Humanoid robot technology has been advancing rapidly since the late 1960s, and today such robots can perform a wide variety of complex tasks. The drive system is one of the most critical components in a humanoid robot, as it must supply high-performance, high-torque-density motors for every joint and moving part. Sintered NdFeB magnets, with their exceptional maximum energy product and high saturation magnetisation, remain the key material for modern servo motors. These motors outperform stepper motors in control accuracy, overload capacity, and velocity response, which makes them especially suitable for robotic applications. However, the high cost of these magnets contributes significantly to the overall expense of humanoid robots. In fact, high-performance sintered NdFeB magnets account for approximately 50% of the motor cost, while the motor itself represents about 20% of the total cost of a humanoid robot. Therefore, developing cheaper yet high-coercivity sintered NdFeB magnets is of great importance for the large-scale deployment of humanoid robots.
In this work I present a systematic study on the preparation and performance improvement of three cost-effective sintered NdFeB magnets designed specifically for humanoid robot motors. I first designed and prepared three families of magnets with different levels of cheap elements, namely a heavy-rare-earth-free (HR-free) magnet, a low-dysprosium (low-Dy) magnet, and a high-cerium (high-Ce) magnet. After identifying the optimal compositions, I introduced six organic modifiers into the production process. By screening three antioxidants (A, B, C) and three lubricants (D, E, F), I found that the combination of lubricant D added before jet milling and antioxidant A added after jet milling gave the greatest improvement in magnetic properties, without changing the base alloy formulation. Finally, I adopted the most economical high-Ce magnet and carried out a pilot-scale production optimisation covering jet milling, orientation pressing, sintering, and heat treatment. The resulting magnets show dense microstructure with continuously distributed rare-earth-rich grain boundary phases, leading to improved densification and coercivity. My results demonstrate that the proposed route can produce reliable low-cost sintered NdFeB magnets suitable for humanoid robot applications.
1. Introduction
The growth of humanoid robotics has raised an urgent demand for motors with high torque density, compact dimensions, and low manufacturing cost. Permanent magnet synchronous motors (PMSMs) are the most widely used motor type in these robots because they offer high efficiency, excellent dynamic response, and high overload capability. The core of such a PMSM is the rotor made of sintered NdFeB magnets. These magnets provide a high remanence \(B_r\), a high intrinsic coercivity \(H_{cj}\), and a record maximum energy product \((BH)_{\max}\). The theoretical maximum energy product of NdFeB can be expressed as:
$$
(BH)_{\max} \approx \frac{B_r^2}{4 \mu_0}
$$
where \(B_r\) is the remanence and \(\mu_0\) is the vacuum permeability. In practice, the actual \((BH)_{\max}\) is lower than this theoretical value because of imperfect alignment, porosity, secondary phases, and oxidation.
Despite their excellent performance, sintered NdFeB magnets rely heavily on scarce and expensive rare-earth elements such as Nd, Pr, Dy, and Tb. High-performance grades require substantial amounts of heavy rare earths to maintain high coercivity at elevated temperatures. Dysprosium and terbium are particularly costly and their supply is fragile, creating a serious bottleneck for the mass production of humanoid robots. The motor in a single humanoid robot typically contains around 3–3.5 kg of sintered NdFeB magnets. With production volumes projected to reach one million units per year, the added demand for NdFeB magnets could exceed 3,500 tonnes annually. This scenario makes the design of low-cost magnets containing more abundant elements, such as Ce and La, highly desirable.
In recent years, researchers have attempted to substitute Nd with Ce in Nd₂Fe₁₄B compounds because Ce is the most abundant rare-earth element. However, the intrinsic magnetic properties of Ce₂Fe₁₄B are inferior to those of Nd₂Fe₁₄B. The saturation polarisation \(J_s\) and the magnetocrystalline anisotropy field \(H_A\) of Ce-based compounds are lower, which leads to lower remanence and coercivity. Another strategy to reduce cost is to eliminate Dy entirely while maintaining high coercivity through microstructural refinement and grain boundary engineering. Both approaches are combined in this work to produce three types of magnets with different element budgets:
- HR-free magnet: \(\text{(PrNd)}_{30.8}\text{Fe}_{\text{bal}}\text{M}_{1.43}\text{B}_1\) where M includes Al (0–0.4 wt%), Co, Zr, Cu, Ga, etc.
