It has long been established that 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, 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, these hot spots may have equivalent temperatures of roughly 5000 K, pressures of about 1000 atmospheres, and heating and cooling rates above 1010 K/s.
The formation, growth, and implosive collapse
of bubbles in liquids irradiated with sound is called ‘‘acoustic cavitation.’’ Flynn suggested two types of cavitation: (1) stable cavitation, when the bubble oscillates several times about its equilibrium radius with small excursion; and (2) transient cavitation, in which the bubble undergoes dramatic volume changes in a few acoustic cycles and violently collapses.
Both types of cavitation may occur at the same time and the bubble undergoing stable cavitation may become a transient cavity.
The bubbles will form a region of active
cavitation, which is known as the cavitation zone. The size of this region depends on the dimensions of the ultrasonic horn and the properties of the liquid. As a rule of thumb, the size is approximately the horn diameter both in height and width.
The bubbles grow during the negative pressure
portion of the sound field, until the sound field pressure turns positive. The resulting inertial implosion of the bubbles can be extremely violent,
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