Richard Dolby joined TWI, then The Welding Institute, in 1966 and spent 14 years in the Materials Department, where he became head in 1978, and Research Manager in 1980. His major interest centred about the welding metallurgy of ferritic steels, specialising in lamellar tearing and reheat cracking, and in HAZ and weld metal toughness problems. He was appointed Director, Research and Technology, his present post, in 1985.
As technical Chief of the Electron Beam Group, Allan Sanderson is concerned with the design and application of EB welding equipment. He obtained his doctorate on the generation and control of high power beams. This pioneering work led to the dramatic breakthrough in single pass thick section EB welding which has subsequently been applied to a wide range of industrial applications.
Philip Threadgill gained his BSc and PhD in Physical Metallurgy from University College, Swansea. He has worked at TWI since 1976 and is currently R&D Manager for the Friction and Forge Processes group.
Friction was first mobilised as a welding resource in 1891 by Bevington in the States. Since then, largely through half a century of study at TWI, it has come a long, long way.
In the final episode of this two part feature on evolutionary developments in electron beam and friction welding Richard Dolby, Allan Sanderson and Phil Threadgill assess recent innovations in both processes. Last issue dealt with EB. Now the authors examine progress in the solid phase process of friction welding.
Friction welding
Innovations for review
The two innovations to be described are developments of rotary friction welding where exciting new machine concepts have emerged within the last 15 years. Friction technology has expanded at an amazing pace in this period and we have seen an extraordinary period of invention and innovation. Both friction stir and linear friction welding, chosen for review, have made a strong impact commercially in a very short period, and enabled existing products to be made at greatly reduced cost and totally new products to be manufactured for the first time.
Friction stir welding
This process (FSW) was invented in 1990 and patented by TWI in 1991. The concept is simple and now well known. A rotating tool with a central probe is passed into the components to be welded and traversed along the joint line. The joint created is a solid phase weld, with no melting involved. The initial work was done on aluminium alloy sheet and plate, and this is where there has been rapid commercial exploitation. However, other metal alloy systems, including lead, zinc, magnesium, copper, titanium and steel can be welded by FSW, although industrial use of FSW for these alloys is still under development.
Since its invention, there have been strong industrial drivers for the wider use of FSW in the welding of Al and its alloys, for example, how to weld faster, weld thicker, weld very thin material, and weld different alloys. In addition, there is the increasing requirement to weld other alloy systems because of the information emerging from aluminium alloy FSW manufacture and the associated cost savings and new product opportunities.
The ability to weld faster at given thicknesses relates crucially to the FSW tool design and at TWI, a new development reported by Dawes et al has been the introduction of a scroll profile on the tool shoulder as shown in Fig.7a. The scroll channel captures most of the material extruded while plunging the tool pin into the workpiece, and when the tool pin is travelling along the joint, a radially inward mechanical advantage is provided by the rotation of the scroll, increasing the compression about the upper threads of the tool pin.
This development was tested on a 5083 Al alloy of 6mm thickness and high quality welds were achieved at double the speed made using FSW tools with no machined scroll. This development has also allowed tools to be used in the 0° tilt position, eliminating the normal 1-3° backward tilt. This simplifies the set-up and, in principle, facilitates welding in the x and y directions.
In the initial development phase with industrial clients, Al alloy thicknesses were in the range 1.5 - 12mm and the tools used contained threaded surfaces on the pin. Heat is generated on the surface by friction between the rotating shoulder and the workpiece surface, and this is the main source of heat for the welding of thin sheets. More heat must be supplied as the sheet/plate thickness increases and, in addition, there are requirements for the probe to create sufficient working of material around the joint line and efficient flow of the material around the tool as the weld proceeds. Using these principles, Thomas and Gittos report the development of two new types of tool known as Whorl TM and Triflute TM. These are shown in Fig.7b and 7c and involve a frustum-shaped probe with auger type threads, with and without the presence of helical flutes.
