Metal Refining by Filtration

Abstract

Melt purification and removal of non-metallic inclusions from the melt prior to casting has much merit. The size, shape, type and distribution of non-metallic inclusions present in the finished metal product are the performance fingerprints of the cast shop. It is advantageous to remove these non-metallic particles as they are known to reduce fluidity of the melt, increase internal porosity in the casting, reduce strength, ductility and fatigue resistance of the product and also result in poor machinability and surface finish. In general, the larger the inclusions are, the greater are their deleterious effects. Non-metallic inclusions act as stress-raisers, and can cause premature failure of a specific component. They provide not only initiation sites for fracture but also play a significant role in the propagation of the crack. In this paper the emphasis will be on inclusion removal by filtration of the melt prior to casting.

The filtration process is not one of physical separation as in screening and separation, but rather melt filtration is a two step serial transport process. First, as a result of bulk fluid flow the inclusions are transported to the filter surface. In the second step inclusion capture occurs due to interfacial or surface forces. In general, particle transport can occur by impingement, interception, sedimentation, diffusion, and other hydrodynamic effects. Particle attachment can be a result of forces developed through pressure, chemical, or Van der Waal effects. The relative dominance of each mechanism is a function of particulate type and size, fluid approach velocity, as well as temperature and media characteristics. As an example, at high temperatures (steel melts) the inclusions sinter to the filter surface, whereas lower temperatures (aluminum melts) the inclusions are attached at the medium by secondary forces.

Accurate mathematical representations of filtration processes can be powerful tools for system design and process optimization. The theoretical basis for inclusion removal by filtration is developed and applied.In addition, inclusion detection techniques will be briefly reviewed since the effect of the filtration process can only be verified by such experimental procedures: by chemical analysis, quantitative metallography, volumetric analysis, and non-destructive ultrasonic techniques.

The experimental work which is described and discussed in this paper includes filtration trials with aluminum, steel, and superalloy melts. In all of the experimental work, the inclusions being removed are well characterized. A distinction is made between solid and liquid inclusions in terms of inclusion capture mechanisms. The solid inclusions (such as Al₂O₃ in steel) are non-deformable, whereas the liquid inclusions (CaO Si0₂ in steel) are deformable. The effect of filter wetting characteristics with respect to filtration performance are discussed. Table 1 lists the melt systems investigated; filtration results for aluminum, superalloy and steel melts are presented and discussed.