Abstract
The twin‐roller casting process is a novel technology used to produce aluminium and other alloys. In this study, the Al‐Fe‐Si alloy was twin‐roller cast into a billet with a thickness of 7.0 mm. This was followed by an annealing process at 480°C for 16 hours. Abnormal grains of more than 15 mm in the nominal diameter were found to have formed on the surface of the billet. Scanning electronic microscopy observation, energy diffraction spectrum, microarea X‐ray diffraction, and electronic backscattered diffraction analysis were performed to study this abnormal grain growth. The results showed that abnormal grain nucleates in the region of (100) <001> texture formed in the twin‐roller casting process. The growth of abnormal grain was governed by the coalescence of the abnormal grain with its surrounding normal grains, with an average orientation angle of 47° between both the types of grains. High‐strain‐concentrated regions on the surface layer of the billet were induced by the heterogeneously distributed Fe particles formed in the twin‐roller casting process. The presence of these regions accelerates the abnormal grain growth in the following annealing process of the billet.
Highlights
The special solidification characteristics of the twin-roller casting (TRC) process result in the special distribution of secondary precipitates and the special grain orientation distribution in the billet [4]. e fine secondary precipitates are resulted from the relatively high cooling rate in the billet in the solidification process. ese precipitates grow slowly because the duration time of the billet at elevated temperatures is short in this solidification process
An annealing process is often necessary before rolling this kind of TRC billet with the heterogeneous microstructure. is raises the question as to how this kind of special microstructure evolved in the annealing process
EDS analysis shows that the main precipitates in the TRC billet are iron enriched ones (Fe precipitates). e microstructure of the TRC billet surface after annealing is presented in Figure 1(b). ese show that an abnormally coarsened grain formed on the surface of the annealed TRC billet. is was much larger than the surrounding grains with a size of 15 mm
Summary
The special solidification characteristics of the TRC process result in the special distribution of secondary precipitates and the special grain orientation distribution in the billet [4]. e fine secondary precipitates are resulted from the relatively high cooling rate in the billet in the solidification process. ese precipitates grow slowly because the duration time of the billet at elevated temperatures is short in this solidification process. The special solidification characteristics of the TRC process result in the special distribution of secondary precipitates and the special grain orientation distribution in the billet [4]. E grain orientation distribution at the surface of the billets may result from both the heterogeneity of the thermal field and the shearing strain from the rollers during the solidification process. An annealing process is often necessary before rolling this kind of TRC billet with the heterogeneous microstructure. E purpose of this article is to understand the effect on the grain growth of the special microstructure in TRC billet in the following annealing process. Following the twin-roller casting process, the billet was annealed at 480°C for 16 hours in a muffle furnace and taken out of the furnace and cooled to room temperature in the air. Samples were sectioned along the central line parallel to the rolling direction with dimensions of 7 mm × 10 mm × 20 mm
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