ON THE MECHANISM OF GRAIN REFINEMENT BY ULTRASONIC MELT TREATMENT IN THE PRESENCE OF TRANSITION METALS

Ultrasonic melt treatment is known to induce grain refining in aluminum alloys. The degree of grain refinement is strongly linked to the stage of solidification when the treatment is applied and on the alloy composition. In the latter case the presence of grain refiners is important. In this paper it is shown that strong grain refining can be achieved in aluminum and its commercial alloys when the ultrasonic treatment is combined with the introduction of Zr and Ti. Ultrasonic processing is performed in the temperature range of the primary solidification of an intermetallic phase, i.e. normal casting temperatures of aluminum alloys. Dual mechanism is discussed involving (1) the dispersion and refinement of primary intermetallic particles that act as solidification sites and (2) growth restriction by the transition metal(s) that are present in the liquid phase.