MICROSTRUCTURAL EVOLUTION DURING PARTIAL REMELTING OF AL-SI7MG ALLOYS

Forming of alloys and composites in the semi-solid state, i.e. within the solidification range, is recognized as a technology offering sever al potential advantages over casting and forging, such as reduction of macrosegregations, reduction of porosity and low forming efforts. The typical non-dendritic microstructure needed is constituted of solid p hase globules suspended in the liquid phase. This paper describes the influence of the thermomechanical history on the obtaining of such a g lobular microstructure during partial remelting and subsequent isother mal holding of Al-Si7Mg thixocasting feedstock alloys. A shape factor capable of a realistic description of the microstructural evolution is introduced. This shape factor is used to describe the influence of in itial solidification conditions (like electromagnetic stirring, solidi fication time, modification of the eutectic and grain refinement) and thermomechanical treatments prior to partial remelting (like solutioni zing and cold rolling) on the microstructural evolution during partial remelting of Al-Si7Mg thixocasting feedstock alloys.