A visco-plastic deformation model is used to predict thermal stresses during casting of binary Mg-Al alloys. The predictions are compared to experiments that allow for the measurement of contraction forces during solidification and cooling. These force measurements, together with estimates from data found in the literature, are used to obtain the high-temperature mechanical properties needed in the stress model. The simulation results show generally good agreement with the measurements. The coherency and yielding behavior of the semi-solid material during solidification is investigated in some detail. It is found that coherency starts at a solid fraction of about 0.5 and that the maximum tensile stress at the final solidification temperature is 1.5 MPa and 4 MPa for Mg-1 and 9wt.%Al alloys, respectively.