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Ô³çèêî-õ³ì³÷íà ìåõàí³êà ìàòåð³àë³â. – 2015. – ¹ 3. – Physicochemical Mechanics of Materials

INFLUENCE OF RRA TREATMENT ON MICROSTRUCTURE
AND STRESS CORROSION CRACKING BEHAVIOR
OF SPRAY FORMED 7075 ALLOY

RUI-MING SU, YING-DONG QU, RONG-DE LI, JUN-HUA YOU
School of Material Science and Engineering, Shenyang University of Technology, Shenyang, China

The effects of retrogression via under pre-aging on microstructure, mechanical properties
and stress corrosion cracking behavior of spray formed 7075 aluminum alloy were investi-
gated by transmission electron microscope, tensile test and slow strain rate test. The results
show that with under aging at 120°C for 16 h as pre-aging, the strength of the alloy can
maintain at a high level and grain boundary precipitates are discrete after retrogression and
re-aging treatment. However, retrogression treatment is uncontrollable by shortened retro-
gression period. After retrogression at 200°C for 8 min and re-aging, the ultimate tensile
strength, elongation and SCC index of the alloy is 791 MPa, 8.5% and 0.155 respectively.
Keywords: spray forming, 7075 alloy, under aging, retrogression, stress corrosion
cracking, strength.
The 7075 (Al–Zn–Mg–Cu) alloys have been widely used in the aerospace industry,
due to their desirable specific mechanical properties [1–4]. Until spray forming has
been used on 7075 alloy in 1990s, the strengths are elevated to over 730 MPa [5, 6].
Many papers have reported the effects of heat treatments on 7075 alloy which belongs
to aging strengthening aluminum alloy. Silva et al. [7] and Ricker et al. [8] report that
both high strength and corrosion sensibility are obtained in 7075 aluminum alloys after
T6 treatment. The authors [9–11] find loss of strength of about 10…15% after T73,
T74 or T76 treatment in their studies of over-aging on corrosion resistance of 7075
alloys.

For the contradiction between strength and corrosion resistance, Cina [12] (Israel
Aircraft Industries Ltd., 1974) present a three-stage treatment (retrogression and re-
aging, RRA). In next researches [13, 14] find that the strength is maintained at T6 level
by RRA treatment and the stress corrosion cracking (SCC) resistance is close to T7 at
the same time. Su et al. [15, 16] report that the RRA treatment also can improve the
intergranular corrosion and exfoliation corrosion sensibility of a spray formed 7075
alloy.
The RRA treatment is divided into pre-aging, retrogression and re-aging. Ohnishi
[17, 18] consider the peak aging to be the best pre-aging in RRA treatment process.
This type of pre-aging has been used until now. In recent years, scholars report some
different opinions. Lin [19] mentions that the peak aging is not a perfect pre-aging in a
U.S. patent and then Han et al. [20] also discover the similar conclusion. With regard to
pre-aging in RRA treatment, some arguments in the academia and studies on the type
and reason of pre-aging have not been reported.
The common retrogression is always treated at high temperature for a short time,
which is only dozens of seconds and even several seconds. Wu et al. [21] and Reda et al.
[13] find that the sufficient retrogressed effects can be received by long-time retrogressi-
ons at a temperature below 200°C but the mechanical properties are lost at the same time.

Corresponding author: YING-DONG QU, e-mail: quingdong@163.com

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