Linear friction welding

by Philip Threadgill

Description

The world-wide industrial acceptance of the economic benefits and high weld quality produced when using conventional rotary friction welding to produce joints in round section metallic components led to the development of linear friction welding (LFW). Non-round or complex geometry components, such as aircraft engine blades to discs, can be welded using LFW, which is a solid phase, machine tool based process.

LFW involves rubbing one component across the face of a second rigidly clamped component, using a linear reciprocating motion. This motion is currently produced using a small amplitude of ±1-3mm, at a frequency of 25-125Hz and a maximum axial welding force of 150kN. The linear reciprocating motion shown in Fig.1 generates frictional heat and softening of material at the weld interface which is expelled as flash as shown in Fig.2.

Linear friction welding

Fig.1.
Linear friction welding
Linear friction welding of titanium at normal atmosphere

Fig.2.
Linear friction welding of titanium at normal atmosphere

The two components are brought into perfect alignment towards the end of the weld cycle, and the welding force is maintained or increased to consolidate the joint.

Current status

A British patent issued in 1969 described a linear reciprocating mechanism for welding mild steel, although no further information was published. In the early 1980s, TWI demonstrated the viability of the LFW technique for metals using modified equipment. The design and build of a prototype electro-mechanical machine with a linear reciprocating mechanism followed in the mid 1980s. Two similar machines are now located at an aircraft engine manufacturer in Europe. Several other machines of alternative designs are operating in the USA and Europe.

Although available for 10 years, the LFW process has only found industrial application in aircraft engine manufacture, in part due to the high cost of the welding machines. It has proved to be an ideal process for joining turbine blades to discs where the high value-added cost of the components justifies the cost of a LFW machine. This approach is more cost-effective than machining blade/disc (blisks) assemblies from solid billets.

LFW has been used successfully to join a range of materials including steel, intermetallic materials, aluminium, nickel and titanium alloys with the greatest emphasis on aircraft engine alloys. The process has also been demonstrated as an effectice way for joining copper to aluminium for electrical conductors.

Current issues

In recent years, LFW research programmes have addressed the following topics:

 

  • Components with irregular cross-section and/or complex shapes.
  • Difficult to weld, heat resisting alloys.
  • Dissimilar material combinations and cast to forged components.
  • Gas shielding of reactive materials.
  • Joining single crystal nickel alloys to polycrystalline alloys.
 

However, to increase greatly the application of LFW in industries such as automotive and power generation, the cost of linear friction welding machines must be drastically reduced. An EU funded study has recently been completed to build a low cost LFW machine. This increases TWI's capability to two machines.

Benefits

The benefits of linear friction welding are:

 

  • A solid state process (no arcs) which gives reproducible, high quality welds.
  • No fume or spatter
  • No filler wire or shielding gas required.
  • Less material loss when compared with flash welding.

Risks

Linear friction welding is regarded as unproven technology for applications outside the aircraft engine industry. Hence, there is a degree of risk associated with the high level of investment in a LFW machine for non-aircraft engine application. However, TWI's experience of welding many materials can minimise this risk, and TWI's machines are available for R&D and prototype development.

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