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Acta Metall Sin  2023, Vol. 59 Issue (5): 657-667    DOI: 10.11900/0412.1961.2021.00237
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Effect of Post-Aging on Microstructure and Mechanical Properties of an Ultrafine-Grained 6061 Aluminum Alloy
LIU Manping1,2(), XUE Zhoulei1, PENG Zhen1(), CHEN Yulin1, DING Lipeng3,4, JIA Zhihong3,4
1School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China
2State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha 410082, China
3Key Laboratory for Light-Weight Materials, Nanjing Tech University, Nanjing 211816, China
4School of Materials Science and Engineering, Chongqing University, Chongqing 400030, China
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LIU Manping, XUE Zhoulei, PENG Zhen, CHEN Yulin, DING Lipeng, JIA Zhihong. Effect of Post-Aging on Microstructure and Mechanical Properties of an Ultrafine-Grained 6061 Aluminum Alloy. Acta Metall Sin, 2023, 59(5): 657-667.

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Al-Mg-Si alloys are widely used in automotive body panels and parts of the engine owing to their low density, medium strength, high specific strength, good corrosion resistance and other characteristics. Currently, there are many studies on the precipitation behavior of undeformed Al-Mg-Si aluminum alloy, but there is a lack of research on the precipitation evolution and precipitation strengthening mechanism of ultra-fine grained 6061 aluminum alloy at different post-aging temperatures. In this study, the microstructure and mechanical properties of an ultrafine grain 6061 aluminum alloy produced by combining the equal channel angular pressing (ECAP) and post aging methods was comparatively evaluated via TEM, XRD, microhardness tests, and tensile tests. The results indicated that the average grain size of the alloy after two ECAP passes was refined to 210 nm. The average grain size of the alloy after the ECAP pass at 80oC and 20 min post aging was 278 nm; moreover, the fine needle β'', L phase, and Q' phase precipitates at nanoscale were dispersed in the matrix. Furthermore, the tensile and yield strengths were 514 and 483 MPa, respectively, while maintaining a remarkably uniform elongation of 15.1%. These results indicate that numerous dislocations introduced by ECAP in the matrix provide a location for the nucleation of the precipitate, which accelerates the precipitation kinetics during the post aging process. The high strength and toughness of the ECAP alloy after low temperature post aging can be attributed to the grain refinement strengthening, dislocation strengthening, and nanoprecipitation strengthening. Thus, the evolution of the aging precipitates during the ECAP and post aging alloy was analyzed.

Key words:  Al-Mg-Si-Cu alloy      ultrafine grain      low temperature post-aging      microstructure      mechanical property     
Received:  02 June 2021     
ZTFLH:  TG146.21  
Fund: National Natural Science Foundation of China(U1710124);National Natural Science Foundation of China(51871035);National Natural Science Foundation of China(52001159);Open Fund of the State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University(32115014)

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Fig.1  XRD spectra of 6061 aluminum alloy at T6 state (a) and ECAPed and post-aged state (b) (ECAP—equal channel angular pressing, RT—room temperature)
nm%1014 m-2
ECAP at RT2100.169.19
ECAP + 80oC, 20 min2110.159.05
ECAP + 80oC, 180 min4880.112.78
ECAP + 110oC, 10 min2570.146.81
ECAP + 170oC, 5 min3950.145.12
ECAP + 170oC, 180 min7700.081.27
Table 1  XRD analyses of the ECAPed and post-aged alloy
Fig.2  Mechanical properties of 6061 aluminum alloy after ECAPed and post-aged states
(a) hardness curves of ECAP + post-aged at 80, 110, and 170oC specimens
(b) engineering stress-strain curves of the ECAPed and post-aged specimens
(c) normalized work hardening rates against true strain
Fig.3  TEM images and grain size distributions of ultrafine-grained 6061 aluminum alloy (a-c) TEM bright field (a) and dark field (b) images, and grain size distribution (c) of the ECAPed specimen (Inset in Fig.3a is the corresponding SAED pattern) (d, e) TEM bright field image (d) and grain size distribution (e) of the ECAP + 80oC, 20 min post-aged specimen
Fig.4  TEM analyses of T6 state (a) and ECAP + 80oC, 20 min post-aged states (b-e) 6061 aluminum alloy (a-c) TEM bright field images (Insets show the corresponding SAED and FFT pattern, respectively) (d) interaction between β'' and dislocation in the post-aged specimen (e) statistical measurements from Fig.4b showing the width and the length of the β'' precipitates
Fig.5  HRTEM images of the ECAPed specimen (a), ECAP + 80oC, 20 min post-aged specimen (c), ECAP + 170oC, 180 min post-aged specimen (d), and inverse fast Fouier transform (IFFT) map of lattice fringe at the black box in Fig.5a (b) (The ellipses show examples of dislocation dipoles. The black circles mark interstitial loops and the white circles mark vacancy loops)
Fig.6  Schematics of the evolution of the internal precipitation of the alloy in different states
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(a) solid-solution treatment (SST) (b) ECAPed
(c) high temperature (> 110oC) post-aging after ECAP (d) low temperature (< 80oC) post-aging after ECAP
Stateσexp / MPaσ0.2 / MPaσgs / MPaσρ / MPaσprec / MPa
ECAPed + 80oC, 20 min48347195 (20.2%)147 (31.2%)229 (48.6%)
ECAPed + 110oC, 10 min43443493 (21.4%)129 (29.7%)212 (48.8%)
ECAPed + 170oC, 5 min36537082 (22.2%)113 (30.5%)175 (47.3%)
Table 2  Contributions and proportions of strengthening mechanisms to the strength of the post-aging alloy
State/treatmentσ0.2 / MPaUTS / MPaEf / %Ref.
6061 ECAP at RT + post-aging48351415.1This work
Al-Mg-Si-Cu rolling+aging45248710.0[10]
6061 ECAP at RT + post-aging41145017.8[13]
6061 under-aging + coldrolling + re-aging5425608.5[27]
6061 HPT at RT6057056.3[34]
6061 Friction stir processing4355052.1[35]
AA6060 HPT at RT4755256.0[36]
6061 HPT at RT6606905.5[37]
6061 HPT at RT + post-aging45548515.0[38]
6061 multi directional forging + aging4094728.9[39]
6101 hydrostatic extrusion + aging3453545.5[40]
6061 accumulative roll bonding + aging450-6.5[41]
Al-Mg-Si-Cu pre-deformation + aging4354507.1[42]
Table 3  Comprehensive comparisons of tensile properties of 6000 series Al alloys[10,13,27,34-42]
Fig.7  Comprehensive comparisons of yield strength and uniform elongation of Al-Mg-Si alloys[10,13,27,34-42]
Color online
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