Abstract
207mm diameter direct chill (DC) cast billets of 6063 aluminium-magnesium-silicon (Al-Mg-Si) alloy were produced with various different primary aluminium (α-Al) grain structures including feathery-dendrites, equiaxed-dendrites and equiaxed-globular morphologies. To control the α-Al grain structure (grain morphology and grain size) an intensive shearing melt conditioning technique and Al-5Ti-1B grain refiner were used. For the first time, due to the variety of controlled microstructures produced in the DC billets, it was possible to study and determine the role of the α-Al grain structure on the iron (Fe) bearing intermetallics (Fe-IMCs). The size, shape and three dimensional (3D) inter-connectivity of the Fe-IMCs were observed to be affected by the modification of the primary α-Al grain morphology and grain size. Although both “αc-AlFeSi” and “β-AlFeSi” phases are present in all billets, β-AlFeSi phase is dominant in the billets with grain refiner addition while αc-AlFeSi is dominant in the billets without grain refiner. This suggests that the addition of Al-5Ti-1B grain refiner plays a more significant role in the intermetallic phase selection than the primary α-Al grain morphology or grain size.
Highlights
Wrought AA6xxx series Al alloys are typically produced in either an ingot or billet form using direct chill (DC) casting, and are suitable for manufacturing roll or extruded products [1]
The straight and ragged boundaries are the result of coherent crystallographic twinning of the primary dendrite trunk [36] and dendrite impingement, respectively
The presence of the heterogeneous nucleation substrates suppresses the growth of the columnar crystals and more equiaxed grains are produced as shown in Fig. 1(c, d) of the GR samples
Summary
Wrought AA6xxx series Al alloys are typically produced in either an ingot or billet form using direct chill (DC) casting, and are suitable for manufacturing roll or extruded products [1]. The appearance and performance of products formed from DC cast billet depends on the nature of “in-soluble” secondary phase particles ( Fe-IMCs) present in the as-cast microstructure [2,3,4,5]. The cast billet is usually subjected to different downstream processes, such as heat treatment, extrusion and rolling [6,7,8,9]. Our previous research revealed that the type, size and distribution of the Fe-IMCs in the final processed product strongly depends on their features in the as-cast billets [10,11,12]. The as-cast IMC particle features dictate whether a pre-heat-treatment step is required for successful rolling/extrusion [12]. It is essential to understand and control the IMCs during solidification
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