D.-Q. SHI et al.: EFFECT OF QUENCHING PARAMETERS ON THE MECHANICAL PROPERTIES OF THE 7A04 ...
95–99
EFFECT OF QUENCHING PARAMETERS ON THE MECHANICAL
PROPERTIES OF THE 7A04 ALUMINIUM ALLOY
VPLIV PARAMETROV GA[ENJA NA MEHANSKE LASTNOSTI
ALUMINIJEVE ZLITINE 7A04
Dequan Shi, Kaijiao Kang, Guili Gao
Harbin University of Science & Technology, Department of Materials Science & Engineering 150040 Harbin, China
shidequan2008@163.com
Prejem rokopisa – received: 2015-08-30; sprejem za objavo – accepted for publication: 2016-01-05
doi:10.17222/mit.2015.267
The effects of quenching parameters, including the solid-solution temperature, water temperature of quenching, transfer time
before quenching and delay time after quenching on the mechanical properties of the 7A04 aluminium alloy were studied. The
experimental results showed that the mechanical properties are relatively stable when the solid-solution temperature is at
460–490 °C, but the over-burn will appears once the temperature exceeds 500 °C. When the water temperature of quenching and
the transfer time before quenching are above 40 °C and 20 s, the strengths of the 7A04 alloy will drop remarkably. Before the
delay time after quenching is 8 h, the strengths will drop gradually while the elongation will rise continuously. In the range of
8–24 h there is an opposite change of mechanical properties compared to the previous 8 h. Once it exceeds 24 h, the mechanical
properties will become stable.
Keywords: quenching parameters, 7A04 aluminium alloy, mechanical properties, delay time, transfer time
Preu~evani so bili vplivi parametrov ga{enja, vklju~no s temperaturo raztopnega `arjenja, ~asa prehoda pred ga{enjem in ~asa
zadr`evanja po ga{enju, na mehanske lastnosti aluminijeve zlitine 7A04. Rezultati eksperimentov so pokazali, da so mehanske
lastnosti relativno stabilne, ~e je temperatura raztopnega `arjenja med 460 °C–490 °C; za`gano pa je, ko temperatura prese`e
500 °C. Ko je temperatura vode za ga{enje nad 40 °C in ~as prehoda pred ga{enjem nad 20 s, se trdnosti zlitine 7A04 ob~utno
zmanj{ajo. Pri ~asu zadr`evanja do 8 h po ga{enju, se trdnost stalno zmanj{uje, medtem ko se raztezek pove~uje. V ~asu od
8–24 h je ravno obratna sprememba mehanskih lastnosti, v primerjavi s predhodnimi pri 8 h. Ko ~as prese`e 24 h, mehanske
lastnosti postanejo stabilne.
Klju~ne besede: parametri ga{enja, aluminijeva zlitina 7A04, mehanske lastnosti, ~as zadr`evanja, ~as prehoda
1 INTRODUCTION
7A04 is a high-strength aluminium alloy of the
Al-Zn-Mg-Cu series. Due to the high strength and
hardness, good corrosion resistance and wear resistance,
it has become a key structural material in the aerospace
field instead of the steels.1,2 Generally speaking,
Al-Zn-Mg-Cu series aluminium alloys are produced by
the method of electromagnetic semi-continuous casting,3
and then their properties were improved by the solution
and aging treatment. Therefore, the heat-treatment
parameters will directly affect the mechanical properties
of the 7A04 aluminium alloy. At present, many experi-
mental researches on optimizing the solution and aging
treatment parameters had also been widely reported. For
example, from the aspects of improving both the mecha-
nical properties and the efficiency of heat treatment, A.
L. Ning et al.4 studied the influences of loading the spe-
cimen at high temperature, rapid short-time progressive
solution and high-temperature short-time aging on the
microstructure and mechanical properties of 7A04
aluminium alloy, respectively. J. J. Liu et al.5 investigated
the influence of the solid-solution conditions on the mi-
crostructure and resistivity of Al-Zn-Mg-Cu series alloy
by in-situ resistivity measurement, optical microscopy,
scanning electron microscopy, transmission electron
microscopy. O. N. Senkov et al.6 studied the effect of Sc
additions on precipitation strengthening in a direct
chill-cast Al-Zn-Mg-Cu alloy after natural and artificial
aging. The microhardness, room-temperature mechanical
properties, and phase composition of the alloys were
determined after different steps of aging, and the
strengthening mechanism was discussed. Y. Lin et al.7
studied the effect of non-isothermal cooling aging on the
microstructure and mechanical properties of an
Al-Zn-Mg-Cu alloy, and the tensile strength, yield
strength and conductivity were increased 2.9 %, 8.1 %
and 8.3 % compared to that of the T6 treatment, respec-
tively. T. Marlaud et al.8 studied the influence of alloy
composition and heat treatment on the precipitate com-
position. However, the effects of the quenching para-
meters on the microstructure and mechanical properties
are rarely studied. Therefore, there is an urgent need to
investigate how the quenching parameters affect the
mechanical properties, which will further improve the
mechanical properties and product quality of the 7A04
aluminium alloy.
