S. W. Kallee, E. D. Nicholas and M. J. Russell
Paper presented at 10 th World Conference on Titanium Ti-2003, Hamburg, Germany, 13-18 July 2003
New methods of friction welding are becoming more widely implemented in the manufacture of aero engines, because these solid phase joining processes provide high weld quality and economic benefits. After the world-wide acceptance of rotary friction welding ( Fig.1), the aero engine industry is now implementing linear friction welding, and is considering friction stir welding, friction taper stitch welding and friction surfacing. Friction welding processesare substantially automated, and reproducibility is high in comparison to manual welding processes. Friction welding processes are generally tolerant to wide changes in the welding parameters without compromising quality, thus reliability is high.
Fig. 1. Rotary friction welding of a Ti-6Al-4V pipe (ø250mm, 17mm wall thickness)
In particular one process, linear friction welding, can be used to join a variety of complex profiles. It is technically, commercially and environmentally a very attractive process. It has been demonstrated for virtually all typesof engineering alloys, and novel solutions have been devised to reduce the manufacturing cost of the equipment, mainly based on using hydraulic actuation and stored energy concepts. This will lead to a very substantial reduction inequipment price.
High-strength titanium alloys are of interest for structures requiring minimum weight, especially in the aerospace industry. Along with the interest in high-strength alloys, there is a growing requirement to join titanium alloycomponents. For high-performance applications an improved strength/toughness combination is needed, and for this reason the solid phase friction welding processes have been developed, as they are likely to have a good balance ofproperties in titanium alloys. Friction welding processes also permit the joining of dissimilar titanium alloys, thus making best use of specific alloy properties at the operating location.
2 Rotary Friction Welding
Titanium alloys in general respond very well to rotary friction welding, and the process is widely used in the aerospace industry for joining many engine components.  Frictional heat isdeveloped by rotating one axially symmetrical component against another stationary component under an applied force. Frictional heating causes the materials to soften at the interface, and after a short time the interface issufficiently hot to allow the rotation to be stopped. A higher forging force is then usually applied to consolidate the joint. Rotary friction welding is very energy efficient compared to most competitive welding processes, and noconsumables such as filler wire, flux or shielding gas are required, even for environmentally sensitive materials such as titanium alloys.
Rotary friction welding can be divided into two distinct variants. In the continuous drive variant, the rotating component is constantly driven by an electric or hydraulic motor, which can be braked as the forge force is applied. Analternative is inertia friction welding, where the rotating component is attached to a flywheel (making use of stored energy), and the non-rotating consumable is used as a brake, thus converting the kinetic energy of the flywheel toheat at the interface. Continuous drive friction welding is more common in Europe, and inertia friction welding is more common in the USA. A feature of inertia friction welding, which can sometimes be an advantage, is that the rate ofenergy transfer is high at the start of the weld. Although there may be subtle metallurgical differences in the welds made by the two processes, the end results are very similar, and either process can be used to make high-quality welds.
Both Ti-6Al-4V ( Fig.1) and Ti-10V-2Fe-3Al alloys ( Fig.2) can be successfully joined using rotary friction welding. [1,2,3] The joints produced exhibit tensile strengthand impact toughness values comparable to the parent metal. Whilst post weld heat treatment may be advisable for the Ti-6Al-4V to stress relieve the joints, there is little measurable benefit to the tensile and impact toughness.However, post weld heat treatment is vital to the Ti-10V-2Fe-3Al to obtain the best balance of properties. It has been reported that friction welds exhibit a superior combination of strength and toughness in comparison fusion welds.Rotary friction welding shows also great potential for joining titanium aluminide intermetallic compounds.
