device
The main purpose of the experimental heating device is to repro-duce on a tensile specimen the thermal history encountered by each point of the heat affected zone during welding of the vessel.The highest temperature to be studied is thus T=560◦C,very close to the solidus temperature of582◦C which should not be reached.To do so,an accurate control of the temperature has been set up.Fur-thermore,the device should be able to reproduce the heating rate observed in the HAZ of an electron beam welding(up to200K/s). This heating rate has been evaluated by measuring it during an instrumented welding experiment.The second aim of the device is to apply a mechanical loading on a
specimen in order to mea-sure the mechanical properties of the material.The mechanical and thermal loadings have to be used simultaneously in order to perform tensile tests at high temperature for further study or to compensate for thermal expansion of the specimen during heating. Therefore,the experimental equipment includes a heating device and a mechanical testing machine.
2.1.1.Design of the device
A convenient method to heat aluminium alloys at very high rate is by Joule effect.Another way would be by induction heating but it is not efficient enough to obtain the required heating rate on alu-minium alloys.For this reason,a resistive heating device has been designed and constructed.In order to measure the temperature of the specimen during heating,a thermocouple has been spot welded on the specimen surface.The strains are measured by means of an extensometer with ceramic tips.
The Joule heating device is a power supply,made of an electrical transformer and a thyristor bridge,providing a continuous current whose intensity is controlled by a thermal controller.Water cooled cables and clamping systems are used to connect the specimen to the heating device.A graphite resistor is added in series in order to increase the potential difference across the generator allowing a good temperature
control.Fig.1.Temperature distribution measured by thermocouples along the tensile specimen.
2.1.2.Specimen design
A specimen heated by using Joule effect reacts as an electrical resistor.Its electrical resistance depend
s on the material electri-cal resistivity and the specimen shape which has to be optimized in order to reach the desired heating rate(up to500K/s).More-over,the temperature must be uniform over the measurement area (between the extensometer tips)and the specimen volume should be large enough for the microstructure to be representative of the alloy in real structures.
A FEM simulation was performed to optimize the size and shape of the specimen.The used software,called Sysweld®was devel-oped by ESI Group.The simulation is carried out by using an electro kinetic model[15].The density d and the thermal conductivity K of the alloy were considered to vary with temperature.A paramet-ric study shows that a diameter of6mm is required to obtain a heating rate up to500K/s.A specimen length of100mm is also required to have a low thermal gradient.Fig.1shows the tempera-ture distribution in the specimen.The gradient has been measured with10thermocouples placed all over the length of a specimen peak-heated to350◦C at a heating rate of15K/s.
2.1.
3.Regulation set-up
The experimental device has been designed to reach high heat-ing rates.An accurate control of the te
mperature is required in order to avoid overshoots.To do so,a PID controller has been used [16–19].The resulting thermal loading is slightly delayed but the heating rate is equal to the desired one.The cooling rate is maxi-mum at the highest temperature(of the order of23K/s at500◦C) and decreases during cooling;it drops to about6K/s when temper-ature becomes lower than150◦C.
2.2.Transmission Electron Microscopy
The experimental device presented previously has been used to heat specimens for both mechanical measurements and TEM observations.Two types of microstructural observations have been carried out during this work.Thefirst one is a detailed observation of the microstructure of the material in the T6temper by means of HRTEM(High Resolution Transmission Electron Microscopy) and the second one by means of classical TEM to compare the microstructure of the alloy for three different states of precipita-tion.They were conducted on a JEOL2010F microscope operating at200kV,which belongs to the Centre Lyonnais de Microscopie (CLYM)located at INSA Lyon(France).
TEM allows only very local observations so it was not intended to measure accurately the volume fraction of the precipitates;also not enough precipitates were analyzed to obtain an accurate mean radius.