In this study, the mechanical properties are measured
using a CSS-44300 electronic universal testing machine,
and the microstructures are observed by optical micro-
Materiali in tehnologije / Materials and technology 51 (2017) 1, 95–99 95
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
UDK 620.171.3:669.715:666.9.04 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(1)95(2017)
scopy. The effect of the solid-solution temperature, water
temperature of quenching, transfer time before quench-
ing and delay time after quenching on the mechanical
properties of 7A04 aluminium alloy were studied,
respectively, and the optimal quenching parameters were
also obtained according to the experimental results.
2 EXPERIMENTAL PART
The experimental materials are hot-rolled 7A04
aluminium alloy plates with 12 mm thickness, and their
chemical compositions are shown in Table 1.
Table 1: Chemical composition of 7A04 aluminium alloy, in mass
fractions (w/%)
Tabela 1: Kemijska sestava aluminijeve zlitine 7A04, v masnih
odstotkih (w/%)
Zn Mg Cu Cr Fe Si Mn Ti
6.05 2.62 1.53 0.18 0.16 0.08 0.45 0.02
Because the 7A04 aluminium alloy is not online
quenched during industrial production, it is very im-
portant to choose optimal quenching parameters. In this
study, through independently altering the solid-solution
temperature, the water temperature of quenching, trans-
fer time before quenching and delay time after quench-
ing, the effects of the quenching parameters on the me-
chanical properties of 7A04 aluminium alloy were
investigated, respectively. If the solid-solution tempera-
ture was too high, the over-burn phenomenon in the
microstructure of 7A04 aluminium alloy might appear.
Therefore, the microstructure was observed by the
OLYMPUS-GX71 optical microscopy when the solid-
solution temperature was studied.
The hot-rolled 7A04 aluminium alloy plates were
homogenized at 460±5°C/24 h, and the standard samples
were prepared by horizontally cutting on the plates
according to the GB/T228-2010 of China. The samples
were heated to different temperatures in the salt bath
furnace and the time of holding at temperature was 60
min. Then the samples were quenched according to the
experimental design, and artificial aging treatment of
130±5 °C/24 h was carried out. In order to study the
effects of the quenching parameters on the mechanical
properties, the following experiments were designed.
1) When the water temperature of quenching, transfer
time before quenching and delay time after quench-
ing were kept at 30 °C, 20 s and 2 h, respectively, the
solid-solution temperature was changed from 400 °C
to 510 °C with the step of 10 °C.
2) When the solid-solution temperature, transfer time
before quenching and delay time after quenching
were kept at 470 °C, 20 s and 2 h, respectively, the
water temperature of quenching was changed from 10
°C to 80 °C with a step of 10 °C.
3) When the solid-solution temperature, water tempe-
rature of quenching and delay time after quenching
were kept at 470 °C, 30 °C and 1 h, respectively, the
transfer time before quenching was changed at 5 s
and from 10 s to 60 s with a step of 10 s.
4) When the solid-solution temperature, water tempera-
ture of quenching and transfer time before quenching
were kept at 470 °C, 30 °C and 20 s, respectively, the
delay time after quenching was set to be 1 h, 2 h, 4 h,
8 h, 16 h, 24 h, 48 h and 72 h.
3 RESULTS AND DISCUSSION
3.1 Effect of solid-solution temperature on the mecha-
nical properties and microstructure
Figure 1 shows the change of the mechanical proper-
ties of the 7A04 alloy with solid-solution temperature.
With the increase of the solid-solution temperature, the
tensile strength and yield strength will first increase and
then decrease, but the elongation will drop monotoni-
cally. Before the solid-solution temperature reaches up to
460 °C, the tensile strength and yield strength will ra-
pidly increase. Once the temperature excess 460 °C, the
increase rate will become very slow.
