The FSW process

What is friction stir welding?

Friction stir welding is a unique and innovative method of joining metals. The friction stir welding process uses frictional heat combined with precisely controlled forging pressure to produce extremely high-strength joints that are virtually defect free, with minimal Heat Affected Zone (HAZ), very low mechanical distortion and excellent surface finish.

The friction stir welding (FSW) process transforms the parent metal from a solid to a plasticised state, mechanically stirring the materials together under precisely controlled process conditions to form a high-integrity, full penetration welded joint.

A purpose-designed rotational probe is set to the required process parameters for the friction stir welding application. The probe is then brought into contact with the metals to be joined at the joint interface.

Under precisely controlled axis conditions, with closed-loop monitoring of load and position, the probe follows the programmed weld path through the material. As the probe rotates, frictionally generated heat plasticises the surrounding material, creating a locally formable region. As the probe travels, the material at the trailing edge is forged under pressure from the probe shoulder and combines to form a cohesive bond.

Unlike fusion welding, during the friction stir welding (FSW) process the metals to be joined are not molten. This means the weld region can retain grain structure integrity and mechanical properties of the original parent metal.

Benefiting from sophisticated sensing and data-logging, the PTG Powerstir friction stir welding process is effective on flat plates, extrusions, cylindrical components and even on parts of irregular thickness.

Typical friction stir welding process applications

  • Electric vehicle battery tray floor assemblies, battery trays and boxes
  • Vehicle chassis and structural components
  • Car and commercial vehicle body panels
  • Locomotive train and carriage panels (aluminium)
  • HGVs, truck bodies and space frames
  • Heat sinks and electronics enclosures
  • Boat and ship panel sections
  • Fuel tanks and bulk liquid containers (flat and cylindrical)
  • Aluminium bridge sections, architectural structures and frames, pipelines, heat exchangers
  • Electrical motor housings

Materials

  • Aluminium
  • Magnesium
  • Copper
  • Titanium
  • Steel
  • Lead
  • Zinc

 

PTG Powerstir dual weld-head friction stir welding for electric vehicle OEMs

In addition to offering conventional single weld-head Powerstir friction stir welding machines, PTG has also developed Powerstir dual weld-head friction stir welding machines. Designed specifically for use in the volume production of automotive battery tray floor assemblies from extruded aluminium panels, the PTG Powerstir dual weld-head process is aimed directly at manufacturers of electric vehicle (EV) skateboard chassis structures and ensures that a tight weld-flatness tolerance is achieved during battery tray floor construction.

A tight weld-flatness tolerance is essential to ensure that each EV battery cell sits perfectly level within its housing. The PTG Powerstir dual weld-head method achieves an even and stable welding process – something that is made possible thanks to PTG’s unique ‘matched’ dual-force control systems and balanced upper and lower head welding parameters. The result is exceptionally stable friction stir welding by both the upper and lower weld heads, producing matched weld seams with balanced heat input. This, in turn, minimises post-weld distortion and equips each welded assembly with a significantly improved flatness tolerance when compared to existing conventional single-side friction stir welding techniques.

PTG Powerstir friction stir welding processes for electric vehicles include:

  • Battery tray floor assemblies
  • Battery trays and boxes
  • Chassis and structural components
  • Control box panels
  • Vehicle body panels
  • Brake regeneration systems
  • Coolant units and heat exchangers

 

Friction stir welding simplicity

The friction stir welding process typically uses just three carefully controlled variables: probe rotational speed, traverse speed and joint pressure, all of which are precisely monitored by the CNC system, with full on-board data logging for traceability. Compare this to standard fusion welding, where quality is governed by multiple parameters that need to be controlled, such as purge gas, voltage and current, wire feed, travel speed, shielding gas and arc clearance gap.

Friction stir welding integrity

The friction stir welding process produces a very clean weld with minimal Heat Affected Zone (HAZ) and spatter. Careful control of the process parameters, combined with the low number of system variables, produces a guaranteed high-quality weld, retaining close to the original tensile and yield strength of the parent material with virtually no porosity. With friction stir welding, the surface finish is good and FSW exhibits excellent retention of key properties, such as hardness, tensile strength, fatigue and corrosion resistance.

Friction stir welding – virtually no distortion

Due to the low welding temperature of the friction stir welding process and method of joining, distortion is practically eliminated with minimal Heat Affected Zone (HAZ). This even applies to extremely long welds in thin material, which are often undertaken under continuous process conditions.

Friction stir welding – health & safety

The friction stir welding process is inherently safe with no risk of arc-light present as in fusion welding processes. Additionally, with friction stir welding, no flammable or shielding gases are needed. The temperature of the metals after friction stir welding is lower in comparison to conventional fusion processes.

Friction stir welding (FSW) was invented and patented by TWI in 1991.

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