Performance improvements in Galfenol laminated rods with stress annealing

Abstract

The recent discovery of Galfenol as a large magnetostrictive material (as high as 400μe) offers a particularly promising transducer material that combines largely desirable mechanical attributes with superior magnetic properties. The high permeability of this material makes it easy to magnetize, however it also causes a relatively low cutoff frequency in dynamic applications, above which eddy currents form and introduce significant power losses. To reduce the eddy current losses, magnetostrictive drivers used in dynamic applications are commonly laminated. A second transducer design consideration is that in materials which exhibit positive magnetostriction, it is common to impose an initial compressive to the material that is sufficient to align the orientation of the magnetic moments within the material to a direction perpendicular to the stress axis. This is done to maximize the magnetostriction realized when a magnetic field applied along the stress axis rotates the moments parallel to the stress axis. An alternative to the application of a compressive prestress is to build-in a uniaxial magnetic anisotropy through stress annealing. Stress annealing is a high temperature process with simultaneous application of a compressive load and subsequent cooling under load in which the magnetic moment alignment developed at temperature is retained upon removal from the stress anneal fixture. The compressive load needed to build in a useful uniaxial magnetic anisotropy in Galfenol is greater than the buckling load for Galfenol laminae sized for use in high frequency dynamic applications. In this study, prior work on stress annealing of solid rods of single and polycrystalline samples of Galfenol is successfully extended to thin laminae of Galfenol by introducing fixtures needed to avoid buckling. The standard stress annealing device uses a hydraulic actuator to apply compressive stress to the sample. Two linear guides have been added to ensure a normal compression load path to reduce the potential for buckling of thin laminations. In addition, a mechanical holding fixture was used to maintain proper alignment of the thin laminations during stress annealing. Data are presented that demonstrate the magnetic uniaxial anisotropy developed by stress annealing of laminated Galfenol rods.