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Friction Stir Welding and Inertia Friction Welding

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There are many options to choose from regarding welding techniques: MIG, TIG, arc and laser are just a few.  Friction welding has been a recent addition to the field of welding technologies available, and it has been gaining in popularity for many applications.

Aluminum is notorious for being difficult to weld because it is a softer, sensitive metal.  Some grades are especially troublesome in this regard, such as 2024 and 7075.

Two types of friction welding are friction stir welding and inertia friction welding. Here’s a primer that should answer some of your friction welding questions.

What is friction welding?

Friction welding is a kind of solid-state welding process in which heat is generated due to mechanical friction between two separate workpieces that are in relative motion to each other.  Additional force is applied laterally and referred to as an upset.  This added force serves to both plastically deform and fuse the materials.

The history of this process dates to the early twentieth century.  The process for linear friction welding was first patented in 1924 by an English engineer, with similar patents filed in Russia and Germany that same decade.  It wasn’t until the 50’s that real experiments took place on the technique and the possibility for commercialization was suggested.

The first commercially viable friction welding technique was known as rotary friction.  More options followed soon thereafter, as companies such as Caterpillar, Rockwell, and American Manufacturing Foundry began experimenting in earnest.  Today, research continues, with new methods of friction welding continually being developed.

What is friction stir welding?

Friction stir welding is one kind of solid-state joining that relies on a tool to combine two facing workpieces.  The tool is rotated and travels along the mating joint of the workpieces being welded, with the resulting friction generating the heat necessary to merge the two pieces.  The tool softens the two pieces of metal enough that mechanical intermixing occurs, forging the metal under the mechanical pressure.

The operator of the tool traverses it along the joint line, applying pressure as necessary. When it was first developed in the early 1990’s, friction stir welding was mainly used with wrought and extruded aluminum.  Its major benefit is that it offers extremely high weld strength, and thus can be used in industries where structural integrity is essential, such as automotive, aerospace, shipbuilding and architecture. It has since been expanded to be used with copper, titanium alloys, mild steel, stainless steel and magnesium alloys.  It’s even possible to weld different kinds of metal together using this process, such as aluminum and magnesium.

The name comes from the fact that the tool is rotated in a cylindrical motion, like stirring a cauldron.  The profiled probe is pushed into a butt joint between the two clamped workpieces, so that the shoulder is in contact with the surface of both pieces.  It is situated such that the probe is a bit shorter than the required weld depth, and the tool shoulder rides atop the work surface.

The tool is made of wear resistant material.  It generates frictional heat along the metal surface, softening the material and forcing it back, leading to the forged consolidation of the weld.

What is inertia friction welding?

An even newer technique is known as inertia friction welding.  Considered at the cutting edge of solid-state welding, it involves using the kinetic energy of the two workpieces to achieve a plasticized state.  One of the pieces is rotated at high speed, and the lateral force that is generated joins them together.

It’s imperative that when preparing a weld, that the type of alloy (or alloys) being used is considered.  The rotational speed of the flywheel, the amount of pressure applied, and the lateral force all must be adjusted to account for the material properties of the metal(s) involved.  A miscalculation will lead to a failed or low quality weld that won’t endure.  Similar to friction stir welding, this process works on different types of metals that otherwise wouldn’t be able to be joined.

The process begins by inserting the workpieces into the welding machine.  One piece goes into the flywheel, while the other is placed in a non-rotating chuck, which allows for it to be moved on a lateral hydraulic axis.  The name of this process stems from the amount of inertia generated by the spinning flywheel.  When it reaches the desired speed, the non-rotating piece is pushed forward, generating extreme friction as the two parts come into contact.

The heat that results softens both pieces of metal without melting them, as with traditional welding.  While they are soft and malleable, the addition thrust of the non-rotating piece causes plastic displacement, with the two pieces of metal flowing into each other.  The final product is a full cross-sectional bond.

 

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