A Comparison of Laser Ultrasonics and EMAT Texture Measurements in Aluminum Alloys

Abstract

BACKGROUND Ultrasonic techniques, which measure elastic anisotropy, have been used to study texture and plastic anisotropy of sheet materials. Ultrasonic velocity measurements can determine the orientation distribution coefficients (ODCs) W 400, W 420, and W 440 which are used to describe crystallographic orientation distributions [1]. For steel sheets, strong correlations have been observed between ultrasonic velocity and the formability parameters i' and Ar [3]. The results for aluminum show a relationship between the ODC W 440 and the degree of earing [2-3]. In addition to a metal sheet's formability and degree of earing, information about individual texture components is also needed in certain cases. A Goss component, for example, is necessary in the manufacturing of quality steel transformers but undesirable in plate forming and canning applications. Recent work suggests that W 440 can indicate the presence of the cube texture verses deformation textures (brass, copper, S, etc.) [4]. Few studies are available, however, which attempt to distinguish between the various textures. This paper presents some initial results on ultrasonic texture identification of aluminum sheets using the distinctive features captured within the slowness (or inverse velocity) curves. We begin with theoretical predictions of Lamb wave slowness curves of perfect textures (cube, Goss, brass) and compare them with measurements taken from two aluminum samples by an electromagnetic acoustic transducer (EMAT) system. A second set of measurements from a laser-based ultrasonic setup (for which theory has not yet been formulated) is then presented and discussed in the context of developing an on-line system used for texture screening.