The low-melting metals Bi, Pb, and Sn are added to several Al alloys to improve machineability. Bi and Pb exhibit miscibility gaps with Al in the liquid phase, and Sn exhibits a metastable liquid miscibility gap with Al. Salient features of the phase diagram Al-Mg-Si-(Bi, Pb, Sn) will be summarized and issues in the thermodynamic modeling will be discussed. A method for modeling solidification paths for these alloys will be described.