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Introduction To Friction Stir Welding

Friction-stir welding (FSW) is a solid-state joining process (the metal is not melted) that uses a third body tool to join two facing surfaces. Heat is generated between the tool and material which leads to a very soft region near the FSW tool. It then mechanically intermixes the two pieces of metal at the place of the joint, then the softened metal (due to the elevated temperature) can be joined using mechanical pressure (which is applied by the tool), much like joining clay, or dough. It is primarily used on aluminium, and most often on extruded aluminium (non-heat treatable alloys), and on structures which need superior weld strength without a post weld heat treatment.

Read more : Friction Welding | Advantages ,Disadvantages and Applications

It was invented and experimentally proven at The Welding Institute UK in December 1991. TWI holds patents on the process, the first being the most descriptive.

Principle Of Operation

In friction stir welding (FSW) a cylindrical, shouldered tool with a profiled probe is rotated and slowly plunged into the joint line between two pieces butted together.
The parts have to be clamped onto a backing bar in a manner that prevents the abutting joint faces from being forced apart.
Frictional heat is generated between the wear resistant welding tool and the material of the work pieces.
This heat causes the material to soften without reaching the melting point and allows traversing of the tool along the weld line.
The maximum temperature reached is of the order of 0.8 of the melting temperature of the material.
It leaves a solid phase bond between the two pieces.
The process can be regarded as a solid phase keyhole welding technique since a hole to accommodate the probe is generated, then filled during the welding sequence.

Important welding parameters

1 ) Tool rotation and traverse speeds

In general, it can be said that increasing the rotation speed or decreasing the traverse speed will result in a hotter weld.
In order to produce a successful weld it is necessary that the material surrounding the tool is hot enough to enable the extensive plastic flow required and minimize the forces acting on the tool.

2) Tool rotation and traverse speeds

If the material is too cold then voids or other flaws may be present in the stir zone and in extreme cases the tool may break.

3) Tool tilt and plunge depth

Plunging the shoulder below the plate surface increases the pressure below the tool and helps ensure adequate forging of the material.
Tilting the tool by 2–4 degrees, such that the rear of the tool is lower than the front, has been found to assist this forging process.
The plunge depth needs to be correctly set, both to ensure the necessary downward pressure is achieved and to ensure that the tool fully penetrates the weld.

Advantages

Low distortion and shrinkage, even in long welds
Excellent mechanical properties in fatigue and tensile tests
No arc or fumes
No porosity
Can operate in all positions (horizontal, vertical, etc.), as there is no weld pool.
Energy efficient
One tool can typically be used for up to 1000m of weld length in 6XXX series aluminium alloys
No filler wire required
No gas shielding is also required for welding

Disadvantages