Grain Refinement and Improvement of Solidification Defects in Direct-Chill Cast Billets of A4032 Alloy by Melt Conditioning

Abstract

Melt-conditioned, direct-chill (MC-DC) casting is an emerging technology to manipulate the solidification process by melt conditioning via intensive shearing in the sump during DC casting to tailor the solidification microstructure and defect formation. When using MC-DC casting technology in an industrial scale DC cast billet of an A4032 aluminum alloy, significant grain refinement and uniform microstructure can be achieved in the primary a-Al phase with fine secondary dendritic arm spacing (SDAS). Improved macrosegregation is quantitatively characterized and correlated with the suppression of channel segregation. The mechanisms for the prevention of channel segregation are attributed to the increased local cooling rate in the liquid-solid phase region in the sump and the formation of fine equiaxed dendritic grains under intensive melt shearing during MC-DC casting. A critical cooling rate has been identified to be around 0.5 to 1 K/s (°C/s) for the channel segregation to happen in the investigated alloy based on quantitative metallographic results of SDAS. Reduction and refinement of microporosity is attributed to the improved permeability in the liquid-solid phase region estimated by the Kozeny-Carman relationship. The potential improvement in the mechanical properties achievable in MC-DC cast billets is indicated by the finer and more uniform forging streamline in the forgings of MC-DC cast billet.

Numerical Studies of Batch and Inline High Shear Melt Conditioning Technology Using Different Rotors

“…High shear melt conditioning technology [8,9] proposes an efficient melt alternative by using "harmful" oxides and other inclusions to strengthen the resulting cast upon treatment [10][11][12]. Intensive mixing deagglomerates the metallic inclusions and redistributes them throughout the melt, thereby reducing the defects in the resulting cast [13,14]. Melt conditioning also modifies the flow pattern in the sump of direct-chill (DC) casting with the increased heat transfer and temperature homogenization around the rotor-stator mixer resulting in finer, equiaxed grains [13,15].…”

Numerical Assessment of In-Line Rotor–Stator Mixers in High-Shear Melt Conditioning (HSMC) Technology

“…High shear melt conditioning (HSMC) technology results in improved thermophysical properties, extrudability, and machinability and reduces the occurrence of defects in the treated light alloy melt, 1,2 without the addition of chemical grain refiners. The beneficial effects of HSMC are attributed to grain refinement by the deagglomeration of inclusions.…”

Melt Conditioned Direct Chill (MC-DC) Casting of AA-6111 Aluminium Alloy Formulated from Incinerator Bottom Ash (IBA)

“…This is typically in the form of an Al-Ti-B master alloy [10]. Due to some disadvantages of chemical refinement like high cost, limitation of using grain refiner with cast alloys and interaction of grain refiner with present solutes, alternative methods such as electromagnetic stirring, ultrasonic cavitation and intensive melt shearing have been investigated [11].…”

“…For in-situ microstructural control and to overcome solidification defects, melt conditioned direct chill (MC-DC) casting technology has been developed at BCAST (Brunel Centre for Advanced Solidification Technology), and has been used for the production of industrial-scale billets by inserting a high shear rotor-stator device in the sump of the DC mould during the casting process [11]. The main advantages of high shear melt conditioning (HSMC) are the homogenisation of melt composition prior to the solidification, enhancement of both diffusion and chemical reaction processes within the melt, dispersion and fragmentation of oxide agglomerates and increased wettability of dispersed oxides.…”