- Low-Dy magnet: \(\text{(PrNd)}_{22.8}\text{Gd}_{1.8}\text{Ce}_7\text{Dy}_{0.3-0.7}\text{Fe}_{\text{bal}}\text{M}_{1.85}\text{B}_{0.92}\).
- High-Ce magnet: \(\text{(PrNd)}_{17}\text{Ce}_{13-16}\text{Fe}_{\text{bal}}\text{M}_{1.55}\text{B}_{0.9}\).
I systematically varied the content of Al, Dy, and Ce to find the best performance-to-cost ratio. In addition, I investigated the influence of six commercial modifiers (three antioxidants and three lubricants) on the magnetic properties of these magnets. The modifiers were introduced at different stages of jet milling and pressing. My aim was to identify a simple, low-cost additive route that could boost coercivity and remanence without changing the alloy composition. Finally, I performed pilot-scale trials on the high-Ce magnet to optimise the process parameters for industrial manufacturing. The results presented here provide an integrative approach to reduce the cost of sintered NdFeB magnets and facilitate their application in humanoid robots.
2. Experimental Methods
2.1. Design of Alloy Compositions
The target compositions of the three magnets are summarised in Table 1. The design principle was to maximise the use of abundant elements (Ce, Gd) and to minimise or eliminate expensive heavy rare earths (Dy and Tb). All compositions are given in weight percent except B, which is expressed as a nominal element. The balance is Fe, with small additions of M that include Al, Co, Zr, Cu, Ga, and Ti in various combinations.
| Magnet type | Composition (wt%) | M (minor elements) |
|---|---|---|
| HR-free | \(\text{(PrNd)}_{30.8}\text{Fe}_{\text{bal}}\text{M}_{1.43}\text{B}_1\) | Al (0–0.4), Co, Zr, Cu, Ga |
| Low-Dy | \(\text{(PrNd)}_{22.8}\text{Gd}_{1.8}\text{Ce}_7\text{Dy}_{0.3-0.7}\text{Fe}_{\text{bal}}\text{M}_{1.85}\text{B}_{0.92}\) | Al, Co, Zr, Cu, Ga, Ti |
| High-Ce | \(\text{(PrNd)}_{17}\text{Ce}_{13-16}\text{Fe}_{\text{bal}}\text{M}_{1.55}\text{B}_{0.9}\) | Al, Co, Zr, Cu, Ga, Ti |
2.2. Preparation Process
All magnets were prepared by a conventional powder metallurgy route. The raw materials – PrNd alloy, Gd-Fe alloy, B-Fe alloy, Dy-Fe alloy, metallic Al, and metallic Ce – were weighed according to the designed formulas. Batches of 50 kg were melted in a vacuum induction melting furnace and then strip-cast into thin flakes at a cooling rate of about \(10^4\,\text{K/s}\). This rapid solidification suppresses the formation of primary \(\alpha\)-Fe dendrites and yields a fine columnar Nd₂Fe₁₄B structure surrounded by a Nd-rich phase.
The strip-cast flakes were subjected to hydrogen decrepitation (HD) in a rotary furnace. During HD, hydrogen is absorbed preferentially by the Nd-rich phase, causing the flakes to fracture along grain boundaries. The resulting coarse powder has a low oxygen content and is suitable for jet milling.
Jet milling was performed with a lab-scale / pilot-scale air classifier mill. The powder particle size was controlled by adjusting the classifier wheel speed. Powder with an average particle size of 3.0–3.3 μm was produced for all experiments. Modifiers were added either before or after jet milling depending on the test, as described later.
The fine powder was then aligned in a magnetic field and pressed isostatically. The green compacts were placed in rubber bags and cold isostatically pressed at about 200 MPa. Sintering was performed in vacuum at temperatures between 1030 °C and 1070 °C for 5–6 hours, followed by two-step heat treatment. The first heat-treatment step was conducted at 860–920 °C for 2–3 hours, and the second step at 650–710 °C for 5–6 hours. The sintered and heat-treated samples were machined into standard test pieces for magnetic measurements.
