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Aero engine improvements through linear friction welding

Michael E Nunn

Paper presented at 1st International Conference on Innovation and Integration in Aerospace Sciences, 4-5 August 2005, Queen's University Belfast, Northern Ireland, UK.

Abstract

Linear friction welding (LFW) is an established niche technology applied by world leading leading gas turbine aero engine manufacturers to fabricate bladed disk (blisk) assemblies. Applicable to joining titanium alloys in the compressor stage and to directionally solidified or single crystal nickel based alloys in the turbine stage, the process is considered a rapid, low cost fabrication route for titanium blisks. Aero engine and other gas turbine manufacturers have and continue to focus considerable attention and investment on this technology. This paper reviews the application of LFW for titanium alloy blisk manufacture.

I. Introduction

The gas turbine industry is bound to continue improving its technical capabilities in terms of achieving higher efficiencies and safety standards and of complying with future environmental legislation. The feasibility of improved aero engines with regard to lower fuel consumption, reduced exhaust gas and noise emission, life cycle costs and enhanced reliability depends on the achievements of research and development activities concerning the processes and materials applied.

Advanced compressor and turbine designs are critical to achieve these goals. New and innovative rotor designs as envisaged for Very High Bypass Ratio (VHBR) engine concepts, are leading to substantially higher rotating speeds in the low pressure compressor and higher end temperatures for each stage of the engine. The reliability, lifetime and temperature capability of today's state-of-the-art designs are inadequate for future VHBR-engines. Conventional fir-tree or dove-tailed disk to blade attachments, as shown in Fig.1, are often the life limiting factor of a rotating stage in a compressor due to fretting fatigue damage at the mechanical joint. By eliminating the mechanical joint and introducing a fully welded blisk or IBR (Integrated Bladed Rotor) the fretting fatigue problem in the attachment region is removed. A further benefit of the blisk design is, that it is lighter compared to the conventional component.

Fig.1. Illustration showing the reduction in material and part count between a conventional mechanically attached blade-disk assembly and a linear friction welded blisk

Fig.1. Illustration showing the reduction in material and part count between a conventional mechanically attached blade-disk assembly and a linear friction welded blisk

This review paper introduces the linear friction welding (LFW) process with respect to its application for blisk manufacture and outlines the key characteristics which make it viable as a manufacturing route, along with the properties of the joints produced between aero engine materials. As titanium alloy blisks are in production today, the welding of these alloys is focused on in this document.

Reciprocating motion for friction welding, and then linear friction welding were proposed respectively in 1929 by the German Richter [1] and in 1959 by the Russian Vill' [2] . When the development and uptake of conventional rotary friction welding was at its height, the potential application of LFW was described as 'very doubtful' by Vill'. In 1969 The Caterpillar Tractor Co. filed a patent [3] giving equipment and process details for making linear friction welds between steel components.

World-wide industrial acceptance of the economic benefits and the high weld quality produced when using rotary friction welding to join round section metallic components led to the development of LFW at TWI Ltd. during the 1980s.

Fig.2. Illustration of the linear friction welding process

Fig.2. Illustration of the linear friction welding process

The solid phase process, illustrated in Fig.2, generates frictional heat by axially pressing, under a predetermined load, the surface of a laterally reciprocating component (i.e. a blade) against the surface of a stationary component (i.e. a disk). The surface and subsurface of both components are heated and the material is transformed into a softened plastic state. To complete the cycle, the amplitude of oscillation is decayed to zero bringing the moving component into perfect alignment under an axial forge loading which consolidates the joint. Softened material is displaced and expelled from the joint during processing in the form of flash. The total cycle time is very short, typically between four and ten seconds when welding titanium alloys.

Having worked extensively in the field of rotary friction welding, TWI explored the use of compound motions to provide the required motion for friction welding non-circular cross sections. The superposition of orbital motion over conventional rotation gave effective linear oscillation [4] .

By 1990 TWI had commissioned a purpose built electro-mechanically actuated LFW machine, similar to that shown in Fig.3, which was subsequently engaged in research and development work for a number of companies including major aero engine manufactures. The process has been shown to give excellent weld quality in many difficult to join materials such as titanium and nickel based alloys as well as aluminides and metal matrix composites [5] . The initial driver for the technology was the repair of severely damaged blades, but development of the process for blisk manufacture was quickly pursued.

a) Overview of the welding equipment
a) Overview of the welding equipment
c) Welding in progress
c) Welding in progress
b) The disk positioned for welding
b) The disk positioned for welding
d) A welded blisk
d) A welded blisk

Fig.3. Linear friction welding of a titanium alloy blisk at MTU Aero Engines using an electro-mechanically actuated LFW machine similar to that employed at TWI
Photographs courtesy of MTU Aero Engines
[6]

The technique can be used to join a variety of complex profiles, giving good functionality. It is technically, commercially and environmentally a very attractive process and is ideally suited to both mass production and to the manufacture of specialised components required in limited batches. Using modern equipment only the tooling to hold the work pieces and machine parameters need to be changed when welding different components.

