Development of an automated non-contact ultrasonic texture measurement system for sheet metal

摘要

We report on the further development of an automated PC-based system for determination of the crystallographic texture of sheet metals by ultrasonic measurement of the zero-order symmetric (S_(0)) Lamb wave mode velocity. Measuring the in-plane anisotropy of S_(0) mode velocities gives information on the crystallographic texture of the sheet. This can be used as an indication of the suitability of the metal for cold forming techniques. Currently, texture information is determined by x-ray pole figure measurements, which are slow, expensive and unsuitable for online quality control. The effect of coil geometry on the electromagnetic acoustic transducer (EMAT) generated S_(0) mode has been investigated. Developments in the EMAT design have led to considerable improvements in performance over our previous work, allowing measurements to be made at increased stand-offs of up to several millimetres on an increased range of metal sheets including up to 3 mm thick steel with pulse repetition rates of up to 60 Hz. Using 360° velocity scans reveals texture asymmetry in certain aluminium sheets which would not be detected by limiting the scan to velocities in one 90° quadrant. This asymmetry is shown to lead to significantly different crystallite orientation distribution coefficients (ODCs) depending which quadrant is looked at.