2 FRICTION PROCESSING TECHNOLOGIES

1 INTRODUCTION

A look back over about 100 years will reveal thatBevington as early as 1891 realised the opportunity touse friction to generate heat for both forming and weld-ing. Of significant importance was the ability to producea product in the solid phase, in other words that did notmelt. The use of friction for welding came to prominencein the 1950’s when Bishop reported many applicationsof Russian origin. While on a more worldwide scale, theprocess gained acceptability for high volume productionand its ability to join a wide range of materials in like anddissimilar combinations from the 1960’s. The automotiveindustry adopted the process to weld bimetallic exhaustvalves, rear axle casings and front wheel drive shafts,while the electrical industry was welding millions of cop-per/aluminium connectors.In general, almost all of the applications involved circu-lar parts, as rotary motion was the primary one available,being the easiest and cheapest to generate. However,changes were to come in the mid 70’s when orbital andlinear reciprocating motions arrived, which then permit-ted the joining of non-round parts with accurate angu-lar alignment.Another major milestone was reached in 1991 whenWayne Thomas of TWI invented Friction Stir Welding,

thus further extending the opportunities to use frictionheating and material flow to join sheets and plates in thesolid phase.

In view of the multiplicity of methods now available toweld and process materials that utilise friction this arti-cle will introduce 16 of them and briefly review themwhen grouped against specific categories.

2 THE BASIC PROCESS

The action of rubbing two objects together causing fric-tion to provide heat is one dating back many centuries.The principles of this method now form the basis ofmany traditional and novel friction welding, surfacingand processing techniques.

The friction process is an efficient and controllablemethod of plasticising a specific area on a material, andthus removing contaminants in preparation for welding,processing, surfacing/cladding or extrusion. The processis environmentally friendly as it does not require con-sumables (filler wire, flux or gas) and produces nofumes.

In friction welding, heat is produced by rubbing compo-nents together under load. Once the required tempera-ture and material deformation is reached, the action isterminated and the load is maintained or increased tocreate a solid phase bond. Friction is ideal for weldingdissimilar metals with very different melting tempera-tures and physical properties.

FRICTION PROCESSING TECHNOLOGIESE.D. Nicholas

Friction and Forge Processes Group, TWI Ltd., (UK)

ABSTRACT

The utilisation of friction as an efficient thermo-mechanical source to both weld and process materials in the solidphase has come a long way since the first patent filing by Bevington in the late 19th Century. It is fair to say that upuntil the early Eighties, rotation was the primary motion used to practice friction welding for most applications on acommercial basis, certainly for metals. Work by Searle in the Seventies with orbital motion gathered momentum topermit the welding of non-round parts. This was followed by the development of a dedicated machine to use linearreciprocating motion for joining. From the late Eighties onwards an ‘explosion’ of friction based technologies wereconceived and promoted. Such processes include friction taper stud and stitch welding, friction hydro pillar pro-cessing, friction extrusion, friction plunge welding, third-body friction welding and not least friction stir welding, whichmust be regarded as the major step change for the welding of aluminium and its alloys. Sandwiched between motionand process developments came more detailed studies of friction surfacing and friction seam welding, which wereboth the subject of a patent filing in 1941. The aforementioned processes are reviewed and selected processes dis-cussed more fully. Attention is drawn to the applications, industrial sectors, etc., to which they can be aligned.

IIW-Thesaurus keywords: Friction welding; Process variants; Friction stir welding; Inertia friction welding; Orbitalfriction welding; Radial friction welding; Friction surfacing; Metal working; Repair; Heat treatment; Stud welding; Plugwelds; Spot welds; Extrusion; History; Surveys; Reference lists.

Welding in the World, Vol. 47, n° 11/12, 2003

Doc. IIW-1637-03 (ex-doc. III-1262-03/III-B-006-03) rec-ommended for publication by Commission III “Resis-tance welding, solid state welding and allied joiningprocesses”Copyright © 2003, TWI Ltd. Published with the kindauthorisation of TWI.

FRICTION PROCESSING TECHNOLOGIES 3

3 FRICTION PROCESSES

The following list identifies the multitude of friction basedprocesses that are now available for commercialexploitation:

Rotary drive friction welding(continuous drive) . . . . . . . . . . . . . . . . Ref. 1

Rotary drive friction welding(stored energy). . . . . . . . . . . . . . . . . . Ref. 2

Friction taper stitch welding . . . . . . . . . Ref. 3

Radial Friction welding . . . . . . . . . . . . Ref. 4

Friction surfacing . . . . . . . . . . . . . . . . Ref. 5

Friction transformation hardening. . . . . Ref. 6

Friction stir welding. . . . . . . . . . . . . . . Refs. 7, 8, 9

Friction plunge welding . . . . . . . . . . . . Ref. 10

Third-body friction welding. . . . . . . . . . Ref. 11

Friction seam welding (Klopstock) . . . . Ref. 12

Friction extrusion and frictionco-extrusion cladding . . . . . . . . . . . . . Refs. 13, 14

