The aerospace industry's high hopes for metal sintering

The last 10 years has seen marked change to the way aircraft are designed and made. While they still largely take on the general shape of a cigar tube with wings, the structure is highly optimised. One of the major changes in recent times has been the introduction of composite materials, which have helped bring about step change improvements in fuel efficiency by significantly reducing the weight of the overall structure. All of this has happened relatively quickly. So, it begs the question: are we at the leading edge of the next big technology? Additive manufacturing technologies are beginning to be used for serious aircraft production. And, it is not just development work, with a bracket connector on the Airbus A350 XWB highlighted as a forerunner application. So could additive manufacturing have a similar impact on the aviation industry over the next 10 years? "Our primary objective is to reduce weight," said Peter Sander, head of emerging technologies and concepts at Airbus. "Additive manufacturing and laser melting metals allows us to design completely new structures that can be more than 30% lighter than conventional designs using a casting or milling process." While it's unlikely to lead the industry back, on mass, to metals, what is likely is that the metal materials used in aircraft, still around 50% of the structure, can be reduced in weight by a significant amount. And this is all possible because of the high degree of geometrical freedom additive manufacturing processes allow. Frank Herzog, chief executive of Concept Laser – an additive manufacturing specialist, said: "It gives the ability to economically keep component density under control and determine the microstructure quality. Another fundamental feature is the ability to define the force distribution within the component, which is often impossible with conventional parts, or is considerably more difficult to achieve." And this design freedom offers near infinite geometric possibilities that enable outside surfaces and also internal structures to be designed, specified and optimised. And this means that parts can be made to be functional. Professor Claus Emmelmann, chief executive of Laser Zentrum Nord, said: "Additive manufacturing offers greater freedom of design since undercuts and interior channels can be used to provide cooling, for example, on a [bigger overall] design." Indeed, additive manufacturing could well bring around a paradigm shift in the way engineers can, in the not too distant future, think about the design and development of aircraft components. Professor Emmelmann added: "The advantage for structural elements used in aircraft are obvious. For the brackets we're currently focussing on, this means a considerable weight reduction, which in turn translates into lower fuel consumption or the potential to increase the load capacity of an aircraft. "In the past, compromise with lightweight construction has been necessary due to the restrictions of conventional manufacturing – restrictions we are now able to elegantly avoid." One such drawback that can be avoided is the ability to control the internal geometry of a part to remove as much weight as possible while keeping the necessary strength. The use of topography optimisation is helping engineers carry out this kind of analysis virtually, but additive manufacturing allows these complex, and often seemingly random, structures to become physical flight-ready parts. Sander said: "With laser melting we can manufacture very fine – even bone-like – porous structures. The aircraft parts of the future will look 'bionic'. Nature has optimised functional and lightweight construction principles over millions of years. We are currently investigating and analysing these solutions found in nature with regard to their applicability. Initial prototypes indicate great potential, and the process could launch a sort of paradigm shift in design and production." How big will this be? Though there are still many technical restrictions to be overcome, such as the cost-effectiveness of particular parts and the industrial availability of metal powders, it is unlikely that complete aircraft will ever be printed, even in 10 years. However, Professor Emmelmann thinks it will be used more ubiquitously in the future. He said: "I'm confident that future laser additive manufacturing will be capable of producing increasingly larger and more complex components in a cost-effective manner. This will be possible thanks to the rapid pace at which the system technology is being developed, and the increased productivity associated with such advances. I see great potential, in particular, for structural components with dimensions of up to 1m, as well as for engine components." As well as the many potential advantages in the design of components, there are also a number of potential areas during production processes that could benefit. The ability to print straight from a 3D CAD file gives a great deal of flexibility of what you make and when. And this could help slash development and procurement cycle times. Herzog said: "Additive manufacturing is generally characterised by various aspects: decentralised, rapid turnaround, quick time from implementation to the finished component. It also allows for lower logistics and warehousing costs. It uses fewer resources than conventional manufacturing methods, which makes it a green technology. "With regard to the safety aspects of components, engineers are discovering that 3D printing offers many answers to problems posed by force absorption, required durability, high quality standards and also bionic design." Additive processes are set to shorten development time by as much as 75%. Airbus, for example, currently budgets around six months to develop