Additive assembly: the 3D printed fastener

Think of 3D printing and you may imagine flimsy, plastic structures used to test initial designs. But the technology is slowly moving away from its prototyping roots, and aerospace engineers are now hoping to prove its potential, one component at a time. 
Now some claim that 3D printing could even create new capabilities in the fasteners and joints that hold together larger structures such as aircraft.

One large aircraft currently requires nearly a million fasteners, which have to withstand extreme and constant forces. As well as being strong, these components must be flexible, lightweight and aerodynamic. Airbus Group has been working on an alternative to traditional fasteners known as ‘hyper joining’ (hybrid penetrative reinforcement). The company claims a hyper joint is around six-and-a-half times stronger than a bonded joint and can absorb around 80 times more energy. The technique also leaves the surface of the joint completely level to reduce drag.

The technology is still at an experimental stage, but Airbus is keen to develop it. The group has been working with Somerset-based manufacturer Charge Bikes to attach seat-post rails to the upper skin of a bicycle saddle using the system. Traditionally, saddle makers use glue or bolts to attach rails to a saddle’s base. A hyper joint, however, uses arrowhead-shaped pins that are an integral part of a 3D-printed component in order to interlock a metal and a composite.

These pins, typically around 1mm wide and 3–4mm tall in length, are inserted into the soft composite structure using ultrasound. The structure is then cured in an autoclave, allowing it to form a bond using composite resin making it much stronger than typical adhesives. Because the pins are 3D-printed, they can also be designed for use with different structures.

‘The Charge project was a useful learning exercise,’ says Philip Parkes, a research engineer at Airbus Group Innovations. ‘The pins we used were much smaller at around 1–2mm tall. The laminate composite was thinner and curved than configurations we had previously tested, so inserting them during the manufacturing process was a bit of a challenge. We had to develop specialist tooling to hold the pins in position and make sure they were at the right angle for consolidation and cure.’

The team at Airbus is using its project with Charge Bikes to test the technology before moving it into an aircraft. ‘We have a lot of options to play with,’ says Parkes. ‘For instance, the pin has a conical head feature that prevents pull-out. We are testing how the size of that, compared with the shaft below, affects its strength. We’re also looking at the angle when inserting them into the composite, as well as how the spacing between the pins affects performance.’

The pins penetrate through 75 per cent into the thickness of a laminate. This means the outer surface of the composite part is perfectly smooth, providing an aerodynamic advantage. ‘We were looking for the best way to use this technology,’ says Parkes. ‘On aircraft, reducing drag is a very large driver and this is one option we can use rather than a bolted connection.’

One potential issue is recycling materials that are meshed together in this way. Airbus is working on developing a solution to separate a hyper joint and Parkes is confident this won’t be a problem going forward. ‘There will be different ways to separate a composite and metallic,’ he says. ‘It is possible to separate the joint and there is existing work going on in those respective areas.’

As with most additive manufacturing technologies, 3D-printed joints are unlikely to replace all traditional components but rather offer engineers a new tool with different capabilities to make use of. 3D printers can more easily create complex shapes that may not otherwise be viable or even possible. One unusual design shape that has already been produced thanks to 3D printers is the ‘Rotite’ non-penetrative system created to, among other applications, attach mechanical and electrical connections 
in aircraft.

The Rotite fastener is something known as ‘helicoidal dovetail’, which is a classic dovetail joint used by carpenters that is rotated around an axis. This means that the two surfaces dovetail together and can be rotated against one another until the required tightness is achieved. Prototypes were creating using 3D printing, but its inventor Stuart Burns believes the technology could also create the end product.

‘In the time that we have been developing Rotite, polymer and metal technologies have also come along,’ says Burns. ‘We are in fact working right now with more than one company to blur these lines between prototyping and production technology. The trick is to understand where the strengths of the technology apply, and embrace them.

‘Post-processing, obviously something we hope to avoid in manufacturing, is now seen as an associated process to additive manufacturing technologies. When items can be metallised, it brings about a whole world of new characteristics and opportunities.’

While it may be some time before full joints are 3D printed, for now, aircraft are already using the technology for non-critical components. ‘We are on the [brink] of a step-change in weight reduction and efficiency — producing aircraft parts that weigh 30 to 55 per cent less, while reducing raw material used by 90 per cent,’ says Peter Sander of the Airbus Innovation Cell. ‘This game-changing technology also decreases total energy used in production by up to 90 per cent compared to traditional methods.’

Flying on a 3D-printed aircraft may sound like a terrifying prospect, but as the materials become stronger and more energy absorbent, the technology may just make aircraft more resilient than anything that can be made today.