Abstract

• High shear melt conditioning in the sump during direct chill casting of A6082 billet. • Novelty of maintaining 2–7 °C above alloy liquidus as the sump thermal condition. • In-situ control of solidification during casting by intensive melt shearing. • Precise tailoring of as-cast microstructure to modify second phase particles. • Potential for industries to eliminate homogenisation and use recycled Al alloys. Controlling the formation of noncompact second phase particles during direct chill (DC) casting of aluminium alloys with grain refiner addition remains challenging, as it results in energy intensive homogenisation and deformation problems. In this work, we employed a novel strategy in the DC casting of A6082 alloy to produce billets with a fine-scale dispersion of second phase particles. The strategy involves maintaining 2–7 °C above alloy liquidus as a thermal condition in the sump by in-situ melt conditioning (MC) using a rotor-stator high-shear device operated at a critical rotation speed. As a result, in-situ control of solidification behaviour is achieved to precisely tailor the as-cast microstructure. The billet grain refinement is attained by MC-DC casting without the deliberate addition of chemical grain refiners. The microstructure of the MC-DC cast billet at the critical rotation speed showed a fine-scale dendritic structure with refined secondary dendrite arm spacing (SDAS). The solidification front proceeded with a shallow sump and a corresponding shorter solidification time, higher cooling rate, higher temperature gradient, and smooth solidification rate profile. The ideal fine-scale dendrites with low SDAS divided the remaining eutectic liquid into fine-scale and isolated liquid pockets, resulting in fine-scale, compact morphology, and uniform distribution of second phase particles in the as-cast microstructure. The MC-DC casting process showed the ability to increase the cast house production rate by increasing the casting speed without bleeding the billet. The present approach could be beneficial for eliminating or reducing the homogenisation practice and may also introduce significant flexibility in using recycled Al alloys in the industry.

Highlights

  • Direct chill (DC) casting has been a prominent technique employed in industries to produce billets of a wide range of aluminium alloys

  • Grain refinement is achieved in the A6082 billet by melt conditioned direct chill casting (MC-DC) without the deliberate addition of grain refiners (GR), which can be attributed to the heterogeneous nucle­ ation of α-Al on the dispersed natural oxide particles in the melt

  • The melt temperature in the sump during DC casting is manipulated by in-situ high-shear melt conditioning within the sump using a rotor-stator device operated at a selected rotation speed, thereby tailoring the as-cast grain structure of the A6082 alloy billet

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Summary

Introduction

Direct chill (DC) casting has been a prominent technique employed in industries to produce billets of a wide range of aluminium alloys. Controlled post-homogenisation cooling is adopted to form precipitates that are dissolved during the subsequent thermomechanical processing. This prevents the complete solutionizing of billets, where individual solute elements can increase the flow stress, as indicated by Birol (2004). Among second phase particles, Fe-bearing intermetallic compounds (IMCs) in the as-cast structure cannot be dissolved during the homogenisation process (Kumar et al, 2016). This creates limita­ tions for using recycled aluminium alloys where the Fe content in the alloy composition is beyond the target levels for downstream processing

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