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Friction Stir Welding - Process
advantages
The process advantages result from the fact that the FSW process (as all
friction welding of metals) takes place in the solid phase below the melting
point of the materials to be joined. The benefits therefore include the
ability to join materials which are difficult to fusion weld, for example 2000
and 7000 aluminium alloys. Other advantages are as follows:
- Low distortion, even in long welds
- Excellent mechanical properties as proven by fatigue, tensile and bend
tests
- No fume
- No porosity
- No spatter
- Low shrinkage
- Can operate in all positions
- Energy efficient
Friction stir welding can use existing and readily available machine tool
technology. The process is also suitable for automation and adaptable for
robot use. Its main advantages are:
- Non-consumable tool
- One tool can typically be used for up to 1000m of weld length in 6000
series aluminium alloys
- No filler wire
- No gas shielding for welding aluminium
- No welder certification required
- Some tolerance to imperfect weld preparations - thin oxide layers can be
accepted
- No grinding, brushing or pickling required in mass production
The limitations of the FSW process are being reduced by intensive research
and development. However, the main limitations of the FSW process are at
present:
- Welding speeds are moderately slower than those of some fusion welding
processes (up to 750mm/min for welding 5mm thick 6000 series aluminium alloy
on commercially available machines)
- Workpieces must be rigidly clamped
- Backing bar required
- Keyhole at the end of each weld
Friction Stir Welding - Materials and
thicknessesFriction stir welding can be used for joining many
types of materials and material combinations, if tool materials and designs
can be found which operate at the forging temperature of the workpieces. Up to
the present day, TWI has concentrated most of its efforts to optimising the
process for the joining of aluminium and its alloys. A major Group Sponsored
Project undertaken for TWI's Industrial Members demonstrated that the
following aluminium alloys could be successfully welded to yield reproducible,
high integrity welds within defined parametric tolerances.
- 2000 series aluminium (Al-Cu)
- 5000 series aluminium (Al-Mg)
- 6000 series aluminium (Al-Mg-Si)
- 7000 series aluminium (Al-Zn)
- 8000 series aluminium (Al-Li)
This work primarily investigated
welding of wrought and extruded alloys. However, subsequent studies have shown
that cast to cast, and cast to extruded (wrought) combinations, in similar and
dissimilar aluminium alloys are equally possible.
Continuing development of the FSW tool, its design and materials have
allowed preliminary welds to be successfully produced in:
- Copper and its alloys
- Lead
- Titanium and its alloys
- Magnesium alloy, Magnesium to aluminium
- Zinc
- MMCs based on aluminium (metal matrix composites)
- Other aluminium alloys of the 1000 (commercially pure), 3000 (Al-Mn) and
4000 (Al-Si) series
- Plastics
- Mild steel Single pass butt joints with aluminium alloys have been made
in thicknesses ranging from 1.2mm to 50mm without the need for a weld
preparation. Thicknesses of up to 100mm can be welded using two passes, one
from each side, with 6082 aluminium alloy. Parameters for butt welding of
most aluminium alloys have been optimised in a thickness range from 1.6mm to
10mm. Special lap joining tools have also been developed for aluminium with
thicknesses of 1.2mm to 6.4mm.
Friction Stir Welding - Superior weld
qualityThe repeatable quality of the solid-phase welds can improve
existing products and lead to a number of new product designs previously not
possible. Welds with the highest quality can be achieved by friction stir
welding. The crushing, stirring and forging action of the FSW tool produces a
weld with a finer microstructure than the parent material. The weld metal
strength can be, in the as welded condition, in excess of that in the
thermo-mechanically affected zone. In the case of annealed materials in the O
condition, tensile tests usually fail in the parent material well away from
the weld and heat affected zone, as the following results show:
Tensile test data for friction stir welds in 6mm thick aluminium alloy
5083 in the O condition (average of three samples taken over 400mm weld
length)
| Test |
0.2% Proof Stress N/mm2 |
Max. Stress N/mm2 |
Elongation % |
Failure location |
| Cross weld tensile |
141 |
298 |
23.0 |
In parent material |
| Parent plate tensile |
148 |
298 |
23.5 |
Not applicable |
The weld properties of fully hardened (cold worked or heat treated) alloys
can be further improved by controlling the thermal cycle, in particular by
reducing the annealing and overageing effects in the thermo-mechanically
affected zone, where the lowest hardness and strength are found after welding.
For optimum properties, it would seem that, for the latter, a heat treatment
after welding is the best choice, although it is recognised that this will not
be a practical solution for many applications. Further studies are necessary
and are currently being conducted to explain the complex microstructural
aspects of friction stir welds and their corrosion properties.
Fatigue tests on friction stir welds made from 6mm thick 5083-O and 2014-T6
were conducted in tension. The preliminary results are quite exceptional in
that they show little scatter and are far better than those of fusion welding
processes such as GTA and MIG. The fatigue performance of friction stir welds
in alloy 5083-O is comparable to that of the parent material when tested using
a stress ratio of 0:1. Despite the fact that the fatigue tested friction stir
welds were single pass from one side, the results have substantially exceeded
BS 8118 class 35 and the European design recommendation ECCS B3 for fusion
welded joints.
