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Article

Binder System Composition on the Rheological and Magnetic Properties of Nd-Fe-B Feedstocks for Metal Injection Molding

by
Vahid Momeni
1,*,
Sorana Luca
2,
Joamin Gonzalez-Gutierrez
1,3,*,
Santiago Cano
1,
Emilie Sueur
2,
Zahra Shahroodi
1,
Stephan Schuschnigg
1,
Christian Kukla
4 and
Clemens Holzer
1
1
Institute of Polymer Processing, Montanuniversitaet Leoben, 8700 Leoben, Austria
2
CEA-Liten, Université Grenoble Alpes, F-38000 Grenoble, France
3
Functional Polymers Research Unit, Luxembourg Institute of Science and Technology (LIST), L-4940 Hautcharage, Luxembourg
4
Industrial Liaison Department, Montanuniversitaet Leoben, 8700 Leoben, Austria
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2024, 14(13), 5638; https://doi.org/10.3390/app14135638
Submission received: 4 June 2024 / Revised: 20 June 2024 / Accepted: 25 June 2024 / Published: 28 June 2024
(This article belongs to the Special Issue Functional Magnetic Materials and Devices: Recent Advances and Applications)
Browse Figures
Versions Notes

Abstract

:
The applications of Nd-Fe-B-based magnets are experiencing significant diversification to achieve efficiency and miniaturization in different technologies. Metal injection molding (MIM) provides new opportunities to manufacture Nd-Fe-B magnets with high geometrical complexity efficiently. In this study, the impacts of the binder system composition and powder loading on the rheological behavior, contamination, and magnetic properties of the Nd-Fe-B MIM parts were investigated. A high-pressure capillary rheometer was used to measure the apparent viscosity and pressure drops for feedstocks with different binder formulations and powder contents. Also, oxygen and carbon contamination, density, and magnetic properties were measured for different feedstock formulations and powder loadings. From the rheological, density, and magnetic properties points of view, the binder system consisting of 45 vol.% LLDPE as backbone was selected as the optimum formulation. The findings indicated that the sample with this binder system and 55 vol.% powder content had a high density (6.83 g/cm3), remanence (0.591 T), and coercivity (744.6 kA/m) compared to other binder compositions. By using 58 vol.% powder loading, the values of density (7.54 g/cm3), remanence (0.618 T), and carbon residue (982 ppm) improved, and a suitable rheological behavior was still observed. Thus, a suitable feedstock formulation was developed.

