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Acta Metall Sin  2017, Vol. 53 Issue (9): 1075-1090    DOI: 10.11900/0412.1961.2017.00047
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Cryogenic Processing High-Strength 7050 Aluminum Alloy and Controlling of the Microstructures and Mechanical Properties
Longgang HOU1(), Mingli LIU1, Xindong WANG2, Linzhong ZHUANG1, Jishan ZHANG1
1 State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China
2 School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China
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The high strength or flow stress as well as low plastic deformability of 7000 series Al alloys makes it difficult to improve their microstructures and mechanical properties by cold processing, and many advanced alloying methods and processing technologies are continually developed for higher mechanical properties and acceptable elongation. In this work, the cryogenic deformation (rolling) was applied to process high-strength 7050 Al alloys, and its effects on the microstructures and mechanical properties were studied. The results showed that after the pre-cooling with liquid nitrogen, the quenched 7050 Al alloy can obtain much higher rolling reduction, similar to that under warm or hot rolling, and a great number of substructures and high-density dislocations were formed which greatly increased the strength. The higher cryogenic deformability would be mainly related with the higher work-hardening ability at low temperature, while the strength enhancement would be largely attributed to the solution strengthening and dislocation strengthening. The cryogenic deformation can obviously stimulate the ageing process of the quenched 7050 Al alloy, but the direct ageing of the cryogenic-rolling 7050 Al alloy can assure higher strength and acceptable elongation, which would be greatly attributed to the precipitation strengthening and dislocation strengthening, while the recovery and ageing-induced precipitates help improving the tensile elongation. During room-temperature rolling, the formation of GP zones and η′ phases caused by the heats transformed from the deformation as well as their interaction with dislocations leads to the appearance of amounts of shear bands (instability areas), which will easily cause the cracking or edge-cracking of the rolling sheets. However, the cryogenic rolling with distinctly impeding the solute diffusion can result in the suppression of precipitation of the strengthening phases so as to decrease the occurrence of the shear instability areas, and uniform and stable plastic deformation or good work-hardening as well as high-quality rolling sheets are obtained. The excellent plastic deformability of high-strength Al alloys at cryogenic temperatures could be suggested as an effective way to improve the processing of high-strength Al alloys.

Key words:  high-strength Al alloy      cryogenic deformation      microstructure      mechanical property      work-hardening     
Received:  15 February 2017     
ZTFLH:  TG146.2  
Fund: Supported by National Natural Science Foundation of China (No.51401016), Fundamental Research Funds for the Central Universities of China (No.FRF-TP-12-137A), Beijing Laboratory of Metallic Materials and Processing for Modern Transportation, and Funds from State Key Laboratory of Advanced Metals and Materials (No.2011Z-05)

Cite this article: 

Longgang HOU, Mingli LIU, Xindong WANG, Linzhong ZHUANG, Jishan ZHANG. Cryogenic Processing High-Strength 7050 Aluminum Alloy and Controlling of the Microstructures and Mechanical Properties. Acta Metall Sin, 2017, 53(9): 1075-1090.

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Fig.1  Room-temperature rolling (RTR) and liquid nitrogen rolling (LN2R) sheets of the quenched 7050 Al alloy (The initial thickness are 15 and 20 mm, respectively, with 10%~15% reduction per pass; CRTR——continued room-temperature rolling; IRTR——interval room-temperature rolling)
Fig.2  Microstructures of the RTR and LN2R 7050 Al alloys

(a) as-quenched (b) 40%RTR (c) 56%RTR (d) 63%RTR (e) 76%LN2R (f, g) 91%LN2R with different magnifications

Fig.3  Dislocation cells and substructures in 70% (a, b) and 91% (c, d) LN2R 7050 Al alloys, and subgrains in the as-quenched alloy (e) (Figs.3b and d are the enlarged images of Figs.3a and c, respectively)
Fig.4  TEM images of microstructures of 7050 Al alloy over-aged at 350 ℃ for 1 h after 60%RTR (a~c), 77%RTR (d~f) and 66%LN2R (g~j) deformations

