So, what could the two possibly have in common? Actually, friction stir welding (FSW) materials can include those made from powder. And in very specific applications, FSW is a viable option for joining powder-metal (PM) parts.

 

What is friction stir welding?

Friction stir welding has only been around since 1991 and originated from The Welding Institute in the U.K. It’s a relatively new joining process, so many metal manufacturers have limited or no understanding of how it works. 

Most people are familiar with basic arc welding, where the engineer uses electricity and an electrode stick to melt two metals to form a bond. Laser welding is also a common practice that achieves the same goal, just at a finer level of detail. 

But FSW doesn’t require the same level of heat as these methods. It uses friction caused by a rotating tool to gently fuse the metals and stir them together, creating a strong bond without melting the metal. This is very similar to the sintering process in PM. 

Notably, FSW has helped build the external tank on space shuttles, including the SpaceX shuttle. As for PM parts, many in the industry are still finding new and exciting applications for this kind of welding.

 

Friction Stir Welding for Powder Metallurgy

In the three or so decades since its invention, FSW has made its way into a handful of PM applications. For now we will cover:

  • Welding PM parts
  • Surface hardening
  • Surface densification
  • Sealing porosity

 

Welding Powder-Metal Parts

In most applications, we recommend sinter bonding, sinter brazing or other bonding processes for joining PM parts. But you can use FSW if the application calls for it. FSW of dissimilar alloys and materials is possible and sometimes even desirable if you don’t want to fully melt the metal with the lower melting point. 

During conventional welding, for example, certain steels will transform to martensite in the area adjacent to the weld (the so-called heat-affected zone, or HAZ) if you heat them to the melting point. That results in a very hard, crackable surface, making conventional welding practices less than ideal for some applications. 

FSW keeps the metal in a state that’s just short of melting. You have an impressive amount of heat input control, which allows for minute adjustments to the end result. The surfaces of the two parts are in very intimate contact and bond well without the risks inherent to melting. The joints are often as strong as the base metal, creating bonds that are reliable enough for use in rotary engine housings.

On the manufacturer’s side, using FSW for welding PM parts also creates a safer work environment because it doesn’t produce toxic fumes. More relevant to the buyer, FSW has minimal tooling needs, which keeps labor and material costs low.

 

PM Part Bonding

 

What is sintering in powder metallurgy?

Let’s start with a basic definition of sintering. Sintering is a post-compaction process that uses high-temperature heat treatment to fuse the powder. This forms a solid mass – but not one so hot that it melts. This heat treatment grants your metal extra strength and hardness.

 

Sinter Bonding

Now let’s get more specific. Sinter bonding is a process in which you place two separately compacted components together during sintering. This causes the two pieces to join. 

Sinter bonding can create a composite structure with unique materials on both the outer and interior diameters of the assembly. This has proven to enhance properties like conductivity.

 

Sinter Brazing

Sinter brazing is a process that bonds two components with the help of a third filler material. A successfully brazed joint relies heavily on the interactions between the braze alloy, adjoining surfaces and sintering conditions.

 

Surface Hardening

You can take a hard particle like silicon carbide and weld it to the original component’s surface with FSW. That leads to a very hard and wear-resistant surface. In this case you’re basically using friction welding as a pseudo-derivative of metal plating. 

If you have a tool you want to make very hard so surface abrasion is not an issue, FSW might be the way to weld.

 

Surface Densification

FSW can be used in a similar fashion to roll densification to get a highly dense surface on a part. The interior of the part remains lower-density, which can be useful if you want to keep your parts lightweight.

 

Sealing Porosity

FSW is seeing use in more industries to seal the joints between aluminum and aluminum alloys. It can be used in the same way to seal PM components. 

Remember that melting increases porosity, allowing contaminants to penetrate the part’s surface. PM parts are already porous by nature, so having a bonding process that decreases or neutralizes porosity is a necessity.

The compaction and sintering processes cannot fully eliminate porosity, which can be a problem if you want to keep a component sealed. Since it doesn’t fully melt the metal, FSW can be used to seal porosity at the surface so nothing infiltrates the component’s pores.

 

Other Methods for Metal Bonding

FSW is a niche application that can replace other PM joining methods on special occasions. Yet those occasions might be too few and far between to merit investing in the necessary equipment. Let’s explore a few more options for bonding your PM parts. The top contenders are sinter bonding and sinter brazing.

 

Combining Powder-Metal Parts Using Sintering

One of the bonuses of sintering is that you can fuse two parts together along the way. You just need to use the right process based on your materials.

With sinter bonding you can make a two-piece assembly that couldn’t normally be made with a single compaction step. The outer part actually shrinks, and the inner part grows during the sintering process. This improves bonding as well as potential shrink fit of the two components.

During sinter brazing, you use a compound that melts and diffuses into two separate pieces to form a single assembly. It actually enables you to skip a step in the PM process (e.g., adhesive bonding or post-sintering welding) because you are simultaneously sintering and bonding.

You can use a slug or paste uniquely designed for PM to accomplish this brazing. But there’s still no sintering together of the two components.

 

Applications for Sinter Bonding and Brazing

While sinter bonding is the most economical way to bring two parts together, it’s not feasible in all applications.

Here’s one example. Say you want to bond a stainless steel component to an iron part. Stainless shrinks dramatically post-sintering, but it also grows tremendously when it is in the furnace. As it heats it will grow away from the other part, making sinter bonding impossible. You need something to bridge that gap, and that’s where sinter brazing comes in.

You might ask, “Why can’t you just weld them?” Yes, welding is an option. However, if you’re welding dissimilar materials, you run the risk of cross-contamination, which results in embrittlement or reduced corrosion resistance. This can make welding certain PM parts more difficult, and it requires much more technical expertise. Sinter brazing reduces the potential cost penalties that welding may cause.

 

Conclusion

Friction stir welding just breaks the surface of all the innovative PM processes available today. Soft magnetic composite materials are revolutionizing the electrification of transportation. Ultrahigh-temperature sintering is making PM parts stronger than ever thought possible.

Still, not all innovations should be considered a one-size-fits-all savior. While friction stir welding has a niche place in powder metallurgy, there are many other reliable ways to improve performance and conserve costs.