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Design for Manufacturability Part 3: Structural Powder Metal Parts.

Posted by Eric Wolfe - October 11, 2021

To raise the bar for performance or cost efficiency in any manufacturing design, you first must learn the basics.

If you’re, for example, looking to replace a structural machined, stamped, or forged part with a powder metal component, there are wide-ranging design-for-manufacturability (DFM) adjustments you’ll need to consider. There are many questions your powder metal parts supplier should ask you before production -- and just as many questions you should ask the supplier. 

What’s in the realm of possibility with powder metallurgy (PM) structural parts? You’d be surprised -- but you’ll never know if you don’t understand the basics first.

Consider this your quick-reference powder metallurgy design manual for structural parts. Just keep in mind there are many exceptions to the rules of powder metallurgy …

The Foundations of Structural Powder Metal Parts

What makes a structural part exactly that?

In powder metal design, guidelines for structural components carry major differences compared with magnetic parts, due to obvious differences in priorities (strength & durability vs. magnetism).

(If your PM part requires high magnetic performance, consult the other entries in this series:

Likewise, powder metallurgy as a whole has unique design considerations compared with other processes, like forging and stamping. No matter your path to getting here, the key mechanical properties of parts made by powder metallurgy for structural use are:

  • Tensile strength -- ability to withstand compressive and tensile stresses due to bearing a load. Components requiring this trait include keyways, compression fitting, and retaining bolts.
  • Fatigue resistance -- endurance against repetitive-use stress applications over time. Components prone to this include gear teeth, connecting rods, & shock absorber guide rods.
  • Corrosion resistance -- with the wrong material or coating, your part may rust and fall apart. Classic wheel hubs that sat on the outside of the shaft were a prime example, as they took the brunt of road salt, sea salt, and other moisture.
  • Hardness/wear resistance -- important for any application where parts are rubbing against each other or another component. Many types of gears require high hardness due to their constant sliding motion.
  • Elongation -- damage tolerance and ability to sustain overstressing without failing. Notably, with today’s advanced sintering techniques you can achieve the same yield strength as conventional PM heat-treated materials, but with improved elongation.

 

7 Design-for-Manufacturability Considerations for Structural Powder Metal Parts

With your target properties identified, you can zero in on the most effective and efficient way to get there.

In structural applications, be on the lookout for these seven parameters that can impact part quality and cost. In worst-case scenarios, ignoring PM design guidelines can mean your model isn’t even producible.

  1. Load conditions
  2. Size
  3. Thickness
  4. Tolerances
  5. Producibility of features
  6. Environment
  7. Prototyping

 

1. Load Conditions

How will customers use your part and overall product, and what are your expectations for performance?

This DFM step includes examining your needs for static strength vs. cyclic stress durability.

In powder metallurgy, the typical fatigue limit is 35% of tensile strength. In a part with a tensile strength of 150,000 psi, your fatigue limit would be approximately 52,500 psi.

Some still carry the misconception that powder metal parts can’t perform well in heavy-duty uses. In truth, with a little creative thinking, PM can meet even the most demanding applications. Options for beefing up the strength of a PM part include:

  • Heat treatment
  • Shot peening & other surface densification techniques

 

2. Size

Planar surface (the surface area of a cross-section of a component) is the most important measurement in regard to DFM. Today, parts requiring a planar surface over 16 sq. in. typically end up as multi-component assemblies. Otherwise, the manufacturer can’t compact your part.

PM also comes with other dimensional requirements, although these depend somewhat on the individual part. Because of die-fill requirements, it can be challenging to form parts over 3" long in the pressing direction. Metal powder usually has a 2:1 compaction ratio, so if you have a 3” part, you need at least a 6” fill in the press.

These are valuable general guidelines, but remember that the rules of powder metallurgy design are broken every day. The most capable metal parts manufacturers are willing to consider minimum and maximum sizes on a part-to-part basis. When in doubt, ask a vendor with experience going beyond conventional techniques.

Powder Metallurgy vs. Competing Methods

3. Wall Thickness

Considering its importance to structural parts, wall thickness deserves a separate mention.

Powder metallurgy processes typically include a 0.06-0.08" minimum thickness. Depending on the part’s geometry, some walls can taper into smaller cross-sections if it’s crucial to your application.

When considering a thinner cross-section, engineers should bear in mind that thin = fragile. Avoid designing long, thin walls -- they require tooling that’s just as fragile and prone to breaking. Thin parts also tend to distort more than thick parts during heat treatment.

In components with length-to-wall-thickness ratios over 8:1, variation in density is an issue. Again, consult an expert if you have a special thickness need.

