Ultrasonic Grain Refinement of Magnesium and Its Alloys

Achievement of a uniform fine grain structure in cast magnesium (Mg) alloys is desired as it improves structural uniformity, reduces segregation and associated casting defects, enhances consistency in performance, and facilitates the solid-state forming of various wrought Mg products. Inoculation is a standard grain-refining practice for commercial cast alloys. However, many commercial alloy systems do not have an established inoculating agent or chemical grain refiner. In the context of Mg alloys, although aluminium-free Mg alloys can be readily grain-refined with zirconium (Emley, 1966; Qian & Das, 2006; Qian, 2006), it has proved difficult to grain-refine the more common Mg-Al based alloys by inoculation (StJohn et al., 2005; Qian & Cao, 2005). Alternative approaches to grain-refining these alloys are desirable. Ultrasonic irradiation during solidification is one such alternative that has proved to be effective for Mg-Al based alloys, first demonstrated in the former USSR countries (Abramov, 1994; Eskin, 1998). It is also effective in grain-refining other metallic materials (Eskin, 1998; Jian et al., 2005; Jian et al., 2006a; Liu et al., 2007; Xu et al., 2008) and ice (Zhang et al., 2001). The recent renewed interest in the process for Mg alloys (Jian et al., 2006b; Ramirez & Qian, 2007; Zhang et al., 2007; Liu et al., 2008; Ramirez et al., 2008; Ramirez et al., 2009; Qian & Ramirez, 2009; Qian et al., 2009; Qian et al., 2010a) is driven by its potent grain-refining capability, ease of operation, and requirement for inexpensive equipment. In addition, the melt surface is left almost undisturbed during irradiation. As a result, there is little concern over contamination by oxidation. In fact, molten Mg and its alloys are particularly suited to ultrasonication for structural refinement, partially because of their low viscosity (η), where ηMg = 1.25 x 10-3 N s m-2 at 651 °C (Brandes & Brook, 1992), being almost the same as that of water, where ηWater = 1.002 x 10-3 N s m-2 at 20°C (Lide, 2001). Low viscosity implies low attenuation (see Eq.(2)), which is important for the structural refinement of a large volume of melt. However, the lack of fundamental understanding of the ultrasonic refining process has largely restricted its development and application.