Al-Mg-Si based alloys have been developed to produce an advanced feedstock material for thixoforming. Effects of excess Si, Mn addition and deformation on microstructure evolutions during reheating and partial remelting were studied. The homogenized microstructures of the Mn-added Al-Mg-Si alloys consist of the Al12Mn3Si2 intermetallic compound along grain boundaries of equiaxed grains with fine-spherical and needle-like morphologies. The deformation by cold-rolling has a strong effect on the semi-solid microstructures of the Mn-added Al-Mg-Si alloys compared with the balanced and excess Si alloys. The Mn-added Al-Mg-Si alloys exhibit good features to produce the feedstock material for thixoforming.