Metallurgical Base

About Friction stir welding.

What is welding?

Welding is a process for joining two similar or dissimilar materials by fusion. Welding uses high temperature to join the materials, whereas soldering and brazing do not allow the base metal to melt. After cooling, the base metal and the filler metal get attached.   

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Introduction of friction stir welding:

FSW was invented by Wayne Thomas at TWI(The Welding Institute) Ltd in UK, December 1991.

It overcomes many of the problems associated with conventional joining techniques. FSW is low energy input, capable of producing very high strength welds in wide ranges of materials at lower cost. FSW process takes place in the solid phase below the melting point of the materials to be joined.  

It is primarily used on wrought or extruded aluminium and particularly for structures which need very high weld strength. FSW is capable of joining aluminium alloys, titanium alloys, mild steel, copper alloys, stainless steel and magnesium alloys. More recently, it was successfully used in welding of polymers.

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Classification of welding:

Main two classifications are:

 Fusion welding

              Heated to molten state

              No pressure required

              Eg: Gas welding, Arc welding

Plastic welding

              Heated to plastic state

              Pressure required

              Eg: friction welding(solid state welding),  forge welding

Solid state welding:

This group of welding processes uses pressure and heat(below the melting temperature) to produce coalescence between the pieces to be joined without the use of filler metal.  

Friction welding:

It is a solid state welding process, where in coalescence is produced by the heat obtained from mechanically induced sliding motion between rubbing surfaces.

Also read: What is corrosion or deterioration of metal.

Friction stirs welding process:

It is one of the new solid joining process. The FSW is performed with a rotating cylindrical tool which has profiled probe(also known as pin) having diameter smaller than the diameter of shoulder. During welding the tool is fed into a butt joint between two clamped workpieces, until the shoulder touches the surface of the workpieces.

Schematic diagram of friction stir welding(Image: springeropen.com)

The probe is slightly shorter than the weld depth required, with the tool shoulder riding atop the work surface. After a short dwell time, the tool is moved forward along the joint line at the preset welding speed.  Friction heat is generated between the tool 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 tool is moved forward, a special profile on the probe forces plasticised material from the leading face to the rear, where the high forces assist in a forged consolidation of the weld.

Also read: What is difference between EN24 and EN8 steel?

Important welding zones:

Friction stir weld in its cross-section consist of four main zones.

Schematic of the cross section of a typical FSW weld: A. base metal, B. heat-affected zone, C. thermo-mechanically affected zone, and D. weld nugget zone (stirred zone). (Image: Springeropen.com)

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Microstructure analysis:  

 The micro-structure can be broken up into the following zones:

  1. the dynamically recrystallised zone (also known as stir zone) is a region of heavily deformed material that roughly corresponds to the location of the pin during welding. The grains within the stir zone are roughly  equiaxed and often an order of magnitude smaller than the grains in the parent material. A unique feature of the stir zone is the common occurrence of several concentric rings, which has been referred to as an “onion-ring” structure. The precise origin of these rings has not been firmly established, although variations in particle number density, grain size and texture have all been suggested.

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2. The flow arm zone is on the upper surface of the weld and consists of material that is dragged by the shoulder from the retreating side of the weld, around the rear of the tool, and deposited on the advancing side.

3. The thermo-mechanically affected zone (TMAZ) occurs on either side of the stir zone. In this region the strain and temperature are lower and the effect of welding on the micro-structure is correspondingly smaller.

4. The heat-affected zone (HAZ) is common to all welding processes. As indicated by the name, this region is subjected to a thermal cycle but is not deformed during welding.

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Applications:

various application of friction stir welding

Ship building and offshore: Friction stir welding was used to prefabricate the aluminium panels of the Super Liner Ogasawara at Mitsui Engineering and Shipbuilding.

Aerospace: Longitudinal and circumferential friction stir welds are used for the Falcon 9 rocket booster tank at the SpaceX factory.

Railways: The high-strength low-distortion body of Hitachi A-train British Rail Class 395 is friction stir welded from longitudinal aluminium extrusions.

Automotive: Friction stir welding used in automotive sector for car assembling.

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 Advantages:

  (a) No consumables

  (b) High superior weld strength

  (c) Operate in all positions

  (d) Low environment impact

Limitations:

  (a) Work piece must be rigidly clamped (b) Slower traverse rate than fusion welding   (c) Large down forces required with heavy-duty clamping necessary to hold the plates together.

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References:

Wikipidea.org/friction-stir-welding

Paul Kah, Richard Rajan, Jukka Martikainen & Raimo Suoranta, Investigation of weld defects in friction-stir welding and fusion welding of aluminium alloys. International Journal of Mechanical and Materials Engineering Article number: 26 (2015).

 Friction stir welding and processing, Materials Science and Engineering. Sciencedirect.com

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K Yashdeep

Hye there..! I am Yashdeep Kamal. I completed my Bachelor in Technology(Materials Science and Metallurgical Engineering). Engineer by passion, writer by choice. I have been writing about Composites, Ceramics, polymers, nanotechnology, advance materials and metallurgy etc. You can read about these topics here. Hope it may help you.

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