Aerospace manufacturers need to throw away the rulebook and review their entire approach to design engineering to take account of recent advances in metal 3D-printing technologies. Failing to do this could mean they miss out on opportunities to reduce cost and drive organisational value.
As well as learning more about the capabilities of new-generation 3D printers, design engineers in the sector need to start thinking differently about how and when to apply them, as they are increasingly being used alongside traditional manufacturing processes.
In the past, manufacturers have been put off from investing in metal 3D-printing systems for a variety of reasons, including: productivity; quality assurance; and the need for post-processing or finishing. Size limitations have also been a factor, as most early metal 3D printers could only produce parts up to 300mm3. For the aerospace industry, concerns about the reliability of 3D-printing machines to produce parts consistently, capable of fulfilling OEM approvals, has been an important factor.
The development of 3D printers that use selective laser melting (SLM) to produce intricate or complex parts to a high standard of repeatability is encouraging engineers in the high-value manufacturing sector to take the technology more seriously. Manufacturers may use their own terminology, but the fundamental process uses a bed of metal powder to create a layer of material on a plate, the metal powder is then melted where needed using a laser. This process is repeated a layer at a time until a complete part is manufactured.
These machines are continuously developing and are increasing in speed by using multiple lasers. They are also increasing in size to be able to produce components that can now be up to 1m long, using scalable architecture, which should allow future expansion.
Historically, even the same additive machine might produce variations in properties from one part to the next. The laser power used is just one of the many factors that affect the properties of the final component. Critical to these properties is the temperature that the material reaches and how quickly it cools. The industry has worked on significantly improving control with manufacturers such as Renishaw able to gather live data from melt pool process emissions to ensure repeatability. As a result, we are seeing more additive components appear, such as the fuel nozzles on GE’s LEAP Engines and wing brackets on the Airbus A350WXB.
The role of the metal powders used in manufacturing is critical to the properties of any part produced and improvements in the understanding of their key characteristics is also improving repeatability. Early specifications for metal powders looked at how quickly the material flowed through a defined funnel. Today’s powders are carefully assessed for particle size distribution, particle shape, chemical composition and impurities.
Another key factor is the ability to re-use powders. Using SLM technology, the majority of metal powder used in the manufacturing process does not actually form part of the final component. Where this powder has been heated as it sits adjacent to molten metal, its properties can change. For this reason, controls are needed regarding the reuse of powders to avoid a deterioration of performance. It would not be sensible to discard all the ‘unused’ powder but equally it cannot be infinitely recycled. This improved definition of the raw material input and recycling controls have been crucial in improving repeatability.
Despite its many advantages, one disadvantage that SLM has is that it cannot be used to repair or add to an existing component. This however, can be done using blown powder technology, which uses a laser to create a melt pool in the initial solid material (as opposed to powder) and then uses a nozzle and a carrier gas to ‘blow’ metal powder into it. By moving the component, or the nozzle (or both), additional material can be added to the original, creating a 3D component. This technology is a derivative of laser cladding and has many similar characteristics to welding, but has the advantage of far greater control, with consequently minimal heat-affected zone and the ability to influence grain size and direction.
It is possible for certain designs to be made much more cheaply than using traditional manufacturing processes. For example, if a company wanted to produce a 300mm diameter stainless steel pipe, which could be available off the shelf from many suppliers, but needed to have integral handles or other additional features, they would normally have to purchase a solid block or perhaps a pipe with far greater wall thickness than required. This would then need to be machined away. Using blown powder techniques, the manufacturer can use standard raw material and simply add the additional features without compromising metallurgical, structural and dimensional integrity.
Designing for the aerospace industry
When it comes to deciding which parts to 3D print, most aircraft makers are reluctant to share information with competitors. Although it is clear that certain parts can create more value added through printing than others, it is sensible to start with parts that are less critical and gain experience of designing for this manufacturing technology, before moving onto more challenging components.
To utilise 3D-printing technologies to the full, a change of mindset is required. To date, much of the innovation activity in the aerospace industry has focused on incremental changes – small steps that are intended to reduce waste, minimise use of space and conserve fuel. This approach is considered ‘safe’, as it can be assumed that parts that have qualified previously are more likely to do so again if modified.
However, the potential of 3D-printing technologies can’t be fully realised by simply copying what has gone before and design engineers need to start rethinking their approach. Modern additive machines are capable of manufacturing increasingly complex metal parts, to a high standard and with improved structural properties. Some of these designs would be impossible to make any other way. With the backing of improved software and a more open-minded approach to design and development, these important benefits should become a standard part of aerospace manufacturing programmes in the future.
Ian Joesbury is a vice president of the Institute of Mechanical Engineers and a senior consultant at Vendigital. For more information visit www.vendigital.com