Proceedings of the 9th International Conference on Aluminium Alloys (2004)
Edited by J.F. Nie, A.J. Morton and B.C. Muddle© Institute of Materials Engineering Australasia Ltd494

Effect of Reversion on the Bake Hardening Responseof Al-Mg-Si(-Cu) Alloy
T. Honma1, 4, K. Matsumoto2, Y. Nagai3, M. Hasegawa3, and K. Hono4, 1
12Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571, Japan
Materials Research Laboratory, Kobe Steel, Ltd., 1-5-5 Takatsukadai, Nishi-ku, Kobe 651-2271, Japan3The Oarai Branch, Instiutte for Materials Research, Tohoku Univerisity, Oarai, 311-1313, Japan
4
National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, 305-0047, Japan

Keywords: Al-Mg-Si-Cu, reversion, precipitation, TEM, 3DAP, positron annihilation

Abstract
The effect of reversion treatment on the bake hardening (BH) properties of Al-Mg-Si alloys
has been investigated. When naturally aged Al-Mg-Si(-Cu) alloys are reversion
heat-treated, bake-hardening response is improved. The GP zones that are formed during
natural aging are completely dissolved by the reversion heat treatment, and coarser GPzones and β” precipitates are formed by the subsequent BH treatment. The change in theBH behavior as well as the effect of Cu addition are discussed based on the detailed
microstructural characterization results by transmission electron microscopy (TEM),
positron annihilation lifetime (PAL) measurement, coincidence ٛ oppler broadening
measurement (CDB) and three dimensional atom probe (3DAP) analysis.

1. Introduction
Due to its excellent formability and medium strength after an age hardening, Al-Mg-Si
based 6000 series alloys are considered to be the best candidate for automotive body
sheet materials. In addition to the excellent formability and corrosion resistance, the alloy
must exhibit age hardening during an automotive paint bake cycle, i.e. aging treatment at
170 – 175˚C for approximately 20 – 30 min.
One serious problem of the Al-Mg-Si based alloy is its negative age hardening response
after natural aging [1,2]. Although as-quenched solution treated sample shows substantial
paint bake response, it does not show any bake hardening (BH) after natural aging. Thistwo step aging process has been a subject of many investigations in 1960s and theresearch interest was recently revived due to its practical importance for automotiveapplications. Many investigations have been recently performed to improve the BH
response such as optimization of alloy composition [3,4], addition of Cu [1,5-7], ٛ opplerٛ g[8, 9] and reversion heat treatment [10]. Among these, this study focuses on themechanism of the enhancement of the BH response by reversion after natural aging. In
addition, we attempt to understand the effect of Cu addition in enhancing the aging kinetics
as well as BH response. For this purpose, we have characterized the microstructures ofcommercial Si-excess Al-Mg-Si alloys with and without Cu by transmission electron
microscopy (TEM), positron annihilation lifetime (PAL) measurement, coincidence ٛ oppler
broadening measurements (CDB) and three dimensional atom probe (3DAP).

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2. Experimental
SampleTernary alloy
Cu-containing
alloy

Table.1: Chemical Composition.
unitMgSiCuFeMn
wt.% 0.6
1.0
0.15 0.08
at.% 0.67 0.96
0.07 0.04
wt.% 0.5 1.05 0.7 0.15 0.08
at.% 0.56 1.01 0.29 0.07 0.04

Cr0.08
0.04

Al
Bal.
Bal.
Bal.
Bal.

The chemical compositions of two alloys used in this study are shown in Table.1. Bothalloys contain excess amount of Si with respect to the stoichiometry of Mg2Si. One sample
contains 0.3 at% Cu (Cu-containing alloy), and the other does not contain Cu (ternary alloy).
The ternary and Cu-containing alloys were cut into square rods and 3 mm disks. The
ternary and Cu-containing samples were solution treated at 550˚C and 535˚C, respectively,
water quenched and then preaged at 70˚C for 2 h following the industrial process. Thesamples were then stored at a room temperature for longer than three months (naturalaging). For reversion treatment, the samples were soaked into an oil bath of 250˚C for 30 s.
These samples were aged at 170˚C for 20 min (BH treatment), simulating the paint bakecycle in an automotive manufacturing process. For comparison, some samples were BH
treated from the as-quenched state and after natural aging without reversion.
CDB method was employed to identify the solute-vacancy interaction by measuring themomentum distribution of the core electrons around the vacancies where positrons aretrapped. The overall energy resolution is about 1.1 keV in full width at half-maximum(FWHM). A total count of more than 2×107 for each measurement was accumulated for 12h.
Positron annihilation lifetime (PAL) was measured simultaneously with CDB measurement
using a conventional fast-fast spectrometer with a time resolution of 190 ps in FWHM. The
source of 22NaCl deposited on a thin Kapton foil was sandwiched between two identicalalloy specimens. The lifetime spectra
were measured at 150K, and the totalcounts
were4×106foreachmeasurement of 12 h duration. TEM
specimens were prepared by theconventional twin jet method. TEM
observations were performed using aPhilips CM200 TEM operated at 200kV. The tomographic atom probe (TAP)and the energy compensated TAP(ECTAP) were employed for thecharacterization
ofelementaldistributions. The samples were cut
to 1×1×10 mm rods by a spark cutter
and needle shaped specimens werepreparedusingthestandard
electropolishingtechnique.
Atom-probe analyses were performedat a specimen temperature of ~20-30K Figure 1: Vickers hardness and positron lifetime withvarious heat treatment conditions. The positron
in a ultrahigh vacuum (10-11 Torr) using lifetime of well annealed pure Al exhibits 168 [ps].
a pulse fraction of 15 %.

