EFFECT OF GRAIN SIZE AND TAYLOR FACTOR ON THE TRANSVERSE MECHANICAL PROPERTIES OF 7050 ALUMINIUM ALLOY EXTRUSION PROFILE AFTER OVER-AGING

doi:10.11900/0412.1961.2015.00163

Acta Metall Sin

2016, Vol. 52

Issue (1): 51-59 DOI: 10.11900/0412.1961.2015.00163

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EFFECT OF GRAIN SIZE AND TAYLOR FACTOR ON THE TRANSVERSE MECHANICAL PROPERTIES OF 7050 ALUMINIUM ALLOY EXTRUSION PROFILE AFTER OVER-AGING

Wei GU,Jingyuan LI(

),Yide WANG

School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China

Cite this article:

Wei GU,Jingyuan LI,Yide WANG. EFFECT OF GRAIN SIZE AND TAYLOR FACTOR ON THE TRANSVERSE MECHANICAL PROPERTIES OF 7050 ALUMINIUM ALLOY EXTRUSION PROFILE AFTER OVER-AGING. Acta Metall Sin, 2016, 52(1): 51-59.

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Abstract

Generally, it is believed that inside the material the smaller grain size is, the higher yield strength is. In addition to this effect, grain refinement method also ensures that the toughness of the material is not reduced. However, it is found that the relationship between the grain size distribution and mechanical properties is contradiction with this law after the properties have been studied in the transverse direction of a large cross-section 7050 aluminum alloy profile. That is, the impact energy and yield strength in the center with a large grain size is higher than those at the edge with the smaller grain size in the thickest section of the profile. Besides that, during the establishment of the yield strength model in over-aging 7050 aluminum alloy, there are two models for the grain boundary strengthening which are Nes model and Hall-Petch model, so the choice from these model is found to affect the final results of the yield strength model. In order to study and understand the reasons for this phenomenon, the difference of mechanical properties distribution in the cross-section of 7050 aluminum extrusion profile has been investigated by impact test and tensile test at normal temperature, meanwhile, the microstructures have been analyzed by OM, EBSD and TEM. The results show that lots of the harder deformation textures, i.e., copper texture in the core of the profile lead to higher yield strength in the core with grain size of 12 mm than that in the edge with grain size of 6 mm. The Taylor factor could be calculated after the solution strengthening by alloying elements, grain boundary strengthening by the sub-grain and the yield stress of the alloy, at last, it reaches to 3.925 in the core, while that is just 2.257 in the edge. Compared with Nes model, the Hall-Petch model is much preferable to the calculation of grain boundary strengthening in yield stress of 7050 aluminum alloys after solid solution treatment. It is established that there is a linear relationship between impact energy and grain size of three over-aging specimens.

Key words: 7050 aluminium alloy grain boundary strengthening Taylor factor impact energy yield strength

Received: 25 March 2015

Fund: Supported by National High Technology Research and Development Program of China (No.2013AA032402) and Project on the Integration of Industry, Education and Research of Guangdong Province (No.2015B090901044)

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https://www.ams.org.cn/EN/10.11900/0412.1961.2015.00163 OR https://www.ams.org.cn/EN/Y2016/V52/I1/51

Fig.1 Schematic of 7050 aluminum alloy extrusion profile and positions of 3 specimens (ND, ED and TD represent normal direction, extrusion direction and transverse direction, respectively. C, M and E stand for the position of center, medium and edge of the thickest section in the extrusion profile, respectively)

Fig.2 OM images of 7050 aluminium alloy extrusion profile in positions E (a~c), M (d~f) and C (g~i) in Fig.1 after T76 (a, d, g), T74 (b, e, h) and T73 (c, f, i) treatments

Fig.3 Grain sizes of different position specimens after T76, T74 and T73 treatments

Fig.4 Impact energies of different position specimens after T76, T74 and T73 treatments

Fig.5 Relationship between impact energy and grain size of specimens after T76, T74 and T73 treatments (k—slope)

Fig.6 Distributions of yield strength (a), yield ratio s_0.2/s_b and elongation d (b) of different position specimens after T76, T74 and T73 treatments

Fig.7 Orientation distribution functions (ODFs) of as-quenched specimens in positions E (a) and C (b) of 7050 aluminum alloy extrusion profile

Fig.8 Misorientation angle distributions of as-quenched specimens in positions E and C of 7050 aluminum alloy extrusion profile

Fig.9 Three fitting schemes to the yield stress of 7050 aluminum alloy in different positions under different periods of the secondary aging (A_i^j and B_i^j are two parameters of Eq.(5). Therein, the subscript i stands for the number of three different schemes, and the superscript j stands for one of the three different positions in the thickest section of profile, including E, M and C)

Table 1 Parameters and computing coefficients in three fitting schemes of yield strength

Fig.10 TEM images of sub-grains in positions E (a) and C (b)

Table 2 Strength of grain boundaries in Nes model and Hall-Petch model

Table 3 Taylor factors calculated by Nes model and Hall-Petch model

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