1. Introduction

Permanent magnets play a significant role in transitioning from non-renewable fuels to sustainable energy sources; therefore, their economic importance across several technologies is increasing [1,2,3,4]. The usage of rare-earth permanent magnets is growing, particularly in wind power production and electric vehicle technology, which necessitate high-performance magnets [5,6,7,8]. Due to their favorable economics and strong magnetic characteristics, Nd-Fe-B permanent magnets are one of the most significant industrial materials used as magnetic field sources for motors, generators, and actuators [9,10]. Their significant brittleness makes producing intricate, tiny, and accurate magnets using traditional production techniques, such as machining, challenging [11,12,13,14].
Within a global concern about reducing the dependency on the critical rare-earth elements, substituting Nd and Pr in the Nd-Fe-B type magnets with more abundant Ce and La elements is nowadays under consideration [15,16]. Nevertheless, using these elements will reduce the intrinsic magnetic properties (saturation magnetization, anisotropy field, and Curie temperature) and, thus, the extrinsic ones (the remanence and the coercivity). However, this can be offset by re-designing some steps of the conventional routes for producing dense anisotropic magnets and optimizing the design of electrical machines. Advanced manufacturing processes, such as Metal Injection Molding (MIM), reduce the material required to produce a magnet and allow for complex shapes with an optimized magnetic architecture [17,18]. They could offer substantial benefits by combining better system performances and material savings. Therefore, Ce-containing powders were used in this research, and a suitable MIM feedstock was developed.
MIM can be a more cost-effective manufacturing approach for high production rates to manufacture Nd-Fe-B-based permanent magnets with complicated geometries, multiple or radial anisotropy, and good magnetic characteristics [19,20]. Making magnets via MIM means having a binder system that performs different tasks at the four critical stages of the process: mixing, injection molding, debinding, and sintering [21,22]. During mixing, the binder is compounded with alloy powder in an optimal ratio to create the feedstock in the initial step. In the course of injection molding, the binder system is a temporary medium to make the injection possible for the metal powder [23,24]. For magnets, magnetic alignment is performed during the injection molding step to produce anisotropic magnets. The injected or green components are then successfully subjected to debinding to eliminate all binder components and sintering to provide superior mechanical and physical properties [25,26]. After sintering, an appropriate annealing treatment is applied, and the magnets are magnetized to produce a permanent magnet.
The binder system for the MIM process typically comprises a polymer as a backbone, waxes as flow enhancers, and additives such as surfactants or lubricants to ensure powder dispersion [27]. Injection molding is one of the crucial stages in the MIM process because several flaws, including voids and powder–binder separation, can arise, especially when the feedstock is inhomogeneous [28]. These flaws may cause cracks in molded or sintered components. MIM feedstocks must conform to a specific rheological behavior to avoid these issues. Among the rheological properties, viscosity is one of the most critical rheological parameters that determine the suitability of a feedstock for a successful MIM process [29]. It is challenging to employ injection molding feedstocks with extremely high viscosities since they can lead to partially filled mold cavities; on the other hand, feedstocks with extremely low viscosities can result in binder–powder separation and have a high sensitivity to temperature, neither of which are desirable for MIM feedstocks [30].
The rheological properties have been the subject of several investigations for various powders and binder systems used in MIM [31,32,33]; such feedstock properties are affected by the binder system and powder characteristics. An optimum powder loading is also crucial for achieving an appropriate rheological behavior. While some researchers have carried out several rheological investigations to study the impacts of powder loading on the MIM process [30,31,32], finding the optimal powder content should be carried out for each powder and binder system since each feedstock behaves differently. Therefore, the rheological properties of the Ce-containing powders produced in this study and the corresponding feedstocks with variable powder contents should be assessed, which was one of the aims of this study.
After thermal debinding, low residual carbon content is one of the most critical desirable characteristics of the binder system [34,35,36,37]. Carbon contamination derived from the binder system and oxygen pickup during the various steps of the MIM process can drastically reduce the magnetic characteristics of a permanent magnet [20,38,39,40,41]. Several studies have been carried out to use different backbones, such as high-density polyethylene (HDPE) [42,43,44], low-density polyethylene (LDPE) [45], ethylene–vinyl acetate copolymer (EVA) [46,47,48], polypropylene (PP) [43,49,50,51], and polymethyl methacrylate (PMMA) [52,53,54,55] in the binder system. Hartwig et al. [56] suggested that a polyoxymethylene (POM) based binder system could be a good choice due to decomposition at lower temperatures, which reduces the risk of reaction between the binder system and the powder particles during debinding and sintering. However, it was later proved that the POM binder system starts degrading during the injection molding step at the processing temperature of 170 °C in the presence of magnetic alloys, which makes it challenging to use this binder system for manufacturing magnets [57]. In this investigation, linear low-density polyethylene (LLDPE) was selected as the backbone since it has a low price, and it decomposes by linearly reducing its molecular weight without crosslinking, as is the case of LDPE.
Although other studies have investigated the impact of carbon residue from the binder system on the magnetic properties [43,58], different binder systems and powder characteristics have unique influences. Therefore, this study aims to provide additional information on minimizing carbon and oxygen contamination of permanent magnets produced using MIM by varying the binder formulation while maintaining adequate rheological properties and selecting LLDPE as a backbone in the binder.
In summary, the current investigation studied the effects of unique binder system formulations and powder loading on the rheological behavior, contamination, and magnetic properties of Ce-containing Nd-Fe-B magnets, which are crucial aspects for the development of economical, reliable permanent magnet production via MIM.

