Friction Stir Welding Process & Its Advantages [micro-stir]

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Wayne Thomas invented friction stir welding at TWI in 1991, and it was patented in the USA, Japan, Europe, and Australia. Soon after the discovery, TWI recognized the potential and started working on ‘Development of the New Friction Stir Technique for Welding Aluminum.’

The process is primarily used in industry to join aluminum alloys of all grades, and it took off pretty quickly. Therefore, FSW (Friction Stir Welding) went faster from invention to industrial application than other welding methods.

Featured image for the friction stir welding article

But what does Friction Stir Welding represent? This article describes mechanical properties, applications, the good, and the bad.

What is FSW?

Friction stir welding (FSW) is a relatively new solid-state joining process. This method uses frictional heat generated by a rotating tool to join materials. Therefore, there are no fusion or filler materials involved. So how does FSW work? 

First, the non-consumable tool, with a profiled probe and shoulder, is rotated and plunged into the interface between two workpieces. Next, it traverses along the joint line, causing the material to heat and soften. Finally, the shoulder also acts to contain this plasticized material, which is mechanically mixed to create a solid weld.

The microstructure of a friction-stir weld depends in detail on the tool design. That’s why there are different rotation and translation speeds, the applied pressure, and the characteristics of the material being joined.

Today, over 40 different types of welding tools have been developed for thin-walled structures, heavy & thick-walled structures, self-adaptive bobbin tools, standard bobbin tools, and retracting pin FSW tooling.

Mechanical Properties of the Welds

As we mentioned, the microstructure of friction stir welds mostly depends on the tools, but the mechanical properties of the welds are proven better than some arc welding processes.

Regarding the properties, we should mention several zones: heat-affected zone (HAZ), nugget zone, and a thermo-mechanically affected zone.

The heat-affected zone (HAZ) is very similar to conventional welds. HAZ is straightforwardly defined as the area between the weld and the base (unaffected) parent metal.

On the other hand, nugget zone and TMAZ are considered separately when it comes to the microstructural features. The central nugget region is the most severely deformed. That’s why the microstructure may consist of equiaxed grains.

diagram of friction stir welding process
image shows the schematics of the process and the resulting welding zone. Source: sciencedirect.com

Meanwhile, the thermomechanically-affected zone lies between the HAZ and nugget. However, TMAZ does not experience dynamic recrystallization. The top surface of the weld has a different microstructure, which is usually affected by the rotating shoulder.

Friction Stir Welding Application

In particular, this process can be used to join high-strength aerospace aluminum alloys and other metallic alloys that are hard to weld by conventional fusion welding. As a result, FSW has been shown to weld aluminum alloy butt joints 0.3mm and 75mm thick in a single pass.

Besides aluminum, FSW can weld magnesium, titanium, copper, nickel, and steel alloys. In addition, research in plastics and metal matrix composites (MMC) has also been concluded and proven positive.

Besides welding, there are a few innovative applications of the friction stir method.

With all the given advantages, Friction stir processing can be used in various industries. The spectrum extends from the automotive industry, including the e-mobility market and aerospace through the electronics industry. FSW is also not rare in shipbuilding and rail industries and in joining EV battery trays.

FSW in Joining Steel

Even though FSW is mainly developed and used for hard-to-weld aluminum alloy applications, it can still be used in joining steel and other metallic and non-metallic materials. However, there are a few drawbacks when it comes to steel welding. 

Due to the mechanical properties of the steel, the FSW tools can wear somewhat faster. Thus, the wear and debris can reach the weld, impacting the strength of the joints.

Since there are many suitable steel-joining methods, many choose traditional welding. Nonetheless, there are specific applications where FSW can be used in steel welding.

Friction Stir Welding vs. Traditional Welding

The frictional heat between the wear-resistant welding tool and the workpieces causes the base material to melt without reaching the actual melting point. Therefore, FSW doesn’t require any shielding gas or filler wire.

When preparing welding procedure specifications, the standard components such as current and voltage are not present. Instead, the heat input is purely mechanical and caused by force, friction, and rotation. As a result, the materials are joined at about 80-90% of the base material’s melting temperature, leading to significantly stronger welds.

With the absence of filler material and shielding gas, the most significant advantage over the traditional is the lack of welding defects such as pores or cracks.

Compared to traditional welding, FSW is more cost-efficient as there are no consumables. In addition, less skill is required to master the technique. One of the noticeable advantages that boost productivity and speed is the excellent single-pass results with low to no downtime. 

image of friction stir rotating tool
image shows the rotating head of the friction stir welding process. The necessary tools and mechanics are radically different compared to arc welding processes. Source: sciencedirect.com

Friction Stir Welding Advantages and Drawbacks

While we might already mentioned some advantages when comparing FSW with traditional welding, here is a list of all the good and the bad sides of this material joining process.

Advantages:

  • Defect-free joining method with no hot cracking, porosity, or solidification cracks,
  • Joining a wide variety of materials, including aluminum, magnesium, titanium, copper, nickel, and steel alloys,
  • Low friction stir temperature reduces the shrinkage and distortion in the base material, which is exceptionally significant when dealing with thin materials,
  • There are no consumables such as filler material, shielding gas, or flux,
  • The economical joining of complex 3D geometries results in material savings through component optimization,
  •  The process produces no fume, spatter, or UV radiation which makes it notably safer than traditional welding,
  • Can join many ‘non-weldable’ aluminum alloys, such as those from the 2xxx and 7xxx series,
  •  The process is easy to automate and highly repeatable, which increases cost-efficiency.

Drawbacks:

  • The tool leaves an exit hole when withdrawn from joined materials,
  • The gaps must be accounted for in the design,
  • All the parts must be clamped sturdily since there are excessive downforce and traverse forces are present,
  • Since there is no filler material, all the gaps between the joined materials must be strictly controlled,
  • High initial or setup cost,
  • It can be less flexible than traditional arc welding processes

Sources:

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Adam Mason

Welder by trade for a decade and more. Now also a web designer and a blog owner. Doing product reviews and writing blogs about welding trade and perks and minuses of being a welder.

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