Competitive markets need every advantage they can get when it comes to product performance and cost efficiency.
That’s in large part why so many manufacturing processes exist – to provide options for finding a path toward faster, better, stronger components. Powder metallurgy (PM) is a metal-forming process that sinters compacted metal powders just below their melting points.
But what exactly does the sintering process do to a part’s properties? Where does it get its strength? Is sintered metal stronger once it’s through the furnace, or more fragile? How does the powder metal process stack up versus other manufacturing methods?
In short, the powder metallurgy (PM) process is capable of producing structural single- and multi-level parts with competitive specs. To get a true idea of what’s possible with PM design-wise, a full explanation is in order:
What Is Sintered Metal?
Sintered metal refers to the final product made from the powder metallurgy process. The powder metal process includes:
- Powder Selection
- Compaction
- Sintering
- (If necessary) Secondary Operations
Sintering is a vital stage in powder metallurgy. It contributes to the production of high-performance, cost-effective, and precisely engineered metal-formed parts and supports structural part making in many major industries, like:
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Industry |
Sintered Parts |
Applications |
|
Automotive |
Fine-pitch & planetary gears, connecting rods, camshafts & crankshafts |
Braking systems, engines, electric vehicle batteries |
|
Industrial & Electric Motors |
Self-lubricating bearings, rotor cores, stators |
Flow control, warehouse equipment, alternative energy |
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Medical |
Motor bearings, cams, hinges, planetary gears, fine-pitch gears |
Patient transport, beds, ventilators |
|
Defense |
Trigger assemblies, ammunition components, gears, rotors, sensor parts |
Firearms, military crafts, surveillance |
|
Appliance |
Cams, motor bearings, hinges, clutch assemblies |
Washers & dryers, refrigerators, ovens, food processors |
Is Sintered Metal Stronger Than Other Material Processes?
The sintered metal process can produce parts with high yield strength (resistance to deformation under pressure) and fatigue strength (resistance to breaking under repeated stress). These properties opened up new competitiveness for sintered metal in products where wrought iron, steel, and other forming technologies were once strong contenders. Let’s look at how the sintering process in powder metallurgy shapes up vs. processes that frequently compete with it:
Casting Process
Casting is the process where molten metal is injected or poured into a mold and allowed to harden to form the part.
The materials used are normally non-ferrous, primarily aluminum, magnesium, and zinc alloys.
Top 3 Advantages of Sintered Metal Versus Casting:
- Design Flexibility: Sintered metal allows for complex shapes and intricate features. This flexibility is limited in casting due to the need for draft angles, parting lines, and wall thickness.
- Material Efficiency: Powder metal process produces minimal waste compared to casting
- Higher Precision: Sintered metal parts are more precise as a result of tightly controlled compaction processes.
Forging Process
Forging is a process that shapes metal using heat and compressive forces. The metal is heated to a specific temperature and then deformed using a hammer or press to achieve the desired shape.
The most common materials are stainless steel, titanium, carbon steels, and other metal alloys.
Top 3 Advantages of Sintered Metal Versus Forging
- Precision and Complexity: Sintered metal excels in producing complex shapes with high precision which may be limited in forging due to the size of the press.
- Material Efficiency: Sintered metal is more material-efficient because it compacts powders into desired shapes, whereas forging, at times, must cut away excess material for part formation.
- Cost-Effective: While the cost of sintered metal parts can vary widely based on part geometry, material, and size, they are often significantly less expensive than the forging process, especially when considering the costs of dies and presses.
Machining Process
Machining involves various processes to shape a solid metal piece by removing material. Common methods include CNC turning, laser cutting, water jet cutting, and milling.
Materials used in the machining process include aluminum, brass, bronze, copper, carbon steel, stainless steel, tool steel, alloys, and in some cases, plastics
Top 3 Advantages of Sintered Metal Versus Machining:
- Precision and Complexity: The sintered metal process enables the production of complex net-shape or near-net-shape parts, eliminating the need for extensive machining. Achieving such intricate designs through traditional machining methods can be challenging and costly.
- Material Efficiency: Sintering is more material-efficient with minimal waste compared to machining which typically has extensive material removal.
- Cost-Effective for High Volume: Sintering is economical for high-volume production runs due to lower material waste and reduced need for secondary operations, which can be more expensive in machining.
Stamping Process
Stamping involves shaping metal using a press machine that applies high pressure to a metal sheet. Common methods include blanking, bending, coining, and embossing.
