This paper describes a finite element model for predicting the temperature field in high power ultrasonic welding aluminum AA6111 to two dissimilar alloys, magnesium AZ31, and low carbon steel DC04. Experimental thermocouple and other evidence are used to infer the magnitude and distribution of the heat input to the workpiece, as a function of time, for each of the material combinations welded. The resulting temperature histories are used to predict the growth of intermetallic phases at the interface in Al-Mg welds. The microstructural model successfully predicts the thickness of the intermetallic layer, but the sensitivity of the results to temperature is demonstrated.