2720  D.Maisonnette et al./Materials Science and Engineering A528 (2011) 2718–2724
The samples used in TEM are thin lamellas.A disk with a thick-ness of about200␮m is extracted from the heated specimen by means of a diamond wire saw.Its diameter is then reduced by punching.The disk is thinned to electron transparency(thickness to about200nm or less)by electropolishing using an electrolytic bath composed of20%of HNO3in methanol.The bath is cooled at−30◦C with liquid nitrogen[20].A Precision Ion Polishing System(PIPS) is used in order to accomplishfinal thinning and cleaning by ion milling.Some EDX(Energy-dispersive X-ray spectroscopy)analy-sis were performed with an Oxford Instruments analyzer,using a nanoprobe(about3nm in diameter)in the TEM to estimate the composition of the precipitates in the T6state.
2.3.Mechanical characterization
Tensile tests have been carried out at room temperature on spec-imens previously heated to peak temperatures of200,300,400,500 and560◦C with various heating rates(0.5,5,15,50,200K/s)in order to measure their mechanical properties.The thermal loadings are representative of the thermal histories encountered in EB welding.
Three parameters have been investigated.Thefirst one is the maximum temperature reached during he
ating at a given heating rate(r=15K/s).The second one is the heating rate for a given max-imum temperature(T=400◦C).The third one is the dwell time at T=560◦C.This last study is not representative of a welding opera-tion but will allow understanding the variation of the mechanical characteristics during holding at a given temperature which cor-responds to the solution treatment of the alloy.For each test,the specimen is heated to the required temperature while compensat-ing for thermal expansion,then it is cooled to room temperature andfinally deformed until fracture at a strain rate of10−2s−1.Dur-ing the test,for a strain close to1.5%,an unloading is performed to measure the elastic modulus.
3.Results
3.1.HRTEM observations of the material in the T6temper
The aim of the HRTEM investigation on the AA6061-T6is to mea-sure the size of some hardening precipitates and to evaluate their composition in order to characterize the microstructure of the ref-erence T6state.The precipitates present in this state are hard to see owing to their very small size and because they are partly coherent with the aluminium matrix.HRTEM is thus mandatory to image the precipitates.
reactor pressure vessel
Fig.2(a)shows a TEM picture at high magnification.Two needle-shaped precipitates can be seen:
•Thefirst one is oriented along the[001]direction.Its cross sec-tion is observed making its diameter measurable accurately.The measure gives a diameter of about4nm.
•The second one is oriented along the[100]direction.It is observed lying in the thin foil.
The diffractogram,obtained by using Fourier transform,asso-ciated to thefirst precipitate is shown in Fig.2(b).In addition to the{200}diffraction spots associated with the aluminium matrix, weak aligned spots prove that the atomic state is partially disor-dered as for pre-␤  phases.
At last,an EDX analysis carried out on the needle-shaped precip-itates by means of a3nm probe gives an atomic ratio X Mg/X Si=1.29 (with a standard deviation of0.3).This value is the average result of measurements onfive precipitates.3.2.Classical TEM observations of the microstructural changes
Following the detailed study of the T6temper,the precipitates for various states were observed by means of classical TEM.The aim is to evaluate the evolution of the microstructure(size and vol-ume fraction of precipitates)as a function of the thermal loading previously submitted to the material.Compared to HRTEM,classi-cal TEM is a better way to evaluate the volume fraction because i
t allows a larger area to be observed at lower magnification.How-ever classical TEM is worse than HRTEM to measure accurately the diameter of the precipitates because the images at high magnifica-tion are often fuzzy(a difficulty inherent to the diffraction contrast in conventional TEM).
3.2.1.Comparison of three precipitation states
The reference microstructure of the T6temper is here compared to states observed after a heating up to300◦C and400◦C at a heat-ing rate of15K/s and no dwell time at the maximum temperature.
Fig.3shows three micrographs obtained from representative sample areas for the three investigated states.In the case of the specimen heated to400◦C,some precipitates with a needle shape are present in the picture.These precipitates are very large,with length between65and170nm and a mean value of112nm,and their diameter ranges between5and11nm with a mean value of 7.35nm.The mean values are calculated by taking into account ten precipitates observed on different pictures.However it should be mentioned that the precipitates could be cut by the sample prepa-ration,consequently the length given above should be considered as indicative only.They will be used to compare the precipitation state.
