There is a lack of specific alloys for thixoforming processes. Usually alloys from conventional casting processes were fitted to the thixoforming purpose, e.g. A356, A357. The aim of this study is to produce new Al-Si-Mg alloys specially tailored for thixoforming purpose. The as-rheocast: Al-7 wt%Si-0.5 wt%Mg, Al-4 wt%Si-0.5 wt%Mg, Al-1 wt%Si-0.5 wt%Mg, Al-7 wt%Si-1 wt%Mg, Al-4 wt%Si-1 wt%Mg and Al-1 wt%Si-1 wt%Mg were produced by grain refining. Thus, the effect of Si and Mg content on structure and rheological behavior of these new alloys was analyzed. To simulate the semi-solid behavior at a high temperature, the samples were re-heated to the semi-solid state to produce 45% solid fraction and their structural evolution was analyzed after 0, 90 and 210 s at this solid fraction. At the same temperature and holding time, the samples were submitted to a compression test using a parallel platen viscosimeter. Results showed that the grain/globule size increased as the Si and Mg content increased. Al-7 wt%Si-0.5 wt%Mg, the Al-4 wt%Si-0.5 wt%Mg and the Al-1 wt%Si-0.5 wt%Mg are suitable for thixoforming purposes. Al-Si alloys with 1.0 wt%Mg showed low apparent viscosity values. (c) 2005 Elsevier B.V. All rights reserved.406416716373