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 Ltd258

Bake-Hardenability of Al-Mg-Cu-X (Ag,Zn,Si) Alloy Sheets
Y. Suzuki1, A. Hibino1, T. Muramatsu2, S. Hirosawa3, T. Sato3
1

Furukawa-SKY Aluminum Corp., Technical Research Division, 1351 Uwanodai, Fukaya-shi, Saitama
366-8511,Japan.
2
Furukawa-SKY Aluminum Corp., Technical Research Division, 21-1 Kurome, Mikuni-cho, Itai-gun,Fukui 913-8588, Japan.
3
Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1 O-okayama,
Meguro-ku, Tokyo 152-8552, Japan.

Keywords: Al-Mg-Cu, Al-Mg-Cu-Ag, Precipitation, Age-hardening, Bake-hardening

Abstract
Age-hardening behaviours of Al-3%Mg-1%Cu (in mass%) alloys with and without Ag, Zn
and Si were investigated. The Al-Mg-Cu ternary alloy showed two-stage hardening at170°C; i.e. the first rapid hardening within 2min and the second hardening after 2000min.
The addition of 0.4%Ag accelerated the age-hardening in both the stages. Precipitates inthe Ag-bearing alloys were much finer than those of the Ag-free alloys after peak-ageing.
0.4%Zn and 0.4%Si additions were less effective than Ag in improving the strengths atshorter ageing times. The yield strength after bake-hardening treatment was improved not
only by Ag addition but also by an optimized heat-treatment.
1. Introduction
Al-Mg alloy sheets are utilized in various applications because of their well balanced
properties such as mechanical strength, ductility and press-formability. In the application
as automotive body sheets, however, unfavourable bake-softening is well-known to occur
during paint-baking after press-forming. In contrast, the strength of Al-Mg-Si alloys can beincreased during paint-baking by taking advantage of the precipitate hardening. Therefore,Al-Mg alloys will be more fascinating if they possess the bake-hardenability like Al-Mg-Si
alloys. Some previous researches [1-3] presented that Cu addition makes Al-Mg alloys
bake-hardenable. However, the yield strengths after stretch-baking (BH-YS) of the Al-MgCu alloy sheets were not so high compared with those of advanced Al-Mg-Si alloy sheets(See Table 1).
Table 1: Previously reported data of bake-hardenability of Al-Mg-Cu alloys [1-3]. The comparison with thoseof some advanced 6XXX alloys for automotive body sheets [4] is also listed. BH: 2% stretching and baking(*170°C,20min, ** 180°C,30min, ***170°C,30min)
T4 tensile properties
BH-YS
Ref.
AlloyComposition
UTSYS
EL
[MPa]
[MPa]
[%]
[MPa]
Al-2.5%Mg-0.5%Cu-(Si)
225
90
30
150*
[1]
Al-Mg-Cu
Al-3.6%Mg-1.0%Cu-(Mn)
271
103
34
188*
[2]
Al-4.2%Mg-0.6%Cu
272
10730170**[3]
6XXXAl-Mg-Si
230
12030195***[4]
body sheets Al-Mg-Si-Cu
255
125
31200***[4]

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In this work, a series of experiments has been conducted to develop Al-Mg-Cu alloy
sheets with excellent bake-hardenability. The effects of small amounts of Ag, Zn and Si on
the age-hardening behaviour and bake-hardenability of Al-3%Mg-1%Cu (in mass%) alloys
has also been investigated.
2. Experimental
Five alloys shown in Table 2 were cast in metal molds, and then hot- and cold-rolled to
1mm thick sheets by using laboratory scale equipments. Solution heat treatment (SHT)
was performed in a salt bath mainly at 520°C for 25sec, followed directly by waterquenching. Artificial ageing behavior of the T4 sheets (1week storage at R.T. after SHT)
was investigated mainly at 170°C, which is a general paint-baking temperature for
automotive body sheets. The ageing treatment was given in an oil bath at 170°C for 1200min or in an electric furnace for longer times than 200min. Micro-hardness test,
electrical resistivity measurement, differential scanning calorimetry (DSC) andtransmission electron microscopy (TEM) observation were performed. The yield strengths
were also measured for the specimens not only aged at 170°C for 20min but also 150°Cfor 20min to determine the values of BH-YS. The effects of higher SHT temperatures; i.e.
520-560°C for 30min, was also investigated. Note that the procedure of 2% stretching andageing treatment at 170°C is considered to well correspond to the actual process of pressforming and paint-baking for body sheet materials.
Table 2: Compositions of Al-Mg-Cu-X alloys. [mass%]
Alloy
3Mg-1Cu
3Mg-1Cu-0.2Ag
3Mg-1Cu-0.4Ag
3Mg-1Cu-0.4Zn
3Mg-1Cu-0.4Si

Mg
3.04
2.98
2.91
3.01
2.93

Cu
0.97
0.99
0.99
1.02
0.99

Ag
--0.200.40-----Zn------0.43
---

Si
0.10
0.10
0.10
0.10
0.39

Fe
0.12
0.13
0.12
0.13
0.12

Ti
0.02
0.02
0.02
0.02
0.02

3. Results and Discussion
3.1 Age-hardening Behavior
Ageing time : 20min3Mg-1Cu

140

0.2Ag
0.4Ag

120

0.4Zn

100

0.4Si

80

T460
00.2Ag60mW/g16040000.4Ag

Heat flow

180

3.0Mg-1.0Cu

5000

3000

0.4Zn

2000

←Endo.