In the two other cases,the precipitates are smaller.Their length is between20and40nm with a mean val
ue of29nm for the T6 temper and between15and40nm with a mean value of25nm for the specimen heated to300◦C.Their diameter ranges between 3.75and4.6nm with a mean value of4.45nm for the T6temper and between2and4nm with a mean value of2.6nm for the specimen heated to300◦C.
3.2.2.Precipitate volume fraction evaluation
The precipitate size can be measured by means of TEM pictures. However,it is much more difficult to determine the precipitate volume fraction.Indeed,projections obtained by TEM correspond to volumetric observations but the thickness of the sample is not known accurately.In order to get a rough estimate of the precipi-tate volume fraction,TEM micrographs were compared to pictures obtained by modeling.A computer software has thus been devel-oped in Matlab to simulate these images.Based on three simple parameters describing the precipitation state,the program can reproduce a needle-shaped precipitate distribution in a sample with a uniform thickness.The three parameters are the volume fraction(f v),the mean radius of the needle precipitates(r avg)and their mean length(L avg).
A Gaussian size distribution is arbitrarily assumed for the radius and the length with a variance of1and36,respectively.The size distributions are discretized in one hundred classes of size.Once the thickness isfixed(illustrations will be given here for a100nm thick material),the total volume is calculate
d and an iterative algo-rithm increases step by step the number of precipitates in each class to obtain the volume corresponding to the desired f v.The pre-cipitates are then shown graphically on a2D view by distributing them uniformly along the three 001 directions of the Al-matrix, which corresponds to the viewing directions of the TEM micro-graphs shown in Figs.2and3.Fig.4compares the precipitation state observed in the specimen heated to300◦C to two modeled states,thefirst one with a volume fraction of3%(Fig.4(a))and the second one with a volume fraction of1.6%(Fig.4(c)).It clearly
D.Maisonnette et al./Materials Science and Engineering A528 (2011) 2718–2724
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Fig.2.HRTEM observations of needle precipitates in AA6061-T6.(a)Lattice image at high magnification;(b)diffractogram(numerical Fourier transform)of the micrograph showing diffraction spots(arrows)arising from the precipitate in addition to the square lattice of the aluminium fcc phase along[001].
appears that f v=3%is not representative of the real precipitation state because it is too dense.The volume fraction of1.6%is obvi-ously closer to the volume fraction observed by TEM.The same type of study carried out for the two other investigated states gives
a similar volume fraction.
3.3.Mechanical characterization
As indicated previously,three parameters have been investi-gated.Thefirst one is the maximum temperature reached at a given heating rate(r=15K/s).The second one is the heating rate for a given maximum temperature(T=400◦C).The third one is the dwell time at T=560◦C.
3.3.1.Influence of the maximum temperature reached at constant heating rate
Thefirst mechanical study carried out at room temperature deals with the influence of the maximum temperature reached at a given heating rate on the mechanical properties of the AA6061-T6.The maximum temperatures are T=200,300,400,450,500and 560◦C at a heating rate of r=15K/s.The variations of temperature with time for these various thermal loadings are shown in Fig.5. The tensile tests are then conducted at room temperature and the corresponding true stress—logarithmic strain curves are shown in Fig.6.The curves obtained for the heated specimens are compared with the curve obtained for the T6temper without thermal loading (black continuous line).It is found that the thermal loading con-siderably influences the mechanical properties of the specimens except for a maximum temperature of200◦C for which the curve (not shown in Fig.6)is exactly the same as that of the T6sample. Indeed,the yield stress Rp0.2decreases from278MPa at T=300◦C to 70MPa at T=500◦C.Increasing the temperature further to560◦C, however,does not change the yield stress.Fig.7illustrates this 75%decrease of the yield stress when the maximum temperature is increased from300to500◦C.The measured values are compared to values from the literature[21]for which the maximum
temper-
Fig.3.Comparison of three precipitation states.(a)T6temper;(b)after heating up to300◦C at15K s−1;(c)after heating up to400◦C at15K s−1.All micrographs were taken along a 100 zone-axis of the aluminium matrix.
