Two-Stage Aging Process of 7A65 Aluminum Alloy Thick Plate Based on In Situ Resistance Method

doi:10.11900/0412.1961.2023.00461

Acta Metall Sin

2025, Vol. 61

Issue (8): 1153-1164 DOI: 10.11900/0412.1961.2023.00461

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Two-Stage Aging Process of 7A65 Aluminum Alloy Thick Plate Based on In Situ Resistance Method

XIAO Wenlong¹^,²^,³(

), ZANG Chenyang²^,³, GUO Jintao²^,³, FENG Jiawen¹, MA Chaoli¹^,²^,³

^1.Tianmushan Laboratory, Hangzhou 311115, China
^2.School of Materials Science and Engineering, Beihang University, Beijing 100191, China
^3.Yunnan Innovation Institute, Beihang University, Kunming 650233, China

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XIAO Wenlong, ZANG Chenyang, GUO Jintao, FENG Jiawen, MA Chaoli. Two-Stage Aging Process of 7A65 Aluminum Alloy Thick Plate Based on In Situ Resistance Method. Acta Metall Sin, 2025, 61(8): 1153-1164.

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Abstract

With the increasing scarcity of traditional energy sources such as petroleum, the concept of sustainable development has prompted strict demands for energy-saving, economical, and environment-friendly industrial production. As the most important lightweight structural material, the requirements for aluminum alloys in modern industries are also increasing. In particular, 7xxx-series ultrahigh-strength aluminum alloy thick plates have been widely used in aerospace and other fields due to its advantages of lightweight and high specific strength. Currently, the problem of nonuniformity of microstructure and mechanical properties in the thickness direction of the thick plates is still prominent. Therefore, developing a new aging process to overcome the shortcomings of the low efficiency of traditional aging processes and difficult-to-control temperature field is urgently needed. Additionally, the research on aging heat treatment mainly focuses on the performance test and static microstructure characterization. Fewer research works have been carried out on the real-time detection of the transformation of the aging dissolution phase byin situ means. To optimize the two-stage aging process of a 7A65 aluminum alloy thick plate, this study systematically analyzed the aging precipitation behavior of the alloy by in situ electrical resistance analysis, formulated the two-stage aging process of the alloy, and explored the influence law of the aging process on the mechanical properties of the thick plate. The study also explored the strengthening and toughening mechanism of the plate through the microstructure observation and mechanical property test. The isothermal transition curve (i.e., time-temperature-transformation (TTT) curve) of the alloy shows that the “C curve” of η-phase dissolution is observed at 120-220 ^oC. According to the TTT curve, the optimal conditions for the two-stage aging process for the thick plate are determined to be as follows: heat at 121 ^oC for 6 h, and then at 152 ^oC for 19 h. After aging, the electrical conductivity of the plate is higher than 38%IACS, the yield strength is higher than 550 MPa, and the elongation reaches 9%.

Key words: 7A65 aluminum alloy two-stage aging precipitation kinetics microstructure mechanical property

Received: 28 November 2023

ZTFLH:

TG 166.3

Fund: Key Research and Development Program of Zhejiang Privince(2024SSYS0078);Complementary Project of Southwest Aluminum (Group) Co. Ltd(JZKG20190099)

Corresponding Authors: XIAO Wenlong, associate professor, Tel: (010)82338631, E-mail: wlxiao@buaa.edu.cn

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	XIAO Wenlong
	ZANG Chenyang
	GUO Jintao
	FENG Jiawen
	MA Chaoli

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https://www.ams.org.cn/EN/10.11900/0412.1961.2023.00461 OR https://www.ams.org.cn/EN/Y2025/V61/I8/1153

Fig.1 Schematic of the in situ resistance test device (NIA—non-isothermal aging, A and B—electrode flanges, T—temperature, t—time)

Fig.2 Schematics of sampling location (a) and geometric size (b) for tensile specimen (RD—rolling direction, ND—normal direction, TD—transverse direction, H—specimen thickness. Unit: mm)

Fig.3 Isothermal transition (time-temperature-transformation, TTT) curves at different thicknesses of 7A65 Al alloy plates during isothermal aging process (α—phase transition fraction)

Fig.4 Precipitation kinetics curves of specimens at surface layer (a), H / 4 (b), and H / 2 (c) positions in 7A65 Al alloy plates under different temperatures

Table 1 Fitting coefficients (K and n) of the precipitation kinetic curves of 7A65 Al alloy plate specimens at surface layer and at different thickness positions under different temperatures

Fig.5 TEM bright field images and selected area electron diffraction (SAED) patterns (insets) (a-e) and average equivalent diameter and volume fraction (f) of the precipitated phase in the surface of the 7A65 Al alloy plate after isothermal aging at 140 ^oC (GP—Guinier-Preston)
(a) α = 10% (b) α = 30% (c) α = 50% (d) α = 70% (e) α = 90%

Fig.6 TEM (a-e) and HRTEM (f) images of grain boundaries in 7A65 Al alloy plates after isothermal aging at 140 ^oC (Insets in Figs.6c and d show high magnified views of the grain boundary precipitates. PFZ—precipitate-free zone)
(a) α = 10% (b) α = 30% (c) α = 50% (d) α = 70% (e) α = 90%

Fig.7 Two-stage aging processing windows of 7A65 Al alloy plates determined by TTT weight curves

Fig.8 Comparison of hardness (a) and conductivity (b), and tensile properties (c) of 7A65 Al alloy plates at different thicknesses under the optimal two-stage aging process (UTS—ultimate tensile strength, YS—yield strength, IACS—internatonal annealed copper standard)

Fig.9 HRTEM image (a) and fast Fourier transform (b) of the precipitated phase in the surface of the 7A65 Al alloy plate after isothermal aging at 140 ^oC for 23 h

Fig.10 Typical engineering stress-strain curves along RD (a) and digital image correlation (DIC) analyses of speciments of surface (b-d), H / 4 (e, f), and H / 2 (g-i) positions in 7A65 Al alloy plates at strains of 0.03 (b, e, g), 0.06 (c, f, h), and 0.09 (d, i) under optimal two-stage aging process

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