Using the electro-mechanical LFW machine, it was demonstrated that excellent weld quality could be achieved. This helped companies such as Rolls Royce, MTU Aero Engines, Pratt & Whitney and General Electric to introduce linear friction welding into their commercial production. Two similar mechanical machines were installed at aircraft engine manufacturing plants in Europe during the 1990s, followed by other machines of alternative design in the USA and Europe. Presently titanium alloy welded blisks are only installed in military engines although significant interest from all sectors is clear.

LFW is most suited to the joining of high value-added components where the significant machine and associated tooling costs can be justified. For large blisk manufacture this approach is considered more cost-effective than machining the form from a solid forging [7] . Novel solutions have been devised to reduce the cost of the equipment, mainly based around the use of more efficient power sources and stored energy concepts.

II. Process fundamentals and terminology

The process cycle can be broken down into three stages, conditioning, friction and forge. A schematic of the time dependant evolution of the weld cycle variables is given in Fig.4.

During the conditioning stage the weld halves are rubbed together under a predetermined load, but no significant axial displacement occurs. Microscopic surface irregularities and tenacious oxide layers are destroyed. The set process variables are:

  • Frequency of oscillation of moving weld half.
  • Amplitude of oscillation.
  • Oscillation wave form - sine, square, etc.
  • Applied axial force.

Frictional heat is generated by the faying surfaces rubbing together during the friction stage. The material close to the interface is softened due to the temperature increase and is expelled in the form of flash. As a result significant axial displacement occurs, in the case of titanium alloys, this may be in the region of 2 to 3mm. As well as those listed for the conditioning stage, the variables in this stage of the process are:

  • Burn-off distance or time period.
  • Period over which amplitude decays to zero.

Once the amplitude of oscillation has decayed to zero, and the weld halves are perfectly aligned, the forge load is applied. This forge stage has the effect of upsetting the plasticised material causing further axial displacement and consolidating the joint. The forge hold time may be in the region of 5 seconds, although it need only be long enough to allow the cooling of the joint to a state where by no further plastic deformation will occur. The variables in this stage of the process are:

  • Applied axial force.
  • Forge force hold time.

In some cases, depending upon machine design, the amplitude of oscillation may not have reached zero before the forge load is applied. In these cases the end of the friction and the beginning of the forge stage overlap. No significant weld property issues have been noted with regard to this machine characteristic.

Fig.4. Schematic of the time dependant evolution of the key weld cycle variables

Fig.4. Schematic of the time dependant evolution of the key weld cycle variables

The process parameters may be varied to effect a high or low heat input by varying the surface rubbing velocity of the materials. This may be achieved by altering the frequency and amplitude of oscillation. Parameter variations in the range 25 to 125Hz, and 1 to 3mm using commercially available equipment are possible. Low conductivity materials may be welded using a wide range of heat conditions, whilst high conductivity materials require a rapid high heat input friction stage. In the former case, the cycle time may vary considerably having an effect on the joint formation and properties.

The axial loading applied during the friction and forge stages of the process must be sufficient to hold the weld half materials in intimate contact during the conditioning stage of the cycle and to displace the plasticised material at the weld interface as it is softened. When welding titanium and nickel based alloys, stresses to the order of 100 and 450N/mm 2 respectively are typical.

The rate at which the amplitude of oscillation is decayed to zero is an important factor in ensuring good weld quality at the extremities of the cross section. Longer decay periods are less severe and may act to massage the softened material at the section extremities, assisting in bond formation. The reactive loads on the equipment used to make the welds are also lower when a long decay is applied. It is normal to bring oscillation to a halt in 0.20 to 1.00 second.

At the end of the friction stage once the oscillation has decayed to zero, the two components are left in a predetermined position. It is a function of the welding machine and associated clamping tooling to ensure the moving weld half is properly positioned when it is halted. Modern production machines are equipped with technology to perform this task repeatable to precision engineering standards.

