Solid State Process – No Melting
Friction Stir Welding (FSW) is a Relatively New Technique for Joining Metals.
The unique characteristics of friction stir welding technology make possible completely new products and greatly improved assemblies that are currently arc welded. The key to exploiting this process is to understand how it works.
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Process Relies on Frictional Heat
Frictional heat generated by force and relative motion between the welding tool and the workpiece raises the workpiece temperature to just below its melting point. Remaining below the melting point allows frictional heating to shear the material while stirring it, resulting in a fine-grain structure with beneficial properties.
The advantage for aluminum is that since there is no significant melting, there is no need for shield gas. Also, since the weld isn’t formed in a liquid state, the shrinkage stress is greatly reduced. These facts bring huge benefits: no porosity, no shrinkage cracking, and greatly reduced distortion.
FSW tools can be thought of as having two functional components: the probe and the shoulder. The probe has special features that facilitate “stirring” the interfaces between workpieces. The shoulder acts as the “cover”, preventing the escape of material as the tool traverses the joint.
This leads to another advantage of friction stir welding over conventional fusion welding techniques. Filler metal isn’t needed, so the workpieces retain their original chemistry. This leads to superior cosmetics since there are no weld beads, eliminating the need for selecting and stocking filler metals.
Expands Product Design Options
The use of Friction Stir Welding makes new things possible.
It’s now possible to weld any aluminum alloy to any other aluminum alloy, in extrusions, forgings, castings, and rolled plates.
Cast magnesium can be welded. Two-inch thick copper can be welded in a single pass. Aluminum composites with embedded hard particles can be joined without segregation of the constituents. Aluminum can be welded to steel, copper, and magnesium. Titanium can be welded – cast to wrought to plate.
Early adopters of FSW were aluminum extrusion producers. Since FSW produces very low distortion and is fully mechanized, they found that relatively narrow extrusions could be welded edge-to-edge to produce very large integrally stiffened panels, with near-perfect flatness. These value-added products were then cut to shape to make ship decks and trailers. Bond produces a very large FSW system specifically for this application, the LS Series, which features dual welding heads that can simultaneously weld both sides of hollow extrusions, producing highly efficient structures.
Fine Grain, Beneficial Microstructure
Another unique characteristic of Friction Stir Welding is the microstructure of the stirred region. Traditional fusion welding leaves a weld zone with the microstructure of a casting – large grain size. Since FSW doesn’t produce significant melting, the plastic work actually refines the grain structure of the base. Fine grains promote high ductility, which is good for formability and corrosion resistance.
This weld microstructure makes possible other new applications. For example, the high weld formability makes it possible to produce tailored blanks, where two different alloys are welded in sheet form and subsequently stamped into shape to give different properties for different areas. The sheets can even be of different thickness, producing a blank with high strength in one area and high corrosion resistance in another.
Superior Product Performance
Friction stir welding has also been shown to greatly increase the ballistic performance of aluminum structures, especially in high-performance alloys. Damage tolerance is important for automotive battery trays, for the containment of nuclear materials, for military vehicles, and for shipbuilding.
FSW has also led to another new technology – friction stir processing. The microstructure of stir welds is so good that it can even be used as a method of reprocessing metals to generate fine grain size and high formability through the thickness or just on the surface. By making repeated, overlapping passes the original material can be transformed.
For example, a casting can be treated with an FSW tool to refine the surface and close any exposed porosity, increasing fatigue performance and making possible post-weld machining to produce a smooth surface for sealing.
Advantages & Benefits Of Friction Stir Welding Include:
- High joint quality
– Solid phase joint, no shielding gas for aluminum alloys
– Autogenous process – no filler metal
- Production friendly
– Fully mechanized process – much lower skill requirement
– “Green” process: lower energy cost, no fumes or visible radiation
- Low distortion
– Solid phase welding greatly reduces residual stress
– Results in lower costs during post-weld processing
- High fatigue strength
– Low residual stress and high joint quality greatly improve fatigue strength
- High corrosion resistance
– Low process temperature reduces localized segregation of solute elements, thus improving corrosion resistance
– Also improves corrosion fatigue properties
- Joins dissimilar alloys
– For example, FSW joins most aluminum alloys to each other. This is not possible with other welding methods. It can even be used to weld aluminum to steel.
- Strength and Ductility
– In aluminum alloys, FSW does not dramatically improve static strength when compared to arc welding for alloys that can be arc welded
– In many alloys, FSW exhibits greatly improved ductility over arc processes
