A metal alloy mass for semi-solid forming is disclosed. After being air-cooled to room temperature from a temperature corresponding to a liquid fraction content of 30-70 %, the mass has a pore ratio of 2-20 % and preferably 3-8 %, as measured by image analysis. A metal alloy mass for semi-solid forming, cast from a liquid metal of which the gassing level, as measured by a solidification test under 80 hPa, is such that the sample has a volume pore ratio of 3-50 % and preferably 4-25 %, is also disclosed. The method for making said mass includes treating the alloy in a liquid state to add a soluble and chemically non-reactive gas such as hydrogen. The rheological properties of the semi-solid metal may thus be improved, whereby parts having a higher quality may be obtained.