Friction Welding: Process, Types, and Advantages

Friction welding, as the name implies, uses friction to weld joints. There is no external heat that is applied in the joining process.

Hence, friction welding is not a fusion weld process but a solid-state weld process where the resultant joint is often as strong as the parent metal. This welding technique is employed in several industries to join parts.

Let’s get into the details of how this technique works and its benefits.

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The process of friction welding

If you rub your palms against each other, you’ll observe that your palms will become hot. As you increase the pressure and speed further, you will start feeling warmer.

The same principle of heat generation by friction is used in friction welding, where the metal parts are made to rub against each other at extremely high speed and pressure.

This interaction between the two surfaces results in mechanical friction. Even if the two materials to be welded together may seem smooth to the naked eye, there are irregularities at the microscopic level. These irregularities are enough to generate friction between their surfaces.

When two materials undergo friction welding, the relative motion between each other, and the pressure applied to them generates heat at the contact points. As the process continues further, the heat generation also rises, and the two materials will start becoming viscous at the contact points.

Again, the motion between the two promotes the mixing of the two parts at their contact points, creating a bond or a weld.

Different types of friction welding

Any welding process that uses friction as a way to create the bond can be termed as friction welding. However, there are four types of friction welding processes fundamentally.

Let's go through each of them briefly to understand the subtle differences between them.

Rotary Friction Welding: Out of the two materials, one is rotated over the surface of the other where the weld is required. The process uses compressive axial force and the high rotation speeds.

This combination causes the plasticizing of the two materials, eventually leading to a bond between the two.

Linear friction welding: In this type of friction welding, one of the materials oscillates in relation with the other at high speeds with high compressive forces in a reciprocating motion. The resulting heat generated at the surfaces causes the metal to plasticize, and the oxides or surface-level contaminants are burned off or expelled at the sides.

Friction Stir Welding: Friction stir welding uses a special tool with a cylindrical shoulder and a profiled pin to create welds. The pin drives along the seam of the two workpieces until the shoulder rests on the seam.

The tool then rotates where the friction between the shoulder and the seam softens the metal. The profiled pin is moved linearly through the line of the seam stirring the soft metal and creating a bond in the process.

Friction Stir Spot welding: Friction stir spot welding is one of the types of friction stir welding with one major difference.

In friction stir welding, the tool is moved along the seam of the workpieces. However, in friction stir spot welding, the tool is rotated at a spot and not moved.

It spins and creates a weld, and the tool is lifted up, creating an exit hole where the profiled pin was injected.

The speed at which the relative motion occurs and the pressure applied on the workpieces depends on the magnitude of the heat needed to create the weld between the two metal parts. For steel, friction welding generates anywhere between 900 and 1300 Celsius.

Inertia welding: is it the same as friction welding?

Many use inertia welding and friction welding synonymously. However, inertia welding is a type of friction welding.

To be precise, inertia welding is a type of rotary friction welding. It gets the name inertia welding from the way the rotation takes place.

In this joining technique, one of the workpieces is kept stationary while the other is mounted on a spindle. The spindle is made to rotate at high speed to generate friction between the two metallic surfaces.

Here, the maximum rpm of the spindle is fixed and is based on the type of material that it holds and the temperature that it should reach to weld the two pieces together.

Once the spindle reaches the top rpm, the drive disengages and the stationary workpiece is trust into the rotating workpiece. The workpiece continues to rotate on its own due to the inertia force resulting from the kinetic energy.

What are the advantages of friction welding?

Not all welding techniques ensure the same joint results. Hence, the type of welding is chosen based on the properties imparted to the joint by the welding process.

Let us discuss some of the advantages of using friction welding:

Enables joining dissimilar metals: One of the major advantages of friction welding is that it can be used to join dissimilar metals.

Some of the common bimetallic friction joints are:

• Aluminum to steel
• Copper to aluminum
• Titanium to copper
• Nickel alloy to steel

As a general rule, every forgeable metal can be friction welded. This gives more freedom to the engineers as they can create bimetallic structures thanks to friction welding.

Copper to aluminum joints are commonly considered unwieldable, but with friction welding, it is possible.

There is no external application of heat or flux: Friction welding does not require any external heat or flux, keeping the process easy and less messy.

Minimal or no defects: One of the advantages of solid-state welding is that it contains minimal or no defects when compared to fusion welding. The same effects are carried over to friction welding.

Very fast process: Friction welding is considered one among the fastest welding methods, clocking up to twice or even 100x faster than normal fusion welds.

Doesn’t need much surface preparation: Machined, saw cut or sheared surfaces can be joined using friction welding. However, the presence of lubricants or oils is not allowed for optimal weld conditions.

Friction welding is an umbrella term that encompasses several types of welding processes. Many industries rely on friction welding to create otherwise unwieldable joints.

It is fast, efficient, and one among the most popular options when it comes to solid-state welding.