Friction Stir Welding (FSW)

Friction Stir Welding - Welding below the Melting Point

Friction Stir Welding (FSW), which was invented by Wayne Thomas at TWI in 1991, operates below the melting point of the work pieces to be joined. This leads to very high strength and minimal distortion. The process is consequently used in all areas of light-weight construction such as ship building, railway rolling stock, automotive and aerospace.


We offer turn-key solutions for close-loop force controlled FSW machines in close co-operation with Stirtec. These can be used for FSW at up to 25 kN (2.5 t) and for high-speed machining at up to 15,000 rpm. The work pieces can be machined before and after welding in the same fixture using these hybrid machine tools. This achieves significant cost savings and very tight tolerances.  


Most metals can be friction stir welded, e.g.

  • Aluminium
  • Magnesium
  • Copper
  • Steel
  • Titanium 

Friction stir welding of aluminium or copper is now state of the art, but even friction stir welding of steels achieves very promising results. 

› More


The parts to be welded can be processed as follows:

  • Extrusions
  • Castings
  • Wrought sheets

Panels are assembled from aluminium extrusions in many industry sectors. Aluminium castings are increasingly used for making battery tubs for e-mobility. 

› More

Material Combinations

Dissimilar materials can be friction stir welded, even if they have differnt melting points:

  • Aluminium-Copper
  • Aluminium-Steel
  • Kupfer-Steel

For FSW of dissimilar metals Special know-how is required regading tool design and set-up of the FSW machine.

› More

Webinar: Friction Stir Welding in 1½ hours

Stephan Kallee (AluStir) reported on 19 April 2021 in this 1½h webinar for 55 students of University of Liège on Friction Stir Welding

© AluStir, Stephan Kallee, CC BY 3.0


Stephan Kallee (AluStir) reported on 19 April 2021 in this 1½h webinar for 55 students of University of Liège on Friction Stir Welding including the principle, applications, materials, FSW tools, CoreFlow™ at TWI, FSW machines (gantry, CNC machine or robot), FSSW, thin sheets and thick plates, accidents, FSP and conclusions. This webinar was organised, hosted and chaired by Dr Yves Marchal (Sonaca) on behalf of Prof Olivier Brüls (University of Liège) and covered the following topics:


FSW Principle

  • Invented in 1991 by Wayne Thomas at TWI
  • Patented for 20 years
  • Hundreds of patents
  • Thousands of scientific publications
  • Developed in a Group Sponsored Project 
  • Initial applications in Scandinavia for Ships
  • Ideal for welding high-strength Al of rockets
  • Now: China pushes forward re E-mobility  

FSW Applications

Norway and Sweden were setting the trend

  • Used since 1995 in Scandinavia (4 years after invention)
  • Pre-manufacture of aluminium panels and large modules

Boeing and Lockheed Martin on behalf of NASA

  • Used for high-strength Al-Li alloys, which are difficult to weld by fusion welding processes such as VPPA
  • Hitachi and Hydro
  • Use of hollow aluminium extrusions
  • Less distortion and better tolerances than MIG welding

Heat Exchangers for Cooling Electronics in Trains

  • IGBT Coolers for insulated-gate bipolar transistors
  • More than 1000 m FSW per day

Automotive Body in White, Suspension Struts and Wheels

  • Ford GT, Panoz Esperante, Lincoln Town Car, Volvo X90
  • So far, very few 3D welds have been published
  • Eclipse 500 and "toe nails" by AJT on behalf of Boeing
  • Business jets, helicopters and military aircraft 
  • Cargo Ramp of Boeing C-17 Globemaster III 
  • Cargo Floor in Airbus A400M

Weldable Materials


  • Extrusions
  • Sheets
  • Castings
  • Dissimilar aluminium alloys
    • Aluminium extrusions to aluminium castings
  • Dissimilar materials
    • Aluminium to copper
    • Aluminium to steel

Steel: Only very few industrial applications

  • 12mm thick (12 % Cr) alloy steel plate, showing parent metal, and first pass and second pass in tension
  • Orbital FSW of pipes and FSP of hunting knives
  • Titanium: Even more difficult to friction stir weld than steel
  • Titanium has low heat conductivity and can easily be joined by rotary and linear friction welding
  • Hot titanium reacts with all gases of the atmosphere  


Copper: Sweden and Japan have been leading the field

  • Encapsulation of nuclear fuel
  • Sputter targets for making flat screen TVs
  • Magnesium: So far only for R&D in UK and Japan
  • FSW roofbox made from non-combustible Mg Alloy at National Institute of Advanced Industrial Science and Technology (AIST) in Japan
  • Plastics: Transparent plastic enables you to watch the tool
  • Sometimes it it better to move the tool up-and-down or forward-and-backward instead of using a rotating tool

FSW tools

  • Early butt welding tool design developed in TWI's GSP 5651
  • The tool with a concave shoulder was tilted by 2.5°
  • A left-handed thread was used with right-handed rotation

The tool is the heart of friction stir welding

  • The pin needs to push plasticised material downwards
  • The scrolled shoulder pushes it inwards

FSW Tools

FSW tools by Hufschmied

CNC- or Robo-Pin?

  • Rotating Shoulder 
  • Stationary Shoulder 

Cylinder or Cone?

  • Wide Root
  • Lower Forces 
  • Higher Speed

Triflat or Triflute?