2. Experimental Procedures

2.1. Materials

Powders with spherical particles and an average particle size of less than 45 μm are typically selected for use in the MIM process [59]. In addition, some investigations indicate that metallic powders with lower particle sizes can increase surface finish, moldability, and sinterability [60,61]. On the other hand, high-performance anisotropic permanent magnet manufacturing requires fine monocrystalline powders with a rare-earth-rich phase at grain boundaries. Here, a compromise has been reached using rare-earth-rich fine powder produced by hydrogen decrepitation.
Strip cast flakes with a (Nd, Pr, Ce, Dy)32.3(Fe, Co, Al, Cu, Ga, Zr)65.5B (wt.%) composition were used for manufacturing the fine powder by hydrogen decrepitation (as described in [61]) and jet milling process under N2 gas, which will be referred to as Nd-Fe-B in the following parts of the paper. The O2 content of 0.35 wt.% was measured on powders sealed in a tin container to minimize powder oxidation during the transfer to the equipment. The size and distribution of the powder particles affect the final properties and can change the flow behavior when interacting with the binder. The particle size distribution can be seen in Figure 1, which had a d50 = 5.12 µm. An SEM image of the powder is also presented in Figure 2, showing particles with an angular morphology.
The binder systems consisted of paraffin wax (PW) supplied by SER Wax Industry (Santena, Italy) as a flow enhancer, linear low-density polyethylene (LLDPE) from Exxon Mobil (Houston, TX, USA) as the backbone, and stearic acid (SA) from Sigma Aldrich (Saint Louis, MO, USA) as the surfactant. Compared to low-density polyethylene (LDPE), LLDPE has more short branches in the molecular chain structure, which gives LLDPE higher tensile strength and impact and puncture resistance. In addition, LLDPE decomposes by chain scission [62], which can be beneficial in minimizing carbon residues in the sintered magnets. For these reasons, LLDPE was selected for this investigation. The PW, LLDPE, and SA densities are 0.9, 0.925, and 0.85 g/cm3 respectively. For feedstock manufacturing, the theoretical density of the Nd2Fe14B was considered, which is 7.6 g/cm3.

2.2. Feedstock and Parts Preparation

The binder components and the powder were mixed in an internal mixer with a chamber volume of 38 cm3 and counter-rotating roller rotors (Plasti-Corder PL2000, Brabender GmbH & Co. KG, Duisburg, Germany) at a rotational speed of 60 rpm, which is required for a uniform distribution of powder particles in the binder system. For all processing stages, the mixing temperature of 160 °C was chosen, as degradation of the low molecular binder components (PW and SA) was observed at higher temperatures, such as 170 °C, during the first mixing evaluations. As explained in [61], the Nd-Fe-B powder must be handled and processed in an inert environment to prevent oxidation and deterioration of its final magnetic characteristics. A 45-minute blending was performed in the internal mixer fitted with a glove box having an argon atmosphere to prevent oxidation and ensure a homogeneous mixture, as demonstrated by a constant torque value at the end of the kneading process.
In this study, four combinations of binder components were considered to assess the effect of the binder system on rheological behavior, carbon and oxygen contamination, and magnetic characteristics. A powder loading of 55 vol.% was used in the first trials to determine the most appropriate binder formulation. Table 1 shows the binder system formulations, which cover a broad range of various percentages of the LLDPE (35–52.5 vol.%), PW (41.25–60 vol.%), and SA (5–8.75 vol.%). LLDPE is the backbone that adds strength to the molded part and helps retain the shape during solvent debinding; a higher content leads to a stronger part but a higher viscosity. PW is the flow enhancer of the binder and the soluble component; therefore, the higher the PW content is, the lower the viscosity of the binder and the less polymer to remove during thermal debinding; however, the molded part becomes more fragile. SA is the dispersant agent and viscosity modifier; the higher the SA content, the fewer agglomerates there could be in the feedstock; however, like PW, the part can become very fragile. As can be seen, choosing the appropriate range of ingredients needs to be experimentally validated. In the next step, feedstocks with the best binder system and different powder loadings (57, 58, 59, and 60 vol.%) were produced to adjust the feedstock formulation and find an optimal powder loading. A higher powder content can lead to a higher density but to a substantial viscosity or unstable flow; therefore, powder content must be verified experimentally.
After mixing, the feedstocks were cooled to room temperature and granulated in a cutting mill (SM200, Retsch GmbH, Haan, Germany). The binder system is expected to coat the alloy particles and prevent oxidation; however, an inert atmosphere is recommended during all process stages. Thus, the grinding was performed under argon, and the ground feedstocks were vacuum-sealed before further processing.
Cylinders, 8 mm in diameter and 15 mm in height, were manufactured from each formulation by uniaxial pressing under vacuum. The pressing was carried out at 50 bar and 160 °C using the hydraulic vacuum press P200PV (Dr. Collin GmbH, Maitenbeth, Germany). Uniaxial pressing was used since insufficient material was available for injection molding trials during feedstock development. Nevertheless, the pressed specimens can be used to determine that the feedstock can be debound and sintered without complications. Although the binder system can protect the produced parts from oxygen, the vacuum bags were used to store the pressed samples before debinding.