Yield strength [MPa]

Exo.→

6000

200

1000

50100

150

200

Ageing temperature [°C]

Figure 1: Yield strength - ageing temperature
curves of the Al-Mg-Cu-X alloys.

0.4Si

02070120

170

220

270

320

Temperature [°C]

Figure 2: DSC curves of the Al-Mg-CuX alloys (Heating rate is 10°C/min).

Figure1 shows the changes in yield strength of the Al-3Mg-1Cu-X alloys aged at varioustemperatures for 20min. No evident age-hardening occurs at 50-70°C in all the260

investigated alloys, whereas considerable hardening takes place at temperatures morethan 90°C.
From the DSC curves in Figure 2, furthermore, the first exothermic peak was found to form
at 90-120°C in all the alloys. This peak is related to the formation of clusters or GPB zones
which contribute to the hardening at a general paint-baking temperature of 170°C. On theother hand, the peak at higher temperatures above 250°C originates from the precipitationof S´ or S phase of Al-Cu-Mg alloy system although the formation of different phases isquite probable in the Ag-bearing alloys.
250

1103Mg-1Cu90
HVYield strength [MPa]

100
200

150

0.2Ag
0.4Ag

800.4Zn
0.4Si

70
100
6050
T4150
10100

1000

10000 100000

T41

Ageing time [min]

10100

1000

10000 100000

Ageing time [min]Figure 3: Changes in yield strength and micro-hardness of the Al-Mg-Cu-X alloys aged at
170 °CThe changes in yield strength and hardness of the Al-Mg-Cu-X alloys during ageing at
170°C are shown in Figure 3. The Al-Mg-Cu ternary alloy exhibits two-stage hardening
behavior; i.e. the first rapid hardening within 2min and the second hardening from~2000min.
It should be noted in Fig.3 that the addition of 0.4%Ag accelerates the hardening in both of
the stages. When compared with the Ag-free and 0.2%Ag-bearing alloys, the 0.4%Agbearing alloy obviously exhibits a faster and larger increases in yield strength andhardness throughout the ageing process. In contrast, the additions of Zn and Si are lesseffective in increasing the strength than the Ag-bearing alloys until prolonged ageing times.
0.3

Changes in ρ [µΩcm]

0.2
0.1
3Mg-1Cu0.0

0.2Ag

-0.1

0.4Ag

-0.2

0.4Zn

-0.3

0.4Si

-0.4
-0.5
-0.6

as quenched
0.1
T4(RT,1week)

1101001000

Ageing time [min]10000

100000(ρ measurement :inliquid nitrogen)Figure 4: Changes in electrical resistivity ρ of the Al-3Mg-1Cu-X alloys during ageing at170°C.

The changes in electrical resistivity of the Al-Mg-Cu-X alloys during ageing at 170°C areshown in Figure 4. The resistivity increase occurs within 1min in all the alloys in relation tothe age-hardening in the initial stage of ageing. The decrease of the resistivity starts earlier

261

in the Ag-bearing alloys than in the other alloys in consistent with the accelerated
hardening to peak hardness by Ag addition.
TEM images of the Al-Mg-Cu-X alloys aged at 170°C for 20000min are shown in Figure 5.
There can be seen an obvious microstructural difference between the Ag-bearing alloys
and the other alloys. Lath-shaped precipitates of several hundred nanometers in length areseen in the Ag-free alloys, whereas fine equiaxed precipitates with the average diameter of~15nm are dispersed in the 0.4%Ag-bearing alloy. The coarsened particles in the Ag-freealloys seems to be S-type precipitates (S or S´ phase) as reported in the previous
researches for the similar Al-Mg-Cu alloys [5,6]. Note that there may be also retained GPBzones in the microstructure. In contrast, the fine particles in the Ag-bearing alloys are
considered to be the same phase as T phase in an Al-4%Mg-1.5%Cu-0.5%Ag alloy [5].
Another possibility of X´ phase [7] may be rejected because the morphology of theparticles in the present study is not identical to that of X´, which is reported as hexagonal
shaped platelet on {111} [7]. It is interesting to know that both of T and S-type phases coexist in the 0.2%Ag-bearing alloy with a tendency that the T particles are not so fine as
those in the 0.4%Ag-bearing alloy. The coarse and inhomogeneous microstructure in the0.2%Ag-bearing alloy must be responsible for the lower peak strength than that of the alloy
with 0.4%Ag. As for the alloys with Zn and Si, furthermore, the precipitates besides the Stype particles probably form because the higher yield strengths were obtained after
prolonged ageing times (Fig.3).
According to the phase diagram of Al-Cu-Mg alloy system at 190°C [4], the composition ofthe ternary alloy in the present study is included in the region of (α+S+T).