Fig. 2. Rotary friction weld and a macrograph of a Ti-10V-2Fe-3Al alloy cylinder (ø60mm, 20mm WT)
3 Linear Friction Welding
3.1 State of the Art of Linear Friction Welding
Linear friction welding (LFW, Fig.3) has been demonstrated for virtually all types of engineering alloys, such as titanium ( Fig.4), nickel based alloys, aluminium, steel, stainless steel and intermetallics. Theprocess can be used to join a variety of complex profiles, giving good functionality. It is technically, commercially and environmentally a very attractive process. It is ideally suited to both mass production and to the manufacture ofspecialised components required in limited numbers. If different components are produced, only the tooling to hold the work pieces needs to be changed and different welding parameters may need to be set.
Fig. 3. Principle of linear friction welding
Fig. 4. Linear friction welded titanium blocks
In the early 1980s the first concepts were developed to use linear reciprocating motion for non-round parts. A major break-through was made by reducing the amplitude at the end of the weld cycle instead of slowing down the frequencyto terminate the friction phase. As a consequence, the workpieces were accurately aligned with respect to each other. In 1990 the first dedicated linear friction welding machine for welding metals was commissioned at TWI ( Fig.5& 6).
Fig. 5. TWI's electro-mechanically actuated linear friction welding machine
Fig. 6. Mechanism to develop linear reciprocating motion with variable amplitude in TWI's electro-mechanical LFW machine
When using this electro-mechanical linear friction welding machine, it was demonstrated that excellent weld quality could be achieved by this process. This helped companies such as Rolls Royce, MTU Aero Engines ( Fig.7) andPratt & Whitney to introduce linear friction welding into their commercial production.
Fig. 7. Electro-mechanical linear friction welding machine built by Blacks Equipment and used by MTU Aero Engines Munich in the series production of aero engine blisks [4,5]
One area for potential application of linear friction welding machines is the manufacture or repair of blisks (blades on disks) of aero engines and stationary turbines. Linear friction welding could be used by producers ofstationary turbines as well as by producers of aero-engines. It seems to be the ideal process for joining blades to disks ( Fig.8), as the melting point of the workpieces is not reached during the operation.
Fig. 8. Joining of blades to disks by linear friction welding to produce blisks
The uptake of linear friction welding by industry may have been impaired by the high capital cost of existing mechanical linear friction welding machines. Novel solutions have been devised to reduce the cost of the equipment, mainlybased around the use of more efficient power sources and stored energy concepts. Several studies  were conducted consisting of a historic review, the selection of the most suitable actuationsystem, a market analysis  and some preliminary welding trials ( Fig.9).
Fig. 9. Metallographic section through a linear friction weld of Ti-6Al-4V
3.2 The LinFric ® project on developing hydraulically actuated LFW machines
The LinFric ® project was conducted to drastically reduce the cost of linear friction welding equipment, in order to make the technology more accessible to potential users especially from the powergeneration, automotive and aerospace industrial sectors. This international project, which was partly funded by the European Community, was conducted by 8 organisations. It started on 1 October 1998 and had a duration of 36 months. Theconsortium consisted of five small and medium sized enterprises (SMEs) from three countries, supported by one larger company, and two research and development organisations. [8,9,10,11]
3.3 The LinFric ® machine specification
A full-size prototype LinFric ® machine was designed, built and commissioned by the project participants. The machine consists of a machine base on which a traverse carriage is mounted via reciprocating ball bearings. Hydrostatic bearings are used to re-act the welding and forging forces. A hydraulic actuator which is fixed in a vertical actuator mounting frame generates the linear reciprocating motion ( Fig.10). A manualguard covers the machine to fulfil health and safety requirements ( Fig.11).
Fig. 10. Artist Impression of the LinFric ® machine
Fig. 11. Fully assembled prototype LinFric ® machine being tested with manual guards at TWI
The machine base and the axial loading system are based on existing typical designs of rotary friction welding machines. The design of the prototype LinFric ® machine is similar to that of fixed headrotary friction welding machines, where the rotating component is not moved in the axial direction, but the non-rotating component is clamped onto a traversing carriage and then pushed against the rotating component. Linear bearingswith reciprocating balls are used to guide the traversing slide. One hydraulic actuator is positioned behind the workpiece in the centreline of the components. A backstop is placed behind the clamp to transfer the forces generated bythe traverse cylinder. It transfers the forces into a traverse cylinder bracket.