4. Dimensional Tolerances

Contrary to old perceptions, the powder metallurgy process can hold tolerances comparable to those in competing metal-forming methods. Still, as with any metal manufacturing process, your tolerances should be no tighter than necessary.

On its own, the powder metal process is a cost-effective forming method for many complex shapes requiring tight tolerances. But if you need tighter tolerances, the manufacturer can improve precision by adding operations like sizing, machining, or grinding.

There are other tolerance considerations you may need to consult your manufacturer on, depending on material and process choices. If you’re building a component with high strength requirements, will it need heat treatment? This may affect tolerances.

5. Producibility of Features

We’ve made the analogy before that powder metal is like cake batter. You put the batter where you need it, rather than cutting a small slice off an already-baked cake (see: machining).

Not only is PM’s net-shaping capability awesome for saving on scrap costs; it also makes most physical features easy to add on a component. Unique geometries, through-holes, and multilevels are within your grasp. 

Here’s a snapshot of the viability of certain common features:

  • Cross-holes -- in the pressing direction, these can be round or D-shaped, and can include keyways, splines, etc. Many hole locations are possible either during forming or via machining afterward.
  • Undercuts -- cuts perpendicular to the pressing direction are impossible, as they prevent ejection from the die.
  • Flanges -- small steps or overhangs are possible via machining. Overly large flanges cause ejection issues.
  • Slots & grooves -- curved grooves must stay within 20% depth of the total part length, while rectangular ones must stay within 15%. Deep, narrow features require fragile tooling; avoid them if possible.

The Metal Powder Industries Federation’s powder metal design guide is a great reference for what can and can’t be done. Note that when using conventional PM, most features are added via secondary machining, which will slightly increase production costs.

6. End-Use Environment 

Chemicals, friction, and other environmental factors may turn a design “want” into a “need.”

In the era of hush-hush NDAs, it’s important to communicate as many end-use details as possible to your manufacturer. What adversity will your part face every day, and what level of protection will it need?

Will your component be:

  • In oil?
  • Next to road or sea salt? 
  • Pressurized?

Simply saying “I need a durable part” isn’t enough, as “durability” carries multiple meanings in manufacturing.

Specific to corrosion durability, PM manufacturers usually add protection through a secondary process. The most common are zinc plating and steam treating, but there are others available depending on the level of protection you need.

7. Prototyping

The way metal part prototyping is carried out differs by manufacturer. The determining factors should be cost and design complexity.

There are three(-ish) ways to prototype parts made by powder metallurgy. These choices exist so you can validate your design in the most cost-effective way possible.

Option #1: Machining From a Blank

Prototyping from a powder metal blank (aka slug) will result in a test component with properties similar to a production-grade, compacted component.

This means you’ll get a uniform-density part, which sounds great in theory but could be an issue if density is important to your project. Since uniform density may not happen during real production (see tip #3), you may want your manufacturer to test multiple densities during prototyping.

This quirk aside, machining from a PM blank is the best option for low-volume prototyping (1-50 parts).

Option #2: Hybrid PM-Machining

The middle-ground option is compressing the part from a set of tools that contain certain defined features, then sintering it. After cooling, the operator machines the rest of the features.

Features that manufacturers tend to add post-compaction include:

  • Inner-diameter splines
  • Gear teeth
  • Hubs

 

Option #3: All-Out PM

The final option follows all the usual steps in powder metallurgy in an attempt to simulate mass-production results. The compaction machine forms the entire design from a set of tools, after which sintering occurs.

This option is best for higher prototyping volumes (50+) or when the part’s configuration is simple. It’s the longest and most costly prototyping method, but the most reliable as well.

Mystery Method?

There’s a hidden, fourth way to prototype a structural powder metal component: 3D printing (aka additive manufacturing).

Today, 3D printing of a PM prototype is only viable in specific applications in which you want to validate the geometry. Additive manufacturing’s uses are slowly expanding, so structural PM and AM may be a better prototyping fit down the line.

Building a New Foundation of Efficiency

By following the seven guidelines below, you’ll get your project out the door sooner and at a lower cost. Even better, you’ll have unlocked your design’s potential through the unique capabilities of advanced powder metallurgy.

Just remember that in powder metallurgy, guidelines are just that -- guidelines. There are exceptions to many so-called rules of PM. Most everything is “manufacturable” … it’s just a matter of what your application calls for, and your budget goals.

If you have a question about adjusting your design for manufacturability, or whether your project can successfully “bend the rules,” click below to ask an engineer.

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Topics: Powdered Metallurgy, Tooling, Materials, Costs, Design, Properties, Processes, sintering, forging, machining


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