a component – but with additive manufacturing it claims the same parts could be developed in just one month. Professor Emmelmann said: "It's now possible to produce functional samples of components that are very close to being ready for series production without incurring the high cost of tools or other pre-production expenses. It means sources of error can be identified in the early stages of the design process, which allows for optimisation much earlier." Production benefits While milling an aircraft part has been a standard process since the early days of flight, it does result in as much as 95% of a solid billet being machined away. So the process has the potential to cut waste and offer economic advantages. Laser melting offers the potential to produce components with near-final contours, and by contrast needs just 5% of its surface to be machined away to form the finished part. This makes the process especially attractive when using expensive materials, such as titanium. Although the resulting swarf is recyclable, it is a process that incurs cost and resource. Professor Emmelmann said: "In aircraft manufacturing, we work with the 'buy-to-fly' ratio, and 90-95% is a fantastic figure. The process generally results in positive effects for manufacturing costs for small to medium-sized unit quantities as the higher investment costs for casting moulds are eliminated, as is the cost of tooling." Initial studies by Airbus show that the number of manufacturing steps necessary could be cut in half, since the process yields near-net-shape parts. The resulting components can also be welded, meaning the constriction of smaller print tray sizes can be overcome. Professor Emmelmann said: "Since we do not require any special tools or clamps for the additive manufacturing process, we can produce the component directly from the 3D CAD data. This time factor ensures that in many cases we can work considerably faster than we can with conventional manufacturing processes. "If the direct cost of manufacturing a milled component is compared with the cost of manufacturing the same component using laser additive manufacturing, the additive process is usually found to be less cost-effective. However, when the components are redesigned, so they are lighter or have a higher functional performance characteristic, there are already many examples of circumstances in which the use of additive manufacturing processes offer cost advantages." Time for a rethink Similar to aircraft structures, Airbus is currently rethinking the entirety of aircraft systems with laser additive manufacturing processes in mind. "We are facing a continent of opportunities and options," said Sander. "We are entering a new territory, one with fascinating opportunities on the horizon. Initial prototypes produced in our development work show significant potential in terms of reducing costs and weight. Functional integration is one of the possible new options." The comparatively small unit quantities involved in aircraft manufacturing could actually favour laser additive manufacturing techniques. The additive manufacturing process does not allow for the advantage of any economies of scale, as is the case for other production methods. It means that unit costs change only very slightly as production volumes increase. Conversely, conventional production methods, such as pressure casting, are more cost-efficient for producing large unit quantities. But additive manufacturing is still a technique in its infancy, with much development still to come. Herzog said: "Laser output has been increased to 1000W while the assembly speed for aluminium parts has been accelerated by a factor of 10 to 15. In my view, this represents fantastic progress. With very large components, the internal stresses in the part increase due to the nature of the process. "The limits can be expanded further with an intelligent joining technique in the sense that assembly could play an important role for large components that are to be manufactured cost-effectively. This makes it possible to develop components with large volumes and extremely long components that extend beyond the assembly space sizes offered by current or future laser melting systems." Though larger parts may well be possible in the future, from a cost-effectiveness standpoint, smaller components that can be manufactured with the systems already available seem to offer the most immediate advantage. Potential for existing aircraft Spare parts for aircraft are considered a necessary cost-intensive and logistical challenge due to the long lifecycles of many components. The problem, to meet the challenges of global availability, warehouse stock, lifecycle and time pressure is one exciting area where additive technologies could make an impact on existing aircraft, not just offer potential to those in the future. Spare parts can be manufactured where they're needed, without tooling, and on demand, instead of having to fill large warehouses to store rarely needed items around the world. Sander said: "Since February of this year, Air Transat in Montreal has been flying with the first spare part printed and delivered by Airbus. "The former manufacturer of the injection-moulded part for a Cabin Attendant Seat in an A300/310 was no longer available and the tools had been scrapped. The question we faced was whether to invest in new tools, at a cost of $36,000 or take advantage of 3D printing? By using the laser melting process, we were able to offer the part at a cheaper price from the outset, without tool costs. As a result, we no longer store hundreds of parts in warehouses, but instead, we will operate decentralised spare parts printing centres and only manufacture plastic spare parts when they are requested. A similar strategy is being pursued for metal components."