The tool, which is shown in Fig.7d, is a conventional tool with an extended pin and an opposing tool shoulder. This second shoulder is the integral backing bar. In recent experiments the bobbin tool design was used with scroll shoulders and a threaded profile which changed direction at the mid-point. This approach produced good welds in 6mm 6081-T6 Al alloy and appears promising.
Experiments with these new tools have shown that for aluminium alloy plates of 25mm or greater, one pass welds can be made with excellent quality as shown in Fig.8a. For example, with 6082 alloy, 25mm thick welds have been made at up to 250mm/min and 7075 alloy, 25mm thick welds at up to 60mm/min. In contrast, when using the same materials and thicknesses along with conventional threaded tools, welds were of very poor appearance and quality.
After only a few years work it is now feasible to weld metre lengths of Ti-6Al-4V alloy, and also Type 304L and Type 316L stainless steel in thicknesses up to 6mm, Fig.8b. Ferritic steels such as 0.03C-12Cr alloy and C-Mn steels have also been satisfactorily welded at greater thicknesses, eg 12mm.
In summary, exciting progress has been made in FSW tool design. Welding speeds have been more than doubled compared to the originally developed tools. One-pass welds can now be made in up to 50mm thickness in 6000 series aluminium alloys using the new tool technology and welding speeds in thin aluminium alloy sheet approach 3m/min. In addition, a range of lead, zinc, magnesium, stainless steels and ferritic steels can be welded. Industrial exploitation is gathering pace and current applications for aluminium alloys are numerous in high speed ships (using aluminium superstructures, bulkheads, floors), railways (Shinkansen and other trains in Japan), aerospace (Delta II rocket fuel tanks and various automotive components. Figure 9 shows two examples.
Linear friction welding
Whilst the idea of linear motion was patented in 1969 the development of suitable machines for welding engineering size components only started just over a decade ago. A consortium of four British companies was formed in the mid eighties led by TWI which designed and built prototype machines with a linear reciprocating mechanism.
Based on an electromechanical system, the machines provided a reciprocating frequency of 5-75 Hz and a reciprocating amplitude of 0-±3mm, with a maximum axial welding force of 150kN. The process opens up new design and manufacturing possibilities for metals, and is capable of welding square and rectangular components in one shot with accurate alignment of parts. With appropriate tooling it can be used for more 'irregular' components such as turbine blades.
Early trials started with Ti-6Al-4V alloy Type 304 stainless steel, 6063 Al alloy and C-Mn steel and were successful in producing sound joints. Since then tooling has improved and section sizes of 20mm x 100mm are now possible. The process seems very well suited to the joining of intermetallics such as Ti aluminide and successful welds can be made provided care is taken to control cooling rates.
To date, only the aircraft engine industry has used the process, eg for fan blade assemblies, including blisks, due mainly to the very high costs involved in machining and tooling. Figure 10 shows a linear friction welded blisk produced by MTU München for Eurofighter, while Fig.11 shows cross sections of a linear friction weld in a single crystal nickel based cast alloy, where the technique is under development in the USA.
However, there has always been strong interest in the process from other sectors, in particular mass production industries such as automotive components, and it is expected that applications will increase as the equipment cost drops. TWI is involved with a consortium of European SMEs to design and build a low cost machine, and this is now being assembled.
Conclusions
This work has dealt with electron beam and friction technologies only, but the innovation involved in the last decade or so has been surprising. It shows that given a concentration of effort and funding, engineers, metallurgists and physicists are capable of making leaps forward in joining technology which can have major impact on the creation of wealth, both in reducing manufacturing costs and facilitating the fabrication of totally new products. Creativity in the scientists and engineers involved must be encouraged and fostered, because the opportunities for further significant developments over the complete range of joining processes, including arcs, lasers, fastening, resistance and adhesives, as well as electron beam and friction, are immense.
