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

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.