TWI Knowledge Summary

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.

Fig.1. Linear friction welding

			Fig.2. Linear friction welding of titanium and steel in air a) and b) titanium c) steel
a)	b)	c)

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 about 20 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, copper and titanium alloys with the greatest emphasis on aircraft engine alloys. The process has also been demonstrated as an effective way for joining copper to aluminium for electrical conductors.

Current issues

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

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 been completed to build a low cost LFW machine. This increases TWI's capability to two machines. The newer machine (Linfric) is designed to enable welds to be made on large structures, and the machine has been fully operational since 2005.

Benefits

The benefits of linear friction welding are:

Further information

You can use the Weldasearch literature database to supplement what you find in JoinIT.