Microstructure ClassificationThe first attempt at classifying
microstructures was made by P L Threadgill (Bulletin, March 1997). This work
was based solely on information available from aluminium alloys. However, it
has become evident from work on other materials that the behaviour of
aluminium alloys is not typical of most metallic materials, and therefore the
scheme cannot be broadened to encompass all materials. It is therefore
proposed that the following revised scheme is used. This has been developed at
TWI, but has been discussed with a number of appropriate people in industry
and academia, and has also been provisionally accepted by the Friction Stir
Welding Licensees Association. The system divides the weld zone into distinct
regions as follows:
A. Unaffected material B. Heat affected zone (HAZ) C.
Thermo-mechanically affected zone (TMAZ) D. Weld nugget (Part of
thermo-mechanically affected zone)
Unaffected material or parent metal: This is material
remote from the weld, which has not been deformed, and which although it may
have experienced a thermal cycle from the weld is not affected by the heat in
terms of microstructure or mechanical properties.
Heat affected zone (HAZ): In this region, which clearly
will lie closer to the weld centre, the material has experienced a thermal
cycle which has modified the microstructure and/or the mechanical properties.
However, there is no plastic deformation occurring in this area. In the
previous system, this was referred to as the "thermally affected zone". The
term heat affected zone is now preferred, as this is a direct parallel with
the heat affected zone in other thermal processes, and there is little
justification for a separate name.
Thermo-mechanically affected zone (TMAZ): In this region,
the material has been plastically deformed by the friction stir welding tool,
and the heat from the process will also have exerted some influence on the
material. In the case of aluminium, it is possible to get significant plastic
strain without recrystallisation in this region, and there is generally a
distinct boundary between the recrystallised zone and the deformed zones of
the TMAZ. In the earlier classification, these two sub-zones were treated as
distinct microstructural regions. However, subsequent work on other materials
has shown that aluminium behaves in a different manner to most other
materials, in that it can be extensively deformed at high temperature without
recrystallisation. In other materials, the distinct recrystallised region (the
nugget) is absent, and the whole of the TMAZ appears to be recrystallised.
This is certainly true of materials which have no thermally induced phase
transformation which will in itself induce recrystallisation without strain,
for example pure titanium, b titanium alloys, austenitic stainless steels and
copper. In materials such as ferritic steels and a-b titanium alloys
(e.g.Ti-6Al-4V), understanding the microstructure is made more difficult by
the thermally induced phase transformation, and this can also make the
HAZ/TMAZ boundary difficult to identify precisely.
Weld Nugget: The recrystallised area in the TMAZ in
aluminium alloys has traditionally been called the nugget. Although this term
is descriptive, it is not very scientific. However, its use has become
widespread, and as there is no word which is equally simple with greater
scientific merit, this term has been adopted. A schematic diagram is shown in
the above Figure which clearly identifies the various regions. It has been
suggested that the area immediately below the tool shoulder (which is clearly
part of the TMAZ) should be given a separate category, as the grain structure
is often different here. The microstructure here is determined by rubbing by
the rear face of the shoulder, and the material may have cooled below its
maximum. It is suggested that this area is treated as a separate sub-zone of
the TMAZ.
Friction Stir Welding - Applications
Shipbuilding and marine industriesThe shipbuilding and marine
industries are two of the first industry sectors which have adopted the
process for commercial applications. The process is suitable for the following
applications:
- Panels for decks, sides, bulkheads and floors
- Aluminium extrusions
- Hulls and superstructures
- Helicopter landing platforms
- Offshore accommodation
- Marine and transport structures
- Masts and booms, e.g. for sailing boats
- Refrigeration plant
Aerospace industryAt present the aerospace industry is welding
prototype parts by friction stir welding. Opportunities exist to weld skins to
spars, ribs, and stringers for use in military and civilian aircraft. This
offers significant advantages compared to riveting and machining from solid,
such as reduced manufacturing costs and weight savings. Longitudinal butt
welds and circumferential lap welds of Al alloy fuel tanks for space vehicles
have been friction stir welded and successfully tested. The process could also
be used to increase the size of commercially available sheets by welding them
before forming. The friction stir welding process can therefore be considered
for:
- Wings, fuselages, empennages
- Cryogenic fuel tanks for space vehicles
- Aviation fuel tanks
- External throw away tanks for military aircraft
- Military and scientific rockets
- Repair of faulty MIG welds
Railway industryThe commercial production of high speed trains made
from aluminium extrusions which may be joined by friction stir welding has
been published. Applications include:
- High speed trains