The machine concept includes the use of a hydraulic actuator similar to those being used in commercial fatigue testing machines ( Fig.12). The machine is not balanced and therefore the machine base might vibrate, as thecentre of gravity will be kept stationary. Thus an up-and-down motion of the oscillating carriage ( Fig.13) is preferred against a movement in the horizontal direction. The machine is fixed onto energy mounts to avoidvibrations being transferred into the workshop floor. To reduce the vertical movement of the machine base, compensating weights artificially increase the mass and inertia of the machine base. Therefore the total weight, excluding thehydraulic power pack, is approximately 12t.
Fig. 12. The prototype LinFric ® machine, suitable for welding cross sections of up to 2000mm 2 at frequencies of up to 125Hz and amplitudes of up to ±3mm
Fig. 13. The automated light-weight tooling of the prototype LinFric ® machine
The oscillating carriage is mounted onto the hydrostatic bearing bracket using hydrostatic bearings. The following parameters were used for dimensioning the parts ( Table 1 ):
Table 1. Basic parameters for the LinFric ® machine design
|High frequency hydraulic actuator
Max in plane force
0 to ±3mm
Max forge force
Max friction force
4 Friction Stir Welding
Friction stir welding is a solid phase process and lends itself to unique joint configurations. It was invented and patented by TWI in 1991. Originally, it was seen as the joining solution for aluminium sheets, but the list ofsuitable materials for friction stir welding grows monthly. As the tool designs are being refined, even the higher melting point materials like steel, stainless steel and titanium  arefinding their way onto the suitable materials list. The EUREKA EuroStir ® project has been devised to establish more industrial applications of the process in Europe. 
5 Other Friction Welding Processes
The Friction and Forge Welding Group of TWI conducts research and development on further friction based welding processes such as friction taper stitch welding, friction surfacing and its variant friction seam welding. These are nowbeing considered for repairing blades of aero engines. A versatile prototype machine was built in the CEMWAM project and has now been installed in TWI's laboratory for conducting feasibility and parameter optimisation studies.
One part of the CEMWAM project had the requirement to further develop friction welding, for alternative joint geometries of aero engine components. Within this work, the concepts of using friction taper stitch welding and frictionseam welding were proven for an aerospace alloy. A machine design was developed to have one machine able to weld with both process variants, which has the advantage of flexibility for the manufacturer and for future applicationdevelopments. The welding trials undertaken in parallel with the machine build, proved that friction stitch welds could be made through previously brazed joints. This gives future advantages for certain aero engine parts and alloys, ofenhancing the strength and performance of brazed joints and of reducing the jigging requirements for the joint which can give significant savings in manufacturing costs and lead time.
It is also being considered to use this machine for attaching bosses to engine casings by friction stud welding to eliminate the need for forging and machining and thus reducing manufacturing cost.
The LinFric ® project 'Development of a Low Cost Linear Friction Welding Machine' was jointly funded by the industrial consortium and the Commission of the European Communities under their CRAFT Initiative(Co-operative Research Action for Technology). The total budget was 1,236,000 Euro and involved an effort of more than 7½ man years. The authors want to thank all LinFric ® participants for theirtechnical contributions and valuable discussions during this project. All participants are very thankful for the financial support provided by the European Commission and for the excellent scientific supervision.
The following conclusions can be drawn:
- Solid phase friction welding processes are used to produce high-quality joints in titanium alloys.
- Rotary and linear friction welding have been implemented by several aero engine manufacturers.
- A prototype hydraulic linear friction welding machine has been assembled, commissioned, tested and demonstrated in a collaborative project, and a strategy for producing and selling these LinFric ® machines has been developed.
- Research and development projects are being conducted on applying other friction welding processes to titanium alloys, e.g. friction stir welding, taper stitch welding and friction stud welding.
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