This can be explained as follows. In general, to im-
prove the solid-solution temperature at a certain range is
an effective means to improve the strength. When the
temperature was increased, the rate of dissolving the
second-phase particles will increase. As a result, it can
promote the number of the strengthening precipitation
phase, thus the tensile strength and yield strength will
increase. However, the increase in the temperature will
make the recrystallization grain grow up, which will
result in the decrease of the strength.9,10 Therefore, when
the temperature was reached to a certain value, the
increase rate of the strength will become slow.
The mechanical properties were satisfied when the
solid-solution temperature was kept at 460–490 °C,
which proves that there is a wide range of solid-solution
temperatures for the 7A04 aluminium alloy. But when
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Figure 1: Mechanical properties changing with solution temperature
Slika 1: Spreminjanje mehanskih lastnosti v odvisnosti od tempera-
ture raztopnega `arjenja
the temperature is more than 500 °C, the tensile strength
and yield strength will drop suddenly.
Figure 2 showed the microstructures at 470 °C and
500 °C, respectively. As shown in Figure 2a, the over-
burn was not found in the microstructure. The partially
recrystallized grains appear and they are elongated along
the deformation direction. Some of the residual phase
and the insoluble phase are broken, and they symmetri-
cally arrange along the deformation direction. However,
it can be seen from Figure 2b that the obvious over-burn
and the complex re-melting grain boundaries appear.
According to the relevant literature11-13 the precipitated
phases continuously distribute at the re-melting grain
boundary. The evenly distributed fine precipitated phases
inside the grains are the coherent GP zone and the small
amount of transition phase. In addition, some of the large
particles randomly distribute in the grain and/or the grain
boundaries, and they are the main strengthening phase
and the impurity phase.
3.2 Effect of water temperature of quenching on me-
chanical properties
The effect of the water temperature of quenching on
the mechanical properties of the 7A04 aluminium alloy
was shown in Figure 3. When the water temperature of
quenching is below 40 °C, the tensile strength, yield
strength and elongation are almost unchanged. However,
when the water temperature is more than 40 °C, the
tensile strength and yield strength decrease remarkably,
while the elongation is found to have little change. This
indicates that the 7A04 alloy is sensitive to the cooling
rate.
The effect of the water temperature on the mechani-
cal properties can be attributed to the cooling rate. It is
well known that the precipitation sequence of the
Al-Zn-Mg-Cu series alloy at the ageing treatment is
super-saturation solid solution, GP zone, 
’ phase and 

phase. So, the size, density and distribution of GP zone
are very important for the forming of the 
’ phase. When
the water temperature is below 40 °C, the cooling rate is
large, and there is not enough time for the second phase
to nucleate and precipitate at the grain boundary, which
makes the concentration of solute atoms become higher.
A lot of stable GP zones can form quickly. During age-
ing, many second phases can uniformly precipitate, and
their sizes are not very different. So the strengthening
effect is good, and the mechanical properties of 7A04
alloy are promoted. In contrast, when the water tempe-
rature increases and the cooling rate becomes small,
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Figure 4: Mechanical properties changing with transfer time before
quenching
Slika 4: Spreminjanje mehanskih lastnosti s ~asom prehoda pred
ga{enjem
Figure 3: Mechanical properties changing with water temperature of
quenching
Slika 3: Spreminjanje mehanskih lastnosti s temperaturo vode pri
ga{enju
Figure 2: Microstructure at different solution temperatures: a) 470 °C,
b) 500 °C
Slika 2: Mikrostruktura pri razli~nih temperaturah raztopnega `arje-
nja: a) 470 °C, b) 500 °C
there is enough time for the second phase to nucleate and
precipitate at the grain boundary, and the formation of
second phases will result in a decrease of the salute atom
concentration at the grain boundary. During ageing,
these second phases can continue to grow by absorbing
the solute atoms near the grain boundary. So the GP
zones will become few and instable. Therefore, the
strengthening effect will become weak, and thus the
mechanical properties of the 7A04 alloy will decrease.
Therefore, the water temperature of quenching
should be kept below 40 °C. Actually, in order to avoid
the quenching crack caused by the excessive stress, the
cooling rate must be small. So, the water temperature
must be high, which is contradictory to the requirement
of a low water temperature due to improving the
strength. Consequently, during practical production, the
water temperature of quenching is as low as possible on
the condition that the stress cracks do not occur.
3.3 Effect of transfer time before quenching on the me-
chanical properties
The effect of the transfer time before quenching on
the mechanical properties of the 7A04 aluminium alloy
was shown in Figure 4. When the transfer time before
quenching is less than 20 s, a small effect can be found.