- Rolling stock of railways, underground carriages, trams
- Railway tankers and goods wagons
- Container bodies
Land transportationThe friction stir welding process is currently
being experimentally assessed by several automotive companies and suppliers to
this industrial sector for its commercial application. A joint EWI/TWI Group
Sponsored Project is investigating representative joint designs for automotive
lightweight structures. Potential applications are:
- Engine and chassis cradles
- Wheel rims
- Attachments to hydroformed tubes
- Tailored blanks, e.g. welding of different sheet thicknesses
- Space frames, e.g. welding extruded tubes to cast nodes
- Truck bodies
- Tail lifts for lorries
- Mobile cranes
- Armour plate vehicles
- Fuel tankers
- Caravans
- Buses and airfield transportation vehicles
- Motorcycle and bicycle frames
- Articulated lifts and personnel bridges
- Skips
- Repair of aluminium cars
- Magnesium and magnesium/aluminium joints
Construction industryThe use of portable FSW equipment is possible
for:
- Aluminium bridges
- Facade panels made from aluminium, copper or titanium
- Window frames
- Aluminium pipelines
- Aluminium reactors for power plants and the chemical industry
- Heat exchangers and air conditioners
- Pipe fabrication
Electrical industryThe electrical industry shows increasing interest
in the application of friction stir welding for:
- Electric motor housings
- Busbars
- Electrical connectors
- Encapsulation of electronics
Other industry sectorsFriction stir welding can also be considered
for:
- Refrigeration panels
- Cooking equipment and kitchens
- White goods
- Gas tanks and gas cylinders
- Connecting of aluminium or copper coils in rolling mills
- Furniture
Friction Stir Welding - EquipmentWith
ever increasing interest in the process from many of the aforementioned
industrial sectors, there is the need for continual process development and
fabrication of prototype assemblies. Consequently, to meet these needs
dedicated equipment has to be developed. A range of modified machine tools now
exists at TWI which are briefly reviewed below:
Modular machine FW22 to weld large size specimens A
laboratory machine was built in October 1996 to accommodate large sheets and
to weld prototype structures. The modular construction of FW22 enables it to
be easily enlarged for specimens with even larger dimensions.
- Sheet thickness: 3mm-15mm aluminium
- Maximum welding speed: up to 1.2m/min
- Current maximum sheet size: 3.4m length x 4m width
- Current maximum working height: 1.15m
Moving gantry machine FW21 The purpose
built friction stir welding machine FW21 was built in 1995. This machine uses
a moving gantry, with which straight welds up to 2m long can be made. It was
used to prove that welding conditions can be achieved which guarantee constant
weld quality over the full length of long welds.
- Sheet thickness: 3mm-15mm aluminium
- Maximum welding speed: up to 1.0m/min
- Current maximum sheet size: 2m length x 1.2m width
Heavy duty Friction Stir Welding machines FW18 and FW14Two existing
machines within TWI's Friction and Forge Welding Group have been modified
exclusively to weld thick sections by FSW. The following thickness range has
been experimentally investigated but the machine are not yet at their limits.
- Sheet thickness: 5mm-50mm aluminium from one side
10mm-100mm
aluminium from two sides 5mm thick titanium from one side
- Power: up to 22kW
- Welding speed: up to 1m/min
High rotation speed machine FW20For welding thin aluminium sheets TWI
equipped one of its existing machines with an air cooled high speed head which
allows rotation speeds of up to 15,000rev/min.
- Sheet thickness: 1.2mm-12mm aluminium
- Maximum welding speed: up to 2.6m/min, infinitely variable
Friction Stir Welding demonstrator FW16TWI's small transportable
machine produces annular welds with hexagonal aluminium alloy discs. It has
been exhibited on fairs in USA, Sweden, Germany, and the United Kingdom in
recent years. It is an eye catcher which enables visitors to produce their
first friction stir weld themselves. It can be operated with 110V or 220V-240V
and has been used by TWI and its member companies to demonstrate the process.
Other machines
Portable CRC machineTWI commissioned a prototype machine which was
designed and manufactured by their CRC partners at the Department for
Mechanical Engineering of the University of Adelaide, Australia. This machine
can be carried and aligned by two operators without the use of a crane or
other lifting device. It has been used to weld curved sheets under site
conditions in a shipyard.
Commercially available FSW machines Purpose built
friction stir welding machines have been designed, manufactured, and
commissioned. One of them, which is installed at Marine Aluminium Aanensen,
Norway, is capable of making 16m long welds. It was built by ESAB in Laxå,
Sweden and is used for the mass production of panels which are made by joining
extruded profiles. The machine and the welding procedure have been approved by
Det Norske Veritas and Germanischer Lloyd. Several shorter machines, some of
them with CNC systems of up to 5 axes, have been built for experiments and for
the production of prototype parts. ESAB demonstrated FSW with a welding speed
of 750mm/min in 5mm thick aluminium (6000 series) at the 14th International
Welding Fair in Essen. Even friction stir welds with very rigid robots were
successful and demonstrated the possibility of non-linear welds.

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