Variation of mechanical properties and microstructure of hot-rolled AA2099 Al-Li alloy induced by the precipitation during preheating process

doi:10.1016/j.jmst.2021.09.035

Abstract

Abstract:

Preheating is the first step for thermal-mechanical processing of the materials. Several preheating schedules were designed based on industrial manufacturing to explore their effect on the microstructure and mechanical properties of AA2099 Al-Li alloy during the following hot rolling. The results show that the mechanical properties of as-rolled sheets are quite sensitive to preheating. Various types of secondary phases, including δ’, T1 and T2 phases, were precipitated during the heating process. The difference among precipitates is the main reason to induce the variation of mechanical properties for as-rolled sheets. A rapid heating rate gave rise to the precipitation of δ’ phase, which largely promoted yield stress and ultimate tensile strength in as-rolled sheets. T1 phase was more favorable in the preheating with a holding period at 150-250 °C, which caused a moderated strain hardening behavior. Precipitates formed during preheating induced different slip behaviors in the following deformation. The coexistence of T1 and δ’ phase could restrict planar slip, which inhibited the formation of the shear band. The difference in the activity of non-octahedral slips and planar slips determined the strain hardening effect, which should be the reason for the variation of mechanical properties of as-rolled sheets.

Key words: Aluminum-lithium alloys, Preheating, Precipitation, Mechanical properties, Shear band

Fei Guo, Weijiu Huang, Xusheng Yang, Haipeng Dong, Hang Yu, Qiuyu Chen, Li Hu, Luyao Jiang. Variation of mechanical properties and microstructure of hot-rolled AA2099 Al-Li alloy induced by the precipitation during preheating process[J]. J. Mater. Sci. Technol., 2022, 110: 198-209.

Figures/Tables 18

Table 1. Chemical content of the alloy used in this study (wt.%).

Sample	Cu	Li	Zn	Mg	Mn	Zr	Fe	Si	Al
AA2099	2.78	1.80	0.67	0.30	0.36	0.09	0.06	0.03	Bal.

Fig. 1. Preheating processes performed in this study.

Table 2. Preheating schedules of the samples before hot rolling.

Group	Sample	Heating stage Ⅰ		Holding stage		Heating stage Ⅱ
Group	Sample	T (°C)	Time (min)	T (°C)	Time (min)	T (°C)	Time (min)
Ⅰ	420-F10m	420 IF*	10
Ⅰ	420-25m	25→420	25
Ⅰ	420-3h	25→420	180
Ⅱ	150-3h	25→150	8	150	180	150→420	17
Ⅱ	190-30m	25→190	11	190	30	190→420	14
Ⅱ	250-20m	25→250	14	150	180	250→420	11

Fig. 2. (a) Mechanical properties and (b-g) microstructure of the samples hot rolled by 70% reduction at 420 °C.

Fig. 3. (a) Strain-stress curves of the preheated samples, comparison of the (b) yield stress and the ratio of yield stress to ultimate tensile stress, (c) ultimate tensile stress and (d) microhardness prior to rolling (PR) and after rolling (AR).

Fig. 4. (a-f) ODFs of the samples after hot rolling, (g) texture intensity distribution of the rolled samples on their β fiber.

Fig. 5. XRD patterns of the (a) preheated samples and (b) hot-rolled samples.

Fig. 6. Microstructure of preheated 420-25m sample: (a) BF image, (b) STEM-HADDF of the same region of (a), (c) core-shell structure phase, (d) DF image obtained at superlattice spot of 1/2{200}Al, (e) HRTEM of the core-shell structure phase highlighted δ’ and β’ phase, (f) FFT pattern of the region in (a) and (g) FFT pattern of the region in (e).

Fig. 7. Microstructure of preheated 420-F10m sample: (a) BF image, (b) STEM-HADDF image, (c) DF image obtained at the spot of 1/2{200}Al. Microstructure of preheated 420-3h sample: (d) BF image with SAED, (e) STEM-HADDF image, (f) HRTEM of a T2 phase with (g) the FFT pattern, (h) DF image obtained at the spot of 1/2{200}Al and (i) HRTEM of a β’ phase.

Fig. 8. Microstructure of preheated 150-3h sample: (a) BF image, (b) STEM-HADDF image, (c) HRTEM shows the details of T1 phases, (d) DF image obtained at the spot of 1/2{200}Al with SAED of the same region, (e) BF of the region contains T1 phase clusters and (f) DF of the same region in (e) with δ’ phase highlighted.

Fig. 9. Microstructure of preheated 190-30m sample: (a) STEM-HADDF image, (b) BF image with SAED, (c) DF image obtained at the spot of 1/2{200}Al and (d) HRTEM highlighted nano-size δ’ phase with FFT pattern.

Fig. 10. Microstructure of preheated 250-20m sample: (a) BF image, (b) the STEM-HADDF image and (c) DF image obtained at the spot of 1/2{200}Al.

Fig. 11. Comparison of T1 phases in the preheated samples (a, b) and (d-f) and (c) thickness of T1 plates.

Table 3. Summary of the precipitates, texture, microstructure and strength of the as-rolled samples.

Sample	Precipitates	Main texture	Microstructure	Mechanical properties
420-F10m	T1(low density)	Bs	-	Medium strength
420-25m	δ', bubble δ', T1	Bs, Cu, S	Shear bands	High strength
420-3h	T2	Cu	-	Low strength
150-3h	T1, a few δ'	Cu	-	Low strength
190-30m	T1, δ'	Bs, S	a few shear bands	High strength
250-20m	T1, T2	S	-	Medium strength

Fig. 12. Schematic figure for the phases in (a) 420-25m, (b) 190-30m and (c) 150-3h samples.

Fig. 13. Microstructure of the hot-rolled 420-25m sample: (a) BF image of the slip bands marked by yellow arrows and (b, c) high magnification image of the slip bands with a slight tilt of the incident beam.

Fig. 14. Schematic figures of the distribution of the precipitates with various slip planes in the aluminum alloys: (a) T1 phase and (b) sphere δ’ phase.

Fig. 15. HRTEM of the rolled sample shows the microstructure of (a) β’ phase and (b) T1 phase debris after rolling.

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