Friction stir welding broadens applications base

Friction stir welding is a machine tool based mechanical joining process that is saving costs and weight for a steadily expanding range of production applications, reports Mike Page.

Today, friction stir welding is most commonly used in the shipbuilding and land-based transportation industries for joining 5000 and 6000 series silicon and magnesium alloyed aluminium alloys.

Generally speaking, the process is used to join those non-ferrous alloys - such as 2000 to 7000 aluminium grades and copper - that are difficult to join with fusion welding processes.

In the laboratory, development is underway to develop friction stir welding (FSW) for joining ferrous metals and high-grade heat- and abrasion-resistant ferrous and non-ferrous alloys.

The attraction of the technique from a design and production standpoint - when compared with available fusion welding processes - is to produce a lighter, more consistent weld joint.

That is, no fillets, a restricted heat affected zone (HAZ) and a 'one-hit' process.

FSW is regarded as a very energy-efficient machine tool technology.

It produces no light or radiation.

Operators need only wear the same safety gear as when operating machine tools.

There is generally an absence of fumes - even when welding chrome-based alloys.

FSW works by rotating a 'bobbin' at high speed.

The bobbin is made of a heat- and abrasion-resistant material.

It is applied and plunged into, say, to a lap joint.

The friction generates local heat to locally fuse the joint area.

The bobbin is so designed to encourage downward pressure of the locally melted metal.

The bobbin also has a shoulder or flange that is maintained flush to the surface of the weldment, to contain the hot metal and ensure a smooth weld surface.

Once fusion has begun, the bobbin is tracked along the joint until the weld joint is complete.

As well as lap joints, the process will do butt- and 'T' joints quite readily.

Commercial applications are in making long, straight or circumferential joints, such as in tanks, vessels, pipe, ship decking, railway rolling stock cantrail and sidewall construction.

Two companies in Sweden and Japan, reports The Welding Institute (TWI) in Abington, Cambridge, UK, are producing heat sinks from aluminium and copper.

These require non-linear friction stir welds on flat panels.

TWI has demonstrated transportable FSW machine to weld concavely curved ship panels.

Several machine builders and welding machine producers have built FSW machines for the aerospace and automotive industries.

In terms of 'thickest' and 'thinnest' FSW weldments, the process has welded commercially 50mm thick aluminium and copper plate using production machines with a capacity of up to 16m length.

So far, the thinnest FSW welds have been made by Airbus using 0.3 to 0.4mm thick foils in laminate construction.

As an aside, the widest rolled aluminium sheet in these thicknesses is 1.2m.

So FSW is being used to produce larger single sheets through splicing.

The smallest aluminium tubes welded with FSW are 25mm diameter, used in the automotive industry.

The shortest FSW welds made are 'spot' or 'linear spot'.

The latter is where the weld head is traversed a few mm to produce a short elongated 'spot'.

On a large scale, the aluminium extruders use spot-FSW to join billets before extruding.

On a smaller scale, the automotive industry is investigating spot-FSW for aluminium panel welding, using industrial robots.

The machines - the first FSW joints were made using modified bed-type millers.

The larger machines are akin to plano-miller structures, in that a moving bed or a moving portal gantry is used to track the FSW head along 2-axis joint paths.

Such machines are now used in shipbuilding and railway rolling stock structures.

Five-axis machines are under development for producing contoured FSW joints.

In terms of reliability and repeatability, Boeing in the US, in the FSW of Delta II and IV rocket fuel tanks, made 2.5km of defect-free welds.

In comparison with fusion welding techniques, the company reported a 71% reduction in weld cycle time and 81% reduction in labour costs.

Cost per foot of FSW weld was $0.14 against $24 on existing circumferential and bolted joints.

Welding speeds - TWI says that FSW equals MIG in butt-welding 6mm thick aluminium.

FSW is slower in thinner materials and faster in thicker materials.

For example, in 1mm thick butt-weld joint, FSW weld speed is 6m/min, but the R and D guys are aiming at 10m/min.

Remember with thick section joints, FSW completes the weld in one pass.

