**1. Introduction**

The fact that by introducing solid metal into the melt one can achieve structure refinement is rather well known. As early as in the 1930s–1950s a series of papers and patents have been published, showing the benefits for grain refinement when dissolving solid metal in the melt before solidification [1,2]. Later on, this way of structure refinement was developed further and dubbed "suspension casting" or introduction of "internal chills" [3,4]. The solid metal was added in a form of cut wire, cut sheet, or powder, with the resulting grain refining, elimination of columnar grain structure in ingots and castings of steel, copper and aluminium alloys. The underlying mechanisms have been suggested as (1) rapid cooling of the melt due to the latent heat consumption upon melting of the solid metal with the resultant melt undercooling and (2) introduction of many solidification substrates in the form of crystal fragments and active non-metallic inclusions [1,5]. A number of patents have been filed where either the same solid alloy or a master alloy with additions (e.g. Ti for Al alloy) is introduced in the amounts up to 50% (typically less than 10%) into the melt close to the liquidus temperature [6–9]. The main reason why this technique is not widely used in industry is the possible incomplete dissolution of the solid parts introduced into the melt with ensuing inhomogeneous as-cast structure and potential defects. In addition, the selection of the temperature range where the technology works the best is not clear.

Another well-documented means of controlling the grain structure of as-cast metals is ultrasonic melt processing (USP) [10–13]. There are well-studied mechanisms through which USP affects the structure, i.e., wetting of non-metallic inclusions, cavitation-assisted heterogeneous nucleation, fragmentation of primary intermetallics and dendrites and enhanced mixing of the liquid volume [13]. Although very powerful in grain refinement, this technology suffers from instrumental issues, i.e.,

the ultrasonic tool (sonotrode) that is used for direct introduction of high-frequency vibration into the melt is subject to cavitation erosion and (if made of metal) gradual dissolution. The choice of the sonotrode material is, therefore, very important and it has been demonstrated that Nb-based alloys are most stable under cavitation condition in the aluminium melt [10]. But even sonotrodes made from Nb alloys suffer eventually from erosion. At the same time, some schemes that use a "consumable" sonotrode have been suggested in ultrasonic welding (wire feeding) and in electro-slag remelting (vibrating electrode) as reviewed elsewhere [11,12]. Such a scheme opens some new avenues in ultrasonic melt processing as it eliminates the issue of sonotrode material selection. It is also well known that ultrasonic vibrations significantly accelerate the dissolution of solid metals in the melt as well as promote rapid mixing of solutes through accelerated diffusion, eliminating the build-up of the solute-rich layer at and facilitating a better excess of the fresh melt to the solid/liquid phase [10,14].

In this study, we attempted to combine these two technologies to achieve a synergetic effect.