III. Materials and joint properties

Linear friction welding may be applied to join many similar and dissimilar material combinations [8] . TWI has demonstrated the process for amongst others:

  • aluminium alloys [9,10]
  • stainless/high strength steels [9,6]
  • titanium alloys [6,11]
  • nickel based super alloys [9]
  • metal matrix composites [12,13]
  • cobalt based super alloys
  • titanium aluminides [14,15,16]
  • nickel aluminides [17]

Many of these have been welded in cast as well as wrought form. Friction welding has also been demonstrated for platinum group metals [8] , which are increasingly suggested for high temperature aero engine applications. Indeed it should be possible to linear friction weld any material which can be conventionally friction welded.

Welds of this type generally have excellent metallographic quality. Due to the weld zone material remaining in the solid phase, and the high loads applied, the joints are normally free from volumetric flaws. In the majority of cases where voids are seen in friction welds, they have originated from the parent material microstructure.

In the case of titanium alloys, as shown in Fig.5, microstructural refinement by dynamic re-crystallisation and phase transformation takes place about the bond line in the thermo-mechanically affected zone (TMAZ), whilst the material in the heat affected zone (HAZ) is thermally affected by the process heat. During the friction stage a proportion of the TMAZ material is moved and expelled as flash at the extremities of the weld section. This flash can take two discrete forms, or a combination of each. Bifurcating flash formations consist of two separate collars of flash protruding from the two original weld halves. In non-bifurcating formations it is difficult to distinguish the flash expelled from each weld half as they are bonded together. These often take the form of wide wings protruding from the weld interface.

a) Neg. No. 1999-5-24-11-17-41-002
a) Neg. No. 1999-5-24-11-17-41-002
b) Neg. No. 1999-5-21-15-26-15-003
b) Neg. No. 1999-5-21-15-26-15-003

Fig.5. Macro and micro photographs of a metallographic section taken from a linear friction weld in Ti-6Al-4V. It is difficult to distinguish the flash expelled from each half in titanium alloy welds due to the characteristic non-bifurcating flash formations that can be seen

Similarly welds made in nickel based alloys, as illustrated by Fig.6, also have a refined hot forged grain structure in the TMAZ and severely deformed parent structure in the HAZ.

a) Neg. No. V4791-92
a) Neg. No. V4791-92
b) Neg. No. N4789
b) Neg. No. N4789

Fig.6 Macro and micro photographs of a metallographic section taken from a linear friction weld in a nickel based alloy. The bifurcated flash formation makes it possible to clearly distinguish the flash expelled from each half. Once again the TMAZ and HAZ can be seen

When the faying surfaces of most materials are cleaned in accordance with best practice, removing oxide and foreign body residues, inclusions such as oxide particles are rare. However when welding reactive materials in air, the air can penetrate under the edges of the section and cause oxidisation that hampers bonding. A suitable inert gas shield can be beneficial in these instances to prevent the formation of lack of bond flaws. It should be noted however that even when welding reactive materials such as titanium alloys in air, inclusions and flaws are normally trapped at the extremities of the welded cross section. Subsequently they may be removed if they fall within the machining allowance.

In certain weld cross sections lack of bond flaws can occur at the extremities. These may or may not be significant depending on the machining allowance on the completed assembly. Lack of bond flaws, like that shown in Fig.7a, are caused predominantly by unequal pressure distribution across faying surfaces of the weld zone. The extremities of a cross section being friction welded are not loaded with the full magnitude of the applied force throughout the weld cycle. Once the friction stage of the process is underway, the heating and subsequent softening of the material, combined with a lack of support around the extremities of the section allow upset of the material to occur. The formation of corner flaws in linear friction welded sections is often attributed to this as the corner material deforms outwards rather than reacting the applied load and forming a consolidated weld. It is sometimes deemed necessary to redesign square section components to incorporate radii at the corners to reduce the amount of unsupported material and hence the likelihood of a lack of bond flaw resulting in that region.

If the initial faying surface area is smaller than that of the final area after welding (due to material burn-off) then lack of bond flaws may occur. The presents of an edge chamfer on a faying surface results in the chamfered region not being in contact with the opposing surface at the start of the weld. As the material which is in contact softens, it may flow around the chamfer resulting in the curvilinear weld interface shown in Fig.7a. This issue is predominant when welding materials with low thermal conductivity as heat saturation away from the interface does not occur.

a) Neg. No. 2003-7-11-11-3-57-003
a) Neg. No. 2003-7-11-11-3-57-003
Neg. No. R4956
Neg. No. R4956

Fig.7. Microphotographs showing sectional views of;

a) A pronounced lack of bond defect in a dissimilar titanium alloy welded joint. This flaw, at the corner of the cross section, was initiated by a small edge chamfer (evident on the left-hand half) and exaggerated by the heat softened material being unsupported and allowed to upset.

b) The edge of a weld between a matensitic and a stainless steel. Cracking at the TMAZ/HAZ interface resulted from weld and flash residual stresses.