2.3. Rheological Measurements

It is essential to comprehend the flow behavior of the feedstock to produce defect-free green parts using the MIM process. The most critical flow property is viscosity, a proportional function between shear stress and shear strain rate. The pressure stability of the feedstock at different shear rates is a further trait that might characterize the flow behavior of the feedstock. Therefore, the high-pressure capillary rheometer Rheograph 2002 (Figure 3) (Göttfert Werkstoff-Prüfmaschinen GmbH, Buchen, Germany) was used to evaluate the rheological properties of the feedstocks.
The apparent shear viscosity based on ISO 11443 [63] was measured at 160 °C at 100 and 1000 s−1 shear rates for the feedstocks with different binder formulations and a 100–2000 s−1 shear rate range for the powder content optimization. The tests used a round die with a diameter and length of 1 mm and 30 mm, respectively. The relation between viscosity and shear rate can be described using Equation (1) for pseudo-plastic behavior at a particular temperature:
η = K · γ ˙ n − 1
where η represents the viscosity, n is the flow behavior index, K is the consistency constant and γ ˙ represents the shear rate [64]. The n index indicates the feedstock viscosity dependence on shear rate, which should generally fall within an acceptable range; higher values of n are preferable for the rheological behavior of MIM feedstocks because they indicate a less drastic viscosity decrease with increasing shear rate. However, it is challenging or impossible to process feedstocks with an extreme value of n = 1 (or close to 1) because the high viscosity is constant and requires very high pressure, even at high shear rates. On the contrary, it was claimed that very high sensitivity (low n value) is undesirable since it might result in flashing during injection molding [26,65].

2.4. Debinding and Sintering

A two-step debinding (solvent and thermal) procedure was conducted for the pressed cylinders. For solvent debinding, the samples were put into cyclohexane at 40 °C for 12 h. Most PW was dissolved during this phase, resulting in a porous structure that facilitates thermal debinding. However, other binder ingredients (especially the SA) were likely also dissolved during this treatment. In the subsequent stage, thermal debinding was carried out in H2 gas flow at a 1 K/min heating rate up to 620 °C. Four plateaus were used at 250, 300, 420, and 620 °C. A 4 h holding time for each plateau allowed the complete removal of the binder components and produced defect-free brown parts.
A continuous thermal debinding and sintering process was used; thus, the sintering was immediately conducted under a secondary vacuum at a 5 K/min heating rate until 1050 °C. One plateau of 4 h holding times was also used at 750 °C to allow the total H2 removal from the powder. Finally, the cooling was carried out under Ar to room temperature at 80 K/min.

2.5. Density, Contamination, and Magnetic Properties

The density of the samples was measured using the Archimedes’ method. Carbon and oxygen contamination measurements were carried out using the gas extraction method using the EMIA-Pro and EMGA-830 (Horiba, Kyoto, Kyoto, Japan), respectively. Room temperature magnetic properties, namely the remanence, and coercivity, were determined using the second quadrant of the hysteresis curves B(H) measured using a hysteresis graph (Laboratorio Elettrofisico srl, Nerviano, MI, Italy).