2722  D.Maisonnette et al./Materials Science and Engineering A
528 (2011) 2718–2724
Fig.4.Modeling of the precipitate distribution for a reached temperature T =300◦C with r avg =2.6nm and L avg =25nm assuming volume fractions of (a)3%and (c)1.6%and comparison with the real precipitate distribution microstructure observed by TEM (b)displayed at the same scale.The volume fraction of 1.6%is obviously closer to
reality.
Fig.5.Thermal loadings used for the study of the influence of the reached temper-
ature.
Fig.6.True stress—logarithmic strain curves for temperatures up to 560◦C.
ature has been held during 30min.It shows that the yield stress at ambient temperature is strongly dependent on the peak tempera-ture reached during the thermal loading,without a dwell time at the highest temperature,for peak temperature higher than 200◦C.No data without dwell time at the maximum temperature have been found in the literature.
The Young modulus has been also measured for each specimen.It has been measured firstly at the origin of the stress–strain curve and then during the elastic unloading.A mean value is then calcu-lated.It decreases from 68.7GPa for the T6temper to 65.0GPa for the specimen heated to 560◦C which represents a 5.4%decrease.3.3.2.Influence of the heating rate
The second mechanical study investigates the influence of the heating rate on the mechanical properties of the AA6061-T6.The maximum temperature applied here is T =400◦C and the studied heating rates are:r =0.5,5,15,50,and 200K/s.The tempera-ture variation obtained for r =50K/s shows an overshoot of 8◦C which results in a slight decrease of the measured stress.Simi-larly,the temperature of the specimen heated with a heating rate of r =200K/s did not reach T =400◦C but T =362◦C.Consequently,the measured stress for this specimen would be higher than expected.The tensile tests give the true stress—logarithmic strain curves shown in Fig.8.They show that the yield stress Rp 0.2
decreases
Fig.7.Yield stress variation versus reached temperature from measurements (with-out temperature holding)and from the literature (with a 30min dwell time).
D.Maisonnette et al./Materials Science and Engineering A528 (2011) 2718–2724
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Fig.8.True stress—logarithmic strain curves for various heating rates up to200K/s. for every heated specimens compared to the T6temper and the lower the heating rate is,the lower the yield stress of the
material is.More precisely Rp0.2decreases from170MPa for a heating rate of r=200K/s to96MPa for a heating rate of r=0.5K/s.These values have not been compared with literature since no data dealing with the influence of the heating rate has been found.
3.3.3.Influence of holding time at560◦C
The last mechanical study accomplished on the material is con-cerned with the influence of a holding time at high temperature before doing the tensile test at room temperature.This last study compares the mechanical properties of the AA6061-T6after a heat-ing at T=560◦C with and without a dwell time at this temperature. The temperature T=560◦C has been chosen because it is close to the solvus temperature of the␤phase in the␣phase.The chosen dwell time is t=30min and the heating rate is r=15K/s.The mechanical properties obtained for both cases are strictly identical.This result indicates that the dwell time at T=560◦C does not influence the mechanical properties measured on the tested specimens.
