Friction welding - the next step forward
TWI Bulletin, November/December 1991
Dave Nicholas is Head of the Forge and Resistance Processes Department which is concerned with research on, and development and application of, resistance welding (spot, seam, projection, flash, etc), friction welding (rotary, linear, stud and surfacing), MIAB and MIAF welding, and explosive welding.
He joined TWI in 1967, after obtaining his degree in metallurgy, working for the first year with resistance welding then in friction welding where he led the Section for almost 20 years. He was responsible for the introduction of friction surfacing and the use of alternative motions such as orbital and linear for joining non-round components. During the last few years he has led a project to develop a linear motion friction welding machine, which was successfully commissioned in January 1990. Another milestone during his time in friction welding was the development of the world's first underwater friction stud welding machine capable of operation at water depths of around 300m.
In his TWI career be has been involved with development projects spanning a wide range of industries including nuclear, automotive, electrical, offshore, chemical and metal winning.
At the turn of the century the door to a new age in joining was pushed ajar with the introduction of friction welding. The process involved round parts and rotary motion. Today that door swings wide open with the advent of linear friction welding of non-round sections. Dave Nicholas takes up the story....
Friction welding with rotary motion is well established as a joining method to provide welds of excellent metallurgical and mechanical properties, with good reproductibility and high productivity. The parts are always of circular symmetry and do not require any particular angular orientation after welding. Both these features have severely limited exploitation of friction welding, especially for joining materials other than thermoplastics.
In the early 70s the concept of using orbital motion was introduced to extend.the applications of the process. Unfortunately the mechanical arrangement to provide the motion involved rotation of both parts off axis which did not lend itself to acceptance for commercial use. However, experimental work carried out in the mid-80s at TWI's laboratories in Abington revealed that the motion was capable of producing high strength welds in a number of engineering steels with reduced surface velocities and welding forces. [1,2]
Also at that time the possibility of using linear reciprocating motion ( Fig.1) was under active consideration. The major obstacle to advancement was the lack of a suitable mechanical system to provide the motion. To maintain the impetus and to establish some understanding of linear motion when applied to non-circular parts a two-axis rotating orbital friction welding machine was modified to provide linear motion.
Fig. 1. The principle of linear friction welding
Experiments quickly identified the conditions needed for welding. Also the successful results prompted a group of interested parties (TWI, Allwood Searle and Timney, Blacks Equipment, Rolls-Royce and the UK Department of Trade and Industry) to pool their resources to develop a linear friction welding machine where only one part is moved.
Early work
The orbital friction welding machine referred to earlier (
Fig.2a) was modified to incorporate two special drive plates and a sliding cruciform (
Fig.2b ).
[3] Linear relative motion was then developed although both parts were still rotated. The machine was capable of operating within the frequency and amplitude ranges of 25-53Hz and ±1 to ±3.4mm respectively whilst a maximum axial force of 100kN was available. Rectangular parts nominally 25 x 6 or 10mm were prepared from mild steel, austenitic stainless steel (316), aluminium alloy (5154A grade) and titanium alloy (Ti-6Al-4V).
Fig. 2. Equipment for linear friction welding: Fig.2a) Orbital friction welding machine
Fig.2b) Additional fixtures - end plates and cruciform
Welds in the titanium alloy ( Fig.3a and b) provided the greatest success. The welds were characterised by the unusual flash produced. It appeared at all four sides ( Fig.3a) but did not divide into the ram's horn shape normally associated with conventional rotary friction welds, and the linear welds produced with the other three metals ( Fig.4 and 5). Nevertheless, the weld interface was fully bonded but there was slight undercut at the flash/parent locations as circled in Fig.3b.
Fig. 3. Linear friction weld in Ti-6Al-4V alloy: Fig.3a) As-welded external appearance
Fig.3b) Macrosection along length
Fig. 4. Linear friction weld in 316 grade austenitic stainless steel: Fig.4a) As-welded external appearance
Fig.4b) Macrosection along half length
Fig. 5. Linear friction weld in mild steel: Fig.5a) As-welded external appearance
Fig.5b) Macrosection along length
As-welded tensile and bend tests demonstrated that the bond line exhibited high integrity even though no gas shielding was used. A tensile strength of 835 N/mm 2 was achieved with failure in the parent material while the bend tested sample withstood a 50° bend before failure in the weld region. The welding machine settings to produce such a weld with 25 x 6mm section were 25Hz frequency, ±2mm amplitude, 5mm burnoff and 12 and 24kN friction and forge forces respectively.
Similar results were observed with the aluminium alloy and austenitic stainless steel welds, as revealed by Fig.4a and b for the latter material. The as-welded appearance shows the normal (bifurcated) flash expected with full interfacial bonding and no undercutting at the edge. A tensile specimen failed at a UTS of 675 N/mm 2, once again in the parent metal. The welding conditions for the 25 x 10mm section were: 42Hz frequency, ±2mm amplitude, 5mm burnoff, 18 and 36kN friction and forge force respectively.
