A stirring work of friction

Alan Mucklow, group product manager Europe, says that friction-stir welding, in combination with CNC milling technology, has the potential to make welded joints a serious option for manufacturers – and also offer enormous production benefits.
Inter-metallic joints and low melting point alloys have traditionally been joined with riveting and other joining technologies, rather than fusion welding. Traditional fusion welding methods, such as arc welding, electric resistance welding and oxy-fuel welding – whilst popular – have clear limitations, most notably their inability to join different types of metals and operate with low melting point alloys.

The result is that many manufacturers, particularly in the aerospace industry, have resorted to other jointing technologies, such as riveting, to cope with the requirement to reduce weight by using ever lighter materials, such as aluminium, titanium and fibre-reinforced thermoplastics.

However, in recent years, ground-breaking work by the Airbus Group has resulted in a significant breakthrough for another type of joining, which offers the potential for joining different metals and low-melting-point alloys, while also offering the possibility of significant quality and production improvements.

Rub up the right way

The DeltaN FS, friction-stir welding technology, which has been developed by Airbus Group Innovations, the research and technology arm of Airbus Group, is a robust joining process that uses frictional heat and process forces that result in a strong and robust joint.

The key design innovation has been the DeltaN FS welding tool which includes a non-rotating shoulder and rotating pin which plunges into the two workpieces generating frictional heat. This leads to a soft, plasticised area near the welding tool which is mechanically formed to produce a joint.

The actual welding process is very simple, fully automated and can be completed in a single pass with no pre-weld preparation. The tool moves along the joint line, stirring and depositing the plasticised material on its trailing edge creating a solid-state weld.

No filler material is required during the process, which means fewer defects and joint damages and low joint or weld distortion.

The non-rotating shoulder itself optimises the heat impact and has no influence on the weld zone. Overall the DeltaN FS friction stir welding method requires approximately 20-30% lower downward process forces as compared to conventional friction stir welding.

Crucially, the metal is not melted during the process, which improves the mechanical properties of the weld and eliminates weld spatter. The result is a superior surface finish combined with robust joints, which can be of similar metals or inter-metallic joints. The process is also significantly greener and safer, with low energy input, no fumes, gases, radiation, plasma or X-rays.

The benefits to manufacturing are equally as profound. The DeltaN FS Friction-stir welding process is able to weld a curved surface with a very small radius (<15mm) and complete a variety of joints, including a butt joint, lap joint and T-joint. The process is also capable of joining workpieces of different thicknesses together (for example a 2mm sheet joined to 5mm plate in a butt joint). What's more, only moderate operator skills are required to work the process.

In terms of mechanical properties, the welded joint carries up to 90% of the base material properties, and in most cases the need for significant post-weld machining is eliminated. The optimised heat input and symmetrical heat distribution throughout the thickness of the work pieces ensures a smaller heat affected zone and significantly lower post-weld distortion compared to other types of welding.

The technology has the potential to be used across multiple industries, from thermal systems through to aerospace, rail and automotive manufacturing.

In the aerospace sector for example, friction-stir welding has been used to join the window frame of a business jet using a welded butt joint configuration as an alternative to a riveted lap-joint design. The change to the manufacturing process resulted in a 10% reduction in overall weight of the component and a 30% cost reduction.

One machine fits all

Perhaps most importantly, the DeltaN FS technology has been integrated into a range of Mazak machining centres to provide a streamlined production process which uses one machine for multiple metalworking operations.

Mazak has recently launched a combined machine tool and friction-stir welding solution incorporated into a Vertical Centre Smart 430A machining centre. The combination has the potential to significantly reduce capital investment compared to a stand-alone robotic solution. The DeltaN FS technology incorporating force control capability has also been incorporated in to Mazak's VTC range of products, which enables both machining and welding functions to be undertaken in the same platform with the help of an automatic tool changing system to switch seamlessly between welding and machining.

The proof, as always, lies in the numbers. In terms of operational costs, the welding cost per unit is significantly reduced due to the lower energy input and no requirement for filler material to make the joint. What's more, a lower rejection/rework rate, combined with the fact that all operations are being concluded on one machine, will result in further improvements to operating margins.

The combination of using a vertical machining centre with the DeltaN FS technology fits perfectly with the Mazak philosophy of using one machine for multiple metalworking operations and offers the potential for manufacturers to streamline their manufacturing processes and make significant production efficiencies.

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