Alnico Magnet Grade Nomenclature and Compositional Core Differences

Alnico (Aluminum-Nickel-Cobalt) magnets are a class of permanent magnets developed in the early 20th century, known for their excellent temperature stability, corrosion resistance, and high magnetic flux density at elevated temperatures. They are composed primarily of iron (Fe), aluminum (Al), nickel (Ni), and cobalt (Co), with minor additions of copper (Cu), titanium (Ti), or niobium (Nb) to refine their microstructure and enhance magnetic properties.

Alnico magnets are classified into different grades based on their composition and magnetic performance, with common designations such as Alnico 2, Alnico 5, Alnico 8, and Alnico 9. Additionally, some specialized grades like 2:17-type SmCo (Samarium-Cobalt) magnets are sometimes mentioned in the same context due to their cobalt content, though they belong to a different family of rare-earth magnets. This paper focuses on Alnico grades and their nomenclature, while briefly distinguishing them from other cobalt-containing magnets.

2. Alnico Grade Nomenclature: Basis and Standards

2.1 MMPA Standard Nomenclature

The most widely recognized naming convention for Alnico magnets is the MMPA (Magnetic Materials Producers Association) standard, which assigns numerical grades (e.g., Alnico 2, Alnico 5, Alnico 8) based on their magnetic properties and composition. These grades are standardized to help users select the appropriate magnet for specific applications.

2.2 Chinese Nomenclature

In China, Alnico grades are often designated using a combination of Chinese pinyin letters and numbers. For example:

• LNG or LNGT prefixes denote casting Alnico (where "LNGT" includes titanium).
• F prefixes denote sintered Alnico.
• The numbers following the prefix represent the minimum value of maximum energy product (BH)max in kJ/m³.
• A suffix "J" may indicate a high-coercivity alloy.

2.3 Other Naming Conventions

Some manufacturers use proprietary names like Columax, Alcomax, Hycomax, or Ticonal, though these are less common in modern technical literature. The numerical MMPA grades remain the most universally understood.

3. Compositional Core Differences Among Alnico Grades

The primary distinction between Alnico grades lies in their cobalt (Co) content, which directly influences their magnetic properties. Other elements like nickel (Ni), aluminum (Al), copper (Cu), and titanium (Ti) also play crucial roles in determining performance. Below is a detailed breakdown of key Alnico grades and their compositions:

3.1 Alnico 2

• Composition: Approximately 5% Al, 15% Ni, 20% Co, 55% Fe, 1% Cu.
• Magnetic Properties:
  • Remanence (Br): Moderate (around 0.7–0.9 T).
  • Coercivity (Hc): Low (around 40–50 kA/m).
  • Maximum Energy Product (BHmax): Low (around 5–10 kJ/m³).
• Characteristics:
  • Isotropic (non-directional magnetic properties).
  • Suitable for low-field applications where cost and ease of magnetization are priorities.
  • Often used in sensors, relays, and low-power motors.

3.2 Alnico 5

• Composition: Approximately 8% Al, 15% Ni, 24% Co, 35% Fe, 4% Cu, 1% Ti.
• Magnetic Properties:
  • Remanence (Br): High (around 1.0–1.3 T).
  • Coercivity (Hc): Moderate (around 48–160 kA/m).
  • Maximum Energy Product (BHmax): Moderate to high (around 25–40 kJ/m³).
• Characteristics:
  • Anisotropic (directional magnetic properties achieved via heat treatment in a magnetic field).
  • Balanced performance with good temperature stability.
  • Widely used in electric motors, generators, loudspeakers, and magnetic separators.

3.3 Alnico 8

• Composition: Approximately 8% Al, 24% Ni, 14% Co, 46% Fe, 2% Ti.
• Magnetic Properties:
  • Remanence (Br): Very high (around 1.1–1.4 T).
  • Coercivity (Hc): High (around 160–240 kA/m).
  • Maximum Energy Product (BHmax): High (around 35–50 kJ/m³).
• Characteristics:
  • Anisotropic with superior magnetic performance compared to Alnico 5.
  • Excellent temperature stability and corrosion resistance.
  • Used in high-performance motors, sensors, and audio equipment where maximum magnetic output is required.

