Direct recycling of high coercivity Fe-Nd-B based sintered magnets by doping with Nd-rich alloy

Summary form only given. The increasing importance of e-mobility will significantly enhance the amount of permanent magnets (pms) required for electric motors. The strongest pms today are Fe-Nd-B based magnets. Due to the scarceness of rare earth metals (RE), the today used RE pms can be regarded as future material source [1]. By direct magnet-to-magnet recycling, savings in energy consumption up to 91% are possible considering energy-intensive mining and purification of RE. Within this value, savings of 46% can be already achieved if strip-casting of new material is not needed [2]. In previous work, direct magnet recycling was mainly performed on low coercivity starting material (H
_c
~ 1200 kA/m) [3-6]. Via powder metallurgical route and NdHx additions, the magnetic properties of the starting material could be approximately fully recovered in the recycled magnets [3, 4, 6]. Using DyH
₃
additions the coercivity of the recycled magnet even exceeds the coercivity of the starting material, however, remanence and maximum energy product are significantly reduced [5]. Higher coercivity starting material (H
_c
~ 2000 kA/m) so far has been tested only by Li et al. [7] and Zakotnik et al. [2]. They produced recycled magnets by mixing with 24 wt% Nd
₃
PrFe
₁₄
B RE rich alloy and 1.9 wt% (Nd,Pr)H
_x
, respectively. Reported processes usually contain jet milling technique, hydrogenation process and isostatic pressing of powders [2-4,7]. Also, low energy ball milling in hexane was used [5,6]. In this work, we focus on the microstructure development during processing of recycled sintered magnets. The resulting microstructure and occurring kind and amount of phase constituents, which can be quantified by means of quantitative image analysis, sensitively depend on chosen parameters for milling and sintering procedure. The parameters partly behave in the opposite direction which is also reflected in the resulting magnetic properties. For magnetto-magnet recycling, a high coercivity Fe-Nd-B based pm (9 wt% Dy) well-suited for elevated temperatures was used as starting material (H
_c
= 2625 kA/m). Solely new Nd-rich Fe-Nd-B as-cast alloy (Fe-26Nd-7B) without intermixing hydride addition (e.g. NdH
_x
) was used for blending. The as-cast alloy was synthesized in an arc furnace operating in a highly clean Ar atmosphere. The chosen as-cast alloy consists of primary solidified T1 (Fe
₁₄
Nd
₂
B, hard magnetic Φ-phase), secondary solidified T1 and T2 (Fe
₄
Nd
_1.1
B
₄
, paramagnetic η-phase) and the ternary eutectic E2 (Nd, T1, T2). This is also in accordance with the liquidus phase projection of the ternary Fe-Nd-B system [8]. For production of the recycled sintered magnets a lab sinter facility was used. The pre-shredded material was ball-milled, mechanically sieved and transversally pressed in a pm field under inert gas atmosphere. The milling time of the starting material was systematically varied between 10 and 45 min. Ball-milling the as-cast alloy for 45 min yielded D50 = 5.7 μm. To mix the powders of starting material and of the as-cast alloy, the method of two-component sintering was used. To obtain optimum magnetic properties, 40 wt% of the as-cast alloy was added to the starting material. Compared to fabrication of new Fe-Nd-B sintered magnets, the sintering temperature for the recycling sintered magnets had to be increased by 40°C. The higher sintering temperature was necessary to reach a sufficient density of ~7.5 g/cm
³
. The obtained density of the recycling magnets was ~ 98 % of the density of the starting material. The magnetic properties of the recycled magnets was measured as H
_c
= 1883 kA/m, Jr = 1.1 T and (BH)
_max
= 230 kJ/m
³
for a milling time of the starting material of 10 min, i.e. 72 % H
_c
, 98 % J
_r
and 98 % (BH)
_max
of the starting material, respectively. Although no Dy was added, 72 % of the original coercivity was recovered in the recycled magnet. In Fig. 1 the typical microstructure and domain structure of the three different recycled magnets is presented compared to the starting pm. In Fig. 2 the content of oxides and average grain sizes of the samples have been quantified by quantitative image analysis of statistically representative areas. The measured room temperature values of the coercivity are also shown. Accordingly, with increasing milling time, the coercivity is lowered. This may be due to both the increasing volume fraction of oxides (3.5 to 6.9 %) and to the simultaneously decreasing grain sizes with increasing milling time, i.e. the increase of surface to volume ratio of the powder particles. Additional grain boundary phase will be needed for complete wetting of the whole grain surfaces for larger milling times. The reduced remanence of the recycled magnets originates from the slightly reduced density. Because of the higher sintering temperature used, recycled magnets show slightly larger mean value of grain sizes compared to the starting material.