2.3. Characterisation
Magnetic properties (remanence \(B_r\), intrinsic coercivity \(H_{cj}\), and maximum energy product \((BH)_{\max}\)) were measured using a NIM-2000 permanent magnet measuring system at room temperature. The density of sintered samples was measured by Archimedes’ method. The microstructure was observed with a scanning electron microscope (SEM) in backscattered electron mode, which provides compositional contrast between the main phase and the intergranular phase. Elemental analysis was carried out using inductively coupled plasma (ICP) spectroscopy to verify the actual composition of the sintered magnets.
2.4. Modifier Screening
Three antioxidant modifiers (labelled A, B, C) and three lubricant modifiers (labelled D, E, F) were purchased from commercial suppliers. In the screening experiments, each modifier was added in two portions: half before the jet milling step and half after the jet milling step. The added amount was varied from 0.6 to 1.4 mL per kilogram of powder. Once the best antioxidant and best lubricant were identified, I tested four different addition schedules:
- Antioxidant A before jet milling + Antioxidant A after jet milling (A+A)
- Lubricant D before jet milling + Lubricant D after jet milling (D+D)
- Lubricant D before jet milling + Antioxidant A after jet milling (D+A)
- Antioxidant A before jet milling + Lubricant D after jet milling (A+D)
- No modifier as control
The total amount of modifier was kept constant at 1.2 mL/kg in all experiments, because the initial screening showed that this dosage gave the best results.
3. Results and Discussion
3.1. Effect of Compositional Variations on Magnetic Properties
3.1.1. HR-free Magnets with Different Al Content
The HR-free magnets were prepared with Al contents from 0% to 0.4%. Figure 1 shows the demagnetisation curves of these magnets, and Table 2 summarises the magnetic properties. The remanence \(B_r\) decreases from 14.64 kGs (1.464 T) at 0% Al to 14.14 kGs at 0.4% Al. Meanwhile, the intrinsic coercivity \(H_{cj}\) increases from 12.20 kOe (971 kA/m) to 13.65 kOe (1086 kA/m). The variation is most significant between 0% and 0.3% Al; beyond 0.3%, the changes become smaller. The addition of Al refines the grain structure and improves the wetting of the Nd-rich phase, resulting in better magnetic isolation of the main phase grains and thus higher coercivity. However, Al also dissolves into the main phase, leading to a dilution of its magnetisation and a reduction in \(B_r\). The optimum Al content in this study was found to be 0.3%, giving \(B_r\) = 14.16 kGs, \(H_{cj}\) = 13.62 kOe, and \((BH)_{\max}\) = 48.07 MGOe.
| Al content (%) | \(B_r\) (kGs) | \(H_{cj}\) (kOe) | \((BH)_{\max}\) (MGOe) |
|---|---|---|---|
| 0 | 14.64 | 12.20 | 51.20 |
| 0.1 | 14.29 | 12.45 | 48.55 |
| 0.2 | 14.18 | 12.90 | 47.67 |
| 0.3 | 14.16 | 13.42 | 48.07 |
| 0.4 | 14.14 | 13.65 | 48.38 |
3.1.2. Low-Dy Magnets with Different Dy Content
The low-Dy magnets were prepared with Dy contents from 0.3% to 0.7%. The measured properties are listed in Table 3. With increasing Dy content, the remanence slowly decreases from 12.32 kGs to 11.94 kGs, while the coercivity increases from 16.06 kOe to 17.21 kOe. The improvement in \(H_{cj}\) saturates beyond 0.6% Dy. Dy partly substitutes for Nd in the main phase, forming \((\text{RE},\text{Dy})_2\text{Fe}_{14}\text{B}\) with a higher anisotropy field. This raises the nucleation field for reverse domains, thereby increasing the coercivity. The small decrease in \(B_r\) is mainly due to the lower saturation magnetisation of Dy₂Fe₁₄B compared to Nd₂Fe₁₄B. The optimum Dy content was chosen as 0.6%, which gives \(B_r\) = 11.95 kGs, \(H_{cj}\) = 17.11 kOe, and \((BH)_{\max}\) = 34.21 MGOe.