Friction hydro pillar processing . . . . . . Ref. 15

Linear and angular friction welding. . . . Ref. 16

Orbital friction welding. . . . . . . . . . . . . Ref. 17

Friction brazing. . . . . . . . . . . . . . . . . . Refs. 18, 19

Friction seam welding (LUC) . . . . . . . . Ref. 20

Unfortunately space will not permit in-depth reviews ofthe relative merits of each process, therefore attentionwill be focused on those processes, which havemade/are making a major impact on industry.

4 ROTARY FRICTION WELDING

This is by far the most common form of friction weldingand accounts for most of the machines in industry today.The main energy variants are those of, continuousdrive, where energy to make the weld is available froman infinite source (Fig. 1), and stored energy, wherethere is only a finite amount of energy available con-served in oil/air under pressure or in a rotating flywheel(Fig. 2). It is worthwhile to note that machines range incapacities to weld ~1mm up to 300 mm diameter (larger

dimensioned tubes) solid bars. The biggest machine isbased on the stored energy variant and can deliver20,000 kN axial thrust. Industries currently employingfriction techniques in their manufacturing processinclude: offshore (drill pipe and underwater stud), petro-chemical, electrical, hydraulics, power generation, rail-way, nuclear, aerospace, automotive, machine tool andwire drawing.

Continual development of rotary fiction welding has seenmachines becoming more lightweight, thus allowing theiruse at site. In this context the opportunity to use themethods of friction stud, friction hydro pillar pro-cessing (FHPP) (Fig. 3) and friction taper stitch weld-ing (FTSW) (Fig. 4) for repair in hostile environmentssuch as underwater, high radiation and explosive atmos-

Fig. 1. Continuous drive rotary friction welding.

Fig. 2. Stored energy (inertia welding).

Fig. 3. Friction hydro pillar processing.

4 FRICTION PROCESSING TECHNOLOGIES

pheres is realised. Already, the offshore, nuclear andaerospace industries are using one or other of theseprocesses. In FHPP, a rotating rod is inserted into ahole and advanced to the bottom under a friction forceto plasticise the material and ensure that the material willtransfer from the rod to fill the hole. To carry out frictiontaper stitch welding, a tapered hole is machined then atapered stud friction welded into it, this procedure is thenrepeated ensuring that there is an overlap between thewelds.

When the need arises to join long pipes/tubulars orattach rings to cylindrical parts then the concept of radialfriction welding can be used. For joining pipes as illus-trated in (Fig. 5), an intermediate ring with a bevelledinternal profile is placed between the “V” preparation ofthe pipe ends, an internal support mandrel is placed inthe bore to prevent flash formation, then the ring isrotated while being subjected to radial compressive

forces to develop friction heating and metal flow andthus produce a weld.

Although friction welding can join a wide range of simi-lar and dissimilar material combinations, there inevitablywill be some combinations that cannot be joined directlyto give high joint integrity. Consequently, processes likefriction plunge welding and third-body friction weld-ing can be considered. With friction plunge welding(Fig. 6) the harder of the material in the combination ismachined to provide “locking” features then it is rotatedand forced into the softer material with the intention ofproducing a metallurgical bond supported by a mechan-ical lock. For third-body friction welding, use is made ofa material that is considered compatible with the pri-mary materials to be joined, and as Fig. 7 shows, onepart is rotated and forced into the third body material tocause plastic flow to effect bond formation. A similarprocess to third-body as just described is that of frictionbrazing (Fig. 8). Here a braze alloy (third-body) isdeposited onto one part, then by careful control of therotation speed and friction force, heat is generated tomelt the braze alloy and produce a bond. A practicalexample of such a combination that has been joined isthat of an aluminium oxide ceramic to pure copper.

Fig. 4. Basic principle of Friction Taper StitchWelding showing reactive taper angle

on both substrate and plug.

Fig. 5. Radial friction welding. Fig. 6. Principle of friction plunge welding.

FRICTION PROCESSING TECHNOLOGIES 5

5 FRICTION SEAM WELDINGAND SURFACING

One concept, that of rotating a consumable rod betweentwo profiled plates to weld the plates together, waspatented by Klopstock and Needlands way back in 1941(Fig. 9). Here the joint was produced via the filler rod andtrials at TWI carried out some years ago demonstratedthat stainless steels, nickel based alloys and aluminiumalloys could be successfully welded. A derivative fromthis idea allowed the deposition of material from therotating consumable rod onto a surface and was thusknown as friction surfacing/cladding (Fig. 10). Such lay-

ers could be deposited again in the solid phase to min-imise/eliminate dilution to provide both corrosion andwear resistant coatings. Equally, a like deposit could bemade to repair a worn part by building up, or to seal acrack.

