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DFM Part 2: Sintered Soft Magnetic Materials in Powder Metallurgy.

Posted by Horizon Technology - July 13, 2021

Different branches of powder metallurgy (PM) materials have very different design considerations. It’s a fact of life for PM manufacturers but sometimes a hard lesson for the OEM buyer/engineer.

In powder metallurgy, there are two branches of magnetic materials. For the sake of simplicity, we’ll say that one serves DC electromagnetic applications, and the other AC.

Soft magnetic composites (SMCs) are an excellent, energy-efficient option for AC electromagnetic applications. However, they're not suitable for DC components because they lack the rapid magnetic response of sintered materials. By the same token, sintered soft magnetic materials are unsuitable for AC applications because they generate significant heat and core losses.

(If your project calls for an AC machine, refer to Part 1 of this series, where we explored designing for manufacturability for soft magnetic composites.)


Part 2 of our “Design for Manufacturing” series will help engineers address the limitations and, more importantly, the advantages of powder metallurgy in DC settings. Sintered powder metal parts have proven themselves in high-performance DC applications because of their high permeability, coercive force, and mechanical strength. 

By applying not just the right material, but also the correct principles of design for manufacturability, you can unlock your part’s potential.

Design Considerations for Powder Metallurgy Sintered Soft Magnetic Materials

Design for manufacturability as a methodology requires that engineers consider the practical limitations from concept to final assembly. In the context of sintered soft magnetic components, this means both sides must understand how the properties of soft magnetic materials impact:

  • The magnetic properties of parts made by powder metallurgy
  • The manufacturing processes necessary for your specific application
  • The mechanical properties of parts made by powder metallurgy

Evaluating your needs in conjunction with the advantages and limitations of sintered powder metal ensures that the finished product is efficient to produce and will perform well in the field. Just as importantly, DFM makes sure that the manufacturing process proceeds without interruption or lost time and money due to defects.

Application

Sintered soft magnetics offer rapid response and customizable performance, the combination of which benefits many DC applications:

  • Solenoids
  • Magnetic stators (flux return paths) for brushed DC motors
  • Rotors for synchronous motors with permanent magnets
  • Almost any DC motor, including brushless DC electric motors

Whatever the specific use, your individual application requirements should remain at the forefront of the design process. As just one example, you'll need to know the frequency of your application before you choose a powder because frequency influences the magnetic performance of sintered soft magnetic materials.

Once we know what you want to do with your part, we can help you compare materials and unlock higher levels of performance.

Shape

Component shape is among the most critical considerations when designing for manufacturability. Your powder metallurgy manufacturer should be capable of helping you fully realize the net-shaping capabilities inherent to PM. In doing so, you can add value, create a more compact design, and maybe even reduce the weight of your product.

Powder metallurgy is a highly efficient process, start to finish. A product whose design has been optimized for powder metallurgy may require no secondary machining whatsoever. However, when compaction can’t capture a product's exact geometry, machining offers a cost-effective means of replicating precise details. Some features that call for machining after compaction include:

  • Cross-holes
  • Undercuts
  • Flanges
  • Grooves

Sintered soft magnetic materials are strong enough to withstand machining, tapping, and drilling when required, so clients can achieve almost any product geometry possible.

Compared to SMC powders for AC applications, it's possible to accomplish complicated geometries with sintered soft magnetics thanks to sinter brazing and sinter bonding. These processes, unique to powder metal, allow you to reduce the number of parts in your assembly. With sinter brazing, for instance, you can join a non-magnetic component to a magnetic one, limiting the magnetic properties to certain regions of the component. 

You can also perform press-fitting onto a shaft, which is impossible with the more brittle SMCs used in AC magnetic projects.

Performance and Powder Metallurgy Secondary Operations

The mechanical properties of parts made by powder metallurgy, sintered or not, will always differ from machined/wrought materials from bar stock. For instance, with powder metallurgy, maximum flux is a function of density. However, a number of secondary processes can improve performance in sintered soft magnetics, and they're far easier to perform on sintered parts than on their non-sintered SMC cousins:

  • Machining, drilling, and tapping for prototyping before long-term production -- all easily executed on sintered soft magnetic components.
  • Ferritic nitrocarburizing introduces both nitrogen and carbon to the surface of the part to increase surface hardness and wear resistance without impacting the core's magnetics.
  • Steam treatment creates a hard surface layer to improve corrosion and wear resistance. (The process may slightly impact permeability and induction.) 
  • Resin and oil impregnation create self-lubricity, seal porosity, and protect the component prior to additional coatings or treatments.
  • Sizing reduces the size variations that arise during sintering, improving tolerance limits by up to 50%. 
  • Annealing is an important secondary process after the machining or sizing of components, as it eliminates stresses and restores soft magnetic properties. 

As such, it's not just the material that impacts performance, but also the manufacturing processes used. Your powder metallurgy company should consider these factors from the outset to improve results and reduce unexpected expenses.


Homogenizing for Customizing

sintered soft magnetic materials resistivity chart

Thanks to ultra-high-temperature sintering (UHTS) breakthroughs, we can now sinter at up to 2500° F.

What impact can UHTS have on soft magnetic components? 

UHTS promotes greater interparticle diffusion with a corresponding increase in the permeability and coercive force. Plus, it facilitates the use of silicon as a magnetic alloying element, giving even greater magnetic response.

These qualities make the alloying process more efficient, adding further value to your product.

What about the mechanical properties? UHTS also yields significant improvements to:

  • Tensile strength
  • Fatigue resistance
  • Impact strength
  • Hardenability


Sintering Resource Center 

DFM can be challenging in powder metallurgy because it requires a thorough understanding of your part’s requirements, PM’s long-underestimated capabilities, and sintered magnetics’ unique properties. For the best (and quickest) results, engineers should communicate with vendors early and often in the design process to optimize their part long before production begins. 

We practically wrote the powder metallurgy design manual when it comes to advanced sintering, so for more guidance on improving your sintered part, download our free guide:

Engineer's Guide to Sintering Download

Topics: Powdered Metallurgy, Materials, Design, Processes


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