3. Results and Discussion

3.1. Rheological Properties

As discussed previously, pseudo-plastic behavior is one of the most significant traits for MIM feedstock, which means that an increase in shear rate leads to a decrease in feedstock viscosity. On the other hand, the viscosity increases with lower temperatures. Then, the required injection temperature can be decreased when pseudo-plastic behavior is present, hence minimizing defects in green components and facilitating the processing of Nd-Fe-B powder, as it is widely acknowledged that lower temperatures lessen the risk of oxidation for magnetic feedstocks. The maximum injection viscosity in the MIM process should not exceed 1000 Pa·s at the molding temperature and shear rates [44,65].
Figure 4 illustrates representative viscosity values at two distinct shear rates for all feedstock formulations, demonstrating that viscosity decreases with increasing shear rates. As can be seen, all feedstocks have apparent viscosities lower than 1000 Pa·s at the measured shear rates, which is within the acceptable range for MIM. Feedstock A shows the highest viscosity values since it contains the highest backbone (LLDPE) content. As established by several studies on different MIM powders, the viscosity of feedstocks is often decreased by reducing the percentage of backbone in the binder system since the backbone polymer is usually a polymer with higher molecular weight and thus a higher intrinsic viscosity [66,67]. In addition, 8.75 vol.% of SA in feedstock C reduced the viscosity to the same level as feedstock D even though feedstock C has 2.5 vol.% more LLDPE and 6.25 vol.% less PW content. Comparing feedstocks C and D at low and higher shear rates suggests that SA is more effective in reducing viscosity than PW. This is because SA has a lower molecular weight than PW and, as such, a much lower viscosity [43].
Although extremely low feedstock viscosity enables lower processing temperatures for Nd-Fe-B feedstocks, it can cause issues during compounding and injection molding, such as flash [68] and powder–binder separation [69]. The observed chamber leakage confirmed the challenges in compounding low-viscosity feedstocks during the preparation of feedstocks C and D. As can be seen by these results, having such a high amount of PW and SA simultaneously is not recommended. From the viscosity and processability standpoints, the 45–52.5 vol.% LLDPE range is acceptable in the binder formulation for the Nd-Fe-B feedstock. Feedstocks A and B have an acceptable apparent viscosity for the MIM process since they have a viscosity lower than 1000 Pa·s and shear thinning behavior. In addition, no leakage during compounding was observed. Therefore, these two feedstocks are good candidates for further optimization. Between feedstocks A and B, generally, it is preferred to use a feedstock with lower viscosity (i.e., feedstock B), which can allow the increase in powder content, resulting in enhanced mechanical and magnetic properties.
The variation of pressure during capillary rheology measurements is a good indication of the viscosity stability and feedstock homogeneity [70]. Figure 5 represents the typical pressure curves during viscosity measurements of the different feedstocks at two distinct shear rates. At low shear rates (100 s−1), the material flow is stable, as shown by the constant pressure between 350 and 450 s for all feedstocks. The pressure behavior at the high shear rate (1000 s−1) for the various formulations is observed between 450 and 575 s in Figure 5. The pressure variation of the A, B, C, and D feedstocks were 0.4%, 0.4%, 0.6%, and 0.6%, respectively. As can be seen, feedstocks A and B had the lowest pressure change (i.e., 0.4%), implying a better homogeneity between the various binder components and the 55 vol.% powder loading at a temperature of 160 °C for Nd-Fe-B feedstock. Although the rheological behavior of feedstock A makes it a proper feedstock formulation, feedstock B was considered the best formulation for further investigation due to its lower viscosity since it has a higher content of PW and SA than feedstock A. This low viscosity makes feedstock B better suited for Nd-Fe-B feedstock because it would allow lower processing temperatures and further increase the powder content.
Since the formulation of feedstock B was determined to be the most promising for this powder, the powder loading adjustment was performed by loading 55, 57, 58, 59, and 60 vol.% of Nd-Fe-B powder in the selected binder system B (B55, B57, B58, B59, and B60).
Figure 6 depicts the representative values of apparent viscosity at different apparent shear rates at 160 °C. As expected, the increase in powder content increases the apparent viscosity of the feedstocks, especially at lower shear rates. Moreover, the apparent viscosity increase with powder content is not linear and is more pronounced at higher contents. As can be seen, the feedstocks with 59 and 60 vol.% had higher viscosities than 1000 Pa·s at lower shear rates (below 250 and 750 s−1, respectively), which are improper formulations from a rheological viewpoint and therefore unsuitable for the MIM process. Both feedstocks with 60 and 59 vol.% solid loadings showed an evident pressure instability (see Appendix A Figure A1). Such pressure instabilities could result from agglomeration at these powder contents and other issues, such as powder–binder segregation at higher shear rates [70]. It must be mentioned that with a powder loading of 65 vol.%, the measured pressure within the capillary rheometer was extremely high, and the viscosity measurement was impossible. Therefore, a higher loading than 60 vol.% was excluded from further investigation.
As mentioned, higher values of the n index are preferable for the MIM process. The n values of feedstocks with 55, 57, 58, 59, and 60 vol.% powder loading can be seen in the insert of Figure 6. The feedstock with 58 vol.% powder content had the highest n value (n = 0.67), indicating the lowest viscosity sensitivity to shear rate. The powder loading higher than 59 vol.% caused a considerable decline in the n value (n = 0.59), indicating that a powder loading around 60 vol.% (n = 0.47) may be assumed as critical solid loading [71]. Therefore, the viscosities of the feedstocks with 57 and 58 vol.% powder content are assumed to be the most appropriate for MIM.
Higher powder loading in MIM feedstocks results in easier densification, less distortion during sintering [71], and less contamination due to a lower amount of the binder system; therefore, higher magnetic properties are expected. Considering the mentioned phenomena and the rheological properties measured, the feedstock with 58 vol.% and the binder system consisting of 45 vol.% LLDPE, 47.5 vol.% PW, and 7.5 vol.% SA could be considered as the optimal feedstock formulation for the selected Nd-Fe-B powder at the processing temperature of 160 °C.