| Dy content (%) | \(B_r\) (kGs) | \(H_{cj}\) (kOe) | \((BH)_{\max}\) (MGOe) |
|---|---|---|---|
| 0.3 | 12.32 | 16.06 | 35.29 |
| 0.4 | 12.19 | 16.43 | 34.43 |
| 0.5 | 12.00 | 16.78 | 34.45 |
| 0.6 | 11.95 | 17.11 | 34.21 |
| 0.7 | 11.94 | 17.21 | 34.24 |
3.1.3. High-Ce Magnets with Different Ce Content
High-Ce magnets were prepared with Ce contents from 12% to 16%. As shown in Table 4, both \(B_r\) and \(H_{cj}\) decrease monotonically with Ce content. The remanence drops from 12.39 kGs to 12.00 kGs, while the coercivity falls from 13.27 kOe to 12.08 kOe. This is expected because Ce₂Fe₁₄B has inferior intrinsic magnetic properties compared to Nd₂Fe₁₄B. Nevertheless, the decline is relatively moderate up to 15% Ce, beyond which the magnetic properties decay more noticeably. Therefore, I selected 15% Ce as the optimum composition for economic reasons. This high-Ce magnet has \(B_r\) = 12.02 kGs, \(H_{cj}\) = 12.20 kOe, and \((BH)_{\max}\) = 32.95 MGOe.
| Ce content (%) | \(B_r\) (kGs) | \(H_{cj}\) (kOe) | \((BH)_{\max}\) (MGOe) |
|---|---|---|---|
| 12 | 12.39 | 13.27 | 35.54 |
| 13 | 12.26 | 12.88 | 34.47 |
| 14 | 12.12 | 12.59 | 33.57 |
| 15 | 12.02 | 12.20 | 32.95 |
| 16 | 12.00 | 12.08 | 32.76 |
3.2. Effect of Modifiers on Magnetic Properties
3.2.1. Screening of Antioxidants and Lubricants
Using the optimum compositions identified above, I then tested the six modifiers. The results for all three magnet types are presented in Tables 5, 6, and 7. For each modifier, the magnetic properties were measured at five addition levels: 0.6, 0.8, 1.0, 1.2, and 1.4 mL/kg. In order to keep the tables concise, I list only the optimum values for each modifier, together with the corresponding addition level.
| Modifier | Optimum dose (mL/kg) | \(B_r\) (kGs) | \(H_{cj}\) (kOe) |
|---|---|---|---|
| A (antioxidant) | 1.2 | 14.43 | 13.76 |
| B (antioxidant) | 1.2 | 14.20 | 13.54 |
| C (antioxidant) | 1.2 | 13.92 | 13.16 |
| D (lubricant) | 1.2 | 14.37 | 13.66 |
| E (lubricant) | 1.2 | 14.19 | 13.51 |
| F (lubricant) | 1.2 | 13.86 | 13.14 |
| Modifier | Optimum dose (mL/kg) | \(B_r\) (kGs) | \(H_{cj}\) (kOe) |
|---|---|---|---|
| A | 1.2 | 12.25 | 17.69 |
| B | 1.2 | 12.20 | 17.53 |
| C | 1.2 | 12.17 | 17.47 |
| D | 1.2 | 12.21 | 17.58 |
| E | 1.2 | 12.19 | 17.41 |
| F | 1.2 | 12.10 | 17.22 |
| Modifier | Optimum dose (mL/kg) | \(B_r\) (kGs) | \(H_{cj}\) (kOe) |
|---|---|---|---|
| A | 1.2 | 12.62 | 12.65 |
| B | 1.2 | 12.54 | 12.50 |
| C | 1.2 | 12.42 | 12.40 |
| D | 1.2 | 12.55 | 12.53 |
| E | 1.2 | 12.44 | 12.48 |
| F | 1.2 | 12.32 | 12.39 |
It is evident that antioxidant A consistently provides the greatest improvement among the three antioxidants, and lubricant D provides the greatest improvement among the three lubricants for all three magnet types. The optimum addition level is 1.2 mL/kg for both. The figure shows the demagnetisation curves of high-Ce magnets with the additives at their optimum dosage. The curve with antioxidant A lies higher than those with B or C, confirming its superior effect.
