High coercivity in bulk Pr-Fe-Cu-B alloys as a stable precursor for permanent magnets by Additive Manufacturing

Abstract

Today, the 3D-printing technology faces a new challenge - the Additive Manufacturing (AM) of functional magnetic materials, especially permanent magnets. AM should provide very good functional properties of the printed magnets, namely locally tailored remanent magnetization Mr and coercivity Hc. The highest magnetic performance is reached for Nd-Fe-B magnets based on the hard magnetic Nd2Fe14B phase, and on extrinsic properties (micro and nanostructure) mastered during the production. Ideal microstructure consists of magnetically decoupled grains of the hard magnetic RE2Fe14B (RE = Rare earth element) phase. The main challenge for Nd-Fe-B magnets, which arises during direct AM techniques, consists of an unsuitable microstructure appearing after re-melting of Nd-Fe-B precursor by the laser, followed by a fast solidification of the alloy. This usually leads to a very low coercivity in magnets made from commonly used commercial Nd-Fe-B alloy powders. Thus, one of the main obstacles in successful 3D printing of full-dense hard magnetic materials, is the engineering of a desired phase composition and the mastering of an appropriate microstructure.In the contrast to conventional Nd-Fe-B-based bulk alloys, a large HC has been found in annealed Pr-Fe-Cu-B compounds. The Pr2Fe14B phase is isostructural to Nd2Fe14B and the Pr-Fe-B and Nd-Fe-B ternary phase diagrams exhibit the same phases. By doping the system with Cu, additional phases can be formed in the sample and some of them affecting drastically the hard-magnetic properties of the magnet. Kajitani et al. [4] found a new grain-boundary phase with tetragonal structure (space group I4/mcm) and Pr6Fe13Cu stoichiometry in hot-pressed Pr17Fe76B5.5Cu1.5 alloys. Annealing at 500 °C leads to an increase in HC of 50 % compared to the hot-pressed state, which was associated to the formation of the antiferromagnetic grain-boundary phase. Further investigations by Marcondes et al. [5] proved the occurrence of the antiferromagnetic Pr3Fe13Cu phase and it was found that annealing at 500 °C improves the HC drastically due to the formation of the Pr3Fe13Cu phase at the expense of the Pr,Cu-rich grain-boundary phase. Recently, the occurrence of the RE6(Fe,Ga)14 within the grain boundary [6]–[8] of Nd-Fe-B permanent magnets was reported. The magnets show good hard- magnetic properties, but the role of the RE6Fe13(Cu,Ga) phase is not clear yet.Our studies explore the applicability of Pr-Fe-Cu-B based alloys for the use in Additive Manufacturing (AM) techniques to produce fully dense permanent magnets. Induction molten Pr17Fe76B5.5Cu1.5 alloys were annealed at high temperature (1000 °C for 5 hours), followed by a low temperature annealing (500 °C for 3 hours). The samples show the formation of soft magnetic Pr2Fe17 phase at high temperature accompanied by low coercivity. After a low temperature annealing, the mentioned Pr6Fe13Cu is forming (see Fig. 1) and an increase of coercivity can be observed.Based on these experiments, the composition has been optimized regarding the coercivity. For this, three pre-alloys (Pr6Fe13Cu, Pr2Fe14B, FeB) were prepared by induction melting and two compositional series by mixing the Pr6Fe13Cu with FeB or Pr2Fe14B were prepared by arc melting. The heat treatments were the same for all samples and the development of coercivity was characterized. The shape of the hysteresis is drastically changing within the sample series. High amount of the Pr6Fe13Cu phase leads to a shoulder and decreasing saturation magnetization, which indicates the non-ferromagnetic properties of the phase. The combination of Pr6Fe13Cu + FeB shows the largest coercvity of ~1250 kA/m which is comparable to a sintered magnet (without heavy Rare Earth elements). In both sample series, the formation of the reported Pr6Fe13Cu phase could be observed by SEM-EDX.The large coercivity in the bulk state makes this material interesting to use as stable precursors for AM of fully dense magnets. The material needs to be in powder form for techniques like SLM. Therefore, the hydrogen decrepitation (HD) process was successfully applied on high coercive samples. Afterwards, the material was suction casted which simulates the remelting and rapid solidification during the SLM process. After the aforementioned annealing, the coercivity is equal to the as-cast condition which demonstrates that the magnetic hardening mechanism is stable throughout the different preparation steps and the material will exhibit large coercivity after SLM.We thank the German Research Foundation (DFG) and the Collaborative Research Center/Transregio 270 HoMMage, Project ID No. 405553726, TRR 270, for making this work possible. **