High-Temperature Stability of AlNiCo Materials
AlNiCo magnets retain their magnetic flux up to 550°C (cast grades) or 450°C (sintered grades), far exceeding NdFeB (80-220°C) and SmCo (250-350°C). The reversible temperature coefficient of Br for AlNiCo is -0.02 to -0.03%/°C, approximately one-fifth that of NdFeB (-0.11 to -0.12%/°C). For applications such as automotive speed sensors near exhaust systems, flow meters in steam lines, or aerospace actuators near engine compartments, AlNiCo is often the only practical choice.
However, AlNiCo has low coercivity (Hcj typically 0.5-1.5 kOe) compared to NdFeB (12-30 kOe). This means AlNiCo magnets are easily demagnetized by reverse fields or improper handling. They must be magnetized after assembly in their final magnetic circuit, not before. Also, AlNiCo's low Hcj makes it unsuitable for motors with strong stator fields or for applications with opposing permanent magnets.
Why Cast AlNiCo is Ideal for Complex Geometries
Cast AlNiCo is produced by pouring molten alloy (approx 1500°C) into sand or ceramic molds. This process allows intricate shapes: horseshoes, U-forms, rings with internal teeth, or custom logos embossed on the pole face. Minimum feature size: 2-3mm. Maximum weight per piece: up to 25kg.
Tooling cost for cast AlNiCo ranges from $800-3000 per pattern, depending on complexity. For small quantities (50-500 pieces), cast AlNiCo is expensive due to machining requirements (the as-cast surface is rough, typically requiring grinding on critical faces). For large volumes (>2000 pieces), cast AlNiCo becomes cost-effective because the tooling amortizes and casting cycle time is short (2-5 minutes per piece).
Applications: magnetic chucks, holding magnets, loudspeaker magnets, magnetic pulleys, sensors for extreme temperatures.
Mechanical Strength Improvements in Sintered AlNiCo
Sintered AlNiCo uses powdered metal pressed into a die (similar to ceramic magnet manufacturing) then sintered at 1200-1300°C. The resulting structure is finer-grained and more homogeneous than cast. Mechanical properties: tensile strength 250-350 MPa vs. cast AlNiCo 150-200 MPa. Sintered AlNiCo also exhibits less porosity, reducing internal corrosion risk in humid environments.
Dimensional tolerances: sintered AlNiCo achieves ±0.1mm on pressed dimensions without secondary grinding. For high-precision applications such as magnetic encoders or Hall sensor targets, sintered AlNiCo is preferred. Maximum size: typically 50-60mm in any dimension due to pressing limitations. Larger than that, casting is required.
Comparative Table: Cast vs. Sintered AlNiCo – Magnetic & Mechanical Specs
| Property | Cast AlNiCo (Grade 5/LNG40) | Sintered AlNiCo (Grade 5S/LNGT40) | Engineering Impact |
|---|---|---|---|
| Residual Induction, Br (kGs) | 10.5 – 11.2 | 8.0 – 9.5 | Cast gives 15-25% higher flux |
| Intrinsic Coercivity, Hcj (kOe) | 0.6 – 0.7 | 0.5 – 0.6 | Both easily demagnetized; handle after assembly |
| Max Energy Product, BHmax (MGOe) | 7.5 – 8.5 | 3.5 – 5.5 | Cast stores ~2x magnetic energy per volume |
| Max Operating Temperature (°C) | 550 | 450 | Cast for ultra-high temp (gas turbines) |
| Tensile Strength (MPa) | 150 – 200 | 250 – 350 | Sintered resists cracking under vibration |
| Dimensional Tolerance (as-made) | ±0.5 – 1.0mm | ±0.1 – 0.2mm | Sintered suitable for precision assemblies without grinding |
| Tooling Cost (one-time) | $800 – 3000 per pattern | $300 – 1000 per die | Cast tooling more expensive; better for high volume |
| Minimum Order Quantity economical | 500 pcs | 200 pcs | Sintered for smaller batches |
| Typical Applications | Chucks, heavy holding, loudspeakers, high-temp sensors | Encoders, sensors, precision holding, magnetostrictive devices |
For design engineers needing AlNiCO magnets with custom shapes – including multi-pole magnetization and threaded holes – please refer to our Cast AlNiCo Magnet product page on our website. We supply both cast and sintered grades with full chemical composition certificates (ISO 9001:2015).
To discuss your application's temperature range, required Br, and dimensional tolerances, contact our AlNiCo engineering team. We provide free demagnetization simulation for your magnetic circuit.
Frequently Asked Questions
Q: Can I magnetize a cast AlNiCO magnet before inserting it into my assembly?
A: No. AlNiCO's low coercivity means it will partially demagnetize during handling and assembly. Always assemble first, then magnetize in place using a pulse magnetizer. We provide on-site magnetization service for volume orders.
Q: Which grade of AlNiCO is best for a magnetic encoder wheel operating at 200°C?
A: Use sintered AlNiCO Grade 5S (LNGT40). It provides sufficient Br (8.5-9.0 kGs) and tighter tolerance (±0.1mm) for precise pole pitch. For temperatures above 450°C, switch to cast Grade 8 (LNG44) with Br up to 11.2 kGs.
Q: Are AlNiCO magnets corrosion-resistant without coating?
A: Yes. AlNiCO naturally forms a passive oxide layer and does not require Ni-Cu-Ni or epoxy coating. However, for saltwater or chemical exposure, we recommend a clear epoxy dip (10-20μm) to prevent alkaline attack.
