The Welding Institute (TWI) has joined a type of steel alloy using friction stir welding that is considered unjoinable with conventional fusion methods.

The U.K.-based research and technology organization friction stir welded several samples of MA956 oxide dispersion-strengthened (ODS) steels. The samples, embedded with thermocouples to monitor process temperatures, were successfully welded at TWI’s Yorkshire Technology Centre and are now undergoing testing at Manchester University to assess their performance in hostile environments, including their susceptibility to irradiation damage.

(Read “Technique Can Join ‘Un-Weldable’ Metals.”)

Friction stir welding in progress on two plates of MA956 ODS steel. Image credit: TWI.Friction stir welding in progress on two plates of MA956 ODS steel. Image credit: TWI.ODS steels are specialized alloys that have been developed to deliver superior strength and creep performance at elevated temperatures, making them suitable for applications in nuclear reactors and power generation equipment. They derive their properties from a distribution of fine particulates, usually ceramic-based, that impede deformation and imbue the steel with its strength and resistance to creep.

However, conventional fusion welding techniques often destroy these strengthening particulates, preventing the steel from being fabricated effectively.

As a solid-state process, friction stir welding does not melt the steel being joined and should therefore allow ODS steels to maintain their properties after fabrication. In addition to being able to successfully weld ODS steel, friction stir welding may also render it less prone to hydrogen cracking—a significant advantage for welds operating in a nuclear environment.

Significant research has been carried out in recent years to develop new welding technologies, such as vaporized foil actuator welding, to overcome some of the limitations of traditional resistance spot welding.

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