Friction Welding

What is Friction Welding?

Friction welding (FRW) is a general solid state welding process with at least five different welding types. It is a solid state process because there is no weld pool like in conventional welding methods. Friction is generated between two parts by rubbing them against each other in relative motion with respect to one another. This creates heat in both components which is plasticizes the two contacting surfaces. Pressure is applied to the parts to displace and fuse the two materials resulting in a very strong welded part. Since the material of both parts is not melted, as in fusion welding processes, a solid state joining process takes place. The great aspect about utilizing a solid state friction welding process is the ability to join dissimilar metals and even thermoplastics.

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How Friction Welding Works

Friction welding technology works by utilizing friction between two parts typically by spinning them together, and an axial force to forge the parts together. There are many ways to generate the friction required to join the materials together explained in detail in the next section. However, the focus is to raise the temperature of the two metals until they are plasticized. The area where the two metals are in contact is called the heat affected zone.

During this time there is an axial force applied causing the friction. A substantially greater axial force applied while the metals are plasticized and soft to forge the two parts together making a solid joint. When the great axial force is applied a flash occurs due to contaminants and oxides on the surface being burnt off. This process is rapid which is great at reducing time for manufacturing the parts.

After the components are welded they are stopped from spinning but the axial force remains until the part is relatively cool. This creates an excellent quality weld that is now complete. There is no consumables used in the process such as filler wire or shield gas. This is especially beneficial for safety and cost savings.

The 5 Types of Friction Welding

1. Inertia Friction Welding (IFW)

The IFW process utilizes part rotation under axial pressure to generate the heat required for the components to bond. This is accomplished through different sized flywheels on the machine that are attached to a chuck and spindle shaft. A motor is connected to the spindle shaft which rotates the part. The required speed is preset to ensure the part heats up to the proper temperature. Energy is stored in the rotating mass of the machine. After the motor spins up and the preset speed is achieved, the motor decouples from the spindle shaft.  The flywheel driven chuck then immediately slows down until the weld zone seizes from its own kinetic mass. The rotational inertia of the part and the axial pressure is responsible for the weld process. This is also sometimes call spin welding.

2. Rotary Friction Welding (RFW)

The RFW process is the classic process for friction welding. The motor in this process is permanently attached to the direct drive spindle shaft. The motor drives the rotating work-piece into the other work-piece which is held stationary. Once there is contact between the surfaces mechanical heat is generated from the friction and welds the two workpieces together creating a strong interface. Unlike using the inertia and mass of the system, the motor spins the work-piece at a decreasing rate as specified in the CNC program. The advantages with the CNC controls make rotary friction welding very consistent with high quality welds. The video below is excellent for demonstrating how rotary friction welding works.

3. Linear Friction Welding (LFW)

LFW shares many aspects to inertia welding except in this case the chuck and spindle shaft does not rotate. In place of rotating the spindle, it oscillates back and forth in a lateral motion. During oscillation, the two work pieces are compressed together with an axial pressure. Frictional heat is generated and the components are joined at the interface of the materials.

A great benefit this offers is the flexibility to bond metals that are irregular shaped versus round. This is commonly used in the aerospace industry for manufacturing turbine engine blades.

4. Friction Stir Welding (FSW)

The friction stir welding process is a solid state joining technology that utilizes a non-consumable tool called a pin. The pin has a shoulder that rotates. The shoulder of the pin is used to join the two work-pieces together. It is lowered down and makes contact with pressure on the joint line between the work-pieces. It rotates at a programmed speed which creates the frictional heat. With downward pressure the shoulder forges the plasticized metal together making welds along the joint line it traverses. FSW was originally used for aluminum extrusions with complex geometries. Now it is used widely in aerospace, automotive, and ship building applications. 

Friction stir spot welding (FSSW) is very similar to FSW and sometimes people mix them up. They both use a non-consumable tool. But, the main difference is FSW traverses the joint line between two metals, and friction stir spot welding does not. FSSW simply plunges into the two work-pieces until the joining materials are welded and then returns.

Friction Stir Spot Welding (FSSW)

5. Orbital Friction Welding

Orbital friction welding shares many similarities with rotary friction welding. Both methods rotate the work-pieces with the same angular velocity. They key difference with orbital friction welding is the axis are offset by up to 3mm.  When the materials are joined the rotation is slowed and the parts are returned to the same axis. The axial pressure is maintained or increased between the surfaces if needed while the materials re-solidify.

The substantial benefit of orbital friction welding is the ability to join work-pieces together with non-round and dissimilar shapes. However, the orbital aspect makes the procedure much more complicated with a higher cost. This has limited the number of companies using the technology.

orbital friction welding

Applications

Friction welding is used in many applications with a diverse range of industries. Rotary friction welding is the most common form of friction welding because many parts are made round. The automotive industry uses rotary friction welding for axles and connecting rods. The packaging industry uses rotary friction welding for rollers and shafts to aid material handling equipment. Linear friction welding as mentioned above is used in aerospace applications. FSW is widely used in the automotive sector to weld aluminum extrusions and panels for cars. This saves costs and reduces the number of processes involved.

Drawbacks of Friction Welding

Friction welding undoubtedly has many benefits going for it. However, there are some drawbacks to using the process. First, getting the weld parameters dialed in with using the feed and speeds of the machine takes time and effort. If the feed and speeds of the machine are not correct a poor quality weld can occur. This usually results in cracking or the two metals simply come apart and fail. Another problem is poor fusion characteristics. This is a concerning problem because it isn’t very see the defect, and the finished part can pass quality control and fail. A simple solution is to run a nondestructive testing method to inspect the weld after the first run of parts are complete. If they pass the nondestructive testing method then the process is set and production can begin.

Another downside of friction welding is the added complexity of setting up the operation. It takes considerable expertise in friction welding to get it or costly mistakes can happen. If you are looking for a friction welding solution for an application you may have feel free to get in touch with us by clicking the link below and filling out the form.

Benefits of Friction Welding

The friction welding process offers many advantages in comparison to conventional fusion methods. Since friction welding is a solid state process the friction welded part does not have to waste time cooling down. This is due in part to the small heat affected zone. In addition, there are no impurities introduced during the weld cycle between the two pieces. That means there is no solidification cracking of the material. This results in a material with excellent mechanical properties. The process is easily automated for high volume manufacturing.

One of the other key benefits is the ability for joining materials together that normally are not weldable. For example, 7000 series aluminum, or dissimilar metals. This opens the door for new possibilities to further enhance the mechanical properties processes of future applications.