Semisolid Metalworking (SSM) is a hybrid manufacturing process that incorporates the advantages of both casting and forging. However, high volume commercial production has suffered from inadequacies in process monitoring and control. A key process parameter is the SSM solid-liquid fraction, which is a sensitive indicator of the deformation and flow behavior of the material during forming. Unfortunately, no sensor currently exists to provide an in-situ measurement of this important parameter. Here we present preliminary results for an advanced ultrasonic sensor to measure solid fraction. With the aid of metal alloy phase diagrams the binary alloy Sn-Bi was chosen to simulate behavior of Al/Si (e.g., 300 series) alloys. The phase diagram was used to provide the value of solid fraction at any given temperature. Measurements with longitudinal ultrasonic waves at 1 MHz in the selected alloy heated to different solid fractions were performed with a combination of commercial transducers and quartz buffers penetrating into a temperature controlled furnace. Five compositions were used ranging from 83% to 100% -Bi in the binary Sn-Bi alloy. Presented are measurements of longitudinal wave velocity as a function of temperature across the entire range from totally solid to semi-solid to liquid states in both heating and cooling runs. The curves show the characteristic phase transitions. A quasi-static viscoelastic model based on Atkinson, Kytömaa and Berryman was prepared and modified for the SSM application. The comparison of the model calculations with the experimental results were in reasonable agreement. With the recent development of high temperature ultrasonics our results provide a potential solution to the development of an ultrasonic sensor for SSM.