Particle dispersion in aluminium and magnesium alloys

Abstract

High shear mixing offers a promising solution for particle dispersion in a liquid with intensive turbulence and high shear rate, and has been widely used in the chemical, food and pharmaceutical industries. However, a practical high shear mixing process has not yet been adapted to solve the particle agglomeration in metallurgy due to the high service temperature and reactive environment of liquid metal. In this study, the effect of high shear mixing using the newly designed rotor-stator high shear device have been investigated with both Al and Mg matrix composites reinforced with SiC particles through casting. The microstructural observation of high shear treated Al and Mg composites show improved particle distribution uniformity in the as-cast state. Increased mechanical properties and reduced volume fraction of porosity are also obtained in the composite samples processed with high shear. With the melt conditioning procedure developed for twin roll casting process, two distinct solutions has been provided for thin gauge Mg strip casting with advanced microstructure and defect control. The melt conditioning treatment activates the MgO as heterogeneous nuclei of α-Mg through dispersion from continuous films to discrete particles. Thus enhanced heterogeneous nucleation in the twin roll casting process not only refines the α-Mg grain size but also eliminates the centre line segregation through equiaxed grain growth and localized solute distribution. The grain refinement of the α-Mg through SiC addition has also been studied through EBSD and crystallographic approaches. Two reproducible and distinct crystallographic orientation relationships between α-SiC (6H) and α-Mg have been determined: [1010]SiC//[2113]Mg, (0006)SiC//(1011)Mg, (1216)SiC//(2202)Mg and [0110]SiC//[1100]Mg, (0006)SiC// (0002)Mg, (2110)SiC//(1120)Mg.