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News

Cti are researching the use of new Friction Stir Welding of castings and associated parts

Castings Technology International : 18 May, 2006  (New Product)
Under a project led by TWI, and including Cti, Corus, AMRC, Technicut and Yorkshire SMEs as partners, Cti are researching the use of Friction Stir Welding of castings and associated parts. FSW has only been around since 1991 but, following development of the process by its inventors at Cambridge based TWI Ltd, it has rapidly been implemented around the world in many applications within the aerospace, shipbuilding, rail and automotive industries. The over-arching project, named FriSC, the Friction Stir welding Collaboration, aims to examine further application of the technology.
Under a project led by TWI, and including Cti, Corus, AMRC, Technicut and Yorkshire SMEs as partners, Cti are researching the use of Friction Stir Welding of castings and associated parts. FSW has only been around since 1991 but, following development of the process by its inventors at Cambridge based TWI Ltd, it has rapidly been implemented around the world in many applications within the aerospace, shipbuilding, rail and automotive industries. The over-arching project, named FriSC - the Friction Stir welding Collaboration, aims to examine further application of the technology.

Generally results have shown a return to as-cast properties in the weld area and heat affected zone, meaning that heat treatment may be required to restore the mechanical properties. The exception to this is ductile iron, which has shown some interesting results and is under further study to understand the phenomena.

FSW utilises the movement of a rotating tool along a joint line, with the heat generated by the friction softening the material. The tool consists of a pin that penetrates the workpiece under high force, and a shoulder section that prevents hot material being expelled from the weld area. The rotation of the tool sweeps the softened material from the front of the tool to the back, effectively creating a solid state weld between the two individual components.

The success of the process in joining aluminium, magnesium and copper alloys is well documented, whilst the application of FSW to higher temperature materials, such as steels, irons, and titanium and nickel alloys, is under development, along with advanced tool materials.

The advantages of the friction stir process is that it forms a weld by intense mechanical deformation, thus producing a refined grain structure, excellent mechanical properties and an absence of porosity. The process can easily be automated and consumes a smaller amount of energy when compared to other welding processes. Welds can be produced in a single pass and no edge profiling or cleaning is necessary prior to welding. For most metals, shielding gas or filler wire are not needed and there is no weld spatter, fume or electromagnetic radiation given off.

The FriSC Project
The work packages currently under investigation are:
Modelling of FSW for process optimisation;
FSW of thin and thick section aluminium alloys;
FSW of Mg, Cu, Ti and Ni alloys;
FSW of steels;
Application of FSW to castings;
Friction stir processing;
FSW of dissimilar materials;
FSW for SMEs.

Cti are mainly involved in the work package on castings. Initial activity at Cti has been to produce a range of cast test plates to evaluate the FSW process when applied to cast materials. To date Cti has produced test plates from a carbon manganese steel, LM 25 aluminium, magnesium, AB2 aluminium bronze and spheroidal graphite iron.Titanium test plates and die steel are currently in production and the project will also address the welding of dissimilar cast parts.

These test plates have been friction stir welded at TWI, following which Cti have investigated the results by determining microstructural and mechanical properties across the resultant weld.

The welds produced have shown good joint with a relatively small heat affected zone. One significant feature was the ability to produce a sound partial weld in S.G cast iron, although further process development is required with this material, and this is in progress.

Work on castings provided by some Cti members has started by initially running a single weld pass through the cast material, and these welds have been limited to longitudinal or circumferential passes. These castings are also being examined for microstructural and mechanical properties.

Further assessments on actual castings will continue. Cti would like to hear from any foundries who would like to learn more about the possibilities of applying the process to their products, current or future, and who may see an application of this technology to their castings and who would like to submit sample castings for evaluation. To obtain the best results from FSW, some design changes to the cast components may be required, but preliminary welding trials can usually be performed on existing castings.

Current areas of interest are:
Welding of castings considered unweldable by current techniques;
Joining of two cast parts too complex to produce as a single component;
Welding of external features onto castings;
Weld repair of surface and through wall defects;
Repair of leaks on pressure tight castings;
Welding of castings to wrought products to make complete fabricated products.
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