1. Introduction to Alnico Magnets
Alnico magnets, composed primarily of aluminum (Al), nickel (Ni), cobalt (Co), and iron (Fe), are a type of permanent magnet known for their excellent thermal stability and high remanence. These magnets have been widely used in various applications, including motors, sensors, loudspeakers, and aerospace components, due to their unique magnetic properties. However, Alnico magnets also exhibit certain characteristics, such as low coercivity, which make them susceptible to demagnetization under specific conditions. Understanding the concepts of reversible and irreversible demagnetization, as well as the critical demagnetization field strength, is crucial for optimizing the performance and reliability of Alnico-based devices.

1. Introduction to Magnetic Permeability
Magnetic permeability (μ) is a fundamental property of magnetic materials that quantifies their ability to support the formation of a magnetic field within themselves. It is defined as the ratio of the magnetic flux density (B) to the magnetizing field intensity (H) (μ = B/H). The permeability of a material determines how effectively it can be magnetized and how it responds to external magnetic fields. In the context of permanent magnets, permeability is crucial for understanding their magnetic circuit behavior, energy storage capacity, and stability under varying operating conditions.

1. Introduction
Alnico (Aluminum-Nickel-Cobalt) magnets are a family of permanent magnet materials known for their excellent thermal stability, making them suitable for high-temperature applications such as aerospace, military, and industrial sensors. Unlike rare-earth magnets (e.g., NdFeB) or ferrite magnets, Alnico exhibits minimal magnetic performance degradation under elevated temperatures due to its unique microstructure and low-temperature coefficients.

1. Introduction to Alnico Magnets
Alnico (Aluminum-Nickel-Cobalt) is a family of permanent magnet materials developed in the 1930s, composed primarily of iron (Fe), aluminum (Al), nickel (Ni), and cobalt (Co), with trace amounts of copper (Cu) and titanium (Ti). Known for its high remanence (Br) and excellent thermal stability, Alnico was once the dominant permanent magnet material before being surpassed by ferrite and rare-earth magnets in the late 20th century. However, it remains indispensable in applications requiring stable magnetic performance under extreme temperatures, such as aerospace, military, and precision instrumentation.

1. Introduction to AlNiCo as a Permanent Magnet
AlNiCo (Aluminum-Nickel-Cobalt) alloys, developed in the 1930s, were among the first commercially viable permanent magnets. Despite having a low intrinsic coercivity (Hcj, typically <160 kA/m)—a trait that would seem disqualifying for a permanent magnet—AlNiCo remains indispensable in applications requiring high remanence (Br), excellent thermal stability, and corrosion resistance. Its unique combination of properties allows it to outperform modern rare-earth magnets in specific niches, such as instrumentation, sensors, and aerospace components, where temperature resilience and long-term stability are paramount.

1. Introduction to AlNiCo Magnets
AlNiCo (Aluminum-Nickel-Cobalt) magnets, developed in the 1930s, were once the dominant permanent magnetic materials due to their exceptional high remanence (Br) and low temperature coefficient, enabling stable performance at temperatures exceeding 600°C. Despite being superseded by rare-earth magnets (e.g., NdFeB) in high-energy applications, AlNiCo remains indispensable in instrumentation, sensors, and aerospace due to its corrosion resistance, thermal stability, and low coercivity (Hcb).

This article explores the microstructural origins of AlNiCo’s high Br and low Hcb, the role of manufacturing processes, and whether these properties can be reversed or tuned via process optimization.

The three core magnetic parameters—Remanence (Br), Coercivity (Hcb), and Maximum Energy Product ((BH)max)—vary significantly among cast oriented (anisotropic) AlNiCo, cast non-oriented (isotropic) AlNiCo, and sintered AlNiCo magnets due to differences in manufacturing processes, microstructures, and alloy compositions. Below is a detailed comparison based on empirical data and material science principles:

1. Introduction
AlNiCo (Aluminum-Nickel-Cobalt) magnets are a class of permanent magnet materials renowned for their exceptional temperature stability, high remanence (Br), and low reversible temperature coefficient. These properties make them indispensable in high-precision applications such as aerospace sensors, automotive instrumentation, and precision motors. However, batch-to-batch performance variability remains a critical challenge in AlNiCo magnet production, leading to inconsistent magnetic properties, reduced yield rates, and increased manufacturing costs.

1. Introduction to AlNiCo Magnets
AlNiCo (Aluminum-Nickel-Cobalt) magnets are a class of permanent magnet materials known for their excellent temperature stability, high remanence (Br), and low reversible temperature coefficient. They are widely used in high-precision applications such as sensors, motors, aerospace components, and precision instruments. However, due to their brittleness, high hardness, and low toughness, AlNiCo magnets are prone to internal defects during manufacturing, which can significantly affect their magnetic performance and reliability.

1. Introduction to AlNiCo Magnets
AlNiCo (Aluminum-Nickel-Cobalt) magnets are a type of permanent magnet material composed primarily of aluminum (Al), nickel (Ni), and cobalt (Co), with small additions of copper (Cu), titanium (Ti), and other elements to enhance performance. They are known for their high remanence (Br), excellent temperature stability, and low reversible temperature coefficient, making them suitable for high-precision applications such as sensors, motors, and aerospace components.

However, AlNiCo magnets also have inherent drawbacks, including low mechanical strength, high hardness, and brittleness, which significantly impact their machinability. This article explores why AlNiCo magnets require large machining allowances and the dimensional accuracy achievable after machining.

1. Stress-Relief Annealing
Objective:
To eliminate residual stresses generated during manufacturing processes such as casting, forging, machining, or welding, thereby improving dimensional stability and reducing the risk of cracking or deformation.

Alnico magnets, as one of the earliest developed permanent magnetic materials, have unique advantages in high-temperature and high-stability magnetic applications. Grain refinement is an important means to improve the magnetic properties of Alnico magnets. This paper provides an in-depth analysis of the grain refinement processes of cast Alnico magnets, including chemical treatment, mechanical vibration and stirring, and external physical field treatment. It also explores the impact of grain refinement on key magnetic performance indicators such as coercivity, remanence, and maximum magnetic energy product, and looks forward to future research directions in this field.