Ultrasonic evaluation of semi-solid metals during processing

Ultrasonic velocity and attenuation measurements for three different semi-solid (SS) aluminum (Al) alloy billets (A356, A357 and 86S) and one dendritic sample (A356) were performed in the temperature range between room temperature and ~575 °C using a non-contact laser-ultrasonic technique. It was found that there are measurable differences in ultrasonic velocity and attenuation between SS and dendritic samples and these differences are larger as the temperature becomes closer to the complete melt state. In a laboratory setup simulating the barrel section of a thixomoulding or a rheomoulding machine, a clad buffer rod (high temperature probe) was used to perform real time ultrasonic monitoring. We have observed 10 MHz ultrasonic signals of 30 dB signal-to-noise ratio reflected from a rotating screw with a distance of 5.5 mm between the probe end and the root of the screw in a viscous liquid Al alloy (A356) at 610 °C. This information should enable us to determine whether the processed part has the desired thixotropic structure or not, and then to properly adjust heating and other process parameters.