Another seam welding method, that of rotating a nonconsumable wheel at very high peripheral velocity whileapplying a light force, was devised by a French lady bythe name of Mrs Luc. This process (Fig. 11) utilised therotating wheel as a third- body to develop friction heat-ing and some plastic flow. The process found successwhen applied to thin gauge materials such as aluminium,copper and plastic.

6 FRICTION STIR WELDING

The friction stir welding technique (invented, patentedand developed by TWI) is a derivative of conventionalfriction welding, which enables the advantages of solidphase welding to be applied to the fabrication of longbutt and lap joints with very little post weld distortion.Also it is useful to remember that friction stir spot weld-ing is possible, indeed one automotive company has

Fig. 7. Principle of “third-body” friction joining.

Fig. 8. Friction brazing.

Fig. 9. Principle of friction seam welding(after Klopstock and Neelands).

Fig. 10. Friction surfacing.

Fig. 11. Friction seam welding (after LUC).

6 FRICTION PROCESSING TECHNOLOGIES

introduced the process into production. Moreover, it isa simple to operate, very cost effective machine tooltechnology offering many advantages.

The joining of aluminium alloys, especially those thatare often difficult to weld, has been the initial target fordeveloping and judging the performance of friction stirwelding. Work to date has concentrated on single passwelds in material thicknesses from 1.6 mm to 50 mm.

A friction stir weld (Fig. 12) is formed by plunging a rotat-ing shouldered pin tool with a pin length slightly lessthan the weld depth required, into the faying faces untilthe tool shoulder is in intimate contact with the work sur-face and then moving the work against the pin, or vice-versa. The rotating pin within the workpiece friction heatsthe metal and produces a plasticised tubular shaft ofmetal around the pin. As the pin is moved in the direc-tion of welding the leading face of the pin, assisted bya special pin profile, forces plasticised material to theback of the pin whilst applying a substantial forging forceto consolidate the weld metal.

Further developments within this process have demon-strated that plate thicknesses up to 75 mm in 6082 alu-minium alloy can be welded using two passes, (Fig. 13).Also other metals such as copper, lead, titanium andmagnesium have been successfully welded, althoughmore studies, particularly to identify the tool material,are needed for steels, nickel based alloys and titaniumalloys.

7 FRICTION PROCESSING OF METALS

The methods that can be considered to: recycle swarf,make new alloys (monolithic and composite), orreprocess existing alloys to improve metallurgical andmechanical properties include: friction extrusion(Fig. 14). Three variants are shown, the first Fig. 14(a)demonstrates how a rod or billet of material is rotatedwhile being forced through a die, the second Fig. 14(b)illustrates how a solid rod can be clad from a tube, wherethey are rotated and forced through a die and the third14(c) shows powder or swarf enclosed in a cylinder,

which is then rotated and forced onto the top of a die tocreate frictional heating and material consolidation forsubsequent extrusion of through a hole in the die, FHPP(Fig. 3), and friction transformation hardening(Fig. 15). In this illustration a non-consumable wheel isrotated at peripheral velocities of ~60-80 mpec while itis forced into contact with and moved across the sub-strate to be processed in this example a shaft, and fric-tion stir processing (a variant of friction stir welding).

To demonstrate the effectiveness of FHPP to reprocesscopper based alloys the reader is referred to Fig. 16,where cast nickel-Al-bronze and cupro-nickel have beenprocessed. Quite clearly the as-cast microstructure hasbeen completely eliminated and replaced by fully dense,fine grained and heavily work microstructures.

8 NON-ROTARY MOTION FRICTIONWELDING

Within this family of processes lies orbital (Fig. 17), heretwo parts are initially aligned on their common axes,then they are rotated in the same direction at the samespeed, to generate the relative orbital motion, their axesare displaced to provide the orbit amplitude while thefriction welding force is applied. The heating sequenceis terminated by returning the orbit amplitude to zero(axes re-aligned) then the forge force is applied. Linear(Fig. 18), the illustration shows one part being set intolinear reciprocating motion then it being forced onto the

Fig. 12. Friction stir welding.

Fig. 13. Friction stir welded 75 mm 6082 Al alloyplate with two passes.

