Published in the Proceedings of the 16th International Aluminum Alloys Conference (ICAA16) 2018
ISBN: 978-1-926872-41-4
by the Canadian Institute of Mining, Metallurgy & Petroleum

EFFECT OF PRE-AGING ON ARTIFICIAL AGE-HARDENING BEHAVIOR OF Al-Mg-Si
ALLOYS WITH Mg + Si = 1.5 MASS%*S. Tsukamoto1, S. Nakamura1, K. Fujiwara1, S. Hirosawa1, Y. Koshino2, and Y. Aruga2
1

Yokohama National University,
Yokohama, Kanagawa, Japan(*Corresponding author: tsukamoto-shuntaro-rm@ynu.jp)
2

Materials Research Laboratory, Kobe Steel, Ltd.
Kobe, Hyogo, Japan
ABSTRACTIn this study, hardness test, differential scanning calorimetry (DSC) and transmission electron
microscopy were carried out to investigate the age-hardening behavior of Al-Mg-Si alloy with a Mg+Si
concentration of 1.5 mass%, and a Mg/Si ratio of 1.0. Pre-aging treatment was conducted for thesolution-treated and then water-quenched specimen at 100, 80, 60, 40, 20 (natural aging), 0 and -20 ˚C for
20 min or 5 h, followed by artificial aging at 170 ˚C for 20 min. It was found that when pre-aging time isprolonged from 20 min to 5 h, the bake-hardening response is deteriorated irrespective of the pre-aging
temperature. In particular, the specimens subjected to pre-aging at 20-40 ˚C for 5 h hardly increased thehardness with the subsequent artificial aging. From the results of DSC measurement, it was suggested thatas pre-aging temperature is lowered, the amount of solute clusters formed during pre-aging, i.e. Mg-Si
clusters and Si-rich clusters, is decreased, but the ratio of Si-rich clusters is increased, resulting in thedeteriorated bake-hardening response.

KEYWORDS
Al-Mg-Si alloy, Bake-hardening response, Pre-aging, Precipitation, Solute clustersINTRODUCTION
In recent years, Al-Mg-Si alloys have been widely used for automotive body panels due to their
excellent characteristics such as high strength-to-weight ratio and good formability. Their strength isincreased not only by artificial aging during paint-bake treatment; i.e. “bake-hardening”, but also when
kept at room temperature (RT); i.e. “natural aging”, in association with the suppressed bake-hardening
response. Such a “negative effect of two-step aging” has been a serious problem because Al-Mg-Si alloy
coils are unavoidably maintained at RT in the manufacturing process of body panels; e.g. shipping andstorage. However, it was reported that pre-aging above 70°C before the two-step aging can restore thebake-hardening response, and such an improvement is related to the formation of solute clusters (Sato,2006; Abid et al., 2010; Takaki et al., 2014; Zandbergen et al., 2015). From the comprehensive analysis by
three-dimensional atom probe (Aruga et al., 2014 and 2015), it is believed that Mg-Si clusters formed
during pre-aging transform into the strengthening β" phase, whereas Si-rich clusters formed during
pre-aging act as a detrimental precursor of β" phase. In this study, it was aimed to investigate the effect ofpre-aging conditions, especially pre-aging temperature and time, on the formation of solute clusters during
pre-aging, in order to elucidate the mechanism of the age-hardening behavior of an Al-Mg-Si alloy.
EXPERIMENTAL
Al-Mg-Si alloy sheet with a Mg+Si concentration of 1.5 mass% and a Mg/Si ratio of 1.0 wasutilized. The chemical composition of the alloy is listed in Table 1 with its abbreviation for mass% of Mg
and Si (i.e. 7M8S indicates ~0.7 mass%Mg and ~0.8 mass%Si). All specimens were subjected to solution
heat treatment at 560°C for 30 min and then water-quenched into RT. Pre-aging treatment at 100, 80, 60, 40,20, 0 and –20°C for 0 min (i.e. directly artificial-aged), 20 min or 5 h was carried out, followed by artificialaging at 170°C for 20 min without natural aging. The applied aging conditions were designated asartificial-aged ‘Az’, pre-aged ‘Px°Cy’ and pre-aged + artificial-aged ‘Px°CyAz’, respectively (The subscripts x,y and z represent pre-aging temperature, pre-aging time and artificial aging time). Vickers hardness of eachspecimen was measured by a Matsuzawa MMT-X1 tester with a load of 4.9 N and a dwell time of 15 s.
Differential scanning calorimetry (DSC) was carried out with a constant heating rate of 10°C/min.
Table 1. Chemical composition of the investigated alloy
Alloy
7M8S

Elements (mass%)
Mg

SiAl

Mg+Si(mass%)

0.73

0.81

Bal.

