Radical development work on three aspects of friction stir welding (FSW) has pushed the process's performance boundaries to new limits - limits deemed a distant objective in the early nineties when TWI invented the process.
Welds up to 75mm thick have just been achieved in aluminium alloys, record speeds in zinc welding have been reached and for the first time friction stir welding has been successfully carried out in steel.
Aluminium alloy plate up to 75mm can now be friction stir welded from both sides. Even more recently welding up to 50mm from one side has been achieved. Although further development is needed to optimise fully the process conditionsfor a range of aluminium alloys from 12.7mm thick and upwards, the preliminary results from thick plate trials show that welds exhibit good mechanical properties and acceptable metallurgical characteristics.
A ferrous first
A growing number of non-ferrous materials have been successfully joined by FSW; recent trials with low carbon steel show that ferrous materials can now join that list. Although still in the embryonic stage, trials show thatacceptable mechanical integrity can be achieved. For relatively thin steel plate the elimination of consumable materials and plate edge preparation is considered commercially attractive.
Early work has shown that FSW can be used to solid-phase weld comparatively thin commercially available zinc roofing sheet; an alternative for a material considered difficult to fusion weld. The welding travel speed for 0.8mm zincsheet is more than 1.8m/min.
Double-sided friction stir weld in 75mm 6082 T6 aluminium alloy showing weld profile cross-sectionTypical surface appearance of FSW butt weld in 3mm low carbon steel showing a regular series of part circular ripples pointing towards the start of the weldThree point bend and tensile test in 75mm aluminium alloy plate double-sided butt weld showing a localised reduction in specimen thickness in the HAZ
Root and face 1800 hammer bend tests in 3mm low carbon steel friction stir butt weld Friction stir butt weld in commercially available 0.8mm zinc roofing sheet Root and face 1800 bend tests in 0.8mm zinc roofing sheet
Usually zinc roof seams are mechanically folded with some seams being soldered. Since FSW is a non-fusion process there is not a great fire hazard. In addition, zinc anode sheet rather than zinc anode bars for sacrificial protectionmay benefit from a butt and lap joint configuration.
Further work is necessary, but already the general prognosis for FSW of thick aluminium, a number of steel and steel alloys and various non-ferrous materials is good.
To find out more, contact Wayne Thomas, Dave Nicholas, Peter Temple Smith, Steve King or Nathan Horrex at TWI. E-mail: wthomas@twi.co.uk or dave.nicholas@twi.co.uk .