FRICTION PROCESSING TECHNOLOGIES 7

stationary part to create frictional heat for plastising thematerials. To terminate the weld, the amplitude is thenreduced to zero and the forge force applied. The rela-tive velocity is determined by the combination of recip-

rocating frequency and amplitude of movement, andangular friction welding. Such systems further extendthe range of components that can be friction welded toinclude non-round, complex and asymmetric geome-tries. For example, the aero-engine industry has shownconsiderable interest in linear friction welding, particularlyfor the fabrication and repair of turbine wheels. Theprocess has proved very successful for welding titaniumand nickel based alloys, widely used in this industry.Today, the opportunity to use linear friction welding tomanufacture components in aluminium and steel alloysis receiving active consideration within the automotiveindustry.

a) Bar or rod to wire b) Cladding

c) Power of swarf to wire

Fig. 14. Friction extrusion.

Fig. 15. Friction transformation hardening.

8 FRICTION PROCESSING TECHNOLOGIES

9 CONCLUDING REMARKS

Of significant importance to remember is that whetherfriction welding or reprocessing, everything takes placein the solid phase, that is, no macroscopic melting. Thebenefits yielded therefore include: refined microstruc-tures, no cast microstructures, no porosity, little distor-tion, the ability to join a wide range of dissimilar mater-ial combinations, the list is endless! Developers close tothe technologies of friction are aware, but it would

appear that the message is just not reaching the widerindustrial population. This paper has attempted toredress that situation.

REFERENCES

1. Vill, V.I, “Friction welding of metals” translated fromRussian and published by the American Welding Society,1962.

2. Wang, K.K., “Friction welding” Welding Research CouncilBull (204), April 1975.

3. Andrews, R.E. & Mitchell, J.S., “Underwater Repair byFriction Stir Welding”, Metals & Materials, pp.796-797,December 1990.

AH1023(a) As-cast cupro-nickel chrome alloy x 100.

AH1023A(b) Re-processed cupro-nickel chrome alloy x 100.

AH1021(c) As-cast nickel-aluminium bronze alloy x 100.

AH1022(d) Re-processed nickel-aluminium bronze x 100.

Fig. 16. Friction hydro pillar processing of copper alloys.

Fig. 17. Orbital friction welding.

Fig. 18. Linear friction welding.

FRICTION PROCESSING TECHNOLOGIES 9

4. Nicholas, E.D., “Radial friction welding” Welding Journal,July 1983.

5. Nicholas, E.D. & Thomas, W.M., Weld Journal No. 5, pp.17-27, 1986.

6. Dobrik, A.V. & Gellerman, M.N.: “Increasing the life ofhot rolling rolls by friction hardening”, KarpenkoPhysicomechanical Institute, Academy of Sciences of theUkrainian SSR, 8 May 1982.

7. Thomas, W.M. et al., “Friction stir butt welding”International Patent Application No. PCT/GB92, GB PatentApplication No. 9125978.8, 6 December 1991.

8. Dawes, C.J., “Seam welding aluminium sheet and plateusing the friction stir welding process”, Proc. Of 6th

International Symposium JWS, 1996, Nagoya.

9. Christner, B.K. & Sylva, G.D., “Friction stir weld devel-opments for aerospace applications”, InternationalConference on Advances in Welding Technology Joining ofHigh Performance Materials, 6-8 November 1996,Columbus, Ohio, USA.

10. Thomas, W.M. & Nicholas, E.D., “Friction takes theplunge”, TWI, Connect, September 1993.

11. Thomas, W.M., Nicholas, E.D. & Jones, S.B., “Third-body friction joining”, Connect, April 1994.

12. Klopstock, H. & Needlands, A.R., British Patent No.572789, IC B 23 K 20/12, “An improved method of joiningor welding metals”.

13. Thomas, W.M. & Nicholas, E.D., “Friction extrusiontechnology”, TWI, Connect, March 1992.

14. Thomas, W.M., “Recycling of non-ferrous powder andmachine swarf by friction extrusion – an introduction”,International Conference – Recycling of Materials,Amsterdam, 19-21 October 1994.

15. Nicholas, E.D., “Friction Hydro Pillar Processing”Advances in Welding Technology, 11th Annual NorthAmerican Welding Research Conference, 7-9 November1995.

16. Nicholas, E.D., “Linear friction welding” DVS BerichteConference on Flash Butt and Friction Welding with AlliedProcesses. Stuttgart, December 1991, pp.18-24.

17. Searle, J., “Friction welding non-circular componentsusing orbital motion” Welding & Metal Fabrication, Vol. 39,No. 8, August 1971, pp. 294-297.

18. “Aluminium brazing” Heat Treating, Vol. 6, No. 1,January 1975, pp. 23-25.

19. Dittman, B., “Friction brazing – an alternative to frictionwelding” Schweisstechnik (Berlin), Vol. 26, No. 2, February1976, p. 80.

20. “Bonding aluminium” British Patent 1 380 558, filed 29October 1970.