Journal of Materials Science and Chemical Engineering, 2013, 1, 57-60
http://dx.doi.org/10.4236/msce.2013.15012 Published Online October 2013 (http://www.scirp.org/journal/msce)
The Heat Tr eatment Behavior o f Super-High Strength
Aluminum Alloys by Spray Forming
Guowei Zhang1,2, Zheng Chen 1, Xiurong Zhu2, Gang Chen2, Jing Zhai2, Anzhen Guo2,
Li Ma2, Wei Chen2, Haiying Xin2
1Northwesten Polytec hnical University, School of Material Science and Technology, Xi’an, China
2The Ningbo Branch of Ordnance Science Institute of China, Ningbo, China
Received June 2013
ABSTRACT
In order to understand the stress corrosion behavior of super-high strength aluminum alloys by spray forming, different
aluminum alloys by differ ent heat treatment was made. The results showed that the alloy with peak aging has the most
sensitive stress corrosion cracking, the crack can even be seen using eyes; the alloys with two step aging were better
than one step aging alloys, the alloys has not been found stress corrosion cracking.
Keywords: Super-High Aluminum Alloys; Spray Forming; Stress Corrosion Crack
1. Introduction
Aluminum alloys especially super-high aluminum alloys
often restrict to use as structural materials because of
stress corrosion cracking. But their properties can be im-
proved by appropriate heat treatment such as two step
aging and retrogression and reaging (RRA). And some
other attempts such as composition modification and use
new processing methods were made to further increase
the properties of the alloys.
Spray forming as a promising method was used to
manufacture high properties aluminum alloys such as
7000 series aluminum alloys. Spray forming processing,
which combines the advantages of rapid solidification,
homogeneous microstructure containing fine grains and
avoiding the macro segregation, and high solute content,
has already been applied to improve the properties of
many alloys. Thus, it is possible to enhance the tensile
strength values of the Al-Zn-Mg-Cu series alloys by
spray forming technique. The addition of Mg, with low
cost and low density, has been used together with Zn to
form the phases of η′ (MgZn2), which can enhance the
strength [5-7]. Furthermore, the content of the Zr, Mn, Ni
between 0.1% and 0.9% is acceptable for increasing me-
chanical properties [8]. However, the stress corrosion
cracking properties limited the alloys to further applica-
tion.
In this paper, the stress corrosion cracking properties
of the alloys by spray forming were researched. And the
microstructure and mechanical properties of the alloys
were introduced.
2. Experiment Procedures
The nitrogen was used of the spray forming equipment as
atomizer gas. The spray pressure was between 0.4 - 0.7
MPa, the atomizer scan frequency was between 20 - 25
Hz, the collector round frequency was between 3 - 5 Hz
and the deposited distance was 650 - 750 mm. The me-
dian frequency induction furnace was used as the melting
furnace. The nominal chemical composition of the Al
alloy was given in Table 1. The N2 gas was used as re-
finer and the spray temperature was 830˚C - 850˚C. Al-
Cu master, pure Mg and Zn were used in this experiment.
Billets of spray forming were shown in Figure 1. The
billets size were between (ф220 - ф280)* (300 - 350)
mm. 2000 t reverse extruding machine was used to ex-
trude the billets. Microstructure and mechanical pro-
perties of the alloy were investigated by S3400-N scan-
ning electron microscopy and Instron Model 5585 re-
spectively.
3. Results and Discussion
Table 2 was the tensile strength and elongations of the
alloys by spray forming with dif ferent heat treatment. We
can see that the tensile strength was 810 MPa and the
elongation was 4% of the alloy with peak aging. The
tensile strength was 710 MPa and the elongation was 8%
of the alloy with two step aging. And the tensile strength
was 800 MPa and the elongation was 6% of the alloy
with retrogression and reaging. Figure 2 was the micro-
structure of the alloys with diff erent heat treatment.

G. W. ZHANG ET AL.
58
Table 1. The main chemical composition of tested alloy.
Elements Zn Mg Cu Zr Mn, Ni Al
Content(%) 11 - 13 2.2 - 2.6 1.0 - 1.3 0.1 - 0.3 0.2 - 0.6 Bal.
