Effect of casting methods on magnetic properties of Sm(CoFeCuZr)z magnet

Abstract

The effect of casting method on the magnetic properties of Sm(Co0.63Fe0.28Cu0.07Zr0.02)7.6 magnet has been investigated. The results show that the plate-mold ingot (PM) and strip-casting flake (SC) have the same phase composition. Compared with PM, the SC has a smaller grain size and larger fine-grain region due to its larger degree of undercooling. The magnetic properties of the PM magnet are Br = 11.63 kG, Hcj = 25.65 kOe, (BH)max = 30.97 MGOe, while the magnetic properties of SC magnet are Br = 11.45 kG, Hcj = 24.00 kOe, (BH)max = 30.03 MGOe. The reason for the low remanence of the SC magnet is revealed by the XRD and EBSD results. It shows that the PM magnet has a better easy-axis orientation than the SC magnet, due to the existence of fine grains in SC that are unfavorable for the alignment under a magnetic field.

Strip Casting of Sm2TM17-Type Alloys for Production of the Metastable SmTM7 Phase

“…As the strip casting parameters were not given and limited microstructural analysis was presented, no direct comparisons can be made regarding the optimisation of the strip casting process. Yang et al and Yang et al [20,21] demonstrated very similar findings to Meng et al [19], showing that slower wheel speeds in strip casting led to better magnetic properties; however, as Meng et al, were utilising the full manufacturing route for magnet processing and were not aiming to circumvent the long heat treatment as would be the aim of this work. Likewise, direct comparison with Zheng et al [22] is limited as they did not optimise the strip casting process; however, they obtained a similar combination of phases of the alloy but were not able to demonstrate directional columnar growth from the wheel side to the free side of the flake as presented in this work.…”

“…For all strip cast alloys, however, the phase distribution changed substantially to consist predominantly of the metastable 1:7H phase with small amounts of 1:5H phase and trace 2:17R phase. Due to the substantially increased cooling rate, the microstructure typically consisted of a nucleation zone where the melt started to solidify in contact with the wheel and columnar growth along the thermal gradient from the wheel side of the flake towards the free side, similar to that demonstrated by Meng et al [19] and Yang et al [20]. However, in a number of flakes, there were regions where the cooling rate was not sufficient to maintain columnar growth, resulting in non-directional growth towards the free side of the flake and elemental segregation similar to that observed in the as-received cast alloy.…”

“…The fine-grained strip cast ribbons also led to lower remanence in fully processed magnets as it was not possible to produce single crystal particles by milling, which may not be a problem with the strip cast alloys presented in this work. Meng et al [19] demonstrated strip cast flakes that were 575 µm thick, where fine grains dominated 445 µm of the thickness and columnar grains the remaining 130 µm. This also led to a reduction in the magnetic properties of processed magnets compared to ingot starting materials, which was attributed to the high proportion of fine-grained material leading to polycrystalline particles after milling.…”

“…When the strip cast alloy was milled, the grain size in these regions was smaller than the average particle size, which led to polycrystalline particles with poor alignment in the final magnet. Therefore, an alloy consisting of columnar grains would likely be more beneficial to magnet production than the fine, equiaxed grains produced in this work [19]. Yang et al [20] investigated the effect of strip casting wheel speed as a part of a larger study on magnet manufacture.…”

“…This work presented by Liu et al led to non-directional growth as the cooling rate was too high to allow for columnar, directional growth along a cooling gradient, which is synonymous with strip casting [18]. Meng et al also investigated strip casting of Sm(CoFeCuZr) z alloys for magnet manufacture but did not detail the strip casting method or parameters used [19]. They found that the fine grains developed formed the majority of the strip cast alloy and were very detrimental to the remanence and energy product of sintered magnets.…”