3.2.2. Combination of Antioxidant and Lubricant
After selecting A and D as the best modifier types, I examined four addition schedules with a fixed total dose of 1.2 mL/kg. The results for the three magnet types are listed in Table 8. In all cases, the schedule D+A (lubricant D before jet milling, antioxidant A after jet milling) gives the highest remanence and coercivity. For example, the HR-free magnet reaches \(B_r\) = 14.46 kGs and \(H_{cj}\) = 13.86 kOe, compared with 14.09 kGs and 13.08 kOe for the unmodified control. The improvement is consistent across all three formulas, indicating that the D+A route is generally applicable.
| Magnet type | Schedule | \(B_r\) (kGs) | \(H_{cj}\) (kOe) | \((BH)_{\max}\) (MGOe) |
|---|---|---|---|---|
| HR-free | A+A | 14.37 | 13.62 | 49.32 |
| D+D | 14.20 | 13.43 | 48.37 | |
| A+D | 14.43 | 13.76 | 49.59 | |
| D+A | 14.46 | 13.86 | 50.00 | |
| None | 14.09 | 13.08 | 46.39 | |
| Low-Dy | A+A | 12.18 | 17.39 | 34.49 |
| D+D | 12.10 | 17.15 | 34.54 | |
| A+D | 12.25 | 17.58 | 35.15 | |
| D+A | 12.29 | 17.65 | 35.36 | |
| None | 12.05 | 16.77 | 34.01 | |
| High-Ce | A+A | 12.42 | 12.34 | 35.72 |
| D+D | 12.29 | 12.21 | 34.64 | |
| A+D | 12.54 | 12.50 | 37.24 | |
| D+A | 12.61 | 12.69 | 37.58 | |
| None | 12.20 | 12.01 | 33.75 |
The improvement caused by the D+A schedule can be explained by the synergistic action of lubricant and antioxidant. The lubricant added before jet milling reduces the adhesion between fine particles, preventing agglomeration and improving powder flowability. This leads to a more uniform particle size distribution and better alignment during pressing. The antioxidant added after jet milling forms a protective layer on the powder surfaces, reducing oxidation during subsequent handling and pressing. Since oxygen is detrimental to the formation of the Nd-rich grain boundary phase, less oxidation means a cleaner and more continuous intergranular phase, which improves both coercivity and squareness of the demagnetisation curve.
3.3. Pilot-Scale Production Optimisation of High-Ce Magnets
Because the high-Ce magnet has the lowest raw material cost, I selected it for pilot-scale production trials. The objective was to optimise the key process parameters – jet milling speed, magnetic orientation field, sintering temperature, and heat-treatment temperature – to obtain the best possible magnetic performance while maintaining economic efficiency. All samples in this section were prepared with the D+A modifier schedule and a fixed total modifier dose of 1.2 mL/kg.
3.3.1. Optimisation of Jet Milling Speed
Jet milling was carried out at classifier wheel speeds of 3800, 3900, 4000, 4100, and 4200 r/min. The resulting average particle sizes and magnetic properties are listed in Table 9. As the speed increases, the powder becomes finer. The remanence first increases, reaching a maximum at 4000 r/min, and then decreases. The intrinsic coercivity follows a similar trend. The optimum speed is 4000 r/min, producing a powder with an average particle size of 3.0–3.1 μm. This gave \(B_r\) = 12.61 kGs, \(H_{cj}\) = 12.69 kOe, and \((BH)_{\max}\) = 37.58 MGOe.
