Directionally solidified electron beam welding: A novel method to optimize soft magnetic properties for welded permalloy

Abstract

Welding for permalloy is potentially applied in forming of permalloy devices in large volume and complex shape. The key point for permalloy welding is to guarantee the soft magnetic properties of joint. The new method of directionally solidified electron beam welding is conducted to optimize soft magnetic properties of 1J85 welded joint achieved by changing the direction of temperature gradient. The growth orientation of columnar crystalline clusters deflects from the original direction perpendicular to the fusion line to that towards weld center, corresponding to the direction of external magnetic field. The initial permeability is consequently increased from 3.54 mH/m (67% of the annealed 1J85) of the ordinary welded joint to 5.86 mH/m (higher than that of annealed 1J85) with the 14% decreased coercivity. The cooling rate of the weld is decreased from 12.5 * 10 3 ℃/s to 9.7 * 10 3 ℃/s, favorable for the growth of cellular grains and the consequently easier movement of magnetic domain boundary during magnetization process. The cellular grain boundary is proved to be element segregation instead of the conventional dislocation-walls in view of the distribution of low-angle grain boundaries. The alleviated distortion deriving from the decrease in stress is the underlying cause of the optimization of magnetic properties.