3Mg-1Cu

100nm

0.2Ag

0.4Zn

100nm

0.4Si

100nm

0.4Ag

100nm

100nm

Figure 5: TEM images of the Al-Mg-Cu-X alloys aged at 170°C for 20000min.

Therefore, T phase seems to be formed in all the investigated alloys, especially after
prolonged ageing times. However, no T phase was observed even in the ternary alloy,
suggesting that Ag plays a role in stabilizing T phase to some extent. In fact, EDX analysis
for the 0.4%Ag-bearing alloy revealed that the concentration of Ag in the T particles (or262

including the adjacent area) is higher than that within the matrix. To understand such aneffect of additional elements, the detection of nanoclusters formed in the shorter ageingtimes is planed by authors using a three-dimensional atom probe (3DAP).

Yield strength [MPa]3.2 Bake-hardenability
200180160140120100806040200T42%Str150°C,20min
170°C,20min3Mg-1Cu

0.2Ag

0.4Ag

0.4Zn

0.4Si

Figure 6:Yield strength before and after stretch-baking (BH-YS) of the Al-Mg-Cu-X alloys (SHT:520°C, 25sec, Stretching: 2%, Baking: 150°C, 20min or 170°C,20min).

Figure 6 shows the bake hardenability of the Al-Mg-Cu-X alloys sheets. The yieldstrengths after T4 treatment around 80-90MPa are increasing to ~140MPa by 2%
stretching and to over 170MPa by baking at 170°C for 20min. The addition of 0.4%Ag
increases the bake-hardenability of Al-Mg-Cu alloys, resulting in the highest BH-YS above
180MPa. The increase in BH-YS by the additions of 0.2%Ag and 0.4%Zn is quite small
although the addition of 0.4%Si shows no positive effect on the bake-hardenability. It isnoteworthy, furthermore, that baking at 150°C still gives considerable hardening to thealloys. Therefore, this type of bake-hardenable Al-Mg-Cu alloys will be preferable if baking
is employed at low temperatures as results of saving energy and optimizing temperatures
for water-based paints.

SHT temp. 520°Ｃ

Yield strength [MPa]Yield strength [MPa]3Mg-1Cu
220
200180160
140120100806040200540°Ｃ 560°Ｃ3Mg-1Cu-0.4Ag
220200180160 T4
140120 Stretch100 baked
806040200520°Ｃ 540°Ｃ 560°Ｃ

T4Stretch-baked

Figure 7: Yield strength after stretch-baking (BH-YS) of the Al-3Mg-1Cu and Al-3Mg-1Cu-0.4Agalloys. (Stretching: 2%, Baking: 170°C, 20min, SHT:520-560, 30min)

The BH-YS of the advanced Al-Mg-Si alloys for body sheets is ~200MPa, which is higher
than that of the Ag-bearing alloys in Figure 6. To achieve further improvement in BH-YS,therefore, additional experiments have been performed. Figure 7 shows the BH-YS of theternary and 0.4%Ag-bearing alloys solution heat-treated at 520-560°C for 30min. The
Higher SHT temperature leads to the higher BH-YS in both alloys. The Ag-bearing alloy
with SHT at 560°C shows the highest BH-YS above 200MPa, which is comparable to that

263

of the advanced 6XXX alloy. Note that even in the Ag-free alloys BH-YS reaches 190MPa
when SHT is employed at higher temperatures.
4. Conclusions
The investigated Al-3%Mg-1%Cu (in mass%) ternary alloy exhibits two-stage hardening
during ageing at 170°C. The addition of 0.4%Ag accelerates and increases hardness andyield strength in both of the stages because of the remarkable change in precipitates. In
the T6 condition, the Ag-free alloys contain lath-type precipitates of S or S´, while the Agbearing alloys show the denser distribution of fine precipitates of T (or X´). In contrast, theadditions of 0.4%Zn and 0.4%Si are less effective than Ag addition to improve thestrengths.
The Ag addition is also effective to improve bake-hardenability. The BH-YS of the 0.4%Agbearing alloy at 170°C is above 180MPa, which is 15MPa higher than that of the ternary
alloy. Considerable bake-hardening is still observed at 150°C in the alloys and the BH-YS
above 200MPa is obtained by an optimized SHT condition even for the Ag-free alloys.
Acknowledgements
This study has been conducted in “Nanotechnology Metal Project” supported by New
Energy and Industrial Technology Development Organization (NEDO) and The Japan
Research and Development Center for Metals (JRCM).
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