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Pushing the boundaries

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.

Shipbuilding, bridge decking, pipe seam welding, and just about any application which requires long uninterrupted steel welds, are extremely attractive target markets for the friction stir process. It's fast, high strength and demands little post weld treatment.

But perhaps its most attractive feature is that it lends itself well to use with metals, such as pipeline steels, which are difficult or impossible to weld using a fusion process. 

To date the process has proved itself capable of welding high-strength pipeline steels such as GRS 550 (X80) and high-strength shipbuilding steels and pressure vessel steels, such as HSLA-65, up to 12mm thick at some 150mm per minute.

A new generation of cBN-based metal composite FSW tool materials has been developed for applications where toughness is required, and predictable wear is desirable. The composite materials combine a ceramic cBN in a tungsten rhenium matrix binder.  The combined effect is that the resultant material is less brittle and still has very good wear resistance. The ultra-hard cubic boron nitride particles are finely dispersed within the metal matrix, forming a tightly bonded structure that takes advantage of both the toughness of the metal matrix and the hardness of the cBN phase. These materials are produced using high pressure high temperature technology.

An Open Day at TWI's Technology Centre in Rotherham is scheduled for mid-October this year. The process is going to be showcased using TWI's PowerStir machine. So it's a unique opportunity to find out more about the process being dubbed the next alternative to arc welding.

For more information, please email

Friction stir welding in steel
Friction stir welding in steel