Soft magnetic materials are essential components in many devices and are indispensable in modern electrical engineering and electronics. The wide range of applications, the steadily rising demands on the quality of components, devices and plants and ever increasing specialization call for a careful selection from the materials available in order to achieve the optimum solution. Development in the field of soft magnetic materials has been rapid in the last few years.
The term "soft" relates to that class of metals or alloys which can be easily magnetized and demagnetized as opposed to "hard" magnetic materials used for permanent magnets.
Today, advanced soft magnetic materials are key components of many devices, which are vital aids in modern electrical engineering and electronics. Because the materials play such a significant role in these applications, even greater care has to go in to choosing the right type of alloy for each application.
In recent years there has been significant progress made in the field of soft magnetic materials. Amorphous and nano-crystalline metals represent an expansion and enhancement of the range of soft magnetic alloys. Due to their structure and composition they have many advantages and innovative properties.
Interestingly however, the silicon steels and ferrites dominate soft magnetic materials. Compared to lamination silicon steels and low carbon steels, powder metallurgy processing offers net shape advantages and to a large extent eliminates secondary operations such as: punching, grinding, honing, drilling, etc.
Over the years, iron powder made by both iron ore reduction and water atomized iron powder, found extensive use in magnetic applications. Three areas are of particular interest namely: powder cores, iron and iron alloys sintered, and insulated iron powder compacts.
Powder cores use iron powder dispersed in a plastic or polymer compacted to different shapes. These cores provide a constant permeability over a wide range of frequencies. Iron powder cores are the lowest cost alternates to ferrites but can provide a higher induction compared to soft ferrites. The applications include switch mode power supplies, inductors and other high frequency broadband applications.
Sintered iron powder and iron-phosphorus alloy powders compete favorably with low carbon steel for variety of applications. Typical properties of powder metal magnetic materials are shown in the Table 1 below.
||0.12 – 0.2
||0.10 – 0.16
||0.02 – 0.08
||0.12 – 0.24
||0.01 – 0.04
Table 1: Typical Properties of PM Magnetic Materials
Iron-Phosphorus Sintered Product
Iron phosphorus powder premixes are sintered typically at 1120°C in a hydrogen or nitrogen –hydrogen atmosphere. Carbon pickup during sintering should be avoided as carbon deteriorates the magnetic properties. Phosphorus content is typically about 0.45%. Higher phosphorus content could be used but should not exceed 0.8%. Care must be exercised while processing higher phosphorus containing materials to avoid embrittlement issues. Application for these types of materials is listed below:
- Higher induction and moderate resistivity
- Used as stators and rotors for low speed stepper motors
- Electric starter motors
- Pole caps
- Actuators for valve control. Anti-locking braking systems (ABS) sensors
- Good strength and hardness
- Ductility allows riveting operations
Iron silicon alloys typically contain about 1.5% to 3.0%. Silicon is not pre-alloyed into iron to avoid the loss of compressibility. A master alloy of iron pre-alloyed with silicon, up to 33% is admixed with pure iron powder and the mixture sintered at 1260°C to achieve diffusion of silicon. The atmosphere generally used is 100% hydrogen. The benefit of this alloy system is shown below:
- Fe-Si sintered parts respond better than Fe-P sintered parts for moderate frequencies
- Fe-Si is used in actuators where impact is involved
- Fe-Si is used in impact printer heads
Table 2: Iron-Silicon PM
Iron –nickel alloys typically contain 50% Nickel, which is pre-alloyed. These powders have low compressibility and can be sintered at 1260°C in a hydrogen or vacuum atmosphere. The benefits of this alloy system are shown below:
- Lower induction
- Higher permeability
- Allows actuation at very low applied fields
- Permendur gives highest saturation
- Difficult to process
- 409L and 434L are stainless grades widely used in magnetic sensor applications where corrosion is important
- Generally have lower induction than Fe or Fe-P systems
In using these materials for magnetic applications care should be exercised in stress relieving the parts if any post sintering operations such as machining or coining is carried out. Annealing the parts at 815C for 15 minutes can relieve this stress.
Toshiba Materials manufactures mainly nickel - and iron-based high-permeability alloys (Permalloy) and degaussing alloys.
Table 3: Iron-Nickel alloys