Microstructure and mechanical properties of Al-Mg-Si alloy fabricated by a short process based on sub-rapid solidification

doi:10.1016/j.jmst.2019.08.053

Abstract

Abstract:

Al-Mg-Si (AA6xxx) series alloys have been used widely in automotive industry for lightweight purpose. This work focuses on developing a short process for manufacturing Al-0.5Mg-1.3Si (wt.%) alloy sheets with good mechanical properties. Hereinto, a preparation route without homogenization was proposed on the basis of sub-rapid solidification (SRS) technique. The sample under SRS has fine microstructure and higher average partition coefficients of solute atoms, leading to weaker microsegregation owing to the higher cooling rate (160 °C/s) than conventional solidification (CS, 30 °C/s). Besides, Mg atoms tend to be trapped in Al matrix under SRS, inducing suppression of Mg₂Si, and promoting generation of AlFeSi phase. After being solution heat treated (T4 state), samples following the SRS route have lower yield strength compared with that by CS route, indicating better formability in SRS sample. After undergoing pre-strain and artificial aging (T6 state), the SRS samples have comparable yield strength to CS samples, satisfying the service requirements. This work provides technological support to industrially manufacture high performance AA6xxx series alloys with competitive advantage by a novel, short and low-cost process, and open a door for the further development of twin-roll casting based on SRS technique in industries.

Key words: Al-Mg-Si alloy, Sub-rapid solidification, Microstructure, Mechanical properties, Short process

Ze-Tian Liu, Bing-Yu Wang, Cheng Wang, Min Zha, Guo-Jun Liu, Zhi-Zheng Yang, Jin-Guo Wang, Jie-Hua Li, Hui-Yuan Wang. Microstructure and mechanical properties of Al-Mg-Si alloy fabricated by a short process based on sub-rapid solidification[J]. J. Mater. Sci. Technol., 2020, 41: 178-186.

Figures/Tables 11

Fig. 1. Schematic diagram for the three different fabrication processes.

Fig. 2. (a) Schematic diagram of temperature measuring system; (b) entire cooling curves of SRS and CS samples; (c) detail of initial stage on SRS cooling curve; (d) detail of initial stage on CS cooling curve.

Fig. 3. Optical micrograph (OM) of (a) as-cast SRS sample, (b) as-cast CS sample and (c) as-homogenized CS sample.

Fig. 4. Backscattered electron imaging (BSE) micrographs observed in as-cast samples of (a) SRS and (b) CS; (line 1), (line 2) corresponds to the composition distribution profiles of Fe, Mg, Si in (a) and (b), respectively.

Table 1 Average partition coefficients of Mg, Si, Fe elements under SRS and CS processes.

Sample	k_Mg	k_Si	k_Fe
SRS	0.65	0.28	0.07
CS	0.51	0.24	0.04

Fig. 5. OM micrographs of cold-rolled (a) SRS and (b) as-homogenized CS samples.

Fig. 6. (a) and (c) EBSD inverse pole figure (IPF) maps of SRS-T4 and CS-T4; (b) and (d) correspond to grain size histogram of (a) and (c).

Fig. 7. TEM micrographs and corresponding selected area electron diffraction (SAED) patterns obtained from the given particles: (a) SRS-T4, (b) CS-T4; TEM micrographs and high resolution TEM images (HRTEM) of the given particles: (c) SRS-T6 and (d) CS-T6.

Fig. 8. Typical EBSD inverse pole figure maps of (a) SRS-T6 and (c) CS-T6; (b) and (d) corresponding misorientation profiles measured along the lines in (a) and (c).

Fig. 9. Typical tensile engineering stress-strain curves of SRS, CS and CSN samples at (a) T4 and (b) T6 states.

Table 2 Mechanical properties of SRS, CS and CSN samples at T4 and T6 states along with those of commercial alloys in other literatures.

Sample	YS (MPa)		UTS (MPa)		El. (%)		Source
Sample	T4	T6	T4	T6	T4	T6	Source
SRS	85 ± 1	248 ± 6	194 ± 4	295 ± 5	28.0 ± 0.5	16.5 ± 1.0	This work
CS	113 ± 2	238 ± 10	225 ± 8	288 ± 11	27.0 ± 0.5	17.5 ± 1.0	This work
CSN	119 ± 6	230 ± 10	230 ± 13	286 ± 9	23.0 ± 1.0	14.0 ± 1.0	This work
AA6022	118	238	228	—	27.5	—	[22]
	157	171	—	—	—	—	[23]
	138	200	—	—	27.5	—	[24]

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