Friction stir welding (FSW) is a solid-state joining process developed at TWI Ltd in 1991. FSW works by using a non-consumable tool, which is rotated and plunged into the interface of two workpieces. The tool is then moved through the interface and the frictional heat causes the material to heat and soften. The rotating tool then mechanically mixes the softened material to produce a solid-state bond. The FSW process is illustrated in Figure 1.
Figure 1. Friction Stir Welding process
FSW is mainly used in industry to join aluminium alloys of all grades, in cast, rolled or extruded condition. Aluminium alloy butt joints with a thickness from 0.3mm to 75mm have been successfully joined in a single pass (dependent on workpiece material, machine power and structural stiffness). Other materials have also been successfully joined, namely magnesium, titanium, copper, and steel alloys. Plastics and metal matrix composites (MMC) have been explored. Dissimilar combinations between these materials have also proven possible.
Since its invention, FSW has become a proven technology in most manufacturing sectors. Some of its known applications include:
Shipbuilding and Marine
- panels for decks, sides, bulkheads and floors
- hulls and superstructures
- helicopter landing platforms
- masts and booms
- fuselage and wing structures
- fuel tanks for space vehicles
- rail stock vehicle floor, side and roof panels, namely for high-speed trains
- railway tankers
- battery trays
- engine chassis cradles
- wheel rims
- tailor welded blanks
- car body structures
- seat frames
- enclosures for circuits
- cooling and thermal management plates
- Apple’s iMac computer body
Figure 2. Friction stir welding applications: (a) wheel rim, (b) joining of aluminium extrusions, and (c) space propellant tank
Microstructure and Mechanical Properties
Friction stir welds typically exhibit three main microstructural regions: a weld nugget, a thermo-mechanically affected zone (TMAZ) and a heat-affected zone (HAZ). Technically, the weld nugget and TMAZ are both “thermo-mechanically affected zones,” but are considered separately for exhibiting distinct microstructural features. The weld nugget experiences dynamic recrystallisation while the TMAZ does not. The extent and microstructural composition of these zones are dependent on the material and processing conditions (parameters and tool design, for example). Figure 1 provides an illustration of these zones.
With regards to the mechanical properties of friction stir welded aluminium alloys, it is now well established that they are generally superior to those obtained by arc welding processes.
There are two main standards that describe the guidelines for use:
AWS D17.3/D17.3M 2021 “Specification for Friction Stir Welding of Aluminum Alloys for Aerospace Applications”
ISO 25239:2020 Friction stir welding — Aluminium
Friction stir welding offers many advantages over fusion-based joining processes, especially when joining aluminium alloys:
- Remaining in the solid-state, avoiding many of the defects associated with melting and solidification during fusion welding, such as pores and solidification cracks.
- The peak temperatures are lower, allowing a reduction in distortion and shrinkage.
- Being able to join many ‘non-weldable’ aluminium alloys, namely from the 2xxx and 7xxx series.
- Producing superior mechanical properties.
- No filler metals, flux or shielding gas are required. No fumes, porosity or spatter are generated.
- Fully automated, making the process highly repeatable.
- Energy efficient.
- Does not require special edge preparation in most applications.
Friction stir welding offers many advantages to the manufacturing sector for a wide range of applications. TWI Ltd has developed extensive knowledge of FSW over many years of research and development. If you would like to know more about the process, please contact us.