Effects of Artificial Ageing on Mechanical Properties and Precipitation of 2A12 Al Sheet
LIANG Mengchao, CHEN Liang(), ZHAO Guoqun
Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan 250061, China
Cite this article:
LIANG Mengchao, CHEN Liang, ZHAO Guoqun. Effects of Artificial Ageing on Mechanical Properties and Precipitation of 2A12 Al Sheet. Acta Metall Sin, 2020, 56(5): 736-744.
2A12 Al alloy has been widely applied in the fields of aviation, aerospace, and vehicles due to its light weight, high specific strength and good corrosion resistance. The solution and ageing treatments are usually performed after the processing on 2A12 Al alloy, and the ageing parameters greatly affect the final mechanical properties. In the present study, the artificial ageing was performed on the cold rolled 2A12 Al sheet under various holding temperatures and holding time. The mechanical properties were evaluated by micro-hardness and tensile tests. Moreover, the evolution of microstructure and precipitations during ageing with different holding time were characterized. The results showed that the 2A12 Al sheet had the sole peak ageing, and the higher the temperature, the shorter time was required for the peak ageing. Both the holding temperature and holding time significantly affected the mechanical properties. The optimal ageing parameters were determined as holding at 185 ℃ for 16 h, and the corresponding yield strength, ultimate tensile strength and elongation along rolling direction were 381 MPa, 476 MPa and 13.6%, respectively. S (Al2CuMg) phase gradually precipitated during ageing process, and the size and distribution of S phase greatly affected the fracture mechanism. At the initial stage of ageing, S phase precipitated near grain boundaries, and the ductility fracture could be observed. With the extension of holding time, the coarsening of S phase took place, and the fracture was gradually transformed to intergranular and transcrystalline modes. Cu-Mg cluster was the main strengthening mechanism at the initial stage of ageing. Both Cu-Mg cluster and GPB zone contributed to the strengthening under the peak ageing, and the precipitations were transformed to stable S phase under the over ageing. Considering the combined effects of homogeneous and inhomogeneous nucleation, the precipitation during ageing of cold rolled 2A12 Al sheet followed the sequence of supersaturated solid solution (SSS)→Cu-Mg cluster+Sinhomo→Cu-Mg cluster+GPB zone+Sinhomo→Cu-Mg cluster +GPB zone+ Shomo+Sinhomo→S.
Table 1 Experimental schemes of the artificial ageing
Fig.1 Shape and dimension of the sample for tensile test (unit: mm)
Fig.2 Schematic of the sampling locations for microstructure observation and tensile test (RD—rolling direction, TD—transverse direction)
Fig.3 Microhardness curves of the 2A12 cold rolled Al sheet during artificial ageing (t—time)
Fig.4 Tensile property curves along RD of the 2A12 Al cold rolled sheet aged under the temperatures of 155 ℃ (a), 170 ℃ (b), 185 ℃ (c) and 200 ℃ (d) (A—elongation, Rm—tensile strength, Rp0.2—yield strength)
Fig.5 Tensile property curves along TD of the 2A12 Al cold rolled sheet aged at 185 ℃ for various holding time
Fig.6 Fracture morphologies along RD of the 2A12 cold rolled Al sheet aged at 185 ℃ for various holding time of 4 h (a), 10 h (b), 16 h (c) and 36 h (d)
Fig.7 TEM images and the selected area electron diffraction (SAED) pattern (inset) of the grain interior (a) and near grain boundary (b) of the 2A12 cold rolled Al sheet aged at 185 ℃ for 4 h (Points 1~7 represent precipitates)
Fig.8 Schematics of the diffraction spots of Al matrix and S phases along [001]Al direction (a) Al matrix (b) S phases of [100]S//[100]Al (A and B) or [100]S// [010]Al (C and D) (c) S phases of [100]S//[001]Al
Fig.9 TEM images of the 2A12 cold rolled Al sheet aged at 185 ℃ for 16 h (a, b) bright-field TEM images of different zones (Insets in Fig.9b are the SAED pattern (up) and diffraction schematic diagram (down) of precipitates) (c) HRTEM images of granular precipitates and SAED pattern (inset) (d) HRTEM images of needle-like precipitates (dashed line) and SAED pattern (inset)
Fig.10 TEM images of the 2A12 cold rolled Al sheet aged at 185 ℃ for 64 h (a) [001]Al zone axis (b) [112]Al zone axis
Fig.11 HRTEM images and corresponding SAED patterns of the granular precipitates of the 2A12 cold rolled Al sheet aged at 185 ℃ for 64 h (a) precipitate with the orientations of [100]S//[001]Al, [010]S//[0]Al, [001]S//[20]Al (b) precipitate with the orientations of [100]S//[00]Al, [010]S//[10]Al, [001]S//[0]Al