The rate of amplitude decay has an effect on the quality of the weld. Tearing in the HAZ may occur if the decay is too rapid. Tears may also occur from the semi-hard flash formations moving back and forth as a result of the weld half motion and causing undue stress on the flash/weld connection. Cracking at the extremities, as shown in Fig.7b, may result at the end of the weld cycle during cooling if the flash formation is unbalanced. One material may cool and gain strength quicker than the other resulting in residual stresses.

Materials with significantly different thermal expansion coefficients may be difficult to weld due to build up of high bond line thermal stresses and subsequent cracking. Welding a titanium aluminide to a titanium alloy may result in this problem. To overcome such issues a third body interlayer may be welded between the two materials with intermediary properties. Similarly material combinations which are likely to form brittle intermetallic alloys at the bond line, with little or no strength may be joined using a compatible third body interlayer. An typical example of this could be a joint between a titanium and a nickel alloy.

Bifurcated flash formations can form sharp vee-notches leading into the weld zone. These may be problematic if significant residual stresses are present, or if the weldment is to be stressed during service. However appropriate post weld heat treatment, and/or flash removal practices can nullify this issue.

Linear friction welds in most aero engine materials have extremely good strength when compared to parent metal. Welds made between titanium alloys will normally fail in the parent material when tensile tested, with some reduction in elongation resulting from localised softening of the weld zone material. It is common for dissimilar joints in titanium alloys to fail in the weaker of the two alloys, well away from the HAZ. Dynamic properties are also good with fracture initiation normally occurring away from the joint area.

IV. Applications

The application of linear friction welding has been considered, and in some cases developed, within the automotive, power generation and distribution, structural engineering and aerospace industries. Welding of single and multiple surfaces simultaneously [18] has been demonstrated. The welding of faying surfaces perpendicular to the axis of forge loading is conventional; however welding of compound loaded faying surfaces has also be successfully demonstrated.

A. Manufacture and repair of blisk, bling and IBR assemblies

To date the only known economically viable application of the process has been the joining of titanium alloy blades to disks and rings for high value added aero engine blisk assemblies. Numerous application based patents are in force which must be observed. These restrict users of the technology with regard to the geometry of blades, blade roots and disk surface features as well as the direction of blade oscillation.

The linear friction welding process offers a number of unique design and manufacturing advantages over other manufacturing routes such as milling from solid and electrochemical machining; these include:

  • Solid phase, autogenous, high integrity, low distortion welding technique.
  • The weld TMAZ has an extremely fine grain microstructure and is free from porosity.
  • Elimination of mechanical blade to disk attachment, which significantly reduces the weight, typically by 20 to 30%, and extends fatigue life [11] .
  • Blisk weight reductions are reflected in the design of shafts and other related parts.
  • Welding of dissimilar alloy/material combinations is possible, hence disk and blade conditions may be optimised.
  • Possibility to weld fine grain blade alloys for high cycle fatigue endurance to a coarse grain disk alloy for low cycle fatigue endurance.
  • Manufacture of large diameter blisks without the need for large forged pancakes.
  • Reduction in engine manufacture and lifetime costs [11] .
  • Welded joints may be made in areas of low stress or temperature where mechanical joints would not be possible.
  • Blades may be fabricated from two pieces - root and airfoil, or may be hollow allowing further material and design optimisation [19] .
  • The mechanical process can achieve high levels of statistical process control.
  • Precision finished forged airfoils with superior metallurgical and mechanical properties may be attached, requiring only minimal finish machining at the root [20] .
  • Positional and angular tolerance of attached blades is excellent when correct tooling is employed [19] .

As the attached blade may be finish forged prior the welding, only limited machining operations are required to remove any surplus material after welding. This surplus will generally include material left in place to facilitate clamping and the flash expelled from the joint during welding. Adaptive machining must be employed to allow for potential variations in airfoil position. Attainable airfoil positional and angular placement tolerances are quoted as being less than 0.2mm and 0.2° respectively in all dimensions [19] .

Full qualification for flight engine application has been achieved for this manufacturing route. Use of the process for repair of thin walled finish machined blisks has been demonstrated [20] . This is a more demanding application of the technology than manufacturing as the blisk is more susceptible to distortion. With the use of suitable support tooling the application has been proven successfully, even when repeated twice in the same location.

The future application of the process for attaching fir-tree style rim up-stands of one material to a disk or ring of another has been proposed [19] . In this way optimum static and dynamic properties may be achieved whilst still attaching airfoil blades with a more traditional fir-tree style mechanical interlock.

