Magnets

High-density metal injection molding (MIM) demonstrates significant potential for soft magnetic applications. While alloying iron with silicon does not substantially enhance magnetic properties, it effectively increases electrical resistivity and reduces eddy current losses—a critical advantage for high-frequency operations up to 1,000 Hz. Ferritic stainless steels are commonly employed in corrosion-prone environments, albeit with compromised magnetic performance.

Ferrous MIM materials require achieving sintered densities exceeding 7.4 g/cm³ to meet stringent specifications for high saturation magnetization (Bs > 1.5 T) and remanence (Br > 0.8 T). Porosity levels below 2% are essential, as interconnected pores degrade permeability (μ) and increase coercivity (Hc). Spherical pore morphology is preferred to minimize magnetic hysteresis.

Figure 1 outlines key guidelines for attaining magnetic properties comparable to wrought materials:

1. Powder selection: Utilize gas-atomized powders with oxygen content < 500 ppm

2. Sintering protocol:

   • Maintain reducing atmospheres (ideally dry H₂ with dew point ≤ -40°C)

   • Maximize sintering temperatures (1,280-1,350°C for Fe-Si systems) to:

     • Promote grain growth (>50 μm)

     • Minimize interstitial contaminants (C/N/O/S < 100 ppm collectively)

3. Post-processing: Implement stress-relief annealing in H₂/N₂ mixtures

Industry best practices involve compacting powders to near-theoretical density (≥95% TD) followed by sintering in dry H₂ at 1,260°C (2,300°F) for 30-60 minutes to optimize domain wall mobility and core loss characteristics (<5 W/kg at 1 kHz).