When considering the composition and design of a component for a new product -- or a redesigned one -- you probably have a certain attribute you’re prioritizing above all else. For some projects, durability is that most-crucial parameter.
In powder metallurgy materials, durability can mean different things depending on the project. When creating a powder metal component for a lawn mower, durability may mean lasting for 400 hours of use. For a car, a durable part would need to last for 200,000 miles or so.
As you can see, the end use of the part defines your parameters.
For those using powder metal parts, it’s useful to have more concrete definitions for the properties of materials. Using these factors, your powder metal parts supplier can aim for a result that meets your required durability priorities (and other desired properties).
Choosing Powder Metallurgy Materials Part 1: Durability
When considering how we’re going to construct a powder metal part, we consider factors like:
- Corrosion resistance
- Tensile strength
- Impact toughness
- Fatigue strength
The following terms are used in powder metallurgy to define the characteristics of a material. A material may have multiple qualities listed here, or only one.
Some applications require a higher level of corrosion resistance. For instance, parts used in marine applications are exposed to salt water regularly, which can be corrosive to standard metal parts. The addition of alloys to create “marine-grade” stainless steel serves to prevent corrosion in these applications.
In mechanical engineering, hardness refers to a material’s ability to withstand denting or abrasion. Wear-resistant alloys tend to have a high hardness level.
Most people have heard how diamonds are the hardest material in the natural world. It’s true that they can withstand considerable friction, more so than just about anything out there. However, diamonds can absolutely be broken through impact, so they actually have mediocre impact resistance.
The strength of a material refers to its ability to withstand force before deforming. Steel is a strong material because it takes a great deal of energy to pull it apart.
The impact toughness of a material refers to its ability to resist fracturing when force is applied. A material can lack strength but be quite tough -- clay is a good example.
It has been said that most parts fail from some sort of fatigue. Fatigue is defined as the application of alternating stresses on a part. Think of an engine connecting rod: It goes through compression on the explosion cycle and then tension on the return to top dead center. Fatigue is a very complex topic, but many powder-based parts are subjected to fatigue loading and provide sufficient fatigue strength to meet your design requirements.
The vast majority of all part failure -- whether the part is made from PM or a competing process -- happens due to fatigue.
As a general rule of thumb, fatigue strength is 30%-35% of the tensile strength.
Materials and Their Qualities
When you make a part, you can utilize various metals to achieve specific qualities. Some of these metals and their qualities include:
- Stainless steel
These materials are among the most popular because they offer a range of different traits, everything from good strength to magnetic properties.
Stainless steel is common in powder metallurgy -- especially in applications where corrosion resistance is a priority.
The 300 and 400 series of stainless steel are particularly popular. Steel is known for its high iron content, good strength, and remarkable impact toughness. It also holds up to wear quite well. Stainless steel also sports high corrosion resistance -- although at the sacrifice of some other qualities. Still, these attributes make stainless steel great for components that’ll be visible to the customer and must be aesthetically pleasing.
Sometimes a stainless steel powder part will show signs of rusting even though it’s “stainless.” Rusting happens when foreign particles get inside the part or the material is processed incorrectly by the part supplier. This is called static corrosion. Sometimes it is advisable to “impregnate” a stainless part to effectively seal the inside of the part from the corrosive environment; thus, improving the overall performance.
Copper powder is another material that’s great for resisting corrosion, which is why you often see copper parts in products like sprinklers.
It’s a nonferrous metal, which means it does not contain iron -- and also means it’s more expensive to produce. For that reason, only use it when the application demands it.
One of the most interesting properties of nickel is that it's magnetic despite not being a ferrous (iron-based) material.
It’s also corrosion-resistant, and like other nonferrous metals, it can be fairly expensive.
Aluminum is much softer than most metals used in powder metallurgy, but it’s also extremely light. In certain applications, its light weight makes up for its lack of strength.
Aluminum powder metal parts can be useful in:
- Other lightweight applications where you can mask its poor strength
Titanium alloys are quite expensive to make, but they have extremely desirable qualities. They can be made to be very tough and have high corrosion resistance.
A common use for titanium is golf clubs -- you need a material that can survive being repeatedly smashed into a golf ball (or thrown into a lake).
Work With Your Powder Metal Manufacturer to Pick a Metal
Each material has its own qualities that can be leveraged to accomplish specific metal manufacturing goals. And when you start to combine powder metallurgy materials, you can generate even more impressive qualities in the final product.
If you want to learn more about the ideal materials for every PM application, check out our Capabilities & Services Handbook. You'll learn more about how powder metal components can change your manufacturing forever.
Editor's Note: This article was first published in May 2019 and has since been updated.