3.2. Density, Contamination, and Magnetic Properties

The average values of density, carbon, and oxygen content of the Nd-Fe-B parts produced with the different binder systems are reported in Table 2. Feedstocks A, B, and C had the same density level. In contrast, feedstock D had slightly lower values, which could be produced by defects during compression molding or worse densification during sintering, resulting from powder binder separation since feedstock D had the lowest SA content, as corroborated by the pressure stability curve in Figure 5. On the other hand, the highest density (6.94 g/cm3) was obtained for feedstock C, which had the highest SA content. It is important to remember that SA acts as a surfactant; thus, SA reduces the surface tension between the binder and the particle and increases the wettability of the binder; therefore, SA helps stabilize the dispersion of the Nd-Fe-B particles in the polymer [72]. As such, higher SA content seems beneficial for achieving a higher density. However, as discussed in the previous sections, high SA content leads to very low viscosity, which causes problems during compounding and molding. Therefore, a lower SA content was used.
All feedstocks had an acceptable range of oxygen and carbon contamination, with feedstock B having the lowest oxygen content but the highest carbon content. These results are directly attributed to its binder composition. The relatively high content of SA in feedstock B chemically adsorbs onto the surface of the alloy particles [72], limiting oxygen pickup. However, feedstock B also has a higher LLDPE content that must be removed during thermal debinding, leading to higher carbon residues compared to feedstocks C and D, which have lower LLDPE contents (Table 1). In this regard, Bioud et al. [73] achieved a lower amount of carbon (800 ppm) by using the same LLDPE (61 vol.%) as the backbone in their binder system. This lower C content could be attributed to the higher powder content (i.e., 61 vol.%), resulting in the decreased binder amount and, consequently, lower carbon contamination. However, the oxygen level was higher (5360 ppm) than all samples in the current study. Thus, there should be a balance between the binder ingredients to prevent oxidation and carbon residues. According to Lee et al. [43], using HDPE as the backbone results in high carbon residue (5000 ppm) in the samples with 50 vol.% NdFeB powder loading, which indicates that employing LLDPE in this study is the reason lower carbon contamination was observed.
On the other hand, the magnetic properties of remanence, coercivity, and maximum energy product were less for the specimens produced with feedstock C than those of feedstock B (Table 3) because feedstock C has a greater variability in the oxygen content after sintering than feedstock B. Meanwhile, feedstock A had the lowest magnetic properties because it had the highest oxygen content after sintering, even though it had low carbon content. These results corroborate that oxygen contamination is more critical than carbon contamination for achieving strong magnetic properties, as has already been mentioned in the literature [39].
In conclusion, and considering these results, careful adjustment of each component of the binder system can increase the densification rate, lower oxygen and carbon contamination, and increase the magnetic properties. Consequently, regarding the contamination results and particularly magnetic properties, feedstock B was selected as the best overall formulation.
The powder loading was increased in feedstock B to improve the densification rate and the magnetic properties. Increasing the powder content from 55 to 57 and 58 vol.% led to a continuous increase in density due to a significant reduction in open porosity, as seen in Table 4. However, as the powder content increased beyond 58 vol.% to 59 and 60 vol.%, the density continuously decreased because the open porosity increased. The very high powder content in these feedstocks (59 and 60 vol.%) could result in powder–binder separation, inhomogeneity, and agglomeration in the feedstock. In addition, the pressure fluctuations are visible during the viscosity measurement at different shear rates for 59 and 60 vol.% (see Appendix A Figure A1). Even if higher magnetic performances can be observed in these two samples compared to the B58 (Table 4 and Figure 7), the inhomogeneity of the feedstock evidenced by the rheological measurements is an issue for the processability of these feedstocks.
As expected, using less binder led to less carbon contamination in the final parts and, thus, to improved physical and magnetic properties. Nonetheless, there was an optimum powder loading regarding the proper distribution of the powder particles in the binder system that affects the final quality of the sintered magnet. Therefore, the highest powder content could not be processed appropriately and 58 vol.% was the optimal powder content for feedstock B. Comparing the carbon contamination of the B58 with the study conducted by Lopes et al. [20], in which they used the same powder content and 45 vol.% backbone (30 vol.% HDPE and 15 vol.% EVA), revealed that using 45 vol.% LLDPE in the B58 formulation led to lower carbon content (982 ppm) compared to employing 30 vol.% HDPE and 15 vol.% EVA as the backbone (1250 ppm), thus indicating that LLDPE is much better than HDPE as a backbone.

