Relationship between blanking performance and microstructure of annealed Fe-Si-B-Cr amorphous alloy sheets

The high saturation magnetic flux density, low coercivity, and high permeability of iron-based amorphous alloys make them excellent magnetic core materials for electrical devices. Their local embrittlement and high fracture strength, however, make them difficult to machine. To overcome these difficulties, we have developed a new machining method with optimized heat treatment to improve the machinability of amorphous alloys. We conducted a series of blanking tests on melt-spun and post-annealed Fe-Si-B-Cr amorphous alloys with as-quenched, relaxed, partially or fully crystallized, and grain-grown microstructures. SEM examinations of the fractured surfaces of the blanked specimens and TEM observations revealed that the blanking resistance and overall machining performance correlated with these microstructures. The as-quenched sample showed the largest blanking resistance, and the fracture surface exhibited a drooped top edge, vertical marks, and the formation of horizontal voids, suggesting that plastic deformation occurred. For the samples annealed up to 763 K, the blanking resistance decreased and fractographic examination indicated that the blanking occurred in a brittle manner. Annealing at higher temperatures (773-873 K) increased the blanking resistance and changed the fracture morphology, exhibiting intragranular and intergranular fractures of crystalline precipitates. After complete crystallization, several burrs appeared at the bottom of the sample. Annealing at 1073 K led to a further decrease in blanking resistance related to the intergranular fracture of large grains. The plastic deformation of the sheet deteriorated the blanking quality. Based on these findings, the optimum microstructure and posttreatment of the amorphous alloy for ideal blanking performance are proposed.