Polymer -bonded magnets produced by laser powder bed fusion Influence of powder morphology filler fraction and energy input on the magn etic and mechanical properties Kilian Schäfera Tobias Brauna Stefan Riegga Jens Musekampb Oliver Gutfleischa

2025-05-02 0 0 2.27MB 41 页 10玖币

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Polymer-bonded magnets produced by laser powder bed fusion: Influence of powder morphology, filler

fraction and energy input on the magnetic and mechanical properties

Kilian Schäfera, Tobias Brauna, Stefan Riegga, Jens Musekampb, Oliver Gutfleischa

a Functional Materials, Institute of Materials Science, Technical University of Darmstadt, 64287 Darmstadt,

Germany

bInstitute for Materials Technology (IfW), Technical University of Darmstadt, Grafenstraße 2, 64283

Darmstadt, Germany

Keywords:

Additive manufacturing

Laser powder bed fusion

Bonded permanent magnets

Particle alignment

Abstract

Bonded permanent magnets are key components in many energy conversion, sensor and actuator devices.

These applications require high magnetic performance and freedom of shape. With additive

manufacturing processes, for example laser powder bed fusion (LPBF), it is possible to produce bonded

magnets with customized stray field distribution. Up to now, most studies use spherical powders as

magnetic fillers due to their good flowability. Here, the behavior of large SmFeN platelets with a high

aspect ratio as filler material and its influence on the arrangement and the resulting magnetic properties

are examined in comparison to a spherical magnetic filler. The 3D distribution and orientation of the

magnetic filler was studied by computed tomography and digital image analysis. The platelet-shaped

particles align themselves perpendicular to the buildup direction during the process, which offers a new

and cost-effective way of producing composites by LPBF with anisotropic structural and functional

properties. The influence of LPBF parameters on the properties of the composites is investigated. Highest

filling fractions are required for high magnetic remanence, however the powder itself limits this maximum

due to particle shape and required minimal polymer fraction to form mechanically stable magnets. The

coercivity decreases for higher filling fractions, which is attributed to increased rotation of insufficiently

embedded magnetic particles in the matrix. It is discussed how filler morphology influences the observed

change in coercivity since the rotation of spherical particles in comparison to platelet-shaped particles

requires less energy. Our work shows the challenges and opportunities of large platelet shaped fillers used

in LPBF for the production of anisotropic functional and structural composites.

1. Introduction:

Bonded permanent magnets are key components for many energy conversion devices including sensor

and actuator applications [1]. These applications require high magnetic performance, customized stray

field distribution and shapeability for complex geometries. Bonded magnets consist of magnetic particles

within a polymer matrix and are conventionally processed via injection or compression molding [2]. The

drawbacks of molding processes are the high price and limited shapes of tooling dies, which makes it

economically only viable for large batch sizes and a reduced degree of freedom of shaping.

Additive manufacturing (AM) can overcome the limitations of conventional bonded magnets

manufacturing techniques [3–5]. AM also brings the potential for manufacturing magnets with specific

stray field distributions for sensor applications and improving them by the combination of soft and hard

magnetic materials [6,7]. In addition, AM is ideal for net-shape production allowing the processing of

magnets in a resource-efficient way due to the omission of additional subtractive machining. For lower

volume parts the resource criticality of the rare-earth elements present in high-performance magnets can

be addressed efficiently by AM [8].

To optimize the magnetic performance, the remanence Jr (which scales to the square for the energy

density (BH)max) and the coercivity Hc must be maximized [1]. A high remanence in bonded magnets can

be achieved with a high magnetic filling fraction. However, the mechanical stability of polymer composites

is reduced for high filling fractions [9] which is detrimental for applications with mechanical loads like

motors or generators. Similar behavior can be observed in AM of bonded magnets [10]. Literature shows

that the morphology of the filler has an influence on the magnetic, microstructural and mechanical

properties of the resulting composites [11,12]. Fim et al. investigated the influence of laser parameters on

the densification of bonded magnets [13]. Moreover, it was shown by Fliegl et al. that the porosity of parts

produced with full metal laser powder bed fusion (LPBF) can vary across the powder bed depending on

the laser incident angle [14].

