Non-Contact Non-Destructive Measurement of Texture Using an Electro-Magnetic Acoustic Transducer (EMAT) Sensor

Abstract

The formability of sheet metals is strongly influenced by, and can be predicted from, crystallographic texture, and is generally assessed in terms of an r-value and/or n-value off-line from tensile test samples. There is interest in the development of a non-destructive, cheap and simple to operate system for texture assessment. Ultrasonic velocity is directly related to a material’s elastic modulus and metal single crystals can have significantly different elastic properties along their principal crystal axes. Hence, if a polycrystalline sample has preferred texture then variations in ultrasonic velocity with angle to the rolling direction are expected. In this work the ultrasonic velocity anisotropy, measured using a noncontact electro-magnetic acoustic transducer (EMAT) system, with respect to sheet rolling direction was determined and compared to calculated elastic modulus anisotropy, using quantified texture components (from X-ray diffraction or EBSD and their known individual anisotropies), and mechanically measured modulus values, at 0°, 45° and 90° to the rolling direction, for aluminium and steel sheets. Predictions of elastic anisotropy based on surface texture determination, as characterised by X-ray diffraction or surface EBSD, gave poor correlations with EMAT velocity anisotropy for aluminium sheets that contained significant through thickness texture variations, however, accounting for this using multiple EBSD scans through thickness gave good correlations. For steel it was found that the EMAT velocity anisotropy matched the measured modulus variation with angle, with differences between samples with different textures (as-rolled and heat treated conditions) being observed. However the predicted modulus variation did not show much difference between samples, resulting in some discrepancies with the EMAT velocity and measured modulus values. Results from this work, and data from the literature, suggest that monitoring the recrystallisation process in aluminium using an EMAT sensor is much more straight forward than for steel due to aluminium showing greater differences in elastic modulus, and hence ultrasonic velocity, anisotropy between the as-rolled and recrystallised textures.