Melt Conditioned Casting of Aluminum Alloys

Abstract

High shear melt conditioning of aluminum alloy melts disperses oxide films and provides potent nuclei to promote non-dendritic solidification leading to refined as cast microstructures for shape castings, semis or continuously cast product forms. A new generation of high shear melt conditioning equipment has been developed based on a dispersive mixer that can condition either a batch melt or can provide a continuous melt feed. Most significantly the melt conditioner can be used directly in the sump of a DC caster where it has a dramatic effect on the cast microstructure. The present goals are to expand the castable alloy range and to increase the tolerance of alloys used in transport applications to impurities to increase the use of recycled metal. The paper will review the current status of the melt conditioning technology across the range of casting options and will highlight development opportunities. Introduction Liquid aluminum alloys oxidize rapidly when exposed to oxygen-containing atmosphere. This means that aluminum alloy melts inevitably contain oxides as inclusions. Such oxides in the from of thin films reduce melt fluidity and cause blockages during die filling [1] and if they are entrained in the melt as bi-films these become cracks in the solidified metal that degrade mechanical properties. Severe oxidation decreases yield and this increases component cost. Consequently, oxide inclusions in alloy melts are usually considered as undesirable. Oxidation is prevented or reduced by using a protective atmosphere or by vacuum casting, or oxide films are removed from melts by either chemical or physical means [2]. The present refining techniques, such as filtration and rotary degassing, can refine melts to some extent by removing inclusions but they are relatively costly and time-consuming. This motivated the development of the Melt Conditioning by Advanced Shearing Technology (MCAST) process [3, 4]. High shear melt conditioning has been achieved using either a twin screw mechanism or a rotor-stator mechanism [3]. In both cases the liquid metal in is subjected to intensive shearing under a high shear rate with a high intensity of turbulence. The conditioned liquid metal has extremely uniform temperature, uniform composition and well dispersed inclusion particles. These individual particles have a both a fine size and a narrow size distribution, and most importantly, are likely to be completely wetted by the liquid metal under the intensive forced convection. In the new generation of melt conditioners the high shear mixer is a rotor-stator unit. This simple device comprises of a rotor, a stator and a housing for holding the stator and the rotor. A motor drives the rotor at speeds up to 10 4 rpm and shears the liquid metal in the gap between the rotor and the stator and also in the openings of the stator. The high shear device provides not only flow for distributive mixing but also intensive shearing for dispersive mixing. The main advantages of the high shear devices include significantly enhanced kinetics for any chemical reactions or phase transformations, well dispersed and uniformly distributed solid particles or gas bubbles, size reduction of solid particles or gas bubbles, improved homogenization of chemical composition and temperature fields, and forced wetting of the usually difficult-to-wet solid particles in the liquid metal. Hence, high shear devices can be used for conditioning alloy melts prior to