Lesser-known Friction-Stir-based AM technologies are aimed at large-scale critical components of simple geometries. Used by the main commercial constructors for the manufacturing of key aircraft structures (floor, wings), they can also be used for cost-effective repairs of legacy piece parts for larger civil infrastructure.

Friction stir-based AM technologies are a set of solid-state processes working below melting temperatures. These technologies benefit from lower residual stresses and lower susceptibility to porosity and hot cracking. They are promising alternatives to conventional forging for manufacturing or repair of large structures for aerospace, naval, nuclear, and automotive applications.

Typical applications

With friction stir AM technologies, mechanical properties can easily be customised, but design flexibility is limited. The most suited applications typically require high performing structural properties combined with simple geometries [1].

Large scale single alloy preforms for aerospace

high strength aluminium alloys [1-3]

High-strength aluminium alloy (> 400 MPa) structures (eg, Al alloys (2XXX and 7XXX)) tend to suffer from hot cracking issues when built using fusion-based additive techniques. As a solid-state process based on plastic deformations occurring below melting temperatures, friction stir AM technologies circumvent these issues.

A study by Boeing [2] showed that using linear, rotational or friction stir welding AM to manufacture large scale pre-forms estimated a volume reduction of nearly five billion pounds of aluminium material and emission of 60 billion pounds of CO2 over the next 25 years.

For the aerospace industry, where aluminium consumption reaches ~630 million pounds / year, accounting for 48% of total raw material used annually, using friction-based AM could help shift the industry towards a more sustainable approach to building large pre-forms [1, 3].

High-performance stiffener/stringer configurations found in different aircraft components can be fabricated using FB-AM:  sheets are assembled to create transverse stiffeners on an I-beam using a combination of friction stir welding for the T-joint and friction stir AM for the stiffener. Integrated stringer can be built on a skin panel for aircraft fuselage or wing spars [1].

The friction mechanisms in FB-AM AM processes naturally induce microstructure refinement (see picture of WE43). This is beneficial for alloys that possess high Hall-Petch strengthening ability (where strength increases as grain size decreases), such as the majority of structurally pertinent lightweight Mg alloys and high temperature titanium alloys and steels.

“The Hall–Petch relationship tells us that we could achieve strength in materials that is as high as their own theoretical strength by reducing grain size. Indeed, their strength continues to increase with decreasing grain size to approximately 20–30 nm where the strength peaks.” [4]

Dissimilar alloys, graded metal structures

It is also possible to build structures made of multiple materials. Intense friction and shearing motions during FBAM contribute to controlling the size and distribution of intermetallics at the interfaces of different materials.  Besides, thermal expansion mismatch is no longer an issue during solid state manufacturing. This is what makes FBAM a technically viable route for graded structures.

Large components repair [5]

Large components, such as hydroelectric turbine runners, moulding dies, power generator gearbox components, and legacy infrastructure (e.g., I-beams, railways, and various struts, frames, and supports) are commonly several cubic meters in volume. They embody large amount of energy and can be difficult to replace. Their manufacturing also requires long lead times.

Shallow surface damage can be repaired by cold spray [link], another solid-state AM method.

If damage is of a much larger scale, Friction Stir Deposition AM can be used to take advantage of large tool sizes and high build rates (up to ~101kg/hour for steel and aluminium). Aluminium ring structures of ~3m in diameter can be built [6]. As a solid-state process, Friction Stir Deposition AM can repair both weldable and nonweldable materials, while limiting residual stresses and without affecting the surrounding materials, which could lead to accelerate the decay of the component post-repair. Often, old infrastructures are made of alloys different to those used in modern design. Even if these alloys may no longer be available, friction AM can still be used for their repair with dissimilar materials.

Friction stirring not only results in strong interfacial mixing and bonding but can also effectively remove the surface dirt, oxides, and corroded materials, possibly reducing the machining efforts. Even when brittle intermetallic phases are formed during dissimilar material deposition, friction stirring likely breaks these phases into globular shapes to mitigate stress concentration.

Future niche applications: underwater repairs

Underwater repairs and manufacturing (proven in a lab environment [7] could prove beneficial for critical applications such as ship and submarine hulls, propellers, etc.

Typically performed using fusion welding, the repair of these parts is liable to high residual stress, hot cracking and short feeding. Automated Friction Stir Deposition AM could not only mitigate technical repair issues but also the risks associated with human underwater operations.

Takeaway

Friction stir based additive manufacturing (AM) are well suited for the manufacturing and repair of large, high-performing structural components with simple geometries. With high throughput and less wastage, FB-AM can also join dissimilar materials. These solid-based methods typically work at temperature below melting points that mitigate residual stresses.

References:

[1] Palanivel, S., Sidhar, H. and Mishra, R.S. (2015b) ‘Friction stir additive manufacturing: route to high structural performance’, JOM, Vol. 67, No. 3, pp.616–621.

[2] Technical Report On: Production of Energy Efficient Preform Structures, The Boeing Company, Huntington Beach, CA.

[3] Sivanesh Palanivel and Rajiv S. Mishra Building without melting: a short review of friction-based additive manufacturing techniques Int. J. Additive and Subtractive Materials Manufacturing, Vol. 1, No. 1, 2017

[4] Ref: S.H. Whang, in Nanostructured Metals and Alloys, 2011

[5] Ryan B. Gottwald, R. Joey Griffiths, Dylan T. Petersen, Mackenzie E. J. Perry, and Hang Z. Yu Accounts of Materials Research 2021 2 (9), 780-792

[6] Yu, H. Z.; Mishra, R. S. Additive Friction Stir Deposition: a Deformation Processing Route to Metal Additive Manufacturing. Mater. Res. Lett. 2021, 9 (2), 71−83.

[7] Griffiths, R. J.; Perry, M. E. J.; Hartley, W. D.; Garcia, D.; Yu, H. Z. Augmented Repair via Additive Manufacturing 2020. https:// http://www.herox.com/iamhydro/round/656/entry/33593 (accessed Apr 20, 2021).