Fig. 1. (a) Schematic diagram of the experimental setup (dimensions are in mm) and (b) actual photograph of the setup during UST.

Fig. 2. Cooling curves and macrostructures in the as-cast condition of (a and c) commercial purity Mg [26] and (b and d) high purity Zn [42].

Fig. 3. Cooling curves and macrostructures after L-UST of (a and c) commercial purity Mg [26] and (b and d) high purity Zn [42].

Fig. 4. Cooling curves and macrostructures after H-UST of (a and c) commercial purity Mg [26] and (b and d) high purity Zn [45].

Fig. 5. Cooling curves and macrostructures after O-UST of (a and c) commercial purity Mg [26] and (b and d) high purity Zn [45].

Fig. 6. Morphology of (a) coarse columnar grains in the as-cast condition, (b) coarse or mixed dendritic structure (H-UST and L-UST) and (c) fine, equiaxed, non-dendritic structure (O-UST). (d) Morphological evolution of grain structure in Zn after UST showing coarse dendrites (L-UST top region), fine dendrites (H-UST) and fine, non-dendritic grains (O-UST, L-UST middle region) [26,42,45] (d) Grain size measurements before and after UST described in (a to c). (e) Number of grains per unit area in the fine equiaxed grains zone with respect to the UST classification listed in Table 1.

Fig. 7. (a) Schematic of thermocouple locations inside and outside mould wall and the corresponding (b) cooling curves of the as-cast pure Zn. (c) Thermal gradients on the wall and inside the solidifying melt. (d) The rate of solidification measured at just above TM and at the solidification temperature. (e) Grain formation by copious nucleation mechanism during UST on the interfaces of mould wall and open top surface marked as an undercooled region (shaded region) producing equiaxed grains. After sufficient equiaxed grains are formed and settled the columnar grains continue to grow from the mould wall towards the centre.

Fig. 8. Grain area mapping for (a) Mg and (b) Zn with respect to UST superheat temperature.

Fig. 9. Fragmentation grain multiplication mechanisms: (a) Bubble erosion and fluid flow disrupting the semisolid region detaching Zn phases in Bi-8Zn alloy [35], (b) bending and cracking of the primary arm detaching the dendrite from the roots [39] and (c) fragmentation of needle shape Zn phase by a pulsating bubble [35]. (d) Zn dendrites in the present work advancing from the mould wall at the rate of 0.025 μm ms-1 during solidification and (e) fragments of grains produced by the mechanical impingements with a growing dendritic tip.

Fig. 10. Columnar to equiaxed interface of (a) Mg and (b) Zn from the side wall of the crucible.

Fig. 11. (a) Schematic of the formation of equiaxed crystals in the undercooled zone below the sonotrode-liquid interface and (b) Experimental results showing the grains captured in Al-2Cu alloy in the cavitation zone [45,65].

Fig. 12. Formation of equiaxed grains (a) in pure Al by moving the stirring frame vertically up and down at a speed of 3 cycles/second for 75 s (b) vibration applied to Al-0.2Cu alloy using a ring shaped vibrator at the surface of the melt and the corresponding macrostructure reported in Ohno experiments (schematic diagrams were redrawn after Ohno for clarity [47]) and (c) O-UST of Mg in the present work (all dimensions are in mm).

Fig. 13. Origin of equiaxed grains during pulsed magneto-oscillation of pure Al with coils placed on the sidewalls of the mould containing stainless steel mesh at (a) horizontal and in (b) vertical position [18]. Electric current pulse method using (c) parallel electrodes placed on the top of the casting [14] and (d) up-down setup of electrodes with a vertical mesh placed close to mould wall [13]. (e) Surface Pulsed Magneto Oscillation technique with coils placed at the top of the casting [55]. (f) O-UST of pure Zn in the present work.

Fig. 14. Actual (TA) and melting temperature (TM) profiles indicating the coolest region of the casting producing equiaxed and columnar grains in the (a) as-cast, (b) low superheat (L-UST or O-UST) and (c) high superheat (H-UST) conditions.