Friction stir in steel - promises of commercial success
Now in its third decade friction stir welding of aluminium is enjoying enormous commercial success in both the aerospace and automotive sectors, but in steel the story is less clear.
Shipbuilding, bridge building, the renewable energy sectors will all benefit from its commercialisation. A soon to be published report aims to establish the current state of the art in the friction stir welding of steel, and draws primarily upon published work. It has been augmented with data from TWI’s own research, both published and unpublished, and information provided by organisations and companies with whom TWI co-operates and who have given their consent for inclusion of their data in the report.
As well as establishing the current status of FSW in steel, this report seeks to identify those applications for which use of the process in steel might first be commercialised, and any obstacles that may need to be overcome in the short term, to facilitate these early applications.
Where any such obstacles are identified, recommendations will be made for programmes of research work to be undertaken to overcome them as potential follow-on phases of this Group Sponsored Project (GSP) or as part of TWI’s rolling Core Research Programme (CRP).
In identifying potential early applications for friction stir welding, note will be taken of the current relative immaturity of the technology for steel and its perceived high cost when compared with fusion welding techniques.
However, as will be seen, the potential benefits that may accrue from successfully transitioning FSW technology from the light metals to ferrous alloys indicate that the process should not be evaluated purely on the basis of the cost per metre of weld made, but on a more comprehensive total lifetime costing.
When such factors as reduced distortion during fabrication and the potential for stronger, tougher welds are brought into the economic consideration the use of friction stir welding for steel, in some applications, becomes considerably more attractive.
It is the identification of these applications, and an assessment of their possible costs and benefits, that forms perhaps the most important strand to this work.
FSW of high temperature materials:
Friction stir welding was developed initially for aluminium, and subsequently other light metals such as magnesium that were considered difficult to weld by traditional means.
The excellent properties attainable by friction stir welding led not only to a demand to develop the process for other materials that were also tough to weld by conventional fusion techniques, for example copper and titanium, but also to develop the process for steel.
Most grades of steel can be welded quite satisfactorily by fusion welding techniques, but the potential for bringing some or all of the proven benefits of FSW to the welding of steel promoted research into this material too.
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 reduced cost, in order to make the FSW of steel an economically viable process.
As better FSW tool materials became available, 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, namely to use the thermo-mechanical nature of the FSW process to alter the microstructure and properties of the steel.
The basic principle of friction stir welding remains unchanged irrespective of the material being welded and thus progressing the friction stir welding process from a technique capable of welding aluminium to one capable of welding steel was essentially a development of enhanced tool materials.
When welding aluminium alloys, the temperatures recorded in the plasticised material around the FSW tool are of the order of 350 to 450ºC depending upon the exact alloy and welding parameters used. The FSW tool used can thus be made of a tool steel such as H13 for those alloys welded at the lower end of the temperature range and nickel based alloys such as MP159 for welds made at the higher end of the range.
However, as the plasticisation temperature of the workpiece material increases these tool materials become unsuitable as they begin to lose their mechanical and chemical properties at elevated temperature and therefore become prone to breakage, wear and dissolution in the workpiece.
The development of the FSW process for copper required a shift to nickel based alloys such as Nimonic 105 or materials based on tungsten carbide as the temperatures in the weld zone increased to between 700 and 850ºC.
Friction stir welding of steel poses a far greater challenge than the FSW of aluminium or copper. Steel FSW is carried out at temperatures calculated to be in excess of 1000°C at the tool work piece interface - direct measurement is difficult as temperature sensors embedded in the steel are destroyed by the friction stir process - and the tool must maintain its strength at these elevated temperatures whilst being subjected to complex bending, rotational and fatigue loads.
Furthermore, many potential tool materials oxidise at these high temperatures, or react with the workpiece material, and frequently both. Iron, the base element in steels, readily alloys with many of the potential candidate tool’s materials, and this high degree of chemical reactivity is further compounded by the presence of numerous other elements in steels, either present deliberately for alloying or as tramp elements.
Current R&D is addressing these points and the commercial exploitation of FSW of steel is coming closer.