3.4 Alnico 9

• Composition: Similar to Alnico 8 but with slightly adjusted proportions (e.g., higher Co or Ti content).
• Magnetic Properties:
  • Remanence (Br): Comparable to Alnico 8.
  • Coercivity (Hc): Higher than Alnico 8.
  • Maximum Energy Product (BHmax): Among the highest in Alnico family.
• Characteristics:
  • Less common than Alnico 5 and 8, but used in specialized applications requiring extreme magnetic performance.

3.5 Key Trends in Composition and Performance

• Cobalt Content: Higher Co content generally increases remanence (Br) and maximum energy product (BHmax) but may reduce coercivity (Hc) slightly unless balanced with other elements like Ti.
• Nickel Content: Higher Ni improves coercivity and temperature stability but may reduce remanence if not balanced with Co.
• Aluminum and Copper: These elements aid in forming the desired microstructure (e.g., elongated NiAl precipitates for shape anisotropy) and enhance magnetic properties.
• Titanium: Added in some grades (e.g., Alnico 8) to refine grain structure and improve coercivity.

4. Comparison with Other Cobalt-Containing Magnets: 2:17-Type SmCo

While not an Alnico grade, the 2:17-type Samarium-Cobalt (SmCo) magnet is worth mentioning due to its cobalt content and high-performance characteristics. Unlike Alnico, SmCo belongs to the rare-earth magnet family and offers superior magnetic properties at the cost of higher material and manufacturing expenses.

4.1 Composition

• 2:17-type SmCo: Contains approximately 23–28% Co, along with Sm (Samarium), Fe (Iron), Cu (Copper), and Zr (Zirconium).
• Magnetic Properties:
  • Remanence (Br): Up to 1.1–1.2 T.
  • Coercivity (Hc): Extremely high (up to 2000–2500 kA/m).
  • Maximum Energy Product (BHmax): Very high (up to 250–300 kJ/m³).
• Characteristics:
  • Exceptional temperature stability (operable up to 350–400°C).
  • Highly resistant to corrosion and demagnetization.
  • Used in aerospace, military, and high-end industrial applications where performance outweighs cost.

4.2 Key Differences from Alnico

• Material Family: Alnico is a metal alloy magnet, while SmCo is a rare-earth magnet.
• Magnetic Performance: SmCo significantly outperforms Alnico in coercivity and energy product but is more expensive.
• Temperature Stability: Both offer excellent stability, but SmCo has a higher upper temperature limit.
• Applications: Alnico is preferred for cost-sensitive, high-temperature applications, while SmCo is used where maximum performance is critical.

5. Cost-Effectiveness and Application-Based Selection

The choice of Alnico grade depends on balancing magnetic performance, temperature stability, corrosion resistance, and cost. Higher cobalt content generally improves performance but increases material and processing costs. The "sweet spot" for cost-effectiveness varies by application:

5.1 Low-Cost, Low-Performance Applications

• Alnico 2: Suitable for sensors, relays, and low-power motors where moderate magnetic output is sufficient.

5.2 Balanced Performance Applications

• Alnico 5: The most widely used grade, offering a good balance of cost and performance for electric motors, generators, and loudspeakers.

5.3 High-Performance Applications

• Alnico 8: Used in high-performance motors, sensors, and audio equipment where maximum magnetic output is required, and cost is less of a concern.

5.4 Specialized Applications

• Alnico 9 or SmCo: Reserved for niche applications requiring extreme performance or temperature stability beyond Alnico's capabilities.

6. Conclusion

Alnico magnets are classified into grades (e.g., Alnico 2, 5, 8, 9) based on their composition and magnetic properties, with cobalt content being the primary differentiator. Higher cobalt grades (e.g., Alnico 8) offer superior magnetic performance but at a higher cost, while lower cobalt grades (e.g., Alnico 2) provide a cost-effective solution for less demanding applications. The MMPA standard nomenclature and Chinese pinyin-based systems help users identify and select the appropriate grade for their needs.

While Alnico magnets remain valuable for their temperature stability and corrosion resistance, they face competition from rare-earth magnets like SmCo in high-performance applications. However, Alnico's unique advantages ensure its continued relevance in industries such as automotive, aerospace, and consumer electronics. The key to optimizing cost-effectiveness lies in matching the magnet grade to the specific requirements of the application, ensuring the best balance of performance and expense.