V. Machine and tooling requirements

Plastics have been welded by linear and angular reciprocating motion for decades using hydraulic, electro-mechanical or ultrasonic driven vibration [21] . The equipment generates high frequency vibration, which is translated to one of the parts to be welded. Although sonotrode based systems have been employed for welding small metallic parts such as electrical wire connections [22] , they cannot operate when forging forces exceed approximately 3kN. The surface rubbing velocity generated by the sonotrode vibration is generally too low for producing high integrity welds in high strength aerospace materials. The loads which may be applied through these systems are between one and two magnitudes too low to be useful for welding aerospace metallics.

The linear friction welding machines developed in the early 1990s were designed to translate rotary drive motion into linear motion. This is done using a pair of coupled crankshafts each incorporating a scotch crank mechanism which converts rotation into short stroke reciprocating motion. The reciprocating motion is translated through two flexible element couplings to each end of a whipple beam. When the two cranks are driven in time with one another, the whipple beam moves in a linear manner. One crank may be shifted out of phase resulting in the whipple beam simply rocking about its mid point. With appropriate tooling linked to the whipple beam mid point the linear motion may be translated to the component for welding. These systems, similar to a mechanically actuated fatigue testing machine, are still employed today in research and development as well as production [11] .

As well as the electromechanical machine, TWI also has a modern lower cost LFW machine. Recently developed under a European 'CRAFT' project 'LinFric ® ' [23] , the machine has a hydraulically actuated motion and uses a stored energy concept. The LinFric ® project was conducted to drastically reduce the cost of linear friction welding equipment, making the technology more accessible to potential users from various industry sectors including aerospace. The machine, shown in Fig.8a, makes use of a hydraulic actuator similar to those used in fatigue testing machines combined with rapid energy release hydraulic accumulators. The machine format was designed to accept a blisk for welding.

The hydraulically actuated systems have driven down the capital and maintenance costs for this type of machine. These machines are generally more compact and versatile than their mechanically actuated counterparts. An alternative machine format marketed by MTS Systems Corporation [24] , is shown in Fig.8b. This machine has a completely open bed that allows access for very large static components.

a) The LinFric ® machine [23]
a) The LinFric ® machine [23]
b) An MTS Systems Corp. machine [24]
b) An MTS Systems Corp. machine [24]

Fig.8, Two example of hydraulically actuated machines designed for blisk welding

In the case of all machines and applications the workpiece holding is of paramount importance. The final geometric alignment of the welded parts and the quality of the joint will both be affected by the performance of the tooling used to grip, support and guide the moving and stationary weld halves. When numerous components are to be attached to one base component, as in blisk manufacture, the tooling used must be compact enough to allow access to each weld location whilst remaining stiff enough to react the substantial forge and in-plane loads. It has been shown that the in-plane force, that equivalent to torque in rotary friction welding, can be greater than half of the applied axial force value [5] . In most applications intimately fitting friction clamps are used to surround and secure the components in perfect alignment. The friction clamps take advantage of the available component surface area and by spreading the clamping pressure over a large area, prevent localised surface damage or bruising. It is considered best practice to extend the tooling so that the components are supported close to the weld location, hence minimising undue vibration and deflection. As with many high value adding processes, it is the case that the short weld cycle time is insignificant in comparison to the time spent fitting and removing the components from the tooling. Total cycle times of between 5 and 15 minutes for blisk airfoil attachment are typical [19] . It has also been shown that special purpose tooling is required for every application, the cost of which for blisk manufacture may rival that of the welding machine itself.

VI. Concluding remarks

With the implementation of LFW the gas turbine industry may take advantage of all the known benefits of blisks machined from solid as well as a number of unique features. Large blisks may be manufactured from a series of smaller forgings giving optimised mechanical and metallurgical properties for blade and disk performance. With this technology the industry may continue improving its technical capabilities in terms of achieving higher efficiencies and safety standards and of complying with future environmental legislation. Aero engines with lower fuel consumption, reduced exhaust gas and noise emission, life cycle costs and enhanced reliability depends on the achievements of research and development activities concerning the processes and materials applied. Linear friction welding many prove to be a significant manufacturing technology when optimising blisk materials and designs.

This paper has focused on the industrially established welding of titanium alloy blisks, but has shown the possibility of welding heat resistant nickel based super alloys. The future application of this technology to the higher temperature regions of the engine will result in large weight and cost savings along with greater design flexibility.

Acknowledgements

The author would like to thank those who contributed to this work with information, ideas and support.

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