  • Tool Life Time
  • Vibrations

Internal voids

• Downward pressure is too low

• Tool position is too high

• Tool rotates in the wrong direction

• Gap between the work pieces 


• Chamfer

• Radius

• Draft (taper on castings)


ISO 25239:2020



CoreFlow™ at TWI

FSW Tools for Lap Welding

Main challenges:

  • Hooking
  • Thinning
  • Remnant joint line

See for more details.


FSW machines

 Panel Welding machine - Prefabrication of FSW panels made from extrusions

  • Clamping is essential, multiple heads are possible
  • The extrusions need to be clean & have good tolerances

Position or Force Control - Proprietary control algorithms are often used

  • A combination of position and force control
  • The key parameters are monitored for quality control

Gantry Machines at TWI - Versatile machines for research & development

  • Sheffield: 15 t welding force, 11 CNC axes, twin heads
  • Cambridge: 5 x 7 m, for 2D non-linear welds in a plane

FSW Machines: Gantry at Eclipse Aviation - Commercial production of business jets

  • Mostly overlap welding to allow for repair by rivetting
  • Fuji Heavy Industries friction stir welded the wings

FSW machines (gantry, CNC machine or robot)

  • One CNC machine welds faster than two robots
  • The best option depends on the size of the work piece
  • The machine should be as rigid as reasonably possible 
  • SS-FSW: Stationary Shoulder FSW

FSSW -  Friction Stir Spot Welding

The visual appearance and hooking are challenging

  • It might be better to make step welds or „squircles“
  • Time is required to position the robot and make a spot

Consoles for FSSW and Refill FSSW

  • The part will be placed in the FSSW machine
  • Several spots can be made on one part with an x-y table

FSSW in production of Mazda RX-8 since 2003

  • Rear doors and impact absorbing aluminium bonnet
  • Low energy consumption (phase shift) and small cables

Non-disclosed prototype by Friction Stir Link in USA

  • A spin-off company by employees of Tower Automotive
  • Excellent robot programming skills making use of open architecture of ABB robots including torque measurment

Aluminium roof panels for high speed trains

  • Kawasaki Heavy Industries implemented FSSW in 2006
  • Low distortion and some other benefits (probably IPR)

Used in Japan, UK, USA etc

  • Good visual appearance
  • Many mechanical parts are affected by wear

For joining billets, FSSW machines are made by two UK based companies

  • RNS Industrial Engineering Ltd, Tottenham (in blue)
  • Otto Junker (UK) Ltd (in purple)


Benefits of FSW

Solid phase pocess

  • No melting
  • Low distortion 
  • No porosity
  • No fume 
  • No spatter 
  • Low shrinkage


Can operate in all positions

  • Non-linear  
  • Non-planar 
  • 3D

Simple machine tools

  • Automation 
  • On-line monitoring
  • Robots 


Limitations of FSW

  • Welding speeds are slower than those of some fusion welding processes
  • Machines are rigid and thus expensive
  • Workpieces must be well clamped, requiring bespoke fixtures
  • Backing bar required, unless you use Bobbin tools
  • Keyhole at the end of each weld, unless you use retractable pin tools
  • Limited access to proprietary know-how
  • Tool wear


Thin sheets and thick plates

FSW of 1.2 mm thick aluminium sheets

  • Flared Triflute Tool with M5 x 0.8mm right hand thread
  • Parallel flutes with 7° flare
  • Whisk feature on tip

FSW of 75 mm thick armored aluminium plates

  • Prototype hull of an aluminium tank designed by the Ground Vehicle Systems Center and welded by Concurrent Technologies Corporation

Thick plate: 50 mm, 75 mm and up to 150 mm thick AA6082-T6

  • TWI's High-Force FSW machine in Yorkshire
    • 15 t of welding force, twin head welding from both sides
    • True 3D welding capability: contoured & complex shapes

Ballistic impact test

  • General Dynamics Land Systems and EWI



Virgin Pendolino derailed on 23 Feb 2007 in Cumbria

  • Margaret Masson, 84, died and 30 others were seriously injured
  • All the partially FSWed carriages remained structurally intact, with damage mainly confined to the crumple zones at their ends.

FSW Balconies melted in Fire onboard ‘Star Princess’

  • Photographs by UK Department of Transport, Marine Accident Investigation Branch


FSP: Friction Stir Processing

FSP: Better microstructure than parent material

  • Idea: Processing of casting to consolidate voids
  • One of the first uses of a parallel kinematics robots

Friction stir processsing of aluminium casting

  • For instance for automotive pistons
  • Early application of articlulated arm robots for FSP 

Patent Application by The Boeing Company

  • Friction stir grain refinement of structural members


Commercial production

  • Aluminium extrusion and heat exchangers
  • Shipbuilding and railway rolling stock panels
  • Aerospace (especially rockets and  space craft)
  • Automotive (batteries and doors and closures)
  • Job shops


  • Ideal for making panels from aluminium extrusions
  • Advantageous for aluminium castings and sheets
  • Industrially used for copper and occasionally steel

Approvals by

  • ABS, BV, DNV, GL, Lloyds, RINA
  • DB, TÜV
  • FAA

Products and services

  • FSW machines
  • FSW tools
  • FSW know-how (consultancy and training)


  • Process variants
  • Difficult to weld materials
  • ai and CAM (artificial intelligence and Computer Aided Manufacturing)