Table 2. The mechanical properties of the alloys with dif-
ferent heat treatment.
Heat treatment Peak aging Two step aging RRA
Rm/ MP a 810 710 800
A/% 4 8 6
Figure 1. Billets of Al alloy by spray forming.
We can see that the main precipitate phases were G.P
zones at peak aging and retrogression and reaging. While
the main precipitate phases were MgZn2 at two step ag-
ing. The G.P zon es are coherence with the matrix, so the
strength of the alloys were enhanced in big extent, but
the MgZn2 phases were coarsen and did not coherence
with the matrix, so the tensile strength decreased in big
extent.
The alloys were put at air condition for 12 months.
And we found that the alloy with peak aging was cracked.
Figure 3 was the crack of the alloy with peak aging.
From the photos we can see that the crack was inter-
granular, and the crack pattern like river. It’s typical
stress corrosion cracking. And the other two alloys didn’t
found crack even in scanning electron microscopy. So
the peak aging heat treatment can not use for high
strength aluminum alloys.
According to the theory of Mg-H composite, the inte-
ractional of Mg-H lead to the increase of H concentration
at grain boundary, th e segregation of H at grain bound ary
would decrease the binding energy of grain boundary, so
the crack would spread quicker. So the interaction of
Mg-H composite maybe the reason of stress corrosion
cracking. In order to improve the stress corrosion crack-
ing resistance of 7000 series alloys, appropriate heat
(a)
(b)
(c)
Figure 2. the microstructure of the alloys by spray forming
with different heat treatment; (a) Peak aging; (b) Two step
aging; (c) Retrogression and reaging.

G. W. ZHANG ET AL.
59
Figure 3. The crack of the alloy with peak aging.
treatment was needed to avoid the segregation of high
concentration Mg. For example, two step aging which
increases the second aging temperature can improve the
stress corrosion cracking resistance of the alloy. That
means the sensitivity of stress corrosion cracking was
low at the second peak aging, this is because the concen-
tration of Mg at the grain boundary was reduced with the
second aging treatment. In addition, the precipitates
phases in the alloy would change, that means α (supersa-
turation solid solution) → G. Pzone → η′ (MgZn2, me-
tastable) → η (MgZn2). The fraction of η′ (MgZn2) phas-
es would increase all the time with the phases change
going, so the free Mg in the grain would deplete, so the
poor Mg zone would form. On the other side, the free Mg
at the grain boundary would change little, so the rich-Mg
formed. Because of the concentration difference, the free
Mg at the grain boundary would diffuse to the inner of
the grain by vacancies, so new η′ (MgZ n 2) phases at the
poor Mg zone would form, at last the free Mg at the grain
boundary would decrease. At the same time, the η′
(MgZn2) phases would form at the grain boundary, so the
free Mg at the grain boundary decreased further. So
there’s three results: 1) η′ (MgZn2) phase was the trap of
free H, so the concentration of H at the grain boundary
was reduced which improve the stress corrosion cracking
resistance; 2) the stress of grain boundary fracture and
the binding energy of grain boundary were increased
because of the reduce of Mg segregation, so the Mg brit-
tle at grain boundary was decreased; 3) because of the
decrease of free Mg, the interaction of Mg-H was re-
duced, so the segregation of H at the grain boundary was
reduced, so the hydrogen brittleness was reduced.
When the second aging temperature was increased,
like two step aging and retrogression and reaging, the
activation energy of the atom diffuse was decreased, so
the hydrogen and Mg composite were interacting more
intensity. So the stress corrosion cracking of the alloys
were improved. So the two step aging and RRA technics
would be used to use the high strength aluminum alloys.
4. Conclusions
1) The tensile strength and elongations were 810 MPa,
4%; 710 MPa, 8%; 800 MPa, 6% respectively at peak
aging, two step aging and retrogression and reaging.
2) The main precipitate phases were G.P zones at peak
aging and retrogression and reaging. While the main pre-
cipitate phases were MgZn2 at two step aging.
3) The alloy with peak aging was appeared stress cor-
rosion cracking while the other two alloys did not appear
stress corrosion cracking.
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