Aircraft wings built with 3D printers could be a step closer thanks to a technique developed by BAE Systems.
The company has developed a process to prevent large metallic structures made using additive manufacturing from distorting or building up internal stress during printing, potentially paving the way for making components strong enough to use in aircraft.
The technique involves an established way of making metal parts stronger by rapidly and repeatedly striking them using an ultrasonic tool – a form of peening – applied as each layer of the component is laid down by the 3D printer in order to relieve stresses and improve the material’s microstructure.
Scaling up the process to large and complex metallic parts could also the enable the creation of more complex shapes – for example hollow and therefore lighter wings – and make production of a small number of these components more cost effective.
But increasing the size of 3D printed parts raises the chance that the internal stresses that build up as the material cools will lead to problems, said Andy Wescott, a senior research scientist at BAE’s Advanced Technology Centre.
‘As the material contracts it creates residual stress that is locked into the part,’ he told The Engineer. ‘This can manifest as distortion but also affects the mechanical performance.’
With BAE’s technique, a layer of material is laid down, melted into shape using a laser and left to cool before being cold worked using the ultrasonic impact treatment (UIT). The next layer is then deposited, heated and the process starts again.
‘It’s not just the cold working step that produces the positive result,’ said Jagjit Sidu, the ATC’s technical leader for additive manufacturing. ‘It’s the combination of the cold working with the heat treatment. It all becomes part of the deposition process.’
UIT is typically used to treat welded areas in metallic structures in the rail and oil and gas industries to increase their fatigue life.
An ultrasonic transducer causes the tool head to vibrate at a very controllable rate and impart powerful compression forces while only placing a small load on the tool itself, allowing it to be handheld – or in this case fitted to a robot, meaning it could be easily integrated with additive manufacturing systems.
The BAE team, which also included group leader for materials engineering Stephen Morgan, developed a feedback system that uses load cells in the base plate, onto which the structure is printed, to measure the strains as they form and adjust the UIT to correct them in real time.
Asked how much time the treatment added to the 3D printing process, Westcott declined to give a specific figure but said: ‘It is a quick manual or automated process. From the measurements obtained from our strain measuring system, the surface quickly becomes saturated and no further amount of treatment has any effect.
‘Obviously, if you treat every layer it is a longer process than if you treat every five layers, but optimising the process will be the subject of further work.’
The company has demonstrated the technique by producing 3D printed structures up to 1m in length and has applied for patents on the UIT process and the feedback system.
Now the researchers plan to further optimise the process and build a better understanding of where to apply the UIT in larger and more complex parts, in order to be able to move towards prototype components.