What is Friction Stir Welding (FSW)?

We dive into the process, how it is used, benefits over other welding methods, plus more.

Friction stir welding (FSW) is a relatively new technology for the welding industry. It was invented at The Welding Institute in late 1991 as an experimental welding process by Wayne Thomas. At first, there were few real world applications for FSW. 

Today, it is used heavily in aerospace applications. Likewise it is gaining traction rapidly in the automotive sector. 

FSW uses a non-consumable devise called a pin. Above the pin is a shoulder. The FSW tool traverses the joint line while rotating between two metal parts. This is where the heat is generated. This area is called the thermo-mechanically affected zone. 

The massive benefit of FSW, is the tool being a non-consumable item. This saves greatly on costs and consumables. 

This is a solid state welding process because the two materials are not melted. The result is, a weld that possesses exceptional mechanical properties.

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Friction Stir Welding Process

The FSW process is much more sophisticated than regular arc welding processes. Advanced controls and automation are required to make sure the tool pin has constant pressure and speed with the work pieces. 

This ensures precise heat input from the tool into the base metal. Plus, it maximizes the mechanical properties of the finished part. The pin or sometimes called probe is a non-consumable part that has a long lifetime. Conversely, tungsten wears away quickly in traditional fusion welding techniques.

The two work pieces are positioned and clamped to the machine. There is a high enough clamp force to ensure they do not move due to the high forces from the welding friction stir process. Next, the tool which spins about its own axis, moves until it touches the surface of the two work pieces. 

Furthermore, there is a programmed momentary pause. Then, the tool moves along the joint line with the tool shoulder on the surface. Consequently, the heat generated by the rotating tool shoulder causes a mixing process. 

This mixing process plasticizes the materials, but benefits the process by keeping the temperature below melting. As the tool moves along the seam the force acting on the work pieces forges the weld. Finally, the result is excellent mechanical properties of the material.

friction stir welding nasa
Friction Stir Welding from NASA

What is Friction Stir Welding Used For?​

The FSW welding process is successfully used in many industries today. Aerospace, ship building, and automotive all use friction stir welding for advanced manufacturing methods where traditional welding methods fall short. Likewise, the automation, high repeatability, and excellent mechanical properties of the welds make FSW an attractive solution for high end applications. In general, high volume parts or critical parts such as for the NASA Space Shuttle are great candidates for friction stir welding. For other great applications of friction stir welding click the link.

Friction Stir Welding at MSU

How Does Friction Stir Welding Work?

The devise used for FSW has two distinct parts to it. First, a cylindrical part called the shoulder rotates on the joint line of the two work pieces. Second, a pin that is profiled, extends down relative to the shoulder. It then drills into the material on the start of the seam. 

The shoulder moves across for example, a butt joint, while rotating with constant speed and pressure. Consequently, this generates considerable heat which softens the two metals at the joint. The metals are joined mechanically by pressure from the pin and shoulder in the heat affected zone from the plasticized material. 

As a result, this is a solid state welding friction stir process because the metals were not heated into the liquid phase as in arc welding. In general, the FSW process is very precise and produces excellent quality welds.

friction stir welding fsw

The Difference Between Friction Welding and Friction Stir Welding​

Friction welding and stir friction welding are similar processes as they both use heat from friction to weld metals. That means they are both solid state processes. The difference is, FSW uses a pin with a shoulder which is applied to two metals where heat is generated to join the materials together.

 

Friction welding uses the two metal parts by moving them in relative motion while contacting each other to generate heat and fuse together the material. First, there are no other tools used. Second, the the work holding apparatus is responsible for creating the heat generation. This is the thermo mechanically affected zone between the two parts.

The Difference Between Friction Welding and Friction Stir Welding​

Friction stir welding machines are custom designed to fit the specific needs of a given application. They are multi-axis CNC controlled machines capable of high volume manufacturing. The control software used can be the popular and proven FANUC or SIEMENS controllers, or custom developed in house which is entirely dependent on the friction stir welding company.

FSW machines are great for welding in longitudinal and circumferential modes. This of course is based on the application of the process. A hydraulic module is used for the main clamp force for the welding process. Precise controls of the hydraulic module allow the user to make fine adjustments for a precise and consistent weld.