1
Chen L, Zhao G Q, Gong J, et al. Hot deformation behaviors and processing maps of 2024 aluminum alloy in as-cast and homogenized states [J]. J. Mater. Eng. Perform., 2015, 24: 5002
2
Liu Z L, Cui H T, Ji S D, et al. Improving joint features and mechanical properties of pinless fiction stir welding of alcald 2A12-T4 aluminum alloy [J]. J. Mater. Sci. Technol., 2016, 32: 1372
3
Zhang Z H, Li W Y, Li J L, et al. Microstructure and anisotropic mechanical behavior of friction stir welded AA2024 alloy sheets [J]. Mater. Charact., 2015, 107: 112
doi: 10.1016/j.matchar.2015.06.039
4
Sha G, Marceau R K W, Gao X, et al. Nanostructure of aluminium alloy 2024: Segregation, clustering and precipitation processes [J]. Acta Mater., 2011, 59: 1659
doi: 10.1016/j.actamat.2010.11.033
5
Bagaryatsky Y A. Structural changes on aging Al-Cu-Mg alloys [J]. Dokl. Akad. Nauk SSSR., 1952, 87: 397
6
Styles M J, Marceau R K W, Bastow T J, et al. The competition between metastable and equilibrium S (Al2CuMg) phase during the decomposition of AlCuMg alloys [J]. Acta Mater., 2015, 98: 64
7
Feng Z Q, Yang Y Q, Huang B, et al. HRTEM and HAADF-STEM tomography investigation of the heterogeneously formed S (Al2CuMg) precipitates in Al-Cu-Mg alloy [J]. Philos. Mag., 2013, 93: 1843
8
Charai A, Walther T, Alfonso C, et al. Coexistence of clusters, GPB zones, S''-, S'- and S-phases in an Al-0.9% Cu-1.4% Mg alloy [J]. Acta Mater., 2000, 48: 2751
9
Sunde J K, Johnstone D N, Wenner S, et al. Crystallographic relationships of T-/S-phase aggregates in an Al-Cu-Mg-Ag alloy [J]. Acta Mater., 2019, 166: 587
10
Wang J, Zhang B, Zhou Y T, et al. Multiple twins of a decagonal approximant embedded in S-Al2CuMg phase resulting in pitting initiation of a 2024Al alloy [J]. Acta Mater., 2015, 82: 22
11
Ringer S P, Hono K, Polmear I J, et al. Nucleation of precipitates in aged Al-Cu-Mg-(Ag) alloys with high Cu∶Mg ratios [J]. Acta Mater., 1996, 44: 1883
doi: 10.1016/1359-6454(95)00314-2
12
Ringer S P, Caraher S K, Polmear I J. Response to comments on cluster hardening in an aged Al-Cu-Mg alloy [J]. Scr. Mater., 1998, 39: 1559
doi: 10.1016/S1359-6462(98)00364-9
13
Wang S C, Starink M J, Gao N. Precipitation hardening in Al-Cu-Mg alloys revisited [J]. Scr. Mater., 2006, 54: 287
doi: 10.1016/j.scriptamat.2005.09.010
14
Wang S C, Starink M J. Two types of S phase precipitates in Al-Cu-Mg alloys [J]. Acta Mater., 2007, 55: 933
doi: 10.1016/j.actamat.2006.09.015
15
Moghanaki S K, Kazeminezhad M. Effects of non-isothermal annealing on microstructure and mechanical properties of severely deformed 2024 aluminum alloy [J]. Trans. Nonferrous Met. Soc. China, 2017, 27: 1
doi: 10.1016/S1003-6326(17)60001-3
16
Moy C K S, Weiss M, Xia J H, et al. Influence of heat treatment on the microstructure, texture and formability of 2024 aluminium alloy [J]. Mater. Sci. Eng., 2012, A552: 48
17
Feng Z Q, Yang Y Q, Huang B, et al. Variant selection and the strengthening effect of S precipitates at dislocations in Al-Cu-Mg alloy [J]. Acta Mater., 2011, 59: 2412
18
Feng Z Q, Yang Y Q, Huang B, et al. STEM-HAADF tomography investigation of grain boundary precipitates in Al-Cu-Mg alloy [J]. Mater. Lett., 2011, 65: 2808
19
Ringer S P, Sakurai T, Polmear I J. Origins of hardening in aged Al-Cu-Mg-(Ag) alloys [J]. Acta Mater., 1997, 45: 3731
doi: 10.1021/es1036332
pmid: 21410193
20
Liu Y, Teng F, Cao F H, et al. Defective GP-zones and their evolution in an Al-Cu-Mg alloy during high-temperature aging [J]. J. Alloys Compd., 2019, 774: 988
21
Tolley A, Ferragut R, Somoza A. Microstructural characterisation of a commercial Al-Cu-Mg alloy combining transmission electron microscopy and positron annihilation spectroscopy [J]. Philos. Mag., 2009, 89: 1095
22
Zuiko I, Kaibyshev R. Aging behavior of an Al-Cu-Mg alloy [J]. J. Alloys Compd., 2018, 759: 108
doi: 10.3390/ma12182907
pmid: 31505758
23
Fu S, Zhang Y, Liu H Q, et al. Influence of electric field on the quenched-in vacancy and solute clustering during early stage ageing of Al-Cu alloy [J]. J. Mater. Sci. Technol., 2018, 34: 335
doi: 10.1016/j.jmst.2017.07.020
24
Li Y, Guo M X, Jiang N, et al. Precipitation behaviors and preparation of an advanced Al-0.93Mg-0.78Si-0.20Cu-3.00Zn alloy for automotive application [J]. Acta Metall. Sin., 2016, 52: 191
doi: 10.11900/0412.1961.2015.00334
Lotter F, Petschke D, De Geuser F, et al. In situ natural ageing of Al-Cu-(Mg) alloys: The effect of In and Sn on the very early stages of decomposition [J]. Scr. Mater., 2019, 168: 104