MIG would need a multi-pass technique.

In thin sections, laser welding is faster, but FSW weld quality is better.

Major FSW advantages, as listed by Dave Nicholas, TWI, include: * Heat generated only in the weld area.

* No high electrical currents.

* Modest stored energy in the system.

* No inefficient energy conversion systems.

* Limited need for heat extraction in weld or environment.

Of interest, said Nicholas, when butt welding 6mm 6082 grade aluminium alloy plate, FSW needed 0.3kJ/mm of weld length (typically 2.5kW gross power at 500mm/min weld speed) against 1.7kJ/mm (8.6kW and 300mm/min) for MIG and 3.1kJ/mm (52kW and 1000mm/min) for a laser.

Limitations of FSW - I mentioned weld speeds.

There are also, according to Nicholas: * Welding speeds must be rigidly clamped.

* Backing bar required.

* Keyhole at the end of a linear weld * Can not make Ofillet1-type welds - though this is being investigated in R and D.

* Workpieces have to be rigidly clamped.

Aerospace - At an Eurostir meeting, Stephen Kallee, TWI, outlined examples of worldwide progress in FSW applications.

Kallee said that FSW offers tremendous potential for the low-cost joining of lightweight aluminium airframe structures for large civil aircraft.

Researchers at Airbus Deutschland see great potential for using FSW for skin-to-skin fuselage connections.

It could lead to a reduction of cost and weight through improved joint quality and new fuselage design strategies.

In the USA, the Phantom Works of the Boeing Company is investigating FSW of thin lap, butt and 'T'-joints as well as thick section butt joints for various aircraft, missile and space applications.

Kallee reported that there is a strong desire for FSW joint configurations on curvilinear paths to produce complex sub-assemblies.

Boeing has already demonstrated curvilinear FSW of a complex landing gear door and the FSW of sandwich assemblies for a fighter aircraft fairing.

The latter involved FSW of fine 'T'-joints, which have since been flight-tested.

Boeing has also started up the production FSW of non-structural parts for its civil aircraft.

Also in the USA, the Eclipse Aviation Corporation of Albuquerque, New Mexico is using FSW to replace traditional riveting and bonding processes.

Eclipse could be the first major user of FSW in volume airspace applications.

The corporation1s objective is to dramatically reduce assembly time and costs.

The first flights were carried out last year (2002).

In June 2002, Eclipse announced that the US Federal Aviation Administration (FAA) had approved FSW specification for the Eclipse 500 business jet, so allowing the corporation to use FSW on a production basis.

Automotive industry - Sapa in Finspang, Sweden, uses two FSW heads to simultaneously weld together hollow aluminium alloy extrusions from both sides for seat frame assemblies.

FSW is carried out on the same site as the extrusion press, using a carousel (dial) type system and OC1-frame type machine structures supporting the welding heads.

Simpler extrusion profiles are therefore readily FSW fabricated into complex assemblies.

Nearby in Norway, Hydro Aluminium in Havik is using an FSW installation to lap or butt weld cast or pressed inner wheel sections to wheel rims.

Weight saving is achieved, as a weld joint flange is not needed (as required for resistance spot welding), so simplifying the press tool, as well as saving weight and shortening process time.

Simmons Wheels in Alexandria, Australia, is using FSW seam-welded cold-roll-formed (CRF) aluminium tube, cut into bands, from which to CRF wheel rims.

Kallee drew delegates' attention to the file being built up in the Eurostir project, concerning lightweight automotive structures.

There is data, for example, on FSW tailored aluminium blanks as it is possible to achieve a smoother transition between different thicknesses when compared with laser welding (C02 or YAG).

Kallee also said that several automotive companies and suppliers are evaluating FSW for the manufacture of components to be used in Ospace-frame1 vehicle structures.

Dr.Jorge dos Santos, GKSS Forschungszentrum, Geesthacht, Germany, drew delegates' attention to the potential of FSW spot welding.

The research institute has been investigating FSW spot welding heads used in various industrial robots, such as the 'tripod' and conventional articulated arm types.

