Powder Metallurgy, Then and Now
The Metal Powder Industries Federation recently republished an interesting article about the early history of powder metallurgy (PM) that was originally printed in a 1944 issue of the Saturday Evening Post. It notes that General Motors began making significant quantities of PM parts during WW2 as a way to cost effectively mass produce many small intricate parts that helped in the war effort. These parts were often difficult to cast or machine; thus, PM offered the ability to mass produce these components while meeting all design strength requirements in a cost-effective manner.
Fast forward 80 years, and the creative minds behind PM have worked continuously to improve the process, producing even stronger and longer lasting parts that are now vital to a myriad of commercial industries in all regions of the globe. Advancements in materials, compaction technology, sintering, and secondary processing have led to many applications that were never dreamed of in PM’s infancy. Yet, modern PM parts share the same challenges as the parts made in 1944; that is, they often have complex geometries that are difficult and costly to produce via other technologies. In addition to the cost effectiveness and ease of production that PM provides, these parts can be expected to last a long time.
PM for Sintered DC Magnetics
One aspect of PM that is sometimes overlooked is its ability to produce sintered DC magnetic components that are customizable to meet a broad range of DC magnetic requirements. If you wonder where DC components are utilized, you can simply sit in your car and see how electric motors, solenoids, relays, and other devices have made modern vehicles much easier and safer to drive while providing a climate-controlled environment.
Sintered DC Magnetic Performance
Typical magnetic properties of interest in the design of DC devices include permeability, induction as a specific applied field (Bmax), and saturation induction (Bsat).
Permeability is loosely defined as how quickly a material or device can be magnetized or how much current is required to achieve a predetermined level of magnetization. Permeability requirements in many DC applications range from approximately 2,000 to a max of 7,000 (and on rare occasions, even higher). When utilizing wrought magnetic steels, your options include low carbon, low silicon steels that will provide permeabilities in the 2,000 to 3,000 range. Many times, these devices are DC motors, low speed actuators, simple flux return paths, speed sensors, and the like. At the high end, these materials are silicon steels typically used in fast acting relays and solenoids.
In a similar fashion, magnetic PM steels can be classified into pure iron or iron-silicon steels. However, utilizing an iron-phosphorous material with magnetic performance intermediate to the pure iron and iron-silicon materials offers added material flexibility. Powder metallurgy is a proven technology to produce unique 3D shapes, and controlling density and sintering conditions to influence the magnetic permeability, induction, and strength of PM DC components provides enhanced flexibility. The chart below (Figure 1) shows the influence of density and sintering atmosphere on the magnetic permeability of both pure iron (F-0000) and iron-phosphorus (FY-4500).
Comparing the permeability effects of density, alloying, and sintering atmosphere from pure iron to the iron-phosphorous materials, we see approximately a 50% improvement from the pure iron at 7.0 g/cm³ in a low hydrogen atmosphere to 7.2 g/cm³ with iron-phosphorus in high hydrogen. As noted previously, higher permeability implies easier magnetization and demagnetization, thus reducing the current required to induce magnetic flux. What does this mean? If permeability is the key component criteria, you can vary combinations of density, alloy, and sintering conditions to achieve your requirements. It should be noted that pure iron is a lower cost raw material and is less abrasive to compaction tooling. Obviously, if permeabilities of more than 3,500 are required, then the iron-phosphorus solution is the best—if not only—choice between these two material systems, and it’s a less costly alternative to iron-silicon materials.
Figure 1: Effect of alloying, part density and sintering furnace purity on maximum DC permeability
The design flexibility discussed above can be expanded further to both lower and higher part densities by utilizing higher performance materials such as iron-silicon steels and iron-nickel alloys. As noted earlier, the data presented in Figure 1 was generated using an industry standard sintering of 2,050 °F (1,120 °C). Horizon Technology has unique capabilities to sinter at temperatures up to 2,450 °F (1,340 °C) coupled with compaction technology enabling higher component densities that produce DC maximum permeabilities approaching 7,000. Figure 2 (below) shows the benefits of the variables just discussed.
Figure 2: Key performance criteria of PM magnetic steels
Through Horizon’s R&D efforts with soft magnetic materials, we are considered the leading authority customers turn to for solutions.
One topic we’ve not yet discussed is the maximum saturation flux density of a PM component. For both pure iron and iron-phosphorous, the maximum saturation flux density can be expressed as follows:
Bsat = 2.15 x (density of the part / 7.85) in Tesla.
For the other PM magnetic materials, the same relationship applies, except the 2.15 is replaced with the theoretical saturation of that specific alloy.
Mechanical Property Performance
Many designs require both magnetic performance and mechanical strength. A good example is the rotor of an alternator: it typically rotates at 300% to 400% of the engine speed. Thus, mechanical strength and fatigue durability are critical. Table 1 (below) provides an overview of the mechanical strength of the most common PM magnetic materials compared to low carbon AISI 1008 steel. Magnetic performance can be met with either a PM pure iron or an iron-phosphorous alloy; however, if comparable strength is necessary, then either an iron-phosphorous or iron-silicon steel is required. If cost is also an issue, then iron-phosphorous is the optimal solution.
Table 1 is just a snapshot of the array of magnetic and physical properties available via PM processing. Consult with Horizon Technology to fully explore the options available for your unique design requirements. Think of pure iron as your base material for DC applications. FY-4500 is a higher performance material , though still less advanced than, say, a 3% silicon material.
Whatever your magnetic application, the name of the game is customized magnetic performance that balances mechanical properties and cost.
Whether you're optimizing for permeability, mechanical strength, or cost, the customization and material versatility Horizon offers is essential in shaping the future of DC magnetic applications. Discover how Horizon Technology can take your components to the next level.