1.54

Mg/Si
1.04

RESULTS AND DISCUSSION
Figure 1a shows the attained Vickers hardness of Px°C5hA20min specimens. The result of A20minspecimen is also shown for demonstrating their negative effect of two-step aging. HV(AQ), ΔHV(P) andΔHV(A) represent the hardness in the as-quenched condition, the increments of hardness during pre-aging
and artificial aging (i.e. bake-hardening response), respectively. It is obvious that the hardness ofP0-100°C5hA20min specimens is smaller than that of A20min specimen and the smallest hardness is obtained
when pre-aged at 20°C. On the other hand, P-20°C5hA20min specimen exhibits the same attained hardness asA20min specimen, suggesting that the storage in refrigerator does not affect the age-hardening response ofthis alloy. From Figure 1b where ΔHV(P) and ΔHV(A) are compared, it was found that as pre-aging
temperature is lowered, ΔHV(P) is decreased in both of two temperature ranges; i.e. higher temperature
range from 100 to 60°C and lower temperature range from 40 to –20°C, whereas ΔHV(A) is increased onlyin lower temperature range (cf. In higher temperature range, higher ΔHV(A), in comparison with
P20°C5hA20min (natural aged) specimen, was obtained irrespective of pre-aging temperature).

Figure 1. (a) Attained Vickers hardness of A20min and Px°C5hA20min (x=100 to –20) specimens withcomposition of 7M8S. The contribution of ΔHV(P) and ΔHV(A) is compared in (b)Figure 2 represents the attained Vickers hardness, ΔHV(P) and ΔHV(A) of A20min and Px°CyA20minspecimens (x = 100, 60 and 40, y = 20 min and 5 h). It is obvious that when pre-aging time is prolonged,
the attained hardness is decreased, ΔHV(P) is increased, and ΔHV(A) is decreased.
Figure 3 shows DSC curves of Px°Cy specimens (x = 100, 60 and 40, y = 20 min and 5 h).
According to previous studies (Edwards et al., 1998; Zhen et al., 1997; Gupta et al., 2001, Serizawa et al.,2008), exothermic peaks (i), (iii) and (iv) and endothermic peak (ii) can be assigned to the formation of
Mg-Si clusters, β” phase and β’ phase, and the dissolution of Si-rich clusters, respectively. Therefore, it islikely that the area of peak (i) decreases if the amount of solute clusters formed during pre-aging isincreased (in other words, the residual solute concentration in the matrix after pre-aging is decreased),
whereas the area of peak (ii) increases if the amount of Si-rich clusters formed during pre-aging isincreased. In this study, to quantify the area of those peaks, a baseline was rationally assumed for each
DSC curve, and an integrated value of heat flow between the peak-start and peak-end temperatures wascalculated (Although the value may vary to some extent depending on how the baseline is drawn, theauthors believe that the order of the magnitude does not change under the investigated pre-aging
conditions). It was confirmed from Table 2 that as pre-aging temperature is lowered, the area of peaks (i)and (ii) increases either in Px°C20min specimen or in Px°C5h specimen. This suggests that the total amount ofsolute clusters formed during pre-aging is decreased, but the amount of Si-rich clusters, and thus the ratioof Si-rich clusters among those clusters are increased with lowering pre-aging temperature (In fact, nopeaks (i) and (ii) in P100°C y specimen confirm that only Mg-Si clusters are formed during pre-aging at100°C). On the other hand, the amount of solute clusters is increased with increasing pre-aging time
because the area of peak (i) decreases in Table 2, and ΔHV(P) is increased in Figure 2b. In P60°C y and P40°C yspecimens, furthermore, the area of peak (ii) increases with increasing pre-aging time, suggesting that theamount of Si-rich clusters is increased as pre-aging time is prolonged (The ratio of Si-rich clusters appears
to be the highest when pre-aged at 40°C because of the most decreased ΔHV(A) in Figure 2b). This
deduces again that Si-rich clusters formed during pre-aging inhibit the formation of β” phase in thesubsequent artificial aging.

Figure 2. (a) Attained Vickers hardness of A20min and Px°CyA20min (x=100, 60 and 40, y=20min and 5h)
specimens with composition of 7M8S. The contribution of ΔHV(P) and ΔHV(A) is compared in (b)Table 2. Area of peaks evaluated by DSC measurement
P100°C 20minP100°C 5h
P60°C 20min
P60°C 5h
P40°C 20min
Area of peak (i) (J/g)
Area of peak (ii) (J/g)

00006.31
7.2108.94

30.1
13.2

P40°C 5h
10.5
25.1

Figure 3. DSC curves of (a) P100°C y, (b) P60℃ y and (c) P40°C y (y=20min and 5h) specimens withcomposition of 7M8S
CONCLUSIONS
1.

2.

3.

When pre-aged at 0 to 100°C, an Al-0.73mass%Mg-0.81mass%Si alloy subjected to artificial
aging at 170°C for 20 min exhibits the attained hardness smaller than that of directly agedspecimen (i.e. negative effect of two-step aging). The smallest hardness was obtained whenpre-aged at 20°C.
When pre-aging time is prolonged from 20 min to 5 h, the increment of hardness during pre-aging
ΔHV(P) is increased in the specimens pre-aged at 0 to 100°C, but their attained hardness andincrement of hardness during artificial aging (i.e. bake-hardening response) ΔHV(A) are bothdecreased.
DSC measurement suggests that only Mg-Si clusters are formed during pre-aging at 100°C, and as4.

pre-aging temperature is lowered, the total amount of solute clusters is decreased, but the ratio ofSi-rich clusters among those clusters is increased.
It is considered that the degree of ΔHV(A) in pre-aged specimens is affected not only by the typeand amount of solute clusters (Mg-Si clusters and Si-rich clusters) formed during pre-aging, butalso by the amount of residual solute concentration in the matrix after pre-aging.
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