EFFECT OF THE THERMO-MECHANICAL TREATMENT OF PRE-AGEING, COLD-ROLLING AND RE-AGEING ON MICROSTRUCTURES AND MECHANICAL PROPERTIES OF 6061 Al ALLOY
LI Hai1,3(), MAO Qingzhong1, WANG Zhixiu1,2,3, MIAO Fenfen1, FANG Bijun1, SONG Renguo1,3, ZHENG Ziqiao2
1 School of Materials Science and Engineering, Changzhou University, Changzhou 213164 2 School of Materials Science and Engineering, Central South University, Changsha 410083 3 Jiangsu Key Laboratory of Materials Surface Technology, Changzhou University, Changzhou 213164
Cite this article:
LI Hai, MAO Qingzhong, WANG Zhixiu, MIAO Fenfen, FANG Bijun, SONG Renguo, ZHENG Ziqiao. EFFECT OF THE THERMO-MECHANICAL TREATMENT OF PRE-AGEING, COLD-ROLLING AND RE-AGEING ON MICROSTRUCTURES AND MECHANICAL PROPERTIES OF 6061 Al ALLOY. Acta Metall Sin, 2014, 50(10): 1244-1252.
In order to improve mechanical properties of age-hardenable Al alloys markedly based on the conventional processing conditions, a novel thermo-mechanical treatment, which was consisted of pre-ageing, cold-rolling and re-ageing, was proposed in the present work. The effects of the thermo-mechanical treatment on microstructure evolution and mechanical properties of the 6061 Al alloy were investigated further by hardness measurement, tensile testing, XRD, TEM and HRTEM. It was shown that both the increased strength and elongation of the 6061 Al alloy were achieved simultaneously by the thermo-mechanical treatment. Especially, after the combination of under-ageing at 180 ℃ for 2 h, cold-rolling with the thickness reduction of 75% and re-ageing at 100 ℃ for 48 h, the ultimate tensile strength and yield strength of the alloy were 560 and 542 MPa, respectively, and the elongation was about 8.5%. The strength increment of the thermo-mechanically treated 6061 Al alloy was attributed mainly to the additional introduction of dislocation strengthening, dislocation cell strengthening and high Taylor factor value except for precipitation strengthening as compared to the conventional peak-aged alloy. Furthermore, the improved elongation of the thermo-mechanically treated 6061 Al alloy was mainly due to both the re-precipitation of strengthening precipitates and slight recovery of dislocations during re-ageing, which increased the accumulation ability of dislocations during the tensile deformation.
Fund: Supported by National Basic Research Program of China (No.2005CB623705), National Natural Science Foundation of China (No.51301027) and Natural Science Foundation of the Jiangsu Higher Education Institutions of China (No.14KJB430002)
Fig.1 Hardening curves of four cold-rolling 6061 Al alloys re-aged at 75 ℃ (a), 100 ℃ (b) and 125 ℃ (c) (ST—solution treatment, UA—under-ageing, PA—peak-ageing, OA—over-ageing, CR—cold-rolling)
Fig.2 Tensile properties of 6061 Al alloy after the thermo-mechanical treatment of pre-treating (a), cold-rolling (b), re-ageing at 75 ℃ (c), 100 ℃ (d) and 125 ℃ (e) for 48 h (sb—ultimate tensile strength, sy—yield strength, d—elongation)
Fig.3 Bright-field TEM images of 6061 alloys under pre-treatment conditions
Fig.4 Microstructures of 6061 alloys under re-ageing conditions
Condition
Cube
Goss
Copper
S
Brass
MA
UA
46.0
19.5
15.5
0
18.5
2.6
UA+CR
10.0
21.5
23.0
16.0
29.5
3.1
UA+CR+RA (100 ℃, 48 h)
10.0
20.0
22.5
15.5
30.5
3.1
Table 1 The texture components and corresponding volume fractions (%) and average Taylor factor MA of the 6061 alloys subjected to UA, UA+CR and UA+CR+RA (100 ℃, 48 h), respectively
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