Ultrasounds: a New Technology for Alloys Degassing, Grain Refinement and Obtainment of a Thixotropic Structure

The aim of the present research is to investigate the improvements induced by the use of power ultrasound to foundry alloys, instead of wrought ones, so as to promote its application in the production of castings. It is a fact that grain refinement and degassing by themselves do not guarantee high performance of cast components. In fact, apart from those defects given by dissolved hydrogen or coarse microstructure, a further problem for casting quality is the po-rosity produced by turbulent flows during die filling, mainly in the case of die-casting. Differ-ent approaches have been studied in order to avoid porosity caused by turbulent filling, as injecting into the die cavity an alloy in semi-solid state, i.e. characterized by a thixotropic microstructure consisting of primary phase globules suspended in a liquid matrix. Power ultrasounds applied to the metal, at a temperature between the liquidus and the soli-dus, enables the fragmentation of the dendrites under development, as well as an increase of the number of active nuclei, resulting in a globular microstructure appropriate for semi-solid applications. Therefore, a further aim of this research was to study the suitability of the US technique for producing semi-solid feedstock directly from molten metal, as an energy effi-cient and easy alternative to the traditional routes.