When applying ultrasonic grain refinement, the formation of a fine, equiaxed grain structure is desirable in castings, because it improves mechanical properties, reduces hot tearing, facilitates feeding to eliminate shrinkage porosity, and gives a more uniform distribution of secondary phases.
Ultimately, ultrasonic grain refinement leads to the formation of a so-called ‘‘nondendritic’’ grain structure.
A distinctive feature of such a structure is the formation of globular grains without segmentation into dendrite arms. In such a case, the grain size will be equivalent to the secondary dendrite arm spacing typical of the given cooling rate.
This is the minimum grain size that one can obtain under given solidification conditions.
There are many techniques available to obtain a fine, equiaxed grain structure: deliberate addition of master alloys containing melt inoculants, the most common of which are based on the Al-Ti-B and Al-Ti-C systems; rapid solidification conditions; physico-mechanical methods, which include mechanical or magneto-hydrodynamic stirring and ultrasonic vibrations.
During ultrasonic melt treatment (UST) waves of
compression and expansion are induced in through liquid metal with a frequency above human hearing, i.e., 2 0to 21 kHz. If the acoustic pressure exceeds a certain value, which is characteristic of a particular liquid, the liquid can fail during the expansion (tensile or negative pressure) portion of the sound field producing cavities, hence the term ‘‘cavitation.’’ Weak sites within the liquid (e.g., pre-existing gas pockets, interfaces, etc. called ‘‘cavitation nuclei’’) are caused to rapidly grow,
thereby forming vapor and gas-filled cavities (bubbles) leading to intense local heating and high pressures with very short lifetimes.
In clouds of cavitating bubbles, ultrasonic grain refinement, these created hot spots may have equivalent temperatures of roughly 5000 K, pressures of about 1000 atmospheres, and heating and cooling rates above 1010 K/s.