The relations between process parameters, filler fraction and morphology on the magnetic and mechanical

performance of bonded magnets produced by AM processes are the subject of current research but are

yet to be fully understood [3,12,13,15,16]. Due to its high flowability, which influences the processability

of the magnetic filler in LPBF, and its easy availability, the focus of previous research on AM of bonded

magnets has mostly been on the utilization of spherical, gas-atomized powders with particles sizes of

around 20 – 50 µm [3]. The behavior of large Sm2Fe17N3 platelets with a high aspect ratio as filler material,

its influence on the distribution and arrangement, as well as the resulting magnetic mechanical properties

has not been studied in detail yet. Up to now, similar studies used significantly smaller particle sizes for

the AM processes with platelet-shaped powders [12,16]. The Sm2Fe17N3 powder used in this manuscript

has median particle size of 91 µm.

The aim of this work is to investigate the trade-off between the magnetic and mechanical properties of

polymer composites produced by LPBF while studying in detail the differences caused by magnetic fillers

with significantly different morphologies. In this study, spherical NdFeB and significantly larger platelet-

shaped SmFeN particles with a high aspect ratio were utilized as filler materials to investigate the potential

of non-spherical particles in AM of bonded magnets. The influence of the laser energy density and

temperature distribution across the build volume on the magnetic and mechanical properties is studied

and compared to the performance of pure polymer polyamide 12 (PA12). Furthermore, the impact of an

increasing filler fraction on the microstructure, magnetic and mechanical properties for both types of

morphologies is explored. This paper also presents the potential of the different used morphologies for an

in situ alignment process of the magnetic filler. This alignment effect can offer new and cost-effective ways

of producing anisotropic bonded magnets by LPBF, which show higher magnetic performance in

comparison to isotropic bonded magnets. The alignment process can also be transferred to AM of other

functional composites produced by LPBF to specifically tune properties like electrical conductivity.

2. Experimental:

Bonded magnets were produced using PA 12 from Sintratec (Sintratec, Switzerland), in combination with

commercially available magnetically isotropic powders: MQP-S (MQP-S-11-9) which is Nd2Fe14B based

from Neo Magnequench (Singapore) and Sm2Fe17N3 (Daido Steel, Japan). The powders were used in

their as received, thermally demagnetized state from the manufacturer. PA12 provides a good

combination of flowability and good mechanical properties which qualifies it very well for the processing

of highly filled composites. Therefore, it is used as the matrix material for the LPBF process in this study.

The morphology of the magnetic powders is presented in Figure 1. The Scanning electron microscopy

(SEM) images show that the MQP-S consist of rounds particles, as produced by atomization, with an

average particle size of 35-55 µm [17]. On the other hand, the Sm2Fe17N3, which is produced by melt

spinning, consist of larger platelet shaped particles with an average size of 91 µm [18]. Latter will be further

referred to as SmFeN. The full hysteresis loops and initial magnetization curves of the two magnetic

powders, measured in a PPMS at 300K up to 4T for fixed powders, are presented in Figure 2. Both powders

are so called exchange-coupled magnetic materials [18,19] which can be seen by the ratio between

remanence polarization (Jr) and saturation (Js). This ratio is larger than 50% of Js. For isotropic magnetic

materials remanence is half of the saturation. In exchange coupled permanent magnets, the coupling

between a hard and soft magnetic phase leads to the stabilization of the soft magnetic phase by the high

coercive phase during the demagnetization process; for this the soft phase should be well-below 100nm

in size [19]. Due to the hindered switching of the soft magnetic phase, an increase of the remanence can

be observed [18]. The properties of the feedstock materials as given by the manufacturer are presented

in Table 1.

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摘要：

Polymer-bondedmagnetsproducedbylaserpowderbedfusion:Influenceofpowdermorphology,fillerfractionandenergyinputonthemagneticandmechanicalpropertiesKilianSchäfera,TobiasBrauna,StefanRiegga,JensMusekampb,OliverGutfleischaaFunctionalMaterials,InstituteofMaterialsScience,TechnicalUniversityofDarmstadt,6428...

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Polymer -bonded magnets produced by laser powder bed fusion Influence of powder morphology filler fraction and energy input on the magn etic and mechanical properties Kilian Schäfera Tobias Brauna Stefan Riegga Jens Musekampb Oliver Gutfleischa

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