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Tuesday 20 March 2012

Friction-stir welding

Friction-stir welding (FSW) is a solid-state joining process (meaning the metal is not melted during the process) and is used for applications where the original metal characteristics must remain unchanged as far as possible. This process is primarily used on aluminium, and most often on large pieces which cannot be easily heat treated post weld to recover temper characteristics 

Principle of operation

Schematic diagram of the FSW process: (A) Two discrete metal workpieces butted together, along with the tool (with a probe).
(B) The progress of the tool through the joint, also showing the weld zone and the region affected by the tool shoulder.
A constantly rotated cylindrical-shouldered tool with a profiled nib is traversely 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 the weld.
This process of the tool traversing along the weld line in a plasticised tubular shaft of metal results in severe solid statedeformation involving dynamic recrystallization of the base material.

Advantages and limitations

The solid-state nature of FSW immediately leads to several advantages over fusion welding methods since any problems associated with cooling from the liquid phase are immediately avoided. Issues such as porositysolute redistribution, solidification cracking and liquation cracking are not an issue during FSW. In general, FSW has been found to produce a low concentration of defects and is very tolerant to variations in parameters and materials.
Nevertheless, FSW is associated with a number of unique defects. Insufficient weld temperatures, due to low rotational speeds or high traverse speeds, for example, mean that the weld material is unable to accommodate the extensive deformation during welding. This may result in long, tunnel-like defects running along the weld which may occur on the surface or subsurface. Low temperatures may also limit the forging action of the tool and so reduce the continuity of the bond between the material from each side of the weld. The light contact between the material has given rise to the name "kissing-bond". This defect is particularly worrying since it is very difficult to detect using nondestructive methods such as X-ray orultrasonic testing. If the pin is not long enough or the tool rises out of the plate then the interface at the bottom of the weld may not be disrupted and forged by the tool, resulting in a lack-of-penetration defect. This is essentially a notch in the material which can be a potent source of fatigue cracks.
A number of potential advantages of FSW over conventional fusion-welding processes have been identified:[5]
  • 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 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.

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