The effect of heat treatment on structure and magnetic properties of additively manufactured Fe-Co-V alloys

In earlier research it was observed that small differences in the alloying and the processing conditions can greatly influence the microstructure and hence the magnetic performance of laser powder bed fusion (PBF-LB) processed soft magnetic Fe-Co-V material. However, a systematic study on the effect of alloy composition and heat treatments on the microstructure and magnetic properties of PBF-LB manufactured Fe-Co-V alloys is pending and the underlying mechanisms remain to be better understood. The microstructure and magnetic properties of two different Fe-Co-V alloys (with and without Nb addition) manufactured by PBF-LB with two sample types (small and large dimensions) and different heat treatments were investigated. PBF-LB processed Fe-Co-V had a fine grain structure that after annealing at 800°C and 850°C for 1, 10 and 24 h developed into a bimodal grain structure. The grain growth kinetics of the alloys varied substantially as the alloy with microalloying addition (Nb) had more homogeneous grain structure and smaller average grain size. The average grain size of specimens annealed for 24 h was approximately 210 µm (Alloy1) without and 45 µm with Nb alloying (Alloy2). Specimen size had an influence on the grain structure evolution as Alloy1 specimens with larger size had higher area fraction of large grains after annealing compared to smaller size specimens. The specimen’s surface temperature was higher for larger specimens based on in-situ thermal imaging resulting in slightly different thermal cycles between the sample types. The magnetic performance of both alloys improved with longer annealing times reaching the highest performance at 24 h anneal (Hc ~ 20 A/m and µmax ~ 15000), but the optimal annealing temperatures were different, i.e., 850°C for Alloy1 and 800°C for Alloy2.