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3. Results and Discussion
Figure 1 shows Vickers hardness and PAL measurements results for various heat treatment
conditions. Both alloys show increase in hardness after natural aging with a slightly higher
hardness in the Cu-containing alloy. The hardness decreases by ∆Hv~20 for both alloys
after the reversion treatment, and the hardness level of the ternary alloy is nearly the sameas that of the as-quenched sample. The hardness of the reverted Cu-containing alloy ishigher than that of the as-quenched alloy by ∆Hv~10. BH treatment right after the reversion
treatment brought an increase in hardness of nearly ∆Hv~6. However, the BH treatment
applied to the naturally aged sample does not bring high BH response, namely, ∆Hv~13 forthe ternary alloy, and ∆Hv~7 for the Cu-containing alloy. For comparison, the hardness ofthe samples that were BH treated immediately after a solution treatment were alsomeasured (AsQ/BH). The Cu-containing alloy shows substantial hardening (∆Hv~56), but
the ternary alloy shows much lower BH response (∆Hv~21).
Positron annihilation lifetime of each stage is also shown in Figure1. Although the solution
treatment temperature for the ternary alloy was higher by 15˚C than that for theCu-containing alloy, the positron lifetimes of both the samples are almost the same (224ps).
This lifetime is much longer than that for the well-annealed pure aluminum, 168ps [11],suggesting that a large number of quenched–in vacancies of the order of 10-5 in atomic
fraction survive. It is interesting to note that the positron lifetime becomes shorter in theCu-containing alloy than in the ternary alloy on aging. After preaging and natural aging, thepositron lifetime of the ternary alloy does not change much, but those for the Cu containing
alloy decreased from 224 to 217 ps. This suggests that a part of quenched-in vacancies
annihilate in sinks in the Cu-containing alloy during preaging and the subsequent naturalaging. On the other hand, in the ternary alloy, vacancies appear to be trapped with somesolute atoms, hindering annihilation at vacancy sinks. After reversion, the positron life timedecreased to 210ps in the Cu-containing alloy, 212ps in the ternary alloy. BH treatment ofboth reverted and naturally aged samples increase the lifetime slightly, suggesting that
positron trap sites were created by the precipitation of the metastable phase that forms by
the BH treatment. However, the lifetime of the Cu-containing sample that was BH treated
immediately after quenching decrease to 208 ps. The decrease in the lifetime of the ternaryalloy is more moderate, from 224ps to 216ps.
3.2 CDB Spectrum

Figure 2: CDB ratio curves for various heat treatment conditions (a) ternary and (b) Cu-containing alloys.
Signals for Pure Mg, Pure Si and Pure Cu are superimposed experimental results.

Figure 2 shows CDB spectra with various heat treatments. The CDB spectra obtained fromboth ternary and Cu-containing alloys are similar and their changes after the heat

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treatments are not so significant. This is because Mg and Si atoms are not sensitive to theCDB method because of their similar electronic shell structures to Al, which are in contrast
to the case of Cu. However, the absence of the characteristic curve for Cu in theCu-containing alloy indicates that vacancies are not bound to Cu atoms. This suggests thatthe rapid recovery of vacancies in the Cu-containing alloy shown in Figure 1 is not due tothe Cu-vacancy interaction. So the kinetics enhancement due to the Cu addition is not
because of the formation of Cu-vacancy complex.
3.3 Room Temperature Aging
TEM images of naturally aged samples are shown in Figure 3. Note that both samples weresubject to preaging at 70°C for 2 h. Tiny spherical clusters or Guinier Preston (GP) zones
are observed. These microstructures are very similar to that of previously observed images[12,13]. The GP zones in the Cu-containing alloy are much smaller in size with a larger
number density than those in the ternary alloy, which would be the reason for the slightdifference in hardness after natural aging.