| Milling speed (r/min) | Average particle size (μm) | \(B_r\) (kGs) | \(H_{cj}\) (kOe) | \((BH)_{\max}\) (MGOe) |
|---|---|---|---|---|
| 3800 | 3.2–3.3 | 12.30 | 12.25 | 35.88 |
| 3900 | 3.1–3.2 | 12.53 | 12.44 | 36.15 |
| 4000 | 3.0–3.1 | 12.61 | 12.69 | 37.58 |
| 4100 | 2.9–3.0 | 12.57 | 12.36 | 35.94 |
| 4200 | 2.8–2.9 | 12.41 | 12.22 | 36.02 |
3.3.2. Optimisation of Orientation Field
The green compacts were pressed under magnetic fields ranging from 1.5 to 2.2 T with a constant holding time of 60 seconds. As shown in Table 10, the best magnetic properties are obtained at 1.8 T. Below 1.8 T, the degree of particle alignment increases with field, improving remanence. Above 1.8 T, the magnetic pressure becomes so strong that it may rotate or even crack the already aligned particles, damaging the orientation quality. Therefore, 1.8 T was selected as the optimum orientation field.
| Orientation field (T) | Holding time (s) | \(B_r\) (kGs) | \(H_{cj}\) (kOe) | \((BH)_{\max}\) (MGOe) |
|---|---|---|---|---|
| 1.5 | 60 | 12.43 | 12.15 | 36.36 |
| 1.8 | 60 | 12.61 | 12.69 | 37.58 |
| 2.0 | 60 | 12.55 | 12.41 | 36.81 |
| 2.2 | 60 | 12.26 | 12.08 | 35.77 |
3.3.3. Optimisation of Sintering Temperature
Sintering was carried out at 1030, 1040, 1050, 1060, and 1070 °C for 6 hours. The density and magnetic properties are listed in Table 11. The density increases with temperature up to 1060 °C and then slightly decreases. At 1060 °C, the magnet reaches near full density (7.58 g/cm³) and exhibits the best magnetic performance: \(B_r\) = 12.61 kGs, \(H_{cj}\) = 12.69 kOe, \((BH)_{\max}\) = 37.58 MGOe. Higher sintering temperatures cause abnormal grain growth, which is detrimental to coercivity. Thus, 1060 °C is the optimal sintering temperature.
| Sintering temperature (°C) | Density (g/cm³) | \(B_r\) (kGs) | \(H_{cj}\) (kOe) | \((BH)_{\max}\) (MGOe) |
|---|---|---|---|---|
| 1030 | 7.48 | 11.64 | 11.28 | 31.66 |
| 1040 | 7.51 | 11.96 | 11.54 | 33.52 |
| 1050 | 7.49 | 12.36 | 12.33 | 35.74 |
| 1060 | 7.58 | 12.61 | 12.69 | 37.58 |
| 1070 | 7.56 | 12.14 | 12.24 | 35.31 |
3.3.4. Optimisation of Two-Step Heat Treatment
After sintering at 1060 °C for 6 hours, the magnets were subjected to a two-step heat treatment. In the first step, the temperature was varied from 860 to 920 °C for 2 hours. The results are listed in Table 12. The coercivity and energy product reach their maxima at 900 °C. At higher temperatures, the Nd-rich phase may penetrate the main phase or become less effective, causing a decline in coercivity. The remanence is almost unchanged in this temperature range.
| First step temperature (°C) | \(B_r\) (kGs) | \(H_{cj}\) (kOe) | \((BH)_{\max}\) (MGOe) |
|---|---|---|---|
| 860 | 12.55 | 11.36 | 36.54 |
| 880 | 12.59 | 11.88 | 36.61 |
| 900 | 12.61 | 12.69 | 37.58 |
| 910 | 12.59 | 12.41 | 36.75 |
The second heat-treatment step was varied from 650 to 710 °C while the first step was fixed at 900 °C. The results are shown in Table 13. The best properties occur at 690 °C. The coercivity improves significantly after the second step, owing to the redistribution of the rare-earth-rich phase and the smoothing of grain boundaries. The optimal second-step temperature is 690 °C, where \(B_r\) = 12.61 kGs, \(H_{cj}\) = 12.69 kOe, and \((BH)_{\max}\) = 37.58 MGOe.
| Second step temperature (°C) | \(B_r\) (kGs) | \(H_{cj}\) (kOe) | \((BH)_{\max}\) (MGOe) |
|---|---|---|---|
| 650 | 12.59 | 12.15 | 37.00 |
| 670 | 12.57 | 12.34 | 37.31 |
| 690 | 12.61 | 12.69 | 37.58 |
| 710 | 12.54 | 11.99 | 36.84 |
3.3.5. Final Production Parameters
Based on the above optimisation, the following process parameters were selected for the pilot-scale production of high-Ce sintered NdFeB magnets:
- Jet milling: classifier wheel speed = 4000 r/min; lubricant D added at 1.2 mL/kg during milling.