Work has included the development of an FSW aluminium fuel tank and the FSW splicing of tailored blanks.

Some of the work has been a joint effort between GKSS and Audi.

Similar work is being carried out between Mazda and Hitachi in Japan, said Dr.Santos.

He reported that FSW lap joints were used to join the tank ends to the bodies, essentially to show what could be done, and the fixturing requirements needed, as well as considerations in joint design.

One of the delegates asked about the formability of FSW tailored blanks.

Dr.Santos replied: "Excellent - if you saw what they (the researchers) did to this metal (in terms of subjecting weldments to metalforming deformation), you would not believe it!" He inferred that laser-welded blanks could not compete against FSW on formability.

Other applications - Kallee of TWI mentioned heat sinks, single skin panels, tubes and thick plates, fuel tanks, hollow panels, housings, replacing riveted joints and 'friction stir processing' (using FSW process to locally modify metallurgical structures - such as on valve facings).

Shipbuilding - Hydro marine Aluminium, Haugesund, Norway, is using FSW to prefabricate light aluminium deck panels by sandwiching CRF corrugated sections between skins and using linear FSW welds to fabricate the sandwich.

In the UK, Aluminium Shipbuilders on the Isle of Wight is fabricating hovercraft bulkhead and deck panels using FSW.

Rolling stock - Panels for passenger rail vehicles are fabricated with FSW, such as roof panels for Alstom made by Hydro Marine Aluminium.

These panels are up to 3m wide and 14.5m long.

Similar panels are being used in the Alstom OPendolino1 trains now being used in the UK.

Other applications include FSW fabrication of electric motor housings and Bang and Olufsen loudspeaker housings.

Linear friction welding - in this process, one component is pressed against another and oscillated at speed until a fusion temperature is reached at the joint interface, then Oforged1 or pressed to complete the weld.

It is similar in effect to the traditional rotary friction welding operation that also uses a Oforge1 phase to complete the weld.

MTU in Germany is using linear friction welding (LFW) to fabricate blisks (blade-integrated-disk) and repair used blisks.

The process considerably reduces machining when compared with the traditional way of machining a blisk from a solid billet.

One drawback of LFW had been the high capital cost of LFW machines, but at the end of 2001; the LinFric project at the TWI produced a prototype low-cost LFW machine using a hydraulic oscillator to impart the linear oscillation.

The oscillator design was based on oscillators widely used in metal fatigue testing rigs.

At Eurostir, a number of delegates from the aerospace industry voiced interest in the potential of FSW and LFW (linear friction welding) to fabricate aircraft structural panels, stiffeners, stringers and spars.

Normally these structural parts are traditionally machined from solid billets and slabs.

Observations - FSW and LFW are machine tool based welding processes.

Both processes have the potential to save on manufacturing and assembly costs, when compared with traditional forming, joining and machining technologies.

Both processes offer consistently defect-free welding when set-up and process requirements are satisfied and maintained.

At the Eurostir meeting held at TWI, it was amusing that while delegates stood around in the lecture theatre assembly area drinking coffee and eating pastries, they were able to watch a quiet, orderly, and highly environmentally friendly demonstration of FSW being given on a free-standing universal milling machine wheeled in for the purpose! Contacts - * TWI (research -UK) - www.twi.co.uk *Eurostir - www.eurostir.co.uk * Crawford Swift (Machine tool builder - UK) - www.crawfordswift.co.uk *GKSS (Research - Germany) - (e-mail) jorge.dos.santos@gkss.de * Fatronik (Research - Spain) - www.fatronik.com *Smart Technology Group (Design - UK) - www.smarttecgroup.com *ESAB (Machine tool builder - Sweden) - www.esab.com *DanStir ApS (subcontractor FSW service - Denmark) - www.DanStir.com * Triton Tooling (FSW bobbin tooling - design and research - UK) - www.tritontooling.co.uk * MagicScope (subcontract fabricator - UK) - www.magicscope.co.uk (Mike Page, Cambridge, September 2003).

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