Materials used in the stamping process include steel, aluminum, brass, copper, and stainless steel.
Top 3 Advantages of Sintered Metal Versus Stamping:
- Precision and Complexity: Sintered metal excels in producing complex shapes with high precision. These intricate designs may be difficult or expensive to achieve through stamping.
- Material Efficiency: Sintering is more material-efficient with minimal waste compared to stamping, which often involves cutting away excess material.
- Cost-Effective for High Volume: Sintering is economical for high-volume production runs due to lower material waste and reduced need for secondary operations, which can be more expensive in stamping
How Does the Metal Sintering Process Add Strength?
At its core, sintering is simply the act of heating a pressed metal to fuse its particles.
Wait – if a part is already pressed, why would you need to stick it in the oven?
Think about the last time you picked up a freshly shaped Christmas cookie. In its raw state, the dough, while fully formed, is fragile and can fold or break, so it needs to be baked in the oven to be completed, making it ready to eat.
Similarly, while powder metal compaction completes the initial forming, sintering bonds, hardens, and strengthens the part, making it ready for industrial use:
The 5 basic steps of sintering are:
Zone 1: Delubing — This step ensures there’s no part contamination.Zone 2: Finish Delube Oxide Reduction — This phase makes sure the green compact is completely free of any remaining lubricants and oxides.
Zone 3: Sintering — The parts are heated to 2050-2150°F, just below their melting point. The manufacturer may bump these numbers up for stainless steel parts, or when it recommends high-temperature sintering (2300+) for meeting demanding specs. The material’s atoms become active, grain size grows, and pores shrink.
Zone 4: Initial Cooling — The green compact undergoes a dramatic transformation into a solid, high-performance component. This phase brings the temperature down to stabilize the newly formed microstructure in a controlled environment.
Zone 5: Final Cooling — The component settles down to room temperature, completing its transformation to a finished part. The parts usually cool slowly to avoid thermal stress. The sinter hardening process is an exception, as its goal is to rapidly (but carefully) cool the parts to increase hardness.
The parameters can be adjusted based on material requirements. Three sintering variables affect its ability to boost mechanical properties:
- Time:
- Temperature:
- Atmosphere: Special furnace additives (i.e. argon, nitrogen) can protect the material’s intrinsic strength from being compromised by impurities or bad reactions.
Other Processes That Impact Sintered Metal’s Strength
As we’ve discussed, sintering is just one step in the press and sinter manufacturing process. Other stages also impact the strength of a sintered metal part:
Material Selection
Once everyone approves a design, the manufacturer selects a material blend, which is crucial for achieving the desired properties and performance of the final component.
Material blends are created by thoroughly mixing elemental or alloy metal powders with additives, such as lubricants or binders to ensure consistent mechanical properties in the parts. The use of lubricants influences the structural development of the material, reduces friction during compaction, and protects the tool during die ejection.
Compaction
Die compaction is the process of pressing the powder into a green compact under high pressure. This step is critical for defining the part's density and porosity.
Higher compaction pressure typically results in higher density, which in turn enhances strength. A solid compaction technique improves structural integrity, ensuring durability and long-term performance.
Together, compaction and sintering are the manufacturing versions of forming a cookie and baking it.
Post-Sintering
It’s possible with powder metal to create a net-shaped part that needs no further processing. In the interest of making your sintered metal even stronger or adding finishing details, you may still opt for a secondary operation.
Your options are many:
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Process |
What It Does |
What It Really Does |
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Quenches, hardens, tempers, or anneals parts to heavily alter microstructure |
Relieves stresses induced during sintering, improving fatigue strength |
|
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Infiltration |
Injects pores with metal that has a lower melting point (i.e. copper, bronze) |
Greatly increases density & structural integrity |
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Surface Finishing |
Applies carburizing, nitriding, or coating to the surface |
Enhances surface hardness & tensile strength (in certain metals) |
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Repressing & Coining |
Applies another round of pressure to further densify the material |
Boosts yield strength & hardness |
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Plating |
Adds a thin metal layer to the part’s surface |
Reduces wear, friction, & erosion; improves hardness |
Is Sintered Metal the Right Fit?
Is sintered metal the right fit? That depends on your design and application needs.
Among competing methods, powder metallurgy can often better meet dimensional requirements and save material (and other costs) at high volumes. The sintering process itself improves strength and durability, ensuring parts are ready for long-term industrial performance.
Ultimately, process fit comes down to the requirements of the application.
To learn more about powder metal and sintering best practices, check out our other educational resources:
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