Formation of intermetallic phases in unrefined and refined AA6082 Al alloys investigated by using SEM-based ultramicrotomy tomography

doi:10.1016/j.jmst.2022.02.007

Abstract

Abstract:

The spatial arrangement, distribution and morphology of Fe-bearing intermetallics in AA6082 alloys depends on the manufacturing process of the alloy and thus influences the macroscopic properties. Here, the microstructure of a near industrial scale casting AA6082 Al alloy fabricated by: (a) direct chill casting, (b) Al-5Ti-1B grain refiner addition and (c) intensive melt shearing has been investigated by three-dimensional visualization using SEM-based serial ultramicrotomy tomography. The formation sequence of phases in AA6082 alloys is generally categorized into four stages: formation of α-Al grains, Fe-bearing intermetallics, Mg₂Si phase, and eutectic rosettes. Results of three-dimensional visualization of the microstructure indicated that TiB₂ particles not only could nucleate Fe-bearing β-intermetallics, but also could provide substrate for the formation of Fe-bearing α-intermetallics and Mg₂Si. A further deep analysis reveals that the essential condition for the formation of secondary phases such as Fe-bearing intermetallics and Mg₂Si phase is the build-up of a supersaturated solute front at the α-Al solid-liquid interface irrespective of the specific nucleation site. In addition, the results indicate that grain refinement processing causes the severe interconnectivity of Fe-bearing α-intermetallics. However, the intensive melt shearing is a better manufacturing process because the intermetallics are more evenly distributed and refined than with the addition of the grain refiner, thereby improving the properties of the alloy.

Key words: Al-Si-Mg alloy, Grain refinement, Al-Fe-Si intermetallic phase, Tomography, Ultramicrotomy, Mg₂Si

J.M. Yu, T. Hashimoto, H.T. Li, N. Wanderka, Z. Zhang, C. Cai, X.L. Zhong, J. Qin, Q.P. Dong, H. Nagaumi, X.N. Wang. Formation of intermetallic phases in unrefined and refined AA6082 Al alloys investigated by using SEM-based ultramicrotomy tomography[J]. J. Mater. Sci. Technol., 2022, 120: 118-128.

Figures/Tables 17

Table 1. Chemical composition of the investigated AA6082 alloy (in wt.%).

Alloy	Si	Mg	Mn	Fe	Cu	Cr	Ti
AA6082	1.05	0.7	0.5	0.18	0.08	0.07	0.02

Fig. 1. SEM images obtained with BSE detector showing the α-Al grains of AA6082 Al alloys (a) unrefined, (b) with Al-5Ti-1B refined and (c) melt conditioned. The secondary phases are distributed along the grain boundaries and inter-dendritic region (arrows indicated some typical intermetallics).

Fig. 2. Three-dimensional visualization of morphology and spatial arrangement of Fe-bearing intermetallics (magenta), Mg2Si (yellow) and eutectic rosettes (Si in cyan) in (a) unrefined Al alloy, (b) Al-5Ti-1B refined Al alloy, (c) MC refined Al alloy.

Fig. 3. Three-dimensional visualization of morphology of Fe-bearing intermetallics in (a) unrefined Al alloy, (b) Al-5Ti-1B refined Al alloy, (c) MC refined Al alloy. Two types of α-Fe intermetallic phases to be distinguished: small isolated and globular α-Fe intermetallic phases within eutectic rosettes; and huge dendritic α-Fe intermetallic phases at grain boundaries and in inter-dendritic regions.

Table 2. Fe-bearing intermetallics, their numbers, volume fraction and the maximum length were calculated from a volume of 400 µm × 400 µm × 250 µm for all investigated alloys. (Note: “Length” is calculated by 3D maximum of the Feret diameters.). The average size of the α-Al grains was calculated from 2D images. (The error given is standard deviation σ.).

Alloy	Numbers of measured intermetallics	Total intermetallics volume fraction (%)	Largest intermetallics length (µm)	Largest intermetallics volume (µm³)	Average Al grain size (µm)
DC	160,717	1.12	280.33	72,221.7	403 ± 89
GR	159,602	1.06	369.25	69,466.3	178 ± 45
MC	107,900	0.94	205.15	12,413.3	211 ± 62

Fig. 4. Typical quantitative analysis of (a) length frequency and volume fraction of Fe-bearing intermetallics and (b) distribution of total volume fraction of Fe-bearing intermetallics corresponding to each length range. The representative significance of Fig. 4(b) is the concentration range of the size distribution of Fe-rich intermetallics. Except for a few large Fe-rich intermetallics for GR and MC, the particle sizes in GR and MC are much smaller than that of DC.

Fig. 5. (a) SEM image of Fe-bearing α-intermetallics at the triple junction of inter-dendritic region of DC alloy; (b) 3D visualization of one of the Fe-bearing α-intermetallics phase (indicated by the rectangle in Fig 5(a)).

Fig. 6. 3D morphology of secondary phases in inter-dendritic boundary in DC alloy: (a) typical morphologies of a complete Fe-bearing α-intermetallics (magenta); (b) 3D visualization of inter-connected secondary Fe-bearing intermetallic phases, Mg2Si phase (yellow) and eutectic rosettes Si (cyan).

Fig. 7. A complete dendritic Fe-bearing α-intermetallics (in magenta) of GR alloy at grain boundary/inter-dendritic boundary initiated from TiB2 particle covered by Si. More details of marked yellow square area are shown in an enlarged square: 2D SEM and 3D images of TiB2 (purplish blue) and Si (cyan) structure.

Fig. 8. Spatial distribution of Fe-bearing α-intermetallics in GR alloy: (a) TiB2 particle on α-intermetallics arm; (b) TiB2 particle wrapped by Mg2Si. (Fe-bearing α-intermetallics in magenta, Mg2Si phase in yellow and TiB2 in purplish blue).

Fig. 9. Spatial distribution of (a) a complete dendritic Fe-bearing α-intermetallics (magenta); (b) Mg2Si (yellow) distribution surrounding dendritic Fe-bearing α-intermetallics; (c) Mg2Si and eutectic rosettes Si (cyan) distribution surrounding dendritic α-intermetallics in GR alloy.

Fig. 10. Formation environment of Mg2Si phases and eutectic rosettes in GR alloy. (Fe-bearing intermetallic phases in magenta, Mg2Si phase in yellow, TiB2 in purplish blue and eutectic rosettes Si in cyan).

Fig. 11. Three-dimensional visualization of intermetallics in GR alloy: (a) distribution of eutectic Si (cell) in cyan and Fe-bearing α-intermetallics (purple) around β-intermetallics (magenta); (b) connected Fe-bearing β-intermetallics; (c) wrapping of Fe-bearing β-intermetallics by Mg2Si phase (yellow); (d) 2D SEM- image of nucleation of Fe-bearing β-intermetallics on TiB2 (in bright white).

Fig. 12. 3D morphology of highly branched Fe-bearing α-intermetallics and surrounding Mg2Si and eutectic rosettes in MC alloy: (a) highly branched Fe-bearing α-intermetallics (magenta) and surrounding Mg2Si (yellow); (b) highly branched α-intermetallics and surrounding eutectic Si rosettes (cyan).

Fig. 13. Scheil simulation of AA6082 alloy using Thermo-Calc software with TTAL5 database.

Fig. 14. Grain structure of (a) DC, (b) GR and (c) MC refined alloys observed using optical microscopy with polarized light and dark field imaging.

Fig. 15. SEM image of microstructure showing a typical evidence that Mg2Si phase forms during ripening of α-Al (marked by arrows).

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