TWI Industrial Member Report Summary 1008/2011
By S R Cater and J G Perrett
Since the invention of friction stir welding (FSW) by TWI in 1991, and its subsequent widespread adoption for welding aluminium in sectors such as aerospace, rail and shipbuilding where high integrity, lightweight, fatigue resistant fabrications are required, interest has been shown in the use of the process for welding steel. Steel's higher melting and therefore plasticisation temperatures, and its greater strength at elevated temperature, pose considerable challenges for FSW, primarily in the development of FSW tool materials that possess the requisite properties of high-temperature strength and toughness, thermal and chemical stability, and oxidation resistance to withstand the more aggressive environment encountered in FSW of steel.
The feasibility of welding steel by the friction stir process was reported by Thomas et al, 1999. Research into the FSW of steel confirmed that though sound welds could be made in steel, the high cost and poor longevity of the available tool materials meant that the process was not economically viable other than for very niche applications. Subsequent research into the FSW of steel has been aimed primarily at improving the available FSW tool materials, seeking to enhance their reliability and longevity, and reduce cost, in order to make the FSW of steel an economically viable process. As better FSW tool materials became available (TWI Members' Reports MR915, 2008 and MR951, 2010), allowing the production of longer and better welds, research also began to be conducted into the properties of the welds, and into the use of the friction stir technique to process steel, ie to utilise the thermomechanical nature of the FSW process to alter the microstructure and properties of the steel.
This report describes work conducted as part of TWI's on-going Core Research Programme (CRP) to develop the FSW process further and assess its applicability to welding steel. The aims of the work reported here were twofold: to establish the performance of a new generation of FSW tools intended for FSW of steel and to assess the feasibility of friction stir welding a range of different grades of structural steels.
- To assess the performance of a new generation of composite refractory metal: ceramic FSW tools.
- To determine the friction stir weldability of a range of different structural steels
- To establish initial mechanical test data for friction stir welds in those steels.