But when the transfer time is more than 30 s, the tensile
strength and yield strength will drop while the elongation
will rise. So the transfer time should be kept within 20 s.
This can be explained as follows. Because of the
additive Mn and Cr, the 7A04 aluminium alloy is sensi-
tive to the quenching. Before the alloy was put into the
water, it was cooled in the air, and the cooling rate is
very small. When the transfer time was extended, there
was plenty of time for the second phase to nucleate on
the grain boundary and grow to a certain extent. Accord-
ing to the Ostwald ripening mechanism14,15 the second
phase can absorb the solute atoms near the grain boun-
dary and continue to grow during aging. It will lead to
the poor area of solute atoms near the grain boundary,
and make it difficult of the precipitation of a new second
phase on the grain boundary. So the distribution of the
precipitated phase on the grain boundary is disconti-
nuous and their size difference becomes too big. On the
other hand, the supersaturated vacancy concentration
after quenching differs in different areas. During the
cooling process, the vacancy will diffuse to the grain
boundary, and it will cause a decrease of the vacancy
concentration near the grain boundary. However, the
vacancy away from the grain boundary has no space to
diffuse, and thus the concentration is relatively higher.
As a result, a concentration gradient was formed. When
the transfer time is longer, more vacancies disappeared
in the grain boundary. During the aging treatment the GP
zone cannot appear in the area that is lower than the
critical vacancy concentration, and thus a precipitation
free zone will form.16–18 So, if the transfer time was
extended, the width of the precipitation free zone would
become large, and the mechanical properties of the alloy
would be deteriorated.
3.4 Effect of delay time after quenching on the mecha-
nical properties
Figure 5 shows the effect of the delay time after
quenching on the mechanical properties of the 7A04
aluminium alloy, which indicated the tensile strength and
yield strength is on the decline as a whole with the delay
time. Within 8 h, the strengths drop gradually and the
elongation rises continuously with the increase of the
delay time. From 8 h to 24 h a rebound of the mechanical
properties appears, and the strengths will rise and the
elongation will drop. When the delay time is more than
24 h, the mechanical properties of 7A04 alloy are stable.
In fact, this obvious delay time effect began once the
quenching was done, and it is related to the dissolution
of the GP zone.19 Along with the extension of the delay
time, a large number of partial poly groups will form,
and thus the concentration of solute elements in solid
solutions is greatly reduced. During the artificial aging,
the GP zones that are less than the critical size will
resolve back to the solid solution, and it will reduce the
number of the precipitation strengthening phases. So, the
strength of the alloy will reduce. But if the delay time
continues to be extended, those GP zones that are less
than the critical size are likely to grow up to the stable
crystal nucleus. Therefore, the delay time effect became
weak, and the mechanical properties of the alloy began
to recover. Considering the practical production, the
delay time after quenching is confined into 2 h. If there
is not enough time, the artificial aging can be performed
after the delay time of 24 h, which also can give ideal
comprehensive performances.
4 CONCLUSIONS
1. In the range of 460–490 °C, the mechanical proper-
ties of the 7A04 alloy are relatively stable. When the
D.-Q. SHI et al.: EFFECT OF QUENCHING PARAMETERS ON THE MECHANICAL PROPERTIES OF THE 7A04 ...
98 Materiali in tehnologije / Materials and technology 51 (2017) 1, 95–99
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
Figure 5: Mechanical properties changing with delay time after
quenching
Slika 5: Spreminjanje mehanskih lastnosti s ~asom zadr`evanja po
ga{enju
solid solution is higher than 500 °C, over-burn may
appear.
2. When the water temperature of quenching exceeds 40
°C or the transfer time before quenching is more than
20 s, the tensile strength and yield strength will drop
remarkably while the elongation will rise.
3. The tensile strength and yield strength will drop
gradually while the elongation will rise continuously
with the delay time after quenching before 8 h. At the
range of 8–24 h there is an opposite change of
mechanical properties compared to the previous 8 h.
Once the delay time exceeds 24 h, the mechanical
properties become stable.
4. The optimal quenching parameters are as follows.
The solid solution temperature is 460–490 °C, and
the water temperature of quenching and the transfer
time before quenching are below 40 °C and 20 s, res-
pectively, and the delay time after quenching is below
2 h or above 24 h.
Acknowledgement
This research was financially supported by Founda-
tion of Heilongjiang Educational Committee (12541107)
and Natural Science Foundation of Heilongjiang Pro-
vince (F201213).
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