Acta Metall Sin  2020, Vol. 56 Issue (5): 736-744    DOI: 10.11900/0412.1961.2019.00293
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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
Abstract

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.

 ZTFLH: TG156
Fund: National Key Research and Development Program of China(2017YFB0306402);National Natural Science Foundation of China(51735008)
Corresponding Authors:  CHEN Liang     E-mail:  chenliang@sdu.edu.cn
 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//[$1ˉ$$2ˉ$0]Al, [001]S//[2$1ˉ$0]Al(b) precipitate with the orientations of [100]S//[00$1ˉ$]Al, [010]S//[1$2ˉ$0]Al, [001]S//[$2ˉ$$1ˉ$0]Al