Identification of key liquid metal flow features in the physical conditioning of molten aluminium alloy with high shear processing

Abstract

Although treating molten alloy with high shear processing (HSP) can dramatically refine the microstructure of solidified aluminium alloys, it was also recently employed as part of an effective route to purification of contaminated aluminium alloy scrap. The key mechanisms of HSP include the dispersion of large aluminium oxide films and clusters into very fine oxide particles by the high shear rate, and the redistribution of bulk melt by the agitation. These fine oxides act as nucleation sites for iron-based intermetallic phases, the formation of which is a precursor to purification of the alloy. Macroscopic flow features of HSP, such as flow rate and shear rate, influence its performance significantly. Simulation based on Computational Fluid Dynamics was used to predict key features of fluid flow during HSP in a static direct chill (DC) caster. It was found that the distribution of shear rate and mass flow rate is highly nonuniform in the caster, and only in the close vicinity of the mixing head is there a relatively high level of shear rate and effective melt agitation. Therefore, effective dispersion of oxide films and clusters, and resulting significant nucleation of the intermetallics and/or primary aluminium phase, can only occur near the mixing head, and not throughout the whole crucible. Confidence in the model validity was built, by comparison with post-solidification microstructures in a previous experiment with similar process parameters and geometry.

Variation improvement of mechanical properties of Mg-9Al-1Zn alloy with melt conditioned high pressure die casting

“…As an improving approach of high shear melt conditioning, the fine solid distribution in the band would reduce the formation possibility of porosity by forcing the band solidification prior to the adjacent regions. The fundamental mechanism of high shearing on grain refinement is that it can disperse the MgO film to MgO particles efficiently by very high shear rate of about 10 5 s -1 [37], and the dispersed MgO particles are the potency heterogeneous nuclei for α-Mg grains during solidification [31]. During high shear processing, the melt was sucked into rotor-stator chamber from the bottom of high shear unit, and forced to enter the shear region to suffer the high shear rate and then pass through the drain holes to complete one cycle of high shearing.…”

Enhancement of chip breakability of aluminium alloys by controlling the solidification during direct chill casting

“…The negative pressure that is generated by the rotor blades sucks the melt upwards and forces flow out of the stator holes. As a result, a turbulent flow with high shear rate is generated in the rotor stator mixer [17]. The advantage of MC-DC casting is that it provides both dispersive and distributive mixing in the sump, while keeping the melt surface relatively stable.…”

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

“…8 (2) The large flow rate through the rotor-stator mixer redistributes the intensively sheared melt in the bulk of the mixing vessel. 9,10 The flow pattern inside the melt bulk is pivotal to both dispersive and distributive processes and affects the efficiency of the mixer. This mixing zone, commonly referred to as a pseudo-cavern for shear thinning liquids, 11 is of paramount interest in the study of rotor-stator mixers.…”

“…13 Optimization of HSMC necessitates understanding of the dynamics of mixing and the flow inside stirred vessels, calling for both experimental investigations 12,[14][15][16] and numerical simulations. 9,10 HSMC technology has proven successful in reducing grain sizes in direct-chill (DC) cast billets, 1,4 a process called melt-conditioned direct-chill (MCDC) casting, and in controlling the size of secondary JOM https://doi.org/10.1007/s11837-020-04320-3 Ó 2020 The Author(s) dendrite arm spacings (SDAS). Widespread extension of HSMC to other continuous casting processes is desirable, with potential benefits including a decrease in per unit cost of production and enhanced homogeneity in the cast products.…”