Temperature model for ultrasonic measurement of titanium welds

Ultrasonic techniques can be used to detect and locate the edge of a molten weld pool. However, to determine the size of the weld pool correctly from transit time of echoes (ultrasonic data) returned from the pool, a correction due to the temperature gradients must be included. No such data for titanium has been reported so far. This data is a must to allow real-time control and monitoring of the weld quality. The present paper investigates how the measured ultrasonic data was affected by temperature. Both plain sheets and weld on plate of 2.3 mm titanium alloy sheet (Ti-6Al-4V) were used in this study. An attempt was then made to provide a methodology to determine the necessary correction when computing the weld pool diameter. It has been shown that the traveling time of the ultrasonic shear wave changes positively and the wave velocity changes inversely with temperature. Moreover, the weld pool diameter as ultrasonically measured was found to be smaller than the actual due to changes in the microstructure in the heat-affected-zone. A compensation value has been determined and applied so as to estimate the real pool diameter. To extend this methodology to a real-time application, a fast-enough data acquisition system is suggested to compute the pool size simultaneously and instantaneously. Also, using the Lagrange interpolation method, a polynomial with power of 3 has been obtained for the ultrasonic response on this titanium alloy at elevated temperatures. However, this non-linearity has no adverse impact on the result given in this paper.