Motion Control Resources
FAQ – Choosing Magnet Materials
Dexter Magnetic Technologies, Inc. Posted 09/26/2017
In a hysteresis coupling, why is Hysterloy the material of choice?
A hysteresis alloy’s composition, orientation and heat treat are optimized to produce maximum hysteresis losses. Generally materials are engineered to reduce losses. Hysterloy is usually supplied in the form of rings with a composition close to that of Alnico 5. The rings are oriented circumferentially during a heat treat cycle that is optimized for maximum losses.
What are the advantages and disadvantages of various materials?
You can see a table that compares the benefits and drawbacks of particular material families, in the Materials section of the Magnetic Products part of our Web site.
What metals are best for use in a magnetic circuit?
Solid steel is generally best, in terms of economics, for the yoke, or frame, of static field devices. The mass of material required to efficiently carry flux between the far poles of an assembly make anything else impossible to justify. The reluctance of this part of the magnetic circuit is relatively low, even without annealing, compared to that of the working gap, so the losses associated with driving flux through the material are a small portion of overall losses. Pole pieces are another story as their purpose is to redistribute and redirect flux over the surface of the gap. Low carbon steel (ASME 1006 – 1018) should be used for pole pieces if possible, and it should be hydrogen annealed after machining. The 400 series stainless steels can be used, but they are not as good magnetically as low carbon steel, so parts may have to be larger to compensate, and/or the magnet stack may have to be larger. Hiperco 50, or an equivalent 50/50 iron cobalt material is needed in the rare case where small size is more valuable than efficiency. This material has a saturation flux density of 24 kG as opposed to around 20kG for good steel. Armco iron, or pure iron may show up on some older drawings. Availability is a concern in smaller volumes and it comes at a higher cost when compared to standard alloy grades. Another factor to consider when choosing Armco iron is the mechanical strength versus standard steel grades. Anodized aluminum is a good non-magnetic material for light weight structures, but non-magnetic (300 series) stainless steels may be needed for environmental protection and strength. Caution: the 300 series stainless steels are austenitic, so cold working will make the cold worked volumes magnetic. This is generally a skin effect.
What’s the difference between Alnico, Sm-Co & Nd-Fe-B magnet materials?
Alnico is an older magnet material that still has important applications. Its maximum energy product is about 1/5 of Sm-Co materials, but it has excellent elevated temperature properties and has better corrosion resistance. Alnico can be cast into different shapes with various magnetic orientations. The rare earth Sm-Co and Nd-Fe-B magnets have high coercivity, so they do not need to be magnetized in circuit and can be used with low permeance coefficients (i.e. thin discs). These materials also lend themselves to Helmholtz coil testing due to their straight line normal curves. This also makes rare earths ideal for motors and high field dipoles. Sm-Co has a good resistance to thermal demagnetization but is brittle. Nd-Fe-B is less brittle, has poor thermal properties, and is prone to corrosion.
Where does a permanent magnet have an advantage over an electromagnet?
Generally, the volume of space needed to produce a given static field will be less for permanent magnets when the working gap is small; electro magnets win in larger devices. However the economic cross over inches up each time a higher energy PM material becomes available. Electromagnet limiting factors are the space consumed by the windings and the power supply, and the heat generated during operation. Permanent magnets do not require a power supply, so they are space and energy efficient. An adjustable power supply allows the magnetic field of an electromagnet to be adjusted easily by simply adjusting the input current. However, adjustable permanent magnets can be used if the field does not have to be adjustment frequently.
Why are rare earth magnets so expensive?
In the case of the rare earth magnets, the heavy metals used to enhance the magnetic properties are difficult to extract. The magnet-related elements are actually a small fraction of the lanthanides mined, so material cannot be produced in huge quantities. Since the fine powders are pyrophoric, production conditions need to be very tightly controlled, and there is a limit to the size of block that can be formed due to the pressure required. Subsequent machining of the magnets adds more cost. Because the magnets are typically very hard and brittle, grinding and slicing operations are slow.