- Orientation pressing: magnetic field = 1.8 T; holding time = 60 s; antioxidant A mixed uniformly into the powder at 1.2 mL/kg.
- Sintering: 1060 °C for 6 h under vacuum or controlled atmosphere.
- Heat treatment: first step at 900 °C for 2 h; second step at 690 °C for 5 h.
3.3.6. Microstructural Analysis
The microstructure of the high-Ce magnets produced with and without the D+A modifier combination was observed by SEM in backscattered electron mode. In the unmodified magnet, the main phase grains are surrounded by a rare-earth-rich phase, but some pores are visible, which reduce the density and harm the magnetic properties. In the modified magnet, the microstructure is more homogeneous, with continuous intergranular phases and virtually no porosity. The grain boundaries are sharp and clear. This confirms that the D+A modifier route improves wetting and densification, leading to better magnetic isolation of the grains and a higher coercivity. The dense microstructure also improves the mechanical strength and corrosion resistance, which are important for the reliability of humanoid robot motors.
4. Conclusion
In this study, I have demonstrated an integrated approach to prepare low-cost sintered NdFeB magnets with tailored magnetic properties for humanoid robot applications. Three magnet families were designed using abundant elements and reduced heavy rare earth content:
- An HR-free magnet with nominal composition \(\text{(PrNd)}_{30.8}\text{Fe}_{\text{bal}}\text{Al}_{0.3}\text{M}_{1.13}\text{B}_1\) showing \(B_r\) = 14.16 kGs, \(H_{cj}\) = 13.62 kOe, and \((BH)_{\max}\) ≈ 48 MGOe.
- A low-Dy magnet containing 0.6% Dy, with \(B_r\) = 11.95 kGs, \(H_{cj}\) = 17.11 kOe, and \((BH)_{\max}\) ≈ 34 MGOe, suitable for applications requiring high-temperature stability.
- A high-Ce magnet with 15% Ce, exhibiting \(B_r\) = 12.02 kGs, \(H_{cj}\) = 12.20 kOe, and \((BH)_{\max}\) ≈ 33 MGOe, which offers the lowest raw material cost.
Systematic screening of six commercial modifiers identified antioxidant A and lubricant D as the most effective. Adding lubricant D before jet milling and antioxidant A after jet milling, both at 1.2 mL/kg, produced significant improvements in remanence and coercivity for all three magnet types, without modifying the base alloy composition. The improvements are attributed to reduced powder agglomeration, better particle alignment, and suppression of oxidation during processing.
Pilot-scale production of the high-Ce magnet was optimised with the following parameters: jet milling at 4000 r/min, orientation field of 1.8 T, sintering at 1060 °C for 6 h, first heat-treatment at 900 °C for 2 h, and second heat-treatment at 690 °C for 5 h. The resulting magnets reached \(B_r\) = 12.61 kGs, \(H_{cj}\) = 12.69 kOe, and \((BH)_{\max}\) = 37.58 MGOe. Microstructural analysis revealed a dense, pore-free structure with distinct and continuous rare-earth-rich grain boundary phases, confirming the positive effect of the modifier route on densification.
The combination of abundant element substitution, simple additive modification, and optimised pilot-scale processing offers a practical and cost-effective route to produce sintered NdFeB magnets for humanoid robots. The performance levels achieved are sufficient for many servo motor applications, and the reduced material cost will accelerate the deployment of humanoid robots by lowering their overall manufacturing expenses. Future work will focus on further enhancing the coercivity of high-Ce magnets, exploring recyclable rare earth sources, and scaling up the process to industrial volumes.