(a) elongated microstructures (b, c) enlarged images of Fig.4a

(d) laminar deformation microstructures (e) subgrains (f) dislocation cells

(g) substructures or subgrains within the micro shear band (h) sharp-shaped equiaxed subgrains

(i) local deformed microstructures with subgrains and elongated subgrains (containing precipitates)

(j) enlarged equiaxed subgrains with precipitates in the triple junction

Rolling process Heat treatment σy / MPa σb / MPa δ / %
Original T6 (120 ℃, 24 h) 507 565 11.6
80% hot rolling[30] T6 518 600 16.8
80%LN2R No 571 624 7.0
(10% per pass)
82.5%LN2R No About 625 676~682 8.6~9.4
(20% per pass)
(10% per pass)
No 650 690 About 3.0
80 ℃, 24 h 578~583 639 11.5~13.6
80 ℃, 48 h 589 642 9.0
80 ℃, 72 h 591 650 12.0
100 ℃, 24 h 601 636 8.2
120 ℃, 24 h 570~573 601~606 5.3~8
475 ℃, 0.5 h+T6 498 590 15.3
PA 602~605 650~653 6.6~7.8
7050 T7651 455 524 8.0
Table 1  Room-temperature tensile properties of the 7050 Al alloy sheets at different states
Fig.5  Room-temperature tensile curves of the LN2R 7050 Al alloys under different states (the 82.5%LN2R curve is for the sheet with 82.5%LN2R reduction (20% reduction per pass), the 91%LN2R curve is for the sheet with 91%LN2R reduction (10% reduction per pass), other curves are for the aged 91%LN2R sheets
Fig.6  Microstructure of the 63%RTR 7050 Al alloy (The arrows show the shear bands or micro shear areas)
Fig.7  DSC curves of the quenched (a) and 91%LN2R (b~g) 7050 Al alloys without ageing (b) and under the ageing states of 80 ℃, 24 h (c), 80 ℃, 48 h (d), 80 ℃, 72 h (e), 100 ℃, 24 h (f) and 120 ℃, 24 h (g)
Fig.8  TEM (a~c) and HRTEM (d) images of the 91%LN2R 7050 Al alloy after ageing at 80 ℃ for 24 h (a) and 48 h (b~d) (Fig.8c is the local magnification of the area in Fig.8b, and the arrows in Fig.8c show the precipitates)
Fig.9  TEM (a, b, e, h) and HRTEM (c, d, f, i) images of the 91%LN2R 7050 Al alloy after different ageing treatments (air furnace), and electron diffraction pattern of Fig.9b (<110>Al) (g) (Circles in Fig.9a indicate some obvious precipitates along subgrain boundaries or within grains; the two arrows in Fig.9g indicate the diffraction patterns at 1/3{220}Al and 2/3{220}Al positions)

(a~d) 100 ℃, 24 h (e~g) 120 ℃, 24 h (h, i) 50 ℃, 5 h+80 ℃, 9 h

Fig.10  XRD spectra of the quenched 7050 Al alloy after RTR (a) and 91%LN2R deformation with ageing treatments (b) (S phase in Fig.10b is the undissolved S phase)
Fig.11  Curves of stress and stress reduction of the quenched 7050 Al alloy at different temperatures and strain rates (The displacement data during the compression were recorded by the testing system without using strain gauge and the strains in Figs.11a and b were for reference; RT—room temperature, LN2—liquid nitrogen, RTC—room-temperature compression)

(a) curves of compressive true stress-strain (The arrows show the starting points of obvious serrated flow)

(b) enlarged serrated flow of RT curves in Fig.11a (0.005 s-1, 213 K)

(c) variations of the stress reduction or increment (Δσ) with strains under RT compression

(d) variation of the average stress reduction or increment (Δσave.) with strain rates

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