1 Aug 2013

Friction Stir Welding

Several years ago I watched a technology show on BBC2 about a new kind of welding process that bonds two sheets of metal without the need for melting, thus leaving the final structure without a heat affected zone and no need for post-weld heat treatment.  The show described the process as the excitement of the atoms into mixing with each other, like mixing grains of sand together.  The effect of this is that the bonded metal sheets appear to be one contiguous extrusion and consequently have superior strength in the area of the weld.

Having spoken of the weld-type several times to anyone who cared to listen, yet having forgotten the terminology used to describe it, I struggled to find examples of it's use on the internet and was ready to file it under the category of conceptual work that had subsequently been binned.

So after a recent visit to TWI (The Welding Institute), I was pleasantly surprised to learn hat not only is the technology actually in existence and being used, but it was developed as far back as 1991 and was not actually as new as I had thought.  And the name is friction stir welding.

From wikipedia:

A constantly rotated non consumable cylindrical-shouldered tool with a profiled nib is transversely fed at a constant rate into a butt joint between two clamped pieces of butted material. The nib is slightly shorter than the weld depth required, with the tool shoulder riding atop the work surface.

Frictional heat is generated between the wear-resistant welding components and the work pieces. This heat, along with that generated by the mechanical mixing process and the adiabatic heat within the material, cause the stirred materials to soften without melting. As the pin is moved forward, a special profile on its leading face forces plasticised material to the rear where clamping force assists in a forged consolidation of the weld.

This process of the tool traversing along the weld line in a plasticised tubular shaft of metal results in severe solid state deformation involving dynamic recrystallization of the base material.

A number of potential advantages of FSW over conventional fusion-welding processes have been identified:
  • Good mechanical properties in the as-welded condition 
  • Improved safety due to the absence of toxic fumes or the spatter of molten material. 
  • No consumables — A threaded pin made of conventional tool steel, e.g., hardened H13, can weld over 1 km (0.62 mi) of aluminium, and no filler or gas shield is required for aluminium. 
  • Easily automated on simple milling machines — lower setup costs and less training. 
  • Can operate in all positions (horizontal, vertical, etc.), as there is no weld pool. 
  • Generally good weld appearance and minimal thickness under/over-matching, thus reducing the need for expensive machining after welding. 
  • Low environmental impact. 

However, some disadvantages of the process have been identified:

  • Exit hole left when tool is withdrawn. 
  • Large down forces required with heavy-duty clamping necessary to hold the plates together. 
  • Less flexible than manual and arc processes (difficulties with thickness variations and non-linear welds). 
  • Often slower traverse rate than some fusion welding techniques, although this may be offset if fewer welding passes are required.

If you would like to see some examples of the weld for yourself, TWI has an impressive gallery of exhibits in the foyer of their Abington site, and even have an entire section of wall comprised of square sheets that were bonded using the process.

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