The effect of ultrasonic treatment on the mechanisms of grain formation in as-cast high purity zinc

[Display omitted] •Ultrasonic treatment (UST) of high purity Zn produced a grain refined structure.•Macrostructures have columnar, refined equiaxed and mixed equiaxed-columnar zones.•The number of grains produced by UST is affected by both time and temperature.•The largest equiaxed zone forms when UST is applied from 440 °C for 3 or 4 min.•High superheat or longer UST time at low superheat reduced the number of grains. The potential for producing a large refined equiaxed zone by ultrasonic treatment (UST) of high purity zinc was investigated in order to improve the mechanical performance and formability. The macrostructure of cast ingots changed from large columnar grains without UST to three zones of fine columnar grains adjacent to the mould walls of the ingot, a refined equiaxed zone and a zone of a mixture of coarse equiaxed and columnar grains. A small zone of equiaxed grains was obtained when UST was applied during cooling from 440 °C to 419 °C for 2 min. The size of the equiaxed zone increased from about 20% of the casting’s cross section to 50% when UST was applied for 3 or 4 min. In contrast, the application of UST for a longer time from a higher temperature (450–419 °C for 4 min) resulted in a smaller equiaxed zone of 18% indicating that a specific combination of UST time and temperature is required for the formation of a large equiaxed zone. The factors affecting the formation of the equiaxed zone throughout the solidification cycle are described.