 

A servo drive is responsible for rotating the friction stir welding tool. This is similar in application to a vertical CNC machine. Precise speeds can be obtained in conjunction with optimal welding parameters for quality welds of the base material and longevity of the machine. As you can see, friction stir welding machine tool technology is heavily borrowed from vertical CNC’s. This benefits from reducing costs and increasing the overall quality of the process. Each friction stir welding machine is custom designed for a client’s specific needs. Do you have a question or need help with your friction welding application? Contact us below.

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Friction Stir Welding Tools

The design of the FSW tool is a crucial factor in the overall quality of the weld. In addition, it directly affects the longevity of the tool life. The tool can further be optimized for maximum welding speed for high volume applications. An FSW tool typically has high toughness and high strength properties. At the same time, the tool must have low wear at the elevated welding temperature for a given material.

Another important factor is the thermal conductivity of the tool. You don’t want thermal conduction of the heat from the devise into the machine. This is a big risk in damaging the machine’s drive train. Hence, these can be costly problems which are difficult to diagnose.

Aluminum is by far the most used metal for friction stir welding. You can utilize a hardened tool steel for the pin and shoulder of the FSW devise. Consequently, this produces outstanding friction stir welded parts and lasts a very long time. The harder materials such as Nickel Alloys and Titanium alloys use much more complex materials for the devise.

The friction stir welding tool typically has a concave shoulder design for most joint welds. This design does several things:

  • Allows the metal to escape that is being displaced by the pin
  • Keeps metal from being pushed out of the shoulder
  • Helps to keep constant pressure onto the work piece
  • Increases the material properties of the weld

 

If you are looking for help in designing a friction stir welding tool for your application or process click the link above to get in touch with us.

stir welding
Stir Welding from 3Dprint.com

Common Friction Stir Welding Metals

Aluminum alloys were the first metal application used for friction stir welding. Welding two different profiles of aluminum extrusion, or welding aluminum extrusion to another aluminum part, are the most used applications for friction stir welding. Since its inception, friction stir welding development has grown to utilize many different materials for a wide range of applications. Below is a list of materials that are common for friction stir welding.

  • Aluminum- nearly all aluminum alloys
  • Steel- Automotive grade steels and stainless steels
  • Copper- Silicon and tin alloyed coppers
  • Nickel- 625, 718, and 725 Nickel alloys
  • Titanium- Pure Titaniums
friction stir weld
Friction Stir Weld from NASA

Benefits of Using FSW

Stir friction welding is a solid state process which inherently has many benefits over other welding processes. First, the obvious benefit is not having to cool down from the liquid phase. Second, there are no issues with porosity, and cracking due to the large temperature delta. Consequently, the introduction of contaminants into the weld process is very minimal. This leads to superior mechanical properties of friction stir welded parts. Below are other beneficial aspects for stir friction welding versus arc welding:

  • Safer due to the absence of fumes, welding splatter, and shielding gas
  • Automated for high volume and high repeatability
  • Capable of any welding position
  • Considerable cost savings for high volume and high end applications
  • Excellent weld mechanical properties and appearance
  • None to negligible consumables needed
  • Environmentally friendly green manufacturing

As you can see stir welding friction excels in many aspects compared to conventional welding methods. However, although it may seem like it is perfect, it certainly is not without its faults.

Disadvantages of Using FSW

Although FSW has many upsides, there are some downsides to using the technology. If the welding friction stir parameters are not dialed in you can get low welding temperatures. Notably, this is most likely due to low rotating speeds or too high of a traverse velocity. The result is improper and inconsistent forging of the weld into the materials. This is a particularly concerning defect, because it is not obvious to detect as it looks like a passable weld. Consequently, nondestructive testing methods must be used to inspect the weld.

 

Another drawback for using stir friction welding is the exit hole. When the FSW tool is finished welding and drawn out, there is a hole left. This must be filled in for most cases. Next, the hole must be cleaned up for proper surface finish requirements. In addition, friction stir welding is less flexible in nature. It requires much more time to setup versus fusion welding processes and conventional arc welding. Finally, this leads to higher costs.