4. Conclusions

This study investigated the effect of using a unique binder formulation on the rheological, density, oxygen, and carbon contamination and the magnetic properties of Ce-substituted Nd-Fe-B feedstocks. The LLDPE backbone content ranged from 35–52.5 vol.%, the PW soluble component from 41.25 to 60 vol.%, and the SA surfactant content from 5 to 8.75 vol.% in the binder system, and the powder loadings between 55 and 60 vol.% were used to understand the impact of the binder formulation on the rheology, density, and magnetic properties of MIM feedstocks. The results demonstrated that a 52.5 vol.% LLDPE backbone content in the binder system results in high viscosity and pressure; however, a 35 vol.% backbone reduces the processability because the extremely low viscosity leads to unstable pressure. However, the rheological results showed that the pressure variation for the binder formulation with the 45 vol.% LLDPE was low, which can be concluded that this binder had a proper flow behavior for the MIM process. The viscosity results for the powder loadings of 55, 57, 58, 59, and 60 vol.% with the binder formulation containing 45 vol.% LLDPE revealed that the viscosity of 60 vol.% was higher than the recommended viscosity of 1000 Pa·s. The feedstock with 58 vol.% powder had acceptable pressure stability at the measured shear rates. In addition, the flow index n of the feedstock with 58 vol.% was the highest; therefore, this feedstock is more suitable for the MIM process since it has the best flow behavior.
The physical characteristics of sintered parts for different feedstock formulations complemented the results of the rheological properties. A high density of 6.83 g/cm3, the remanence (Br) of 0.591 T, and the coercivity (Hcj) of 744.6 kA/m were obtained for feedstocks having the best rheological behavior. Increasing the powder loading to 58 vol.% decreased the porosity to 0.7% and increased the density to 7.54 g/cm3. Although the feedstock with 59 vol.% solid loading has the highest magnetic properties, the 58 vol.% powder loading was considered the optimized formulation due to having better flowability and homogeneity. Therefore, the binder system components significantly influence the magnetic properties of MIM magnets, and a tailored formulation can be used to maximize their magnetic properties.

Author Contributions

V.M. and S.L.: conception, experimental design, carrying out measurements and manuscript composition; J.G.-G.: conception and manuscript composition; S.C.: experimental design and manuscript composition; E.S.: carrying out measurements; Z.S.: manuscript composition; S.S.: manuscript composition; C.K.: manuscript composition; C.H.: manuscript composition. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported under Project Agreement No. 19120 and has received funding from the European Institute of Innovation and Technology (EIT), a body of the European Union, under the Horizon 2020, the EU framework program for research and innovation.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data that support the findings of this study will be available on request.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

Applsci 14 05638 g0a1
Figure A1. The representative pressure curves for different powder loadings.
Figure A1. The representative pressure curves for different powder loadings.
Applsci 14 05638 g0a1