Problems were encountered when welding mild steel; however, a weld of reasonable appearance and strength was possible ( Fig.5a and b). The as-welded appearance showed a very ragged flash whilst the macrosection identified undercut at the edges on both sides of the weld interface. A closer look at the macrosection reveals the presence of vertical striations perpendicular to the bond interface.
The inability to produce uniform flash and heating profiles together with no undercut is thought to be associated with inadequate gripping of the parts and the relatively low surface velocity at the interface (0.5 m/sec). The former could be associated with heavy vibrations set up during welding, while for mild steel surface velocities need to be greater than 1 m/sec to promote even heating and stable metal flow.
These preliminary investigations demonstrated that sound high strength linear motion friction welds were possible with the four metals studied, albeit that both parts were rotated. The information obtained on the welding conditions formed the basis of a specification for a dedicated single part motion machine.
The new machine
The specification for TWI's linear friction welder was:
| Reciprocating frequency | 5-75Hz |
| Reciprocating amplitude | 0-±3mm |
| Welding force | 0-150kN |
| Transmission power | 89kW |
A decision was taken to produce the reciprocating motion by a mechanical system rather than one based on hydraulics. The operating principle ( Fig.6) is based on use of a rotating crankshaft with two cranks, which can be phase shifted. Bending elements connect the cranks to a beam which will rock about its centre point when the cranks are 180° out of phase, such that the centre remains stationary. Reciprocating motion is developed, i.e. the centre of the beam is displaced, when the cranks are in phase. The frequency of reciprocation is related to the rotational speed of the crankshaft, which is driven by an 89kW DC variable speed motor.
Fig. 6. Linear friction welding - machine operating (Courtesy of Allwood, Searl and Tironey)
To ensure smooth operation all the mechanism is fully balanced with counterbalance weights. Use of bending elements rather than conventional pin and hole type bearings produces the lightest possible partially rotating joints without problems of lubrication, wear and backlash. The motion thus becomes a solidly bolted together construction subject to cyclically variable stresses. This resulted in all parts being either forged or machined from solid to ensure a long fatigue life. The reciprocating mechanism was designed to handle a tooling mass of 100kg.
The machine ( Fig.7a) consists of a main structure which houses the reciprocating mechanism and tooling to hold the parts for welding; the DC drive with its associated electrical control console; the hydraulically actuated phase changer ( Fig.7b); the hydraulic lubrication power pack; the main hydraulic power pack to drive the axial force cylinder and power the phase changer; tooling ( Fig.7c) and the main control console.
Fig. 7. Linear friction welding facility Fig.7a) Main machine
1-structure housing mechanism and tooling
2-DC drive
3-electrical controls
4-hydraulic lubrication power pack
5-main power pack
6-main controls
7-weld area
Within the console is a GEM 80 PLC which handles all the machine's functions input from a keyboard. An accompanying VDU displays all the machine settings used during a weld. To augment development with this machine, a multi-channel recorder was interfaced to obtain dynamic recordings of welding pressure (hydraulic), axial displacement, amplitude displacement and frequency. It is intended to supplement this instrumentation facility with a PC data acquisition/processing system and a special reciprocating ram to monitor in-plane forces (equivalent to torque with rotary friction welding) during the welding sequence. The machine was installed in late '89 and successfully made its first weld on 8 January 1990. To fulfil one of the objectives of the machine development program, TWI was targeted to weld a titanium alloy of 50 x 20mm rectangular section (1000mm 2 cross sectional area). The machine functioned perfectly to produce this weld in approximately five seconds ( Fig.8).
Fig. 8. Linear friction weld in titanium alloy (Ti-6Al-4V) - 50 x 20mm rectangular section
Applications
The new machine has greatly extended the potential applications of friction welding. Now non-round and round parts can be joined with precise angular alignment. Figure 9 shows that both multiple individual joints and complex multi-welded parts can be accommodated. Trials so far also indicate that linear motion will produce sound high strength bonds in like and dissimilar combinations similar to those weldable by rotational friction welding. Specific applications could include fabrication of gears, chain links, turbine wheels, electrical busbars, and bimetal cutter blades and broaches.
Fig. 9. Linear friction welds in complex austenitic stainless steel parts
In conclusion
The machine installed in TWI's friction welding laboratories will be used for continuing development of non-rotary friction welding to gain a deeper understanding of the technology. The facility is also available for Industrial Members to sponsor experimental work on specific applications relevant to their business interests.
It is hoped to extend the machine's specification to allow higher welding forces to be used (which will permit joining of larger cross-sections). Also, development of a more versatile suite of tooling will be pursued, to provide an ability to tackle real component parts.
For more information Industrial Members are invited to contact Edward Watts or Dave Nicholas.
References
| N° | Author | Title | |
| 1 | Cousens A K and Camping M J: | 'An initial investigation of the orbital friction welding of mild steel'. TWI Members Report 270/1985. | Return to text |
| 2 | Dunkerton S B: | 'Properties of 25mm diameter orbital friction welds in three engineering steels'. TV/1 Members Report 272/1985. | |
| 3 | Nicholas E D: | 'Friction welding non-circular sections with linear motion - A preliminary study'. TWI Members Report 332/1987. | Return to text |