4.Discussion
4.1.Precipitation
According to literature[5–9,13,14],the precipitates which are normally present in the T6temper of the AA6061alloy are very thin and their density is quite high.They are small needles of␤  (or pre-␤  )type.They are oriented following the three 100 matrix directions.Some authors[6,10,22]have carried out a detailed study of the␤  phase.It appears that the X Mg/X Si atomic ratio is very often close to1as reported in Table1.However,other authors[23]man-aged to measure a X Mg/X Si ratio higher than1for GP zones and co-clusters contained in an aged6061.In addition,the observed precipitates are only partially coherent as for the pre-␤  phase. Based on these results,it can be assumed that the precipitates con-tained in the studied reference material are pre-␤  or␤  phases (although the X Mg/X Si atomic ratio measured here to1.29is slightly higher than1).Otherwise,Andersen et al.[6]measured needle pre-cipitates with a size of about4nm×4nm×50nm for the␤  phase and20nm×20nm×500nm for the␤ phase.Furthermore,Don-nadieu et al.[8]measured the size of the precipitates contained in a 6065-T6alloy.They obtained a mean diameter of2.86nm.By com-paring these values to those presented in Sections3.1and3.2it can be assumed that the precipitates contained in the studied AA6061 after heating at400◦C are composed of the␤ phase.On the con-trary,the precipitates contained in the6061-T6and in the6061 after a heating at300◦C are smaller.Therefore,the precipitates are probably remaining␤  precipitates for the6061alloy after heating at300◦C.
In addition to that,large intermetallics are visible in the micro-graphs at low magnification,as shown in Fig.9.The size of the intermetallics ranges from50to300nm.These intermetallics formed during the elaboration of the material do not contribute to the hardening of the alloy.An energy dispersive X-ray spectrometry analysis(EDX)proved that their composition type is(Fe–Cr–Mn–Si) and not(Al–Mg–Si)as for hardening precipitates.The structure of these intermetallics was not investigated further.However,it is important to note that the intermetallics do contain silicon,so that the corresponding quantity will not be available for hardening precipitation.
4.2.Mechanical properties
Fig.6showed that the behavior of the material after heating at500◦C is strictly identical to the behavior of the material after heating at560◦C.Thus,it can be assumed that the microstructure is the same in both cases.Furthermore,a tensile test carried out on a specimen heated to560◦C during30min gives exactly the same behavior.This behavior corresponds to the O temper.It is commonly accepted that a long holding time at T=560◦C(solvus temperature of the␤phase in the␣phase)is required to dis-solve the precipitates.Comparison of the true stress—logarithmic strain curves obtained with and without dwell time shows that the mechanical properties are identical.This means that the dwell time at T=560◦C does not change the mechanical properties.The microstructure is therefore identical correspo
nding to the annealed state(or O temper)for which no precipitate is present in the mate-rial.This last result shows that for the heating rate and for the specimens used in this study,it is not necessary to apply a dwell time to reach the O temper.This conclusion is probably not valid in the case of a large structure since the peak temperature at each point within the material would depend on its distance from the closest surface.Another result of this investigation is that the heat-ing rate has an influence on the mechanical properties.By using a higher heating rate,the O temper could not be obtained without a dwell time.
The hardening is due to the precipitates contained within the material.They hinder dislocation glide.For a given volume fraction, hardening is most effective if the precipitates are small(and there-fore more numerous).These small precipitates have been observed by TEM for the T6temper.This microstructure leads to more favor-able mechanical properties than the other investigated states.The behavior observed here is quite close to the one observed by Zain-ul-Abdein et al.[24]on a6056-T4.Then,the microstructure of the specimen heated to300◦C seems to be close to the one observed for the T6temper,which explains the small difference of mechanical properties.If the maximum temperature is further increased,the yield stress Rp0.2decreases significantly as shown in Fig.7.The TEM observations show that this decrease is due to a strongly enhanced growth of the precipitates.The volu
me fraction of the precipitates remains identical so that the precipitate number is decreased.This results in a sharp decrease of the mechanical properties,as high-lighted by the tensile tests.
Concerning the study of the influence of the heating rate,no microstructural observations have been carried out.However,Fig.8 shows a decrease of the mechanical properties for every thermal loading up to400◦C compared to the mechanical properties of the T6temper.This means that the material has encountered a microstructural change for every investigated heating rate.If the heating rate is very low,the microstructural changes as dissolution and growth of precipitates,have more time to occur.Consequently, less precipitates are present(for a constant volume fraction)and the mechanical properties are lower.
The Young modulus has been measured and it has been shown that it decreases slightly compared to the T6state when

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