Friction stir welding in steel - longer than ever before
A friction stir weld performance, believed unattainable a decade ago, has been achieved at TWI's Technology Centre in Rotherham. The FSW team has created a continuous weld, in 6mm thick steel, no less than six metres long.
Part of the success, on its PowerStir machine has been attributable to the use of a new high-temperature composite tool material developed by MegaStir Technologies Inc in the USA. The material is manufactured from cubic boron nitride (cBN) with a tungsten rhenium (W-Re) second phase. Traditionally high-temperature strength and wear characteristics of friction stir welding tools have been the limiting factors to weld performance in steels.
But, by using the new material, tool lives in excess of 45 metres have been consistently achieved with low tool wear rates. It is important that the tool shoulder temperature is kept below 750°C, by controlling the welding parameters, for a high quality weld and low tool wear rate.
Unlike the tool materials predecessor, polycrystalline boron nitride (PcBN), this newly developed material is more tolerant to poor set-up and spindle run-out. Its characteristics comprise a fine balance of high-temperature strength, hardness and ductility.
The finished welds have excellent mechanical strength with tensile test failures occurring in the parent material, and have very low distortion across the weld and along its length. It's believed this is attributable to the process's relatively low welding temperature and good clamping.
It is the process's ability to create long uninterrupted high strength, low distortion welds that puts it head and shoulders above competing processes. Arc welding for instance, whether it be manual metal arc, submerged arc or metal inert gas would create considerable distortion which would be costly to eliminate.