Figure 3: Bright field images and corresponding [001] SAED patterns ofnaturally aged (a) ternary and (b) Cu-containing alloys.

3.4 Bake Hardening Behavior
Figure 4 shows the Hv change of as-quenched, naturally aged and reverted samples
plotted as functions of isothermal aging time at 170˚C. The vertical dotted line shows theBH condition (20 min at 170˚C).
Industrial requirement is to obtain thehighest hardness (strength) in the BH
condition from the lowest initialhardness. The age hardening response
within 20 min of aging is the highest in
the as-quenched Cu-containing alloy.
On the other hand, as-quenched
ternary alloy shows rather slow
age-hardening response in the initialstage. This clearly demonstrates thatCu enhances initial age hardeningresponse. Naturally aged samples bothshow softening in the initial stage, andno bake hardening response is Figure 4: Isothermal age hardening curves foras-quenched, reversion treated and naturally aged
obtained in the BH condition. The samples, which were aged at 170˚C.

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Cu-containing sample shows more adverse effect. On the other hand, the reverted samples
show improved BH response (∆Hv~26 for the ternary alloy and ∆Hv~21 for theCu-containing alloy).

Figure 5: Bright field images and corresponding [001] SAED
patterns of ternary alloy (a) reversion treated, (b) naturally aged
followed by BH, and of Cu containing alloy (c) reversion treatedand (d) naturally aged followed by BH.

Figure 5 shows TEM bright field images of the sample that was BH treated after reversion
and the one that was BH treated without reversion after natural aging. The precipitate
particles observed in the BH treated samples after reversion are larger than those in the BH
treated samples after natural aging. The SAED pattern in Figure 5 (a) indicates that β”
precipitates are present. Since TEM image show spherical particles, we conclude that both
GP zones and β” precipitates coexist in this specimen
The Cu-containing alloy that was BH treated after reversion shows much finer
microstructure than the ternary alloy with the same heat treatment. The SAED patternshows that the precipitates are β”, although bright field image does not show clear rod-likefeature of β” precipitates. Thus we can conclude that the precipitates in the reversion/BH
treated Cu-containing alloy are mostly β”. The BH treated ternary alloy without reversion
show fine spherical particles. No diffraction evidence for the presence of β” precipitates isseen, hence we conclude that the sample contains only GP zones. On the other hand, theSAED pattern of Cu-containing alloy shows the presence of β” precipitates in addition to theGP zones. Streaks are observed along the <100> matrix directions in Figure.5 (a), (c), and(d), which have been attributed to the β“ phase [1,12,13,14-17].

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Figure 6: Elemental mapping of Mg, Si and Cu in (a) ternary and (b) Cu-containing alloys, aged at 170˚C for
20min (BH treated) after reversion treatment.

Figure 6 shows 3DAP map of Mg, Si, and Cu atoms obtained from (a) ternary and (b)Cu-containing alloys that were BH treated after reversion. The box size is approximately ~8×8×40 nm, and a probe direction is parallel to the matrix [022] direction. The TEM images
for these samples are shown in Figure 5 (a) and (b), respectively. The size of the sphericalparticles observed in Figure 6 (a) is approximately 4 nm, which is consistent with the BF
image in Figure 5 (a). In addition to the spherical particles, precipitates elongated to the[001] direction are also seen in this map, which are believed to be needle like β“ phase. On
the other hand, only elongated β“ precipitates are observed in the Cu-containing alloy
parallel to the [001] matrix direction. Note that Cu is partitioned into β“ phase in this stage,in agreement with the previous atom probe results [12].

4. Summary
Transmission electron microscopy, three dimensional atom probe and positron annihilation
techniques were employed to study the effect of reversion on the BH response of Si-excess
Al-Mg-Si(-Cu) alloys. Although the vacancy concentrations were estimated to be nearly thesame in the as-quenched state of both ternary and Cu-containing alloys based on thepositron annihilation lifetimes, the vacancy concentrations are lower in all heat treated
Cu-containing alloys. Rapid decrease of vacancies in the Cu-containing alloy during
preaging and natural aging as well as BH treatment from the as-quenched state explainsobserved pronounced BH response from the as-quenched Cu-containing alloy. However,CDB results did not indicate that this rapid decrease of vacancies in the Cu-containing alloy
was due to the formation of Cu-vacancy complex. Both alloys did not show any BH
response after natural aging, but the hardness of the Cu-containing alloy after BH wasslightly higher because of higher initial hardness after natural aging. Reverted samples
showed pronounced BH response in both samples compared to the naturally aged ones. Inthe Cu-containing alloy, the precipitates formed after BH treatment were all β“, while GPzones and β“ precipitates coexist in the BH treated ternary alloy.
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