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Figure 1. Particle size distribution of the Nd-Fe-B powder used in this study.
Figure 1. Particle size distribution of the Nd-Fe-B powder used in this study.
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Figure 2. SEM image of the jet-milled powder used for this study.
Figure 2. SEM image of the jet-milled powder used for this study.
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Figure 3. Capillary rheometer used in the study.
Figure 3. Capillary rheometer used in the study.
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Figure 4. The representative curve of apparent viscosity versus the apparent shear rate for feedstocks (A, B, C, and D refer to the formulations shown in Table 1).
Figure 4. The representative curve of apparent viscosity versus the apparent shear rate for feedstocks (A, B, C, and D refer to the formulations shown in Table 1).
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Figure 5. The representative pressure curve of feedstocks.
Figure 5. The representative pressure curve of feedstocks.
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Figure 6. Representative apparent viscosity versus apparent shear rates for different powder loadings.
Figure 6. Representative apparent viscosity versus apparent shear rates for different powder loadings.
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Figure 7. Representative values for magnetic properties of the feedstock formulation of B at different powder content.
Figure 7. Representative values for magnetic properties of the feedstock formulation of B at different powder content.
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Table 1. Different binder system formulations investigated in this study.
Table 1. Different binder system formulations investigated in this study.
FeedstockLLDPE (vol.%)PW (vol.%)SA (vol.%)
A52.541.256.25
B4547.57.5
C37.553.758.75
D35605
Table 2. Average values of the density, carbon, and oxygen contamination content of sintered parts for different feedstock formulations at 55 vol.% powder loading.
Table 2. Average values of the density, carbon, and oxygen contamination content of sintered parts for different feedstock formulations at 55 vol.% powder loading.
Feedstock Density (g/cm3)Oxygen (ppm)Carbon (ppm)
A6.836097 ± 9221356 ± 63
B6.854468 ± 2922012 ± 182
C6.945121 ± 13451949 ± 91
D6.614966 ± 5871165 ± 104
Table 3. The magnetic properties of the different formulations at 55 vol.% powder content.
Table 3. The magnetic properties of the different formulations at 55 vol.% powder content.
FeedstockRemanence (Br) (T)Coercivity (Hcj (kA/m))Maximum Energy Product (BH)max (MGOe)
A0.572356.66.35
B0.591744.67.11
C0.587461.46.5
D0.537499.35.43
Table 4. The average density, contamination, and magnetic properties of feedstock B with different powder loadings.
Table 4. The average density, contamination, and magnetic properties of feedstock B with different powder loadings.
FeedstocksDensity (g/cm3)Oxygen (ppm)Carbon (ppm)Remanence (Br) (T)Coercivity (Hcj (kA/m))
B556.83446820120.591744.6
B577.27838410450.556418.8
B587.5471779820.618740.1
B597.4769179970.637828.3
B607.42682510640.578852.8
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Momeni, V.; Luca, S.; Gonzalez-Gutierrez, J.; Cano, S.; Sueur, E.; Shahroodi, Z.; Schuschnigg, S.; Kukla, C.; Holzer, C. Binder System Composition on the Rheological and Magnetic Properties of Nd-Fe-B Feedstocks for Metal Injection Molding. Appl. Sci. 2024, 14, 5638. https://doi.org/10.3390/app14135638

AMA Style

Momeni V, Luca S, Gonzalez-Gutierrez J, Cano S, Sueur E, Shahroodi Z, Schuschnigg S, Kukla C, Holzer C. Binder System Composition on the Rheological and Magnetic Properties of Nd-Fe-B Feedstocks for Metal Injection Molding. Applied Sciences. 2024; 14(13):5638. https://doi.org/10.3390/app14135638

Chicago/Turabian Style

Momeni, Vahid, Sorana Luca, Joamin Gonzalez-Gutierrez, Santiago Cano, Emilie Sueur, Zahra Shahroodi, Stephan Schuschnigg, Christian Kukla, and Clemens Holzer. 2024. "Binder System Composition on the Rheological and Magnetic Properties of Nd-Fe-B Feedstocks for Metal Injection Molding" Applied Sciences 14, no. 13: 5638. https://doi.org/10.3390/app14135638

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